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Reverse Osmosis of Dairy Liquids
Reverse Osmosis of Dairy Liquids J. H I D D I N K , R. de BOER, and P.F.C. NOOY Netherlands Institute for Dairy Research (NIZO) P. O. Box 20 6710BA Ede, The Netherlands ABSTRACT
Permeate flux during reverse osmosis of skim milk, Gouda cheese whey at various pH, desalted whey, ultrafiltration permeate, and lactose solutions were studied. The flux-limiting factors of the feed appeared to be osmotic pressure and fouling agents. Protein in Gouda whey at pH 6.6 did not give rise to fouling; however, after desalting or decrease of pH to 4.6, fouling by whey protein occurred. If Gouda whey was concentrated at 30 C over a concentration ratio of 1.6:1, calcium-phosphate precipitation caused strong fouling of the membrane. This fouling can be prevented by removing calcium from the whey or by acidifying to a pH of about 6.0. During reverse osmosis of skim milk, casein is the principal fouling material. To understand better the influence of the process conditions on the permeate flux, the effect of concentration polarization and fouling was calculated for various conditions. It appeared possible to achieve a reasonably low concentration polarization and fouling by applying, in a tubular system, flow velocities of 2 m/s for whey and of 2.5 m/s for skim milk. Furthermore, for skim milk an optimum process pressure was about 3.5 MPa. INTRODUCTION
In the dairy industry reverse osmosis (RO) is used more and more as a concentration technique (6, 11, 13, 26). The main advantage of RO over evaporation is the low energy consumption (2). In the Netherlands RO has been in use for some years for the concentration of whey and ultrafiltration permeate. The main reason whey is concentrated at the cheese
Received April 2, 1979, 1980 J Dairy Sci 63:204--214
factory is savings on costs of transport from the cheese factory to a central whey processing plant. At the moment, seven RO plants with a water removal capacity ranging from 3.5 to 15 m3/h have been installed, four of them using tubular membranes and three of them using flat membranes. In principle, two processes can be distinguished. One process involves concentration of whey at 30 C in a tubular single-pass system, to a concentration ratio of about 2:1. In the second process whey is cooled to 10 or 17 C and then concentrated, batchwise or in a continuous system, to a concentration ratio of 2:1 or 3:1. An important factor for RO is the permeate flux, this factor governing to a large extent the economy of the process. The factors affecting the permeate flux, in addition to the properties of the membrane, are the composition of the feed and the process conditions. Though RO membranes can be made of a variety of materials (cellulose acetate, polyamide, cellulose nitrate, sulfonated polyphenylene oxide, polybenzimidazolone), up to now only cellulose acetate (CA) has proved to be commercially acceptable for RO of food and dairy liquids. For process conditions, temperature is of particular importance. Raising the temperature of the feed increases the permeate flux, but to work under completely safe bacteriological conditions a temperature below 10 C or above 50 C should be used. Cellulose acetate (CA) membranes should not be operated above 35 C since these membranes are subject to hydrolysis and compaction at higher temperatures. This consideration should lead to a process temperature of 10 C. In practice, however, temperatures of 30 C (temperature of the Gouda cheese whey) and 17 C (temperature of the whey after cooling with cold milk in a heat exchanger) are also used. At these temperatures extensive precautions, such as a short residence time and intermediate disinfection, must be taken to prevent bacteriological spoilage. 204
REVERSE OSMOSIS OF DAIRY LIQUIDS Concerning the properties of the feed, osmotic pressure and the fouling tendency must be considered. The osmotic pressure of dairy liquids is determined mainly by the low molecular weight solutes such as salts and lactose. It must be overcome by the applied process pressure. Fouling can be caused by precipitation of certain salts and by build up of a deposit of colloidal material. It should be prevented by a proper pretreatment of the feed and by applying proper process conditions. A number of investigations referred to in the literature deal with the fouling properties of the various components of whey and skim milk and with the composition of the deposit. Lira et al. (15), Peri and Dunkley (18), and Mehta (16) concluded that during concentration of cottage cheese whey, the principal fouling material is protein, and particularly casein. The selective concentration of casein was explained by its low diffusion coefficient. Smith and McBean (28)investigated membrane fouling in RO of cheddar cheese whey and hydrochloric acid casein whey. They found that calcium plays an important part in membrane fouling, in particular precipitation of insoluble calcium salts. The pretreatments developed by Hayes et al. (12) for reduction of fouling in uhrafiltration led to increased fouling in RO. This was ascribed to the higher ionic concentration that with RO takes place in the surface layer on the membrane. Glover and Brooker (10) and Skudder et al. (27) studied by electron micrographs the structure of deposits formed during RO of whole milk. The deposit consisted mainly of casein micelles while Ca-phosphate was precipitated in the deposit during concentration. Concerning the process conditions which should be applied for RO of whey and skim milk much work has been done. The essential difference between the early work (8, 15, 18) and more recent work (5, 25, 26, 27) is the difference in applied flow velocity. In the early work the flow velocity was .10 to .55 m/s (at a tube diameter of 1.25 cm), but in the more recent work flow velocities of 2 to 3 m/s were applied at a similar tube diameter. With these higher velocities much better permeate fluxes were achieved. Typical applied pressures in both cases were 3 to 5 MPa and temperatures between 10 and 35 C. Our work was aimed at acquiring a better
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understanding of the effect of both composition of the feed and process conditions (temperature, flow velocity, and pressure) on the permeate flux. In particular we tried to quantify the effect of concentration polarization and fouling. Our study was restricted to skim milk, Gouda cheese whey, and their derivatives. MATERIALS AND METHODS Experimental Procedure
A laboratory recirculation RO plant was used, consisting of two modules type B1 (length 3.66 m) from Paterson Candy International Ltd., England. The modules were provided with tubular CA membranes type T2/15W with an inner diameter of 1.3 cm and a salt rejection of about 93%. The membrane area per module amounted to 2.55 m 2. The experiments were by batch; from a tank the liquid was pumped through the system by a piston pump and returned to the tank while the permeate was drained. Each experiment took 2 to 4 h. The process temperature was maintained at 10 or 30 C. If a process temperature of 30 C was used, the liquid was heated in the tank before starting the experiment. Pressures were between 2.5 and 4.5 MPa and flow velocities between 1 and 2.6 m/s. The flow velocity was calculated from the rate of flow divided by the area of cross-section of the membrane tube. Before use, the skim milk and the whey were separated (6800 × g) and pasteurized at 72 C/15 s. Dairy Liquids
Composition of the dairy liquids tested is in Table 1. When whey was desalted, a strong acid cation exchange resin and a weak base anion exchange resin were used. About 95% of the mineral matter was removed. The ultrafiltration permeate (UF-permeate) was obtained by UF of whey at 10 C. The pH of all liquids referred to in Table 1 was 6.5 to 6.7 or was adjusted to that pH. In some of our experiments we used Gouda cheese whey acidified with HCI to pH 6.0 or 4.6. In some experiments whey was used from which 90% or more of the calcium was removed by ion exchange with a weak cation exchange resin, which was regenerated with HC1 and NaOH. After regeneration the resin is in Na + form. In Table 1 the osmotic pressure for the Journal of Dairy Science Vol. 63, No. 2, 1980
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TABLE 1. The composition, the osmotic pressure at 10 C and the dynamic viscosity at 10 C of the dairy liquids used. Total solids
Lactose
Ash
Whey protein
8.9 5.5 5.1 5.0 4.0
4.6 4.1 4.1 4.1 4.0
.7 .5 .03 .5 . . . .
Osmotic pressure
Casein
(%)
Skim milk Gouda whey pH 6.6 Desalted whey pl-I 6.6 UF-permeatc Lactose solution
Dynamic viscosity
(MPa) .7 .7 .6 .2 a . . . .
2.8 .. .. ..
(mPa.s)
.72
2.8 1.6 1.6 1.5 1.4
.61
.35 .61 .30
aNonprotein nitrogen × 6.38.
various liquids is given also. T h e o s m o t i c pressure was d e t e r m i n e d b y m e a s u r i n g t h e freezing p o i n t d e p r e s s i o n w i t h a K n a u e r l lalbm i k r o - O s m o m e t e r t y p e M. T h e o s m o t i c pressures at 10 C a n d 30 C were c a l c u l a t e d f r o m t h e freezing p o i n t d e p r e s s i o n a c c o r d i n g t o t h e m e t h o d d e s c r i b e d b y Reid (21). T h e viscosity o f t h e various liquids was d e t e r m i n e d b y an E M I L A R h e o m e t e r ( T a b l e 1). Samples were a n a l y z e d a c c o r d i n g to t h e f o l l o w i n g m e t h o d s : t o t a l solids b y d r y i n g in an oven at 105 C; t o t a l p r o t e i n b y t h e Kjeldahl p r o c e d u r e ( p r o t e i n f a c t o r 6.38); lactose a c c o r d ing t o L u f f - S c h o o r l ( 2 4 ) ; ash b y d r y i n g in an o v e n at 105 C a n d h e a t i n g in an electric m u f f l e f u r n a c e at 550 C for at least 8 h; a n d calcium b y a f l a m e p h o t o m e t e r .
also. T h e increasing o s m o t i c pressure o f t h e r e t e n t a t e caused a decreasing driving f o r c e for w a t e r p e r m e a t i o n (see A p p e n d i x A, e q u a t i o n [1] ). T h e d i f f e r e n c e b e t w e e n t h e p u r e w a t e r flux and t h e calculated curve A in Figure 1 r e p r e s e n t s t h e effect of t h e r e d u c t i o n o f t h e driving force b y t h e o s m o t i c pressure. Curve A w o u l d be f o u n d if we h a d to cope o n l y w i t h an e f f e c t o f increasing o s m o t i c pressure. Curve B in Figure 1 w o u l d be f o u n d if t h e effect o f c o n c e n t r a t i o n p o l a r i z a t i o n also is t a k e n into a c c o u n t . C o n c e n t r a t i o n p o l a r i z a t i o n is d e f i n e d as t h e b u i l d - u p o f s o l u t e s n e a r t h e m e m b r a n e , w h i c h as a result causes an increase
9Pr'leate fix ,1121} 31/
~_
pure waler flux . . . . . . . . . . . . %. . . . . . . . . . . . . . . . .
RESULTS A N D DISCUSSION Influence of Composition of Feed at 10 C
A series o f R O e x p e r i m e n t s was w i t h liquids in T a b l e 1. The liquids were c o n c e n t r a t e d d u r i n g t h e e x p e r i m e n t s a t a process t e m p e r a t u r e o f 10 C, an average pressure of 4 MPa, a n d a flow v e l o c i t y o f 2 m/s. A t this flow v e l o c i t y t h e R e y n o l d s n u m b e r s were 5 0 0 0 to 2 0 0 0 0 , d e p e n d i n g o n t h e viscosity o f t h e liquid. The experimentally determined permeate fluxes are in Figure 1 as a f u n c t i o n o f t h e o s m o t i c pressure o f t h e b u l k liquid. During c o n c e n t r a t i o n t h e o s m o t i c pressure o f t h e r e t e n t a t e increased, a p p r o x i m a t e l y in p r o p o r t i o n w i t h t h e c o n c e n t r a t i o n ratio, w h i l e t h e p e r m e a t e f l u x decreased. In Figure 1 t h e p u r e w a t e r flux u n d e r t h e r e l e v a n t process c o n d i t i o n s is i n d i c a t e d Journal of Dairy Science Vol. 63, No. 2, 1980
1
\
effect o~ (on{enlrahon p31arh'dtl )n
\ \
x•
5
e e 0 0
~
~a n
Z
4
6
8
10
1.2
n9
f ~ tollllng ~deseltedwhey~
I4
16
18
"~ 20
22 2.4 20 0sm0hc pressure ,~Pal
Figure I. Permeate f l u x versus osmotic pressure o f the b u l k liquid f o r RO o f various liquids. The experimental fluxes are compared with those calculated assuming no concentration polarization and no f o u l i n g (curve A) and with those calculated assuming o n l y concentration polarization (curve B). Process conditions: T = 10 C, P = 4 MPa, v = 2 m/s. • lactose,
o UF-permeate, X sweet whey, o desalted whey, and skim milk.
REVERSE OSMOSIS OF DAIRY LIQUIDS of osmotic pressure at the membrane surface (see Appendix A). Curve B was achieved by the evaluation of equation [6] of Appendix A. In Figure 1 the experimental permeate fluxes for RO of the lactose solution, the UF-permeate, and the whey resemble more or less the calculated curve B. The experimental permeate fluxes for desalted whey are lower than those calculated for only concentration polarization. Here fouling also plays a role. Fouling is defined as the deposition of colloidal material on the membrane surface, giving rise to an additional hydraulic resistance, acting in series with the membrane resistance. The difference between curve B and the experimental permeate flux in Figure 1 is a measure of the degree of fouling. For desalted whey the cause of the fouling may be due to the high degree of desalting, about 95%, because the stability of the whey protein is affected, which may cause a tendency to aggregate at the membrane surface. For skim milk the differences between experimental and expected (assuming only concentration polarization) permeate fluxes are even bigger. The casein in skim milk is responsible for considerable fouling, because its high concentration and low diffusion coefficient cause formation of a deposit (gel layer) on the membrane surface (10, 27). Under these conditions, the osmotic pressure was the main governing factor for Gouda whey, UF-permeate, and the lactose solution. In the case of desalted whey and skim milk a fouling tendency also has to be considered.
207
strong flux decline was not found, and the permeate flux resembled that of the calculated curve B. Based on this information it was assumed that during the RO of whey and UF-permeate, precipitation of Ca-salts on the membrane occurred. Solubility of Ca-phosphate decreases with increasing temperature (1, 20, 23). Apparently during the RO of whey and UF-permeate at 30 C at a concentration ratio of about 1.6:1 fouling of the RO membrane by precipitated Ca-phosphate started. This fact agrees with observations of Roger et al. (22) that in UF-permeate heated to temperatures above 30 C a fine mineral precipitate is formed which causes fouling of UF membranes. For the effect of process temperature on permeate flux, a comparison of Figure 1 and Figure 2 shows that the pure water flux increases by about 3% per degree. The initial permeate flux for whey and UF-permeate at 30 C was also about 60% higher than at 10 C, but due to the strong flux decline at a concentration ratio of about 1.6: 1, the permeate flux at 30 C decreased to below that at 10 C. For skim milk we see that at 30 C, just as at 10 C, the permeate flux was much lower than the calculated flux, for only concentration polarization (curve B), so a severe fouling had to be considered. Probably the fouling was
permeate flux dim 2. h) 50 h -. . .-. . -. . . . .
pu_re__waterfI_u_x. . . . . . . . . . .
Influence of Composition of Feed at 30 C
A second series of experiments was at 30 C. In Figure 2 are plotted the experimental permeat fluxes versus the osmotic pressure of the bulk liquid. The permeate flux calculated according to equation [6] of Appendix A, for only concentration polarization, is presented also (curve B). For Gouda whey and UFpermeate, the permeate fluxes initially resemble those calculated. However, at a concentration ratio of about 1.6:1 (osmotic pressure about 1 MPa) a sudden decline in permeate flux was found. Obviously considerable fouling of the membrane t o o k place. Since both whey and UF-permeate show this behavior, it is not likely that the whey protein was responsible for this effect. During the RO of decalcified whey the
0
L
L
.4
.8
I
1.2
I
1.6
L
I
I
2.0 2.4 2.8 osmotic pressure (MPa}
Figure 2. Permeate flux versus osmotic pressure of the bulk liquid for RO of skim milk, whey and UFpermeate at 30 C, P = 4 MPa, and v = 2 m/s. Curve B was calculated according to Appendix A assuming only concentration polarization (no fouling). • Gouda cheese whey pH = 6.6, × decalcified whey, a UFpermeate, obtained at 55 C, o UF-permeate, obtained at 10 C, and o skim milk. Journal of Dairy Science Vol. 63, No. 2, 1980
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caused mainly by casein. Since milk is saturated with Ca-phosphate (20), during heating and concentration a certain precipitation of Caphosphate will take place. The fact that this precipitation of Ca-phosphate did not give such a strong flux decline as in the case of whey and UF-permeate may be attributed to the precipitation of Ca-phosphate on casein micelles, and therefore less precipitation on the membrane. In experiments in which UF-permeate, obtained by UF at 55 C, was concentrated by RO, the strong flux decline did not occur, the permeate flux being similar to that for decalcified whey (see Figure 2). During UF of whey or skim milk at those high temperatures a precipitation of Ca-phosphate occurred in the UF retentate (23). As a result, the UF permeate had a lower Ca and phosphate content. In our experiments UF-permeate obtained at 10 C contained 33 mg Ca and 30 mg P/100 g while UF-permeate obtained at 55 C had 25 mg Ca and 25 mg P/100 g, which means a reduction of about 25%. In the latter UF-permeate during RO to 30 C apparently no more membrane fouling took place. Roger et al. (22) reported a similar observation. They found an improvement in flux of UF-permeate in an enzyme reactor by removing 15% of the Ca. Solubility of Ca-phosphate increases at lower pH (1, 20). At a pH of about 4.9 phosphate appears to be completely in solution in milk (20). Some RO experiments were with whey at 30 C at various pH. From Figure 3 when the whey was acidified to pH 4.6 or even to pH 6.0, the strong flux decline, pH 6.6, did not occur. At pH 4.6 Ca-phosphate is supposed to be completely soluble so we do not expect Ca-phosphate fouling. At pH 6.0 more Caphosphate is soluble than at pH 6.6. Solubility is improved enough to make possible a fourfold concentration of whey at 30 C without a strong Ca-phosphate fouling. The permeate flux (Figure 3) at pH 6.0 was less than the calculated permeate flux (curve B); probably fouling is not avoided completely. However, the permeate flux at pH 4.6 was considerably lower than curve B. This effect should probably be attributed to the effect of pH on the behavior of the whey protein (9, 14). In the neighborhood of the iso-electric point, the protein has a low electrical charge. Under these conditions aggregation takes place easily and accumulation of protein at the membrane Journal of Dairy Science Vol. 63, No. 2, 1980
surface may occur. This p h e n o m e n o n agrees with that for UF of whey at various pH (9). Influence of Process Conditions
The process conditions such as temperature, flow velocity, and pressure have a considerable effect on permeate flux, and time also can be an important factor. To make the discussion easier, three resistances to permeation often can be distinguished: the membrane resistance Rm, the fouling resistance Rf, and the apparent resistance which can be attributed to concentration polarization Rp (see Appendix B). The membrane resistance especially is affected by temperature; the higher the temperature, the lower the Rm. For the other resistances the matter is more complex as they are particularly dependent on flow velocity and pressure. In the interpretation of our experiments it is assumed that the effect of time on the permeate flux can be neglected. This assumption can be justified since each experiment lasted only a few hours. During longer periods the effect of time on the permeate flux certainly becomes important. Effect of Flow Velocity
According to the method in Appendix B, for
permeate flux (I/m 2. h) . . . . . .
50
p._ur e._w_ater _flEx . . . . . . . . . . . .
40
30
20
10
0
Z
l 0
14
4
3
~
i
,
~
.8
1.2
1.6
2.0
5 concentration
ratio
2.4 2.8 osmotic pressure (MPa)
Figure 3. Permeate flux versus osmotic pressure of the bulk liquid (concentration ratio) for Re of whey at various pH-levels. T = 30 C, P = 4 MPa, v = 2 m/s. Curve B was calculated according to Appendix A assuming only concentration polarization (no fouling). • Gouda cheese whey pH = 6.6, DGouda cheese whey acidified to pH = 6.0, and o Gouda cheese whey acidified to pH = 4.6.
REVERSE OSMOSIS OF DAIRY LIQUIDS a range o f e x p e r i m e n t s with skim milk and whey, R f and Rp were calculated. In Figure 4 t h e e f f e c t o f the flow velocity on R f and Rp is given. The main trend is a decrease of Rp and R f with an increase of the f l o w velocity. F o r w h e y (2× c o n c e n t r a t e d ) at 10 C, an R f (curve A) of a b o u t .8 TPa.s/m is f o u n d at a f l o w velocity of 1 m/s; this is a b o u t 1.7 × the m e m b r a n e resistance R m at 10 C. At a flow velocity of 2 m/s, R f has decreased to about 1/5 o f R m . As in Figure 4, a further increase in flow velocity does not diminish R f to any extent. F r o m Figure 4 we see t h a t for skim milk R f is o f m u c h m o r e i m p o r t a n c e than for whey. Again d e p e n d e n c e of R f on flow velocity is strong. At 30 C R f for t w o f o l d c o n c e n t r a t e d skim milk b e c o m e s less t h a n R m at a flow velocity o f about 2.4 m/s. An increase in flow velocity o v e r 2.6 m/s m a y have o n l y a marginal effect. Also presented is R f for a c o n c e n t r a t i o n ratio of 1.5 and 3. At higher c o n c e n t r a t i o n
resistance (TPa. s/ml
c\ 3.5 3.0 2.5
3.0x
20 .
B
1.5
\ \\\
1,5x
A\
1.0
.5 .._R_mat 10 C
Rmat30 C
o
~,
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~-x
1.0
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-,----~---, .5
~+.
20 .x? \ \ \ \ \
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" "°~.~'~.
\"-,..'~-~''---,-
......... 1.5
~.
-,----:==:=- ---T-~ 2.0
2.5
flow rate qmls)
Figure 4. Various resistances to water permeation as a function of the flow velocity. A) Fouling resistance (Rf) for whey, 2X concentrated, T = 10 C, P = 4 MPa; B) Fouling resistance (Rf) for skim milk, 1.5, 2.0, and 3.0X concentrated, T = 30 C, P = 3.5 MPa; C) Fouling resistance (Rf) for skim milk, 2X concentrated, T = 10 C, P = 3.5 MPa; and D) Resistance due to concentration polarization (Rp), o for whey 2× concentrated, T = 10 C, P = 4 MPa, a for skim milk, 2X concentrated, T = 30 C, P = 3.5 MPa (1 TPa.s/m = 1012 Pa.s/m).
209
ratios R f increases. For 2 × c o n c e n t r a t e d skim milk, Rf at 10 C is m u c h higher than at 30 C; at a flow v e l o c i t y o f 2.6 m / s we still have an Rf o f a b o u t 1.0 TPa.s/m, which is m o r e t h a n twice R m at 10 C. To reach reasonably small Rf at 10 C, m u c h higher flow velocities should be used. The effect o f f l o w velocity on c o n c e n t r a t i o n polarization, expressed as Rp, also is in Figure 4. Curve D represents Rp for w h e y at 10 C and skim milk at 30 C (both t w o f o l d c o n c e n t r a t e d ) . The Rp is in b o t h cases a b o u t the same since the p e r m e a t e fluxes are at the same level. The Rp are considerably less than R f and Rm. It can be c o n c l u d e d that high flow velocities are n o t necessary for reducing Rp b u t are particularly so for reducing Rf. Lim et al. (15) evaluated Rp and R f for RO of cottage cheese whey, and c a m e to similar conclusions a b o u t the e f f e c t o f flow rate on Rp and Rf. F r o m e x p e r i m e n t s at flow velocities between 15 and 50 cm/s, t h e y c o n c l u d e d that R f w o u l d b e c o m e negligible at a b o u t 1 m/s. Since t h e y had to d e t e r m i n e this by e x t r a p o l a t i o n , it is understandable that this figure cannot be accurate. For skim milk the effect o f the flow v e l o c i t y on R f and, thus, on the p e r m e a t e flux is stronger than for whey. Since Rp is small c o m p a r e d with Rf, it may be assumed that R f is the main governing factor for the p e r m e a t e flux. If so, the situation is similar to that e n c o u n t e r e d for U F . F r o m theoretical considerations o f UF (19) 1 . 1 . the p e r m e a t e flux J v or -Rf - (since J v a ,Rf - - ) .Is p r o p o r t i o n a l to the mass transfer coefficient K, which is for t u r b u l e n t flow in tubes proportional to Re.8 (see A p p e n d i x A). However, for UF often the effect of flow velocity is m u c h greater than when calculated according to this relationship (17, 19). An evaluation o f our results with skim milk and of the results of Skudder et al. (27) showed a p r o p o r t i o n a l i t y to Re 1-3. Due to the relatively narrow range o f variation in flow velocity this is not accurate, but it emphasizes the similarity b e t w e e n RO of skim milk and U F of protein solutions. In both situations a protein layer is f o r m e d on the membrane. Effect of Pressure
Particularly for RO of skim milk applied pressure is a critical factor. Skudder et al. (27) p e r f o r m e d a range o f e x p e r i m e n t s b e t w e e n 2.4 and 4.1 MPa and f o u n d that the p e r m e a t e flux Journal of Dairy Science Vol. 63, No. 2, 1980
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passed t h r o u g h a m a x i m u m in this region. W e carried o u t a similar r a n g e o f e x p e r i m e n t s in w h i c h t h e applied pressures were varied f r o m 2.5 to 4.5 MPa at 10 C a n d 30 C. F r o m t h e p e r m e a t e fluxes t h e f o u l i n g resistance R f was c a l c u l a t e d . In Figure 5, R f is p r e s e n t e d as a f u n c t i o n o f t h e a p p l i e d pressure. A t 10 C ( c o n c e n t r a t i o n ratio 1.5:1 a n d 2 : 1 ) R f a t 2.5 MPa are c o n s i d e r a b l y l o w e r t h a n R m . F u r t h e r m o r e , linear increase o f R f w i t h pressure is strong. A t 30 C a n d a t a p r e s s u r e o f 2.5 MPa, R f a g a i n are small, a n d a b o v e 3 MPa increase o f R f w i t h pressure is a l m o s t p r o p o r t i o n a l . Also, R f d e p e n d s o n t h e c o n c e n t r a t i o n ratio (Figure 5); t h e h i g h e r t h e c o n c e n t r a t i o n r a t i o t h e h i g h e r t h e Rf. This e f f e c t is m o r e p r o n o u n c e d at 10 C t h a n at 30 C. In Figure 6 t h e p e r m e a t e flux for skim m i l k at a c o n c e n t r a t i o n ratio 1.5:1 is p l o t t e d versus t h e a p p l i e d pressure. T h e a c t u a l p e r m e a t e flux increased to a pressure o f a b o u t 3.0 MPa, a n d for h i g h e r pressures a c o n s t a n t p e r m e a t e flux was r e a c h e d , w i t h a slight t e n d e n c y to
foulin( resistance (TPa.s/m) 1.4
9/ A /
/
decrease a t pressures a b o v e 4 MPa. In Figure 6 t h e p u r e w a t e r f l u x also is p r e s e n t e d , a n d t h e p e r m e a t e f l u x w h i c h c o u l d be e x p e c t e d if o n l y t h e o s m o t i c pressure were t a k e n i n t o a c c o u n t (curve B) a n d t h e p e r m e a t e f l u x if t h e e f f e c t o f c o n c e n t r a t i o n p o l a r i z a t i o n were also c o n s i d e r e d (curve C). T h e p h e n o m e n o n t h a t t h e p e r m e a t e f l u x is i n d e p e n d e n t o f t h e applied pressure a b o v e 3 MPa s h o u l d , a c c o r d i n g t o o u r calculations, b e a t t r i b u t e d t o increasing f o u l i n g resistance. A t a p r o c e s s t e m p e r a t u r e o f 10 C t h i s e f f e c t is even m o r e s e r i o u s t h a n at 30 C, a c o n s t a n t p e r m e a t e flux over t h e w h o l e p r e s s u r e range in this case. T h e r e s p o n s e o f t h e p e r m e a t e f l u x to p r e s s u r e v a r i a t i o n s can be u n d e r s t o o d as follows. In a s t e a d y s t a t e s i t u a t i o n as m u c h s o l u t e ( i n c l u d i n g p r o t e i n ) is m o v e d b a c k i n t o t h e b u l k liquid b y d i f f u s i o n a n d t u r b u l e n c e - as is b r o u g h t to t h e m e m b r a n e surface u n d e r t h e i n f l u e n c e o f p e r m e a t i o n t h r o u g h t h e m e m b r a n e . T h e transp o r t b a c k i n t o t h e b u l k is g o v e r n e d b y t h e mass t r a n s f e r c o e f f i c i e n t w h i c h again is d e t e r m i n e d b y t h e flow velocity. If at a c o n s t a n t flow v e l o c i t y a h i g h e r pressure is applied, d u e to an initial higher p e r m e a t e flux, t h e t h i c k n e s s o f t h e fouling layer increases, w h i l e at t h e same time a certain compaction of the deposit may t a k e place. T h e s e e f f e c t s will r e s u l t in a h i g h e r
/
1.2
// //
permeate flux (l/m 2. h)
/
l.O
2/
.- A
// /
.8
/
// /
/
/
9/ .6
/ /
Rmat]O
C
---
.4 -
/
//
/
/+" /
//
Rm at
-;0-c
/ .
//
tO
2.5
,
+,.- B
/
//
..,..~/
3.0
C
/
+//
/ " o
3.5
x-
4.0
D
E
4.5
applied pressure (NtPa)
Figure 5. Fouling resistance as a function of the applied pressure for RO of skim milk at a flow velocity of 2.6 m/s. A) T = 10 C, concentration ratio 2.0:1; B) T = 10 C, concentration ratio 1.5:1; C) T = 30 C, concentration ratio 2.0:1; D) T = 30 C, concentration ratio 1.5:1; and E) T = 30 C, concentration ratio 1.05:1 (1 TPa.s/m = 1012 Pa.s/m). Journal of Dairy Science Vol. 63, No. 2, 1980
0
/~ 2.5 ~
~ 3.0
a 3.5
i 40
i 45 pressure (/VlPa)
Figure 6. Permeate flux as a function of the applied pressure for RO of skim milk (1.5X concentrated) at T = 30 C, v = 2.6 m/s. A) pure water flux; B) permeate flux to be expected without concentration polarization and fouling; C) permeate flux to be expected without fouling; and D) actual permeate flux.
REVERSE OSMOSIS OF DAIRY LIQUIDS
211
2 the results of our e x p e r i m e n t s are given. F o r b o t h layers a fairly strong d e p e n d e n c e on c o n c e n t r a t i o n ratio, pressure, and flow v e l o c i t y was f o u n d , which makes it d o u b t f u l that there was a characteristic difference b e t w e e n t h e t w o layers. We think it is m o r e useful to consider the fouling layer as a whole and to distinguish within the fouling layer a certain gradient in c o n c e n t r a t i o n and firmness. This m o d e l agrees with that for c o n v e n t i o n a l filtration with a p o r o s i t y gradient (30). The effects in Table 2 can be u n d e r s t o o d as follows. The higher the f l o w velocity (shear rate), the m o r e will be scraped o f f o f the fouling layer and the lower the remaining fouling resistance after water rinsing. High pressures will cause a greater c o m p a c t i o n of the fouling layer, which again causes a firmer and m o r e fouling resistance after water rinsing.
Rf, and this process continues until such a high R f is built up t h a t equilibrium exists again b e t w e e n the a m o u n t o f solute transported to and f r o m the m e m b r a n e surface. Ultimately this results in an unchanged p e r m e a t e flux. The slight decrease in p e r m e a t e flux at higher pressures can be explained by a m o r e serious c o m p a c t i o n o f the fouling layer. Skudder et al. (27) report that the position o f the m a x i m u m in p e r m e a t e flux d e p e n d s b o t h on f l o w velocity and t e m p e r a t u r e . The higher the f l o w velocity and t e m p e r a t u r e , the lower the R f and the higher the o p t i m u m pressure. In our e x p e r i m e n t s with w h e y an effect of pressure on p e r m e a t e flux was similar but only at pressures above 4.5 MPa. Such an effect for w h e y also was f o u n d by D o n n e l l y et al. (5). The l o w protein c o n t e n t o f w h e y and the high diffusion coefficient for w h e y protein c o m p a r e d to casein m a y m e a n that for w h e y at higher pressures o n l y (i.e., higher p e r m e a t e fluxes) a considerable fouling resistance is built.
CONCLUSIONS
For reverse osmosis of dairy liquids, b o t h the c o m p o s i t i o n o f the feed and process conditions have an i m p o r t a n t effect on p e r m e a t e flux. F l u x limiting factors include o s m o t i c pressure and fouling tendency. F o r G o u d a whey, processed at 30 C, fouling o f the m e m branes due to Ca-phosphate precipitation is an i m p o r t a n t factor, while for skim milk, desalted w h e y , and acid whey, protein is the i m p o r t a n t fouling agent. Fouling by G o u d a w h e y can be limited by decreasing the p H or by decalcifying the whey. F u r t h e r m o r e , it is possible, by p r o p e r process conditions in which the flow v e l o c i t y plays an i m p o r t a n t role, to decrease the resistance due to fouling and c o n c e n t r a t i o n polariza-
Structure of Fouling Layer
Lim et al. (15) distinguished within the fouling layer ( e x p e r i m e n t s with cottage cheese whey), a cake layer which should be a c o m p a c t layer " a t t a c h e d " to the m e m b r a n e surface, and a viscous layer b e t w e e n the cake layer and the circulating bulk. The characteristic difference b e t w e e n those t w o layers is that the viscous layer is removed by rinsing with water while the cake layer remains a t t a c h e d to the m e m b r a n e . In our e x p e r i m e n t s with skim milk there was also a proteinaceous fouling layer as for Lim et al. (15). This layer was divided into a cake layer and a viscous layer (see A p p e n d i x B). In Table
TABLE 2. Distribution of the fouling resistance over a viscous layer and a cake layer for RO of skim milk. Rcake = fouling resistance a f t e r w a t e r rinsing. Rviscou s = Rf - Rcake. (1 TPa.s/m) = 1012 Pa.s/m). Rviscous (TPa.s/m)
Rcake (TPa.s/m) Concentration ratio
1.05
2.0
3.0
1.05
2.0
3.0
P = 2.5 MPa
v = 2.6 m/s
.01
.01
. . . .
02
.03
...
3.5 MPa
v = 1 . 4 m/s v = 2.0 m/s v = 2.6 m/s
.16 .12 .05
.88 .29 .10
. . . . .54 .24
63 .18 .04
.66 .21 .07
...
v = 2.6 m/s
.07
.22
. . . .
13
.20
...
P =
P = 4.5 MPa
Journal
.24 .09
of Dairy Science Vol. 63, No. 2, 1980
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HIDDINK ET AL.
tion considerably below the resistance of the membrane. An important step needed to reach a h i g h e r p e r m e a t e f l u x is t h e d e v e l o p m e n t o f membranes with higher water permeabilities and resistance to high temperatures.
14
15 REFERENCES
1 Brule, G., E. Real del Sol, J. Fauquant, and C. Fiand. 1978. Mineral salts stability in a q u e o u s phase o f milk: Influence o f heat treatments. J. Dairy Sci. 61:1225. 2 de Boer, R., J. N. de Wit, and J. Hiddink. 1977. Processing of whey by m e a n s o f m e m b r a n e s and applications o f whey protein concentrate. J. Soc. Dairy Technol. 30:112. 3 Dejmek, P. 1975. Permeability o f the concentration polarization layer in ultrafiltration o f macromolecules. Page A2.26 in Separation processes by m e m b r a n e s , ion exchange and freeze-concentration in food industry. A.P.RA.A., Paris. 4 D e l a n e y , R.A.M., and J. K. Donnelly. 1977. Applications o f reverse osmosis in the dairy industry. Page 417 in Reverse osmosis and synthetic m e m b r a n e s . S. Sourirajan, ed. Nat. Res. Counc. Can., Ottawa. 5 Donnelly, J.K., A. C. O'Sullivan, and R.A.M. Delaney. 1974. Reverse osmosis-concentration applications. J. Soc. Dairy Technol. 27:128. 6 Eriksson, P. 1977. Concentration of whey by reverse osmosis-inventory and operating experiences. Nordeuropaeisk Mejeritidsskrift. 43 : 238. 7 Evans, E. W., and F. A. Glover. 1974. Basic principles of reverse osmosis and ultrafiltration. J. Soc. Dairy Technol. 27:111. 8 Fenton May, R. I., C. G. Hill, C. H. A m u n d s o n , M. H. Lopez, and P. D. Auclair. 1972. Concentration and fractionation of skim milk by reverse osmosis and ultrafiltration. J. Dairy Sci. 55 : 1561. 9 Forbes, F. 1972. Considerations in the optimisation of ultrafiltration. Chem. Eng.:21. 10 Glover, F. A., and B. E. Brooker. 1974. The structure of the deposit formed on the m e m b r a n e during the concentration of milk by reverse osmosis. J. Dairy Res. 41:89. 11 Glover, F. A., P. J. Skudder, P. H. Stothart, and E. W. Evans. 1978. Reviews of the progress of dairy science: reverse osmosis and ultrafiltration in dairying. J. Dairy Res. 45:291. 12 Hayes, J. F., J. A. Dunkerley, and L. L. Muller. 1974. Studies on whey processing by ultrafiltration. Austr. J. Dairy Technol. 29:132. 13 J o h n s t o n , A. N. 1977. How European cheese
APPENDIX A Calculation of Concentration Polarization
16
17 18
19
20
21
22
23
24 25
26
27
28
29 30
plants save costs in w h e y processing t h r o u g h reverse osmosis system. Dairy and Ice Cream Field 160:1160. Lee, N. D., and R. L. Merson. 1976. Chemical t r e a t m e n t s o f cottage cheese w h e y to reduce fouling of ultrafiltration m e m b r a n e s . J. Food Sci. 41:778. Lim, T. H., W. L. Dunkley, and R. L. Merson. 1971. Role of protein in reverse osmosis o f cottage cheese whey. J. Dairy Sci. 54: 306. Mehta, B. 1973. Processing o f model compositional whey solutions with pressure driven membranes. Ph.D. thesis, The Ohio State University, Columbus. Muller, L. L. 1976. Whey utilization in Australia. Australian J. Dairy Technol. 31 : 92. Peri, C., and W. L. Dunkley. 1971. Reverse osmosis of cottage cheese whey. 1. Influence o f composition o f the feed. J. Food Sci. 36:25. Porter, M. C. 1972. Concentration polarization with m e m b r a n e ultrafiltration. Ind. Eng. Chem. Prod. Res. Develop. 11 : 234. Pyne, G. T. I 9 6 2 . Reviews o f the progress o f dairy science. Some aspects o f the physical chemistry of the salts o f milk. J. Dairy Res. 29:101. Reid, C. E. 1972. Principles of reverse osmosis. Page 109 in Industrial processing with membranes. R. E. Lacey and S. Loeb, ed. Wiley-Interscience, New York. Roger, R., J. L. T h a p o n , J. L. Maubois, and G. Brule. 1976. Hydrnlyse du lactose c o n t e n u dans l'ultrafiltrat de lair ou de lactosdrum en r~acteur e n z y m a t i q u e ~ m e m b r a n e . Le Lait 5 5 1 - 5 5 2 : 5 6 . Rose, D., and H. Tessier. 1959. Composition of ultrafiltrates from milk heated at 80 to 230 F in relation to heat stability. J. Dairy Sci. 42:969. Schoorl, N. 1929. Suiker titratie. Chem. Weekblad 26:130. Short, J. L., and R. K. Doughty. 1976. An evaluation of m o d u l e s and m e m b r a n e s for the concentration of cheddar cheese whey by reverse osmosis. New Zealand J. Dairy Sci. Technol. 11 : 237. Short, J. L., and I. R. Hughes. 1978. The concentration of separated milk by reverse osmosis. New Zealand J. Dairy Sci. Technol. 13:114. Skudder, P. J., F. A. Glover, and M. L. Green. 1977. An e x a m i n a t i o n of the factors affecting the reverse osmosis o f milk with special reference to deposit formation. J. Dairy Res. 44: 293. Smith, B. R., and R. D. Macbean. 1978. Fouling in reverse osmosis. Australian J. Dairy Technol. 33:57. Sourirajan, S. 1970. Reverse osmosis. Academic Press, New York. Tiller, F. M., and J. R. Crump. 1977. Solid liquid separation: An overview. Chem. Eng. Progr. 73:65.
somewhat equation:
simplified
form,
by
Jv = (AP- ATrw)/Rm The permeate (water) flux through a reverse osmosis membrane is u s u a l l y d e s c r i b e d , in a Journal of Dairy Science Vol. 63, No. 2, 1980
where
the following
[11
REVERSE OSMOSIS OF DAIRY LIQUIDS flux through m e m b r a n e ( l / m 2 . h ) or (m/s) hydrostatic pressure difference across AP= the m e m b r a n e (Pa) ATrw = o s m o t i c pressure difference across the m e m b r a n e (Pa) R m = resistance to water p e r m e a t i o n o f the m e m b r a n e (Pa.s/m) Jv
--
If restricted to high rejection membranes, the o s m o t i c pressure at the p e r m e a t e side m a y be neglected; then &Trw equals Zrw, which is the osmotic pressure at the m e m b r a n e surface at retentate side. As a result of the rejection of the solutes by the m e m b r a n e , at the m e m b r a n e surface a local c o n c e n t r a t i o n (and local o s m o t i c pressure) is found which is higher than in the bulk of the solution. This c o n c e n t r a t i o n polarization is c o u n t e r a c t e d by diffusion of solutes back into the bulk, by t u r b u l e n t eddies, and by shearing of the fluid near the m e m b r a n e . At equilibrium the transport o f solutes back into the bulk occurs as fast as solutes are brought up to the m e m b r a n e . A t equilibrium the concentration o f solutes ( o s m o t i c pressure) at the m e m b r a n e surface is given by (7, 19): C w / C b = Ztw/~ b = exp (Jv/k)
[2]
where c o n c e n t r a t i o n o f solutes at the m e m brane surface (kg/kg) C b = c o n c e n t r a t i o n of solutes in the bulk (kg/kg) Ti'b = o s m o t i c pressure in the bulk (Pa) k= mass transfer c o e f f i c i e n t (m/s)
CW
=
It is assumed that the o s m o t i c pressure is directly p r o p o r t i o n a l to the solute concentration. A c o m b i n a t i o n of equations [1] and [2] results in: Jv = (AP - 7rw)/Rm
[3] = (& P / R m ) - ( z r b / R m ) exp (Jv/k) If k is k n o w n , the p e r m e a t e flux J v can be calculated. F o r a calculation o f k, for t u r b u l e n t flow in tubes, generally the following dimensionless correlation is used (19): Sh = .023 Re.8 Sc.33
[41
213
where
Sh Re Sc and d v ID v
= kd/ID ( S h e r w o o d n u m b e r ) = dv//) ( R e y n o l d s n u m b e r ) = u/ID ( S c h m i d t n u m b e r ) = = = =
d i a m e t e r of the t u b e (m) f l o w velocity in the t u b e (m/s) diffusion coefficient (m2/s) kinematic viscosity (m2/s)
For the calculation of k, e q u a t i o n (4) can be written as: k = .023 ID '67 v ' 8 / ( d "2 /).47)
[5]
Equation [31 was evaluated for the liquids referred to in Table 1. For such an evaluation the physical properties o f the liquids m u s t be k n o w n . The viscosity and o s m o t i c pressure were d e t e r m i n e d e x p e r i m e n t a l l y , the values o f the diffusion coefficient were taken f r o m the literature (Table A.1). In our calculations concerning c o n c e n t r a t i o n polarization, we o n l y a c c o u n t e d for those c o m p o n e n t s which are contributing to the o s m o t i c pressure (i.e., lactose and salts). Since lactose and salts have different diffusion coefficients and, thus, different k-values, e q u a t i o n [31 was e x t e n d e d to two c o m p o n e n t s as follows: Jv = (AP/Rm) - (/Tbl/RM)'exp (Jv/kl) [6] - ( r r b 2 / R m ) - e x p (Jv/k2) It appeared that an evaluation o f e q u a t i o n [3] resulted in a p e r m e a t e flux versus o s m o t i c pressure of the bulk liquid which nearly coincides for w h e y , U F - p e r m e a t e , and lactose, see curve B in Figure 1. The variations were less than 1 liter/m 2.h. This can be u n d e r s t o o d f r o m the fact that the mass transfer c o e f f i c i e n t k for these liquids is about equal which again is caused by t h e fact that the process c o n d i t i o n s are the same and the viscosity, density and diffusion coefficients do not differ much. The calculated p e r m e a t e flux for skim milk tends to be s o m e w h a t lower (.5 to 2.0 l i t e r / m 2 . h ) than curve B in Figure 1. This can be u n d e r s t o o d f r o m the higher viscosity o f skim milk. Journal of Dairy Science Vol. 63, No. 2, 1980
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HIDD1NK ET AL.
TABLE A . I . Solute-water diffusion coefficients for some components of milk at 20 C. Diffusion coefficient (m2/s) Lactose 5% Ash(KCI.74%) Whey protein (/3-1actoglobulin 1%) Casein 3%
APPENDIX B Evaluation of Resistance due to Fouling and Concentration Polarization from the Experiments
Strictly speaking, c o n c e n t r a t i o n polarization reduces the driving force. However, the effect of c o n c e n t r a t i o n polarization o f t e n is translated into an additional resistance to p e r m e a t i o n o f the m e m b r a n e , when the original driving force is considered. However, a fouling layer on the m e m b r a n e m a y cause a real additional hydraulic resistance acting in series with the m e m b r a n e resistance (28). The m e m b r a n e resistance R m can be determined f r o m the pure water flux for the clean membrane. F r o m the e x p e r i m e n t a l p e r m e a t e flux for the liquid concerned, after an evaluation of the mass transfer coefficient k, the c o n c e n t r a t i o n polarization modules Cw/Cb = rrw/Zrb can be calculated f r o m A p p e n d i x A, e q u a t i o n [21. If k n o w n nw the value of the fouling resistance R f can be calculated since
Journal of Dairy Science Vol. 63, No. 2, 1980
3.8 18.4 .64 .19
× 10-10 X 10 -1° X 10 -10 × 10 -1°
Reference (4) (29) (4) (4)
Jv = (AP - rrw)/(Rm + Rf)
[71
Next the c o n c e n t r a t i o n polarization resistance Rp can be calculated f r o m the following equation: Jv = (AP - "trb)/(Rm + Rf + Rp) To divide the fouling resistance (Rf) in a resistance of a cake layer (Rcake) and the resistance of a viscous layer (Rviscou s) the following procedure was followed. The bulk liquid was r e m o v e d by water; then the pure water flux was measured under the same c o n d i t i o n as previously used for t h e liquid. F r o m this pure water flux Jw, Rcake was calculated according: J w = A P / ( R m + Rcake )
[8]
N e x t Rviscuu s can be calculated f r o m : Rviscou s = R f - Rcake
[9]
Permeate flux during reverse osmosis of skim milk, Gouda cheese whey at various pH, desalted whey, ultrafiltration permeate, and lactose solutions were studied. The flux-limiting factors of the feed appeared to be osmotic pressure and fouling agents. Protein in Gouda whey at pH 6.6 did not give rise to fouling; however, after desalting or decrease of pH to 4.6, fouling by whey protein occurred. If Gouda whey was concentrated at 30 C over a concentration ratio of 1.6:1, calcium-phosphate precipitation caused strong fouling of the membrane. This fouling can be prevented by removing calcium from the whey or by acidifying to a pH of about 6.0. During reverse osmosis of skim milk, casein is the principal fouling material. To understand better the influence of the process conditions on the permeate flux, the effect of concentration polarization and fouling was calculated for various conditions. It appeared possible to achieve a reasonably low concentration polarization and fouling by applying, in a tubular system, flow velocities of 2 m/s for whey and of 2.5 m/s for skim milk. Furthermore, for skim milk an optimum process pressure was about 3.5 MPa. INTRODUCTION
In the dairy industry reverse osmosis (RO) is used more and more as a concentration technique (6, 11, 13, 26). The main advantage of RO over evaporation is the low energy consumption (2). In the Netherlands RO has been in use for some years for the concentration of whey and ultrafiltration permeate. The main reason whey is concentrated at the cheese
Received April 2, 1979, 1980 J Dairy Sci 63:204--214
factory is savings on costs of transport from the cheese factory to a central whey processing plant. At the moment, seven RO plants with a water removal capacity ranging from 3.5 to 15 m3/h have been installed, four of them using tubular membranes and three of them using flat membranes. In principle, two processes can be distinguished. One process involves concentration of whey at 30 C in a tubular single-pass system, to a concentration ratio of about 2:1. In the second process whey is cooled to 10 or 17 C and then concentrated, batchwise or in a continuous system, to a concentration ratio of 2:1 or 3:1. An important factor for RO is the permeate flux, this factor governing to a large extent the economy of the process. The factors affecting the permeate flux, in addition to the properties of the membrane, are the composition of the feed and the process conditions. Though RO membranes can be made of a variety of materials (cellulose acetate, polyamide, cellulose nitrate, sulfonated polyphenylene oxide, polybenzimidazolone), up to now only cellulose acetate (CA) has proved to be commercially acceptable for RO of food and dairy liquids. For process conditions, temperature is of particular importance. Raising the temperature of the feed increases the permeate flux, but to work under completely safe bacteriological conditions a temperature below 10 C or above 50 C should be used. Cellulose acetate (CA) membranes should not be operated above 35 C since these membranes are subject to hydrolysis and compaction at higher temperatures. This consideration should lead to a process temperature of 10 C. In practice, however, temperatures of 30 C (temperature of the Gouda cheese whey) and 17 C (temperature of the whey after cooling with cold milk in a heat exchanger) are also used. At these temperatures extensive precautions, such as a short residence time and intermediate disinfection, must be taken to prevent bacteriological spoilage. 204
REVERSE OSMOSIS OF DAIRY LIQUIDS Concerning the properties of the feed, osmotic pressure and the fouling tendency must be considered. The osmotic pressure of dairy liquids is determined mainly by the low molecular weight solutes such as salts and lactose. It must be overcome by the applied process pressure. Fouling can be caused by precipitation of certain salts and by build up of a deposit of colloidal material. It should be prevented by a proper pretreatment of the feed and by applying proper process conditions. A number of investigations referred to in the literature deal with the fouling properties of the various components of whey and skim milk and with the composition of the deposit. Lira et al. (15), Peri and Dunkley (18), and Mehta (16) concluded that during concentration of cottage cheese whey, the principal fouling material is protein, and particularly casein. The selective concentration of casein was explained by its low diffusion coefficient. Smith and McBean (28)investigated membrane fouling in RO of cheddar cheese whey and hydrochloric acid casein whey. They found that calcium plays an important part in membrane fouling, in particular precipitation of insoluble calcium salts. The pretreatments developed by Hayes et al. (12) for reduction of fouling in uhrafiltration led to increased fouling in RO. This was ascribed to the higher ionic concentration that with RO takes place in the surface layer on the membrane. Glover and Brooker (10) and Skudder et al. (27) studied by electron micrographs the structure of deposits formed during RO of whole milk. The deposit consisted mainly of casein micelles while Ca-phosphate was precipitated in the deposit during concentration. Concerning the process conditions which should be applied for RO of whey and skim milk much work has been done. The essential difference between the early work (8, 15, 18) and more recent work (5, 25, 26, 27) is the difference in applied flow velocity. In the early work the flow velocity was .10 to .55 m/s (at a tube diameter of 1.25 cm), but in the more recent work flow velocities of 2 to 3 m/s were applied at a similar tube diameter. With these higher velocities much better permeate fluxes were achieved. Typical applied pressures in both cases were 3 to 5 MPa and temperatures between 10 and 35 C. Our work was aimed at acquiring a better
205
understanding of the effect of both composition of the feed and process conditions (temperature, flow velocity, and pressure) on the permeate flux. In particular we tried to quantify the effect of concentration polarization and fouling. Our study was restricted to skim milk, Gouda cheese whey, and their derivatives. MATERIALS AND METHODS Experimental Procedure
A laboratory recirculation RO plant was used, consisting of two modules type B1 (length 3.66 m) from Paterson Candy International Ltd., England. The modules were provided with tubular CA membranes type T2/15W with an inner diameter of 1.3 cm and a salt rejection of about 93%. The membrane area per module amounted to 2.55 m 2. The experiments were by batch; from a tank the liquid was pumped through the system by a piston pump and returned to the tank while the permeate was drained. Each experiment took 2 to 4 h. The process temperature was maintained at 10 or 30 C. If a process temperature of 30 C was used, the liquid was heated in the tank before starting the experiment. Pressures were between 2.5 and 4.5 MPa and flow velocities between 1 and 2.6 m/s. The flow velocity was calculated from the rate of flow divided by the area of cross-section of the membrane tube. Before use, the skim milk and the whey were separated (6800 × g) and pasteurized at 72 C/15 s. Dairy Liquids
Composition of the dairy liquids tested is in Table 1. When whey was desalted, a strong acid cation exchange resin and a weak base anion exchange resin were used. About 95% of the mineral matter was removed. The ultrafiltration permeate (UF-permeate) was obtained by UF of whey at 10 C. The pH of all liquids referred to in Table 1 was 6.5 to 6.7 or was adjusted to that pH. In some of our experiments we used Gouda cheese whey acidified with HCI to pH 6.0 or 4.6. In some experiments whey was used from which 90% or more of the calcium was removed by ion exchange with a weak cation exchange resin, which was regenerated with HC1 and NaOH. After regeneration the resin is in Na + form. In Table 1 the osmotic pressure for the Journal of Dairy Science Vol. 63, No. 2, 1980
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HIDDINK ET AL.
TABLE 1. The composition, the osmotic pressure at 10 C and the dynamic viscosity at 10 C of the dairy liquids used. Total solids
Lactose
Ash
Whey protein
8.9 5.5 5.1 5.0 4.0
4.6 4.1 4.1 4.1 4.0
.7 .5 .03 .5 . . . .
Osmotic pressure
Casein
(%)
Skim milk Gouda whey pH 6.6 Desalted whey pl-I 6.6 UF-permeatc Lactose solution
Dynamic viscosity
(MPa) .7 .7 .6 .2 a . . . .
2.8 .. .. ..
(mPa.s)
.72
2.8 1.6 1.6 1.5 1.4
.61
.35 .61 .30
aNonprotein nitrogen × 6.38.
various liquids is given also. T h e o s m o t i c pressure was d e t e r m i n e d b y m e a s u r i n g t h e freezing p o i n t d e p r e s s i o n w i t h a K n a u e r l lalbm i k r o - O s m o m e t e r t y p e M. T h e o s m o t i c pressures at 10 C a n d 30 C were c a l c u l a t e d f r o m t h e freezing p o i n t d e p r e s s i o n a c c o r d i n g t o t h e m e t h o d d e s c r i b e d b y Reid (21). T h e viscosity o f t h e various liquids was d e t e r m i n e d b y an E M I L A R h e o m e t e r ( T a b l e 1). Samples were a n a l y z e d a c c o r d i n g to t h e f o l l o w i n g m e t h o d s : t o t a l solids b y d r y i n g in an oven at 105 C; t o t a l p r o t e i n b y t h e Kjeldahl p r o c e d u r e ( p r o t e i n f a c t o r 6.38); lactose a c c o r d ing t o L u f f - S c h o o r l ( 2 4 ) ; ash b y d r y i n g in an o v e n at 105 C a n d h e a t i n g in an electric m u f f l e f u r n a c e at 550 C for at least 8 h; a n d calcium b y a f l a m e p h o t o m e t e r .
also. T h e increasing o s m o t i c pressure o f t h e r e t e n t a t e caused a decreasing driving f o r c e for w a t e r p e r m e a t i o n (see A p p e n d i x A, e q u a t i o n [1] ). T h e d i f f e r e n c e b e t w e e n t h e p u r e w a t e r flux and t h e calculated curve A in Figure 1 r e p r e s e n t s t h e effect of t h e r e d u c t i o n o f t h e driving force b y t h e o s m o t i c pressure. Curve A w o u l d be f o u n d if we h a d to cope o n l y w i t h an e f f e c t o f increasing o s m o t i c pressure. Curve B in Figure 1 w o u l d be f o u n d if t h e effect o f c o n c e n t r a t i o n p o l a r i z a t i o n also is t a k e n into a c c o u n t . C o n c e n t r a t i o n p o l a r i z a t i o n is d e f i n e d as t h e b u i l d - u p o f s o l u t e s n e a r t h e m e m b r a n e , w h i c h as a result causes an increase
9Pr'leate fix ,1121} 31/
~_
pure waler flux . . . . . . . . . . . . %. . . . . . . . . . . . . . . . .
RESULTS A N D DISCUSSION Influence of Composition of Feed at 10 C
A series o f R O e x p e r i m e n t s was w i t h liquids in T a b l e 1. The liquids were c o n c e n t r a t e d d u r i n g t h e e x p e r i m e n t s a t a process t e m p e r a t u r e o f 10 C, an average pressure of 4 MPa, a n d a flow v e l o c i t y o f 2 m/s. A t this flow v e l o c i t y t h e R e y n o l d s n u m b e r s were 5 0 0 0 to 2 0 0 0 0 , d e p e n d i n g o n t h e viscosity o f t h e liquid. The experimentally determined permeate fluxes are in Figure 1 as a f u n c t i o n o f t h e o s m o t i c pressure o f t h e b u l k liquid. During c o n c e n t r a t i o n t h e o s m o t i c pressure o f t h e r e t e n t a t e increased, a p p r o x i m a t e l y in p r o p o r t i o n w i t h t h e c o n c e n t r a t i o n ratio, w h i l e t h e p e r m e a t e f l u x decreased. In Figure 1 t h e p u r e w a t e r flux u n d e r t h e r e l e v a n t process c o n d i t i o n s is i n d i c a t e d Journal of Dairy Science Vol. 63, No. 2, 1980
1
\
effect o~ (on{enlrahon p31arh'dtl )n
\ \
x•
5
e e 0 0
~
~a n
Z
4
6
8
10
1.2
n9
f ~ tollllng ~deseltedwhey~
I4
16
18
"~ 20
22 2.4 20 0sm0hc pressure ,~Pal
Figure I. Permeate f l u x versus osmotic pressure o f the b u l k liquid f o r RO o f various liquids. The experimental fluxes are compared with those calculated assuming no concentration polarization and no f o u l i n g (curve A) and with those calculated assuming o n l y concentration polarization (curve B). Process conditions: T = 10 C, P = 4 MPa, v = 2 m/s. • lactose,
o UF-permeate, X sweet whey, o desalted whey, and skim milk.
REVERSE OSMOSIS OF DAIRY LIQUIDS of osmotic pressure at the membrane surface (see Appendix A). Curve B was achieved by the evaluation of equation [6] of Appendix A. In Figure 1 the experimental permeate fluxes for RO of the lactose solution, the UF-permeate, and the whey resemble more or less the calculated curve B. The experimental permeate fluxes for desalted whey are lower than those calculated for only concentration polarization. Here fouling also plays a role. Fouling is defined as the deposition of colloidal material on the membrane surface, giving rise to an additional hydraulic resistance, acting in series with the membrane resistance. The difference between curve B and the experimental permeate flux in Figure 1 is a measure of the degree of fouling. For desalted whey the cause of the fouling may be due to the high degree of desalting, about 95%, because the stability of the whey protein is affected, which may cause a tendency to aggregate at the membrane surface. For skim milk the differences between experimental and expected (assuming only concentration polarization) permeate fluxes are even bigger. The casein in skim milk is responsible for considerable fouling, because its high concentration and low diffusion coefficient cause formation of a deposit (gel layer) on the membrane surface (10, 27). Under these conditions, the osmotic pressure was the main governing factor for Gouda whey, UF-permeate, and the lactose solution. In the case of desalted whey and skim milk a fouling tendency also has to be considered.
207
strong flux decline was not found, and the permeate flux resembled that of the calculated curve B. Based on this information it was assumed that during the RO of whey and UF-permeate, precipitation of Ca-salts on the membrane occurred. Solubility of Ca-phosphate decreases with increasing temperature (1, 20, 23). Apparently during the RO of whey and UF-permeate at 30 C at a concentration ratio of about 1.6:1 fouling of the RO membrane by precipitated Ca-phosphate started. This fact agrees with observations of Roger et al. (22) that in UF-permeate heated to temperatures above 30 C a fine mineral precipitate is formed which causes fouling of UF membranes. For the effect of process temperature on permeate flux, a comparison of Figure 1 and Figure 2 shows that the pure water flux increases by about 3% per degree. The initial permeate flux for whey and UF-permeate at 30 C was also about 60% higher than at 10 C, but due to the strong flux decline at a concentration ratio of about 1.6: 1, the permeate flux at 30 C decreased to below that at 10 C. For skim milk we see that at 30 C, just as at 10 C, the permeate flux was much lower than the calculated flux, for only concentration polarization (curve B), so a severe fouling had to be considered. Probably the fouling was
permeate flux dim 2. h) 50 h -. . .-. . -. . . . .
pu_re__waterfI_u_x. . . . . . . . . . .
Influence of Composition of Feed at 30 C
A second series of experiments was at 30 C. In Figure 2 are plotted the experimental permeat fluxes versus the osmotic pressure of the bulk liquid. The permeate flux calculated according to equation [6] of Appendix A, for only concentration polarization, is presented also (curve B). For Gouda whey and UFpermeate, the permeate fluxes initially resemble those calculated. However, at a concentration ratio of about 1.6:1 (osmotic pressure about 1 MPa) a sudden decline in permeate flux was found. Obviously considerable fouling of the membrane t o o k place. Since both whey and UF-permeate show this behavior, it is not likely that the whey protein was responsible for this effect. During the RO of decalcified whey the
0
L
L
.4
.8
I
1.2
I
1.6
L
I
I
2.0 2.4 2.8 osmotic pressure (MPa}
Figure 2. Permeate flux versus osmotic pressure of the bulk liquid for RO of skim milk, whey and UFpermeate at 30 C, P = 4 MPa, and v = 2 m/s. Curve B was calculated according to Appendix A assuming only concentration polarization (no fouling). • Gouda cheese whey pH = 6.6, × decalcified whey, a UFpermeate, obtained at 55 C, o UF-permeate, obtained at 10 C, and o skim milk. Journal of Dairy Science Vol. 63, No. 2, 1980
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HIDDINK ET AL.
caused mainly by casein. Since milk is saturated with Ca-phosphate (20), during heating and concentration a certain precipitation of Caphosphate will take place. The fact that this precipitation of Ca-phosphate did not give such a strong flux decline as in the case of whey and UF-permeate may be attributed to the precipitation of Ca-phosphate on casein micelles, and therefore less precipitation on the membrane. In experiments in which UF-permeate, obtained by UF at 55 C, was concentrated by RO, the strong flux decline did not occur, the permeate flux being similar to that for decalcified whey (see Figure 2). During UF of whey or skim milk at those high temperatures a precipitation of Ca-phosphate occurred in the UF retentate (23). As a result, the UF permeate had a lower Ca and phosphate content. In our experiments UF-permeate obtained at 10 C contained 33 mg Ca and 30 mg P/100 g while UF-permeate obtained at 55 C had 25 mg Ca and 25 mg P/100 g, which means a reduction of about 25%. In the latter UF-permeate during RO to 30 C apparently no more membrane fouling took place. Roger et al. (22) reported a similar observation. They found an improvement in flux of UF-permeate in an enzyme reactor by removing 15% of the Ca. Solubility of Ca-phosphate increases at lower pH (1, 20). At a pH of about 4.9 phosphate appears to be completely in solution in milk (20). Some RO experiments were with whey at 30 C at various pH. From Figure 3 when the whey was acidified to pH 4.6 or even to pH 6.0, the strong flux decline, pH 6.6, did not occur. At pH 4.6 Ca-phosphate is supposed to be completely soluble so we do not expect Ca-phosphate fouling. At pH 6.0 more Caphosphate is soluble than at pH 6.6. Solubility is improved enough to make possible a fourfold concentration of whey at 30 C without a strong Ca-phosphate fouling. The permeate flux (Figure 3) at pH 6.0 was less than the calculated permeate flux (curve B); probably fouling is not avoided completely. However, the permeate flux at pH 4.6 was considerably lower than curve B. This effect should probably be attributed to the effect of pH on the behavior of the whey protein (9, 14). In the neighborhood of the iso-electric point, the protein has a low electrical charge. Under these conditions aggregation takes place easily and accumulation of protein at the membrane Journal of Dairy Science Vol. 63, No. 2, 1980
surface may occur. This p h e n o m e n o n agrees with that for UF of whey at various pH (9). Influence of Process Conditions
The process conditions such as temperature, flow velocity, and pressure have a considerable effect on permeate flux, and time also can be an important factor. To make the discussion easier, three resistances to permeation often can be distinguished: the membrane resistance Rm, the fouling resistance Rf, and the apparent resistance which can be attributed to concentration polarization Rp (see Appendix B). The membrane resistance especially is affected by temperature; the higher the temperature, the lower the Rm. For the other resistances the matter is more complex as they are particularly dependent on flow velocity and pressure. In the interpretation of our experiments it is assumed that the effect of time on the permeate flux can be neglected. This assumption can be justified since each experiment lasted only a few hours. During longer periods the effect of time on the permeate flux certainly becomes important. Effect of Flow Velocity
According to the method in Appendix B, for
permeate flux (I/m 2. h) . . . . . .
50
p._ur e._w_ater _flEx . . . . . . . . . . . .
40
30
20
10
0
Z
l 0
14
4
3
~
i
,
~
.8
1.2
1.6
2.0
5 concentration
ratio
2.4 2.8 osmotic pressure (MPa)
Figure 3. Permeate flux versus osmotic pressure of the bulk liquid (concentration ratio) for Re of whey at various pH-levels. T = 30 C, P = 4 MPa, v = 2 m/s. Curve B was calculated according to Appendix A assuming only concentration polarization (no fouling). • Gouda cheese whey pH = 6.6, DGouda cheese whey acidified to pH = 6.0, and o Gouda cheese whey acidified to pH = 4.6.
REVERSE OSMOSIS OF DAIRY LIQUIDS a range o f e x p e r i m e n t s with skim milk and whey, R f and Rp were calculated. In Figure 4 t h e e f f e c t o f the flow velocity on R f and Rp is given. The main trend is a decrease of Rp and R f with an increase of the f l o w velocity. F o r w h e y (2× c o n c e n t r a t e d ) at 10 C, an R f (curve A) of a b o u t .8 TPa.s/m is f o u n d at a f l o w velocity of 1 m/s; this is a b o u t 1.7 × the m e m b r a n e resistance R m at 10 C. At a flow velocity of 2 m/s, R f has decreased to about 1/5 o f R m . As in Figure 4, a further increase in flow velocity does not diminish R f to any extent. F r o m Figure 4 we see t h a t for skim milk R f is o f m u c h m o r e i m p o r t a n c e than for whey. Again d e p e n d e n c e of R f on flow velocity is strong. At 30 C R f for t w o f o l d c o n c e n t r a t e d skim milk b e c o m e s less t h a n R m at a flow velocity o f about 2.4 m/s. An increase in flow velocity o v e r 2.6 m/s m a y have o n l y a marginal effect. Also presented is R f for a c o n c e n t r a t i o n ratio of 1.5 and 3. At higher c o n c e n t r a t i o n
resistance (TPa. s/ml
c\ 3.5 3.0 2.5
3.0x
20 .
B
1.5
\ \\\
1,5x
A\
1.0
.5 .._R_mat 10 C
Rmat30 C
o
~,
ON \
~-x
1.0
\\ \ \
~
-,----~---, .5
~+.
20 .x? \ \ \ \ \
\
" "°~.~'~.
\"-,..'~-~''---,-
......... 1.5
~.
-,----:==:=- ---T-~ 2.0
2.5
flow rate qmls)
Figure 4. Various resistances to water permeation as a function of the flow velocity. A) Fouling resistance (Rf) for whey, 2X concentrated, T = 10 C, P = 4 MPa; B) Fouling resistance (Rf) for skim milk, 1.5, 2.0, and 3.0X concentrated, T = 30 C, P = 3.5 MPa; C) Fouling resistance (Rf) for skim milk, 2X concentrated, T = 10 C, P = 3.5 MPa; and D) Resistance due to concentration polarization (Rp), o for whey 2× concentrated, T = 10 C, P = 4 MPa, a for skim milk, 2X concentrated, T = 30 C, P = 3.5 MPa (1 TPa.s/m = 1012 Pa.s/m).
209
ratios R f increases. For 2 × c o n c e n t r a t e d skim milk, Rf at 10 C is m u c h higher than at 30 C; at a flow v e l o c i t y o f 2.6 m / s we still have an Rf o f a b o u t 1.0 TPa.s/m, which is m o r e t h a n twice R m at 10 C. To reach reasonably small Rf at 10 C, m u c h higher flow velocities should be used. The effect o f f l o w velocity on c o n c e n t r a t i o n polarization, expressed as Rp, also is in Figure 4. Curve D represents Rp for w h e y at 10 C and skim milk at 30 C (both t w o f o l d c o n c e n t r a t e d ) . The Rp is in b o t h cases a b o u t the same since the p e r m e a t e fluxes are at the same level. The Rp are considerably less than R f and Rm. It can be c o n c l u d e d that high flow velocities are n o t necessary for reducing Rp b u t are particularly so for reducing Rf. Lim et al. (15) evaluated Rp and R f for RO of cottage cheese whey, and c a m e to similar conclusions a b o u t the e f f e c t o f flow rate on Rp and Rf. F r o m e x p e r i m e n t s at flow velocities between 15 and 50 cm/s, t h e y c o n c l u d e d that R f w o u l d b e c o m e negligible at a b o u t 1 m/s. Since t h e y had to d e t e r m i n e this by e x t r a p o l a t i o n , it is understandable that this figure cannot be accurate. For skim milk the effect o f the flow v e l o c i t y on R f and, thus, on the p e r m e a t e flux is stronger than for whey. Since Rp is small c o m p a r e d with Rf, it may be assumed that R f is the main governing factor for the p e r m e a t e flux. If so, the situation is similar to that e n c o u n t e r e d for U F . F r o m theoretical considerations o f UF (19) 1 . 1 . the p e r m e a t e flux J v or -Rf - (since J v a ,Rf - - ) .Is p r o p o r t i o n a l to the mass transfer coefficient K, which is for t u r b u l e n t flow in tubes proportional to Re.8 (see A p p e n d i x A). However, for UF often the effect of flow velocity is m u c h greater than when calculated according to this relationship (17, 19). An evaluation o f our results with skim milk and of the results of Skudder et al. (27) showed a p r o p o r t i o n a l i t y to Re 1-3. Due to the relatively narrow range o f variation in flow velocity this is not accurate, but it emphasizes the similarity b e t w e e n RO of skim milk and U F of protein solutions. In both situations a protein layer is f o r m e d on the membrane. Effect of Pressure
Particularly for RO of skim milk applied pressure is a critical factor. Skudder et al. (27) p e r f o r m e d a range o f e x p e r i m e n t s b e t w e e n 2.4 and 4.1 MPa and f o u n d that the p e r m e a t e flux Journal of Dairy Science Vol. 63, No. 2, 1980
210
HIDDINK ET AL.
passed t h r o u g h a m a x i m u m in this region. W e carried o u t a similar r a n g e o f e x p e r i m e n t s in w h i c h t h e applied pressures were varied f r o m 2.5 to 4.5 MPa at 10 C a n d 30 C. F r o m t h e p e r m e a t e fluxes t h e f o u l i n g resistance R f was c a l c u l a t e d . In Figure 5, R f is p r e s e n t e d as a f u n c t i o n o f t h e a p p l i e d pressure. A t 10 C ( c o n c e n t r a t i o n ratio 1.5:1 a n d 2 : 1 ) R f a t 2.5 MPa are c o n s i d e r a b l y l o w e r t h a n R m . F u r t h e r m o r e , linear increase o f R f w i t h pressure is strong. A t 30 C a n d a t a p r e s s u r e o f 2.5 MPa, R f a g a i n are small, a n d a b o v e 3 MPa increase o f R f w i t h pressure is a l m o s t p r o p o r t i o n a l . Also, R f d e p e n d s o n t h e c o n c e n t r a t i o n ratio (Figure 5); t h e h i g h e r t h e c o n c e n t r a t i o n r a t i o t h e h i g h e r t h e Rf. This e f f e c t is m o r e p r o n o u n c e d at 10 C t h a n at 30 C. In Figure 6 t h e p e r m e a t e flux for skim m i l k at a c o n c e n t r a t i o n ratio 1.5:1 is p l o t t e d versus t h e a p p l i e d pressure. T h e a c t u a l p e r m e a t e flux increased to a pressure o f a b o u t 3.0 MPa, a n d for h i g h e r pressures a c o n s t a n t p e r m e a t e flux was r e a c h e d , w i t h a slight t e n d e n c y to
foulin( resistance (TPa.s/m) 1.4
9/ A /
/
decrease a t pressures a b o v e 4 MPa. In Figure 6 t h e p u r e w a t e r f l u x also is p r e s e n t e d , a n d t h e p e r m e a t e f l u x w h i c h c o u l d be e x p e c t e d if o n l y t h e o s m o t i c pressure were t a k e n i n t o a c c o u n t (curve B) a n d t h e p e r m e a t e f l u x if t h e e f f e c t o f c o n c e n t r a t i o n p o l a r i z a t i o n were also c o n s i d e r e d (curve C). T h e p h e n o m e n o n t h a t t h e p e r m e a t e f l u x is i n d e p e n d e n t o f t h e applied pressure a b o v e 3 MPa s h o u l d , a c c o r d i n g t o o u r calculations, b e a t t r i b u t e d t o increasing f o u l i n g resistance. A t a p r o c e s s t e m p e r a t u r e o f 10 C t h i s e f f e c t is even m o r e s e r i o u s t h a n at 30 C, a c o n s t a n t p e r m e a t e flux over t h e w h o l e p r e s s u r e range in this case. T h e r e s p o n s e o f t h e p e r m e a t e f l u x to p r e s s u r e v a r i a t i o n s can be u n d e r s t o o d as follows. In a s t e a d y s t a t e s i t u a t i o n as m u c h s o l u t e ( i n c l u d i n g p r o t e i n ) is m o v e d b a c k i n t o t h e b u l k liquid b y d i f f u s i o n a n d t u r b u l e n c e - as is b r o u g h t to t h e m e m b r a n e surface u n d e r t h e i n f l u e n c e o f p e r m e a t i o n t h r o u g h t h e m e m b r a n e . T h e transp o r t b a c k i n t o t h e b u l k is g o v e r n e d b y t h e mass t r a n s f e r c o e f f i c i e n t w h i c h again is d e t e r m i n e d b y t h e flow velocity. If at a c o n s t a n t flow v e l o c i t y a h i g h e r pressure is applied, d u e to an initial higher p e r m e a t e flux, t h e t h i c k n e s s o f t h e fouling layer increases, w h i l e at t h e same time a certain compaction of the deposit may t a k e place. T h e s e e f f e c t s will r e s u l t in a h i g h e r
/
1.2
// //
permeate flux (l/m 2. h)
/
l.O
2/
.- A
// /
.8
/
// /
/
/
9/ .6
/ /
Rmat]O
C
---
.4 -
/
//
/
/+" /
//
Rm at
-;0-c
/ .
//
tO
2.5
,
+,.- B
/
//
..,..~/
3.0
C
/
+//
/ " o
3.5
x-
4.0
D
E
4.5
applied pressure (NtPa)
Figure 5. Fouling resistance as a function of the applied pressure for RO of skim milk at a flow velocity of 2.6 m/s. A) T = 10 C, concentration ratio 2.0:1; B) T = 10 C, concentration ratio 1.5:1; C) T = 30 C, concentration ratio 2.0:1; D) T = 30 C, concentration ratio 1.5:1; and E) T = 30 C, concentration ratio 1.05:1 (1 TPa.s/m = 1012 Pa.s/m). Journal of Dairy Science Vol. 63, No. 2, 1980
0
/~ 2.5 ~
~ 3.0
a 3.5
i 40
i 45 pressure (/VlPa)
Figure 6. Permeate flux as a function of the applied pressure for RO of skim milk (1.5X concentrated) at T = 30 C, v = 2.6 m/s. A) pure water flux; B) permeate flux to be expected without concentration polarization and fouling; C) permeate flux to be expected without fouling; and D) actual permeate flux.
REVERSE OSMOSIS OF DAIRY LIQUIDS
211
2 the results of our e x p e r i m e n t s are given. F o r b o t h layers a fairly strong d e p e n d e n c e on c o n c e n t r a t i o n ratio, pressure, and flow v e l o c i t y was f o u n d , which makes it d o u b t f u l that there was a characteristic difference b e t w e e n t h e t w o layers. We think it is m o r e useful to consider the fouling layer as a whole and to distinguish within the fouling layer a certain gradient in c o n c e n t r a t i o n and firmness. This m o d e l agrees with that for c o n v e n t i o n a l filtration with a p o r o s i t y gradient (30). The effects in Table 2 can be u n d e r s t o o d as follows. The higher the f l o w velocity (shear rate), the m o r e will be scraped o f f o f the fouling layer and the lower the remaining fouling resistance after water rinsing. High pressures will cause a greater c o m p a c t i o n of the fouling layer, which again causes a firmer and m o r e fouling resistance after water rinsing.
Rf, and this process continues until such a high R f is built up t h a t equilibrium exists again b e t w e e n the a m o u n t o f solute transported to and f r o m the m e m b r a n e surface. Ultimately this results in an unchanged p e r m e a t e flux. The slight decrease in p e r m e a t e flux at higher pressures can be explained by a m o r e serious c o m p a c t i o n o f the fouling layer. Skudder et al. (27) report that the position o f the m a x i m u m in p e r m e a t e flux d e p e n d s b o t h on f l o w velocity and t e m p e r a t u r e . The higher the f l o w velocity and t e m p e r a t u r e , the lower the R f and the higher the o p t i m u m pressure. In our e x p e r i m e n t s with w h e y an effect of pressure on p e r m e a t e flux was similar but only at pressures above 4.5 MPa. Such an effect for w h e y also was f o u n d by D o n n e l l y et al. (5). The l o w protein c o n t e n t o f w h e y and the high diffusion coefficient for w h e y protein c o m p a r e d to casein m a y m e a n that for w h e y at higher pressures o n l y (i.e., higher p e r m e a t e fluxes) a considerable fouling resistance is built.
CONCLUSIONS
For reverse osmosis of dairy liquids, b o t h the c o m p o s i t i o n o f the feed and process conditions have an i m p o r t a n t effect on p e r m e a t e flux. F l u x limiting factors include o s m o t i c pressure and fouling tendency. F o r G o u d a whey, processed at 30 C, fouling o f the m e m branes due to Ca-phosphate precipitation is an i m p o r t a n t factor, while for skim milk, desalted w h e y , and acid whey, protein is the i m p o r t a n t fouling agent. Fouling by G o u d a w h e y can be limited by decreasing the p H or by decalcifying the whey. F u r t h e r m o r e , it is possible, by p r o p e r process conditions in which the flow v e l o c i t y plays an i m p o r t a n t role, to decrease the resistance due to fouling and c o n c e n t r a t i o n polariza-
Structure of Fouling Layer
Lim et al. (15) distinguished within the fouling layer ( e x p e r i m e n t s with cottage cheese whey), a cake layer which should be a c o m p a c t layer " a t t a c h e d " to the m e m b r a n e surface, and a viscous layer b e t w e e n the cake layer and the circulating bulk. The characteristic difference b e t w e e n those t w o layers is that the viscous layer is removed by rinsing with water while the cake layer remains a t t a c h e d to the m e m b r a n e . In our e x p e r i m e n t s with skim milk there was also a proteinaceous fouling layer as for Lim et al. (15). This layer was divided into a cake layer and a viscous layer (see A p p e n d i x B). In Table
TABLE 2. Distribution of the fouling resistance over a viscous layer and a cake layer for RO of skim milk. Rcake = fouling resistance a f t e r w a t e r rinsing. Rviscou s = Rf - Rcake. (1 TPa.s/m) = 1012 Pa.s/m). Rviscous (TPa.s/m)
Rcake (TPa.s/m) Concentration ratio
1.05
2.0
3.0
1.05
2.0
3.0
P = 2.5 MPa
v = 2.6 m/s
.01
.01
. . . .
02
.03
...
3.5 MPa
v = 1 . 4 m/s v = 2.0 m/s v = 2.6 m/s
.16 .12 .05
.88 .29 .10
. . . . .54 .24
63 .18 .04
.66 .21 .07
...
v = 2.6 m/s
.07
.22
. . . .
13
.20
...
P =
P = 4.5 MPa
Journal
.24 .09
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HIDDINK ET AL.
tion considerably below the resistance of the membrane. An important step needed to reach a h i g h e r p e r m e a t e f l u x is t h e d e v e l o p m e n t o f membranes with higher water permeabilities and resistance to high temperatures.
14
15 REFERENCES
1 Brule, G., E. Real del Sol, J. Fauquant, and C. Fiand. 1978. Mineral salts stability in a q u e o u s phase o f milk: Influence o f heat treatments. J. Dairy Sci. 61:1225. 2 de Boer, R., J. N. de Wit, and J. Hiddink. 1977. Processing of whey by m e a n s o f m e m b r a n e s and applications o f whey protein concentrate. J. Soc. Dairy Technol. 30:112. 3 Dejmek, P. 1975. Permeability o f the concentration polarization layer in ultrafiltration o f macromolecules. Page A2.26 in Separation processes by m e m b r a n e s , ion exchange and freeze-concentration in food industry. A.P.RA.A., Paris. 4 D e l a n e y , R.A.M., and J. K. Donnelly. 1977. Applications o f reverse osmosis in the dairy industry. Page 417 in Reverse osmosis and synthetic m e m b r a n e s . S. Sourirajan, ed. Nat. Res. Counc. Can., Ottawa. 5 Donnelly, J.K., A. C. O'Sullivan, and R.A.M. Delaney. 1974. Reverse osmosis-concentration applications. J. Soc. Dairy Technol. 27:128. 6 Eriksson, P. 1977. Concentration of whey by reverse osmosis-inventory and operating experiences. Nordeuropaeisk Mejeritidsskrift. 43 : 238. 7 Evans, E. W., and F. A. Glover. 1974. Basic principles of reverse osmosis and ultrafiltration. J. Soc. Dairy Technol. 27:111. 8 Fenton May, R. I., C. G. Hill, C. H. A m u n d s o n , M. H. Lopez, and P. D. Auclair. 1972. Concentration and fractionation of skim milk by reverse osmosis and ultrafiltration. J. Dairy Sci. 55 : 1561. 9 Forbes, F. 1972. Considerations in the optimisation of ultrafiltration. Chem. Eng.:21. 10 Glover, F. A., and B. E. Brooker. 1974. The structure of the deposit formed on the m e m b r a n e during the concentration of milk by reverse osmosis. J. Dairy Res. 41:89. 11 Glover, F. A., P. J. Skudder, P. H. Stothart, and E. W. Evans. 1978. Reviews of the progress of dairy science: reverse osmosis and ultrafiltration in dairying. J. Dairy Res. 45:291. 12 Hayes, J. F., J. A. Dunkerley, and L. L. Muller. 1974. Studies on whey processing by ultrafiltration. Austr. J. Dairy Technol. 29:132. 13 J o h n s t o n , A. N. 1977. How European cheese
APPENDIX A Calculation of Concentration Polarization
16
17 18
19
20
21
22
23
24 25
26
27
28
29 30
plants save costs in w h e y processing t h r o u g h reverse osmosis system. Dairy and Ice Cream Field 160:1160. Lee, N. D., and R. L. Merson. 1976. Chemical t r e a t m e n t s o f cottage cheese w h e y to reduce fouling of ultrafiltration m e m b r a n e s . J. Food Sci. 41:778. Lim, T. H., W. L. Dunkley, and R. L. Merson. 1971. Role of protein in reverse osmosis o f cottage cheese whey. J. Dairy Sci. 54: 306. Mehta, B. 1973. Processing o f model compositional whey solutions with pressure driven membranes. Ph.D. thesis, The Ohio State University, Columbus. Muller, L. L. 1976. Whey utilization in Australia. Australian J. Dairy Technol. 31 : 92. Peri, C., and W. L. Dunkley. 1971. Reverse osmosis of cottage cheese whey. 1. Influence o f composition o f the feed. J. Food Sci. 36:25. Porter, M. C. 1972. Concentration polarization with m e m b r a n e ultrafiltration. Ind. Eng. Chem. Prod. Res. Develop. 11 : 234. Pyne, G. T. I 9 6 2 . Reviews o f the progress o f dairy science. Some aspects o f the physical chemistry of the salts o f milk. J. Dairy Res. 29:101. Reid, C. E. 1972. Principles of reverse osmosis. Page 109 in Industrial processing with membranes. R. E. Lacey and S. Loeb, ed. Wiley-Interscience, New York. Roger, R., J. L. T h a p o n , J. L. Maubois, and G. Brule. 1976. Hydrnlyse du lactose c o n t e n u dans l'ultrafiltrat de lair ou de lactosdrum en r~acteur e n z y m a t i q u e ~ m e m b r a n e . Le Lait 5 5 1 - 5 5 2 : 5 6 . Rose, D., and H. Tessier. 1959. Composition of ultrafiltrates from milk heated at 80 to 230 F in relation to heat stability. J. Dairy Sci. 42:969. Schoorl, N. 1929. Suiker titratie. Chem. Weekblad 26:130. Short, J. L., and R. K. Doughty. 1976. An evaluation of m o d u l e s and m e m b r a n e s for the concentration of cheddar cheese whey by reverse osmosis. New Zealand J. Dairy Sci. Technol. 11 : 237. Short, J. L., and I. R. Hughes. 1978. The concentration of separated milk by reverse osmosis. New Zealand J. Dairy Sci. Technol. 13:114. Skudder, P. J., F. A. Glover, and M. L. Green. 1977. An e x a m i n a t i o n of the factors affecting the reverse osmosis o f milk with special reference to deposit formation. J. Dairy Res. 44: 293. Smith, B. R., and R. D. Macbean. 1978. Fouling in reverse osmosis. Australian J. Dairy Technol. 33:57. Sourirajan, S. 1970. Reverse osmosis. Academic Press, New York. Tiller, F. M., and J. R. Crump. 1977. Solid liquid separation: An overview. Chem. Eng. Progr. 73:65.
somewhat equation:
simplified
form,
by
Jv = (AP- ATrw)/Rm The permeate (water) flux through a reverse osmosis membrane is u s u a l l y d e s c r i b e d , in a Journal of Dairy Science Vol. 63, No. 2, 1980
where
the following
[11
REVERSE OSMOSIS OF DAIRY LIQUIDS flux through m e m b r a n e ( l / m 2 . h ) or (m/s) hydrostatic pressure difference across AP= the m e m b r a n e (Pa) ATrw = o s m o t i c pressure difference across the m e m b r a n e (Pa) R m = resistance to water p e r m e a t i o n o f the m e m b r a n e (Pa.s/m) Jv
--
If restricted to high rejection membranes, the o s m o t i c pressure at the p e r m e a t e side m a y be neglected; then &Trw equals Zrw, which is the osmotic pressure at the m e m b r a n e surface at retentate side. As a result of the rejection of the solutes by the m e m b r a n e , at the m e m b r a n e surface a local c o n c e n t r a t i o n (and local o s m o t i c pressure) is found which is higher than in the bulk of the solution. This c o n c e n t r a t i o n polarization is c o u n t e r a c t e d by diffusion of solutes back into the bulk, by t u r b u l e n t eddies, and by shearing of the fluid near the m e m b r a n e . At equilibrium the transport o f solutes back into the bulk occurs as fast as solutes are brought up to the m e m b r a n e . A t equilibrium the concentration o f solutes ( o s m o t i c pressure) at the m e m b r a n e surface is given by (7, 19): C w / C b = Ztw/~ b = exp (Jv/k)
[2]
where c o n c e n t r a t i o n o f solutes at the m e m brane surface (kg/kg) C b = c o n c e n t r a t i o n of solutes in the bulk (kg/kg) Ti'b = o s m o t i c pressure in the bulk (Pa) k= mass transfer c o e f f i c i e n t (m/s)
CW
=
It is assumed that the o s m o t i c pressure is directly p r o p o r t i o n a l to the solute concentration. A c o m b i n a t i o n of equations [1] and [2] results in: Jv = (AP - 7rw)/Rm
[3] = (& P / R m ) - ( z r b / R m ) exp (Jv/k) If k is k n o w n , the p e r m e a t e flux J v can be calculated. F o r a calculation o f k, for t u r b u l e n t flow in tubes, generally the following dimensionless correlation is used (19): Sh = .023 Re.8 Sc.33
[41
213
where
Sh Re Sc and d v ID v
= kd/ID ( S h e r w o o d n u m b e r ) = dv//) ( R e y n o l d s n u m b e r ) = u/ID ( S c h m i d t n u m b e r ) = = = =
d i a m e t e r of the t u b e (m) f l o w velocity in the t u b e (m/s) diffusion coefficient (m2/s) kinematic viscosity (m2/s)
For the calculation of k, e q u a t i o n (4) can be written as: k = .023 ID '67 v ' 8 / ( d "2 /).47)
[5]
Equation [31 was evaluated for the liquids referred to in Table 1. For such an evaluation the physical properties o f the liquids m u s t be k n o w n . The viscosity and o s m o t i c pressure were d e t e r m i n e d e x p e r i m e n t a l l y , the values o f the diffusion coefficient were taken f r o m the literature (Table A.1). In our calculations concerning c o n c e n t r a t i o n polarization, we o n l y a c c o u n t e d for those c o m p o n e n t s which are contributing to the o s m o t i c pressure (i.e., lactose and salts). Since lactose and salts have different diffusion coefficients and, thus, different k-values, e q u a t i o n [31 was e x t e n d e d to two c o m p o n e n t s as follows: Jv = (AP/Rm) - (/Tbl/RM)'exp (Jv/kl) [6] - ( r r b 2 / R m ) - e x p (Jv/k2) It appeared that an evaluation o f e q u a t i o n [3] resulted in a p e r m e a t e flux versus o s m o t i c pressure of the bulk liquid which nearly coincides for w h e y , U F - p e r m e a t e , and lactose, see curve B in Figure 1. The variations were less than 1 liter/m 2.h. This can be u n d e r s t o o d f r o m the fact that the mass transfer c o e f f i c i e n t k for these liquids is about equal which again is caused by t h e fact that the process c o n d i t i o n s are the same and the viscosity, density and diffusion coefficients do not differ much. The calculated p e r m e a t e flux for skim milk tends to be s o m e w h a t lower (.5 to 2.0 l i t e r / m 2 . h ) than curve B in Figure 1. This can be u n d e r s t o o d f r o m the higher viscosity o f skim milk. Journal of Dairy Science Vol. 63, No. 2, 1980
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HIDD1NK ET AL.
TABLE A . I . Solute-water diffusion coefficients for some components of milk at 20 C. Diffusion coefficient (m2/s) Lactose 5% Ash(KCI.74%) Whey protein (/3-1actoglobulin 1%) Casein 3%
APPENDIX B Evaluation of Resistance due to Fouling and Concentration Polarization from the Experiments
Strictly speaking, c o n c e n t r a t i o n polarization reduces the driving force. However, the effect of c o n c e n t r a t i o n polarization o f t e n is translated into an additional resistance to p e r m e a t i o n o f the m e m b r a n e , when the original driving force is considered. However, a fouling layer on the m e m b r a n e m a y cause a real additional hydraulic resistance acting in series with the m e m b r a n e resistance (28). The m e m b r a n e resistance R m can be determined f r o m the pure water flux for the clean membrane. F r o m the e x p e r i m e n t a l p e r m e a t e flux for the liquid concerned, after an evaluation of the mass transfer coefficient k, the c o n c e n t r a t i o n polarization modules Cw/Cb = rrw/Zrb can be calculated f r o m A p p e n d i x A, e q u a t i o n [21. If k n o w n nw the value of the fouling resistance R f can be calculated since
Journal of Dairy Science Vol. 63, No. 2, 1980
3.8 18.4 .64 .19
× 10-10 X 10 -1° X 10 -10 × 10 -1°
Reference (4) (29) (4) (4)
Jv = (AP - rrw)/(Rm + Rf)
[71
Next the c o n c e n t r a t i o n polarization resistance Rp can be calculated f r o m the following equation: Jv = (AP - "trb)/(Rm + Rf + Rp) To divide the fouling resistance (Rf) in a resistance of a cake layer (Rcake) and the resistance of a viscous layer (Rviscou s) the following procedure was followed. The bulk liquid was r e m o v e d by water; then the pure water flux was measured under the same c o n d i t i o n as previously used for t h e liquid. F r o m this pure water flux Jw, Rcake was calculated according: J w = A P / ( R m + Rcake )
[8]
N e x t Rviscuu s can be calculated f r o m : Rviscou s = R f - Rcake
[9]