IE43317B1

IE43317B1 - Concentrating aqueous solutions

Info

Publication number: IE43317B1
Application number: IE1874/76A
Authority: IE
Original assignee: British Vinegars Ltd
Priority date: 1975-09-05
Filing date: 1976-08-23
Publication date: 1981-01-28
Also published as: ES451228A1; AT363052B; BE845866A; IE43317L; DE2639594A1; FR2322636A1; SE7609535L; NL7609825A; ZA765105B; JPS5232882A; CH624854A5; GB1535336A; DK381876A; CA1090650A; NO762921L; LU75715A1; ATA642476A

Abstract

An aqueous solution is contacted with a hydrate-forming fluid at a temperature beneath the maximum temperature at which the hydrate-forming fluid forms a solid hydrate in the presence of the solution, and at a temperature at which the amount of dissolved material which is present in the original aqueous solution exceeds the solubility of the dissolved material in any solution remaining after the hydrate formation. In this case, a magma is formed from solid hydrate, solid dissolved material, any unreacted hydrate-former still present and any unreacted aqueous solution still present. Thereupon, there are separated: 1) the hydrate-former and at least a part of the aqueous constitutents of the solid hydrate and 2) at least a part of the dissolved material, producing an essentially hydrate-former-free product and any residual water still present. The separation is carried out by fractional sublimation and/or evaporation and/or elution.

Description

This invention relates to the removal of water from aqueous solutions.

Xt has previously been known that solid hydrates may be formed between certain hydrate forming fluids, referred to hereinafter as hydrate formers, and water from aqueous solutions such as sea water and that after separation of the solid hydrate by filtration or similar mechanical handling processes, pure water may be obtained from the separated hydrate by decom10 position thereof. However, in the case of solutions wherein the solute solidifies at the temperatures used to form the solid hydrate, techniques such as filtration cannot be used to separate the solid hydrate from the remainder of the mixture. .5 Accordingly it is an object of the present invention to provide a process for the removal of water from aqueous solutions of the type in which a solid hydrate is formed wherein the solid hydrate and at least part of the solid phase containing solid solute are separated from each other, preferably by non-mechanical means.

The present invention provides a process for removing water from an aqueous solution which comprises: (a) contacting an aqueous solution with a hydrate former at a temperature below the maximum temperature at which said hydrate former forms a solid hydrate - 3 in the presence of the solution, and at a temperature at which there is precipitation of solid solute so as to form a magma comprising solid hydrate, solid solute, any unreacted hydrate former and any unreacted aqueous solution; and (b) separating (i) the hydrate former and at least part of the aqueous constituents of the solid hydrate, and (ii) at least part of the solute, from each other, by fractional sublimation, evaporation and/or elution, so as to produce a substantially hydrate former-free product comprising the solute and any remaining water.

In a preferred separation process, the solid mixture resulting from the treatment of the aqueous solution with hydrate former is subjected to temperature and pressure conditions that result in the decomposition of the solid hydrate in which the hydrate breaks down into ice and hydrate former, the hydrate former is removed by vacuum evaporation, and the mixture of ice and solid solute is preferably separated by differential or fractional sublimation.

In the case where the solute is acetic acid and the hydrate forming fluid is trichlorofluoromethane (also known under the Trade Mark Freon 11 but hereinafter referred to as TCFM), decomposition of the hydrate former is usually effected at about 0°C or below and at a pressure of about 750mm of mercury or lower at 0°C or a correspondingly lower pressure at lower temperatures. Fractional sublimation removing the ice may then be carried out at about 0°C and 4.5mm of mercury or lower, for example at O.OO75°C. and 4.5mm of mercury, though other suitable combinations may readily be determined by trial and error and/or by the use of vapour pressure data at various temperatures to select conditions under which at the chosen temperature the vapour pressure of the ice exceeds that of the chosen pressure whilst the vapour pressure of the solute is less than the chosen pressure and vice versa. If difficulty is experienced in selecting suitable conditions, for example, when the solute has a similar triple point to water, high yields can be obtained by passing the combined solute and water vapours through a column of water vapour absorbing material such as silica gel or anhydrous copper sulphate (which can be regenerated) the non-aqueous vapour being collected.

Irt a further aspect the present invention provides a process for removing water from an aqueous solution which comprises: a) contacting an aqueous solution with a hydrate former at a temperature below the maximum temperature at which said hydrate former forms a solid hydrate !0 in the presence of the solution and above the maximum temperature at which ice forms in the aqueous solution so that said hydrate former forms a solid hydrate with water from the aqueous solution; and b) decomposing the solid hydrate so as to produce hydrate former and ice, removing the hydrate former, and separating at least part of the ice and at least part of the solute, from each other, by fractional sublimation of the mixture, so as to produce a substantially hydrate former-free product comprising ) the solute and any remaining water.

As used herein the term sublimation is a process wherein a solid is converted directly into - 5 a vapour and includes such processes wherein the solid hydrate is decomposed without passing through a liquid phase into hydrate former gas and water vapour or ice.

Another separation process is differential elution using a solvent or solvents in which the solute is soluble but in which the solid hydrate of the hydrate former used is substantially insoluble at the temperatures at which the hydrate is formed and is stable. In the case where the solute is acetic acid suitable elution solvents for the solute include, ethanol, formaldehyde and butanol, as well as trichlorofluoromethane and dichloromethane.

A preferred method of differential elution is when the magma of solid hydrate and solid solute is separated from the liquid component which may contain one or more of unreacted hydrate former, unreacted aqueous solution, and unreacted minor constituents that may be present in the solution being concentrated. The latter fraction may be distilled to recover any such minor constituents that may be present, as for example, in the case of vinegar, any dissolved solute.

Although the minor constituents also comprise solutes of the aqueous solution from which water is being removed they will be referred to herein as the minor constituents whilst the major constituent(s), will be referred to herein as the solute, for convenience.

Conveniently the solid phase is then rapidly dried by evaporation of the unseparated unreacted hydrate former and then by increasing the vacuum - 6 (i.e. decreasing pressure) so that the solid hydrate becomes unstable and is broken down to ice and hydrate former. The hydrate former is at once evaporated and may be collected for recycling. This leaves a mixture of ice and solid solute (e.g. acetic acid).

The ice will start to melt at below 0°C, due to the depression of the freezing point by the solute solution. However if the solute is highly soluble in water at this temperature it is possible to obtain LO a significant degree of concentration if the solute is dissolved and the solution removed prior to the ice all melting, the solute in this case being effectively preferentially eluted in water as an elution solvent. The unmelted ice recovered then .5 represents the amount of water extracted from the original solution. An alternative method is to treat the ice/solid solute mixture before much melting occurs with another solvent which gives rapid solution of the solute but little effect on the ice (examples are T.C.F.M. or Methylene dichloride when the solute is acetic acid). It will be noted that in the case of concentration of aqueous acetic acid T.C.F.M. can be used as both hydrate former and subsequently as an elution solvent. The solution is then separ5 ated from the ice by filtration and the solute recovered by evaporation.

Although T.C.F.M. is a particularly valuable hydrate former, especially for use with aqueous acetic acid solutions, on account of its non-toxicity, ) commercial availability, low cost and ease of handling due to the fact that it is a liquid at ambient temperatures and pressures hence avoiding the need 3 317 - 7 for costly pressurised storage and reaction vessels, and due to the fact that it can form a solid hydrate (also sometimes referred to in the art as clathrates) at ambient pressures when the temperature is sufficiently reduced, other hydrate formers may also be used. Known hydrate formers together with their hydrate formulae are shown in Table I wherein M represents any of the individual hydrate forming molecules in the given section.

Table I Hydrate Formula Hydrate Former, M M.5.75 H20 A, Kr, N2,02, H2 S, H2 Se, C02 N?0,ph?,AsH^,CH?F,CHsC1,CH4 M.5.75 H20 or M.7.66 H„,0SO2'CH2F2'C2H2'C2H4 M.7.66 H20 Xe,Br2,NF3.CHF3,CF4,CH3Br c2h3f, ch3chf2, c2h6 M.17 H20 CH2C12, CHC13, CC1 , CH I.CJLCl 3 2 5 CH CF.,CHCHC1 ,CHBrF_,CC1F3 3 3 2 2 3 CC12F2,CBr2F2,CBrClF2Cycl° C5H10 The selection of operating conditions for formulation of the solid hydrate are well known and understood in the art. Briefly the actual operating conditions for a given system are based on the pressure-temperature equilibrium line data for a given hydrate former as pre-determined and calculated for a desired solution concentration, operating temperature and pressure limit utilising the formula: 311' £lt° = where: t° = preselected temperature of operation for formation of the solid hydrate and the concentrated aqueous solution.

P^ = minimum absolute pressure of the hydrate former to be exerted at temperature t° to achieve the desired final concentration of the aqueous solution through solid hydrate formation. = absolute pressure of the hydrate former to be exerted at temperature t° to achieve formation of solid hydrate with pure water. (Assumes water is saturated with hydrate former, but contains substantially no other solute), x = mole fraction of water at desired final concentration of the concentrated aqueous solution. n = number of water molecules associated with one molecule of hydrate former in the solid gas hydrate. >5 The values of Ρθ at a preselected t° can be obtained from experimental and published data.

Although ordinarily the formula will be utilised to determine the operating pressure (P^) for a preselected temperature (t°), known pressure (Ρθ) of formation of solid hydrate with pure water and for a desired final solution concentration as represented by a residual mole fraction of water (x) in the solution, generally it is to be understood that if 3317 - 9 any two of the operating variables, i.e. Ρ^,Ρ^, and x at a preselected t° are known, the third can be found by calculation using the above formula.

In the case where T.C.F.M. is used to remove water from acetic acid solutions it has been found that substantially complete reaction of the available water to form solid hydrate can be obtained in the presence of an excess of T.C.F.M. over the stoichiometrically required amount, at atmospheric pressure provided a sufficiently low temperature, preferably below 5°C, is used.

The selection of the hydrate former will depend on various factors such as safety, cost and availability but is primarily determined by the particular solute which it is desired to concentrate and its properties as well as on the process selected for the separation of the solute concentrate from the hydrate or vice versa. Thus in general the hydrate former is chosen for maximising ease of the separation process, subject to the other above-mentioned criteria, for example where differential sublimation is employed the hydrate former is chosen to provide a solid hydrate which may be readily sublimed preferentially to the solute. On the other hand where the solid hydrate is first decomposed and the released hydrate former then separated off by differential evaporation leaving the solute behind, then the hydrate former selection will take into account the need for the hydrate former to be evaporated preferentially to the solute. In the case of aqueous acetic acid solutions where the separation process used is differential sublimation, especially suitable hydrate formers other than T.C.F.M. include dichloromethane, trichloromethane and dichlorofluoromethane.

The amount of hydrate former used in the initial solid hydrate formation stage of the process of the invention may be varied within broad limits. Desirably though the amount used will be at least an amount that is sufficient to react with all the water present in the solution to be concentrated and advantageously an excess of the hydrate former is used especially . when vinegar is to be concentrated since in that case the minor constituents of the vinegar (which contribute to its flavour and character) may be conveniently recovered in the excess unreacted hydrate former.

In practice the reaction of hydrate formers, such as T.C.F.M. with water to form solid hydrates, is substantially stoichiometric so that the required amount of hydrate former may usually be readily calculated, though it will be appreciated that if some of the water solidifies before reaction with the hydrate former, it will no longer be available to react with the latter.

Thus in the case where the hydrate former is T.C.F.M. the amount of T.C.F.M. theoretically required to react with all the water initially present is at least 1 molecule of T.C.F.M. for every 17 molecules of water i.e. at least about 1 part of T.C.F.M. to 2 parts of vinegar by weight. Conveniently approximately equal parts of vinegar and T.C.F.M., by volume, are used.

□ The above processes of the present invention have been found to be especially valuable for the concentration of vinegar. Vinegar is essentially - 11 an aqueous acetic acid solution with an acetic acid content of the order of 5 to 10% w/v depending on its source and method of manufacture which solution contains small amounts of various other natural products constituents which contribute to the flavour of the particular vinegar. Particular vinegars that may be mentioned include distilled malt vinegar, alcohol (spirit) vinegar, grain vinegar, wine vinegar, cider vinegar and flavoured vinegars. Malt vinegar in England usually contains at least 4% w/v acetic acid and wine vinegar in France and Italy is required to contain at least 6 and at least 7% w/v acetic acid, respectively.

From the above it will be apparent that in general vinegars comprise some 90 - 95% w/v of water.

It is therefore clearly desirable that if vinegar transportation costs are to be significantly reduced, the vinegar should be substantially concentrated.

On the other hand it must be borne in mind that many of the minor natural products constituents of vinegar which are essential to the flavour of the vinegar are susceptible to denaturation at elevated temperatures and under Other severe conditions.

It is therefore a further object of the present invention to provide a process for the concentration of vinegar which process does not substantially denature the vinegar constituents or result in any substantial loss of the vinegar constituents - other than water .

Accordingly a further aspect of the present invention provides a process for producing a vinegar concentrate by removing water from vinegar comprising - 12 a) contacting the vinegar with a hydrate former at a temperature below the maximum, temperature at which said hydrate former forms a solid hydrate in the presence of the vinegar and at a temperature at which there is precipitation of solid acetic acid, so as to form a magma comprising solid hydrate, solid acetic acid, any unreacted hydrate former, any unreacted aqueous vinegar solution, and minor vinegar constituents; and b) separating (i) the hydrate former and at least part of the aqueous constituents of the solid hydrate and any unreacted hydrate former, and (ii) at least part of the acetic acid, from each other, so as to produce a substantially hydrate former-free acetic acid concentrate, and, where the minor vinegar constituents are not separated with the acetic acid concentrate; recovering the minor vinegar constituents and recombining them with the acetic acid concentrate, so as to produce a vinegar concentrate.

A particularly preferred hydrate former for use !5 in this process is trichlorofluoromethane.

Preferably the solid hydrate is separated from the concentrated vinegar and any solid acetic acid that has precipitated out of the vinegar solution, by sublimation or solution of solid hydrate, optionally with decomposition of the latter, under temperature and pressure conditions at which any solid acetic acid is substantially not vapourised or dissolved and - 13 substantially not denatured.

In another preferred process though, where the solute is separated from the solid hydrate by elution with the solvent separation may be conveniently carried out under ambient temperature and pressure.

On the other hand in some cases the minor constituents of the vinegar are conveniently separated off from the solute, for example, in solution in any excess hydrate former present, and after recovery may be recombined with the solute concentrate obtained after completion of the concentration process (providing the required final concentration).

It will of course be appreciated that where the highest degrees of concentration are required it may be necessary or more convenient to carry out the concentration in more than one step i.e. by repeating the concentration process one or more times. In this case where any minor constituents have been separated from the solute (e.g. in solution in excess hydrate former), they need not be recombined with the concentrated aqueous solution of the solute until the final concentration cycle has been completed.

The degree of concentration obtainable by the process of the present invention will depend on various factors such as the nature of the solute and of the hydrate former used, the particular separation process used and the number of concentration cycles carried out. Nevertheless concentrations of at least 40%, 60% or even 80% w/v acetic acid may be achieved by the selection of suitable conditions in the case of vinegar concentration by a process of the present invention, the concentrated vinegar being, after 3 317 - 14 reconstitution with water, substantially indistinguish able, for practical purposes, from untreated vinegar.

Example 1.

Concentration of aqueous acetic acid.

A. Solid hydrate formation.

To aqueous acetic acid solution (1 litre of 10% w/v) was added liquid trichlorofluoromethane (TCFM, 1 litre) and the mixture cooled to 3°C. Vigorous agitation together with external cooling and internal cooling, by the addition of solid carbon dioxide, was then carried out so as to ensure that the temperature of the mixture remained below 5°C throughout the hydrate formation step. After a few minutes a magma comprising unreacted TCFM, solid acetic acid, residual aqueous acetic acid i.e. acetic acid solution in unreacted water and solid hydrate, was obtained.

B. Separation of Hydrate The magma was subjected to vacuum evaporation (3 mm Hg at 0°C) for 60 minutes until no more hydrate sublimed. The unused TCFM was first removed by this step and was subsequently recovered together with the TCFM trapped in the hydrate. The removal of the TCFM and later removal of the hydrate reduces the temperature to 0°C and this temperature is maintained !5 thereafter by suitable heat application for rapid TCFM removal. This process yielded a mixture of solid glacial acetic acid and residual aqueous acetic acid which at ambient temperatures gave 82% w/v aqueous acetic acid solution.

Example 2 Concentration of malt vinegar.

A. Solid Hydrate Formation Liquid trichlorofluoromethane (TCFM, 500 ml) was added to malt vinegar (500 ml.) and the mixture stirred vigorously for 3 minutes at 3°C. Excess TCFM and other liquids were then drained off from the solid hydrate and acetic acid which were then divided into two equal parts.

B. Separation of Hydrate (1) One part of the solids was subjected to vacuum evaporation (3.4 mm Hg at - 0.5°C) until no more TCFM or water was removed. This step yielded aqueous acetic acid solution containing 69% w/v acetic acid. (2) The other part of the solids was homogenised with a little ice cold water for 10 minutes in a cold room at -4°C. The liquid phase was then filtered off under vacuum. The liquid phase yielded under ambient conditions an aqueous solution containing 42.5% w/v acetic acid.

The liquids from the first stage were then added, after removal of the TCFM from them by evaporation, to the acetic acid solutions obtained after the separation of the hydrate to finally yield concentrated malt vinegar.

Example 3 Concentration of Spirit Vinegar.

A. Solid Hydrate Formation Liquid trichlorofluoromethane (TCFM, 250 ml) was added to spirit vinegar (250 ml.) and the mixture stirred vigorously for 3 minutes at 3°C. Excess TCFM and 3317 - 16 other liquids were then strained off from the solid hydrate and acetic acid.

B. Separation of Hydrate The solids resulting from the first stage were subjected to vacuum evaporation (10 mm Hg at -5°C) until no more TCFM came off. The pressure was then reduced to 3 mm Hg and the temperature increased to -1°C. Initially ethanol and acetaldehyde were removed and collected. Evaporation was then continued until no more water could be removed. This process yielded a residue which at ambient temperatures gave an aqueous solution containing 68% w/v acetic acid.

To the aqueous acetic acid solution were then added the recovered ethanol and acetaldehyde and the L5 liquids from the first stage after the TCFM has been evaporated from them, finally yielding concentrated spirit vinegar.

Example 4 Concentration of Spirit Vinegar Solid Hydrate Formation a) 50 ml. of Spirit Vinegar was added to 50 ml. of T.C.F.M. and the mixture attempered at 0°C. and agitated with an air bleed until most or all the aqueous phase had reacted. The excess T.C.F.M. was then drained off and stored for recovery of solutes.

The solid magma was similarly stored at 0°C. until required for evaporation and sublimation. b) A second method of solid hydrate formation was used: to 100 ml. of spirit vinegar at 0°C., 75 ml. ) of T.C.F.M. was added dropwise over a period of 6 hours. o The temperature was maintained at 0 C. and agitation - 17 was achieved by an air bleed. After 6 hours the excess T.C.F.M. and dissolved constituents were filtered off from the solid magma constituents and stored until required for recovery of solutes. The solid magma was stored at 0°C. until required for evaporation and sublimation.

Separation of Hvdrate The solid magmas from hydrate formations (a) and (b) were placed in a freeze dryer and a vacuum applied.

The excess T.C.F.M. was initially removed and the vacuum was then increased until the solid hydrate decomposed and the T.C.F.M. was released. The temperature was maintained just below 0°C. by infra red radiation. As the hydrate decomposes, ice and gaseous T.C.F.M. are formed. The T.C.F.M. was condensed for later re-use. After a substantial part of the T.C.F.M. was removed the thin layer of magma consisted of ice crystals and solid acetic acid. The pressure was now reduced so as to sublime the ice (3 mm. Hg at 0°C.), leaving a substantial portion of the acetic acid which was allowed to regain room temperature at normal atmospheric pressure.

The excess T.C.F.M. from the solid hydrate formation stages was carefully distilled at 20°C. until all traces of T.C.F.M. were removed as shown by gas liquid chromatography (G.L.C.) examination of the product. Hydrate formation method (b) gave higher yields of dissolved acetic acid than method (a): after combining the distillates and the sublimed samples the former method yielded a concentrate with 84% w/v acetic acid and the latter gave a concentration of 89%.

Example 5 A 100 ml. aliquot of Malt Vinegar was reacted with 100 ml. T.C.F.M. as in Example 4. The excess T.C.F.M. containing the minor constituents of the malt vinegar was drained from the solid magma and the dissolved acetic acid and minor constituents Were recovered by distillation at 20°C. until no T.C.F.M* was detected by G.L.C. analysis of the residue.

The solid magma was then subjected to vacuum evaporation under mild conditions until the excess unreacted T.C.F.M.. was removed. The vacuum was then increased until the hydrate decomposed leaving solid acetic acid and ice. Some melting ice was observed at -1°C. and in addition to this 1 ml. of ice cold water was added. The slurry was agitated briefly and the liquid drained off by vacuum filtration.

This liquid was concentrated acetic acid and When recombined with the solutes from the excess T.C.F.M., concentrated vinegar was obtained, the level of !0 concentration of the acetic acid in the aqueous solution being governed by the amount of ice that had melted into the liquid eventually drained off from the slurry. After the addition of the T.C.F.M. dissolved acetic acid and minor constituents the acetic acid content of the samples was 54% w/v.

Example 6 Hydrate formation was carried out as in Example 4a). The solid magma was then subjected to an initially low vacuum to remove excess T.C.F.M. and ) then the hydrate was rapidly broken down to give ice and solid acetic acid. To the mixture of ice and acetic acid, at a recorded temperature of -4°C, - 19 an equal volume of T.C.F.M., at 1°C, was added and the mixture agitated. The T.C.F.M. was then removed by filtration from the ice and liquid was evaporated at 20°C. until no more T.C.F.M. was detected. This method yielded after recombination with the minor constituents recovered from the distilled T.C.F.M. an acetic acid content of 89% w/v.

Analysis of reconstituted concentrated vinegars by gas liquid chromatography (G.L.C.) showed that the concentrated product resembled the original vinegar very closely. The compounds acetaldehyde and ethanol found in vinegar were determined individually and the remainder of the volatile constituents were grouped together. The determinations of quantity were based on the G.L.C. peak areas.

The following table gives the acetic acid content (% w/v) and the percentages (by weight) of the minor constituents recovered in five vinegar concentrates obtained by the method of Example 6 but carried out on a small scale using 10 ml. vinegar samples. The error in the minor constituent determinations is of the order of + 7 to 8%.

Proportions of minor constituents retained in vinegar concentrates Acetic Acid Cone. Acetal- Other Vinegar Type % w/v + dehyde Ethanol Volatiles Spirit 67 94 89 93 Wine 71 64 76 77 Wine (white) 61 81 73 82 Malt 82 92 98 102 Cider 58 84 86 91 - 20 + The acetic acid concentrations given were determined by acid-base titration and include any minor acidic constituents recovered that may be present in the vinegar being concentrated.

Claims

1. CLAIMS:1. A process for removing water from an aqueous solution comprising: a) contacting an aqueous solution with a hydrate former at a temperature below the maximum temperature at which said hydrate former forms a solid hydrate in the presence of the solution, and at a temperature at which there is precipitation of solid solute so as to form a magma comprising solid hydrate, solid solute, any unreacted hydrate former and any unreacted aqueous solution; and b) separating (i) the hydrate former and at least part of the aqueous constituents of the solid hydrate and (ii) at least part of the solute, from each other, by fractional sublimation, evaporation and/or elution, so as to produce a substantially hydrate former-free product comprising the solute and any remaining water.

2. A process for removing water from an aqueous solution comprising a) contacting an aqueous solution with a hydrate former at a temperature below the maximum temperature at which said hydrate former forms a solid hydrate in the presence of the solution and above the maximum temperature at which ice forms in the aqueous solution so that said hydrate former forms a solid hydrate with water from the aqueous solution; and decomposing the solid hydrate so as to produce hydrate former and ice; b) removing the hydrate former; and separating at least part of the ice and at least part of the solute - 22 from each other so as to produce a substantially hydrate former-free product comprising the solute and any remaining water.

3. A process as claimed in claim 1 or claim 2 5 wherein the solid hydrate is decomposed into ice and hydrate former by increasing temperature and/or reducing pressure to provide temperature and pressure conditions at which said solid hydrate decomposes and at which the solute is substantially unaffected. LO

4. A process as claimed in claim 3 when dependent on claim 1 wherein decomposition is effected at temperature and pressure conditions at which the solute remains substantially in the solid phase.

5. A process as claimed in any one of claims 2 .5 to 4 wherein the solute and at least part of the ice are separated from each other by fractional sublimation at temperature and pressure conditions such that one of the ice and the solute is volatilised whilst the other of said ice and the solute remains substantially 0 in the liquid and/or solid phase.

6. A process as claimed in any of claims 2 to 4 wherein the solute and at least part of the ice are separated from each other by sublimation at temperature and pressure conditions such that both the ice and 5 solute are volatilised and the resulting vapour contacted with a water vapour absorbing material to absorb the water vapour entrained with the solute vapour .

7. A process as claimed in claim 6 wherein the ) water vapour absorbing material is selected from silica gel and anhydrous copper sulphate. 49317 - 23

8. A process as claimed in claim 1 wherein the solute is separated from the solid hydrate by differential elution using a solvent in which the solute is substantially more soluble than is the solid hydrate.

9. A process as claimed in claim 8 wherein when the solute is acetic acid, the elution solvent is selected from ethanol, formaldehyde and butanol.

10. A process as claimed in claim 8 wherein when the solute is acetic acid, the elution solvent is dichloromethane.

11. A process as claimed in claim 8 wherein when the solute is acetic acid, the elution solvent is trichlorofluoromethane.

12. A process as claimed in claim 11 wherein the trichlorofluoromethane elution solvent containing dissolved acetic acid is separated off by filtration.

13. . A process as claimed in claim 11 or claim 12 wherein the trichlorofluoromethane is removed from the solution of acetic acid in trichlorofluoromethane by fractional evaporation.

14. A process as claimed in claim 3 when dependent on claim 1 or in claim 4 wherein at least part of the ice is maintained in the solid state and the solute is mechanically separated from said ice in the solid state, in the form of a concentrated agueou s solution.

15. A process as claimed in claim 14 wherein part of the ice is allowed to melt into liquid water in which solute dissolves to form a concentrated aqueous solution of the solute.

16. A process as claimed in claim 14 or claim 15 wherein liquid water is added and solute allowed - 24 to dissolve in said added liquid water to form a concentrated aqueous solution of the solute.

17. A process as claimed in any of claims 14 to 16 wherein the concentrated aqueous solution is 5 separated from the solid ice by filtration.

18. A process as claimed in claim 3 when dependent on claim 1 or in claim 4 wherein at least part of the ice is maintained in the solid state and a non-aqueous solvent, in which ice is substantially 10 less soluble than is the solute, is added to form a solution of the solute, which solution is then mechanically separated from the solid ice, and the solute is then recovered from said solution by separation from said elution solvent. 15

19. A process as claimed in claim 18 wherein the solute is acetic acid and the non-aqueoUs solvent is trichlorofluoromethane and said acetic acid is recovered by evaporation of said non-aqueous solvent .

20. 20. A process as claimed in claim 18 wherein the solute is acetic acid and the non-aqueous solvent is dichloromethane and said acetic acid is recovered by evaporation of said non-aqueous solvent.

21. A process as claimed in any of claims 1 to 25 20 wherein at least the stoichiometric amount of hydrate former required to react with all the water in the aqueous solution, is used.

22. A process as claimed in any of claims 1 to 21 wherein the hydrate former is dichloromethane, !0 trichloromethane or dichlorofluoromethane.

23. A process as claimed in any of claims 1 to 20 wherein the hydrate former is trichlorofluoromethane - 25

24. A process as claimed in claim 23 wherein at least 1 mole of trichlorofluoromethane is used for every 17 moles of water in the aqueous solution from which the water is being removed.

25. A process as claimed in claim 23 or claim 24 wherein the hydrate formation is carried out at atmospheric pressure and at a temperature below 5°C.

26. A process as claimed in any of claims 1 to 25 wherein an excess of hydrate former over the amount required to react with all the water in the aqueous solution is used and wherein the liquid phase remaining after solid hydrate formation and containing the excess unreacted hydrate former is filtered off from the solid phase and then evaporated to recover any minor constituents of the solution being concentrated which remain in the liquid phase during the solid hydrate formation, said minor constituents being recombined with the concentrated aqueous solution of the solute.

27. A process as claimed in any of claims 1 to 8, 15 to 20 and 21 to 26 when dependent on any of claims 1 to 8 and 15 to 20 wherein the aqueous solution from which water is to be removed is aqueous acetic acid.

28. A process as claimed in claim 27 when dependent on claim 25 wherein hydrate decomposition is carried out at a temperature not higher than 0°C. and a pressure not higher than 750 mm of mercury so that said hydrate is decomposed and the trichlorofluoromethane volatilised whilst the solid acetic acid is maintained in substantially solid form. - 25

29. A process as claimed in claim 28 wherein the hydrate decomposition is carried out at a temperature and pressure such that the hydrate is decomposed into ice and hydrate former and that the ice is then fraction5 ally sublimed off by reducing the pressure to a value not greater than 4.5 mm of mercury when a temperature of about 0°C. is used.

30. A process as claimed in claim 29 wherein ice sublimation is carried out at about 3 mm of mercury LO and about 0°C.

31. A process as claimed in any of claims 27 to 30 wherein the aqueous acetic acid solution is vinegar.

32. A process for producing a vinegar concentrate by removing water from vinegar comprising .5 a) contacting the vinegar with a hydrate former at a temperature below the maximum temperature at which said hydrate former forms a solid hydrate in the presence of the vinegar and at a temperature at which there is precipitation of solid acetic acid, so as 0 to form a magma comprising solid hydrate, solid acetic acid, any unreacted hydrate former, any unreacted aqueous vinegar solution, and minor vinegar constituents ; and b) separating 5 (i) the hydrate former and at least part of the aqueous constituents of the solid hydrate and any unreacted hydrate former, and (ii) at least part of the acetic acid, from each other and, where minor vinegar constituents are not 3 separated with the acetic acid concentrate, recovering at least part of the minor vinegar constituents? and recombining them with the acetic acid concentrate so as to produce a vinegar concentrate. - 27

33. A process as claimed in claim 32 wherein the hydrate former is trichlorofluoromethane.

34. A process as claimed in any of claims 31 to 33 wherein the vinegar is selected from spirit vinegar, grain vinegar, wine vinegar, cider vinegar and a flavoured vinegar.

35. A process as claimed in any of claims 31 to 33 wherein the vinegar is malt vinegar.

36. A process as claimed in any of claims 31 to 35 wherein the vinegar is concentrated to provide a final concentration of acetic acid of at least 40% w/v.

37. A process as claimed in claim 31 wherein the final concentration is at least 60% w/v.

38. A process as claimed in claim 32 wherein the final concentration is at least 80% w/v.

39. A process as claimed in any of claims 1 to 38 wherein the water removal process is carried out at least twice on the aqueous solution from which water is to be removed.

40. A concentrated aqueous solution when prepared by a process as claimed in any of claims 1 to 39 .

41. Concentrated vinegar when prepared by a process according to any of claims 1 to 39.

42. A process for concentrating vinegar substantially as described hereinbefore with particular reference to any of Examples 1 to 6.