WO1999026911A1

WO1999026911A1 - Condensation method

Info

Publication number: WO1999026911A1
Application number: PCT/EP1998/007491
Authority: WO
Inventors: Michael-Dieter Ulbrich; Armin Schraut; Ulrich Hammon; Volker Schliephake
Original assignee: Basf Aktiengesellschaft
Priority date: 1997-11-25
Filing date: 1998-11-20
Publication date: 1999-06-03
Also published as: DE19752199A1

Abstract

The invention relates to a method for condensing one or more components of a gas mixture in a condenser, wherein the difference in temperature between the gas mixture and the cooling surfaces at every point in the condenser is less than 25 °C, especially less than 20 °C. The method is particularly suitable for separating condensable reaction products and/or their by-products from reaction gases, preferably for separating (methyl)acrylic acid and/or (meth)acrolein and/or glyoxal and/or ethylene oxide and/or aniline.

Description

Condensation process

The invention relates to a method for the condensation of one or more components from a gas mixture.

Such processes are used, for example, in the processing of gaseous reaction mixtures, with condensable by-products or impurities first being removed by condensation. The main product remaining in the gas phase is then taken up in a suitable absorbent and separated therefrom, for example by stripping. Such a sequence of work-up steps is common, for example, in the production of (meth) acrolein or (meth) acrylic acid.

EP-B 0 559 227 discloses a process for the preparation of acrolein by working up a gas mixture which is formed in the catalytic gas phase oxidation of propene. The condensation conditions are set here so that the condensate from the bottom of the column has a temperature between 35 ° and 50 ° C., the gas drawn off at the top has a temperature between 35 ° and 55 ° C., and that the temperature of the gas separated off at the top is approximately the same or is greater than the temperature of the condensate in the column bottom. In order to achieve these process conditions, for example, the condensate stream to be fed to the column is separated into two partial streams, a first partial stream being fed to the lower column part after prior cooling and a second partial stream to the upper column part without being cooled. As a further process condition, the residence time of the condensate in the cooling column and the circulating section is limited to 0.5 to 3 hours. The procedure described is intended to form acrolein hydrate in the condensate which is obtained when the product mixture is worked up for the production of acrolein. be inhibited. A disadvantage of this procedure is that, due to the large temperature difference between supercooled condensate and saturated gas, fog droplets can form in the presence of condensation nuclei. In the mist droplets, by-products are carried off into the downstream separation of the reaction product, as a result of which the reaction product is contaminated. Another disadvantage is that the condensate at the bottom of the column is relatively cold and thus dissolves a larger amount of the gaseous reaction product acrolein, which leads to product losses.

The object of the invention is therefore to provide a process for working up gaseous reaction products which avoids the formation of mist, reduces the loss of gaseous reaction product in the condensate and reduces the formation of by-products.

The solution is based on the known method for condensing one or more components from a gas mixture in a condenser. The method according to the invention is then characterized in that the temperature difference between the gas mixture and the cooling surface at each point of the condenser is less than 25 ° C, in particular less than 20 ° C.

A condenser is known in a known manner in which condensation takes place on a cooling surface. The cooling surface can be generated as a cold wall, for example in a tube bundle or plate heat exchanger, or as a liquid film, for example in a column with falling film internals.

Hot gaseous reaction products are often worked up by evaporating a solvent, for example water, in a quench column in a first cooling step, the heat of vaporization being used for gas cooling. To remove reaction products which are poorly soluble in the solvent, the reaction gas saturated with the solvent must be cooled further become. In this further cooling step, the evaporated solvent and also gaseous by-products are condensed from the reaction gas by contact with supercooled condensate. If the temperature difference between supercooled condensate and reaction gas is too large, the reaction gas is cooled down faster than the solvent and by-products can condense. The heat transfer is then faster than the mass transfer belonging to the thermodynamic equilibrium. An imbalance in the gas phase occurs, which is called supersaturation. In the presence of condensation nuclei, this supersaturation is reduced by the condensation of solvents and by-products on these nuclei. With bacterial counts, such as those already present in the ambient air, it is possible that droplets of fog that are difficult to separate can be formed by reducing the supersaturation. In the mist droplets, by-products are carried off into the downstream separation of the reaction product, which leads to contamination of the reaction product. If the mist droplets containing by-products or impurities are separated in downstream compressors, they can lead to deposits and corrosion.

It has now been found that during the cooling of the hot gas mixture with a maximum temperature difference of 25 ° C., in particular of 20 ° C., there is no mist formation between the gas mixture and the cooling surface in direct contact with it.

The method is not restricted in terms of temperature: it can be carried out in the range from 50 Kelvin to 600 Kelvin, preferably from 200 Kelvin to 400 Kelvin, usually in the range from 270 to 370 Kelvin. Of the components of the gas mixture, as a rule only the portion corresponding to the thermodynamic equilibrium at the temperature of the process is condensed, so partial condensation generally takes place.

The method is particularly suitable for condensation in the presence of at least 100 condensation nuclei per cm ³ , preferably at least 1000 nuclei per cm, particularly preferably of at least 10,000 germs per cm ³ .

The method is particularly suitable for condensation from gas mixtures with a Lewis index of 0.8 to 5.0, preferably 0.9 to 3.0, particularly preferably 5 0.9 to 1.5. The Lewis index is defined as the ratio between the temperature conductivity a of the gas mixture and the diffusion coefficient of the condensing substance A in the gas phase:

Le = a _g as DstoffA with

Cp, _g as [J / (kg. K)] -Pkgkg / m

15 (see also A. Mersmann: Stofi Transfers, Springer Verlag, 1986, page 105). The Lewis index expresses the relationship between heat transfer and material transfer in a gas mixture. To calculate the Lewis key figures to be taken into account according to the method according to the invention, the value used as the coefficient of diffusion D is that of the components of the gas mixture

20 with the smallest diöusion coefficient. In particular with higher Lewis indicators, that is to say when condensing components with larger molecules and thus smaller diffusion coefficients, it is advantageous to avoid oversaturation if the temperature difference between the gas phase and the cooling surface at each point of the condenser is less than 15 ° C., in particular less than

25 to keep 10 ° C. For the condensation of water vapor from a gas mixture that mainly contains air, the Lewis index is approximately 0.9. With a concentration of condensation nuclei of at least 1000 nuclei per cm, fog formation at a temperature gradient of maximum 25 ° C between the gas mixture and the cooling surface is avoided.

30

A liquid film, in particular an aqueous liquid film and / or a cold wall, is equally suitable as the cooling surface. Another application of the process is the partial water vapor condensation from water vapor-saturated flue gases with supercooled washing solutions. The supersaturated water vapor forms very quickly on the fine dust usually present in flue gases.

If a liquid film is used as the cooling surface, the process can be carried out in such a way that the gas phase and liquid film are carried out in cocurrent or in countercurrent.

The condensation can take place in one or more stages, preferably in two stages. In the case of two-stage or multi-stage cooling and condensation, in the case of countercurrent flow of gas and liquid, a first partial flow of the condensate from the bottom of the column into the lower part of the column and a further partial flow of condensate from the bottom of the column after prior cooling in the upper part of the column as cooling medium to be led back.

The process is particularly suitable for the separation of condensable organic substances from gas mixtures which have been produced in particular in gas phase reactions, preferably for the separation of (meth) acrylic acid and / or (meth) acrolein and / or glyoxal and / or ethylene oxide and / or aniline from gas mixtures .

The invention is explained in more detail below with the aid of exemplary embodiments and drawings.

The individual shows:

Figure 1 shows the dependence of the fog formation in the condensation of

Water and methacrylic acid from a reaction gas of the temperature difference between the gas phase and the cooling surface, which are in direct contact with one another, Figure 2 shows a process scheme for the production of acrolein, the

Condensation of water and by-products takes place according to the method according to the invention,

5

3 shows a section of the process diagram for the production of acrolein, which relates to the condensation from the reaction gas, which is the focus of the present invention (FIG. 3a), FIG. 3b shows the diagram of a condensation according to the prior art,

FIG. 4 shows the temperature difference between the gas phase and the liquid phase, which are in direct contact with one another, plotted over the column height h, curve a representing the method according to the invention and curve b representing the prior art

Example 1:

The invention is explained below using the example of the condensation of water and methacrylic acid from a reaction gas

20th

In a full-body column with a diameter of 0.1 m, a water-saturated reaction gas stream of approximately 70 ° C. with a temperature of approximately 70 ° C. was converted from the methacrylic acid oxidation of 22 m / h with a water stream of 30 l / h in countercurrent at different temperatures in the cooling water cooled down

25

5 3

The reaction gas contained about 3 x 10 nuclei per cm with an average diameter of 0.13 μm, the number and size of nuclei using an SMPS device (Scanning Mobile Particle Sizer) according to the "Scanning Electrical Mobility Spectrometer" in SC Wang and RC Flagen ", Aerosol Science and Technology 13, pages 230 to 30 240, (1990). The signal of a transmitted light probe was determined depending on the temperature difference between the reaction gas and with measured in this cooling water in direct contact. The signal of the transmitted light probe in mV is a measure of the amount of fog formed. At temperature differences between the reaction gas and cooling water below about 20 ° C, the transmitted light probe no longer showed any fog.

The measurement results for the signal of the transmitted light probe at different values for the temperature difference between reaction gas and cooling water are listed in Table 1 below and shown in FIG. 1:

In addition to water vapor, the reaction gas contained nitrogen, oxygen, carbon dioxide, carbon monoxide and the starting material methacrolein.

During the cooling, water vapor and methacrylic acid were condensed out of the reaction gas, which has a thermal conductivity a of approx. 1.6 x 10 ^" ms ^" .

These measurement results were obtained for the reaction gas mentioned by way of example from the methacrylic acid oxidation, but they generally apply to the condensation from gas mixtures, with Lewis key figures from 0.8 to 5.0. The predominantly condensing component in Example 1 is water vapor. For components that condense more heavily than water vapor, for example methacrylic acid with Le = 2.5, maintaining a temperature difference of at most 25 ° C., in particular 20 ° C., is even more important.

Example 2:

Example 2 relates to a two-stage procedure for the condensation of water and by-products from the reaction gas which is particularly suitable for the reaction gas cooling in the production of acrolein. Acrolein is technically produced by the heterogeneously catalyzed oxidation of propene with atmospheric oxygen:

The reaction gas from the propene oxidation is worked up in a system shown schematically in FIG. 2:

The approximately 220 ° C hot reaction gas, which mainly contains nitrogen (76.6% by weight), acrolein (8.9% by weight) and water (4.7% by weight), is fed 1 in via a feed line the upper part of a quench column K 1, and brought into contact with a quench circulation solution 2, which consists predominantly of water, and also acrolein, acrylic acid, acetic acid and aldehydes. The reaction gas is cooled to about 70 ° C. at a pressure of 1.8 bar by partial evaporation of the aqueous quench circuit solution.

The approximately 70 ° C hot, water vapor-saturated reaction gas 3 is fed from the bottom of the quench column K 1 to the bottom part of the absorption column K 2. The steam-saturated reaction gas 3 predominantly contains nitrogen (71.5% by weight), water (10.8% by weight) and acrolein (8.7% by weight).

The water vapor-saturated reaction gas 3 is cooled in two stages to about 30 ° C. in the lower part I of the absorption column K 2.

The work-up step of the saturated reaction gas in the lower part 1 of the absorption column K 2 by cooling and condensation is essential for the present invention: Therefore the lower part 1 of the absorption column K 2 with the associated process parameters was shown separately in FIG. 3a. The condensate flowing out of the absorption column K 2 is separated into two partial streams, a partial stream 4 being fed to the second condensation stage 7 of the lower column part I after cooling in a cooler 5. The rest of the still hot condensate 6 is led to the first condensation stage 8 of the lower column part I and mixed there with the colder condensate stream flowing out of the second condensation stage 7.

In the upper part II of the absorption column K 2, the acrolein is absorbed from the reaction gas with a circulated aqueous solution 9. The exhaust gas 10 is removed via the top of the absorption column K 2.

The circulating water 9 loaded with acrolein is separated in the distillation column K 3 into the acrolein / water azeotrope 11 at the top of the column K 3 and the circulating water 12 containing the high boilers at the bottom of the column. The circulating water 12, which has largely been freed from acrolein, is used again after absorption to about 30 ° C. for the absorption of acrolein in the absorption column K 2. Fresh water 13 is fed to the circulating water 9 to discharge by-products. Via the cycle water purge 14, loaded cycle water reaches the lower part I of the absorption column K 2. From the condensation circuit, this purge runs together with the condensate as the condensate purge 15 into the quench circuit. The final removal of by-products and water from the process is carried out via a purge 16 in the quench circuit.

The essential process step for working up the reaction gas from the production of acroiein takes place in the lower part I of the absorption column K 2. This part of the system from FIG. 2 is shown in FIG. 3a together with the corresponding process parameters. FIG. 3b, on the other hand, represents the process control according to the prior art.

In a absorption column K 2 with a diameter of 0.1 m and two packed beds of 0.5 m with metal pall rings of 15 mm in diameter, a reaction tion gas stream of 31.5 m / h cooled from 72 ° C to 30 ° C in two stages. A first partial stream 4 of the condensate circuit was cooled from 180 I / h in a cooler 5 to 30 ° C and in the second condensation stage 7 with the Reaction gas brought into contact from the first condensation stage 8 In the first condensation stage 8, the remaining partial stream 6 of the condensate of 300 l / h was passed uncooled to the first condensation stage 8 and mixed there with the colder condensate stream running out of the second condensation stage 7

For comparison, the same gas flow of 31.5 m / h was cooled and condensed in accordance with the method known from EP-B-0 559 227 in such a way that the entire condensate circuit was cooled from 480 l / h to 30 ° C. in the cooler 5 and on the second Fullkoφerschuttung 7 was brought into contact with the reaction gas

The following temperatures in the gas and liquid phases were measured at several points in the absorption column K 2 for the two process procedures according to FIGS. 3a and 3b. The measured values are given in Table 2 below and shown graphically in FIG. 4 TG and the corresponding number each indicate the temperatures of the gas phase, and TL and the corresponding number indicate the temperatures of the liquid phase at the points designated in FIGS. 3a and 3b

In FIG. 4, curve a shows the course of the temperature difference for the procedure according to FIG. 3a, and curve b shows the course of the temperature difference for the procedure according to FIG. 3b.

From the measured temperature values it follows that the condensate TL1 drawn off at the bottom of the column is hotter according to the inventive method (57 ° C.) compared to the known method (41 ° C.); therefore less product in the gas phase, for example acrolein, can dissolve in the condensate and less product is available in the liquid phase for undesired reactions of the product, for example the acid-catalyzed hydrolysis of acrolein.

For a reaction gas with approx. 4 vol% acrolein, the following acrolein contents were found in the condensate drawn off at the bottom of column K 2 at a pressure of 1.8 bar measured: 1% by weight of acrolein in the process according to the invention (FIG. 3a) compared to 1.6% by weight in the known process (FIG. 3b). The loss of product in the condensate was reduced.

It is particularly serious, however, that when the reaction is carried out according to FIG. 3b, a clearly visible formation of mist occurs, which leads to a transfer of high-boiling by-products into the upper part II of column K 2. The acrylic acid content in the circulating water 9 rose from approximately 2000 ppm with fog-free cooling in accordance with FIG. 3a to approximately 5500 ppm with cooling with fog formation (FIG. 3b). The higher acrylic acid content is particularly critical because of the aforementioned acid-catalyzed hydrolysis of acrolein to 3-hydroxypropanal and subsequent hydrate formation.

Claims

claims

1. A method for condensing one or more components from a gas mixture in a condenser, characterized in that the temperature difference between the gas mixture and the cooling surface at each point of the condenser is less than 25 ° C, in particular less than 20 ° C.

2. The method according to claim 1, characterized in that condensation nuclei are present in the condenser in a concentration of at least 100 nuclei per cm, in particular of at least 1000 nuclei per cm, particularly preferably of at least 10,000 nuclei per cm.

3. The method according to claim 1 or 2, characterized in that the gas mixture has a Lewis index from 0.8 to 5.0, preferably from 0.9 to 3.0, particularly preferably from 0.9 to 1.5.

4. The method according to any one of claims 1 to 3, characterized in that the temperature difference between the gas phase and the cooling surface at each point of the

Capacitor is less than 15 ° C, in particular less than 10 ° C.

5. The method according to any one of claims 1 to 4, characterized in that the cooling surface is a liquid film, in particular an aqueous liquid film and / or a cold wall.

6. The method according to any one of claims 1 to 5, characterized in that the gaseous component which is condensed from the gas mixture, an inorganic substance, in particular water and / or an organic substance, in particular a by-product and / or main product from one

Is gas phase reaction.

7. The method according to any one of claims 1 to 6, characterized in that the cooling takes place in one or more stages, in particular in two stages.

8. Use of the method according to one of claims 1 to 7 for the separation of condensable organic substances from gas mixtures, which preferably in

Gas phase reaction were prepared.

9. Use according to claim 8 for the separation of (meth) acrylic acid and / or (meth) acrolein and / or glyoxal and / or ethylene oxide and / or aniline.

10. Use according to claim 8 for the separation of by-products and / or main products of (meth) acrolein synthesis and / or (meth) acrylic acid synthesis.