PROCESS FOR HYDROGENATING SILICON TETRACHLORIDE

Abstract

The invention relates to a process bit hydrogenating silicon tetrachloride in a reactor, wherein a reactant gas containing hydrogen and silicon tetrachloride is heated to a temperature between 850 C. and 1600 C. by means of at least one heating element, which comprises a graphite surface, wherein the temperature of the heating element is between 850 C. and 1600 C. The process is characterized in that a nitrogen compound is added to the reactant gas in a substance amount fraction of 0.1 to 70% based on hydrogen.

Claims

1. A method for hydrogenating silicon tetrachloride in a reactor, comprising: heating a feed gas containing hydrogen and silicon tetrachloride to a temperature in the range from 850 C. to 1600 C. using at least one heating element comprising a graphite surface, the temperature of the heating element being in the range 850 C. to 1600 C., wherein nitrogen is added in a molar proportion of from 0.1 to 10% based on hydrogen to the feed gas.

2. The method of claim 1, wherein nitrogen is added in a molar proportion of from 0.5 to 5%, preferably from 1 to 4%, more preferably from 1.5 to 3%, based on hydrogen.

3. The method of claim 1, wherein the temperature of the at least one heating element is from 850 to 1500 C., preferably from 900 to 1400 C., more preferably from 900 to 1300 C.

4. The method of claim 1, wherein the temperature of the feed gas is from 850 to 1500 C., preferably from 900 to 1400 C., more preferably from 900 to 1300 C.

5. The method of claim 1, wherein the method is carried out at a pressure in the range from 2 to 18 bar, preferably from 3 to 17 bar, more preferably from 4 to 16 bar, in particular from 5 to 15 bar.

6. The method of claim 1, wherein silicon tetrachloride and hydrogen are present in a molar ratio of from 1:1.3 to 1:2.5, preferably from 1:1.5 to 1:2.3, more preferably from 1:1.6 to 1:2.2.

7. The method of claim 1, wherein the graphite surface is coated with silicon carbide.

8. The method of claim 1, wherein a boron compound is additionally present in the feed gas.

9. The method of claim 8, wherein the boron compound is selected from the group consisting of diborane, higher boranes, boron-halogen compounds, borosilyl compounds and mixtures thereof.

10. The method of claim 9, wherein the boron compound is added in a molar proportion of from 0.1 ppmv to 100 ppmv based on the amount of feed gas.

11. The method of claim 1, further comprising installing at least one heat exchanger preferably comprising a graphite surface upstream and/or downstream of the at least one heating element.

12. The method of claim 1, wherein nitrogen is introduced together with hydrogen into the reactor.

13. The method of claim 1, wherein nitrogen is introduced together with silicon tetrachloride into the reactor.

14. The method of claim 1, wherein the hydrogen used includes nitrogen.

Description

[0053] FIG. 1 shows an apparatus for carrying out the process of the invention

[0054] FIG. 2 shows the effect of N.sub.2 on the electrical resistance of heating elements during the process of the invention

[0055] FIG. 3 shows the effect of a boron compound on the electrical resistance of heating elements during the process of the invention

[0056] FIG. 4 shows the economic advantages of the process of the invention

[0057] FIG. 5, 6 in each case show a mass spectrum of a heating element surface.

[0058] The process of the invention can, for example, be carried out in an apparatus as shown schematically in FIG. 1.

[0059] The apparatus 1 comprises a reactor 2 for hydrogenating STC, a condensation unit 3 for separating off TCS and unreacted STC and also a separation unit 4 for separating hydrogen (H.sub.2), nitrogen (N.sub.2) and hydrogen chloride (HCl). The reactor 2 comprises a resistance heating element 5 and a heat exchanger 6 and also a feed conduit 11 for STC, a feed conduit 8 for N.sub.2 and H.sub.2 and a discharge conduit 9 for product gas. Both the resistance heating element 5 and the heat exchanger 6 have a graphite surface. The reactor 2 can have a construction as described in U.S. Pat. No. 4,536,642 A.

[0060] The separation unit 4 connected by a conduit 7 to the condensation unit 3 forms, together with the feed conduit 8 and the discharge conduit 9, a recycle process 10 for working up product gas taken from the reactor 2. The product gas comprises unreacted H.sub.2 and STC in addition to TCS and HCl.

[0061] The arrows denoted by an asterisk (*) represent possible feed conduits for the nitrogen compound, which is by way of example N.sub.2.

[0062] To carry out the process, STC is fed via the feed conduit 11 into the reactor 2 and H.sub.2 is fed via the feed conduit 8. The N.sub.2 can in principle be introduced via any of the feed conduits denoted by an asterisk (*). However, in the present example an N.sub.2-containing hydrogen stream which is, in particular, obtained at another place in an integrated plant and is utilized in the recycle process 10 is employed. Additional process safety devices and regulating devices, for example for precise metering, are avoided thereby and the costs are reduced. A further preferred method of introducing N.sub.2 is the introduction of N.sub.2 dissolved in STC. The solubility of N.sub.2 in STC is determined by Henry's law.

[0063] Both the heat exchanger 6 and the resistance heating element 5 are arranged so that they are in direct contact with the gases STC, H.sub.2 and N.sub.2 fed in. The graphite surfaces of the resistance heating element 5 have a temperature in the range from 850 to 1600 C., and those of the heat exchanger 6 have a temperature in the range from 850 to 1600 C. The N.sub.2 leads to passivation of the hot graphite surfaces. Methanation of the surfaces is prevented thereby and the wear experienced by heat exchanger 6 and resistance heating element 5 is slowed.

[0064] To achieve the passivating effect, the way in which the nitrogen compound, in particular the N.sub.2, is introduced into the process is in principle inconsequential.

[0065] The product gas is discharged from the recycle process 10 via the discharge conduit 9. Firstly, the product TCS is largely separated off from unreacted STC in the product stream in the condensation unit 3, for example by condensation. The STC obtained can be fed to the reactor 2, for example via the feed conduit 11. The TCS obtained can, for example, be employed directly for the production of polysilicon.

[0066] The gas leaving the condensation unit 3 via the conduit 7 is fed to the separation unit 4. Here, HCl is separated off, for example by means of one or more absorption and desorption steps, with H.sub.2 and N.sub.2 being returned to the recycle process 10. A method for separating off HCl is disclosed in DE 31 39 705 A1.

[0067] The recycle process 10 results in the advantage that the amount of N.sub.2 added decreases only slightly during the course of the process. It has been found that significant losses of N.sub.2 occur exclusively via discharges during the process.

[0068] Samples of reactor internals based on carbon which have been examined using different analytical methods (infrared spectroscopy IR (attenuated total reflection (ATR) on diamond), scanning electron microscopy SEM, energy-dispersive X-ray spectroscopy EDX, X-ray diffraction XRD, elemental analysis) have confirmed this balancing relationship. Analyses of the offgas during the process of the invention have revealed no nitrogen species in the offgas from the reactor.

[0069] However, examination of carbon-based component surfaces does detect isolated nitrogen species. No nitrogen species could be detected by means of the abovementioned infrared-spectroscopic and X-ray-diffractometric measurement methods since these nitrogen species are apparently present in amounts below the respective detection limits. However, the existence of nitrogen could be detected in a few examined areas of the component surface by means of a combination of SEM/EDX analysis. Owing to the local limitation of this analytical method, it was, however, not possible to draw any conclusions in respect of the total component surface.

[0070] Traces of various nitrogen species could likewise be detected by means of time-of-flight secondary ion mass spectrometry (ToF-SIMS) (cf. Example 3). However, these nitrogen species are detectable only on the component and not in the product.

[0071] Overall it can be presumed from the detection of nitrogen or nitrogen species on the component surface that the nitrogen compound, in particular N.sub.2, introduced is, due to formation of surface-specific bods, active in inhibiting the methanation reaction.

EXAMPLES

[0072] The examples were carried out in an apparatus as shown in FIG. 1. A feed gas containing 33 mol % of STC and 67 mol % of H.sub.2 was used.

[0073] N.sub.2 was added in an amount of from 0.1 to 5 mol % to the H.sub.2 and the effects on the graphite surfaces (inhibition of the methanation reaction) was observed over a prolonged period of time.

Example 1

[0074] The following operating parameters were selected: [0075] Molar ratio of STC:H.sub.2=1:1.9 [0076] Reaction temperature=975 C. [0077] Pressure=12 bar

[0078] The arrangement of the resistance heating elements having a graphite surface was as has been described in DE 10 2011 077 967 A1. The temperature of the heating elements could be regulated or controlled either in pairs or in defined groups of the elements.

[0079] The electric resistance (R) can be calculated for the respective heating element from the electric current (I) and the electric potential (U) according to the general formula

[00001]$R=\frac{U}{I}.$

The resistance served as indicator of damage to the heating element.

[0080] Is a significant increase in the resistance is found in a short time, damage to the heating element is also to be expected. Such damage leads to premature failure of the heating elements.

[0081] This method of observing the damage to heating elements has been described in DE 10 2012 218 741 A1.

[0082] FIG. 2 shows the course of the resistance of a heating element as a function of the time on stream for seven different N.sub.2 concentrations (0 mol %, 0.1 mol %, 0.5 mol %, 1.0 mol %, 1.5 mol %, 3 mol %, 5 mol % in each case based on H.sub.2). The addition of N.sub.2 was carried out after a time on stream of 200 hours.

[0083] It can clearly be seen that the resistance curve flattens after addition of N.sub.2. In other words, the resistance of the heating element increases to a lesser extent after the addition of N.sub.2 to the feed gas than would have been the case in the absence of N.sub.2. The increase in the resistance is reduced. This trend continues up to an N.sub.2 concentration of 3 mol %. Above a concentration of >3 mol %, no further reduction in the resistance increase could be determined.

[0084] FIG. 3 shows the course of the resistance of a heating element as a function of the time on stream after a joint addition of diborane and N.sub.2. Diborane was added to the feed gas at a constant concentration of 4 ppmv based on the total volume flow after a time on stream of 200 hours. Immediately afterward, a reduction in the increase in the resistance was observed. After a time of 300 hours, N.sub.2 was added in a concentration of 1.5 mol % based on H.sub.2. A further reduction in the resistance increase and thus a reduction in damage to the heating elements can clearly be seen. The positive effect can thus also be realized by addition of a boron compound and a nitrogen compound.

Example 2

[0085] The inhibition of the methanation reaction by addition of N.sub.2 was tested over a period of one year on two apparatuses A and B for hydrogenation of STC. The two apparatuses each had a construction as shown in FIG. 1. Both apparatuses were operated under the same conditions. The only difference was the N.sub.2 concentration in the feed gas.

[0086] The two apparatuses each comprised ten reactors having resistance heating elements, heat exchangers and further construction components composed of graphite which were all in contact with feed gas and/or product gas.

[0087] N.sub.2 was introduced into the system via a nitrogen-containing stream of hydrogen. The nitrogen-containing stream of hydrogen was introduced into the recycle process 10 at the separation unit 4 to set the N.sub.2 concentration in the feed gas (cf. FIG. 1).

[0088] The main operating parameters of the apparatus A and B were: [0089] Molar ratio of STC:H.sub.2=1:1.9 [0090] Reaction temperature=975 C. [0091] Pressure=12 bar

[0092] Nitrogen concentration based on H.sub.2: [0093] Apparatus A: 0.2 mol % [0094] Apparatus B: 1.5 mol %

[0095] It was found that over the course of a year significantly less damage to the elements having a graphite surface occurred in the case of apparatus B. Consequently, apparatus B was able to be operated significantly longer without replacement of the elements having a graphite surface. The annual production of TCS could be increased. The maintenance costs could be considerably reduced.

[0096] The presence of N.sub.2 had no adverse effects, for example on the yield or the specific energy consumption of the apparatuses.

[0097] A comparison of the two apparatuses A and B in respect of time on stream and maintenance is shown in FIG. 4.

Example 3

[0098] The inhibition of the methanation reaction by addition of diborane and N.sub.2 was tested over a period of about one year on one of the apparatuses as shown in FIG. 1.

[0099] The following operating parameters were selected: [0100] Molar ratio of STC:H.sub.2=1:1.9 [0101] Reaction temperature=975 C. [0102] Pressure=12 bar

[0103] Diborane was added to the feed gas in a constant concentration of 4 ppmv based on the total volume flow. N.sub.2 was added in a concentration of 1.5 mol % based on H.sub.2. Diborane and N.sub.2 were introduced together at the commencement of operation. The time on stream of the reactor after the combined introduction was 350 days.

[0104] After the apparatus was shut down, the resistance heating elements were taken out and prepared for analysis by means of ToF-SIMS (preparation of polished sections of the surface). Two mass spectra were recorded, and these are shown in FIGS. 5 and 6.

[0105] The yield or intensity (y-axis of the spectra) of the ablated or detached ions is significantly dependent on the total matrix which basically has to be calibrated beforehand for quantitative information. This calibration was not carried out in the measurement. For this reason, only qualitative conclusions can be drawn.

[0106] The ablated ions can have either a positive charge or a negative charge and are shown as a function of charge in separate spectra. To obtain an overall view of the sample surface, both the spectrum of the cations and also that of the anions have to be looked at.

[0107] The x axis of the spectra shows the unit m/z (mass per elementary charge).

[0108] FIG. 5 shows an extract of the anion spectrum in an x axis range of 25-80 m/z.

[0109] FIG. 6 shows an extract of the cation spectrum in an x axis range of 10-35 m/z.

[0110] Small traces of nitrogen-containing species can be seen in addition to clear chlorine, silicon and oxygen species both in the anion spectrum and in the cation spectrum of the ToF-SIMS analysis. As examples, mention may be made here of the (CN.sup.) and (BN.sup.) fragments at 25 and 26 m/z, respectively (cf. FIG. 5), and the (BNH.sub.2.sup.+) and (NH.sub.4.sup.+) fragments at 26 and 18 m/z, respectively (cf. FIG. 6).

[0111] Overall, it can be concluded from the detection of these nitrogen and boron species on the component surface that the introduced nitrogen and/or boron compound does not display inert behavior but is, due to formation of surface-specific bonds, actively involved in the inhibition of the methanation reaction.

[0112] In the case of an apparatus which was operated under comparable conditions but without combined addition of N.sub.2 and diborane, replacement of the heating elements had to be carried out after only 160 days.