Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
lU90~;50
~his invention relates to the removal of water from
aqueous solutions. ;
It previously has been known that solid hydrates may be
formed between certain hydrate-forming fluids, referred to here-
inafter 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 decomposition
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. ~
Accordingly, it is an object of the present invention -~ -
to provide, for the removal of water from aqueous solutions, a
process 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
- ~ : - . . .
~20 water from an aqeuous 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 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 (il) at least part
30 of the solute, from each other, by fractional sublimation, eva- -
~' ~
- 1 -
B
.
~osvtjso
poration and/or elution, so as to produce a substantially
hydrate former-free product comprising the solute and any re-
maining 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 condi-
tions 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 mix-
ture of ice and solid solute is preferably separated by dif- ~-
ferential or fractional sublimation. In the case where the `
solute is acetic acid and the hydrate forming fluid is trichlo- ~-
rofluoromethane (also known under the Trade Mark Freon II but
hereinafter referred to as ~.C.F.M.), decomposition of the
hydrate formed usually is effected at about 0C or below and at
a pressure of about 750mm of mercury or lower at 0C or a cor-
respondingly lower pressure at lower temperatures. Fractional ~
sublimation removing the ice then may be carried out at about ~-
0C and 4.5mm of mercury or lower, for example at 0.0075C and
4.5mm of mercury, though other suitable combinations readiiy
may 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
1090~i5U
sulphate (which can be regenerated), the non aqueous vapour
being collected.
In 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 in the presence of the solution
and above the maximum temperature at which ice forms in the
aqueous solution so that the 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 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 fluid used is substan-
tially 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.
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1090~;50
A preferred method of differential elution is when the
magma of solid hydrate and solid solute i5 separated from the
liquid component which may contain one or more of unreacted
hydrate former, unreacted a~ueous solution, and unreacted minor
constituents that may be present in the solution being concen-
trated. The latter fraction may be distilled to recover any
such minor constituents that may be present, as for example,
in the case of vinegar, and 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 then is rapidly dried
by evaporation of the unseparated unreacted former and then by
increasing the vacuum (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 0C 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 a signifi-
cant 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 elu-
tion solvent. The unmelted ice recovered then represents the
amount of water extracted from the original solution. An al-
- ternative method is to treat the ice/solid solute mixture be-
fore much melting occurs with another solvent which gives rapid .
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10S'~;50
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 form-
er and subsequently as an elution solvent. The solution is then
separated 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 ;
for costly pressurised storage and reaction vessels, and due to
the fact that it can form a solid hydrate (also sometimes re-
ferred 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 1 wherein M represents any
of the individual hydrate forming molecules in the given section.
Table 1 ~
Hydrate Formula Hydrate Former, M -
M.5.75 H2O A'Kr'N2'2H2S'H2Se'C2
2o~pH3~AsH3~cH3F~cH3cl~cH4
M.5.75 H2O or M.7.66 H2O SO2,CH2F2~C2H2'c2 4
M.7.66 H2O Xe,Br2,NF2,CHF3,CF4,CH3Br
C2H3F~ CH3CHF2' C2H6
M.17 H2O CH2C12,CHC13,CC14,CH3I,C2H5C1
CH3cF3~c~3cHcl2~cHBrF2~cclF3
CC12F2 ' CBr2F2 ~ CBrClF2
3 ,C3H6,C3H8~ cyclo C5H10
B 5 _
10906S0
The selection of operating conditions for formulation
of tlle solid hydrate are well known and understood in the art.
Briefly the actual operating conditions for a given system are
based on the press~re-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:
Plto = (PO/xn-)to
where: -
t = preselected temperature of operation for forma-
tion of the solid hydrate and the concentrated
aqueous solution. ~-
P = minimum absolute pressure of the hydrate former
--1
- to be exerted at temperature t to achieve the
desired final concentration of the aqueous solu-
tion through solid hydrate formation.
PO = 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 concen-
tration of the concentrated aqueous solution.
n = num~er of water molecules associated with one
; molecule of hydrate former in the solid gas
~ hydrate.
; The values of PO at a preselected t can be obtained
from experimental and published data.
Although ordinarily the formula will be utilized to
determine the operating pressure (Pl) for a preselected
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~ ` 1090650
temperature (t), known pressure (PO) of formation of solid
hydrate with pure water and for a desired final solution con-
centration as represented by a residual mole fraction of water
(x) in the solution, generally it is to be understood that if
any two of the operating variables, i.e. Pl,Pt, 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 an excess of T.C.F.M. over the
stoichiometrically required amount, at atmospheric pressure
provided a sufficiently low temperature, preferably below 5C,
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 abovementioned criteria, for example -
where differential sublimation is employed the hydrate former
is chosen to provide a solid hydrate which may be readily ~;
sublimed pre~erentially 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
-- 7 --
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.. . .
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acetic solutions where the separation process used is differen- -
tial sublimation, especially suitable hydrate former other than
T.C.F.M. include dichloromethane, trichloromethane and dichloro-
fluoromethane.
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 advan-
tageously an excess of the hydrate former is used especiallywhen 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 unreact-
ed 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
usually may be readily calculated though it will be appreciated
that if ~ome of the water solidifies before reaction with the
~0 hydrate former, it no longer will 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 invcntion have been
found to be especially valuable for the concentration of vinegar.
10~)~' jO
Vinegar is essentially 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 le~ast 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 transporation costs are to be
significantly reduced, the vinegar should be substantially con-
centrated. 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 suscept-
ible to denaturation at elevated temperatures and under other --~
severe conditions.
It is, therefore, another object of the present inven-
tion to provide a process for the concentration of vinegar
which process does not substantially denature the vinegar con-
stituents or result in any substantial loss of the vinegar
constituen~s - other than the water.
Accordingly, a further aspect of the present inven-
tion provides a process for producing a vinegar concentrate by
removing water from vinegar comprising:
(a) contacting the vinegar with a hydrate-forming fluid at a
temperature below the maximum temperature at which said hydrate
B~
lO9V~iSO
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 unreated hydrate former, any unreacted aqueous
vinegar solution, and minor vinegar constituents; and
(b) separating (i) the hydrate former and at least part of the
agueous, constituents of the- solid hydrate an~ 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 consti-
tuents are not separated with the acetic acid concentrate; re-
covering 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 in -;
this process is trichlorofluoromethane.
Preferably the solid hydrate is separated from the
concentrated vinegar and any solid acetic acid that has pre-
cipitated 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 substantially not denatured. In another preferred process
though, where the solute is separated from the solid hydrate by
elution of the solvent, separation conveniently may be 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 ~e recombined with the solute concentrate
-- 10 --
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lOS~V~ O
1,
¦ obtained after completion of the concentration process (pro-
viding the required final concentration).
j It will of course be appreciated that where the
highest degrees of concentration are required it may be neces-
sary or more convenient to carry out the concentration in more
j 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 concen- -~
trated aqueous solution of the solute until the final concentra-
tion 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 concentra-
tion 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 con-
centrated vinegar being, after reconstitution with water, sub-
stantially indistinguishable, for practical purposes, from un-
treated 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 3C. Vigorous agitation together with external cool-
ing and internal cooling, by the addition of solid carbon
dioxide, was then carried out so as to ensure that the tempera-
ture of the mixture remained below 5C throughout the hydra-
-- 11 --
109{~0
., ,
tion formation step. After a few minutes a magma comprising
unreacted TCFM, solid acetic acid, residual aqueous acetic acid
~i.e. liquid or solid 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 0C)
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 tempera-
ture to 0C and this temperature is maintained thereafter by
~f` 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.
i, Example 2. Concentration of malt vinegar.1
A ~olid Hydrate Formation.
Liquid trichlorofluoromethane (TCFM, 500 ml) was added to malt
vinegar (500 ml. j and the mixture stirred vigorously for 3
minutes at 3C. 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 - 0.5C) 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 -4C. The
~0 liquid phase was then filtered off under vacuum. The liquid
:
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~9.
.~.,,~ . ~ `
5U
'~
phase yielded under ambient conditions an aqueous solution con-
taining 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 hy-
drate 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 3C. Excess TCFM and other liquids were then
strained off from the solid hydrate and acetic acid.
~ B. Separation of Hydrate
s
The solids resulting from the first stage were subjected to
vacuum evaporation (10 mm Hg at -5C) until no more TCFM came
' off. The pressure was then reduced to 3 mm Hg and the tempera-
ture increased to -1C. 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 contain-
~` ing 68% w/v acetic acid.
To the aqueous acetic acid solution were then added
the recovered ethanol and acetaldehyde and the 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
i' ~a) 50 ml. of Spirit Vinegar was added to 50 ml. of T.C.F.M.
and the mixture attempered at 0C. and agitated with an air
bleed until most or all the aqueous phase had reacted. The
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^, . ~ ~ . . . .. ..
~,:., - - - ` . `
lO~t)~'jO
exce$s T.C.F.M. was then drained off and stored for recovery of -~
solutes. The solid magma was similarly stored at 0C. until
,f required for evaporation and sublimation.
(b) A second method of solid hydrate formation was used: to
, 100 ml. of spirit vinegar at 0C., 75 ml. of T.C.F.M. was added
dropwise over a period of 6 hours. The temperature was main-
tained at 0C. and agitation 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 0C. until required for evaporation and sublimation.
Separation of Hydrate
.'
f The solid magmas from hydrate formations (a) and ~b)
were placed in a freeze dryer and a vacuum applied. The excess
5. 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 0C. by infra red
1,
j 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
0C.), 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
i stages was carefully distilled at 20C. 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
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1~901i50
L~
~,
(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 mls. T.C.F.M. as in Example 4. The excess T.C.F.M. contain-
ing the minor constituents of the malt vinegar was drained from
the solid magma and the dissolved acetic acid and minor con-
stituents were recovered by distillation at 20C. until no
T.C.F.M. was detected by G.L.C. analysis of the residue.
The solid magma was then subjected to vacuum evapora-
tion 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 -1C. and in addition to this 1 ml. of ice
cold water was added. T~e slurry was agitated briefly and the
liquid drained off by vacuum filtration. This liquid was con-
centrated acetic acid and when recombined with the solutes from ~;
the excess T.C.F.M., concentrated vinegar was obtained, the
level of concentration of the acetic acid in the aqueous solu-
tion 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 con-
stituents the acetic acid content of the samples was 54% w/v.
Example 6
i
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 -4C, an equal
-- 15 --
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~0!~0~;50
volume of T.C.F.M., at 1C, was added and the mixture agitated.
The T.C.F.M. was then removed by filtration from the ice and
liquid was evaporated at 20C. until no more T.C.F.M. was de-
tected. 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 com-
pounds acetaldehyde and ethanol found in vinegar were determinedindividually and the remainder of the volatile constituents were
groups together. The determinations of quantity were based on
the G.L.C. peak areas.
The following table gives the acetic acid content
(% w/v) of and the percentages (by Weight) of the minor con-
stituents 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 i 7 to 8%.
Proportions of minor constituents
retained in vinegar concentrates
% % Other
Vinegar Type Acetic Acet-aldehyde Ethanol Volatiles
Acid Conc.
% w/v*
Spirit 67 94 89 93
~i Wine 71 64 76 77
I Wine (White) 61 81 73 82
Malt 82 92 98 102
Cider 58 84 86 91
* The acetic acid concentrations given were determined by acid-
base titration and include any minor acidic constituents re-
covered that may be present in the vinegar being concentrated.
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