METHOD AND APPARATUS FOR SEPARATING LIQUID COMPOSITIONS ON THE BASIS OF MOLECULAR WEIGHT

Abstract: An ultrafiltration device comprising hollow fibers or thin channels through which ultrafiltration of blood plasma is effected is operated under conditions of tangential flow and low conversion. A recirculation stream of high molecular weight fraction of the liquid stream is fed to the device and a permeate stream from the device is recovered. The optimum module aspect ratio is designed and the corresponding maximum shear rate of incoming liquid is specified and controlled so as to provide maximum separation efficiency (selectivity and flux) allowing for separation between species of similar size or molecular weight. Shear rate is controlled by controlling the recirculation stream flow rate. The ratio of the transmembrane pressure at the channel outlet to the transmembrane pressure at the channel inlet resulting from the optimal design and operation is also specified.

Description: Background of the invention This invention relates to a method and apparatus for separating plasma into a high molecular weight plasma component and a low molecular weight plasma component utilizing porous hollow fiber on thin channels having membrane walls. Presently, liquids can be separated into a high molecular weight component and a lower molecular weight component by being passed along thin channels having membrane walls or porous hollow fibers when the membranes of the hollow fibers have a pore size which exclude the high molecular weight component while permitting passage therethrough of the low molecular weight component. In human blood plasma, the separation of the mixture by ultrafiltration is limited by the layer of the high molecular weight component concentrated on the membrane or fiber surface as a result of concentration polarization. Membranes are selected which allow substantial permeation of the albumin portion of the plasma while simultaneously achieving substantial rejection of globulins. It has often been observed that many membranes have these properties when tested with albumin alone or globulin alone. However, in the vast majority of cases, when the albumin and globulins are mixed together, the permeation of albumin is reduced while the globulins continue to be substantially rejected. This undesirable effect becomes more pronounced the higher the flux and/or the higher the concentration of globulins in the mixture. For this reason, ultrafiltration has been used almost exclusively as a concentration tool, since systems that reliably separate species based on the molecular size have not been possible especially when the species are of similar molecular size as in the case of albumin and globulins. To circumvent the effects of concentration polarization, several processes have been developed which modify the feed plasma source to improve liquid flux and selectivi­ ty. For example, Ohno et al in U.K. patent application number 2,065,l29 discloses a separation process in which serum is diluted to reduce total protein and salt concen­ tration while the pH is adjusted to between 3.8 and 4.7 prior to ultrafiltration. Baeyer et al in the Journal of Membrane Science, 22 (l985) 297-3l5, discloses a process in which the plasma is first diluted by a factor of l2 prior to ultrafiltration. Malchesky et al in U.S. Patent 4,350,l56 discloses a process for removing macromolecules from plasma by cooling the plasma to about l0° C and then filtering the macromolecules from the cooled plasma to form a filtered low molecular weight plasma stream. This process does not utilize an ultrafiltration membrane. It would be desirable to provide an ultrafiltration process utilizing open thin channels which effects passage of a lower molecular weight species through an ultrafiltration surface from a mixture of high molecular weight species and low molecular weight species at a level approaching that achieved with solutions containing a single low molecular weight species. In the case of plasma treatment wherein albumin is separated and recy­ cled to a patient, it would be desirable to attain the highest albumin permeate flow rate through the separation membrane with a minimum extracorporeal volume (membrane area). Brief description of the drawings Fig. l is a schematic view of a plasmapheresis and plasma separation system utilized in the present invention. Fig. 2 is an exploded view of an ultrafiltration unit which can be utilized in the process of this invention. Fig. 3 is a top view of an ultrafiltration membrane and the first spacer of Fig. 2 in contiguous relationship. Fig. 4 is a cross-sectional view of a rectangular channel of the apparatus of the Fig. l, 2 and 3. Fig. 5 is a cross-sectional view of an ultrafiltration hollow fiber which can be utilized in the present in­ vention. Summary of the invention This invention is based upon the discovery that in the design and operation of ultrafiltration modules it is necessary to control both the aspect ratio (ratio of channel length to channel height or fiber radius) of the separation module and the operating conditions of fluid shear rate in order to achieve maximum selectivity between the high molecular weight species and the low molecular weight species while minimizing membrane area. Selectivity is defined as the ratio of the concentration of low molecular weight species to that of the high molecular weight species that passes through the ultrafiltration surface. The process of this invention can be conducted in any ultrafiltration application which is operated at low volumetric conversions, defined as the ratio of the volume of fluid removed as permeate to the volume traversing the membrane surface in any single pass. As a consequence of this low conversion the concentration of species in the bulk retentate remains nearly constant, (within plus or minus l0 percent from the ideal value). The process of this invention includes a recirculation stream wherein a portion of the separated high molecular weight fraction is recirculated to the channel or fiber inlets The desired shear rate is attained by controlling (a) the ratio of transmembrane pressure at the channel or fiber outlet to the transmembrane pressure at the channel or fiber inlet and (b) the ratio of recirculation stream flow rate to the permeate stream flow rate. At the desired concentration of species in the bulk retentate this invention makes it possible to accurately maintain the flux of the low molecular weight component flow through the separation membrane by the proper control of the recirculating flow rate and permeate flow rate. Description of specific embodiments Simultaneous control over both the module aspect ratio and the corresponding module operating conditions enable strict control over the extent of concentration polarization to be achieved. For a given channel dimension of length and height a shear rate which is too low results in significant concentration polarization over the entire membrane surface and a loss in both flux and separation selectivity. When utilizing a shear rate which is too high, fluid backflow will result. Thus by simultaneously optimizing both the module aspect ratio and the operating shear rate the ultrafiltration module performance, high volumetric flux, low membrane area and high selectivity is controllable, reproducible and stable. The separations attainable with the present invention are not limited to a concentration process wherein the low molecular weight species differs in molecular weight from the high molecular weight species by a factor of 8 or more. The separations attainable include separations of species wherein the molecular weights are not widely different such as albumin from IgG. The ultrafiltration membrane utilized in the process of this invention has a thin skinned surface while the remainder acts as a very open - pore support structure. These membranes as well as the hollow fibers have retention cut-offs from about l,000 to about l,000,000 nominal molecular weight limit (NMWL). The optimum module aspect ratio is dependent upon the type of separation channel or fiber utilized and is dependent upon proportionality constants, as defined below, and the hydraulic permeability of the membrane being used. The equations set forth below include constants that are determined experimentally by optimiz­ ing the recirculation and permeate flow rates, by the procedure set forth below. The procedures for determining the aspect ratio and shear rates give optimum values. It is to be understood that it is within the scope of this invention to include values within plus or minus l0 percent of these optimum values since it is difficult to operate at exactly the optimum values due to variance of conditions such as variances in pump output or possible channel blockage. As used in the claims, the phrase "substantially the aspect ratio" is meant the L/h calculated by Equation l, plus or minus l0 percent. By the phrase "substantially the shear rate" as used in the claims is meant the maximum shear rate calculated by Equation 2, plus or minus l0 percent. For optimal performance, the module aspect ratio is given by: K is a function of the ratio of the transchannel pressure drop to the average pressure in the channel. K is obtained experimentally by the procedure set forth below. h is the channel height or the hollow fiber radius, ρ is the ratio of the recirculation stream flow rate, Q R , to the permeate stream flow rate, Q P , µ is the viscosity of the incoming plasma being separated, L is the length of the channel or fiber and L P is the membrane hydraulic permeability after the membrane is wet with the liquid to be ultrafiltered. For channels of rectangular geometry having at least one wall formed of a porous membrane, h is the distance between the membranes which define the height of the channel shown in Figure 4. Generally h for the rectangu­ lar channels is between about .0l00 and .030 cm. In hollow fiber cylindrical geometry, h is the radius of the fiber lumen shown in Figure 5 and is generally between about .0l50 and .025 cm. In the present invention, the module aspect ratio, L/h, can range between about 50 and 5000, preferably between about l00 and 300. The unique features of this invention is that the module aspect ratio and module operating shear rate are simultaneously optimized to achieve the desired selectivity at the largest possible flux. Unlike conventional systems, the system of this invention does not operate either at an excessively large shear rate or an excessively low volumetric flux, but at conditions which maximize the desired selectivity which in the case of plasma is the plasma protein selectivity. An exact relationship has been found between the module aspect ratio and module operating shear rate which gives optimal separation performance. The maximum shear rate to be utilized in the apparatus is defined by equation 2: wherein is the maximum shear rate to obtain optimal selectivity performance. The proportionality constant, D*, is obtained empirically by the following procedure: A prototype module is provided containing a plurality of thin channels or hollow fibers of the type of ultrafil­ tration membrane to be utilized in the final apparatus. The channels or fibers in the prototype can be of any dimension. A prototype having channels with an L/h of about 200 has been found to be useful. Pumps and con­ duits are provided to form a flux stream to recover the low molecular weight component and to control the flow rate of the permeate, Q p . This permeate stream is recombined in a feed reservoir with a recirculation stream comprising the high molecular weight component obtained from the ultrafiltration channels or fibers. The recirculation stream flow rate (Q R ) is controlled. A plurality of runs are made with the apparatus with either Q P being varied and Q R being maintained constant or Q R being varied and Q P being maintained constant. After each run, the separation performed (selectivity) between the species of interest is measured. After each run the system also is thoroughly flushed such as with saline or water to remove all treated liquid from the system. A standard hydraulic permeability, e.g. water or saline, L P , is then measured by standard methods. The value then is multiplied by the ratio of µ of the treated liquid to µ of the standard fluid wherein µ is viscosity in centipoises and wherein L P is used in equation l. From the selectivity values obtained, the optimum selectivity is identified and the Q P and Q R values which correspond to the optimum selectivity can be determined. The selected optimum value refers to the Q P and Q R values when both the flux and selectivity are maximized simulta­ neously. The constant K can be calculated using the optimum value of Q R /Q P using Equation l. Using the shear rate corresponding to the optimum Q R , The constant D* then can be calculated from equation 2. D* is a property of the solution being ultrafiltered and is between about l × l0⁻⁷cm²/sec and 25 × l0⁻⁷ cm²/sec and preferably between about 5 × l0⁻⁷ cm²/sec and l2 × l0⁻⁷ cm²/sec The limits on ρ reflect the limits on the ratio, Q R /Q P . The upper limit is set by the size of the recirculation pump whereas the lower limit is set by the maximum increase in concentration which is allowable down the length of the ultrafilter at low values of Q R /Q P , i.e. at high conversions, (the retained species become more concentrated as permeate is removed). A maximum increase of l0 percent above the concentration at the inlet is permitted for the high molecular weight species in the retentate stream. When an ultrafiltration device is designed and operated in accordance with equations l and 2, the total membrane area in the device which provides optimal separation efficiency is given by equation 3 wherein A is the total membrane surface area. Furthermore, the maximum transchannel pressure drop which can be measured directly is also given by equation 4 for optimal separation conditions wherein ΔP C is the transchannel pressure drop. Control of the concentration polarization in a tangential flow module depends both upon a combined match between the module aspect ratio and operating shear rate. Therefore only a restricted range of module designs and of operating shear rate are feasible. This results in an upper and lower limit for the factors, L/h, and ρ. The ratio, ρ, of recirculation stream flow rate, Q R to permeate stream flow rate Q P , is between about 5 and l00, preferably between about l0 and 50 and most preferably 30. Finally, with the optimal design, L/h, and operating con­ ditions, , the ratio of the transmembrane pressure drop at the channel outlet to the transmembrane pressure drop at the channel inlet, is significantly different from l.0. The value of derived from this invention lies between 0.0 and 0.85, most typically 0.75. The following example illustrates the present invention and is not intended to limit the same. EXAMPLE l The module aspect ratio and operating conditions were optimized by the above-described procedure for the separation of IgM from albumin in human plasma using a polyvinylidine fluoride ultrafiltration membrane having an NMWL of 500,000. The optimum value of D* determined emperically is 8.5 × l0⁻⁷ cm²/sec. yielding an optimum aspect ratio of = 276 and maximum shear rate = 4250 sec⁻¹ for h = l25 µm. This module would operate near 0.033 cm/min flux, require 0.l4 m² membrane area and achieve a selectivity of Δ R = 0.8 (R ALB = 0.l0 and R IgM = 0.90) for plasmapheresis applications from which permeate flow rates of about 40 ml/min are required. This albumin rejection coefficient is equal to that measured for solutions of albumin alone with this membrane. The separation between albumin and immune complexes in human plasma can also be achieved with the polyvinylidine fluoride membrane (assuming immune complexes to be on average twice the size of an IgG molecule). The optimum values for this separation using the preferred value of D* = 8.5 × l0⁻⁷cm²/sec yielding = 385 and = 3700 sec⁻¹ for h = l50µm. This module would operate near 0.025 cm/min flux and achieve a selectivity of ΔR = 0.7 (R ALB = 0.l and R IgG = 0.8) Finally, the separation between albumin and IgG in human plasma can also be achieved with the polyvinylidine fluoride membrane. The optimum values for this sepa­ ration again using D* = 8.5×l0⁻⁷ cm²/sec are = 276 = 4250 sec⁻¹ for h = l25 um This module would operate near 0.033 cm/min flux and achieve a selectivity of ΔR=0.4 (R ALB = 0.l and R IgG = 0.5). Referring to Fig. l, a typical system in which the present invention is useful is shown. A catheter which communicates with conduit l2 is inserted into the patient l0 in order to pump the blood from the patient l0 by means peristaltic pump l4. An anticoagulant is introduced into the system through conduit l6 into conduit l8. The whole blood containing anticoagulant is introduced into a cell separator 20 such as a centrifuge adapted to separate whole blood into its plasma component and into its cellular component. Any suitable means for separating the whole blood into its plasma component and cellular component can be utilized in the present invention. Typically, the plasma-cell separation can be effected by passing the whole blood into contact with a hollow channel having a membrane generally having a pore size of about between 0.05 and l.0 microns, particularly membranes having a pore size of about .06 microns. When utilizing the volumetric flow rates set forth above, typical pressures across the ultrafiltration membrane in step 20 are usually between about l0 to about l00 mm Hg. The cellular blood compo­ nent is directed through conduits 24 and 26 and reinject­ ed into the patient l0. The plasma component is removed from cell separator 20 through conduit 28 by means of peristaltic pump 30 and passed through conduit 32 and to reservoir 34. A physiologically acceptable diluent is added to the reservoir 34 through conduit 38. The mixture of diluent and plasma is removed from reservoir 34 by means of peristaltic pump 36 and directed to a second separation step 40 wherein low molecular weight plasma fraction is separated from a high molecular weight plasma fraction by means of membrane 42. The high molecular weight plasma fraction is recycled to reservoir 34 by means of conduit 44. The low molecular weight plasma fraction containing albumin is removed from separation step 40 through conduit 46 by means of peristaltic pump 48 and is recycled to the patient l0 by means of conduit 50 and conduit 26. The rate of addition of diluent to reservoir 34 and the control of peristaltic pump 30, and 48 regulate the flow rates of the various plasma strength containing streams. In an alternative embodiment of this invention, a bleed stream 52 can be utilized in order to remove a portion of the high molecu­ lar weight plasma fraction from the system. Typical operating conditions for the system shown sche­ matically in Figure l are as follows: The peristaltic pump l4 is operated in order to effect whole blood removal from the patient at a flow rate between about 20 and about l80. The anticoagulant such as heparin, ACD or anticoagulant citrate dextrose typically is added in aqueous solution at a flow rate through conduit l6 of about between l0 and about 50. The pump 30 which effects removal of the plasma fraction from separator 20 generally is operated to effect a flow rate in conduit 28 of about 5 and about 60. The pump 48 which effects removal of the low molecular weight plasma fraction generally is operated to effect a flow rate in conduit 50 between about l0 and about 50 ml/min. diluent through conduit 38 to reservoir 34 at a rate required to maintain nearly constant protein concentrations in reservoir 34. For example, when the albumin return desired is 50 percent of that originally in the patient, typical flow rates are whole blood in conduit l2 of about 80 ml/min. anticoagulant in conduit l6 of about l0 ml/min., plasma flow rate in conduit 28 of about 30, recirculated plasma in conduit 44 of about l,000, low molecular weight plasma fraction in conduit 50 of about 30 and flow rate of diluent initially of about 30 and gradually to 0 over a time period of about l.5 hours. The process also can be operated without a diluent or anticoagulent with a liquid stream other then blood plasma such as fermentation baths. Referring to Figures 2 and 3, a typical structure utiliz­ ing rectangular hollow channels (Fig. 4) is shown. This structure is disclosed by U.S. Patent 4,540,942 which is incorporated herein by reference. A filter unit l0 comprises, a first membrane l2, a second membrane l4, a first spacer l5, and a second spacer l8 which, when joined together from a plurality of rectangu­ lar channels (Fig. 4). The apparatus includes a plurali­ ty of filter units l0 which are positioned contiguous to each other and form a stack of filter units l0. Both the first membrane l2 and the second membrane l4 are of identical construction and are formed from an ultrafiltration membrane. Each membrane l2 and l4 is provided with two longitudinal channels 20 and 22 and a widthwise channel 24. The widthwise channel 24 is not in fluid communication with either of the channels 20 or 22. The first spacer comprises of plurality of channels 32 which extend from edge 26 to edge 28 an outlet channel 30. When membranes l2 and l4 are contiguous to spacer l5, the edges 26 and 28 coincide with the edges 25 and 27 respectively. The second spacer l8 is provided with a plasma inlet channel 34, a high molecular weight plasma outlet 36 and a widthwise low molecular weight plasma outlet channel 38. The second spacer l8 also is provided with interior channels 40 which provide fluid communica­ tion with channel 42 which in turn is in fluid communica­ tion with low molecular weight plasma outlet channel 38. When spacer l8 is juxtaposed to membrane l4, edges 37 and 39 coincide respectively with edges 25 and 27 of spacer l4. The spacer strips 33 between channels 32 and the spacer strips 3l between the channels 40 are bonded to the next adjacent membrane (not shown) and provide the necessary support for the membranes adjacent the channels so that membrane flexibility is controlled to maintain the desired channel height. The channels 32 in first spacer l6 should have a particu­ lar height, width and length in order to accommodate the plasma produced from the normal blood flow rate from a donor so that blood can be removed continuously while enriched blood from which plasma has been removed and the low molecular weight plasma stream can be reintroduced continuously to the donor. While the module structure shown in Figure 2 and 3 is useful in the present in­ vention, it is to be understood that any ultrafiltration module having their channels formed with ultrafiltration membranes can be utilized in the present invention so long as the aspect ratio and operating condition are controlled in accordance with this invention as set forth above. Referring to Fig. 3, the channels 32 of first spacer l6 are shown to overlap into channel 20 and 22 of membrane l4. This overlap permits introducing of whole blood into channel 20, passage of the whole blood lengthwise along channels 32 while being in contact with membrane l4 and removal of plasma depleted blood from channels 32 through widthwise channel 22.

Claims: 1. A system for separating blood plasma into a high molecular weight stream and a low molecular weight permeate stream which comprises a separation module having separation means compris­ ing a plurality of thin channels or hollow fibers having walls through which ultrafiltration is effected, said separation module having substantially the aspect ratio defined by Equation l and being between about 50 and 5,000, means for introducing said blood plasma into an inlet portion of said separation module at substantially the shear rate defined by Equation 2 and means for removing said high molecular weight stream and said low molecular weight stream from said separation module, means for recirculating at least a portion of said high molecular weight stream to an inlet portion of said separation module thereby to form a recirculation stream, means for controlling the ratio of the recirculation stream flow rate to the permeate stream flow rate to between about 5 and l00 and so that the ratio of the transmembrane pressure at the outlet to said separation means to the transmembrane pressure at the inlet of said separation means to between about 0.0 and 0.85. 2. The system of claim l wherein the aspect ratio is between about l00 and 300. 3. The system of claim l wherein the ratio of recircu­ lation stream flow rate to permeate stream flow rate is controlled to between about l0 and 50. 4. The system of claim l wherein the ratio of transmem­ brane pressure at the inlet to said separation means to the outlet of said separation means is controlled to between about 0.7 and 0.85. 5. A process for separating a blood plasma into a high molecular weight stream and a low molecular weight permeate stream utilizing a separation module having separation means comprising a plurality of thin channels or hollow fibers having walls through which ultrafiltra­ tion is effected, said separation module having substan­ tially the aspect ratio defined by Equation l and being between about 50 and 5,000 which comprises, introducing said blood plasma into an inlet portion of said separation module and removing said high molecu­ lar weight stream and said low molecular weight stream from said separation module, recirculating at least a portion of said high molecular weight stream to said inlet portion of said separation module thereby to form a recirculation stream controlling the ratio of the recirculation stream flow rate to the permeate stream flow rate to between about 5 and l00 so that the ratio of the transmembrane pressure of the outlet to said separation means to the transmembrane pressure at the inlet of said separation means to between about 0.0 and .85. 6. The process of claim 5 wherein the ratio of recircu­ lation stream flow rate to permeate stream flow rate is controlled to between about l0 and 50. 7. The process of claim 5 wherein the ratio of trans­ membrane pressure at the outlet to said separation means to the inlet of said separation means is controlled to between about .70 and .85.

Inventors: DE LOS REYES, GASTON, DILEO, ANTHONY J.
Applicants: MILLIPORE CORPORATION
Application year: 1987
Main classification: B01D61
IPC codes: B01D61, A61M1