‘
84
<
:
;
,
:FREEZBDRYNG
enter blind pOres, ana traverses“ “omy the mre*arrea paths, Which
'
may be regarded as regular, cylindrical pores. The ratio of the permeability of the porous
'
.
.
.
'
L
2
product to the permeability of a product, assummg regular cylmdncalpores,
(f) >_
thus A1 should be réduCedbÿ such a factor.
'
.
Solvi}; equation [12]
v and multiplying by the
MP/RT, the dry1ng
rate in this case becomes :
_
_
_
_
G
RT …)%
(1
)
[14]
on integrating this equation, the mass”sublimed is
.…
'
:
m : ocpA [—ä— (%> 2]
':
[15]
_
Here a. is obtained from oc2.=
-
———1.—7
& and & are the penods forjtheî g%îŸ
-PRT _“‘z tl + Ï2
‘
Î
cvl:
,
low and—high pressure parts of
vapeur
_
transfer in the lowïÏ-ÿpressure;Î part of the cycle; which
the only _vapour
in each cycle.
*ï
.
”‘i
“'
;
‘
'
ÂÉ
In an example, aïsmall slab of cookéd meat product—was freeze dried from two s1des ‘
byradiaht heat, using a vacuum—pressure cycle of 0.2 torr for 74
followed by 20 torr
for 222 Sec. Thé parameters &,
150 cfm2,
and 0.03—f'secî1 approx., respectively. T he;densitÿtof ice”, p, Was taken “as 0.92 g/cm3. Observed
and calculated [equation 15]=Values of the mass bf ice sublimed gïfora" Slab of:rneat oriiginälly
containing 58.2 g ice are tabulated (”fable 2).
”TABLE 2. MASS OF
“
Time
subhmedm
t%
Calculated
1
.
_
.
60…
272
287
…
…
-
.,
z.
.
.
.
.
,
84.8
4142
'
41.8
,
—
…;
3. . . . .*
103.8
“
50.4
51.7
'
mess…cyclesashoerclrymeabout30/lessthanmthecom==spemüng<=fmstan’£ ‘
.
_
,
Furthemtuäœs aremprogress toextendtherangeoffreeædrymgbYthlsmethod
andthetheorylsbeingrened5
-
’