DE2210857A1 - Process for the production of anhydrous hydrogen chloride - Google Patents

Description

9 9 1 Π ft R 7

DR.-ING. FROM CIRCLE i.F.i < DR.-ltiG. NICE! WAID DK.-ING. TH. MEYER DR. FlJi = S UfFL.-CHEM. ALEK VO ^ KBEfSLKR DIPL-CKEM. CAROLA KELLER DR.-IMG. KLCfPSCH DIPL-ING. SELTINCS

5 COLOGNE 1, DLICMMANNHAUS

Cologne, February 21, 1972 Ke / Ax

HOL

Haifa Bay, nea

Traversing s part to Piers clothes of wag se ^ rf ^ io re η Chlorv / ater

material

The invention relates to the production of gaseous hydrogen chloride-iKO mixtures containing at least 75% by weight of HCl from aqueous HCl solutions and the recovery of anhydrous hydrogen chloride from these mixtures. The term "anhydrous hydrogen chloride" used here denotes an HCl gas which does not contain _{more than 1% by weight of H 2 O.}

In the end, anhydrous chlorine acid has considerable properties Significance in various processes of organic synthesis, e.g. for oxychlorination processes, won, so that a great need for this FrocUüct at a reasonable price. The known processes for the production of anhydrous hydrogen chloride from its aqueous solutions turned out to be economical unsatisfactory. The idea »divides the Object to provide an improved method for this purpose to make available.

When an aqueous solution of hydrogen chloride through Distillation can freeze, forms after initial Removal of some water or some HCl

209839/1086

azeotropic HCl-JULO mixture, which contains 20 to 22 Gevi .-% HCl and cannot be broken down by ordinary distillation. Therefore, all processes for obtaining anhydrous hydrogen chloride from its aqueous solutions are based on distillation in the presence of a dehydrating agent contained in the solution that is able to separate the azeotropic equipment. Examples of such dehydrating agents are calcium chloride, magnesium chloride, lithium chloride, ferric chloride and zinc chloride. It is also known that the anhydrous hydrogen chloride is best obtained from a ternary mixture of water, hydrogen chloride and a dissolved dehydrating agent by stripping with superheated steam and, if necessary, by fractional condensation of a resulting steam-HCl mixture. The effect of the dehydrating agent in this operation is based on the change in the relative volatilities of HCl and EUO, and this procedure is commonly referred to as extractive distillation. The above-mentioned process for obtaining HGl from its aqueous solutions by stripping with steam in the presence of a dehydrating agent is referred to below as "extractive distillation of the type defined". _v

During the extractive distillation of those defined above Kind takes the one containing the dehydrating agent Solution water up. The methods thus comprise two main operations, which are carried out separately and simultaneously will:

In the first step, aqueous hydrochloric acid is mixed with a concentrated solution of the dehydrating agent brought together and the HCl is distilled off with steam from the dilute solution formed in this way. Of the HCl sbromine escaping from this process can cause a contain some amount of water that condenses and

209839/1086

agree with some of the HCl using a reflux condenser is returned. With this one. Operation also falls into a dilute solution of dehydration? " by means of.

The second step is that of the first step Incident prediluted solution of the hydrating agent by removing what was absorbed in the first step Water by any known method of Brine concentration concentrated again.

All known processes for extractive distillation of the type defined above are based on this basic scheme. For example, in a process described in US Pat. No. 2,557,095, anhydrous hydrogen chloride is extracted from an aqueous solution containing 10 to 40% by weight of HCl by treating the solution in a fractionating column with a JO- to 35 / ^ 3 gen aqueous CaCl ^ solution, which is fed to the column at a temperature of 20 to 55 ° G, prepared. The CaClp solution is drawn off hot from the column and must therefore be cooled to the desired temperature of 20 to 55 ° C by indirect heat exchange, whereby sensible heat is consumed and lost. The HCl solution is fed to a central section of the column, the calcine chloride solution to the upper section and steam to the lower section. The upper section is cooled by a cooling jacket, whereby the condensation heat and the heat of dilution of the CaCl2 solution is absorbed, so that the water vapor pressure in this area should not be higher than 1 % of the working pressure. In this method som.it a relatively ke.lte solution of the dehydration agent is used, but it follows from the disclosure in the description that this is a loss; from JIGl to a height of 9 to 10 % of the infl. Servant VoiOi: v'- is quite relaxed. »! I,

209839/1086

8 AD ORlGiN * L

and given dietsee loss and the above The entire process is uneconomical due to the loss of sensible heat.

In another, in US Pat method of extractive distillation of the kind defined above described 2 764 532 is cooled, the HCl solution used to below ⁰ ° C. Their concentration must be at least 32% by weight HCl. This cooled and concentrated feed solution is applied to the top of a fractionation column. A concentrated CaClo solution is fed to the middle section and water vapor in turn to the lower section of the column. In this process, the cold acid used not only serves as a source of HCl, but also to cool the vaporous mixture emerging from the column and to recycle the water content. The 32% HCl concentration required for the feed in this process is higher than the HCl concentration in an azeotropic HCl-HpO mixture. Such a solution is not easily available and must be manufactured in special plants. Furthermore, this process requires expensive cooling of the HCl solution to be used.

Another method of extractive distillation of the type defined above is described in US Pat. No. 2,765,049. In this method, a cold HCl solution is supplied to the central portion of a fractionating column, while a teiße CaClg concentrated solution is placed with ο he boiling temperature or in the vicinity thereof (120 to 140 ⁰ C) to the upper portion of the column. As with the other methods, water vapor is added at the foot and thus the lowest temperature is in the middle section of the column, where the cold HCl solution is introduced, and the temperature both towards the top and halfway through the column increases. One HCl-H - /) -

209839/1086

The mixture emerges from the top of the column and is fed into a reflux condenser, from which an HCl-H ^ O mixture is returned to the middle part of the column, while anhydrous hydrogen chloride is the product deducted v / ird. This method has the disadvantage that a high heat load on the radiator vrf.rd and the HCl losses are high.

Another method for extractive distillation of the type defined above is described in the Canadian patent 490 523. Here, too, an HCl solution is used a higher concentration than an azeotropic HCl-HpO mixture placed on top of a fractionation column, while boiling CaClp soles the middle part the column and steam is fed to the lower part of the column. An HCl-H-O mixture emerges from the Column off and is fed into a reflux condenser, from which an aqueous HCl solution is returned to the column while anhydrous hydrogen chloride is withdrawn as a product. Just like the procedure of the USA patent 2,764,532, this process has the specialist part that an HCl solution is required as the starting material whose concentration is higher than in an azeotropic HCl-HpO mixture. Then there are the heat and HCl losses considerable.

The known processes described above can be divided into two groups: In the processes of one group, a relatively cool solution of the dehydrating agent is used, which reduces the heat load on the condenser, but results in considerable HCl losses. Most of the heat extracted from the cooled water vapor is lost in an unusable manner. In the other group of processes, the solution is dehydrated at or near the boiling point (120 to 14O ⁰ C in the case of a calcium chloride solution)

209839/1086

-G-

and these processes can in certain cases be satisfactory from the standpoint of HCl recovery be, however, are unsatisfactory due to the high energy requirements.

An analysis of all known methods leads to the conclusion that one can choose between a satisfactory one Energy utilization at the expense of HCl losses or a satisfactory HCl recovery at the expense of the higher Energy requirements can choose. According to the invention it has now surprisingly been found that there are conditions are, among which an extractive distillation of the defined type can be carried out so that the combined effect of HCl recovery and a relatively low energy requirement is higher than at all known procedures.

The invention relates to the recovery of anhydrous hydrogen chloride from its aqueous solutions by a process which is characterized in that an aqueous hydrochloric acid solution containing 15 to 35% by weight of HCl is introduced into the upper part of a fractionating column and into the column in the same Height at which this hydrochloric acid solution is introduced, or up to 4 trays above it, an aqueous dehydrating agent solution is applied at a temperature at which it generates a water vapor pressure of 50 to 300 mm Hg, introduces superheated steam into the lower part of the column, from the top of the Column withdraws a mixed stream of HCl-H ₂ O vapor containing at least 75% by weight of HCl. contains, and this stream dries "

According to one embodiment of the invention, this; Drying by condensation of the water vapor, for example carried out with the help of a reflux cooler, the is downstream of the column head.

In another embodiment of the invention, 209839/1086 is done

this drying is carried out by treating the HCl-H ₀ O vapor with a dehydrating agent, for example a concentrated HCT solution, a concentrated HpSCV solution, a concentrated Η, ΡΟ ^ solution and other dehydrating agents.

According to the invention, it is also possible to carry out the drying by a combined operation of condensation and Make dehydration.

The conditions that the Wapserdair.pfd ^ uck the Solution of the dehydrating agent in the range of 50 must be up to 500 mm Hg is essential. She means that the temperature of this EinatζStroms on a Range is limited, which lies between the temperatures of the very hot and very cold solutions, which are at the known methods can be used. According to the invention it has been found that due to this choice of Temperature of the solution of the de-hydrating agent. HCl losses are considerably lower than with all known ones Process, while at the same time the total energy consumption of the process is also reduced.

From an operational standpoint, the fractionation column can be used as can be viewed as consisting of two sections: An upper contact section in which the aqueous hydrochloric acid solution and the concentrated solution of the dehydrating agent in the presence of an ascending HpO-BOl stroma mixed, and a lower stripper distillation section, into which the watery stream from the contal cut in countercurrent to that from the foot of the Column rising steam flows.

The working conditions of the method according to the invention based on the following considerations. · η to which in the course of the examinations leading to the Invention led to:

"AD OF" S; * t * l

209839/1086

In the case of an extractive distillation of the type defined, the superheated steam introduced at the foot of the column has as is well known, a twofold task. On the one hand, its partial condensation provides the necessary pan than Compensation for the enthalpy difference between the inlet flow and the outlet flow, since the heat of dilution of the dehydrating agent is too low, to meet this heat demand. On the other hand, the superheated steam causes the HCl concentration to decrease in the vapor phase, whereby the vapor / liquid volume ratio increases and the stripping effect is brought about. The one used for this second purpose The amount of water vapor is an operational variable: the more steam that is used, the less it is HCl losses because there is less HCl in the liquid phase a finite number of countercurrent stages remains solved. At the same time, however, an excessively large amount of water vapor worsens the heat balance of the process.

According to the invention it was found that the water vapor amount that was previously required to the amount of HCl that was diluted in the withdrawn from the bottom of the column Solution of the dehydrating agent remains to reduce to acceptable economic limits, is significantly higher than the amount of water vapor, which would be required by the enthalpy equilibrium for the first purpose. As a result, water vapor occurs from the top of the column together with HCl gas and has to be condensed, as a result of which its content is lost goes. This in turn results in the following economic disadvantages:

a) losses of thermal energy,

b) Need for an expensive warm transition area in the condenser and waste of cooling water .;

c) since only HCl and Wa sr in the condenser. he exist,

209839/1086

the condensed liquid phase is more concentrated of HCl (30-40%), so that a large amount of HCl is returned to the column as reflux together with the water. This recirculation of HCl provides also a loss of heat in the form of the heat of desorption of the circulating HCl.

The scope of these disadvantageous features is according to the invention significantly reduced. It has been found that by introducing the solution of the dehydrating agent at a temperature at which its vapor pressure has the above-mentioned value of 50 to JOO mm Hg, i.e. at a temperature which is considerably below the boiling point at that prevailing inside the column Pressure is, a sharp temperature gradient arises in the upper half of the column. As a result of this temperature gradient most of the water vapor that is introduced at the foot of the column for »stripping, condensed inside the column so that it does not reach the condenser. It was found that this Working method has the following advantages:

1) The heat recovered in this way in the column can be used outside the column during the renewed concentration to evaporate water by flash evaporation under vacuum. In other words, while, according to the prior art, the hot and dilute CaCl ^ solution withdrawn from the bottom of the column is further heated to re-concentrate (U.S. Patents 2,764 and 2,765,049 and Canadian Patent 490,323) and thereby is brought to the boiling point or close to the boiling point at which the concentrated Löfjung is fed to the column, or ala alternative, the re-concentrated solution is ^{cooled to 20 bio 35 0} C (UGA patent 2 357 095), in each case a considerable amount of work to

209839/1086

additional energy is required, is invented accordingly the sensible warmth of the withdrawn thinned ones Solution of the dehydration agent for Concentration of this solution used, whereby the required amount of foreign heat considerably is decreased.

2) La most of the water vapor within the Column is condensed is the costly heat transfer surface of the indirect contact working reflux condenser, where this is used, much less than in the known processes, where a reflux condenser is used.

5) The condensation of the HCl-HpO mixture in the contact zone of the column is more selective, ie the HCl-H ₂ O-DaInPf obtained as product has a higher HCl concentration.

4-) Surprisingly, it was found that the process according to the invention, the HCl gradient in the Liquid phase within the column is much steeper than in known processes, so that the HCl losses as a result of the dissolution of the remaining HCl in the Solution of the dehydrating agent can be reduced to a minimum.

The dehydrating agent used for the purposes of the invention is preferably a calcium chloride solution which has a concentration of 50 to 60%, preferably about 55% by weight and is fed to ^{the column at a temperature of 75 to 95 °} C., preferably 80 to 90 ^{° C.} will.

The invention is described below in connection with the figures.

Figure 1 is an HCl loss / heat loss dingram. 209839/1086

Figure 2 shows schematically a system for implementation of the method according to the invention.

FIG. 1 shows the experimental values for HCl losses in an extractive distillation of the type defined above using economically acceptable amounts of superheated steam and a reflux condenser as a function of the heat consumed in the condenser. Curve (a) indicates the results obtained when a known method was used _{with a GaCl 2 solution.} Curve (b) is representative of the method according to the invention. In both test groups, the hydrochloric acid solution fed in contained 20% by weight of HCl. In both groups, a 55% CaCl ^ solution was used as the dehydrating agent, and the solution withdrawn from the bottom of the column contained 44% by weight CaClo. that CaClp solution was fed at 13O ⁰ C, while according to the invention performed for the, experiments shown by the curve (b) was carried out feeding the GaCIp solution at 85 ° C.

FIG. 1 shows that for a given IiCl loss dex ^1, heat loss in the known method is higher than in the method according to the invention. For example, with an HCl loss of 5 %, the heat consumption in the known method is 400 kcal, while it is only 160 kcal in the method according to the invention. Conversely, the HG.l loss in the known methods for a given heat loss is considerably higher than in the method according to the invention. For example, with a heat loss of 'f / 10 kcal UfV HCl loss in the known process is about 5.5%, while it is only about 2.1% in the case of the previous method according to the invention, (Kh. Uni. R. Half lie ^ t.

209839/1086

Another notable feature of Figure 1 is the fact that curve (a) (known method) shows no HCl loss below 2.5 'fo within the acceptable range of steam consumption in question. This means that even by increasing the heat losses in the practical range in question or, in other words, by increasing the energy input in the process, a reduction in the HCl loss below 2.5 % is impossible. In contrast, curve (b) (method according to the invention) shows an HCl loss down to 0.7 % *. This means that in the method according to the invention, the HCl losses can be reduced to negligible amounts during the Heat loss remains in an economically viable range.

The plant shown schematically in FIG. 2 for carrying out the process according to the invention consists of a multistage fractionation column 1 to which an HCl solution is fed at 2 and a concentrated solution of the dehydrating agent at a slightly higher point at 3. In the vicinity of the column foot at M , the column is supplied with superheated waste. At the top of the column, an HCl-HpO stream is withdrawn at 5 and introduced into a reflux condenser 6, from the upper end of which a dry HCl gas stream is withdrawn as a product at 8, while the condensate, which consists of a dilute HCl solution, is withdrawn. 7 in the column 1? .Iiriir..k {;, ef ührt, at the foot of the column ie 1 a dilute solution of the dehydrating agent, which contains only a small amount of HCl, is drawn off at 9.

The invention is carried out by the following example explained. This example spells out some of the orientations dvu'c]) (', - efii) vrte attempts and some

209839/1086

Comparative experiments which were carried out according to a known method and illustrates the advantages of the invention ,

Some attempts have been made with a distillation column carried out, which had a diameter of 10.2 cd « and was provided with 14 bell bottoms. The column was continuously used as a concentrated CaClp solution Dehydrating agent, an HCl solution and superheated Steam all supplied with controlled concentration and temperature * The heat load on the cooler was registered. Furthermore, the compositions and temperature of all streams were in several places continuously registered within the column. The system was designed in the manner shown in FIG.

A 55% aqueous CaCl3 solution was used as a dehydrating agent in all experiments. The HCl solution used contained 20 wt .- ^ HGL and was introduced in an amount of 20 kg / hour at a temperature of 20 ⁰ C. The sequence of a foot of the column contained 45 wt .-% CaCIp and had when leaving a temperature of 125 ⁰ C. water vapor was introduced in measured amounts at 175 ⁰ C. Major attempt Wr. 1 was carried out according to the invention, while tests 2 and 'j> were carried out in a known manner * The results are given in Table 1 below.

209839/1086

1 2 3 9.5 8.9 4.7 175 175 175 115 112 93 83 129 129 95 590 590 137 114

Table 1 elle Erm ittclte ^ Versuchsä at eη

Experiment no ^

Supplied water vapor, kg / hour

What evaporates emperο door, ° C amount of solution (55% CaCl ₃ ), kg / hour ^d

Solution temperature, ⁰ C

Vapor pressure of the incoming solution, ram Hg

escaping solution, kg / hour

% HCl in the exiting

Solution 0.068 0.075 0.320

Heat consumption in the cooler,

kcal / hour 1000 3400 900

The following comparative figures can be derived from the above data:

Table 2 Comparative results

Attempt no.

Relative HCl Loss,% Heat consumed at the cooler, kcal / kg produced HCl amount of steam supplied per kg HCl generation

Calorific value of the sensible heat, calculated as the equivalent of kg steam / kg HCl generation

pure heat consumption for dehydration, calculated as Equivalent kg steam / kg HCl-Krzeugurig

209839/1086

2.4 2 , 6 9.1 256 870 248 P, 43 2 , 28 1.29 1.31 - - 1.21 2 , 28 1.29

221Q857

The values given in Table 1 and their answering in table 2 are to be explained as follows:

Experiment 1 represents the typical working conditions according to the invention.

Experiment 2 represents the typical * work of the known processes with practically the same BCl losses as in experiment 1, but with a higher heat loss.

Experiment 5 is an experiment, the same to achieve pure heat loss as in Experiment 1, while taking the solution of the dehydrating agent the conditions of experiment 2 was supplied. The results show that the HCl loss is excessive is.

In experiment 2, only about 70 % of the water vapor was used as a carrier gas for stripping. This vapor did not add to the enthalpy differences between the inlet and outlet streams, but instead reached the reflux condenser where its heat was consumed.

In experiment 1, only about 19 % of the water vapor (which was introduced in a slightly higher amount than in experiment 2) reached the cooler. In addition corresponds to the difference in the tactile V / Slee the concentrated solution of DehydratiGierungs: eans more than 55% of Donpfeinsatzes. This sensible heat can be made usable outside the column and therefore credited as water vapor equivalent.

Another aspect that does not appear in Table 1 -!;., Is the fact that the heat transfer area necessary for operation under the conditions of experiment 2 , about 5.5 -fochf.? is the area that is used, in which -under dt ·: · conditions according to öc: v

209839/1086

Invention is worked. This also results in a higher consumption of cooling water.

The pure heat consumption in experiment 1 is approximately half of the heat consumption in attempt 2. Da however, the palpable that was recovered in Experiment 1 Heat, for example, in a multiple evaporator straight passage of the insert (forward feed multiple effect evaporator) can be exploited is yours pure value higher than their actual calorific value. This is another advantage of the invention.

Experiment 3 shows that in the attempt to reduce the direct heat losses in the cooler to the level of experiment 1 and at the same time to keep the concentrated solution of the dehydrating agent fed to the column close to its normal boiling point, the relative HCl loss due to the insufficient volume of stripping vapors increased from 2.4 to 9.1 % .

The sensible heat recovered from the concentrated solution of the dehydrating agent according to the invention can, as already mentioned, be used for evaporation of water by flash evaporation under vacuum or for evaporation of water in a multiple evaporator with straight passage of the insert (the last stage of which works under vacuum) or for Transfer of heat by indirect heat exchange to another stream that is to be heated can be exploited.

209839/1086

Claims (1)

Godfather η t _ι _aη s ρ r ü c _h e 1) Process for obtaining anhydrous hydrogen chloride from aqueous hydrogen chloride solutions, characterized in that one in the upper section a fractionation column introduces an aqueous Balzic acid solution containing 15 to 35 wt .-% HCl contains, in the column at the same level as the aqueous hydrochloric acid is introduced, or up to 4 floors above an aqueous solution of a dehydrating agent gives up at a temperature by which the solution has a vapor pressure of 50 to 300 mm Hg, introducing superheated steam into the lower section of the column from the top of the column withdraws a vaporous HCl-HoO mixture, containing at least 75% by weight HCl, and this mixture is dried. 2) Method according to claim 1, characterized in that the drying is carried out by condensation of the water vapor takes place with the aid of a reflux condenser connected downstream of the column head. 5) Method according to claim 1, characterized in that the drying by treating the HCl ^ O-Geuiisches made with a dehydrating agent will. 4) Method still claim 1, characterized pekomifceiahnet, that drying is achieved through a combination of condensation and treatment of the vaporous HCl-IIpO-Genischea made with a dehydrating agent will. 5) Method according to claim 1, 3 and 4, characterized in that ,, diifi as Dtih. '/ Dratinier'ungsmitfcel a; o 50 to 60 / ii ge CaCl ₀ solution is used. 209839/1086 6) Method according to claim 1 to 5, characterized in that that the CaClo solution of the column at a Temperature of 7 !? up to 95 ° C. 209839/1086 Blank page