.
FREEZE—DRYING
.
,
concentration of salt is eventually reached where theremaining
mixture of salt and water
3
freeze as_ one (gure 1). The “‘ëôëéiitfät‘i0n
is known as the eutectic
;
concentration and the temperature at
solidication
mixture occurs ——
_
the“ euteCtic tempe;{atûëglfbthcät salts"are'î‘pres‘entiWith lower—eutëctiôiÆtemperatures, complete
sodiCation will iiot, occur until the lowest temperature is reached, though each eutectic
'
mixture will separate at its own eutec‘tic temperature.
'
Corisider now-a dilute solution of sodium chloride in water at a temperature at which ice
begins to form. As'soon as "the bulk of_ thewater has separated as ice the sample looks
completely solid
the eutectiC temperature of sodium chloride—water mixture
(21.6°C)‘has not yet béé *‘f5adhèa.
that when the proportion of a eutectic
miXt1‘1‘reËis very small compared With the bulk of the material to be dried it is diîcult
to forecast the optimum temperature for drying. In order to determine the eutectic temper—
ature within a sample to be dried, Greaves (1954) introduced a technique whereby he
measured the conductivity Within the sample. The conduction of electricity through a
solution is dependent upon the mobility of ions within that solution. Provided there is
liquid present the electric current can pass unhindered but when a solution is completely
solidied, i.e. at the lowest eutectic temperature, the ions are immobilised and the resistance
of the sample will be very high. Similay, during cooling, if the viscosity of the sample
increases so the mobility of ions is decreased and the conductivity will fall progressively.
Using this technique it is therefore possible to state the condition of the sample and to
temperature within
Although it is now possible to state
at_What teipèratiire…a product Ïs_hould
dried, this is often too lowto be
,,,(—,
_:
,
:
':
..
«
’
:
m
‘
<‘
’
'
+L-qu-d
“
“
:.“Lä‘
;
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-
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+Ïq…d…… ....Z......
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_
.
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«
_FIGURE1Phasedmgramforabmarysystem ofasaltm waterrmxtureTheplotshowsthefræzmg
'
pointsfor ag1ven concentrationofsaltmwater
FREEZE—DRYING
.
,
concentration of salt is eventually reached where theremaining
mixture of salt and water
3
freeze as_ one (gure 1). The “‘ëôëéiitfät‘i0n
is known as the eutectic
;
concentration and the temperature at
solidication
mixture occurs ——
_
the“ euteCtic tempe;{atûëglfbthcät salts"are'î‘pres‘entiWith lower—eutëctiôiÆtemperatures, complete
sodiCation will iiot, occur until the lowest temperature is reached, though each eutectic
'
mixture will separate at its own eutec‘tic temperature.
'
Corisider now-a dilute solution of sodium chloride in water at a temperature at which ice
begins to form. As'soon as "the bulk of_ thewater has separated as ice the sample looks
completely solid
the eutectiC temperature of sodium chloride—water mixture
(21.6°C)‘has not yet béé *‘f5adhèa.
that when the proportion of a eutectic
miXt1‘1‘reËis very small compared With the bulk of the material to be dried it is diîcult
to forecast the optimum temperature for drying. In order to determine the eutectic temper—
ature within a sample to be dried, Greaves (1954) introduced a technique whereby he
measured the conductivity Within the sample. The conduction of electricity through a
solution is dependent upon the mobility of ions within that solution. Provided there is
liquid present the electric current can pass unhindered but when a solution is completely
solidied, i.e. at the lowest eutectic temperature, the ions are immobilised and the resistance
of the sample will be very high. Similay, during cooling, if the viscosity of the sample
increases so the mobility of ions is decreased and the conductivity will fall progressively.
Using this technique it is therefore possible to state the condition of the sample and to
temperature within
Although it is now possible to state
at_What teipèratiire…a product Ïs_hould
dried, this is often too lowto be
,,,(—,
_:
,
:
':
..
«
’
:
m
‘
<‘
’
'
+L-qu-d
“
“
:.“Lä‘
;
L'CE‘
-
°—
+Ïq…d…… ....Z......
...---...
Ï
;…
_
.
Ë
ICE+ sur…xrua:
‘
«
_FIGURE1Phasedmgramforabmarysystem ofasaltm waterrmxtureTheplotshowsthefræzmg
'
pointsfor ag1ven concentrationofsaltmwater