CONCENTRATION OF AQUEOUS PSEUDOPLASTIC SOLUTIONS BY MEMBRANE ULTRAFILTRATION

Abstract: A method of concentrating an aqueous, highly-­ pseudoplastic biopolymer solution or preconcentrate having an apparent viscosity of at least about 3,000 centipoise comprising subjecting said solution or preconcentrate to membrane ultrafiltration at a linear flow velocity of at least about 40 cm/sec across said membrane surface, while exerting an average pressure differential across said mem­ brane of at least about 2.0 atmospheres.

Description: CONCENTRATION OF AQUEOUS PSEUDOPLASTIC SOLUTIONS BY MEMBRANE ULTRAFILTRATION Ultrafiltration is the term applied to the separation of relatively high-molecular weight solutes (e.g., proteins, natural gums, polymers, other complex organic compounds) and colloidally dispersed substances such as clays, pigments, minerals, latex particles, microorganisms, etc. from their solvents (usually water). In these systems, the osmotic pressure of the solute is usually negligible and plays no important role in the separation process. Membrane ultrafiltration, the hydraulic pressure activated separation of solutions into their individual components by passage through synthetic semi-permeable membranes, constitutes a continuous separation process which does not involve a phase change or interphase mass transfer. The process utilizes specially structured and constituted polymeric films as molecular screens which discriminate between solute and solvent molecules on a basis of differences in molecular size, shape or chemical structure. Representative of the many and extensive reviews of ultrafilter membranes and ultrafiltrations are those of Ferry, J. D., Chemical Reviews, 18, 373-455 (1937) and Michaels, A. S., Chemical Engineering Progress, 64, No. 12, 31-43 (1968). Operationally, membrane ultrafiltration is a simple process. A feed solution under pressure flows over the surface of a suitably supported membrane, and solvent and certain solute species pass through the membrane and are collected as ultrafiltrate (permeate). Other solute species and suspended matter are retained by the membrane and a concentrated solution or suspension of these species, the retentate, is recovered from the upstream chamber or feed side of the membrane. Hence, by proper membrane selection, it is possible to concentrate, purify and fractionally separate many solutions, the sole energy requirement being that required to pump the feed liquid through the equipment at the desired pressure. Ultrafiltration as a separation technique has found wide industrial application for the removal and/or recovery of solutes from liquids by physical means, both in quantity and at low cost. There are only a few reports or accounts of the demonstration of membrane ultrafiltration for the separation or concentration of biopolymers in aqueous solution. These high molecular weight materials form true, single-phase solutions in water. In most of these reports the primary objective was to illustrate the membrane retention characteristics using high molecular weight materials but in low viscosity solutions. None of these reports have dealt with highly viscous biopolymers (viscosity of greater than 3000 centipoise) because up to the present time this has seemed an impossibility and in fact such separation or concentration is impossible unless the material in question is of a highly pseudoplastic nature. U.S. Patent 3,541,006 claims the introduction of inert particles to improve flux in a polysaccharide solution in batch-stirred vessels and ultra-filtration of batch cells. U.S. Patent 3,567,810 claims a process for forming a polymeric anisotropic membrane. A 1% dextran solution was used to illustrate the rejectivity of the membrane. U.S. Patents 2,128,551 and 3,856,569 are both concerned with recovering alginous material from seaweed. Both the '551 and the '569 patents use ultrafiltration for concentration but they deal with materials of low viscosity and not having pseudoplastic behavior. Furthermore, both deal with seaweed-derived materials and not with microbial-derived biopolymers. Of emerging and growing interest are the highly pseudoplastic xanthan biopolymers. These biopolymers are produced by the cultivation in suitable aqueous nutrient media of microorganisms belonging to the genus Xanthomonas. The hydrophillic colloids produced by Xanthomonas species are polysaccharides which contain mannose, glucose, glucuronic acid, 0-acetyl radicals and ketal-linked pyruvic acid. These gums and their derivatives have found wide food and industrial applications. Of special interest is the increasing focus on the use of Xanthomonas gums in displacement of oil from partially depleted reservoirs. Typically, oil is recovered from underground reservoirs via a series of sequential operations. A new well will generally produce a limited amount of oil as a result of release of internal pressure in the well. As this pressure becomes depleted, it is necessary to pump further quantities of oil by mechanical means. These measures recover only about 25% of the total oil stored in the reservoir. A great deal of oil is still trapped within the pores of the formation. Further enhancement of recovery can then be effected by secondary recovery. In one method of recovery, a waterflood is carried out by pumping water into a well or series of wells, displacing part of the trapped oil from surrounding wells. However, waterflooding still leaves about 55-60% of the available oil trapped in the formation. The explanation for this phenomenon is that the water has a very low viscosity compared to the crude oil and tends to follow the path of least resistance, fingering through the oil and leaving large pockets untouched. In addition, surface forces in the formation tend to bind the oil and prevent its displacement. A number of processes have been developed in recent years to recover further quantities of oil from these reservoirs by the use of mobility control solutions which enhance oil displacement by increasing the viscosity or permeability of the displacing fluid. Of interest are those enhanced recovery processes employing flooding with a polysaccharide or polyacrylamide to increase the viscosity of the displacing fluid. Variations of this process include the use of surfactants and cosurfactants to release the oil from the rock formation. Polyacrylamides have been found to suffer such deficiencies as viscosity loss in brines and severe shear sensitivity. Since, as was well documented in the prior art, xanthan gum is relatively insensitive to salts (does not precipitate or lose viscosity under normal conditions), is shear stable, thermostable and viscosity stable over a wide pH range, xanthan gum is a good displacing agent Moreover, the gum is poorly adsorbed on the elements of the porous rock formations and it gives viscosities useful in enhanced oil recovery (5 to 90 centipoise units at 1.32 sec. 1 shear rate) at low concentrations (100 to 3000 ppm). The use of solutions of xanthan gum or derivatives of xanthan gum for oil recovery is described in U.S. Patents 3,243,000; 3,198,268; 3,532,166; 3,305,016; 3,251,417; 3,319,606; 3,319,715; 3,373,810; 3,434,542 and 3,729,460. It is suggested in U.S. Patent 3,305,016 that aqueous solutions containing heteropolysaccharide in sufficient quantity to increase the viscosity be employed as the thickening agent in preparing viscous water flooding solutions. The polysaccharide may be prepared, separated, purified and then added. Alternatively, according to this reference, the entire culture after adding a bactericide (e. #., formaldehyde) to kill the bacteria, may be added to the flood water. This invention concerns a method of concentrating an aqueous, highly-pseudoplastic biopolymer solution or preconcentrate having an apparent viscosity of at least about 3,000 centipoise comprising subjecting said solution to membrane ultrafiltration at a linear flow velocity of at least about 40 cm/sec across said membrane surface while exerting an average pressure differential across said membrane of at least 2.0 atmospheres. Note that the desired minimum linear flow velocity across the membrane can be achieved by a combination of pumping rate and/or flow velocity promoters contained within the ultrafiltration device. The method is preferred in which the solution is a Xanthomonas biopolymer and especially a Xanthomonas fermentation broth, most especially that derived from Xanthomonas campestris, The method is also preferred in which the activity of solution is concentrated at least 2-fold. Especially preferred is the method in which the apparent viscosity of the solution rises to at least about 10,000 centipoise. The preferred temperature for the method is from about 200-500C. There is an increasing trend toward the use of Xanthomonas fermentation broths for use in enhanced oil recovery. However, the transportation of these fermentation broths from the fermentation plants to widely dispersed oil fields involves considerable freight costs because of the tremendous volumes of broths involved. In order to reduce these volumes and shipping costs, efforts have been directed toward volume reduction by evaporation under vacuum or by precipitation of the xanthans by addition of large volumes of water-miscible solvents. These methods, unfortunately, add to process costs and may alter physical properties of the xanthan biopolymer. It has been found, surprisingly, that highly pseudoplastic biopolymer solutions including gelatinous and viscous Xanthomonas fermentation broth can be readily and economically concentrated by the ultrafiltration process. In the case of Xanthomonas broths, such concentration is achieved without altering xanthan activity or affecting the injectivity of the reconstituted solution. The process is adaptable to the concentration of viscous Xanthomonas solutions and preconcentrate, including undiluted and diluted whole or filtered Xanthomonas fermentation broths. Another important feature of the ultrafiltration process is that the biopolymer solutions can be concentrated without altering the polymer structure and other physical properties of the solutions. Pseudoplastic materials are non-Newtonian fluids in which viscosity decreases with increasing shear rate. Some materials, such as xanthan, can be termed highly pseudoplastic"; i.e., their viscosity decreases drastically with a relatively small increase in shear rate. Other pseudoplastic materials, such as pullulan biopolymer, have less pseudoplastic behavior in that their viscosity decrease is not as pronounced as with xanthan. Thus, under identical conditions of ultrafiltration, pullulans can only achieve about one-tenth of the flux rate of xanthan and they can only be concentrated to a viscosity of below about 3000 Eentipoise. The novel ultrafiltration application of this invention is practical for concentrating viscous broths and preconcentrates up to 20% xanthan concentration, which has a viscosity of greater than 20,000 centipoise and even higher, while at the same time retaining the xanthan activity, injectability and other important physical properties of Xanthomonas fermentation broths for use in enhanced oil recovery. The starting materials may be of a wide range of viscosities depending on the biopolymer and its concentration. In general, a Xanthomonas fermentation broth of about 3% concentration will have an apparent viscosity of 3000-6000 centipoise, by definition. The present invention is concerned with a method of concentrating in this uncommonly high viscosity range. Apparent viscosity is defined as the viscosity measured by the Brookfield Synchro-Lectric viscometer with LV4 spindle at 30 rpm and 250C. The process is carried out at a linear flow velocity of at least 40 cm/sec across the membrane surface and preferably at between 50 and 200 cm/sec. The process is operated at an average pressure differential of at least 2.0 atmospheres, preferably between 4 and 14 atmospheres. Average pressure differential is defined as the numerical average of the difference in pressure across the membrane as measured at the inlet and outlet sections of the ultrafiltration device. Linear velocity across the membrane is defined as the linear velocity which the ultrafiltration membrane "sees", and is dependent upon pumping rate and internal baffling within the UF device. The ultrafiltration equipment is preferably selected from any of those specially equipped to operate with relatively large flow channels in either turbulent or laninar flow and under sufficiently high pressure and linear flow velocities for achieving reasonable flux rate. Examples would include the tubular configuration manufactured by Western Dynetics, Inc. (Newbury Park, Ca) and the spiral wound configuration manufactured by Abcor, Inc. (Wilmington, Mass.). While the process may be satisfactorily operated at temperatures between 10 and 600C., it is preferably operated between about 200-500C. so that the physical properties of the retentate are substantially unaltered. The ultrafiltration method may be applied to the concentration of any pseudoplastic aqueous solution. In the special case of xanthan solutions, the fermentation broth may be any of those produced employing standard production media. The preferred Xanthomonas fermentation broths are those described in U.S. Patent 4,119,546 and co-pending application, Serial No. 81,879, filed November 10, 1979. These Xanthomonas fermentation broths are substantially free of insoluble matter having a particle size greater than about 3 microns. Broths derived by fermenting Xanthomonas campestris are especially preferred. Mobility control solutions (100-3000 ppm xanthan) prepared from these whole fermentation broths can be injected into subterranean oil reservoirs without the need for prior filtration. Ultrafiltration provides a simple and economical process for concentrating these fermentation broths. The highly viscous retentates can be economically shipped to oil field sites for subsequent dilution to mobility control solutions with unaltered injectability for use in enhanced oil recovery. This concentration process can be also applied to reconcentrating xanthan solutions which have been diluted for heat and/or other treatment. The active Xanthomonas polymer concentration is defined as that weight percent content of xanthan which could yield a final concentration of 500 ppm activity, if diluted to 10 centipoise (under 6 rpm, 250C, Brook field synchron viscometer) with a 500 ppm salt solution (NaCl:CaCl2 =10:1). Injectability is an important property of mobility control solutions. It is correlated with a Millipore test which is an experimental procedure that measures flow rate through a Millipore filter (0.45 to 3.0 u pore size) as a function of volume under a constant pressure of 40 psig. The filter ratio (F.R.) is the ratio of the time to collect the fourth 250 ml of mobility control solution to the time to collect the first 250 ml of mobility control solution (see U.S. Patent 4,119,546). A filter ratio of 1.0 indicates that the solution has no plugging tendencies. Example 1 Whole Xanthomonas fermentation broth (2.82% xanthan) was concentrated about threeZfold in a laboratory ultrafiltration unit (Model 4 SFPP manufactured by Western Dynetics, Inc. employing a 0.37 m2 polysulfone membrane (10,000 Mol. Wt. cut-off) at an average flow velocity of 122 cm/sec. and pressure differential of 7-10 atm. and a temperature of 250C. Active Apparent Xanthan Time Retentate Flux Rate Xanthan Visc. Content (hrs) Vol (liters) (lsmd)* Conc.(%) (cps) (kg) F.R. 0 14.74 442 2.82 4200 0.42 1.53 30 11.35 435 3.7 0.42 50 9.2 407 4.6 0.42 70 7.43 407 5.6 0.42 100 5.26 406 7.8 8200 0.41 1.54 * lsmd = liter per square meter per 24 hour day. Example 2 Whole Xanthomonas fermentation broth (3.15% xanthan) was concentrated about three-fold at an average temperature of 30-360C and pressure of 18.520 atm. and flow velocity of 122 cm/sec by the ultrafiltration method of Example 1. Apparent Active Xanthan Time Retentate Flux Rate Visc. Xanthan Contents (hrs) Vol. (liters) (lsmd) (cps) Conc. (%) (kg) F.R. 0 160 697 5100 3.15 5.04 1.34 2 136 737 5700 3.8 5.2 3 126 725 -- 4 116 689 6100 4.3 5.0 5 106 40--- --- 6 96 627 -- 8 77.2 587 --10 67 485 9000 8.3 5.56 1.40 12 59.2 407 11000 9.6 5.68 1.37 13 54 310 13300 11.5 5.72 1.38 Example 3 Whole Xanthomonas fermentation broth of an apparent viscosity of 4800 centipoise units (4% xanthan) was concentrated to an apparent viscosity of greater than 20,000 (estimated 50,000) centipoise (to 22% xanthan) at a temperature of 25-35 C at a pressure of 8-14 atmospheres and at an average linear flow velocity of 190 cm/sec by the ultrafiltration method of Example 1, using a larger tl-2m2) model of the Western Dynetics' ultrafilter described in Example 1 Example 4 Whole fermentation broth produced from yet a different strain of Xanthomonas campestris having an initial concentration of 2.58 xanthan and an initial viscosity of 8100 cps was concentrated five-six fold to a terminal concentration of 13.3% xanthan and greater than 20,000 cps (estimated viscosity 80,000) at a temperature of 25-350C., at a pressure of 8-14 atmospheres and at an average linear flow velocity of 190 cm/sec by the ultrafiltration method of Example 3. Example 5 Whole fermentation broth was produced as in Examples 1-3 above and was diluted and heat-treated to a viscosity of 450 centipoise. The solution was then partially concentrated using a common ultrafiltration system (Romicon HFMl.l-43-PM50 2 cartridge unit) to 3% with a viscosity of 3600 centipoise at a temperature of 250C, an average pressure differential of 2-2.5 atmospheres and an average linear viscosity of 210 cm/sec. The preconcentration process was terminated as the flux rate decreased from 2500 to 200 lpmd while the inlet pressure reached the maximum pressure allowable for the membrane system (3 atmospheres). This preconcentrate can be further concentrated by the method of Example 1 to a concentration of about 9% or above at a viscosity of about 10000 centipoise or above. Whole fermentation broth produced as in Examples 1-3, having an initial concentration of 3.4% xanthan, and an initial viscosity of about 5,000 cps, was concentrated to a terminal concentration of 9.4% xanthan and about 10,000 cps viscosity in a spiral wound ultrafiltration device 2 (manufactured by Abcor, Inc.), employing 5 ft2 of a nominal 20,000 molecular weight cut-off membrane, at a temperature of 250-300C, an average pressure differential of 4-5 atmospheres, and an average cross flow velocity (based on flow rate and internal turhulance generators) of approximately 50 cm/sec.

Claims: CLAIMS 1. A method of concentrating an aqueous, highly pseudoplastic biopolymer solution or preconcentrate having an apparent viscosity of at least 3000 centipoise comprising subjecting said solution or preconcentrate to membrane ultrafiltration at a linear flow velocity of at least about 40 cm/sec across said membrane surface while exerting an average pressure differential across said membrane of at least about 2.0 atmospheres. 2. The method of claim 1 in which said aqueous, highly pseudoplastic solution is a Xanthomonas biopolymer. 3. The method of claims 1 or 2 in which the apparent viscosity of said solution is increased to at least 10,000 centipoise. 4. The method of claims 1 or 2 in which said solution is a fermentation broth. 5. The method of claim 4 in which said broth is Xanthomonas campestris fermentation broth. 6. The method of claims 1 or 2 in which the activity of said solution is concentrated at least 2-fold. 7. The method of claims 1 or 2 in which said solution is maintained at a temperature of from about 200-500C.

Inventors: HO, LEWIS, TAYLOR, ROY JASPER
Applicants: PFIZER INC.
Application year: 1982
Main classification: B01D13
IPC codes: C12P19, B01D61