System for producing trioxane by heat pump assisted reaction rectification coupled with pressure swing distillation and energy-saving process thereof

Abstract

The application discloses a system and an energy-saving process for producing trioxymethylene by means of a heat pump assisted reaction rectification coupled variable pressure rectification, and the process comprises the following steps: feeding a formaldehyde aqueous solution into a pre-rectification tower to enrich, feeding the dilute formaldehyde collected from the top of the tower into a heat pump unit C1 to increase the pressure and temperature, and then taking the dilute formaldehyde as a tower bottom heat source of a high-pressure rectification tower; feeding the concentrated formaldehyde collected from the bottom of the pre-rectification tower into a reaction rectification tower, and converting the concentrated formaldehyde into trioxymethylene under the catalysis of a catalyst; taking the azeotrope of formaldehyde, water and trioxymethylene from the top of the reaction rectification tower as a tower bottom heat source of the pre-rectification tower, feeding the azeotrope into a low-pressure rectification tower after heat exchange and condensation, taking the azeotrope vapor from the top of the low-pressure rectification tower as a heat source of the tower bottom of the reaction rectification tower after increasing the pressure and temperature by means of a heat pump unit C2, feeding the azeotrope into the high-pressure rectification tower after heat exchange and condensation, feeding the dilute formaldehyde collected from the bottom of the low-pressure rectification tower back to the pre-rectification tower to enrich, and collecting the trioxymethylene product from the bottom of the high-pressure rectification tower. The application can improve the single-pass conversion rate of trioxymethylene, simplify the operation, and reduce the operation cost and energy consumption.

Description

TECHNICAL FIELD

[0001] The present application relates to a manufacturing technology of trioxane. BACKGROUND

[0002] In order to convert formaldehyde into trioxane, the existing process mainly uses a kettle reactor to react formaldehyde aqueous solution and catalyst in the kettle reactor. However, the reaction of formaldehyde into trioxane is greatly limited by chemical equilibrium, and the equilibrium concentration of the product trioxane is extremely small. Therefore, the rectification process after the reaction needs to circulate a large amount of unreacted formaldehyde into the reactor, which is low in reaction efficiency and high in energy consumption. In addition, due to the complex azeotropy of formaldehyde-water-trioxane, a conventional rectification method cannot obtain trioxane with high purity, and the existing process mainly uses an extractive rectification method to obtain trioxane. However, the extractant has strong toxicity and has a great influence on the subsequent use of trioxane, and the separation and purification of the extractant is also relatively complex. Furthermore, the formaldehyde-water-trioxane system has complex azeotropy, and the concentration of formaldehyde in the wastewater should not be too high to avoid affecting the subsequent wastewater treatment process. The harsh rectification conditions make the system have high energy consumption. The existing process mainly uses steam heating to provide heat for the system, which has large energy consumption and does not fully utilize the heat in the system. SUMMARY

[0003] The technical problem to be solved by the present application is to provide an energy-saving process for producing trioxane by heat pump assisted reaction rectification coupled with pressure swing rectification, which can improve the single-pass conversion rate of trioxane, simplify the operation, reduce the operating cost and energy consumption, and achieve the effect of economic energy saving.

[0004] Another technical problem to be solved by the present application is to provide a system for producing trioxane by heat pump assisted reaction rectification coupled with pressure swing rectification.

[0005] According to an aspect of an embodiment of the present application, an energy-saving process for producing trioxane by heat pump assisted reaction rectification coupled with pressure swing rectification is provided, which comprises the following steps:

[0006] The formaldehyde aqueous solution raw material is introduced into a pre-rectification column T1 for concentration, and the dilute formaldehyde collected from the top is pressurized and heated by a heat pump unit C1 and then used as the heat source of the column bottom of a high-pressure rectification column T4. The concentrated formaldehyde collected from the bottom of the pre-rectification column T1 is sent into a reaction rectification column T2, which is converted into trioxane under the catalysis of the catalyst in the reaction rectification column T2;

[0007] The azeotrope vapor of formaldehyde, water and trioxane formed at the top of the reaction rectification column T2 is used as the heat source of the pre-rectification column T1, and after heat exchange and condensation, is introduced into the middle of the low-pressure rectification column T3; the azeotrope vapor taken from the top of the low-pressure rectification column T3 is pressurized and heated by the heat pump unit C2, and is used as the heat source of the bottom of the reaction rectification column T2, and after heat exchange and condensation, is introduced into the middle of the high-pressure rectification column T4; the dilute formaldehyde solution taken from the bottom of the low-pressure rectification column T3 is sent back to the pre-rectification column T1 for concentration;

[0008] The trioxane product is taken from the bottom of the high-pressure rectification column T4.

[0009] According to another aspect of the embodiment of the present application, a system for producing trioxane by heat pump assisted reaction rectification coupled with pressure swing rectification is provided, which comprises a reaction rectification unit, a pressure swing rectification unit and a heat pump unit; the reaction rectification unit comprises a pre-rectification column T1, a reaction rectification column T2, a reflux tank V1, a reflux tank V4, a column top condenser E5, a column top condenser E14, a condensation reboiler E4, a condensation reboiler E7, a reboiler E3 and a reboiler E6; the pressure swing rectification unit comprises a low-pressure rectification column T3, a high-pressure rectification column T4, a medium-pressure rectification column T5, a reflux tank V2, a reflux tank V3, a reflux tank V5, a column top condenser E11, a column top condenser E17, a condensation reboiler E10, a condensation reboiler E12, a condensation reboiler E15, a reboiler E9, a reboiler E13 and a reboiler E16; the heat pump unit comprises a heat pump unit C1 and a heat pump unit C2; the gas phase outlet at the top of the pre-rectification column T1 is communicated with the air inlet of the heat pump unit C1, the air outlet of the heat pump unit C1 is communicated with the hot side inlet of the condensation reboiler E12 at the bottom of the high-pressure rectification column T4, and the liquid phase outlet at the bottom of the pre-rectification column T1 is communicated with the middle of the reaction rectification column T2; the gas phase outlet at the top of the reaction rectification column T2 is communicated with the hot side inlet of the condensation reboiler E4 at the bottom of the pre-rectification column T1, the hot side outlet of the condensation reboiler E4 is communicated with the inlet of the reflux tank V1, the liquid phase outlet of the reflux tank V1 is divided into two paths, one of which is communicated with the reflux port at the upper part of the reaction rectification column T2, and the other of which is communicated with the middle of the low-pressure rectification column T3; the gas phase outlet at the top of the low-pressure rectification column T3 is communicated with the air inlet of the heat pump unit C2, the air outlet of the heat pump unit C2 is communicated with the hot side inlet of the condensation reboiler E7 at the bottom of the reaction rectification column T2, the hot side outlet of the condensation reboiler E7 is communicated with the reflux tank V2, and the liquid phase outlet of the reflux tank V2 is divided into two paths, one of which is communicated with the reflux port at the upper part of the low-pressure rectification column T3, and the other of which is communicated with the middle of the high-pressure rectification column T4; the liquid phase outlet at the bottom of the low-pressure rectification column T3 is communicated with the feed inlet of the middle of the pre-rectification column T1; the bottom of the high-pressure rectification column T4 has a liquid phase outlet for discharging.

[0010] The present application has at least the following advantages:

[0011] 1、the embodiment of the present application replaces the kettle type reactor with the reaction rectification tower, adopts the reaction rectification to realize the conversion of formaldehyde and the separation of formaldehyde-water-trioxane, improves the single pass conversion rate of trioxane, and the product is separated and purified in the reaction rectification tower, so that the load of the subsequent rectification tower is reduced, the subsequent equipment investment and operating cost are saved, and the energy consumption is reduced;

[0012] 2、the embodiment of the present application adopts the pressure swing rectification to realize the purification of trioxane, does not introduce other substances such as extractants, and simplifies the operation process;

[0013] 3、the embodiment of the present application adopts the heat pump recovery system to recover heat to meet the heat demand, uses less compression work to realize the reboiling of the tower kettle, reduces the heat load of the tower kettle reboiler, saves the amount of tower top cooling circulating water and tower kettle steam, saves the operating cost, and reduces the energy consumption;

[0014] 4、since the top temperature and the kettle temperature of different towers are quite different, the embodiment of the present application adopts the internal heat coupling process to heat the kettle liquid of other towers with the top vapor, and good energy saving effect is realized. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 A schematic diagram of a system for producing trioxane by heat pump assisted reaction rectification coupled with pressure swing rectification according to an embodiment of the present application is shown.

[0016] Figure 2 A schematic diagram of a heat pump unit according to an embodiment of the present application is shown. DETAILED DESCRIPTION

[0017] The present application will be described in detail below with reference to the drawings and specific embodiments.

[0018] Please refer to Figure 1 . A system for producing trioxane by heat pump assisted reaction rectification coupled with pressure swing rectification according to an embodiment of the present application includes a reaction rectification unit, a pressure swing rectification unit and a heat pump unit.

[0019] The reaction rectification unit includes a pre-distillation tower T1, a reaction rectification tower T2 and a reflux tank V1, the pressure swing rectification unit includes a low-pressure rectification tower T3, a high-pressure rectification tower T4 and a reflux tank V2, and the heat pump unit includes a heat pump unit C1 and a heat pump unit C2.

[0020] The gas phase outlet of the top of the pre-distillation tower T1 is in communication with the gas inlet of the heat pump unit C1, the exhaust port of the heat pump unit C1 is in communication with the hot side inlet of the condenser reboiler E12 at the kettle of the high-pressure rectification tower T4, and the liquid phase sampling outlet of the kettle of the pre-distillation tower T1 is in communication with the middle part of the reaction rectification tower T2.

[0021] The overhead gas phase outlet of the reaction rectification column T2 is communicated with the hot side inlet of the condensation reboiler E4 at the column bottom of the pre-rectification column T1, the hot side outlet of the condensation reboiler E4 is communicated with the inlet of the reflux tank V1, and the liquid phase outlet of the reflux tank V1 is divided into two paths: one path is communicated with the reflux port of the upper part of the reaction rectification column T2, and the other path is communicated with the middle part of the low-pressure rectification column T3.

[0022] The overhead gas phase outlet of the low-pressure rectification column T3 is communicated with the gas inlet of the heat pump unit C2, the exhaust port of the heat pump unit C2 is communicated with the hot side inlet of the condensation reboiler E7 at the column bottom of the reaction rectification column T2, the hot side outlet of the condensation reboiler E7 is communicated with the reflux tank V2, and the liquid phase outlet of the reflux tank V2 is divided into two paths: one path is communicated with the reflux port of the upper part of the low-pressure rectification column T3, and the other path is communicated with the middle part of the high-pressure rectification column T4; the liquid phase outlet at the column bottom of the low-pressure rectification column T3 is communicated with the feed inlet of the middle part of the pre-rectification column T1;

[0023] The column bottom of the high-pressure rectification column T4 has a liquid phase outlet for discharging products.

[0024] According to an embodiment of the present application, an energy-saving process for producing trioxane by a heat pump-assisted reaction rectification coupled with pressure swing rectification comprises the following steps:

[0025] The formaldehyde aqueous solution raw material is fed into the pre-rectification column T1 for concentration, the dilute formaldehyde taken from the column top is pressurized and heated by the heat pump unit C1 and then used as the heat source for the column bottom of the high-pressure rectification column T4, and the concentrated formaldehyde taken from the column bottom of the pre-rectification column T1 is fed into the reaction rectification column T2, which is converted into trioxane under the catalysis of the catalyst in the reaction rectification column T2;

[0026] The azeotrope vapor of formaldehyde, water and trioxane taken from the column top of the reaction rectification column T2 is used as the heat source for the column bottom of the pre-rectification column T1, is condensed by heat exchange and then fed into the middle part of the low-pressure rectification column T3, the azeotrope vapor taken from the column top of the low-pressure rectification column T3 is pressurized and heated by the heat pump unit C2 and then used as the heat source for the column bottom of the reaction rectification column T2, is condensed by heat exchange and then fed into the middle part of the high-pressure rectification column T4, and the dilute formaldehyde solution taken from the column bottom of the low-pressure rectification column T3 is fed back to the pre-rectification column T1 for concentration; wherein: the overhead vapor of the reaction rectification column T2 is fed into the condensation reboiler E4 at the column bottom of the pre-rectification column T1 as the heat source, and is fed into the reflux tank V1 after being condensed into condensate by heat exchange, a part of the condensate taken from the reflux tank V1 is refluxed to the upper part of the reaction rectification column T2, and the other part of the condensate is fed into the middle part of the low-pressure rectification column T3; the azeotrope vapor pressurized and heated by the heat pump unit C2 is fed into the condensation reboiler E7 at the column bottom of the reaction rectification column T2 as the heat source, and is fed into the reflux tank V2 after being condensed into condensate by heat exchange, a part of the condensate taken from the reflux tank V2 is refluxed to the upper part of the low-pressure rectification column T3, and the other part of the condensate is fed into the middle part of the high-pressure rectification column T4;

[0027] The trioxane product is taken from the column bottom of the high-pressure rectification column T4.

[0028] The catalyst of the reaction rectification column T2 described above can be a heterogeneous catalyst such as molecular sieve, bentonite, or a homogeneous catalyst such as sulfuric acid, phosphoric acid, etc., in which case a catalyst and wastewater separation device needs to be provided in the column still.

[0029] Further, the pressure swing rectification unit of the embodiment of the present application further comprises a medium-pressure rectification column T5, a reflux tank V3 and a reflux tank V5.

[0030] The overhead gas phase outlet of the high-pressure rectification column T4 is connected to the hot side inlet of the condensation reboiler E15 at the column still of the medium-pressure rectification column T5, the hot side outlet of the condensation reboiler E15 is connected to the inlet of the reflux tank V5, the liquid phase outlet of the reflux tank V5 is divided into two paths: one path is connected to the reflux port of the upper part of the high-pressure rectification column T4, and the other path is connected to the middle part of the medium-pressure rectification column T5; the overhead gas phase outlet of the medium-pressure rectification column T5 is connected to the hot side inlet of the condensation reboiler E10 at the column still of the low-pressure rectification column T3, the hot side outlet of the condensation reboiler E10 is connected to the inlet of the reflux tank V3, the liquid phase outlet of the reflux tank V3 is divided into two paths: one path is connected to the reflux port of the upper part of the medium-pressure rectification column T5, and the other path is connected to the middle part of the low-pressure rectification column T3; the column still of the medium-pressure rectification column T5 is provided with a liquid phase outlet.

[0031] Correspondingly, the energy-saving process for producing trioxane by a heat pump assisted reaction rectification coupled pressure swing rectification according to an embodiment of the present application further comprises the following steps: using the overhead vapor of the high-pressure rectification column T4 as the heat source for the column still of the medium-pressure rectification column T5, the overhead vapor of the high-pressure rectification column T4 is condensed by heat exchange and then enters the medium-pressure rectification column T5, and the medium-pressure rectification column T5 is used to concentrate formaldehyde and trioxane; using the ternary mixture taken from the overhead of the medium-pressure rectification column T5 as the heat source for the column still of the low-pressure rectification column T3, the ternary mixture is condensed by heat exchange and then enters the low-pressure rectification column T3, and the wastewater is taken from the column still of the medium-pressure rectification column T5. Among them: the overhead vapor of the high-pressure rectification column T4 is used as the heat source to enter the condensation reboiler E15 at the column still of the medium-pressure rectification column T5, and after being condensed into condensate by heat exchange, it is introduced into the reflux tank V5, a part of the condensate taken from the reflux tank V5 is used to reflux the upper part of the high-pressure rectification column T4, and the other part of the condensate is introduced into the middle part of the medium-pressure rectification column T5. The ternary mixture vapor taken from the overhead of the medium-pressure rectification column T5 is used as the heat source to enter the condensation reboiler E10 at the column still of the low-pressure rectification column T3, and after being condensed into condensate by heat exchange, it is introduced into the reflux tank V3, a part of the condensate taken from the reflux tank V3 is used to reflux the upper part of the medium-pressure rectification column T5, and the other part of the condensate is introduced into the middle part of the low-pressure rectification column T3.

[0032] Preferably, the final condensation temperature of the overhead gas phase of all the rectification columns of the embodiment is at least 80℃, so as to avoid trioxane crystallization and affect the normal operation of the device.

[0033] Optionally, the pressure swing rectification unit comprises a top condenser E8, a top condenser E11 and a top condenser E17. The top condenser E8 is arranged between the condensing reboiler E7 and the reflux tank V2, and the hot side inlet and the hot side outlet of the top condenser E8 are in communication with the hot side outlet of the condensing reboiler E7 and the inlet of the reflux tank V2, respectively. The top condenser E11 is arranged between the condensing reboiler E10 and the reflux tank V3, and the hot side inlet and the hot side outlet of the top condenser E11 are in communication with the hot side outlet of the condensing reboiler E10 and the inlet of the reflux tank V3, respectively. The top condenser E17 is arranged between the condensing reboiler E15 and the reflux tank V5, and the hot side inlet and the hot side outlet of the top condenser E17 are in communication with the hot side outlet of the condensing reboiler E15 and the inlet of the reflux tank V5, respectively.

[0034] Further, the reaction rectification unit of the embodiment of the present application comprises a reflux tank V4. The hot side outlet of the condensing reboiler E12 is in communication with the inlet of the reflux tank V4, and the liquid phase outlet of the reflux tank V4 is divided into two paths: one path is in communication with the reflux port of the upper part of the pre-rectification tower T1, and the other path is in communication with the middle part of the medium-pressure rectification tower T5.

[0035] Correspondingly, the energy-saving process for producing trioxane by a heat pump assisted reaction rectification coupled with pressure swing rectification according to an embodiment of the present application further comprises the following steps: the dilute methylol of the top of the pre-rectification tower T1 is pressurized and heated by the heat pump unit C1, and then is used as a heat source to enter the condensing reboiler E12 at the bottom of the high-pressure rectification tower T4, and after heat exchange, the condensate is used to enter the reflux tank V4, and a part of the condensate obtained from the reflux tank V4 is used to reflux the upper part of the pre-rectification tower T1, and the other part of the condensate is used to enter the middle part of the medium-pressure rectification tower T5.

[0036] Optionally, the reaction rectification unit comprises a top condenser E5 and a top condenser E14; the top condenser E5 is arranged between the condensing reboiler E4 and the reflux tank V1, and the hot side inlet and the hot side outlet of the top condenser E5 are in communication with the hot side outlet of the condensing reboiler E4 and the inlet of the reflux tank V1, respectively; the top condenser E14 is arranged between the condensing reboiler E12 and the reflux tank V4, and the hot side inlet and the hot side outlet of the top condenser E14 are in communication with the hot side outlet of the condensing reboiler E12 and the inlet of the reflux tank V4, respectively.

[0037] Optionally, the system for producing trioxane by a heat pump assisted reaction rectification coupled with pressure swing rectification comprises a first-stage pre-heater E1 and a second-stage pre-heater E2; the cold side inlet of the first-stage pre-heater E1 is used for feeding, the cold side outlet of the first-stage pre-heater E1 is in communication with the cold side inlet of the second-stage pre-heater E2, the cold side outlet of the second-stage pre-heater E2 is in communication with the feeding port of the middle part of the pre-rectification tower T1; the liquid phase outlet of the bottom of the reaction rectification tower T2 and the liquid phase outlet of the bottom of the medium-pressure rectification tower T5 are both in communication with the hot side inlet of the first-stage pre-heater E1, and the liquid phase outlet of the bottom of the high-pressure rectification tower T4 is in communication with the hot side inlet of the second-stage pre-heater E2.

[0038] In the present embodiment, reboilers E3, E6, E9, E13 and E16 are provided at the bottoms of pre-distillation column T1, reaction distillation column T2, low-pressure distillation column T3, high-pressure distillation column T4 and medium-pressure distillation column T5, respectively, and are heated by external heat sources. The overhead condensers E5, E14, E8, E11, E17, reboilers E3, E6, E9, E13 and E16 in the reaction distillation unit and the pressure swing distillation unit are standby equipment and are activated when the heat coupling and heat pump system is insufficient in energy or the heat source is shut down.

[0039] The condensing reboiler described herein is distinguished from a reboiler in that the heat source of the condensing reboiler is overhead gas or high-temperature gas output by a heat pump unit, and the heat source of the reboiler is external heating steam.

[0040] In the present embodiment, the working pressure of reaction distillation column T2 is greater than that of pre-distillation column T1, and the working pressures of pre-distillation column T1 and reaction distillation column T2 are both less than that of high-pressure distillation column T4. Preferably, the working pressure of low-pressure distillation column T3 is the same as that of pre-distillation column T1, the working pressure of reaction distillation column T2 is the same as that of medium-pressure distillation column T5, the working pressure of reaction distillation column T2 is greater than that of low-pressure distillation column T3 and pre-distillation column T1, and the working pressure of high-pressure distillation column T4 is greater than that of medium-pressure distillation column T5.

[0041] In the present embodiment, pre-distillation column T1, reaction distillation column T2, low-pressure distillation column T3, high-pressure distillation column T4 and medium-pressure distillation column T5 are plate columns, and in another embodiment, pre-distillation column T1, reaction distillation column T2, low-pressure distillation column T3, high-pressure distillation column T4 and medium-pressure distillation column T5 are packed columns.

[0042] The heat pump unit can be a direct compression heat pump unit, an indirect heat pump unit or a flash heat pump unit. Please refer to Figure 2In the present embodiment, each heat pump unit (i.e. heat pump unit C1 and heat pump unit C2) comprises a compressor 21 and a gas-liquid separator 22, the gas inlet of the compressor 21 constitutes the gas inlet of the heat pump unit, the gas outlet of the compressor 21 is communicated with the inlet of the gas-liquid separator 22, and the gas outlet of the gas-liquid separator 22 constitutes the gas outlet of the heat pump unit. The first liquid outlet of the gas-liquid separator 22 is communicated with the gas inlet of the compressor 21 to provide the gas inlet of the compressor 21 with liquid injection, and the second liquid outlet of the gas-liquid separator 22 is used to discharge liquid. The gas-liquid separator 22 traps the liquid phase at the outlet of the compressor and injects it into the gas inlet of the compressor, and the gas outlet is saturated. In other embodiments, the outlet of the compressor is not provided with a gas-liquid separator, and the outlet of the compressor is superheated vapor. Preferably, the compressor 21 is an oil-free double-screw compressor, which has good stability and a wide operating range.

[0043] In another embodiment, the compressor 21 is a centrifugal compressor, in which case the liquid injection pipeline is changed to inject liquid into the gas-liquid separator 22, and the liquid phase no longer returns to the gas inlet of the compressor. At the same time, a heater needs to be arranged in front of the compressor 21 to prevent the liquid phase from entering the compressor 21. In other embodiments, the compressor 21 can also be a reciprocating compressor.

[0044] The working process of the energy-saving process for producing trioxymethylene by heat pump-assisted reactive rectification coupled with pressure swing rectification according to the embodiments of the present application will be further described below in combination with a specific embodiment.

[0045] In this embodiment, the working pressures of the pre-distillation column T1, the reaction distillation column T2, the low-pressure distillation column T3, the high-pressure distillation column T4 and the medium-pressure distillation column T5 are 80 kPa, 300 kPa, 80 kPa, 700 kPa and 300 kPa, respectively. The formaldehyde aqueous solution raw material (in other embodiments, the raw material can be formaldehyde aqueous solution with other concentrations, and the raw material can be obtained by hydrolysis of polyformaldehyde or directly obtained from formaldehyde solution) obtained by oxidation of methanol and containing formaldehyde at a concentration of 37 wt.% is preheated to 70°C by a primary preheater E1 and a secondary preheater E2, and then introduced into the middle of the pre-distillation column T1. After being concentrated to 60 wt.% in the pre-distillation column T1, the dilute formaldehyde at the top is pressurized and heated to 199°C and 1900 kPa by a heat pump unit C1, and then used as the heat source for the bottom of the high-pressure distillation column T4. After being condensed, the dilute formaldehyde is pumped by a top pump P1 into the middle of the medium-pressure distillation column T5, and the concentrated formaldehyde is discharged from the liquid phase outlet at the bottom of the pre-distillation column T1, pressurized to 350 kPa by a bottom pump P2, and then introduced into the middle of the reaction distillation column T2. Under the catalysis of the catalyst in the reaction section of the reaction distillation column T2, the formaldehyde is converted into trioxane. In this embodiment, the catalyst is a solid acid resin catalyst, and the resin is a heterogeneous catalyst. The catalyst does not need to be separated from the reaction material, and the structure of the resin is stable and not easy to be lost. Since formaldehyde-water-trioxane forms a minimum azeotrope, the unreacted formaldehyde, the product trioxane and water are discharged from the top of the reaction distillation column T2. The pre-distillation column T1 is a vacuum distillation column, and the reaction distillation column T2 is a pressurized distillation column. The vapor temperature at the top of the reaction distillation column T2 (123°C) is higher than the bottom temperature of the pre-distillation column T1 (96°C), so the vapor at the top of the reaction distillation column T2 is used to heat the liquid at the bottom of the pre-distillation column T1. When the heat of the vapor at the top of the reaction distillation column T2 is insufficient, a reboiler E3 is started to supplement the heat by external heating steam. The vapor condensed in the condenser E4 enters a reflux tank V1. Part of the condensed liquid obtained from the reflux tank V1 is returned to the reaction distillation column T2, and the other part enters the low-pressure distillation column T3. If the stream after heat exchange is in a gas-liquid two-phase state, a top condenser E5 is started to further cool the stream to the bubble point 123°C, and then the stream enters the reflux tank V1. The top condenser E5 uses cooling water as the cooling medium. The single-pass conversion rate of formaldehyde in the reaction distillation column T2 is 79.55%.

[0046] For the azeotrope formed by formaldehyde, water and trioxane, the content of trioxane in the azeotrope increases with the increase of pressure. The material from the top of the reactive distillation column T2 enters the middle of the low-pressure rectification column T3, and the concentration of trioxane in the mixture is further increased in the low-pressure rectification column T3, and the content of trioxane in the overhead vapor is increased from 62wt.% to 70wt.%. The azeotrope taken from the top of the low-pressure rectification column T3 is pressurized and heated to 145℃ and 550kPa by the heat pump unit C2, and then used as the heat source of the reactive distillation column T2. After heat exchange and condensation, it enters the high-pressure rectification column T4. The dilute formaldehyde solution with a concentration of 23.6wt.% taken from the bottom of the low-pressure rectification column T3 is pressurized to 100kPa by the pump P4, and then sent back to the pre-distillation column T1 for concentration. The azeotrope from the low-pressure rectification column T3 is pressurized to 350kPa by the top pump P3, and then sent to the middle of the high-pressure rectification column T4. Due to the increase of pressure, the content of trioxane in the azeotrope composition decreases from 70wt.% to 65.8wt.%. Therefore, the azeotrope is taken from the top of the high-pressure rectification column T4, and the high-purity trioxane with a purity of 99.99wt.% is taken from the bottom. The overhead vapor temperature of the high-pressure rectification column T4 is 153℃, which is higher than the bottom liquid temperature of 133℃ of the medium-pressure rectification column, and can be used as the heat source of the bottom of the medium-pressure rectification column T5. After heat exchange and condensation, it enters the medium-pressure rectification column T5, and the concentration of formaldehyde and trioxane is increased in the medium-pressure rectification column T5. The overhead vapor temperature of the medium-pressure rectification column T5 is 127℃, which is higher than the bottom temperature of 93℃ of the low-pressure rectification column, and can be used as the heat source of the bottom of the low-pressure rectification column T3. After heat exchange and condensation, it enters the low-pressure rectification column T3 for circulation. The bottom of the medium-pressure rectification column T5 takes out the waste water with a purity of 99.99wt.%. Due to the high temperature of the materials taken from the bottoms of the reactive distillation column T2, the high-pressure rectification column T4 and the medium-pressure rectification column T5, which are 134℃, 188℃ and 134℃ respectively, the waste water taken from the bottoms of the reactive distillation column T2 and the medium-pressure rectification column T5 is mixed and used as the heat source of the first preheater E1 to preheat the feed, and the trioxane taken from the bottom of the high-pressure rectification column T4 is used as the heat source of the second preheater E2 to preheat the feed, so that the heat is fully recovered.

[0047] The overhead vapor of the pre-distillation column T1 and the low-pressure rectification column T3 enters the compressor 21 for pressurization and heating. The pressurized vapor enters the gas-liquid separator 22 to separate the liquid phase, and the liquid phase is sent to the inlet of the compressor as the liquid spray. The gas phase enters the condensing reboiler to heat the corresponding column bottom liquid. The pressurized vapor is condensed into liquid phase and then enters the reflux tank.

[0048] The energy-saving process for producing trioxane by heat pump assisted reactive distillation coupled with pressure swing distillation in the embodiment of the present application is improved in the following four aspects compared with the prior art:

[0049] 1. Reaction rectification is used to realize the conversion of formaldehyde and the separation of formaldehyde-water-trioxane: In the embodiment of the present application, a reaction rectification column is used to replace the tank reactor, and a reaction rectification process is used to improve the single-pass conversion rate of trioxane from 4% to 79.55%. Meanwhile, the product is separated and purified in the reaction rectification column, and the circulation rate of the low-pressure rectification column T3 to the pre-rectification column is reduced from 95.43% to 39.73%, thereby reducing the load of the subsequent rectification column and saving the subsequent equipment investment and operating costs.

[0050] 2. Pressure swing rectification is used to realize the purification of trioxane: In the embodiment of the present application, a pressure swing rectification process is used without introducing other substances such as extractants, thereby avoiding the complex process of separating and purifying the extractant and simplifying the operation process.

[0051] 3. Heat pump recovery system is used to recover heat: In the embodiment of the present application, a heat pump rectification process is used to recover system heat to meet its own heat demand, thereby saving all heating steam for the column still of the reaction rectification column T2 and the high-pressure rectification column T4, saving 30% of the circulating cooling water for the column top of the pre-rectification column T1 and the low-pressure rectification column T3, and saving the operating costs.

[0052] 4. Heat coupling is used to realize system energy saving: Since the column top temperature and the column still temperature of different columns differ greatly, an internal heat coupling process is used in the embodiment of the present application to heat the column still liquid of other columns with the column top vapor, thereby saving 17% of the heating steam for the column still of the pre-rectification column T1, the low-pressure rectification column T3 and the medium-pressure rectification column T5, saving 84% of the circulating cooling water for the column top of the reaction rectification column T2, the high-pressure rectification column T4 and the medium-pressure rectification column T5, and realizing good energy saving effect.

Claims

1. An energy-saving process for producing trioxane by heat pump-assisted reactive rectification coupled with pressure swing distillation, characterized in that, The method comprises the following steps: The formaldehyde aqueous solution raw material is introduced into the pre-distillation column T1 for concentration, the dilute formaldehyde taken from the top of the column is pressurized and heated by the heat pump unit C1 and then used as the heat source for the column bottom of the high-pressure distillation column T4, and the concentrated formaldehyde taken from the bottom of the pre-distillation column T1 is sent into the reaction distillation column T2 to be converted into trioxane under the catalysis of the catalyst in the reaction distillation column T2; wherein the dilute formaldehyde at the top of the pre-distillation column T1 is pressurized and heated by the heat pump unit C1 and then introduced into the condensation reboiler E12 at the column bottom of the high-pressure distillation column T4 as the heat source, and after heat exchange to become condensate, is introduced into the reflux tank V4, and part of the condensate taken from the reflux tank V4 is refluxed to the upper part of the pre-distillation column T1, and the other part of the condensate is introduced into the middle part of the medium-pressure distillation column T5; The azeotrope vapor of formaldehyde, water and trioxane at the top of the reaction distillation column T2 is used as the heat source for the column bottom of the pre-distillation column T1, is condensed after heat exchange and then introduced into the middle part of the low-pressure distillation column T3, the azeotrope vapor taken from the top of the low-pressure distillation column T3 is pressurized and heated by the heat pump unit C2 and then used as the heat source for the column bottom of the reaction distillation column T2, is condensed after heat exchange and then introduced into the middle part of the high-pressure distillation column T4, and the dilute formaldehyde solution taken from the column bottom of the low-pressure distillation column T3 is sent back to the pre-distillation column T1 for concentration; wherein the vapor at the top of the reaction distillation column T2 is introduced into the condensation reboiler E4 at the column bottom of the pre-distillation column T1 as the heat source, and after heat exchange to become condensate, is introduced into the reflux tank V1, part of the condensate taken from the reflux tank V1 is refluxed to the upper part of the reaction distillation column T2, and the other part of the condensate is introduced into the middle part of the low-pressure distillation column T3; the azeotrope vapor pressurized and heated by the heat pump unit C2 is introduced into the condensation reboiler E7 at the column bottom of the reaction distillation column T2 as the heat source, and after heat exchange to become condensate, is introduced into the reflux tank V2, part of the condensate taken from the reflux tank V2 is refluxed to the upper part of the low-pressure distillation column T3, and the other part of the condensate is introduced into the middle part of the high-pressure distillation column T4; The trioxane product is taken from the column bottom of the high-pressure distillation column T4, the vapor at the top of the high-pressure distillation column T4 is used as the heat source for the column bottom of the medium-pressure distillation column T5, the vapor at the top of the high-pressure distillation column T4 is condensed after heat exchange and then enters the medium-pressure distillation column T5, the formaldehyde and trioxane are concentrated in the medium-pressure distillation column T5, the ternary mixture taken from the top of the medium-pressure distillation column T5 is used as the heat source for the column bottom of the low-pressure distillation column T3, the ternary mixture is condensed after heat exchange and then enters the low-pressure distillation column T3, and waste water is taken from the column bottom of the medium-pressure distillation column T5; wherein the vapor at the top of the high-pressure distillation column T4 is introduced into the condensation reboiler E15 at the column bottom of the medium-pressure distillation column T5 as the heat source, and after heat exchange to become condensate, is introduced into the reflux tank V5, part of the condensate taken from the reflux tank V5 is refluxed to the upper part of the high-pressure distillation column T4, and the other part of the condensate is introduced into the middle part of the medium-pressure distillation column T5; the vapor of the ternary mixture taken from the top of the medium-pressure distillation column T5 is introduced into the condensation reboiler E10 at the column bottom of the low-pressure distillation column T3 as the heat source, and after heat exchange to become condensate, is introduced into the reflux tank V3, part of the condensate taken from the reflux tank V3 is refluxed to the upper part of the medium-pressure distillation column T5, and the other part of the condensate is introduced into the middle part of the low-pressure distillation column T3.

2. The energy saving process for production of trioxane by heat pump assisted reactive rectification coupled with pressure swing distillation as claimed in claim 1 wherein, The formaldehyde aqueous solution raw material sequentially passes through a first pre-heater E1 and a second pre-heater E2 and then enters a pre-distillation column T1; Waste water collected from the column bottom of the reaction distillation column T2 and the column bottom of the medium-pressure distillation column T5 is mixed and used as a heat source of the first pre-heater E1 to pre-heat the feed of the pre-distillation column T1, and trioxane collected from the column bottom of the high-pressure distillation column T4 is used as a heat source of the second pre-heater E2 to pre-heat the feed of the pre-distillation column T1.

3. The energy saving process for production of trioxane by heat pump assisted reactive rectification coupled with pressure swing distillation as claimed in claim 1 wherein, The pre-distillation column T1 is a reduced-pressure column, and the reaction distillation column T2 is a pressurized column.

4. A system for producing trioxane by heat pump-assisted reactive rectification coupled with pressure swing distillation, characterized in that, The reaction distillation unit, the pressure swing distillation unit and the heat pump unit are included; The reaction distillation unit includes the pre-distillation column T1, the reaction distillation column T2 and a reflux tank V1, the pressure swing distillation unit includes the low-pressure distillation column T3, the high-pressure distillation column T4, the medium-pressure distillation column T5, reflux tanks V2, V3, V4 and V5, and the heat pump unit includes heat pump units C1 and C2; The gaseous phase outlet at the top of the pre-distillation column T1 is communicated with the air inlet of the heat pump unit C1, the air outlet of the heat pump unit C1 is communicated with the hot side inlet of a condensing reboiler E12 at the column bottom of the high-pressure distillation column T4, and the liquid phase collection outlet at the column bottom of the pre-distillation column T1 is communicated with the middle part of the reaction distillation column T2; the hot side outlet of the condensing reboiler E12 is communicated with the inlet of the reflux tank V4, and the liquid phase outlet of the reflux tank V4 is divided into two paths, one of which is communicated with the reflux port of the upper part of the pre-distillation column T1, and the other of which is communicated with the middle part of the medium-pressure distillation column T5; The gaseous phase outlet at the top of the reaction distillation column T2 is communicated with the hot side inlet of a condensing reboiler E4 at the column bottom of the pre-distillation column T1, the hot side outlet of the condensing reboiler E4 is communicated with the inlet of the reflux tank V1, and the liquid phase outlet of the reflux tank V1 is divided into two paths, one of which is communicated with the reflux port of the upper part of the reaction distillation column T2, and the other of which is communicated with the middle part of the low-pressure distillation column T3; The gaseous phase outlet at the top of the low-pressure distillation column T3 is communicated with the air inlet of the heat pump unit C2, the air outlet of the heat pump unit C2 is communicated with the hot side inlet of a condensing reboiler E7 at the column bottom of the reaction distillation column T2, the hot side outlet of the condensing reboiler E7 is communicated with the reflux tank V2, and the liquid phase outlet of the reflux tank V2 is divided into two paths, one of which is communicated with the reflux port of the upper part of the low-pressure distillation column T3, and the other of which is communicated with the middle part of the high-pressure distillation column T4; the liquid phase collection outlet at the column bottom of the low-pressure distillation column T3 is communicated with the feed inlet of the middle part of the pre-distillation column T1; The column bottom of the high-pressure distillation column T4 has a liquid phase outlet for discharge, the gaseous phase outlet at the top of the high-pressure distillation column T4 is communicated with the hot side inlet of a condensing reboiler E15 at the column bottom of the medium-pressure distillation column T5, the hot side outlet of the condensing reboiler E15 is communicated with the inlet of the reflux tank V5, and the liquid phase outlet of the reflux tank V5 is divided into two paths, one of which is communicated with the reflux port of the upper part of the high-pressure distillation column T4, and the other of which is communicated with the middle part of the medium-pressure distillation column T5; The overhead vapor outlet of the medium-pressure rectifying column T5 is communicated with the hot side inlet of the condensation reboiler E10 at the column bottom of the low-pressure rectifying column T3, the hot side outlet of the condensation reboiler E10 is communicated with the inlet of the reflux tank V3, the liquid phase outlet of the reflux tank V3 is divided into two routes: one is communicated with the reflux port of the upper part of the medium-pressure rectifying column T5, and the other is communicated with the middle part of the low-pressure rectifying column T3; the column bottom of the medium-pressure rectifying column T5 is provided with a liquid phase outlet.

5. The system for producing trioxane by heat pump auxiliary reactive rectification coupled pressure swing rectification according to claim 4, characterized in that, The reaction rectifying unit comprises a top condenser E5 and a top condenser E14; The top condenser E5 is arranged between the condensation reboiler E4 and the reflux tank V1, and the hot side inlet and the hot side outlet of the top condenser E5 are respectively communicated with the hot side outlet of the condensation reboiler E4 and the inlet of the reflux tank V1; The top condenser E14 is arranged between the condensation reboiler E12 and the reflux tank V4, and the hot side inlet and the hot side outlet of the top condenser E14 are respectively communicated with the hot side outlet of the condensation reboiler E12 and the inlet of the reflux tank V4.

6. The system for producing trioxane by heat pump auxiliary reactive rectification coupled pressure swing rectification according to claim 4, characterized in that, The pressure swing rectifying unit comprises a top condenser E8, a top condenser E11 and a top condenser E17; The top condenser E8 is arranged between the condensation reboiler E7 and the reflux tank V2, and the hot side inlet and the hot side outlet of the top condenser E8 are respectively communicated with the hot side outlet of the condensation reboiler E7 and the inlet of the reflux tank V2; The top condenser E11 is arranged between the condensation reboiler E10 and the reflux tank V3, and the hot side inlet and the hot side outlet of the top condenser E11 are respectively communicated with the hot side outlet of the condensation reboiler E10 and the inlet of the reflux tank V3; The top condenser E17 is arranged between the condensation reboiler E15 and the reflux tank V5, and the hot side inlet and the hot side outlet of the top condenser E17 are respectively communicated with the hot side outlet of the condensation reboiler E15 and the inlet of the reflux tank V5.

7. The system for producing trioxane by heat pump auxiliary reactive rectification coupled pressure swing rectification according to claim 4, characterized in that, The system for producing trioxane by the heat pump auxiliary reaction rectifying coupled pressure swing rectifying comprises a first preheater E1 and a second preheater E2; The cold side inlet of the first preheater E1 is used for feeding, the cold side outlet of the first preheater E1 is communicated with the cold side inlet of the second preheater E2, the cold side outlet of the second preheater E2 is communicated with the feeding port of the middle part of the pre-rectifying column T1; the liquid phase outlets of the column bottom of the reaction rectifying column T2 and the column bottom of the medium-pressure rectifying column T5 are both communicated with the hot side inlet of the first preheater E1, and the liquid phase outlet of the column bottom of the high-pressure rectifying column T4 is communicated with the hot side inlet of the second preheater E2.

8. The system for producing trioxane by heat pump auxiliary reactive rectification coupled pressure swing rectification according to claim 4, characterized in that, Each heat pump unit comprises a compressor and a gas-liquid separator, the gas inlet of the compressor constitutes the gas inlet of the heat pump unit, the gas outlet of the gas-liquid separator constitutes the gas outlet of the heat pump unit.

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