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The resins all have high exchange capacities and can be used in aqueous and organic solvents. The cyclic tests showed good stability against chemical degradation and physical attrition of the particles. The chloromethyl intermediates can be converted by conventional reactions into a variety of resins, to be used in water purification, hydrometallurgy, and catalysis. L i t e r a t u r e Cited Anderson, R. E. I d . Eng. Chem. 1964, 3, 85. Belfer, S.; Glozman, R. J. Appl. folym. Sci. 1979, 24, 2147. Belfer, S.; Deshe, A.; Glozman. R.; Warshawsky, A. J. Appl. folym. Sci. 1980, in press. Davankov, V. A.; Rogozhln. S. V.; Tsyurupa, M. P. “Ion Exchange and Solvent Extraction”, Martnsky, J. A.; Marcus, Y.; Ed.; Marcel Dekker Inc.: New York, 1977; Vol. 7, Chapter 2. Dinaburg, V. A.; Koiomieltsev, 0. P.; Vansheldt. A. A. USSR Patent 209 741. 1968. Galazzi, H., Germen Offen 2 455 946, 1975.
Hagge, W.; Quaedolbg, M.; Selfert, H. German Patent 1065 174, 1959. Hauptmann, R., Schwa&&, G. Z. Chem. 1968, 6 , 227. Hauptmann, R.; Schwachula, G. Kunstharz-Ionenaeistusher, Symposiumsbericht, Akademic Verlag: Berlin, 1970: p 196. Hauptmann, R.; Schwachula, G.; Reuter, H.East German Patent 8996, 1972. Kun, K. A.; Kunin, R. J. fowm. Scl., A - I 1988, 6. 2689. Rusting, Ir. N.; Frielink, J. G. Netherlands Patent Appl. 6414948, 1965. Schwachula, G.; Hauptmann, R.; Kain. J. J. folym. Scl., Polym. Symp. 1974, 47, 103. Sederel, W. L.; de Yong, G. Y. J. Appl. folym. Scl. 1973, 17, 2835. Tager, A. A.; Tsilipotkiva, M. V., Makovskaya, E. B.; Lyustgarten, E. I.;Pashkov, A. B.; Lagunova, M. A. Vysokomol. Soed. 1971, A13, 2370. Vansheidt, A.; Okhrimenko, 0. I. USSR Patent 114174, 1958. Warshawsky, A.; Kallr, R. J. Appl. folym. 1979, 24(4), 1125. Waters, J. G.; Smith, Th. G. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 591. Zabicky-Zissmann, J. S.; Oren, J.; Katchalsky, E. German Patent 2 041 915, 1971.
Received for review July 7, 1980 Accepted November 12, 1980
Tubular Cellulose Acetate Reverse Osmosis Membranes for Treatment of Oily Wastewaters 0. Kutowy, W. L. Thayer, J. Tlgner, and S. Sourlrajan’ Division of Chemistry, National Research Council of Canada, Ottawa, Canada, K I A OR9
G. K. Dhawan Electrohome Ltd., Kltchener, Ontario, Canada N2G 4J6
This paper reviews briefly a specific application oriented product research and development program for making tubular cellulose acetate reverse osmosis membranes suitable for ultrafiltration treatment of oily wastewaters arising in automotive and similar industries. Details of production, specification, and typical performance of such membranes are given. A commercial version of the membranes developed is currently available as Electrohome ultrafiltration modules. These modules show an average permeate productivity of 1.22 m3/m2day (30 gal/ft2 day) at a mean operating pressure of about 345 kPag (-50 psig) with an oil concentration in the range 5 to 35% on the feed side of membrane in the entire module: the permeate water contains less than 1% of solute (emulsifiers and washlng chemicals), with practically no oil. Based on savings arising from oil recovery and water reuse, the payback time for ultrafiltration units involving the above modules could be as low as six months.
Introduction
The use of enormous quantities of cutting oils and drawing oils in metal working industry has resulted in a difficult oily wastewater disposal problem. These oils are usually mineral oils combined with a variety of ionic and nonionic surfactants including petroleum sulfonates and amine soaps which provide emulsification and rust inhibition. The cutting oil is usually diluted to a 1:20 oil/water ratio for use as coolant during metal working; the drawing oil sticking to the surface of metal parts is removed by subsequent washing with water. Discharge of spent coolants and drawing oil washwaters into public waterways or municipal sewer systems is becoming increasingly unacceptable. In the past, stable oil-water emulsions have been treated by first destabilizing the emulsion by acidification and coagulation and subsequent separation of oil by air-floatation, centrifugation, filtration, or suitable agglomeration techniques. Aside from being expensive, these processes are generally inadequate to meet the
current environmental requirements of practically negligible oil content in the final effluents. Ultrafiltration using reverse osmosis membranes can be used to concentrate the oil in the oily wastewaters yielding membrane-permeated waters which (with or without further treatment) can either be reused in the industry or discharged into public waterways with no harmful effect on the environment. The concentrated oil resulting from ultrafiltration can either by upgraded, reutilized, or readily incenerated with no additional fuel. There are several reports on the subject (Cruver, 1974; Eykamp, 1975; Goldsmith, 1974; Goldsmith and deFillippi, 1973; Goldsmith et al., 1974; Gollan et al., 1975; Hockenberry and Lieser, 1976; Markind et al., 1974; Milstead and LOOS,1972; Nordstrom, 1974; Priest, 1978; Young, 1974). This paper is concerned with a specific application oriented product research and development program for making tubular cellulose acetate reverse osmosis membranes suitable for ultrafiltration treatment of oily
0196-4321/81/1220-0354$01.25/00 1981 American Chemical Society
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wastewaters arising in automotive or similar industries. In this particular application, the drawing oil on the metal parts is removed by a mixture of water and emulsifiers in a washer tank. The oil concentration in the washer tank is maintained at less than 5 % . Therefore, the ultrafiltration system was required to remove oil from the oily waters in the washer tank and maintain the oil concentration in the tank always under 5%. In particular, it was required of the ultrafiltration membrane to retain the oil essentially completely and let a q-iajor part of the emulsifiers to pass through under the operating conditions, so that the permeate water from the ultrafiltration system could be returned back directly to the washer tank for reuse. The ultrafiltration membranes developed in this work satisfied the above requirements, and they have now been commercialized by Electrohome Limited (Kitchener, Ont.), whose ultrafiltration systems are currently in operation at Budd Automotive (Kitchener, Ont.) and many other industries.
Cellulose Acetate Material-Cutting Oil-Water System In both reverse osmosis and ultrafiltration, the preferential sorption characteristics at the membrane-solution interface govern both solute and solvent transport through membrane pores. It has been shown that liquid-solid chromatography (LSC) data on relative retention volumes (V,’) (= retention time X flow rate) of different solutes can offer some definitive indication of the preferential sorption characteristics at polymer material-solution interfaces when the polymer material is used in the chromatographic column (Matsuura and Sourirajan, 1978a). For example, considering that in LSC the data on retention volume of DzOare essentially the same as those of ordinary water, it can be expected that when water is preferentially sorbed at the polymer material-solution interface, the retention volume of solute is relatively lower than that of DzO, and when solute is preferentially sorbed at the interface, the retention volume of solute is relatively higher than that of DzO.In addition to data on retention volumes, the corresponding data on peak areas with and without the polymer column in the chromatography apparatus are also of interest, particularly in ultrafiltration transport. If the solute is either rejected or only very weakly adsorbed at the polymer material-solution interface, and if the solute does not undergo any physical change, then one may expect that the peak area ratio (= peak area with column/peak area without column) is close to unity. However, if the solute is strongly adsorbed on the surface of the polymer material or if the solute tends to aggregate, the mobility of the solute through the column will be slowed down resulting in a peak area ratio of significantly less than unity. Using DzO and Dromus B cutting oil (pH = 9.0) as solutes, LSC data were obtained with a cellulose acetate (E-398) column (Sourirajan et al., 1979). The experimental details used were the same as those reported earlier (Matsuura et al., 1976; Matsuura and Sourirajan, 1978a,b). Even though the cutting oil used is not a single compound, the chromatogram obtained showed essentially a single peak. Further, the VR’values for DzO and the cutting oil were 1.837 and 0.963 cm3, respectively, and the corresponding peak area ratios were -1.0 and 0.53, respectively. These results indicated that the cutting oil had components, some of which were preferentially rejected and some others of which were preferentially adsorbed at the polymer material-solution interface. These results are consistent with the observations made earlier (Matsuura and Sourirajan, 1972, 1973; Matsuura et al., 1977) that basic
solutes are rejected, and hydrocarbons and long chain alcohols are preferentially adsorbed in the polymer material-solution interface. Since the cutting oil consists predominantly of hydrocarbons and they are preferentially adsorbed on the surface of the cellulose acetate material, it is reasonable to expect a significant pore-blocking effect during ultrafiltration. Further, when solute is preferentially sorbed at the membrane-solution interface, solute separation tends to decrease and the pore-blocking effect tends to increase with increase in operating pressure (Matsuura and Sourirajan, 1973);further, more of the basic (emulsifier) components in the feed waters may be expected to pass through the membrane at lower operating pressures. For these reasons, the operating pressure for the ultrafiltration treatment of oily wastewaters must be as low as possible; for economic operation, however, the water flux through the membrane must also be as high as possible at such low operating pressure even under the prevailing pore blocking conditions. These requirements indicated the special need for the development of high flux cellulose acetate membranes suitable for the ultrafiltration treatment of oily wastewaters.
Choice of Composition for Film Casting Solution It has been shown that the solution structure-evaporation rate (i.e., solvent removal rate) approach to membrane development for reverse osmosis (Sourirajan and Kunst, 1977) is generally applicable for the development of cellulose acetate ultrafiltration membranes (Kunst and Sourirajan, 1974; Kutowy and Sourirajan, 1975). With respect to casting solutions involving cellulose acetate (polymer,P), acetone (solvent, S) and aqueous magnesium perchlorate (nonsolvent, N), it has been shown that decrease in S/P ratio, increase in N/S ratio, and increase in N / P ratio in the casting solution composition tend to increase the average size of pores on the surface of the resulting membranes in the as-cast condition; further, increase in S/P ratio in the casting solution composition tends to increase the effective number of such pores on the membrane surface. These conclusions arise from the correlations of experimental data presented in Figure 1. Further, the membranes made from casting solution compositions indicated in Figure 1are subject to significant initial compaction during reverse osmosis operation as indicated by the drop in pure water permeation rate (PWP) as a function of time; the extent of compaction is most in the first hour of operation. The data on the difference between the initial PWP and that after 1 h of operation (8pwp) with pure water feed at 690 kPag (100 psig) for membranes made from different casting solution compositions indicated in Figure 1 are shown in Figure 2. These results show that Jpwp passes through a minimum with increase in S/P for each value of N/P. On the basis of data given in Figures 1and 2, it is clear that in order to increase both the average size and number of pores on the membrane surface, and in order for the resulting membrane to have the least compaction characteristic, the casting solution composition for the membrane should have a value of S / P neither too low nor too high, the value of N / P should be as high as possible, and the composition should preferably lie in the locus of the minumum value of 8pwp values. Based on the above considerations, the following casting solution composition (wt %), called the K1 composition, was chosen for the purpose of this work: cellulose acetate (Eastman 400-25), 14.8; acetone, 63.0; water, 19.9; magnesium perchlorate, 2.3. The above composition has S/P, N/S, and N/P ratios of 4.25, 0.352, and 1.5, respectively, and it lies in the locus of the minimum JpWpvalues shown in Figure 2.
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1°C
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Figure 1. Effects of S/P, N/S, and N/P ratios in casting solution composition on performance of resulting membranes. Polymer (P), cellulose acetate (E-398-3); solvent (S), acetone; nonsolvent (N), aqueous magnesium perchlorate (Mg(C104)2:H20= k8.5); temperature of film casting soldtion, 0 OC; temperature of film casting atmosphere, 20 O C ; solvent evaporation period, minimum (2 to 5 s); solute concentration in feed, 200 ppm; operating pressure, 690 kPa gauge (100psig); effective film area, 13.2 cm2. Data of Kutowy and Sourirajan (1975). Reprinted with permission from Kutowy and Sourirajan (1975). Copyright 1975 John Wiley and Sons, Inc.
n a 40
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I
Figure 2. Effect of casting solution composition on membrane compaction. The numbers indicating compositions, fiim casting, and test conditions are the same as those in Figure 1. 6pwp is in g/h; membrane area, 13.2 cm2.
Choice of Gelation Conditions During Film Formation Ethyl alcohol-water mixtures at different temperatures have been shown to be useful gelation media for obtaining high flux cellulose acetate ultrafiltration membranes (Kutowy et al., 1978; Tweddle and Sourirajan, 1978;Gildert et d., 1979). Figure 3 shows the effects of temperature and ethyl alcohol concentration in the EtOH-H,O gelation
Figure 3. Effects of temperature and ethyl alcohol concentration in the gelation medium on membrane perfbrmarice. Casting solution composition, same as K1; operating pressure, 690 kPa gauge (100 psig); feed solution, 300 ppm MgS04-H20; 1 gal/ft2 day = 0.0407 m3/m2day. Data of Kutowy et al. (1978). Adapted with permission. Copyright 1975, Elsevier Scientific Publishing Co.
medium on reverse osmosis performance of cellulose acetate membranes obtained from the casting solution composition specified above. From Figure 3, it is clear that by using EtOH-H20 gelation media with an alcohol concentration greater than that corresponding to the initial minimum in water flux, the effective average pore size on the membrane surface can be increased resulting in useful ultrafiltration membranes. These results were used in the development of the ultrafiltration membranes needed in this work. Production of Tubular Cellulose Acetate Ultrafiltration Membranes The foregoing results gave rise to practical techniques for producing 2.54 cm diameter tubular cellulose acetate ultrafiltration membranes either as free unsupported membranes or as membranes integrally cast on porous supports. The initial work was on the development of technique for producing unsupported membranes. In this technique, the membrane was cast at the laboratory temperature on the inside surface of a glass tube using an appropriate mixture of ethyl alcohol and water as the gelation medium. The apparatus used for producing such membranes is described in detail in the literature (Thayer et al., 1977). In particular, the casting-bob housing used in the above apparatus was essentially the same as that described below for producing supported membranes; in addition, the apparatus included an electrical water probe at the bottom of the casting bob for maintaining in the casting tube any desired length of air-zone for the freshly cast membrane. The membranes obtained were wrapped with soft nylon cloth and then loaded into perforated fiberglass tubes in ultrafiltration modules for subsequent practical operation in the field. The work on the technique for producing integrally supported membranes was a later development. In this technique, the membrane was cast, again at the laboratory temperature, directly on the inside surface of a porous
Ind. Eng. Chem. Prod. Res. Dev., VoI. 20. NO. 2. 1981 357
Figure 4. Apparatus for producing supported tubular membranes.
support tube using the differential gelation technique (Kutowy et al., 1980). The latter technique consists in using an EtOH-H20 mixture at a composition corresponding to a water flux in region 1or region 2 in figure 3 as gelation medium in contact with one side of the membrane, and simultaneously using another similar liquid mixture at a composition corresponding to an order of magnitude higher water flux in region 4 in Figure 3 as the gelation medium in contact with the other side of the membrane; this technique results in an asymmetrically porous membrane with the relatively smaller size pores on the former side of the membrane. Since such supported membranes are currently the preferred ones for use in industry their production technique is described below in detail. The apparatus for producing supported tubular membranes is shown schematically in Figure 4. The support tubes used in this development work were porous (lo-” voids) high density polyethylene tubes 3.81 cm (1.5 in.) outside diameter and 0.635 cm (0.25 in.) wall thickness and 1.22 m (4 ft)long. The membranes were cast integrally on the inside surfaces of these tubes. The casting bob aasembly used consisted of a permanently located overhead pressure vessel for storing the film casting solution connected to a movable casting-bob housing by a flexible nylon tubing (Figure 4a). A brief description of the casting-bob assembly (Figure 4) is as follows. The overhead pressure vessel is fitted with a plastic bag containing the casting solution. The object of the bag is to prevent direct contact of the film casting solution with the walls of the pressure vessel and the high pressure gas (air or nitrogen) used to drive the solution into the casting-bob housing. The casting solution in the plastic bag is driven to the casting-bob housing through the flexible nylon tubing by applying gas pressure to the bag through the top of the pressure vessel. The viscosity of the casting solution, the width of the casting solution exit into the casting-bob housing, the film casting speed, and the required extent of penetration of the casting solution into the voids of the porous support tube together determine the pressure needed to drive the casting solution into the casting-bob housing; under the experimental conditions
used in this work, the above pressure was 517 kPag (75 psig). The sue of the pressure vessel itself could be as big as necessary to hold the quantity of the casting solution needed for the manufacture of the required number of tubular membranes. The movable casting-bob housing consists of a castingbob, centering-bob and a tapered buckebtrailer as shown in Figure 4c. The casting-bob has five independent vertical passages as indicated in Figure 4b. The central passage, connected to the overhead pressure vessel through the nylon flexible tubing, is for the flow of the casting solution. This passage extends down to the bottom section of the casting-bob where it connects with four perpendicular horizontal openings through which the casting solution can be ejected uniformly around the space between the porous support tube (casting tube) and the casting-bob during film casting. The other four vertical passages serve as air vents during film casting. The centering bob is screwed onto the casting bob from its top. The centering bob centers the casting-bob in the casting tube during film casting, regulates the rate of flow of the casting solution by controlling the width of the opening for the casting solution exit, and also serves as a cap to seal the casting solution within the casting bob by closing the above exit completely when it is not in use. The tapered bucket trailer hangs from the bottom of the casting-bob, and the inside gelation medium is below the trailer as shown in Figure 4c. The trailer squeezes the casting solution into the voids of the porous support tube, elimiites air bubbles left by the casting-bob, and respreads the casting solution on the support surface; it also serves as a receptacle for the excess casting solution scrapped from the support surface. In general, the incorporation of the trailer in the casting-bob assembly makes the process of casting integrally supported membranes practically successful. An EtOH-H20 mixture, at laboratory temperature (23-27 “ C )at a composition corresponding to mole fraction of EtOH in the range 0.1 to 0.15 was the gelation medium for the inside surface of the tubular membrane; similarly, another EtOH-H,O mixture at laboratory (or higher) temperature at a composition corresponding to mole fraction of EtOH equal to or greater than 0.8 was the gelation medium for the casting solution that had penetrated into the void spaces of the porous tube and also for that underneath the inside surface layer of the tubular membrane. These gelation liquids are referred here as “inside gelation medium” and “outside gelation medium”, respectively. The membranes were cast by holding the casting-bob stationary at a level higher than the upper level of the outside gelation medium in the liquid applicator, and letting the casting tube (Le., the porous support tube) move down into the inside gelation medium at controlled speed (Kutowy et al., 1978). During membrane casting, the outside gelation medium continuously percolated into the voids of the porous support tube by capillary action aided by, if necessary, a slight external pressure (device not shown in Figure 4). After the gelation step, the outside gelation medium, which penetrated into the voids of the support tube, was leached out completely along with the solvent and nonsolvent components in the gelled casting solution by pumping water under pressure through the membrane. Specification of Membranes Suitable for the Treatment of Oily Wastewaters Dromus B cutting oil was used for test purposes in thii work. For the treatment of wastewaters containing the above oil, it was found that the suitable cellulose acetate
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10
tubular membranes were those capable of giving at least 10% solute separation in reverse osmosis for a 100 ppm MgS04-H20 feed solution at 138 Wag (20 psig) at the feed flow rate used for the wastewater treatment; such membranes were found to give essentially complete separation for oil during ultrafiltration in the operating pressure range 138 to 690 kPag (20 to 100 psig) in the temperature range 25 to 65 "C. Laboratory Tests a n d Typical Test Results Experimental Details. The apparatus used for testing membranes was a simple one as shown schematically in Figure 5. The oil-water mixture was drawn from the bottom of a storage tank and pumped under pressure through the tubular membrane which divided the feed stream into a permeate stream (membrane permeated product) which was essentially free from oil and a concentrate stream which was enriched in oil. In most experiments, the latter stream was returned back to the storage tank as indicated in Figure 5. In all experiments, the feed, permeate, and the concentrate samples were analyzed for carbon content by means of a Beckman total carbon analyzer; in a few cases, more detailed analysis were carried out using the standard APHA method no. 137. In all the experiments reported in this paper, the permeate samples were clear and slightly colored; in particular, they had no visible turbidity. The smallest concentration of oil in water capable of showing visible turbidity by the turbidity meter (HF Instruments, Model DRT 100) was -15 ppm. Therefore it was concluded that, in the permeate samples obtained in all the experiments reported here, oil concentrations were always less than 15 ppm, and the color and carbon content in the samples were primarily due to the surfactant additives in the cutting oil. Figure 6 shows a calibration of turbidity reading as a function of cutting oil content in Dromus oil-water emulsions. The correlation is essentially linear up to 120 ppm of cutting oil content (which includes emulsifiers) in water. On the basis of this calibration, a typical permeate sample obtained in this work showed only an oil content of less than 10 ppm. The data on overall solute separations reported in this paper are based on total carbon content and they do not differentiate between oil and surfactant in the permeate. The permeate flux through the membrane depended very much on the experimental conditions of operating pressure, concentration of oil in feed, temperature of feed solution, and the feed flow rate. With the type of centrifugal pump used in this work for pumping the feed fluid through the membrane tube, the operating pressure and the feed flow rate could not be controlled independently; when the operating pressure increased, the feed flow rate decreased. For example, at the operating pressures of 76,97, and 152 Wag (11, 14, and 22 psig), the feed flow rates were 7.99,7.27, and 4.09 m3/h (2112, 1920, and 1080 gal/h), respectively.
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Figure 7. Effect of pressure, temperature, and oil concentration in feed on overall solute separation and permeation rate.
Typical Test Results with Unsupported Membranes. As indicated already, the unsupported membranes were supported by perforated fiberglass tubes during ultrafiltration operation. In one set of experiments, the initial pure water permeation rate for the membrane was 0.928 m3/m2 day (22.8 gal/ft2 day) at 34 "C and 138 kPag (20 psig) operating pressure. This membrane was used to concentrate oil in a feed water from 12 to 24 wt % by ultrafiltration operation at the above temperature and pressure conditions. The overall solute separation was 99% throughout. At the end of the concentration process, the tubular membrane was cleaned by pumping soap water through it for a few minutes, after which it was found that the pure water permeation rate through the membrane at the initial test conditions was again 0.928 m3/m2 day. These results showed that the porous structure of the membrane remained unaffected under the test conditions of the ultrafiltration process, and that the membrane could be easily cleaned after use with oily wastewaters. Another sample of the tubular membrane was used to study the effect of pressure, temperature, and oil concentration in feed on overall solute separation and permeation rate. In this test, the oil concentration in the feed ranged from 2.5 to 40 wt % , the operating pressure was either 97 or 152 kPag (14 or 22 psig, respectively), and the operating temperature was 20 or 45 "C. The feed flow rates were 7.27 m3 (1920 gal)/h at 97 Wag (14 psig), and 4.09 m3 (1080 gal)/h at 152 kPag (22 psig). The results obtained, given
Ind. Eng. Chem. Prod. Res. Dev., Vol. 20, No. 2, 1981 359
40.C, 20 PSIG
9 . 7 WT. X OIL IN FEE0
SOLUTE SEPARATION
MEMBRANE
ID€ GELATION MEDIUM
2 4 WT.
Figure 8. Oil concentration, solute separation, and permeate flux aa a function of permeate water recovery. Operating pressure, 138 Wag (20psig); feed flow rate, 5 m3 (1320 gal)/h; permeate rate data are for 30 "C.
in Figure 7, showed that the overall solute separations were greater than 99% in all cases, they tended to decrease with increase in oil concentration in feed, and the permeate rates were higher at 45 O C than at 20 O C at each operating pressure primarily due to the effect of temperature on viscosity of permeate water. The combined effect of pressure and feed flow rate on the rate of permeate water is particularly interesting. The data given in Figure 7 show that a t lower levels of oil concentration in the feed, the permeate rate is higher at the higher operating pressure (and lower feed flow rate), whereas at higher levels of oil concentration in the feed, the permeate rate is higher at the lower operating pressure (and higher feed flow rate) at each operating pressure studied; further, the oil concentration in the feed for which the permeation rates at the two operating pressures studied become identical is lower at the lower operating temperature. These results are understandable on the basis that the oil which is preferentially adsorbed at the membrane-solution interface tends to block the pores and decrease the rate of permeate flow through the membrane, and this pore blocking effect is relatively more at a lower feed flow rate, lower operating temperature, and higher oil concentration in the feed. A third sample of a tubular membrane had a pure water permeation rate of 0.810 m3/m2day (19.9 gal/ft2 day) at 30 O C and 138 kPag (20 psig) pressure. This membrane was used to concentrate the oil content in an oily feed water from 0.32 to 50 wt %, recirculating the concentrate through the system until the final concentration was reached. The operating pressure in this test was 138 Wag (20 psig) and the feed flow rate was 5.0 m3 (1320 gal)/h, and the temperature of the feed varied from 29 to 62 "C during the process which took 18 h in the apparatus used in this work. Figure 8 gives the data on oil concentration in feed, overall solute separation (%) and permeate flux as a function of water recovery (volume percent of initial feed) as permeate. The permeate was clear throughout the process. These results showed that at 138 Wag (20 psig) pressure, even with 50 wt % oil in feed, permeate rate through the membrane was 0.12 m3/m2day (2.95 gal/ft2 day), and at temperatures up to 62 "C, the permeate obtained with the membrane tested was clear with no visible turbidity. Typical Test Results with Integrally Supported Membranes. The integrally supported tubular membrane used in this particular test was made using 35 vol % EtOH-H20 as the inside gelation medium and 50 vol %
-
*/a
OIL I N F
1
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Figure 9. Effect of time on pure water permeation rate (in m3/mz day), and product rate during concentration of oily wastewaters using integrally supported membranes.
EtOH-H20 as the outside gelation medium. During the test, the operating pressure was 138 Wag (20 psig) at which the feed flow rate was 5.0 m3 (1320 gal)/h, and the operating temperature was kept at 40 "C. The machine-oil used in this test was supplied by Budd Automotive Co. The experimental test results obtained are given in Figure 9. Figure 9 shows the effect of time on pure water permeation rate, which represents the initial compaction characteristics of the membrane. The above rate decreased from 5.05 m3/m2day (124 gal/ft2 day) to 4.28 m3/m2day (105 gal/ft2 day) during the first 11 h of continuous operation with pure water feed. After this period, the pressure was released and the membrane allowed to relax overnight. Figure 9 also shows the productivity of the membrane starting with an oily water containing 9 w t % oil. This feed was recirculated in the system until the oil concentration increased to 24 wt %, which took about 12 h; the permeate rate steadily decreased during this period as shown in the figure primarily due to increase in oil concentration in feed, and also partly due to further membrane compaction. The overall solute separation was over 99% throughout the test, and the permeate water was clear with no visible turbidity. Figure 9 illustrates that the integrally supported membrane has a much higher productivity (higher permeate rate for a given level of overall solute separation under otherwise identical experimental conditions) compared to the unsupported membranes tested earlier. For example, Figure 9 shows that the permeate rate was 1.63 m3/m2day (40 gal/ft2 day) with a feed water containing 24% oil under the experimental conditions used; under the same experimental conditions, the permeation rate obtainable from a comparable unsupported membrane was only about one-half of the above rate. For this reason (and also for other reasons concerned with ease of handling), integrally supported membranes became the preferred ones for industrial use. The increased productivity of the integrally supported membrane may be attributed to the effect of simultaneous gelation of both sides of the membrane during formation as a result of the differential gelation technique. Industrial Performance of Tubular Membranes in Electrohome Modules The tubular membrane technology developed in this work has now been commercialized by Electrohome Lim-
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ited (Kitchener, Ont.). Their earlier modules had unsupported membranes which were installed with nylon cloth backing into perforated fiberglass tubes; their current modules have integrally supported membranes cast on porous polyethylene tubes wrapped with stainless steel screens to provide additional support to the membrane tube. The effective area of membrane surface in each tube is 0.1022 m2 (1.1ft2) in both types of modules. With unsupported membranes, each module had six membrane tubes connected in series; with integrally supported membranes, each module has only three membrane tubes (because of their higher productivity) designed for parallel flow within the module. These modules have been in satisfactory continuous service in several industrial locations for the treatment of oily wastewaters. Just for illustration, experimental details and typical data on membrane performance of the above two types of modules in Electrohome ultrafiltration systems operating at Budd Automotive Co. (Kitchener, Ont.) for the treatment of oily wastewaters are given below. As indicated earlier, in this particular application, washwaters containing emulsified oil and cleaning chemicals were treated by the ultrafiltration unit. The washwater is first pumped through a vibrating screen to remove lint and other suspended matter, The screen unit has an automatic steam clean cycle to prevent blinding of the screen by oil. Following screening, the wastewater (pH -8 to 10) is pumped to the process tank where the required amount of sulfuric acid is injected to bring the pH of the oily water to -7. The ultrafiltration unit itself consists of two banks of modules. Each bank has 18 modules arranged as three parallel columns of six modules in series. This modular arrangement is the same whether the membrane tubes in the modules have unsupported or integrally supported membranes. It may be noted here that in order to do essentially the same job, the entire ultrafiltration unit needs to contain either 216 membrane tubes using unsupported membranes or only 108 membrane tubes using integrally supported membranes, because of the intrinsic high productivity of the latter membranes. A centrifugal pump for each bank pumps the liquid from the process tank through the modules. The feed flow rate at the bank inlet is maintained at 18.17 m3 (4800 gal)/h at 517 kPag (75 psig). The average pressure for the ultrafiltration unit as a whole is -345 kPag (50 psig). The unit operates continuously 24 h each day. A typical daily operation involves ultrafiltration treatment of 13.6 m3 (3600 gal) of washwater (at -43 "C) containing about 5% emulsified oil, to produce 11.36 m3 (3000 gal) of permeate containing a total of less than 1% emulsifiers and cleaning chemicals but practically no oil, and 2.27 m3 (600 gal) of concentrate containing about 35% oil. The permeate is continuously returned back to the rinse stage from where it overflows into the washer. The oil concentrate is collected continuously in one of two settling tanks. After standing for about 24 h, the concentrate settles into two layers. The bottom layer containing mostly water is fed back to the ultrafiltration unit; the top layer containing 50 to 95% oil is pumped into emulsion tanks where a make-up package of additives is mixed with the recovered oil to form an oil-water emulsion which is then reused in the plant. The permeate rate for the entire ultrafiltration unit decreases steadily with time due to the continuous pore blocking effect of the preferentially sorbed oil at the membrane-solution interface, and also due to accumulation of lint and dirt on the membrane surface; this decrease, however, mostly vanishes by periodic (once a week) cleaning of the membrane surface by pumping a special
PERFORMANCE OF UF-SYSTEM AT BUDD AUTOMOTIVE, KITCHENER 20
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Figure 10. (a) Performance of Electrohome modules using unsupported membranes, for the treatment of oily wastewaters. Average operating pressure, -345 Wag (50 psig); operating temperature -43 O C ; oil concentration in the module, -5 to 35 wt %; 1 gal/ft2 day = 0.0407 m3/m2day. (b) Performance of Electrohome modules using integrally supported membranes for the treatment of oily waste waters. Operating conditions, same as for part a.
cleaning liquid through the membrane tubes just for about 30 min. Figure 10 gives a typical set of data on overall permeate rates obtained during a period of 8 consecutive weeks for two ultrafiltration units, one involving modules with unsupported membranes (Figure loa) and the other involving modules with integrally supported membranes (Figure lob). These data show that periodic cleaning restores the original productivity of the membranes practically completely, and hence the drop in permeation rate during continuous operation is primarily due to a poreblocking effect, and the contribution of membrane compaction to decline in membrane flux is relatively small, if any. The above results also show that the average productivity of integrally supported membranes in the industrial modules under the specified operating conditions is about 1.22 m3/m2day (30 gal/ft2 day), which is about twice that obtained from modules using unsupported membranes. The performance of Electrohome ultrafiltration units at Budd Automotive has demonstrated the cost effectiveness of the modules involving integrally supported membranes for the treatment of oily wastewater, and, based on savings arising from oil recovery and water reuse, the pay-back time for the above ultrafiltration modules could be as low as six months. Conclusion This report is a brief review of the sequence of some of the major steps taken in the development of tubular cellulose acetate ultrafiltration membranes for a specific application. The practical success of this program of research and development also testifies to the validity of the basic approach to ultrafiltration research represented by this work. The essence of this approach is that the science of reverse osmosis and reverse osmosis membranes (Sourirajan, 1978; Sourirajan and Matsuura, 1979; Sourirajan et al., 1979)is a natural and fruitful basis for research and development of ultrafiltration and ultrafiltration membranes.
Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 361-365
Literature Cited Cruver, R. E. Trans. ASME, Ser. B: J . Eng. Ind. 1974, Paper No. 74ENAS-41. Eykamp, W. Paper presented at the 79th National Meeting, AIChE, Houston, Texas, March 1975. Gildert, 5. R.; Matsuura, T.; Sourlrajan, S. J. Appl. Polym. Sci. 1979, 24, 305
Goldsmith, R. L. Trans. ASME, Ser. 8 : J . Eng. Ind. 1974, Paper No. 74ENAS-26. Goldsmith, R. L.; de Filippi, R. P. In “Membranes In Separation Processes-A Workshop Symposium“, Case Western Reserve University, Cleveland, Ohlo. 1973. Goldsmith. R. L.: Roberts, D. A.; Burre. D. L. J. Water Pollut. Control Fed. 1974, 46, 2183. Goilan, A.; Grant, D.; Goldsmith, R. L. ASME Reprint No. 75-ENAS-57, 1975. Hockenberry, H. R.; Lieser, J. E. ASLE, Preprint No. 76-AM-2B-2, 1976. Kunst, B.; Sourirajan, S. J. Appl. Polym. Sci. 1974, 18, 3423. Kutowy, 0.; Sourlrajan, S. J. Appl. Polym. Sci. 1975, 19, 1449. Kutowy, 0.;Thayer, W. L.; Sourirajan, S. Desalination 1978, 26, 195. Kutowy, 0.; Thayer, W. L.; Capes, C. E.; Sourirajan, S. J. Sepn. Process Techno/. 1980, 1(3), 28. Markind, J.; Minard, P.; Neri, J.; Stone, R. AIChE Symp. Ser. No. 144 1974, 70, 157. Matsuura, T.; Baxter, A. G.; Sourirajan, S. Ind. Eng. Chem. Process Des. Dev. 1977, 16, 82. Matsuura, T.; Biais, P.; Sourirajan, S. J. Appl. Polym. Sci. 1976, 20, 1515. Matsuura. T.; Sourirajan, S. J. Appl. Polym. Sci. 1972, 16, 1663.
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Matsuura, T.; SourIraJan,S. J. Appl. Polym. Scl. 1973, 77, 3683. Matsuura, T.; Sourirajan, S. J. Collokj Interface Sclence 1978r, 66, 589. Matsuura, T.; Sourlrajan, S. Ind. Eng. Chem. Process Des. Dev. 1978b, 17, 419. Milstead, C. E.; Loos, J. F. U.S. Nat. Tech. Infor. Serv., AD Rep& No. 758321, 1972. Nordstrom, R. P., Jr. Pollut. Eng. Oct 1974, 46. Priest, W. Wlre Ind. Feb 1978, 106. Sourirajan, S. Pure Appl. Chem. 1978. 50, 593. Sourlrajan, S.; Kunst, B. I n “Reverse Osmosis and Synthetlc Membranes”, Sourlrajan, S., Ed.; Natlonal Research Council Canada: Ottawa, 1977; Chapter 7. Sourlrajan, S.; Matsuura, T. “Proceedlngs of the Symposium on Textlle Industry Technology, U.S. Environmental Protection Agency, EPA-600/2-79104, May 1979, pp 73-106. Sourirajan, S.; Matsuura, T.; Hsieh, F. H., Gildert, G. R. Paper presented at the ACS Symposium on UltrafiltrationMembranes and Applications, Washington, D.C., Sept 1979. Thayer, W. L.; Pageau, L.; Sourirajan, S. Desallnatlon 1977, 21, 209. Tweddle, T. A.; Sourirajan, S. J. Appl. Polym. Sci. 1978, 22, 2265. Young, W. S., Jr. Prod. Finlsh. 1974, 39(1), 62.
Received for review June 10,1980 Accepted December 1, 1980
Issued as N.R.C. No. 18828.
Properties of the Amphoteric, Thermally Regenerable Ion-Exchange Resins Amberlite XD-2 and Amberlite XD-5 Tah-Ben Hsu and Robert L. Plgford’ Depaflment of Chemical Engineering, University of Delaware, Newark, Delaware 1971 1
The amphoteric ion-exchange reaction is endothermic in both Amberlite XD-2 and Amberllte XD-5 resins. Both show significant capacity decrease for several solutes when temperature increases owing to the increase of water ionization with increasing temperature. Amberllte XD-5 has about 50% higher resln capacity than Amberllte XD2. Amberlite XD-2 is more porous and stiffer in structure. Bath resins swell with an increase of equilibrium llquld concentration up to a certain point beyond which swelling stops. In the process off swelling, which is reverslble, approximately 80% of the volume change is due to the expansion of the pore volume; the rest Is due to the increase in volume of the solid resin phase. Temperature is not a critlcal factor for swelling. The lntrapartlcle porosity is increased by swelling. The solid resin densities for both resins are also reported.
Introduction Different from the conventional anionic or cationic ion-exchange resins, Amberlite XD-2 and Amberlite XD-5 (products of the Rohm and Haas Co., Philadelphia, Pa.) are amphoteric. Both resins are synthesized by introducing a gelular “guest copolymer” into a matrix of macroreticular “host copolymer”, such as styrene-divinylbenzene or POlyvinylbenzyl chloride (Barrett and Clemens, 1976). After proper procedures of chloromethylation, aminolysis, and hydrolysis, both carboxylic groups and amine groups offer the amphoteric functionality. Typically, the “guest copolymer” contains divinylbenzene (DVB) which is also a cross-linking agent. The major difference between Amberlite XD-2 and Amberlite XD-5 lies in their DVB content which changes the extent of cross-linking (Helfferich, 1962). These two resins virtually possess highly porous structure with reasonably good mechanical strength. The thermally regenerable property of the two resins may make them superior to the common chemically regenerable ion-exchange resins from the viewpoint of cost in pollution prevention appplications. Using weakly acidic carboxylic groups and basic secondary amine groups to 0196-4321/81/1220-0361$01.25/0
exchange with sodium and chloride ions as an example, the chemistry of these ion-exchange resins (Ackerman et al., 1976) can be described as follows. cation exchange R-COO--H+
+ Na+ F? R-COO-Na+ + H+
(1)
anion exchange RiNH
+ C1- + HzO F? R,’NH,+Cl-+
OH-
(2)
water ionization
HzO F? H+ + OH-
(3) As temperature increases, the degree of water ionization also increases, driving the equilibrium of (1)and (2) from the loaded forin (right-hand side) to the regenerated form (left-hand side). Mass transfer phenomena in Amberlite XD-2 have been studied by Knaebel et al. (1979a). Processes using Amberlite XD-2 for salt removal have also been developed (Dabby et al., 1976; Knaebel and Pigford, 1979b). Little is published for the newer Amberlite XD-5 resin. 0 1981 American Chemical Society
