equivalent weight polyols. Properties are usually the best when the NCO/OH ratio is 1.5 or greater. A wide range of physical properties were built into the plastics by properly selecting the poly01 and the amounts of isocyanate and starch. The range of properties these relatively inexpensive materials provide suggests that starch and starch-derived polyols have potential for the commercial production of urethane plastics. Acknowledgment
We thank A. J. Ernst, R. G. Fecht, and D. E. Smith for their assistance with physical measurements, and V. F. Pfeifer and L. H. Burbridge for milling the starch and determining its particle size. literature Cited
Bennett, F. L., Otey, F. H., Mehltretter, C. L., J. Cell. Plast. 3, 369 (1967). Boggs, F. W. (to United States Rubber Co.), U. S. Patent 2,908,657 (Oct. 13, 1959).
Buchanan, R. A., Weislogel, 0. E., Russell, C. R., Rist, C. E,, IND. ENG. CHEM.PROD.RES. DEVELOP.7, 155 (1968). Otey, F. H., Bennett, F. L., Mehltretter, C. L., U. S. Patent 3,405,080 (1968a). Otey, F. H., Bennett, F. L., Zagoren, B. L., Mehltretter, C. L., IND. ENG. CHEM.PROD.RES. DEVELOP.4, 228 (1965). Otey, F. H., Mehltretter, C. L., Rist, C. E., Cereal Sci. Today 13 (5), 199 (1968b). Patton, T. C., Ehrlich, A., Smith, M. K., Rubber Age 86,639 (1960). RECEIVED for review December 16, 1968 ACCEPTED May 19, 1969 Division of Organic Coatings and Plastics Chemistry, 157th Meeting, ACS, Minneapolis, Minn., April 1969. The Northern Laboratory is part of the Northern Utilization Research and Development Division, Agricultural Research Service, U. S. Department of Agriculture. Mention of firm names or commercial products does not constitute an endorsement by the U. S. Department of Agriculture.
PERFORMANCE OF POROUS CELLULOSE
ACETATE MEMBRANES IN SOME REVERSE OSMOSIS EXPERIMENTS J.
K O P E C E K ' A N D
s.
S O U R I R A J A N
Division of Applied Chemistry, flational Research Council of Canada, Ottawa, Canada
The effects of surface layer-side and backside reverse osmosis operations on the performance of the Loeb-Sourirajan type porous cellulose acetate membranes have been studied at the operating pressures of 100 and 250 p.s.i.g. The mechanism of reverse osmosis transport is the same in both operations. The relative selectivity of the membrane for different solutes changes with operating pressure and membrane pretreatment. Backpressure treatment offers an effective method of improving membrane performance. The results of cyclic experiments at pressures up to 250 p.s.i.g. indicate the possibility of utilizing the backpressure treatment technique for simultaneous brackish water conversion.
THEselectivity of the Loeb-Sourirajan type porous cellulose acetate membranes, and the effects of compaction and backpressure treatment on their performance at operating pressures of 600 p.s.i.g. or higher, have been discussed (Agrawal and Sourirajan, 1969; Kimura and Sourirajan, 1968; KopeEek and Sourirajan, 1969). This paper gives similar results for operating pressures of 100 and 250 p.s.i.g. with membranes initially pressure-treated a t 120 and 300 p.s.i.g., respectively. This work is of interest from the point of view of both the effect of porous structure and backpressure treatment on the performance of reverse osmosis membranes, and the design of low pressure reverse osmosis units for brackish water conversion. Present address, Institute of Macromolecular Chemistry, Prague, Czechoslovakia 274
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
Experimental Details
Reagent grade chemicals and porous cellulose acetate membranes (designated here as CA-NRC-18 type films), made in the laboratory, were used. The films were cast a t -10" C. in accordance with the general method described earlier (Sourirajan and Govindan, 1965), using the following composition (weight per cent) for the film casting solution: acetone 68.0, cellulose acetate (acetyl content = 39.8%) 17.0, water 13.5, and magnesium perchlorate 1.5. Membranes shrunk a t different temperatures were used to give different levels of solute separation a t preset operating conditions. The apparatus and the experimental procedure have been reported (KopeEek and Sourirajan, 1969; Sourirajan, 1964). All experiments were carried out a t the laboratory
temperature. Unless otherwise stated, the experiments were of the short-run type, each lasting about 2 hours; membranes were initially subjected to pure water pressures of 120 and 300 p.s.i.g., respectively, for 1 to 2 hours on the surface layer side of the film (normal pressure treatment) for subsequent reverse osmosis operation a t 100 and 250 p.s.i.g.; the feed solution concentration was about 0.0172M (equivalent to about 1000 p.p.m. of solute in the NaCl-HZO system); and the feed rate was maintained at about 300 cc. per minute. Sodium chloride, sodium sulfate, magnersium chloride, and magnesium sulfate were used as solutes. Reverse osmosis experiments were carried out either in the usual manner with the surface layer side of the membrane facing the feed solution (surface layer-side operation), or with the backside of the film facing the feed solution (backside operation) using either the normally pressure-treated or the backpressure-treated membrane. The backpressure treatment (KopeEek and Sourirajan, 1969) consisted of pumping pure water a t the laboratory temperature past the backside of the normally pressure-treated membrane a t the intended reverse osmosis operating pressure-Le., 100 or 250 p.s.i.g.-for 80 hours. The product rates reported are those corrected to 25”C. using the relative viscosity and density data for pure water. In each experiment, the solute separation, f, defined as
f=
molality of feed (ml) - molality of product (ma) molality of feed (ml)
the product rate, and the pure water permeability [PWP], in grams per 7.6 sq. cm. of effective film area, were determined at preset operating conditions. In all cases, the terms “product” and “product rate” refer to the membrane-permeated sdutions. The concentrations of the solute in the feed and product solutions were determined by specific resistance measurements. The accuracy of the separation data is within 1% and that of product rate and [PWP] data is within 3% in all cases. The reverse osmosis experimental data were subjected to the Kimura-Sourirajan analysis (Kimura and Sourirajan, 1967, 1968; Sourirajan and Kimura, 1967) and the solute transport parameters, (DaM/K6), were calculated under the operating conditions using the osmotic pressure data of Sourirajan (1969). The parameter ( D A M I K ~ ) (expressed in centimeters per second) plays the role of a mass transfer coefficient with respect to solute transport through the membrane, and hence it is treated as a single quantity for purposes of analysis. A represents simply the [PWP] data in units of gram moles of HzO per (sq. cm.) (sec.) (atm.).
F I L M TYPE CA N R C - 1 8 SYSTEM SODIUM CHLORIDE-WATER FEED CONCENTRATION 0.1 11 % FEED RATE 3 0 0 ~ 6/ m l n u l e I SURFACE LAYER SIDE OPERATION 210 p i , II BACK SIDE OPERATION 250 p s
I V BACK SIDE OPERATION 100 P I I p
F I L M AREA 7.6 (4 c m
I-
v a n
-
0
0
0.2
0.4
0.6
0.8
SOLUTE SEPARATION, f Figure 1. Effect of surface layer-side and backside reverse osmosis operations on membrane performance
backside operation would not be expected a t operating pressures less than 14 atm. for the type of films used in this work. During backside operation, both the pure water permeability constant, A , and the solute transport parameter, ( D A M / K increase, ~), as illustrated in Table I. These data constitute definite experimental proof that, as stated earlier (KopeEek and Sourirajan, 1969), pores open wider during backside operation. One may suggest concentration polarization within the membrane as an alternative explanation for the results obtained during backside operation. This suggestion is unacceptable for the following reasons. Concentration polarization must reduce product rate because of increase of osmotic pressure of the boundary solution. The experimental observation is that product rate increases during backside operation. Concentration polarization does not necessarily mean reduced solute separation. The latter depends on feed concentration, and the mass transfer coefficient on the high pressure side of the membrane. At low feed concentrations ( < 0 . 5 M ) , solute separations may actually increase
Table 1. Effect of Surface layer-Side and Backside Reverse Osmosis Operations on A and ( DAM/KG) Film type. CA-NRC-18 System. Sodium chloride-water Feed concentration. 1000 p.p.m. NaCl Feed rate. 300 cc./minute Operating pressure. 100 p.s.i.g.
Results and Discussion
Results of Surface Layer-Side and Backside Reverse Osmosis Operations. These are illustrated in Figure 1, where line 1 gives the solute separation us. product rate data for a set of films during surface layer-side operation, and line I1 gives corresponding data for the same films during backside operatiion at 250 p.s.i.g.; lines I11 and IV give similar data for another set of films at 100 p.s.i.g. At both operating pressures, backside operation decreases solute separation and increases product rate. The effect, however, is more at 250 than a t 100 p.s.i.g. This is consistent with the observation made earlier (KopeEek and Sourirajan, 1969) that drastic change in performance on
g
Ill SURFACE LAYER SIDE OPERATION I O O p i I o
Surface LayerSide Operation
Backside Operation A x lo‘,
Film No. 1 2 4 6
A x lo6, (g.mole
(g. mole ( D A M I K ~ ) H10)l (sq. cm. H 2 0 ) i(sq. cm. x lo”, sec. atm.) em.jsec. see. atm.)
1.190 1.283 3.621 5.043
0.90 0.975 17.25 24.57
1.479 1.503 3.928 5.649
( D + M / K x~ ) lo”, cm./see.
2.476 2.520 47.18 104.61
VOL. 8 N O . 3 S E P T E M B E R 1 9 6 9
275
with increase in feed concentration (Agrawal and Sourirajan, 1969; Kimura and Sourirajan, 1967, 1968; Sourirajan and Govindan, 1965). That this is so under the conditions of the experiments used in this work is illustrated in Figure 2. Thus while concentration polarization does not necessarily mean reduced solute separation, reduced solute separation during the backside operation under the experimental conditions used in this work can occur only if there is pore widening. The solute transport parameter (DAMIKB) is independent of concentration polarization for the system sodium chloride-water (Kimura and Sourirajan, 1967; Sourirajan, 1969). Figure 3 illustrates that this is the case in both surface layer-side and backside operations. Consequently, the increase in ( D a ~ l K 6 obtained ) during backside operation (Table I) cannot be due to concentration polarization.
- OPERATING
I .c
On the other hand, an increase in both A and (DAM/ K6) must necessarily indicate pore widening (Agrawal and Sourirajan, 1969; Kimura and Sourirajan, 1968). Results of Continuous Test Runs. Using NaC1-HzO feed solutions (- 1000 p.p.m. of NaCl), continuous 3-day test runs were conducted both with surface layer-side and backside reverse osmosis operations. A number of films covering a wide range (20 to 90%) of solute separations were used in these experiments. The results obtained at operating pressures of 100 and 250 p.s.i.g. were similar. Most of the compaction took place in the first 24 hours. For all films giving more than 50% solute separation, the product rate obtained a t the end of 72 hours of continuous operation was more than 90% of their initial product rates. For less dense membranes with more porous surface structures, the compaction effect was more. The analysis
OPERATING PRESSURE 2 5 0 p s.1.g
PRESSURE: 100 p . s . i . g
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SURFACE L A Y E R SIDE OPERATION BACK SIDE OPERATION
I
I
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-
F I L M AREA: 7.6 sq,cm.
----A
4
n
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I
FILM T Y P E C A - N R C - 1 8 S Y S T E M SODIUM CHLORIDE -WATER F E E D R A T E 300 c c / m i n u t e
8t-
t
F I L M AREA
: 7.6
sq. c m .
---e-- -- 2I -------e
-,----
4-
h
I
I
IO00
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I
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I
I
1500
I
I
IYI
17,-
2000
2500
FEED CONCENTRATION p.p.m. NaCl
FEED CONCENTRATION p . p . m . NaCl
Figure 2. Effect of feed concentration on membrane performance during surface layer-side and backside reverse osmosis operations 276
I & E C PRODUCT RESEARCH A N D DEVELOPMENT
c
L
/
4
-
i
A
0 6 4 F I L M TYPE
I
F I L M NO. 6 d
c
n
t
d
A
CA-NRC-18
(*)K 6
S Y S T C M SODIUM CHLORIDE-WATER OPERATING PRESSURE 100 p 3 D
0 0 A
0
- SURFACE
,
SOLUTE lo-
41
LAYER
SIDE OPERATION
crn. see
A e - BACK SIDE OPERATION
‘0-5k
t
,
o
-
6
0.02
’
1
F I L M T Y P E C 4 - N R C - 18 I I I I 1, I S Y S T E M 0 MAGNESIUM CHLORIDE-WATER
A
0.03
0.04
A SODIUM S U L F A T E - W A T E R
FEE0 M O L A L I T Y
0 MAGNESIUM SULFATE - W 4 T E R
Figure 3. Effect of surface layer-side and backside reverse osmosis operations on solute transport parameter VS. feed molality correlation
10.3
.UTE
of data obtained in th,e continuous test runs with surface layer-side operation showed that ( D A MK6) / remained constant throughout the test period for all the films tested (Kimura and Sourirajan, 1968). Membrane Selectivity. The variation in the values of ( D A ~ / K 6for ) different solutes obtained with a given membrane offers a method of expressing membrane selectivity for different solutes. This has been illustrated and discussed (Agrawal anjd Sourirajan, 1969). Figure 4 gives a log-log plot of (DAM/Ks) for NaCl US. (DAM/Kd) for MgSO.,, Na2S04, and MgCL at the operating pressures of 100 and 250 p.s.i.6;. Agrawal and Sourirajan (1969) found that similar selectivity plots were essentially straight lines a t 1500 p.s.i.g. The Kimura-Sourirajan analysis itself does not require that such plots should be straight lines. Figure 4 illustrates that such plots are not straight lines a t low operating pressures, and, further, the relative mag) are different for different nitudes of ( D A M I K ~values operating pressures. Since selectivity data are of great practical importance, Figure 4 points out the need for obtaining such data experimentally for different operating pressures with membranes prepared under different conditions and subjected to different initial pressure treatments. Performance of Backpressure-Treated Membranes. In Figure 5 , line I gives the product rate us. solute separation data for a set of five films shrunk a t different temperatures (covering solute separations in the range 20 to 95%) without backpressure treatment, and line I1 gives the corresponding data for the same films with backpressure treatment. All the results given in Figure 5 are for surface layer-side operation a t 250 p.s.i.g. For membranes giving solute separation greater than about 30%, line I1 is considerably above line I. With respect to the performance of individual membranes, backpressure treatment resulted in an increase in solute separation in all cases, indicating the preferential opening of the smaller size pores on the membrane surface; but the product rates increased only for membranes giving greater than about 50% solute separation. With the less dense and more porous membranes,
OPERATING PRESSURE
-
loops I g
---
( h M ) KB
N ~ C I
cm. sec.
Figure 4. Relative scale of membrane selectivity for operating pressures of 100 and 250 p.s.i.g.
I20
TYPE C L - N R C - I 8 SYSTEM SODIUM CHLORIDEW4TER FEED CONCENTRATION 0 I wt % FEED RATE 3 0 0 c C / m i n u f 0 OPERATING PRESSURE 2 5 0 P s 1 0 F I L M AREA 7 6 s q ~ r n FlCM
100-
. I
c?
80-
W
5
e
60-
u 0
g
40-
20-
I
0
I
I
I
I
0.2
0.4
0.6
0.8
I
Figure 5. Effect of backpressure treatment on membrane performance
backpressure treatment resulted in a decrease in product rate in the subsequent surface layer-side operation. This is due to the effect of membrane compaction brought about during backpressure treatment, as seen from the data given in Figure 6, which shows considerable membrane compaction for less dense and more porous membranes during backside operation. VOL. 8 NO. 3 S E P T E M B E R 1969
277
=
0.7
15
-
e
0
u
BACK SIDE OPERATION F I L M TYPE CA-NRC-18 SYSTEM SODIUM. CHLORIDE-WATER FEED CONCENTRATION 0.1 I t h FEED RATE 300 c c / m i n u t e OPERATING PRESSURE 2 5 0 P S I 9 F I L M 4REA 7.6 S q Cm LY
a
fi
n
n
0.4
0
Y
A
0
,-+
0.1+8/:I-++-f
i
I
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I
\
501
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a
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W
1 200
I
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+
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:
+
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+-+-
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I
Membrane Performance in Cyclic Experiments. These experiments consisted in determining the [PWP], product rate, and solute separation with 1000-p.p.m. NaCl feed solution as a function of increasing or decreasing pressure during alternate surface layer-side and backside operations. The operating pressure extended up to 250 p.s.i.g. During the experiment, data were taken a t each operating pressure after the film had been under that pressure for 15 minutes. Operating pressures were changed from one to the next without stopping the run. The run was stopped only when the side of the film facing the feed solution was changed. A typical set of results is shown in Figure 7. The data on the top right side of each plot are for the surface layer-side operation, and those on the bottom left side of each plot are for the backside operation. Numbers I, 11, 111, etc., represent the sequence of the cyclic operation and the arrows indicate the direction of pressure change in each operation. These data are essentially similar to those reported earlier (Banks and Sharples, 1966; KopeEek and Sourirajan, 1969). There was practically no hysteresis in the [PWP] and product rate data because of the low operating pressures. The widening of the surface pores as a result of the backside operation is clearly shown by the increase in the [PWP] and product rate data; the slight decrease in solute separation obtained after the backside operation shows the transient effect of the short duration of such operation. These results confirm the observations made earlier on the possible structural changes taking place in the membrane during the backpressure treatment. Conclusions
Figure 6. Membrane compaction during backside operation
At operating pressures less than 100 p.s.i.g., there seems no difference in the solute separation us. product rate
OPERATING PRESSURE otm
SYSTEM WATER
S Y S T E M . SODIUM CHLORIDE-WATER
SYSTEM SODIUM C H L O R I D E - W A T E R
Figure 7. Membrane performance during cyclic experiments 278
l & E C PRODUCT RESEARCH A N D DEVELOPMENT
Banks, W., Sharples, A., J . A p p l . Chem. 16, 28 (1966). Kimura, S., Sourirajan, S., A.I.Ch.E. J . 13, 497 (1967). Kimura, S., Sourirajan, S., Ind. Eng. Chem. Process Design Develop. 7, 197 (1968). KopeEek, J., Sourirajan, S., J.A p p l . Polymer Sci., in press, 1969. Sourirajan, S., I n d . Erg. Chem. Fundamentals 3,206 (1964). Sourirajan, S., “Reverse Osmosis,” Logos Press, London, England, 1969 (in press). Sourirajan, S., Govindan, T. S., Proceedings of First International Symposium on Water Desalination, Washington, D. C., 1965; Office of Saline Water, U. S. Dept. Interior, Washington, D. C., Vol. 1, pp. 251-74. Sourirajan, S., Kimura, S., I n d . Eng. Chem. Process Design Develop. 6, 504 (1967).
curves obtained by surface layer-side and backside operations, especially with films giving greater than 60% solute separation for sodium chloride in low feed concentrations. Details of pretreatment and operating pressure affect the relative selectivity of a membrane for different solutes. I t may be possible to carry out backpressure treatment with brackish waters to give simultaneously potable water and an improved membrane for subsequent surface layerside reverse osmosis op’erations. Acknowledgment
The authors are grateful Pageau, and A. G. Baxter investigations. One of the National Research Council a postdoctoral fellowship.
to Nguyen N. Hung, Lucien for their assistance in these authors (J. K.) thanks the of Canada for the award of
literature Cited
RECEIVED for review November 29, 1968 ACCEPTED April 14, 1969
Agrawal, J. P., Sourirajan, S., I n d . E n g . Chem. Process Design Develop. 8 , No. 4, not published (1969).
Issued as N.R.C. No. 10903.
EFFECT OF TEMPERATURE AND CONCENTRATION ON THE CHLOROHYDRINATION OF ALLY LTRIMETHY LAMMONIUM CH LORlDE C .
G L A S S ,
A N D
Northern Regional Research Laboratory, U . S . Department of Agriculture, Peoria, Ill.
1. M E H L T R E T T E R ,
T .
A .
M C G U I R E ,
C .
A .
61604
C .
A .
W I L H A M
Reaction of allyltrimethylammonium chloride with chlorine in aqueous solution in the temperature range of 25’ to 32’C. gave a nearly quantitative yield of CI 3 to 2 mixture of N-( 2-chloro-3-hydroxypropyl)- and N-( 3-chloro-2hydroxypropyl)trimethylammonium chlorides. Chlorohydrination a t 42’ to 46’ C. formed an appreciable quantity of the chlorination product, N( 2,3dichloropropyl)trimethylammonium chloride. A change in concentration of allyltrimethylammonium chloride from 15 to 35% did not affect the course of chlorohydrination at lower temperatures. Proton magnetic resonance spectra established the structure and composition of the products. The improved synthesis should allow more efficient production of cationic starches and ion exchange cellulose.
PARTHEIL (1892) made allyltrimethylammonium chloride
ogous synthesis in which allyltrimethylammonium chloride in aqueous solution was treated with chlorine a t 69°C. This chlorohydrination procedure, although more practical, introduced competitive chlorination, since the isolated product was a mixture of 68% MCC and 32% N-(2,3dichloropropyl)trimethylammonium chloride (DCC). The importance of MCC for producing cationic starches
(Weiss, 1892) react with hypochlorous acid in the cold and obtained a mixture of monochlorohydroxypropyltrimethylammonium chlorides (MCC), as shown below, which were separated and isolated as crystalline platinum chloride double salts. In 1967, Shildneck and Hathaway described an anal-
+ [(CH3)3NCHzCH=CH*]Cl- + HOC1
-
+
[(CH3)3N-CH2CHOH-CH2Cl]Cl-
+
[(CH,),N-CHLCHCl-CH?OH]Cl-
(I) (11)
VOL. 8 NO. 3 SEPTEMBER 1969
279
