IL36399A - Production of concentrated hydrogen chloride from aqueous solutions thereof - Google Patents

Description

Production of concentrated hydrogen chloride from aqueous solutions thereof FOR RESEARCH The inventors EDI EDWARD 34241 present invention concerns the production of a hydrogen chloride gaseous mixture containing at least weight of from aqueous HCl Such mixtures are suitable for recovering therefrom dry HCl by any conventional drying operation such as condensation or water absorption by means conventional water absorbing In the context of the present specification and the appended claims the term hydrogen means an HCl gas containing not more than by weight of Anhydrous hydrogen chloride has recently gained considerable importance in various organic synthesis processes such for ox and there is a demand for this product at a reasonable Known processes for the production of anhydrous hydrogen chloride from aqueous solutions thereof turned out to be economically unsatisfactory and it is the object of the present invention to provide an improved process for this Where an aqueous solution of hydrogen chloride is concentrated by distillation there after the initial removal of some water or HCl an azeotropie mixture containing by weight of HCl and this mixture cannot be broken up by ordinary Therefore all processes the recovery of anhydrous hydrogen chloride from an aqueous solution thereof are based on the distillation in the presence of a dehydratation agent which is capable of breaking up the Examples of such agents are calcium magnesium lithium ferric chloride and zinc It is further known that from a ternary mixture comprising hydrogen chloride and a dehydratation and where necessary fractionated condensation of a emerging from such action of the dehydratant component in such an operation is based on the change in the relative volatility between HCl and and the operation is usually referred to as extractive Hereinafter the above described method for the recovery of HCl from an aqueous solution thereof by steam stripping in the presence of a dehydratation agent will be referred to distillation of the kind During extractive distillation of the kind specified the dehydratant solution takes up Thus the processes involve two main operations conducted separately and simultaneousl In the first operation aqueous hydrochloric acid is contacted with a concentrated dehydratant solution and the HCl is stripped out of the resulting diluted solution with The HCl vapour stream emerging from this operation may contain a certain amount of water which is condensed and refluxed together with some of the HCl by means of a reflux There also emerges from this operation a dilute dehydratant the second operation the dilute dehydratant solution emerging from the first one is reconcentrated by elimination of the water absorbed in the first operation by any of the known brine concentration Known methods of the extractive distillation of the kind specified are all based on this basic Thus in accordance with one method described in patent specification aqueous solution containing 10 to by weight of HCl by contacting the solution in a fractionation column with a 30 to weight aqueous solution which is fed to the column at a temperature of 20 to solution is withdrawn hot from the column and to be cooled by indirect heat exchange to the desired temperature of 20 to whereby sensible heat is dissipated and The HCl solution is fed to a median region of the the calcium chloride solution is fed to the top region and steam is fed to the bottom region The top region is cooled by a cooling jacket whereby the heat of condensation and the heat of dilution of the is absorbed so that the water vapour pressure in that region should not exceed of the operational This method then uses a relatively Gold dehydratant solution but it follows from the disclosure in the specification that this entails a loss of HCl at a rate of from 9 to of the This loss is quite substantial and in view of it and the above referred to loss of sensible heat the entire process is In accordance with another method of extractive distillation of the kind specified described in patent specification the feed HCl solution is cooled to below and its concentration must be at least by weight of cooled and concentrated feed is fed to the top of a fractionation a concentrated solution is fed to the median region and steam is again d to the bottom region In this operation the cold acid in addition to serving as an HCl source also serves for cooling the vapourous from the column and refluxing the water is higher than the HCl concentration in an Such a solution not readily available and has to he produced by special this requires expensive refrigeration of the feed Yet another method of extractive distillatio of the kind specified is described in patent specification In accordance with this process cold HCl solution is fed to the median region of a fractionation while a hot concentrated solution at boiling or near boiling temperature is fed to the top portion in the other methods steam is introduced at the bottom and in this way lowest temperature prevails in the median portion of the column where the cold HCl solution is introduced and the temperature rises in the direction o both the top and the An mixture emerges from the top of the column and is charged into a reflux condenser from where an mixture is recycled to the centre portion of the column while anhydrous aqueous HCl is withdrawn as Shis method has the disadvantage that a large heat load is given to the condenser and the HCl losses are A still further method of extractive distillation of the kind specified is described in Canadian patent specification Here again an HCl solution at a concentration higher than the one is introduced at the top of a fractionation column while boiling g brine is introduced to the median region of the column and steam the bottom region An mixture emerges from the column and is charged into a reflux condenser from where an aqueous HCl solution is returned to the column while anhydrous specification this has the disadvantage of requiring as starting material an HCl of a concentration higher than Furthermore the hea and HCl losses are The above described known methods may be divided into two in one kind a relatively cool dehydratant solution is which although reducing the heat load on the condenser entails substantial HCl Most of the heat removed from the cooled dehydratant stream is dissipated in a In the other kind the dehydratan solution is introduced at boiling or near boiling temperature in case of a calcium chloride These methods nay in some cases be satisfactory from the point of view of HCl recovery but they are unsatisfactory because of high energetic An analysis of these methods leads to the conclusion that one may opt between a satisfactory energetic regime for the price of HCl losses or a satisfactory recovery for price of higher energetic In accordance with the present invention it has now surprisingly found that there exist conditions at which an extractive distillation of the kind specified can be conducted in such a way that the combined effect of HCl recovery and comparatively low energetic requirements is superior to what it is in any known In accordance with the invention there is provided a method for the recovery of anhydrous hydrogen chloride from an aqueous solution thereof comprising introducing into the upper region of a fractionation an aqueous hydrochloric acid solution containing from 15 to by weight of gives it a vapour pressure of 50 to 300 introducing into the bottom region of the column superheated withdrawing from the top of the column a mixed stream of vapour containing at leas by weight of and subjecting said stream to a drying In accordance with one embodiment of the invention said drying operation is effected by condensation of the water by means of a reflux eondenser associated with the column In accordance with another embodiment of the invention the said drying operation is performed by contacting the vapour with a dehydratant such as a concentrated HCl a concentrated a concentrated solution and It is also possible in accordance with the invention to effect said drying operation by a combined operation comprising condensation and The requirement that the water vapour pressure of the aqueous dehydratant solution must be within the range of is It means that the temperature of this feed is confined to a range that lies between the temperatures of the very hot and very cold solutions that are used in known It has been found in accordance with the invention that due to this selection of the temperature of the dehydratant solution the HCl losses are considerably lower than in any known method while at the same time the overall energetic requirements of the process are also From an operational point of view the fractionation column may be considered as comprising two an and the concentrated dehydratant solution are mixed in the presence of a rising and a lower distillation section into which the aqueous stream from the contact section flows countercurrently to the steam rising from the bottom of the The operative conditions of the process according to the invention are based on the following considerations which were discovered in the course of investigations which led to the present inventions It is known that in an extractive distillation of the kind specified the function of the superheated steam introduced at the bottom of the column is On the one hand its partial condensation provides the necessary heat to compensate the difference in enthalpy between the inlet and outlet the heat of desorption of the HC1 and the dilute difference between the sensible heat of solution leaving the column and the dehydratant solution and introduced The heat of the dilution of the is too small to provide for such a heat On the other hand the superheated steam acts to dilute the concentration in the vapour increasing the volumetric ratio and thus providing the stripping The of steam used for this second purpose is an operational the more steam is the lower the losses since less remains dissolved in the liquid phase for a finite number of countercurrent At the same an excessive amount of steam reduces the heat economy of the It has been recognised in accordance with the present invention that the amount of steam needed to reduce the 4 considerably higher than the amount of steam which would be required by enthalpy balance for the first In consequence steam is discharged from the column top together with HCl gas and must be condensed whereby its heat content is Shis in turn introduces the following economic losses of hea the need of expensive heat transfer area in the condenser and of cooling since in the condenser only HCl and water are present the condensed liquid phase is very concentrated in HCl and much HCl is thus refluxed together with the water to the This recirculation represents also a loss of heat in the form of the heat of desorp of the recirculated The extent of these features considerably reduced in accordance with the present It was found that by the introduction of the dehydratant solution at a temperature at which its vapour pressure is from sua Hg as considerably below the boiling temperature at the pressure prevailing inside the there results a sharp temperature gradient in the upper half of the column in consequence of which most of the steam introduced at the bottom of the column for stripping purposes is condensed inside the column and does not reach the It was found that this of operation produces the following The heat recovered inside the column in this manner can he utilised outside the column in the reconoentration operation for the purpose of evaporating water by flashing under In other whereas in accordance with the prior art the hot end dilute solution withdrawn from the of the column is further heated for reconoentration patents and 2 and Canadian patent thereby to bring it to the boiling or near boiling temperature at which the solution is fed to the columns or alternatively the reeoncentrated solution is cooled to patent all of which involves considerable expenditure of added in accordance with the invention the sensible heat of the withdrawn dilute dehydratant solution is utilised for the reconoentration of the latter whereby the required amount of extraneous heat is considerably Because of the fact that most of the steam is condensed inside the column the expensive heat transfer area of the indirect contact reflux condenser where such is is much than in known methods in which reflux condensers are The condensation of the mixture in the contact zone the column is more selective which means that the product vapour is more concentrated in Quite surprisingly it has been found in accordance with the invention that the HCl gradient in the liquid phase within the column is much steeper than in known in consequence of which the HCl losses due to the remaining HCl dis olved in the eh d n so ution a e m nm The preferred dehydratant used in accordance with the invention is a and preferably about by weight calcium chloride solution which is fed into the column at a temperature of from C and preferably The invention will now be described with reference to the annexed drawings in 1 is an loss and 2 is a diagrammatic representation of an installation for carrying out the method according to the In 1 experimental values of losses in an extractive distillation of the kind specified and using economically practical quantities of steam and employing a reflux condenser are plotted against heat dissipated in the Curve describes the results obtained when operating with a solution at in accordance with a known and curve describes the of the In both sets of experiments the feed contained by weight of H01 and in both the sets the dehydratant was a by weight solution and the effluent solution withdraw from the bottom of the column contained 44 by weight of In the set of experiments represented by curve the solution was fed at while in the set of experiments according to the invention represented by curve the solution was fed at It is seen from 1 that for a given loss the heat loss according to the known method is higher than in accordance with the For for a loss the heat dissipation according to the known method is 400 for a given heat loss the loss in aeeordanee with the known method is considerably higher than in accordance with the For for a heat loss of 350 the HC1 loss in accordance with the known method is about while in accordance with the invention it is only about less than A further noteworthy feature apparent from 1 is that curve does not show an loss rate lower than within the practical range of steam consumption This means that even by increasing the heat losses in the practical range or in other words by putting in more energy into the it is impossible to reduce the loss below Against this curve according to the indicates loss value down to which means that in accordance with the invention the HC1 losses may be reduced to negligible quantities while keeping the heat dissipation within the economically practical The installation for carrying out the process according to the invention diagramiaatically shown in 2 comprises a multistage fractionation 1 fed at 2 with an solution and at a somewhat higher at with a concentrated dehydratant Near the at the column is d with superheated At the top a mixed stream is withdrawn at 5 and conducted into a reflux condenser 6 from whose upper end a dry is withdrawn at 8 as while the which a dilute HG1 is returned at 7 to column From the bottom of column 1 a dilute dehydratant solutioi containing a small amount of ECl is withdrawn at The invention is further illustrated in the following with a prior art and illustrates the advantages of the present Some tests were with distillation column of consisting of 14 and fed continuously with concentrated solution as a an HCl solution and superheated all of controlled concentration and The heat load of the condenser was Also the compositions and temperature of all streams and of several points inside the column were continuously The installation was of the design of In all by weight solution of was used as dehydratant and the HCl feed solution contained by weight of HCl and was fed at the rate of 20 at a temperature of The bottom effluent contained weight of and its temperature at discharge was Steam was introduced in measured quantities at Test 1 was according to the invention while tests 2 and 3 were according to prior results are tabulated in Table TABLE 1 Experimental Data obtained By interpretation of the above data the following figures are TABLE 2 Comparative results The data in fable 1 and their interpretation as given in Table 2 can be explained as Test 1 represents typical conditions according to the present Test 2 represents typical conditions according to known with substantially the same HCl losses as in Test 1 but a higher heat 3 represents an attempt to duplicate the net heat loss of Test 1 while feeding the dehydratant solution in the conditions of Test It can be seen that the HCl loss is excessively In Test about of the steam was needed only as a stripping gas and did not contribute to the enthalpy differences between the inlet and outlet but reached the condenser where its heat was In Test only about of the steam in slightly higher quantity than in Test reached the In the sensible heat difference of the concentrated dehydratant solution is equivalent to more than of the steam As mentioned this sensible heat can be usefully utilised outside the and therefore credited for its steam equivalent Another aspect which does not appear in Table 1 is that the considerable heat transfer area needed for operation under the conditions of test 2 is about times that needed when working under the conditions of the present This entails also a larger consumption of cooling heat difference is about half in Test 1 as compared with Test since the sensible heat recovered in Test 1 can be for example in a forward feed multiple effect its net value is higher than its actual heating This is a further advantage in favour of the present shows that if it is attempted to reduce the direct heat losses in the condenser to the level of Test while keeping the concentrated dehydratant solution fed to the column near its normal boiling the relative HCl loss increased from to due to insufficient volume of stripping The sensible heat recovered from the concentrated dehydratant solution in accordance with the invention can be as mentioned to evaporate water by flashing under or to evaporate water in a forward feed multiple effect evaporator last effect of which is operating under i or to transmit heat by indirect heat exchange to another stream which it is desired to insufficientOCRQuality

Claims (6)

1. Method for the recovery of anhydrous hydrogen chloride from an aqueous solution thereof comprising introducing into the upper region of a fractionation column an aqueous hydrochloric acid solution containing from 15 to 3 ?» by weight of ECl, introducing into the column at the same level or up to 4 plates above it an aqueous dehydratant solution at a temperature which gives it a vapour pressure of 50 to 300 mm Hg, introducing into the bottom region of the column superheated steam, withdrawing from the top of the column a mixed stream of HCl-H^O vapour containing at least 75?a by weight of HOI and subjecting said stream to a drying operation.

2. Method according to Claim 1, wherein said drying operation comprises condensation of the water vapour by means of a reflux condenser associated with the column head,

3. « Method according to Claim 1, wherein said drying operation comprises contacting the said ECl-BgO stream with a dehydratan .

4. Method according to Claim 1, wherein said drying operation comprises in combination condensation and contacting of the HCl-HgO vapour stream with a dehydratant.

5. · Method according to Claim 1» wherein the dehydratant is an aqueous 50-60$ CaClg solution,

6. Method according to Claim 2, wherein the CaCl2 solution is fed to the column at a temperature of 75-95°C.