Method and device for obtaining acrylic acid

Abstract

The present invention relates to a method for obtaining acrylic acid from a reaction gas. The method comprises quenching the reaction gas with a quenching fluid in a first section (2), wherein a fluid flow containing acrylic acid and a gas flow containing acrylic acid are obtained, and cooling the gas flow by bringing the gas flow into contact with an aqueous fluid in a second section (3), in that the aqueous fluid is collected and removed at the bottom end and is indirectly cooled, and at least a portion of the cooled aqueous fluid is fed back to the second section (3). In the process, according to the invention, the aqueous fluid is cooled in a plurality of successive heat exchangers (8-1 to 8-4), wherein a first portion of the aqueous fluid downstream of the last heat exchanger (8-4) is fed back to the second section (3) above the removal position, and a second portion of the aqueous fluid is discharged downstream of a first heat exchanger (8-1) and upstream of the last heat changer (8-4) and fed back to the second section (3). The invention also relates to a corresponding device for obtaining acrylic acid from a reaction gas.

Description

[0001] Process and apparatus for the production of acrylic acid

[0002] The present invention relates to a process for obtaining acrylic acid from a reaction gas containing acrylic acid, acrolein, water, and impurities, which is formed during the catalytic gas-phase oxidation of propylene and acrolein. The process comprises quenching the reaction gas with a quench liquid in a first section, thereby obtaining a liquid stream containing acrylic acid and a gas stream containing acrylic acid. The process further comprises cooling the gas stream by contacting it with an aqueous liquid in a second section, in which the aqueous liquid is collected and withdrawn at a withdrawal position at the lower end of the second section, indirectly cooled, and at least a portion of the cooled aqueous liquid is returned to the second section.Furthermore, the invention relates to an apparatus for obtaining acrylic acid from a reaction gas, comprising a first section for quenching the reaction gas with a quenching liquid to obtain a liquid stream containing acrylic acid and a gas stream containing acrylic acid, and a second section for cooling the gas stream by bringing it into contact with an aqueous liquid.

[0003] DE 197 40 253 A1 discloses a process for the fractional condensation of a hot gas mixture containing a high proportion of non-condensable components. The process relates in particular to the production of acrylic acid by heterogeneously catalyzed gas-phase oxidation. In the process, the hot gas mixture, which contains a high proportion of non-condensable components, is fractionally condensed by being fed from the bottom into a column with separating internals, and the condensable components are condensed out by cooling. In the lower part of the column, a high-boiling fraction is condensed by distillation and condensing out the high-boiling fraction from the gas stream conveyed upwards in countercurrent. The medium-boiling fraction containing acrylic acid, which is also condensed out from the gas mixture conveyed upwards in countercurrent, is discharged via a side offtake of the column.An external cooling circuit is provided in the upper section of the column. In the cooling circuit, the light-boiling fraction is condensed from the gas stream, which is conducted upwards in countercurrent. The heat of condensation is removed externally in the cooling circuit by means of a heat exchanger. The condensed light-boiling fraction is withdrawn via a line, cooled, and a portion of the cooled, condensed light-boiling fraction is returned to the column via a line arranged above the withdrawal line. In this process, a portion of the condensed light-boiling fraction is discharged. The uncondensed gases are finally withdrawn from the top of the column. A similar process is described in DE 102 35 847 A1. In this case, a gas mixture is withdrawn at the top of the column and subjected to partial condensation in a spray cooler. The resulting sour water is returned to the top of the column.

[0004] EP 1 097 916 A2 discloses another process and apparatus for producing acrylic acid. In this process, the reaction gas is fed to an absorption column and brought into contact there with an acrylic acid absorption solvent, whereby the reaction gas is cooled and absorbed. This results in a liquid stream containing acrylic acid. In the bottom section, the liquid stream containing acrylic acid is fed to a cooling device, cooled there, and then recirculated to a central section of the absorption column.

[0005] In order to achieve a substantial recovery of acrylic acid from the gas stream containing acrylic acid, cooling of the aqueous liquid to low temperatures of e.g. 20 - 45 °C is necessary.

[0006] The present invention is based on the object of providing a method and a device of the type mentioned at the outset, the energy requirement for cooling the withdrawn aqueous liquid is reduced and in which an optimization is achieved with regard to the energy consumption for cooling and the acrylic acid losses.

[0007] According to the invention, this object is achieved by a method having the features of claim 1 and a device having the features of claim 8. Advantageous embodiments and further developments emerge from the dependent claims.

[0008] Accordingly, the process according to the invention for obtaining acrylic acid from a reaction gas containing acrylic acid, acrolein, water and impurities that is formed during the catalytic gas phase oxidation of propylene and acrolein comprises the following steps: a) quenching the reaction gas with a quench liquid in a first section, whereby a liquid stream containing acrylic acid and a gas stream containing acrylic acid are obtained, and b) cooling the gas stream by bringing it into contact with an aqueous liquid in a second section, in which the aqueous liquid is collected and removed at a removal position at the lower end of the second section, indirectly cooled and at least a portion of the cooled aqueous liquid is fed back to the second section.

[0009] According to the invention, the aqueous liquid is cooled in several successive heat exchangers, wherein a first portion of the aqueous liquid is returned to the second section after the last heat exchanger at a first temperature at a first return position above the removal position, and a second portion of the aqueous liquid is discharged at a first discharge position after a heat exchanger and before the last heat exchanger at a second temperature and is returned to the second section at a second return position which is below the first return position.

[0010] Since in the second section a low-boiling fraction is condensed by direct cooling from the gas stream which is conducted upwards in countercurrent to the aqueous liquid and is enriched in the aqueous liquid by distillation, the second section can also be referred to as a quench or backwash section.

[0011] In the process according to the invention, the aqueous liquid withdrawn from the second section is thus cooled in several stages, each stage comprising at least one separate heat exchanger. The successive heat exchangers thus form a multi-stage cooling system in which the aqueous liquid is cooled to a lower temperature in each cooling stage. In the first or first cooling stages, readily available coolants such as air or surface water are expediently used. It has been found that the energy requirement of the heat exchangers can be reduced by discharging a second portion of the aqueous liquid before the last heat exchanger and returning it to the second section, with essentially the same or only insignificantly higher acrylic acid losses.At the same time, however, the energy requirement for cooling the aqueous liquid is reduced, since the last heat exchanger only cools the first portion of the aqueous liquid and no longer cools the discharged second portion of the aqueous liquid.

[0012] According to a further embodiment of the method according to the invention, the cooling medium used by the heat exchanger(s) located downstream of the first discharge position is cooled using energy. The cooling medium used by the heat exchanger(s) located upstream of the first discharge position, in contrast, is taken from the environment without cooling.

[0013] In this document, the terms downstream and upstream refer to the flow direction of the extracted aqueous liquid.

[0014] The method according to the invention takes into account that the energy required to cool the extracted aqueous liquid varies between successive heat exchangers. To cool the aqueous liquid from a temperature level significantly above ambient temperatures, the cooling medium can be taken from the environment without cooling. For example, ambient air can be drawn in or surface water can be pumped into the heat exchanger. If the aqueous liquid is to be cooled to a temperature that is close to or below ambient temperature, energy, for example electrical energy, must be applied for the cooling. For example, the cooling medium must be cooled using electrical energy.

[0015] Advantageously, in the method according to the invention, the cooling capacity is shifted to heat exchangers, which require less energy, particularly electrical energy, for the cooling capacity because they can be operated with a cooling medium that can be taken from the environment without prior cooling. If the amount of cooled volume flow is increased in the method according to the invention, an energy saving can advantageously be achieved while maintaining the same overall cooling capacity.

[0016] For this reason, the method according to the invention can be used particularly in an environment with relatively high temperatures, since in this case a relatively high proportion of the cooling capacity must be provided by heat exchangers that use cooling media that must be cooled by electrical energy. Shifting the cooling capacity to heat exchangers that use cooling media from the environment advantageously leads to energy savings in this case, even if a larger volume flow of the aqueous liquid is cooled to achieve essentially the same total cooling capacity.

[0017] According to a further embodiment of the method according to the invention, a third portion of the cooled aqueous liquid is discharged at a second discharge position through at least one further heat exchanger upstream of the withdrawal of the second portion at a third temperature and mixed with the second portion of the cooled aqueous liquid. The third portion is then returned to the second section at a second return temperature at the second return position. In this case, a first return temperature corresponds to the first temperature at which the first portion of the aqueous liquid is returned to the second section after the last heat exchanger.The second recirculation temperature is between the second and the third temperature, since the cooled aqueous liquid of the third portion, which has the third temperature, is mixed with the cooled aqueous liquid of the second portion, whose temperature is lower than the third temperature.

[0018] By dividing the aqueous liquid to be cooled into a total of three portions with different temperature levels and recirculating the cooled portions to at least two different return points, the energy requirement for cooling the extracted aqueous liquid can be optimized while taking acrylic acid losses into account. Losses during acrylic acid production, which may be slightly higher, are offset by significant energy savings for cooling the aqueous liquid.

[0019] According to a further embodiment of the method according to the invention, the cooling medium used by the heat exchanger(s) arranged upstream of the second discharge position is removed from the environment without cooling.

[0020] Advantageously, this embodiment of the method according to the invention shifts the cooling capacity even further to heat exchangers, in which less energy, in particular electrical energy, has to be applied for the cooling capacity, since they can be operated with a cooling medium which can be taken from the environment without prior cooling.

[0021] In the process according to the invention, the reaction gas passes through a first section, in which a liquid stream containing acrylic acid and a gas stream containing acrylic acid are obtained by quenching the reaction gas with a quench liquid. In the subsequent second section, the gas stream is also cooled by contacting it with an aqueous liquid. In the second section, the aqueous liquid is collected, for example, at the withdrawal position at the lower end, and removed there.The withdrawn aqueous liquid is then indirectly cooled by means of successive heat exchangers and at least a portion of the cooled aqueous liquid is returned to the second section, namely a first portion having a first return temperature at a first, upper return position and a second portion having a second, higher return temperature at the second return position located between the first return position and the withdrawal position.

[0022] The two sections can be designed separately; however, they can also be integrated, for example, in a condensation column. If the two sections are formed in a common condensation column, the first section is arranged, in particular, below the second section. In this case, the two sections are separated, in particular, by a collecting tray, which thus forms the lower end of the second section. The collecting tray can have at least one chimney through which the reaction gas can rise from the first section and reach the second section. The aqueous liquid condensed in the second section collects on the collecting tray.In particular, the removal position is located slightly above the collecting tray at the lower end of the second section, but below the liquid level of the condensate, so that the aqueous liquid collected on the collecting tray can be led out of the second section to be indirectly cooled by the heat exchangers.

[0023] A fourth portion of the aqueous liquid withdrawn at the withdrawal position is returned to the first section to prevent it from running dry. For example, this fourth portion of the aqueous liquid from the condensation column can be fed below the collecting tray to the upper region of the first section, so that the rising reaction gas flows countercurrently to this aqueous liquid returned to the first section.

[0024] Furthermore, in the process according to the invention, a fifth portion of the withdrawn aqueous liquid can be discharged from the second section for further extraction of the acrylic acid still contained in the aqueous liquid. This fifth portion can be further processed in a downstream extraction column to recover acrylic acid portions. It has been found that by developing the process according to the invention, in which the third portion is discharged from the cooling circuit and mixed with the second portion, the acrylic acid portion in the fifth portion of the withdrawn aqueous liquid is higher compared to the acrylic acid portion in a process in which no third portion of the cooled aqueous liquid is discharged into at least one further heat exchanger upstream of the withdrawal of the second portion and mixed with the second portion.

[0025] In the method according to the invention, the first temperature is in particular in a range from 5 °C to 21 °C. The first temperature is, for example, in a range from 18 °C to 21 °C. The second temperature is in particular in a range from 21 °C to 30 °C. For example, the second temperature is in a range from 23 °C to 27 °C. However, the second temperature is higher than the first temperature. The third temperature is in particular in a range from 36 °C to 50 °C, preferably in a range from 35 °C to 45 °C. For example, the third temperature can be in a range from 40 °C to 44 °C. The third temperature is higher than the second temperature.

[0026] In the process according to the invention, the second recycle temperature is, in particular, higher than the second temperature. It is, in particular, in a range from 37°C to 50°C, for example, in a range from 25°C to 42°C.

[0027] By selecting these temperatures generated by the serially arranged heat exchangers and by recirculating the cooled aqueous liquid at the various recirculation positions, the temperature level in the second section can be shifted compared to a conventional recirculation with only one recirculation position. In particular, it is possible to increase the temperature level, i.e., by mixing the second and third portions of the cooled aqueous liquid, the temperature at the second recirculation position can be raised.

[0028] Furthermore, the method according to the invention allows the withdrawal rate of the withdrawn aqueous liquid to be varied. If the cooled aqueous liquid is returned at a higher temperature to the second return position, the method according to the invention allows the withdrawal rate of the aqueous liquid to be increased and, at the same time, the rate of the cooled aqueous liquid returned to the two return positions to be increased accordingly. In this way, an optimization can be carried out in which a higher temperature level is compensated by a larger volume of returned cooled aqueous liquid.

[0029] By at least two separate recirculations of the cooled aqueous liquid at different positions of the second section and by discharging cooled aqueous liquid from different sections of the multi-stage cooling, the flexibility for optimizing the energy requirement for cooling and any associated changes in losses of acrylic acid in the process for producing acrylic acid is advantageously increased.

[0030] In the second section, the gas stream containing acrylic acid is cooled by bringing it into contact with the aqueous liquid. This results in the condensation of so-called sour water, which, in addition to water, also contains significant amounts of acrylic acid and, in smaller proportions, other carboxylic acids. In order to dissolve the largest possible amount of condensed components in the sour water, the aqueous liquid taken from the second section and then recycled, i.e. the sour water, can be cooled as much as possible. However, this has the disadvantage that the energy consumption for cooling increases. If the aqueous liquid is returned at a higher temperature, the energy consumption for cooling is lower. At the same time, the temperature increase shifts the temperature profile of the column upwards. This increases the proportion of acrylic acid that reaches the upper condensation zone and can be condensed.

[0031] Therefore, the acid water extracted from the second section and the cooling circuit and fed to a downstream extraction column can advantageously have a higher acrylic acid content. Overall, the process according to the invention thus allows for very flexible optimization with regard to energy consumption, the acrylic acid content in the discharged acid water, and acrylic acid losses.

[0032] The device according to the invention for obtaining acrylic acid from a reaction gas comprises a first section for quenching the reaction gas with a quench liquid to obtain a liquid stream containing acrylic acid and a gas stream containing acrylic acid. Furthermore, the device comprises a second section for cooling the gas stream by contacting it with an aqueous liquid. The second section has a withdrawal opening at a withdrawal position at the lower end of the second section and at least one first and one second return opening at a first and one second return position, each above the withdrawal position, with the second return position being below the first return position.Furthermore, the device according to the invention comprises a plurality of successive heat exchangers arranged in series between the withdrawal opening and the second return opening for cooling aqueous liquid that has been withdrawn from the second section through the withdrawal opening. A first return line connects the last heat exchanger to the first return opening for returning a first portion of the aqueous liquid to the second section at a first temperature, and a second return line connects, at a first discharge position, a first region downstream of the first heat exchanger and upstream of the last heat exchanger to the second return opening for discharging and returning a second portion of the aqueous liquid to the second section at a second temperature. The device according to the invention is particularly designed to carry out the method according to the invention.It therefore has the same advantages as the method according to the invention.

[0033] According to a further development of the device according to the invention, it has a third return line which, in a second discharge position, connects a second region, which is arranged at least one further heat exchanger upstream of the first region, to the second return line for discharging a third portion of the aqueous liquid at a third temperature and for mixing the third portion with the second portion. Advantageously, this embodiment of the device according to the invention makes it possible, in at least two different regions separated at least by a heat exchanger, to discharge aqueous liquid at different temperatures from the cooling circuit in addition to the first portion of the aqueous liquid returned via the first return line, and to return it to the second section via the second return line.This arrangement of the return lines allows for optimization of the energy requirements of the heat exchangers when using the device for extracting acrylic acid.

[0034] The heat exchangers are primarily indirect heat exchangers, in which thermal energy is transferred from the extracted aqueous liquid to another material stream, the cooling medium. The series of heat exchangers further cools the aqueous liquid extracted from the second section through each heat exchanger. This allows for multi-stage cooling, with the aqueous liquid being cooled to a lower temperature at each cooling stage.

[0035] According to one embodiment of the device according to the invention, the heat exchanger(s) arranged upstream of the second discharge position are selected from an air cooler and a water cooler. For example, the first heat exchanger in the device according to the invention is an air cooler. A second heat exchanger arranged downstream of the first heat exchanger is, for example, a water cooler, for example, a surface water cooler. The water used for cooling is, for example, taken from a river without being cooled.

[0036] According to a further embodiment of the device according to the invention, at least one further heat exchanger, e.g., a third heat exchanger, arranged downstream of the second discharge position and upstream of the first discharge position, is a cold water cooler. In this case, water is used as the cooling medium, which has been cooled compared to the water used in the water cooler. Energy, in particular electrical energy, must be applied for this purpose. For example, the cold water cooler can also be a brine-cooled heat exchanger. Alternatively, a glycol-water mixture is used as the cooling medium of the third heat exchanger.

[0037] According to yet another embodiment of the device according to the invention, two heat exchangers are arranged downstream of the second discharge position and upstream of the first discharge position. For example, one of these heat exchangers uses a glycol-water mixture as the cooling medium, and the other of these heat exchangers uses water, which has been cooled compared to the water used in the water cooler, as the cooling medium.

[0038] According to a further embodiment of the device according to the invention, at least one further heat exchanger, e.g., a fifth heat exchanger, arranged downstream of the first discharge position, is a liquid gas evaporator, by which the sour water is further cooled after the first discharge position.

[0039] According to one embodiment of the device according to the invention, the second return line connects the outlet of the cold water cooler to the second return opening.

[0040] According to a further embodiment of the device according to the invention, the third return line connects the outlet of the water cooler to the second return line.

[0041] According to a further embodiment of the device according to the invention, a mixer is arranged between the third return line and the second return line, which mixer mixes the third portion of the aqueous liquid with the second portion of the aqueous liquid.

[0042] The invention will now be explained using exemplary embodiments with reference to the drawings:

[0043] Figure 1 shows a first embodiment of the device according to the invention and

[0044] Figure 2 shows a second embodiment of the device according to the invention.

[0045] A first exemplary embodiment of the device according to the invention is described with reference to Fig. 1: The basic structure of the device corresponds to an arrangement as described in DE 197 40 253 A1. The device comprises a condensation column 1 with a first section 2 and a second section 3. In the first section 2, reaction gas containing acrylic acid, acrolein, water and impurities which are formed during the catalytic gas-phase oxidation of propylene and acrolein is introduced into a bottom region. There, the hot reaction gas is cooled by quenching, i.e. by direct cooling. This produces a liquid stream containing acrylic acid and a gas stream containing acrylic acid flowing upwards in countercurrent. At the upper end of the second section 2, a collecting tray 4 is arranged, through which the gas stream containing acrylic acid can pass, e.g. through at least one chimney.The collecting tray 4 separates the first section 2 from the second section 3. In the embodiment described here, the two sections 2 and 3 are formed in a condensation column 1. However, it would also be possible to form the second section 3 separately from the first section in a separate condensation column.

[0046] The gas stream rising in the second section 3 is also cooled by contacting it with an aqueous liquid. Quenching, i.e., direct cooling of the rising gas stream by means of an aqueous liquid flowing countercurrently, also occurs in the second section 3. The gas stream is finally discharged from the condensation column 1 via the outlet opening 5.

[0047] The aqueous liquid collecting on the collecting tray 4, which contains condensed components of the rising gas stream, is also called sour water. In addition to water, this liquid also contains significant amounts of acrylic acid and, in smaller amounts, other carboxylic acids.

[0048] The sour water is withdrawn from the second section 3. For this purpose, a withdrawal opening 6 is provided at a withdrawal position just above the upper surface of the collecting tray 4. Via this withdrawal opening 6, the sour water is subjected to multi-stage cooling. This multi-stage cooling comprises several heat exchangers, generally designated 8, and a plurality of lines, generally designated 9. The sour water is withdrawn via line 9-1 at the withdrawal opening 6 by a pump 7 and fed via line 9-2 to a first heat exchanger 8-1. The first heat exchanger 8-1 is an air cooler. The first heat exchanger 8-1 cools the withdrawn sour water. Via another line 9-3, the withdrawn sour water is fed to a second heat exchanger 8-2, where it is further cooled.The second heat exchanger 8-2 is designed as a water cooler, which uses surface water as the cooling medium.

[0049] The extracted sour water is fed via another line 9-4 to a third heat exchanger 8-3, which further cools the sour water. The third heat exchanger 8-3 uses a glycol-water mixture as the cooling medium, which has a temperature of, for example, 32 °C. This mixture can serve as a heating medium in the process and extract heat from the sour water circuit.

[0050] From the third heat exchanger 8-3, the cooled sour water flows via line 9-5 to a fourth heat exchanger 8-4, which is designed as a cold water cooler. In this case, chilled water is used as the cooling medium, which has a lower temperature than the surface water used by the water cooler.

[0051] After the fourth heat exchanger 8-4, the cooled acid water enters line 9-6. There, the acid water is divided into a first portion and a second portion at a first discharge position 16-1.

[0052] The first portion is further passed through line 9-6 to a fifth heat exchanger 8-5, which is designed as a liquid gas evaporator. In the fifth heat exchanger 8-5, the extracted sour water is further cooled. From the fifth heat exchanger 8-5, the first portion of the further cooled sour water returns to the second section 3 via a first return line 11-1 through a first return opening 10-1 at a first return position. The first return position is located in the upper region of the second section 3, i.e., at the top of the condensation column 1. From there, the cooled sour water is conducted downward, i.e., in countercurrent to the ascending reaction gas, whereby the reaction gas is cooled and, among other things, acrylic acid is enriched in the aqueous liquid. The aqueous liquid then collects on the collecting tray 4.

[0053] For discharging the second portion of the cooled sour water at the first discharge position 16-1, a second return line 11-2 is provided, which connects a region downstream of the first heat exchanger 8-1 and upstream of the last heat exchanger 8-5 with a second return opening 10-2 for returning the second portion of the sour water to the second section 3. In the first embodiment described here, the second return line 11-2 branches off at the first discharge position 16-1 from the line 9-6, which connects the fourth heat exchanger 8-4 to the fifth heat exchanger 8-5.

[0054] The second recirculation opening 10-2 is arranged at a second recirculation position, which is located between the withdrawal position and the first recirculation position. The second temperature at which the second portion of the sour water is recirculated is higher than the first temperature at which the first portion of the sour water is recirculated, since the second portion of the sour water has not been cooled by the fifth heat exchanger 8-5. The second section 3 of the condensation column 1 thus has a plurality of recirculation openings, generally designated 10, through which a cooled aqueous liquid is recirculated at different temperatures at different recirculation positions.

[0055] To prevent the first section 2 of the condensation column 1 from running dry, a portion of the acid water withdrawn at the withdrawal opening 6 is fed back into the upper region of the first section 2 via the column line 14. This portion of the acid water returned to the first section 2 is also referred to in this document as the fourth portion of the aqueous liquid withdrawn at the withdrawal position.

[0056] Furthermore, a further fifth portion of the extracted acid water is discharged from line 9-2 via an extraction line 13 and fed to a downstream extraction column 15. The extracted acid water is further processed in the extraction column 15 to recover acrylic acid fractions.

[0057] In the following, a first embodiment of the method according to the invention is explained with reference to Fig. 1:

[0058] In this process, acrylic acid is obtained from a reaction gas containing acrylic acid, acrolein, water, and impurities, which is produced during the catalytic gas-phase oxidation of propylene and acrolein. The reaction gas is fed to the first section 2 of the condensation column 1, where it is cooled by quenching with a quench liquid. This results in a liquid stream containing acrylic acid and a countercurrent gas stream containing acrylic acid, which rises in the condensation column 1.

[0059] A portion of the acrylic acid-containing liquid stream is withdrawn from the first section 2 via a side draw (not shown), and acrylic acid is recovered therefrom. The acrylic acid-containing gas stream passes through a chimney in the collecting tray 4 into the second section 3 of the condensation column 1. There, the gas stream is cooled by bringing it into contact with an aqueous liquid. The aqueous liquid is conducted countercurrent to the ascending gas stream, so that acrylic acid is enriched in the aqueous liquid. It collects on the collecting tray 4. The aqueous liquid, i.e. the sour water, is withdrawn at a withdrawal position at the withdrawal opening 6 at the lower end of the second section 3 and cooled in several successive heat exchangers 8. The withdrawal position is therefore arranged below the liquid level of the sour water collecting on the collecting tray 4.

[0060] In the method according to the invention, a volume flow of 865 m 3 / h through the discharge opening 6 at a temperature of 62.7 °C.

[0061] A volume flow of 36.70 m 3 / h is fed back to the first section 2 of the condensation column via line 9-2 and column line 14. Furthermore, acid water with a volume flow of 14.51 m 3 / h and fed to the extraction column 15.

[0062] The remaining sour water is passed through the first heat exchanger 8-1 and cooled there to 50 °C. The required cooling capacity of the first heat exchanger is 10,486 kW.

[0063] The sour water is then passed through the second heat exchanger 8-2, where it is cooled to 42 °C. The required cooling capacity of the second heat exchanger 8-2 is 6,612 kW.

[0064] The sour water is then fed to the third heat exchanger 8-3 and cooled to 39 °C. The required cooling capacity of the third heat exchanger 8-4 is 2,450 kW. Since the third heat exchanger 8-3 is a cold water cooler, energy must be applied for this purpose.

[0065] The sour water is then fed to the fourth heat exchanger 8-4 and cooled to 24.7 °C. The required cooling capacity of the fourth heat exchanger 8-4 is 11,756 kW. Since the fourth heat exchanger 8-4 is a cold water cooler, electrical energy is required for this purpose. After the fourth heat exchanger 8-4, the cooled sour water is divided into a first portion and a second portion. The first portion, approximately 42% of the volume flow, is fed to the fifth heat exchanger 8-5, where it is cooled to a first return temperature of 20.7 °C and returned to the second section 3 at the first return position through the first return opening 10-1. The power consumption required for cooling by the fifth heat exchanger 8-5 is 1,400 kW. Since the fifth heat exchanger 8-5 is a liquid gas evaporator, it also extracts this energy from the sour water.

[0066] The second portion, approximately 58%, is returned to the second section 3 of the condensation column 1 via the second return opening 10-2 at the second return temperature of 24.7 °C at the second return position.

[0067] By recycling the second portion of the acid water at a second recycle position with a second temperature that is higher than the first temperature, the temperature profile in section 3 of the condensation column 1 is increased compared to a process in which no portion of the acid water is removed and recycled at a higher temperature. This increases the acrylic acid content in section 3. In the first embodiment of the process according to the invention, a volume flow of 14.51 m3 results in section 3. 3 / h at 62.7°C with 9.97% acrylic acid.

[0068] In the following, a second embodiment of the device according to the invention is explained with reference to Fig. 2:

[0069] The device of the second embodiment essentially corresponds to the device of the first embodiment, which is why only the differences from the device of the first embodiment will be discussed below.

[0070] In the device of the second embodiment, a third portion of the sour water cooled at least by the first heat exchanger 8-1 is discharged, specifically to at least one further heat exchanger upstream of the withdrawal of the second portion at the first discharge position 16-1. In the first embodiment, this second portion was withdrawn between the fourth heat exchanger 8-4 and the fifth heat exchanger 8-5 at line 9-6. While this second portion is also discharged at this line 9-6 in the second embodiment, in the second embodiment the third portion is branched off between the second heat exchanger 8-2 and the third heat exchanger 8-3 at a second discharge position 16-2 at line 9-4 and fed to a mixer 12 via a third return line 11-3. In the mixer 12, the third portion is mixed with the second portion, which is withdrawn at line 9-6.The mixture is then returned to the second section 3 via the second return line 11-2, as in the first embodiment, at the second return position through the second return opening 10-2.

[0071] The first heat exchanger 8-1 and the second heat exchanger 8-2 are located upstream of the second discharge position. These two heat exchangers 8-1, 8-2 are characterized by the fact that the cooling medium they use, i.e., the ambient air and the surface water, is taken from the environment without cooling and, in particular, does not require separate cooling for use in the heat exchangers 8-1, 8-2.

[0072] In the following, a second embodiment of the method according to the invention is explained with reference to Fig. 2:

[0073] The second embodiment of the method according to the invention essentially corresponds to the first embodiment of the method according to the invention, which is why only the differences from the first embodiment of the method according to the invention will be discussed below.

[0074] In the second embodiment, the sour water is supplied to the second section 3 at the withdrawal position through the withdrawal opening 6 with a volume flow of 1,227 m 3 / h at a temperature of 63.2 °C. The volume flow of the sour water returned to the first section 2 via the column line 14 and the volume flow of the sour water removed via the extraction line 13 correspond to the volume flows of the first embodiment.

[0075] The first heat exchanger 8-1 cools the remaining volume flow of sour water to a temperature of 50 °C. The required power consumption for this is 15,587 kW.

[0076] The second heat exchanger 8-2 cools the volume flow of sour water to a temperature of 42°C. The required power input for this is 9,464 kW. In line 9-4, approximately 70% of the sour water is discharged at the second discharge point 16-2 at a temperature of 42°C. The remaining portion, approximately 30%, is fed to the third heat exchanger 8-3, where it is cooled to 35°C. This requires a power input of 2,453 kW.

[0077] The sour water is then fed to the fourth heat exchanger 8-4, where it is cooled to 25 °C. This requires an electrical power of 3,291 kW.

[0078] At line 9-6, acid water is then discharged via line 9-7 at the first discharge position 16-1. In the mixer 12, the discharged portions are then mixed with the portion discharged at the discharge position 16-2 and returned to the second section 3 via the second return line 11-2 at a second return temperature between 25 and 42 °C at the second return position via the second return opening 10-2.

[0079] As in the first embodiment of the process according to the invention, the first portion of the sour water is cooled to 20.9 °C in the fifth heat exchanger 8-5 and returned to the second section 3 through the first return opening 10-1 at the first return position. The power consumption of the fifth heat exchanger 8-5 is 1,400 kW, as in the first embodiment.

[0080] Compared to the first embodiment of the process according to the invention, the acid water is discharged after the second heat exchanger 8-2 at a higher temperature, e.g., 42 °C. At the mixer 12, this portion is then variably mixed with another portion of the acid water, which has been brought to a lower temperature by the second and third heat exchangers 8-2, 8-3. In this way, it is possible to return the acid water in a variable amount and at a variable temperature via the second return opening 10-2 to the second section 3. This enables process optimization in favor of energy savings while controlling acrylic acid losses.

[0081] Compared to the first embodiment of the method according to the invention, 72% less electrical energy is consumed for cooling in the second embodiment of the method according to the invention, assuming that essentially the same total cooling power is applied.

[0082] By recycling the second and third portions of the acid water at the second recycle position at a second temperature that is higher than the first temperature, a further increase in the temperature profile in section 3 of the condensation column 1 results in this case compared to the process of the first embodiment. This further increases the acrylic acid content in section 3. In the second embodiment of the process according to the invention, a volume flow of 14.54 m³ results in section 3. 3 / h at 63.1°C with 11.9% acrylic acid.

[0083] Furthermore, the acid water extracted through the extraction line 13 has a 1.9% higher acrylic acid content compared to the process of the first embodiment.

[0084] List of reference symbols:

[0085] 1 condensation column

[0086] 2 first section

[0087] 3 second section / quench section

[0088] 4 collecting tray

[0089] 5 Outlet opening

[0090] 6 Removal opening

[0091] 7 Pump

[0092] 8 heat exchangers

[0093] 8-1 first heat exchanger (air cooler)

[0094] 8-2 second heat exchanger (water cooler)

[0095] 8-3 third heat exchanger

[0096] 8-4 fourth heat exchanger (cold water cooler)

[0097] 8-5 fifth heat exchanger (liquefied gas evaporator)

[0098] 9 lines

[0099] 9-1 Line

[0100] 9-2 Line

[0101] 9-3 Line

[0102] 9-4 Line

[0103] 9-5 line

[0104] 9-6 Line

[0105] 9-7 line

[0106] 10 return openings

[0107] 10-1 first return opening

[0108] 10-2 second return opening

[0109] 11 return lines

[0110] 11-1 first return line

[0111] 11-2 second return line 11-3 third return line

[0112] 12 mixers

[0113] 13 Extraction line

[0114] 14 Column line 15 Extraction column

[0115] 16 discharge positions

[0116] 16-1 first discharge position

[0117] 16-2 second discharge position

Claims

Patent claims:

1. A process for obtaining acrylic acid from a reaction gas containing acrylic acid, acrolein, water, and impurities, which is formed during the catalytic gas-phase oxidation of propylene or acrolein, comprising: a) quenching the reaction gas with a quench liquid in a first section, thereby obtaining a liquid stream containing acrylic acid and a gas stream containing acrylic acid; b) cooling the gas stream by contacting it with an aqueous liquid in a second section, in which the aqueous liquid is collected and removed at a removal position at the lower end of the second section, indirectly cooled, and at least a portion of the cooled aqueous liquid is returned to the second section, characterized in that the aqueous liquid is cooled in several successive heat exchangers,wherein a first portion of the aqueous liquid is returned to the second section after the last heat exchanger at a first temperature at a first return position above the removal position, and a second portion of the aqueous liquid is discharged at a first discharge position after a first heat exchanger and before the last heat exchanger at a second temperature and is returned to the second section at a second return position which is below the first return position.

2. Method according to claim 1, characterized in that the cooling medium used by the heat exchanger(s) arranged downstream of the first discharge position is cooled with energy expenditure and the cooling medium used by the heat exchanger(s) arranged upstream of the first discharge position is removed from the environment without cooling.

3. Method according to claim 1 or 2, characterized in that a third portion of the cooled aqueous liquid at a second discharge position passes at least one further heat exchanger upstream of the removal of the second portion with a third temperature and mixed with the second portion of the cooled aqueous liquid and then returned to the second section at the second return position with a second return temperature.

4. Method according to claim 3, characterized in that the cooling medium used by the heat exchanger(s) arranged upstream of the second discharge position is taken from the environment without cooling.

5. Method according to one of the preceding claims, characterized in that the first temperature is in a range from 5 °C to 21 °C.

6. Method according to one of the preceding claims, characterized in that the second temperature is in a range from 21 °C to 30 °C.

7. Method according to one of claims 3 to 6, characterized in that the third temperature is in a range from 26 °C to 50 °C, in particular in a range from 35 °C to 45 °C.

8. An apparatus for recovering acrylic acid from a reaction gas, comprising: a first section for quenching the reaction gas with a quench liquid to obtain a liquid stream containing acrylic acid and a gas stream containing acrylic acid; a second section for cooling the gas stream by contacting it with an aqueous liquid, the second section having a withdrawal opening at a withdrawal position at the lower end of the second section and at least a first and a second return opening at a first and a second return position, each above the withdrawal position, the second return position being below the first return position; and a plurality of successive heat exchangers arranged in series between the withdrawal opening and the second return opening for cooling aqueous liquid withdrawn from the second section through the withdrawal opening. wherein a first return line connects the last heat exchanger to the first return opening for returning a first portion of the aqueous liquid to the second section at a first temperature, and wherein a second return line, at a first discharge position, connects a first region downstream of a first heat exchanger and upstream of the last heat exchanger to the second return opening for discharging and returning a second portion of the aqueous liquid to the second section at a second temperature.

9. Device according to claim 8, characterized by a third return line which, at a second discharge position, connects a second region which has at least one further heat exchanger arranged upstream of the first region to the second return line for discharging a third portion of the aqueous liquid at a third temperature and for mixing the third portion with the second portion.

10. Device according to claim 8 or 9, characterized in that the heat exchanger(s) arranged upstream of the second discharge position are selected from an air cooler and a water cooler.

11. Device according to one of claims 8 to 10, characterized in that at least one further heat exchanger, which is arranged downstream of the second discharge position and upstream of the first discharge position, is a cold water cooler.

12. Device according to one of claims 8 to 11, characterized in that at least one further heat exchanger arranged downstream of the first discharge position is a liquid gas evaporator.

13. Device according to one of claims 8 to 12, characterized in that the second return line connects the outlet of the cold water cooler to the second return opening.

14. Device according to one of claims 9 to 13, characterized in that the third return line connects the outlet of the water cooler to the second return line.

15. Device according to one of claims 9 to 14, characterized in that a mixer is arranged between the third return line and the second return line, which mixer mixes the third portion of the aqueous liquid with the second portion of the aqueous liquid.

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