GB2034291A

GB2034291A - Process for Obtaining Gaseous Hydrogen Chloride from Dilute Aqueous Hydrochloric Acid

Info

Publication number: GB2034291A
Application number: GB7927056A
Authority: GB
Original assignee: Chemische Werke Huels AG
Current assignee: Huels AG
Priority date: 1978-08-04
Filing date: 1979-08-03
Publication date: 1980-06-04
Also published as: JPS5946884B2; GB2034291B; NL7905986A; IL57973A; DE2834252B2; IT7949940A0; IL57973A0; IT1119813B; BE878078A; DE2834252C3; FR2432476B1; DE2834252A1; EP0008611A1; JPS5523095A; FR2432476A1

Description

SPECIFICATION Process for Obtaining Gaseous Hydrogen Chloride from Dilute Aqueous Hydrochloric Acid In numerous industrial processes, hydrogen chloride arises as a by-product in the form of dilute aqueous hydrochloric acid, for which there is only a limited industrial field of use. In order to be able to utilise the hydrogen chloride, present in the form of such a dilute, aqueous hydrochloric acid, as a chemical raw material, for example for the preparation of vinyl chloride, ethyl chloride, chloropropene or chlorosulphonic acid, the hydrogen chloride must be isolated as an anhydrous gas. The working up of dilute acid by distillation, required to achieve this, is ruled out for economic reasons, because the amounts of energy which have to be provided to evaporate the water are too great. Extractive isolation of the HCI gas has hitherto failed because of the unfavourable partition coefficients between HCI and extractants, such as, for example, pentanols. Relatively long-chain amines, which must be water-insoluble both as such and in the form of their hydrochlorides, suggest themselves as extractants having far more advantageous partition coefficients. Using such amines, it is possible to separate off the hydrogen chloride almost completely from a dilute hydrochloric acid. The subsequent thermal cleavage of the amine hydrochlorides thus obtained, and the isolation of the anhydrous hydrogen chloride, can then be carried out in accordance with the processes described in German Offenlegungsschrift 2,633,640 and in our copending GB Patent Application No. 7904824, Serial No. 2012235 corresponding to German Patent Application P 28 05 933.8. The process of German Offenlegungsschrift 2,633,640 is characterised in that a) the aqueous hydrochloric acid is extracted with an amine or a mixture of an amine and an inert, water-immiscible solvent which boils below the amine used, the latter being a tertiary alkylamine, a tertiary aryldialkylamine, a secondary arylalkylamine, a primary alkylarylamine or a mixture thereof, which contains 14 to 36 carbon atoms in the nitrogenlinked carbon chains, at most one of the nitrogenlinked chains being a methyl group and at least one being an aliphatic radical containing at least 6 carbon atoms, the acid constant Ka of the amine being less than 10- , b) an inert, water-immiscible solvent which boils below the amine used is added to the extract, unless the solvent has already been added in stage a), c) the extract is distilled, the resulting vapour is condensed,the water is continuously removed from the two-phase condensate, and the organic phase is returned to the distillation process, and d) after removing the water, the extract is distilled under reflux at a temperature of from 100 to 250[deg]C at the column bottom, and the gaseous hydrogen chloride liberated at the top of the column is taken off. According to the process of our GB Patent Application No. 7904824 Serial No. 2012235 and German Patent Application P 28 05 933.8, the gaseous hydrogen chloride is obtained-in a modification of the process of German Offenlegungsschrift 2,633,640-by heating the mixture of amine hydrochloride and solvent to a temperature below the boiling point of the solvent and separating off the hydrogen chloride liberated by passing an inert gas stream through the mixture. It is true that the two processes mentioned for the first time permit the isolation of gaseous hydrogen chloride from dilute aqueous hydrochloric acid by means of special amines and subsequent thermolysis of the amine hydrochlorides formed, but they still suffer from shortcomings. Thus, the principal shortcoming is that the tertiary aliphatic amine decomposes, dependent on its structure and dependent on how high is the temperature to which it is exposed, within a more or less short period, inter alia with the formation of a primary or secondary aliphatic amine or alkyl chloride, and the efficiency of the cleavage as a result increasingly declines, since, though primary and secondary aliphatic amines extract hydrogen chloride effectively from dilute hydrochloric acid, their hydrochlorides are barely capable of thermal cleavage.There is therefore an initially insignificant formation of the interfering by-products, which however progressively becomes greater as the decomposition proceeds. The process for isolating cheap hydrogen chloride by extracting dilute hydrochloric acid with amines can however only operate economically if the relatively expensive amine can be recycled as far as possible without losses. The present invention therefore seeks to provide a process for obtaining gaseous hydrogen chloride by extracting dilute aqueous hydrochloric acid with a tertiary aliphatic amine and subsequent cleavage of amine hydrochloride, in which the formation of undesired decomposition products is either prevented or at least substantially repressed. We have now found that good results may be achieved if the process described in the abovementioned Offenlegungsschrift and Patent Application is improved by using in stage a) a tertiary amine of a high degree of purity and the accumulation of impurities and decomposition products is prevented by passing part or all of the recycled stream of amine and solvent over an adsorbent which selectively retains the decomposition products, especially secondary and primary amines, out of the mixture. According to the present invention there is provided a process for obtaining gaseous hydrogen chloride from dilute aqueous hydrochloric acid by an amine extraction process in which a) the aqueous hydrochloric acid is extracted with an amine or a mixture of an amine and an inert, water-immiscible solvent boiling at not less than 120[deg]C, the amine being a tertiary alkylamine which contains a total of 14 to 36 carbon atoms in the nitrogen-linked carbon chains, at most one of the nitrogen-linked carbon chains being a methyl group and at least one being an aliphatic radical containing at least 6 carbon atoms, the acid constant Ka of the amine being less than 10-3, b) an inert, water-immiscible solvent boiling at not less than 120[deg]C is added to the extract, unless all the solvent required has already been present in stage a), c) the extract is distilled,the resulting vapour is condensed, the water is continuously removed from the two-phase condensate, and the organic phase is returned to the distillation process, d) the extract is distilled, under reflux, after the water has been removed, and the gaseous hydrogen chloride liberated at the top of the column is taken off, or the hydrogen chloride is separated off at below the boiling point of solvent by passing an inert gas stream through the mixture, and e) the solvent containing the amine is recycled to the extraction process, wherein part or all of the amine and any solvent and impurities is passed over an adsorbent which selectively removes impurities before being employed in stage a). The amines suitable for the process according to the invention are tertiary alkylamines which possess a total of 14 to 36 carbon atoms in the nitrogen-linked carbon chains. One of these chains must contain at least 6 carbon atoms, that is to say must be at least a cyclohexyl, hexyl or iso-hexyl radical, and the amino group must not carry more than one methyl group. The acid dissociation constant K., of the equilibrium reaction NR3H++H2O=NR3+H3O+ must be less than 10- to ensure adequate extraction of the hydrogen chloride. The dissociation constant Ka for numerous amines is to be found in the relevant literature, for example Handbook of Chemistry and Physics, 51 st edition (1970-71), page D 117 et seq. Examples of suitable amines are trihexylamine, tricyclohexylamine, triheptylamine, trioctylamine, cyclohexyldiisooctylamine, cyclohexyl-4-heptyloctylamine, cyclohexyl-2-ethylhexyl-octylamine, 2-ethylhexyl-4-heptyl-octylamine, tri-2ethylhexylamine, di-2-ethylhexyl-methylamine, didecylethylamine, tridodecylamine, didodecylmethylamine, dodecyl-diisopropylamine, dodecyldibutylamine, dodecyl-diisobutylamine, dodecylisobutyl-methylamine, diisopentadecylmethylamine, diisopentadecyl-ethylamine and diisopentadecyl-isopropylamine. The solvents used for the amines in the process are organic liquids which under the process conditions are inert towards water, hydrogen chloride and the amine and towards exposure to heat and which furthermore do not permit elution of the impurities. Compounds which are as nonpolar as possible, such as straight chain or branched, cycloaliphatic, araliphatic or aromatic hydrocarbons, and which have at least 6, preferably 6 to 20, carbon atoms and boil above 120.C, for example, xylenes, cumene, cymenes, ethylbenzene, trimethyl-benzenes, 1,2,4triethylbenzene, 1,3,5-triethylbenzene, 1,2,3,4tetramethylbenzene, 1,2,3,5-tetramethylbenzene, 3-phenylpentane, dodecylbenzene, nonane, decane, undecane, dodecane, tetradecane, decalin and tetralin, are particularly suitable for the process. The optimum concentration of the tertiary amine in the solution depends on the nature of the amine and the amount of the impurities to be separated off. In general, an amine-solvent ratio of 1 :1 to 1:10 is used. Examples of adsorbents for use in the process according to the invention are aluminium oxides of high specific surface area (100 to 400 m /g) in granular, tableted or crushed form. It has been found that the basicity of the aluminium oxide is not a deciding factor, and superficially basic, neutral or acidic aluminium oxide can be employed. On the other hand the level of activity is important for best results. Products with activity levels 1 to 3, preferably activity levels 1 and 2, are recommended. Aluminium oxide used as a chromatographic adsorbent is classified under 5 activity levels, according to the Brockmann scale. The aluminium oxides standardised by the Brockmann method differ in respect of their water content which, in per cent by weight, is as follows: 1 (0%), 2 (3%), 3 (4.5%), 4 (9.5%), 5 (15%), (O.A.Neumuller, Rompps Chemie-Lexikon, page 427 (1972), where further literature may be found). Further adsorbents which may be used are silica gels having specific surface areas of 500 to 700 m /g, silanised silica gels (specific surface area 300 to 400 m /g) and molecular sieves of the calcium zeolite type (specific surface area 500 to 600 m /g). In the silica gels, the silica is present in the form of highly condensed polysilicic acids having a foliated structure of large surface area. Silica gels wherein the hydrophilic surfaces have been rendered hydrophobic by reaction with chlorosilanes are referred to as silanised silica gels. The specific surface area is determined by the BET method (S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc. 60, 309 (1938)). In general, the adsorption step of the process according to the invention is carried out at room temperature (15 to 30[deg]C), but it can also be advantageous to work at higher temperatures. The adsorbent can be regenerated, without difficulties, by means of a polar solvent. The purity of the purified tertiary amine for use in stage a) should if possible be above 99%, preferably above 99.5% and especially preferentially above 99.9%, by weight. Important advantages of the process according to the invention are that not only are amine losses avoided but also, in the case of less stable amines, the invention makes it possible for the first time to carry out the HCI extraction and cleavage of the hydrochloride in practice. This is exemplified by the trioctylamine-dodecane system. In other cases it becomes possible to operate the cleavage economically over a substantially greater temperature range, namely from 100 to 300[deg]C. The examples and comparative examples which follow illustrate the process according to the invention. All percentage data are percentages by weight, unless stated otherwise. The Comparative Examples A to D show that on addition of secondary amine hydrochloride to the tertiary amine hydrochloride an autocatalytic decomposition of the tertiary amine is observed during thermolysis. Comparative Example A 77.9 g (0.2 mol) of tri-(2-ethylhexyl)-amine hydrochloride were dissolved in an equal amount by weight of xylene in a 500 ml three-necked flask equipped with a gas inlet tube, thermometer and water separator surmounted by a reflux condenser. The contents of the flask were rapidly brought to the boil by dipping the flask into an oil bath preheated to 220[deg]C, and at the same time a uniform stream of nitrogen of 30 l/hr was introduced into the flask. The hydrogen chloride split off was trapped in two receivers arranged in series and charged with sodium hydroxide solution, and was determined titrimetrically. In the course of 6 hours, 97.3% of the theoretically possible amount of hydrogen chloride were split off from the hydrochloride.The bottom product was treated with sodium hydroxide solution and its composition was then examined by gas chromatography. <0.1 % of secondary amine was found as a decomposition product. Comparative Example B

hydrochloride were subjected to thermolysis in an apparatus according to Comparative Example A, after having added 7.8 g (0.028 mol) of di-(2ethylhexyl)-amine hydrochloride in 78 g of xylene. In the course of 6 hours, 92.7% of the theoretically possible amount of hydrogen chloride were split off. The bottom product, after treatment with sodium hydroxide solution, contained-calculated as solvent-free materials-84.9% of tri-(2-ethylhexyl)-amine and 14.1 % of di-(2-ethylhexyl)-amine. Since the starting product contained 8.8% of secondary amine, an additional 5.3% of decomposition product had accordingly formed during the cleavage of the hydrochloride. Comparative Example C A solution of 60 g (0.2 mol) of tricyclohexylamine hydrochloride in 275 g of tert.butyl-m-xylene was rapidly brought to the boil, in an apparatus according to Comparative Example A, by means of an oil bath preheated to 280[deg]C, whilst passing a uniform stream of nitrogen of 30 I/hr into the apparatus. In the course of 6 hours, 93.7% of the theoretically possible amount of hydrogen chloride were split off from the hydrochloride. The bottom product, after treatment with sodium hydroxide solution, contained <0.1% of secondary amine as a decomposition product. Comparative Example D 60 g (0.2 mol) of tricyclohexylamine hydrochloride were subjected to thermolysis in an apparatus according to Compararative Example A, after addition of 6 g (0.028 mol) of dicyclohexylamine hydrochloride in 275 g of tert.butyl-m-xylene. In the course of 6 hours, 98.6% of the theoretically possible amount of hydrogen chloride were liberated. The bottom product, after treatment with sodium hydroxide solution, consisted of 89.3% of tricyclohexylamine and 10.7% of dicyclohexylamine (calculated as solvent-free materials). Since the starting material contained 8.7% of secondary amine, an addition 2.0% of decomposition product had accordingly formed during the cleavage of the hydrochloride.

Example 1 The experiment was carried out in a continuously operated installation for the extraction of hydrochloric acid with an amine and cleavage of the resulting amine hydrochloride. The extraction section consisted of a 2.5 m high column ( 30 mm) filled with glass beads (4 mm), in which a hydrochloric acid flowed downwards in counter-current to a mixture of solvent and amine. At the top of the extraction column, the extract containing hydrochloride was taken off and fed to the cleavage column (height: 2,500 mm, 40 mm). The water entrained with the extract was removed, in the form of concentrated hydrochloric acid, at the top of the column with the aid of a water separator surmounting the column, the solvent serving as the entraining agent for the water. (The water could also be removed, without loss of HCI, in an upstream distillation stage).The hydrogen chloride formed during the cleavage of the hydrochloride was taken off via the top of the column and in the course thereof passed through the water separator, whilst the HCI-free aminesolvent mixture was removed from the bottom of the column via a cooler and was returned to the extraction. A part-stream was regenerated by passing it over acidic aluminium oxide (activity level 1; specific surface area 200 m /g), with which an adsorption column (height 800 mm/ 45 mm) was packed. The installation was operated under nitrogen as a blanketing gas. Using a mixture of 1,155 g of tri-(2ethylhexyl)-amine and 1,155 g ofxylene (850 ml/hr), 505.4 kg of 9.85 per cent strength hydrochloric acid containing sodium chloride (250 ml/hr) was extracted in counter-current at 50.C in the course of 1,913 hours. The extract was subjected to hydrochloride cleavage in the cleavage column at 148-152[deg]C; it gave a total of 46.9 kg of hydrogen chloride, of which about one-third was obtained as concentrated hydrochloric acid, alongside hydrogen chloride gas. Since the cleavage took place virtually quantitatively, an extraction rate of 94.2% could be calculated from the stated amount of HCI. To regenerate the amine, 100 ml per hour of the bottom product were constantly passed over the adorption column filled with aluminium oxide.After completion of the experiment, 1,036 g out of the 1,155 g of tri-(2-ethyl-hexyl)-amine employed were recovered unchanged; this corresponds to a loss of 119 g of 10.3%, relative to amine employed. In addition, di-(2-ethylhexyl)amine in an amount corresponding to 40 g of tertiary amine preparable therefrom could be isolated from the regeneration process. If this is taken into account, the amine loss is reduced to 6.8%. Relative to hydrogen chloride produced, the amine loss is calculated to be 0.17%. Comparative Example E In an apparatus according to Example 1, 309.3 kg of 10.6 per cent strength hydrochloric acid containing sodium chloride were extracted in counter-current with a mixture of 1,150 g of tri(2-ethylhexyl)-amine and 1,150 g of xylene in the course of 1,175 hours. The extract, after cleavage of the hydrochloride, gave 32.0 kg of hydrogen chloride; this corresponds to an extraction rate of 97.6%. The amine formed in the bottom of the cleavage column was recycled, without prior regeneration, to the extraction. After completion of the experiment, 827 g of the 1,150 g of tri-(2ethylhexyl)-amine employed were recovered unchanged; this corresponds to a loss of 323 g or 28.1 %, relative to amine employed. On working up, di-(2-ethylhexyl)-amine and 2ethylhexylamine could additionally be isolated in an amount corresponding to 186 g of tertiary amine preparable therefrom.If this is taken into account, the amine loss is reduced to 11.9%. Relative to hydrogen chloride produced, the amine loss is calculated to be 0.43%.

Example 2 In an apparatus according to Example 1, 279.1 kg of 9.74 per cent strength hydrochloric acid containing sodium chloride were extracted in counter-current at 80-90[deg]C with a mixture of 1,750 g of trioctylamine and 7,000 g of dodecane in the course of 1,078 hours. The extract, after cleavage of the hydrochloride at 222-224[deg]C, gave 14.5 kg of hydrogen chloride; this corresponds to an extraction rate of 53.3%. The amine obtained at the bottom of the cleavage column was regenerated completely by being passed over a large-sized column (height 1,000 mm, 100 mm), packed with acidic aluminium oxide (as described in Example 1), before being returned to the extraction. After completion of the experiment, 1,438 g out of 1,750 g of trioctylamine employed were recovered unchanged; this corresponds to a loss of 312 g or 17.8% relative to amine employed.Dioctylamine and octylamine, in an amount corresponding to 183 g of tertiary amine preparable therefrom, could be isolated from the regeneration process. If this figure is taken into account, the amine loss is reduced to 7.4%. Relative to hydrogen chloride produced, the amine loss is calculated to be 0.89%. Comparative Example F An approximately 10 per cent strength hydrochloric acid containing sodium chloride was extracted in counter-current at 80 to 90[deg]C with a mixture of 1 part by weight of trioctylamine and 4 parts by weight of dodecane in the apparatus according to Example 1, but without regeneration. The extract was subjected to hydrochloride cleavage at 227[deg]C. The experiment had to be abandoned after a short time because the pipelines and extraction column became blocked by copiously formed dioctylamine hydrochloride.

Example 3 and Comparative Example G In an apparatus according to Example 1, 127.6 kg of 10.95 per cent strength hydrochloric acid containing sodium chloride were extracted in counter-current at 80 to 90[deg]C with a mixture of 1,900 g of trioctylamine and 7,500 g of tetradecane in the course of 481 hours. The extract, after cleavage of the hydrochloride at 260 to 262[deg]C, gave 9.4 kg of hydrogen chloride; this corresponds to an extraction rate of 67.3%. The amine obtained at the bottom of the cleavage column was regenerated completely by being passed over a large-sized column (height 1,000 mm, if) 100 mm), packed with acidic aluminium oxide, before being returned to the extraction. After completion of the experiment, 1,373.6 g out of 1,900 g of trioctylamine employed were recovered unchanged; this corresponds to a loss of 526.4 g or 27.7% relative to amine employed. Dioctylamine and octylamine, in an amount corresponding to 441.4 g of tertiary amine preparable therefrom, could be isolated from the regeneration process. If this figure is taken into account, the amine loss is reduced to 4.5%. Relative to hydrogen chloride produced, the amine loss is calculated to be 0.9%. If, however the process is carried out without regeneration (Comparative Example G), the experiement fails for the reasons given under Comparative Example F.

Claims

1. A process for obtaining gaseous hydrochloride chloride from dilute aqueous hydrochloric acid by an amine extraction process in which a) the aqueous hydrochloric acid is extracted with an amine or a mixture of an amine and an inert, water-immiscible solvent boiling at not less than 120[deg]C, the amine being a tertiary alkylamine which contains a total of 14 to 36 carbon atoms in the nitrogen-linked carbon chains, at most one of the nitrogenlinked carbon chains being a methyl group and at least one being an aliphatic radical containing at least 6 carbon atoms, the acid constant Ka of the amine being less than 10-3, b) an inert, water-immiscible solvent boiling at not less than 120[deg]C is added to the extract, unless all the solvent required has already been present in stage a), c) the extract is distilled, the resulting vapour is condensed, the water is continuously removed from the two-phase condensate, and the organic phase is returned to the distillation process, d) the extract is distilled, under reflux, after the water has been removed, and the gaseous hydrogen chloride liberated at the top of the column is taken off, or the hydrogen chloride is separated off at below the boiling point of the solvent by passing an inert gas stream through the mixture, and e) the solvent containing the amine is recycled to the extraction process, wherein part or all of the amine and any solvent and impurities is passed over an adsorbent which selectively removes impurities before being employed in stage a).

2. A process according to claim 1, wherein a straight-chain or branched, aliphatic, cycloaliphatic, araliphatic or aromatic, entirely or substantially non-polar, hydrocarbon which has 6 to 20 carbon atoms and boils over 120[deg]C is used as the inert solvent.

3. A process according to claim 1 or 2, wherein aluminium oxide having a specific surface area of 100 to 400 m /g and an activity level on the Brockmann scale of from 1 to 3, silica gel having a specific surface area of 500 to 700 m /g, silanised silica gel having a specific surface area of 300 to 400 m /g or calcium zeolite having a specific surface area of 500 to 600 m /g, the surface area in each case being measured by the BET method, is employed as the adsorbent.

4. A process according to any of claims 1 to 3, wherein the distillation in stage (d) is carried out at a temperature of 120 to 300[deg]C at the column bottom.

5. A process according to any of claims 1 to 4, wherein the amine used in stage a) has a purity of at least 99.5% by weight.

6. A process according to claim 1 carried out substantially as described in any of the foregoing Examples 1 to 3.

7. Gaseous hydrochloride when obtained by a process according to any of claims 1 to 6.

8. The use of gaseous hydrogen chloride according to claim 7 for the manufacture of vinyl chloride, ethyl chloride, chloroprene or chlorosulphonic acid.