Process and apparatus for separating methanol and dimethyl carbonate from transesterification process dimethyl carbonate product

Abstract

This invention relates to a method and apparatus for separating methanol and dimethyl carbonate products from transesterification. By introducing a reactive distillation column heat pump and a low-pressure separation column steam heat pump into the process, steam consumption can be significantly reduced by more than 33% compared with the process without the introduction of heat pumps.

Description

Technical Field

[0001] This invention relates to a method and apparatus for separating methanol and dimethyl carbonate from dimethyl carbonate products obtained by transesterification. Background Technology

[0002] Dimethyl carbonate (DMC) is hailed as the "new cornerstone" of 21st-century organic synthesis. Due to the presence of multiple active functional groups such as carbonyl, methyl, methoxy, and carbonylmethoxy in its molecular structure, it can undergo various organic synthesis reactions including carbonylation, methylation, methoxylation, and carbonylmethoxylation. Its excellent chemical properties allow it to replace certain highly polluting and toxic chemicals, making it an environmentally friendly chemical raw material that meets the requirements of modern clean processes and has broad application prospects.

[0003] The main synthesis processes for dimethyl carbonate (DMC) include the phosgene-methanol method, methanol oxidative carbonylation method, transesterification method, urea alcoholysis method, and direct synthesis from carbon dioxide and methanol. Among these, the transesterification method is the most widely used. Specifically, propylene oxide or ethylene oxide is used as a raw material to first synthesize propylene carbonate or ethylene carbonate. Then, propylene carbonate or ethylene carbonate undergoes a transesterification reaction with methanol in the presence of a catalyst to produce DMC. During the synthesis process, excess methanol is usually added to ensure the conversion rate of propylene carbonate or ethylene carbonate, but methanol forms an azeotrope with the generated DMC. Despite this, the reactive distillation column, atmospheric separation column, and pressurized separation column account for 80% of the total energy consumption of the unit, which does not change the main problem of high energy consumption in the methanol transesterification to dimethyl carbonate production unit.

[0004] The dimethyl carbonate (DMC) fractionation process is a direct sequence of three conventional distillation columns: a reactive distillation column, a low-pressure separation column, and a high-pressure separation column. Azeotropic distillation is performed using varying azeotropic ratios with methanol at different pressures to separate transesterification products. This process route requires a large amount of steam to separate DMC. The separation process is as follows: Figure 1 As shown, an azeotrope of methanol and DMC (31% DMC content) is first obtained from the top of the dimethyl carbonate reaction tower T1. The azeotrope enters the low-pressure separation tower T2, where a mixture of methanol and DMC with different azeotropic ratios is obtained at the top, and a high-purity DMC product is obtained at the bottom, which enters the high-pressure separation tower T3. The high-pressure separation tower further separates the product, and a high-purity DMC is obtained at the bottom. A certain azeotropic ratio of methanol and DMC is returned to the low-pressure separation tower at the top. The bottom temperatures of both the low-pressure and high-pressure separation towers are above 100°C. The energy consumption mainly comes from the 2.5 MPa steam and 1.0 MPa steam consumed by the reboilers of these two towers. Taking a 100,000-ton / year methanol-to-DMC transesterification unit as an example, a single high-pressure fractionation tower consumes more than 30 tons of 2.5 MPa steam per hour.

[0005] Table 1 System material composition and parameters

[0006] CN101357890A, "Synthesis and Refining Process and Apparatus of Dimethyl Carbonate Using Heat Pump Technology," introduces an energy-saving method using heat pumps. The reactive distillation column, pressurized separation column, and methanol recovery column each utilize heat pumps to recover heat. The pressurized top of the pressurized separation column is used for heat exchange in the reboiler, requiring a temperature at least 5°C higher than the reboiler temperature. For example, with a pressure of 1.6 MPaG, the top temperature is compressed to 217°C, resulting in an outlet pressure of 3.06 MPaG. Achieving such high compressor pressure is difficult, and the system investment is large. Excessively high compression ratios lead to extremely high power consumption, and high temperatures cause dimethyl carbonate decomposition, affecting product yield. Summary of the Invention

[0007] The purpose of this invention is to solve the problems of the prior art mentioned above, and to provide a method and apparatus for separating methanol and dimethyl carbonate from dimethyl carbonate products produced by transesterification.

[0008] The apparatus of the present invention includes: a reactive distillation column, a low-pressure separation column, and a high-pressure separation column. The reactive distillation column is equipped with a feed pipe for the reaction raw materials (usually located at the top or middle of the column). The top outlet pipe of the reactive distillation column is divided into two branches, which are respectively connected to the top gas inlet (heat medium inlet) of the reactive distillation column heat pump and the inlet of the reactive distillation column condenser. After cooling, the liquid phase pipe at the outlet of the reactive distillation column heat pump (heat medium outlet) is divided into two paths: one path connects to the top of the reactive distillation column, and the other path connects to the feed inlet of the low-pressure separation column. The outlet pipe of the reactive distillation column condenser is also divided into two paths: one path returns to the top of the reactive distillation column, and the other path connects to the feed inlet of the low-pressure separation column. The outlet pipe of the reactive distillation column is divided into two paths. One path connects to the liquid phase inlet of the reactive distillation column heat pump, and the liquid phase outlet of the reactive distillation column heat pump connects to the bottom of the reactive distillation column. The other path is further divided into two branches. One branch connects to the liquid inlet of the reboiler of the reactive distillation column, and the liquid outlet of the reboiler of the reactive distillation column connects to the bottom of the reactive distillation column. The other branch connects to the propylene glycol separation section. The top outlet pipe of the low-pressure separation tower is divided into two branches. One branch connects to the heating medium inlet of the reboiler of the reactive distillation tower. The heating medium outlet of the reboiler of the reactive distillation tower is further divided into two branches. One branch connects to the upper part of the low-pressure separation tower, and the other branch is the top product. The other branch connects to the top gas inlet of the low-pressure separation tower steam heat pump. The top gas outlet of the low-pressure separation tower steam heat pump is also divided into two branches. One branch connects to the upper part of the low-pressure separation tower, and the other branch is the top product. The bottom outlet of the low-pressure separation tower is connected to the liquid phase inlet of the reboiler of the low-pressure separation tower, and the liquid phase outlet of the reboiler of the low-pressure separation tower is connected to the bottom of the low-pressure separation tower. The bottom of the low-pressure separator is connected to the feed inlet of the high-pressure separator. The top outlet of the high-pressure separator is connected to the top gas inlet of the reboiler of the low-pressure separator. The top gas outlet pipeline of the reboiler of the low-pressure separator is divided into two paths: one path returns to the upper part of the high-pressure separator, and the other path is used as the top product. The bottom outlet pipeline of the high-pressure separator is divided into two paths: one path connects to the reboiler of the high-pressure separator and returns to the bottom of the high-pressure separator after exiting the reboiler; the other path is used as the product collection pipeline for dimethyl carbonate.

[0009] In the apparatus of the present invention, the overhead gas outlet pipe of the reactive distillation column heat pump is divided into two paths, the bottom outlet pipe of the reactive distillation column is divided into two paths, and the other path is further divided into two branches, the overhead outlet pipe of the low-pressure separation column is divided into two paths, the heating medium outlet of the reboiler of the reactive distillation column is further divided into two branches, and the overhead gas outlet pipe of the reboiler of the low-pressure separation column is divided into two paths, etc. Valves are installed on these branches to switch the direction of the material.

[0010] This invention further provides a method for separating methanol and dimethyl carbonate from dimethyl carbonate products obtained by transesterification. The method of this invention includes the following steps: Phase 1 (A) Propylene carbonate, methanol, and sodium methoxide are fed into a reactive distillation column in a certain proportion, where they undergo transesterification and distillation. The vapor phase at the top of the column is cooled in the condenser of the reactive distillation column. Part of the vapor phase is used as reflux for the reactive distillation column, and part is used as feed for the low-pressure separation column. The reflux ratio of the reactive distillation column can be 1-4, preferably about 2. The pressure at the top of the reactive distillation column is 60-100 kPa (A), preferably about 80 kPa (A), and the temperature is 55-62°C, preferably 57-60°C, and further about 58°C. The ratio of dimethyl carbonate to methanol in the product at the top of the column is about 31:29. The temperature of the bottom product is controlled at 65-70°C. The ratio of propylene glycol to methanol in the bottom product is about 1:1. The bottom product is then sent to another section for separation.

[0011] (B) Top pressure of the low-pressure separation tower: 0.8-1.2 The pressure is MPa (G), preferably about 1.0 MPa (G), the top temperature is 135-145℃, preferably about 140.5℃, the ratio of dimethyl carbonate to methanol in the top product is 12.6:87.4, the top vapor phase enters the reboiler of the reactive distillation column to heat the bottom material of the reactive distillation column, after the top vapor phase of the low-pressure separation column is cooled, part of it is used as reflux, and part of it is used as the top product for the separation of dimethyl carbonate and methanol, for example, the product is sent to the atmospheric separation column for azeotropic distillation to further separate dimethyl carbonate and methanol, the reflux ratio of the low-pressure separation column is 0.8-3, preferably about 1.5, the bottom temperature of the low-pressure separation column is controlled at 138-145℃, preferably about 142℃ or 143℃, the ratio of dimethyl carbonate to methanol in the bottom product is about 39:61, the bottom material of the low-pressure separation column (for example, according to the production scale of 29±10t / h) is used as feed for the high-pressure separation column.

[0012] (C) The pressure at the top of the high-pressure separation tower is 1.0-2.0 MPa, preferably about 1.5 MPa, and the temperature at the top of the tower is 150-160℃, preferably about 156℃. The ratio of dimethyl carbonate to methanol in the top product is about 11.2:88.8. The gas phase at the top of the tower enters the reboiler of the low-pressure separation tower to heat the bottom of the low-pressure separation tower. After the gas phase in the high-pressure separation tower is cooled, part of it is used as reflux in the high-pressure separation tower, and part of it is used as the top product for the separation of dimethyl carbonate and methanol. For example, the product is sent to the atmospheric separation tower for azeotropic distillation to further separate dimethyl carbonate and methanol. The reflux ratio of the high-pressure separation tower is 1-4, preferably about 1.5. The reboiler at the bottom of the high-pressure separation tower is heated by 2.5 MPa (G) steam, and the bottom temperature is about 205-215℃, preferably about 210℃. The bottom product of the high-pressure separation tower is high-purity dimethyl carbonate (≥99%). Phase Two After the reactive distillation column and the low-pressure separation column reach total reflux operation, the heat pump for the reactive distillation column and the steam heat pump for the low-pressure separation column are put into operation. After the heat pump system is put into operation, the original thermal coupling system is taken out of operation. At this time, the process is as follows: (A') Propylene carbonate, methanol, and sodium methoxide are fed into a reactive distillation column in a certain proportion, where they undergo transesterification and distillation. The vapor phase at the top of the column is compressed by a heat pump to a pressure of 150-200 kPa (A), preferably 170-190 kPa (A), and further to about 180 kPa (A), at a temperature of 75-85°C, preferably about 80°C, to heat the bottom of the reactive distillation column. After the vapor phase at the top of the reactive distillation column is heated and cooled, part of it is used as reflux for the reactive distillation column, and part of it is used as feed for the low-pressure separation column. The reflux ratio of the reactive distillation column is 1-4, preferably about 2.

[0013] Preferably, the pressure at the top of the column is 60-100 kPa(A) (absolute pressure), preferably about 80 kPa(A), and the temperature at the top of the column is 55-62°C, preferably 57-60°C, and further about 58°C. The product at the top of the column generally has a mass ratio of dimethyl carbonate to methanol of 31:29. The temperature of the bottom of the reactive distillation column is controlled at 65-70°C, and the product at the bottom of the column generally has a mass ratio of propylene glycol to methanol of about 1:1. The material at the bottom of the column enters another section (22) for separation.

[0014] (B') The gas phase at the top of the tower enters the steam heat pump system, first generating low-pressure steam. The low-pressure steam is compressed by the heat pump into, for example, 1.0 MPa (G) steam for use by other systems. After being cooled by the steam heat pump system, part of the gas phase at the top of the low-pressure separation tower is used as reflux for the low-pressure separation tower, and part is used as the top product for the separation of dimethyl carbonate and methanol. For example, the product is sent to the atmospheric separation tower for azeotropic distillation to further separate dimethyl carbonate and methanol. The reflux ratio of the low-pressure separation tower is 1-3, preferably about 1.5. The bottom material of the low-pressure separation tower is used as feed for the high-pressure separation tower.

[0015] Preferably, the top pressure of the low-pressure separation tower is 0.8-1.2 MPa (G), more preferably about 1.0 MPa (G), the temperature is 135-145℃, more preferably about 140℃, the ratio of dimethyl carbonate to methanol in the top product is about 12.6:87.4, the bottom temperature of the low-pressure separation tower is controlled at 136-145℃, more preferably about 142-143℃, and the ratio of dimethyl carbonate to methanol in the bottom product is 39:61.

[0016] (C') The pressure at the top of the high-pressure separation tower is 1.0-2.0 MPa, preferably 1.5 MPa, and the temperature is 152-160℃, preferably about 156℃. The ratio of dimethyl carbonate to methanol in the product at the top of the high-pressure separation tower is about 11.2:88.8. The reboiler of the low-pressure separation tower in the gas phase at the top of the tower heats the bottom of the low-pressure separation tower. After the gas phase of the high-pressure separation tower is cooled, part of it is used as reflux for the high-pressure separation tower, and part of it is used as the top product for the separation of dimethyl carbonate and methanol. For example, the product is sent to the atmospheric separation tower for azeotropic distillation to further separate dimethyl carbonate and methanol. The reflux ratio of the high-pressure separation tower is 1-4, preferably about 1.5. The bottom of the high-pressure separation tower is heated by 2.5 MPa (G) steam, and the bottom temperature is 205-215℃, preferably about 210℃. The bottom of the high-pressure separation tower produces high-purity dimethyl carbonate (≥99%).

[0017] Based on the control of the top vapor temperature and pressure of the reactive distillation column and the reboiler temperature, the required heat pump technical parameters are as follows: Table 2 Heat Pump Technical Parameters

[0018] The heat source for the reactive distillation column comes from the condensation heat of the vapor phase at the top of the low-pressure separation column, and ultimately from the reboiler heat source of the high-pressure separation column. When the reactive distillation column heat pump is put into operation, the latent heat of the vapor phase at the top of the low-pressure separation column is used to evaporate and generate saturated water vapor at 145°C. The saturated water vapor generated by the evaporator is pressurized to 1.0 MPaG by the heat pump compressor to obtain saturated steam, which is then connected to the 1.0 MPaG steam network for use by other systems.

[0019] Based on the control of the gas phase temperature and pressure at the top of the low-pressure separation tower and the temperature control of the tower bottom, the required heat pump technical parameters are as follows: Table 3 Compressor operating parameters

[0020] Advantages of the present invention Using the method and apparatus of the present invention, steam consumption can be significantly reduced by more than 33% compared with that without the introduction of a heat pump. Attached Figure Description

[0021] Figure 1Here is a flow chart of the existing dimethyl carbonate separation process; Figure 2 This is a flow chart of the dimethyl carbonate separation process of the present invention; Figure 3 This is a schematic diagram of a steam heat pump system.

[0022] Figure label: T1: Reactive distillation column; T2: Low-pressure separation column; T3: High-pressure separation column; P1: Reactive distillation column heat pump; P2: Low-pressure separation column steam heat pump; L1-L24: Pipelines 1 to 24; 1-Steam pipeline network; 2-Tower top gas; 3-Condensate; 4-Heat pump compressor; 5-Evaporator; 6-Water cooler; 7-Heat exchanger; 8-Circulating pump. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings.

[0024] like Figure 2 As shown, the apparatus of the present invention includes: a reactive distillation column T1, a low-pressure separation column T2, and a high-pressure separation column T3. The reactive distillation column T1 is equipped with a feed pipe for the reaction raw materials (generally located in the upper or middle part of the column, the 21st pipeline L21). The top outlet pipe of the reactive distillation column is divided into two branches, namely the first pipeline L1 and the seventh pipeline L7, which are respectively connected to the top gas inlet of the reactive distillation column heat pump P1 and the inlet of the reactive distillation column condenser E1. After cooling, the liquid phase pipeline (the second pipeline L2) at the outlet of the reactive distillation column heat pump P1 is divided into two paths. One path (the ninth pipeline L9) is connected to the upper part of the reactive distillation column, and the other path (the tenth pipeline L10) is connected to the feed inlet of the low-pressure separation column T2 (usually located at the bottom of T2). The outlet pipe of the reactive distillation column condenser E1 is also divided into two paths. One path returns to the upper part of the reactive distillation column, and the other path is connected to the feed inlet of the low-pressure separation column T2 (usually located at the bottom of T2). The outlet pipeline of the T1 column of the reactive distillation column is divided into two lines. One line (the third line L3) is connected to the liquid phase inlet of the heat pump of the reactive distillation column, and the liquid phase outlet of the heat pump of the reactive distillation column is connected to the bottom of the reactive distillation column. The other line is further divided into two branches. One branch (the fifth line L5) is connected to the liquid inlet of the E2 column reboiler of the reactive distillation column, and the liquid outlet of the E2 column reboiler of the reactive distillation column is connected to the bottom of the reactive distillation column. The other branch (the twenty-second line L22) is connected to the propylene glycol separation section. The top outlet pipeline of the low-pressure separation tower T2 is divided into two branches. One branch (the eighth pipeline L8) connects to the heating medium inlet of the reboiler E2 of the reactive distillation tower. The heating medium outlet of the reboiler E2 of the reactive distillation tower is further divided into two branches. One branch (the twelfth pipeline L12) connects to the upper part of the low-pressure separation tower, and the other branch (the twenty-third pipeline L23) is the top gas outlet. The other branch (the eleventh pipeline L11) connects to the top gas inlet of the low-pressure separation tower steam heat pump P2. The top gas outlet of the low-pressure separation tower steam heat pump P2 is also divided into two branches. One branch connects to the upper part of the low-pressure separation tower, and the other branch is the top gas outlet (the twenty-third pipeline L23). The bottom outlet of the low-pressure separation tower (the fifteenth pipeline L15) connects to the liquid phase inlet of the reboiler E3 of the low-pressure separation tower. The liquid phase outlet of the reboiler E3 of the low-pressure separation tower (via the fourteenth pipeline L14) connects to the bottom of the low-pressure separation tower. The bottom of the low-pressure separator T2 (via the thirteenth pipeline L13) is connected to the feed inlet of the high-pressure separator T3 (located in the lower middle part of T3). The top outlet of the high-pressure separator is connected to the top gas inlet of the reboiler E3 of the low-pressure separator. The top gas outlet pipeline of the reboiler E3 of the low-pressure separator is divided into two paths: one path (the seventeenth pipeline L17) returns to the upper part of the high-pressure separator T3, and the other path is used as the top product (the twenty-fourth pipeline L24). The bottom outlet pipeline of the high-pressure separator is divided into two paths: one path (the eighteenth pipeline L18) connects to the reboiler E4 of the high-pressure separator, and returns to the bottom of the high-pressure separator T3 after exiting the reboiler E4; the other path (the twentieth pipeline L20) is used as the dimethyl carbonate product.

[0025] like Figure 3 As shown, the overhead gas 2 enters the evaporator 5, and after evaporation, the gas phase enters the heat pump compressor 4 for pressurization. After pressurization, it enters the steam network 1. The steam in the steam network 1 can be used for heat exchange in the heat exchanger 7. After heat exchange, it is transported back to the evaporator 5 through the circulating pump 8. The liquid phase after the overhead gas is condensed enters the water cooler 6 for cooling, and then is discharged as condensate 3.

[0026] In the apparatus of the present invention, the overhead gas outlet pipe of the reactive distillation column heat pump is divided into two paths, the bottom outlet pipe of the reactive distillation column is divided into two paths, and the other path is further divided into two branches, the overhead outlet pipe of the low-pressure separation column is divided into two paths, the heating medium outlet of the reboiler of the reactive distillation column is further divided into two branches, and the overhead gas outlet pipe of the reboiler of the low-pressure separation column is divided into two paths, etc. Valves are installed on these branches to switch the direction of the material. Example 1

[0027] Phase 1 Propylene carbonate, methanol, and sodium methoxide are fed into reactive distillation column T1 (via pipeline L21) in a certain ratio, where they undergo transesterification and distillation simultaneously. The pressure at the top of the column is 80 kPa(A), and the temperature is 58°C. The ratio of dimethyl carbonate to methanol is approximately 31:29. The vapor phase from the top of the column is cooled in the condenser E1 of the reactive distillation column via pipeline L7. A portion of the vapor is used as reflux for reactive distillation column T1 (via pipeline L9), and a portion is used as feed for low-pressure separation column T2 (via pipeline L10). The reflux ratio for reactive distillation column T1 is 2. The temperature of the T1 bottom is controlled at 65-70°C, and the ratio of propylene glycol to methanol is 1:1. The bottom material enters the propylene glycol separation section (L22) for separation.

[0028] The top pressure of the low-pressure separation tower T2 is 1.0 MPa (G), and the temperature is 140.5℃. The dimethyl carbonate:methanol ratio is 12.6:87.4. The vapor phase from the top of the tower enters the reboiler E2 of the reactive distillation tower via the eighth pipeline L8 to heat the material in the bottom of the reactive distillation tower T1. After the vapor phase from the top of the low-pressure separation tower T2 is cooled, part of it is used as reflux (L12) and part of it is used as the top product (L23) to another separation system. The reflux ratio of T2 is 1.5. The bottom temperature of the T2 tower is controlled at 142.8℃, and the dimethyl carbonate:methanol ratio is 39:61. The bottom product of the T2 tower is used as feed for the high-pressure separation tower T3.

[0029] The high-pressure separation tower T3 has a top pressure of 1.5 MPa and a temperature of 156°C. The ratio of dimethyl carbonate to methanol is approximately 11.2:88.8. The vapor phase from the top of the tower enters the reboiler E3 of the low-pressure separation tower via the sixteenth pipeline L16 to heat the bottom of tower T2. After the vapor phase from tower T3 is cooled, part of it is used as reflux (L17) and part is used as the top product (L24) to another separation system, with a reflux ratio of 1.5. The bottom of tower T3 is heated by 2.5 MPa (G) steam, with a bottom temperature of 210°C. The bottom product of tower T3 is high-purity dimethyl carbonate (≥99%).

[0030] Phase Two The original thermal coupling system was used to run T1 and T2 under full reflux. Then, the heat pump P1 of the reactive distillation column and the steam heat pump P2 of the low-pressure separation column were slowly switched in. After the T1 and T2 heat pumps were switched in, the original thermal coupling system was taken out of operation.

[0031] Propylene carbonate, methanol, and sodium methoxide are fed into reactive distillation column T1 (L21) in a certain ratio for transesterification and distillation. The pressure at the top of the column is 80 kPa (A) and the temperature is 58°C. The ratio of dimethyl carbonate to methanol is approximately 31:29. The vapor phase (1) at the top of the column is compressed to a pressure of 180 kPa (A) and a temperature of 80°C by a heat pump and used to heat the bottom of column T1. After the vapor phase at the top of column T1 is heated and cooled, part of it is used as reflux for column T1 (L9) and part of it is used as feed for column T2 (L10). The reflux ratio of column T1 is 2. The temperature of the bottom of column T1 is controlled at 65-70°C. The ratio of propylene glycol to methanol is 1:1. The bottom material is then fed into another section (L22) for separation.

[0032] The low-pressure separation tower has a top pressure of 1.0 MPa (G) and a temperature of 140.5℃. The ratio of dimethyl carbonate to methanol is approximately 12.6:87.4. The top vapor phase enters the steam heat pump system via L11, where low-pressure steam is generated. This low-pressure steam is then compressed by the heat pump into 1.0 MPa (G) steam for use in other systems. The top vapor phase of T2 is cooled by the steam heat pump system, with part of it used as T2 reflux (L19) and part as the top product (L23) sent to another separation system. The T2 reflux ratio is 1.5. The bottom temperature of T2 is controlled at 142.8℃, with a dimethyl carbonate to methanol ratio of approximately 39:61. The bottom product of T2 is used as feed for T3.

[0033] The high-pressure separation tower T3 has a top pressure of 1.5 MPa and a temperature of 156 °C. The ratio of dimethyl carbonate to methanol is 11.2:88.8. The vapor phase from the top of the tower passes through L16 and enters the reboiler E3 of the low-pressure separation tower to heat the bottom of tower T2. After the vapor phase from T3 is cooled, part of it is used as reflux (L17) and part is used as the top product (L24) to another separation system. The reflux ratio of T3 is 1.5. The bottom of tower T3 is heated by 2.5 MPa (G) steam, and the bottom temperature is 210 °C. The bottom product of tower T3 is high-purity dimethyl carbonate (≥99%).

[0034] Table 4 Comparison of Energy Saving Effects Before and After the Application of Heat Pump Technology

[0035] After applying heat pump technology to reactive distillation column T1, low-pressure separation column T2, and high-pressure separation column T3, the top production of low-pressure separation column T2 is 16 t / h of steam; the heat pumps in T1 and T2 increase power consumption by 3110 kWh. The higher the unit price of steam, the more economical the compression heat pump becomes. With a steam unit price of 300 yuan / t and an electricity unit price of 1.0 yuan / kW, the hourly energy saving benefit is 1690 yuan / h, and the annual energy saving benefit is 13.52 million yuan / year, demonstrating a significant energy-saving effect.

Claims

1. An apparatus for separating methanol and dimethyl carbonate from dimethyl carbonate products obtained by transesterification, comprising: The reactive distillation column (T1), the low-pressure separation column (T2), and the high-pressure separation column (T3) are all part of the system. The reactive distillation column (T1) is equipped with a feed pipe for the reaction raw materials. The top outlet pipe of the reactive distillation column is divided into two branches, which are respectively connected to the top gas inlet of the reactive distillation column heat pump (P1) and the inlet of the reactive distillation column condenser (E1). After cooling, the liquid phase pipe at the outlet of the reactive distillation column heat pump (P1) is divided into two paths: one path connects to the upper part of the reactive distillation column (T1), and the other path connects to the feed inlet of the low-pressure separation column (T2). The outlet pipe of the reactive distillation column condenser (E1) is also divided into two paths: one path returns to the upper part of the reactive distillation column (T1), and the other path connects to the feed inlet of the low-pressure separation column (T2). The outlet pipe of the reactive distillation column (T1) is divided into two paths. One path connects to the liquid inlet of the reactive distillation column heat pump (P1), and the liquid outlet of the reactive distillation column heat pump (P1) connects to the bottom of the reactive distillation column (T1). The other path is further divided into two branches. One branch connects to the liquid inlet of the reboiler (E2) of the reactive distillation column, and the liquid outlet of the reboiler (E2) connects to the bottom of the reactive distillation column. The other branch connects to the propylene glycol separation section. The top outlet pipe of the low-pressure separation tower (T2) is divided into two branches. One branch connects to the heating medium inlet of the reboiler (E2) of the reactive distillation tower, and the outlet of the heating medium of the reboiler (E2) of the reactive distillation tower is further divided into two branches. One branch connects to the upper part of the low-pressure separation tower, and the other branch is the top outlet. The other branch connects to the top gas inlet of the low-pressure separation tower steam heat pump (P2). The top gas outlet of the low-pressure separation tower steam heat pump (P2) is also divided into two branches. One branch connects to the upper part of the low-pressure separation tower (T2), and the other branch is the top outlet. The bottom outlet of the low-pressure separation tower is connected to the liquid phase inlet of the reboiler (E3) of the low-pressure separation tower, and the liquid phase outlet of the reboiler of the low-pressure separation tower is connected to the bottom of the low-pressure separation tower (T2). The bottom of the low-pressure separator (T2) is connected to the feed inlet of the high-pressure separator (T3). The top outlet of the high-pressure separator (T3) is connected to the top gas inlet of the reboiler (E3) of the low-pressure separator. The top gas outlet pipeline of the reboiler (E3) of the low-pressure separator is divided into two paths: one path returns to the upper part of the high-pressure separator (T3), and the other path is used as the top gas outlet. The bottom outlet pipeline of the high-pressure separator (T3) is divided into two paths: one path connects to the reboiler of the high-pressure separator and returns to the bottom of the high-pressure separator after exiting the reboiler; the other path is used as the dimethyl carbonate product outlet pipeline.

2. A method for separating methanol and dimethyl carbonate from dimethyl carbonate products obtained by transesterification, comprising the following steps: Phase 1 (A) Propylene carbonate, methanol, and sodium methoxide are fed into a reactive distillation column in a certain proportion to undergo transesterification and distillation. The vapor phase at the top of the column is cooled in the condenser of the reactive distillation column. Part of the vapor phase is used as reflux for the reactive distillation column, and part of the vapor phase is used as feed for the low-pressure separation column. The bottom material is sent to another section for separation. (B) The top pressure of the low-pressure separation tower is 0.8-1.2 MPaG, preferably about 1.0 MPaG, and the top temperature is 135-145℃, preferably about 140.5℃. The gas phase at the top of the tower enters the reboiler of the reactive distillation tower to heat the bottom material of the reactive distillation tower. After the gas phase at the top of the low-pressure separation tower is cooled, part of it is used as reflux and the other part is used as the top product for the separation of dimethyl carbonate and methanol. The reflux ratio of the low-pressure separation tower is 0.8-3, preferably about 1.

5. The bottom temperature of the low-pressure separation tower is controlled at 138-145℃, preferably about 142℃ or 143℃. The bottom material of the low-pressure separation tower is used as the feed for the high-pressure separation tower. (C) The pressure at the top of the high-pressure separation tower is 1.0-2.0 MPa, preferably about 1.5 MPa, and the temperature at the top of the tower is 150-160℃, preferably about 156℃. The gas phase at the top of the tower enters the reboiler of the low-pressure separation tower to heat the bottom of the low-pressure separation tower. After the gas phase in the high-pressure separation tower is cooled, part of it is used as reflux in the high-pressure separation tower, and the other part is used as the top product for the separation of dimethyl carbonate and methanol. The reflux ratio of the high-pressure separation tower is 1-4, preferably about 1.5, and the bottom temperature is about 205-215℃, preferably about 210℃. The bottom product of the high-pressure separation tower is ≥99wt% high-purity dimethyl carbonate. Phase Two After the reactive distillation column and the low-pressure separation column reach total reflux operation, the heat pumps of the reactive distillation column and the low-pressure separation column are slowly switched in. After the switch-in is complete, the original thermal coupling system is shut down. The process at this time is as follows: (A') Propylene carbonate, methanol, and sodium methoxide are fed into a reactive distillation column in a certain proportion, where they undergo transesterification and distillation. The vapor phase at the top of the column is compressed by a heat pump to a pressure of 150-200 kPaA, preferably 170-190 kPaA, and further to about 180 kPaA, at a temperature of 75-85°C, preferably about 80°C, to heat the bottom of the reactive distillation column. After the vapor phase at the top of the reactive distillation column is heated and cooled, part of it is used as reflux for the reactive distillation column, and part of it is used as feed for the low-pressure separation column. The reflux ratio of the reactive distillation column is 1-4, preferably about 2. (B') The gas phase at the top of the tower enters the steam heat pump system, first generating low-pressure steam. The low-pressure steam is compressed by the heat pump and supplied to other systems. After being cooled by the steam heat pump system, part of the gas phase at the top of the low-pressure separation tower is used as reflux of the low-pressure separation tower, and part is taken out as the top of the tower and sent to another separation system. The reflux ratio of the low-pressure separation tower is 1-3, preferably about 1.

5. The material taken out of the bottom of the low-pressure separation tower is used as feed for the high-pressure separation tower. The top pressure of the low-pressure separation tower is 0.8-1.2 MPaG, preferably about 1.0 MPaG, and the temperature is 135-145℃, preferably about 140℃. The bottom temperature of the low-pressure separation tower is controlled at 136-145℃, preferably about 142-143℃. (C') The pressure at the top of the high-pressure separation tower is 1.0-2.0 MPa, preferably 1.5 MPa, and the temperature is 152-160℃, preferably about 156℃. The gas phase at the top of the tower enters the reboiler of the low-pressure separation tower to heat the bottom of the low-pressure separation tower. After the gas phase in the high-pressure separation tower is cooled, part of it is used as reflux of the high-pressure separation tower, and part of it is taken out as the top of the tower to another separation system. The reflux ratio of the high-pressure separation tower is 1-4, preferably about 1.5, and the bottom temperature is 205-215℃, preferably about 210℃. The bottom of the high-pressure separation tower produces high-purity dimethyl carbonate.

3. The method according to claim 2, wherein, In step (A), the reflux ratio of the reactive distillation column is 1-4, preferably about 2; the pressure at the top of the reactive distillation column is 60-100 kPaA, preferably about 80 kPaA; the temperature is 55-62°C, preferably 57-60°C, further about 58°C; and the temperature at the bottom of the column is controlled at 65-70°C.

4. The method according to claim 2, wherein, In step (A'), the pressure at the top of the reactive distillation column is 60-100 kPaA, preferably about 80 kPaA, and the temperature at the top of the column is 55-62°C, preferably 57-60°C, and further about 58°C; the temperature of the bottom of the reactive distillation column is controlled at 65-70°C, and the material in the bottom of the column enters another section for separation.

5. The method according to claim 2, characterized in that, In steps (B) and (B'), after the vapor phase at the top of the low-pressure separation tower is cooled, another portion is taken out as the top of the tower and sent to the atmospheric pressure separation tower for azeotropic distillation to further separate dimethyl carbonate and methanol.

6. The method according to claim 2, characterized in that, In steps (C) and (C'), the remaining portion after the high-pressure separation tower is cooled in the gas phase is taken out as the top of the tower and sent to the atmospheric separation tower for azeotropic distillation, which is then used for the separation of dimethyl carbonate and methanol.

7. The method according to claim 2, characterized in that, In step (C), the reboiler of the high-pressure separator is heated with 2.5 MPaG steam, and / or In step (C'), the reboiler of the high-pressure separation tower is heated by 2.5 MPaG steam.

Images