Process for separating an offgas mixture

Abstract

This invention relates to a method for separating a waste gas mixture containing HCl, H2, chlorosilane, and a C-containing component. The method comprises: contacting the waste gas mixture with an absorbent at a specific temperature and pressure in an absorption tower; as the loaded absorbent forms, at least HCl and chlorosilane are absorbed, and a first gas phase containing H2 is removed; transferring the loaded absorbent to a desorption tower and establishing a specific temperature at its bottom; as unloaded absorbent is obtained, a second gas phase is desorbed, and the unloaded absorbent is returned to the absorption tower. The HCl content and the amount of the C-containing component are measured in the first gas phase and / or in the second gas phase. A ratio K1 is established in the first gas phase and / or a ratio K2 is established in the second gas phase by controlling at least one manipulated variable. K1, K2, and the manipulated variable are detailed in the specification.

Description

[Technical Field]

[0001] This invention relates to a method for separating an offgas mixture containing hydrogen chloride, hydrogen, chlorosilane and carbon (C) components.

[0002] Polycrystalline silicon is the starting material for producing monocrystalline silicon via the Czochralski process or the float-zone process. Monocrystalline silicon can be used in the semiconductor industry to manufacture electronic components (chips) in wafer form. Furthermore, polycrystalline silicon is required for the production of polycrystalline silicon via the block casting method. Polycrystalline silicon can be used to manufacture solar cells. [Previous Technology]

[0003] Polycrystalline silicon can be produced using the Siemens process, a chemical vapor deposition method. This involves heating silicon filaments directly by passing an electric current through a reactor (a Siemens reactor) and introducing a reaction gas containing silicon-containing components and hydrogen (H2). The silicon-containing components used can comprise monosilanes (SiH4) or halosilanes with a general composition of SiHnX4-n (n = 0, 1, 2, 3; X = Cl, Br, I). It typically contains chlorosilanes or mixtures of chlorosilanes, and more particularly trichlorosilanes (SiHCl3, TCS). A typical Siemens reactor design is described, for example, in US 2009 / 0136408 A1.

[0004] Another production mode is to deposit polycrystalline silicon on heated granular silicon particles in a fluidized bed reactor (particle method), as described in, for example, US 2013 / 0295385A1.

[0005] Regardless of the production method, the starting materials used all contain silicon-containing components, usually TCS. For example, as described in WO 2016 / 198264A1, there are three methods for producing TCS. (1) SiCl4 + H2 → SiHCl3 + HCl + Byproduct (2) Si + 3SiCl4 + 2H2 → 4SiHCl3 + Byproduct (3) Si + 3HCl → SiHCl3 + H2 + Byproduct

[0006] The generated byproducts may include other chlorosilanes, such as monochlorosilanes (H3SiCl), dichlorosilanes (DCS, H2SiCl2), silicon tetrachloride (STC, SiCl4), as well as disilanes and oligomeric silanes. Additionally, impurities such as hydrocarbons, organochlorosilanes, and metal chlorides may be present as byproducts.

[0007] Each of the three methods produces a waste gas mixture that, after condensation of chlorosilane, contains not only hydrogen chloride (HCl) and H2, but also trace amounts of uncondensed chlorosilane and hydrocarbons.

[0008] In addition, during the deposition of polycrystalline silicon by the Siemens process or the particle process, a waste gas mixture containing chlorosilane, HCl, H2 and hydrocarbons is formed.

[0009] For environmental and economic reasons, waste gases are typically treated and H2 and HCl are recovered. They can then be fed back into the integrated production of polysilicon, as described in US 4,454,104 A.

[0010] In addition to recovering H2 by adsorption onto activated carbon or other solids, as disclosed in US 2013 / 0011558A1, when the fraction of HCl is relatively high, a combination of adsorption and absorption / desorption can be used to separate and purify H2 and HCl. This type of combination is described in US 2012 / 198998 A1. In the case of pure absorption separation (including absorption and desorption) disclosed in CN 102431972 A and CN102614741 A, HCl is adsorbed onto an absorbent at low temperature and high pressure. Subsequently, HCl is discharged again in a column called a desorption column by increasing the temperature and / or decreasing the pressure.

[0011] For the recycling of H2 and HCl, there are specific quality requirements regarding impurities that must be met. For example, in particular, the HCl content in H2 is related to the equilibrium of the reaction of STC with H2 to produce TCS according to reaction formula (1).

[0012] It is also known from WO 2015 / 140027 A1 that the amount of C impurities (e.g., methane) in H2 affects the lifetime of graphite-based and SiC-based materials.

[0013] When recycling back to the Siemens process, the requirements for H2 purity are particularly stringent.

[0014] The upstream operations of the waste gas treatment that generate the waste gas mixture (see equations (1) to (3)) experience fluctuations in production volume, reaction conditions and conversion rate. This results in fluctuations in the volume and composition of the waste gas.

[0015] For example, according to US 2005 / 0226803 A1, silicon reactants (see equations (2) and (3)) can directly affect the composition of the products and the conversion rate in the reaction.

[0016] In addition, fluctuations in the integrated system (e.g., plant shutdowns and readjustments) may cause fluctuations in the composition, pressure and volume of the exhaust gas mixture.

[0017] In order to ensure the consistent high purity of the recovered HCl and H2 gases, the waste gas treatment is often operated under unnecessarily high loads (especially with low waste gas volume and amount of impurities), resulting in high energy consumption and high costs.

[0018] The separation of the exhaust gas mixture and the recovery of H2 and HCl are also described in PCT / EP2023 / 051875. No instruction is given to control multiple variables for the purpose of affecting the quality of H2 with respect to C impurities or compensating for short-term fluctuations in the composition and / or volume of the exhaust gas mixture.

[0019] CN 206444413 U proposes controlling the reflux ratio of the desorption tower based on the quality of the absorbent and the quality of the recovered H2, with the aim of ensuring consistently high quality of the recovered H2. This aims to avoid the adverse effects of high HCl content on subsequent adsorption purification of H2. Before using H2 for silicon production, it is necessary to purify it again by adsorption. The purpose of this control system is to adjust the quality of the recovered H2 by regenerating the absorbent. However, it does not control the amount of absorbent or the amount of vapor entering the desorption tower. Combined with adsorption purification, the methane content in H2 cannot be variably adjusted. Furthermore, due to the high time delay in measurement, it is impossible to intercept short-term fluctuations in the composition and quantity of the waste gas using this system.

[0020] CN 113797715 A discloses a stepwise control of the absorbent amount based on simulation data, adapting the absorbent amount to the amount of supplied waste gas. The aim is to obtain high-purity H2, which undergoes a second adsorption purification. By simulating operation at partial load (100% to 60%), the absorbent amount can be gradually adapted to the simulated reduced demand. However, the quality of the regenerated H2 is not continuously captured. Therefore, it is not possible to directly, continuously, and flexibly control the quantity, especially based on the fluctuating composition of the waste gas in the short term. There is also no control based on the multivariate quality of the regenerated gas, including other controlled variables such as the amount of vapor entering the desorption tower. Therefore, it is impossible to affect the methane content in the H2. [Summary of the Invention]

[0021] The present invention, based on the described drawbacks, aims to provide an energy-optimized operating mode for purifying waste gas from polysilicon production, taking into account fluctuations in the amount and composition of the waste gas.

[0022] This objective is achieved by a method for separating a waste gas mixture G3 containing HCl, H2, chlorosilane and C components, the method comprising the following steps: a) in an absorption tower, the waste gas mixture is contacted with an absorbent at a temperature of -70 to -10°C and a pressure of 0.5 MPa to 2 MPa, wherein, with the formation of a loaded absorbent, at least HCl and chlorosilane are absorbed, and a first gas phase G1 containing H2 is removed; b) the loaded absorbent is transferred to a desorption tower and a temperature CV3 of 50°C to 150°C is established at the bottom of the desorption tower by supplying power MV2 to a heater, wherein, with the acquisition of an unloaded absorbent, a second gas phase G2 is desorbed and the unloaded absorbent is transferred back to the absorption tower.

[0023] The method is characterized in that the HCl content CV1a and the amount of C component CV2a are measured in the first gas phase G1 and / or the HCl content CV1b and the amount of C component CV2b are measured in the second gas phase G2, wherein by adjusting the manipulation variable, a ratio K1 of 0.02 to 1000 is established in G1, K1 = CV1a / CV2a and / or a ratio K2 of 10 to 10000 is established in G2, K2 = CV1b / CV2b.

[0024] The manipulation variable (MV) includes at least - the amount of unloaded absorbent returned from the desorption tower MV1, and / or - the power supplied to the heater MV2.

[0025] The C-containing component in step a) is usually partially absorbed and partially removed along with G1.

[0026] This process requires predictive, model-based, multivariate control.

[0027] The method of the present invention has the particular advantage of ensuring consistent quality of the recirculated gases (H2 and HCl) in G1 and G2. The method can also be customized to meet the requirements of downstream operating steps. This takes into account not only the fraction of HCl in H2 or the fraction of H2 in HCl, but also, in particular, the fraction of C-containing components, as the latter can significantly affect downstream operations.

[0028] Surprisingly, a consistent quality of the recirculated gas can be stably and optimally established using a total control system. The total control system can be implemented by using predictive, model-based multivariate control with a multivariate controller. This method allows for stable and optimal operating modes by overall control of the target variable (controlled variable (CV)) and taking into account all relevant variables affecting operation (manipulated variable (MV), disturbance variable (FF), see Table 1).

[0029] Here, the disturbance variable (FF) is usually a variable that cannot be directly affected, while the controlled variable (CV) is a variable that changes due to the adaptation of the manipulated variable (MV) and / or under the influence of the disturbance variable (FF). The manipulated variable (MV) is a variable manipulated by the method of the present invention.

[0030] Not only the manipulating variables but also the controlled variables usually have a cost function for which they are recorded, an example being the steam cost for supplying steam to apply the power MV2 supplied to the heater, and thus, for each combination of manipulating and controlling variables, the total cost can be calculated.

[0031] In this context, a multivariable controller allows for minimizing total cost by incorporating adherence to limits on the controlled variable (CV).

[0032] In order to implement the method of the present invention, as a predictive model-based multivariable controller, commercial software can be used, preferably from the available DMC (Dynamic Matrix Control) software product portfolio, such as "DMCplus" (from Aspentech).

[0033] For example, US 6,453,308 B1 describes software for performing predictive multivariable control. In this case, nonlinear models and algorithms are employed in particular.

[0034] The ratio K1 in the first gas phase G1 preferably has a value range of 0.04 to 200, more preferably 0.1 to 100, and more particularly 0.4 to 50.

[0035] The ratio K2 in the second gas phase G2 preferably has a value of 10 to 2500, more preferably 40 to 1500, and very preferably 80 to 1000.

[0036] The HCl content CV1a in the first gas phase is preferably 0.01 vol% to 1.0 vol% (100 to 10000 ppmv), more preferably 0.02 vol% to 0.8 vol% (200 to 8000 ppmv), and very preferably 0.05 vol% to 0.7 vol% (500 to 7000 ppmv).

[0037] The content of C component CV2a in the first gas phase is preferably 0.005 vol% to 0.25 vol% (50 to 2500 ppmv), more preferably 0.01 vol% to 0.1 vol% (100 to 1000 ppmv), and very preferably 0.015 vol% to 0.075 vol% (150 to 750 ppmv). These values ​​are based on the carbon fraction of the C component.

[0038] The C-containing components are calculated based on methane equivalents having one carbon atom; therefore, for example, the figure for ethane having two carbon atoms is twice the methane equivalent. For example, one mole of ethane corresponds to two moles of methane equivalent, and therefore, one mole of propane corresponds to three moles of methane equivalent.

[0039] The HCl content CV1b in the second gas phase is preferably 70 vol% to 99.9 vol% (7*105 to 9.99*105 ppmv), more preferably 75 vol% to 99.5 vol% (7.5*105 to 9.95*105 ppmv), and more particularly 85 vol% to 99.0 vol% (8.5*105 to 9.9*105 ppmv).

[0040] The content of C component CV2b in the second gas phase is preferably 0.01 vol% to 5.0 vol% (100 to 50000 ppmv), more preferably 0.02 vol% to 2.5 vol% (200 to 25000 ppmv), and more particularly 0.03 vol% to 1.0 vol% (300 to 10000 ppmv).

[0041] CV1a / b and CV2a / b can be determined using methods selected from infrared spectroscopy, Raman spectroscopy, and gas chromatography. Suitable detectors for process gas chromatographs are PDD (pulse discharge detector), TCD (thermal conductivity detector), and / or FID (flame ionization detector). Measurements are preferably performed via infrared or Raman spectroscopy because these methods allow for the simultaneous determination of HCl and hydrocarbons (especially methane) in H2. The measurement frequency is preferably from 0.5 seconds to 90 seconds, more preferably from 1 second to 60 seconds. MIR (mid-infrared) spectrometers are particularly useful.

[0042] The C-containing component is preferably selected from organosilanes, organochlorine silanes, saturated and unsaturated hydrocarbons having no more than three carbon atoms, saturated and unsaturated hydrochlorocarbons having no more than three carbon atoms, and mixtures thereof.

[0043] The organic groups of organosilicones and organochlorine silanes can be methyl, ethyl, and methoxy. Methyl is preferred.

[0044] The component discussed is preferably an organochlorine monosilane and / or an organosilane of general formula I: HxSiRyCl(4-xy) (formula I), where x = 0, 1, 2 or 3; y = 1, 2, 3 or 4, and R = methyl and / or methoxy.

[0045] In addition, the C-containing component can be a saturated hydrocarbon and / or a chlorinated hydrocarbon of general formula II: CnH(2n+2-m)Clm (formula II), where n = 1, 2 or 3; m = 0, 1 or 2.

[0046] In addition, the component may be an unsaturated hydrocarbon and / or a chlorinated hydrocarbon of general formula III: CqH(2q-r)Clr (formula III), where q = 2 or 3; r = 0, 1 or 2.

[0047] In particular, the C-containing component is selected from methane, ethane, acetylene, propane, and mixtures thereof. More particularly preferably, the C-containing component is methane.

[0048] The absorbent is preferably selected from STC, TCS, DCS, organochlorosilanes, and mixtures thereof. More preferably, the absorbent is STC, TCS, or DCS. More specifically, it comprises a mixture of the above three silanes.

[0049] The composition of the absorbent may fluctuate depending on upstream operations. This fluctuation may affect the temperature CV3 at the bottom of the desorption tower, and thus the temperature distribution of the desorption tower. Therefore, typically, in addition to the composition of the regeneration gas (first and second gas phases), the temperature distribution of the desorption tower is also captured and adapted where appropriate. The temperature distribution of the desorption tower is typically composed of its top and bottom temperatures as well as the temperature composition defined within the tower packing.

[0050] The bottom temperature CV3 is preferably 50°C to 150°C, more preferably 75°C to 140°C, and particularly 95°C to 125°C.

[0051] Furthermore, preferably, the temperature CV4 in the first packing unit of the capture desorption tower is adjusted to 45°C to 140°C, preferably 70°C to 130°C, and more preferably 90°C to 120°C.

[0052] The first packing unit refers to the first of several (typically two, three, or more) zones equipped with conventional packing (e.g., structured packing or random packing). Random packing is preferred (e.g., Raschig rings from Raschig). Upward (in the direction of the column top), each zone is typically terminated by a liquid distributor (a typical column design can be found in: Henry Z. Kister, Distillation Operation, 1990, McGraw-Hill Inc.).

[0053] A preferred embodiment uses a desorber having a packing unit (such as the random or structured packing described in Henry Z. Kister, Distillation Operation, 1990, McGraw-Hill Inc.). The packing unit preferably comprises random packing, which preferably comprises Raschig rings, more particularly Raschig ring No. 2.

[0054] The absorption tower may also be equipped with the packing unit described above.

[0055] Preferably, a temperature difference ΔT1 (CV3 – CV4) of ≤ 5°C, more preferably ≤ 2.5°C, and best ≤ 1.7°C is established, and this difference is always ≥ 0°C.

[0056] The top temperature CV5 of the desorption tower is preferably 50°C to 100°C, more preferably 55°C to 95°C, and particularly 60°C to 90°C.

[0057] The amount of unloaded absorbent returned from the desorption tower, MV1, is preferably from 0 kg / h to 300,000 kg / h, more preferably from 50,000 kg / h to 280,000 kg / h, and more particularly from 125,000 kg / h to 260,000 kg / h.

[0058] Preferably, the ratio CV11 (Equation 4) of MV1 (the amount of unloaded absorbent returned from the desorption tower) to the sum of the volumetric flow rates (VzG) of the first gas phase (G1) and the second gas phase (G2) is 1 kg / m3 to 15 kg / m3, more preferably 2 kg / m3 to 12 kg / m3, and more particularly 3 kg / m3 to 10 kg / m3, CV11 = MV1 / VzG (Equation 4), where VzG = Vg1 + Vg2, where Vg1: volumetric flow rate G1 [m3 / h], typically 0 m3 / h to 45000 m3 / h, and Vg2: volumetric flow rate G2 [m3 / h], typically 0 m3 / h to 5000 m3 / h.

[0059] The unit [m3 / h] here represents the gas volume flow rate under standard conditions (according to DIN1343, 273.15 K and 101 325 Pa), and is often also expressed as [Nm3 / h].

[0060] MV1 here is reported in units of [kg / h] and is typically from 0 kg / h to 300,000 kg / h.

[0061] Therefore, VzG is the sum of the volumetric flow rates of the recovered (regenerated) gases (G1 and G2) and is typically from 500 m3 / h to 50,000 m3 / h, preferably from 1,000 m3 / h to 45,000 m3 / h, and even more preferably from 1,500 m3 / h to 40,000 m3 / h.

[0062] By capturing temperatures CV3, CV4, and CV5 and simultaneously controlling the temperatures with manipulation variables MV1 and MV2, a multivariate control system is generated. This system can be implemented to flexibly adapt the power MV2 of the heater in the desorption tower and the amount of absorbent MV1 independent of it, thereby establishing temperatures CV3, CV4, and CV5 in the desorption tower and achieving the desired composition K1 and / or K2 of the recovered gases G1 and G2.

[0063] Generally, no additional measurement of the composition of the loaded and / or unloaded absorbent from step b) is required. Nor is adaptation of the reflux ratio (as required in CN 206444413 U) necessary. The reflux ratio (R) represents the ratio of the returned amount (liquid flow rate L) to the distillate output amount (top product flow rate D): R = L / D. (See Henry Z. Kister, Distillation Operation, 1990, McGraw-Hill Inc. (Chapter 2)).

[0064] The power MV2 of the heater supplied to the desorption tower is preferably in the range of 100 kW to 5000 kW, more preferably 200 kW to 4300 kW, and more particularly 300 kW to 3700 kW.

[0065] In a preferred variation of the method, MV2 can be provided by supplying steam. This can be done using steam at a pressure of 2 to 16 bar, preferably 2 to 8 bar, and even more preferably 2 to 4 bar.

[0066] For example, for a pressure of 2 bar, the amount of steam can be from 0.25 t / h to 8 t / h, more preferably from 0.5 t / h to 7 t / h, and even more preferably from 0.75 t / h to 6 t / h.

[0067] In another preferred variation of the method, as another manipulation variable, an amount of cooling water MV3 can be provided, wherein the amount of cooling water MV3 is supplied to a heat exchanger for cooling the exhaust gas mixture before it comes into contact with the absorbent in step a) of the method.

[0068] MV3 is supplied after the exhaust gas mixture is compressed in the compressor as needed (see MV5 below).

[0069] The following cooling water volume flow rates are based on standard conditions (according to EN 14511-1:2013) compared to the gas volume flow rate: 293.15 K and 101,325 Pa.

[0070] The cooling water volume MV3 here can vary from 10 m3 / h to 100 m3 / h, preferably from 15 m3 / h to 80 m3 / h, and even more preferably from 20 m3 / h to 60 m3 / h.

[0071] Therefore, a temperature difference CV6 of 2°C to 30°C, preferably 5°C to 20°C, and more preferably 10°C to 15°C can be established between the outlet and inlet of the cooling water at the heat exchanger.

[0072] Another controllable variable that can be provided is the amount of cooling water MV4 supplied to the condenser at the top of the desorption tower.

[0073] The volume of this cooling water MV4 can be from 10 m3 / h to 120 m3 / h, preferably from 20 m3 / h to 100 m3 / h, and even more preferably from 25 m3 / h to 80 m3 / h.

[0074] Furthermore, preferably, the temperature difference CV7 between the cooling water outlet and the inlet of the cooling water at the top of the desorption tower can be controlled by adjusting the amount of cooling water MV4.

[0075] Therefore, a cooling water temperature difference CV7 of 5°C to 25°C, more preferably 8°C to 20°C, and more particularly 10°C to 18°C ​​can be achieved between the outlet and inlet of the cooling water at the condenser at the top of the desorption tower.

[0076] Another controllable variable is the power consumption MV5 of the compressor used to compress the exhaust gas mixture. MV5 can be from 100 kW to 1000 kW, preferably from 125 kW to 700 kW, and even more preferably from 150 kW to 600 kW.

[0077] According to one preferred embodiment of the method, control is performed for the optimal coordination of time-fluctuation costs and cooling water availability with the power costs generated in the integrated system used for polysilicon production. In this case, for the optimal coordination of cooling water costs, cooling water availability, and power costs (which fluctuate according to the time of day and season), the power consumption MV5 of the exhaust gas compressor, the amount of absorbent MV1, the power MV2 supplied to the desorption tower heater, and the amounts of cooling water MV3 and MV4 can be adapted.

[0078] This optimization is better performed in terms of cooling water availability, cost and inlet temperature.

[0079] In a preferred embodiment of the method of the present invention, the N2 content CV12 is also controlled in G1, because the content may affect downstream operations, particularly the hydrogenation of tetrachlorosilane according to reaction equation (1). This effect is disclosed, for example, in US2019322534 A1.

[0080] By adjusting at least two of the manipulation variables MV1, MV2, MV3, MV4 and MV5, the controlled variables controlled by the predictive model-based multivariate control of the present invention are preferably as follows: - ratio K1, - ratio K2, - bottom temperature CV3 of the desorption tower, - temperature in the first packing unit of the desorption tower CV4, - top temperature of the desorption tower CV5, - temperature difference CV6 between the outlet and inlet of the cooling water at the heat exchanger used to cool the exhaust gas before contact in step a), - temperature difference CV7 between the outlet and inlet of the cooling water at the condenser at the top of the desorption tower, - temperature of the exhaust gas mixture at the gas inlet of the absorption tower CV8, - temperature of the absorbent when supplied to the absorption tower (or when in contact with the exhaust gas mixture) CV9, - temperature difference ΔT1 between temperatures CV3 and CV4, - ratio CV11 (see Equation 4), - N2 content in the first gas phase CV12.

[0081] Preferably, when adjusting at least two of the manipulation variables MV1, MV2, MV3, MV4 and MV5, the predictive model-based multivariate control of the present invention is used to control K1, K2, CV3, CV4, CV6 and CV7.

[0082] Predictive, model-based multivariate control dynamically maintains the controlled variables within their respective expected ranges. The ranges are summarized as follows: - Preferably 0.01 vol% to 1 vol%, more preferably 0.02 vol% to 0.8 vol%, and more particularly 0.05 vol% to 0.7 vol% of the HCl content in the first gas phase CV1a. - Preferably 70 vol% to 99.9 vol%, more preferably 75 vol% to 99.5 vol%, and more particularly 85 vol% to 99.0 vol% of the HCl content in the second gas phase CV1b. - Preferably 0.005 vol% to 0.25 vol%, more preferably 0.01 vol% to 0.1 vol%, and more particularly 0.015 vol% to 0.075 vol% of the C-containing component content in the first gas phase CV2a. - Preferably, the C-containing component content in the second gas phase is 0.01 vol% to 5.0 vol%, more preferably 0.02 vol% to 2.5 vol%, and more particularly 0.03 vol% to 1.0 vol%, CV2b. - Preferably, the bottom temperature of the desorption tower is 50°C to 150°C, more preferably 75°C to 140°C, and more particularly 95°C to 125°C, CV3. - Preferably, the temperature in the first packed unit of the desorption tower is 45°C to 140°C, more preferably 70°C to 130°C, and more particularly 90°C to 120°C, CV4. - Preferably, the top temperature of the desorption tower is 50°C to 100°C, more preferably 55°C to 95°C, and more particularly 60°C to 90°C, CV5. - A preferred temperature difference CV6 between the inlet and inlet of the cooling water at the heat exchanger used to cool the exhaust gas mixture before contact with the absorbent, ranging from 2°C to 30°C, more preferably from 5°C to 20°C, and more particularly from 10°C to 15°C. - A preferred temperature difference CV7 between the outlet and inlet of the cooling water at the condenser at the top of the desorption tower, ranging from 5°C to 25°C, more preferably from 8°C to 20°C, and more particularly from 10°C to 18°C. CV6 and CV7 are preferably kept constant. - A preferred temperature CV8 between the exhaust gas mixture at the gas inlet of the absorption tower, ranging from -70°C to 20°C, more preferably from -65°C to 0°C, and more particularly from -55°C to -25°C. - A preferred temperature CV9 of the absorbent supplied to the absorption tower at -70°C to -10°C, more preferably -60°C to -20°C, and more particularly -50°C to -30°C. - A preferred temperature difference ΔT1 (CV3 – CV4) of ≤ 5°C, more preferably ≤ 2.5°C, and more particularly ≤ 1.7°C, where ΔT1 ≥ 0°C. - A preferred ratio CV11 of 1 kg / m³ to 15 kg / m³, more preferably 2 kg / m³ to 12 kg / m³, and more particularly 3 kg / m³ to 10 kg / m³ (see Equation 4). - A ratio K1 of 0.02 to 1000, more preferably 0.04 to 200, more preferably 0.1 to 100, and more particularly 0.4 to 50.- A ratio of K2 of 10 to 10000, preferably 10 to 2500, even better 40 to 1500, and more particularly 80 to 1000. - A N2 content CV12 in G1 of 0.01 vol% to 10 vol%, preferably 0.5 vol% to 8.0 vol%, and more particularly 1.5 vol% to 5.0 vol%.

[0083] The method of the present invention, namely, predictive model-based multivariate control, preferably takes into account disturbance variables FF. These variables preferably include at least one of the following: - the amount of waste gas supplied to the absorber (FF1), - the temperature of the loaded absorbent when it enters the desorber (FF2), - the pressure in the desorber (FF3), - the energy content FF4 (e.g., vapor pressure) of the energy carrier that can be used to generate power MV2, - the composition of the waste gas mixture G3 in terms of HCl (FF5), - the composition of the waste gas mixture G3 in terms of C-containing components (FF6), - the composition of the waste gas mixture G3 in terms of H2 (FF7).

[0084] As mentioned above, the interference variable (FF) is a variable that cannot be directly affected. The manipulation variable (MV) is a variable that is adjusted in the method of the present invention to cause a change in the controlled variable (CV).

[0085] It is preferable to adjust the control variables simultaneously within a time interval of 0 to 270 seconds, particularly preferable to 10 to 180 seconds, and even more preferable to 20 to 90 seconds.

[0086] Measurement data of controlled variables and manipulated variables are also captured within this range.

[0087] For predictive control of the method of the present invention, the relationship shown in the model matrix (Table 1) can be adopted.

[0088] CV1a CV1b CV2a CV2b CV3 CV4 CV5 CV6 CV7 CV8 CV9 ΔT1 CV11 CV12 K1 K2 MV1 - + - + - - - (+) + - - + MV2 (-) (+) (-) (+) + + + + + - - (-) (-) (+) MV3 (-) (+) (+) (-) (+) (-) (-) - (-) (-) (-) (+) (-) (+) MV4 (-) (+) (+) (-) (+) (-) - (-) (+) (-) (+) MV5 - + - + + - - - (+) + (-) - - + FF1 + - + - + - - + - + + + - + + - FF2 - + - + + + + + (-) - - + FF3 + - + - + + + + (+) (+) + + - FF4 (-) (+) (-) (+) + + + + + - - (-) (-) (+) FF5 + + + - - - + + (+) + + FF6 + + FF7 - - - - - Table 1: Model Matrix

[0089] In Table 1, the parentheses indicate known dependencies that are not typically considered in the multivariate control of the method of the present invention. Although these dependencies can be additionally integrated into multivariate control, they still require additional computational power from the multivariate controller and, in general, do not lead to any significant improvement in the economy or quality of exhaust gas mixture separation.

[0090] In the table, "+" indicates positive feedback, and "-" indicates negative feedback. This is shown below:

[0091] The model matrix is ​​read in such a way that, for example, an increase in the amount of unloaded absorbent MV1 leads to a decrease in the HCl content CV1a in the first gas phase, while an increase in MV1 leads to an increase in the HCl content CV1b in the second gas phase. Accordingly, a decrease in MV1 leads to an increase in the HCl content CV1a in the first gas phase and a decrease in the HCl content CV1b in the second gas phase. As described above, the signs of the positive and negative feedback of the controlled variables are related to the increase of the manipulated variables and the disturbance variables (MV1 to MV5 and FF1 to FF7) in each case.

[0092] The disturbance variable FF is preferably taken over into control (model matrix). The response to the disturbance variable FF is achieved by predictively adapting the manipulation variables MV1 to MV5. This adaptation can be based on the overall system cost and / or by adapting the quality of the recirculated gas (first and second gas phases), thereby minimizing the overall system cost.

[0093] Predictive control is usually based on operational and functional relationships, which are pre-captured through empirical adaptation of the response of the controlled variable CV in the system to changes in the manipulator variable MV and the disturbance variable FF.

[0094] This can be accomplished by gradually changing the manipulated variable MV and determining the system response to the controlled variable CV through a so-called ladder test. Therefore, by using a ladder test, the manipulated variable MV is gradually increased or decreased, and the effect on the controlled variable CV in the system is observed.

[0095] For example, a gradual increase in the amount of unloaded absorbent MV1 leads to a gradual decrease in the HCl content CV1a in the first gas phase and a gradual increase in the HCl content CV1b in the second gas phase. These dependencies are qualitatively shown in the model matrix (see Table 1).

[0096] In addition to the direct system responses described above, predictive control also allows for the description of secondary effects. For example, increasing the evaporator power MV2 has no direct effect on the ratio CV11. However, an increase in MV2 leads to a decrease in CV1a (see Table 1). Since the manipulated variables MV1 and MV2 change independently of each other, increasing MV2 allows MV1 to decrease, which in turn leads to a corresponding decrease in CV11. In some cases, this indirect effect may lead to improved control quality.

[0097] The combination of relationships between manipulating variables, controlled variables and disturbance variables in the model matrix and their realization in the form of multivariable control usually cannot be performed by a process control system (PCS). Due to its high complexity, the process control system usually contains one or at most two control loops to be combined.

[0098] Based on the difference between the measured value and the target value, the control loop corresponds to the measurement of the controlled variable until the variable is successfully changed.

[0099] Therefore, by using multivariate control, which relies on cost functions (e.g., steam costs and / or electricity costs) to simultaneously optimize and predictively combine all manipulated variables, optimal economic efficiency can be achieved at any time.

[0100] It is preferable to separate the exhaust gas mixture so that the resulting costs, together with downstream operations (e.g., hydrogenating STC using regenerated H2 (G1) according to reaction equation (1) or (2) or producing chlorosilane using regenerated HCl (G2) according to equation (3), are minimized. In this document, with the reduction of energy use in the separation of the exhaust gas mixture, the quality of the resulting first and / or second gas phases may not be optimal for use in subsequent operations. Therefore, it may be necessary to perform these operations at a slightly higher cost. However, overall, the cost is thus minimized in all systems. Conversely, it is also possible to increase energy use in separating the exhaust gas mixture to produce the first and / or second gas phases to achieve optimal performance of downstream operations and the resulting optimization of economic efficiency.

[0101] One example is reducing energy usage in the separation of exhaust gas mixtures and intentionally accepting a limited increase in the HCl content CV1a in the first gas phase. According to thermodynamic equilibrium, this results in a decrease in the yield of TCS in the reduction of STC using H2 (see reaction equation (1)). However, there are cases where the increase in operating costs gained from the reduction of STC using H2 is less than the savings from separating the exhaust gas mixture. By means of the variable control of the present invention, this overall optimization in terms of production costs is consistently ensured.

[0102] The method of the present invention can be used in an integrated system for the production of polycrystalline silicon to variably adapt the optimal operating point to fluctuating electricity costs, steam costs, and cooling water costs and / or the availability of these factors. Here, the optimal operating point can be automatically adapted to electricity costs that fluctuate over the course of a day, preferably even to short-term fluctuations of less than one hour. This can be achieved by assigning a time-dependent cost function to the manipulated variables MV1 to MV5 in a multivariate control system.

[0103] In particular, a system comprising separating the exhaust gas mixture and using regenerated H2 to hydrogenate STC can be operated in this way at the best cost (see Equation (1)).

[0104] For example, for the hydrogenation of STC, a very low fraction of HCl in the first gas phase (CV1a) may be advantageous, while according to WO 2015 / 140027 A1, an increased fraction of C-containing components (especially methane) in the first gas phase (CV2a) is preferred to achieve longer execution times and therefore lower costs. Therefore, this system will require a first gas phase (regenerated H2) with high HCl purity and an increased fraction of C-containing components.

[0105] Surprisingly, the method of the present invention makes these different quality requirements possible.

[0106] Another aspect of the invention relates to the use of the method of the invention for purifying waste gas mixtures derived from the production of chlorosilanes or polysilicon.

[0107] The waste gas mixture preferably originates from the production of chlorosilane according to reaction equations (1), (2) or (3). In particular, it originates from the reduction of STC according to equation (1).

[0108] In addition, there are other controlled variables, manipulation variables and disturbance variables that can be integrated into the control concept to perform the method of the present invention.

[0109] For example, cooling power for reducing the absorbent temperature, which is a function of the composition of the regeneration gases (G1 and G2), can be introduced as an additional manipulated variable in the multivariate control system. An example of an additional disturbance variable is the introduction of the chlorosilane content in the exhaust gas mixture (G3) and / or the N2 content in the exhaust gas mixture.

[0110] Preferably, more than one apparatus is used in an integrated system for producing polycrystalline silicon to implement the method of the present invention. In this case, two or more of these apparatuses preferably each produce a first gas phase with different compositions (quality).

[0111] In this way, by combining downstream systems (e.g., the generation of chlorosilane, the reduction of STC, and the deposition of polycrystalline silicon), the economic optimization of the integrated system can be achieved. Therefore, the entire integrated system for producing polycrystalline silicon can operate at the lowest possible cost.

Implementation Method

[0114] Figure 1 illustrates the scheme of the method of the present invention. A waste gas mixture G3 from the production of polysilicon, preferably from STC hydrogenation, is compressed to 0.5 MPa to 2.0 MPa by a power consumption (MV5) via compressor 17, and then cooled to -70°C to 20°C by a condenser and heat exchanger section 6. It is then sent to an absorption tower 4, where it is contacted with an absorbent (e.g., a mixture of STC and TCS). As the loaded absorbent is formed, at least HCl and chlorosilanes are absorbed, and a first gas phase G1 containing H2 is removed. The waste gas mixture G3 may contain, for example, 7.3 vol% HCl, 87.0 vol% H2, 2.1 vol% N2, and 3.5 vol% uncondensed chlorosilanes (monochlorosilanes, DCS, TSC, and STC), and 0.05 vol% methane (G3 typically contains less than 0.1 vol% methane). The difference from 100% is due to rounding of the individual components. The first gas phase G1 taken from the top of the absorption tower 4 after absorption typically contains at least 95% by volume H2.

[0115] The loaded absorbent is heated to a temperature of 20°C to 100°C by passing it through the heat exchanger 7, according to the countercurrent principle, and then sent to the desorption tower 5.

[0116] In desorption tower 5, the loaded absorbent undergoes a further temperature increase to 50°C to 150°C. This temperature increase is generated by power 15 (MV2) supplied to heater 9 in vapor form. Here, the loaded absorbent is at least partially converted to a gaseous state. The temperature increase leads to the desorption of a second gas phase G2, which is removed at the top of desorption tower 5 and cooled to 35°C via a (possibly multi-stage) condenser 10 (via a countercurrent device and / or condenser). The second gas phase G2 typically contains at least 88% by volume HCl, as well as H2, N2, trace amounts of chlorosilanes (monochlorosilanes, DCS, TCS, STC) and methane.

[0117] The amount of unloaded absorbent (MV1) returning from the desorption tower 5 is cooled (countercurrent principle) by means of heat exchanger 7, and further cooled to -70°C to -10°C in condenser 7a as needed, and then supplied back to the absorption tower 4. Pump 8 is provided for this recirculation. As needed, the fraction 32 of unloaded absorbent can be taken out and supplied to, for example, a distillation apparatus.

[0118] The heat exchanger 6 typically includes a heat exchanger section comprising a primary water condenser and a countercurrent device for recovering energy from the exhaust gas mixture G3 and the first gas phase G1. Furthermore, the exhaust gas mixture G3 can be cooled to -70°C to 20°C in a separate condenser 6a (particularly low temperatures can be achieved using a cryogenic heat transfer agent such as Frigen). In this way, the flow of the first gas phase G1 can countercurrently pre-cool the flow of the exhaust gas mixture G3 in the heat exchanger 6 (this is not explicitly shown; however, generally, the flow of the second gas phase G2 can also be used for this purpose).

[0119] The cooling water flow rate 16 (MV3) to the heat exchanger 6 is varied, so that the temperature difference (CV6) between the measuring points 12 and 11 reaches a value of 2°C to 30°C.

[0120] The amount of cooling water 21 (MV4) supplied to the condenser 10 is controlled so that the temperature difference (CV7) between temperature measurement points 31 and 30 reaches a value of 5°C to 25°C.

[0121] For example, in each case, the amount of cooling water taken out, 28 and 29, is supplied to the cooling circuit.

[0122] Furthermore, the temperature 18 (CV3) at the bottom of the desorption tower 5, the temperature 19 (CV4) in the first packing unit of the desorption tower 5, and the temperature 20 (CV5) at the top of the desorption tower are captured by thermocouples (e.g., type K) and maintained within the desired target range by adapting the amount of unloaded absorbent returned to the desorption tower 5 to MV1 and by adapting the power 15 (MV2) supplied to the heater 9.

[0123] The composition of the first gas phase G1 at measurement point 13 (HCl content CV1a, C-containing component, especially methane content CV2a) is determined using high time resolution (e.g., via an IR spectrometer). By controlling multiple variables (see the matrix in Table 1), and by changing the manipulated variable MV to respond to short-term fluctuations in the disturbance variable FF, stable quality of CV1a and CV2a of the first gas phase G1 can be ensured. Therefore, at least currently the amount of unloaded absorbent returned from the desorption tower 5 via the mass flow controller 14 (MV1) and the power 15 (MV2) supplied to the heater 9 are varied with high time resolution.

[0124] The composition of the exhaust gas mixture G3, particularly its HCl content and the content of C-containing components, was determined with high time resolution using an instrument (e.g., an IR spectrometer) at measurement point 22.

[0125] For the measurement of N2 content and / or H2 content in G1, G2 and G3, at the corresponding measurement points 13, 22 and 25, for example, process chromatography using PDD (pulse discharge detector) or TCD (thermal conductivity detector) or Raman spectroscopy may be used.

[0126] The temperature of the loaded absorbent is measured at measuring point 23 at the inlet of the desorption tower 5 and at measuring point 24 at the outlet of the absorption tower 4 in the liquid phase, for example, via a thermocouple (e.g., type K).

[0127] The pressure of the first gas phase G1 at measurement point 13 at the gas outlet of the absorption tower 4 can be captured by a pressure sensor. Similarly, the pressure at measurement point 25 can also be captured. The pressure sensor can also be incorporated into a system for analyzing the gas components at measurement points 13 and 25. The gas composition of the second gas phase G2 can be measured at measurement point 25 via IR spectroscopy.

[0128] The volumetric flow rates at measurement points 13 (first gas phase, Vg1), 22 (exhaust gas mixture, FF1) and 25 (second gas phase, Vg2) can be captured by a float-type flow meter (rotameter) or calculated based on the pressure and temperature at these measurement points by measuring the mass via a mass flow meter and capturing them separately.

[0129] For example, the temperature of the waste gas mixture entering the absorption tower 4 in the gas phase at inlet 26 and the temperature of the unloaded absorbent returning to the absorption tower 4 in the liquid phase via inlet 27 can be captured by a thermocouple (e.g., type K).

[0130] Furthermore, the packing units present in the desorption tower 5 are represented as shaded rectangles (e.g., three packing units). The absorption tower may also contain installed packing units (not shown) for improving mass transfer.

[0131] Figure 2 illustrates a variant of the method of the present invention. Notably, this variant features a separate outgassing unit 33. Here, the loaded absorbent is transferred from the absorber tower 4 to the outgassing unit 33, where the pressure is reduced (e.g., due to the height difference shown). Degassing then occurs (forming a third gas phase G4). The third gas phase G4, typically having an H2 fraction of at least 60% by volume, is removed at the top of the outgassing unit 33 and can be resupplyed to an integrated system for the production of polycrystalline silicon, or it can be used for the production of dispersed silicon dioxide.

[0132] As a result, the HCl content in the second gas phase G2 increased.

[0133] Example

[0134] All examples were carried out using exhaust gas mixtures from a method for producing chlorosilane by reduction of tetrachlorosilane (STC; see equation (1)).

[0135] The unloaded absorbent used is a mixture of trichlorosilane (TCS) and STC: 69 moles of STC and 31 moles of TCS.

[0136] For all embodiments, the conditions established at the absorption tower and desorption tower are as follows: Pressure (absorption tower, measurement point 13): 1.57 MPa, Temperature (absorption tower, measurement point 27): -50°C, Pressure (desorption tower, measurement point 25): 0.36 MPa. The bottom temperature CV3 of the desorption tower is reported in the embodiments.

[0137] The heater used is a convection evaporator (shell-and-tube heat transfer device). Power MV2 is applied to the shell-and-tube heat transfer device via a 2-bar pressure steam supply. The compressor's power consumption MV5 is in the form of electrical energy.

[0138] The contents of C-containing components (CV2a, CV2b) and HCl contents (CV1a, CV1b) in the first gas phase G1 and the second gas phase G2 are measured by an analytical system using an MIR or process gas chromatograph. Temperature is measured by a thermal sensor (e.g., a type K thermocouple), and pressure is measured by a pressure sensor (e.g., a capacitive or piezoresistive pressure sensor). The contents of chlorosilane, N2, and H2 are determined, for example, by process chromatography using a PDD (pulse discharge detector) or TCD (thermal conductivity detector) or by Raman spectroscopy.

[0139] Based on standard conditions (DIN definition: DIN 1343: 0°C and 101325 Pa), report all gas volumetric flow rates (VzG, Vg1, Vg2, FF1) in standard volumetric flow rate [m3 / h].

[0140] In this embodiment of the invention, the manipulation variables MV1, MV2, MV3, MV4 and MV5, including the composition (CV1a and CV2a) of the first gas phase, are established within 30 seconds via MIR and online analysis.

[0141] Therefore, by changing the amount of unloaded absorbent MV1 within 30 seconds (e.g., via a mass flow controller), the deviation of the controlled variable CV1a from the target range (e.g., determined via MIR) can be responded to. This high temporal resolution optimizes the observation of the target range even in the presence of many interfering variables.

[0142] Embodiments of the Invention

[0143] Table 2 summarizes the embodiments of the invention.

[0144] It should be noted that under the conditions of absorption in the first gas phase G1 and desorption in the second gas phase G2, the volumetric flow rates (Vg1 and Vg2) of the first and second gas phases are essentially free of condensable components (e.g., chlorosilane), thus making the total volumetric flow rate VzG (Vg1 + Vg2) lower than the volumetric flow rate (Vgm) of the waste gas mixture. Example 1a Example 1b Example 1c Example 1d Example 1e Example 1f MV1 [kg / h] 168000 144800 205000 137200 105000 220000 MV2 [kW] 1564 917 1894 1222 611 1772 CV2a [volume ppm] 490 369 303 683 594 158 CV1a [volume%] 0.25 0.21 0.66 0.31 0.09 0.23 K1 [-] 5.1 5.7 21.8 4.5 1.5 14.6 K2 [-] 1438 982 794 651 671 2850 CV2b [volume ppm] 682 937 1151 1352 1348 319 CV1b [Volume%] 98.07 92 91.4 88 90.5 90.9 CV11 [kg / m 3 ] 7.3 9.6 7.0 8.5 4.7 7.8 Vg1 [m 3 / h] 21430 14277 27339 15000 21900 25990 Vg2 [m 3 / h] 1742 801 1942 1227 450 2081 CV3 [°C] 110.2 110.5 100.2 97.2 93.5 104.3 CV4 [°C] 108.6 109.1 99.1 96.4 91.8 103.3 CV6 [°C] 12 12 12 12 12 12 CV7 [°C] 12 12 12 12 12 12 ΔT1 [°C] 1.6 1.4 1.1 0.8 1.7 1.0 CV12 [%] 2.35 1.82 2.57 4.50 4.30 2.91 FF1 [m 3 / h] 24006 15576 30247 16730 23043 28941 FF5 [Volume %] 7.3 4.9 6.5 6.7 1.9 6.7 FF7 [Volume %] 87.0 90.1 87.9 86.1 90.9 87.5 FF6 [Volume in ppm] 487 386 348 712 590 165 N2 in exhaust gas [volume %] 2.1 1.7 2.4 4.1 4.2 2.7 CS in exhaust gas [volume %] 3.5 3.2 3.2 3.0 3.0 3.0 MV5 [kW] 303 195 375 210 290 360 Table 2: Embodiments of the Invention

[0145] Comparative Example

[0146] Table 3 summarizes the comparative examples. Example 2a Example 2b Example 2c MV1 [kg / h] 240700 168000 174000 MV2 [kW] 1503 1100 1271 CV2a [volume ppm] 3 4 366 CV1a [volume%] 1.3 0.3 1.5 K1 [-] 4333 750.0 41.0 K2 [-] 202 6 1555 CV2b [volume ppm] 4436 122989 582 CV1b [Volume%] 89.5 75 90.5 CV11 [kg / m 3 ] 20.4 98.0 6.6 V g1 [m 3 / h] 10849 1565 24531 V g2 [m 3 / h] 945 150 1863 CV3 [°C] 100.2 102.8 104.3 CV4 [°C] 82.7 99.7 92.5 CV6 [°C] 12 12 12 CV7 [°C] 12 12 12 DT1 [°C] 17.5 3.1 11.8 CV12 [%] 2.57 0.00 2.20 FF1 [m 3 / h] 12136 1749 27212 FF5 [Volume%] 8.1 6.7 7.5 FF7 [Volume %] 86.6 90.3 87.3 FF6 [volume in ppm] 348 10550 370 N2 in exhaust gas [volume %] 2.4 0.0 2.1 CS in exhaust gas [volume %] 2.8 2.0 3.0 MV5 [kW] 152 50 340 Table 3: Comparative Examples

[0147] As can be clearly seen from Table 3, Example 2a failed to achieve the required fraction CV2a of C component in the first gas phase. The HCl content CV1a and K1 exceeded the required economic range. The ratio CV11 and temperature difference ΔT1 were outside the control range. The combination of MV1 and MV2 with the exhaust gas composition and volume shown in the Comparative Example did not allow for maintaining these controlled variables (CV1a, CV2a, K1, CV11, and ΔT1). It is impossible to maintain all control ranges for every combination of manipulated variables. By using a predictive, model-based controller to ensure control range and optimal cost, those impossible combinations shown in Table 3 were avoided.

[0148] Example 2b illustrates a situation where excessively high purity is achieved in the first gas phase G1, i.e., excessively low C-containing component fraction CV2a. K1 and K2 are outside the economic range. The N2 content CV12 in the first gas phase is also outside the target range. The ratio CV11 is outside the control range. The compressor power MV5 is too low. Conversely, in Example 2c, the HCl content CV1a in the first gas phase is exceeded, and the temperature difference ΔT1 exceeds the control range. These parameter combinations are also prevented by implementing the method according to the invention using a predictive model-based controller.

[0149] These embodiments provide a good illustration of the fact that even within the expected range of each individual manipulated variable, not every combination of manipulated variables will result in compliance with all target ranges and optimal costs for the controlled variables. Avoiding combinations that lead to deviations from the target range of the controlled variables and / or increase costs is not easy and can only be achieved through predictive, model-based control structures. System operators cannot see all possible relationships, and due to the high complexity, this is no longer possible in process control systems (PCS) that contain only one or two control loops to be combined.

[0150] Cost optimization of the method implemented according to the present invention and while adhering to the target range of controlled variables. Example 3a Example 3b Example 3c MV1 [kg / h] 168000 168000 202100 MV2 [kW] 1564 1564 1880 MV3 [m 3 / h] 27 108 27 MV4 [m 3 / h] 30 110 30 MV5 [kW] 303 303 252 CV6 [°C] 12 5 12 CV7 [°C] 12 5 12 Relative cost [%] 100 130 95 Table 4: Impact of Manipulated Variables on Costs

[0151] As shown in Table 4, according to the present invention, in Example 3a, the controlled variables CV6 and CV7 remain within the control range at 12°C. Higher temperature differences for CV6 and CV7 would conflict with environmental limitations. At the relatively lower temperature difference (5°C) shown in Comparative Example 3b, an economically unfavorable operating mode was achieved by increasing the cooling water supply (by changing the manipulated variables MV3 and MV4), resulting in an increase in operating costs of 1.3 times. The remaining controlled parameters (CV) (except for the specified CV6 and CV7) remain unchanged within the target range.

[0152] Another means of optimizing cost (see Example 3c) is to change the compression power MV5, thereby changing the power requirements of the compressor, i.e., the power cost. Based on the dependencies shown in Table 1, it is necessary to adapt the variables MV1 and MV2 to observe the target range of the controlled parameters, such as the constant target group composition of the first and second gas phases when the compression power changes.

[0153] Based on the cost conditions of electricity and steam, a particularly economical operating mode relative to Example 3a (as shown in Example 3c) can therefore be achieved via a predictive, model-based multi-component controller. Additional requirements of downstream systems may need to be considered, such as the pressure required for the first gas phase in the case of STC hydrogenation.

[0154] In this economic consideration, relevant cost factors that affect the overall cost, such as the power of the compressor, are automatically taken into account by a predictive, model-based multi-component controller. [Simplified Explanation of the Diagram]

[0112] Figure 1 illustrates the scheme of the method of the present invention.

[0113] Figure 2 shows a variant of the method of the present invention.

Claims

1. A method for separating an offgas mixture G3 containing hydrogen chloride, hydrogen, chlorosilane, and a carbon-containing component, wherein the carbon-containing component is selected from organosilanes, organochlorosilanes, hydrocarbons having no more than three carbon atoms, hydrochlorocarbons having no more than three carbon atoms, and mixtures thereof, the method comprising the steps of: a) contacting the offgas mixture with an absorbent in an absorption tower at a temperature of -70°C to -10°C and a pressure of 0.5 MPa to 2 MPa, the absorbent being selected from silicone tetrachloride, trichlorosilane, dichlorosilane, organochlorosilanes, and mixtures thereof, wherein, As the loaded absorbent is formed, at least the hydrogen chloride and the chlorosilane are absorbed, and the first gas phase G1 containing hydrogen is removed; b) The loaded absorbent is transferred to a desorption tower and a temperature CV3 of 50°C to 150°C is established at the bottom of the desorption tower by supplying power to the heater, wherein as the unloaded absorbent is obtained, the second gas phase G2 is desorbed and the unloaded absorbent is returned to the absorption tower, wherein the HCl content CV1a measured in the first gas phase G1 is 0.01 vol% to 1.0 vol% and the amount of carbon components CV2a is 0.005 vol% to 0.25 vol%, and / or the HCl content CV1b measured in the second gas phase G2 is 70 vol% to 99.9 vol% and the amount of carbon components CV2b is 0.01 vol% to 5.0 vol%. The ratio K1, K1 = CV1a / CV2a, is established in G1 from 0.02 to 1000 by adjusting at least one manipulated variable, and / or the ratio K2, K2 = CV1b / CV2b, is established in G2 from 10 to 10000, wherein the manipulated variable includes - the amount of unloaded absorbent returned from the desorption tower MV1, and / or - the power MV2 supplied to the heater.

2. The method as described in request item 1, wherein, The ratio K1 in G1 includes values ​​ranging from 0.04 to 200.

3. The method as described in claim 1 or 2, wherein, The ratio K2 in G2 has values ​​ranging from 10 to 2500.

4. The method as described in claim 1 or 2, wherein, The amount of hydrogen chloride in G1, CV1a, ranges from 0.02 vol% to 0.8 vol%.

5. The method as described in request item 1 or 2, wherein, On a carbon basis, the amount of carbon component in G1, CV2a, is 0.01 vol% to 0.1 vol%.

6. The method as described in claim 1 or 2, wherein, The amount of hydrogen chloride in G2, CV1b, ranges from 75% to 99.5% by volume.

7. The method as described in request item 1 or 2, wherein, The amount of carbon component in G2, CV2b, ranges from 0.02 vol% to 2.5 vol%.

8. The method as described in claim 1 or 2, wherein, The carbon-containing component is selected from methane, ethane, acetylene, propane, and mixtures thereof.

9. The method as described in claim 1 or 2, wherein, CV3 ranges from 75°C to 140°C.

10. The method as described in claim 1 or 2, wherein, Temperature CV4 is captured in the first packing unit of the desorption tower and regulated to 45°C to 140°C.

11. The method as described in claim 10, wherein, The temperature difference ΔT1 (CV3 – CV4) is ≤ 5°C, where ΔT1 is always ≥ 0°C.

12. As in request item 1 or 2, where, The top temperature CV5 of the desorption tower was captured and adjusted to 50°C to 100°C.

13. The method as claimed in claim 1 or 2, wherein the ratio CV11 of MV1 to the sum of the volumetric flow rates VzG of G1 and G2 is from 1 kg / m3 to 15 kg / m3.

14. The method as described in claim 1 or 2, wherein, The power MV2 of the heater supplied to the desorption tower is in the range of 100 kW to 5000 kW.

15. The method as described in claim 1, wherein, The amount of cooling water MV3 is provided as another manipulation variable, wherein MV3 is supplied to the heat exchanger for cooling the exhaust gas mixture prior to contact in step a).

16. The method as described in claim 1, wherein, The amount of cooling water MV4 supplied to the condenser at the top of the desorption tower is provided as another control variable.

17. The method as described in claim 1, wherein, The power consumption MV5 of the compressor used to compress the exhaust gas mixture is provided as another control variable.

18. The method as claimed in any one of claims 1 and 15 to 17, wherein, The controlled variables, which are controlled by adjusting at least two of the manipulation variables MV1, MV2, MV3, MV4 and MV5, are as follows: - ratio K1, - ratio K2, - bottom temperature CV3 of the desorption tower, - temperature in the first packing unit of the desorption tower CV4, - top temperature of the desorption tower CV5, - temperature difference CV6 between the outlet and inlet of the cooling water at the heat exchanger used to cool the exhaust gas before contact in step a), - temperature difference CV7 between the outlet and inlet of the cooling water at the condenser at the top of the desorption tower, - temperature of the exhaust gas mixture at the gas inlet of the absorption tower CV8, - temperature of the absorbent when supplied to the absorption tower CV9, - temperature difference ΔT1 between temperatures CV3 and CV4, - ratio CV11 of the sum of the volumetric flow rates VzG of MV1 and G1 and G2, - N2 content in the first gas phase CV12.

19. The method as described in claim 18, wherein, K1, K2, CV3, CV4, CV6 and CV7 are controlled by adjusting at least two of the manipulation variables MV1, MV2, MV3, MV4 and MV5.

20. The method as described in claim 19, wherein, The interference variables are taken into consideration, including at least one of the following: - the amount of waste gas supplied to the absorber FF1, - the temperature of the absorbent at the pressure in the desorber FF2 when it enters the desorber, - the pressure in the desorber FF3, - the energy content of the energy carrier that can be used to generate power MV2 FF4, - the composition of the waste gas mixture G3 in terms of HCl FF5, - the composition of the waste gas mixture G3 in terms of the carbonaceous component FF6, - the composition of the waste gas mixture G3 in terms of H2 FF7.

21. The method as described in claim 1 or 2, wherein, The manipulation variable is adapted within an interval of 0 to 270 seconds.

22. Use of the method as claimed in any one of claims 1 to 21 for purifying a mixture of waste gases from the production of chlorosilane or polysilicon.

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