A method for the coupled purification of isopropanol by molecular sieve adsorption dehydration and distillation

Abstract

This invention discloses a coupled molecular sieve adsorption dehydration and distillation purification method for isopropanol, belonging to the field of chemical separation and purification. The method includes feeding hydrous isopropanol feedstock into a distillation column for distillation separation, and defining a feed section and a product discharge section within the distillation column. By identifying a water content slow-decreasing section within the distillation column, and extracting a side stream from this section for molecular sieve dehydration before returning it to the side near the product discharge section, the water content of the isopropanol feedstock near the product discharge section continues to decrease and remains stable. This solves the problem in existing molecular sieve dehydration and distillation processes that do not address the slow-decreasing water content location within the column, leading to fluctuations in the product discharge water content.

Description

Technical Field

[0001] This invention relates to the field of chemical separation and purification technology, and more specifically, to a method for the coupled purification of isopropanol by molecular sieve adsorption dehydration and distillation. Background Technology

[0002] Isopropanol is commonly used in electronic cleaning, precision component cleaning, and chemical formulation. These applications require a high water content in the isopropanol, especially during the isopropanol refining process, where the stable removal of trace amounts of water directly affects the batch stability of the product.

[0003] In existing isopropanol refining processes, distillation is typically used to remove light and heavy components, and molecular sieve adsorption beds are used for dehydration.

[0004] Molecular sieves can adsorb water from isopropanol feed, and distillation columns can regulate the component distribution within the column through vapor-liquid separation. Therefore, the two are often used in series, for example, distillation followed by molecular sieve dehydration, or molecular sieve dehydration followed by distillation column purification.

[0005] However, in the actual refining process, the water content does not decrease at the same rate as the isopropanol feed solution moves from the feed tower section to the product discharge tower section.

[0006] Due to the interaction between isopropanol and water, reflux within the tower, and changes in the load on the tower sections, the water content in some sections may decrease significantly at a slower pace.

[0007] After this point, even if the reflux ratio is further increased, the moisture content of the product discharge tower section may still experience a slight increase or periodic fluctuations.

[0008] Existing processes typically focus only on the moisture content of raw materials, the moisture content of the product output, or the moisture content at the molecular sieve bed outlet, while paying less attention to which section of the distillation column begins to show a slower decrease in moisture content.

[0009] If molecular sieve dewatering is performed directly on the entire raw material, the molecular sieve load will be too high, and the bed will easily approach saturation in advance. If molecular sieve dewatering is performed only after the product is discharged, it will be difficult to improve the water content distribution inside the distillation column, and trace amounts of water may still accumulate near the product discharge section. Summary of the Invention

[0010] To address the problems mentioned in the background section, the present invention provides the following technical solution: A method for purifying isopropanol by coupling molecular sieve adsorption dehydration with distillation includes the following steps: Aqueous isopropanol feedstock is fed into a distillation column for distillation separation, and the feed section and product discharge section are determined within the distillation column. At least three continuous testing towers are selected between the feed tower section and the product discharge tower section. The water content of the isopropanol liquid in each testing tower section is continuously measured, and the average water content of each testing tower section is obtained. Along the direction from the feed tower section to the product discharge tower section, calculate the moisture content difference between adjacent testing tower sections, and obtain the slow-descent ratio of the testing tower section based on the moisture content difference before and after the testing tower section located in the middle position. When the slow-descent ratio of the detection tower section is less than the set slow-descent ratio, and the isopropanol content of the isopropanol feed liquid in the detection tower section is within the preset range, the detection tower section is determined as the water content slow-descent section. The side stream liquid is extracted from the moisture content reduction section and sent to the current molecular sieve adsorption bed for dehydration; The dehydrated side stream liquid is sent back to the return tower position on the side of the moisture content slow-decreasing section near the product discharge tower section, and the return tower position is located between the moisture content slow-decreasing section and the product discharge tower section. During the continuous extraction, dehydration, and return of the side stream feed liquid, the position of the water content slow-decreasing section is redefined; when the water content of the isopropanol feed liquid in the product discharge tower section increases, and the redefined water content slow-decreasing section moves towards the product discharge tower section, the side stream feed liquid is switched to another molecular sieve adsorption bed, and the cut-out current molecular sieve adsorption bed is regenerated. When the water content of the isopropanol liquid in the product discharge tower section is lower than that in the water content reduction section, and the change in water content of the isopropanol liquid in the product discharge tower section within a set time does not exceed a set value, high-purity isopropanol is obtained from the product discharge tower section.

[0011] Furthermore, the three continuous detection tower sections are sequentially referred to as the (i-1)th tower section, the ith tower section, and the (i+1)th tower section along the direction from the feed tower section to the product discharge tower section; The difference in water content before the i-th tower section is Cw,i-1-Cw,i, and the difference in water content after the i-th tower section is Cw,i-Cw,i+1. Wherein, Cw,i-1 is the average water content of the isopropanol feed in the (i-1)th column section, Cw,i is the average water content of the isopropanol feed in the ith column section, and Cw,i+1 is the average water content of the isopropanol feed in the (i+1)th column section. The descent ratio Ri of the i-th tower segment satisfies: Ri=(Cw,i-Cw,i+1) / (Cw,i-1-Cw,i); When Ri is less than the set descent ratio, the i-th tower segment is selected as a candidate descent tower segment.

[0012] Furthermore, the isopropanol content of the isopropanol feed solution in the candidate slow-descent tower section was detected. When the isopropanol content of the candidate slow-descent column section is in the range of 99.60% to 99.95%, the candidate slow-descent column section is designated as the column section to be extracted. When there are two or more sections to be extracted, the section with the smallest slow-descent ratio is selected as the slow-descent section for water content.

[0013] Furthermore, the water content of the isopropanol feed solution in each detection tower section was determined using a Karl Fischer moisture analyzer. Each testing tower section was measured at least three times consecutively, and the average of the three measurements was taken as the average water content of that testing tower section.

[0014] Furthermore, the amount of liquid extracted from the side stream is 2% to 15% of the amount of water-containing isopropanol feedstock. When the water content of the isopropanol liquid in the product discharge tower section increases, increase the extraction rate of the side stream liquid. When the water content of the isopropanol liquid in the product discharge tower section remains stable within a set time, and the water content of the isopropanol liquid decreases in the water content slow-decreasing section, the extraction rate of the side stream liquid is reduced.

[0015] Furthermore, the molecular sieve adsorption bed uses one or two of 3A molecular sieves and 4A molecular sieves; After the side stream feed liquid enters the molecular sieve adsorption bed, the water in the side stream feed liquid is adsorbed by the molecular sieve, and the water content of the side stream feed liquid at the outlet of the molecular sieve adsorption bed is lower than that at the inlet of the molecular sieve adsorption bed.

[0016] Furthermore, the return tower position is the position corresponding to 1 to 5 theoretical plates on the side of the moisture content slow-descent section near the product discharge tower section; The dehydrated side stream liquid is returned to the return tower location in liquid phase and mixed with the descending liquid phase in the return tower location.

[0017] Furthermore, the molecular sieve adsorption bed includes a first molecular sieve adsorption bed and a second molecular sieve adsorption bed; When the side stream feed liquid is dehydrated by the current molecular sieve adsorption bed and sent back to the distillation column, if the water content of the isopropanol feed liquid in the product discharge column section still increases, or if the newly determined water content slow-decline section moves at least one theoretical plate towards the product discharge column section compared to the previously determined position, the side stream feed liquid is switched to another molecular sieve adsorption bed. The cut-out current molecular sieve adsorption bed is regenerated to obtain an aqueous isopropanol regenerated condensate; The regenerated condensate of the aqueous isopropanol is returned to a position on the side of the slow-decreasing water content section away from the product discharge section, or returned to the bottom discharge position of the distillation column.

[0018] In summary, the present invention has the following beneficial effects: By identifying a slow-decreasing section of water content within the distillation column, and extracting a side stream feed from this section for molecular sieve dehydration before returning it to the side near the product discharge column, the water content of the isopropanol feed near the product discharge column continues to decrease and remains stable. This solves the problem of fluctuating product discharge water content caused by the lack of treatment of the slow-decreasing water content location within the column in existing molecular sieve dehydration and distillation series processes. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the overall process flow of the present invention; Figure 2 This is a schematic diagram illustrating the determination of the moisture content gradual decrease section in this invention; Figure 3 This is a schematic diagram of the molecular sieve adsorption bed switching and regeneration condensate return to the tower according to the present invention. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0022] Example 1 The following is in conjunction with the appendix Figure 1-3 The present invention will be described in further detail below.

[0023] Please see Figure 1-3 This invention provides a technical solution: a method for the coupled purification of isopropanol by molecular sieve adsorption dehydration and distillation, comprising the following steps: Aqueous isopropanol feedstock is fed into a distillation column for distillation separation, and the feed section and product discharge section are determined within the distillation column. Select at least three continuous testing tower sections between the feed tower section and the product discharge tower section, and obtain the water content of the isopropanol liquid in each testing tower section. Compare the moisture content differences between adjacent testing tower sections along the direction from the feed tower section to the product discharge tower section; When the moisture content difference after a certain detection tower section is less than the moisture content difference before that detection tower section, that detection tower section is defined as a moisture content slow-decreasing section. The side stream liquid is extracted from the moisture content reduction section and sent to the molecular sieve adsorption bed for dehydration. The dehydrated side stream liquid is sent back to the return tower position on the side of the water content slow-decreasing section near the product discharge tower section; During the continuous extraction, dehydration, and return of the side stream liquid, when the water content of the isopropanol liquid in the product discharge tower section is lower than that in the water content reduction section, and the change in water content of the isopropanol liquid in the product discharge tower section within a set time does not exceed a set value, isopropanol is obtained from the product discharge tower section. This embodiment is used to process aqueous isopropanol feedstock, wherein the aqueous isopropanol feedstock has an isopropanol mass fraction of 99.2%, a water content of 0.75%, and a feedstock flow rate of 1000 kg / h.

[0024] The aqueous isopropanol feedstock enters the distillation column from the middle section. The distillation column is equipped with a packed section or a tray section. In this embodiment, the section corresponding to the feedstock inlet is designated as the feed section, and the section corresponding to the isopropanol product outlet is designated as the product outlet section. The distillation column operates at atmospheric pressure, with a reboiler temperature of 82℃~86℃, a top temperature of 78℃~82℃, and a reflux ratio controlled at 2.0~4.0.

[0025] Between the feed tower section and the product discharge tower section, sections 9, 10, 11, and 12 were selected as continuous monitoring tower sections along the direction from the feed tower section to the product discharge tower section. Each monitoring tower section was equipped with a sampling port, and the water content of the sampled isopropanol solution was determined using a Karl Fischer moisture analyzer after cooling.

[0026] After 30 minutes of stable operation, the water content of the isopropanol feed solution in the 9th, 10th, 11th and 12th tower sections was 280 ppm, 170 ppm, 140 ppm and 134 ppm, respectively.

[0027] The moisture content difference between section 9 and section 10 is 110 ppm, and the difference between section 10 and section 11 is 30 ppm. Since the moisture content difference after section 10 is significantly smaller than that before section 10, it indicates that the rate of dehydration of the isopropanol feed solution decreases after section 10. Therefore, section 10 is designated as the moisture content slow-decreasing section.

[0028] Side stream liquid is extracted from the 10th tower section at a rate of 80 kg / h.

[0029] The side stream feed liquid is cooled to 25℃~35℃ and then enters the molecular sieve adsorption bed. The molecular sieve adsorption bed is filled with 3A molecular sieve with a particle size of 1.6mm~3.2mm and a bed height of 1.2m.

[0030] The residence time of the side-stream feed liquid in the molecular sieve adsorption bed is 8 min to 15 min.

[0031] After dehydration by the molecular sieve adsorption bed, the water content of the side stream feed liquid decreased from 170 ppm to below 25 ppm.

[0032] The dehydrated side stream liquid is returned in liquid phase to the 11th column section, which is located near the product discharge column section of the 10th column section, so that the isopropanol liquid with lower water content enters the liquid phase flow path in the column after the water content slows down section.

[0033] Before returning the dehydrated side stream liquid to the tower, it can be heated to near the liquid phase temperature at the tower return location via a heat exchanger, or the return flow rate can be controlled to not exceed 15% of the raw material feed rate, in order to reduce the impact on the vapor-liquid balance at the tower return location.

[0034] After the side stream feed liquid was continuously extracted, dehydrated, and returned for 60 minutes, the water content of the isopropanol feed liquid in the 10th, 11th, and product discharge sections was measured again. The water content in the 10th section was 155 ppm, the water content in the 11th section was 42 ppm, and the water content in the product discharge section ranged from 18 ppm to 22 ppm.

[0035] When the water content of the isopropanol solution in the product discharge tower section is lower than that in the 10th tower section, and the change in water content of the isopropanol solution in the product discharge tower section within 30 minutes is no greater than 5 ppm, isopropanol is drawn out from the product discharge tower section.

[0036] The obtained isopropanol has an isopropanol mass fraction of not less than 99.99% and a water content of not more than 30 ppm.

[0037] In this embodiment, by comparing the difference in moisture content between the continuous detection tower sections, the tower section where the moisture content decreases more slowly is determined, and the side stream liquid is extracted from the tower section for molecular sieve dehydration. The dehydrated side stream liquid is then sent back to the side of the tower section near the product discharge section, so that the moisture content of the product discharge tower section is kept at a low level.

[0038] Example 2 like Figure 1-3 As shown, the three continuous detection tower sections along the direction from the feed tower section to the product discharge tower section are respectively denoted as the (i-1)th tower section, the ith tower section, and the (i+1)th tower section; The difference in water content before the i-th tower section is Cw,i-1-Cw,i, and the difference in water content after the i-th tower section is Cw,i-Cw,i+1. Wherein, Cw,i-1 is the water content of the isopropanol feed in the (i-1)th column section, Cw,i is the water content of the isopropanol feed in the ith column section, and Cw,i+1 is the water content of the isopropanol feed in the (i+1)th column section. When Cw,i-Cw,i+1 is less than Cw,i-1-Cw,i, the i-th tower section is determined as the water content gradually decreasing section.

[0039] The descent ratio Ri of the i-th tower segment satisfies: Ri=(Cw,i-Cw,i+1) / (Cw,i-1-Cw,i); When Ri is less than the set slow-descent ratio, the i-th tower section is determined as the slow-descent section of water content.

[0040] When the slow-descent ratio of two or more detection tower sections is less than the set slow-descent ratio, the isopropanol content of the isopropanol feed liquid in the two or more detection tower sections is obtained respectively. The detection tower section with a slow-fall ratio less than the set slow-fall ratio and isopropanol content within the preset range is designated as the extraction tower section; When there are two or more sections to be extracted, the section with the smallest slow-descent ratio is selected as the slow-descent section for water content.

[0041] This embodiment further explains the method for determining the gradual decrease in water content, based on Embodiment 1.

[0042] After the aqueous isopropanol feedstock enters the distillation column, the column top temperature, column bottom temperature, and reflux ratio are stabilized for 30 minutes. Then, a continuous monitoring section is selected from the feed section to the product discharge section. In this embodiment, sections 7 to 13 are selected as monitoring sections, and each monitoring section is equipped with a sampling port. During sampling, the residual liquid in the sampling tube is first vented, then 50 mL of sample is taken, cooled to 25°C, and then sealed for testing. The water content is determined using a Karl Fischer moisture analyzer, and three consecutive measurements are taken for each section, and the average value is recorded.

[0043] The water content of the isopropanol feed solution in each detection tower section is as follows:

[0044] Along the direction from the feed tower section to the product discharge tower section, calculate the moisture content difference between adjacent testing tower sections. The moisture content difference between tower sections 7 and 8 is 209 ppm, between tower sections 8 and 9 is 106 ppm, between tower sections 9 and 10 is 43 ppm, between tower sections 10 and 11 is 11 ppm, between tower sections 11 and 12 is 3 ppm, and between tower sections 12 and 13 is 2 ppm.

[0045] In this embodiment, the slow-descent ratio is set to 0.35. The slow-descent ratio R10 is calculated for the 10th column section: R10 = (162 - 151) / (205 - 162) = 0.26; the slow-descent ratio R11 is calculated for the 11th column section: R11 = (151 - 148) / (162 - 151) = 0.27. The slow-descent ratios for both the 10th and 11th column sections are less than 0.35, indicating that the rate of decrease in water content after these sections has become smaller.

[0046] To avoid using sections with low isopropanol content as extraction points, the isopropanol content of sections 10 and 11 was verified. Gas chromatography analysis showed that the isopropanol content in section 10 was 99.72%, and in section 11 it was 99.90%, both within the preset range of 99.60%–99.95%. Because the slow-decay ratio of section 10 was lower than that of section 11, section 10 was designated as the slow-decay section for water content.

[0047] After determining that section 10 is the section with a slow decrease in water content, the side stream feed liquid is drawn from section 10 and enters the molecular sieve adsorption bed. This extraction point is located where the decrease in water content begins to slow down, the water content in the side stream feed liquid is higher than that in the feed liquid near the product discharge section, and the isopropanol content still meets the requirements for entering the molecular sieve dehydration path.

[0048] Example 3 like Figure 1-3 As shown, the amount of liquid extracted from the side stream is 2% to 15% of the amount of water-containing isopropanol feedstock. When the moisture content of the product discharge tower section increases twice consecutively, and the moisture content slow-decreasing section does not move away from the product discharge tower section, increase the side stream liquid extraction rate. When the water content of the isopropanol liquid in the product discharge tower section remains stable within a set time, and the water content of the isopropanol liquid decreases in the water content slow-decreasing section, the extraction rate of the side stream liquid is reduced.

[0049] The molecular sieve adsorption bed uses one or two types of 3A molecular sieve and 4A molecular sieve. After the side stream feed liquid enters the molecular sieve adsorption bed, the water in the side stream feed liquid is adsorbed by the molecular sieve, and the water content of the side stream feed liquid at the outlet of the molecular sieve adsorption bed is lower than that at the inlet of the molecular sieve adsorption bed.

[0050] The return tower position is the location corresponding to 1 to 5 theoretical plates on the side of the moisture content slow-decreasing section near the product discharge tower section; The dehydrated side stream liquid is sent back to the return tower position as a liquid phase and mixed with the descending liquid phase in the return tower position; Based on the determination of the water content slow-decreasing section in Example 2, this embodiment further explains the extraction rate of the side stream feed liquid, molecular sieve dehydration, and return position to the tower.

[0051] After the distillation column was operating stably, section 10 was designated as the water content reduction section according to the method in Example 2. The feed rate of hydrous isopropanol was 1000 kg / h, the water content of the isopropanol solution in section 10 was 158 ppm to 165 ppm, and the water content of the isopropanol solution in the product discharge section was 42 ppm to 48 ppm. Since the water content in the product discharge section was still higher than the target control value, a side stream was first extracted from section 10 at a rate of 60 kg / h.

[0052] The side-stream feed solution is cooled to approximately 30°C before entering the molecular sieve adsorption bed. The molecular sieve adsorption bed is filled with 3A molecular sieves with a particle size of 1.6mm–3.2mm, a bed diameter of 160mm, a bed height of 1.2m, and a molecular sieve loading of 18kg. Before use, the molecular sieve adsorption bed is activated with hot nitrogen at 220°C for 4 hours, and then cooled to room temperature for later use. When the side-stream feed solution enters the molecular sieve adsorption bed, the inlet pressure is 0.15MPa–0.20MPa, and the residence time of the feed solution in the bed is controlled to be 10–15 minutes.

[0053] When the side stream feed rate is 60 kg / h, the water content of the side stream feed at the inlet of the molecular sieve adsorption bed is approximately 160 ppm, and the water content of the side stream feed at the outlet is 28 ppm to 35 ppm. The dehydrated side stream feed is returned as liquid to section 11, which is located on the side of section 10 near the product discharge section, corresponding to the position one theoretical plate above section 10. The returned side stream feed mixes with the descending liquid phase in section 11.

[0054] After 40 minutes of continuous operation, the water content of the isopropanol feed in the product discharge section decreased to 34 ppm–38 ppm, but still showed a slow upward trend. The extraction rate of the side stream feed in section 10 was increased from 60 kg / h to 80 kg / h, while other operating conditions remained unchanged. After increasing the extraction rate, the water content of the side stream feed at the molecular sieve adsorption bed outlet was 30 ppm–38 ppm, and the water content of the isopropanol feed in the product discharge section stabilized at 24 ppm–28 ppm within 60 minutes.

[0055] During continued operation, the moisture content of the 10th column section and the product discharge column section was measured every 20 minutes. When the moisture content of the isopropanol feed solution in the product discharge column section was consistently below 30 ppm, and the moisture content of the isopropanol feed solution in the 10th column section decreased from approximately 160 ppm to 135 ppm–145 ppm, the side stream feed rate was reduced from 80 kg / h to 70 kg / h. After reducing the feed rate, the moisture content of the isopropanol feed solution in the product discharge column section remained between 26 ppm and 31 ppm, without a significant increase.

[0056] In this embodiment, the amount of side stream liquid extracted is 6% to 8% of the raw material feed, falling within the range of 2% to 15%. The dehydrated side stream liquid is sent back to the 11th column, which is located near the product discharge column in the 10th column, thereby reducing and stabilizing the water content of the isopropanol liquid in the product discharge column.

[0057] Example 4 like Figure 1-3 As shown, the molecular sieve adsorption bed includes a first molecular sieve adsorption bed and a second molecular sieve adsorption bed. When the side stream feed liquid is dehydrated by the current molecular sieve adsorption bed and sent back to the distillation column, the water content of the isopropanol feed liquid in the product discharge column section still increases. When the newly determined water content slow-decrease section moves more than one theoretical plate away from the product discharge column section compared to the previous determined position, the side stream feed liquid is switched to another molecular sieve adsorption bed, and the cut-out current molecular sieve adsorption bed is regenerated to obtain hydrous isopropanol regenerated condensate. The hydrous isopropanol regenerated condensate is returned to the position of the water content slow-decrease section away from the product discharge column section, or returned to the bottom discharge position of the distillation column. This embodiment, based on Embodiment 3, further explains the switching of the molecular sieve adsorption bed and the recovery method of the regenerated condensate.

[0058] After the distillation column is operating stably, section 10 serves as a slow-decreasing section for water content, with the side stream feed rate controlled at 70-80 kg / h. The molecular sieve adsorption bed employs a two-bed switching method, with both the first and second molecular sieve adsorption beds filled with 3A molecular sieves, each bed containing 18 kg of molecular sieve. During normal production, the side stream feed first enters the first molecular sieve adsorption bed, while the second molecular sieve adsorption bed remains in standby mode.

[0059] After approximately 7 hours of continuous treatment of the side stream feed by the first molecular sieve adsorption bed, the water content of the inlet side stream feed was 145 ppm–165 ppm, and the water content of the outlet side stream feed increased from 28 ppm–35 ppm to 52 ppm–60 ppm. At this time, the water content of the isopropanol feed in the product discharge section increased from 26 ppm–31 ppm to 36 ppm–42 ppm, and upon retesting, it was found that the water content gradually decreased from section 10 to section 11. Since section 11 is closer to the product discharge section, it indicates that the current dehydration capacity of the molecular sieve adsorption bed is insufficient to maintain a stable water content on the product discharge side.

[0060] The side stream feed was switched from the first molecular sieve adsorption bed to the second molecular sieve adsorption bed. After the switch, the water content of the side stream feed at the inlet of the second molecular sieve adsorption bed was approximately 150 ppm, and the water content of the side stream feed at the outlet decreased to 25 ppm–32 ppm. After continuing operation for 50 minutes, the water content of the isopropanol feed in the product discharge section dropped back to 24 ppm–29 ppm. The 10th section remained a section with a slow decrease in water content and did not continue to move closer to the product discharge section.

[0061] After the first molecular sieve adsorption bed is removed, any remaining side stream liquid in the bed is first drained, and then nitrogen gas is introduced for purging. The nitrogen temperature is raised from room temperature to 180°C over 1 hour; subsequently, the nitrogen temperature is increased to 220°C and held for 3 hours. The gas containing isopropanol and water discharged during the regeneration process is condensed in a condenser to obtain a regenerated isopropanol condensate containing water. The water content of the regenerated condensate is 0.8%–1.5%, and the isopropanol mass fraction is 97.5%–98.8%.

[0062] The regenerated condensate is not fed into the product discharge section, nor is it mixed with the dehydrated side stream feed. Instead, it is returned to section 8, located on the side of section 10 furthest from the product discharge section. Section 8 is situated on the feed side of the section with a gradual decrease in water content. Upon entering this section, the regenerated condensate continues to participate in the distillation separation along with the liquid phase inside the column, with the water content accumulating along the bottom of the column. For regenerated condensate with a water content higher than 1.5%, it is sent to the bottom discharge position of the distillation column and discharged or recycled along with the water-containing stream from the bottom.

[0063] After the first molecular sieve adsorption bed is regenerated, it is cooled to below 35°C by continuously purging with nitrogen gas at room temperature, and the bed is replaced with a small amount of intermediate-purity isopropanol. Once the water content of the replacement solution at the outlet is below 80 ppm, the first molecular sieve adsorption bed is used as a standby bed, awaiting the next switchover.

[0064] In this embodiment, the side stream liquid is switched to the second molecular sieve adsorption bed when the dehydration capacity of the first molecular sieve adsorption bed decreases; the first molecular sieve adsorption bed after switching is regenerated by hot nitrogen, and the resulting aqueous isopropanol regenerated condensate is returned to the side of the water content slow-decreasing section away from the product discharge tower section to prevent water in the regenerated condensate from entering the product discharge side.

[0065] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0066] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the present invention. Various changes and modifications can be made to the present invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed.

Claims

1. A method for purifying isopropanol by coupling molecular sieve adsorption dehydration with distillation, characterized in that, Includes the following steps: Aqueous isopropanol feedstock is fed into a distillation column for distillation separation, and the feed section and product discharge section are determined within the distillation column. At least three continuous testing towers are selected between the feed tower section and the product discharge tower section. The water content of the isopropanol liquid in each testing tower section is continuously measured, and the average water content of each testing tower section is obtained. Along the direction from the feed tower section to the product discharge tower section, calculate the moisture content difference between adjacent testing tower sections, and obtain the slow-descent ratio of the testing tower section based on the moisture content difference before and after the testing tower section located in the middle position. When the slow-descent ratio of the detection tower section is less than the set slow-descent ratio, and the isopropanol content of the isopropanol feed liquid in the detection tower section is within the preset range, the detection tower section is determined as the water content slow-descent section. The side stream liquid is extracted from the moisture content reduction section and sent to the current molecular sieve adsorption bed for dehydration; The dehydrated side stream liquid is sent back to the return tower position on the side of the moisture content slow-decreasing section near the product discharge tower section, and the return tower position is located between the moisture content slow-decreasing section and the product discharge tower section. During the continuous extraction, dehydration, and return of the side stream feed liquid, the position of the water content slow-decreasing section is redefined; when the water content of the isopropanol feed liquid in the product discharge tower section increases, and the redefined water content slow-decreasing section moves towards the product discharge tower section, the side stream feed liquid is switched to another molecular sieve adsorption bed, and the cut-out current molecular sieve adsorption bed is regenerated. When the water content of the isopropanol liquid in the product discharge tower section is lower than that in the water content reduction section, and the change in water content of the isopropanol liquid in the product discharge tower section within a set time does not exceed a set value, high-purity isopropanol is obtained from the product discharge tower section.

2. The method for coupled molecular sieve adsorption dehydration and distillation purification of isopropanol according to claim 1, characterized in that, The three continuous detection tower sections are sequentially denoted as the (i-1)th tower section, the ith tower section, and the (i+1)th tower section along the direction from the feed tower section to the product discharge tower section; The difference in water content before the i-th tower section is Cw,i-1-Cw,i, and the difference in water content after the i-th tower section is Cw,i-Cw,i+1. Wherein, Cw,i-1 is the average water content of the isopropanol feed in the (i-1)th column section, Cw,i is the average water content of the isopropanol feed in the ith column section, and Cw,i+1 is the average water content of the isopropanol feed in the (i+1)th column section. The descent ratio Ri of the i-th tower segment satisfies: Ri=(Cw,i-Cw,i+1) / (Cw,i-1-Cw,i); When Ri is less than the set descent ratio, the i-th tower segment is selected as a candidate descent tower segment.

3. The method for coupled molecular sieve adsorption dehydration and distillation purification of isopropanol according to claim 2, characterized in that, The isopropanol content of the isopropanol feed solution in the candidate slow-descent tower section was detected. When the isopropanol content of the candidate slow-descent column section is in the range of 99.60% to 99.95%, the candidate slow-descent column section is designated as the column section to be extracted. When there are two or more sections to be extracted, the section with the smallest slow-descent ratio is selected as the slow-descent section for water content.

4. The method for coupled molecular sieve adsorption dehydration and distillation purification of isopropanol according to claim 3, characterized in that, The water content of the isopropanol feed solution in each detection tower section was determined using a Karl Fischer moisture analyzer. Each testing tower section was measured at least three times consecutively, and the average of the three measurements was taken as the average water content of that testing tower section.

5. The method for coupled molecular sieve adsorption dehydration and distillation purification of isopropanol according to claim 1, characterized in that, The amount of liquid extracted from the side stream is 2% to 15% of the amount of water-containing isopropanol feedstock. When the water content of the isopropanol liquid in the product discharge tower section increases, increase the extraction rate of the side stream liquid. When the water content of the isopropanol liquid in the product discharge tower section remains stable within a set time, and the water content of the isopropanol liquid decreases in the water content slow-decreasing section, the extraction rate of the side stream liquid is reduced.

6. The method for coupled molecular sieve adsorption dehydration and distillation purification of isopropanol according to claim 1, characterized in that, The molecular sieve adsorption bed uses one or two types of 3A molecular sieve and 4A molecular sieve. After the side stream feed liquid enters the molecular sieve adsorption bed, the water in the side stream feed liquid is adsorbed by the molecular sieve, and the water content of the side stream feed liquid at the outlet of the molecular sieve adsorption bed is lower than that at the inlet of the molecular sieve adsorption bed.

7. The method for coupled molecular sieve adsorption dehydration and distillation purification of isopropanol according to claim 1, characterized in that, The return tower position is the position corresponding to 1 to 5 theoretical plates on the side of the moisture content slow-decreasing section near the product discharge tower section; The dehydrated side stream liquid is returned to the return tower location in liquid phase and mixed with the descending liquid phase in the return tower location.

8. The method for coupled molecular sieve adsorption dehydration and distillation purification of isopropanol according to claim 1, characterized in that, Molecular sieve adsorption beds include a first molecular sieve adsorption bed and a second molecular sieve adsorption bed; When the side stream feed liquid is dehydrated by the current molecular sieve adsorption bed and sent back to the distillation column, if the water content of the isopropanol feed liquid in the product discharge column section still increases, or if the newly determined water content slow-decline section moves at least one theoretical plate towards the product discharge column section compared to the previously determined position, the side stream feed liquid is switched to another molecular sieve adsorption bed. The cut-out current molecular sieve adsorption bed is regenerated to obtain an aqueous isopropanol regenerated condensate; The regenerated condensate of the aqueous isopropanol is returned to a position on the side of the slow-decreasing water content section away from the product discharge section, or returned to the bottom discharge position of the distillation column.

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