Process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride

Abstract

This invention discloses a process for the cold hydrogenation of silicon tetrachloride to produce trichlorosilane. Silicon tetrachloride and hydrogen are fed into a bubbling slurry bed reactor for catalytic cold hydrogenation. The bubbling slurry bed reactor uses molten salt as a medium and metal chloride as a catalyst, operating at a temperature of 450–500°C and a reaction pressure of 2.5–4.0 MPaG. This invention features a simple process, meets the requirements for large-scale and continuous production, and boasts low investment, low operating costs, and high safety and reliability. The silicon tetrachloride conversion rate and process flow are simple, and the reactor investment is significantly lower than existing cold hydrogenation reactors, solving the problems of high equipment investment and low reaction conversion rates in existing cold hydrogenation processes.

Description

Technical Field

[0001] This invention belongs to the field of polycrystalline silicon technology, specifically relating to a process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride. Background Art

[0002] In the field of polysilicon production technology, cold hydrogenation is a core technology. Its main principle is that, in the presence of silicon powder, catalyst, and hydrogen, at 2.0–3.0 MPaG and 500–600°C, silicon tetrachloride reacts with hydrogen and silicon powder to produce trichlorosilane and a small amount of dichlorosilane. This process achieves the conversion of silicon tetrachloride to trichlorosilane, significantly reducing the production cost of polysilicon.

[0003] Current cold hydrogenation reactors are large in size, have low space utilization efficiency, and because of the high temperature, the equipment materials need to be nickel-based alloys (NO8120 / NO8810, etc.), making the equipment cost very high.

[0004] Currently, the single-pass conversion rate of cold hydrogenation processes in industrial plants is mostly between 19% and 24%. Some newly built plants have also achieved certain conversion rate improvements due to the use of more distribution grids and improvements to the bottom nozzles; for example, some companies claim to have reached a conversion rate of 28%. However, there is still considerable room for improvement in the overall conversion rate (estimated at 10%, with greater potential at lower temperatures). Improving the conversion rate can significantly reduce the overall energy consumption and investment of cold hydrogenation plants. To improve the conversion rate of silicon tetrachloride, many companies, research institutes, and design institutes have conducted extensive research and practical work, but most of this has focused on the type and number of grids inside the reactor, the type of bottom distributor, and its control. No research has yet been found on changing the type of reactor.

[0005] In current industrial cold hydrogenation reactors, a mixture of silicon tetrachloride and hydrogen enters from the bottom of the reactor and is then injected into the silicon powder bed through gas nozzles. Under the influence of the gas flow, the silicon powder bed undergoes localized fluidization. However, due to the large reactor diameter and low empty tower gas velocity (below the critical fluidization velocity), the silicon powder cannot be effectively fluidized, potentially leading to gas short-circuiting and a low reaction conversion rate. While adding grids within the reactor can help break up large bubbles and improve silicon powder distribution, the effect is limited. Increasing the fluidization gas velocity would cause severe abrasion to the reactor and its internal components due to the high hardness of the silicon powder, so engineers are hesitant to do so. Therefore, cold hydrogenation reactions can only proceed under these low gas velocity and insufficient gas-solid contact conditions, which is the current predicament.

[0006] Regarding catalysts for cold hydrogenation reactions, existing studies have shown that various chlorides have certain catalytic effects, such as ferric chloride and cuprous chloride. Currently, cuprous chloride is mainly added industrially, at a dosage of 1-3‰. This portion of catalyst is discharged along with the slag at the bottom of the reactor. To reduce costs, the dosage is relatively low. The uneven distribution of solids within the reactor also makes it difficult for the catalyst to achieve uniform distribution, thus limiting its effectiveness.

[0007] The problems with existing cold hydrogenation reactors can be summarized as follows:

[0008] (1) The high hardness of silicon powder makes it impossible to increase the fluidization gas velocity, so silicon powder and catalyst cannot be evenly distributed.

[0009] (2) Due to uneven gas-solid contact, silicon tetrachloride has a low single-pass conversion rate.

[0010] (3) Due to the low space utilization efficiency and low single-pass conversion rate of the reactor, the investment in the equipment is large and the energy consumption is high.

[0011] Therefore, how to further improve the cold hydrogenation reaction and reduce the investment and energy consumption of the cold hydrogenation unit is a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0012] The purpose of this invention is to solve the above-mentioned technical problems and provide a process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride, which is simple, has a high product yield, requires little equipment investment, is easy to inspect and maintain, and has low investment and operating costs.

[0013] To achieve the above objectives, the present invention provides a process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride. Silicon tetrachloride and hydrogen are fed into a bubbling slurry bed reactor for catalytic cold hydrogenation reaction. Molten salt is used as the medium and metal chloride as the catalyst in the bubbling slurry bed reactor. The operating temperature is 450-500℃ and the reaction pressure is 2.5-4.0 MPaG.

[0014] Furthermore, the molten salt is KNO3 or NaNO2.

[0015] Furthermore, the metal chloride is cuprous chloride, and the amount of cuprous chloride added is 1 to 3% of the amount of silicon powder added.

[0016] Furthermore, the bubbling slurry bed reactor is equipped with a bottom feed gas distributor and a middle gas redistributor, with a self-rotating impeller installed on the upper part of the gas redistributor.

[0017] Furthermore, the gas redistributor includes 5 to 10 layers of sieve plate gas redistribution disks, with an opening ratio of 60 to 90%.

[0018] Furthermore, a silicon powder inlet is provided at the top of the bubbling slurry bed reactor, and the silicon powder inlet is located below the molten salt layer.

[0019] Furthermore, the self-rotating blade is located below the silicon powder inlet.

[0020] Furthermore, the reaction product gas produced by the catalytic cold hydrogenation reaction of silicon tetrachloride and hydrogen gas is discharged from the top of the bubbling slurry bed reactor and then enters the product gas filter for separation of molten salt droplets and gas. The separated molten salt liquid flows back to the gas phase pipeline at the top of the bubbling slurry bed reactor.

[0021] Furthermore, a slag discharge filtration and molten salt recovery system is installed at the bottom of the bubbling slurry reactor. The slag discharge filtration and molten salt recovery system includes a residue filter, a molten salt tank, a molten salt pump, and a waste residue packaging system connected to the bottom outlet of the bubbling slurry reactor. The discharged material enters the residue filter, where solids are filtered out, and the molten salt liquid enters the molten salt tank. The molten salt recovered in the molten salt tank is returned to the bubbling slurry bed reactor by the molten salt pump. After filtration, high-temperature gas is used to continue to pressurize the molten salt liquid in the residue filter to the molten salt tank. Then, the solid residue in the residue filter is discharged, and the filtered solid residue is packaged by the waste residue packaging system and transported out of the plant area.

[0022] Furthermore, the bubble slurry bed reactor is equipped with multiple density gauges on its sidewalls and a level gauge on its upper part.

[0023] To address the problems existing in the background art, the inventors made the following improvements:

[0024] (1) A slurry bed is used instead of a gas-solid semi-fluidized bed. This is because the density of molten salt is 1.8–2.0 g / cm³. 3 Between these values, the silicon powder particle density is 2.33 g / cm³. 3 The difference between the two is small, and the density of the gas is only 0.02 g / cm³. 3 The density difference between the solid silicon powder and the catalyst is significant. Therefore, in the molten salt liquid phase, a small amount of gas flow is sufficient to keep the silicon powder in suspension, while in the gas-solid semi-fluidized bed, a higher gas flow velocity is required for the fluidization of the silicon powder. This achieves a uniform distribution of both solid silicon powder and catalyst, and also avoids the abrasive effects of high-speed flowing silicon powder on the reactor and its internal components. The more uniform distribution of catalyst and silicon powder creates favorable conditions for improving the reactor's volume utilization rate and reaction conversion rate.

[0025] (2) Based on the bubbling slurry bed reactor, in order to enhance gas distribution and avoid the accumulation of bubbles to form large bubbles, multiple sieve plates and gas redistribution disks are set in the reactor; high porosity is to reduce the ineffective accumulation of solids on the plates.

[0026] (3) A slurry bed is adopted, in which silicon powder is suspended in molten salt liquid. Therefore, there is no need for a gas-solid separation cyclone separator, while traditional cold hydrogenation reactors require a built-in cyclone separator, which results in a large waste of reactor volume. The slurry bed has a higher effective volume utilization rate, which greatly reduces the reactor volume and lowers reactor investment.

[0027] (4) The original gas-solid reaction is transferred to a molten salt liquid. Molten salt may hinder the contact between gas and solid, but the molten salt of many metal chlorides can itself act as a catalyst. In addition, the catalyst exists in liquid form, so the contact between gas and liquid will be more uniform and sufficient. The hydrogen chloride generated by the catalytic hydrogenation of silicon tetrachloride reacts with silicon powder at a relatively fast rate. Although the molten salt liquid hinders the reaction between the two, it will not become the controlling step of the entire cold hydrogenation process.

[0028] (5) By adopting the present invention, the volume of the cold hydrogenation reactor is expected to be reduced by 50-60%, and the single-pass conversion rate of silicon tetrachloride is expected to be increased to 32-35%.

[0029] The present invention has a simple process that meets the requirements of large-scale and continuous production, and has low investment, low operating costs, and is safe and reliable. The silicon tetrachloride conversion rate and process flow are simple, and the reactor investment is much smaller than that of existing cold hydrogenation reactors, solving the problems of high equipment investment and low reaction conversion rate in existing cold hydrogenation processes. Attached Figure Description

[0030] Figure 1 This is a flow chart of the process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to the present invention. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0032] like Figure 1 The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride (catalytic cold hydrogenation process) involves feeding silicon tetrachloride and hydrogen into a bubbling slurry bed reactor R01 for catalytic cold hydrogenation. Molten salt is used as the medium in the bubbling slurry bed reactor, which allows silicon powder to be suspended in the molten salt liquid under relatively low gas velocity agitation. The catalyst in the bubbling slurry bed reactor is a metal chloride, the operating temperature is 450-500℃, and the reaction pressure is 2.5-4.0 MPaG.

[0033] The molten salt used is KNO3 or NaNO2, and the working temperature of the molten salt is 150-600℃, which must meet the requirement of being higher than the melting point of the catalyst. The metal chloride used is cuprous chloride, which has a melting point of 426℃. It exists in liquid form in the molten salt and will not be lost in large quantities during the slag discharge process. Therefore, the addition ratio can be increased, that is, the amount of cuprous chloride added is 1-3% of the amount of silicon powder added, thereby improving the catalytic effect and reducing the reaction temperature.

[0034] Using composite molten salt as a liquid-phase carrier can effectively suspend silicon powder; on the other hand, the catalyst, melted in the molten salt, is more uniformly dispersed, resulting in better catalytic effect. This is because the density of the molten salt is between 1.8 and 2.0 g / cm³. 3 Between these values, the silicon powder particle density is 2.33 g / cm³. 3 The difference between the two is small, and the density of the gas is only 0.02 g / cm³. 3 The density difference between the slurry and silicon powder is significant. Therefore, in a molten salt liquid phase, a small amount of gas flow is sufficient to keep the silicon powder in suspension, while in a gas-solid semi-fluidized bed, a higher gas flow velocity is required for silicon powder fluidization. In conventional gas-solid phase cold hydrogenation reactors, it is difficult to achieve uniform gas-solid dispersion, and the catalyst cannot function effectively. In contrast, a slurry bed can provide sufficient contact area and catalytic effect for the cold hydrogenation reaction.

[0035] The bubble slurry bed reactor R01 is equipped with a bottom feed gas distributor c and a middle gas redistributor. The upper part of the gas redistributor is equipped with a self-rotating blade a. The self-rotating blade a rotates under the action of rising bubbles, thereby playing a certain role in stirring the solid and liquid. The gas redistributor includes 5 to 10 layers of sieve plate gas redistribution disks b, and the opening ratio of the sieve plate gas redistribution disks b is 60 to 90%.

[0036] Silicon powder is added from the top of the bubbling slurry bed reactor and distributed in the molten salt liquid by the agitation of the rotating impeller. A mixture of hydrogen and silicon tetrachloride enters from the bottom of the reactor, passes through a built-in gas distributor for uniform distribution, and then slowly rises. Due to the multi-layered sieve plate gas redistribution disk within the bubbling slurry bed reactor, the rising mixed gas can be distributed multiple times. Simultaneously, the upward movement of the mixed gas also agitates the molten salt liquid. Because the density difference between the molten salt and silicon powder particles is much smaller than the density difference between the mixed gas and silicon powder particles, the silicon powder remains suspended in the molten salt liquid, thus achieving sufficient gas-solid contact. The mixture of silicon tetrachloride and hydrogen comes into contact with the silicon powder and catalyst in the molten salt liquid from bottom to top, undergoing a cold hydrogenation reaction to generate some trichlorosilane.

[0037] Again Figure 1As shown, a silicon powder inlet d is provided at the top of the bubbling slurry bed reactor, and the silicon powder inlet d is located below the molten salt layer to avoid dust generation during feeding. Simultaneously, a self-rotating impeller is located below the silicon powder inlet, and the self-rotating impeller rotates automatically under the action of the rising airflow, dispersing the incoming silicon powder. Furthermore, to avoid fluctuations in the liquid level of the bubbling slurry bed reactor, silicon powder is continuously fed using a rotary feed valve. In this embodiment, a silicon powder feeding system Z01 is used. The silicon powder feeding system Z01 is a conventional facility and is only used to better illustrate the principle of the present invention.

[0038] The reaction product gas is discharged from the top of the bubbling slurry bed reactor (the reaction product gas may contain some molten salt droplets), and then enters the product gas filter FI01 to separate the molten salt droplets from the gas. The separated molten salt liquid flows back to the gas phase pipeline at the top of the bubbling slurry bed reactor, which plays a certain role in flushing.

[0039] The bottom of the bubbling slurry reactor is equipped with a slag discharge filtration and molten salt recovery system. This system includes a residue filter FI02, a molten salt tank V01, a molten salt pump P01, and a waste packaging system Z02, all connected to the bottom outlet of the bubbling slurry reactor. The discharged material mainly contains silica powder that is difficult to react further, as well as deposited impurities such as calcium chloride (melting point 772℃). These solids are discharged along with the molten salt liquid and enter the residue filter FI02, where they are filtered out. The molten salt liquid then enters the molten salt tank V01. The molten salt recovered in the molten salt tank V01 is then returned to the bubbling slurry bed reactor via the molten salt pump P01. After filtration, high-temperature gas is used to further pressurize the molten salt liquid in the residue filter FI02 back into the molten salt tank. The solid residue in the residue filter FI02 is then discharged, and the filtered solid residue is packaged by the waste packaging system Z02 and transported out of the plant.

[0040] Multiple densitometers DI are installed on the sidewalls of the bubbling slurry bed reactor to monitor the solid content in the liquid phase; a level gauge LI is installed at the top of the bubbling slurry bed reactor to monitor the molten salt level.

[0041] Compared to existing gas-solid semi-fluidized beds, the bubbling slurry reactor of this invention eliminates the need for a cyclone separator, thus significantly reducing the reactor volume. Because the silica powder is well dispersed in the slurry bed, a large silica powder storage capacity is no longer required, and the silica powder flow rate is greatly reduced, decreasing reactor wear. The product gas contains almost no solid powder, eliminating the need for multi-stage cyclone filtration and reducing equipment investment. Enhanced catalysis allows for a lower reaction temperature, improving single-pass conversion and reducing overall energy consumption. Simultaneously, it increases the catalyst quantity and lowers the temperature. Traditional gas-solid cold hydrogenation reactors operate at temperatures between 550 and 580°C, while this invention operates at temperatures between 450 and 500°C.

[0042] Example

[0043] The raw materials are silicon powder, silicon tetrachloride and hydrogen. The flow rate of the mixed gas of hydrogen and silicon tetrachloride is 100 t / h, of which hydrogen is 70% mol and silicon tetrachloride is about 30% mol. The temperature of the mixed gas is 580℃ and the pressure is 2.8 MPaG. The industrial silicon feed flow rate is 1.92 t / h and the purity is >99% wt.

[0044] A mixture of hydrogen and silicon tetrachloride enters from the bottom of the bubbling slurry bed reactor. After passing through a built-in gas distributor, it is evenly distributed and then slowly rises at a velocity between 0.3 and 0.5 m / s. The reactor contains multiple layers of sieve plates and gas redistribution disks, allowing the rising gas to be distributed multiple times. Simultaneously, the upward movement of the gas also agitates the molten salt liquid, keeping the silicon powder in suspension. Hydrogen reacts with silicon tetrachloride under the action of a catalyst to produce silicon trichloride and hydrogen chloride, which then reacts with the silicon powder. The overall reaction is the reaction of silicon tetrachloride, silicon, and hydrogen to produce trichlorosilane. A silicon powder inlet is located at the top of the reactor, below the liquid layer to prevent dust generation during feeding. A self-rotating impeller is installed below the inlet, automatically rotating under the influence of the rising airflow to disperse the incoming silicon powder. To avoid fluctuations in the reactor's liquid level, a rotary feed valve is used for continuous silicon powder feeding.

[0045] The bubbling slurry bed reactor of this invention has a diameter of 2000 mm and a cylinder height of 14000 mm. In comparison, the conventional cold hydrogenation process of the same scale has a reactor diameter of 3800 mm and a cylinder height of 18000 mm.

[0046] The product gas is drawn from the top of the reactor and then enters the product gas filter. After filtration, the gas phase goes to the downstream heat exchange equipment. The product gas contains 11.24% mol of trichlorosilane, 20.4% mol of silicon tetrachloride, and the remainder is mainly hydrogen.

[0047] After the reaction system has been running for a period of time, the content of difficult-to-react silicon powder and impurities gradually increases at the bottom of the reactor. At this point, appropriate slag removal is required. The slag removal flow rate should be controlled to be ≤5t / h, and the slag removal time should be determined based on the density change of the liquid in the bubbling slurry bed reactor. The slag enters the filtration system, the molten salt is recovered, and the waste residue is discharged. After cooling, it is packaged and shipped out.

[0048] The process method of this invention has a high silicon tetrachloride conversion rate, a simple process flow, low investment, small footprint, and low operating energy consumption. The reactor investment is much smaller than that of existing cold hydrogenation reactors, which solves the problems of high equipment investment and low reaction conversion rate in existing cold hydrogenation processes.

[0049] Comparison between the examples and the comparative examples

[0050]

Claims

1. A process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride, characterized in that: Silicon tetrachloride and hydrogen are fed into a bubbling slurry bed reactor for catalytic cold hydrogenation. Molten salt is used as the medium and metal chloride as the catalyst in the bubbling slurry bed reactor. The catalyst exists in liquid form in the molten salt. The operating temperature is 450~500℃ and the reaction pressure is 2.5~4.0MPaG.

2. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 1, characterized in that: The molten salt is KNO3 or NaNO2.

3. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 1, characterized in that: The metal chloride used is cuprous chloride, and the amount of cuprous chloride added is 1-3% of the amount of silicon powder added.

4. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 1, characterized in that: The bubbling slurry bed reactor is equipped with a bottom feed gas distributor and a middle gas redistributor, with a self-rotating impeller on the upper part of the gas redistributor.

5. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 4, characterized in that: The gas redistributor includes 5 to 10 layers of sieve plate gas redistribution disks, with an opening ratio of 60 to 90%.

6. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 1, characterized in that: A silicon powder inlet is provided at the top of the bubbling slurry bed reactor, and the silicon powder inlet d is below the molten salt layer.

7. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 4, characterized in that: The self-rotating blade is located below the silicon powder inlet.

8. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 1, characterized in that: The silicon tetrachloride and hydrogen are fed into the bubbling slurry bed reactor for catalytic cold hydrogenation. The resulting product gas is discharged from the top of the bubbling slurry bed reactor and then enters the product gas filter for separation of molten salt droplets from the gas. The separated molten salt liquid flows back to the gas phase pipeline at the top of the bubbling slurry bed reactor.

9. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 1, characterized in that: The bottom of the bubbling slurry bed reactor is equipped with a slag discharge filtration and molten salt recovery system. The slag discharge filtration and molten salt recovery system includes a residue filter, a molten salt tank, a molten salt pump, and a waste residue packaging system connected to the bottom outlet of the bubbling slurry bed reactor. The discharged material enters the residue filter, where solids are filtered out, and the molten salt liquid enters the molten salt tank. The molten salt recovered in the molten salt tank is returned to the bubbling slurry bed reactor by the molten salt pump. After filtration, high-temperature gas is used to continue to pressurize the molten salt liquid in the residue filter to the molten salt tank. Then, the solid residue in the residue filter is discharged, and the filtered solid residue is packaged by the waste residue packaging system and transported out of the plant area.

10. The process for producing trichlorosilane by cold hydrogenation of silicon tetrachloride according to claim 1, characterized in that: Multiple density gauges are installed on the sidewall of the bubbling slurry bed reactor, and a level gauge is installed on the upper part of the bubbling slurry bed reactor.

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