System and method for separating methanol-cyclohexane heterogeneous azeotropic system
By using a combination system of water washing tower and partition tower in the heterogeneous azeotropic system for separating methanol and cyclohexane, and using countercurrent water washing and distillation separation technology, the problems of low separation efficiency and high energy consumption in the prior art are solved, and the separation and low energy consumption operation of high-purity products are achieved.
Patent Information
- Application Number
- CN202510390104.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-03-31
- Publication Date
- 2025-05-30
AI Technical Summary
It is difficult to effectively separate heterogeneous azeotropic systems for methanol and cyclohexane in the prior art. Especially when high-purity products are needed, conventional distillation methods have problems such as high energy consumption, high equipment costs and difficult extraction agent recovery.
The combined system of the washing tower and the partition tower is adopted. By setting up the washing tower and the partition tower, and using countercurrent washing and distillation separation technology, the reflux ratio and the output amount in the tower top and tower are controlled to achieve efficient separation of methanol and cyclohexane.
It has achieved high-quality separation of methanol and cyclohexane, with product purity reaching more than 99.9%, low energy consumption, stable system operation, and recycling of water washing water.
Smart Images
Figure CN120054015A_ABST
Abstract
Description
Technical Field
[0001] The present invention relates to a system for separating a methanol-cyclohexane heterogeneous azeotropic system, and also relates to a method for separating a methanol-cyclohexane heterogeneous azeotropic system based on the above system. Background Art
[0002] In scenarios such as chemical synthesis, solvent extraction, and paint manufacturing, a mixture of methanol and cyclohexane is formed in the final waste liquid. However, the two form an azeotrope and cannot be effectively separated by conventional distillation methods. Currently, the distillation methods for treating heterogeneous azeotropes include: extractive distillation, combination of a two-tower and a phase separator, membrane separation technology, etc.
[0003] The extractive distillation technology is to add a certain high-boiling extractant to the mixed system to increase the relative volatility between components, and the extractant is recovered and recycled through an additional extractant recovery tower. Extractive distillation requires the introduction of a third component as an extractant. For the methanol-cyclohexane azeotropic system, the difficulty of recycling the extractant is high, and the purity of the product after extractive distillation is low. Currently, there is no reported method for separating this azeotropic system by extractive distillation.
[0004] The combination technology of a two-tower and a phase separator is for the partially miscible system of methanol and cyclohexane. By setting up two azeotropic distillation towers and adding a layer separator at the top of the towers; the azeotrope after the gas phase condensation at the top of the two azeotropic distillation towers enters the layer separator for static separation, and then returns to the distillation towers respectively for re-distillation. After stabilization through circulation, the bottoms of the two azeotropic distillation towers can respectively produce products meeting the quality requirements. However, the method of separating azeotropes by the combination of a two-tower and a phase separator is only applicable to partially miscible systems and requires maintaining an appropriate residence time to ensure the separation effect. When facing the demand for high-purity products, due to the relatively small concentration difference of the products in the two systems after static phase separation, the required circulation volume of the stable system is large, resulting in high energy consumption for separation; and after introducing the two-tower process, the investment costs such as the equipment cost and floor area will increase significantly.
[0005] The membrane separation technology uses a selectively permeable membrane as the separation medium. When there is a driving force (concentration difference, pressure difference, temperature difference, etc.) on both sides of the membrane, the mixture components selectively permeate through the membrane to achieve the purpose of separation and purification. However, when facing the high-quality demand for solvent recovery products, the selectivity requirement for the membrane is extremely high, and at the same time, the high-throughput requirement also needs to be met (the two characteristics of selectivity and flux are usually opposite). Therefore, the R & D and manufacturing process of the membrane itself is extremely difficult and costly (the service life of the membrane is generally short and it needs to be frequently cleaned and replaced). Summary of the Invention
[0006] Object of the Invention: The object of the present invention is to provide a system for separating a heterogeneous azeotropic system of methanol - cyclohexane. Another object of the present invention is to provide a method for separating a heterogeneous azeotropic system of methanol - cyclohexane based on the above system. By setting up a water - washing tower and a dividing wall column, the azeotropic mixture of methanol and cyclohexane can be effectively separated, and the quality and yield of the solvent recovery product obtained are high. At the same time, the energy consumption of the system operation is low, and the washing water can be recycled.
[0007] Technical Solution: The system for separating a heterogeneous azeotropic system of methanol - cyclohexane according to the present invention includes a water - washing tower and a dividing wall column; the bottom product outlet of the water - washing tower is connected to the feed inlet of the dividing wall column, and a cyclohexane product outlet is provided at the top of the water - washing tower; the overhead product of the dividing wall column is an azeotrope of methanol - cyclohexane, the middle product of the column is methanol, and the bottom product of the column is water; there are also two circulation pipelines; one circulation pipeline mixes the cooled overhead product of the dividing wall column with the raw material feed and sends it into the water - washing tower, and the other circulation pipeline sends the bottom product of the dividing wall column back into the water - washing tower as washing water again.
[0008] Among them, a heating device for heating the dividing wall column is further included.
[0009] Among them, the number of trays of the water - washing tower is not less than 10 trays; the number of trays of the dividing wall column is not less than 30 trays.
[0010] Among them, an interface meter or a sight glass is provided in the upper space of the water - washing tower. The position of the phase interface in the water - washing tower is observed through the interface meter or the sight glass to see if it is at the corresponding position, and the position of the phase interface is continuously controlled to be at the corresponding determined position (overflow line position) by controlling the bottom product flow rate of the water - washing tower. The position of the phase interface in the water - washing tower is determined, so that the product extraction position is also determined.
[0011] The method for separating a heterogeneous azeotropic system of methanol - cyclohexane based on the above system includes the following steps:
[0012] (1) First, inject a certain amount of washing water into the water - washing tower; then first open the water - washing inlet valve, and then synchronously open the raw material feed valve and the bottom product valve; the washed oil layer (cyclohexane) is overflowed from the top of the tower; the washed water layer is taken out from the bottom of the tower; the bottom product flow rate is approximately the sum of the water - washing tower inlet flow rate and the methanol flow rate in the raw material feed.
[0013] (2) The water layer after washing (a mixture containing water, methanol, and a small amount of cyclohexane) withdrawn from the bottom of the water washing tower is transported into the dividing wall column, and is preliminarily separated through the pre-fractionation section. The liquid phase flow rate ratio of the pre-fractionation section to the main column is controlled at about 10:1. The methanol-water heavy components enter the common stripping section, and the cyclohexane-methanol and a small amount of water enter the common rectification section. Then, further separation is carried out through rectification (the intermediate product after preliminary separation (the intermediate product is a composition of methanol and water, cyclohexane, a composition of methanol and a small amount of water) is further separated). During the rectification separation process, the top temperature of the column is about 56.5 - 56.8 °C, the temperature of the draw from the middle of the column is about 64.2 - 64.5 °C, and the bottom temperature of the column is about 101.2 - 101.5 °C. An azeotrope of methanol-cyclohexane is withdrawn from the top of the column, methanol is withdrawn from the middle of the column, and water is withdrawn from the bottom of the column; a part of the azeotrope withdrawn from the top of the column is cooled and then mixed with the raw material feed and enters the water washing tower again for water washing, and a part is refluxed into the dividing wall column for rectification again. The water withdrawn from the bottom of the column is used as the raw water of the water washing tower for recycling.
[0014] Among them, in step (1), the ratio of the feed amount of the washing water to the raw material is 0.5 - 0.6, ensuring that there is no methanol residue in the cyclohexane product withdrawn from the top of the column.
[0015] Among them, in step (1), the temperature of the water washing tower is 25 - 30 °C.
[0016] Among them, in step (2), counting from top to bottom, the water layer after washing withdrawn from the bottom of the water washing tower enters the dividing wall column at the 6th - 8th tray position of the dividing wall column, which can ensure that the top product does not contain water, the bottom product does not contain cyclohexane, and due to the reduction of backmixing in the column, the energy consumption of the rectification column can be maintained at a relatively low level.
[0017] Among them, in step (2), after the azeotrope withdrawn from the top of the dividing wall column is cooled, a part is mixed with the raw material feed and enters the water washing tower again for water washing, and a part is refluxed into the dividing wall column for rectification again; among them, the reflux ratio is not less than 20.
[0018] Among them, in step (2), the draw amount from the top of the column is the flow rate of cyclohexane in the feed of the dividing wall column divided by the proportion of cyclohexane in the methanol-cyclohexane azeotrope; the draw amount of methanol from the middle of the column is the flow rate of methanol in the feed of the dividing wall column minus the flow rate of methanol in the methanol-cyclohexane azeotrope.
[0019] Among them, in step (2), counting from top to bottom, the position where methanol is withdrawn from the middle of the dividing wall column is between the 4th - 6th trays of the dividing wall column; setting it like this can ensure that the contents of cyclohexane and water in the methanol product from the middle of the column are at a relatively low level.
[0020] When the raw material enters the water washing tower, due to the high solubility of methanol in water and the insolubility of cyclohexane in water (at 25 °C, the solubility of cyclohexane in water is only 0.008 g / 100 gH 2O), with a density lower than that of water, so cyclohexane can be obtained by water washing and layering (the water content in cyclohexane is about 60 ppm); however, since the water drawn from the bottom of the tower contains methanol, a part of cyclohexane will also be dissolved in the water. Therefore, the bottom draw is a mixture of methanol-water and a small amount of cyclohexane.
[0021] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: The present invention adopts countercurrent water washing. After the raw materials enter the water washing tower for water washing, a cyclohexane product with a high content can be directly obtained; the present invention adopts a dividing wall column. By controlling the reflux ratio and the draw amounts at the top and middle of the tower, a methanol product with a high content can be obtained in the middle of the tower; the present invention separates the mixed system by water washing and rectification, and the product qualities of cyclohexane and methanol are well controlled. Moreover, the entire system effectively separates the methanol and cyclohexane products without generating waste liquid. Description of the Drawings
[0022] Figure 1 is the schematic diagram of the system in Example 1;
[0023] Figure 2 is the schematic diagram of the system in Example 2. Detailed Embodiments
[0024] Example 1
[0025] As Figure 1 shown, the system for separating the methanol-cyclohexane heterogeneous azeotropic system of the present invention includes a water washing tower T1 and a dividing wall column T2; it also includes a reboiler E1 for heating the dividing wall column T2, a condenser E2 for condensing the top azeotrope, a top draw cooler E3 and a bottom draw cooler E4; the mixture drawn from the bottom of the water washing tower T1 is pumped into the dividing wall column T2 by a draw pump P1 for rectification separation, and a cyclohexane draw port is provided at the top of the water washing tower T1; the bottom draw of the dividing wall column T2 is pumped into the water washing tower T1 by a draw pump P2; there are two reflux pipelines in the system of the present invention. One reflux pipeline is that the draw from the top of the dividing wall column T2 is cooled and then mixed with the raw material feed and enters the water washing tower T1 again from the bottom raw material inlet of the water washing tower T1 for water washing. The other reflux pipeline is that the bottom draw of the dividing wall column T2 is used as the original water of the water washing tower T1 and enters the water washing tower again from the top water inlet of the water washing tower.
[0026] A chemical enterprise produces a solvent waste liquid which is a methanol-cyclohexane solution (in this example, other heavy components or other impurities are not considered. If they actually exist, they can be removed by pre-distillation or pre-steaming after water washing first). The temperature is 25°C. In the methanol-cyclohexane solution, the mass percentage of methanol is 60%, and the mass percentage of cyclohexane is 40%; it is required to recover the methanol and cyclohexane therein, and it is required that the product methanol purity ≥ 99.9%, the cyclohexane residue < 500 ppm, the product cyclohexane purity ≥ 99.9%, and the methanol residue < 500 ppm.
[0027] Method for separating methanol-cyclohexane heterogeneous azeotropic system based on the above system, specifically:
[0028] (1) The number of trays of water wash tower T1 is 10 trays. Raw water enters from the top plate of water wash tower T1, and the methanol-cyclohexane solution is pumped in from the bottom plate of water wash tower T1; before the raw material methanol-cyclohexane solution enters the water wash tower, a certain amount of water is injected into the water wash tower first, and the injected water volume accounts for about 80% of the tower volume to ensure that there is sufficient water in the tower, reduce the methanol residue in the cyclohexane product, then open the raw water feed valve of the water wash, and then synchronously open the raw material feed valve and the bottom draw valve to gradually draw out the bottom mixture. At this time, control the ratio of the feed rate so that the water wash water: the ratio of the feed rate of water to the raw material = 0.5, that is, the feed rate of the methanol-cyclohexane solution is 2000 kg / h, and the water feed rate is 1000 kg / h; during the process, control the temperature of the water wash tower at 25 °C by controlling the water inlet temperature; when overflow appears at the top of the water wash tower, draw out the product cyclohexane with a content > 99.98%, almost no methanol residue, and the water content is about 0.01% (the quality of the cyclohexane product can be further improved by water removal later); an interface meter or a sight glass can be set in the upper space of the water wash tower. After the system is stable, the position of the phase interface can be controlled by the bottom draw flow rate of the tower, so as to control the draw flow rate of the top oil layer (the draw flow rate is about 800 kg / h of the cyclohexane flow rate in the raw material feed); the bottom draw flow rate of the tower is about the sum of the water inlet flow rate of the water wash tower and the methanol flow rate in the raw material feed ((1000 + 1200) kg / h); through sampling and testing, among them, in the mixture drawn from the bottom of the water wash tower, the mass percentage of water is about 45%, the mass percentage of methanol is about 54%, and the mass percentage of cyclohexane is about 1%;
[0029] (2) The mixture drawn from the bottom of the water washing tower enters the dividing wall column for rectification separation. The dividing wall column T2 has 30 trays. Counting from top to bottom, the water layer after water washing drawn from the bottom of the water washing tower enters the dividing wall column at the 6th to 8th tray positions of the dividing wall column, and is preliminarily separated through the pre-fractionation section. During the process, the liquid phase flow ratio of the pre-fractionation section / main column is controlled by an external flow meter and a regulating valve to be about 10:1 (at this flow ratio, cyclohexane can be prevented from entering the common stripping section while the amount of water entering the common rectification section is small enough). The methanol-water heavy components enter the common stripping section, and the cyclohexane-methanol and a small amount of water enter the common rectification section. Then, the components after preliminary separation are subjected to rectification separation. During the rectification separation process, the top temperature is about 56.5 °C, the temperature of the draw in the middle of the column is about 64.2 °C, and the bottom temperature is about 101.2 °C. The top product is a mixture of methanol and cyclohexane (where methanol is about 85.6% and cyclohexane is about 14.4%). The flow rate of the top distillate is about 153 kg / h (about the amount of fed cyclohexane (2200 kg / h multiplied by 1%) divided by the proportion of cyclohexane in the methanol-cyclohexane mixture composition (14.4%)), and the reflux ratio is controlled to be 20; the methanol product in the middle of the column is controlled by a draw flow meter and a regulating valve to have a draw amount of about 1057 kg / h (about the methanol feed flow rate (2200 kg / h multiplied by 54%) minus the flow rate of methanol in the top azeotrope (153 kg / h multiplied by 85.6%)). Counting from top to bottom, the position of the methanol draw in the middle of the dividing wall column is at the 4th to 6th trays of the dividing wall column; the material drawn from the top is cooled by a condenser and then mixed with the raw material feed and re-entered into the water washing tower for water washing; while the bottom product is water, which is pumped back into the water washing tower as the raw water of the water washing tower.
[0030] After the system is stable, the quality of the methanol product in the middle of the column > 99.9%, and the cyclohexane residue is about 150 ppm; there is no cyclohexane residue in the bottom material, the water content > 99.6%, and the recovery rates of methanol and cyclohexane can theoretically reach 99.9% (without considering process losses such as tail gas).
[0031] The system of the present invention can be controlled by DCS or PLC as needed.
[0032] Example 2
[0033] As Figure 2As shown in the figure, the system for separating the methanol-cyclohexane heterogeneous azeotropic system of the present invention includes a phase separator V1 and a dividing wall column T2; it also includes a reboiler E1 for heating the dividing wall column T2, a condenser E2 for condensing the overhead azeotrope, an overhead product cooler E3 and a bottom product cooler E4; the heavy-phase substance after static phase separation in the phase separator V1 is pumped into the dividing wall column T2 by a pump P1 for rectification separation, and the light-phase substance after static phase separation is taken out from the top of the phase separator V1; the bottom product of the dividing wall column T2 is pumped into a pipeline mixer X1 by a pump P2; in the two reflux pipelines of the system of the present invention, one reflux pipeline is that the overhead product of the dividing wall column T2 is cooled and then mixed with the raw material feed and enters the pipeline mixer X1, and the other reflux pipeline is that the bottom product of the dividing wall column T2 is pumped into the pipeline mixer X1, and water and the raw material are mixed in the pipeline mixer X1 and then enter the phase separator V1 for static phase separation.
[0034] A chemical enterprise produces solvent waste liquid as a methanol-cyclohexane solution (in this embodiment, other heavy components or other impurities are not considered. If they actually exist, they can be removed by pre-distillation or pre-steaming after water washing first). The temperature is 25°C. In the methanol-cyclohexane solution, the mass percentage of methanol is 60%, and the mass percentage of cyclohexane is 40%; it is required to recover the methanol and cyclohexane therein, and it is required that the purity of the product methanol is ≥99.9%, the cyclohexane residue is <500 ppm, the purity of the product cyclohexane is ≥99.5%, and the methanol residue is <5000 ppm.
[0035] The method for separating the methanol-cyclohexane heterogeneous azeotropic system based on the above system is specifically as follows:
[0036] (1) Open the water inlet valve, and make it enter the phase separator V1 after passing through the pipeline mixer X1. Control the water inlet volume to be about 2000 kg / h (the initial flow rate is large to prevent poor layering effect when the raw material feed volume fluctuates greatly), and then slowly open the raw material feed valve. After fully mixing the raw material with water through the pipeline mixer, enter the phase separator, and control the raw material feed volume to be 2000 kg / h (the feed volume of the methanol-cyclohexane solution is 2000 kg / h); after the feed is stable, gradually and slowly reduce the raw water inlet volume to 1000 kg / h.
[0037] An interface meter is set on the phase separator to control the relative stability of the phase interface by extracting the heavy phase (aqueous phase). After the phase separator is full, the oil layer (cyclohexane) is extracted by overflow. After stabilization, the extraction flow rate of the oil layer can be controlled by the extraction flow rate of the aqueous layer; or a strip-shaped sight glass is set on the phase separator, and the interface change is observed through the sight glass to adjust the extraction flow rate of the oil layer in a timely manner. After the phase separator is stable, the cyclohexane product content > 99.5%, the methanol residue < 5000 ppm, and there is almost no water residue; the flow rate of the aqueous layer extracted from the phase separator is 2214 kg / h. After sampling and testing, among the aqueous layer extracted from the phase separator, the mass percentage of water is 45.2%, the mass percentage of methanol is 54%, and the mass percentage of cyclohexane is 0.8%; in the oil layer extracted from the phase separator, the mass percentage of water is 0.01%, the mass percentage of methanol is 0.45%, and the mass percentage of cyclohexane is 99.54%.
[0038] (2) The aqueous layer extracted from the phase separator enters the dividing wall column for rectification separation. The dividing wall column T2 has 30 trays. The aqueous layer extracted from the phase separator enters the dividing wall column from the 6th to the 8th tray position from top to bottom and is preliminarily separated through the pre-fractionation section. During the process, the liquid phase flow ratio of the pre-fractionation section / main column is controlled at about 10:1 by an external flow meter and a regulating valve. The methanol-water heavy components enter the common stripping section, and the cyclohexane-methanol and a small amount of water enter the common rectification section. Then, the components after preliminary separation are rectified and separated. During the rectification separation process, the top temperature is about 56.5 °C, the extraction temperature in the column is about 64.2 °C, and the bottom temperature is about 101.2 °C. The top product is a mixture of methanol-cyclohexane (where methanol is about 85.9% and cyclohexane is about 14.1%), the flow rate of the top distillate is about 125.6 kg / h, and the reflux ratio is controlled at 20; the methanol extraction amount in the column is about 1087.7 kg / h; the material extracted from the top is cooled by a condenser and then mixed with the raw material feed and enters the pipeline mixer; the water extracted from the bottom is pumped into the pipeline mixer, and the water and the raw material are mixed again in the pipeline mixer and then enter the phase separator for static phase separation.
[0039] After the system is stable, the methanol product quality in the column > 99.9%, and the cyclohexane residue is about 150 ppm; there is no cyclohexane residue in the bottom material, and the water content > 99.6%.
[0040] The system of the present invention can be controlled by DCS or PLC as needed.
Claims
1. A system for separating a methanol-cyclohexane heterogeneous azeotropic system, characterized in that: It includes a water washing tower and a bulkhead tower; the water washing tower kettle outlet is connected to the feed inlet of the bulkhead tower, and the top of the water washing tower is provided with a cyclohexane outlet; the bulkhead tower top outlet is a methanol-cyclohexane azeotrope, the tower outlet is methanol, and the bottom outlet is water; it also includes two circulation pipelines; one of the circulation pipelines mixes the cooled bulkhead tower top outlet with the raw material feed and sends it to the water washing tower, and the other circulation pipeline sends the bulkhead tower kettle outlet as washing water to the water washing tower again.
2. The system for separating the methanol-cyclohexane heterogeneous azeotropic system according to claim 1, characterized in that: Also included is a heating device for supplying heat to the adjacent tower.
3. The system for separating the methanol-cyclohexane heterogeneous azeotropic system according to claim 1, characterized in that: The number of plates of the water washing tower is not less than 10 plates; the number of plates of the adjacent tower is not less than 30 plates.
4. The system for separating the methanol-cyclohexane heterogeneous azeotropic system according to claim 1, characterized in that: An interface meter or sight glass is provided in the upper space of the water washing tower, through which the position of the phase interface in the water washing tower is observed to be at a corresponding position, and the position of the phase interface is controlled to be continuously at a corresponding determined position by the extraction flow rate of the water washing tower kettle.
5. The method for separating a methanol-cyclohexane heterogeneous azeotropic system according to claim 1, characterized in that: The steps include: (1) First, inject washing water of no less than 80% of the volume of the washing tower into the washing tower; then open the washing water inlet valve, and then simultaneously open the raw material feed valve and the tower kettle extraction valve; the tower top overflows to extract the oil layer after washing; the tower kettle extracts the water layer after washing; the tower kettle extraction flow rate is the sum of the washing tower inlet flow rate and the methanol flow rate in the raw material feed; (2) The water layer after washing taken out from the bottom of the water washing tower is transported to the next tower for preliminary separation through the pre-fractionation section. By controlling the liquid phase flow rate ratio of the pre-fractionation section / the main tower, the methanol-water heavy component enters the common stripping section, and the cyclohexane-methanol and a small amount of water enter the common distillation section. Then, the preliminary separated intermediate product is distilled and separated. During the distillation and separation process, the top temperature of the next tower is 56.5-56.8°C, the extraction temperature in the tower is 64.2-64.5°C, and the bottom temperature is 101.2-101.5°C. The methanol-cyclohexane azeotrope is taken out from the top of the tower, methanol is taken out from the tower, and water is taken out from the bottom of the tower. The quality of the methanol taken out from the tower is improved by controlling the reflux ratio and the extraction amount from the top and the tower. After cooling, a part of the azeotrope taken out from the top of the tower is mixed with the raw material feed and enters the water washing tower again for water washing, and a part of it is refluxed to the next tower for distillation again. The water taken out from the bottom of the tower is recycled as the raw water of the water washing tower.
6. The method according to claim 5, characterized in that: In step (1), the ratio of the feed amount of washing water to the feed amount of raw material is 0.5 to 0.
6.
7. The method according to claim 5, characterized in that: In step (2), counting from top to bottom, the water layer after washing extracted from the kettle of the water washing tower enters the next-door tower from the 6th to 8th tower plates of the next-door tower.
8. The method according to claim 5, characterized in that: In step (2), the reflux ratio of the overhead product of the next-door tower is not less than 20.
9. The method according to claim 5, characterized in that: In step (2), the top extraction amount is the flow rate of cyclohexane in the feed of the next-door tower divided by the proportion of cyclohexane in the methanol-cyclohexane azeotrope; the methanol extraction amount in the tower is the flow rate of methanol in the feed of the next-door tower minus the flow rate of methanol in the methanol-cyclohexane azeotrope.
10. The method according to claim 5, characterized in that: In step (2), counting from top to bottom, the position where methanol is extracted from the next-door tower is between the 4th and 6th tower plates of the next-door tower.