DE10336386A1 - Absorptive separation of acrylic acid, useful for the production water absorbents and adhesives, comprises recycling organic raffinate from the acid water extraction to the absorption unit at least one theoretical stage below its head - Google Patents

Abstract

Process for the absorptive separation of acrylic acid from a product gas mixture, resulting from a heterogeneously catalyzed gas phase oxidation of propene comprises cooling of the product mixture and feeding to an absorption unit with recycling of the organic raffinate from the acid water extraction to the absorption unit whereby the recycle is carried out at least one theoretical stage below the head of the absorption unit. Process for the absorptive separation of acrylic acid from a product gas mixture, resulting from a heterogeneously catalyzed gas phase oxidation of propene to acrylic acid and containing acrylic acid and water vapor as well as by-products comprising low boiling material, middle boiling material, high boiling material and material that is difficult to condense, comprises cooling of the product mixture; passage of the cooled product mixture by counter current flow into an absorption unit containing an ascending high boiling point liquid hydrophobic organic absorption agent with avoidance of the formation of a liquid aqueous phase; cooling of the non-absorbed residual product mixture in a condensation unit with condensation of the water vapor and low boiling point material as acid water; recycling of a partial quantity as a diluent gas to the gas phase oxidation of propene with a partial quantity used as a stripping gas after passage through a washing unit to remove acrylic acid and disposal of the remaining gas; passage of a small amount of the absorbed liquid containing acrylic acid to a distillation unit as a high boiling point outlet that is recycled, as a vapor phase by heating, back to the absorption unit and the remaining high boiling point material is disposed of; the liquid outlet from the absorption unit containing acrylic acid is fed to a desorption unit and low boiling point material removed by the stripping gas with recycle of the stripping gas containing low boiling point material to the absorption unit; passage of the absorption agent stream containing acrylic acid to a distillation column having a concentrating and a distillation compartment with condensation of the absorption agent and high boiling point material in the sump of the distillation column; separation of crude acrylic acid via a side take-off from the concentrating compartment of the distillation column; recycle of the low boiling point materials from the head of the column to the absorption unit; removal of the absorption agent from the sump of the distillation column with optional addition of fresh absorption agent as required and passage through the wash unit in counter-current flow to the recycle gas flow to remove acrylic acid; partition of the absorption agent leaving the wash unit into two portions having the same composition whereby the larger portion is recycled to the head of the absorption unit and the smaller portion is mixed with acid water in an extractive exchange in order to take up the acrylic acid contained in the acid water into the absorption agent and the polar middle boiling material contained in the absorption agent into the acid water; recycle of the organic raffinate from the acid water extraction to the absorption unit whereby the recycle is carried out at least one theoretical stage below the head of the absorption unit.

Description

• – das Produktgasgemisch auf die für die Absorption gewünschte Temperatur abkühlt,
• – das abgekühlte Produktgasgemisch in einer Absorptionseinheit zum Zweck der absorptiven Abtrennung der Acrylsäure zu einem in der Absorptionseinheit absteigenden hochsiedenden flüssigen hydrophoben organischen Absorptionsmittel unter Vermeidung des Anfalls einer flüssigen wässrigen Phase im Gegenstrom führt,
• – das nicht in das Absorptionsmittel absorbierte verbleibende Restproduktgasgemisch in einer Kondensationseinheit abkühlt und darin enthaltenen Wasserdampf sowie Leichtsieder als Sauerwasser auskondensiert,
• – das bei der Sauerwasserkondensation verbleibende, insbesondere die Schwerkondensierbaren enthaltende Restgas als Kreisgas in Teilmengen gleicher Zusammensetzung aufteilt,
• – eine Kreisgasteilmenge als Verdünnungsgas in die heterogen katalysierte partielle Gasphasenoxidation von Propen zu Acrylsäure rückführt, eine andere Kreisgasteilmenge in in einer Wascheinheit von Acrylsäure frei gewaschenen Form als Strippgas verwendet und die verbleibende Kreisgasmenge entsorgt,
• – eine kleine Teilmenge des die Acrylsäure absorbiert enthaltenden Flüssigkeitsablaufs der Absorptionseinheit einer Destillationseinheit als Schwersiederauslaß zuführt, die durch Erwärmen des Flüssigkeitsablaufs in selbiger erzeugte Dampfphase in die Absorptionseinheit rückführt und die dabei flüssig verbleibenden Schwersieder entsorgt,
• – aus dem übrigen der Absorptionseinheit effektiv entnommenen, die Acrylsäure absorbiert enthaltenden Flüssigkeitsablauf der Absorptionseinheit in einer Desorptionseinheit durch Strippen mit der in der Wascheinheit von Acrylsäure frei gewaschenen Kreisgasteilmenge Leichtsieder entfernt,
• - mit Leichtsiedern beladenes Strippgas in die Absorptionseinheit rückführt,
• – den aus der Desorptionseinheit ablaufenden mit Acrylsäure beladenen Absorptionsmittelstrom einer Rektifikationseinheit mit Verstärkungs- und Abtriebsteil zuführt,
• – im Abtriebsteil der Rektifikationseinheit das Absorptionsmittel und Schwersieder in den Sumpf der Rektifikationseinheit kondensiert,
• – im Verstärkungsteil der Rektifikationseinheit über Seitenabzug eine rohe Acrylsäure abtrennt,
• – den über Kopf des Verstärkungsteils abgetrennten Leichtsiederstrom in die Absorptionseinheit rückführt,
• – aus dem Sumpfbereich der Rektifikationseinheit das von Leichtsiedern und Acrylsäure befreite Absorptionsmittel entnimmt, bei Bedarf mit frischem Absorptionsmittel ergänzt und in der Wascheinheit im Gegenstrom zur noch Acrylsäure enthaltenden Kreisgasteilmenge führt, um diese von Acrylsäure frei zu waschen,
• – das in der Wascheinheit ablaufende Absorptionsmittel, gegebenenfalls nachdem es über die als Schwersiederauslaß eingesetzte Destillationseinheit geführt wurde, in zwei Teilmengen gleicher Zusammensetzung aufteilt, die größere der beiden Teilmengen auf den Kopf der Absorptionseinheit rückführt und die kleinere Teilmenge mit Sauerwasser in extraktive Wechselwirkung bringt, um im Sauerwasser enthaltene Acrylsäure in das Absorptionsmittel und im Absorptionsmittel enthaltene polare Mittelsieder ins Sauerwasser aufzunehmen und
• – das bei der Sauerwasserextraktion anfallende organische Raffinat in die Absorptionseinheit rückführt.

The present invention relates to a process for the absorptive basic separation of acrylic acid from the product gas mixture of a heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid which, in addition to acrylic acid and water vapor, contains low boilers, medium boilers, high boilers and heavy condensables as secondary constituents, in which

• - the product gas mixture cools to the temperature desired for absorption,
• The cooled product gas mixture leads in an absorption unit for the purpose of absorptively separating the acrylic acid to a high-boiling liquid hydrophobic organic absorption medium descending in the absorption unit while avoiding the occurrence of a liquid aqueous phase in countercurrent,
• The residual product gas mixture not absorbed into the absorbent cools in a condensation unit and the water vapor and low boilers contained therein are condensed out as acid water,
• Divides the residual gas remaining in the acid water condensation, in particular containing the heavy condensables, as recycle gas into portions of the same composition,
• - recirculates a part of the circulating gas as dilution gas into the heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid, uses another part of the circulating gas in a form which has been freed of acrylic acid in a washing unit as stripping gas and disposed of the remaining amount of circulating gas,
• Feeds a small portion of the liquid drain containing the acrylic acid absorbed to the absorption unit of a distillation unit as a high boiler outlet, which returns to the absorption unit by heating the liquid drain in the vapor phase it produces and disposes of the high boilers remaining liquid,
• - from the rest of the absorption unit, the liquid outlet of the absorption unit, which contains acrylic acid absorbed, is removed in a desorption unit by stripping with the partial gas amount of circulating gas which has been washed free of acrylic acid in the washing unit,
• - returns stripping gas loaded with low boilers to the absorption unit,
• Feeds the stream of absorbent loaded with acrylic acid from the desorption unit to a rectification unit with a reinforcing and stripping section,
• - In the stripping section of the rectification unit, the absorbent and high boilers are condensed into the bottom of the rectification unit,
• - in the reinforcement part of the rectification unit, a crude acrylic acid is separated off via a side draw,
• - recycle the low boiler flow separated off at the top of the reinforcement section into the absorption unit,
• - from the sump area of the rectification unit removes the absorbent freed from low boilers and acrylic acid, if necessary, replenishes it with fresh absorbent and in the washing unit countercurrently leads to the portion of circulating gas still containing acrylic acid in order to wash it free of acrylic acid,
• - The absorbent running off in the washing unit, if necessary after it has been passed through the distillation unit used as the high boiler outlet, is divided into two portions of the same composition, the larger of the two portions is returned to the top of the absorption unit and the smaller portion is interacting with acidic water in order to acrylic acid contained in the acid water in the absorbent and polar medium boilers contained in the absorbent in the acid water and
• - The organic raffinate obtained during the acid water extraction is returned to the absorption unit.

Acrylic acid is very much because of it reactive Double bond and their acid function a valuable monomer for the production of polymers that e.g. as water-absorbent resins, as binders for aqueous emulsion paints or in watery Medium disperse are suitable as adhesives.

Among other things, acrylic acid is accessible through heterogeneously catalyzed partial gas phase oxidation of propene and / or acrolein with gases containing oxygen or oxygen in the presence of solid catalysts and at elevated temperature. Especially to cope with the high heat of reaction, the reactants in the reaction gas mixture are preferably diluted with inert gases and / or water vapor. The propene used in large-scale industrial processes is usually a propene of the “chemical grade” or “polymer grade” purity, as is commercially available on the propene market. To improve the selectivity, the heterogeneously catalyzed partial oxidation of propene to acrylic acid, which naturally occurs in two successive steps (the first reaction step converts the propene to acrolein and the second reaction step converts acrolein to acrylic acid) are often carried out in two spatially separated reaction stages, in which the respective reaction conditions and catalytic active materials are ideally adapted to the respective reaction step can be (in principle, the aforementioned partial oxidation can also be carried out in one reaction stage). In the first reaction stage, in which the heterogeneously catalyzed partial oxidation of propene to acrolein takes place, the catalytic active materials in generally oxidic multi-component systems based on molybdenum, bismuth and iron, such as z. B in EP-A 1 005 908, EP-A 700714 and in DE-A 19501325 are used. In the second reaction stage, in which the heterogeneously catalyzed partial oxidation of acrolein to acrylic acid takes place, oxidic multicomponent systems based on molybdenum, vanadium, tungsten, copper and optionally antimony, as described, for example, in EP-A 1, are generally used as catalytic active compositions 071 507 and described in EP-A 700893 are used.

In this process, however, not pure acrylic acid but a product gas mixture is obtained which, in addition to acrylic acid as secondary components, contains unreacted acrolein and / or propene as well as components such as water vapor, carbon oxides (CO; CO ₂ ), nitrogen, oxygen, propane, methane, and lower saturated carboxylic acids such as Contains formic acid, acetic acid and propionic acid, lower aldehydes such as formaldehyde, benzaldehyde and furfural and higher carboxylic acids or their anhydrides such as benzoic acid, phthalic anhydride and maleic anhydride. Some of these secondary components are more volatile than acrylic acid (eg acetic acid) and are referred to here as low boilers. Another part is less volatile than acrylic acid (e.g. phthalic anhydride) and is referred to in this document as a high boiler (but this term also includes compounds that only form during the removal of acrylic acid, such as acrylic acid oligomers formed in the liquid phase by Michael addition ), while the term middle boilers (e.g. maleic acid) means secondary components that have a similar volatility to acrylic acid. The term "heavy condensable" summarizes the secondary components which, like nitrogen, are much more volatile than water. The acrylic acid then has to be separated from this product gas mixture.

The basic separation of acrylic acid from the gaseous product gas mixture is usually carried out by sorptive and distillative processes. For example, by countercurrent absorption of acrylic acid in a (preferably inert) high-boiling liquid hydrophobic organic solvent or solvent mixture, avoiding the formation of a liquid aqueous phase, and several subsequent distillative workup steps, as described, for example, in the documents DE-A 21 36 396 . DE-A 24 49 780 . DE-A 43 08 087 . DE-A 44 36 243 . DE-A 196 06 877 and EP-A 1 125 912 is described. According to the scriptures DE-A 21 21 123 . DE-A 34 29 391 , EP-A 0 009 545, EP-A 1 041 062, EP-A 1 065 197, EP-A 1 097 916 and US-A 5 154 800 is first absorbed with water or aqueous acrylic acid in countercurrent and then distilled extractively or azeotropically (the term distillation is used in this document as a generic term and is also intended to include the rectifications having more than one theoretical plate).

EP-A 117146 relates to a method for the separation of acrylic acid from the product gas mixture of the partial oxidation by absorption of acrylic acid in an absorption column operated with water. By extraction with ethyl acetate the acrylic acid from the watery liquid drain separated from the absorption column and from the acrylic acid extract over the sump rectified.

According to Ullmann's Encyclopedia of Industrial Chemistry, Ed. 5, vol. A1, pages 166 and 167 and in accordance with EP-A 1 125 912 is the use of a high boiling liquid hydrophobic Absorbent particularly advantageous.

Suitable high-boiling (inert) liquid hydrophobic organic absorbents (absorbent) (also for the process according to the invention) are, for example, those of EP-A 722926 and those of DE-A 4436243 into consideration. These are liquids whose boiling point at normal pressure (1 atm) is above the boiling point of acrylic acid and which consist of at least 70% by weight of those molecules that do not contain any polar group that acts on the outside and are therefore unable, for example, to form hydrogen bonds to build. Particularly favorable absorbents are described as Diphyl ^® mixtures of diphenyl ether (70 to 75 wt .-%) and biphenyl (25 to 30 wt .-%). A very particularly favorable absorption medium is a mixture of a mixture of 70 to 75% by weight of diphenyl ether and 25 to 30% by weight of diphenyl and, based on this mixture, 0.1 to 25% by weight of dimethyl phthalate.

In the DE-A 19606877 Such high-boiling (inert) liquid hydrophobic organic absorbents (absorbent) are also used in a process as described at the outset for the basic separation of acrylic acid from the product gas mixture of the heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid.

Characteristic of the procedure of the DE-A 19606877 is that according to all the figures the DE-A 19606877 the organic raffinate obtained during the acid water extraction is returned to the top of the absorption unit.

A disadvantage of this procedure is that with it too high acrylic acid losses go hand in hand with the sour water extract.

The object of the present invention therefore consisted of an improved as described at the beginning Processes for basic separation of acrylic acid are available too put.

• – das Produktgasgemisch auf die für die Absorption gewünschte Temperatur abkühlt,
• – das abgekühlte Produktgasgemisch in einer Absorptionseinheit zum Zweck der absorptiven Abtrennung der Acrylsäure zu einem in der Absorptionseinheit absteigenden hochsiedenden flüssigen hydrophoben organischen Absorptionsmittel unter Vermeidung des Anfalls einer flüssigen wässrigen Phase im Gegenstrom führt,
• – das nicht in das Absorptionsmittel absorbierte verbleibende Restproduktgasgemisch in einer Kondensationseinheit abkühlt und darin enthaltenen Wasserdampf sowie Leichtsieder als Sauerwasser auskondensiert,
• – das bei der Sauerwasserkondensation verbleibende, insbesondere die Schwerkondensierbaren enthaltende Restgas als Kreisgas in Teilmengen gleicher Zusammensetzung aufteilt,
• – eine Kreisgasteilmenge als Verdünnungsgas in die heterogen katalysierte partielle Gasphasenoxidation von Propen zu Acrylsäure rückführt, eine andere Kreisgasteilmenge in in einer Wascheinheit von Acrylsäure frei gewaschenen Form als Strippgas verwendet und die verbleibende Kreisgasmenge entsorgt,
• – eine kleine Teilmenge des die Acrylsäure absorbiert enthaltenden Flüssigkeitsablaufs der Absorptionseinheit einer Destillationseinheit als Schwersiederauslaß zuführt, die durch Erwärmen des Flüssigkeitsablaufs in selbiger erzeugte Dampfphase in die Absorptionseinheit rückführt und die dabei flüssig verbleibenden Schwersieder entsorgt,
• – aus dem übrigen der Absorptionseinheit effektiv entnommenen, die Acrylsäure absorbiert enthaltenden Flüssigkeitsablauf der Absorptionseinheit in einer Desorptionseinheit durch Strippen mit der in der Wascheinheit von Acrylsäure frei gewaschenen Kreisgasteilmenge Leichtsieder entfernt,
• – mit Leichtsiedern beladenes Strippgas in die Absorptionseinheit rückführt,
• – den aus der Desorptionseinheit ablaufenden mit Acrylsäure beladenen Absorptionsmittelstrom einer Rektifikationseinheit mit Verstärkungs- und Abtriebsteil zuführt,
• – im Abtriebsteil der Rektifikationseinheit das Absorptionsmittel und Schwersieder in den Sumpf der Rektifikationseinheit kondensiert,
• – im Verstärkungsteil der Rektifikationseinheit über Seitenabzug eine rohe Acrylsäure abtrennt,
• – den über Kopf des Verstärkungsteils abgetrennten Leichtsiederstrom in die Absorptionseinheit rückführt,
• – aus dem Sumpfbereich der Rektifikationseinheit das von Leichtsiedern und Acrylsäure befreite Absorptionsmittel entnimmt, bei Bedarf mit frischem Absorptionsmittel ergänzt und in der Wascheinheit im Gegenstrom zur noch Acrylsäure enthaltenden Kreisgasteilmenge führt, um diese von Acrylsäure frei zu waschen,
• – das in der Wascheinheit ablaufende Absorptionsmittel, gegebenenfalls nachdem es über die als Schwersiederauslaß eingesetzte Destillationseinheit geführt wurde, in zwei Teilmengen gleicher Zusammensetzung aufteilt, die größere der beiden Teilmengen auf den Kopf der Absorptionseinheit rückführt und die kleinere Teilmenge mit Sauerwasser in extraktive Wechselwirkung bringt, um im Sauerwasser enthaltene Acrylsäure in das Absorptionsmittel und im Absorptionsmittel enthaltene polare Mittelsieder ins Sauerwasser aufzunehmen und
• – das bei der Sauerwasserextraktion anfallende organische Raffinat in die Absorptionseinheit rückführt, gefunden,

das dadurch gekennzeichnet ist, dass

• – die Rückführung des bei der Sauerwasserextraktion anfallenden organischen Raffinats in die Absorptionseinheit wenigstens eine theoretische Trennstufe unterhalb des Kopfs der Absorptionseinheit erfolgt.

Accordingly, a process of the absorptive basic separation of acrylic acid from the product gas mixture of a heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid which, in addition to acrylic acid and water vapor, contains low boilers, medium boilers, high boilers and heavy condensables as secondary constituents, in which

• - the product gas mixture cools to the temperature desired for absorption,
• The cooled product gas mixture leads in an absorption unit for the purpose of absorptively separating the acrylic acid to a high-boiling liquid hydrophobic organic absorption medium descending in the absorption unit while avoiding the occurrence of a liquid aqueous phase in countercurrent,
• The residual product gas mixture not absorbed into the absorbent cools in a condensation unit and the water vapor and low boilers contained therein are condensed out as acid water,
• Divides the residual gas remaining in the acid water condensation, in particular containing the heavy condensables, as recycle gas into portions of the same composition,
• - recirculates a part of the circulating gas as dilution gas into the heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid, uses another part of the circulating gas in a form which has been freed of acrylic acid in a washing unit as stripping gas and disposed of the remaining amount of circulating gas,
• Feeds a small portion of the liquid drain containing the acrylic acid absorbed to the absorption unit of a distillation unit as a high boiler outlet, which returns to the absorption unit by heating the liquid drain in the vapor phase it produces and disposes of the high boilers remaining liquid,
• - from the rest of the absorption unit, the liquid outlet of the absorption unit, which contains acrylic acid absorbed, is removed in a desorption unit by stripping with the partial gas amount of circulating gas which has been washed free of acrylic acid in the washing unit,
• - returns stripping gas loaded with low boilers to the absorption unit,
• Feeds the stream of absorbent loaded with acrylic acid from the desorption unit to a rectification unit with a reinforcing and stripping section,
• - In the stripping section of the rectification unit, the absorbent and high boilers are condensed into the bottom of the rectification unit,
• - in the reinforcement part of the rectification unit, a crude acrylic acid is separated off via a side draw,
• - recycle the low boiler flow separated off at the top of the reinforcement section into the absorption unit,
• - from the sump area of the rectification unit removes the absorbent freed from low boilers and acrylic acid, if necessary, replenishes it with fresh absorbent and in the washing unit countercurrently leads to the portion of circulating gas still containing acrylic acid in order to wash it free of acrylic acid,
• - The absorbent running off in the washing unit, if necessary after it has been passed through the distillation unit used as the high boiler outlet, is divided into two portions of the same composition, the larger of the two portions is returned to the top of the absorption unit and the smaller portion is interacting with acidic water in order to acrylic acid contained in the acid water in the absorbent and polar medium boilers contained in the absorbent in the acid water and
• - the organic raffinate obtained during the acid water extraction is returned to the absorption unit, found,

which is characterized in that

• - The return of the organic raffinate obtained during the acid water extraction into the absorption unit takes place at least one theoretical separation stage below the head of the absorption unit.

According to the invention, the return of the organic raffinate resulting from acid water extraction into the absorption unit 1 to 5, particularly preferably 2 to 4, theoretical plates below the head of the absorption unit.

In the following the method according to the invention is intended without limitation its generality are described as examples.

1 - The first stage of the reaction (Step 1)

To manufacture the basic separable acrylic acid containing product gas mixture is usually a mainly Propene, air and cycle gas (and possibly water vapor) existing Reaction gas starting mixture in one consisting of two reaction stages Reaction unit implemented on oxidic catalysts.

Normally, a reactor with a bundle of contact tubes which are filled with the oxidic catalyst and through which the reaction gas starting mixture flows, preferably from top to bottom, is used as the first reaction stage. The contact tubes are usually made of ferritic steel, preferably from the material 1.0425 (according to DIN EN 10020), and preferably have wall thicknesses of 1 to 3 mm, inner diameters of 20 to 30 mm and tube lengths of 2 to 4 m. The number of Contact tubes in the reactor are frequently in the range from 5000 to 40,000, preferably in the range from 10,000 to 30,000. The contact tubes are generally arranged in an evenly distributed manner in the reactor, the center-to-center distance between adjacent contact tubes, the so-called tube pitch, preferably being 35 to 45 mm.

The contact tubes are on both ends in tube sheets attached sealingly, preferably welded, and open at both pipe ends one hood each. about the hood is fed or discharged the reaction gas mixture. to Dissipation of the heat of reaction the pronounced exothermic reaction of propene to acrolein is caused by a heat transfer medium passed the space around the contact tubes. The inflow of Contact tubes through the heat transfer medium can both in longitudinal countercurrent, in longitudinal direct current, take place in cross-countercurrent or in cross-cocurrent. The liquid heat transfer medium takes off the heat of reaction as it passes through the reactor. Outside of the reactor Heat transfer medium completely or partly in a cooler cooled indirectly (evaporative cooling with water under pressure) and fed to the reactor again. The cooler to be used are known per se to the person skilled in the art and are not subject to any particular ones Restriction. Preferably, the heat transfer circuit along to Tube length, divided into several segments, particularly preferably 2 to 3, and the temperature of the heat transfer medium in regulated separately for each segment. One then speaks of a multi-zone reactor.

It can be used as the first reaction stage also several reactors, preferably 2 to 3, are operated in parallel.

The waste heat is particularly preferred for Saturated steam generation used. The vapor pressure of the saturated steam generated has in an expedient manner 15 to 80 bar, preferably 20 to 55 bar. The term saturated steam means that the water vapor partial pressure is at least 99% of the total pressure is. The saturated steam, which typically has a temperature of 250 ° C, can be used for the purpose of trace heating or for the operation of indirect heat exchangers in the method according to the invention be used. Alternatively, it can also be used to generate electricity in steam turbines become.

Temperature-stable are suitable as heat carriers Temperature control agents with high heat capacity, in particular Melting of salts such as potassium nitrate, potassium nitrite, sodium nitrate or sodium nitrite, or mixtures of two or more of the aforementioned salts or of low-melting metals, such as sodium, mercury as well as metal alloys. The heat transfer medium must liquid under the conditions of use be, that is, in an application-related manner a melting point in the range of 50 to 250 ° C, preferably from 150 to 200 ° C.

The oxidic catalysts contain the elements molybdenum, bismuth, Iron and cobalt. The reaction temperature in the contact tubes is many times 280 to 450 ° C. in the the incoming Reaction gas starting mixture facing section of the contact tubes the oxidic catalyst can be replaced by an inert material such as, for example Steatite, diluted become. Preferably is the proportion of diluent 10 to 50 wt .-% based on the mixture.

This dilution zone often extends over 10 to 50% of the pipe length and prevents overheating due to insufficient heat dissipation.

The reaction gas starting mixture exists mostly from fresh propene (usually the purity “chemical grade "or" polymer grade ", cf. DE-A 10131297), preferably 5 to 15% by weight of the total amount, Air, preferably 30 to 60 wt .-% of the total amount, and cycle gas and possibly water vapor. The oxygen content in the reaction gas starting mixture is regularly 5 to 15% by weight of the total. The reaction gas starting mixture is preferably set so that there is no ignitable mixture. Height Oxygen levels and low water levels in the reaction gas starting mixture are advantageous and lower the propionic acid and acetic acid content in the product gas mixture.

The air is preferably taken from the atmosphere and filtered beforehand as a component of the reaction gas starting mixture. For example, the filtering can be carried out using a roller belt filter (the filter surface continues to move automatically when it is exhausted, a predetermined pressure loss is exceeded) with the following degree of separation of the solid particles it may contain:

Air is very particularly preferred with a salt content, in particular inorganic and / or organic Chlorides (especially on alkali chlorides), less than 10 Gew.ppm used.

The propene is usually kept in liquid form and for the purpose of its use as a component in the reaction gas mixture in a heat exchanger evaporated in advance. In this regard, are suitable heat exchangers known to the person skilled in the art and are not subject to any particular restriction. preferably, becomes a shell-and-tube heat exchanger used. Can be used as a heat transfer medium for example sour water can be used. This cools it down and can be used for direct cooling and extraction of fresh sour water can be used.

The reaction gas starting mixture can are preheated in a heat exchanger before entering the reactor, preferably to 250 to 300 ° C. Suitable heat exchangers are known per se to the person skilled in the art and are not subject to any particular ones Restriction. A shell-and-tube heat exchanger is preferably used. The shell and tube heat exchanger can, for example, with the saturated steam from the waste heat Propene oxidation are operated.

The reactor hood contains for better mixing inert material in the entrance area of the reaction gas starting mixture, preferably steatite balls with a diameter of 50 to 200 mm (on the tube sheet).

The exit gas from the first reaction stage typically contains the following components (the proportions are based on the total amount):
Nitrogen 70-80% by weight,
Oxygen 5-15% by weight,
Carbon oxides 1-5% by weight,
Water 2-10% by weight,
Acrolein 5-15% by weight,
Acetic acid 0.05-1% by weight,
Acrylic acid 0.1-2% by weight,
Diphyl 0.001-0.01% by weight,
Formic acid 0.001-0.005% by weight,
Propane 0.1-1% by weight and
Propene 0.01-0.2% by weight.

The contained in the exit gas mixture Acrolein is thermally unstable and should be cooled, preferably on 200 to 300 ° C. The cooling the outlet gas mixture can be, for example, by heat exchangers, which are known per se to the person skilled in the art and are not subject to any particular restriction, respectively. A shell and tube heat exchanger is preferably used, wherein the gas in the pipes and the heat transfer medium around the Pipes flows. Internals in the pipes such as packings and / or fillings improve the effect of the heat exchanger. From the fillings are those with balls, rings, spirals, saddle bodies, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids preferred.

Steatite balls are particularly preferred. Usually used as a heat transfer medium used a molten salt. For the purpose of cooling the molten salt e.g. in an indirect heat exchanger preheated the water be used for the production of saturated steam.

2 - The second stage of the reaction (Level 2)

The reaction gas starting mixture for the second reaction stage can from secondary air and that cooled exit gas from the first reaction stage. The acrolein contained in it is partially oxidized to acyl acid in the second reaction stage using oxidic catalysts.

Preferably, the second reaction stage also used a reactor with a bundle of contact tubes, which are filled with the oxidic catalyst and those of the reaction gas mixture, preferably from top to bottom. The contact tubes are common made from ferritic steel, preferably from the material 1.0425 (according to DIN EN 10020), and preferably have wall thicknesses of 1 to 3 mm, inner diameter of 20 to 30 mm and tube lengths of 2 to 4 m. The number of contact tubes in the reactor is often included 5000 to 40,000, preferably 10,000 to 30,000. The contact tubes are usually evenly distributed in the reactor arranged, the center point distance between adjacent contact tubes the so-called pipe pitch, preferably 35 to 45 mm.

The contact tubes are on both ends in tube sheets attached sealingly, preferably welded, and open at both pipe ends one hood each. about the hood is fed or discharged the reaction gas mixture. to Dissipation of the heat of reaction the pronounced exothermic reaction of acrolein to acrylic acid is caused by a heat transfer medium passed the space around the contact tubes. The inflow of Contact tubes through the heat transfer medium can both in longitudinal countercurrent, in longitudinal direct current, in Cross-countercurrent or cross-cocurrent. The liquid heat transfer medium takes off the heat of reaction as it passes through the reactor. Outside of the reactor Heat transfer medium completely or partially in a cooler cooled indirectly (evaporative cooling with water under pressure) and fed to the reactor again. The cooler to be used are known per se to the person skilled in the art and are not subject to any particular ones Restriction. Preferably, the heat transfer circuit along to Tube length! divided into several segments, particularly preferably 2 to 3, and the temperature of the heat transfer medium in regulated separately for each segment. One then speaks of a multi-zone reactor.

The second reaction stage can also several reactors, preferably 2 to 3, are operated in parallel.

The waste heat is particularly preferred for Saturated steam generation used. The vapor pressure of the saturated steam generated has 5 to 50 bar, preferably 20 to 40 bar (compared to the first reaction stage lower temperature causes the lower pressure). The term saturated steam means that the water vapor partial pressure is at least 99% of the total pressure.

As heat transfer media are suitable as in the first reaction stage temperature-stable tempering media with high Heat capacity, in particular Melting of salts such as potassium nitrate, potassium nitrite, sodium nitrate or sodium nitrite, or mixtures of two or more of the the aforementioned salts or of low-melting metals, such as sodium, Mercury and metal alloys. The heat transfer medium must be used under the conditions of use liquid be that means in an application-related manner have a melting point in the range from 50 to 250 ° C., preferably 150 to 200 ° C.

The oxidic catalysts contain the elements molybdenum and vanadium are often present in their oxidic active materials and antimony. The reaction temperature in the contact tubes is often 220 to 380 ° C. in the the incoming Reaction gas starting mixture facing section of the contact tubes the oxidic catalyst can be replaced by an inert material such as, for example Steatite, diluted become. Preferably is the proportion of diluent 10 to 50 wt .-% based on the mixture. This dilution zone extends over many times 10 to 50% of the pipe length and prevents overheating due to insufficient heat dissipation.

The reaction gas starting mixture for the second The reaction stage preferably consists of air, preferably 5 to 15 % By weight of the total amount, and the exit gas from the propene partial oxidation in the first reaction stage. The oxygen content in the reaction gas starting mixture the second reaction stage usually 5 to 15 wt .-%. The reaction gas starting mixture for the second The reaction stage is preferably set so that no ignitable mixture is present.

The air is preferably taken from the atmosphere and filtered beforehand as a component of the reaction gas starting mixture. For example, the filtering can be carried out using a roller belt filter (the filter surface continues to move automatically when it is exhausted, a predetermined pressure loss is exceeded) with the following degree of separation of the solid particles it may contain:

Air is very particularly preferred with a salt content, in particular inorganic and / or organic Chlorides (especially on alkali chlorides), less than 10 Gew.ppm used.

The reactor hood contains for better mixing the second reaction stage in the input area of the reaction gas starting mixture Inert material, preferably steatite balls with a diameter of 50 to 200 mm (on the tube sheet).

The product gas mixture leaving the second reaction stage typically contains the following constituents (the proportions are based on the total amount):
Nitrogen 70-85% by weight,
Oxygen 2-10% by weight,
Carbon oxides 2-10% by weight,
Water 2-10% by weight,
Acrylic acid 5-20% by weight,
Acetic acid 0.1-1% by weight,
Acrolein 0.01-2% by weight,
Diphyl 0.001-0.005% by weight,
Formic acid 0.01-0.2% by weight,
Formaldehyde 0.1-2% by weight,
Propionic acid 0.001-0.01% by weight,
Furfural 0.001-0.01% by weight,
Benzaldehyde 0.001-0.01% by weight,
Maleic anhydride 0.01-0.2% by weight,
Benzoic acid 0.001-0.02% by weight,
Phthalic anhydride 0.001-0.005% by weight,
Propane 0.05-0.2 wt% and
Propene 0.1-2% by weight.

Oxygen levels are preferred in the product gas mixture of over 3.5% by weight based on the total amount. Such increased oxygen levels in the product gas mixture in particular reduce the formation of polymers Deposits in the absorption unit and in the desorption unit of the method according to the invention.

The propene partial oxidation and the Acrolein partial oxidation as well as intercooling can also be done in a single Reactor can be carried out with at least two separately controllable cooling circuits.

3 - Cooling of the product gas mixture (Level 3)

The one leaving the acrolein oxidation stage Product gas mixture, which usually consists mainly of acrylic acid, diphyl, Dimethyl phthalate (absorbent), water, carbon oxides and oxygen exists, can in principle both indirectly in a corresponding heat exchangers as well directly on one for the absorption step according to the invention cooled to a suitable temperature become. Preferably according to the invention the cooling directly in a direct cooler (Direct condenser) by partial evaporation of the absorbent. The latter is preferred according to the invention a mixture of diphyl and dimethyl phthalate, particularly preferred a mixture of 75 to 90% by weight of diphyl and 10 to 25% by weight of dimethyl phthalate. The weight ratio from to direct cooling Absorbent to be used to be cooled product gas mixture is advantageously 2 to 10, preferably 4 to 6.

The direct condenser (usually a distillation unit) can be of a type known per se be and have the usual internals. However, internals are preferably dispensed with. The direct cooler is usually made from austenitic steel, preferably from material 1.4571 (according to DIN EN 10020).

Usually becomes the column body of the direct capacitor is not isolated. All measures are preferred which the heat losses over the Increase column wall, For example, perforated plates welded parallel to the column surface or cooling fins.

The temperature of the acrolein oxidation level leaving product gas mixture is typically 200 to 350 ° C, preferably 250 to 300 ° C.

This is preferred in the direct cooler Product gas mixture and finely sprayed absorbent in cocurrent guided. The product gas mixture becomes by partial evaporation of the absorbent cooled. High boilers contained in the product gas mixture condense into the non-evaporated absorbent.

The cooled product gas mixture is together with the non-evaporated absorbent in the sump the subsequent absorption unit.

The temperature of the one leaving the direct cooler Product gas mixture as well as the non-evaporated leaving the direct cooler Solvent mixture is typically 120 to 170 ° C, preferably 150 to 160 ° C.

4 - absorption (level 4)

From e.g. leaving the product gas mixture direct cooling cooled Product gas mixture that mainly Acrylic acid, Contains diphyl, dimethyl phthalate, water, carbon oxides and oxygen according to the invention in a Absorption unit acrylic acid with a high-boiling liquid hydrophobic absorbent (preferably a mixture of diphyl and dimethyl phthalate, particularly preferably a mixture of 75 to 90% by weight of diphyl and 10 to 25% by weight of dimethyl phthalate).

The absorption unit is from on known design (usually an absorption column) and has the usual internals. As In principle, column internals are all common internals. These are floors, for example, Packs and / or fillings. Under the floors are bell bottoms, sieve trays, Valve trays, Thormann trays and / or dual flow floors preferred, are among the fillings those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, barrel or Intalox saddles, Top-Pak etc. or braids preferred. Dual-flow floors are cheap.

Dual-flow trays and / or are particularly preferred Valve trays. Under the valve bottoms the 2 to 4 flow valve bottoms are preferred. Using of dual flow floors and valve trays the dual flow bottom are most preferred in the lower section of the absorption unit and the valve bottoms in the upper section of the Absorption unit used.

They are usually for the absorptive Separation task 10 to 20 theoretical plates sufficient. The absorption will be convenient with increased Printing done, preferably at a head pressure of 1013 to 1400 mbar. The swamp pressure results from the head pressure, the number and type of column internals and the fluid dynamic requirements of absorption and is preferably 1400 to 2000 mbar.

When using dual flow floors as separating installations the diameter of the hole is advantageous from 10 to 80 mm, preferably 25 to 50 mm. The hole diameter preferably increases in the floors the column from top to bottom. The ground clearance (preferably the floors are equidistant arranged) is typically 300 to 800 mm, preferably 400 to 750 mm and particularly preferred 500 to 700 mm.

The absorption unit is common Made from austenitic steel, preferably from the material 1.4571 (according to DIN EN 10020).

Usually becomes the column body not isolated. Preference is given to all measures involving heat losses via the Increase column wall, For example, perforated plates welded parallel to the column surface or cooling fins.

The bottom floor of the absorption unit is designed as a catch floor (chimney floor, only permeable upwards), preferably other collecting trays are preferred, more preferably further 1 to 2, installed.

In the case of direct cooling of the product gas mixture, the gas mixture leaving it is separated from the absorbent not evaporated in the cooling device below the bottom tray (both are led together into the bottom of the absorption column). The bottom temperature in the absorption column is often 130 to 170 ° C, preferably 140 to 160 ° C. The non-evaporated absorbent obtained in the absorption column sump typically contains the following constituents (the proportions are based on the total amount):
Diphyl 40-70% by weight,
Dimethyl phthalate 20-50% by weight,
Diacrylic acid 0.5-3% by weight,
Acrylic acid 1-10% by weight,
Phenothiazine 0.1-2% by weight,
Phthalic anhydride 0.1-2% by weight,
Benzoic acid 0.1-2% by weight,
Maleic anhydride 0.01-0.2% by weight,
Benzaldehyde 0.01-0.2% by weight,
Formic acid 0.001-0.02% by weight,
Acrolein 0.001-0.02% by weight,
Water 0.005-0.05% by weight,
Acetic acid 0.01-0.2% by weight and
Furfural 0.001-0.005 wt%.

The proportion of heavy metal ions is usually in the absorption column sump less than 1 ppm by weight per heavy metal.

The sump liquid (liquid drain the absorption unit, the absorption column) is predominant as a coolant in direct cooling of the product gas mixture. A Partial amount, preferably 0.1 to 2% by weight, based on the total amount, is removed and fed to a distillation unit (stage 9).

The inlet of the absorbent into the absorption column in the top area of the same. In the case of one Bottom column preferably on its top floor. According to the invention arises the total inflow from the one to be described below for cycle gas washing washing unit used (stage 7), preferably 80 to 95% by weight based on the total amount (this subset contains polymerization inhibitor), and from the acid water extraction (stage 8), preferably 5 to 20 wt .-% based on the total amount. According to the invention, the organic Raffinate from acidic water extraction advantageous 1 to 5 theoretical floors, preferably 2 to 4 theoretical trays, below the inlet for the Absorbent from the cycle gas scrubbing into the absorption column guided.

The absorbent loaded with acrylic acid (the absorbate) is removed from the bottom tray of the absorption unit, which is designed as a collecting tray (as a further liquid drain from the absorption column). The removal temperature is 100 to 140 ° C, preferably 110 to 120 ° C. The absorbate typically contains the following components (the proportions are based on the total amount)
Diphyl 40-70% by weight,
Dimethyl phthalate 5-20% by weight,
Diacrylic acid 0.5-3% by weight,
Acrylic acid 20-40% by weight,
Phenothiazine 0.005-0.05% by weight,
Phthalic anhydride 0.1-1% by weight,
Benzoic acid 0.1-2% by weight,
Maleic anhydride 0.1-2% by weight,
Benzaldehyde 0.1-2% by weight,
Formic acid 0.01-0.1% by weight,
Acrolein 0.001-0.05% by weight,
Water 0.1-1% by weight,
Acetic acid 0.1-2% by weight,
Propionic acid 0.001-0.2% by weight,
Formaldehyde 0.001-0.005% by weight,
Allyl acrylate 0.001-0.01 wt% and
Furfural 0.01-0.05 wt%.

Some of the absorbate becomes one Desorption unit (stage 5) supplied, preferably 10 to 30 wt .-% based on the total amount withdrawn, in some cases directly to that Bottom fed to the absorption column, preferably 15 to 40 % By weight based on the total amount withdrawn, and in some cases over one heat exchangers (in which it cooled is returned to the absorption unit (preferably 30 to 75 % By weight based on the total amount removed). Preferably done this return 2 up to 5 theoretical floors above the removal floor. The use of several heat exchangers, connected in series or in parallel is possible. In the heat exchangers, which are known per se to the person skilled in the art and are not subject to any particular restriction, the absorbate is preferably cooled by 10 to 25 ° C. Preferred is the in the heat exchanger dissipated Heat to Partial evaporation of the condensation unit to be described resulting acid water used.

The same return can be removed from further catch trays, preferably 1 to 2, attached in the lower third of the absorption unit. The return flow is partially directly below the floor drained half of the collecting tray, preferably 5 to 20 wt .-% based on the total amount withdrawn, and partially returned via a heat exchanger to the absorption unit, preferably 2 to 5 theoretical trays above the sampling tray. It is possible to use several heat exchangers, connected in series or in parallel. In the heat exchangers, which are known per se to the person skilled in the art and are not subject to any particular restriction, the return is cooled, preferably by 10 to 25 ° C.

These collecting trays are also suitable as feed points for the overhead of the reinforcement part the low boiler flow taken from the rectification unit (stage 6) or generally as a feed point for acrylic acid-containing material flows, such as for example crude acrylic acid that does not meet specifications.

4a - Acid water condensation (level 4a))

The remaining one, not in the absorbent absorbed residual product gas mixture, especially low boilers and Contains non-condensable is cooled in a condensation unit.

The cooling can be indirect, for example through heat exchangers, which are known per se to the person skilled in the art and are not subject to any particular restriction, or directly, for example by a quench. Preferred according to the invention is direct cooling. For this purpose, the residual product gas mixture is already condensed aqueous low boiler fraction using a suitable heat exchanger chilled (temperatures of around 10 ° C are typical) and the cooled liquid sprayed in the vapor above the condensate removal point. This spraying can in a separate apparatus or preferably in one of the absorption units mounted condensation unit. In the latter case the condensate removal point is advantageously designed as a collecting tray. By internals, the mixing of the cooled aqueous low-boiling condensate with the brothers improve, the effect of direct cooling can be increased. For this basically all common come Internals into consideration. These are, for example, floors, packs and / or fillings. Under the floors are bell bottoms, sieve trays, Valve trays, Thormann trays and / or dual flow floors preferred, among the fillings are those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids prefers. Valve bottoms are particularly preferred.

2 to 4 flow valve bases are preferred among the valve heads.

As a rule, 2 to 5 theoretical plates are sufficient here. The acid water condensation can also be carried out in several stages (more than one quench circuit with an increasing temperature). The acid water at the top of the condensation unit essentially contains the following components (the proportions are based on the total amount):
Acrylic acid 2-5% by weight,
Acetic acid 4-10% by weight,
Water 60-80% by weight,
Diphyl 5-20% by weight,
Dimethyl phthalate 1-5% by weight,
Formaldehyde 2-10% by weight,
Diacrylic acid 0.1-2% by weight,
Allyl acrylate 0.0005-0.005% by weight,
Furfural 0.01-0.005 wt%,
Propionic acid 0.0005-0.005% by weight,
Formic acid 0.2-2% by weight,
Benzaldehyde 0.1-1% by weight,
Maleic anhydride 0.01-0.2% by weight,
Benzoic acid 0.05-0.5% by weight,
Maleic acid 0.2-2% by weight,
Phthalic anhydride 0.01-0.1% by weight,
Phenothiazine 0.002-0.02 wt% and
Acrolein 0.01-0.1 wt%.

The sour water is partially discharged, preferably 0.5 to 5% by weight based on the total amount withdrawn, and partly over a heat exchanger into the condensation unit, preferably 2 to 5 theoretical plates above the tapping point. The Use of several heat exchangers, connected in series or in parallel, preferably in series, is possible. The Repatriation of the chilled watery Low boiler condensate can also be found in various places above its tapping point (each cooled again with decreasing temperature). In the heat exchangers, the specialist are known per se and are not subject to any particular restriction, the condensate is cooled, preferably around 5 to 25 ° C.

The effectively discharged amount of acid water is fed to the acid water extraction (stage 8). Their temperature is often 20 up to 60 ° C, preferably 30 to 50 ° C.

The residual gas mixture remaining in gaseous form in the acid water condensation essentially contains the following components (the proportions are based on the total amount):
Nitrogen 80-95% by weight,
Oxygen 2-15% by weight,
Carbon oxides 2-10% by weight,
Water 0.5-5% by weight,
Acetic acid 0.01-0.2% by weight,
Acrylic acid 0.01-2% by weight,
Acrolein 0.01-0.5% by weight,
Diphyl 0.001-0.01% by weight,
Formic acid 0.001-0.01% by weight,
Propane 0.01-0.2 wt% and
Propene 0.1-2% by weight.

This residual gas (circulating gas) is expediently partially, preferably 30 to 50% by weight based on its total amount, discharged, partially, preferably 10 to 25% by weight based on its total amount, in stage 5 from acrylic acid washed form used as stripping gas and partially, preferably 40 to 60 wt .-% based on its total amount, returned to the propene oxidation (stage 1). The discharged residual gas is burned together with the other production residues, preferably after washing with the aqueous extract from the acid water extraction (stage 8). The waste heat generated in this way is preferably used to generate steam, as is the case, for example, in DE-A 196 24 674 is described.

The is very particularly preferred dissipated from the absorption unit Heat to Partial evaporation of the sour water used. This increases the steam yield in the energetic utilization of production residues (acid water combustion).

The one with acid water condensation remaining gas flow is preferably overheated by 4 to 10 ° C (e.g. with 4 bar steam). This will prevent possible condensation in the Exhaust gas pipes avoided.

The recycle gas returned to the process is reduced to one Pressure from 1.5 to 5 bar, preferably to 2 to 3.5 bar, particularly preferably on a print that is dimensioned so that no further Compressor in the cycle gas system is required, compressed.

Of course, the absorption unit can also be operated with a low vacuum at the top of the column. In In this case it is advantageous to use an additional compressor (if necessary a blower) to discharge residual gas to the atmospheric pressure to overcome.

5 - Desorption (level 5)

That from the bottom catchment floor of the absorption unit (Level 4) effectively removed absorbate, mainly from Acrylic acid, Diphyl and dimethyl phthalate, becomes a desorption unit fed around to free it from the low boilers it still contains.

The desorption unit (usually a column) is of a type known per se and has the usual Internals on. In principle, all common internals come as column internals into consideration, for example floors, Packs and / or fillings. Under the floors are bell bottoms, sieve trays, Valve trays, Thormann trays and / or dual flow floors preferred, among the fillings are those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids preferred. Are particularly preferred Dual flow floors and / or Valve trays. Under the valve bottoms the 2 to 4 flow valve bottoms are preferred. Using of dual flow floors and valve bottoms the dual flow floors very particularly preferably in the lower section of the desorption unit and the valve bottoms used in the upper section of the desorption unit.

Usually there are 10 to 20 here theoretical floors sufficient. The desorption unit can be operated under reduced pressure are operated, but preferably the partial pressures of the desorbable components by dilution with one under desorption conditions non-condensable and inert gas, particularly preferably with the circulating gas washed in the washing unit (stage 7). The Conditions are chosen so that the temperature of the vapor at the head of the desorption unit does not exceed 140 ° C, preferably 130 ° C.

When using dual flow floors as separating internals in the desorption column is the hole diameter (the diameter of the passage openings in the floors) usually 8 to 50 mm, preferably 10 to 35 mm. Particularly preferred the hole diameter increases from top to bottom. The ground clearance (usually the floors are equidistant arranged) is frequently 300 to 800 mm, preferably 400 to 600 mm and very particularly preferred 500 mm.

The desorption unit is common Made from austenitic steel, preferably from the material 1.4571 (according to DIN EN 10020).

The column feed to be stripped Absorbate takes place in the upper area of the desorption unit, preferably on their top floor. The inlet is preferably preheated (typical to 130 to 140 ° C) and partially vaporized.

The heat supply in the swamp of the Desorption column can over internal and / or external heat exchanger conventional Type and / or over Double wall heating take place (as a heat carrier is advantageous with the waste heat water vapor obtained from the partial oxidation is used). Preferably it is done via external Circulation evaporator with natural or forced circulation. Particularly preferred be outside Circulation evaporator with forced circulation used. The use of several Evaporators, connected in series or in parallel, are possible.

When using one under desorption conditions non-condensable and inert stripping gas, it is advantageous introduce this directly into the sump of the desorption unit and the evaporator circulation on one of the lower column trays (before preferably in the field of theoretical soils 2 to 4 from below). The inert, which is not condensable under the desorption conditions Gas can enter the desorption unit with the bottom product mixed and preheated (remove partial flow from the circulation evaporator and collectively with the stripping gas over a separate heat exchanger to lead). Washing in the washing unit (stage 7) is particularly preferred Recycle gas is used as a non-condensable and inert gas.

The bottom temperature in the desorption column is usually 110 to 150 ° C., preferably 120 to 140 ° C. The absorbent accumulating in the bottom of the desorption column, which is essentially only loaded with acrylic acid and medium boilers, contains the following components (the proportions are based on the total amount)
Diphyl 55-75% by weight,
Dimethyl phthalate 10-25% by weight,
Diacrylic acid 0.5-3% by weight,
Acrylic acid 15-25% by weight,
Acetic acid 0.01-0.2% by weight,
Formic acid 0.001-0.02% by weight,
Propionic acid 0.001-0.02,
Phenothiazine 0.01-0.1% by weight,
Phthalic anhydride 0.1-1% by weight,
Benzoic acid 0.2-2% by weight,
Maleic anhydride 0.1-2% by weight,
Benzaldehyde 0.1-1% by weight and
Furfural 0.01-0.05 wt%.

The bottom liquid (that from the desorption unit running, especially absorbent stream loaded with acrylic acid) the desorption unit is about a heat exchanger guided, partially removed, preferably 65 to 90 wt .-% based on the feed fed into the desorption column, and partially returned to the lower area of the desorption unit. The discharged amount is fed to the rectification unit (stage 6).

The strip gas loaded with low boilers can indirectly, for example by heat exchangers, the expert are known per se and are not subject to any particular restriction, or cooled directly, for example by a quench. Some of the low boilers condensing can be used as reflux, the rest are returned to the absorption unit (stage 4). Using a non-condensable and inert stripping gas, this is in his remaining with the described low boiler condensation Form also returned to the absorption unit (level 4).

In a preferred embodiment the aforementioned condensation is not carried out and that at the top of the desorption unit led out loaded stripping gas as such into the absorption (stage 4), preferably below the first floor (from below) of the absorption unit, returned (if applicable the return takes place via the direct cooling of the product gas mixture (stage 3)).

As a rule, a loaded stripping gas returned in this way contains the following components (the proportions are based on the total amount):
Nitrogen 30-60% by weight,
Oxygen 1-5% by weight,
Carbon oxides 1-5% by weight,
Acrylic acid 30-60% by weight,
Diphyl 1-5% by weight,
Water 0.5-2% by weight,
Acetic acid 0.2-2% by weight,
Dimethyl phthalate 0.01-0.2% by weight,
Formaldehyde 0.001-0.01% by weight,
Acrolein 0.01-0.1% by weight,
Formic acid 0.02-0.2% by weight,
Propionic acid 0.005-0.05% by weight,
Allyl acrylate 0.001-0.02% by weight,
Diacrylic acid 0.001-0.02% by weight,
Phthalic anhydride 0.001-0.01% by weight,
Benzoic acid 0.002-0.02% by weight,
Maleic anhydride 0.1-0.5% by weight,
Benzaldehyde 0.05-0.2% by weight and
Furfural 0.005-0.02 wt%.

A liquid trace from the top The bottom of the desorption unit is possible and does not interfere.

6 - Rectification of the from the Desorption unit discharged liquid drain (level 6)

The one from the desorption unit (level 5) discharged sump drain, mainly from acrylic acid, diphyl and dimethyl phthalate is used in a reinforcing and a rectification unit comprising a stripping section.

The rectification unit is from design known per se (typically a rectification column) and assigns the usual Internals on. In principle, all common internals come in as column internals Consideration, for example floors, Packs and / or fillings. Under the floors are bubble trays, sieve trays, valve trays, Thormann trays and / or Dual-flow trays preferred, among the fillings are those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, or barrels Intalox saddles, Top-Pak etc. or braids preferred. Dual-flow trays are particularly preferred.

As a rule, 10 to 25 are theoretical Floors in the rectification unit is sufficient. The rectification is usually carried out under reduced pressure, preferably at a head pressure of 70 to 140 mbar. The swamp pressure results from the head pressure, the number and type of column internals and the fluid dynamic requirements of rectification and is preferably 300 to 400 mbar.

When using dual flow floors as separating installations the hole diameter in the bottoms usually 5 to 25 mm, preferably 10 to, above the inlet 20 mm. The hole diameter in the bottoms below the inlet is usually 15 to 80 mm, preferably 25 to 50 mm and is very particularly preferred graded as in German patent application DE - A 10156988 described. The ground clearance (usually the floors are equidistant arranged) is typically 300 to 700 mm, preferably 350 to 400 mm and very particularly preferably 380 mm.

The upper part of the Rectification unit heated. With a rectification unit with n theoretical floors this concerns the area above the n / 2nd theoretical floor. The temperature of the trace heating is chosen so that on the inner wall of the column no acrylic acid can condense. This temperature is preferably 5 to 10 ° C. above the boiling point of acrylic acid with the one present in the rectification unit in the area under consideration Print.

The rectification unit is common Made from austenitic steel, preferably from the material 1.4571 (according to DIN EN 10020).

The inflow into the rectification unit conveniently takes place in your lower area. It is preferably 2 to 5 theoretical plates above the bottom of the rectification column. The inlet temperature is typical 150 ° C.

The heat is supplied via internal and / or external heat exchangers (heat transfer medium is again preferably steam) of conventional design and / or via double-wall heating. It is preferably carried out via external circulation evaporators with natural or forced circulation. External circulation evaporators with forced circulation are particularly preferred. The use of several evaporators, in series or in parallel, is possible. Preferably 2 to 4 evaporators are operated in parallel. The bottom temperature of the rectification unit is typically 180 to 210 ° C, preferably 190 to 200 ° C. The high boiler fraction condensing in the bottom of the rectification column generally contains the following constituents (the proportions are based on the total amount):
Diphyl 70-85% by weight,
Dimethyl phthalate 10-25% by weight,
Diacrylic acid 0.5-5% by weight,
Acrylic acid 0.2-2% by weight,
Phenothiazine 0.01-0.1% by weight,
Phthalic anhydride 0.1-1% by weight,
Benzoic acid 0.2-2% by weight,
Maleic anhydride 0.1-2% by weight,
Benzaldehyde 0.1-0.5% by weight and
Furfural 0.01-0.05 wt%.

The one taken from the rectification unit containing the high-boiling absorbent encoded, bottoms liquid is partially discharged, preferably based on 75 to 90 wt .-% on the inflow into the rectification unit, and partly via a heat exchangers returned to the bottom of the rectification column. The High boiler fraction is removed, preferably via a Solids separator (cyclone) and, if necessary, with fresh absorbent added, essentially the washing unit (stage 7) and in a small one Partial amount fed to the swamp area of absorption (stage 4). In usually is the aforementioned ratio 30 to 1 to 300 to 1, preferably 60 to 1 to 150 to 1.

Alternatively, a subset of the High boiler fraction of the rectification unit above the heat exchanger and below the separating internals by means of a side trigger gaseous be removed. The gaseous removed high boiler fraction, which is free in a special way Is solid particles, is then condensed and, if necessary added by fresh absorbent, fed to the washing unit (stage 7). The Condensation of these gaseous withdrawn high boiler fraction can indirectly, for example in heat exchangers, which are known per se to the person skilled in the art and are not subject to any particular restriction, or directly, for example by a quench. Prefers is indirect cooling. additionally then another part of the high boiler fraction is the swamp area the rectification column liquid removed and the sump area of the absorption unit (level 4) fed. The mass ratio of gaseous too fluid removed high boiler fraction is expediently 30 in this variant 1 to 300 to 1, preferably 60 to 1 to 150 to 1. This process variant the formation of deposits (fouling) in the absorbent circuit significantly reduced.

A crude acrylic acid, preferably 8 to 20 theoretical plates above the bottom of the column, is removed via a side draw above the feed into the rectification column. The crude acrylic acid is removed in the customary manner and is not subject to any restrictions. Removal via a collecting tray is suitable, whereby the entire return is collected and some is discharged and the other part is used as a return below the collecting tray, or via a tray with an integrated extraction option, preferably via a dual-flow tray with an integrated extraction option. The crude acrylic acid removed normally contains the following constituents (the proportions are based on the total amount):
Acrylic acid 98-99.9% by weight,
Acetic acid 0.05-0.3% by weight,
Water 0.001-0.01% by weight,
Formic acid 0.001-0.005% by weight,
Propionic acid 0.01-0.05% by weight,
Furfural 0.01-0.05% by weight,
Allyl acrylate 0.001-0.01% by weight,
Benzaldehyde 0.001-0.01% by weight,
Maleic anhydride 0.002-0.02% by weight,
Diacrylic acid 0.01-0.05% by weight and
Phenothiazine 0.01-0.05 wt%.

The crude acrylic acid removed by means of a heat exchanger chilled (as a coolant are e.g. Surface water). The Use of several heat exchangers, connected in series or in parallel is possible. In the heat exchangers, which are known per se to the expert and are not subject to any particular restriction, becomes the raw acrylic acid preferably around 40 to 70 ° C cooled.

The crude acrylic acid removed, preferably 10 to 25% by weight, based on the feed into the rectification column, is discharged and partly as a solvent for the polymerization inhibitor used.

The low-boiling stream separated off at the top of the rectification column can be cooled indirectly, for example by heat exchangers (surface water, for example, can be used as a coolant), which are known per se to the person skilled in the art and are not subject to any particular restriction, or directly, for example by a quench. It is preferably carried out by direct cooling. For this purpose, already condensed low boiler fraction is cooled using a suitable heat exchanger and the cooled liquid is sprayed into the vapor above the point of use. This spraying can take place in a separate apparatus or in the rectification unit itself. When spraying in the rectification unit, the removal point of the low boiler fraction is advantageously designed as a collecting tray. The effects of direct cooling can be increased by internals which improve the mixing of the cooled low boiler fraction with the vapor. In principle, all common installations are considered, for example floors, packings and / or fillings. Bell bottoms, sieve bottoms, valve bottoms, Thormann bottoms and / or dual-flow bottoms are preferred among the bottoms. Among the fillings, those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids are preferred. Dual-flow trays are particularly preferred. As a rule, 2 to 5 theoretical floors are sufficient here accordingly. These soils are not taken into account in the previous information on the number of theoretical soils of the rectification unit. The direct condensation of the low boiler fraction can also be carried out in several stages, with the temperature decreasing upwards. The low boiler stream separated off at the top of the rectification column essentially contains the following components (the proportions are based on the total amount):
Acrylic acid 90-99% by weight,
Acetic acid 0.5-3% by weight,
Water 0.5-2% by weight,
Phenothiazine 0.02-0.1% by weight,
Diacrylic acid 0.01-0.05% by weight,
Allyl acrylate 0.01-0.1% by weight,
Furfural 0.002-0.01% by weight,
Propionic acid 0.01-0.05% by weight,
Formic acid 0.1-1% by weight and
Acrolein 0.001-0.002 wt%.

Part of the as a low boiler fraction withdrawn liquid, preferably 30 to 50% by weight, based on the feed into the rectification column, is called a rewind used, the rest of the low boiler fraction, preferably 0.5 to 2 wt .-% based on the feed into the rectification column removed and returned to the absorption unit (level 4) (ins lower third).

The waste gas from the rectification unit generally contains the following constituents (if the air inhibition that is still to be carried out below is used) (the proportions are based on the total quantity):
Nitrogen 70-80% by weight,
Oxygen 20-25% by weight,
Acrylic acid 1-5% by weight,
Acetic acid 0.05-0.5% by weight,
Water 0.1-0.5% by weight,
Acrolein 0.001-0.005% by weight,
Formic acid 0.02-0.2% by weight,
Allyl acrylate 0.001-0.005 wt% and
Carbon oxides 0.02-0.06 wt%.

The exhaust gas is shared with the others Production residues burned.

As a polymerization inhibitor in the rectification column alkylphenols, for example o-, m- or p-cresol (methylphenol), 2-tert-butyl-4-methylphenol, 6-tert-butyl-2,4-dimethyl-phenol, 2,6-di-tert-butyl-4-methylphenol, 2-tert-butylphenol, 4-tert-butylphenol, 2,4-di-tert-butylphenol, 2-methyl-4-tert-butylphenol, 4-tert-butyl-2,6-dimethylphenol, or 2,2'-methylene-bis- (6-tert-butyl-4-methylphenol), Hydroxyphenols, for example hydroquinone, 2-methylhydroquinone, 2,5-di-tert-butylhydroquinone, pyrocatechol (1,2-dihydroxybenzene) or benzoquinone, aminophenols, e.g. para-aminophenol, nitrosophenols, e.g. para-nitrosophenol, alkoxyphenols, for example 2-methoxyphenol (Guaiacol, catechol monomethyl ether), 2-ethoxyphenol, 2-isopropoxyphenol, 4-methoxyphenol (hydroquinone monomethyl ether), mono- or di-tert-butyl-4-methoxyphenol, Tocopherols, e.g. O-tocopherol and 2,3-dihydro-2,2-dimethyl-7-hydroxybenzofuran (2,2-dimethyl-7-hydroxycoumaran), N-oxyls such as 4-hydroxy-2,2,6,6-tetramethyl-piperidine-N-oxyl, 4-oxo-2,2,6,6-tetramethyl piperidin-NoxyI, 4-acetoxy-2,2,6,6-tetramethyl-piperidine-N-oxyl, 2,2,6,6-tetramethyl-piperidine-noxyl, 4,4 ', 4 "tris (2,2,6,6-tetramethyl-piperidine-N-oxyl) phosphite or 3-oxo-2,2,5,5-tetramethyl-pyrrolidine-N-oxy1, aromatic amines or phenylenediamines, e.g. N, N-diphenylamine, N-nitrosodiphenylamine, N, N'-dialkyl-para-phenylenediamine, where the alkyl radicals are the same or can be different and each independently consist of 1 to 4 carbon atoms and are straight-chain or can be branched Hydroxylamines, e.g. N, N-diethylhydroxylamine, phosphorus-containing Connections such as Triphenylphosphine, triphenylphosphite, hypophosphorous Acid or Triethyl phosphite, sulfur-containing compounds such as e.g. diphenyl or phenothiazine, optionally in combination with metal salts, such as the chlorides, dithiocarbamates, sulfates, salicylates or acetates of copper, manganese, cerium, nickel or chromium are used become. Of course can mixtures of the polymerization inhibitors mentioned are also used become.

Preference is given to the rectification unit Phenothiazine used as a polymerization inhibitor or another Polymerization inhibitor that has the same effectiveness as phenothiazine shows.

The polymerization inhibitor is preferably added to the condensed low boiler reflux as a solution in relatively pure acrylic acid, particularly preferably as a solution in the crude acrylic acid separated off via a side draw. The total concentration of aldehydic impurities in the acrylic acid used for the stabilizer batch is preferably less than 500 ppm by weight. The stabilizer (inhibitor) concentration in the stabilizer mixture (in the stabilizer solution) depends on the stabilizer used and the used solvents. The optimal stabilizer concentration in the stabilizer batch can be determined by dissolution experiments and is 0.1 to 2% by weight, preferably 1.2 to 1.7% by weight, in the case of phenothiazine as stabilizer and crude acrylic acid as solvent. The stabilizer is dosed so that the stabilizer concentration in the discharged crude acrylic acid is 50 to 500 ppm by weight, preferably 150 to 350 ppm by weight.

The form of the solid polymerization inhibitors, into the stabilizer neck container is in principle arbitrary. For example, come Dandruff, granules or pellets. Because of the lesser Proportion of dust and the cheaper Pellets are preferred for free-flowing behavior. The solid polymerization inhibitors can for example by means of screw conveyors from a silo into the stabilizer batch container.

To further support the Stabilization (polymerization inhibition) can be an oxygen-containing gas, preferably air or a mixture of air and nitrogen (lean air), air is particularly preferred to be present.

This oxygen-containing gas will preferably in the bottom region of the rectification column and / or metered into a circulation evaporator.

The air expediently becomes immediate the atmosphere taken, preferably at one or more sampling points.

According to the invention, preference is given to that which is also used Air in advance of their use as well, as already related described with the partial oxidation, filtered.

Air is very particularly preferred with a salinity, e.g. of organic and inorganic chlorides, especially on alkali chlorides of less than 10 ppm by weight.

The oxygen-containing gas becomes like this doses that the partial pressure of oxygen at the head of the rectification unit is at least 0.5 mbar, preferably at least 2 mbar.

In addition, a surfactant can be metered into the rectification unit, such as in the DE-A 19810962 is described. The inhibition of the entire process according to the invention ultimately takes place via the inhibition of polymerization of the rectification column on the way of carrying inhibitors. Only in the acid water circuit does no polymerization inhibitor arrive (except through entrainment in the gas stream from the lower areas of the absorption (absorber) column). However, the acrylic acid is so diluted in the acid water that further inhibition is not necessary.

7 - washing unit (level 7)

When using cycle gas as under Desorption conditions non-condensable and inert stripping gas (Level 5) it is to increase the desorption performance expedient, this Recycle gas prior to its use as a pretreatment strip gas to undergo.

The cycle gas is mainly from Consisting of nitrogen, oxygen, carbon oxides and water in one Washing unit of absorbable (especially acrylic acid residues, acetic acid and acrolein) volatile Shares exempted (preferably in countercurrent).

The washing unit is common of known design and has the usual internals. As In principle, column internals are all common internals. These are floors, for example, Packs and / or fillings.

Bell bottoms, sieve bottoms, valve bottoms, Thormann bottoms and / or are among the bottoms Dual-flow trays prefers. Under the fillings are those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids prefers. Dual-flow trays are particularly preferred.

Usually there are 10 to 20 here theoretical floors sufficient. The cycle gas wash will be useful at increased Printing done, preferably at a head pressure of 1400 to 2000 mbar. The swamp pressure results from the head pressure, the number and type of column internals as well as the fluid dynamic requirements.

When using dual flow floors as separating installations whose hole diameter is usually 10 to 50 mm, preferably 20 up to 40 mm, very particularly preferably 30 mm. The ground clearance (in usually equidistant Arrangement) typically 300 to 700 mm, preferably 350 to 400 mm and very particularly preferably 380 mm.

The washing unit is common Made of austenitic steel, preferably made of 1.4571 (according to DIN EN 10020).

The sump drain from the rectification unit (stage 6) is placed on the top floor of the washing unit as washing liquid. The application temperature is suitably from 20 to 100.degree. C., preferably from 30 to 60.degree. For this purpose, the bottom effluent from the rectification unit (stage 6) is cooled. The use of several heat exchangers, in series or in parallel, preferably in series, is possible. The heat exchangers are known per se to the person skilled in the art and are not subject to any restriction. Possible coolant is Surface water.

Below the first floor (from below) of the washing unit is the appropriate discharge point for that the absorption gas (stage 4) coming cycle gas. Whose inlet temperature is regularly 20 to 100 ° C, preferably 30 to 60 ° C. For this purpose, the (stage 4) recycle gas coming from the absorption unit is cooled. The Use of several heat exchangers, connected in series or in parallel, preferably in series, is possible. The heat exchangers are known per se to the person skilled in the art and are not subject to any particular ones Restriction. potential coolant is surface water. The amount of cycle gas is normally 10 to 50% by weight, preferably 20 to 40% by weight to that supplied from level 6 Amount of washing liquid.

The liquid obtained in the column bottom essentially contains the following components (the proportions are based on the total amount):
Diphyl 70-85% by weight,
Dimethyl phthalate 10-25% by weight,
Diacrylic acid 0.5-5% by weight,
Acrylic acid 0.2-2% by weight,
Maleic anhydride 0.2-1% by weight,
Phenothiazine 0.02-0.1% by weight,
Phthalic anhydride 0.1-2% by weight,
Benzoic acid 0.2-2% by weight,
Acrolein 0.01-0.05% by weight,
Water 0.02-0.2% by weight,
Acetic acid 0.01-0.05% by weight,
Benzaldehyde 0.1-0.5% by weight and
Furfural 0.01-0.05 wt%.

The one in the washing unit bottoms liquid is removed and in two subsets of identical composition divided up. The two subsets become the absorption unit (Level 4) and the acid water extraction (level 8) usually in mass ratio 1 to 1 to 20 to 1, preferably in a mass ratio of 5 to 1 to 15 to 1, fed. The feeder into the absorption unit is done on its head.

The scrubbed cycle gas essentially still contains the following components (the proportions are based on the total amount):
Nitrogen 80-95% by weight,
Oxygen 2-15% by weight,
Carbon oxides 2-10% by weight,
Water 0.5-3% by weight,
Acrylic acid 0.01-0.05% by weight,
Diphyl 0.01-0.1% by weight,
Benzaldehyde 0.001-0.01% by weight,
Maleic anhydride 0.002-0.2% by weight,
Propane 0.05-0.3 wt% and
Propene 0.1-2% by weight.

The scrubbed cycle gas is discharged and passed as a stripping gas into the desorption unit (stage 5).

8 - Acid water extraction (level 8th)

The acid water from the acid water condensation (Level 4a), mainly from acrylic acid, Diphyl, acetic acid, Consisting of water, formaldehyde and dimethyl phthalate, is in one Extraction unit with the part of the liquid drain supplied to it extracted from the washing unit (stage 7).

The extraction unit can be one Extraction column. The extraction column is in itself known design and can be the usual Have internals. In principle, all common internals come as column internals into consideration. Examples are floors, Packs and / or fillings. Under the floors are sieve trays and / or dual flow floors prefers. Under the fillings are those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids prefers. Dual-flow trays are particularly preferred. Usually are here 1 to 5 theoretical floors sufficient.

Of course, a stirred tank with a settling tank can also be used as the extraction unit. In principle, all common stirrers come into consideration as stirrers, for example disc stirrers, impeller stirrers, cross-bar stirrers, grid stirrers, blade stirrers, anchor stirrers, paddle stirrers, propeller stirrers, spiral stirrers and multi-stage impulse countercurrent stirrers. The stirring can also be multi-stage, that is, several stirring are arranged one above the other on a common axis. A two-stage impeller is preferred stirrer used. In principle, all common containers can be considered as settling containers. A horizontal container is preferably used. The inlet and outlet of the container are preferably separated by internals transverse to the direction of flow. In principle, all common installations are considered as installations, for example perforated sheets, packings and / or fillings. Of the fillings, those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids are preferred. Perforated sheets are particularly preferred. The residence time in the settling tank is 0.1 to 2 hours, preferably 20 to 50 minutes.

The extraction unit is common Made from austenitic steel, preferably from the material 1.4571 (according to DIN EN 10020).

The acid water from the acid water condensation (Step 4a) is added to the extraction unit. The task temperature is usually 20 to 70 ° C, preferably 30 to 60 ° C. For this purpose, the acid water from the acid water condensation (stage 4a) cooled. The use of several heat exchangers, connected in series or in parallel, preferably in series, is possible (as cooling medium surface water comes in ) Account. The usable heat exchangers are known per se to the person skilled in the art and are not subject to any particular ones Restriction.

The relevant amount of sump drainage Washing unit (stage 7) is also fed to the extraction unit. The mass ratio of the amount of sump drain from the washing unit (level 7) and acid water from the acid water condensation (stage 4a) is advantageously 0.5 to 1 to 10 to 1, preferably 1 to 1 to 3 to 1.

The aqueous extract obtained in the extraction unit (it takes up polar constituents such as diacrylic acid (Michael adduct) and maleic anhydride from the bottom outlet of the washing unit; the latter is hydrolyzed) usually contains the following components (the proportions are based on the total amount):
Water 70-95% by weight,
Acetic acid 2-10% by weight,
Acrylic acid 1-5% by weight,
Formaldehyde 2-10% by weight,
Maleic acid 1-5% by weight,
Diacrylic acid 1-5% by weight,
Diphyl 0.01-0.05% by weight,
Dimethyl phthalate 0.1-1% by weight,
Allyl acrylate 0.0005-0.002% by weight,
Furfural 0.001-0.001% by weight,
Formic acid 0.2-2% by weight,
Benzaldehyde 0.01-0.05% by weight,
Benzoic acid 0.05-0.2% by weight,
Maleic acid 0.5-3% by weight and
Phthalic anhydride 0.01-0.1% by weight.

The discharged aqueous phase is usually burned together with the other production residues. Before it is burned, the sour water is advantageously partially evaporated. The heat required for this can for example the cooling circuits be taken from the absorption unit.

The organic phase obtained in the extraction unit (the raffinate) generally contains the following components:
Diphyl 70-85% by weight,
Dimethyl phthalate 10-25% by weight,
Acrylic acid 0.2-2% by weight,
Maleic anhydride 0.05-0.3% by weight,
Diacrylic acid 0.2-2% by weight,
Phenothiazine 0.01-0.1% by weight,
Phthalic anhydride 0.1-1% by weight,
Benzoic acid 0.2-2% by weight,
Maleic anhydride 0.1-0.3% by weight,
Benzaldehyde 0.1-1% by weight,
Formic acid 0.02-0.1% by weight,
Acrolein 0.01-0.1% by weight,
Water 0.2-2% by weight,
Acetic acid 0.1-2% by weight and
Furfural 0.01-0.05 wt%.

The removed organic phase is in the inventive manner fed to the absorption unit (stage 4).

9 - Distillation unit (stage 9)

A subset of the swamp discharge of the Absorption unit (level 4), mainly made of acrylic acid, diphyl and dimethyl phthalate is a distillation unit supplied and in this by warming in a high boiler fraction and a low boiler fraction separated.

The distillation unit is common of known type (usually a column) and points the usual Internals on. In principle, all come as such column internals common Installations, for example floors, packings and / or fillings. Under the floors are bubble trays, sieve trays, valve trays, Thormann trays and / or Dual-flow trays prefers. Under the fillings are those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, barrel or Intalox saddles, Top-Pak etc. or braids preferred. Dual-flow trays are particularly preferred.

Usually there are 1 or 2 theoretical ones Soils sufficient. Due to the low separation task, there is also no need for internals become. The distillation is expedient in terms of application technology carried out under reduced pressure. Preferably is the head pressure 20 to 150 mbar, particularly preferably 40 to 100 mbar.

The distillation unit is common Made from austenitic steel, preferably from the material 1.4571 (according to DIN EN 10020).

The column is fed in at the bottom Area of the distillation unit, preferably directly in the column bottom. The feed amount is 0.1 to 2 wt .-%, preferably 0.2 to 1 wt .-%, based on the from the absorption unit (stage 4) to the product gas mixture cooling (stage 3) returned quantity of swamp liquid.

The heat can be supplied via internal and / or external heat exchangers of conventional design and / or via double-wall heating. External circulation evaporators with natural or forced circulation are preferred. External circulation evaporators with forced circulation are particularly preferred. The use of several evaporators, in series or in parallel, is possible. The temperature in the bottom of the distillation unit is advantageously 180 to 210 ° C, preferably 190 to 200 ° C. The high boiler fraction remaining in the column bottom of the distillation unit normally essentially contains the following components (the proportions are based on the total amount):
Diphyl 20-40% by weight,
Dimethyl phthalate 40-70% by weight,
Phenothiazine 10-30% by weight,
Acrylic acid 0.1-0.5% by weight,
Diacrylic acid 0.5-2% by weight,
Phthalic anhydride 0.5-2% by weight,
Benzoic acid 0.2-2% by weight,
Maleic anhydride 0.005-0.02% by weight and
Benzaldehyde 0.001-0.05 wt%.

The high boiler fraction becomes partial discharged, preferably 1 to 10 wt .-% based on that of Distillation unit fed Inflow amount, and partly over a heat exchanger returned to the bottom of the column. The residence time in the swamp the distillation unit is preferred over the viscosity of the cleavage residue regulated so that the cleavage residue remains pumpable at the discharge temperature. Is particularly preferred the gap residue clocked ejected. The discharged high boiler fraction is for better eligibility optionally with a suitable diluent, for example Dimethylformamide, diluted and usually burned together with the other production residues.

The low boiler fraction converted into the vapor phase in the distillation unit can be cooled and condensed indirectly, for example in heat exchangers which are known per se to the person skilled in the art and are not subject to any particular restriction, or directly, for example by a quench. A combination of direct and indirect cooling is preferably used. For this purpose, already condensed low boiler fraction is sprayed in the vapor and cooled together with the uncondensed vapor by means of a suitable heat exchanger and the cooled liquid is sprayed in the vapor above the heat exchanger. The effects of direct cooling can be increased by internals which improve the mixing of the cooled low boiler fraction with the vapor. In principle, all common internals, such as floors, packings and / or fillings, come into consideration. Bell bottoms, sieve bottoms, valve bottoms, Thormann bottoms and / or dual-flow bottoms are preferred among the bottoms. Among the fillings, those with rings, spirals, saddle bodies, Raschig, Intos or Pall rings, Barrel or Intalox saddles, Top-Pak etc. or braids are preferred. The condensation of the low boiler fraction can also be carried out in several stages (usually with an increasing temperature). The low boiler fraction obtained at the top of the distillation unit essentially contains the following components (the proportions are based on the total amount):
Acrylic acid 2-10% by weight,
Diphyl 50-70% by weight,
Dimethyl phthalate 20-40% by weight,
Diacrylic acid 0.5-5% by weight,
Acetic acid 0.01-0.05% by weight,
Water 0.005-0.05% by weight,
Formic acid 0.001-0.01% by weight,
Phenothiazine 0.05-0.2% by weight,
Benzaldehyde 0.01-0.1% by weight,
Maleic anhydride 0.05-0.2% by weight,
Benzoic acid 0.2-2% by weight,
Phthalic anhydride 0.2-2% by weight,
Furfural 0.001-0.005 wt% and
Acrolein 0.001-0.005 wt%.

Part of the as a low boiler fraction withdrawn liquid, preferably 90 to 99 wt .-% based on that of the distillation unit supplied Inflow, is discharged and in the lower section of the Absorption unit (level 4), preferably below the first Bottom (from below).

The waste gas leaving the distillation unit in gaseous form usually contains the following components (the proportions are based on the total amount):
Acrylic acid 40-70% by weight,
Acetic acid 0.5-5% by weight,
Water 10-30% by weight,
Diphyl 10-30% by weight,
Dimethyl phthalate 1-10% by weight,
Formaldehyde 0.5-2% by weight,
Acrolein 0.2-1% by weight,
Propionic acid 0.005-0.02% by weight,
Furfural 0.01-0.05% by weight,
Benzaldehyde 0.1-0.5% by weight,
Maleic anhydride 0.1-1% by weight,
Benzoic acid 0.05-0.2% by weight,
Phthalic anhydride 0.02-0.2% by weight,
Diacrylic acid 0.01-0.05% by weight,
Formic acid 0.1-0.5% by weight and
Allyl acrylate 0.002-0.01 wt%.

This exhaust gas is combined with the other production residues burned.

In the thermal separation units of the method according to the invention, the liquids supplied to and / or returned to the thermal separation units are generally applied as uniformly as possible to the separating internals. This is usually done using special distributors and / or nozzles. The distributors are either rectangular or ring-shaped above the separating internals and have a rectangular cross-section. Several distributors, for example two rectangular distributors next to one another, can also be used. The distributors are usually 10 to 50 cm, preferably 20 to 40 cm, above the separating internals. Each distributor has one or more feed lines, preferably one or two. The arrangement of the inlets is expediently chosen so that uniform distribution is ensured, for example in the case of rectangular distributors with a central inlet. The distance of the distributors from the inner wall of the column is usually 5 to 30%, preferably 10 to 20%, of the column diameter. The top edge of the manifold normally forms a point weir. The overflow openings usually have the shape of equilateral triangles with an edge length of often 1 to 5 cm, preferably 2 to 4 cm. The shape of the spike weir allows a uniform distribution of the liquid over the separating internals over a large area. The depth of the manifold to the overflow is often 10 to 30 cm, preferably 15 to 20 cm. The width of the distributors is often 10 to 30 cm, preferably 15 to 20 cm. The distributors are preferably provided with idle openings, as described in the German patent application DE-A 10218616 to be discribed.

When using tray columns becomes an undesirable Cross mixing on the floors is advisable Spray weirs prevented. The spray weirs are on the floors and are usually 10 to 30 cm, preferably 15 to 25 cm high. The spray weirs usually have openings on the side facing the floor. These openings typically have a height from 10 to 60 mm, preferably from 30 to 50 mm and a length of 10 to 60 mm, preferably 30 to 50 mm. The bridge between the openings regularly has one length of 10 to 60 mm, preferably 30 to 50 mm.

The advantages of the method according to the invention are an increased yield of acrylic acid, a ver better use of energy and less formation of unwanted polymer.

The specified in this document Unless otherwise stated, percentages are% by weight.

Finally, it should be noted that the process according to the invention can also be applied to a product gas mixture containing acrylic acid, as is the case in a three-step process according to WO 01/96270 starting from propane, or in a one-step process according to DE-A 10046672 starting from propane. Ie, it is particularly applicable to all acrylic gas-containing product gas mixtures, which according to the process of DE-A 10245585 as well as the DE-A 10246119 are available starting from propane.

Examples

example 1

propene

The first stage reaction tube was 3900 mm long and had an inner diameter of 29 mm. The catalyst bed was 2700 mm high. A catalyst like the one in the EP-A 1 005 908 is described. The bath temperature was 298-300 ° C; the hotspot was 364-373 ° C and 70-80 cm. Through the reaction tube 1235-1400 Nl / h cycle gas from the Absorption unit, 930-970 Nl / h air and 124-128 Nl / h propene ("polymer grade ").

acrolein

The second stage reaction tube was 3900 mm long and had an inner diameter of 29 mm. The catalyst bed was 3000 mm high. A catalyst like the one in the EP-A 1 071 507 is described. The bath temperature was 245-248 ° C; the hotspot was 295-301 ° C and 80-130 cm. In addition to Exit gas mixture from the propene oxidation was 265-290 Nl / h Air passed through the reaction tube.

Product gas mixture cooled

The device for product gas mixture cooling was an empty pipe with a length of 700 mm and an inner diameter of 80 mm. The swamp liquid The absorption column was pumped into the upper one Part of the empty pipe injected. The capacity the pump was 1000 l / h. By means of a heat exchanger in the pump circuit the temperature in the product gas mixture cooling was set to 165 ° C. Hourly became 90 g of swamp liquid discharged and 1100 ml of sump liquid from the absorption column added. The product gas mixture from acrolein oxidation was from the top passed below through the product gas mixture cooling.

absorption

The absorber column was a 30-plate bubble tray column with a length of 2160 mm and an inner diameter of 50 mm. The bubble tray column had an electric protective heater. The temperature of the protective heater was at 100 ° C (Bottom 1 to 10), 74 ° C (Bottom 11 to 20) and 58 ° C (Floor 21 to 30) set. The bottom temperature was 108 ° C and the Head temperature was 40 ° C. The absorber column was operated at a top pressure of 1155 mbar.

The one leaving the product gas mixture cooling Product gas mixture was in the absorber column below the first tray passed and countercurrent to 2240 ml / h absorbent from the Washing unit and organic raffinate from acid water extraction guided. The inlet floor for the absorbent from the washing unit was floor 23 from below. This feed also contained 0.7% by weight of acrylic acid. The inlet floor for the raffinate from the acid water extraction was floor 19 from below. The acrylic acid content in the organic raffinate was 1.5% by weight.

The bottom of the absorption column withdrawn sump liquid was through a heat exchanger to the second tray (from below) in the absorption column. The second floor was also the inlet for the low boilers from the Rectification column. The heat exchanger was adjusted so that the bottom temperature was 103 ° C. 3300 ml / h of the bottom liquid were removed.

From the 11th tray of the absorption column were 13000 ml / h reflux removed, in a heat exchanger to 44 ° C chilled and returned to the 14th tray of the absorption column. The 12th floor was also the inflow for the low boilers from the distillation unit.

Sour water condensation

The vapor of the absorber column was in a quench cooler condensed. The quench cooler was an empty pipe with a length of 700 mm and an inner diameter of 80 mm. Already condensed vapors was injected into the upper part of the empty pipe using a pump. The output the pump was 1000 l / h. By means of a heat exchanger in the pump circuit the temperature in the quench cooler to 17 ° C set. Hourly 200 ml of acid water were removed.

The uncondensed portions were reused as cycle gas or discarded.

desorption

The desorber column was a 25-tray bell-bottom column with a length of 1695 mm and an inner diameter of 50 mm. The bubble tray column had a double coat. The temperature of the double jacket was using a heat transfer oil to 115 ° C (bottom 1 to 12) and 110 ° C (Floor 13 to 25) set. The bottom temperature in the desorption column was at 110 ° C and the head temperature was 73 ° C. The desorber column was operated at a top pressure of 1195 mbar.

The from the absorption column in the desorber column led bottoms liquid was led to the top of the desorber column.

In front of the swamp of the desorber column there was a cycle gas preheater. The cycle gas preheater was an empty pipe with a length of 700 mm and an inner diameter of 80 mm. The washed Circulating gas from the washing unit was passed through the top of the unit Kreisgasvorwärmer directed. Bog liquid out the bottom of the desorber column was pumped into the upper one Part of the empty pipe injected. The output was 1000 l / h. The evaporate bottoms liquid was together with the cycle gas below the first floor (from below) fed to the desorber column. Using a heat exchanger the temperature in the bottom of the desorber column was in the pump circuit to 110 ° C set. Hourly were 3200 ml desorber sump liquid discharged.

The exhaust gas from the desorber column was together with the product gas mixture leaving the product gas mixture cooling led into the absorber column below the first tray (from below).

Rectification column: The rectification column was a 35-flush Bell-bottom column with a length of 3245 mm and an inner diameter of 80 mm. The bubble tray column had a double coat. The temperature of the double jacket was using a heat transfer oil at 150 ° C (bottom 1 to 11), 119 ° C (Bottom 12 to 25) and 100 ° C (Floor 26 to 35) set. The bottom temperature in the rectification column was 160 ° C and the head temperature was 76 ° C. The rectification column was at a top pressure of 80 mbar operated.

From the bottom of the desorption column led into the rectification column Swamp liquid was metered onto the 11th tray (from below) of the rectification column.

The bottom liquid of the rectification column was heated to 160 ° C. with a forced circulation evaporator, the circulation amount was 500 l / h. additionally 5 Nl / h of air were metered into the forced-circulation flash evaporator. Hourly were 2800 ml of sump liquid from the bottom of the rectification unit discharged.

about a side trigger on the 25th floor (from below) of the rectification unit were hourly 400 g of crude acrylic acid decreased. Another 800 g / h of raw taken from the side trigger acrylic acid were used as rewind metered onto the 24th tray (from below) of the rectification column.

The vapors of the rectification column was in a quench cooler condensed. The quench cooler was an empty pipe with a length of 700 mm and an inner diameter of 80 mm. Vapor has already been condensed injected into the upper part of the empty pipe by means of a pump. The output the pump was 1000 l / h. By means of a heat exchanger in the pump circuit the temperature in the quench cooler to 17 ° C set. The liquid in the quench cooler was with 200 ml / h of a 0.3 wt .-% solution of phenothiazine in crude acrylic acid stabilized. Hourly 150 ml of condensed low boilers were discharged and 3600 ml / h condensed low boilers as reflux used.

washing column

The washing column was a 15-tray bubble tray column with a length of 1015 mm and an inner diameter of 50 mm. The temperature was 55 ° C.

520 l of the Circulation gas leaving absorption column below the first Bottom (from below) passed into the washing column and in countercurrent with bottom liquid discharged from the rectification column washed.

Acid water extraction

The acid water extraction took place in a 20-flush Sieve tray column with a length of 2000 mm and an inner diameter of 50 mm. The temperature was 60 ° C.

560 ml per hour were added to the Absorbent removed from the distillation unit below the first Soil (from below) into the acid water extraction and in Countercurrent washed with acid water.

The aqueous extract was discarded and the organic raffinate was returned to the absorption column the 19th floor from below). Hourly became 172 ml of aqueous Extract with an acrylic acid content of 2.5% by weight. The acrylic acid loss via the aqueous extract was 4.3 g / h.

distillation unit

The distillation unit was one 5-bödige Bell-bottom column with a length of 470 mm and an inner diameter of 80 mm. The bubble tray column had a double coat. The temperature of the double jacket was adjusted to 120 ° C using a heat transfer oil. The bottom temperature was 178 ° C and the head temperature was 120 ° C. The distillation unit was operated at a head pressure of 80 mbar.

The one leaving the washing unit bottoms liquid was metered into the bottom of the distillation unit.

The bottom liquid of the distillation unit was heated with a forced circulation evaporator, the circulation amount was 4000 l / h. Hourly were 2 g of swamp liquid discharged.

From the first floor (from below) 2800 ml / h absorbent removed.

The vapors of the distillation unit were in a quench cooler condensed. The quench cooler was an empty pipe with a length of 700 mm and an inner diameter of 80 mm. Vapor has already been condensed injected into the upper part of the empty pipe by means of a pump. The output the pump was 1000 l / h. By means of a heat exchanger in the pump circuit the temperature in the quench cooler to 17 ° C set. Hourly 25 ml of low boiler distillate were discharged and fed into the absorption column.

Comparative example

The procedure was as in Example 1. The inlet floor for the organic raffinate from the acid water extraction in the absorption column however, was floor 23 (from below).

Hourly, 172 ml became watery Extract from acid water extraction with an acrylic acid content of 3.0% by weight. The acrylic acid loss via the aqueous extract was 5.2 g / h.

Example 2 It was like in example 1 proceed. The inlet floor for the organic raffinate from the acid water extraction in the absorption column however, was floor 20 (from below).

Hourly, 172 ml became watery Extract from acid water extraction with an acrylic acid content of 2.6% by weight. The acrylic acid loss via the extract was 4.5 g / h.

Example 3 It was like in example 1 proceed. The inlet floor for the organic raffinate from the acid water extraction in the absorption column however, was floor 17 (from below).

Hourly, 172 ml became watery Extract from acid water extraction with an acrylic acid content of 2.7% by weight. The acrylic acid loss via the extract was 4.6 g / h.

Claims (3)

Process of basic absorptive separation of acrylic acid from the product gas mixture of a heterogeneously catalyzed partial gas phase oxidation of propene to acrylic acid, which, in addition to acrylic acid and water vapor, contains low boilers, medium boilers, high boilers and heavy condensables as secondary components, in which - the product gas mixture is cooled to the temperature desired for absorption, - The cooled product gas mixture in an absorption unit for the purpose of absorptively separating the acrylic acid leads to a high-boiling liquid hydrophobic organic absorption medium descending in the absorption unit while avoiding the occurrence of a liquid aqueous phase in countercurrent, - The remaining product gas mixture not absorbed into the absorption medium cools in a condensation unit and the water vapor and low boilers contained therein condensed out as acid water, in particular the remaining water in the acid water condensation e divides the heavy condensable residual gas as circulating gas into partial amounts of the same composition, - a circulating gas partial amount as dilution gas into the heterogeneously catalyzed partial gas phase oxidation of Recycle propene to acrylic acid, use a different amount of circulating gas in a form that has been freed of acrylic acid as stripping gas and dispose of the remaining amount of circulating gas, - a small portion of the liquid drain containing the acrylic acid absorbed to the absorption unit of a distillation unit as a high boiler outlet, which is heated by heating the liquid drain in the vapor phase generated in this way is returned to the absorption unit and the high boilers which remain liquid are disposed of, - from the rest of the absorption unit, which has been effectively removed from the absorption unit and contains the acrylic acid absorbed, the liquid unit is removed in a desorption unit by stripping with the partial gas amount of circulating gas which has been washed free of acrylic acid in the washing unit, returns stripping gas loaded with low boilers to the absorption unit, - the absorber loaded with acrylic acid, which runs out of the desorption unit feed agent flow to a rectification unit with reinforcement and stripping section, - in the stripping section of the rectification unit the absorbent and high boilers are condensed into the sump of the rectification unit, - crude acrylic acid is separated off in the rectification section of the rectification section, - the low-boiling stream separated over the top of the reinforcement section is returned to the absorption section , - from the sump area of the rectification unit removes the absorbent from which low boilers and acrylic acid have been removed, if necessary supplemented with fresh absorbent and in the washing unit in countercurrent to the portion of the circulating gas still containing acrylic acid in order to wash it free of acrylic acid, - the absorbent running off in the washing unit , if necessary after it was passed over the distillation unit used as the high boiler outlet, divided into two portions of the same composition, the larger of the be returns the partial quantities to the top of the absorption unit and brings the smaller partial quantity into interaction with acidic water in order to absorb acrylic acid contained in the acidic water into the absorption medium and polar middle boilers contained in the absorption medium into the acidic water and - returns the organic raffinate obtained during the acidic water extraction to the absorption unit, characterized in that - the return of the organic raffinate obtained during the acid water extraction into the absorption unit takes place at least one theoretical separation stage below the head of the absorption unit. A method according to claim 1, characterized in that the Repatriation of the organic raffinate of acidic water extraction at least two theoretical plates below the head of the absorption unit he follows. A method according to claim 1 or 2, characterized in that the organic raffinate of the acid water extraction at most 5% by weight acrylic acid contains.