Ind. Eng. Chem. Process Des. Dev., Vol. 17,No. 2, 1978
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The Recovery of Protein from Potato Juice Waste Water by Foam Separation D. C. Weijenberg, J. J. Mulder, A. A. H. Drinkenburg,' and S. Stemerding Laboratory for Chemical Engineering, University of Groningen, Groningen, the Netherlands
The production of potato starch implies the release of large quantities of protein dissolved in the potato juice. Efforts have been made to retrieve the protein. In this contribution the extent will be discussed to which protein from potato juice can be collected at air-water interfaces, thus enabling the separation of protein by foam fractionating methods. The surface loading was measured as a function of the bulk concentration, temperature, pH, and sodium chloride addition. The rate of loading was determined as well as the stability and density of the foam.
Introduction Potato starch is a much used carbohydrate in Western Europe for direct consumption and as a basic chemical for derivates. In its production process large quantities of potato juice are released containing 1-2% protein. T h e juice, after dilution with process water, is discharged largely upon open water, giving rise to extensive pollution problems. For example, in Holland a BOD, equivalent to 25 million inhabitants on a yearly basis, is dumped within a few months. T h e waste water problem has been of great concern lately to government and potato starch companies. Efforts have been made to reduce the quantity of process water that is led to waste, thereby concentrating the protein. Methods of separating the protein by coagulation or reversed osmosis (de Noord, 1976) can then be followed. In our laboratory another approach has been undertaken. From observations in the environment, it became clear that the waste water produced a very persistent type of foam. Consequently it has been attempted to separate the proteins by means of foam fractionation (Brady, 1949; Lemlich, 1972), a method applied during World War I1 for retrieving protein from sugar beets (Ostwald and Siehr, 1937). Other applications are the removal of enzymes from solutions (Bikerman, 1953). This paper deals with results of our investigation into the surface concentration process; it describes the physical loading of the surface, the liquid-gas interface. Many parameters of influence have been studied, viz. concentration, pH, temperature, and salt addition. Also the stability of the foam has been measured as well as the rate of loading. An investigation into the technology of the loading process has also been initiated. Excess Surface Concentration. Gibbs Relation The surface characteristics of the fluid are reflected in the value of its surface tension. Soluble organic compounds added to water will break up the surface structure and thereby lower the surface tension, or better the surface energy. Moreover, surface-active components tend to aggregate a t the liquid-gas interface, thereby creating a much larger influence than corresponds to their bulk concentration. It is therefore not surprising that there exists a relationship between the surface tension and the amount of organics present a t the liquid-gas boundary in excess of the bulk concentration of the liquid. This excess surface concentration, r, measured in mol/m2 of surface, can be thermodynamically related to the bulk concentration as has been shown by Gibbs (1948).
in which y is the surface tension, c is the concentration of the active component in the bulk, R is the universal gas constant, and T i s the absolute temperature. Especially for large molecules the concentration c must be modified. In this case in its place an activity is introduced, to be determined empirically
Note that this relation is only valid under equilibrium circumstances. Since the surface tension of aqueous solutions will generally decrease exponentially with bulk concentration, this means t h a t for a certain value of the bulk concentration a maximum in excess surface concentration r is reached, after which maximum r decreases to zero again. For values of the bulk concentrations well below this value a linear relationship between bulk concentration and surface excess concentration exists. In Figure 1 the experimental relation between y and In c is given for potato juice; c is expressed here in units corresponding to natural potato juice. For c = V the potato juice is not diluted; for c = 10-2V one volume unit of potato juice is diluted with 99 volume units of pure water. Now from this diagram a peak value of the surface concentration a t a concentration equal to or lower than V would be expected. However, as will be shown later on, the peak value will be found a t larger concentration value, the activity coefficient being larger than 1. Also it is seen that the surface tension decreases with standing time, due to deterioration of the organic compounds.
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Figure 1. Surface tension vs. logarithmic concentration.
0019-7882/78/1117-0209$01.00/0
0 1978 American Chemical Society
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Figure 4. Excess surface concentration of protein vs. logarithmic dilution of potato juice.
tion.
The investigation reported here had a threefold aim: (1) to measure the excess surface concentration, r, as a function of the parameters mentioned above, thereby selecting a set of parameters at which an optimal result can be expected in a technical separation process; ( 2 ) to measure the rate with which protein is collected at the gas-liquid interface; (3) to measure the foam stability, which is a very important parameter. When a foam bubble, or a lamella, collapses the aggregated protein is released into the bulk again, thereby locally increasing the bulk concentration so much that in the adjacent underlying surface layers the protein will be stripped, etc., causing a cone of protein-free foam surface.
Experimental Section In the first instance air was bubbled through a well defined diluted potato juice solution. The foam on top was sampled and analyzed. However, i t soon became clear t h a t the residence time of the air bubbles in the solution was far too small for reaching t h e equilibrium excess surface concentration. Choosing a higher level for the solution in the aerated vessel would have given higher surface concentrations, but, as was proved later on, equilibrium would have required liquid heads of approximately 2 m. It was decided instead to recycle L e
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bulk liquid over the foam. Therefore the equipment was modified as shown in Figure 2 . Air is bubbled through water (1) and thereby is saturated with water vapor. The flow is measured by a pressure difference over a capillary tube (4). Needle valve 3 regulates the air flow. Column 5 has a thermostated wall; the air here is brought to the desired temperature by bubbling through water again. I n vessel 7 foam is made by introducing bubbles of uniform size. As soon as the bubbles pass the liquid surface they are caught in a concentric tube which rises from the surface with t h e same speed as the foam layer. Therefore, no slip between foam and glass container will occur. The uniform bubbles are made by using a membrane pump (6) as a pulsator on the air flow. The membrane is excited with 5 0 - H ~ac current and thus produces 50 pulses/s which create 50 bubble& in the foam vessel (7). The liquid in this vessel is (diluted) potato juice. The solution is pumped around and after thermostating tube 10, is distributed on top of the foam layer. The foam layer is irrigated for a certain time in the recycle loop after foam formation has been stopped. After irrigation the foam is allowed to drain for 5 min. Then the foam is removed, frozen, broken, and after adding 0.1 N NaOH to keep the proteins in solution, stored for analysis. By using a high-pressure liquid chromatograph for analysis only very small samples are needed, so that the laboratory equipment can be kept within reasonable proportions.
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Figure 5. Excess surface concentration of protein vs. pH.
Ind. Eng. Chem. Process Des. Dev., Vol. 17,No. 2, 1978
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Figure 6. Stability of foam as a function of excess surface concentration (a) and pH (b).
Experimental Results a. Surface Tension Measurements. Surface tension measurements were made by a modified Wilhelmy plate method. Since the value of the surface tension was time dependent, the plate was directly attached to the arm of an electronic balance. During the first 20 s after a fresh surface has been formed (e.g., through stirring gently) the surface tension drops sharply from a value near the surface tension of pure water (72 dyn/cm) to values of approximately 50 dyn/cm. During the first time interval the process is controlled by molecular diffusion of protein toward the gas-liquid interface. Thereafter the kinetics of adsorption are of the greatest importance and a much slower further decrease of the
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surface tension is found to the final value. Moreover, after some time chemical degradation of the protein sets in, although this can be depressed by adding sulfite. b. Excess Surface Concentration Measurements. 1. Influence of Irrigation Time. A number of experiments were performed for different irrigation times. Figure 3 gives the results. Although there is quite some scattering, the conclusion may be drawn that for times longer than 20 s saturation of the foam with protein is reached. T h e 20 s roughly correspond to one complete refreshing of the liquid holdup in the foam, as was recorded separately by color tracing tests. Note also the close correspondence with the 20 s required for reaching the point of inflection in surface tension measurements. After these tests, it was concluded t h a t an irrigation time of 150 s was sufficient for reaching the equilibrium excess surface concentration, especially after 5 min more of draining the foam. 2. Influence of Potato Juice Dilution. In Figure 4 the outcome of experiments is shown in which the concentration of the bulk liquid was varied a t neutral conditions (pH 7 ) and 20 "C. A very steep excess surface concentration peak is shown for c 0.1V. From the deviation of the value of the bulk concentration a t which the maximum excess surface concentration is found from the inflection point in the surface tension graph, the conclusion could be drawn t h a t instead of concentration an activity must be taken t h a t has roughly one tenth of the concentration value. I t must be borne in mind, however, t h a t the potato juice solution contains many different surface active components, some of which may be very small molecules (e.g. peptides) but with a marked influence on the surface tension. Therefore not much theoretical value can be attached to the thus calculated activity, although its numerical value provides information for the fractionating process to be applied. T h e top value of r corresponds to a protein layer of approximately 20 A thickness, roughly equivalent with a monolayer of protein. 3. Influence of the Hydrogen Ion Concentration. I t can be expected that the hydrogen ion concentration of the solution is of importance because the structure of protein is dependent upon the p H of the bulk. Proteins can be rolled up in spheres or be stretched out. The structure is closely connected to the isoelectric point, measured to be near p H 7 . Indeed as is shown in Figure 5 , the excess surface concentration reaches its highest value near neutral conditions. Moreover, r is very dependent upon the pH. Below a p H of 4.5 coagulation of the protein occurs and values of r become meaningless. 4. Influence of Temperature and Salt Addition. The influence of the temperature was measured by thermostating the column at two more temperatures, 15 and 40 "C. Excess concentrations a t 40 "C were slightly higher, but well within the possible experimental error. Addition of NaCl up to 1
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Figure 8. Density of foam vs. bulk concentration for two percolation times through potato juice.
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Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 2, 1978
kg/mt3also did not produce a significant change, although the measured values of r were somewhat higher than without salt added to the solution. c. Foam Density and Foam Stability. As has been shown above, the stability of the foam is of extreme importance during the process, due to the stripping effect of protein which is set free to t h e bulk after a collapse of a lamella. T h e foam stability is connected with the thickness of the lamella (Ross, 1943); in the literature, values less than 200 8, are given. At values of around 50 8, rupture will occur due to molecular forces. Also for thin lamellae the number of foam cells may be reduced because of diffusion of the gas from smaller cells to larger ones (de Vries, 1958), which are thermodynamically more stable. Knowing the size of the bubbles and the air flow rate provides us with a method for calculating the thickness from overall foam density. Again we can expect that the stability and the density of t h e foam is directly related to the excess surface concentration and thus to the bulk concentration and pH. More protein a t the interface will certainly slow down the draining and therefore tend to increase the density. The stability will be positively affected by the excess surface concentration of protein. The last argument can easily be shown to be correct: suppose that by any chance, an instability in the foam is created. Then we may expect t h a t locally t h e surface concentration of protein is diminished (e.g., stretching of the interface). But then the surface tension, also locally, increases, causing the surrounding interface to contract again, thereby acting against the instability (Marangoni, 1871).This inherent stability is lost for very thin lamellae due t o the orientation of the hydrating water molecules to the absorbed protein layers. T h e stability has been measured qualitatively as follows. For a series of experiments a t different bulk concentrations t h e foam was allowed to drain for 10 min, during which time the number of collapsed lamellae was counted. When none were found, the stability was given the value 5; when less than 20 lamellae collapsed, t h e value given was 4. For more than 20 collapses but without any larger voids present in the foam, the value given was 3; when one large void was visible the value was 2. Sometimes more than one void was found and then the value 1 was attached. Ultimately for completely unstable and collapsing foam we gave the value 0. The total number of foam cells in the tube can be calculated and varied between 2000 and 5000. Now in Figure 6 the stability thus defined is given as a function of t h e excess surface concentration (part a ) and as a function of p H (part b). T h e scatter in results in Figure 6a is partly due to the arbitrary division in only five stability classes b u t especially through the scatter of the protein analysis results. The curve drawn in Figure 6a must therefore only be considered as a visual expression of the trend that the stability of the foam increases with a higher coverage of the surface with protein. I t is clear that for the highest excess surface tension the most stable foam is created, corresponding t o c = 0.1V and nearly neutral pH. Because care was taken that uniform bubbles were made a t the gas orifice in the liquid the foam cells were also uniform in size and no net interdiffusion of gas between the cells took place as was experimentally observed for stable foams. Collapsing foams will return t h e absorbed protein to t h e draining liquid. This may locally lead to concentrations far above the bulk concentration of 0.1 V, and may consequently lead to stripping of protein from the foam in a cone below the point of collapse. In all experiments the weight of the foam in the catching tube has been determined. The density was calculated in kg of water/m" of foam. In Figure 7 this density is given as a function of the bulk concentration and the pH. The left part of Figure 7a is puzzling. One would expect that a t bulk concentrations below c = 0.1 V the protein coverage of t h e lam-
ellae would become smaller and therefore would allow the water t o drain more rapidly. This is not observed. Explanations can be thought of b u t are very speculative, e.g. hydratation of protein by water or disturbing effects of other components. Instead it was decided to prove the consistence of the phenomenon in a different experiment. Two series of tests were performed in which no recirculation of the solution over the foam was applied. In series 1 the contact time between bubbles and solution was 0.4 s; in the second series, 1.9 s. Therefore in t h e second series more protein was present in the foam. Figure 8 illustrates the results. Indeed the density of the protein rich foam is larger, but here also t h e foam density increases with decreasing bulk concentration. T h e experimental error in these experiments was estimated to be small enough to justify the drawing of the curves. In separate experiments the influence of salt addition on foam density has been proven to be negligible; on the contrary, the influence of temperature was very marked. Increasing the temperature to 40 "C decreased the density with a factor 2 most certainly caused by a decrease in (surface) viscosity.
Discussion Maximum excess surface concentration was obtained a t a hydrogen ion concentration of and a bulk protein concentration of 1 to 10 diluted potato juice. T h e foam is very stable under these conditions due t o the structure of the interface a t which a monolayer of protein is absorbed. T h e gas-liquid interface has to be in contact with the bulk liquid for at least 20 s, as was concluded from the experiments where the surface tension of a fresh interfare was measured as a function of time. The same time constant was found from the foam enrichment measurements (Figure 3). This time interval is also found when the enrichment of the bubble surface is calculated according to Higbie's instationary mass transfer theory. According to this theory the mean mass partial liquid mass transfer coefficient will'be equal to (Higbie, 1935).
in which D = molecular diffusion coefficient in water ( 5 7 X IO-" m2/s) and t = time of contact between a bubble and a liquid element passing the bubble during its rise; thus t is equal to the ratio between the bubble diameter and the bubble rise velocity. We calculated for bubbles of 1.5 mm a E of lo-* m/s. From an unsteady-state mass balance and using the experimentally determined equilibrium relationship we can calculate the time t h a t is needed to reach 90%of t h e equilibrium excess surface concentration. This time is then calculated to be 18 s, also very close to the experimentally found 20 s. In what concerns the technological implications it became clear that further investigations will be necessary. The first emphasis must be laid upon the reduction of the water content in the foam lamellae. This reduction of water content will mean a higher enrichment factor for the protein in the foam on basis of weight. If a lamellae thickness of 400 8, can be reached, then a solution of 10%protein by weight will be recovered. The draining section must then be separated from t h e foam irrigation section and preferably be horizontal if gravity is to be used as the draining agent. Centrifugal draining can also be considered. Another implication of the equilibrium relationship found between excess surface concentration and the bulk concentration will be that the concentration of the solution entering the foam separation column must be kept below 0.1V. Therefore, if undiluted potato juice is used, care must be taken that the concentration is artificially lowered, which can be done by recycling t h e irrigating solution. Furthermore, a t -
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 2, 1 9 7 8
tention must be paid to keep t h e gas-bubble volume as small as possible since air-compressing costs will increase with t h e bubble volume size to the exponent 2Jj, and also since t h e size of the equipment will increase considerably with t h e bubble size.
Nomenclature a = activity, mol/m" c = concentration of t h e bulk liquid, mol/m:3 D = molecular diffusion coefficient in liquid, m*/s k = mean mass transfer coefficient, m/s R = universal gas constant, J/mol K t = contact time, s T = absolute temperature, K V = concentration in units of potato juice
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y = surface tension, N/m r = surface excess concentration, mol/m2
Literature Cited Bikerman, J. J., "Foams", Reinhold, New York, N.Y., 1953. Brady, A. P., J. Phys. Chem., 53, 56 (1949). de Noord, K. G., World Congress on Chemical Engineering, Amsterdam, 1976. de Vries, A. J., R e d Trav. Chiffl. Pays Bas, 76, 81 (1958). Gibbs, J. W., "Collected Works", Vol. I, p 219, Yale University, 1948. Higbie, R. W., Trans. Am. Inst. Chem. f n g . , 31, 365 (1935). Lemlich, R., "Adsorptive Bubble Separation Techniques", Academic Press, New York, N.Y., 1972. Marangoni, C., Nuovo Cimento, 2, 239 (1871). Ostwald, W.. Siehr, A.. KoIIoid Z.,76, 33 (1937). Ross, S., J. Phys. Chem.. 47, 266 (1943).
Receitied f o r reuieu! October 2 2 , 1976 Accepted November 26, 1977
