WO2008120996A1

WO2008120996A1 - Process for production of silicon tetrachloride by reaction of silicon metal and chlorine with internal cooling

Info

Publication number: WO2008120996A1
Application number: PCT/NO2008/000107
Authority: WO
Inventors: Per Bakke; Terje Fuglerud; Patrick Müller; Christian Rosenkilde; Jorild Margrete Svalestuen
Original assignee: Norsk Hydro Asa
Priority date: 2007-04-02
Filing date: 2008-03-17
Publication date: 2008-10-09
Also published as: TW200902443A; NO20071765L

Abstract

Process for producing silicon tetrachloride, SiCI4 by reaction of silicon metal, Si and chlorine, Cl2 in a reactor. The temperature and rate of reaction is controlled by simultaneous, direct injection of chlorine and a cooling medium to the reaction zone of the reactor where the cooling medium is introduced as a liquid at a temperature lower than the reactor temperature and evaporates inside the reactor utilizing its heat of evaporation and its specific heat capacity for cooling. The cooling medium may preferably be silicon tetrachloride or an inert gas.

Description

Process for production of silicon tetrachloride by reaction of silicon metal and chlorine with internal cooling

The present invention relates to a process for producing silicon tetrachloride by reaction of silicon metal and chlorine.

The reaction of silicon metal with chlorine gas is characterized by an extremely high positive reaction enthalpy; at room temperature 687kJ/mol SiCI₄. This high release of energy causes major challenges.

For a high reaction rate, which is needed to get a commercial scale output rate, the temperature can rise to 1200⁰C and above. Under these conditions it is possible that the silicon metal begins to melt leading to an uncontrolled reaction. It is a significant challenge to get rid of this generated energy (heat). Due to the restricted surface of a reactor and thereby small convection areas, an outside cooling wall-system may not be sufficient. Small size reactors (i.e. greater surface) may be used, but in such case the output will be small and hence this is not a commercially interesting option. An internal cooling of the reactor is, because of the corrosive chlorine atmosphere and the high reaction temperature, considered to be dangerous. Another problem is related to the requirements of the construction-materials. Due to the corrosive medium chlorine represents and the high temperature of the process there are hardly materials available which would meet the strong requirements of the reaction of silicon and chlorine.

To overcome these problems, the known SiCI₄-production processes are based mainly on the reaction of HCI with silicon metal. In this process, the reaction enthalpy is only less than one third (ca. 27%) of the direct chlorine process, 185 kJ/mol, and therefore less demanding in terms of temperature control and material selection. This reaction can take place in a fluidized bed reactor or a solid bed reactor. However, this process has the following disadvantages:

• Chlorine has to be converted to HCI in a separate process.

• Beside the primary product SiCI₄ hydrogen -chlorosilanes are formed, in fractions of 10 - 50% depending on the reaction temperature. These products are dangerous due to the risk of self-ignition.

The present invention is represented by a novel process for direct chlorination of silicon metal with chlorine. The basic and novel feature of the invention is the use of a cooling medium, preferably the end product of the process as an energy reduction and reaction modifying agent which enables full control of the reaction temperature and speed of the process.

The invention is characterized by injection of a liquid cooling medium, preferably SiCI₄ directly into the chlorination reaction, as defined in the attached independent claim 1.

Independent claims 2 - 7 define advantageous embodiments of the invention.

The invention will be further described in the following by way of example and with reference to the enclosed Fig. 1.

The present invention relates as initially stated to a process for producing silicon tetrachloride by reaction of silicon metal and chlorine, and as further explained above, the invention is based on the use of a cooling medium, preferably the end product of the process, SiCI₄, as an energy reduction and reaction modifying agent by injecting such medium directly into the reaction zone of the process. The injection can be done with liquid and/or with gaseous SiCI₄. The injection of gaseous SiCI₄ has the advantage of selecting an optimum mixture of the reactant (chlorine) with the cooling component (SiCI₄). The components are preferably mixed before injection into the reactor. The liquid silicon tetrachloride injection has a double-effect: thus the secondary heat obtained by evaporation of SiCI₄ comes in addition to the energy consumed to heat the SiCI₄ to the temperature in the reactor. The liquid SiCI₄-injection may preferably be sprayed with nozzles directly into the reaction zone of reactor in the vicinity of where the chlorine is injected. The amount of SiCI₄ may, based on calculations, be in the range of one to twenty moles per produced mole of SiCI₄, preferably in the range 4 - 12 depending on the reaction rate, the heat loss through the reactor walls and the outlet gas temperature.

The process according to the invention has two important and advantageous effects.

1. The evaporation energy (secondary heat) of the liquid SiCI₄ and the energy to heat the gaseous silicon tetrachloride are used to decrease and thereby enable adjustment of the reaction temperature, (heat of evaporation is 28 kJ/mol; specific heat capacity: 95 J/mol ⁰K). Evaporation and heating to for example 500⁰C require approximately 70 kJ/mole.

2. According to the law of mass action the reaction rate (speed) is also reduced.

Consequently, these two effects make the chlorination process easily controllable.

In the process according to the invention inert gasses such as Ar and He may also be used. The mixing of chlorine with inert gases like Ar and He in this process are, however, far less efficient since the specific heat capacity of Ar and He both are only 21 J/mol ⁰K which is far lower than for SiCI₄. As a result, with the same volume flow of Cl₂ significantly larger volume flows of Ar or He must be used to provide the same effect. This can lead to entrapment of fine particles in the gas flow leading to deposition downstream of the reactor or contamination of the product as a result. The secondary heat (heat of evaporation) effect cannot be easily utilized with Ar or He since the boiling temperatures for Ar and He are -186⁰C and -269⁰C, respectively.

Example:

Fig. 1 shows the volume flow ratio of SiCI₄ to CI₂ on the left vertical axis as a function of volume flow of SiCI₄ to achieve constant temperature 500⁰C in an industrial scale tube type reactor with 1.5m inner diameter and 6m height. On the right vertical axis the corresponding output of SiCI₄ assuming 100% conversion of the injected Cl₂ gas is plotted. Solid lines indicate a situation where liquid SiCI₄ is introduced to the reactor hence the heat of evaporation is utilized for cooling. Dotted lines indicate a situation where SiCI₄ is introduced as a gas at 100⁰C. In both situations, a constant heat loss through the reactor walls of 1kW/m² is assumed. As can be seen from the figure, the volume flow ratio of SiCI₄ to Cl₂ has to be increased if SiCI₄ is introduced as a vapour and not as a liquid. If the outlet gas temperature is chosen to be lower than 500⁰C, the volume flow ratio of SiCI₄ to Cl₂ has to be increased. Likewise, if the heat loss through the walls is lower than 1kW/m² the volume flow ratio of SiCI₄ to Cl₂ has to be increased.

Additional cooling of the reaction can be supplied by external cooling at the reactor walls. With this cooling the amount of SiCI₄-injection can be reduced. With the injection procedure, keeping the reaction temperature low (e. g. < 40O⁰C), internal cooling with heat exchangers (pipe systems) can be an option.

In principle, as a cooling medium that is a gas or liquid at the temperature of injection and a gas at the selected reactor temperature can be used. An additional requirement is that the cooling medium cannot decompose to any product that is considered to contaminate the reactor or the reaction product. For safety reasons the preferred cooling agent is SiCI₄ or similar compounds like Si₂CI₆ or liquid silicone-organic polymers. The latter compounds have the advantage of higher boiling points. (SiCI₄ - boiling point: 57,6⁰C; Si2Cl6-boiling point: 146⁰C; Silicone-organic polymers about 22O⁰C).

For energy saving, after condensation, the amount of SiCI₄ used for cooling can be directly recycled to the chlorination-reactor. The produced SiCI₄ may be transferred to a distillation process for further purification.

The proposed process can take place in a solid-bed, as well as in a fluidized bed- reactor. A fluid bed can offer higher production rates since the Si particles are relatively small and thus provide a large surface to volume ratio. The reaction rate is largely governed by the available surface. With the high reaction rates the need for cooling increases. In a fixed bed larger particles are used, and the particle size can be selected so as to control the reaction rate and temperature. In order to obtain the same production rate of SiCI₄, a fixed bed reactor need to be of a significantly larger size.

Claims

1. Process for producing silicon tetrachloride, SiCI₄ by reaction of silicon metal, Si and chlorine, Cl₂ in a reactor, characterized by simultaneous, direct injection of chlorine and a cooling medium to the reaction zone of the reactor where the cooling medium is introduced as a liquid at a temperature lower than the reactor temperature and evaporates inside the reactor utilizing its heat of evaporation and its specific heat capacity for cooling, whereby the temperature control of the reaction and the reaction rate is provided by controlling the volume flow ratio of the cooling medium to chlorine.

2. Process according to claim 1 , characterized in that the reactor is either a fluid bed or fixed bed reactor.

3. Process according to claims 1 and 2, characterized in that the cooling medium does not decompose at temperatures at or below the reactor temperature to products that contaminate the reactor or the silicon tetrachloride product.

4. Process according to claims 1-3, characterized in that the cooling medium is silicon tetrachloride.

5 Process according to 1 - 3 and 4,

Characterized in that the cooling medium in addition to silicon tetra chloride is an inert gas such as argon or helium.