Patent Application
20130078176
The invention relates to a process for reacting silicon tetrachloride with hydrogen to give trichlorosilane in a hydrodechlorination reactor which is operated under pressure and comprises one or more reactor tubes consisting of ceramic material. The invention further relates to the use of such a 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, such that temperatures of 1100° C. and 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 was thus an object of the present invention to provide a process for reacting silicon tetrachloride with hydrogen to give trichiorosilane, which works more efficiently and can achieve a higher conversion with comparable reactor size, i.e. increases the space-time yield of TCS. Furthermore, the process according to the invention should enable a high selectivity for TCS.
The problem has been solved by finding that a mixture of STC and hydrogen can be conducted through a pressurized tubular reactor which may preferably be equipped either with a catalytic wall coating or with a fixed bed catalyst. The combination of the use of a catalyst for improving the reaction kinetics and enhancing the selectivity and a pressurized reaction ensure an economically and ecologically very efficient process regime. By suitable setting of the reaction parameters, such as pressure, residence time, ratio of hydrogen to STC, it is possible to implement a process in which high space-time yields of TCS are obtained with a high selectivity.
The use of a suitable catalyst in conjunction with pressure constitutes a special feature of the process, since it is thus possible to obtain sufficiently high amounts of TCS even at comparatively low temperatures of significantly below 1000° C., preferably below 950° C., without having to accept significant losses as a result of thermal decomposition.
In this context, it has been found that it is possible to use particular ceramic materials for the reaction tubes of the reactor, 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, for example, being subject to a phase conversion which would damage the structure and hence adversely affect the mechanical durability. In this context, it is necessary to use gas-tight tubes. Gas-tightness and inertness can be achieved by means of high-temperature-resistant ceramics which are specified in detail below.
The reactor tube material may be provided with a catalytically active inner coating. As an additional measure, the reactor tube may be filled with an inert bed, in order to optimize the flow dynamics. The bed may consist of the same material as the reactor material. The beds used may be random packings, such as rings, spheres, rods, or other suitable random packings. In a particular embodiment, the random packings may additionally be covered with a catalytically active coating. In this case, it is optionally possible to dispense with the catalytically active inner coating.
The dimensions of the reactor tube and the design of the complete reactor are determined by the availability of the tube geometry, and by the requirements regarding the introduction of the heat required for the reaction regime. It is possible to use either a single reaction tube with the corresponding periphery or a combination of many reactor tubes. In the latter case, it may be advisable to arrange many reactor tubes in a heated chamber, in which the amount of heat is introduced, for example, by means of natural gas burners. In order to avoid a local temperature peak in the reactor tubes, the burners should not be directed onto the tubes. They may, for example, be aligned into the reactor chamber indirectly from above and be distributed over the reactor chamber, as shown by way of example in
The inventive achievement of the abovementioned object is described in detail hereinafter, including different or preferred embodiments.
The invention thus provides a process for reacting silicon tetrachloride with hydrogen to give trichlorosilane in a hydrodechlorination reactor, characterized in that the hydrodechlorination reactor is operated under pressure and comprises one or more reactor tubes consisting of ceramic material.
More particularly, the process according to the invention is a process wherein the reaction is that of a silicon tetrachloride-containing reactant gas and a hydrogen-containing reactant gas in a hydrodechlorination reactor by supply of heat to form a trichlorosilane-containing and HCl-containing product gas, characterized in that the silicon tetrachloride-containing reactant gas and/or the hydrogen-containing reactant gas are conducted as pressurized streams into the pressure-operated hydrodechlorination reactor, and the product gas is conducted out of the hydrodechlorination reactor as a pressurized stream. The product stream may possibly also comprise by-products such as dichlorosilane, monochlorosilane and/or silane. The product stream generally also comprises as yet unconverted reactants, i.e. silicon tetrachloride and water.
The equilibrium reaction in the hydrodechlorination reactor is typically performed at 700° C. to 1000 ° C., preferably at 850° C. to 950° C., and at a pressure in the range from 1 to 10 bar, preferably from 3 to 8 bar, more preferably from 4 to 6 bar.
In all variants of the process according to the invention described, the silicon tetrachloride-containing reactant gas and the hydrogen-containing reactant gas can also be conducted as a combined stream into the pressurized hydrodechlorination reactor.
The ceramic material for the one or more reactor tubes is preferably selected from Al2O3, AIN, Si3N4, SiCN and SiC, more preferably selected from Si-infiltrated SiC, isostatically pressed SiC, isostatically hot-pressed SiC and SiC sintered under ambient pressure (SSiC).
Particularly reactors with SiC-containing reactor tubes are preferred, since they possess particularly good thermal conductivity, which enables homogeneous heat distribution and good heat input for the reaction. It is especially preferred when the one or more reactor tubes consist of SiC sintered under ambient pressure (SSiC).
It is envisaged in accordance with the invention that the silicon tetrachloride-containing reactant gas and/or the hydrogen-containing reactant gas is 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.
The heat for the reaction can be supplied in the hydrodechlorination reactor by means of a heating space in which the one or more reactor tubes are arranged. For example, the heating space can be heated by electrical resistance heating. The heating space may also be a combustion chamber which is operated with combustion gas and combustion air.
According to the invention, it is particularly preferred that the reaction in the hydrodechlorination reactor is catalysed by an inner coating which catalyses the reaction in the one or more reactor tubes. The reaction in the hydrodechlorination reactor can additionally be catalysed by a coating which catalyses the reaction on a fixed bed arranged in the reactor or in the one or more reactor tubes. In the case of use of a catalytically active fixed bed, it is possible if desired to dispense with the catalytically active inner coating. However, it is preferable to include the inner wall of the reactor, since the catalytically usable surface area is thus increased compared to purely supported catalyst systems (for example by a fixed bed).
The catalytically active coating(s), i.e. for the inner wall of the reactor and/or any fixed bed used, consist 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 hydrodechlorination reactor is operated under pressure and comprises one or more reactor tubes which consist of ceramic material. The hydrodechlorination reactor for use 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 shown in
The plant shown in
Reaction in an inventive reactor: The reaction tube used was an SSiC tube with a length of 1100 mm and an internal diameter of 5 mm.
The reactor tube was placed into an electrically heatable tub furnace. First, the tube furnace containing the particular tube was brought to 900° C., in the course of which nitrogen at 3 bar absolute was passed through the reaction tube. After two hours, the nitrogen was replaced by hydrogen. After a further hour in the hydrogen stream, likewise at 3 bar absolute, 36.3 ml/h of silicon tetrachloride were pumped into the reaction tube. The hydrogen stream was adjusted to a molar excess of 4.2 to 1. The reactor discharge was analysed by online gas chromatography, and this was used to calculate the silicon tetrachloride conversion and the molar selectivity to give trichlorosilane. The only secondary component found was dichlorosilane. The hydrogen chloride formed was not excluded from the calculation and not assessed. The results are shown in table 1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2010 000 978.4 | Jan 2010 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP2010/069799 | 12/15/2010 | WO | 00 | 11/21/2012 |
