Method for separating an exhaust-gas mixture containing hydrogen chloride, hydrogen and chlorosilanes

Abstract

The invention relates to a method for separating an exhaust-gas mixture containing HCL, H2 and chlorosilanes, the method comprising the following steps: - bringing the exhaust-gas mixture into contact with an absorbent in an absorption column at a temperature of -70 to -10°C and a pressure of 0.5 to 2 MPa, wherein HCL and chlorosilanes are absorbed, forming a laden absorbent; - desorbing a gas stream from the laden absorbent in a desorption column at a temperature of 50 to 150°C and / or at a pressure which is reduced by 0.1 to 1 MPa in relation to step a), wherein, after step a) and before step b), the laden absorbent is expanded in at least one degassing unit at a temperature which is increased in relation to step a) and / or at a pressure which is reduced, at a head end of the degassing unit, in relation to step a), and separation of a resulting gas phase has the result that hydrogen is separated from the laden absorbent, wherein the desorbed gas stream in step b) has a hydrogen content of ≤ 10 mol.%, a hydrogen chloride content of ≥ 89 mol.% and a chlorosilane content of ≤ 1 mol.%. The invention furthermore relates to an apparatus for separating a mixture containing HCL, H2 and chlorosilanes.

Description

[0001] Process for separating an exhaust gas mixture containing hydrogen chloride, hydrogen, and chlorosilanes

[0002] The invention relates to a method for separating a

[0003] Exhaust gas mixture containing hydrogen chloride, hydrogen and chlorosilanes.

[0004] Polycrystalline silicon (polysilicon) is the starting material in the production of single-crystal silicon using crucible pulling (Czochralski process) or float zone melting (float zone process). Single-crystal silicon can be used in the form of wafers in the semiconductor industry for the production of electronic components (chips). Polysilicon is also required for the production of multicrystalline silicon using the ingot casting process. Multicrystalline silicon can be used in the production of solar cells.

[0005] Polysilicon can be produced using the Siemens process – a chemical vapor deposition process. In a reactor (Siemens reactor), thin silicon filament rods are heated by direct current flow, and a reaction gas containing a silicon-containing component and hydrogen (H2) is introduced. The silicon-containing component can be monosilane (SiH^2) or a halosilane with the general composition SiH. n X4n (n = 0, 1, 2, 3; X = Cl, Br, I) can be used. This is usually a chlorosilane or chlorosilane mixture, especially trichlorosilane (SiHCl, TCS). The design of a typical Siemens reactor is described, for example, in US 2009 / 0136408 Al.

[0006] Another method of production is the deposition of polysilicon on heated silicon granules in a fluidized-bed reactor (granule process), as described, for example, in US 2013 / 0295385 A1. Regardless of the production method, silicon-containing components, usually TCS, are used as the starting material. TCS can be produced using three processes, as described, for example, in WO 2016 / 198264 A1.

[0007] (1) SiC14 + H2 --> SiHCls + HCl + by-products

[0008] (2) Si + 3SiC14 + 2H2 --> 4S1HC13 + by-products

[0009] (3) Si + 3HC1 --> SiHCls + H2 + by-products

[0010] Other chlorosilanes may be produced as by-products, such as monochlorosilane (HsSiCl), dichlorosilane (DCS, H2SiCl2), silicon tetrachloride (STC, SiCl-i), and di- and oligosilanes. Furthermore, impurities such as hydrocarbons, organochlorosilanes, and metal chlorides may occur as by-products.

[0011] In each of the three processes, an exhaust gas mixture is produced which, after condensation of the chlorosilanes, contains both hydrogen chloride (HCl) and H2 as well as traces of uncondensed chlorosilanes.

[0012] Furthermore, the deposition of polysilicon using the Siemens or granulate process produces an exhaust gas mixture containing chlorosilanes, HCl and H2.

[0013] For ecological and economic reasons, the exhaust gas is generally treated and H2 and HCl are recovered. These can be recycled into the polysilicon production network.

[0014] In addition to the recovery of H2 by adsorption on activated carbon or other solids, as disclosed, for example, in US 2013 / 0011558 A1, a combination of adsorption and absorption with desorption can be used to separate and purify H2 and HCl at higher HCl contents. Such a combination is described, for example, in US 2012 / 198998 A1. In purely absorptive separation (comprising absorption and desorption), which is disclosed, for example, in CN 102431972 A and CN102614741 A, HCl is absorbed in an absorbent at low temperatures and elevated pressure. HCl is then expelled by increasing the temperature and / or reducing the pressure in a so-called desorption column.

[0015] A disadvantage of absorptive separation is that, in addition to HCl, a significant amount of H2 is desorbed with the absorbent. Thus, in the desorption step, H2 is also transferred into the gas phase and entrained with the HCl. In downstream process steps, the H2 entrained into the HCl can have adverse effects, for example, by promoting the formation of byproducts, reducing capacity, and requiring higher compressor power.

[0016] CN 201567231 U discloses a separation of HCl and H2 after desorption by condensing the HCl. However, this procedure has the disadvantage that the exhaust gas must be condensed after desorption, which requires energy, and the separated, liquid HCl must be re-evaporated, which requires energy, before further use in the production network.

[0017] The described disadvantages gave rise to the object underlying the invention, namely the provision of a more efficient process for cleaning exhaust gases from polysilicon production.

[0018] This object is achieved by a process for separating an exhaust gas mixture containing HCl, H2 and chlorosilanes, comprising the steps of: a) bringing the exhaust gas mixture into contact with an absorbent in an absorption column 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 hydrogen-containing first gas phase is removed at the top end of the absorption column; b) desorption of a 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).

[0019] The loaded adsorbent is expanded after step a) and before step b) in at least one outgassing unit at a temperature higher than in step a) and / or at a pressure at the top end of the outgassing unit that is lower than in step a). By separating a resulting second gas phase at the top end, further H2 is separated from the loaded adsorbent. The desorbed gas stream in step b) then has an H2 content of 10 mol%, an HCl content of 89 mol%, and a chlorosilane content of

[0020] 1 mol%. The proportions add up to 100 mol%.

[0021] The compositions of the gas phases and streams do not take into account any impurities that may have been introduced from previous process steps (e.g., N2, CO2, CH4). These typically make up no more than 0 to 6 mol% of the exhaust gas mixture and are irrelevant to the feasibility of the invention.

[0022] Preferably, the desorbed gas stream has a H2 content of

[0023] 8.2 mol%, an HCl content of > 91.0 mol%, and a chlorosilane content of ≤ 0.8 mol%. Particularly preferably, the desorbed gas stream has an H2 content of < 7.3 mol%, an HCl content of > 92.0 mol%, and a chlorosilane content of ≤ 0.7 mol%.

[0024] In particular, the desorbed gas stream has an H2 content of < 6.5 mol%, an HCl content of 93.0 mol% and a chlorosilane content of < 0.5 mol%.

[0025] The proportions of the components in the desorbed gas stream can be determined using a gas chromatograph (GC) with helium as the carrier gas. An alternative analytical method is Raman spectroscopy. The proportion of H2 in HCl can be analyzed, for example, using a GC-TCD (thermal conductivity detector). HCl is generally assumed to be 100% residual in the determination.

[0026] For contacting in step a), the exhaust gas mixture can be fed via pipelines to the absorption column containing the absorbent. HCl, chlorosilanes, and, to a very small extent, H2, are absorbed at least partially, preferably until the absorbent is saturated. The absorbent is preferably a chlorosilane selected from the group consisting of tetrachlorosilane, TCS, DCS, and mixtures thereof.

[0027] Typically, the exhaust gas mixture is brought into contact with the absorbent at a temperature of -60 to -20°C, preferably -50 to -30°C. The pressure is preferably 1.2 to 1.8 MPa, particularly 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 determined in the first gas phase above the absorbent. Temperature and pressure in the absorption column are determined at the top of the column.

[0029] The head end of a column, or more generally of a vessel / apparatus, is usually understood to be the upper part, especially a closure or end piece (cover). For example, the pressure can be determined by a manometer in an exhaust line (through which the first gas phase is discharged) at the head end. The temperature can be determined by a thermocouple in a supply line at the head end.

[0030] To expand the loaded absorbent, it can be fed from the absorption column via pipelines to at least one outgassing unit. The expansion, in conjunction with the resulting Fh-containing second gas phase, is thus at least one intermediate step provided between steps a) and b).

[0031] The loaded absorbent can also be expanded in two or more sequentially arranged outgassing units, with expansion taking place in each outgassing unit and a downstream outgassing unit having a higher temperature and / or lower pressure than an upstream one. In other words, a pressure and / or temperature gradient exists between a first and a last outgassing unit (decreasing pressure, increasing temperature, each within the specified ranges).

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

[0033] For desorption of the gas stream in step b), the loaded absorbent can be fed from the (possibly the last of two or more) outgassing units via pipelines to the desorption column. The temperature is determined in the bubble (at the low point) of the desorption column in the liquid phase (e.g., using a temperature sensor), and the pressure at the top of the desorption column is determined in the gas phase (e.g., in an exhaust line using a manometer). 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 outgassing unit (intermediate step) and the desorption column (step b)), the requirement is that within the specified pressure and temperature ranges, there is a temperature gradient (increase) and / or a pressure gradient (decrease) between the individual steps.

[0035] It has been shown that at least one-stage expansion in at least one outgassing unit downstream of the absorption column allows entrained (absorbed) H2 to be separated with high selectivity from the absorbent loaded with HCl and chlorosilanes. As a result, the H2 content of the gas stream desorbed in step b) can be significantly reduced, thus improving the economic efficiency of the process.

[0036] Furthermore, it has been shown that multi-stage relaxation generally offers little or no advantage over single-stage relaxation.

[0037] The pressure at the head end of the outgassing unit is preferably reduced by 0.1 to 1.3 MPa, particularly preferably by 0.3 to 1.2 MPa, in particular by 0.5 to 1.1 MPa (differential pressure) compared to step a). The pressure at the head end of the outgassing unit is preferably 0.1 to 1.4 MPa, particularly preferably 0.2 to 1.2 MPa, in particular 0.3 to 1.0 MPa.

[0038] The pressure at the head of the degassing unit can be adjusted by a relative height difference between the degassing unit and the absorption column, using a hydrostatic pressure drop across the liquid column. The degassing unit can therefore be positioned at a higher elevation than the absorption column. The pressure drop caused by the height can lead to the desired differential pressure. This eliminates the need for pressure reduction devices.

[0039] For example, an inlet opening provided for the supply of the loaded absorbent can be arranged laterally on the outgassing unit 5 to 25 m, preferably 7 to 20 m, particularly preferably 10 to 18 m, above an outlet opening of the absorption column provided for the loaded absorbent.

[0040] The pressure can also be adjusted by a valve-controlled volume expansion at an outlet opening of the absorption column.

[0041] In general, for effective separation of HCl and H2, both the pressure and temperature should be as low as possible within the specified ranges. At the same time, cost-effectiveness requires direct incorporation of the outgassed H2 into the relevant composite material (e.g., polysilicon production) without additional compression or cooling. The temperature of the loaded absorbent in the outgassing unit is preferably -70 to -10°C, more preferably -60 to -20°C, and especially -50 to -30°C.

[0042] The average hydrodynamic residence time r of the loaded absorbent in the outgassing unit is usually 1 to 40 s, preferably 2 to 30 s, particularly preferably 5 to 20 s, in particular 6 to 15 s.

[0043] The average hydrodynamic residence time r of the second gas phase in the outgassing unit is usually 10 to 1150 s, preferably 25 to 850 s, particularly preferably 40 to 250 s, in particular 45 to 150 s.

[0044] Furthermore, r of the absorbent in the absorption column can be from 100 to 3500 s, preferably from 200 to 3000 s, particularly preferably from 250 to 2500 s.

[0045] T is calculated as follows:

[0046] V R : Fluid volume in the respective apparatus (e.g. volume filled with loaded absorbent or gas mixture in the outgassing unit or column) in [m 3 ].

[0047] V : Volume flow of the fluid (e.g. absorbent or exhaust gas mixture) at operating conditions (p, T) in [m 3 / s].

[0048] T for the absorbent in the absorption column is determined from the associated liquid volume and the absorbent volume flow. Similarly, r in the gas phase is determined from the gas-filled volume and the gas volume flow.

[0049] Typical apparatus volumes of the absorption and desorption column are 5 to 50 m 3, where typically 10 to 80% of the volume is filled with absorbent. This results in the corresponding fluid volumes for the gas and liquid phases.

[0050] The volume of the outgassing unit is preferably 0.01 to 5 m 3 The proportion filled with absorbent is typically between 10 and 70%.

[0051] Preferably, by separating the second gas phase at the head end of the outgassing unit, a volume flow of > 30 Nm 3 / h, particularly preferably > 75 Nm 3 / h, especially from

[0052] > 125 Nm 3 / h, discharged.

[0053] The second gas phase separated from the outgassing unit usually has a hydrogen content of > 60%, particularly preferably > 70%, in particular > 80%.

[0054] The degassing unit may be a separator, preferably a gravity separator. It is particularly preferred to be a vertical gravity separator.

[0055] The apparatus volume is determined by the preferred residence times. In a preferred embodiment, the volume is kept as small as possible within the specified residence times.

[0056] For example, the diameter of a typical vertical gravity separator may be 300 to 1000 mm, preferably 400 to 900 mm, particularly preferably 500 to 800 mm.

[0057] A typical overall height for a vertical gravity separator can be in the range of 900 to 3000 mm, preferably 1400 to 2700 mm, in particular 1800 to 2500 mm. The ratio of diameter to overall height of the vertical gravity separator can be 0.13 to 0.43, preferably 0.17 to 0.39, particularly preferably 0.21 to 0.35.

[0058] The lateral inlet opening of the vertical gravity separator can be located at a height corresponding to 26 to 87%, preferably 43 to 78%, particularly preferably 56 to 74%, of its total height. The starting point for the measurement is the bottom of the separator.

[0059] The height of the vertical gravity separator above its side inlet opening preferably corresponds to its diameter.

[0060] The liquid level (absorbent) in the degassing unit can be determined and adjusted using, for example, guided radar measurement or a measurement via communicating tubes. Furthermore, the liquid level can be determined via the pressure in the degassing unit.

[0061] In a preferred embodiment, the diameter of the outlet of the outgassing unit is selected such that the dimensionless Froude number Fr is in the range from 0.2 to 4.0, particularly preferably from 0.5 to 3.0, particularly preferably from 0.8 to 2.0.

[0062] Fr is calculated as Fr = -^=, where v L : Flow velocity of the loaded absorbent in the outlet [m / s] (e.g. 0.4 to 3.0 m / s) g : Acceleration due to gravity (9.81 m / s 2 )

[0063] D: Diameter of the discharge line (e.g. 0.01 to 0.7 m) Typical ratios of the liquid level of the loaded absorbent in the outgassing unit to the diameter of the outlet are 0.7 to 10.0, preferably 0.9 to 9.0, particularly preferably 1.0 to 8.0.

[0064] The maximum average gas velocity of the gas phase in the vertical gravity separator can be < 0.1 m / s, preferably < 0.075 m / s, particularly preferably < 0.05 m / s.

[0065] The maximum average flow velocity of the loaded absorbent in the vertical gravity separator can be < 0.2 m / s, preferably < 0.15 m / s, particularly preferably < 0.13 m / s.

[0066] The exhaust gas mixture to be treated with the process according to the invention can arise in a polysilicon production plant. In particular, the exhaust gas mixture is formed during the production of TCS, which is preferably a production process using silicon, HCl, and optionally H2.

[0067] By the process according to the invention, the H2 content in HCl can be reduced by > 50%, preferably by 65%, particularly preferably by > 80%. The separated H2 is typically recycled and can be fed back into the polysilicon production process. For example, it can be used to produce highly dispersed silica.

[0068] This allows for increased capacity in TCS production (by reducing the proportion of H2 and the associated increase in HCl throughput while maintaining a constant residence time in the reactor). This results in increased space-time yield. Alternatively, energy costs can be reduced while maintaining the same space-time yield, as the amount of gas recycled and thus the compressor power can be reduced (reducing the proportion of H2).

[0069] In a preferred embodiment, the second gas phase from the outgassing unit is recombined with the exhaust gas mixture before it is fed to the absorption column.

[0070] In a further embodiment, at least a part of the loaded absorbent is passed through a degassing unit, while the other part can be fed directly to the desorption column.

[0071] Typically, heat exchange occurs between the cold, loaded absorbent and the heated absorbent after desorption to minimize the system's energy requirements at high throughputs. Furthermore, the offgas mixture usually undergoes multi-stage cooling before entering the absorption column, e.g., through coolers and / or countercurrent systems and, if necessary, additional heat transfer paths, in order to achieve the low temperature for absorption as energy-efficiently as possible. The separated gas streams from the outgassing unit and the desorption column can also undergo multi-stage heat exchange to reduce energy losses. In principle, complete separation of HCl and H2 is not achieved under economically relevant process conditions.

[0072] A further aspect of the invention relates to a device for separating an exhaust gas mixture containing HCl, H2, and chlorosilanes. The device is particularly suitable for carrying out the process according to the invention. The device comprises the following components:

[0073] - An absorption column 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, the absorption column comprising an outlet for discharging a hydrogen-containing first gas phase,

[0074] - At least one outgassing unit downstream of the absorption column for expanding the absorbent loaded with the exhaust gas mixture in the absorption column at a temperature which is higher than the temperature in the absorption column and / or at a pressure which is lower than the pressure in the absorption column, the outgassing unit comprising an outlet for discharging a hydrogen-containing second gas phase;

[0075] - A desorption column downstream of the outgassing unit for desorbing a gas stream from the loaded absorbent at a temperature of 50 to 150°C and / or a pressure reduced compared to the absorption column in a range of 0.1 to 1 MPa, wherein the desorption column comprises an outlet for discharging the desorbed gas stream.

[0076] Two or more outgassing units can be arranged sequentially (in series) between the absorption and desorption columns. However, it is particularly preferred to use only one outgassing unit.

[0077] Preferably, the outgassing unit is arranged separately from the absorption and desorption column. This should be understood in particular that the outgassing unit is connected to the two columns essentially only via pipelines.

[0078] The outgassing unit can therefore be a component that can be retrofitted into an existing exhaust gas separation system without having to make fundamental modifications to the entire system. In a preferred embodiment, a demister is provided at the top of the outgassing unit to prevent the entrainment of liquid into the gas phase.

[0079] For further details of the individual components, please refer to the process description and the following figure description.

[0080] Fig. 1 shows a diagram of the process according to the invention.

[0081] Fig. 2 shows a diagram of a variant of the method according to the invention.

[0082] Fig. 3 shows a degassing container.

[0083] Figure 1 shows a schematic of the process according to the invention. An offgas from polysilicon production, indicated by arrow 1, is passed through a condensation and heat exchanger section 6 into an absorption column 4, where it is brought into contact with an absorbent (e.g., a mixture of STC and TCS). The offgas contains, for example, 7 mol% HCl, 92 mol% H2, and 1 mol% uncondensed chlorosilanes (silane, monochlorosilane, DCS, TSC, and STC). In the condensation and heat exchanger section 6, it is compressed or cooled to 0.5 to 2.0 MPa and to -70 to -10°C.

[0084] The first gas phase discharged from the head end of the absorption column after absorption, indicated by arrow 2, typically contains at least 95 mol% H2 and a maximum of 5 mol% HCl.

[0085] The loaded absorbent is fed to a separate outgassing unit 12, where the pressure decreases due to the indicated height difference, resulting in outgassing (a second gas phase). The resulting second gas phase, which typically has an H2 content of at least 60 mol%, is discharged at the top of the outgassing unit 12 (arrow 11) and can be fed back into a plant for the production of polysilicon or used to produce dispersed silica.

[0086] The loaded absorbent is then fed to a desorption column 5, where it first passes through one (or more) heat exchangers (or countercurrent devices) 7 and then experiences a further temperature increase to 60 to 150°C in the desorption column 5 by a heater. This temperature increase desorbs a gas stream, which is discharged at the top of the desorption column 5 and cooled to approximately 5 to 35°C by means of a multi-stage heat exchanger (via countercurrent devices and / or coolers 10). This gas stream (arrow 3) typically contains at least 89 mol% HCl, a maximum of 10 mol% H2 and a maximum of 1 mol% chlorosilanes (usually silane, monochlorosilane, DCS, TCS, STC).

[0087] The unloaded absorbent recovered from the desorption column 5 is cooled via the heat exchanger 7 (countercurrent principle) and returned to the absorption column 4. A pump 8 is provided for this recirculation.

[0088] Figure 2 shows a diagram of another embodiment of the process according to the invention, wherein, in contrast to the embodiment according to Figure 1, an additional outgassing unit 13 is provided. This is connected downstream of a first outgassing unit 12. In both outgassing units 12, 13, outgassing takes place, wherein in the additional outgassing unit 13 the pressure is typically reduced compared to the outgassing unit 12, while the temperature level preferably remains constant. Figure 3 shows a vertical gravity separator 20 as a special embodiment of an outgassing unit. The gravity separator 20 has a lateral inlet opening (inlet) 22 and an outlet 24 for the loaded absorbent. At the head end of the gravity separator 20, an outlet 26 for the second gas phase is provided. Line 21 indicates the fill level of the absorbent. The following dimensions are marked with corresponding arrows.ht: total height (wall thicknesses are generally negligible) d: diameter d&: inner diameter of the outlet 24 hr. Filling height (liquid level) of the absorbent hu'. Height below the inlet 22 h. o : Height above inlet 22

[0089] Examples

[0090] All examples were carried out at constant exhaust gas composition: HCl: 7.5 mol% H2: 89 mol% Chlorosilanes (silane, monochlorosilane, DCS and STC): 3.5 mol% A mixture of STC (69 mol%) and TCS (31 mol%) was used as absorbent.

[0091] For all examples, the following conditions were set on the absorption and desorption column: Pressure (absorption column): 1.57 MPa Temperature (absorption column): -50°C Pressure (desorption column): 0.36 MPa Temperature (desorption column): 90°C

[0092] The gas composition can be measured using gas chromatography with a thermal conductivity detector or a RAMAN measurement. Temperatures were determined using a thermocouple, and pressures were measured using a pressure sensor (e.g., capacitive or piezoresistive pressure sensor).

[0093] The exhaust unit (if used) was identical in all examples.

[0094] Example 1

[0095] A single-stage degassing was carried out in a vertical gravity separator (see scheme Fig. 1) at -40°C (measured in the liquid phase in the inlet for the absorbent). The pressure was adjusted by hydrostatic pressure reduction by positioning the separator above the absorption column. The pressure was measured in the gas outlet at the top of the separator using a pressure sensor: 0.6 MPa. The average hydrodynamic residence time of the loaded absorbent in the vertical gravity separator was 8 s. The maximum average flow velocity of the loaded absorbent was 0.12 m / s. The maximum average gas velocity of the gas phase was 0.015 m / s. A gas flow (second gas phase) of 146 Nm³ was achieved. 3 / h with an H2 content of 84.0 mol% was removed from the separator. After desorption of the loaded absorbent in the desorption column, an H2 content of 1.77 mol% was obtained in the recycled HCl (reduction of H2 slip by > 82%).

[0096] Comparison example 1

[0097] The device was essentially the same as in Example 1, except that an intermediate gravity separator was omitted. The absorption and desorption parameters were the same as in Example 1. The H2 content in the HCl without separate outgassing was 12.3 mol%. Example 2

[0098] The vertical gravity separator is operated at 0.4 MPa and -50°C. The pressure is measured at the top of the degassing vessel in the gas flow outlet using a pressure transducer, and the temperature in the liquid phase in the loaded absorbent inlet is measured using a temperature sensor. The average hydrodynamic residence time of the loaded absorbent in the vertical gravity separator was 8 s. A gas flow of 183 Nm3 was measured. 3 / h separated with a fh content of 83.8 mol% (arrow 11 in Fig. 1).

[0099] Comparison example 2

[0100] The vertical gravity separator is operated at 0.4 MPa and 0 °C. The average hydrodynamic residence time of the loaded absorbent in the vertical gravity separator was 8 s. The pressure is measured at the top of the degassing vessel in the gas flow outlet using a pressure sensor, and the temperature in the liquid phase in the loaded absorbent inlet using a temperature sensor. A gas flow of 202 Nm³ was used. 3 / h. The H2 content of the discharged gas phase was only 46.2 mol%. Here, the outgassing was 202 Nm 3 / h, but with a lower H2 content, ie a lot of HCl would be recycled, which is undesirable.

[0101] It is shown that under non-inventive temperature conditions (operating the gravity separator at 0°C), a significantly higher proportion of HCl is obtained in the discharged gas stream (or a significantly lower proportion of H2), whereby the goal of the most effective hydrogen separation possible is not achieved (not selective enough). Example 3

[0102] The vertical gravity separator is operated at 0.4 MPa and -40°C. The mean hydrodynamic residence time of the loaded absorber in the vertical gravity separator was 8 s. The pressure and temperature were determined as described.

[0103] A total gas flow of 186 Nm 3 / h with an H2 content of 77.3 mol% from the gravity separator.

[0104] Comparison example 3

[0105] The vertical gravity separator is operated at 1.5 MPa and -40 °C. The mean hydrodynamic residence time of the loaded absorbent in the vertical gravity separator was 8 s. The pressure and temperature were determined as described.

[0106] A total gas flow of 23 Nm 3 / h with an H2 content of 92.4 mol% from the vertical gravity separator.

[0107] It is shown that under pressure conditions not according to the invention (operating the gravity separator at 1.5 MPa), only a small hydrogen volume flow (calculated from gas volume flow multiplied by the hydrogen content) can be separated, whereby the goal of the most effective possible separation of H2 is not achieved (volume flow too low).

[0108] The examples according to the invention clearly show that large amounts of H2 can be selectively removed by reduced pressure at low temperatures and thus the H2 slip in HCl can be greatly reduced.

Claims

Patent claims 1. A process for separating an exhaust gas mixture containing hydrogen chloride, hydrogen and chlorosilanes, comprising the steps of: a) bringing the exhaust gas mixture into contact with an absorbent in an absorption column 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 hydrogen-containing first gas phase is removed;b) desorption of a 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), characterized in that the loaded absorbent is expanded after step a) and before step b) in at least one outgassing unit at a temperature which is elevated compared to step a) and / or at a pressure which is reduced at a head end of the outgassing unit compared to step a), and hydrogen is separated from the loaded adsorbent by separating off a second gas phase thus formed, wherein the desorbed gas stream in step b) has a hydrogen content of < 10 mol%, a hydrogen chloride content of > 89 mol% and a chlorosilane content of < 1 mol%.; 2. Process according to claim 1, characterized in that the desorbed gas stream has a hydrogen content of 8.2 mol%, a hydrogen chloride content of 91.0 mol% and a chlorosilane content of 0.8 mol%.

3. Process according to claim 1 or 2, characterized in that the loaded absorbent is expanded in two or more sequentially arranged outgassing units, wherein an expansion takes place in each outgassing unit and a downstream outgassing unit has an increased temperature and / or a reduced pressure compared to an upstream one.

4. Process according to one of the preceding claims, characterized in that the pressure at the head end of the outgassing unit is reduced by 0.1 to 1.3 MPa, preferably by 0.3 to 1.2 MPa, particularly preferably by 0.5 to 1.1 MPa, compared to step a).

5. Process according to one of the preceding claims, characterized in that the pressure at the head end of the outgassing unit is 0.1 to 1.4 MPa, preferably 0.2 to 1.2 MPa, particularly preferably 0.3 to 1.0 MPa.

6. Process according to one of the preceding claims, characterized in that the pressure at the head end of the outgassing unit is adjusted by means of a height difference between the outgassing unit and the absorption column by hydrostatic pressure reduction across the liquid column.

7. Process according to one of the preceding claims, characterized in that the temperature of the loaded absorbent in the outgassing unit is -70 to -10°C, preferably -60 to -20°C, particularly preferably -50 to -30°C.

8. Process according to one of the preceding claims, characterized in that the average hydrodynamic residence time of the loaded absorbent in the outgassing unit 1 to 40 s, preferably 2 to 30 s, particularly preferably 5 to 20 s, in particular 6 to 15 s.

9. Method according to one of the preceding claims, characterized in that by separating the second gas phase at the head end of the outgassing unit, a volume flow of > 30 Nm 3 / h, preferably > 75 Nm 3 / h, particularly preferably > 125 Nm 3 / h is discharged.

10. Process according to one of the preceding claims, characterized in that the second gas phase has a hydrogen content of > 60%, preferably > 70%, particularly preferably > 80%.

11. Method according to one of the preceding claims, characterized in that the outgassing unit is a separator, preferably a gravity separator, particularly 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, particularly 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, particularly preferably 0.13 m / s.

14. Device for separating an exhaust gas mixture containing hydrogen chloride, hydrogen and chlorosilanes, in particular for carrying out the process according to at least one of claims 1 to 13, comprising — an absorption column 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, the absorption column comprising an outlet for discharging a hydrogen-containing first gas phase; — at least one outgassing unit downstream of the absorption column for expanding the absorbent loaded with the exhaust gas mixture in the absorption column at a temperature which is higher than the temperature in the absorption column and / or at a pressure which is lower than the pressure in the absorption column, the outgassing unit comprising an outlet for discharging a hydrogen-containing second gas phase; — a desorption column downstream of the outgassing unit for desorbing a gas stream from the loaded absorbent at a temperature of 50 to 150°C and / or a pressure reduced compared to the absorption column in a range of 0.1 to 1 MPa, wherein the desorption column comprises an outlet for discharging the desorbed gas stream.

15. Device according to claim 14, characterized in that two or more outgassing units are arranged sequentially between the absorption and desorption columns.

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