GB2067090A - Process and apparatus for the isolation of vapours from gas mixtures - Google Patents

Abstract

Gas mixtures (1) containing the vapours of volatile compounds, e.g. industrial solvents and volatile oils, are contacted with an aqueous solution of cyclodextrins and/or cyclodextrin derivatives, preferably in an absorber (2), the volatile compounds forming inclusion complexes with the cyclodextrins. The cyclodextrin inclusion complex obtained may if desired be separated, e.g. in settling tank (5) from the aqueous solution phase. It is then subjected (8) to thermal dissociation and the volatile compound released is isolated from the vapour phase, preferably by condensation (12). The mother liquor (14) left behind after separating the cyclodextrin inclusion complex and the solution (9) obtained from the thermal dissociation, containing cyclodextrin as the main component, are combined, and, if the process is carried out in an absorber, the combined solution is recycled (16) into that absorber. Water can be added to the combined solution, to ensure that the gas mixtures which are passed through the absorber are saturated with water. <IMAGE>

Description

SPECIFICATION Process and apparatus for the isolation of vapours from gas mixtures The present invention relates to a process for the isolation of vapours from gas mixtures. More particularly, the invention concerns a process for isolating volatile compounds from gas mixtures containing their vapours. According to the invention gas mixtures containing the vapours of voltaile compounds are first contacted with aqueous solution of cyclodextrins and/or cyclodextrin derivatives, preferably in an absorber, in which a great liquid-gas interface can be formed. The cyclodextrin inclusion complex obtained if desired, is separated from the aqueous solution phase. It is then subjected to reversible thermal dissociation and the released volatile compound is isolated, preferably from the vapour phase by condensation.The mother liquor left behind after separating the cyclodextrin inclusion complex and the solution obtained by reversible thermal dissociation of the cyclodextrin inclusion complex, containing cyclodextrin as a main component, are combined and, if the process is carried out in an absorber, the combined solution is recycled into that absorber. If desired, before recycling or simultaneously water can be added to the combined solution, to ensure that the gas mixtures containing volatile compounds, which are passed through the absorber are saturated with water.

The invention also relates to an equipment in which the above process can be carried out.

In the context of the present invention the term "volatile compounds" relates to various, industrially used solvents and chemicals having a low boiling point, which get easily to the atmosphere and may be poisonous, as well as volatile oils and mixtures thereof.

According to the invention an aqueous solution of cyclodextrins or cyclodextrin derivatives is contacted with gas mixtures containing vapours of volatile compounds. The aqueous solutions of cyclodextrins optionally may contain also other additives, which influence the ion strength and pH of the solution, the hydratability and solubility of cyclodextrin or its derivative and/or the crystallizability of the inclusion complex obtained, etc.

When large amounts of volatile compounds or their vapours are to be manufactued, e.g. distillation or regeneration of solvents, exchange of air in containers, the vapours of volatile compounds unavoidably get into the atmosphere. The amount of the volatile compounds contaminating the atmosphere has so far been reduced by intensively cooled condensers, or adsorbers filled with activated carbon. The efficiency of cooling is strongly limited by the quantity of the cooling water available, and the temperature and in extreme cases quality thereof. The artifical cooling, due to its extremely high equipment and energy requirements, is hardly used in the practice.

On the other hand, the efficiency of water cooling is generally insufficient. For example when regenerating solvents in vacuo, from the vacuum pump a considerable amount of solvent vapour is exhausted, in spite of the fact that water cooled condensers are employed. In this way the loss in solvent is increased and the environment is contaminated. According to the other method referred to above, volatile compounds are separated from vapour phase by active carbon. Since the solid phase has to be dispersed on a great surface to minimize the resistance to the gas or vapour stream, very large equipments are required. The regeneration of active carbon is also rather complicated and can be carried out only by a step-by-step method.

Up to the present time there has been no process known in the art by which vapours of volatile compounds could have been isolated from gas mixtures with a good efficiency, under low operation costs.

It has now surprisingly been found that if a gas containing vapours of volatile compounds is contacted with equeous solutions of cyclodextrins, a cyclodextrin inclusion complex containing the volatile compound precipitates from the aqueous cyclodextrin solution. The precipitation starts at a temperature, which is characteristic of the volatile compound and other experimental conditions.

Inclusion complexes are only formed, if the volatile compounds are less polar than water. If this condition is fulfilled, the molecule of the volatile compound can easily replace water in the cave inside the cyclodextrin, since the apolar conditions inside the cyclodextrin give preference to the molecules less polar than water. According to a further precondition the size and shape of the volatile molecule should harmonize with the size of the cyclodextrin cave. In view of the fact that the inside cave of a-cyclodextrin has a diameter of about 6 A, while the diameters of the inside caves in p- and gamma-cyclodextrins amount to 8 and 10 , respectively, almost all aliphatic compounds and aromatic compounds containing one, two or three fused benzene rings can form inclusion complexes with cyclodextrins.

Evaluation ofthetemperature-dependence of crystallization revealed a histeresis in the crystallisation diagram, and it was found that the crystalline complex could be redissolved at a temperature exceeding the temperature of crystallization, and dissociates into its components. This temperature is also a function of the compound included and the other experimental conditions. Cyclodextrin remains in the aqueous phase, while the volatile compound is partitioned between the solution and vapour phase. The partition ratio depends on the temperature. Starting from this fact, a high volume of a gas mixture containing vapours of volatile compounds to be eliminated, is contacted with a cyclodextrin solution along a large surface. The temperature of the solution is lower than the temperature at which the cyclodextrin inclusion complex crystallizes.The temperature of crystallization depends on the vol atile compound, the concentration of cyclodextrin solution and the concentration of further compo The drawing originally filed were informal and the print here reproduced is taken from a later filed formal copy.

nents present, such as organic or inorganic salts.

When these conditions are fulfilled, the formation of inclusion complexes starts instantaneously, and after saturation of the solution the crystals of inclusion complexes immediately appear. The inclusion complexes generally contain cyclodextrin and the volatile compound in a molar ration 1:1 /see Table 1/.

Table I Properties of ss-cyclodextrin volatile compound inclusion complexes Volatile M.p. Amount included Approximate Starting compound 1 CI compound and analysis composition temperature, of method of inclusion crystallization complex l"Clin a 3% sscyclodextrin solution Diisopropyl ether 68 gas chromatography/5%/ 1:1 55 toluene 110, a gaschromatographyl4 /O/ 0,54:1 46 carbon tetrachloride 76,7 elementary analysis Cl=9,11 # f :1 > 60 chloroform 61,3 gas chromatography /4# 1:1 > 60 benzene 80,1 gas chromatography /2, 5# 0,4::1 > 60 n-hexane 68,7 gas chromatography /5# 1:1 39 1,2-dichloroethane 83, 5 elementary analysis, Cl=7, 79% 1:1 > 60 dichloromethane 40, 2 elementary analysis, Cl=5, 42% 1:1 > 60 ethylchloroformate 94-95 elementary analysis, Cl=5, 65 0,68:1 < 5 trichloroethylen 87 elementary analysis Cl=8, 64% 1:1 > 60 a mixture of 26,5% diphenyl and 73,5% diphenyl ether 254-259 0.5:1 < 5 The simplest way of contacting the gas phase with the cyclodextrin solution consists in bubbling the gas mixtures through the cyclodextrin solution.

Alternatively, the gas containing the volatile compounds can be passed through a tower filled with a substance having a high surface, in counterstream with the cyclodextrin solution. In the latter case the crystalline inclusion complex is washed offthe column by the cyclodextrin solution.

The volatile compounds are always obtained after washing the gas to be purified as inclusion complex crystals floating in the cyclodextrin solution. The heterogeneous system formed is easily decomposed and the crystalline inclusion complex can be separated for example by sedimentation or centrifugation. The crystallization inclusion complex contains the volatile compound separated from the gas phase in a small volume. According to the invention the volatile compound is released from the inclusion complex by subjecting itto thermal dissociation. As a consequence of thermal dissociation the volatile compound is generally obtained as a vapour, which can then be condensed in a conventional way.The other product obtained by thermal dissociation is a concentrated cyclodextrin solution, which can be combined with the mother liquor left behind after separation of the inclusion complex and, after adjusting its temperature to the desired value can be recycled to the absorption step.

If the boiling point of the volatile compound exceeds 100 C, upon thermal dissociation of the cyclodextrin inclusion complex the volatile compound is obtained in the liquid phase, instead of the vapour phase. The layers of the bi-phase reaction mixture are separated by conventional techniques.

As a cyclodextrin, depending on the size of the molecules to be separated, ow, p- or gammacyclodextrin or a derivative thereof can be used. If the volatile molecules are small, cu-cye lodextrin is preferably used, but especially p-cyclodextrin is capable of binding a large variety of volatile compounds. In certain cases it can be disadvantageous that water-solubility of p-cyclodextrin is very poor; at200C in 100 ml. of water merely 1.8 g. of pcyclodextrin can be solved. The solubility can be increased by modifying the p-cyclodextrin molecule chemically or by adding a surface active agent into the p-cyclodextrin solution.

More particularly, the chemical modification is performed by introducing substituents into the p-cyclodextrin molecule, which improve the solutility in water. Suitablesubstituents include the methyl, carboxymethyl and sulfopropyloxy groups.

The solubility of these derivatives is considerably better than that of p-cyclodextrin, while the complex forming properties remain unchanged. Accordingly, using the'solutions of p-cyclodextrin and its derivatives in the same volume, a considerably higher efficiency can be obtained, when these derivatives are employed.

The moleculeofp-cyclodextrin contains 21 methylatable hydroxyl groups, i.e. 14 secondary and 7 primary hydroxyls. The totally methylated derivative is insoluble in water, but if only 1 to 7 hydroxyls are methylated, the water-solubility is improved without influencing the complex forming capability, moreover, the inclusion complexes of partially methylated p-cyclodextrin derivatives have an improved stability. /B. Case, M. Reggiani, G. R. Sanderson: Abstracts of Vllth International Symposium on Carbohydrate Chemistry, B ratislava, August 75-9, 1974, p. 1241 I.

The various methylated p-cyclodextrin derivatives can be prepared by conventional methods, well known in the chemistry of carbohydrates. It has been found that the water solubility of the p-cyclodextrin derivative, which contains on the average 1.5 methyl groups4substitution grade = 1.5, if the substitution grade of heptakis4trimethyl/-p-cyclodextrin is 21/, at 200C amounts to 7 9./100 ml. of water, i.e. this partially methylated derivative is about four-times as soluble as p-cyclodextrin. This methyl p-cyclodextrin derivative forms inclusion complexes for example with the following solvents: carbon tetrachloride, diisopropyl ether, benzene, n-hexane, dichloromethane.The molar ration of the cyclodextrin derivative to the included solvent is about 1:1 in these inclusion complexes.

The solubility data of the sulfopropyloxy - p - cyclodextrin derivatives, prepared by literature known methods from p-cyclodextrin and propanesulfon /J.

N. J. Lammers, J. L. Koole, J. Hurkmans: Sta#rke, 23, 167,1971/are shown in the Table 2.

Table II sulfapropyloxy watersolubilityat20 C relative solubility group in one Molar solubility at50 C pcyclodextrin weight g/lOOml. molellit related to /g/100 ml/ molecule p-cyclodextrin lmolellitl 0 1134 1.8 0.017 1.0 2 1422 4 0.028 1.6 9 4 1710 8.3 0.048 2.8 20.5 6 1998 13.0 0.065 3.8 30.0 7 2142 24.6 0.115 6.8 53.6 This p-cyclodextrin derivative forms inclusion complexes for example with n-hexane, benzene, chloroform, carbon tetrachloride and dichlormethane. The molar ratio of the p-cyclodextrin derivative to the included compound is about 1:1 Carboxymethyl derivatives of p-cyclodextrin are prepared by methods known in the art [ J. N. J.

Lammers,J. L. Koole,J. Hurkmans: Sta#rke,23, 167 /1971/ ] , using monochloroacetic acid. But the substitution grade of this compound should be less than 1 to obtain inclusion complexes, which instantaneously precipitate from an aqueous solution. If the substitution grade exceeds 1, the inclusion complex precipitates only after keeping the mixture at a temperature of about 0 C for more hours. The solubility of a carboxymethyl - p - cyclodextrin having a substitution grade of 0.5 is already considerably bettes than that of p-cyclodextrin, while the complex forming properties of the two compounds are identical.

A great advantage of the substituted derivatives consists in their improved solubility, which allows to reach a higher capacity in the same volume. On the other hand, the cyclodextrin derivatives are less sensitive to enzymatic effects. The latter property is especially important if the equipment, in which the isolation takes place is out of work for a longer time.

During operation the volatile compounds to be isolated generally inhibit the propagation of any kind of bacteria, or fungi in the equipment, and the regeneration carried out at higher temperatures also sterilizes the equipment.

The solubility of p-cyclodextrin can also be increased by adding a suitable surface active agent to the solution. Suitable surface active agent is for example cetyltrimethylammonium bromide.

The solubility of p-cyclodextrin in the presence of cetyltrimethylammonium bromide is considerably increased in the presence of cetyltrimethylammonium bromide. At a temperature of 20 to 300C a solubility of 15-20% can also be achieved without substantial foaming of the solution. This is due to the fact that also cetyltrimethylammonium bromide is incorporated into the complex. In this way the p-cyclodextrin efficiently inhibits the decrease of the surface tension of the solution, i.e. the foaming. Due to the polarity of its molecule, the inclusion complex of cetyltrimethylammonium bromide with p-cyclodextrin is very labile.When this inclusion complex is contacted with another molecule with better complex forming properties, e.g. with a solvent molecule, it decomposes and a p- cyclodextrin-solvent inclusion complex is formed, since the stability of the latter complex considerably surpassed the stability of the complex formed with the above-mentioned surface active agent. Accordingly, the presence of cetyltrimethylammonium bromide does not disturb the formation of complexes between p-cyclodextrin and the volatile compounds present.When to an aqueous solution containing 7% p-cyclodextrin and 0.45% cetyltrimethylammonium bromide diisopropyl ether is added, the crystalline inclusion complex obtained contains 5% of diisopropyl ether, which is identical with the diisopropyl ether content of the inclusion complexes prepared in the absence of cetytrimethylammonium bromide.

Further details of our invention are to be found in the following examples, which are, however, not intended to limit the invention in any way.

Example 1 Into a gas washing bottle 50 ml. of dichloromethane are filled, and the bottle is placed into a thermostate of 40 C. This gas washing bottle is con nected with another, unthermostated gas washing bottle, containing 100 ml. of a 2.5% aqueous p-cyclodextrin solution. Air is pumped through the first, cichloromethane-containing bottle and then through the second bottle, filled with the aqueous ss-cyclodextrin solution. The air bubbled through dichloromethane brings the vapours thereof into the aqueous p-cyclodextrin solution. The formation of dichloromethane - p - cyclodextrin inclusion complex starts immediately after starting the operation of pump and a white, crystalline substance precipitates from the solution.Practically the whole amount of p-cyclodextrin is converted into such a complex, which deposits in a very short time on the bottom of the gas washing bottle. The molar ratio of p-cyclodextrin to dichloromethane in the inclusion complex is 1 :1, which corresponds to 7% of dichloromethane, calculated for dry substance. The inclusion complex is identical with the product given in Table I.

Example 2 When manufacturing the active ingredient of the insecticide composition Trichlofon in an industrial equipment having an annular output of 600 tones, as a last step of the technology the mother liquor remained after obtained at crystallization must be concentrated. Concentration is practically carried out by vacuum evaporation. From the vacuum pump about 600 kg. of diisopropyl ether are exhausted in a year, which highly contaminates the environment and also results in a sufficient loss in chemicals.

The use of the process according to the invention for this purpose is illustrated hereinbelow, referring to Fig. l, on which the corresponding apparatus is illustrated.

The gas exhausted from the vaccum pump, containing diisopropyl ether is introduced into the absorber 2 filled with charge 3 through the pipe line 1. In the absorbing tower the gas is led in counterstream with the thermostated p-cyclodextrin solution, which is introduced into the equipment through the pipe line 16 and the quenchers 17. The diisopropyl ether-containing gas is in this way contacted with pcyclodextrin solution, and diisopropyl ether is absorbed in the p-cyclodextrin solution. The exhausted gas is then led through the drip pan 18 washed with water, which is introduced into the system through the feeding valve 19, and is exhausted into the open air through the chimney 20.The pcyclodext(in solution containing diisopropyl ether is discharged from the absorbing tower 2 through the cooler 4. In the cooler4the p- cyclodextrin-diisopropyl ether inclusion complex precipitates in crystalline form. The crystals are then deposited in the settling tank 5. The solution disoliarged from the tank through the pipe line 14. The preciptated crystal slurry is transported by the pump 6, through the pipe line 7 into the heater 8, where the dissociation of the inclusion complex takes place.

The released diisopropyl ether vapour is exhausted from the heater 8 through the pipe line 11, is condensed in the cooler 12 and collected in the container 13. The pure p-cyclodextrin solution is discharged from the heater 8 through the pipe line 9, whereupon is admixed with the cool p-cyclodextrin solution ledthroughthe pipe line 14 from the settling tank 5. The mixture is transported by the pump 10 into the thermostating tank 15, where it is thermostated to the optimum temperature for the formation in inclusion complexes, and it is recycled into the absorbing tower 2. The water removed from the system by the purified gas, exhausted through the chimney 20 is supplemented by adding the neces sary amount of fresh water through the feeding valve 19.

The same process can be carried out also in an equipment illustrated on Figure 2. In this system the gas phase containing a large amount of diisopropyl ether vapor is introduced into the system through the pipe line 1. It is then bubbled through the cyclodextrin solution contained in the absorbing tank 2.

The p - cyclodextrin - diisopropyl ether inclusion complex crystallizes out from the saturated cyclodextrin solution, and the crystals are deposited at the bottom of the absorbing tank 2. The obtained crystalline slurry, having a dry substance content of 20% /1:1 diisopropyl ether - p - cyclodextrin inclusion complex/, is transported by the slurry pump 6, through the pipe line 7 into the heating unit$ By proper choice of the pumping speed it is ensured that only the precipitated crystals are transported.

The lower part of the heating unit 8 is a counterflow heat-exchanger and the diisopropyl ether included in the pcyclodextrin inclusion complex is evaporated in the upper part thereof, which is heated with a pipe coil 21. The warm cyclodextrin solution released from the diisopropyl ether is recycled into the absorber 2 through the pipe line 9. In the absorber 4 the temperature of the solution is adjusted to an optimum value for the formation of complexes by the thermostating pipe coil 15, and the whole procedure is repeated. The gas released from diisopropyl ether is exhausted from the equipment through the chimney 20, directly into the open air.

From the heater8the diisopropyl ethervapouris introduced through the pipe line 11 into the cooler 12. The condensate is collected in the container 13.

When the operation of the equipment is started, it isfilledwith a solution prepared from 1000 lit. of water having a temperature of 35 C and 40 kg. of p-cyc{#dextrin. The solution can repeatedly be used until it is not contaminated with oil, tar or other impurities, present in the gas to be purified. The solution is not sensitive to the changes in the pH.

Example 3 Into a 50-lit. dry glass bottle 0.5 g. of trichloroethylene are injected. The bottle is equipped with a double-bore stopper. In the bores glass tubes are placed, one up to the bottom of the bottle and another one up to the bottom of the stopper.

Through the shorter glass tube air is sucked from the upper part of the bottle and is then bubbled through 200 ml. of a 2.5% aqueous pcyclodextrin solution, in a gas washing bottle. The air led through the gas washing bottle is recycled to the bottom of the glass bottle through the longer glass tube. After four cycles the whole amount of trichloroethylen gets to the inclusion complex, precipitated and deposited in the washing bottle. In this way the concentration of solvent vapours can considerably be reduced in the atmosphere of working sites, where evaporation of solvents is unavoidable. In such cases the atmosphere of the room is circulated through an absorber containing an aqueous solution of cyclodextrins or cyclodextrin derivatives. The aqueous solution of the complexes of cyclodextrins and their derivatives can be regenerated by boiling.In this way the continuous exchange of air for example in rooms, where chemical cleaning equipments are operated, can be avoided, and this results in a considerable reduction in the heating costs in the wintertime. In a chemical cleaning equipment the average loss in perchloroethylen per hour is about 2 kg., therefore such equipments can generally be operated only in rooms, where the solvent vapours can efficiently be eliminated through a suitable chimney system.

Example 4 The formation of p - cyclodextrin - benzene inclusion complex is examined in the presence of cetyltrimethylammonium bromide. 10,15 and 20% aqueous p-cyclodextrin solutions are prepared in the presence of 1,2,5 and 10% cetylmethylammonium bromide. After separating the precipitated crystalline product by centrifugation and drying the separated product, the benzene concentration /UV spectrophotometer/ and the cetyltrimethylammonium bromide concentration /chemical analysis of bromine content/ are determined. The inclusion complex contains 5 to 6% benzene, just as like in absence of cetyltrimethylammonium bromide, while the cetyltrimethylammonium bromide content amounts to 0 to 4.3%.The cetyltrimethylammonium bromide optionally present, is presumable only adsorbed on the surface of the inclusion complexes, since the product was washed only with its own mother liquor. The complexes formed can be decomposed by heating and benzene can be regained in a conventional manner. If the p-cyclodextrin content is 10, 15 and 20%, respectively, 0.5, 0.75 and 1.0% of benzene can be absorbed by the cyclodextrin solution. The experimental results are given in Table Ill.

Table III - Preparation Analysis of the product ss-cyclo- cetylmethyl- cetyltri oc dextrin ammonium metylammo- benzene ill 100 mil bromide nium /3/ bromide 1%1 20 10 1 0 3.87 20 10 2 1.8 3.42 20 10 5 3.6 4 20 10 10 4.3 5.28 60 15 1 0 5.63 40 15 2 1.24 5.8 30 15 5 2.6 4.93 70 20 1 0 4 50 20 2 0.75 5.4 30 20 5 1.5 4.4 30 20 10 2.3 3.9 Example 5 Following the procedure and using the equipment described in Example 2, but employing an aqueous solution containing 10% of cyclodextrin and 1% of cetyltrimethylammonium bromide for the absorb tion, a continuous solvent stream is led through the absorber, the complexformed is decomposed and the solvent is regenerated.

Example 6 Through an aqueous solution containing 10% of pcyclodextrin and 1% of cetyltrimethylammonium bromide diphyl /a mixture of 26.5% of diphenyl and 73.5% of diphenyl ether is passed. In the solution the diphyl vapour is bound in an inclusion complex. By heating the solution containing the complex formed up the 80 to 90 C, the complex is decomposed, the diphyl oil separates as a lower layer and can be discharged from the bottom of the equipment. From diphyl-containing vapours and diphyl about 70% of the input amount can be regained.

Claims (8)

1. Process for the isolation of volatile compounds from gas mixtures containing their vapours, which comprises contacting a gas mixture containing vapours of the volatile compounds with an aqueous solution of cyclodextrins and/or cyclodextrin derivatives, optionally containing also further additives for adjusting the pH, ion strength and/or surface tension, along a great liquid-gas interface, and if desired, separating the inclusion complex of cyclodextrins and/or cyclodextrin derivatives with the volatile compounds obtained, from the mother liquor, after or simultaneously with cooling the solution, subjecting the inclusion complex to thermal dissociation, separating the volatile compounds released, and recycling the remaining solution of cyclodextrins or cyclodextrin derivatives into the absorption phase, optionally after combining it with the mother liquor obtained after separation of the inclusion complex.

2. A process as claimed in claim 1, which comprises using Bcyclodextrin as a cyclodextrin.

3. A process as claimed in claim 1 or claim 2, which comprises adding a surface active agent to the solution of a cyclodextrin and/or cyclodextrin derivative.

4. A process as claimed in claim 3, which comprises using cetyltrimethylammonium bromide as a surface active agent.

5. A process as claimed in any one of claims 1 to 4, which comprises subjecting the inclusion complex of a cyclodextrin and or cyclodextrin derivative with a volatile compound to thermal decomposition, without separation from the mother liquor.

6. A process as claimed in any one of claims 1 to 4, which comprises subjecting the inclusion complex of a cyclodextrin and/or cyclodextrin derivative with a volatile compound to thermal decomposition after separation from the mother liquor.

7. Equipment for carrying out the process claimed in claims 1 to 6, which comprises an absorber, in which an aqueous solution of cyclodextrins and/or cyclodextrin derivatives are contacted with a gas mixture containing the vapours of volatile compounds on a great surface, optionally a settling tank in which the crystals of the inclusion complex are deposited, and a heating reactor, which is connected in series with the main- or side-stream of the liquid phase by which the volatile compounds are absorbed.

8. An equipment as claimed in claim 7,which comprises a sedimentator between 1 the absorber, and the heating reactor.