NO762921L - - Google Patents

Description

Procedure for removing water from

an aqueous solution.

The present invention relates to a method for

remove water from aqueous solutions.

It is previously known that solid hydrates can form between certain hydrate-forming fluids and water from aqueous solutions such as e.g. seawater and that after separation of the solid hydrate by filtration or similar mechanical treatment processes, clean water can be obtained from the separated hydrate by splitting it. However, in the case of solutions in which the solute becomes solid at the temperatures used to form the solid hydrate, a technique such as e.g. filtration is used to separate the solid hydrate from the rest of the mixture.

It is therefore an object of the present invention to provide a method for removing water from aqueous solutions of the type in which a solid hydrate is formed and in which the solid hydrate is separated from the solid phase, preferably by means of non-mechanical devices.

The present invention thus relates to a method for

to remove water from an aqueous solution, and the distinctive feature of the method according to the invention is that: a) an aqueous solution is brought into contact with a hydrate-forming fluid at a temperature below the maximum temperature at which

said hydrate-forming fluid forms a solid hydrate in the presence of the solution, and at a temperature where the amount of solute present in the original aqueous solution exceeds the solubility of the solute in any solution remaining after the hydrate formation, so that a magma is formed comprising solid hydrate, solid solute, any unreacted hydrate former and any unreacted aqueous solution,

b) separation

(i) the hydrate former and at least a portion of the aqueous components

in the solid hydrate is separated

(ii) at least part of the solute, by fractional sublimation and/or elution, so that an essentially hydrate-forming-free product comprising the solute and any remaining water is formed.

In a preferred separation process, the solid mixture resulting from the treatment of the aqueous solution with hydrate-forming fluid is subjected to conditions of temperature and pressure which result in the cleavage of the solid hydrate into ice and hydrate former, the hydrate former is removed by vacuum evaporation and the mixture of ice and solid solute are preferably separated by means of differentiated or fractional sublimation. In the case where the solute is acetic acid and the hydrate-forming fluid is trichlorofluoromethane

(also known under the trade name "Freon 11", but hereafter simply referred to as TCFM) the cleavage of the hydrate former is usually carried out at about 0°C or lower and at a pressure of about 750 mmHg or less at 0°C or a correspondingly lower pressure at lower temperatures.

Fractional sublimation to remove the ice can be carried out at about 0°C and 4.5 mm Hg or lower, e.g. at 0.0075°C and 4.5 mm Hg, although other suitable combinations can be easily determined by experiment and/or using vapor pressure data at different temperatures, choosing conditions under which at the selected temperature the vapor pressure of the ice exceeds the selected pressure while the vapor pressure of the solute is less than the selected pressure and vice versa. If difficulties arise when choosing suitable conditions, e.g. when the solute has a similar triple point as water, high yields can be obtained by passing the vapors through a column of water vapor-absorbing material such as e.g. silica gel or anhydrous copper sulfate (which can be regenerated) as the non-aqueous vapor is collected.

In a particular embodiment of the invention, water is removed from an aqueous solution by a) bringing an aqueous solution into contact with a hydrate-forming fluid at a temperature below the maximum temperature at which said hydrate-forming fluid 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 the aforementioned hydrate-forming fluid forms a solid hydrate with water from the aqueous solution, and

b) the solid hydrate is split so that hydrates and ice are produced, and then at least a part of the ice and at least a part are separated

of the solute from each other by fractional sublimation of the mixture, so that an essentially hydrate-forming-free product comprising the solute and any remaining water is produced.

As used herein, the term "sublimation" is a process in which a solid is converted directly to a vapor and includes such processes in which the cladrate hydrate is decomposed without passing through a liquid phase into hydrate forming gas and water vapor 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 fluid is approximately insoluble at the temperatures where the hydrate is formed and is stable. In the case where the solute is acetic acid, suitable elution solvents include ethanol, formaldehyde and butanol.

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 unreacted formers, unreacted aqueous solution, and unreacted minor constituents which may be present in the solution being concentrated. The last fraction may be distilled to recover any such inconsequential constituents which may be present, as e.g. in the case of vinegar, and any dissolved solute.

Even if the non-essential components also include solutes of the aqueous solution from which water is removed, they shall be referred to herein as non-essential components, while the main component or components shall, for the sake of simplicity, be referred to herein as the solute.

The solid phase is then dried quickly by evaporation of unseparated forms

and then increasing the vacuum (ie reducing the pressure) the formed hydrate becomes unstable and splits into ice and forms. The former evaporates immediately and can be collected for recycling. This leaves a mixture of ice and solid acetic acid. The ice will start to melt at below 0°C due to the lowering of the freezing point of the solute solution. If, however, the solute is very soluble in water at this temperature, it is possible to achieve a significant degree of concentration if the solute is dissolved and the solution is removed before

all the ice melts. The unmelted ice that is then obtained will then represent the amount of water that is extracted from the original solution. An alternative method is to treat the ice/solid solute mixture before melting occurs with a solvent that provides rapid dissolution of the solute, but has little effect on the ice (examples are T.e.F.M. or methylene dichloride when the solute is acetic acid). The solution is then separated from the ice by filtration and the solute is recovered by evaporation.

Although T.C.F.M. is a particularly valuable hydrate-forming fluid, especially for use with aqueous acetic acid solutions, because of its non-toxicity, easy commercial availability, low cost, and ease of processing due to the fact that it is a liquid at ambient temperature and pressure so as to avoid the need for expensive pressure storage and pressure/reaction vessels, and due to the fact that it can form a solid cladrate hydrate at ambient pressure when the temperature is lowered sufficiently, so

other hydrate-forming fluids can also be used. Known hydrate-forming fluids together with their hydrate formulas are shown in Table 1 where M represents the individual hydrate-forming molecules in the given section.

The choice of working conditions for formation of the solid hydrate is well known and understood by the person skilled in the art.

In short, the relevant working conditions for a given system are based on the pressure-temperature-equilibrium data for a given hydrate former which has been previously determined and calculated for a desirable solution concentration, working temperature and pressure limit using the formula:

in which:

t° = previously selected working temperature for formation of the solid hydrate

and the concentrated aqueous solution

P-^= minimum absolute pressure for the hydrate former exerted at the temperature t° to achieve the desired final concentration of the

aqueous solution by solid hydrate formation

PQ = absolute pressure of the hydrate former exerted at the temperature t° to achieve formation of solid hydrate with pure water.

(Assuming water saturated with hydrate formers, but containing essentially no other solute)

x = mole fraction of water at the desired final concentration of it

concentrated aqueous solution

n = number of water molecules associated with one molecule of hydrate former

in the solid hydrate.

The values of PQ at a preselected t° can be obtained from experimental and published data.

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

In the event that T.C.F.M. is used to remove water from acetic acid solutions, it has been found that almost complete reaction of the water to form solid hydrate can be achieved in the presence of an excess of T.G.F.M. beyond the stoichiometrically required amount, at atmospheric pressure if only a sufficiently low temperature is used, preferably below 5°C.

The selection of the hydrate-forming fluid will depend on various factors such as e.g. safety, costs and easy availability, but is determined primarily by the particular solute that is desired to be concentrated and its properties as well as the process chosen for separating the solute concentrate from the hydrate or vice versa. The hydrate-forming fluid is thus usually chosen for maximum ease of carrying out the separation process, taking into account the other above-mentioned criteria, e.g. where differentiated sublimation is used, the hydrate-forming fluid is chosen so that a solid hydrate is provided which is easily sublimated and preferred in relation to the solute. In the case of aqueous acetic acid solutions where the separation process used is differentiated sublimation, particularly suitable hydrate-forming fluids include beyond T.C.F.M. dichloromethane, trichloromethane and dichlorofluoromethane.

The amount of hydrate-forming fluid used in the first

solid hydrate formation steps in the method according to the invention can be varied within wide limits: Although it is desirable that the amount used should be at least an amount that is sufficient to react with all the water present in the solution to be concentrated, nevertheless, an excess of the hydrate former is advantageous, especially when vinegar is to be concentrated, then in this case the unimportant constituents in the vinegar (which contribute to its aroma and character) can easily be recovered in excess of unreacted hydrate former.

In practice, the reaction of hydrate formers, such as e.g. T.C.F.M.,

with water to form solid hydrates, approximately stoichiometrically so

that the required amount of hydrate can easily be calculated.

In the case where..the hydrate former is T.C.F.M. is thus the required amount of T.C.F.M. at least one molecule of T.C.F.M. for 17 molecules of water, i.e. at least about .1 part by weight T.C.F.M. to 2 parts by weight of vinegar. Appropriately, approximately equal parts by volume of vinegar and T.C.F.M.

The method according to the present invention has been found to be particularly advantageous for. concentration of vinegar. Vinegar consists mainly of an aqueous acetic acid solution with an acetic acid content of approximately 5" to 10% w/v depending on the origin and method of production, as the solution contains small amounts of various other natural product components that contribute to the aroma of the particular vinegar.. Special types of vinegar that can be mentioned includes distilled, malt vinegar, alcohol vinegar, grain vinegar, wine vinegar, cider vinegar and flavored vinegar types. In England, malt vinegar usually contains at least 4% w/v acetic acid and

In France and Italy, wine vinegar contains at least 6 smallest

7% weight/volume acetic acid.

From the foregoing it will be clear that common types of vinegar comprise approximately 90 - 95% weight/volume water. It is therefore clearly desirable that if the transport costs for vinegar are to be reduced to a noticeable degree, the vinegar should be highly concentrated. On the other hand, one must be aware that many of the non-essential natural product constituents in vinegar which are of significant importance for the vinegar's aroma are exposed to denaturation at elevated temperatures and under other severe conditions.

It is consequently a further object of the present invention to provide a method for concentrating vinegar in which the vinegar components are not denatured to a particular extent or lead to any significant loss of vinegar components other than the water.

The present invention provides, in a particular embodiment, a method for producing a vinegar concentrate by removing water from vinegar comprising

a) the vinegar is brought into contact with a hydrate-forming fluid at a temperature below the maximum temperature at which it mentioned

hydrate-forming fluid forms a solid hydrate in the presence of the vinegar and at a temperature where the amount of acetic acid in the vinegar exceeds the solubility of the acetic acid in any solution remaining after hydrate formation so that a magma is formed comprising solid hydrate, solid' acetic acid, any unreacted hydrate former, any unreacted aqueous vinegar solution, and non-essential vinegar constituents, and

b) One separates

(i) the hydrate former and at least a portion of the aqueous components

in the solid hydrate and any unreacted hydrate formers, and

(ii) at least part of the acetic acid from each other, so that an essentially hydrate-forming-free acetic acid concentrate is produced and when the non-essential acetic components are not separated with the acetic acid concentrate, these non-essential acetic components are recovered and recombined with the acetic acid concentrate so that a vinegar concentrate.

A particularly preferred hydrate-forming fluid for use in this process is trichlorofluoromethane.

The solid hydrate is preferably separated from the concentrated vinegar and any solid acetic acid that has precipitated from the vinegar solution by means of sublimation or dissolution of solid hydrate under temperature and pressure conditions where any solid acetic acid is essentially not evaporated or dissolved and essentially not denatured.

On the other hand, in some cases the unimportant components in the vinegar are advantageously separated from the solute, e.g. in resolution

in the hydrate former, and after recovery can be recombined with the solute concentrate obtained after completion of the concentration process

(subject to the required final concentration).

< r

It should of course be understood that when the highest levels of concentration are required it may be necessary or easier 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 unimportant constituents are separated from the solute (e.g. in solution in excess of hydrate formers) they do not need to be recombined with the concentrated aqueous solution of the solute before the final concentration cycle is completed. The degree of concentration that can be achieved by the method according to the invention will depend on various factors such as e.g. the nature of the solute and hydrate former used, the particular separation process used and the number of concentration cycles carried out. Nevertheless, concentrations of 40, 60 or even 80% weight/volume of acetic acid can be achieved by choosing suitable conditions in the case of vinegar concentration by the method according to the present invention, the concentrated vinegar after reconstitution with water for practical purposes approximately not can be rarefied from untreated vinegar.

Example 1 Concentration of aqueous acetic acid.

A. F§st_hydrate formation

To aqueous acetic acid solution (1 liter 10% w/v) was added liquid trichlorofluoromethane (TCFM, 1 liter) and the mixture was cooled to 3°C. Vigorous stirring along with external cooling and internal cooling by addition of solid carbon dioxide was then carried out to ensure that the temperature of the mixture remained below 5°C during the hydrate formation step. After a few minutes,

a magma comprising unused TCFM, solid acetic acid, residual aqueous acetic acid and solid hydrate was obtained.

B_ Sep_arerin3_av_hydratet.

The magma was subjected to vacuum evaporation (3 mm Hg at 0°C) i

60 minutes until no more hydrate sublimed. The unused TCFM was first removed at this step and then recovered together with the TCFM bound in the hydrate. The removal of TCFM and subsequent removal of the hydrate reduces the temperature to 0°C and this temperature is then maintained by suitable heat input for rapid TCFM removal. This process produced a mixture of solid glacial acetic acid and residual aqueous acetic acid which at room temperature gave an 82% w/v aqueous acetic acid solution.

Example 2 Concentration of malt vinegar.

A. Fast_hydratdannel.se

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

B_ §§E§rering_av_Hydratet

(1) A portion of the solids was subjected to vacuum evaporation (3.4 mm Hg at -0.5°C) until no more was removed

TCFM or water. This step gave an aqueous acetic acid solution containing 69% w/v acetic acid. (2) The second part of the solids was homogenized 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 remaining solids gave under the ambient conditions an aqueous solution containing 4 2.5% weight/volume acetic acid.

The liquids from the first step were then added, after removal of TCFM therefrom by evaporation, to the acetic acid solution obtained after separation of the hydrate to finally give concentrated malt vinegar.

Example 3 Concentration of alcohol vinegar.

A. F_st__Y_ra_formation_

Liquid trichlorofluoromethane (TCFM, 250 mL) was added to ethyl acetate (250 mL) and the mixture stirred vigorously for 3 minutes at 3°C. Excess TCFM and other liquids were then suctioned off from the solid hydrate and the acetic acid.

B_ Separerin2_av_Hydratet

The solids that precipitated from the first step were subjected to vacuum evaporation (10 mm Hg at -5°C) until TCFM was no longer emitted. 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 gave a residue which at ambient temperature gave an aqueous solution containing 68% weight/volume acetic acid.

To the aqueous acetic acid solution was then added the recovered ethanol and acetaldehyde and the liquids from the first step after that. TCFM had been evaporated from them, whereby concentrated spirit vinegar was finally obtained.

Example 4. Concentration of alcohol vinegar.

Solid_hydrate formation

a) 50 ml of spirit vinegar was added to 50 ml of TCFM and the mixture was allowed to assume a temperature of 0°C and was stirred with an air stream until most

or all the aqueous phase had reacted. The surplus of TCFM was then siphoned off. and stored for recovery of the solutes. The fixed

magma was correspondingly stored at 0°C until it was needed for evaporation and sublimation.

b) Another method of cladrate hydrate formation was used:

to 100 ml of spirit vinegar at 0°C, 75 ml of TCFM was added dropwise

within 6 hours. The temperature was kept at 0°C and stirring was ensured by means of an air bubble. After 6 hours, excess TCFM and dissolved constituents were filtered from them

solid magma constituents and stored until they are needed for the recovery of solutes. The solid magma was stored at 0°C until required for evaporation and sublimation.

The solid magmas from the hydrate formations (a) and (b) were placed in a freeze dryer and a vacuum applied. Excess molds were initially removed and the vacuum was then increased until the clathrate hydrate split and the former was released. The temperature was kept immediately below 0°C by infrared irradiation. As the hydrate breaks down, ice and gaseous TCFM are formed. TCFM was condensed for later renewed use. After a substantial part of the former had been removed, the thin layer of magma consisted of ice crystals and solid acetic acid. The pressure was now reduced for sublimation of the ice (3 mm Hg at 0°C) leaving a significant portion of the acetic acid which was allowed to remain at room temperature at normal atmospheric pressure.

Excess TCFM from the clathrate formation step was carefully distilled at 20°C until all traces of TCFM were removed as shown by examination (G.L.C.) of the product. The hydrate formation method (b)

gave higher yields of dissolved acetic acid than method (a). After combining the distillates and the sublimed samples, the first method gave a concentrate with 84% w/v acetic acid and the last method gave a concentration of 89% w/v.

Example 5

A 100 ml sample portion of malt vinegar was reacted with 100 ml of TCFM as in Example 4. Excess TCFM containing the non-essential components in the malt vinegar was suctioned from the solid magma and the dissolved acetic acid and the non-essential components were recovered. by distillation at 20°C until no TCFM was detected by analysis (G.L.C.) of the residue.

The solid magma was then subjected to vacuum distillation under mild conditions until excess TCFM was removed. The vacuum was then increased until the hydrate split to leave a 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 briefly stirred and the liquid sucked off by vacuum filtration. The liquid was concentrated acetic acid and was recombined with the solutes from the excess of TCFM whereby concentrated vinegar was obtained, where the concentration level of the acetic acid in the aqueous solution was controlled by the amount of ice that had melted into the liquid that had possibly been suctioned from the slurry. After the addition of the acetic acid dissolved in TCFM and unimportant components, the acetic acid content in the samples was 54% weight/volume.

Example 6

Hydrate formation was carried out as in example 4a). The fixed

the magma was then subjected to an initially small vacuum to remove excess TCFM and then the hydrate was rapidly decomposed to give ice and solid acetic acid. To the mixture of ice and acetic acid, at the noted temperature of -4°C, an equal volume of TCFM at 0°C was added and the mixture stirred. TCFM was then removed by filtration from the ice and the liquid was evaporated at 20°C until no more TCFM was observed. This method gave, after recombination with the non-essential constituents removed from the distilled TCFM, 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 closely resembled the original vinegar; The compounds acetaldehyde and ethanol found in vinegar were determined separately and the rest of the volatile components as a grouping. The quantity determinations were based on the peak areas at G.L.C.

The following table gives the acetic acid content (% w/v) and the percentages (weight-based) of the total components removed in 5 vinegar concentrates obtained by the method according to example

6 carried out on a smaller scale using a 10 ml vinegar sample. The error in the determinations of the non-essential components is of the order of magnitude + 7 to 8%.

PATENT CLAIMS

1. Process for removing water from an aqueous solution wherein an aqueous solution is brought into contact with a hydrate-forming fluid at a temperature lower than the maximum temperature at which said hydrate-forming fluid forms a solid hydrate in the presence of the solution, and at a temperature at which the amount of solute present in the original aqueous solution exceeds the solubility of the solute in any solution remaining after hydrate formation, so that a magma comprising solid hydrate, solid solute, any unreacted hydrate former and any unreacted aqueous solution is formed, and separation of

(i) the hydrate former and at least a portion of the aqueous constituents of the solid hydrate and (ii) at least a portion of the solute from each other, to produce a essentially hydrate-forming-free product and any remaining water, characterized in that the separation is carried out by fractional sublimation and/or elution. 2. Method as stated in claim 1, where an aqueous solution is brought into contact with a hydrate-forming fluid at a temperature lower than the maximum temperature at which the hydrate-forming fluid 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-forming fluid forms a solid hydrate with water from the aqueous solution, and separating a) the hydrate former and at least a part of the ice constituents in the hydrate and b) at least a part of the solute from each other so that an approximately hydrate-forming product is produced comprising the solute and any remaining water, characterized in that the solid hydrate is split before the separation to produce hydrate forming and ice; the hydrate former is removed and the separation is carried out by fractional sublimation. 3. Procedure as specified in claim 1 or 2, characterized by the solid hydrate is decomposed before the separation by increasing the temperature and/or reducing the pressure to provide temperature and pressure conditions where the said solid hydrate is decomposed and where the solute is essentially unaffected. 4. Procedure as stated in claims 1 and 3, characterized in that the cleavage is carried out at temperature and pressure conditions where the solute remains essentially solid. Method as stated in claims 1-4, characterized in that the solute and at least part of the ice are separated from each other by fractional sublimation at temperature and pressure conditions so that the ice. is volatilized while the solute remains essentially in the liquid and/or solid phase. 6. Procedure as stated in claims 1 - 4, characterized in that the solute and at least part of the ice are separated from each other by sublimation at temperature and pressure conditions so that both the ice and the solute are volatilized bg are then brought into contact with a water vapour-absorbing material. 7. Procedure as stated in claim 1, characterized in that the solute is separated from the solid hydrate by differential elution using a solvent in which the solute is essentially more soluble than the solid hydrate. 8. Procedure as stated in claim 7, characterized in that when the solute is acetic acid, the elution solvent is selected from ethanol. formaldehyde and butanol. 9. Procedure as stated in claim 1, 3 or 4, characterized in that at least part of the ice is maintained in a solid state and the solute is separated mechanically from the ice in the solid state, in the form of a concentrated aqueous solution. 10. Procedure as specified in claim 9,

characterized in that part of the ice is allowed to melt into liquid water in which the solute dissolves to

form a concentrated aqueous solution of the solute.

11. Method as stated in claim 9 or 10, characterized in that liquid water is added and that the solute is allowed to dissolve in the aforementioned added liquid water to form a concentrated aqueous solution of the solute. 12. Method as stated in claims 9-11, characterized in that the concentrated aqueous solution is separated from the solid ice by filtration. 13. Procedure as stated in claim 1, 3 or 4,

characterized in that at least part of the ice is maintained in a solid state and a non-aqueous solvent,

wherein ice is substantially less soluble than the solute, is added to form a solution of the solute, the solution then mechanically separated from the solid ice, and the solute recovered from the solution by separation from the elution solvent. 14. Method as stated in claim 13, characterized in that the solute is acetic acid and that the non-aqueous solvent is selected from trichlorofluoromethane and methylene dichloride, and acetic acid is recovered by evaporation of the non-aqueous solvent. 15. Method as stated in claims 1-14, characterized in that dichloromethane, trichloromethane or dichlorofluoromethane is used as hydrate former. 16. Procedure as stated in claims 1-14,

characterized in that trichlorofluoromethane is used as hydrate former.

17. Procedure as stated in claims 1-16,

in which an excess of hydrate formers beyond the amount required for reaction with all the water in the aqueous solution is used, characterized by the fact that the liquid phase remaining after solid hydrate formation and containing an excess of unreacted forms is filtered off from the solid phase and then evaporated for recovery of any non-essential components in the solution which are concentrated and which remain in the liquid phase during the solid hydrate formation, the non-essential components being recombined with the concentrated aqueous solution of the solute. 18. Procedure as stated in claims 1-17,

characterized in that the aqueous solution from which water is to be removed is vinegar.,

19. Procedure as specified in claim 18,

in which the hydrate former is trichlorofluoromethane, characterized in that the hydrate cleavage is carried out at a temperature not higher than 0°C and at a pressure not higher than about 750 mm Hg so that the hydrate

is split and the trichlorofluoromethane is volatilized while the solid acetic acid is maintained essentially in solid form. 20. Method as set forth in claim 19, characterized in that hydrate splitting is not carried out at a temperature and pressure such that the hydrate splits into ice and forms hydrate and that the ice is then fractionally sublimated by reducing the pressure to a value of no greater than 4.5 mm Hg when a temperature of approximately 0°C is used. 21. Procedure as stated in claim 20,

characterized in that the ice sublimation is carried out at about 3 mm Hg and about 0°C.

22. Procedure as stated in claim 1,

for the production of a vinegar concentrate by removing water from vinegar, characterized by

a) the vinegar is brought into contact with a hydrate-forming fluid at a temperature below the maximum temperature at which it mentioned

hydrate-forming fluid forms a solid hydrate in the presence of vinegar and at a temperature at which the amount of acetic acid in the vinegar exceeds the solubility of the acetic acid in any solution remaining after hydrate formation, so that a magma is formed comprising a solid hydrate, solid acetic acid, any unreacted hydrate forms, any unreacted aqueous vinegar solution and non-essential vinegar constituents; and

b) separation of

(i) the hydrate former and at least a portion of the aqueous components

in the solid hydrate and any unreacted hydrate formers, and

(ii) at least part of the acetic acid from each other, where the non-essential acetic components are not separated together with the acetic acid concentrate, the non-essential acetic components are recovered and recombined with the acetic acid concentrate so that a vinegar concentrate is produced. 23. Method as stated in claim 22, characterized in that trichlorofluoromethane is used as hydrate former. 24. Method as stated in claim 22 or 23, characterized in that the vinegar is concentrated to give a final concentration of acetic acid of at least 40% weight/volume. 25. Method as stated in claim 24, characterized in that the final concentration is at least 60% weight/volume. 26. Procedure as stated in claim 25, characterized by the final concentration being at least 80% weight/volume. 27. Procedure as specified in claims 1-26,

characterized in that the water removal process is carried out at least twice with the aqueous solution from which water is to be removed.