METHOD AND DEVICE FOR THE PRODUCTION OF HYDROCHLORIC ACID

Abstract: A method and a device for producing hydrochloric acid by contacting a hydrochloric-acid-containing gas with an aqueous washing liquid are disclosed. A chloride-forming substance is added to the washing liquid, so that the hydrochloric acid in the gas not only is bound by the chloride-forming substance as a chloride salt, but also is absorbed by the washing liquid as 'free' hydrochloric acid. Optionally, the washing liquid is then concentrated by stripping of water. Subsequently, sulphuric acid is added to the washing liquid to drive off hydrochloric acid from the chloride salt. Finally, hydrochloric acid is distilled from the washing liquid. The device includes a scrubber (1) having inlets (2, 10, 11, 12) and outlets (5, 13), a distillation unit (25) having inlets (27, 24) and an outlet (29), and optionally an evaporation unit (14) having an inlet (16) communicating with the scrubber outlet (13), and an outlet (24) communicating with the distillation-unit inlet.

Description: METHOD AND DEVICE FOR THE PRODUCTION OF HYDROCHLORIC ACID The present invention relates to a method and a device for producing hydrochloric acid. More specifical¬ ly, the invention concerns a method and a device for pro¬ ducing hydrochloric acid from hydrochloric-acid-contain¬ ing flue gases resulting from the combustion of waste, such as household refuse. The combustion of waste, such as industrial waste and household refuse, produces flue gases containing hydrochloric acid. This acid is formed e.g. when the burnt waste contains plastics, such as polyvinyl chloride. Before the flue gases are emitted into the atmosphere, the hydrochloric acid has to be removed by washing the flue gases with water in a so-called scrubber or washer. The hydrochloric acid dissolves in the water, resulting in a diluted aqueous solution of hydrochloric acid. The concen¬ tration of hydrochloric acid in the aqueous solution is about 2-10% by weight, depending among other things on the design of the scrubber and the concentration of hydro¬ chloric acid in the flue gases. Here and in the following, the hydrochloric acid concentration is indicated in per cent by weight of HC1, based on the total amount of HC1 and water. Although this takes care of the problem of removing hydrochloric acid from the flue gases, another problem arises, namely how to remove the diluted hydrochloric acid solution obtained. One way of doing this is to subject the hydrochloric acid solution to distillation. In the distillation, water is stripped at the beginning, but by and by a weak hydro¬ chloric acid solution is instead stripped, the concentra¬ tion of which rapidly increases to the concentration of the azeotrope formed by hydrochloric acid and water (about 20% by weight). The azeotrope makes it practically impos¬ sible to obtain hydrochloric acid with a higher concentra- tion than about 20% by weight by common distillation. Fur¬ ther, the hydrochloric acid resulting from the distilla¬ tion contains impurities, which considerably reduces its market potential. Thus, the hydrochloric acid produced by today's techniques has but a slight value or no value at all, and is to be regarded as waste. In another method for cleaning hydrochloric-acid-con¬ taining flue gases to obtain a usable product, a sodium- base or calcium-base chemical is added to the washing liquid in the scrubber, whereby to bind the chlorine of the flue gases as sodium chloride or calcium chloride. Then, this product is recovered by evaporation of the washing liquid. However, also this method is disadvanta¬ geous in that use has to be made of a sodium-base or cal- cium-base chemical which in itself is more expensive than the salt constituting the end product. Thus, the produc¬ tion costs for the salt become unreasonably high, and the salt production thus has to be looked upon primarily as a way of avoiding deposition problems. EP 0,393,402 discloses a prior-art method for clean¬ ing hydrochloric-acid-containing flue gases by absorption in water. The absorption is performed in several steps, the concentration of hydrochloric acid being increased to above 50 g of HCl/1. The thus-obtained hydrochloric-acid- containing aqueous solution is distilled to separate the hydrochloric acid. As also appears from EP 0,393,402, it is known to add carbonates or hydroxides of alkali metals or alkaline earth metals to the washing liquid, and then evaporate the washing liquid to recover e.g. sodium chlo- ride or calcium chloride. The object of the present invention is to obviate the disadvantages of the prior art and provide a method and a device enabling the production of hydrochloric acid with a high concentration and a high degree of purity from hydrochloric-acid-containing gases. The object of the invention is achieved by adding to the washing liquid a chloride-forming substance, so that the hydrochloric acid in the flue gases not only is bound by the chloride-forming substance as a chloride salt, but also is absorbed by the washing liquid in the form of 'free' hydrochloric acid. The thus-obtained washing liquid is optionally concentrated by stripping of water. There¬ after, sulphuric acid is added to the washing liquid to drive off hydrochloric acid from the chloride salt in the washing liquid. Finally, hydrochloric acid is distilled from the washing liquid. More specifically, the invention provides a method for producing hydrochloric acid by contacting a hydro¬ chloric-acid-containing gas with an aqueous washing liquid, characterised by a) adding a chloride-forming substance selected from oxides, hydroxides and carbonates of alkali metals and alkaline earth metals to the washing liquid before this is contacted with the hydrochloric-acid-containing gas, b) branching off at least some of the washing liquid after this has been contacted with the hydrochloric-acid- containing gas, c) optionally concentrating the washing liquid branched off, d) adding sulphuric acid to the washing liquid branched off, and e) thereafter distilling hydrochloric acid from the washing liquid branched off. The invention also provides a device for producing hydrochloric acid from a hydrochloric-acid-containing gas, characterised by a scrubber having inlets for hydrochloric-acid-con¬ taining gas, for aqueous washing liquid, and for a chlo¬ ride-forming substance, as well as outlets for cleaned gas, and for hydrochloric-acid-containing washing liquid, a distillation unit having inlets for sulphuric acid and for hydrochloric-acid-containing washing liquid, as well as an outlet for distilled hydrochloric acid, and optionally an evaporation unit having an inlet which communicates with the scrubber outlet for hydrochloric- acid-containing washing liquid, as well as an outlet for concentrated hydrochloric-acid-containing washing liquid, this outlet communicating with the distillation-unit inlet for hydrochloric-acid-containing washing liquid. Further distinctive features of the invention are recited in the appended claims. Since the hydrochloric acid absorbed by the washing liquid is present as hydrochloric acid and as a chloride salt, the total amount of chlorine in the washing liquid can be made higher than in the case of only hydrochloric acid absorption. Further, by releasing the chlorine in the chlorid -salt as hydrochloric acid by adding sulphuric acid, the concentration of hydrochloric acid in the wash¬ ing liquid can be increased more than is possible with only hydrochloric acid absorption. Consequently, the distillation yields hydrochloric acid with a concentration exceeding that of the azeotrope. As mentioned above, the chloride-forming substance added to the washing liquid is an oxide, a hydroxide or a carbonate of an alkali metal or an alkaline earth metal. Potassium and especially sodium are preferred among the alkali metals. Of the alkaline earth metals, preference is given to calcium. It is especially preferred to use cal¬ cium carbonate as chloride-forming substance. The form in which the calcium carbonate is added is not critical, but it is usually added in the form of grains having a size of about 1-2 mm. The method and the device according to the invention will be described below with reference to the accompanying drawing where Fig. 1 schematically illustrates a preferred embodiment of the device according to the invention. This embodiment is a device for cleaning flue gases from a refuse incinerator plant (not shown), but it will be appreciated that the invention is by no means restricted to such an application, but indeed is applicable to hydro¬ chloric-acid-containing gases in general and the produc- tion of hydrochloric acid therefrom. It should especially be pointed out that in some cases, as in the embodiment described in Example 3, it is not necessary to concentrate the washing liquid further prior to the distillation of hydrochloric acid. In such cases, the evaporation unit shown in the drawing is dispensed with, and the washing liquid branched off from the scrubber is conducted direct¬ ly to the distillation-unit inlet for hydrochloric-acid- containing washing liquid. Fig. 1 shows a scrubber or washer 1 having an inlet 2 for flue gases 3 containing hydrochloric acid. The flue gases 3 are conducted through the scrubber 1, where they are contacted with an aqueous washing liquid 4 absorbing hydrochloric acid from the flue gases, which thus leave the scrubber through the outlet 4 in the cleaned state at 6. In the illustrated embodiment, the washing liquid 4 is contacted with the flue gases 3 by being supplied through a conduit 7 provided with a number of nozzles 8. Conve¬ niently, the scrubber includes several nozzle-equipped conduits 7 arranged on different levels above one another in the scrubber to provide maximum contact between the flue gases and the washing liquid. In principle, the wash¬ ing liquid 4 can be supplied from a tank (not shown) for fresh washing liquid, but is, in the illustrated embodi¬ ment, pumped by a pump 9 from the lower part of the scrub- ber, to which water is supplied through conduits 10 and 11 and a chloride-forming substance through a conduit 12. As mentioned earlier, the chloride-forming substance preferably is calcium carbonate. The amount of chloride-forming substance in the wash- ing liquid should be sufficient to bind about 10-50%, pre¬ ferably 20-40%, most preferred about 30-35%, of the HC1 taken up by the washing liquid. Since the acid is taken up by the washing liquid both as absorbed 'free' hydrochloric acid and as 'bound' chlo¬ ride, the separation of hydrochloric acid is very effec¬ tive and practically all the hydrochloric acid in the flue gases is taken up by the washing liquid. Generally, the 3 cleaned flue gases contain less than 10 g of HCl/Nm . Some of the washing liquid, preferably about 1-7% by weight, is branched off from the conduit 7, which with the aid of the pump 9 circulates the washing liquid from the lower part of the scrubber to the upper part, and conducted through a conduit 13 to an evaporator 14 where it is concentrated. The washing liquid is fed to the eva¬ porator by a pump 15 and a conduit 16, and is concentrat¬ ed in conventional manner. The heat transfer medium used in the evaporator suitably is vapour having a pressure of about 1-2 bar, which corresponds to a temperature of about 100-115°C. The vapour is introduced into the evapo¬ rator as indicated by the arrow 17, and is drawn off in the form of a condensate at the arrow 18. In the separa- tor 19 associated with the evaporation unit, the concen¬ trated washing liquid is separated from evaporated water, which is conducted through the conduit 20 to a condenser 21, where the vapour, under heat exchange, is cooled and condensed to water, which is recycled to the scrubber 1 via the conduit 22. The washing liquid, initially having a concentration of hydrochloric acid of about 2-10% by weight, has been concentrated, by the evaporation, to have a concentration of 'free' hydrochloric acid of about 13-18% by weight. It is difficult to attain a concentration of 'free' hydro¬ chloric acid exceeding about 18% by weight, since this comes too close to the azeotrope of hydrochloric acid with water, which has a concentration of hydrochloric acid of about 20% by weight. From the separator 19, the concentrated washing liquid is recycled to the evaporator via a conduit 23, prior to which some of the washing liquid has been drawn off and conducted to a distillation unit 25 via a conduit 24. This unit includes a distillation tower 26, into the lower part of which the concentrated washing liquid is fed. As indicated by the arrow 27, also sul- phuric acid is fed to the lower part of the distillation tower. The sulphuric acid can be supplied either sepa¬ rately or together with the concentrated washing liquid through the conduit 24. As mentioned earlier, the purpose of adding sulphuric acid is to release as hydrochloric acid the chlorine which is bound as chloride in the washing liquid. To this end, the sulphuric acid should be concentrated, and use is nor¬ mally made of sulphuric acid of about 98% by weight. It is • actually possible to use some other strong acid for driv- ing off the bound hydrochloric acid, but the use of sul¬ phuric acid is preferred. Preferably, the amount of sul¬ phuric acid added is adjusted so as to be essentially equivalent to the amount of chlorine bound as chloride in the washing liquid. When the sulphuric acid is added, the chlorine bound as chloride in the washing liquid is released as hydro¬ chloric acid, and the washing liquid will instead contain a hydrogen sulphate or sulphate of the chloride-forming agent, e.g. sodium or, preferably, calcium. In the distillation tower 26, the concentrated wash¬ ing liquid is distilled in conventional manner. Distil¬ lation takes place at atmospheric pressure or at a reduced pressure and preferably at a reduced pressure as vacuum distillation. In the distillation, hydrochloric acid leaves in vapour phase, which is condensed in a separator 28 yielding liquid, concentrated hydrochloric acid 29. Since the hydrochloric acid which is distilled off in¬ cludes both the 'free' hydrochloric acid in the concen¬ trated washing liquid (about 13-18% by weight of HC1) and the hydrochloric acid released by the addition of sulphu¬ ric acid, the hydrochloric acid 29 obtained from the sepa¬ rator 28 will have a concentration exceeding that of the azeotrope. In general, the concentration of the hydrochlo¬ ric acid obtained is about 20-36% by weight under the con¬ ditions indicated above (about 2-10% by weight of hydro¬ chloric acid in the the original washing liquid, about 10-50% of the hydrochloric acid being bound as chloride by the addition of a chloride-forming substance). The distillation residue is drawn off the lower part of the distillation column through a conduit 30, and is recycled to the distillation tower 26 via the heat ex- changer 21 by means of a pump 31. In the heat exchanger 21, the distillation residue acts as cooling medium for condensating the water vapour which is leaving when the washing liquid is evaporated. When the distillation resi¬ due is recycled, a certain amount is branched off from the conduit 30 via a conduit 32. This amount is treated at 33 by neutralisation and precipitation of heavy metals. Pre¬ ferably, neutralisation is performed by adding calcium oxide or calcium hydroxide, resulting in the precipitation of calcium sulphate, and the precipitation of heavy metals is preferably effected by adding sodium sulphide, so that the heavy metals are precipitated as sulphides. After hav¬ ing thus cleaned the distillation residue and separated the impurities in the form of a sludge 34, the purified distillation residue, which essentially consists of pure water, is conducted to the scrubber 1 via the conduit 35 for preparation of new washing liquid. The illustrated design gives a mainly closed flue- gas-cleaning system which, apart from taking in contami¬ nated flue gases and discharging cleaned flue gases, only discharges hydrochloric acid in the form of a highly con¬ centrated and useful product 29 and a heavy-metal sludge 34 to be stored. There is, in addition, required an addi¬ tion of a chloride-forming substance, such as calcium car¬ bonate, through the conduit 12. If need be, additional water can be supplied to the system, either directly to the scrubber 1 or at any other suitable point, e.g. to the conduit 35, as indicated by the arrow 36 in the drawing. The invention will now be further illustrated in a few Examples. All contents and percentage figures indicate per cent by weight, unless otherwise stated. EXAMPLE 1 In a refuse incineration plant, about 10 tonnes of household refuse was burnt every hour. The resulting 3 amount of flue gases was about 60,000 Nm /h, and the flue gases contained about 60 kg of HCl/h. The flue gases were treated in a device of the type illustrated in the drawing in order to remove the hydrochloric acid and produce a concentrated hydrochloric acid solution. In the treatment, the flue gases were conducted through a scrubber and contacted with a washing liquid to which was added a chloride-forming substance in the form 3 of about 27.5 kg of CaC0 3 /h per 100 m of washing liquid. By the contact with the washing liquid, essentially all the hydrochloric acid in the flue gases was absorbed, some being absorbed as 'free' hydrochloric acid in the washing liquid and the remainder being bound as calcium chloride (CaCl 2 ) in the washing liquid. From the scrubber a certain amount of the washing liquid was branched off, which con¬ sisted of about 1300 kg of H 2 0/h, about 40 kg of HCl/h and about 30 kg of CaCl^/h, i.e. the washing liquid contained about 3% HC1. The thus-removed washing liquid was fed to an evaporator, where about 1120 kg of H 2 0/h was evaporated and, after condensation, recycled to the scrubber, while a concentrated washing liquid containing about 180 kg of H 2 0/h, about 40 kg of HCl/h and about 30 kg of CaCl 2 /h was obtained. The concentrated washing liquid, which thus had a concentration of hydrochloric acid of about 18%, was then conducted to a distillation plant. In connection with the supply of the concentrated washing liquid to the distillation tower, about 53 kg of 98% sulphuric acid/h was added, so that the chlorine bound as chloride was released as hydrochloric acid. As a result, the supply of concentrated washing liquid to the distillation tower con- tained about 25% of hydrochloric acid which was distilled off and, after condensation, recovered as a product. In the manner described above, the distillation resi¬ due was treated with calcium oxide and sodium sulphide to neutralise and precipitate heavy metals, whereupon the thus-cleaned distillation residue was recycled to the scrubber for the production of new washing liquid. EXAMPLE 2 Example 1 was repeated, but to the washing liquid was added a chloride-forming substance in the form of about 11 3 kg of CaC0 3 /h per 100 m of washing liquid. The washing liquid branched off from the scrubber consisted of about 5,200 kg of H 2 0, about 52 kg of HC1 and about 12 kg of CaCl 2 per hour. During evaporation, about 4,965 kg of H 2 0/h was evaporated, and the resulting flow of washing liquid contained about 235 kg of H 2 0, about 50 kg of HC1 and 12 kg of CaCl 2 per hour. About 21 kg of 98% sulphuric acid/h was added to the concentrated washing liquid, and the washing liquid then contained about 20% of HC1 which was distilled off and recovered as a product. EXAMPLE 3 Example 1 was repeated, but to the washing liquid was added a chloride-forming substance in the form of about 3 41 kg of CaCO^/h per 100 m of washing liquid. The washing liquid branched off from the scrubber consisted of about 135 kg of H 2 0, about 30 kg of HC1 and about 45.6 kg of CaCl 2 per hour. In this case, no evaporation was required, and the washing liquid branched off was therefore con¬ ducted from the scrubber directly to the distillation unit. About 80 kg of 98% sulphuric acid/h was added to the washing liquid, and the resulting washing liquid then contained about 30% of HC1 which was distilled off and recovered as a product.

Claims: CLAIMS 1. A method for producing hydrochloric acid by con- tacting a hydrochloric-acid-containing gas with an aqueous washing liquid, c h a r a c t e r i s e d by a) adding a chloride-forming substance selected from oxides, hydroxides and carbonates of alkali metals and alkaline earth metals to the washing liquid before this is contacted with the hydrochloric-acid-containing gas, b) branching off at least some of the washing liquid after the contact with the hydrochloric-acid-containing gas, c) optionally concentrating the washing liquid branched off, d) adding sulphuric acid to the washing liquid branched off, and e) thereafter distilling hydrochloric acid from the washing liquid branched off. 2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d by condensing and recycling as new washing liquid the water evaporated during concentration of the washing liquid. 3. A method as claimed in claim 1 or 2, c h a r - a c t e r i s e d by adding CaC0 3 as chloride-forming substance. 4. A method as claimed in any one of preceding claims, c h a r a c t e r i s e d by treating the resi¬ due from the distillation of hydrochloric acid with a calcium-containing substance to precipitate gypsum, and with a sulphur-containing substance to precipitate heavy metals as sulphides. 5. A method as claimed in claim 4, c h a r a c ¬ t e r i s e d by treating the distillation residue with CaC0 3 and Na 2 S. 6. A method as claimed in claim 4 or 5, c h a r ¬ a c t e r i s e d by recycling as new washing liquid the distillation residue after precipitation and separation of gypsum and heavy-metal sulphides. 7. A device for producing hydrochloric acid from a hydrochloric-acid-containing gas, c h a r a c t e r i s ¬ e d by a scrubber (1) having inlets (2, 10, 11, 12) for hydrochloric-acid-containing gas (3), for aqueous washing liquid, and for a chloride-forming substance, as well as outlets (5, 13) for cleaned gas (6) and for hydrochloric- acid-containing washing liquid, a distillation unit (25) having inlets (27, 24) for sulphuric acid and for hydrochloric-acid-containing wash- ing liquid, as well as an outlet (29) for distilled hydro¬ chloric acid, and optionally an evaporation unit (14) having an inlet (16) which communicates with the scrubber outlet (13) for hydrochloric-acid-containing washing liquid, as well as an outlet (24) for concentrated hydrochloric-acid-containing washing liquid, this outlet communicating with the distil¬ lation-unit inlet for hydrochloric-acid-containing washing liquid. 8. A device as claimed in claim 7, c h a r a c - t e r i s e d in that the evaporation unit (14) includes a condenser (21) adapted for condensing evaporated water and connected to the scrubber (1) in order to recycle the condensed water as washing liquid. 9. A device as claimed in claim 7 or 8, c h a r - a c e r i s e d in that the distillation unit (25) is connected to the scrubber (1) in order to recycle the distillation residue to the scrubber. 10. A device as claimed in claim 9, c h a r a c ¬ t e r i s e d in that the distillation residue from the distillation unit (25) is recycled via a cleaning device (33) for precipitating and separating heavy-metal sul¬ phides and gypsum from the distillation residue.

Inventors: RAGNARSSON SVEN, RAGNARSSON, SVEN
Applicants: FLAEKT AB, ABB FLAEKT AB
Application year: 1992
Main classification: C01B7
IPC codes: C01B7