Method for separating exhaust gas mixture

Abstract

The invention relates to a method for separating an off-gas mixture containing HCl, H2 and chlorosilane, comprising the following steps. -contacting the exhaust gas mixture with an absorbent in an absorption column at a temperature of-70 to-10 DEG C and a pressure of 0.5 to 2 MPa, where HCl and chlorosilane are absorbed, forming a supported absorbent; -desorbing the gas stream from the loaded absorbent in a desorption column at a temperature of 50 to 150 DEG C and / or at a reduced pressure of 0.1 to 1 MPa relative to step a), whereby after step a) and before step b) in at least one degassing unit at an increased temperature relative to step a) and / or at a reduced pressure relative to step a), the gas stream is desorbed from the loaded absorbent at a temperature of 50 to 150 DEG C and / or at a reduced pressure relative to step a). The supported adsorbent is decompressed at the top end of the degassing unit and hydrogen is removed from the supported adsorbent by removing the resulting gas phase, in which the desorption gas stream in step b) has a hydrogen fraction < = 10 mol%, a hydrogen chloride fraction > = 89 mol% and a chlorosilane fraction < = 1 mol%. The invention further relates to a device for separating a mixture containing HCl, H2 and chlorosilane.

Description

Technical Field

[0001] The invention relates to a method for separating an exhaust gas mixture containing hydrogen chloride, hydrogen and chlorosilanes. Background Art

[0002] Multicrystalline silicon (polysilicon) is the starting material for producing single-crystalline silicon using the Czochralski method or the float zone method. Single-crystalline silicon can be used in the semiconductor industry in the form of wafers to manufacture electronic components (chips). Furthermore, polycrystalline silicon is required for the production of multicrystalline silicon using bulk casting. Polycrystalline silicon can be used in the manufacture of solar cells.

[0003] Polysilicon can be produced by the Siemens process, a chemical vapor deposition method. This involves heating a thin silicon filament rod by directly passing electricity through a reactor (Siemens reactor) and introducing a reaction gas containing a silicon-containing component and hydrogen (H2). The silicon-containing component used can contain monosilane (SiH4) or a general composition of SiH n X 4-n Halosilanes (n = 0, 1, 2, 3; X = Cl, Br, I). These typically include chlorosilanes or chlorosilane mixtures, more particularly trichlorosilane (SiHCl3, TCS). The design of a typical Siemens reactor is described, for example, in US 2009 / 0136408 A1.

[0004] Another mode of production is the deposition of polycrystalline silicon on heated granular silicon particles in a fluidized bed reactor (granular process), as described, for example, in US 2013 / 0295385 A1.

[0005] Regardless of the production method, the starting material used comprises a silicon-containing component, typically TCS. As described, for example, in WO 2016 / 198264 A1, there are three methods by which TCS can be produced.

[0006] (1)SiCl4+H2-->SiHCl3+HCl+byproducts

[0007] (2)Si+3SiCl4+2H2-->4SiHCl3+byproduct

[0008] (3)Si+3HCl-->SiHCl3+H2+byproduct

[0009] The by-products produced may include additional chlorosilanes, such as monochlorosilane (H3SiCl), dichlorosilane (DCS, H2SiCl2), silicon tetrachloride (STC, SiCl4), as well as disilazane and oligomeric silanes. In addition, impurities such as hydrocarbons, organochlorosilanes, and metal chlorides may be present as by-products.

[0010] Each of the three processes produces an off-gas mixture which, after condensation of the chlorosilanes, contains not only hydrogen chloride (HCl) and H 2 but also traces of uncondensed chlorosilanes.

[0011] Furthermore, during the deposition of polycrystalline silicon by the Siemens process or the pellet process, an exhaust gas mixture containing chlorosilanes, HCl and H 2 is formed.

[0012] For environmental and economic reasons, the waste gases are usually treated and H2 and HCl are recovered. They can be fed back into the integrated polysilicon production.

[0013] In addition to recovering H2 by adsorption on activated carbon or other solids, as disclosed, for example, in US 2013 / 0011558 A1, when the HCl fraction is relatively high, a combination of adsorption and absorption with desorption can be used to separate and purify H2 and HCl. This type of combination is described, for example, in US 2012 / 198998 A1. In the case of a purely absorptive separation (comprising absorption and desorption), as disclosed, for example, in CN 102431972 A and CN 102614741 A, HCl is adsorbed in an absorbent at low temperature and high pressure. The HCl is then removed again in a column known as a desorption column by increasing the temperature and / or reducing the pressure.

[0014] A disadvantage of absorptive separation is that a significant amount of H2 is desorbed from the absorbent along with the HCl. Consequently, during the desorption step, the H2 is also transferred to the gas phase and entrained with the HCl. In downstream process steps, the H2 entrained in the HCl can have adverse effects, for example, by promoting the formation of by-products, reducing production capacity, and requiring higher compressor output.

[0015] CN 201567231U discloses separating HCl and H2 by condensing HCl after desorption. However, a disadvantage of this method is that after desorption, the exhaust gas must be condensed using energy, and the removed liquid HCl must be evaporated again using additional energy before being further used in the integrated production process. Summary of the Invention

[0016] The disadvantages described lead to the object on which the present invention is based, namely to provide a more efficient method for purifying exhaust gases from polysilicon production.

[0017] This object is achieved by a method for separating an off-gas mixture containing HCl, H2 and chlorosilanes, the method comprising the following steps:

[0018] a) contacting the waste gas mixture with an absorbent in an absorption tower at a temperature of -70 to -10°C and a pressure of 0.5 to 2 MPa, wherein HCl and chlorosilanes are absorbed to form a loaded absorbent, and a first gaseous phase containing hydrogen is discharged at the top of the absorption tower;

[0019] b) desorbing the gas stream from the loaded absorbent in a desorption column at a temperature of 50 to 150° C. and / or a pressure of 0.1 to 1 MPa which is reduced compared to step a).

[0020] Here, after step a) and before step b), the loaded absorbent is depressurized in at least one degassing unit at the top of the degassing unit at an elevated temperature and / or a reduced pressure relative to step a). Further H₂ is removed from the loaded absorbent by removing the resulting second gas phase at the top. The desorbed gas stream in step b) has an H₂ fraction of ≤10 mol %, an HCl fraction of ≥89 mol %, and a chlorosilane fraction of ≤1 mol %. These fractions add up to 100 mol %.

[0021] In the composition of the gas phase and the gas stream, impurities that may have been introduced from previous process steps (e.g., N2, CO2, CH4) are neglected. They generally constitute no more than 0 to 6 mol % of the off-gas mixture and are insignificant to the operability of the invention.

[0022] The desorbed gas stream preferably has an H2 fraction of ≤8.2 mol %, an HCl fraction of ≥91.0 mol % and a chlorosilane fraction of ≤0.8 mol %.

[0023] More preferably, the desorbed gas stream has an H2 fraction of ≤7.3 mol %, an HCl fraction of ≥92.0 mol % and a chlorosilane fraction of ≤0.7 mol %.

[0024] The desorbed gas stream more particularly has an H2 fraction of ≤6.5 mol %, an HCl fraction of ≥93.0 mol % and a chlorosilane fraction of ≤0.5 mol %.

[0025] The fractions of the components in the desorbed gas stream can be determined using a gas chromatograph (GC) with helium as the carrier gas. As an alternative analytical technique, the measurement can be performed using Raman spectroscopy. The H2 fraction in the HCl can be analyzed, for example, using a GC-TCD (thermal conductivity detector). In the determination, it is generally assumed that HCl represents the remainder of 100%.

[0026] For the contacting in step a), the off-gas mixture can be supplied via a conduit to an absorption column containing an absorbent. HCl, chlorosilanes, and very small amounts of H₂ are also absorbed at least partially, preferably to the saturation point of the absorbent. The absorbent is preferably a chlorosilane selected from tetrachlorosilane, TCS, DCS, and mixtures thereof.

[0027] The exhaust gas mixture is usually contacted with the absorbent at a temperature of -60 to -20° C., preferably -50 to -30° C. The pressure here is preferably 1.2 to 1.8 MPa, more preferably 1.3 to 1.6 MPa.

[0028] The temperature at which the exhaust gas mixture comes into contact with the absorbent is determined as the inlet temperature of the absorbent in the liquid phase. The pressure is usually measured above the absorbent in the first gas phase. The temperature and pressure in the absorption column are each measured at the top of the column.

[0029] The top of the column or generally the top of the container / equipment is usually the upper part, more specifically the hood or end piece (cover). For example, the pressure can be measured by a pressure gauge in the exhaust gas line at the top (through which the first gas phase is discharged). The temperature can be measured by a thermal sensor in the inlet line at the top.

[0030] In order to depressurize the loaded absorbent, it can be supplied from the absorption column to at least one degassing unit via a conduit. The depressurization associated with the H2-containing second gaseous phase formed in this case thus comprises at least one intermediate step between steps a) and b).

[0031] The loaded absorbent can also be depressurized in two or more degassing units arranged in series, wherein the degassing is carried out in each degassing unit and the downstream degassing unit has a reduced pressure and / or increased temperature relative to the upstream degassing unit. In other words, there is a pressure and / or temperature gradient between the first and last degassing unit (pressure reduction, temperature increase, in each case within the stated ranges).

[0032] Particularly preferably, there is only one decompression step between steps a) and b).

[0033] For desorption of the gas stream in step b), the loaded absorbent can be supplied from a degassing unit (optionally from the last of two or more such units) via a pipeline to a desorption column. The temperature is measured in the liquid phase (e.g., with the aid of a temperature sensor) in the still (at the low point) of the desorption column, and the pressure is measured in the gas phase at the top of the desorption column (e.g., with the aid of a pressure gauge in the exhaust gas line). The pressure in the desorption column is 0.1 to 1 MPa.

[0034] For the sequential process steps in the absorption column (step a)), the degassing unit (intermediate step) and the desorption column (step b)), provision is made for a temperature gradient (increase) and / or a pressure gradient (decrease) within the stated pressure and temperature ranges between the individual steps.

[0035] It has been found that by at least a single-stage degassing in at least one degassing unit downstream of the absorption column, entrained (absorbed) H 2 can be removed with high selectivity from the absorbent loaded with HCl and chlorosilanes. As a result, the H 2 fraction of the gas stream desorbed in step b) can be significantly reduced, and thus the profitability of the process can be increased.

[0036] It has further been shown that multi-stage decompression generally offers little or no advantage over single-stage decompression.

[0037] The pressure at the top of the degassing unit is preferably reduced by 0.1 to 1.3 MPa, more preferably by 0.3 to 1.2 MPa, more particularly by 0.5 to 1.1 MPa (pressure difference) relative to step a).

[0038] The pressure at the top end of the degassing unit is preferably from 0.1 to 1.4 MPa, more preferably from 0.2 to 1.2 MPa, more particularly from 0.3 to 1.0 MPa.

[0039] The pressure at the top of the degassing unit can be established by utilizing the relative height difference between the degassing unit and the absorption tower through the hydrostatic pressure drop across the liquid column. Therefore, the degassing unit can be located at an elevated position compared to the absorption tower. The pressure drop caused by the height difference can achieve the desired pressure differential. This approach can also save on pressure relief equipment.

[0040] For example, the inlet opening intended for supplying the loaded absorbent may be arranged laterally on the degassing unit 5 to 25 m, preferably 7 to 20 m, more preferably 10 to 18 m above the outlet opening in the absorption column intended for the loaded absorbent.

[0041] The pressure can also be built up by means of a valve-controlled volume expansion at the outlet opening of the absorption column.

[0042] In general, for efficient separation of HCl and H2, both pressure and temperature should be kept as low as possible within specified limits. At the same time, profitability requires direct incorporation of the degassed H2 into relevant integrated systems (e.g., polysilicon production) without additional compression / cooling.

[0043] The temperature of the loaded absorbent in the degassing unit is preferably from -70 to -10°C, more preferably from -60 to -20°C, more particularly from -50 to -30°C.

[0044] The average hydrodynamic residence time τ of the loaded absorbent in the degassing unit is generally from 1 to 40 seconds, preferably from 2 to 30 seconds, more preferably from 5 to 20 seconds, more particularly from 6 to 15 seconds.

[0045] The average hydrodynamic residence time τ of the second gas phase in the degassing unit is generally from 10 to 1150 seconds, preferably from 25 to 850 seconds, more preferably from 40 to 250 seconds, more particularly from 45 to 150 seconds.

[0046] Furthermore, the absorbent in the absorption tower may have a τ of 100 to 3500 seconds, preferably 200 to 3000 seconds, more preferably 250 to 2500 seconds.

[0047] τ is calculated by

[0048] in

[0049] V R : The volume of fluid in the corresponding equipment (e.g. the volume in a degassing unit or a column filled with loaded absorbent or gas mixture), in [m 3 ].

[0050] V: Volume flow rate of fluid (e.g. absorbent or exhaust gas mixture) under operating conditions (p, T), in [m 3 / s].

[0051] The τ of the absorbent in the absorption column is obtained via a specified liquid volume and the volume flow rate of the absorbent. Similarly, τ in the gas phase is a function of the gas filling volume and the gas volume flow rate.

[0052] Typical equipment volumes for absorbers and desorbers range from 5 to 50 m 3 , typically the portion filled with absorbent is 10% to 80% of the volume. This therefore gives the corresponding fluid volumes for the gas phase and the liquid phase.

[0053] The volume of the degassing unit is preferably 0.01 to 5 m 3 Here, the absorbent filling fraction is typically 10% to 70%.

[0054] As a result of the removal of the second gas phase at the top of the degassing unit, the volume flow discharged is preferably >30 Nm 3 / h, more preferably >75Nm 3 / h, more particularly >125Nm 3 / h.

[0055] The second gas phase removed from the degassing unit generally has a hydrogen fraction of >60%, more preferably >70%, more particularly >80%.

[0056] The degassing unit can be a separator, preferably a gravity separator. Particularly preferably, it is a vertical gravity separator.

[0057] The volume of the apparatus is a function of the preferred residence time. In a preferred embodiment, the volume here is as low as possible given the residence time.

[0058] For example, a typical vertical gravity separator may have a diameter of 300 to 1000 mm, preferably 400 to 900 mm, more preferably 500 to 800 mm.

[0059] A typical total height of a vertical gravity separator may be in the range of 900 to 3000 mm, preferably in the range of 1400 to 2700 mm, in particular in the range of 1800 to 2500 mm.

[0060] The ratio of the diameter to the total height of the vertical gravity separator may be from 0.13 to 0.43, preferably from 0.17 to 0.39, more preferably from 0.21 to 0.35.

[0061] The height of the lateral inlet opening of the vertical gravity separator may be at a height corresponding to 26% to 87%, preferably 43% to 78%, more preferably 56% to 74% of its total height. The starting point for measurement is the base of the separator.

[0062] The height of the vertical gravity separator above its lateral inlet opening preferably corresponds to its diameter.

[0063] The liquid level (absorbent) in the degassing unit can be determined and regulated, for example, via guided radar measurement or via measurement through a connecting pipe. In addition, the liquid level can be determined via the pressure in the degassing unit.

[0064] In a preferred embodiment, the diameter of the outlet of the degassing unit is selected such that the dimensionless Froude number Fr is in the range of 0.2-4.0, more preferably 0.5-3.0, more preferably 0.8-2.0.

[0065] Fr by Calculate, where

[0066] v L : Flow rate of the loaded absorbent at the outlet [m / s] (e.g. 0.4 to 3.0 m / s)

[0067] g: acceleration due to gravity (9.81 m / s 2 )

[0068] D: Diameter of the discharge pipe (e.g. 0.01 to 0.7 m)

[0069] A typical ratio of the level of loaded absorbent in the degassing unit to the diameter of the outlet is from 0.7 to 10.0, preferably from 0.9 to 9.0, more preferably from 1.0 to 8.0.

[0070] The maximum average gas velocity of the gas phase in the vertical gravity separator may be ≤ 0.1 m / s, preferably ≤ 0.075 m / s, more preferably ≤ 0.05 m / s.

[0071] The maximum average flow velocity of the loaded absorbent in the vertical gravity separator may be ≤ 0.2 m / s, preferably ≤ 0.15 m / s, more preferably ≤ 0.13 m / s.

[0072] The exhaust gas mixture treated by the method of the invention can be generated in an integrated plant for the production of polysilicon. The exhaust gas mixture is more particularly formed in the production of TCS, in which case the process is preferably a production process from silicon, HCl and optionally H2.

[0073] The method of the present invention allows the H2 content of HCl to be reduced by ≥50%, preferably by ≥65%, and more preferably by ≥80%. The removed H2 is typically recovered and can be returned to the integrated plant for use in the production of polysilicon. For example, it can be used to produce finely divided silicon dioxide.

[0074] Thus, it is possible to increase the production capacity of TCS (by reducing the H2 fraction and the associated increase in HCl production at a constant reactor residence time). This results in an increased space-time yield. Alternatively, for the same space-time yield, energy costs can be reduced because the amount of gas in circulation can be reduced, thereby reducing the compressor output (reduced H2 fraction).

[0075] In a preferred embodiment, the second gas phase from the degassing unit is recombined with the off-gas mixture before said mixture is supplied to the absorption column.

[0076] In another embodiment, at least a portion of the loaded absorbent is conveyed via a degassing unit, while another portion can be supplied directly to the desorber.

[0077] After desorption, heat exchange is typically performed between the cold, loaded absorbent and the heated absorbent to minimize the system's energy requirements at high throughputs. Furthermore, before entering the absorption column, the exhaust gas mixture is typically cooled in multiple stages, for example, using condensers and / or countercurrent components, and possibly other heat transfer paths, to achieve a low temperature for absorption in the most energy-efficient manner possible. The separated gas streams from the degassing unit and the desorption column are also subjected to multiple heat exchanges to reduce energy losses. In principle, under economically relevant operating conditions, HCl and H₂ are not completely separated.

[0078] Another aspect of the present invention relates to an apparatus for separating an off-gas mixture containing HCl, H2 and chlorosilanes. The apparatus is particularly suitable for carrying out the method of the present invention. The apparatus comprises the following components:

[0079] an absorption tower for contacting the off-gas mixture with an absorbent at a temperature of -70 to -10°C and a pressure of 0.5 to 2 MPa, wherein the absorption tower comprises an outlet for discharging a first gaseous phase containing hydrogen,

[0080] at least one degassing unit downstream of the absorption column for decompressing the absorbent loaded with the off-gas mixture in the absorption column at a temperature elevated relative to the temperature in the absorption column and / or at a pressure reduced relative to the pressure in the absorption column, wherein the degassing unit comprises an outlet for discharging the second gaseous phase containing hydrogen;

[0081] a desorption column downstream of the degassing unit for desorbing the gas stream from the loaded absorbent at a temperature of 50 to 150° C. and / or at a reduced pressure in the range of 0.1 to 1 MPa relative to the absorption column, wherein the desorption column comprises an outlet for discharging the desorbed gas stream.

[0082] There may be two or more degassing units arranged in series between the absorption column and the desorption column. However, more preferably, only one degassing unit is used.

[0083] The degassing unit is preferably arranged separately from the absorption column and the desorption column. This more particularly means that the degassing unit is essentially connected to the two columns only via a pipeline.

[0084] The degassing unit can thus be a component which, where appropriate, can also be retroactively inserted into existing waste gas separation plants without requiring fundamental modifications to the entire plant.

[0085] In a preferred embodiment, a demister is provided on top of the degassing unit to prevent liquid from being carried into the gas phase.

[0086] For further details on the individual components, please refer to the method description and the illustrations below. BRIEF DESCRIPTION OF THE DRAWINGS

[0087] Figure 1 The mechanism of the method of the present invention is shown.

[0088] Figure 2 The mechanism of a variant of the method of the invention is shown.

[0089] Figure 3 A degassing vessel is shown. DETAILED DESCRIPTION

[0090] Figure 1The mechanism of the method of the present invention is shown. In this method, the waste gas from polysilicon production, indicated by arrow 1, is passed through a condensation and heat exchange section 6 into an absorption tower 4, where it comes into contact with an absorbent (e.g., a mixture of STC and TCS). The waste gas contains, for example, 7 mol % HCl, 92 mol % H₂, and 1 mol % of uncondensed chlorosilanes (silane, monochlorosilane, DCS, TSC, and STC). In the condensation and heat exchange section 6, it is compressed to 0.5 to 2.0 MPa and cooled to -70 to -10°C.

[0091] The first gaseous phase which is withdrawn from the absorption column at the top thereof after absorption and is marked by arrow 2 generally comprises at least 95 mol % of H 2 and not more than 5 mol % of HCl.

[0092] The loaded absorbent is fed to a separate degassing unit 12, where the pressure is reduced due to the height difference shown, and degassing (second gas phase) is carried out accordingly. The resulting second gas phase, which typically has an H2 fraction of at least 60 mol%, is discharged at the top of the degassing unit 12 (arrow 11) and can be fed back to the integrated plant for the production of polysilicon or for the production of finely divided silicon dioxide.

[0093] The loaded absorbent is then fed to the desorption column 5, where it first passes through one (or more) heat exchangers (or countercurrent elements) 7 and then undergoes a further increase in temperature in the desorption column 5 to 60 to 150° C. by means of a heater. This temperature increase results in the desorption of a gas stream which is discharged at the top of the desorption column 5 and cooled to about 5 to 35° C. by means of multi-stage heat exchange (via countercurrent elements and / or condenser 10). This gas stream (arrow 3) typically contains at least 89 mol % of HCl, not more than 10 mol % of H 2 and not more than 1 mol % of chlorosilanes (typically silane, monochlorosilane, DCS, TCS, STC).

[0094] Via a heat exchanger 7, the unloaded absorbent withdrawn from the desorption column 5 is cooled (countercurrent principle) and returned to the absorption column 4. A pump 8 is provided for this return.

[0095] Figure 2 The mechanism of the different embodiments of the method of the present invention is shown, Figure 1 Compared to the configuration of the first degassing unit 12, this mechanism has an additional degassing unit 13. This unit 13 is downstream of the first degassing unit 12. Degassing takes place in both degassing units 12, 13; in the additional degassing unit 13, the pressure is generally reduced compared to the degassing unit 12, while the temperature level is preferably kept constant.

[0096] Figure 3A vertical gravity separator 20 is shown as a specific embodiment of a degassing unit. The gravity separator 20 has a lateral inlet opening (feed) 22 for the loaded absorbent and a discharge opening 24. At the top of the gravity separator 20 there is an outlet 26 for the second gas phase. Line 21 indicates the absorbent filling level. The following dimensions are marked with corresponding arrows:

[0097] h t : Total height (wall thickness is usually negligible)

[0098] d: diameter

[0099] d A : Inner diameter of discharge port 24

[0100] h l : Absorbent filling level (liquid level)

[0101] h u : Lower than the height of the feed port 22

[0102] h o : Higher than the height of the feed port 22

[0103] Example

[0104] All examples were carried out under constant exhaust gas composition:

[0105] HCl: 7.5 mol%

[0106] H2: 89 mol%

[0107] Chlorosilanes (silane, monochlorosilane, DCS and STC): 3.5 mol%

[0108] The absorbent used was a mixture of STC (69 mol %) and TCS (31 mol %).

[0109] For all examples, the conditions set at the absorber and desorber were as follows:

[0110] Pressure (absorption tower): 1.57MPa

[0111] Temperature (absorption tower): -50℃

[0112] Pressure (desorption tower): 0.36MPa

[0113] Temperature (desorption tower): 90°C

[0114] The gas composition can be measured via gas chromatography with a thermal conductivity detector or via RAMAN measurement. The temperature is determined by means of a thermal sensor, the pressure using a pressure sensor (eg a capacitive or piezoresistive sensor).

[0115] The degassing unit (if used) had the same structure in all examples.

[0116] Invention Example 1

[0117] Single-stage devolatilization was carried out in a vertical gravity separator at -40 °C (see Figure 1 The mechanism of the absorption column is described in detail below (measured in the liquid phase in the absorbent feed). The pressure is regulated by positioning the separator above the absorption column via hydrostatic pressure reduction. The pressure is measured using a pressure sensor in the gas outlet at the top of the separator: 0.6 MPa. The average hydrodynamic residence time of the loaded absorbent in the vertical gravity separator is 8 seconds. The maximum average flow velocity of the loaded absorbent is 0.12 m / s. The maximum average gas velocity of the gas phase is 0.015 m / s. 146 Nm2 of H2 with a H2 fraction of 84.0 mol% are discharged from the separator. 3 / h of gas flow (second gas phase). After desorption of the loaded absorbent in the desorption column, an H2 content of 1.77 mol% was obtained in the returned HCl (reduction of H2 slip>82%).

[0118] Comparative Example 1

[0119] The apparatus essentially corresponds to that of Inventive Example 1, but without the intervening gravity separator. The absorption and desorption parameters correspond to those of Inventive Example 1. The H2 content of the HCl not separately degassed is 12.3 mol %.

[0120] Invention Example 2

[0121] The vertical gravity separator was operated at 0.4 MPa and -50°C. The pressure was measured in the gas discharge line by means of a pressure sensor at the top of the degassing vessel, while the temperature was measured in the feed line for the loaded absorbent by means of a temperature sensor in the liquid phase. The average hydrodynamic residence time of the loaded absorbent in the vertical gravity separator was 8 seconds. 183 Nm was removed. 3 / h of gas flow, discharged at a H2 fraction of 83.8 mol% ( Figure 1 Arrow 11).

[0122] Comparative Example 2

[0123] The vertical gravity separator was operated at 0.4 MPa and 0°C. The average hydrodynamic residence time of the loaded absorbent in the vertical gravity separator was 8 seconds. The pressure was measured in the gas stream discharge line by means of a pressure sensor at the top of the degassing vessel, while the temperature was measured in the feed line for the loaded absorbent by means of a temperature sensor in the liquid phase. 202 Nm removed 3 / h of gas flow. In this case, the H2 fraction of the discharged gas phase is only 46.2 mol%. The degassing here is 202Nm 3 / h, despite having a lower H2 fraction, which means that large amounts of HCl are undesirably sent back.

[0124] It is obvious that under non-inventive temperature conditions (gravity separator operated at 0°C), a much higher HCl fraction (or a much lower H2 fraction) is obtained in the exiting gas stream, so that the goal of maximum efficient hydrogen removal is not achieved (inadequate selectivity).

[0125] Invention Example 3

[0126] The vertical gravity separator was operated at 0.4 MPa and -40°C. The average hydrodynamic residence time of the loaded absorbent in the vertical gravity separator was 8 seconds. The pressure and temperature were measured as described.

[0127] 186 Nm2 of H2 with a H2 fraction of 77.3 mol% was discharged from the gravity separator. 3 / h total airflow.

[0128] Comparative Example 3

[0129] The vertical gravity separator was operated at 1.5 MPa and -40°C. The average hydrodynamic residence time of the loaded absorbent in the vertical gravity separator was 8 seconds. The pressure and temperature were measured as described.

[0130] 23 Nm2 of H2 fraction with 92.4 mol % was discharged from the vertical gravity separator. 3 / h total airflow.

[0131] Obviously, under non-inventive pressure conditions (gravity separator operated at 1.5 MPa), only a low hydrogen volume flow rate (calculated as gas volume flow rate multiplied by hydrogen fraction) can be removed, which means that the goal of maximum effective H2 removal cannot be achieved (the volume flow rate is too low).

[0132] The examples according to the present invention clearly show that by reducing the pressure at low temperature, a large amount of H 2 can be selectively discharged, thereby greatly reducing the H 2 slip in HCl.

Claims

1. A method for separating a waste gas mixture containing hydrogen chloride, hydrogen and chlorosilanes, comprising the following steps: a) contacting the waste gas mixture with an absorbent in an absorption tower at a temperature of -70 to -10°C and a pressure of 0.5 to 2 MPa, wherein the hydrogen chloride and the chlorosilanes are absorbed to form a loaded absorbent, and a first gas phase containing hydrogen is discharged; b) desorbing the gas stream from the loaded absorbent in a desorption column at a temperature of 50 to 150° C. and / or a pressure of 0.1 to 1 MPa which is reduced with respect to step a), characterised in that, after step a) and before step b), the loaded absorbent is depressurised in at least one degassing unit at the top of the degassing unit at an elevated temperature and / or a reduced pressure relative to step a), and hydrogen is removed from the loaded absorbent by removing the resulting second gaseous phase, wherein the desorbed gas stream in step b) has a hydrogen fraction of ≤10 mol %, a hydrogen chloride fraction of ≥89 mol % and a chlorosilane fraction of ≤1 mol %.

2. The method according to claim 1, characterized in that The desorbed gas stream has a hydrogen fraction of ≤8.2 mol %, a hydrogen chloride fraction of ≥91.0 mol % and a chlorosilane fraction of ≤0.8 mol %.

3. The method according to claim 1 or 2, characterized in that The loaded absorbent is depressurized in two or more sequentially arranged degassing units, wherein the depressurization is performed in each degassing unit and the downstream degassing unit has an increased temperature and / or a reduced pressure relative to the upstream degassing unit.

4. The method according to any one of the preceding claims, characterized in that The pressure at the top of the degassing unit is reduced by 0.1 to 1.3 MPa, preferably by 0.3 to 1.2 MPa, more preferably by 0.5 to 1.1 MPa, relative to step a).

5. The method according to any one of the preceding claims, characterized in that The pressure at the top end of the degassing unit is 0.1 to 1.4 MPa, preferably 0.2 to 1.2 MPa, more preferably 0.3 to 1.0 MPa.

6. The method according to any one of the preceding claims, characterized in that The pressure at the top of the degassing unit is established by means of the height difference between the degassing unit and the absorption column by means of the hydrostatic pressure drop via the liquid column.

7. The method according to any one of the preceding claims, characterized in that The temperature of the loaded absorbent in the degassing unit is from -70 to -10°C, preferably from -60 to -20°C, more preferably from -50 to -30°C.

8. The method according to any one of the preceding claims, characterized in that The average hydrodynamic residence time of the loaded absorbent in the degassing unit is from 1 to 40 seconds, preferably from 2 to 30 seconds, more preferably from 5 to 20 seconds, more particularly from 6 to 15 seconds.

9. The method according to any one of the preceding claims, characterized in that By removing the second gas phase at the top of the degassing unit, >30 Nm 3 / h, preferably >75Nm 3 / h, more preferably >125Nm 3 / h volume flow rate.

10. The method according to any one of the preceding claims, characterized in that The second gas phase has a hydrogen fraction of >60%, preferably >70%, more preferably >80%.

11. The method according to any one of the preceding claims, characterized in that The degassing unit is a separator, preferably a gravity separator, more preferably a vertical gravity separator.

12. The method according to claim 11, characterized in that The maximum average gas velocity of the gas phase in the vertical gravity separator is ≤0.1 m / s, preferably ≤0.075 m / s, more preferably ≤0.05 m / s.

13. The method according to claim 11 or 12, characterized in that The maximum average flow velocity of the loaded absorbent in the vertical gravity separator is ≤ 0.2 m / s, preferably ≤ 0.15 m / s, more preferably ≤ 0.13 m / s.

14. An apparatus for separating an offgas mixture containing hydrogen chloride, hydrogen and chlorosilanes, more particularly for carrying out the process according to at least one of claims 1 to 13, comprising an absorption tower for contacting the exhaust gas mixture with an absorbent at a temperature of -70 to -10°C and a pressure of 0.5 to 2 MPa, wherein the absorption tower comprises an outlet for discharging a first gaseous phase containing hydrogen; at least one degassing unit downstream of the absorption tower for decompressing the absorbent loaded with the off-gas mixture in the absorption tower at a temperature elevated relative to the temperature in the absorption tower and / or at a pressure reduced relative to the pressure in the absorption tower, wherein the degassing unit comprises an outlet for discharging a second gaseous phase containing hydrogen; a desorption tower downstream of the degassing unit for desorbing a gas stream from the loaded absorbent at a temperature of 50 to 150° C. and / or at a reduced pressure in the range of 0.1 to 1 MPa relative to the absorption tower, wherein the desorption tower comprises an outlet for discharging the desorbed gas stream.

15. The device according to claim 14, characterized in that Two or more degassing units are arranged in sequence between the absorption tower and the desorption tower.

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