A method and an apparatus for separating chlorine gas from a gaseous anode outlet stream of an electrochemical reactor The invention relates to a method for separating chlorine from a gaseous anode out- let stream mass flow of an electrochemical cell reactor. In a first aspect, the method makes use of an absorption step, wherein an anode outlet stream mass flow of the electrochemical cell reactor is exposed to an organic solvent for achieving an exergy- efficient separation of chlorine and hydrogen chloride by dissolving the chlorine in the organic solvent. In a further aspect, the method makes use of absorption step, wherein the anode outlet stream mass flow is exposed to an ionic liquid, wherein the hydrogen chloride is dissolved in said ionic liquid, thereby forming a gas flow contain- ing essentially chlorine and a solution mass flow comprising the ionic liquid and the hydrogen chloride. The hydrogen chloride is desorbed from the solution mass flow in a desorption step. In another aspect, the method makes use of a distillation step, wherein the anode outlet stream mass flow is separated at a static pressure of at least 1 bar, like at least 2 bar for an exergy-efficient separation.

Prior art

Chlorine is needed as a base chemical for numerous synthesis methods, e.g. for the industrial production of polycarbonate and isocyanate, such as methylene diisocyanate and toluene diisocyanate. In these synthesis methods, phosgene is em- ployed in reaction steps, thereby consuming chlorine on the one hand and emerging hydrogen chloride as a byproduct. The hydrogen chloride as a byproduct exceeds the need in other chemical processes, therefore recycling of the chlorine from the hy- drogen chloride is favorable.

In the prior art, recycling of the chlorine is performed by heterogeneously cata- lyzed conversion of hydrogen chloride to chlorine, having the disadvantage of high investment costs. An electrochemical conversion of hydrogen chloride provides the advantages of both modularity in small-scale plants and flexibility concerning the pro- duction capacity. One variant for electrochemical conversion of the hydrogen chloride is a diaphragm electrolyzer technology, converting the hydrogen chloride being fed as an aqueous hydrochloric acid feed stream.

US 8153838 B2 discloses a more exergy-efficient variant of the electrolyzer technology with a membrane, said technology being already applied at industrial scale by Bayer and Uhdenora. Further details of the process are disclosed in“Hydro- chloric acid electrolysis sustainable chlorine production” by ThyssenKrupp Uhde GmbH in the year 2012.

US 5580437 A discloses a further variant for the electrochemical conversion in a gas phase process, wherein gaseous hydrogen chloride is lead to an anode at a membrane permeable for protons. An anode outlet stream comprising chlorine and unconverted hydrogen chloride is formed by oxidation of the hydrogen chloride at the anode, whereby the protons diffuse through the membrane to a cathode and react with a cathode gas comprising oxygen or an oxygen-comprising gas, thereby forming water. Separation of chlorine from the unreacted hydrogen chloride in the anode out- let stream is proposed by a separator, the separation involving generally e.g. distilla tion, adsorption, extraction or membrane separation.

DE 10 2013 009 230 A1 discloses an electrochemical cell for the conversion of hydrogen chloride to chlorine and water in a gas-phase reaction and proposes to hu- midify the cathode gas comprising oxygen or an oxygen-comprising gas for better proton conductivity of the membrane.

Description of the present invention

The object of the present invention is to provide a method and an apparatus for separating chlorine from a gaseous anode outlet stream mass flow of an electro- chemical cell reactor with a small exergy demand.

It has been found out that a reduction of the exergy demand is achievable by applying a separation step making use of an organic solvent in an absorption step. Therefore, the invention provides a method for separating chlorine from a gaseous anode outlet stream mass flow of an electrochemical cell reactor, said anode outlet stream mass flow essentially comprising chlorine and anhydrous hydrogen chloride. The method comprises an absorption step a), wherein the anode outlet stream mass flow is exposed to an organic solvent which is essentially immiscible with water. Thereby, a solution mass flow comprising the organic solvent is formed, said organic solvent containing the chlorine and a part of the hydrogen chloride. The chlorine and the part of hydrogen chloride both are dissolved in said organic solvent. Furthermore, a gas flow containing essentially hydrogen chloride is formed. The use of the organic solvent according to the invention makes the solution applicable to exergy-efficient separation of the chlorine in a subsequent desorption step.

The organic solvent according to the present invention is an organic solvent be- ing essentially immiscible with water. In an aspect, the organic solvent is a non-polar solvent. E.g., non-polar solvents are defined as having a dielectric constant of less than 15.

The electrochemical cell reactor is to be understood as to comprises an anode and a cathode being separated by a membrane, said membrane being permeable for protons and, of course, water. Therein, an electrical potential is applied between the cathode and the anode by an electrical power source. The membrane does split up the electrochemical cell into two half-cells generally being connected only with re- spect to protons due to the permeability for said protons. A gaseous hydrogen chlo- ride steam is fed into the anode-side half-cell of the electrochemical cell reactor, the anode by means of the power source oxidizes the hydrogen chloride forming chlorine and protons. The protons diffuse through the membrane to the cathode. A cathode gas is fed into the cathode-side half-cell of the electrochemical cell reactor. For ex- ample. In one embodiment, the cathode gas comprises oxygen which may be humid- ified. At the cathode, the oxygen of the cathode gas is reduced by the electrical power source, thereby forming water with the diffused protons from the anode side as a product of the cathode-side half-cell. Alternatively, the protons are reduced at the cathode to hydrogen.

In the electrochemical cell, the hydrogen chloride being fed at the anode-side of the electrochemical cell reactor is essentially dry. This allows for an exergy saving combination with the absorption step according to the invention with an advanta- geous balance of exergy.

The absorption step is to be understood for example to be conducted in an ab- sorption column, particularly an absorption column with bubble trays, sieve bottoms, or a packed absorption column. The absorption column is arranged such as to allow for a good mass transfer from the anode outlet stream mass flow into the organic sol- vent concerning the chlorine being contained in the anode outlet stream mass flow. Usually, the gas flow does leave an absorption column at the head. The solution mass flow usually leaves an absorption column at the bottom. The anode outlet stream mass flow usually will be led into the absorption column’s bottom for ascend- ing the column. The organic solvent mass flow usually is led into the column’s head. The anode outlet stream mass flow and the organic solvent mass flow by these means form a countercurrent flow for a good mass transfer of the chlorine into the or- ganic solvent.

The absorption step according to the invention allows for direct recycling of the gas flow containing essentially hydrogen chloride into the electrochemical cell reac- tor. Thereby, a high overall conversion of hydrogen chloride can be achieved with dif ferent possible single-pass conversions in the electrochemical cell reactor, hence a low cell potential is possible.

The absorption step, due to the lack of dissociation of the hydrogen chloride in an organic solvent, therefore implies a reduced production of solution entropy and therefore a reduced loss of Gibbs enthalpy of the hydrogen chloride which can be re- cycled without a further cost or energy consuming processing. Thus, also investment expenses are low.

In the absorption step, a further saving of exergy can be achieved by adjust- ment of the organic solvent’s mass flow to a minimum being just necessary for achieving the absorption of essentially all the chlorine into the solution mass flow.

This allows for direct recycling of a large fraction of the hydrogen chloride from the anode outlet stream mass flow.

The necessary amount of organic solvent in the absorption step can be reduced by compressing the anode outlet stream mass flow prior to exposing it to the organic solvent. This allows for additionally savings of exergy. In spite of the higher consump- tion of electricity for the compression, the exergy consumption in total has been found out to be reduced when taking into account the necessity of boiling a corre- spondingly smaller amount of organic solvent. An advantageous static pressure of the absorption has turned out to be at least 1 bar, at least, 2 bar, like at least 8 bar.

Further, the method with step a) involving an organic solvent additionally corn- prises an extraction step b) and a desorption step c). In the extraction step b), the so- lution mass flow obtained in step a) is exposed to a water mass flow, thereby forming an aqueous hydrogen chloride mass flow comprising essentially the water mass flow and hydrogen chloride extracted from said solution. Thus, a purified solution mass flow is formed, said purified solution mass flow being composed of the organic sol- vent and the chlorine. In the desorption step c), the chlorine is desorbed from the pu- rified solution mass flow obtained in the extraction step b). The combination of the extraction step and the desorption step c) leads to a high purity of the desorbed chlo- rine, allowing for recycling of said chlorine and for its usage as product for chemical synthesis steps, e.g. in synthesis processes for the production of polycarbonate and isocyanates, such as Methylene diphenyl diisocyanateand toluene diisocyanate.

The extraction step can be performed in a device for extraction. The device for extraction is to be understood as a device for exposing the solution mass flow to a water mass flow with a large interphase. The organic solvent is essentially not solu- ble in water and vice versa. Therefore, the entire extraction step can be performed for example in a mixer-settler device, wherein a mixing component of the mixer-set- tier device applies said interphase for a good mass transport of the hydrogen chloride from the solution mass flow into the water mass flow. After said mass transport is achieved, separation of the aqueous hydrogen chloride mass flow, which is non-solu- ble in the solution mass flow and the purified solution mass flow, and the purified so- lution mass flow is performed due to differences in density in the settling component of the mixer-settler device. The purified solution mass flow and the aqueous hydro- gen chloride mass flow are discharged from the device for extraction.

The purified solution mass flow is exposed to a desorption step, for example in a desorption column, wherein the purified solution mass flow is boiled, thereby forrn- ing a liquid organic solvent mass flow and an essentially gaseous chlorine mass flow.

In addition, the present inventors recognized that usage of an ionic liquid in an absorption step for separating chlorine from the gaseous anode outlet stream mass flow is surprisingly exergy-efficient. Thus, the present invention discloses a method for separating chlorine from a gaseous anode outlet stream mass flow of an electro- chemical cell reactor, said anode outlet stream mass flow essentially comprising chlorine and anhydrous hydrogen chloride, comprising an absorption step a) and a desorption step b). In the absorption step a), the anode outlet stream mass flow is exposed to an ionic liquid, thereby forming a solution mass flow. The solution mass flow comprises the ionic liquid, said ionic liquid containing the hydrogen chloride dis- solved in said ionic liquid. Thereby, a gas flow is formed, said gas flow containing es- sentially chlorine. In the desorption step b), the hydrogen chloride is desorbed from the solution mass flow obtained by reduction of pressure and/or raising the tempera- ture.

As used herein, the term“ionic liquid” refers to a salt in a liquid state. Ionic liq uids are also known as ionic melts, liquid salts, fused salts or ionic glasses. The ionic liquids are selected to not react with at least one of HCL, Cl_2 or H2O. Examples of ionic liquids include diethyl-methylsulfonium-methanesulfonate, triethylsulfonium-thio- cyanate, triethylsulfonium-dicyanamide, diethyl-methylsulfonium-thiocyanate, ethyl- dimethylsulfonium-thiocyanate, diethyl-methylsulfonium-dicyanamide, ethyl-dimethyl- sulfonium-dicyanamide, ethyl-dimethylsulfonium-methylsulfate, diethyl-methyl- sulfonium-methylsulfate, triethylsulfonium-ethylsulfate, diethyl-methylsulfonium-ethyl- sulfate, ethyl-dimethylsulfonium-ethylsulfate, triethylsulfonium-methoxyethylsulfate, triethylsulfonium-2-(2-methoxyethoxy)ethylsulfate, trihexyl(methoxymethyl)phospho- nium-methylsulfate, triethylsulfonium-tricyanomethane, diethyl-methylsulfonium-tricy- anomethane, ethyl-dimethylsulfonium-tricyanomethane, hexadecyl-trihexyl-phospho- nium-hydrogensulfate, tributylmethylphosphonium-tricyanomethane, butyl-trihexyl- phosphonium-tricyanomethane, trihexyl(methoxymethyl)phosphonium-tricyano- methane, trihexyl(methoxymethyl)phosphonium-trifluoromethane-sulfonat e, tetra- hexyl-phosphonium-tricyanomethane, heptyl-trihexyl-phosphonium-tricyanomethane, tributyl-tetradecylphosphonium-tricyanomethane, trihexyl-octyl-phosphonium-tricy- anomethane, decyl-trihexyl-phosphonium-tricyanomethane, dodecyl-trihexyl-phos- phonium-tricyanomethane, trihexyl-tetradecyl-phosphonium-tricyanomethane, hexa- decyl-trihexyl-phosphonium-tricyanomethane, triethylsulfonium-trifluoromethyl- sulfonylimide, diethyl-methylsulfonium-trifluoromethylsulfonylimide, ethyl-dimethyl- sulfonium-trifluoromethylsulfonylimide, triethylsulfonium-bis(pentafluoroethyl- sulfonyl)amide, diethyl-methylsulfonium-bis(pentafluoroethylsulfonyl)amide, ethyl-di- methylsulfonium-bis(pentafluoroethylsulfonyl)amide, trihexyl(methoxymethyl)phos- phonium-trifluoromethylsulfonylimide, trihexyl(methoxymethyl)phosphonium-bis(pen- tafluoroethylsulfonyl)amide.

Then, the ionic liquid and the hydrogen chloride can be compressed to atmos- pheric pressure. The desorption step can be carried out at a low pressure, in particu- lar at a pressure in the range of 0 to 100 mbar. The compression of the desorbed hy- drogen chloride can be performed in several steps with intermediate cooling. The temperature in the desorption step is chosen to be below the decomposition temper- ature of the ionic liquid. Thus, between the device for absorption and the desorption column, whereby the desorption column basically can be a single stage column (flash), there must be installed a pressure equipment for decreasing the pressure, such as a throttle and/or a vacuum pump or another pump (e.g. a membrane or piston pump). Usually, the pressure change is applied by one or more compressors and/or one or more pumps discharging the streams from the desorption device. When reducing the pressure of the solution mass flow comprising the ionic liquid containing the hydrogen chloride, heat must be supplied at the same time in order to avoid cooling down the ionic liquid below its freezing point. Thus, if the ionic liquid has a freezing point close to room temperature, the pressure decrease by expansion cannot be adiabatic but a low tem- perature heat supply may be necessary. This can be simply water at room tempera- ture, which does not lead to any additional exergy costs.

The absorption of the hydrogen chloride in the ionic liquid can be carried out at pressures between 0.5 and 3 bar, like 0.5 to 2 bar, in particular 0.8 to 1.2 bar, like 1.0 bar or 1.0 atm.

The ionic liquid mass flow should be chosen in a way that it is sufficient to ab- sorb essentially all of the hydrogen chloride, while not being too high, since then also chlorine dissolves in significant amounts. If for some reasons there is too much chlo- rine absorbed, the hydrogen chloride being desorbed from the ionic liquid later on will also contain small amounts of chlorine. For reactor recycling this is not a problem. However, if parts of the hydrogen chloride are absorbed in water to form aqueous hy- drogen chloride, then the contained chlorine will mostly not be absorbed but will leave the absorber as a humid chlorine gas stream. This gas stream could then be dried in a subsequent drying step, e. g. with sulfuric acid. The chlorine thus obtained can be combined with the chlorine product stream from the absorption column. This option is feasible when the selectivity of the absorption of hydrogen chloride is not high enough in that small amounts of chlorine will be absorbed in the ionic liquid as well.

Surprisingly, it has been found out that a distillation step at an increased static pressure allows for an exergy-efficient separation of chlorine and hydrogen chloride. Therefore, according to the invention a method for separating chlorine from a gase- ous anode outlet stream mass flow of an electrochemical cell reactor, said anode outlet stream mass flow essentially comprising chlorine and anhydrous hydrogen chloride is described, said method comprising a distillation step, wherein the anode outlet stream mass flow is separated at a static pressure of at least 1 bar, like at least 2 bar. Thereby, an overhead product mass flow comprising the hydrogen chloride and a bottom product mass flow comprising the chlorine is formed. The static pres- sure of at least 1 bar, like at least 2 bar allows for an exergy-efficient separation. In particular, implementation of an ammonia cooling cycle for condensing the overhead product is possible at a static pressure in the range between 3 to 5 bar.

The distillation step also allows for flexible alteration of the intensity of the an- ode outlet stream mass flow without changing the design parameters of the distilla tion process except the throughput of the distillation device being used for the distilla tion step. Thus, the distillation step allows for variation of the overall conversion by varying the parameters of the electrochemical reactor. For this reason, the method according to the present invention allows for variation of the load of hydrogen chlo- ride being subject to the conversion by electrolysis without modification of the plant. This makes the method quantitatively flexible for the load of hydrogen chloride.

The distillation device is to be understood to be for example a distillation col- umn, in particular an absorption column with bubble trays, sieve bottoms, or a packed distillation column. The anode outlet stream mass flow is usually fed into the column between the head and the bottom or sump. The chlorine and the hydrogen chloride are then separated due to their difference in fugacity, forming the higher boil- ing liquid bottom product mass flow at the bottom and the lower boiling overhead product mass flow at the head of the column. In an embodiment, cooling of the head for condensing the overhead product can be achieved by an ammonia cooling cycle. In this case, a pressure of at least 3 bar is desired.

In one embodiment of a method according to the present invention comprising the distillation step including a distillation device, like a distillation column, the con- denser temperature in the distillation column is at or above 195 K, like above 200 K. Thus, a reduction of exergy demand is achieved. Furthermore, cooling at the conden- ser can be implemented easier and with less technical efforts.

In an advantageous embodiment of the method according to the present inven- tion comprising the distillation step, the static pressure in the distillation device is of at least 1 bar, like at least 2 bar, e.g. at least 4, at least 6, at least 8 bars. Further, the static pressure should not exceed 12 bar. This allows for a reduced exergy demand coming along with a higher pressure. With respect to the distillation column of the present distillation process base on calculating purity of at least 99,5%, it is preferred that said distillation column has a theoretically plate number of at least 10, e.g. between 10 and 20, like between 10 to 17, including 11 to 17 plate numbers. Moreover, the reflux ratio in the column, namely, the molar reflux ratio is between 3.2 and 3.5, like 3.24 to 3.3, in case of an 80 % single pass conversion in the electrochemical reactor In case of 60 % single pass conversion, the reflux ratio may be in the range of 1.3 to 1.5, like 1.35 to 1.4. In case of 30 % single pass transfer in the reactor, the molar reflux ratio may be in the range of 0.5 to 0.6, like 0.55 to 0.58.

In one embodiment of the method according to the present invention comprising step a) involving an organic solvent according to invention, the organic solvent is a non-polar organic solvent selected from aromatic solvents, halogenated alkanes, al- kanes and cycloalkanes including chloroform, benzene, toluene and octane. In an embodiment, the organic solvent is an alkane or a cycloalkane and derivatives thereof being derivatized with halogens. Further, aromatic compounds are preferred including benzene or toluene. Typically, alkane or cycloalkane which may optionally be substituted with halogens represent a preferred embodiment. It is preferred that the boiling point of the organic solvent is below or not significantly above the boiling point of water. Further, short chain alkanes have an increased solubility in water, thus, it is preferred that the alkane solvents are alkane solvents having at least four, five or at least six C-atoms. It is preferred that the organic solvent is a C6, C7 or Cs al- kane including hexane, heptane and octane. In further embodiments of the method comprising step a) involving an organic solvent according to invention, the organic solvent is or comprises carbon disulfide or derivatives thereof. Further, alkenes in- cluding halogenated alkenes may be used.

In one embodiment of the methods according to the invention, the separated hydrogen chloride is at least partly recycled to the inlet of the anode of the electro- chemical cell reactor. This allows for a reduction of disposal hydrochloric acid and for an economization with respect to the overall net consumption of hydrogen chloride.

In one embodiment of the methods according to the invention, the separated hydrogen chloride is at least partly converted into a concentrated hydrochloric acid solution. This allows for an economic utilization of at least a part of the unconverted hydrogen chloride. In particular the aqueous hydrogen chloride mass flow can be en- riched with hydrogen chloride until a concentrated hydrogen chloride solution is achieved for economic utilization.

In one embodiment of the method comprising step a) involving an organic sol- vent according to the invention, the desorption step c.) is a distillation step. This al- lows for separation by application of heat energy. In particular a combination with an increased static pressure in the absorption step a) as mentioned above does allow for a low exergy consumption.

In an embodiment of the method including an organic solvent, the distillation is conducted at atmospheric pressure or below atmospheric pressure.

In one embodiment according to the invention, the conversion rate of hydrogen chloride to chlorine in the electrochemical reactor is in between 30 % to 90 %, partic- ularly between 50 % to 80 %. These have turned out to be ranges of the conversion rate with an exceptional low overall exergy consumption for an integrated process with an electrochemical cell reactor and a method for separating chlorine according to the invention.

In one embodiment according to the invention, the purity of the separated chlo- rine is at least 98 %, like 99 %, e.g. at least 99.5%.

Further with respect to immiscible, the term“essentially” is to be understood as to refer to an miscibility of less than 5 %, like less than 1 %, e.g. 1000 ppm at most of one solvent present in the other solvent. For example, the water is present at most in 1000 ppm in the organic solvent, e.g. at most 500 ppm, like at most 100 ppm.

As used here with respect to impurities, the term“essentially” is to be under- stood as to involve a degree of impurities of less than 1 %, in particular of less than 1000 ppm, like less than 100 ppm.

The invention further relates to an apparatus for separating chlorine from a gas- eous anode outlet stream mass flow. The apparatus comprises an electrochemical reactor having an anode outlet of a gaseous anode outlet stream comprising chlorine and hydrogen chloride, a cathode outlet, an anode inlet and a cathode inlet.

The apparatus further comprises a first absorber connected with the outlet of a gaseous anode outlet stream by a gaseous anode outlet stream line allowing absorp- tion of chlorine in an organic solvent introduced into the absorber by a solvent provid- ing inlet. The absorber has an inlet for introducing the gaseous anode outlet stream into the absorber and a first outlet for an organic solvent containing essentially chlo- rine and a part of hydrogen chloride. The absorber also has a second outlet for es- sentially the gaseous hydrogen chloride.

The apparatus also comprises a device for applying an extraction step to the or- ganic solvent containing essentially chlorine and a part of hydrogen chloride, thereby forming i) an aqueous hydrogen chloride mass flow and ii) a purified solvent mass flow containing chlorine. Moreover, the apparatus comprises a desorption device for separating the chlorine from the organic solvent present in the purified solvent mass flow.

Optionally, the aforementioned apparatus comprises a second absorber con- nected by a line with the first absorber and/or the device for extraction for absorbing at least part of hydrogen chloride obtaining concentrated aqueous hydrogen chloride.

Optionally, the aforementioned apparatus comprises a recycling line for the transport of hydrogen chloride from the first absorber to the anode inlet of the electro- chemical reactor.

The apparatus - comprising or not comprising the aforementioned optional components - is adapted for conducting the method according to the invention, thereby bringing out the advantages discussed for the method according to the in- vention.

The invention also relates to an apparatus for separating chlorine from a gase- ous anode outlet stream mass flow. The apparatus comprises an electrochemical re- actor having an anode outlet of a gaseous anode outlet stream comprising chlorine and hydrogen chloride, a cathode outlet, an anode inlet and a cathode inlet.

The apparatus furthermore comprises a distillation device connected with the outlet of a gaseous anode outlet stream by a gaseous anode outlet stream line allow- ing distillation of the gaseous anode outlet stream into an overhead product mass flow essentially comprising hydrogen chloride and a bottom product mass flow es- sentially comprising chlorine. If necessary, a compressor or pumps is present be- tween the outlet and the distillation device altering and controlling the pressure in the distillation device, e.g. the distillation column.

The distillation device, e.g. the distillation column, is designed to have a theoret- ical plate number in between 10 to 20 theoretical plate numbers, like 10 to 17, e.g. 11 to 17 taking a purity of 99.5 % as a basis. Further, depending on the single pass transfer. For example, in case of 80 % single pass conversion in the reactor, the plate number is in between 10 to 20, like 11 to 17 with a molar reflux ratio of in be- tween 3.0 to 3.8, like 3.24 to 3.3. In case of 60 % single pass conversion in the reac- tor, the plate number is in between 10 to 20, like 11 to 17 with a molar reflux ratio of in between 1.3 to 1.5, like 1.35 to 1.4. In case of 30 % single pass conversion in the reactor, the plate number is 10 to 20, like 10 to 17 with a molar reflux ratio of in be- tween 0.5 to 0.6, like 0.55 to 0.58.

Optionally, the apparatus comprises an absorber connected by a line with the head of the distillation device for absorbing at least part of hydrogen chloride present in the overhead product mass flow obtaining concentrated aqueous hydrogen chlo- ride.

Optionally, the apparatus comprises a recycling line for the transport of hydro- gen chloride to the anode inlet of the electrochemical reactor.

The apparatus - comprising or not comprising the aforementioned optional components - is adapted for conducting the method involving an ionic liquid accord- ing to the invention, thereby bringing out the advantages being discussed for the method according to the invention.

The invention also relates to an apparatus for separating chlorine from a gase- ous anode outlet stream mass flow based on the use of an ionic liquid. The appa- ratus comprises an electrochemical reactor having an anode outlet of a gaseous an- ode outlet stream comprising chlorine and hydrogen chloride, a cathode outlet, an anode inlet and a cathode inlet.

The apparatus furthermore comprises a first absorber connected with the outlet of a gaseous anode outlet stream by a gaseous anode outlet stream line allowing ab- sorption of hydrogen chloride in an ionic liquid introduced into the absorber by an ionic liquid providing inlet. The absorber has an inlet for introducing the gaseous an- ode outlet stream into the absorber and a first outlet for the ionic liquid containing hy- drogen chloride and a second outlet for essentially the gaseous chlorine.

The apparatus comprises a desorption device for separating the hydrogen chlo- ride from the ionic liquid present in the solvent mass flow. For example, the desorp- tion device is a heating device wherein desorption of hydrogen chloride after or dur- ing heating is affected. In particular, the heating device may be a flash or a series of flash. In particular, the heating device can be a vapor.liquid separator, like a flash- drum or a series of flashdrums. Optionally, the apparatus comprises a recycling line for the transport of hydro- gen chloride from the desorption device to the anode inlet or the electrochemical re- actor, adapted for conducting a method according to the invention.

In a further embodiment of one of the aforementioned apparatuses, the appa- ratus comprises a pressure equipment for applying pressure in the distillation device, heating device or the absorber, if necessary. The pressure equipment is to be under- stood as to be devices for increasing or lowering a pressure, for example compres- sors, pumps, in particular vacuum pumps.

The pressure equipment, in particular the vacuum pumps, can be arranged such that a pressure decrease of the solution mass flow involving an ionic liquid is applied in the desorption step. Thus, a good mass transport during desorption is achieved. In an advantageous embodiment, the pressure is reduced to a low vacuum by the pressure equipment for conduction of the desorption step. After performance of the desorption step, the pressure of the desorbed hydrogen chloride gas and of the ionic liquid can be raised by one or more compressors or pumps (for the ionic liq- uids) and optionally one or more heat exchangers being installed after or between the compressors or pumps, generating a pressure of the ionic liquid and/or of the de- sorbed hydrogen chloride gas of e.g. 1 atm, typically similar to the pressure present in the absorption device e.g. the absorption column and the reactor.

A high overall exergy-efficiency is achieved in all apparatuses by energy-inte- gration of heat flows, e.g. of heat flows at the heads and sumps of columns and/or at heat exchangers for changing properties of streams.

In the following, examples of embodiments of the invention are discussed, mak- ing reference to the following drawings. The drawings show as follows:

Fig. 1 : Fig. 1 shows a first example of an embodiment according to the invention,

Fig. 2: Fig. 2 shows a second example of an embodiment according to the inven- tion,

Fig. 3: Fig. 3 shows a third example of an embodiment according to the invention.

Fig. 1 shows an apparatus 1 for separating chlorine from a gaseous anode out- let stream mass flow. The apparatus comprises an electrochemical reactor 2. The electrochemical reactor 2 comprises an anode-side half-cell 3 and a cathode-side half-cell 4 being separated by a membrane 5. The membrane 5 is permeable for pro- tons and impermeable for hydrogen chloride and chlorine. Such membranes are known and commercially available, for example under the trade mark NAFION®. The membrane 5 carries out the function of an electrolyte for the transport of protons be- tween the half-cells 3, 4.

The electrochemical cell reactor 2 comprises an anode inlet 6a for introducing a gaseous hydrogen chloride mass flow 7 into the anode-side half-cell 3 and a cathode inlet 6b for introducing a cathode-gas mass flow 8 into the cathode-side half-cell 4. The cathode-gas mass flow 8 may comprise or consist of oxygen. Alternatively, pro- tons are reduced to hydrogen. In the anode-side half-cell 3, the gaseous hydrogen chloride mass flow 7 is oxidized by an electrical power source (not shown), thereby forming chlorine. The chlorine is discharged from the anode-side half-cell 3 together with unconverted fraction of the gaseous hydrogen chloride mass flow and the chlo- rine forming a gaseous anode outlet stream 9. In the cathode-side half-cell 4 at least a part or all of the oxygen and the protons form water which is discharged in the form of a cathode outlet stream 4a through a cathode outlet stream line 4b. Alternatively, the hydrogen is obtained from reducing the protons at the cathode

The apparatus 1 further comprises a first absorber 10 in form of an absorption column 10. The absorption column 10 comprises an inlet 11 , the gaseous anode out- let stream 9, being directed through an anode outlet stream line 12, is introduced into the absorber 10 through inlet 11. The absorber 10 also comprises a solvent providing inlet 13.

The gaseous anode outlet stream 9 is compressed by a pressure equipment 14 in the form of a compressor 14 between the outlet of the anode-side half-cell 3 and the inlet 11 of the absorption column 10, by which compressor the pressure of the gaseous anode outlet stream mass 7 flow is adjusted for the absorption step.

Due to absorption of chlorine and hydrogen chloride from the anode outlet stream 9 into the solvent being introduced through the solvent providing inlet, inside the column a gas flow 15 of hydrogen chloride and a solution mass flow 16 are formed. The solution mass flow 16 comprises the organic solvent containing, both dissolved, the chlorine and a part of the hydrogen chloride. The absorber 10 also comprises a first outlet 17a at the bottom of the column for detaching the solution mass flow 16. At the column’s head, the absorber 10 corn- prises a second outlet 17b for detaching the gas flow 15 containing the hydrogen chloride.

The apparatus 1 also comprises a device for extraction 18 in the form of a mixer-settler device. The device for extraction 18 comprises a water mass flow inlet 19 for introduction of a water mass flow 19a into the extraction device 18. Further- more, the device for extraction 18 comprises a solution mass flow inlet 20 being con- nected to the first outlet 17a of the column 10 for introducing the solution mass flow 16 into the device for extraction 18. Due to an extraction step, the hydrogen chloride from the solution mass flow 16 is extracted into the water mass flow 19a, thereby forming an aqueous hydrogen chloride mass flow 21 and a purified solution mass flow 22. The purified solution mass flow 22 and the aqueous hydrogen chloride mass flow 21 are discharged from the device for extraction 18.

The detached purified solution mass flow 22 is charged through line 23 to a de- sorption device 24. Before being introduced into the desorption device 24, a pressure decrease can be applied to the purified solution mass flow 22, e.g. by a throttle. The purified solution mass flow 22 is separated by application of a distillation inside the desorption device 24, thereby forming a liquid organic solvent mass flow 25 and a gaseous chlorine mass flow 26. The organic solvent mass flow 25 is recycled through a recycling line 25a and introduced into the absorption column 10 again through the solvent providing inlet 13.

The apparatus 1 further comprises a second absorber 27 being connected by line 28 with the device for extraction 18, whereby the aqueous hydrogen chloride mass flow 21 is introduced into the second absorber 27. Furthermore, at least a part of the gas flow 15 of hydrogen chloride, being a hydrogen chloride byproduct mass flow 28a, is introduced into the second absorber 27 by a line 28b, thereby augment- ing the concentration of the aqueous hydrogen chloride mass flow 21 by an addi- tional absorption of gaseous hydrogen chloride from the hydrogen chloride byproduct mass flow 28a. By these means, a concentrated hydrochloric acid mass flow 29 is obtained as a byproduct.

The gas flow 15 of hydrogen chloride, or respectively its part not being intro- duced into the second absorber 27, is transported through a recycling line 30 as a re- cycled hydrogen chloride mass flow 30a from the second outlet 17b to the anode in- let 6a of the anode-side half-cell 3 of the electrochemical reactor 2. Formation of the recycled hydrogen chloride mass flow 30a and of the hydrogen chloride byproduct mass flow 28a is performed by splitting up the gas flow 15 by a splitter 30b.

Fig. 2 shows an apparatus 31 according to the invention. The apparatus 31 comprises an electrochemical reactor 2 according to the apparatus 1 of the first ex- ample of an embodiment. As far as the same reference numbers are used, the com- ponents referred to in this embodiment are configured in the same way as in the em- bodiment according to Fig. 1.

The apparatus 31 further comprises a distillation device 32 in the form of a dis- tillation column. In this embodiment, the distillation column 32 is connected to the gaseous anode outlet stream line 12 for introducing the gaseous anode outlet stream 9 into the distillation column. The distillation step being performed in the distillation column 32 is performed at a pressure of at least 1 bar, like at least 2 bar. A pressure increase between the anode-side half-cell 4 and the distillation column 32 is applied by compressing the anode outlet stream 9 in a compressor 14. Thus, the gaseous anode outlet stream 9 is cooled, wherein the chlorine is liquefied and forms a bottom product mass flow 33 which is detached from the bottom of the distillation column 32 by line 34. The not liquefied gaseous hydrogen chloride is detached at the head of the distillation column 32. The hydrogen chloride not being liquefied forms an over- head product mass flow 35 being detached through a line 36.

Line 36 is connected with a splitter 37 allowing for transporting a part or all of the hydrogen chloride of the overhead product mass flow 35 to the anode inlet 6a of the electrochemical reactor 2 through a recycling line 38. By means of the splitter 37, a part or all of the hydrogen chloride can be introduced into an absorber 39, into which absorber also water is introduced for formation of a concentrated hydrochloric acid mass flow 40 as a byproduct.

Fig. 3 shows an apparatus 41 for separating chlorine from a gaseous anode outlet stream mass flow 9. The apparatus 41 comprises an electrochemical reactor 2 according to the apparatuses 1 and 31 of the aforementioned examples of an em- bodiment. As far as the same reference numbers are used, the components referred to in this embodiment are configured in the same way as in the embodiments accord- ing to Figures 1 and 2. According to a further example of an embodiment, a gaseous anode outlet stream 9 is introduced into an absorber 10 through an inlet 11. In this example of an embodiment, an ionic liquid solvent mass flow 42 is introduced into the absorber 10 for absorbing the hydrogen chloride of the gaseous anode outlet stream 9, thus forrn- ing a ionic liquid solution mass flow 43 containing essentially all the hydrogen chlo- ride. The ionic liquid solution mass flow 43 is discharged from the absorber 10 by the first outlet 17a. The gaseous chlorine of the gaseous anode outlet stream 9, being non-soluble in the ionic liquid, is detached from the absorber 10 in the form of a puri fied gas flow 44 at the head of the absorber 10 by the second outlet 17b.

The purified gas flow 44 can be used for chemical synthesis steps as mentioned above.

The ionic liquid solution mass flow 43 comprising the ionic liquid containing the hydrogen chloride is transported to a desorption device 45 for removing the hydrogen chloride from the ionic liquid solution mass flow 43. At least a part of the hydrogen chloride can be recycled by being transported through a recycling line 46 as a recy- cled hydrogen chloride mass flow 46a and being introduced into the electrochemical cell reactor 2 again.

The desorption step in the desorption device 45 is performed at a lower static pressure than the pressure in the absorber 10. The pressure decrease may be per- formed by a throttle or by a vacuum pump 47 between the absorber 10 and the de- sorption device 45. The pressure decreased ionic liquid solution mass flow 48 is in- troduced through a line 49 into an inlet 50 of the desorption device 45. The desorp- tion device comprises a first outlet 51 a and a second outlet 51 b.

By heating the pressure decreased ionic liquid solution mass flow 43 inside the desorption device, the hydrogen chloride in the ionic liquid solution mass flow 43 is desorbed and can be discharged from the desorption device 45. The purified ionic liquid mass flow 53 being formed during the desorption step is detached from the de- sorption device 45 by the first outlet 51 a and through a line 52. The pressure of the purified ionic liquid mass flow 53 is increased by a pump 54 and the purified ionic liq- uid mass flow 53 is recycled through a line 55 for getting introduced into the ionic liq uid solvent mass flow 42 by a splitter 56. Thus, a circulation of the ionic liquid is achieved in the apparatus 41.

The hydrogen chloride being desorbed from the pressure decreased ionic liquid solution mass flow 48 is detached from the desorption device 45 through the second outlet 51 b and through a line 57 as a gaseous purified hydrogen chloride mass flow 58. The purified hydrogen chloride mass flow 58 is compressed in a compressor 59 and introduced into a splitter 60. From the splitter 60, a part of the purified hydrogen chloride mass flow 58 may be recycled in the form of the recycled hydrogen chloride mass flow 46a through the line 46. Another part of the purified hydrogen chloride mass flow 58 may be transported from the splitter 60 through a line 61 , forming a gaseous hydrogen chloride byproduct mass flow 62. The gaseous hydrogen chloride byproduct mass flow 62 is introduced into a second absorber 27 and is exposed to a water mass flow 63. The water mass flow 63 absorbs the gaseous hydrogen chloride byproduct mass flow 62, thereby forming a concentrated hydrochloric acid mass flow 64 as a byproduct.

Examples of working conditions

Given below, for each of the three embodiments according to the figures 1 to 3 one set of working conditions is presented as sets of exemplary data. These data are based on a MDI production plant similar to the production plant of BASF in Antwerp applying a FICL stream of about 0.362 kmol/s.

The following exemplary data were obtained from flowsheet simulations with the pro- gram Aspen Plus V.8.8. In these simulations, the single pass conversion of the elec- trochemical cell reactor, as well as the overall conversion of hydrogen chloride in the process was set to 80 %. Therefore, in these specific examples, no hydrogen chlo- ride recycle towards the electrochemical reactor was employed, but the remaining 20 % were dissolved in water forming concentrated hydrochloric acid. The amount of Oxygen fed into the cathode-side half-cell of the electrochemical cell reactor was enough to stochiometrically convert all the employed hydrogen chloride to chlorine. Since only a hydrogen chloride conversion of 80 % is simulated, this means that an oxygen surplus of ca. 10 % is employed in these specific examples

Flydrogen chloride mass flow 7:

Cathode gas mass flow 8:

Gaseous anode outlet stream 9:

Cathode outlet stream 4a

In the following examples, the hydrogen chloride mass flow 7, the cathode gas mass flow 8, the gaseous anode outlet stream 9 and the cathode outlet stream 4a have the same properties.

The exergy consumption of the electrochemical reactor in each example is about 25.04 MW.

Example 1

For the present example of the process involving an absorption step with an or- ganic solvent, n-octane was chosen as organic solvent. The example relates to a setup according to Fig. 1. The streams according to the process flow chart in Fig. 1 are as follows using 02 as cathode inlet gas:

Gas flow 15:

Solution mass flow 16:

Water mass flow 19a:

Aqueous hydrogen chloride mass flow 21 :

Purified solution mass flow 22:

Chlorine mass flow 26:

Organic solvent mass flow 25:

Concentrated aqueous hydrogen ch oride mass flow 29:

In this example, the absorber 10 is an absorption co umn with 16 stages (or an equivalent height in the case of a packing column). The absorption step is carried out at 8.10 bar and with a top stage temperature of 298.3 K and a bottom stage tempera- ture of 331.50 K.

The extraction step in the extraction device 18 is carried out with a temperature of the water and hydrogen chloride phase of 308.15 K and a temperature of the oc tane phase of 300 K.

The desorption step is carried out in the desorption device 24 with a condenser temperature of 244 K and a sump temperature of 395 K.

Absorption of hydrogen chloride in the second absorber 27 is carried out with a temperature of 308.15 K.

In this embodiment, the exergy consumption of the method for separating the chlorine and the production of the hydrochloric acid is about 2 MW.

The exergy consumption of the overall process is about 27.04 MW which is about 36 % lower than the exergy consumption of about 42,27 MW of the Bayer UHDENORA process as described in US 8153938 B2 and based on the public data sheets by Thyssen, e.g. as published in“Hydrochloric acid electrolysis sustainable chlorine production”, ThyssenKrupp Uhde GmbH, 2012. Example 2

For the present example of the process involving an absorption step with an ionic liquid, diethyl-methylsulfonium methanesulfonate ([S221][MeS03]) was chosen as ionic liquid. The example relates to a setup according to Fig. 3. The most im- portant streams according to the process flow chart in Fig. 3 are as follows:

Ionic liquid solvent mass flow 42:

lonic liquid solution mass flow 43:

Purified gas flow 44:

Pressure decreased ionic liquid solution mass flow 48:

Purified ionic liquid mass flow 53:

Purified hydrogen chloride mass flow 58:

Gaseous hydrogen chloride byproduct mass flow 62:

Since no recycled hydrogen chloride mass flow 46a is provided, the composi- tion of the gaseous hydrogen chloride byproduct mass flow 62 corresponds to the composition of the purified hydrogen chloride mass flow 58 but may have a higher temperature and a higher pressure due to compression by the compressor 59 and eventually additional heating by a heat exchanger 65.

Water mass flow 63:

Flydrogen chloride mass flow 64:

In this example absorption step is carried out at a temperature of 298.15 K in a single stage absorber. The vacuum pump 47 and/or a throttel valve in combina- tion with the compressor 59 does provide a vacuum with a pressure of 0.05 bar at its outlet.

The compressor 54 provides a pressure of 1 .013 bar for recycling the ionic liq uid through line 55 and into the splitter 56.

The compressor 59 provides a four-stage compression of the purified hydrogen chloride mass flow 58, i.e. from 0.05 bar to 0.1 bar by the first compression stage, from 0.1 to 0.2 bar by the second compression stage, from 0.2 to 0.4 bar by the third compression stage and from 0.4 to 1 .013 bar by the fourth compression stage. After being compressed to a pressure of 1 .013 bar or 1 atm, the compressed purified hy- drogen chloride mass flow 58 is cooled by the heat exchanger 65 before being intro duced into the second absorber 27.

In the second absorber 27, absorption of hydrogen chloride is carried out at a temperature of 308.15 K and at a pressure of 1 .013 bar.

In this embodiment, the exergy consumption of the method for separating the chlorine and the production of the hydrochloric acid is about 1 ,96 MW.

The exergy consumption of the overall process is about 27.00 MW which is about 36 % lower than the exergy consumption of about 42,25 MW of the Bayer UHDENORA process.

Example 3

The example relates to a setup according to Fig. 2. For the process involving a distillation step, the distillation was carried out at 1 atmospheric pressure. The most important streams according to the process flow chart in Fig. 2 are as follows:

Bottom product mass flow 33 being discharged from distillation column 32: distillation column 32:

Concentrated aqueous hydrogen chloride mass flow 40 being discharged from distillation column 32:

In this example, the distillation device 32 is a distillation column with 16 stages (or in the case of a packed column with a height being equivalent to 16 stages). The distillation column 32 is operated with a condenser temperature of 187.3 K at the col umn’s head and with a sump temperature of 239.4 K at the bottom of the column.

In this embodiment, the exergy consumption of the method for separating the chlorine and the production of the hydrochloric acid is about 1 ,22 MW.

The exergy consumption of the overall process is about 26.26 MW which is about 38 % lower than the exergy consumption of about 42,25 MW of the Bayer UHDENORA process

References:

1 apparatus

2 electrochemical reactor

3 anode-side half-cell

4 cathode-side half-cell

4a cathode outlet stream 4b cathode outlet stream line

5 membrane

6a anode inlet

6b cathode inlet

7 gaseous hydrogen chloride mass flow

8 cathode gas mass flow

9 gaseous anode outlet stream

10 absorber

11 inlet

12 anode outlet stream line

13 solvent providing inlet

14 compressor

15 gas flow

16 solution mass flow

17a first outlet

17b second outlet

18 device for extraction

19 water mass flow inlet

20 solution mass flow inlet

21 aqueous hydrogen chloride mass flow

22 purified solution mass flow

23 line

24 desorption device

25 organic solvent mass flow

25a recycling line

26 chlorine mass flow

27 second absorber

28 line

28a hydrogen chloride byproduct mass flow

28b line

29 concentrated aqueous hydrogen chloride mass flow

30 recycling line

30a recycled hydrogen chloride mass flow

30b splitter

31 apparatus

32 distillation device

33 bottom product mass flow

34 line

35 overhead product mass flow

36 line

37 splitter

38 recycling line

39 absorber

40 concentrated aqueous hydrogen chloride mass flow

41 apparatus

42 ionic liquid solvent mass flow

43 ionic liquid solution mass flow

44 gas flow

45 desorption device

46 recycling line

46a recycled hydrogen chloride mass flow 47 vacuum pump

48 pressure decreased ionic liquid solution mass flow

49 line

50 inlet

51 a first outlet

51 b second outlet

52 line

53 purified ionic liquid mass flow

54 pump

55 line

56 splitter

57 line

58 purified hydrogen chloride mass flow

59 compressor

60 splitter

61 line

62 hydrogen chloride byproduct mass flow

63 water mass flow

64 concentrated hydrochloric acid mass flow

65 heat exchanger