Methods for recovering hydrogen chloride from exhaust gases

Abstract

In the chemical industry, various processes generate exhaust gas streams contaminated with hydrogen chloride. Using conventional state-of-the-art methods, the hydrogen chloride is removed from the exhaust gas by absorption in water and recovered as aqueous hydrochloric acid. High-purity hydrogen chloride with >99.99 wt% HCl can then be produced from this by extractive rectification or pressure swing rectification. The present invention relates to a method for recovering hydrogen chloride from exhaust gas streams in which the hydrogen chloride is removed from the exhaust gas by adiabatic absorption in dilute hydrochloric acid and recovered as concentrated aqueous hydrochloric acid. This concentrated hydrochloric acid is then separated by pressure rectification into high-purity, concentrated hydrogen chloride and dilute hydrochloric acid, and the dilute hydrochloric acid produced is used again for the absorption of the hydrogen chloride. The method is characterized in that... a) the concentrated hydrochloric acid flowing from an absorption column (K201) is fed into the head of a stripping column (K202) and b) a heat exchanger (W202) is installed at the sump of this stripping column (K202), through which energy is indirectly introduced into the system.

Description

[0001] In the chemical industry, various processes generate exhaust gas streams contaminated with hydrogen chloride. Using state-of-the-art methods, the hydrogen chloride is removed from the exhaust gas by absorption in water and recovered as aqueous hydrochloric acid. Dry hydrogen chloride gas with >99.99% HCl by mass can then be produced from this by extractive rectification or pressure swing rectification.

[0002] GB 102 1963 A describes a process for the treatment of exhaust gas mixtures containing hydrogen chloride, phosgene and possibly other impurities, in which hydrogen chloride is separated from the exhaust gas mixture as concentrated hydrochloric acid by adiabatic absorption with water and / or dilute hydrochloric acid, the mixture of water vapor and phosgene released in this process is passed over activated carbon at temperatures above the dew point of the mixture, the hydrogen chloride produced by the reaction of phosgene and water vapor is condensed and the hydrochloric acid is separated from the remaining components of the gas mixture.

[0003] German patent GB 1 032 806 A describes the production of hydrochloric acid by adiabatic absorption of a fluctuating amount of impure hydrogen chloride in water, where the concentration of the outflowing acid is kept constant by controlling the water supply according to the concentration. The temperature of the adsorption zone is high enough to carry away impurities in the generated vapor. Should the hydrogen chloride concentration drop, the water supply is increased to form a dilute acid, which is withdrawn, stored, and fed back into the process in a subsequent phase of high hydrogen chloride concentration in the feed gas instead of water.

[0004] US Patent 2 220 570 A describes a process for the absorption of hydrochloric acid from a moist gas, using an aqueous absorbent, in which the heat generated by the absorption is removed by adding the amount of dry ambient air required for cooling.

[0005] In US 3 807 139 A, high-quality 32 to 33 percent hydrochloric acid is recovered from hydrogen chloride produced in organic reactions by a combination of isothermal and adiabatic absorption steps.

[0006] CN 116 062 695 A describes a heat-coupled hydrochloric acid pressure-swing distillation process with low energy consumption. In this process, hydrated hydrogen chloride gas is driven off at the top of a pressurized column. Hydrochloric acid is then concentrated and condensed from this gas with energy recovery. Further condensation and the use of a separator yield highly pure, concentrated hydrogen chloride. The dilute hydrochloric acid obtained at the bottom of the pressurized column is concentrated in a separate, vacuum-operated column by evaporating water, and the concentrated hydrochloric acid is then recycled back to the pressurized column.

[0007] DE 2542127 C2 describes a process for producing sulfur-free hydrohalic acid from a mixture of hydrohalic acid, SO2 and optionally water, characterized in that the volatile part of the hydrohalic acid is absorbed with water and subjected to a stripping process, which drives SO2 out of the mixture by means of stripping gas and also subjects the resulting mixture of stripping gas, SO2 and volatile components of the hydrohalic acid to absorption.

[0008] US 2765873 A describes a process for recovering pure chlorine from a gaseous mixture of chlorine and inert atmospheric gases, in which the mixture is introduced into an absorption zone operated at overpressure and in which chlorine is absorbed in countercurrent flow in carbon tetrachloride.

[0009] EP 619268 B1 describes a process for producing hydrogen chloride (HCl) from chlorine-containing waste, comprising the following steps: combustion of the waste, production of crude acid from the flue gas formed during combustion by means of gas scrubbing, treatment of wastewater, some of which is generated during flue gas scrubbing, producing residual waste with a high solids content and a chloride-containing solution, return of the chloride-containing solution to the combustion process for chlorine recovery, distillation of the crude acid with an entrainment agent to obtain an HCl-rich gas while simultaneously removing impurities soluble in the entrainment agent, optionally separating hydrogen fluoride from the HCl-rich gas, drying of the HCl-rich gas by means of sorption, whereby the sorbent is regenerated and any aqueous condensate produced is fed to wastewater treatment.and furthermore, processing of the entraining agent by evaporating water and removing impurities by means of precipitation and filtration, whereby on the one hand any aqueous condensate is returned to the gas scrubbing process and on the other hand filtered-out solids are fed to wastewater treatment for chlorine recovery.

[0010] A review of the various processes according to the prior art reveals to a person skilled in the art a process for recovering hydrogen chloride from exhaust gases by absorption of the hydrogen chloride in dilute hydrochloric acid and process water, producing concentrated hydrochloric acid. This concentrated hydrochloric acid is then separated by rectification in a pressure rectification column (K203) under pressure into high-purity, concentrated hydrogen chloride with >99.99 wt% HCl and dilute hydrochloric acid, which is then recirculated for absorption. This prior art process is described in Fig.depicted.

[0011] Hydrogen chloride exhaust gas [1] is introduced at the bottom of an absorption column (K101). In the middle section of the absorption column [K101], recycled dilute hydrochloric acid [2] is fed countercurrently to the exhaust gas. At the top of the absorption column (K101), process water [3] is fed countercurrently to the exhaust gas. Concentrated hydrochloric acid [4] exits the absorption column (K101) at the bottom, flows into a feed tank (B101), and is pumped by a pump (P101) to a pressure rectification column (K102). Due to the heat of absorption released during absorption, water evaporates along with the exhaust gas at the top of the absorption column (K101) [5]. The water vapor is condensed in a heat exchanger (W101). A partial stream of the condensed water [6] is fed at the top of the absorption column [K101]. The remaining condensate [7] is discharged as wastewater. The cleaned exhaust gas [8] is discharged from the heat exchanger [W101].

[0012] The concentrated hydrochloric acid [4] is fed into the middle section of a pressure rectification column (K102). The energy required for the evaporation of the hydrogen chloride is supplied by indirect heating via a heat exchanger (W102) at the bottom of the pressure rectification column (K201). The dilute hydrochloric acid [2] obtained at the bottom of the pressure rectification column (K201) is returned to the absorption column (K101). Hydrogen chloride gas [9] containing water is driven off at the top of the pressure rectification column (K102). In a heat exchanger (W103), concentrated hydrochloric acid

[10] is condensed, thereby reducing the water content of the hydrogen chloride gas

[11] . In another heat exchanger (W104), concentrated hydrochloric acid

[12] is condensed again, yielding highly pure, concentrated hydrogen chloride

[13] . The condensed concentrated hydrochloric acids

[10] and

[12] are recirculated to the head of the pressure rectification column (K102).

[0013] This state-of-the-art procedure has the following shortcomings.

[0014] All the water introduced with the hydrogen chloride exhaust gas [1] and all the process water [3] added for absorption must be discharged as wastewater [7] to ensure the overall mass balance for the process is correct.

[0015] Table 1 shows an example 1 with a simplified mass balance for a state-of-the-art method. Table 1 1 2 3 HCl 100.0 kg / h 50,00% 68.8 kg / h 18,00% H2O 2.0 kg / h 1,00% 313.5 kg / h 82,00% 10.0 kg / h 100,00% Air 98.0 kg / h 49,00% Total 200.0 kg / h 100,00% 382.3 kg / h 100,00% 10.0 kg / h 100,00% Drunk 0.20 MPa 0.40 MPa 0.20 MPa Temperature 20,0 °C 140,0 °C 25,0 °C Enthalpy 14.6 kW 44.9 kW 0.3 kW 4 5 6 HCl 168.7 kg / h 35,00% 0.4 kg / h 0,25% 0.3 kg / h 0,81% H2O 313.3 kg / h 65,00% 43.9 kg / h 30,86% 31.7 kg / h 99,19% Air 98.0 kg / h 68,89% Total 482.1 kg / h 100,00% 142.3 kg / h 100,00% 32.0 kg / h 100,00% Drunk 0.10 MPa 0.10 MPa 0.10M Pa Temperature 86,0 °C 101,0 °C 50,0 °C Enthalpy 26.2 kW 35.4 kW 1.9 kW 7 8 9 HCl 0.1 kg / h 0,81% 131.6 kg / h 70,00% H2O 12.2 kg / h 99,19% 56.4 kg / h 30,00% Air 98.0 kg / h 100,00% Total 12.3 kg / h 100,00% 98.0 kg / h 100,00% 188.0 kg / h 100,00% Drunk 0.10 MPa 0.10 MPa 0.40 MPa Temperature 50,0 °C 50,0 °C 135,0 °C Enthalpy 0.7 kW 1.4 kW 69.1 kW 10 11 12 HCl 29.3 kg / h 36,00% 102.3 kg / h 96,00% 2.4 kg / h 36,00% H2O 52.2 kg / h 64,00% 4.3 kg / h 4,00% 4.3 kg / h 64,00% Air Total 81.5 kg / h 100,00% 106.6 kg / h 100,00% 6.7 kg / h 100,00% Drunk 0.40 MPa 0.40 MPa 0.40 MPa Temperature 40,0 °C 40,0 °C 0,0 °C Enthalpy 1.7 kW 18.1 kW 0.0 kW 13 HCl 99.9 kg / h 99,99900% H2O 0.001 kg / h 0,00100% Air Total 99.9 kg / h 100,00% Drunk 0.40 MPa Temperature 0,0 °C Enthalpy 12.3 kW

[0016] For Example 1 described in Table 1, a cylindrical absorption column with a diameter of 250 mm is selected. The liquid loading of the absorption column (K101) is calculated as 0.84 m³ from the process water flow rate of 10 kg / h [3] and the recirculated partial flow of condensed water of 32 kg / h [6]. 3 / m 2 h.

[0017] In state-of-the-art processes, the liquid loading is limited and only minimally controllable. As the exhaust gas volume increases, a larger column diameter is required, but the available liquid volume remains the same, thus reducing the liquid loading. If, with the same exhaust gas volume, the hydrogen chloride content in the hydrogen chloride exhaust gas [1] is low, less absorption energy is available for water evaporation, and therefore less process water [3] can be fed in. The same applies if the water content in the hydrogen chloride exhaust gas [1] is higher. For the operation of the absorption column (K101), a minimum liquid loading at the top is required to achieve the desired separation efficiency and to minimize hydrogen chloride losses in the wastewater [7]. A higher liquid loading in the absorption column K101 can minimize the loss of hydrogen chloride with the wastewater.

[0018] The present invention was therefore based on the objective of modifying the process in such a way that, even with an unfavorable composition of the hydrogen chloride exhaust gas [1], highly pure, concentrated hydrogen chloride with >99.99 wt% HCl can be recovered with reasonable effort and at the same time minimize the losses of hydrogen chloride with the wastewater.

[0019] This problem can surprisingly be solved simply according to the invention by additionally installing a stippling column (K202) with a sump heater (W202) below an absorption column (K201). The process according to the invention is described in Fig. depicted.

[0020] The hydrogen chloride exhaust gas

[21] is introduced at the bottom of an absorption column (K201). In the middle section of the absorption column (K201), recirculated dilute hydrochloric acid

[22] is fed countercurrently to the exhaust gas. At the top of the absorption column (K201), process water

[23] is fed countercurrently to the exhaust gas. Concentrated hydrochloric acid

[24] exits the absorption column (K201) at the bottom and flows into the stripping column (K202) according to the invention. A heat exchanger (W202) for indirect heating is installed at the bottom of the stripping column according to the invention. This allows additional energy to be introduced into the absorption column by indirect heating, so that more water evaporates at the top

[25] and is then returned to the absorption column as reflux

[26] .Since concentrated hydrochloric acid with an HCl concentration above the azeotropic composition is obtained at the outlet of the absorption column, the indirect heating in the stripping column according to the invention causes more HCl than water to evaporate, thus reducing the concentration of the concentrated hydrochloric acid

[24] flowing into a feed vessel (B201). This increases the quantity

[24] pumped by a pump (P201) to a pressure rectification column (K203) and consequently also the quantity of dilute hydrochloric acid

[22] returned to the absorption column (K201) for hydrogen chloride absorption. Since this stream of dilute hydrochloric acid

[22] is not cooled according to the invention, the energy input into the absorption column (K201) is further increased, thus increasing water evaporation

[25] and the corresponding reflux volume

[26] .Compared to the prior art, in addition to the heat of absorption released during absorption, the energy from indirect heating via a heat exchanger

[202] and the additional energy from the larger quantity of recycled dilute hydrochloric acid

[22] are available for the evaporation of water together with the exhaust gas

[25] at the top of the absorption column (K201). The water vapor is condensed in a heat exchanger (W201). A partial stream of the condensed water

[26] is fed at the top of the absorption column [K201]. The remaining condensate

[27] is discharged as wastewater. The purified exhaust gas

[28] is discharged from the heat exchanger [W201].

[0021] The concentrated hydrochloric acid

[24] , which has a lower HCl concentration compared to the prior art, is fed into the middle section of a pressure rectification column (K203). The energy required for the evaporation of the hydrogen chloride is supplied by indirect heating via a heat exchanger (W203) at the bottom of the pressure rectification column (K203). The dilute hydrochloric acid

[22] obtained at the bottom of the pressure rectification column (K203) is returned to the absorption column (K201). Hydrogen chloride gas

[29] containing water is driven off at the top of the pressure rectification column (K203). Concentrated hydrochloric acid

[30] is condensed in a heat exchanger (W204), thereby reducing the water content of the hydrogen chloride gas

[31] . In a further heat exchanger (W205) concentrated hydrochloric acid

[32] is condensed again, resulting in highly pure, concentrated hydrogen chloride

[33] .The condensed concentrated hydrochloric acids

[30] and

[32] are recirculated to the head of the pressure rectification column (K202).

[0022] Table 2 shows an example 2 with a simplified mass balance for the method according to the invention.

[0023] In Example 2, the composition of the hydrogen chloride exhaust gas is identical to Example 1. In Example 2, a heat quantity of 6.7 kW is introduced at the bottom of the stripping column (K202) via the heat exchanger (W202) according to the invention. As described above, this reduces the concentration of the concentrated hydrochloric acid

[24] to 30% HCl compared to 35% HCl in Example 1. This increases the amount of dilute hydrochloric acid

[22] returned to the absorption column (K201) for hydrogen chloride absorption to 583.3 kg / h compared to 382.3 kg / h in Example 1. The reflux rate

[26] is therefore 63.4 kg / h. For Example 2 described in the table, a cylindrical absorption column with a diameter of 250 mm is also selected, since the amount of hydrogen chloride exhaust gas is identical to Example 1.The liquid load achieved in Example 2 with the method according to the invention is calculated from the mass flow of process water 10 kg / h

[23] and the recirculated partial flow of the condensed water 63.4 kg / h

[26] divided by the cross-sectional area of ​​the absorption column (K101) to 1.5 m. 3 / m 2 h. The 75% higher liquid load results in a better cleaning effect of the absorption column and minimizes hydrogen chloride losses with the wastewater

[27] . This also increases the recovery rate. This is a further advantage of the process according to the invention. Table 2 21 22 23 HCl 100.0 kg / h 50,00% 105.0 kg / h 18,00% H2O 2.0 kg / h 1,00% 478.3 kg / h 82,00% 10.0 kg / h 100,00% Air 98.0 kg / h 49,00% Total 200.0 kg / h 100,00% 583.3 kg / h 100,00% 10.0 kg / h 100,00% Drunk 0.20 MPa 0.40 MPa 0.20 MPa Temperature 20,0 °C 140,0 °C 25,0 °C Enthalpy 14.6 kW 68.5 kW 0.3 kW 24 25 26 HCl 205.0 kg / h 30,00% H2O 478.3 kg / h 70,00% 75.4 kg / h 43,48% 63.4 kg / h 100,00% Air 98.0 kg / h 56,52% Total 683.3 kg / h 100,00% 173.4 kg / h 100,00% 63.4 kg / h 100,00% Drunk 0.10 MPa 0.10 MPa 0.10 MPa Temperature 86,0 °C 101,0 °C 50,0 °C Enthalpy 37.8 kW 56.1 kW 3.7 kW 27 28 29 HCl 131.8 kg / h 70,00% H2O 12.0 kg / h 100,00% 56.5 kg / h 30,00% Air 98.0 kg / h 100,00% Total 12.0 kg / h 100,00% 98.0 kg / h 100,00% 188.2 kg / h 100,00% Drunk 0.10 MPa 0.10 MPa 0.40 MPa Temperature 50,0 °C 50,0 °C 135,0 °C Enthalpy 0.7 kW 1.4 kW 69.2 kW 30 31 32 HCl 29.4 kg / h 36,00% 102.4 kg / h 96,00% 2.4 kg / h 36,00% H2O 52.2 kg / h 64,00% 4.3 kg / h 4,00% 4.3 kg / h 64,00% Air Total 81.6 kg / h 100,00% 106.7 kg / h 100,00% 6.7 kg / h 100,00% Drunk 0.40 MPa 0.40 MPa 0.40 MPa Temperature 40,0 °C 40,0 °C 0,0 °C Enthalpy 1.7 kW 18.2 kW 0.0 kW 33 HCl 100.0 kg / h 99,99900% H2OLair 0.001 kg / h 0,00100% Total 100.0 kg / h 100,00% Drunk 0.40 MPa Temperature °C0.0°C TemperatureEnthalpy 0.0°C12.3 kW

[0024] The present invention relates to a process for recovering hydrogen chloride from exhaust gas streams in which hydrogen chloride is removed from the exhaust gas by adiabatic absorption in dilute hydrochloric acid and recovered as concentrated aqueous hydrochloric acid, which is then separated by rectification in a pressure rectification column (K203) under pressure into high-purity, concentrated hydrogen chloride and dilute hydrochloric acid, and the dilute hydrochloric acid produced is used again for the absorption of the hydrogen chloride, characterized in that a) the concentrated hydrochloric acid flowing from an absorption column (K201) is fed into the head of a stripping column (K202) and b) a heat exchanger (W202) is installed at the sump of this stripping column (K202), through which energy is indirectly introduced into the system.

[0025] The absorption column (K201) according to the invention is operated at atmospheric pressure 0.1 MPa + / - 0.02 MPa.

[0026] The stripping column (K202) according to the invention is also operated at atmospheric pressure 0.1 MPa + / - 0.02 MPa.

[0027] The pressure rectification column (K203) according to the invention is operated at a pressure between 0.2 MPa and 0.6 MPa.

[0028] According to the invention, the concentration of the concentrated aqueous hydrochloric acid flowing from the stripping column (K202) is between 28 and 34 wt% HCl.

[0029] According to the invention, the concentrated hydrochloric acid flows out of the stripping column (K202) at its boiling point. This point is between 70 °C and 100 °C, depending on the acid concentration.

[0030] According to the invention, the concentration of the dilute aqueous hydrochloric acid flowing from the pressure rectification column (K203) is between 16 and 20 wt% HCl.

[0031] According to the invention, the dilute hydrochloric acid flows out of the pressure rectification column (K203) at its boiling point. This boiling point is between 110 °C and 160 °C, depending on the acid concentration.

[0032] To recover energy, the concentrated hydrochloric acid could be preheated against the dilute hydrochloric acid. According to the invention, this step is omitted, as the aim is to transfer as much energy as possible into the absorption column (K201) with the dilute hydrochloric acid.

[0033] The process according to the invention has the advantage over the prior art that, with the stripping column (K202) according to the invention, volatile impurities such as organic solvents or inorganic gases are also removed from the concentrated hydrochloric acid and thus do not enter the highly pure, concentrated hydrogen chloride.

Claims

[1] Process for the recovery of hydrogen chloride from exhaust gas streams in which hydrogen chloride is removed from the exhaust gas by adiabatic absorption in dilute hydrochloric acid and recovered as concentrated aqueous hydrochloric acid, which is then separated by rectification in a pressure rectification column (K203) under pressure into high-purity, concentrated hydrogen chloride and dilute hydrochloric acid and the dilute hydrochloric acid produced is used again for the absorption of the hydrogen chloride, characterized by , that a) the concentrated hydrochloric acid flowing from an absorption column (K201) is fed into the head of a stripping column (K202) and b) a heat exchanger (W202) is installed at the sump of this stripping column (K202), through which energy is indirectly introduced into the system. [2] Method according to claim 1 characterized by , that the absorption column (K201) according to the invention is operated at atmospheric pressure 0.1 MPa + / - 0.02 MPa. [3] Method according to claim 1 characterized by , that the stripping column (K202) according to the invention is operated at atmospheric pressure 0.1 MPa + / - 0.02 MPa. [4] Method according to claim 1 characterized by , that the pressure rectification column (K203) according to the invention is operated at a pressure between 0.2 MPa and 0.6 MPa. [5] Method according to claim 1 characterized by , that the concentration of the concentrated aqueous hydrochloric acid flowing from the stripping column (K202) is between 28 and 34 mass % HCl. [6] Method according to claim 1 characterized by , that the concentrated hydrochloric acid with a boiling point between 70 °C and 100 °C flows out of the stripping column (K202). [7] Method according to claim 1 characterized by , that the concentration of the dilute aqueous hydrochloric acid flowing from the pressure rectification column (K203) is between 16 and 20 mass % HCl. [8] Method according to claim 1 characterized by, that the dilute hydrochloric acid with a boiling point between 110 °C and 160 °C flows out of the pressure rectification column (K203). [9] Method according to claim 1 characterized by , that with the stripping column (K202) according to the invention, volatile impurities are removed from the concentrated hydrochloric acid and thus do not enter the highly pure, concentrated hydrogen chloride.

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