CROSS-FLOW MICROFILTRATION PROCESS

Abstract: A method of establishing an essentially steady state cross-flow filtration flow in a cross-flow membrane filtration module by starting at a low transmembrane pressure (TMP) and steady state operation and then increasing the TMP by an iterative procedure until the performance starts to become unstable and then backing down the TMP slightly until the performance restabilizes. One preferred embodiment employs a filtration module that describes a flow length such that the permeate pressure is equal to the concentrate pressure.

Description: CROSS-FLOW MICROFILTRATION PROCESS BACKGROUND OF THE INVENTION The present invention relates to cross-flow microfiltration for removal of suspended and colloided solids and/or emulsified oil from liquids, particularly from water, waste-water, industrial waste, and industrial process streams. Cross-flow filtration is described in detail in U. S. Patent 5,047, 154 issued September 10, 1991, to Comstock et al., which is incorporated herein by reference. As described in Comstock et al., column 2, lines 21-31, cross-flow filtration systems are plagued by the problem of declining filtration fluxes. Such problems are especially acute when the material to be removed from an aqueous system comprises suspended proteinaceous solids as are typically found in process streams such as raw beet juice or high fructose corn syrup (HFCS). Comstock et al. recognizes the desirability of operating in a steady state with nearly constant flux rate when the driving pressure differential is held constant, but acknowledges its inability to achieve such conditions in practice. Since Comstock et al. was unable to solve this problem, it proposed a method of increasing the time-averaged cross-flow filtration flux of a liquid through a porous microfiltration medium by maintaining the filtration flux rate at a pre-selected substantially constant value during the entire filtration run by applying a variable throttling pressure on the filtrate side and reducing the throttling pressure during the run to control the instantaneous value of the pressure differential as required to maintain the pre-selected flux rate, the flux rate being greater than the equilibrium flux rate. In Comstock et al., as the filtration run progresses, the throttling pressure is continually reduced, thus increasing the pressure differential in order to maintain the flux rate at the desired constant value. At the end of a filtration run, the Comstock et al. method resembles a conventional system. (Column 8, lines41-49). Comstock acknowledges that at this point, it is necessary to shut the system down for cleaning. It remains desirable to attain in practice the steady state operation of a cross-flow filtration system. SUMMARY OF THE INVENTION The present invention provides a further improvement in the art by providing a method of establishing such a steady state cross-flow filtration flux in a cross-flow membrane filtration system. The essence of the invention is determining a maximum transmembrane pressure by an iterative process whereby the transmembrane pressure is maintained at an initially low rate at which the process is in steady state and gradually increased to the point where process parameters such as concentrate pressure or permeate pressure just begin to vary and is then decreased slightly to the steady state condition. The system is then maintained at this point. The advantages of this process include steady state operation which permits prolonged system operation without the necessity of shutdown for backf lushing and/or other cleaning and operating at the highest transmembrane pressure that permits steady state operation. In one embodiment, the filtration module employed in the method of this invention describes a flow length such that the permeate pressure is equal to the concentrate pressure, thus defining a maximum recovery per pass achievable during steady state operation. The latter requires that there is no net fouling along the entire membrane length. That is, at any point, the rate of deposition of foulant caused by the permeate flow to the membrane surface is exactly opposed by the rate of foulant removal induced by tangential shear forces of the cross flowing stream. Implicit in the criteria of a maximum recovery per pass is the relationship that maximum steadystate flux or permeate flow will increase with increasing cross-flow velocity. A particular advantage of the present system isthat by operating under the steady state conditions, it is possible for the system to operate for extended periods of time without the membrane becoming fouled to the extent that it is necessary to shut down the system to clean the membrane. The system is commercially practical because it can be operated for days or weeks without the necessity of backf lushing and/or other cleaning. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing the feed, concentrate, and permeate pressures obtained in the steady state operation of this invention. DETAILED DESCRIPTION OF THE INVENTION The process or method of this invention is independent of the filtration membrane or apparatus employed. Filtration modules may be employed in any configuration, but are generally employed in series, parallel, or a combination of both, with parallel being generally employed, and oriented in any manner, but typically either horizontally or vertically. Typical filtration membranes that may be employed in the practice of this invention include, for example, membranes described in U. S. Patent 4,798,847 issued January 17, 1989, to Roesink et al., incorporated herein by reference. Other useful membranes are described in U. S. Patent 5,076,925 issued December 31 , 1991, to Roesink et al. and U. S. Patent 5,096,585 issued March 17, 1992, to Nguyen. Most preferred are hydrophobic membranes that are modified for hydrophilic character by either blending or coating modifications. Membranes may be tubular, capillary, flat sheet, hollow fiber, or spiral wound with capillary bore feed being generally preferred. It is desirable that the effective pore size of the membranes be in the range of from about 0.01 to about 10 microns, generally 0.1 to 1.0 microns, i.e., the mean flow pore diameters as defined by ASTM method F316-86 are from 0.01 to 10 microns. The following terms as used herein have the following meaning: Feed Pressure (Pf) is the pressure of the feed as it enters the filtration module; Concentrate is the portion of the feed that does not permeate through the membrane contained within the filtration module; Concentrate pressure (Pς) is pressure of the concentrate as it exits the filtration module; Permeate pressure (P p ) is pressure of the permeate as it is collected at the exterior « of the tube; 5 Transmembrane pressure (TMP) is the pressure across the membrane contained f within the filtration module and is 1/2(Pf - ?_) - (P p - P__). As discussed above, the present invention is a cross-flow microfiltration process in which steady state operation is obtained by an iterative process. This process comprises beginning operation under conditions which result in a transmembrane pressure which is 10 significantly lower than used in heretofore conventional systems. If rapid membrane fouling as shown by sharply decreasing permeate flow is observed, a lower TMP is used. Otherwise, the TMP is gradually increased to a point where process parameters begin to vary with time. At this point, the transmembrane pressure is decreased to a point at which process parameters are essentially constant and the system is maintained at essentially that transmembrane pressure. 15 Various techniques exist for controlling transmembrane pressure. In a preferred embodiment, this is accomplished by controlling the permeate pressure by, for example, use of a valve in the permeate line. Other techniques will be suitable as will be recognized by those skilled in the art. In one preferred embodiment, the TMP is maintained such that the permeate 20 pressure is equal to or greater than the concentrate pressure. It is preferred that the permeate pressure is maintained as close as equal to the concentrate pressure as possible. In some systems, such as process streams of raw beet juice or high fructose corn syrup, this means that for a single pass, recovery is maximized to a level no greater than about 6 percent. For purposes of this invention, steady state means that the various process 25 parameters, including flux, concentrate pressure, permeate pressure, feed pressure, temperature, etc. remain relatively constant over an extended time period. Of particular importance is that the rate of membrane fouling is zero. By this is meant that rate of deposition of foulant carried by the permeate flow to the membrane surface is opposed by the rate of foulant removal induced by tangential shear forces of the cross-flowing stream. When 30 these parameters remain relatively constant and equal, it is possible to maintain continuous operation of the cross-flow microfiltration system without the need to shut down the system to backflush or chemically clean the membrane. It should be recognized that "essentially * constant" does not mean that the process parameters will not vary at all. In a practical situation, for example, the feed conditions such as concentration, temperature, etc. will vary 35 routinely. A particular advantage of this process is that minor disruptions to the system do not lead to extensive membrane fouling which results in the need to shut down the system to clean the membrane. While it is recognized that system parameters will depend on the nature of the feed and other factors, the present process results in far fewer disruptions to the system due to membrane fouling that would be present in systems operated conventionally at higher initial flux rates and in the absence of the steady state conditions. While each system will vary, the steady state process of this invention permits the operation of a cross-flow microfiltration system for extended periods of time, i.e., days or weeks, without the need to stop the process for backflushing or other cleaning. When compared to a system which operates under higher pressure conditions and obtains higher initial flux rates, but must shut down frequently, i.e., in terms of minutes or seconds, for backflushing, etc., the present process obtains higher average flux rates. The present invention is useful in treatment of various streams containing components to be separated by microfiltration. Examples of such streams include effluent steams from operations for the production of sugar such as raw beet effluent streams and high fructose corn syrup streams. Additionally the process is useful in the treatment of waste streams from laundry applications such as exist in stone washing operations. The following examples are provided to illustrate the invention and should not be interpreted as limiting it. Example 1 In an illustration of an operation in accordance with the present invention, a microfiltration module (FilmTec part #90006, one square meter surface area, 0.2 micron pore size) is used to filter 30 percent dissolved solids dextrose solution containing 0.4 percent suspended solids (mainly proteinaceous) at a driving pressure differential of about 27.6 kPa, a single pass recovery rate of less than about 5 percent and a temperature of about 68°C Filtration is continued in an essentially steady state condition for several days with substantially no change in permeate flux rate. The turbidity of the feed is about 150 NTU and the turbidity of the permeate is about 0.37 NTU (Nepholometer Turbidity Units) and the total yield of dextrose is greater than 90 percent. Example 2 As a further illustration of the present invention, a 30 percent dissolved solids saccharifi cation effluent containing 0.4 percent suspended solids (mainly proteinaceous) is filtered employing a cross-flow microfiltration module having a polyethersulfone-polyvinyl pyrrolidone membrane having a pore size of 0.2 micron and a surface area of 1 square meter. The feed flow rate is about 0.69 liters per second (yielding an average feed velocity of about 1.7 meters/second) and the flow direction is vertical. The filtration is continued for about 14 days with the results being indicated in the following Table 1. The initial feed turbidity in this experiment is about 108 NTU. The permeate turbidity of all samples tested is 0.32 or less. The purified yield of dextrose is greater than 99 percent without diafiltration. I Ul I TABLE I (continued) I en I TABLE I (continued) PRESSURES Cumulative Feed Temp Concentrate Days (C) Turbidity Feed Concentrate Permeate (NTU) (kPa) (kPa) (kPa) 10.93 68 346 265 288 N.D. 12.06 43**-** 353 265 291 N.D. 13.02 49**** 350 265 290 N.D. 14.06 54**** 353 271 293 N.D. I --J I * - % OF FEED PERMEATED PER PASS - NOT TO BE CONFUSED WITH TOTAL DEXTROSE PERMEATED N.D. - NOT DETERMINED ** - PROBABLY NEEDS A BACKFLUSHING *** - BACKFLUSHED - CLOSED PERM VALVE/REVERSE FEED FLOW (5 MIN.) **** - TEMPERATURE DECLINE DUE TO CONDENSER WATER SHUTDOWN AND RESULTING BIOLOGICAL GROWTH It is noted in Table I that at three days it appeared that the membrane might be fouling as indicated by an increase in driving pressure differential. However, backflushing was not carried out for nearly 18 hours, during which time the system continued to operate and, in the end, no backflushing was needed. It is further noted that at around the twelfth day of operation, the feed temperature declined significantly due to an accidental shutdown of the condenser water, resulting in biological growth and subsequent decline in the percent feed permeated. Despite the biological growth, the system continued to produce permeate. Example 3 In this example, a sample of unfiltered corn syrup containing 38 percent dissolved solids and 36 dextrose equivalents (DE) is treated. The feed turbidity is 800 NTU (Nepholometer Turbidity Units) and is coarse filtered through a 600 micron filter prior to introduction into the cross flow microfiltration membrane system. The membrane used is one having 10 square meter surface area and 0.2 micron pore size. The results obtained are shown in Figure 1 and in Table II. As can be seen in Table II, the feed temperature varies from 175 to 185°F and the transmembrane pressure is at 34.83 kPa at the beginning of the run and is at 37.48 kPa at the end of the run. During the course of the run the pressure varies from a low of 32.84 kPa to a high of 38.96 kPa. It should also be noted that F p starts at 4.00 gallons per minute and ends at 4.02 gallons per minute which shows a relatively constant flow rate over the 15 hours of this particular run without any need to stop the process for backflushing or other cleaning. Table II CMF FILTRATION OF 36 DE CORN SYRUP I KD I Pp - Feed Pressure (kPa) FC - Concentrate Flow rate (GPM) Pp - Permeate Pressure (kPa) Fp - Permeate Flow rate (GPM) PC - Concentrate Pressure (kPa) FR - Recirculation Flow rate (GPM) ΔP - Transmembrane pressure (kPa) Table II (continued) CMF FILTRATION OF 36 DE CORN SYRUP I o-> I p - Feed Pressure (kPa) c - Concentrate Flow rate ( GPM) Pp - Permeate Pressure (kPa) Fp - Permeate Flow ra te ( GPM) C - Concentrate Pressure (kPa) FR - Recirculation Fl ow rate (GPM) TMP - Transmembrane pressure (kPa) When operating in accordance with this invention, it has been found that filtration is continued without backflushing for many weeks without a significant decrease in permeate flow rate. Backflushing may be achieved by reversing feed and concentrate flow direction with or without the permeate valve closed. Increases in feed flow velocity during backflushing may be practiced but are generally unnecessary. Other traditional types of backflushing, for example, pumping permeate product back through the module, will also regenerate the membrane as will other traditional methods. When backflushing is necessary or desirable, the original flow rate is generally achieved unless the filter is biologically fouled. Backflushing may be carried out with water, if desirable. Total product yields have been demonstrated by the method of this invention of greater than 99 percent without the need for diafiltration, which is a significant improvement over the prior art where even yields up to 90 percent are considered excel lent. In addition, the turbidity of the product stream is greatly improved over prior art methods. While it is typically not necessary to stop the filtration for cleaning, it is possible to clean the membrane when the system is shut down for other reasons. Cleaning of the module after operation in accordance with this invention has been demonstrated with both cold and hot tap water. When dealing with streams subject to biological attack, a 250 parts per million bleach solution as well as other biocides may be necessary to regenerate to the original flow rate. It is also apparent that according to the method of this invention increases in module feed velocities allow higher membrane fluxes such that the ratio of maximized flux to cross-flow velocity remains essentially constant. Various modifications may be made in the present invention without departing from the spirit or scope thereof as will be understood by those skilled in the art.

Claims: C LA I MS : 1. A method of establishing an essentially steady state cross-flow filtration rate in a cross-flow membrane filtration module comprising maintaining an initially low transmembrane pressure and gradually increasing the transmembrane pressure to the point where process parameters, including concentrate pressure and permeate pressure, begin to vary with time and then decreasing the transmembrane pressure to a point at which the parameters no longer vary with time. 2. A method of Claim 1 wherein transmembrane pressure is maintained so that the permeate pressure is equal to the concentrate pressure. 3. A method of the Claims 2 wherein a single pass recovery is maximized up to a level no greater than about 6 percent of the feed flow. 4. Method of Claim 1 wherein the membrane is a microfiltration membrane comprising a blend of polyethersulfone and polyvinyl pyrrolidone. 5. Method of Claim 4 wherein the membrane has an average pore diameter of from about 0.01 to about 10 microns. 6. Method of Claim 1 wherein the feed comprises an aqueous solution of at least one sugar which can permeate through the membrane and suspended solids. 7. The process of Claim 6 wherein said solid impurities include suspended proteinaceous solids which are substantially incapable of permeating the membrane.

Inventors: MARSH ALLYN R III, MCREYNOLDS KENT B, MARSH, ALLYN, R., III, MCREYNOLDS, KENT, B.
Applicants: DOW CHEMICAL CO, MARSH ALLYN R III, MCREYNOLDS KENT B, MARSH, ALLYN, R., III, MCREYNOLDS, KENT, B., THE DOW CHEMICAL COMPANY
Application year: 1993
Main classification: B01D61
IPC codes: B01D61, B01D65, B01D69, B01D71, C13D3, C13K11