Countercurrent washing or drying and shaking completely removes the clogging particles. The substitution of l P r for the filter resistance, in the classical filtration equation. includes the effect of the clogging of filter media in the clacssical equations. This correction is independent of the mechanism of clogging. I t is probable that the correlation of the effective resistance is valid a t higher pressures. This postulate will be tested in the near future. Acknowledgment
The help of Y . Gad and M. Cohen in obtaining some of the experimental data is gratefully acknowledged. Nomenclature U
=
A b
= = = = =
B C C’
sc
=
Ah
=
I
= =
k
K L m n
= = = =
A T
=
A’,
= =
pap
=
7
=
ra
=
constant in Equation 7 , sq. cm. effective filtration area, sq. cm. geometrical Igctor in Equation 13 dimensionless parameter, defined by Equation 14 concentration of feed suspension, g./liter concentration of particles that can clog holes by complete blocking, g./liter gravitation conversion factor, 980 g. cm./g. force, sec.2 head of water on filter, cm. integral function of p, defined by Equation 29 constant in E;quations 2 and 3 ratio of r / r o thickness of filter medium, cm. average mas:, of single particle, g . number of cycles number of unclogged holes per unit area of filter, cm.? number of initial holes per unit area of filter, cm.-2 probability clf particle clogging hole pressure differential across filter and cake, g. force/ sq. cm. effective resistance of filter cloth during cake filtration, cm.? effective resistance of filter cloth a t beginning of filtration cycle, cm.?
reduction in 7, caused by bleeding, cm.-l reduction in 7, caused by bridging, cm.-’ resistance of filter cloth a t beginning of first filtrarl tion cycle, cm.? R = free radius of partly clogged hole, cm. = radius of hole, cm. R, = average radius of particle, cm. R, = number of layers of particles required for complete S bridging SO = specific surface of particles, sq. cm./cc. t = time from start of filtration cycle, sec. tl = time of beginning of cake filtration, sec. V = total volume of filtrate a t time t , liters VI = total volume of filtrate a t time tl liters V, = volume of filtrate equivalent to bleed, liters ( t / V ) O = inverse of flow rate a t beginning of cycle, sec./liter ( t / V ) ’ = extrapolation of inverse of flow rate for cake filtration. to zero filtrate volume, sec./liter = = =
fo
TC
GREEKLETTERS ff
=
p
=
E
cc
= = =
Ps
=
?1
specific resistance of cake, cm./g. dimensionless parameter, defined by Equation 26 porosity of cake within holes soaking factor, defined by Equation 36 viscosity of water a t the operating temperature, centipoise density of particles, g./cc.
literature Cited
(1) Dickey, G. D., “Filtration,” pp. 30, 32, Reinhold, New York, 1961. ( 2 ) Grace, H. P., A.2.Ch.E. J . 2, 307 (1956). (3) Heertjes, P. M., Haas, H.v.d., Rec. Trau. Chim. 68,361 (1949). (4) Hermans, P. H., BredCe, H. L., J . Soc. Chem. Ind. 55T, 1
,- ,--,.
(IC)?&)
( 5 ) Jahreis, C. A., Chem. Eng. 70 (23), 237 (1963). (6) Miller, S. A,, Chem. Eng. Progr. 47, 497 (1951). (7) Perry, J. H., ed., “Chemical Engineers’ Handbook,” 4th ed., McGraw-Hill, New York, 1963. ( 8 ) Smith, E. G., Chem. Eng. Progr. 47, 545 (1951). (9) Wrotnowski, A. C., Zbid., 53, 313 (1957). RECEIVED for review August 4, 1965 ACCEPTED July 28, 1966
ION EXCLUSION EQUILIBRIA IN THE SYSTEM SU CROSE-POTASSI U M CH LORID EWATER-DOWEX 5 0 W X 4 General Correlation of Ion Exclusion Data WALTER
MEYER,’
R I C H A R D S. OLSEN, AND S. L. KALWANI2
Chemical Engineering Department, Oregon State University, Coruallis, Ore.
Wheaton ancl Bauman (77) introduced the principle of ion exclusion, they examined the possible application of the process to sugar purification. However, early attempts failed because of mutual exclusion of both sucrose and salt (NaC1) from the ion exchange resin. T h e process was later re-examined by Asher (2) who, using chromatographic columns, noted the effect of liquid flow rate, resin particle size, feed volume, column temperature, and degree of resin crossHEN
1 Present address, Nuclear Engineering Department, Kansas State University, Manhattan, Kan. 2 Present address, Chemical Engineering Department, Washington State University, Pullman, Wash.
linkage on the separation of sugar and salt. With a suitable choice of these variables, a well defined chromatographic separation was achieved. O n the basis of Asher’s work plus continued work by the Sugar Beet Laboratory, Western Regional Laboratory, U. 3. Department of Agriculture (75), 50- to 100-mesh (U. S. screen size) Dowex 50W X4 (a cation exchange resin of moderate crosslinkage) was shown to produce satisfactory separations of salt and sucrose in fixed bed ion exclusion columns. I t has also been suggested (75) that KCl is the simple salt most representative of the salts present in commercial sugar beet VOL. 6
NO. 1
JANUARY 1 9 6 7
55
The effect of temperature and sugar and salt concentration on equilibria in the system sucrose-KCI-waterDowex 50W X4 a t sucrose concentrations from 0 to 60 weight % and KCI concentrations of 0, 1 , 3, and 6 weight % has been measured. For 60 weight % sucrose, 0 weight % KCI, increasing temperature increased the amount of sucrose sorbed. From 60" to 90" C. the increase was 4.3 mg. of sucrose per dry gram of K+' resin per degree C. Equilibrium measurements a t 90" C. showed that the amount of sucrose sorbed decreased with increasing KCI concentration; a t fixed solution KCI concentration the increasing sohtion sucrose concentration increased the amount o f salt sorbed b y the resin. The average distribution ratio of sugar to salt in the resin and solution was 2.32 for solutions containing 1 weight salt; it decreased to 1.65 for concentrations of 3 and 6 weight %. These results show that sucrose can be separated from the salt b y successive batch or continuous countercurrent columnar treatment. The sucrose sorption data were correlated b y the n form of the BET multimolecular adsorption equation. This equation was applied to the correlation of other ion exclusion data and found to be generally applicable to systems where the sorbed solute shows a limited solubility in the solvent phase.
yo
molasses. Therefore Dowex 50W X4 resin and KCl were chosen for stud:7.
for the particular case of a 1-1 electrolyte then, the following relation applies (Y
Theory of Ion Exclusion Separation
In separating a n electrolyte and a nonelectrolyte by ion exclusion, the role of the ion exchange resin appears to be dual. With regard to the electrolyte the resin may be described as a second solution phase separated from the other solution by a hypothetical membrane which is permeable to all species present except RR, the fixed ionic groups of the resin matrix. T o the nonelectrolyte, however, the resin appears as a solid sorbent phase in contact with the solution phase. Taking up the electrolyte case first, based on the above system definition, the equilibria are described by the Donnan theory. This theory, described in detail by Helfferich ( 6 ) , shows that when a strongly ionized but dilute electrolyte, A,Y,, is brought into contact with a cation exchange resin whose ion exchange sites have previously been saturated with ion A , the resin will take on a negative charge with respect to the solution phase. This negative charge is the result of a slight excess migration of the coion or anion, Y , into the resin and a similar slight migration of the cation, A , into the solution phase. Both migrations are diffusion-controlled phenomena. I n effect, when the resin is placed in the electrolyte solution, the higher relative concentration of anions in the solution causes anion diffusion into the resin; the higher relative concentration of cations in the resin causes cation migration into the solution phase. T h e magnitude of the potential difference, E, between solution and resin phases due to the excess migration of anion into the resin phase is given by the Donnan potential as
where a y = activity of species Y in the solution, S, and resin, R, phases, respectively [activity as defined here is modified to include resin swelling effects ( 6 )1
F = Faraday constant E
=
valance of ion Y (negative by definition)
At equilibrium this potential will counterbalance the diffusion forces and thus prevent further concentration differencecontrolled migration of electrolyte ions bet\veen phases. Applying the Donnan theory further, a t equilibrium the activity product of the mobile electrolyte species in the resin phase will be equal to its activity product in the solution phase; 56
l & E C P R O C E S S D E S I G N A N D DEVELOPMENT
. m ) A R (Y . m ) Y R =
(Y
. m)As
(Y
.
m)Yg
(2)
where y = single ion activity coefficient and m = ion molality. Bearing in mind that the amounts of excess anion in the resin and excess cation in the solution are extremely small, the following relations can be written: mAR = mYR
+
mRR
(3)
and mA,q
= mYg
(4)
Using Equations 3 and 4, Equation 2 can be rearranged to give
where
If we assume for the moment that K = 1 (values of K are discussed further below), Equation 5 shows that for a monovalent salt a t an equilibrium solution concentration equivalent to mRR (minimum of about 3.5 mmoles per ml. of resin-contained water) the resin electrolyte concentration would attain a value no greater than 0.62 mRR. Thus the Donnan potential leads to an exclusion of an ionized species from the resin phase; this exclusion is favored by high values of mRR relative to mys. The nonelectrolyte in contact with the ion exchange resin will see no Donnan potential barrier to its solution within the resin phase. If the resin phase is viewed as simply a second solution phase entirely similar to the outer solution but separated from it by a boundary through which the nonelectrolyte can freely pass, the equilibrium concentration of solute in both solutions would be expected to be the same. I t also follows that all nonelectrolytes would show identical distributions between the resin and solution. Experimental measurements of nonelectrolyte distribution factors. however, have shown this reasoning to be faulty for many nonelectrolyte solutions ( 7 7 , 77). Correlation of Nonelectrolyte Sorption Data
There appears to be no one factor determining resin-nonelectrolyte-solvent equilibria ; factors which may be important have been outlined by Kolthoff and Elving ( 7 7 ) . These include attraction forces between the polar group of the solute and the fixed and mobile ions of the resin and between the
hydrocarbon part of the solute and the hydrocarbon resin matrix. There may be other important factors, such as the relative activity of \vat.er in the resin and solution phases, but those cited above are similar to those acting in physical adsorption and hence nonelectrolyte sorption data may be expected to follow the correlatiions of physical adsorption theory. If one thinks of the resin as a polyelectrolyte gel or second solution phzse, the application of adsorption theory (originally derived to explain the equilibria between a solid sorbent and a gas and later applied to the equilibria between a solid and a sorbed liquid) to the correlation of ion exclusion data \vould seem to be a considerable extension of adsorption theory. However, as Helfferich (8) notes, the ion exchanger in ion exclusion acts Lvith respect to the nonelectrolyte merely as a sorbent. Thus Helfferich notes that separation by ion exc!usion may be compared with conventional chromatography. As in chromatography, two solutes are separated by ion exclusion by a difference in the sorption strength of the two solutes; in ion exclusion the unique factor is the mechanism which causes this difference ( 8 ) . I n correlating ion exclusion data, the general practice has been to present them in the form of Freundlich or Langmuir isotherms. As long as i:he data fit the so-called type 1 isotherm (76): these correlations may be expected to be applicable. However, it is difficult to attach physical significance to a correlation by the Freundlich equation and if the data deviate from so-called monoinolecular adsorption the Langmuir chemical-kinetic equation will be inapplicable as well. If limited ion exclusion equilibrium data are to be extrapolated to conditions beyond those a t \vhich they were particularly measured, a generally applicable method of correlation is required. A more general data correlation ivould be in the form of the BET equation ( 3 ) ,which is also based on a kinetic adsorption mechanism
a,cX
a = ---
1-
x
+
+
1 - (n l ) X n nXn+' ~1 (c - 1)X - CX""
+
(7)
where a = amount adsorbed, grams/gram resin a, = amount of a.dsorbate to form a monolayer X = relative solution concentration, C/C, C = solution concentration C, = solution saturation concentration a t isotherm temperature c = temperature- and energy-dependent system constant n = number of monolayers adsorbed Equation 7 with n = 1 becomes
This is the Langmuir equation and thus Equation 7 is suitable for correlation of monomolecular adsorption data. If n becomes large (n > 5), Equation 7 becomes
which can be rearranged to the more common linear form, C a(C,
-
1 c-1c - -+-C) a,c a,c C,
the so-called m form of the BET multimolecular layer adsorption equation. Equation 7 is thus generally applicable and can be expected to give a better physical description of the adsorption process.
Measurement of Ion Exclusion Equilibrium Data
The original Dowex 50W X4 50- to 100-mesh resin was received in the wet potassium form. To prepare it for the equilibrium experiments it was rinsed with distilled water and placed in a buret where it was in contact with a stream of 57c KC1 which was sloizly passed through the buret for several hours. At the end of this time it was assumed that the resin was completely in the potassium form; the resin was again rinsed with distilled water and air-dried. The air-dried resin was further dried a t about 80' C. in a desiccator under vacuum until constant weight was attained (about 30 hours' drying time was required). Samples of this dry resin were then Lveighed into sample cells with care to prevent moisture adsorption by the resin. T h e equilibrium sample cells consisted of a n inner tube which held the resin supported on a 20 X 150 mesh stainless steel wire screen and a n outer concentric tube through \+?hi& water was passed from a temperature-controlled bath. T h e dried resin in the cells \vas further prepared for use by re\vetting it xvith a saturated KC1 solution (the use of a saturated salt solution in the rewetting process prevented shattering of the resin by too rapid swelling). T h e saturated solution was next gradually rinsed out of the resin with distilled water; the resin was then considered ready for the equilibrium experiments. The experimental solution of sucrose and KC1 was heated to the temperature of the experiment and then slowly passed through the experimental cell, which was maintained a t the same temperature. A sufficient volume of solution \vas passed through the cell to bring the resin and solution into equilibrium, determined in several initial experiments as that volume beyond which no further increase in solution volume would increase the amount of sucrose on the resin. I n later experiments this volume, plus a suitable volume increment to ensure an equilibrium state, was used. After equilibration the resin cell was disconnected from the controlled temperature bath and the cell quickly placed in a centrifuge, T h e cell and its contents were then subjected to an average force of 337 G's for 4 minutes. This force and time bvere sufficient to remove the interstitial sucrose-KC1 solution from the resin bed. To determine the amount of sucrose and KC1 in the resin, the cell was reconnected to the temperature bath and the sugar and salt were eluted with distilled \vater a t the same temperature a t \vhich the resin was loaded. Distilled water \vas passed through the resin until the eluent showed no trace of sucrose or KC1. T h e weight of the eluent solution \vas determined and the chloride concentration of the solution was measured by Mohr's method. Attempts to use a polarimeter for sucrose determination in the presence of KC1 failed because of salt interference. Salts could be removed for polarimeter analysis, but the procedure is laborious and the results obtained in any case with the polarimeter \rere not considered accurate enough to warrant its use. Therefore the sucrose concentration of the eluent was determined by a precision optical refractometer and standard tables, Measurements shoived that no significant sugar inversion occurred and that the presence of KC1 did not interfere with the refractometer measurements. Presentation and Correlation of Equilibrium Data
T h e effect of temperature on the adsorption of sucrose from a 60 weight % sucrose-0 weight KC1 solution is shown in Figure 1. I t was known in chromatographic separations of sucrose and salt that the separation improved with temperature (2). This result is now shown to be a result not simply of kinetic effects but also of an improved equilibrium condition. T h e reaction equilibrium describing the sorption of species I from solution onto the resin may be written as f o l l o ~ s :
Is
+ RR * IR
(10) As in other types of equilibria, the temperature dependence of Equation 10 can be related to the standard enthalpy change which accompanies the reaction as follows: VOL. 6
NO. 1
JANUARY 1967
57
z X
I
I
-+Y Y
> a
. Y
0 VI U
0 U
60
I
I
I
70
80
DO
VI VI
4 I
TEMPERATURE, DEGREES C.
0
Figure 1. Resin loading as a function of temperature for 60% sucrose solutions
W
g
0
a
Y
0 6 WEIGHT PERCENT
0 02
The only reported calorimetric measurements on ion exchange resin have involved the ion exchange process; measurements show (7) that the heat effect in this case is less than 2 kcal. per gram mole exchanged. I t is probable that the forces involved in ion exclusion are less specific or weaker than in ion exchange and therefore the heat effect will be even smaller. If AH" is small, the temperature effect will also be small, but if Equation 11 is the only controlling factor, an increase in temperature in any case will decrease the amount adsorbed. T h e controlling factor then, rather than Equation 11, may be an increase in the flexibility and volume of the resin matrix with temperature, which permits a greater number of sucrose molecules to squeeze into the resin structure as the temperature increases. A systematic study of this temperature effect was not made here but would seem to be warranted, since the effect of temperature on sorption equilibria is complex and a t present not well understood (6). On the basis of the data of Figure 1 and the fact that ion exclusion column kinetics will improve with increasing temperature, it was decided to measure further equilibrium data a t 90' C. The temperature was limited to 90°, since this appeared to be an optimum operating temperature for planned future column work. Figure 2 shows Freundlich-type sucrose adsorption isotherms a t 90" for KC1 solution concentrations of 0, 1, 3, and 6 weight yo. At these same KC1 solution concentrations, Table I shows the resin KC1 sorption as a function of sucrose solution Concentration. I n terms of grams of sucrose per gram of resin the mere presence of the salt seriously affects sucrose adsorption; however, increasing the weight per cent salt sixfold decreases
Table 1.
Solution Concn., Weight Sucrose 10
20 30 40
50 '60
58
KCI Sorption Isotherm Data for System KCI-SucroseWater-Dowex SOW X4 at 90" C. 7
Solution Salt Concn.. W e i e h t 7 0 3 6 Resin concn., gram KCl/gram resin
0.00264 0.00331 0.00360 0.00370 0.00376 0.00419
0.0123 0.0127 0.0135 0.0139 0.0160 0.0177
0.0312 0.0332 0.0363 0.0374 0.0375 0.0391
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
sucrose adsorption an average of only 21 weight yo. Lsing the data of Figure 2 and Table I, Figure 3 was drawn; this figure shows that the distribution factor, D, where
D =
(grams sucrose/gram KCl). (grams sucrose,/gram KCI),
is greater than 1. D increases with increasing sucrose solution concentration but decreases with increasing salt solution concentration. Over the linear portions of the curves shown in Figure 3, D,,for 1 weight 70 KC1 is 2.32; for 3 and 6 weight % KC1 it is 1.65. These results show that simple batch elution of the equilibrium resin contents would lead to a n increase in sucrose purity. T h e decrease in resin sucrose capacity with increasing KC1 solution concentration can be explained by two phenomena. T h e first is a decrease in resin volume with increasing solute concentration due to a reduction of resin water content. As solute concentration in the solution phase increases, the solvent water activity falls relative to solvent activity in the resin. T h e relative decrease in solvent activity, however, is in turn balanced by a water loss by the resin, which tends to bring solvent activity in the solution and resin phase back into balance. T o examine the effect of resin swelling on sucrose adsorption, the bulk volume, uB, of the resin as a function of sucrose and KC1 concentration was determined as shown in Figure 4. Using Figure 4, the information presented in Figure 2 was redetermined to eliminate the swelling effect due to resin water loss in solutions of increasing KC1 and sucrose concentration. These data, in terms of grams of sucrose adsorbed per milliliter of bulk resin, showed a closer grouping; however, the data a t the four KCI concentrations still fall on four distinct curves. This result may have been due to only a partial elimination of the resin swelling effect. To decrease further resin swelling effects the data noted above were corrected by eliminating the resin matrix volume. I n this case the data would be reported as grams of sucrose adsorbed per unit volume of resin-contained liquid. The volume of this liquid, uR, was computed as
Y
ties (moles of KCl per liter of resin-contained liquid) exceed the solution molalities. This result is, of course, not what would be typically expected in terms of Donnan theory, but is no violation of theory in that it merely reflects a drastic increase in the KC1 activity coefficient ratio of Equation 6 in the presence of the nonelectrolyte. To correlate the sucrose adsorption data the general form of the BET equation (Equation 7) was applied. I n applying this relation, which contains three unspecified constants, the equation was rearranged to the following linear form suggested by Joyner et a / . (9):
20
0
y
100
v) v)
eo
[L
u- 6 0 (0
I
. I 40
z v1
y
20
0 00
30
20
10
SOLLJTION
40
SO
PHASE, GRAMS SUCROSE I GRAM K C I
Figure 3. Ratio of sucrose t o KCI in resin vs. ratio of sucrose to KCI in solution a t 90" C. 0 1 weight 0 3 weight d 6 weight
SOLUTION CONC
+(n,X)
70
KCI
0
0 WEIGHT PERCENT
0
I
WEIGHT PERCENT
3 WEIGHT PERCENT 6 WEIGHT PERCENT
;$I
00
X(l =
% %
25
0
where
,
,
,
I
02
04
OB
on
I
IO
LIQUID FRACTION, GRAMS SUCROSE IGRAM WATER
Figure 4. Resin lbulk vo!urne a s a function of solution solute concentration a t 90" C.
where 0.62 = solids fraction in the wet settled resin bed (measured resin bed porosity is 0.38) u B = wet settled resin bulk volume, ml. per gram dry K+1 resin u . , ~ = specific volume of the resin matrix, 0.584 ml. per gram dry K+' resin Figure 5 shows the data of Figure 2 in terms of sucrose adsorbed per milliliter of resin-contained liquid. T h e data obtained in the presence of KCl now all fall close to one curve, but are still distinctly different from those obtained in the absence of KC1. Thus some factor other than swelling is influencing the system equilibria. This second factor appears to be a strong interaction between sucrose and KC1 in solution. Some interaction was expected, since it was known that KCl increases the aqueous solubility of sucrose (70, 73). To examine this interaction further, the K values of Equation 0 were computed from Equation 5 and compared with data in the literature (5) for KCl aqueous solutions (Figure 6). In the absence of sucrose, K values are typically on the order of 1 to 10. IVith the sugar present, although the K values show the same typical trend with KC1 solution molality ( 5 ) , they are two orders of magnitude higher and increase with sucrose concentration. I n fact, the interaction between the sucrose and KC1 is strong enough that KCl resin phase molali-
- Xn) - nXn(I - X) (1 - X)*
Equation 14 has previously been programmed (72) and the best value of n within a deviation of 0.062 was determined with this computer code. Figure 7 is a plot of +(n,X)/a us. e(n,X) for the best values of n for the data shown in Figure 2. From this figure the values of a, (from the reciprocal of the slope) and c (from the reciprocal of the intercept) were determined by linear least squares analysis (Table 11). The adsorption data are satisfactorily correlated a t intermediate and high sucrose concentration ; however, a t low concentrations, the rapid drop in a , the weight of sucrose adsorbed, causes $ ( n , X ) / a to increase toward 0 5 . This effect becomes more pronounced as KC1 concentration increases and is probably due to the strong KC1-sucrose interaction noted earlier. If, as described above, the volume of the resin shrinks with increasing KCl concentration, the general decrease in a, shown in Table I I A would be expected. T h e effect of resin shrinkage can be partially eliminated by treating the sucrose
Table II.
linear least Squares Analysis of BET Data am,
Grams Sucrose/ W e i g h t 70 Salt Gram Resin n C A. Amount Adsorbed in Grams Sucrose per Dry Gram Resin at 90" C.
B.
0 1
3 6 C.
0 1
3 6
Amount Adsorbed in Grams Sucrose per M1. Wet Settled Resin at 90" C. Grams Sucrose/ MI. Resin 1.625 1.625 1.625 1.750
0.4132 0,4286 0.4484 0.4184
2.85 1.96 1.77 1.82 Amount Adsorbed in Grams Sucrose per M1. Resin-Contained Liquid at 90' C.
1.625 1.625 1.625 1.750
Grams Sucrose/ MI. Resin Fluid 0.961 1.041 1.150 1,150
VOL. 6
NO. 1
2.89 1.88 1.64 1.61
JANUARY 1 9 6 7
59
a W
600
-
B K i 400\
& 300ln
w' 200-
SOLUTION
CONC.
KCI
ln
-
LL W
LIQUID FRACTION SUCROSE, GRAMS SUCROSE/GRAM WATER
Figure 5.
Isotherms a t
90" C. for system sucrose-KCI-water-Dowex
% weight %
0 0 weight 0 1 300 I
50W X4
% 0 6 weight % 8 3 weight
I
IO0
0 0 0
10
20 30
00
0
"
t NOTE:
6.
"
6 WEIGHT PERCENT
WEIGHT PERCENT WEIGHT PERCENT
0,
02
03
04
05
0 0
Figure 7. Solution of n form of BET equation for system KCI-sucrose-water-Dowex 50W X4 at 90" C.
CURVE ESTIMATE0 FROM DATA OF DAVIES AND YEOMAN FOR K C I IN AQUEOUS SOLUTION (5).
Equation
6
OF KCI
IN SOLUTION PHASE
values as a function of
KCI
molality
adsorption data in terms of grams of sucrose adsorbed per milliliter of wet settled resin; the BET analysis of the data in this form is shown in Table IIB. I n this case, however, the a, values show a n inconsistent trend. T o eliminate further resin shrinkage effects the adsorption data in terms of grams of sucrose adsorbed per milliliter of resin-contained liquid were analyzed by the BET technique (Table I I C ) . I n this latter case the a, values again show a consistent trend with increasing KCl conceniration. 60
0
8(n , X )
MOLALITY
Figure
3
PERCENT
"
"
60
I
0
"
40 n 50 0
1
WEIGHT
0
I & E C PROCESS D E S I G N A N D DEVELOPMENT
I n the light of the data shown in Figure 2, a comparison of the am values in Tables 11, A and C, indicates that the interaction of t w o factors is controlling the degree of resin sucrose adsorption. The first factor is resin water content; as the water content falls with increasing KC1 concentration the amount of adsorbed sucrose also decreases. T h e relation between resin water content and KC1 solution concentration has been noted earlier. As resin water content falls, however, the a, values in Table I I C show that the amount of adsorbed sucrose required to form a monolayer over the adsorption sites does not fall in proportion to the water loss. In fact, the amount of adsorbed sucrose required to form the monolayer increases and then becomes constant with increasing KC1 concentration. This result may be due to a change in the resin adsorption site structure. As the resin shrinks with
increasing salt concentration, the adsorption site structure is compressed and conr,olidated. The trend of a, values in Table IIB also indicates that such a structural change occurs. Thus the second factor affecting sucrose adsorption is, as it must be under the initial considerations laid down in this analysis, the attraction forces between the resin matrix and the sucrose molecules. Unlike the typical adsorbent, however, the resin in ion exclusion is subject to shrinkage effects which are determined by the character and concentration of the components of the solution phase. Referring to Table I1 again, values of the BET constant, c, are listed. This constant is generally regarded (3) to be absorption energy, E'& and temperature-dependent; G is approximately given by the relation I'
(15)
= ,-Eads/RT
The general drop in c found in Table I I C can be interpreted as an increase in the energy barrier to sucrose adsorption, with Eads increasing with salt concentration. T o test the applicability of Equation 14 further, it was applied to the ion exclusion data of Anderson and Hansen (7) for the sorption of phenol and p-nitrophenol on Dowex 2C1-, to the data of Shurts and White (74) for the sorption of glycerol and sodium chloride on Dowex-50, and to the data of Davies and Thomas (4) for the sorption of five organic acids on two ion exchangers. In each case a satisfactory correlation was obtained for those data where the solute showed a limited solubility in the solvent phase. For those systems where the solute was infinitely soluble in the solvent (the saturation concentration, C, in this case \vas taken as 100% solute), Equation 14 did not apply. This failure in completely miscible systems is probably a result of the fact that the variations in solution activity with concentration can no longer be adequately represented by the ratio C/Co.
Figure 8 shows a plot of +(n,X)/a us. O(n,X) for the phenol and p-nitrophenol sorption data of Anderson and Hansen; Table I11 presents the least squares analysis of the Figure 8 data. The correlations shown in Figure 8 are typical of those noted for the adsorption of vapors on solid adsorbents. The curvature noted for the phenol data (too great amounts of adsorbate sorbed a t low relative concentrations) by ana!ogy to the experience with vapor-solid adsorption is believed to be due to accelerated adsorption on the more active resin adsorption sites at small values of X.
Table 111. linear Least Squares Summary of Data for Phenol and p-Nitrophenol Adsorption on Dowex 2 CIa,,,, Grams/ n Gram Resin C
Phenol p-Nitrophenol
1.000 1.875
1.484 1.258
1.96 1.93
Conclusions
Equilibrium measurements for the system sucrose-KC1water-Dowex 50W X 4 a t 90' C. show that separations of sucrose and KC1 by batchwise or continuous processing is possible. Increasing solution salt concentration causes the amount of sorbed sucrose and water to decrease; conversely, increasing sucrose solution at fixed solution salt concentration leads to increased resin salt adsorption. The average ratio of sucrose to KC1 distribution in resin and solution is 2.32 a t 1% KCl but decreases to 1.65 a t 3 and 6% KC1. The effect of KC1 on the equilibria appears to be twofold. First, the salt tends to dehydrate the resin. Secondly, the salt interacts strongly with sucrose solutions to reduce sucrose sorption in the resin. Sucrose adsorption increases with temperature, 4.3 mg. of sucrose per dry gram of K+1 resin per degree C. This result is not expected in the light of enthalpy considerations but may be a result of an increasing volume and flexibility of the resin matrix with temperature. The sucrose sorption data have been correlated by application of the n limited form of the BET equation. Application of this same relation to other ion exclusion data shows that it is generally applicable to the correlation of such data where the solute shows a limited solubility in the solvent phase. Nomenclature a
=
a,
= = = =
al C
C,
= = E = F = AHo =
c
D
I K 00
01
02
03
04
05
06
07
08
09
10
I1
8(n,xl
Figure 8.
Solution of n form of BET equation
12
k, mr n
=
= =
= =
amount of solute sorbed by resin, grams sorbed/gram dry K+' resin, or grams sorbed/ml. of resin, or grams sorbed/ml. of resin-contained liquid amount of sorbed solute required to form monolayer activity of component I solution concentration solution saturation concentration at temperature of experiment temperature-dependent constant of BET equation distribution factor Donnan potential Faraday constant standard enthalpy change of reaction sorbed species activity coefficient ratio (see Equation 6) thermodynamic equilibrium constant molality of component I, moles per liter of solution constant of BET equation proportional to number of monolayers sorbed VOL. 6
NO. 1 J A N U A R Y 1 9 6 7 61
R
=
T
= absolute temperature
u
= = = =
X y
gas law constant specific volume, ml./gram relative concentration, C/C, molal activity coefficient ionic valence of anion Y
SUBSCRIPTS B M
R S
= bulk resin
resin matrix resin phase = solution phase = =
literature Cited
(1) Anderson, R. E., Hansen, R. D., Znd. Eng. Chem. 47,71 (1955). (2) Asher, D. R., Ibid., 48, 1465 (1956). (3) Brunauer, S., Emmett, P. H., Teller, E. S., J . Am. Chem. SOC. 60, 309 (1938). (4) Davies, C. W., Thomas, G. G., J . Chem. Soc. 1951, p. 2624. ( 5 ) Davies, C. W., Yeoman, G. D., Trans. Faraday SOC. 49, 968 (1953). (6) Helfferich, F., “Ion Exchange,” pp. 132-46, McGraw-Hill, New York, 1962.
(7) Zbid., p. 166. (8) Ibid.. D. 433. (9) JoynLr, L. G., Weinber er, E. B., Montgomery, C. W., J . Am, Chem. Soc. 67. 2182 (19457. (10) Kelly, F. H. C., J . Appl. Chem. 4, 401 (1954). (11) Kolthoff, I. M., Elving, P. J., “Treatise on Analytical Chemistry,” Part I, Vol. 3, Interscience, New York, 1961. (12) Mever. W.. “Solution of the Limited Form of the BET Multimolecular Adsorption Equation,” Computer Manual No. 9, A.I.Ch.E., New York, 1961. (13) Rorabaugh, G., Norman, L. W., J . Am. SOC.Sugar Beet Technologists 9, 238 (1956). (14) Shurts, E. L., White, R. R., A.I.Ch.E. J . 3,183 (1957). (15) Stark, J. B., “Ion Exclusion Purification of Molasses,” unpublished work, Western Regional Research Laboratory, USDA, Albany, Calif., 1964. (16) Weiser, H. B., “Colloid Chemistry,” 2nd ed., p. 57, Wiley, New York, 1958. (17) Wheaton, R. M., Bauman, W. C., Ann. N . Y.Acad. Sci. 57, 159 (1953). I - - - -
\ - -
- I
RECEIVED for review November 22, 1965 ACCEPTED September 28, 1966 Work supported by the Western Regional Laboratory, U. S. Department of Agriculture, Engineering Experiment Station, Oregon State University, and Engineering Computer Laboratory and Nuclear Engineering Department, Kansas State University.
EFFICIENCY M E A S U R E M E N T S ON A N ALL-GLASS WIPED-FILM S T I L L The performance of an all-glass laboratory-scale wiped-film evaporator has been evaluated with a dodecane-octadecene mixture at varying feed rates, temperatures, pressures, and rotating speeds of the glass wiper. Separation sharpness, split ratios, coefficients of heat transfer, and residence times were determined. Analyses of both the overhead and bottom product indicate that the observed separations correspond closely to theoretically computed values obtained from the smoothed out data of Jordan and Van Winkle using the equations for a simple batch distillation. Based on analogous findings of Kirschbaum and Dieter on a commercial-size wiped-film evaporator, the data obtained on these laboratory-scale stills can, therefore, be used to demonstrate the feasibility of extrapolating to commercial-size film evaporators.
T. H .
G O U W A N D R . E. J E N T O F T
Chevron Research Co., Richmond, Calif.
W I P E D - f i l m evaporators are currently being used more often both in the laboratory and in industrial practice for the stripping of lighter boiling material from a heat-sensitive feed. Operatio’n is usually carried out under high vacuum and a t relatively low temperatures. Application is usually restricted to systems with key components of widely different boiling points. In the processing of new products where a wiped-film evaporator may be incorporated in the plant design, it is necessary to know whether this apparatus would yield the desired separation sharpness a t the expected split ratios. T h e absence of thermodynamic and physical data on these components and the availability of probably only a small quantity of material make it necessary to test the mixture physically on a laboratory-scale film evaporator. A large number of laboratory-scale wiped-film evaporators have been described and are commercially available. However, poor temperature control is evident in many of these stills. In addition, in some stills metal surfaces come in contact with the hot product, with resultant danger of catalytic decomposition of the product material. The described unit does not have these disadvantages. These studies were carried out to investigate the possibility of extending data obtained on a laboratory-scale wiped-film evaporator to plant-size operation. Besides confirming the 62
l&EC PROCESS DESIGN AND DEVELOPMENT
feasibility of using this type of evaporator in the commercial design, additional data may be obtained on the properties of the tested products. I t is evident that this knowledge would be very helpful in designing and operating commercial-size film evaporators. Apparatus
A schematic diagram of the wiped-film still and the accessory equipment is given in Figure 1, A glass spiral (7) is used to form the thin film on the evaporating surface. The wiped tube has a n effective evaporating surface of 255 sq. cm. and is kept a t the desired temperature by the heating fluid, which is recirculated from a thermostat. DC-710 silicone oil (Dow Corning Corp., Midland, Mich.) is used as the heat transfer medium. T o ensure a good heat control, a large thermostat (4-liter liquid content) is used together with a centrifugal liquid recirculating pump rated a t 250 ml. per minute. One-inch thick foamed insulation is used around the lines between the thermostat and the still. T h e binary mixture is pumped from the feed graduate by a variable-feed pump into the still. The check valves have ratings slightly higher than the operating vacuum in the still to ensure a positive pressure a t the outlet of the pump. This is necessary for a fine adjustment and reproducible operation of the feed rate. The feed is preheated in the small heat exchanger outside the evaporator proper before being deposited on the wiped
