Method and apparatus for obtaining acrylic acid

Abstract

The present invention relates to a method for obtaining acrylic acid from a reaction gas. The method includes the steps of rapidly cooling the reaction gas with a quenching liquid in a first compartment 2 to obtain an acrylic acid-containing liquid stream and an acrylic acid-containing gas stream, and cooling the gas stream by contacting it with an aqueous liquid in a second compartment 3, in which the aqueous liquid is collected and removed at the lower end of the second compartment 3, cooled indirectly, and at least a portion of the cooled aqueous liquid is returned to the second compartment 3. According to the present invention, the aqueous liquid is cooled in a plurality of consecutive heat exchangers 8-1 to 8-4, the first portion of the aqueous liquid is recycled to the second compartment 3 above the removal position after the final heat exchanger 8-4, and the second portion of the aqueous liquid is discharged after the first heat exchanger 8-1 and before the final heat exchanger 8-4 and recycled to the second compartment 3. The present invention further relates to a corresponding apparatus for obtaining acrylic acid from a reaction gas.

Description

Technical Field

[0001] The present invention relates to a method for obtaining acrylic acid from a reaction gas containing acrylic acid, acrolein, water and impurities formed in the catalytic gas-phase oxidation of propylene and acrolein. The method includes quenching the reaction gas with a quench liquid in a first section to obtain an acrylic acid-containing liquid stream and an acrylic acid-containing gas stream. The method further includes cooling the gas stream by contacting it with an aqueous liquid in a second section, where the aqueous liquid is collected and withdrawn at a withdrawal position at the lower end of the second section, and it is indirectly cooled, and at least a part of the cooled aqueous liquid is returned to the second section. The present invention further relates to an apparatus for obtaining acrylic acid from a reaction gas, having a first section for quenching the reaction gas with a quench liquid to obtain an acrylic acid-containing liquid stream and an acrylic acid-containing gas stream, and a second section for cooling the gas stream by contacting it with an aqueous liquid.

Background Art

[0002] German Patent Application Publication No. 19740253 discloses a method for fractionally condensing a high-temperature gas mixture with a high proportion of non-condensable components. This method relates, in particular, to the production of acrylic acid by heterogeneous catalytic gas-phase oxidation. In this method, a high-temperature gas mixture with a high proportion of non-condensable components is introduced from below into a column containing a separation internal structure, and the high-temperature gas mixture with a high proportion of non-condensable components is fractionally condensed by condensing and separating the condensable components by cooling. In the lower part of the column, the high-boiling-point fraction is condensed by distillation concentration of the high-boiling-point fraction from the counter-flowing, upward-moving gas flow and then condensing and separating it. The acrylic acid-containing medium-boiling-point fraction, which is similarly condensed and separated from the counter-flowing, upward-moving gas mixture, is discharged through a side outlet of the column. An external cooling circuit is provided in the upper region of the column. In the cooling circuit, the low-boiling-point fraction is condensed from the counter-flowing, upward-moving gas flow. The heat of condensation is dissipated to the outside within the cooling circuit by a heat exchanger. This is done by extracting the condensed low-boiling-point fraction through piping, cooling it, and recycling a portion of the cooled and condensed low-boiling-point fraction to the tower through piping located above the extraction piping. Simultaneously, a portion of the condensed low-boiling-point fraction is discharged. The uncondensed gas is finally extracted from the top of the tower.

[0003] A similar method is described in German Patent Application Publication No. 10235847. In this case, the gas mixture is removed at the top of the column and subjected to partial condensation in a spray cooler. The resulting acidic water is recycled at the top of the column.

[0004] European Patent Application Publication No. 1097916 discloses an alternative method and apparatus for producing acrylic acid. In this method, a reaction gas is supplied to an absorption tower, where it comes into contact with an acrylic acid absorption solvent, resulting in the cooling and absorption of the reaction gas. This yields an acrylic acid-containing liquid stream. In the bottom region, the acrylic acid-containing liquid stream is supplied to a cooling device, where it is cooled, and then returned to the intermediate section of the absorption tower.

[0005] To achieve substantial recovery of acrylic acid from an acrylic acid-containing gas stream, the aqueous liquid needs to be cooled to a low temperature, for example, 20-45°C. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] German Patent Application Publication No. 19740253 [Patent Document 2] German Patent Application Publication No. 10235847 [Patent Document 3] European Patent Application Publication No. 1097916 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The object of the present invention is to specify the aforementioned type of method and apparatus, in which the energy demand for cooling the extracted aqueous liquid is reduced and optimization is achieved with respect to energy consumption for cooling and loss of acrylic acid.

[0008] This objective is achieved according to the present invention by a method having the features of claim 1 and an apparatus having the features of claim 8. Advantageous embodiments and variations will become apparent from the dependent claims. [Means for solving the problem]

[0009] Therefore, the present invention's method for obtaining acrylic acid from a reaction gas containing acrylic acid, acrolein, water, and impurities formed in the catalytic gas-phase oxidation of propylene and acrolein comprises the following steps: a) A step of rapidly cooling the reaction gas in the first compartment with a quenching liquid to obtain an acrylic acid-containing liquid stream and an acrylic acid-containing gas stream, b) A step of cooling a gas flow by bringing it into contact with an aqueous liquid in a second compartment, wherein the aqueous liquid is collected in the second compartment and taken out at a take-out position at the lower end of the second compartment, where it is indirectly cooled and at least a portion of the cooled aqueous liquid is returned to the second compartment.

[0010] According to the present invention, an aqueous liquid is cooled in a plurality of consecutive heat exchangers, where a first portion of the aqueous liquid is recycled at a first temperature to a second compartment at a first recycling position above the outlet position after the final heat exchanger, and a second portion of the aqueous liquid is discharged at a second temperature at a first discharge position after a heat exchanger and before the final heat exchanger, and recycled to a second compartment at a second recycling position below the first recycling position.

[0011] In the second section, the low-boiling-point fraction is condensed by direct cooling from a gas flow that flows countercurrently upward relative to the aqueous liquid, and is then concentrated into the aqueous liquid by distillation. Therefore, the second section can also be called the quenching section or the backwashing section.

[0012] In the method of the present invention, the aqueous liquid withdrawn from the second compartment is thus cooled in multiple stages, each stage comprising at least one independent heat exchanger. Thus, the successive heat exchangers form multi-stage cooling, in which the aqueous liquid is cooled to a lower temperature in each cooling stage. In the first cooling stage, it is convenient to use an easily available coolant, such as air or surface water. It has been found that by pre-discharging the second portion of the aqueous liquid before the final heat exchanger and recycling it back into the second compartment, the energy demand of the heat exchanger can be reduced with virtually no change or only a slight increase in acrylic acid loss. However, at the same time, the energy demand for cooling the aqueous liquid is reduced because the final heat exchanger cools only the first portion of the aqueous liquid and no longer cools the discharged second portion of the aqueous liquid.

[0013] According to a further embodiment of the method of the present invention, the cooling medium used by the heat exchanger downstream of the first discharge point is cooled by energy consumption. In contrast, the cooling medium used by the heat exchanger upstream of the first discharge point or at least one of the heat exchangers upstream of the first discharge point is removed from the environment without being cooled.

[0014] In this specification, the terms “downstream” and “upstream” refer to the direction of flow of the aqueous liquid being drawn.

[0015] The method of the present invention takes into account the fact that the energy consumption for cooling the extracted aqueous liquid differs in a continuous heat exchanger. To cool the aqueous liquid from a temperature level substantially higher than the ambient temperature, the cooling medium can be removed from the environment without cooling. For example, ambient air can be drawn into the heat exchanger, or surface water can be pumped into the heat exchanger. When cooling the aqueous liquid to a temperature in the range of ambient temperature or below ambient temperature, energy, such as electrical energy, needs to be supplied for cooling. For example, the cooling medium needs to be cooled by electrical energy.

[0016] Advantageously, the method of the present invention allows the heat exchanger to be operated with a cooling medium that can be removed from the environment without prior cooling, thus transferring the cooling capacity to a heat exchanger that requires less energy, especially electrical energy, to be input for the cooling capacity. By increasing the volumetric flow rate being cooled using the method of the present invention, energy can be advantageously saved when the method is implemented with the exact same total cooling capacity.

[0017] Therefore, the method of the present invention can be used especially in an environment having a relatively high temperature. This is because, in that case, the proportion of the cooling capacity to be input that is due to the heat exchanger using a cooling medium that needs to be cooled by electrical energy becomes relatively high. In this case, transferring the cooling capacity from the environment to the heat exchanger using the cooling medium advantageously leads to energy savings even when cooling an aqueous liquid with a larger volume flow rate in order to achieve substantially the same total cooling capacity.

[0018] According to a further embodiment of the method of the present invention, a third portion of the cooled aqueous liquid is discharged at a third temperature at a third discharge position that is upstream of at least one further heat exchanger relative to the withdrawal of the second portion, is mixed with the second portion of the cooled aqueous liquid, and is then recycled to the second compartment at a second recycle temperature at the second recycle position. In this case, the first recycle temperature corresponds to the first temperature when the first portion of the aqueous liquid is recycled to the second compartment after the final heat exchanger. The second recycle temperature is between the second temperature and the third temperature. This is because the third portion of the cooled aqueous liquid having the third temperature is added to the second portion of the cooled aqueous liquid having a temperature lower than the third temperature.

[0019] By dividing the aqueous liquid to be cooled into a total of three portions having different temperature levels and recycling the cooled portions at at least two different recycle positions, it is possible to optimize the energy demand for cooling the withdrawn aqueous liquid while taking into account the loss of acrylic acid. The loss in the production of acrylic acid can be slightly higher, but it is offset by a considerable energy savings regarding the cooling of the aqueous liquid.

[0020] According to a further embodiment of the method of the present invention, the cooling medium used by the heat exchanger upstream of the second discharge position is taken out from the environment without being cooled.

[0021] Advantageously, this embodiment of the method of the present invention enables the heat exchanger to be operated with a cooling medium that can be taken out from the environment without prior cooling, resulting in a further shift in cooling capacity to the heat exchanger, which requires less energy, particularly electrical energy, to be input for cooling capacity.

[0022] In the method of the present invention, the reaction gas passes through the first section. In this first section, an acrylic acid-containing liquid stream and an acrylic acid-containing gas stream are obtained by quenching the reaction gas with a quench liquid. In the subsequent second section, the gas stream is also cooled by contact with an aqueous liquid. In the second section, the aqueous liquid is collected, for example, at the withdrawal position at the lower end and taken out there. The withdrawn aqueous liquid is then indirectly cooled by a continuous heat exchanger, and at least a part of the cooled aqueous liquid is returned to the second section, specifically, a first part at the first recycle temperature at the first upper recycle position and a second part at a second recycle temperature higher than that at a second recycle position between the first recycle position and the withdrawal position.

[0023] These two sections may be independent, or may be integrated, for example, in a single condensation column. When these two sections are formed in a composite condensation column, the first section is particularly arranged below the second section. In this case, these two sections are particularly separated by a collection tray that forms the lower end of the second section as described above. The collection tray may have at least one chimney through which the reaction gas can rise from the first section and enter the second section. What collects on the collection tray is the aqueous liquid condensed in the second section. The withdrawal position is particularly slightly above the collection tray at the lower end of the second section so that the aqueous liquid collected on the collection tray can be taken out of the second section and indirectly cooled by a heat exchanger, but below the liquid level of the condensate.

[0024] The fourth portion of the aqueous liquid extracted at the extraction point is returned to the first compartment to prevent depletion of the liquid. For example, this fourth portion of the aqueous liquid in the condensing tower can be supplied to the upper region of the first compartment below the collection tray so that the rising reaction gas flows countercurrently to this aqueous liquid recycled into the first compartment.

[0025] Furthermore, in the method of the present invention, the fifth portion of the withdrawn aqueous liquid may be discharged from the second compartment for further extraction of acrylic acid still present in the aqueous liquid. This fifth portion can be further processed in a downstream extraction column to recover the acrylic acid component. In an advanced form of the method of the present invention in which the third portion is discharged from the cooling circuit and mixed with the second portion, it has been found that the acrylic acid content in the fifth portion of the withdrawn aqueous liquid is higher than that in a procedure in which the cooled third portion of the aqueous liquid is not discharged upstream of the withdrawal of the second portion by at least one further heat exchanger and mixed with the second portion.

[0026] In the method of the present invention, the first temperature is particularly in the range of 5°C to 21°C. For example, the first temperature is in the range of 18°C ​​to 21°C. The second temperature is particularly in the range of 21°C to 30°C. For example, the second temperature is in the range of 23°C to 27°C, provided that the second temperature is higher than the first temperature. The third temperature is particularly in the range of 36°C to 50°C, preferably in the range of 35°C to 45°C. For example, the third temperature may be in the range of 40°C to 44°C. The third temperature is higher than the second temperature.

[0027] The second recycling temperature is particularly higher than the second temperature in the method of the present invention. This is particularly within the range of 37°C to 50°C, for example, within the range of 25°C to 42°C.

[0028] By selecting these temperatures generated by a series of heat exchangers and recycling the cooled aqueous liquid at various recycling locations, it becomes possible to shift the temperature level in the second section compared to conventional recycling where the recycling location is limited to one. In particular, it becomes possible to raise the temperature level. That is, by mixing the second and third portions of the cooled aqueous liquid, it becomes possible to increase the temperature at the second recycling location.

[0029] Furthermore, the method of the present invention allows for variations in the extraction rate of the aqueous liquid. In the method of the present invention, by recycling the cooled aqueous liquid at a higher temperature at the second recycling location, the extraction rate of the aqueous liquid can be increased, and at the same time, the rate of the cooled aqueous liquid recycled at the two recycling locations can be increased accordingly. This makes it possible to optimize the process by offsetting the higher temperature level through the cooling and recycling of a larger volume of aqueous liquid.

[0030] Advantageously, the two distinct examples, which recycle cooled aqueous liquid at different locations in the second compartment and discharge cooled aqueous liquid from different compartments of multi-stage cooling, provide greater flexibility in optimizing the energy demand for cooling and possibly the resulting changes in acrylic acid loss in the method for obtaining acrylic acid.

[0031] In the second section, the acrylic acid-containing gas stream is cooled by contact with the aqueous liquid. This causes condensation of so-called acidic water, which contains not only water but also a considerable amount of acrylic acid and, to a smaller proportion, other carboxylic acids. By cooling the aqueous liquid, i.e., the acidic water, which is removed from the second section and then recycled, as much as possible, it is possible to dissolve as many condensation components as possible into the acidic water. However, this has the disadvantage of increasing energy consumption for cooling. If the aqueous liquid is recycled at a high temperature, energy consumption for cooling is reduced. At the same time, the temperature rise shifts the temperature profile of the tower upward. This increases the proportion of acrylic acid that can enter the upper condensation region and condense.

[0032] Therefore, the acidic water taken from the second compartment and cooling circuit and supplied to the downstream extraction column can, advantageously, have a higher acrylic acid content. Thus, overall, the method of the present invention allows for highly flexible optimization with respect to energy consumption, acrylic acid content in the discharged acidic water, and acrylic acid loss.

[0033] The apparatus of the present invention for obtaining acrylic acid from a reaction gas comprises a first compartment for rapidly cooling the reaction gas with a quenching liquid to obtain an acrylic acid-containing liquid flow and an acrylic acid-containing gas flow. Furthermore, the apparatus comprises a second compartment for cooling the gas flow by contact with an aqueous liquid. The second compartment has an outlet opening at an outlet position at the lower end of the second compartment, and at least a first recycle opening and a second recycle opening at a first recycle position and a second recycle position, respectively, located above the outlet position, with the second recycle position being below the first recycle position. Furthermore, the apparatus of the present invention comprises a plurality of continuous heat exchangers arranged in series between the outlet opening and the second recycle opening for cooling the aqueous liquid extracted from the second compartment through the outlet opening. The first recycling piping connects the final heat exchanger to the first recycling opening in order to recycle the first portion of the aqueous liquid into the second compartment at a first temperature, and the second recycling piping connects the first region behind the first heat exchanger and before the final heat exchanger at the first discharge point to the second recycling opening in order to discharge the second portion of the aqueous liquid and recycle it into the second compartment at a second temperature.

[0034] The apparatus of the present invention is particularly designed to carry out the method of the present invention. Therefore, it has the same advantages as the method of the present invention.

[0035] According to one advanced form of the apparatus of the present invention, it has a third recycling pipe that connects to a second region at a second discharge location, where at least one further heat exchanger portion upstream from the first region is a second region, in order to discharge a third portion of aqueous liquid at a third temperature and mix the third portion with the second portion. Advantageously, this embodiment of the apparatus of the present invention allows for the discharge of aqueous liquid at different temperatures from the cooling circuit in at least two different regions separated by at least one heat exchanger, in addition to the first portion of aqueous liquid recycled via the first recycling pipe, and to recycle it into a second region via the second recycling pipe. This arrangement of the recycling pipe allows for the optimization of the energy demand of the heat exchanger when the apparatus is used to obtain acrylic acid.

[0036] A heat exchanger is, in particular, an indirect heat exchanger that transfers thermal energy from a withdrawn aqueous liquid to another material flow, a cooling medium. Since the heat exchangers are arranged in series, the aqueous liquid withdrawn from the second compartment is further cooled by each heat exchanger. Therefore, what can be performed is multi-stage cooling, in which the aqueous liquid is cooled to a lower temperature in each cooling stage.

[0037] According to one embodiment of the apparatus of the present invention, the heat exchanger upstream of the second discharge position is selected from an air-cooled heat exchanger and a water-cooled heat exchanger. For example, the first heat exchanger in the apparatus of the present invention is an air-cooled heat exchanger. The second heat exchanger downstream of the first heat exchanger is, for example, a water-cooled heat exchanger, for example, a surface water-cooled heat exchanger. The water used for cooling is, for example, taken from a river without being cooled.

[0038] According to a further embodiment of the apparatus of the present invention, at least one further heat exchanger located downstream of the second discharge position and upstream of the first discharge position, for example, a third heat exchanger, is a chilled water-cooled heat exchanger. In this case, the cooling medium used is water that has been cooled compared to the water used in the water-cooled heat exchanger. This requires the input of energy, particularly electrical energy. For example, the chilled water-cooled heat exchanger can also be a saltwater-cooled heat exchanger. Alternatively, the cooling medium used in the third heat exchanger is a glycol / water mixture.

[0039] According to a further embodiment of the apparatus of the present 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 a cooling medium, and the other of these heat exchangers uses water that has been cooled compared to the water used in a water-cooled heat exchanger as a cooling medium.

[0040] According to a further embodiment of the apparatus of the present invention, at least one further heat exchanger, for example, a fifth heat exchanger located downstream of the first discharge position, is a liquefied gas evaporator that further cools the acidic water after the first discharge position.

[0041] According to one embodiment of the apparatus of the present invention, the second recycling piping connects the outlet of the chilled water-cooled heat exchanger to the second recycling opening.

[0042] According to a further embodiment of the apparatus of the present invention, the third recycling pipe connects the outlet of the water-cooled heat exchanger to the second recycling pipe.

[0043] According to a further embodiment of the apparatus of the present invention, a mixer is placed between the third recycling pipe and the second recycling pipe to mix the third portion of the aqueous liquid with the second portion of the aqueous liquid.

[0044] The present invention will be described below with reference to the drawings and based on exemplary embodiments. [Brief explanation of the drawing]

[0045] [Figure 1] This figure shows a first exemplary embodiment of the apparatus of the present invention. [Figure 2] This figure shows a second exemplary embodiment of the apparatus of the present invention. [Modes for carrying out the invention]

[0046] A first exemplary embodiment of the apparatus of the present invention will be described with reference to Figure 1.

[0047] The basic structure of the apparatus corresponds to the arrangement described in German Patent Application Publication No. 19740253. The apparatus comprises a condensing tower 1 having a first compartment 2 and a second compartment 3. In the first compartment 2, a reaction gas containing acrylic acid, acrolein, water, and impurities, formed in the catalytic gas-phase oxidation of propylene and acrolein, is introduced into the bottom region. Here, the high-temperature reaction gas is cooled by rapid cooling, i.e., direct cooling. As a result, an acrylic acid-containing liquid flow and an acrylic acid-containing gas flow flowing countercurrently upward are produced. A collection tray 4 is located at the upper end of the second compartment 2, through which the acrylic acid-containing gas flow can pass, for example, through at least one chimney. The collection tray 4 separates the first compartment 2 and the second compartment 3. In the exemplary embodiment described herein, the two compartments 2 and 3 are formed within one condensing tower 1. However, it would also be possible to form the second compartment 3 separately from the first compartment in a separate condensing tower.

[0048] The gas flow rising through the second section 3 is also cooled by contact with the aqueous liquid. In this way, rapid cooling, or direct cooling of the rising gas flow by the counterflowing aqueous liquid, occurs in the second section 3 as well. Finally, the gas flow exits the condensation tower 1 through the outlet opening 5.

[0049] The aqueous liquid that collects on collection tray 4 contains components of the condensed rising gas stream, and is also called acidic water. This liquid contains not only water, but also a considerable amount of acrylic acid and, in smaller proportions, other carboxylic acids.

[0050] Acidic water is extracted from the second compartment 3. For this purpose, an extraction opening 6 is provided at an extraction position just above the top surface of the collection tray 4. The acidic water is subjected to multi-stage cooling through this extraction opening 6. The multi-stage cooling includes a plurality of heat exchangers, collectively indicated by 8, and a plurality of pipes, collectively indicated by 9. The acidic water is extracted by a pump 7 from the extraction opening 6 via pipe 9-1 and supplied to the first heat exchanger 8-1 via pipe 9-2. The first heat exchanger 8-1 is an air-cooled heat exchanger. The first heat exchanger 8-1 cools the extracted acidic water.

[0051] The extracted acidic water is supplied to the second heat exchanger 8-2 via further piping 9-3, where it is further cooled. The second heat exchanger 8-2 is a water-cooled heat exchanger that uses surface water as a cooling medium.

[0052] The extracted acidic water is supplied via further piping 9-4 to a third heat exchanger 8-3, which further cools the acidic water. The cooling medium used by the third heat exchanger 8-3 is a glycol / water mixture (for example, having a temperature of 32°C). This acts as the heat transfer medium in the method and can extract heat from the acidic water circuit.

[0053] The cooled acidic water is sent from the third heat exchanger 8-3 through piping 9-5 to the fourth heat exchanger 8-4, which is a chilled water-cooled heat exchanger. The cooling medium used in this case is cooled water that is at a lower temperature than the surface water used in the water-cooled heat exchanger.

[0054] The cooled acidic water enters piping 9-6 after the fourth heat exchanger 8-4. Here, the acidic water is divided into a first part and a second part at the first discharge position 16-1.

[0055] The first portion is further sent through piping 9-6 to the fifth heat exchanger 8-5, which is a liquefied gas evaporator. In the fifth heat exchanger 8-5, the extracted acidic water is further cooled. The first portion of the further cooled acidic water is returned from the fifth heat exchanger 8-5 to the second compartment 3 via the first recycling piping 11-1 and the first recycling opening 10-1 of the first recycling location. The first recycling location is located in the upper region of the second compartment 3, i.e., at the top of the condensing tower 1. From here, the cooled acidic water flows downward, i.e., countercurrent to the rising reaction gas, and as a result the reaction gas is cooled, and in particular the acrylic acid is concentrated in the aqueous liquid. The aqueous liquid then collects on the collection tray 4.

[0056] A second recycling pipe 11-2 is provided to discharge the second portion of the cooled acidic water at the first discharge location 16-1. This pipe connects the area after the first heat exchanger 8-1 and before the final heat exchanger 8-5 to a second recycling opening 10-2 for recycling the second portion of the acidic water to the second compartment 3. In the exemplary embodiment described herein, the second recycling pipe 11-2 branches off at the first discharge location 16-1 from pipe 9-6 connecting the fourth heat exchanger 8-4 to the fifth heat exchanger 8-5.

[0057] The second recycling opening 10-2 is located at the second recycling position, situated between the extraction position and the first recycling position. The second temperature at which the second portion of the acidic water is recycled is higher than the first temperature at which the first portion of the acidic water is recycled, because the second portion of the acidic water is not cooled by the fifth heat exchanger 8-5. Thus, the second section 3 of the condensing tower 1 as a whole has multiple recycling openings, indicated by 10, where cooled aqueous liquid is recycled at different temperatures at different recycling positions.

[0058] To prevent the first section 2 of the condensing tower 1 from running out of liquid, a portion of the acidic water removed at the outlet opening 6 is returned to the upper region of the first section 2 via the tower piping 14. In this specification, this portion of the acidic water recycled to the first section 2 is also referred to as the fourth portion of the aqueous liquid removed at the outlet.

[0059] Furthermore, a fifth portion of the extracted acidic water is discharged through the extraction pipe 13 in pipe 9-2 and supplied to the downstream extraction column 15. The extracted acidic water is further treated in the extraction column 15 to obtain the acrylic acid component.

[0060] A first exemplary embodiment of the method of the present invention will be described below with reference to Figure 1.

[0061] In this method, acrylic acid is obtained from a reaction gas containing acrylic acid, acrolein, water, and impurities, which is formed in the catalytic gas-phase oxidation of propylene and acrolein. The reaction gas is supplied to the first compartment 2 of the condensing tower 1, where it is cooled by rapid cooling with a quenching liquid. As a result, an acrylic acid-containing liquid flow and an acrylic acid-containing gas flow rising in the countercurrent direction within the condensing tower 1 are generated.

[0062] A portion of the acrylic acid-containing liquid stream is withdrawn from the first compartment 2 via a side outlet (not shown), from which acrylic acid is obtained.

[0063] The acrylic acid-containing gas stream enters the second compartment 3 of the condensing tower 1 through the chimney of the collection tray 4. Here, the gas stream is cooled by contact with an aqueous liquid. The aqueous liquid is flowed countercurrent to the rising gas stream so that the acrylic acid is concentrated in the aqueous liquid. It then collects on the collection tray 4. The aqueous liquid, i.e., acidic water, is removed at the removal position of the removal opening 6 at the lower end of the second compartment 3 and cooled in a series of heat exchangers 8. Therefore, the removal position is located below the liquid level of the acidic water collected on the collection tray 4.

[0064] In the method of the present invention, 865 m 3A volumetric flow rate of / h is extracted by the pump 7 at a temperature of 62.7°C through the outlet opening 6.

[0065] 36.70 m 3 The volumetric flow rate of / h is returned to the first section 2 of the condensing tower via the tower piping 14 in piping 9-2. Furthermore, the volumetric flow rate of 14.51 m³ 3 Acidic water at a concentration of / h is discharged through extraction pipe 13 in pipe 9-2 and supplied to extraction column 15.

[0066] The remaining acidic water is led to the first heat exchanger 8-1, where it is cooled to 50°C. The required cooling capacity of the first heat exchanger is 10486 kW.

[0067] Next, the acidic water is 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 6612 kW.

[0068] Next, the acidic water is supplied to the third heat exchanger 8-3 and cooled to 39°C. The required cooling capacity of the third heat exchanger 8-4 is 2450 kW. Since the third heat exchanger 8-3 is a chilled water-cooled heat exchanger, it requires energy input.

[0069] Next, the acidic water is supplied 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 11756 kW. Since the fourth heat exchanger 8-4 is a chilled water-cooled heat exchanger, it requires the input of electrical energy.

[0070] After the fourth heat exchanger 8-4, the cooled acidic water is divided into a first and a second part. The first part, which accounts for approximately 42% of the volumetric flow rate, is supplied to the fifth heat exchanger 8-5, where it is cooled to a first recycling temperature of 20.7°C and recycled to the second section 3 through the first recycling opening 10-1 at the first recycling location. The capacity consumption of the fifth heat exchanger 8-5 required for cooling is 1400 kW. Since the fifth heat exchanger 8-5 is a liquefied gas evaporator, it also draws energy from the acidic water.

[0071] The second portion, which accounts for approximately 58%, is recycled through the second recycling opening 10-2 to the second section 3 of the condenser 1 at the second recycling location, at a second recycling temperature of 24.7°C.

[0072] By recycling the second portion of the acidic water at a second recycling location at a second temperature higher than the first temperature, a temperature increase occurs in the temperature profile of section 3 of the condensing tower 1 compared to a method in which a portion of the acidic water is discharged beforehand and not recycled at a higher temperature. This increases the acrylic acid content in section 3. In a first exemplary embodiment of the method of the present invention, 14.51 m of acrylic acid containing 9.97% is recycled at 62.7°C. 3 A volumetric flow rate of / h is obtained in section 3.

[0073] A second exemplary embodiment of the apparatus of the present invention will be described below with reference to Figure 2.

[0074] Since the apparatus of the second exemplary embodiment is substantially equivalent to the apparatus of the first exemplary embodiment, only the differences between the two will be discussed below.

[0075] In the apparatus of the second exemplary embodiment, a third portion of the acidic water cooled by at least the first heat exchanger 8-1 is discharged (specifically, at least one further heat exchanger upstream from the extraction of the second portion at the first discharge position 16-1). In the first exemplary embodiment, this second portion was extracted in piping 9-6 between the fourth heat exchanger 8-4 and the fifth heat exchanger 8-5. In the second exemplary embodiment, if this second portion is also discharged in piping 9-6, the third portion in the second exemplary embodiment branches off at the second discharge position 16-2 in piping 9-4 between the second heat exchanger 8-2 and the third heat exchanger 8-3, and is supplied to the mixer 12 via the third recycling piping 11-3. In the mixer 12, the third portion is mixed with the second portion extracted in piping 9-6. Next, the mixture is recycled back to the second compartment 3 at the second recycling location through the second recycling opening 10-2 via the second recycling pipe 11-2, similar to the first exemplary embodiment.

[0076] Upstream of the second discharge point are the first heat exchanger 8-1 and the second heat exchanger 8-2. These two heat exchangers 8-1 and 8-2 are distinguished by the fact that the cooling medium they use, namely ambient air and surface water, is taken from the environment without being cooled, and in particular, its use in heat exchangers 8-1 and 8-2 does not require separate cooling.

[0077] A second exemplary embodiment of the method of the present invention will be described below with reference to Figure 2.

[0078] Since the second exemplary embodiment of the method of the present invention is substantially equivalent to the first exemplary embodiment of the method of the present invention, only the differences between the two will be discussed below.

[0079] In the second exemplary embodiment, acidic water is dispensed at the dispensing position through the dispensing opening 6, at a rate of 1227 m 3The acidic water is drawn from the second compartment 3 at a volumetric flow rate of / h and a temperature of 63.2°C. The volumetric flow rate of the acidic water recycled to the first compartment 2 via the tower piping 14 and the volumetric flow rate of the acidic water drawn via the extraction piping 13 correspond to the volumetric flow rates of the first exemplary embodiment.

[0080] The first heat exchanger 8-1 cools the remaining volumetric flow rate of acidic water to a temperature of 50°C. The required capacity consumption is 15587 kW.

[0081] The second heat exchanger 8-2 cools the volumetric flow rate of acidic water to a temperature of 42°C. The required capacity consumption is 9464 kW. In piping 9-4, approximately 70% of the acidic water is discharged at a temperature of 42°C at the second discharge point 16-2.

[0082] The remaining portion, approximately 30%, is supplied to the third heat exchanger 8-3, which cools it to 35°C. The required capacity is 2453 kW.

[0083] Next, the acidic water is supplied to the fourth heat exchanger 8-4, where it is cooled to 25°C. The required power is 3291 kW.

[0084] Next, in piping 9-6, acidic water is discharged through piping 9-7 to the first discharge position 16-1. Then, in mixer 12, the discharged portion is mixed with the portion discharged at discharge position 16-2 and recycled through the second recycling piping 11-2 to the second section 3 at the second recycling position at a second recycling temperature of 25-42°C, through the second recycling opening 10-2.

[0085] Similar to the first exemplary embodiment of the method of the present invention, the first portion of the acidic water is cooled to 20.9°C in the fifth heat exchanger 8-5 and recycled to the second compartment 3 through the first recycling opening 10-1 at the first recycling location. Similar to the first exemplary embodiment, the capacity consumption of the fifth heat exchanger 8-5 is 1400 kW.

[0086] Compared to the first exemplary embodiment of the method of the present invention, the acidic water is pre-discharged at a higher temperature, for example, 42°C, after the second heat exchanger 8-2. Then, in the mixer 12, this portion is variably mixed with further portions of the acidic water whose temperature has been lowered by the second and third heat exchangers 8-2 and 8-3. In this way, the acidic water can be recycled to the second compartment 3 through the second recycling opening 10-2 in varying amounts and temperatures. This allows for method optimization to obtain the advantage of saving energy while suppressing the loss of acrylic acid.

[0087] Compared to the first exemplary embodiment of the method of the present invention, the second exemplary embodiment of the method of the present invention consumes 72% less electrical energy for cooling, assuming substantially the same total cooling capacity is applied.

[0088] In this case, by recycling the second and third portions of the acidic water at a second recycling location at a second temperature higher than the first temperature, a further temperature increase in the temperature profile of section 3 of the condenser 1 occurs compared to the method of the first exemplary embodiment. This further increases the acrylic acid content in section 3. In a second exemplary embodiment of the method of the present invention, 14.54 m of acrylic acid at 63.1°C containing 11.9% acrylic acid is used. 3 A volumetric flow rate of / h is obtained in section 3.

[0089] Furthermore, the acidic water extracted through the extraction pipe 13 has a 1.9% higher acrylic acid content compared to the method of the first exemplary embodiment. [Explanation of Symbols]

[0090] 1. Condensation tower 2. Section 1 3. Section 2 / Rapid Cooling Section 4 Collection trays 5 Outlet opening 6. Removal opening 7 Pumps 8 Heat exchanger 8-1 First heat exchanger (air-cooled heat exchanger) 8-2 Second heat exchanger (water-cooled heat exchanger) 8-3 Third heat exchanger 8-4 4th heat exchanger (chilled water cooling type heat exchanger) 8-5 Fifth heat exchanger (liquefied gas evaporator) 9 Piping 9-1 Piping 9-2 Piping 9-3 Piping 9-4 Piping 9-5 Piping 9-6 Piping 9-7 Piping 10 Recycling openings 10-1 First Recycling Opening 10-2 Second Recycling Opening 11. Recycled Piping 11-1 First Recycling Piping 11-2 Second Recycling Piping 11-3 Third Recycling Piping 12 Mixer 13 Extraction piping 14 Tower Piping 15 Extraction tower 16 Discharge position 16-1 1st discharge position 16-2 2nd discharge position

Claims

1. A method for obtaining acrylic acid from a reaction gas containing acrylic acid, acrolein, water, and impurities formed in the catalytic gas-phase oxidation of propylene or acrolein, a) The reaction gas is rapidly cooled in a first compartment with a quenching liquid to obtain an acrylic acid-containing liquid stream and an acrylic acid-containing gas stream. b) A step of cooling the gas flow by bringing it into contact with an aqueous liquid in a second compartment, wherein the aqueous liquid is collected and taken out at a takeout position at the lower end of the second compartment, is indirectly cooled, and at least a portion of the cooled aqueous liquid is returned to the second compartment. Includes, The aqueous liquid is cooled by a plurality of consecutive heat exchangers. The first portion of the aqueous liquid is recycled to the second section at a first temperature at a first recycling position located above the extraction position after the final heat exchanger. The second portion of the aqueous liquid is discharged at a second temperature at a first discharge position after the first heat exchanger and before the final heat exchanger, and recycled to the second section at a second recycling position located below the first recycling position. The aforementioned method.

2. The method according to claim 1, wherein the cooling medium used by the heat exchanger located downstream of the first discharge point is cooled by energy consumption, and the cooling medium used by the heat exchanger located upstream of the first discharge point or at least one of the heat exchangers located upstream of the first discharge point is removed from the environment without being cooled.

3. The method according to claim 1 or 2, wherein the cooled third portion of the aqueous liquid is discharged at a third temperature at a second discharge position located at least one further heat exchanger upstream of the extraction position of the second portion, mixed with the cooled second portion of the aqueous liquid, and then recycled to the second section at a second recycling temperature at the second recycling position.

4. The method according to claim 3, wherein the cooling medium used by the heat exchanger located upstream of the second discharge position is removed from the environment without being cooled.

5. The method according to any one of claims 1 to 4, wherein the first temperature is in the range of 5°C to 21°C.

6. The method according to any one of claims 1 to 5, wherein the second temperature is in the range of 21°C to 30°C.

7. The method according to any one of claims 3 to 6, wherein the third temperature is in the range of 26°C to 50°C, particularly in the range of 35°C to 45°C.

8. An apparatus for obtaining acrylic acid from a reaction gas, A first section for rapidly cooling the reaction gas with a quenching liquid to obtain an acrylic acid-containing liquid stream and an acrylic acid-containing gas stream, A second compartment for cooling the gas flow by bringing it into contact with an aqueous liquid, Here, the second compartment has an extraction opening at an extraction position at the lower end of the second compartment, and at least a first recycling opening and a second recycling opening at a first recycling position and a second recycling position, respectively, which are located above the extraction position, with the second recycling position being below the first recycling position. A plurality of continuous heat exchangers arranged in series between the extraction opening and the second recycling opening for cooling the aqueous liquid extracted from the second compartment through the extraction opening, To recycle the first portion of the aqueous liquid into the second compartment at a first temperature, a first recycling pipe connects the final heat exchanger to the first recycling opening, To discharge the second portion of the aqueous liquid and recycle it in the second section at a second temperature, a second recycling pipe is provided that connects the first region after the first heat exchanger and before the final heat exchanger to the second recycling opening at the first discharge position. The apparatus, including the above.

9. The apparatus according to claim 8, further comprising a third recycling pipe connecting a second region, at a second discharge position, to the second region, where at least one further heat exchanger component is upstream of the first region, in order to discharge the third portion of the aqueous liquid at a third temperature and mix the third portion with the second portion.

10. The apparatus according to claim 8 or claim 9, wherein the heat exchanger located upstream of the second discharge position is selected from an air-cooled heat exchanger and a water-cooled heat exchanger.

11. The apparatus according to any one of claims 8 to 10, wherein at least one further heat exchanger located downstream of the second discharge position and upstream of the first discharge position is a chilled water-cooled heat exchanger.

12. The apparatus according to any one of claims 8 to 11, wherein at least one further heat exchanger downstream of the first discharge position is a liquefied gas evaporator.

13. The apparatus according to any one of claims 8 to 12, wherein the second recycling piping connects the outlet of the chilled water-cooled heat exchanger to the second recycling opening.

14. The apparatus according to any one of claims 9 to 13, wherein the third recycling pipe connects the outlet of the water-cooled heat exchanger to the second recycling pipe.

15. The apparatus according to any one of claims 9 to 14, wherein a mixer for mixing the third portion of the aqueous liquid with the second portion of the aqueous liquid is arranged between the third recycling pipe and the second recycling pipe.

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