An efficient energy-saving cold hydrogenation device and method

Abstract

The present invention discloses an efficient energy-saving cold hydrogenation device and method, belonging to the technical field of polysilicon production. The system includes a fluidized bed reactor and an STC vaporization tower. The output end at the top of the fluidized bed reactor is connected to a quench tank through a multi-stage mixer heat exchanger, and the output end at the top of the quench tank is connected to a reflux tank through a quench tower. In the present invention, two process gases before and after the reaction exchange heat, fully recovering the reaction heat, and saving energy consumption by optimizing the heat exchange network.

Description

Technical Field

[0001] The present invention belongs to the technical field of polysilicon production, and relates to an energy-efficient cold hydrogenation device and method. Background Art

[0002] At present, the production of polysilicon mainly adopts the improved Siemens method. This method consumes 19 - 24 tons of trichlorosilane (TCS) for every 1 ton of polysilicon produced, and simultaneously by-produces 15 - 20 tons of silicon tetrachloride (STC). The by-product reacts with air to generate silicic acid and hydrogen chloride, seriously polluting the environment and harming human health. For the recovery of silicon tetrachloride, the most effective current method is to use the cold hydrogenation technology to convert it into trichlorosilane, thereby realizing the closed-loop circulation of materials inside the device, optimizing the process flow, and achieving the effect of zero pollutant discharge.

[0003] The cold hydrogenation device is the core part of polysilicon production. Usually, silicon tetrachloride, silicon powder, hydrogen, and hydrogen chloride are used as raw materials, and copper chloride is used as a catalyst. The reaction occurs in a fluidized bed at a specific temperature and pressure to produce trichlorosilane products. The main reactions are as follows: SiCl4 + H2 = SiHCl3 + HCl, Si + 3HCl = SiHCl3 + H2.

[0004] The cold hydrogenation process mainly includes a silicon powder feeding system, an STC vaporization system, a fluidized bed reaction system, a washing, dust removal and quenching system, and a condensation and recovery system. Its general process is as follows: The raw materials hydrogen and silicon tetrachloride are preheated respectively and then enter the STC vaporization tower for mixing and vaporization. The vaporized raw material mixture gas is heated in multiple stages and reacts with silicon powder in a fluidized bed reactor at 500 - 600 °C and 3.0 - 3.2 MPa to carry out a hydrogenation reaction. The generated process gas contains substances such as silicon tetrachloride, trichlorosilane, and dichlorodihydrogen silane. The process gas is subjected to cyclone separation, cooling, and quenching washing to remove its overheat, solids, and soluble metal chlorides. The slurry is periodically discharged from the bottom of the quench tank to prevent the accumulation of solids and metal chlorides. The process gas is then subjected to multi-stage condensation and recovery to obtain a crude chlorosilane liquid, which is sent to a rough separation system for primary separation. The separated silicon tetrachloride is returned to cold hydrogenation, the crude trichlorosilane is sent to a rectification unit, and the non-condensable gas hydrogen is returned to cold hydrogenation for recycling.

[0005] The main energy consumption of the cold hydrogenation process is the heating of raw materials and the condensation and cooling of process gas. Therefore, the energy consumption can be effectively reduced through energy-saving optimization design. At the same time, since a large amount of silicon powder and soluble metal chlorides are carried by the process gas into the subsequent system after the reaction, it causes wear and blockage of equipment and pipelines, affecting the stable production of the device. Therefore, a reasonable selection of washing and energy-saving methods is a crucial factor for the cold hydrogenation process to reduce energy consumption and balance operation. Summary of the Invention

[0006] The object of the present invention is to provide an energy-efficient cold hydrogeneration process, mainly to solve the problems existing in the current cold hydrogeneration process, such as the decrease in conversion rate in the later stage of the reaction, the damage of the STC vaporizer due to vibration, and the poor dust removal effect of the process gas, resulting in unstable cold hydrogeneration operation and decreased product purity. By adopting this technology, the reaction conversion rate can be effectively improved, and the stable operation of the device can be ensured. At the same time, the energy consumption is further reduced by optimizing the process heat exchange network.

[0007] The object of the present invention can be achieved by the following technical solutions:

[0008] An energy-efficient cold hydrogeneration device and method, the method comprising the following steps:

[0009] 1) Silicon powder automatic feeding system

[0010] The silicon powder is transported to the low-pressure silo in a dense phase, and hot nitrogen at 180 - 210 °C is introduced into it to remove the moisture in the silicon powder and the catalyst. The dried silicon powder flows by gravity into the high-pressure silo, and after pressurization, the silicon powder enters the fluidized bed reactor in small batches by gravity;

[0011] 2) STC heating and evaporation system

[0012] The silicon tetrachloride from the tank farm and rough separation enters the STC buffer tank, is pressurized by the STC transfer pump and sent to the STC preheater, preheated by the top gas of the quench tower and then enters the STC heater, heated by steam as the heat source and sent to the STC vaporization tower. At the same time, hydrogen is added to the bottom of the vaporization tower to lower the vaporization temperature of silicon tetrachloride. The vaporized STC and hydrogen raw gas enter the multi-stage heat exchanger in sequence, and finally enter the electric heater for heating;

[0013] 3) Reaction system

[0014] The raw gas heated by the electric heater enters the fluidized bed reactor from the bottom, and the raw gas is evenly distributed across the cross-section of the reactor through the gas distributor, and then the silicon powder bed layer in the reactor is made to boil and be in a fluidized state; in the reactor, silicon tetrachloride, hydrogen, hydrogen chloride react with the silicon powder to generate gas, and the reacted gas enters the mixed gas heat exchanger for heat exchange from the top discharge port and then enters the quench tank;

[0015] 4) Quench system

[0016] In the quench tank, the condensate from the bottom of the quench tower is used to wash and remove excess silicon powder and solid metal chlorides. The process gas from the quench tank enters the quench tower, and the gas flows upward through the packing section, where it is further cooled by the downward-flowing reflux liquid. The packing section is used to remove heavy component chlorosilanes and volatile metal chlorides in the product. The quenched gas is discharged from the top of the tower and enters the hydrogen preheater and STC preheater in sequence for cooling. The condensate is further recovered to the reflux tank. The cooled gas then enters the air cooler, and the chlorosilane condensate after air cooling is collected in the quench tower reflux tank. Part of the condensate in the tank is refluxed to the quench tower through the quench reflux pump, and part of the chlorosilanes go to the rough separation system for rectification and purification. The non-condensable gas enters the exhaust gas recovery system.

[0017] In the above method: A fluidized bed cyclone separator system is provided in the fluidized bed reactor. After the reaction, the process gas passes through the silicon bed, flows through the free space of the reactor, and enters the fluidized bed cyclone separator system. The cyclone separator system consists of two parallel-operated cyclone separators and their downcomers. The cyclone separators capture the large particle silicon powder entrained in the gas and return it to the reactor bed through the downcomers. The counterweight valve on the downcomer sends the captured silicon powder to the free space of the reactor above the high material level. After the process gas is discharged from the cyclone separator system at the top of the reactor, it enters the multi-stage mixed gas heat exchanger to cool down and then enters the quench tank.

[0018] In the above method: The drying step of the silicon powder and the catalyst is to transport the silicon powder and the catalyst to the low-pressure silo and dry the silicon powder in the low-pressure silo; then it is transported to the high-pressure silo and then to the fluidized bed reactor to participate in the reaction.

[0019] Furthermore, hot hydrogen at 160 - 180 °C is added to the bottom of the vaporization tower to reduce the vaporization temperature of silicon tetrachloride.

[0020] A system for implementing the above high-efficiency and energy-saving cold hydrogenation method, which system includes a fluidized bed reactor and an STC vaporization tower. The output end at the top of the fluidized bed reactor is connected to the quench tank through a multi-stage mixer heat exchanger, and the output end at the top of the quench tank is connected to the reflux tank through the quench tower.

[0021] In the above system: The system includes a low-pressure silo and a high-pressure silo connected in sequence. The output end of the silicon powder feed is connected to the low-pressure silo, and the high-pressure silo is connected to the fluidized bed reactor;

[0022] Furthermore, in this system, there is a first-stage low-pressure silo and a first-stage high-pressure silo connected thereto, a second-stage low-pressure silo and a second-stage high-pressure silo connected thereto. The first-stage high-pressure silo is connected to the upper part of the fluidized bed reactor, and the second-stage high-pressure silo is connected to the lower part of the fluidized bed reactor.

[0023] In the above system: The system further includes an STC vaporization tower. The hydrogen feed end is connected to the STC vaporization tower after passing through a hydrogen preheater and an H2 heater in sequence. The STC is connected to the STC vaporization tower after passing through an STC preheater and an STC heater. The output end at the top of the STC vaporization tower is connected to an electric heater through a multi-stage mixer heat exchanger, and the electric heater is connected to the bottom of the fluidized bed reactor.

[0024] In the above system: One output end at the bottom of the reflux drum is connected to the quench tower, and the other output end is adjacent to the distillation system; the output end at the top of the reflux drum is connected to the slurry flash drum through an STC preheater, a hydrogen preheater, the quench tower, and a quench drum in sequence.

[0025] Advantages of the present invention:

[0026] (1) The silicon powder feeding adopts automatic unpacking and pneumatic conveying, reducing the number of staff and the problem of dust pollution of silicon powder.

[0027] (2) The silicon powder feeding adopts two-stage feeding. The first-stage feeding is that the silicon powder is added from the middle dilute-phase area, and the second-stage feeding is added in the bottom dense-phase area, reducing the deposition of silicon powder on the bottom bubble breaker and avoiding the accumulation of silicon powder at the bottom feeding port; at the same time, an adjustable cyclone separator is adopted to improve the reaction conversion rate in the later stage and ensure that the conversion rate in the later stage is maintained at a relatively high level.

[0028] (3) The wet dust removal by the quench tower is adopted to avoid the generation of dry silicon powder and reduce the risk of contact between dry silicon powder and air, increasing the operating safety of the device.

[0029] (4) The two process gases before and after the reaction are subjected to heat exchange to fully recover the reaction heat and save energy consumption.

[0030] (5) In the exhaust gas recovery system, the low-temperature exhaust gas at the top of the tail gas absorption tower is used to condense the reaction tail gas, further reducing the energy consumption. At the same time, hydrogen and chlorosilane in the tail gas are recovered, reducing the material consumption. Description of the drawings

[0031] Figure 1 It is a schematic diagram of the device of the present invention. Detailed implementation manners

[0032] The present invention will be further described below in conjunction with embodiments, but the protection scope of the present invention is not limited thereto:

[0033] Such as Figure 1 , a system for an efficient energy-saving cold hydrogenation method, the system includes a fluidized bed reactor 5 and an STC vaporization tower 12. The output end at the top of the fluidized bed reactor 5 is connected to a quench drum 19 through a multi-stage mixer heat exchanger, and the output end at the top of the quench drum 19 is connected to a reflux drum 22 through a quench tower 20.

[0034] The system includes a low-pressure silo and a high-pressure silo connected in sequence. The output end of the silicon powder feeding is connected to the low-pressure silo, and the high-pressure silo is connected to the fluidized bed reactor.

[0035] Furthermore, in this system, there is a first-stage low-pressure silo and a first-stage high-pressure silo connected thereto, a second-stage low-pressure silo and a second-stage high-pressure silo connected thereto. The first-stage high-pressure silo is connected to the upper part of the fluidized bed reactor, and the second-stage high-pressure silo is connected to the lower part of the fluidized bed reactor.

[0036] The system also includes an STC vaporization tower 12. The hydrogen feed end is connected to the STC vaporization tower 12 after passing through a hydrogen preheater 10 and an H2 heater 11 in sequence. The STC is connected to the STC vaporization tower 12 after passing through an STC preheater 8 and an STC heater 9. The output end at the top of the STC vaporization tower 12 is connected to an electric heater 18 through a multi-stage mixer heat exchanger, and the electric heater 18 is connected to the bottom of the fluidized bed reactor 5.

[0037] One output end at the bottom of the reflux drum 22 is connected to the quench tower 20, and the other output end is adjacent to the distillation system; the output end at the top of the reflux drum 22 is connected to the slurry flash tank 24 through an STC preheater 8, a hydrogen preheater 10, a quench tower 20, and a quench tank 19 in sequence.

[0038] The specific method is as follows:

[0039] 1) Silicon powder automatic feeding system

[0040] The silicon powder ton bag is transported to the automatic bag breaker by the overhead crane. After the bag is broken, the silicon powder and the catalyst copper chloride or cuprous chloride are jointly added to the silicon powder sending tank according to a certain mass ratio (1000:5), and are transported to the first-stage low-pressure silo 1 and the second-stage low-pressure silo 2 by dense-phase transportation, and the silicon powder is dried in the low-pressure silo; the drying is carried out by intermittent fixed-bed drying, and hot nitrogen at 195 - 205 °C is introduced to remove the moisture of the silicon powder and the catalyst. At the same time, after detecting that the oxygen content and dew point of the exhaust gas meet the requirements, the silicon powder and the catalyst flow into the first-stage high-pressure silo 3 and the second-stage high-pressure silo 4 by gravity. The silicon powder in the high-pressure silo enters the fluidized bed reactor 5 in small batches by gravity to participate in the reaction. Within one cycle, the controller adjusts the silicon powder feeding amount according to the silicon powder level in the reactor. At the same time, a small-flow hydrogen purging system is set on the silicon powder feeding pipeline between the high-pressure silo and the fluidized bed to prevent the hot gas in the reactor from flowing back to the silo system.

[0041] 2) STC heating and evaporation system

[0042] Silicon tetrachloride from the tank farm enters the STC buffer tank 6, is pressurized by the STC transfer pump 7 and sent to the STC preheater 8. After being preheated by the top gas of the quench tower, it enters the STC heater 9, is heated to 175°C with 1.0 MPa steam and then enters the STC vaporization tower 12. At the same time, hot hydrogen gas at 168 - 172°C is added to the bottom of the vaporization tower to lower the vaporization temperature of STC. The liquid silicon tetrachloride at the bottom of the tower kettle enters the circulating liquid heater 14 through the STC vaporization tower circulation pump 13, is heated with 1.0 MPa steam and then sent to the top of the vaporization tower. Part of the liquid STC and hydrogen gas are in countercurrent contact in the packing and continuously vaporize. The vaporized STC and hydrogen gas mixture enters the three-stage mixed gas heat exchanger, the two-stage mixed gas heat exchanger and the one-stage mixed gas heat exchanger in sequence to recover the reaction heat, and finally enters the electric heater 18, and is heated to the reaction temperature of 550 - 570°C.

[0043] 3) Reaction system

[0044] The heated feed gas enters the fluidized bed reactor 5 from the bottom. The feed gas is evenly distributed across the cross-section of the reactor through the gas distributor, and then the silicon powder bed layer in the reactor is made to boil and be in a fluidized state. In the reactor, silicon tetrachloride, hydrogen gas, hydrogen chloride react with silicon powder to generate gases such as trichlorosilane and dichlorodihydrogen silane. After the reaction, the process gas passes through the silicon bed, flows through the free space of the reactor, and enters the fluidized bed cyclone separator system. The cyclone separator system consists of two parallel-operating cyclone separators and their downcomer legs. The cyclone separators capture the large-particle silicon powder entrained in the gas and return it to the reactor bed layer through the downcomer legs. The counterweight valve on the downcomer leg sends the captured silicon powder to the free space above the high material level in the reactor. The process gas is discharged from the top cyclone separator system of the reactor, enters the one-stage mixed gas heat exchanger, the two-stage mixed gas heat exchanger and the three-stage mixed gas heat exchanger to cool down, and then enters the quench tank 19. Heat exchange is carried out between the two process gases before and after the reaction to fully recover the reaction heat, greatly reducing the production energy consumption of the device and saving 2 - 4 t / h of steam consumption.

[0045] 4) Quench system

[0046] The cooled product gas enters the quench tank 19, where it is washed with the condensate from the bottom of the quench tower to remove excess silicon powder and solid metal chlorides. The slurry periodically discharged from the tank is further concentrated in the slurry flash tank and then sent to the slurry treatment unit; The process gas from the quench tank enters the quench tower 20. The gas flows upward through the packing section and is further cooled by the downward flowing reflux liquid. The packing section is used to remove the heavy component chlorosilanes and volatile metal chlorides in the product; The quenched gas is discharged from the top of the tower and sequentially enters the hydrogen preheater 10 and the STC preheater 8 for cooling. The condensate is further recovered to the reflux tank 22. The cooled gas then enters the air cooler 21. The chlorosilane condensate after air cooling is collected in the quench tower reflux tank 22. Part of the condensate in the tank is refluxed to the quench tower through the quench reflux pump 23, and part of the chlorosilanes go to the rough separation system for rectification and purification. The non-condensable gas enters the off-gas recovery system.

[0047] 5) Process off-gas recovery system

[0048] The off-gas containing hydrogen and chlorosilanes from each storage tank enters the bottom of the tail gas absorption tower after being cooled by circulating water water cooling, low-temperature gas at the top of the tower and low-temperature chlorosilane at the bottom of the tower in sequence. It contacts and cools and condenses reversely with the low-temperature circulating liquid pumped by the circulating pump and cooled by the refrigerant at -45°C, and then enters the recovered hydrogen compression system. The recovered hydrogen is recycled as raw material hydrogen. The chlorosilanes condensed in the tail gas enter the rough separation system. This process uses the liquid drawn from the bottom of the tail gas absorption tower as a cold source to condense the tail gas, saving 6 - 10 t / h of refrigerant Freon consumption and further reducing the material consumption.

[0049] This process flow has the advantages of high safety, low energy consumption, and simple process route. The two-stage silicon powder feeding method is beneficial to maintaining the conversion rate of the fluidized bed reactor at a stable level all the time. The improved wet dust removal scheme effectively reduces the content of silicon powder and metal chlorides, thus ensuring the stable operation of the whole set of equipment.

Claims

1. An efficient and energy-saving cold hydrogenation method, characterized in that: The method includes the following steps: 1) Automatic feeding system for silicon powder The silicon powder and the catalyst are transported in dense phase to a low-pressure silo, and hot nitrogen at 180 - 210 °C is introduced to remove the moisture in the silicon powder and the catalyst. After drying, the silicon powder flows by gravity into a high-pressure silo. After pressurization, the silicon powder enters the fluidized bed reactor (5) by gravity; 2) STC heating and evaporation system Silicon tetrachloride in the STC buffer tank (6) is heated successively through the STC preheater (8) and the STC heater (9), and then sent to the STC vaporization tower (12) for vaporization. The vaporized STC and the hydrogen raw gas enter a multi-stage heat exchanger in sequence, and finally enter the electric heater (18) for heating; 3) Reaction system The raw gas heated by the electric heater (18) enters the fluidized bed reactor (5) from the bottom. The raw gas is evenly distributed across the cross-section of the reactor through a gas distributor, and then the silicon powder bed layer in the reactor is made to boil and be in a fluidized state. In the reactor, silicon tetrachloride, hydrogen, hydrogen chloride react with the silicon powder. The reacted gas exits from the top discharge port, enters a multi-stage heat exchanger, and then enters the quench tank (19); 4) Quench system In the quench tank (19), it is washed by the condensate from the bottom of the quench tower. The process gas from the quench tank enters the quench tower (20). The gas flows upward through the packing section and is further cooled by the downward flowing reflux liquid. The packing section is used to remove the heavy component chlorosilanes and volatile metal chlorides in the product. The quenched gas exits from the top of the tower and enters the hydrogen preheater (10) and the STC preheater (8) in sequence for cooling. The condensate is further recovered to the reflux tank (22). The cooled gas then enters the air cooler (21). The chlorosilane condensate after air cooling is collected in the quench tower reflux tank (22). Part of the condensate in the tank is refluxed to the quench tower through the quench reflux pump (23), and part of the chlorosilane goes to the rough separation system for rectification and purification. The non-condensable gas enters the exhaust gas recovery system.

2. The high-efficiency energy-saving cold hydrogenation method according to claim 1, characterized in that: A fluidized bed cyclone separator system is provided in the fluidized bed reactor (5). After the reaction, the process gas passes through the silicon bed, flows through the free space of the reactor, and enters the fluidized bed cyclone separator system. The cyclone separator system consists of two parallel-operated cyclone separators and their downcomers. The cyclone separators capture the large particle silicon powder entrained in the gas and return it to the reactor bed layer through the downcomers. The counterweight valve on the downcomer sends the captured silicon powder to the free space of the reactor above the high material level; The process gas exits from the cyclone separator system at the top of the reactor, enters a multi-stage mixed gas heat exchanger to cool down, and then enters the quench tank (19).

3. The high-efficiency energy-saving cold hydrogenation method according to claim 1, characterized in that: The drying step of the silicon powder and the catalyst is to transport the silicon powder and the catalyst to a low-pressure silo and dry the silicon powder in the low-pressure silo; then it is transported to a high-pressure silo and then to the fluidized bed reactor (5) to participate in the reaction.

4. The high-efficiency energy-saving cold hydrogenation method according to claim 1, characterized in that: Hot hydrogen at 160 - 180 °C is added to the bottom of the vaporization tower to reduce the vaporization temperature of silicon tetrachloride.

5. A system for implementing the high-efficiency energy-saving cold hydrogenation method described in claim 1, characterized in that: The system includes a fluidized bed reactor (5) and an STC vaporization tower (12). The output end at the top of the fluidized bed reactor (5) is connected to the quench tank (19) through a multi-stage mixer heat exchanger. The output end at the top of the quench tank (19) is connected to the reflux tank (22) through the quench tower (20).

6. The system according to claim 5, wherein: The system includes a low-pressure silo and a high-pressure silo connected in sequence. The output end of the silicon powder feed is connected to the low-pressure silo, and the high-pressure silo is connected to the fluidized bed reactor.

7. The system according to claim 6, characterized in that: In this system, there is a first-stage low-pressure silo (1) and a connected first-stage high-pressure silo (3), a second-stage low-pressure silo (2) and a connected second-stage high-pressure silo (4). The first-stage high-pressure silo (3) is connected to the upper part of the fluidized bed reactor (5), and the second-stage high-pressure silo (4) is connected to the lower part of the fluidized bed reactor (5).

8. The system according to claim 5, wherein: The system further includes an STC vaporization tower (12). The hydrogen feed end is connected to the STC vaporization tower (12) after passing through a hydrogen preheater (10) and an H2 heater (11) in sequence. STC is connected to the STC vaporization tower (12) after passing through an STC preheater (8) and an STC heater (9). The output end at the top of the STC vaporization tower (12) is connected to an electric heater (18) through a multi-stage mixer heat exchanger, and the electric heater (18) is connected to the bottom of the fluidized bed reactor (5).

9. The system according to claim 5, characterized in that: One output end at the bottom of the reflux drum (22) is connected to the quench tower (20), and the other output end is connected to the distillation system; the output end at the top of the reflux drum (22) is connected to the slurry flash tank (24) through an STC preheater (8), a hydrogen preheater (10), a quench tower (20), and a quench tank (19) in sequence.

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