Patent Application
20130224098
The invention relates to a process for reacting silicon tetrachloride with hydrogen to give trichlorosilane in a modified hydrodechlorination reactor. The invention further relates to the use of such a modified hydrodechlorination reactor as an integral part of a plant for preparing trichlorosilane from metallurgical silicon.
In many industrial processes in silicon chemistry, SiCl4 and HSiCl3 form together. It is therefore necessary to interconvert these two products and hence to satisfy the particular demand for one of the products.
Furthermore, high-purity HSiCl3 is an important feedstock in the production of solar silicon.
In the hydrodechlorination of silicon tetrachloride (STC) to trichlorosilane (TCS), the industrial standard is the use of a thermally controlled process in which the STC is passed together with hydrogen into a graphite-lined reactor, known as the “Siemens furnace”. The graphite rods present in the reactor are operated in the form of resistance heating, and so temperatures of 1100° C. or higher are attained. By virtue of the high temperature and the hydrogen component, the equilibrium position is shifted toward the TCS product. The product mixture is conducted out of the reactor after the reaction and removed in complex processes. The flow through the reactor is continuous, and the inner surfaces of the reactor must consist of graphite, being a corrosion-resistant material. For stabilization, an outer metal shell is used. The outer wall of the reactor has to be cooled in order to very substantially suppress the decomposition reactions which occur at the high temperatures at the hot reactor wall, and which can lead to silicon deposits.
In addition to the disadvantageous decomposition owing to the necessary and uneconomic very high temperature, the regular cleaning of the reactor is also disadvantageous. Owing to the restricted reactor size, a series of independent reactors has to be operated, which is economically likewise disadvantageous. The present technology does not allow operation under pressure in order to achieve a higher space-time yield, in order thus, for example, to reduce the number of reactors.
A further disadvantage is the performance of a purely thermal reaction without a catalyst, which makes the process very inefficient overall.
It is likewise disadvantageous that, in conventional systems, heat exchanger systems and reactors are separated, and so an increased level of losses has to be accepted in the efficiency of these spatially separate systems.
Furthermore, in the case of use of ceramic tubes, the maximum permissible temperature in the sealing region of ceramic to metal is limited to the maximum permissible temperature of sealing materials, such that there is generally only very inefficient utilization of the hot reaction discharge.
It was thus an object of the present invention to provide a process for reacting silicon tetrachloride with hydrogen, which works more efficiently and with which a higher conversion can be achieved with comparable reactor size, which means that the space-time yield of TCS is increased significantly. In addition, the process according to the invention should enable a high selectivity for TCS.
To solve the problem, it has been found that a mixture of STC and hydrogen can be conducted through a pressurized reaction chamber, preferably a tubular reactor, which may preferably be equipped with a catalytic wall coating and/or with a fixed bed catalyst, preference being given to providing a catalytic wall coating, and the use of a fixed bed catalyst being merely optional.
The inventive configuration with a second tube which is within the reaction chamber and through which the STC and H2 reactants flow and are also heated by the reaction chamber enables a comparatively compact design, it being possible to dispense with expensive inert materials or catalytically coated supports which may bind a high proportion of noble metals.
The combination of the use of a catalyst to improve the reaction kinetics and enhance the selectivity, and a pressurized reaction with integrated flow tube for heat exchange, ensures an economically and ecologically very efficient process regime. Suitable adjustment of the reaction parameters, such as pressure, residence time, ratio of hydrogen to STC, can give a process in which high space-time yields of TCS are obtained with a high selectivity.
The utilization of a suitable catalyst in conjunction with pressure constitutes a special feature of the process, since sufficiently high amounts of TCS can thus be obtained at comparatively low temperatures of distinctly below 1000° C., preferably below 950° C., without having to accept significant losses as a result of the thermal decomposition.
It has been found that particular ceramic materials can be used for the reaction chamber and the integrated heat exchanger since they are sufficiently inert and ensure the pressure resistance of the reactor even at high temperatures, for example 1000° C., without the ceramic material passing through a phase conversion, for example, which would damage the structure and thus adversely affect the mechanical durability. In this context, it is necessary to use a gas-tight reaction chamber. Gas-tightness and inertness can be achieved by high-temperature-resistant ceramics which are specified in detail below.
The reaction chamber material and the heat exchanger material can be provided with a catalytically active internal coating. An inert bulk material for improving the flow dynamics can be dispensed with.
The dimensions of the reaction chamber with integrated heat exchanger and the design of the complete hydrodechlorination reactor are determined by the availability of the reaction chamber geometry, and by the requirements regarding the introduction of the heat required for the reaction regime. The reaction chamber may be either a single reaction tube with the corresponding peripheral equipment or a combination of many reactor tubes. In the latter case, the arrangement of many reactor tubes in a heated chamber may be advisable, in which the amount of heat is introduced, for example, by natural gas burners. In order to avoid a local temperature peak on the reactor tubes, the burners should not be directed at the tubes. They can, for example, be aligned indirectly into the reactor space from above and be distributed over the reactor space. To enhance the energy efficiency, the reactor system is connected to a heat recovery system by the integrated heat exchanger.
The inventive solution to the abovementioned problem is described in detail hereinafter, including different or preferred embodiments.
The invention thus provides a process in which a silicon tetrachloride-containing reactant stream and a hydrogen-containing reactant stream are reacted in a hydrodechlorination reactor by supplying heat to form a trichlorosilane-containing and HCl-containing product mixture, characterized in that the process has the following further features: the silicon tetrachloride-containing reactant stream and/or the hydrogen-containing reactant stream are conducted under pressure into the pressurized hydrodechlorination reactor; the reactor comprises at least one flow tube which projects into a reaction chamber and through which one or both of the reactant streams is/are conducted into the reaction chamber; the product mixture is conducted out of the reaction chamber as a pressurized stream; the reaction chamber and optionally the flow tube consist(s) of a ceramic material; the product mixture formed in the reaction chamber is conducted out of the reaction chamber in such a way that the reactant/product stream in the interior of the reaction chamber is conducted at least partly along the outside of the flow tube which projects into the reaction chamber; heat is supplied through a heating jacket or heating space which at least partly surrounds the reaction chamber; and the reaction chamber comprises, downstream of the region of the reaction chamber heated by the heating jacket or heating space, an integrated heat exchanger which cools the heated product mixture, the heat removed being used to preheat the silicon tetrachloride-containing reactant stream and/or the hydrogen-containing reactant stream.
The equilibrium reaction in the hydrodechlorination reactor is performed typically at 700° C. to 1000° C., preferably at 850° C. to 950° C., and at a pressure in the range between 1 and 10 bar, preferably between 3 and 8 bar, more preferably between 4 and 6 bar.
In all described variants of the process according to the invention, the hydrodechlorination reactor may comprise a single flow tube through which both of the reactant streams are conducted together, or the reactor may comprise more than one flow tube through which both of the reactant streams are optionally conducted together into the reaction chamber in each of the flow tubes, or the different reactant streams can be conducted separately into the reaction chamber, each in different flow tubes.
The ceramic material for the reaction chamber, the integrated heat exchanger tubes and optionally the flow tube is preferably selected from Al2O3, AlN, Si3N4, SiCN and SiC, more preferably selected from Si-infiltrated SiC, isostatically pressed SiC, hot isostatically pressed SiC and SiC sintered at ambient pressure (SSiC).
In particular, reactors with an SiC-containing reaction chamber (for example one or more reactor tubes), riser tube(s) and precisely such integrated heat exchanger tubes are preferred, since they possess particularly good thermal conductivity, and enable homogeneous heat distribution and good heat input for the reaction, and also good thermal shock stability. It is particularly preferred when the reaction chamber, the riser tube(s) and the integrated heat exchanger tubes consist(s) of SiC sintered at ambient pressure (SSiC).
It is envisaged in accordance with the invention that the silicon tetrachloride-containing reactant stream and/or the hydrogen-containing reactant stream is/are preferably conducted into the hydrodechlorination reactor with a pressure in the range from 1 to 10 bar, preferably in the range from 3 to 8 bar, more preferably in the range from 4 to 6 bar, and with a temperature in the range from 150° C. to 900° C., preferably in the range from 300° C. to 800° C., more preferably in the range from 500° C. to 700° C.
In the case that the silicon tetrachloride-containing reactant stream is conducted into the hydrodechlorination reactor separately from the hydrogen-containing reactant stream, the silicon tetrachloride-containing reactant stream may be liquid or gaseous depending on the pressure applied and the temperature, while the hydrogen-containing reactant stream is typically gaseous. For instance, the liquid silicon tetrachloride-containing reactant stream can be supplied to the reactor chamber via a flow tube. However, the liquid silicon tetrachloride-containing reactant stream can also first be converted to the gas phase, preferably by means of heat exchangers, especially by utilizing the waste heat present, and conducted into the reactor chamber via a flow tube. In addition, the hydrogen-containing reactant stream can be passed into the reactor chamber via a separate flow tube. However, the hydrogen-containing reactant stream can also be supplied to a silicon tetrachloride-containing reactant stream which is preferably already present in gaseous form, and the mixture can be passed into the reactor chamber via a flow tube. In the case that both reactant streams are conducted together into the hydrodechlorination reactor, the combined reactant stream is preferably gaseous.
Heat can be supplied for the reaction in the hydrodechlorination reactor through a heating jacket which is heated by electrical resistance heating, or by means of a heating space. The heating space may also be a combustion chamber which is operated with combustion gas and combustion air.
It is particularly preferred in accordance with the invention that the reaction in the hydrodechlorination reactor is catalysed by an internal coating which catalyses the reaction in the reaction chamber (for example of the reactor tube(s)) and/or by a coating which catalyses the reaction in a fixed bed arranged within the reactor chamber.
The catalytically active coating(s), i.e. for the inner wall of the reactor and/or any fixed bed used, consist(s) preferably of a composition which comprises at least one active component selected from the metals Ti, Zr, Hf, Ni, Pd, Pt, Mo, W, Nb, Ta, Ba, Sr, Ca, Mg, Ru, Rh, Ir and combinations thereof, and silicide compounds thereof, especially Pt, Pt/Pd, Pt/Rh and Pt/Ir.
The inner wall of the reactor and/or any fixed bed used may be provided with the catalytically active coating as follows: by providing a suspension, also referred to hereinafter as coating material or paste, comprising a) at least one active component selected from the metals Ti, Zr, Hf, Ni, Pd, Pt, Mo, W, Nb, Ta, Ba, Sr, Ca, Mg, Ru, Rh, Ir and combinations thereof, and silicide compounds thereof, b) at least one suspension medium, and optionally c) at least one auxiliary component, especially for stabilizing the suspension, for improving the storage stability of the suspension, for improving the adhesion of the suspension to the surface to be coated and/or for improving the application of the suspension to the surface to be coated; by applying the suspension to the inner wall of the one or more reactor tubes and, optionally, by applying the suspension to the surface of random packings of any fixed bed provided; by drying the suspension applied; and by heat-treating the applied and dried suspension at a temperature in the range from 500° C. to 1500° C. under inert gas or hydrogen. The heat-treated random packings can then be introduced into the one or more reactor tubes. The heat treatment and optionally also the preceding drying may, however, also be effected with already introduced random packings.
The suspension media used in component b) of the inventive suspension, i.e. coating material or paste, especially those suspension media with binding character (also referred to as binders for short), may advantageously be thermoplastic polymeric acrylate resins as used in the paints and coatings industry. Examples include polymethyl acrylate, polyethyl acrylate, polypropyl methacrylate or polybutyl acrylate. These are systems customary on the market, for example those obtainable under the Degalan® brand name from Evonik Industries.
Optionally, the further components used, i.e. in the sense of component c), may advantageously be one or more auxiliaries or auxiliary components.
For instance, the auxiliary component c) used may optionally be solvent or diluent. Suitable with preference are organic solvents, especially aromatic solvents or diluents, such as toluene, xylenes, and also ketones, aldehydes, esters, alcohols or mixtures of at least two of the aforementioned solvents or diluents.
A stabilization of the suspension can—if required—advantageously be achieved by inorganic or organic rheology additives. The preferred inorganic rheology additives as component c) include, for example, kieselguhr, bentonites, smectites and attapulgites, synthetic sheet silicates, fumed silica or precipitated silica. The organic rheology additives or auxiliary components c) preferably include castor oil and derivatives thereof, such as polyamide-modified castor oil, polyolefin or polyolefin-modified polyamide, and polyamide and derivatives thereof, as sold, for example, under the Luvotix® brand name, and also mixed systems composed of inorganic and organic rheology additives.
In order to achieve an advantageous adhesion, the auxiliary components c) used may also be suitable adhesion promoters from the group of the silanes or siloxanes. Examples for this purpose include—though not exclusively—dimethyl-, diethyl-, dipropyl-, dibutyl-, diphenylpolysiloxane or mixed systems thereof, for example phenylethyl- or phenylbutylsiloxanes or other mixed systems, and mixtures thereof.
The inventive coating material or the paste may be obtained in a comparatively simple and economically viable manner, for example, by mixing, stirring or kneading the feedstocks (cf. components a), b) and optionally c)) in corresponding common apparatus known per se to those skilled in the art. In addition, reference is made to the present inventive examples.
The invention further provides for the use of a hydrodechlorination reactor as an integral part of a plant for preparing trichlorosilane from metallurgical silicon, characterized in that the reactor is operated under pressure; the reactor comprises at least one flow tube which projects into a reaction chamber for the entering reactant streams; the reaction chamber and optionally the flow tube consist(s) of a ceramic material; the reactant/product stream is conducted within the reaction chamber such that the reactant/product stream is conducted at least partly along the outside of the flow tube which projects into the reaction chamber; heat is supplied through a heating jacket or heating space which at least partly surrounds the reaction chamber; and the reaction chamber comprises, downstream of the region of the reaction chamber heated by the heating jacket or heating space, an integrated heat exchanger for cooling the heated product mixture. The hydrodechlorination reactor to be used in accordance with the invention may be as described above.
The plant for preparing trichlorosilane, in which the hydrodechlorination reactor can preferably be used, comprises:
The hydrodechlorination reactor 3 shown in
The plant shown in
The reaction tube used was a tube of SSiC with a length of 1400 mm and an internal diameter of 16 mm. The reaction tube was equipped on the outside with an electrical heating jacket. The temperature measurement showed a constant temperature of 900° C. over a tube length of 400 mm. This region was considered to be the reaction zone. The reaction tube was covered with a Pt-containing catalyst layer. The reaction tube was charged with rings of SSiC, which had a diameter of 9 mm and a height of 9 mm. For catalyst forming, the reactor tube was brought to a temperature of 900° C., in the course of which nitrogen was passed through the reaction tube at 3 bar absolute. After two hours, the nitrogen was replaced by hydrogen. After a further hour in the hydrogen stream, likewise at 4 bar absolute, silicon tetrachloride was pumped into the reaction tube. The amount (“STC feed flow rate”) was varied in comparative examples CE1 to CE3 according to Table 1. The hydrogen flow rate was set to a molar excess of 4 to 1. The reactor output was analysed by online gas chromatography and this was used to calculate the silicon tetrachloride conversion and the molar selectivity for trichlorosilane. The results (“STC conversion” and “TCS in the product”) are reported in Table 1 and additionally shown graphically in
The reaction tube used was a tube of SSiC with a length of 1400 mm and an internal diameter of 16 mm. The reaction tube was equipped on the outside with an electrical heating jacket. The temperature measurement showed a constant temperature of 900° C. over a tube length of 400 mm. This region was considered to be the reaction zone. The reaction tube was covered with a Pt-containing catalyst layer. A second tube of SSiC which was conducted into the reaction tube had an external diameter of 5 mm and a wall thickness of 1.5 mm. This tube was uncoated. Through this inner tube, the STC and the hydrogen were introduced from the bottom. The reactant mixture flowed upward within the inner tube and was heated. Through the opening of the inner tube, it then flowed into the reaction zone. The product mixture was conducted out of the reaction tube at the bottom. For catalyst forming, the reactor tube was brought to a temperature of 900° C., in the course of which nitrogen was passed through the reaction tube at 3 bar absolute. After two hours, the nitrogen was replaced by hydrogen. After a further hour in the hydrogen stream, likewise at 4 bar absolute, silicon tetrachloride was pumped into the reaction tube. The amount (“STC feed flow rate”) was varied in examples 1 to 3 according to Table 1. The hydrogen flow rate was set to a molar excess of 4 to 1. The reactor output was analysed by online gas chromatography and this was used to calculate the silicon tetrachloride conversion and the molar selectivity for trichlorosilane. The results (“STC conversion” and “TCS in the product”) are reported in Table 1 and additionally shown graphically in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 039 267.7 | Aug 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2011/061911 | 7/13/2011 | WO | 00 | 5/16/2013 |
