DE2842092C2 - - Google Patents

Description

The invention relates to a method for separating Methacrolein and / or methacrylic acid from their aqueous solution solutions, in particular an effective method of separation and recovering methacrylic acid and methacrolein from the Production of methacrylic acid, in the methacrolein and / or a compound that is under the reaction conditions Methacrolein gives, and molecular oxygen a gas phase oxidation in the presence of a catalyst be opened.

Examples of compounds under the reaction conditions form methacrolein (hereinafter: "Precursors [of Methacrolein] ") are isobutene and tert-butanol.

In the oxidation of methacrolein or its precursor in the gas phase together with water vapor and molecular acid substance (usually air) in the presence of an oxidation catalyst lysator gives a gaseous reaction mixture that in addition to methacrylic acid, acetic acid, carbon monoxide, carbon di oxide, unreacted methacrolein or its precursor,  Contains oxygen, nitrogen and the like. It is therefore required, not only methacrylic acid, but also meth to remove acrolein from the reaction mixture in an effective manner separate.

There are various commercially available catalysts for the Oxidation of methacrolein or its precursor to meth acrylic acid known, but not in technical operation fully satisfied.

Enable most currently known catalysts under reaction conditions where the methacrolein low sales, high selectivity for methacrylic acid while operating under reaction conditions where the Sales is high, a significantly reduced selectivity surrender. In the case of a relatively low methacroleinum sentence is therefore the development of effective procedures for Ab separation of unreacted methacrolein from the aq gene methacrylic acid solution and for recycling methacrolein in the oxidation reactor an important factor that the Economy of the production of methacrylic acid Gas phase oxidation of methacrolein or its precursor certainly.

In JP-OS 1 11 017/1975 z. B. a procedure for separating methacrolein from the gaseous reaction gene mixed described, which consists essentially in that the gas mixture containing methacrolein in 50 to 500 mol Water, based on 1 mole of methacrolein, absorbed and the obtained aqueous solution, which is the separated methacrolein contains, then distilled or stripped to the methacrolein to win. To get the methacrolein out by this procedure To separate the gaseous reaction mixture, the Ab sorption of methacrolein under high pressure or never temperature or with a large amount of water or in an absorption tower with an exceptionally large floor number can be carried out. Because methacrolein in water only  is very slightly soluble (6.1% by weight at 25 ° C) the absorption of methacrolein in water was extremely difficult rig. If you use a large amount of water as an absorption Solvents for methacrolein, not only have to Methacrolein absorption tower, but also the distilla tion and stripping tower that follow the absorption tower, be enlarged. To contribute to the absorption of methacrolein high pressure, the operating pressure of all Plants, including the reactor, which the methacrolein Absorption tower are upstream, with the increase in Operating pressure of the absorption tower also increased who the. However, this is very difficult Lich the pressure resistance and safety of the systems as linked to economy. Absorption too of methacrolein at low temperatures or with Ab sorption towers that have a large number of soils economically disadvantageous. For these reasons, the Ver use of solvents in which methacrolein little is soluble, e.g. B. water, disadvantageous.

Given the low solubility of methacrolein in what Ser is in JP-OS 92 007/1974 the use of Alko get instead of water as a solvent for the meth acrolein absorption suggested. Alcohols are suitable for this purpose; on the return of methacrolein in the oxidation system these organic solutions are tel, if only in traces, together with the separated one Methacrolein is introduced into the oxidation system where it various difficulties, e.g. B. poisoning of Ka talysators or oxidation inhibition, as well as unwanted Re actions, e.g. B. Explosions caused by contamination of the steam with the organic solvent. It is there not always advantageous, organic solvents Use solvents for methacrolein absorption even if they absorb methacrolein very well.  

When looking for solvents for the absorption of methacrolein, which absorb methacrolein very well and even when it is returned to the oxidation system together with the separated methacrolein, no difficulties, e.g. B. cause no catalyst poisoning, it has now been found that an aqueous methacrylic acid solution has far better solubility for methacrolein than water itself; see. Fig. 1.

The US-PS 35 13 632 describes a method for separation of acrylic acid and acrolein by washing the gas mixture with water.

The washing solution containing the acrylic acid becomes one Subject to stripping while the acrolein ent holding gas washed with water in a second absorber with the acrolein being completely absorbed.

On the alleged applicability of the separation procedure of methacrolein of methacrylic acid is pointed out. This Note is in no way by description of a Procedure or demonstrated by examples and it is not of course, that the reaction conditions without further on the separation of methacrolein and methacrylic let acid transfer.

DE-OS 14 43 695 is based on the task, a procedure ren to create acrylic acid, in which the gases leaving the reaction vessel are quickly quenched be, causing the undesirable polymerization of the acrylic acid monomers is reduced (p. 2, lines 1 to 5).  

The process is that after the oxidation of Acrolein obtained gaseous reaction mixture with a ge cooled liquid from acrylic acid or an aqueous solution solution is quenched by acrylic acid. The cooling takes place by direct contact of the gases with the pre-cooled rivers liquidity.

The object of the invention is to provide a method completely separated from methacrylic acid with methacrolein can be, the concentration of the acid in aqueous Solution is kept as high as possible.

The invention relates to a method for separating Methacrolein and / or methacrylic acid from gaseous reak tion mixtures used in catalytic gas phase oxidation of methacrolein, which is characterized by that you have the gas mixture from the oxidation reactor in which the catalytic gas phase oxidation has been carried out in initiates a condenser in which the gas mixture 100 ° C or less in direct countercurrent contact with one Part of a condensed liquid that brings up the bottom End of the capacitor has been obtained, mainly Lich the water contained in the gas mixture and the Condense methacrylic acid. If this condensation in one stage, the operating temperature is ture 100 ° C or less, preferably 30 to 80 ° C. If the Condensation is carried out in two stages, the Operating temperature in the first stage 100 ° C or less, preferably 30 to 80 ° C, and is in the second stage 10 up to 50 ° C below the temperature range of the first condenser sators. Operating temperatures above 100 ° C should be avoided because they lead to a polymerization of methacrolein and Methacrylic acid can lead.

The way in which the gas mixture is in direct counter brought into electrical contact with the condensed liquid is not subject to any particular limitation and it can  NEN any conventional equipment, e.g. B. packing, Sieve tray, bubble tray or spray towers can be used.

The methacrolein-containing gas obtained from the top of the The condenser is then at the bottom of a methacrolein Ab sorption tower initiated at a temperature of 30 ° C or less, preferably 0 to 15 ° C is operated. In the tower, the gas is in direct countercurrent contact with brought an aqueous solution, the 5 weight percent or more, preferably 10 weight percent or more, methacrylic contains acid and is fed in at the top of the tower, the methacrolein contained in the gas in the solution is absorbed. At the top end you get an almost methacrolein free gas, while at the bottom an aqueous one Methacrylic acid solution is obtained, the absorbed methacrolein contains. Operating temperatures of the methacrolein absorption Avoid towers above 30 ° C as they are the methacro impair linen absorption. In addition, methacrylic acid Concentrations of less than 5 percent by weight lig, since then the solubility of methacrolein decreases and thus the absorption performance is impaired.

The type of methacrolein absorption tower is not subject to any certain limitation and it can be any conventional Towers are used, e.g. B. packing, sieve bottom, gloc kenboden- or spray towers.

The aqueous methacrylic acid solution is used for the methacrolein absorption used in an amount that the molar ratio the solution to the 8 or 8 entering the absorption tower less, preferably 1 to 5.  

Aqueous methacrylic containing the absorbed methacrolein Acid solution is released from the lower end of the absorption tower drawn and at the top of a methacrolein stripping tower initiated at 30 to 100 ° C, preferably 50 to 80 ° C, is operated. At the same time, the condenser obtained in the condenser is also condensed liquid in the middle or at the top of the stripping tower. An inert gas - e.g. B. nitrogen, Air, carbon dioxide, exhaust gas that is produced during the oxidation of meth acrolein or its precursor arises, exhaust gas that at the combustion of the first-mentioned exhaust gas occurs, or a mixture of these inert gases - is at the bottom of the tower initiated, the amount of gas is adjusted so that the molar ratio of the total discharged into the tower Liquid to the gas 3 to 30, preferably 5 to 20, be wearing. That in the aqueous solution and the condensed Liquid methacrolein is removed from it separated and at the top of the tower in the form of a gasge mix subtracted while that of methacrolein is practical freed aqueous methacrylic acid solution abge at the lower end is leading.

At an operating temperature of the methacrolein stripping tower below 30 ° C is the methacrolein vapor pressure low, so that the stripping effect is impaired. At Temperatures above 100 ° C can lead to polymerisation sation of methacrolein and methacrylic acid.

The type of stripping tower is not subject to any particular loading restriction and any conventional towers can be used be used, the gas-liquid contact possible chen, e.g. B. packing, sieve tray, bubble tray or Spray towers.

The operating temperature of the absorption tower and stripp towers can be set in different ways depending on the type of tower. For example, you can see the tower  provided with a jacket, or a heating medium through the Feed the jacket or a spiral.

The aqueous solution containing 5 weight percent or more meth contains acrylic acid and as an absorption solvent for Methacrolein at the top of the methacrolein absorption towers is fed, an aqueous methacrylic acid solution be obtained independently of the method according to the invention or part of the bottom liquid of the methacrolein Stripping tower. In the latter case, the floor liquid cooled and returned to the absorption tower leads.

The methacrolein-containing inert gas mixture from the upper end of the Stripping towers can be used to make methacrolein or for used for other purposes or together with a Inert gas to the reactor for the catalytic gas phase oxides tion of methacrolein or its precursor the.

In the drawing, the invention is explained in more detail: Fig. 2 is a flow diagram of an embodiment of the separation of methacrolein and methacrylic acid, in which the condensation of the gas mixture from the reactor is carried out in two stages.

The gas mixture from the reactor 3 for the catalytic gas phase oxidation of methacrolein and / or its precursor is fed via line 4 to the lower end of a first capacitor 5 . Then the gas is brought into direct countercurrent contact with a liquid containing methacrylic acid, which is circulated via line 6 , cooler 8 and line 9, the methacrylic acid contained in the gas mixture and the water vapor condensing. The methacrylic acid-containing liquid is fed from the lower end of the first condenser 5 via the line 7 , the heat exchanger 24 and the heating device 25 to the middle part of the methacrolein stripping tower 31 . The top gas of the first capacitor 5 is fed via line 10 to the lower end of the second capacitor 11 , which is operated at a lower temperature than the first capacitor 5 . The gas is then brought into direct countercurrent contact with a liquid containing methacrylic acid, which is circulated through line 12 , cooler 14 and line 15 , with large portions of the methacrylic acid or water vapor remaining in the head gas from the first condenser being condensed . The methacrylic acid-containing liquid from the lower end of the second capacitor 11 is then fed via the line 13 and the heat exchanger 23 to the middle part of the stripping tower 31 . The line 7 for withdrawing the first condensed liquid and the line 13 for withdrawing the second condensed liquid can (as shown in FIG. 2) be connected separately to the stripping tower 31 or brought together and connected to the stripping tower 31 .

The overhead gas from the second condenser 11 , from which large proportions of methacrylic acid and water vapor in the first condenser 5 and the second condenser 11 have been removed, is then fed via line 16 to the lower end of the methacrolein absorption tower 17 . At the upper end of the absorption tower 17 , an aqueous methacrylic acid solution as a solvent for methacrolein absorption is introduced via line 26 . The solution is brought into countercurrent contact with the rising gas containing methacrolein so that it absorbs methacrolein. The methacrolein-free overhead gas from the absorption tower 17 is drawn off via the line 20 and blown off into the atmosphere via an exhaust gas treatment zone. The aqueous methacrylic acid solution containing absorbed methacrolein is drawn off via line 19 and fed to the upper end of stripping tower 31 via heat exchanger 21 and heating device 22 .

Preferably, the absorption tower 17 is operated at the lowest possible temperature (namely at 30 ° C. or less, preferably before 0 to 15 ° C.) in order to increase the amount of methacrylic acid absorbed. For this purpose, the temperature must be controlled by providing the tower with a jacket and / or a cooling coil inside and allowing a cooling medium to circulate. The supplied through the line 26 aqueous methacrylic acid solution can be part of the circulating soil liquid from the stripping tower 31 or else an aqueous methacrylic acid solution which was obtained independently of the process according to the invention.

In the stripping tower 31 , an inert gas is introduced via line 32 , e.g. As air, nitrogen, carbon dioxide, exhaust gas, which was obtained in the oxidation of methacrolein and / or its precursor, or exhaust gas, which arises in the combustion of the former exhaust gas. Methacrolein is therefore stripped from the liquids supplied through lines 19 , 13 and 7 , while a methacrolein-containing gas mixture is drawn off via line 27 at the upper end.

This gas mixture can be used for the production of methacrolein or for other purposes or via the Heizvorrich device 28 and the gas supply line 1 in the reactor 3 leads back. The soil liquid from the stripping tower 31 is passed through line 29 , sheared in the heat exchangers 23 and 24, and drawn off via line 30 as a practically methacrolein-free aqueous methacrylic acid solution. The solution is fed to a cleaning stage for methacrylic acid or other stages. Part of the bottom liquid from the stripping tower 31 can be supplied to the absorption tower 17 as a solvent for the methacrolein absorption via the heat exchanger 21 , the line 26 and the cooler 18 .

In the methacrolein stripping tower 31 , the operating temperature drops because a large part of the methacrolein and some water evaporate. In order to keep the operating temperature within the required range, the tower must be provided with a jacket or a heating spiral through which a heating medium circulates. The arrangement of the heat exchangers 21 , 23 and 24 , the heaters 22 and 25 and the cooler 18 in the jacket system or the heating coil system of the absorption tower and the stripping tower need not necessarily correspond to the flow picture of FIG. 2 when the operating temperatures of the absorption tower and Stripping tower to be kept within the required areas. These devices can optionally be omitted or used in addition.

If the condensation of the gas mixture is not carried out as in FIG. 2 at two temperature levels, but only at one temperature level, this is done with the omission of the second condenser 11 , the lines 12 , 13 , 15 and 16 , the cooler 14 and the heat exchanger 23 , by connecting the first condenser 5 directly to the absorption tower 17 via the overhead gas line 10 .

The operating temperature of the condenser is preferably selected so that ordinary cooling water can be used as the cooling medium in the coolers 8 and 14 which are connected to the condenser. Furthermore, the lowest possible operating temperature is preferred for economic reasons, since in this way the polymerization of methacrylic acid in the condensed liquid can be prevented, the amount of water vapor in the top gas from the second (or first) condenser, which is the methacrolein absorption tower 17 is supplied via line 16 (or 10 ), is ent, decreases and thus the cooling capacity of the absorption tower 17 and the cooler 18 can be reduced. Too low operating temperatures are disadvantageous, however, because if the average temperature difference between the operating temperature and the cooling medium is too low, an extraordinarily large heat transfer surface in the coolers 8 and 14 is required. When using the first capacitor alone, an operating temperature of 30 to 80 ° C is preferred. When using two capacitors, the operating temperature of the first capacitor is preferably 30 to 80 ° C, while that of the second capacitor is 10 to 50 ° C below the temperature range of the first capacitor. The choice of a single-capacitor system, two-capacitor system or multi-capacitor system is therefore based on the following factors:
Required heat transfer area of the coolers 8 and 14 connected to the respective condensers and cooling capacity of the absorption tower and the cooler 28 . The respective advantages of the two systems cannot therefore be stated from the outset.

The operating pressure of the condensers 5 and 11 , the meth acrolein absorption tower 17 and the methacrolein stripping tower 31 is selected depending on the operating pressure of the reactor 3 for the catalytic gas phase oxidation of methacrolein or its precursor. However, it is preferred to select operating pressures that can be achieved without having to provide a line 16 or 27 with a compressor to increase the pressure; usually 101 to 811 kPa, preferably 152 to 507 kPa.

Figure 1 shows the Henry coefficients of methacrolein in water and aqueous methacrylic acid solution.

Fig. 2 shows an embodiment of the separation of meth acrolein and methacrylic acid according to the inventive method.

The examples illustrate the invention. Reference numerals used in the examples refer to FIG. 2.

example 1

A 310 ° C hot gas mixture, the 1.62 percent by volume Methacrolein, 1.57 volume percent methacrylic acid, 0.52 vol lumen percent acetic acid, 42.75 volume percent steam and 53.53 percent by volume of non-condensable gases such as Sauer contains substance, nitrogen, carbon monoxide and carbon dioxide, is in an amount of 4528 Nl / h. one with Raschig Rings filled porcelain tower as the first condenser guided. In the first capacitor 1698 g / h. one Obtained condensate whose temperature be 45 to 50 ° C wearing. The concentrations of methacrylic acid, methacrolein and acetic acid in the liquid are 15.24 Ge percent by weight, 0.18 percent by weight or 3.37 percent by weight cent.

The gas drawn off at the upper end of the first condenser 5 contains 2.65 percent by volume methacrolein, 0.13 percent by volume methacrylic acid and 8.08 percent by volume steam and is used in an amount of 2723 Nl / h. (Total volume) fed to a porcelain tower filled with Raschig rings as a second capacitor 11 . 94.66 g / h. obtained a condensate whose temperature is 32 to 35 ° C. The concentrations of methacrylic acid, methacrolein and acetic acid in the liquid are 10.19 percent by weight, 0.74 percent by weight and 4.567 percent by weight. The gas drawn off at the upper end of the second condenser 11 contains 2.75 volume percent methacrolein, 0.04 volume percent methacrylic acid and 4.62 volume percent steam and is in an amount of 2619 Nl / h. (Ge total volume) fed to the lower end of a stainless steel tower filled with Raschig rings as a methacrolein absorption tower 17 . As a solvent for methacrolein absorption, part of the soil liquid from methacrolein stripping tower 31 (which contains 15.1 percent by weight methacrylic acid, 3.46 percent by weight acetic acid and 0.20 percent by weight methacrolein) is cooled to 8 to 10 ° C. and fed to it in an amount of 6378 g / h at the top of the absorption tower 17 .

At the lower end of the absorption tower 17 , 6689 g / h. obtained a liquid containing methacrolein containing 3.55% by weight of methacrolein, 14.46% by weight of methacrylic acid and 3.40% by weight of acetic acid. At the upper end of the absorption tower 17 2451 Nl / h. withdrawn from a gas containing 60 ppm methacrolein. The methacrolein absorption tower (diameter 55 mm; height of the packing area 6 m) is provided with a jacket and is operated at 5 to 15 ° C. Both the first and the second condensate are introduced into the central part of the methacrolein stripping tower 31 , while the obtained liquid containing absorbed methacrolein is supplied to the upper end. At the lower end of stripping tower 31 , 692 Nl / h. Nitrogen introduced. At the upper end of the stripping tower 847 Nl / h. obtained a gas containing 8.48 volume percent stripped methacrolein and 10.0 volume percent steam. This gas is heated to 280 ° C and returned to the oxidation reactor 3 . At the lower end of the methacrolein stripping tower 31 , 8183 g / h are obtained. a bottom liquid containing 15.1 percent by weight methacrylic acid, 3.46 percent by weight acetic acid and 0.20 percent by weight methacrolein. Part of the soil liquid (6378 g / h) is recycled as a solvent for methacrolein absorption, while the rest (1805 g / h) is withdrawn as product. The stripping tower (diameter 55 m, height of the packing area 3 m) is a stainless steel tower filled with Raschig rings, which is provided with a jacket and is operated at 60 to 70 ° C.

Example 2

The process is carried out as in Example 1, but the second condenser 11 is omitted and the overhead gas from the first condenser 5 is fed directly to the lower end of the methacrolein absorption tower 17 , which has a height of the packing area of 6.5 m. A gas containing 65 ppm methacrolein is obtained at the upper end of the absorption tower 17 , while at the lower end of the tower 6778 g / h. a liquid containing 3.52 percent by weight of absorbed methacrolein, 14.41 percent by weight of methacrylic acid and 3.34 percent by weight of acetic acid. The compositions and throughputs in the other areas essentially correspond to those of Example 1.

Claims (4)

1. A process for the separation of methacrolein and / or methacrylic acid from methacrylic acid-containing gaseous reaction mixtures obtained in the gas phase reaction of methacrolein and / or a compound which gives methacrolein under the reaction conditions, and molecular oxygen at elevated temperature in the presence of an oxidation catalyst have been characterized in that the gaseous reaction mixture from the reactor in which the gas phase reaction has been carried out is brought into direct countercurrent contact with a portion of a condensed liquid in a condenser at a temperature of not more than 100 ° C. The lower end of the condenser has been obtained, so the methacrylic acid and the water vapor of the reaction mixture are condensed and the greater part of the methacrolein and non-condensable gases are separated from the reaction mixture, the gas obtained at the lower end of a methacroline absorption tower, which at e is operated in a temperature of not more than 30 ° C, initiates, the methacrolein contained in the gas at a liquid / gas molar ratio of 8 or less in an aqueous solution containing not less than 5 weight percent methacrylic acid, and which one feeds at the upper end of the absorption tower, the methacrolein-containing aqueous methacrylic acid solution obtained is drawn off at the lower end of the tower and the upper end of a methacrolein stripping tower, which is operated at a temperature of 30 to 100 ° C., is supplied simultaneously by condensation the condensed liquid obtained in the gaseous reaction mixture is introduced into the middle part or at the upper end of the stripping tower and an inert gas in a liquid / gas molar ratio of 3 to 30 is supplied at the lower end of the tower and from the methacrolein-containing aqueous methacrylic acid solution or from the condenser to the condensed liquid, the methacrolei n separates and draws off at the upper end of the stripping tower and draws off the methacrylic acid as an aqueous solution at the lower end of the stripping tower. 2. The method according to claim 1, characterized in that the direct countercurrent contact between the gaseous reaction mixture and part of the previously in the Condenser obtained condensed liquid 100 ° C or less in several stages. 3. The method according to claim 1, characterized in that a part of the aqueous methacrylic acid solution from lower end of the methacrolein stripping tower as an absorber tion solvent for the methacrolein absorption tower starts. 4. The method according to claim 1, characterized in that the gaseous mixture containing methacrolein from the top End of the stripping tower of methacrolein circulating to the inlet of the oxidation reactor.