Reverse Osmosis and Ultrafiltration Applied to the Processing of Fruit

25 Reverse Osmosis and Ultrafiltration Applied to the Processing of Fruit Juices

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DAVID J. PAULSON, R I C H A R D L. WILSON, and D. D E A N SPATZ Osmonics, Inc., Minnetonka, M N 55343 Use of crossflow membrane separation technology for food and beverage manufacture has come of age. The feasibility of processing many types of fruit juices has been demonstrated and several production size units are currently operating. Applications such as replacement of troublesome diatomaceous earth (DE) filters for juice clarification have been highly successful. Employing reverse osmosis for apple juice concentration produces potential shipping cost savings of over $2000/day for a processor handling juice at a rate of 15,000 gallons per day. This paper discusses the membranes and membrane configurations available, and the steps necessary to properly develop applications for crossflow membrane technology. Apple, grapefruit, lemon, cranberry, and grape juices, as well as wine test results are presented. Clarification, pectin removal, and several concentration applications are discussed. Advances in membrane, membrane elements and hardware design and construction to allow operation under the increased temperature and pressure requirements for juice processing are also presented. Crossflow Membrane Filtration Crossflow membrane filtration is the separation of the components of a fluid performed by polymeric semi-permeable membranes through the application of pressure. The pressure required varies depending upon the size of the pores in the membranes. Reverse osmosis (RO) membranes have pores from 5 Angstroms (A) to 20A in diameter, and will reject most ions as well as most organics bver 150 molecular weight. Ultrafiltration (UF) membranes have pore sizes of 10A to 0.2 micron, rejecting larger organics such as proteins and viruses while passing most ions. Microfiltration (MF) has pore sizes ranging from 0.05 micron to 2 microns, and can replace traditional depth filtration for high volume removal of bacteria and very small suspended particulates.

0097-6156/85/028I-0325$06.00/0 © 1985 American Chemical Society

In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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REVERSE OSMOSIS AND ULTRAFILTRATION

Figure 1 shows a chart of separation from the ionic to the particulate range. The crossflow membrane classes overlap each other somewhat, since neither the marketplace nor the technology have yet developed sharp distinctions. Crossflow membrane filtration is fundamentally different in design from the older technologies of depth and surface barrier filtration. In crossflow, the influent stream is separated by the membrane into two effluent streams: the "permeate", which passes through the membrane, and the "concentrate", which retains the solutes or suspended solids which have been rejected by the membrane. Figure 2 illustrates the process of crossflow filtration. There are several advantages to operating in the crossflow design mode. First, the process is self-cleaning, with solutes and solids swept away by the pressurized concentrate stream which runs parallel to the membrane (hence, the term crossflow). Crossflow systems can operate economically on large fluid volumes with minimal operator attention and system downtime, since constant filter media changeouts are not required. Second, depending upon the application and process requirements, the permeate, the concentrate or both may be the product. Third, crossflow filtration membranes produce the finest separation available economically, and the most selective separation in the molecular range. Crossflow filtration systems can be designed to selectively pass some components of a feed stream while rejecting others. This "fractionation" can be performed at the ionic, molecular and macro-molecular levels. Crossflow Membrane Polymers The wide range of juice processing applications comprise a variety of fluid environments, some of which may be detrimental to certain polymeric materials. Operating conditions such as temperature, pressure, solution pH and chemical compatibility must therefore always be considered in relationship to the membrane. No one polymer will withstand the environments and perform adequately in all the possible juice processing applications where crossflow membrane processing can be applied, so several different membrane materials should be investigated to select the optimum for each application. RO polymers currently available commercially include the asymmetric membranes: cellulose acetate (CA) and aromatic polyamides (PA) which are homogeneous. Also available are the newer thin-film composite membranes (TFC), usually considered polyamides but integral with a polysulfone substrate. UF polymers currently available include: polysulfone (PS), CA-blends, a fluorinated polymer (VF), and Osmonics' patented polymer (PA type)• MF polymers commercially available are the most numerous, due to the relative ease of achieving the required pore size for MF compared to UF and RO membranes. Polypropylene, acrylonitrile, nylon and PTFE are among the more common MF polymers, and their broad chemical compatibility characteristics make the MF range of membranes the most chemically stable.


PAULSON ET AL.

RO and UF Applied to the Processing of Fruit Juices

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Figure 2.

A r t i s t ' s c o n c e p t i o n of c r o s s f l o w membrane

filtration.


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Developmental RO polymers which show some long term promise are acrylonitrile copolymers and polyethersulfone, although neither is beyond the laboratory stage yet, TFC and grafted ultrafiltration membranes are also being investigated in the labs. In choosing a polymer membrane/ the following variables must be evaluated: Operating Pressure. MF membranes are typically operated at transmembrane pressures (feed side pressure less permeate pressure) from 20 to 345 kPa (3 to 50 psi). UF membranes from 345-1380 kPa (50 to 200 psi) and RO membranes from 1380-6890 kPa (200 to 1000 psi). Under the higher operating pressure conditions/ membranes may be subject to mechanical compression (compaction) and other mechanical deformation which can alter performance over time. Increased temperature intensifies the compaction effect. Understanding this characteristic and selecting the most appropriate membrane for the pressure required is an important consideration. Temperature. Since some processes in the juice industry may be performed at elevated temperatures, consideration of the polymer membrane's temperature limits is essential. Membrane separation processes are currently operating at a temperature range from 0 to 85°C (32-185°F). Membrane cleaning and steam sterilization of the system occur up to 110°C and higher. It is often preferable to operate the membrane system at elevated temperatures due to pasteurization or other processing requirements to further save process energy costs. The best understood and least expensive RO membrane/ cellulose acetate (CA)/ has often had an upper temperature limit assigned to it in the 35-40°C (95-104°F) range. Recent data from controlled experiments show that CA membrane retains its separation performance quite well at much higher temperatures and is effective in processes to 65°C (149°F). Polyamide (PA) type RO membranes definitely have higher temperature limits and can operate successfully at 82°C (180°F) at 400-600 psi and even higher temperatures under lower operating pressure. UF and MF membranes/ operated under much lower pressures and typically made of more temperature resistant polymers, can often be operated up to 100°C and in some cases are autoclavable. Under these conditions/ it is common for the other membrane element components to be the limiting factors/ not the membrane itself. Solution pH. Although pH is often considered in the context of the overall solution chemistry/ the degree of acidity or basicity itself also affects membrane life. The generally accepted pH range for CA membrane is 2 to 8/ although higher and lower pH values are feasible if the economics of the application allow more frequent membrane replacement/ or if ionic rejection is not critical to the application. PA type RO membranes are generally rated as compatible up to pH 11 or 12/ yet many on the market are not rated below pH 3 or 4 in the acid range. For UF membranes, pH range tolerances are usually greater, with polysulfone (PS) membrane frequently rated as compatible from 0.5 to 13. MF membranes are generally even more pH tolerant. In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


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Chemical Compatibility. Along with pH range, general chemical compatibility is also a major requirement for choosing a membrane polymer. Chlorine attack, even at very low levels, has been a major obstacle to the application of TFC-PA type membranes. Attack of PS membrane by many types of hydrocarbons has resulted in the use of CA membrane in many UF applications, despite its lower hydrolytic stability. A frequent limiting factor for MF membranes is extreme hydrophobicity, resulting in the requirement for chemical additives to lower surface tension or operation at much higher hydraulic pressures. When chemical compatibility limitations are combined with the fact that the most desirable polymers can only be manufactured into certain types of membrane (with limited range of pore size, morphology and production rate or flux), the requirement for a wide array of membranes becomes even more apparent. An example of how membrane and solution chemistry understanding can affect overall system performance comes from the dairy industry. Lab scale studies on the ultrafiltration of acid cheese whey showed that lowering the acid whey pH to 2, or raising the pH to 7 followed by centrifugation or 5 micron filtration, resulted in a 100% increase in membrane flux compared to whey pretreated with centrifugation at its normal pH of 4.5. The calcium and phosphorous contained in the whey are much more soluble at pH 2 and readily pass through the membrane rather than foul the membrane surface and pore walls. At pH 7, these minerals are very insoluble and therefore subject to removal by centrifugation or 5 micron filtration( JJ . The result is an overall system design that is less costly yet has superior membrane performance. Membrane Element Configurations As the membrane industry has grown and developed, several different configurations for supporting and containing the membrane were developed, with UF and MF membranes, the relatively low operating pressures allow more configurations than with RO. Commercially available configurations for UF include: tubular, larger internal-flow hollow fiber or "spaghetti bundle", plate and frame, and spiral wound. Tubular units are generally produced with inside diameters of 12.5 or 25 mm (0.5 or 1 inch) and in lengths of 150 to 610 cm (5 to 20 ft.). The feed solution flows through the inside of these tubes, whose inside walls contain the membrane. Hollow 'fat1 fiber membranes can be thought of as selfsupporting, miniature tubular units, with inside diameters of the fibers ranging from 0.5 to 1.1 mm (0.020 to 0.043 inches). Like the tubular units, the feed stream flows through the inside of the fibers and the permeate is collected on the outside. The "plate and frame" configuration is a third style for housing membrane. Flat sheets of membrane are placed between plates with heights of approximately 0.5 to 1.0 mm (0.020 to 0.039 inches) which in turn are stacked in parallel groups. This configuration is similar to a conventional plate and frame filter press.



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The "spiral wound" element also uses economical flat sheet membrane; when wound around a central permeate collection tube and separated by thin spacer materials/ this design yields a high membrane packing density(_2). The feed passes over the membrane through the spacer material, which also acts as a turbulence promoter to keep the membrane clean at relatively low velocities. A recent innovation for spiral elements is the use of channelized spacer materials. One new spacer material creates channels with much the same fluid dynamics as small tubes, combining the high membrane area of the spiral wound configuration with the fouling resistance of the tubular design. Although relatively uncommon, reverse osmosis membrane is also available in the tubular configuration. As with ultrafiltration, the feed and concentrate streams flow through the inside of the tube, commonly available in 12.5 mm diameters (0.5 inch), with several tubes in a common housing. For RO, the hollow fiber configuration has the pressurized flow on the outside of the fiber, not inside as for UF. The extremely small size of RO fibers (typically 42 microns I.D., 85 microns O.D.) results in their description as "hollow fine fibers" (HFF). This small size allows an extremely high element packing density, however, this is largely offset by the lower permeate production rate (flux values) inherent in membrane in the fiber form when compared to flat sheet membrane. The plate and frame configuration for RO is basically the same as that applied in UF, with stronger support materials required due to higher operating pressures. RO elements of the spiral wound configuration are of the same basic design as spiral UF. The flow channel height is generally smaller than in UF, ranging from 0.63 to 0.86 mm (0.025 to 0.034 inches). This results in increased packing density compared to UF (approximately +30%). The fluid flow dynamics are similar in the plate and frame and spiral wound configurations. For the application of pure water production, the volume of permeate produced worldwide by HFF and spiral wound elements are roughly equal (3»), and both of these designs are far more common than the tubular or plate and frame. The spiral design is used in process and waste water treatment, whereas the HFF is not due to its tendency to foul or "plug" rapidly. Crossflow MF membrane elements are the most recent entry into the crossflow membrane field and have been available commercially on a limited basis. These typically have been modified pleated cartridge filters and have been applied largely in the pharmaceutical and medical industries where high cost, low volume solutions have been processed. However, spiral-wound MF membrane elements have been in the developmental stage since early 1983. In-house application tests and field test results have shown promise for spiral wound MF use in clarification, color removal and macro-molecular fractionation applications. The important differences between element configurations which must be considered include the pumping energy costs to maintain efficient operation (fouling resistance), pretreatment requirements, cost of replacement membrane, labor and the capital cost of the equipment. The literature cited contains more indepth discussion of this subject.


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Equipment and System Design Considerations Quality systems engineering is as important as membrane and membrane element selection in providing the juice processor with effective long term membrane system performance. Supplying a crossflow membrane system that is the most effective design in terms of both capital and operating cost, contains the necessary controls and support equipment, meets the various FDA and USDA requirements for food contact and sanitary construction, and will meet customer separations objectives with long term system performance is the goal of both the customer and systems supplier. Advances in product and component engineering are also enhancing the prospects for successful crossflow filtration applications in the juice industry. A new multi-stage centrifugal pump has been designed to be in conformance with sanitary standards. Such a pump should bring the efficiencies of the multi-stage centrifugal design - energy efficiency, modular design to allow custom sizing, smooth flow - to the numerous applications in the beverage industry requiring sanitary design hardware. Advances in sanitary design of other hardware components and the element itself continue. Elements, pumps, meters and plumbing hardware which can withstand higher pressures and higher temperatures are under development, driven by the needs of an increasing range of membrane applications. Crossflow Membrane Filtration Applications in the Juice Industry Membrane applications fall into three general categories: process, waste treatment and pure water production. In the juice processing industry, potential for all three areas exists, with the process area perhaps holding the most potential overall. Processing. Membrane processing is often a more cost effective unit process than older technologies, totally replacing, or in some cases supplementing these existing processes. Membranes can also effect separations that are unique to crossflow filtration, or can make separations economically viable that were previously not. Replacement of evaporation as a technique for water removal is one of the larger potential areas for membrane systems. Even with the increased efficiency of multiple-effect and MVR evaporators, evaporation is an energy-intensive process which requires about 940 BTU per pound of water removed. Product dewatering with crossflow membranes requires no phase change and no added heat, not only requiring a fraction of the operating energy but eliminating heat-induced product damage. Concentration of a valuable product is performed at reduced cost while often increasing the final product quality. Producing juice concentrates is only one of several potential "concentration" applications. Others include the concentration of water soluble dyes from grape skins, pectin and fruit flavorings for jam, jelly and flavorings manufacture, increasing concentration of dilute juices to eliminate the need for blending with concentrates (for example, taking 8-10° Brix apple juice to 12°), and removal of water to save on transportation costs.



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In addition to concentration of the majority of juice constituents, flavor enhancement can be achieved by selective passage of off-tasting solutes- Tests have shown that RO membrane can remove bitterness from juices, finished wines and grape juice prior to wine-making. New RO/UF membranes can remove a portion of juice sugars to manufacture "light" beverages for marketing as low calorie alternatives. The same type membranes can increase the sugar content and reduce the acidity of juices for improved cider, wine, cognac, and other beverage production. Increasing solute concentrations can result in reduced fermentation time and also reduced waste loading by the process by-products. Figure 3 shows a production scale sanitary RO/UF system operating to fractionate a valuable beverage by-product. Waste Treatment. The most commmon waste treatment application for crossflow membrane technology is the reduction of BOD/COD in byproduct and waste rinse streams. Although the main impetus to apply membrane in these applications is pollution control, the reuse of valuable materials such as reclaimed sugars and the reuse of purified water often add an economic incentive not originally considered. Reclaimed sugars and other nutrients can be used as animal feed or as fermentation starter materials. Depending upon the molecular size of the BOD-contributing materials, both UF and RO have provided cost-effective pollution control processes. Water Purification. RO and UF are commonly used for water purification in many industries, including medical, pharmaceutical and electronics. The benefits of salt and organic removal via crossflow filtration are well understood. For juice reconstitution, RO provides a low cost, controlled source of water which is 99.9+% bacteria-free and has virtually all undesirable taste, odor, and color causing solutes removed. The following are several examples of applications in the juice industry which are currently in commercial production, undergoing field-site testing or appear quite viable based on application tests run on actual fluid samples provided to Osmonics by processors: Juice Concentration. Pear and orange juice have been concentrated to 21 20° Brix with SEPA-97(CA) RO membrane. Diatomaceous earth (DE) filtered pear juice was concentrated from 11° to 20+° Brix, and the permeate stream had no detectable sugar even at the high concentration level. A slight, bitter taste was noticed in the permeate stream, indicating the passage of an off-flavor through the membrane which would be beneficial for the final product (concentrate) quality. Fresh squeezed orange juice was concentrated from 11.75° to 23.0° Brix and neither sugars nor flavors were detectable in the permeate stream, according to processors attending the test. They judged the quality of the concentrated juice as quite good. Figure 4 shows flux performance obtained during concentration of fresh orange juice.



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Figure 3.

P r o d u c t i o n s c a l e s a n i t a r y RO/UF s y s t e m .

DISSOLVED SOLIDS CONCENTRATION (°Brix) Figure 4. Concentration of fresh orange juice with SEPA-97(CA) RO membrane. Operating conditions: 600 psig operating pressure; ambient temperature; filtered to 5 micron.


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'Light1 Apple Juice. An apple juice processor investigating the manufacture of a "light" apple juice participated in a batch test which demonstrated good potential for the application. SEPA-50(CA) membrane (with a pore size falling between RO and UF) produced a permeate ranging from 3.3° to 4.5° Brix from an 11.2° Brix apple juice feed. The permeate contained reduced smell and flavor characteristics, but the low sugar level will allow blending with full strength juice to produce an acceptable tasting "light" product. Although not available at the time of that test, a recently developed small-pore UF membrane, SEPA-O(PA), would also be an excellent choice for performing partial removal of mono- and disaccharides. Pectin Processing. Pectin processing is unusual in that pectin is regarded as both a contaminant and a valuable product to be separated and concentrated. Because pectin occurs naturally in apple juice but can cause post-bottling cloudiness and off-taste problems, removal is usually considered a necessary processing step. Yet pectin from citrus peels (such as lemons) is often recovered as a product after a complicated precipitation process which requires the use of several chemicals. Where essential oil recovery from citrus peels is performed, pectin again can be a contaminant. Crossflow membrane systems can be used in all pectin removal and recovery applications. Pectin removal from apple juice via membrane processing can eliminate the need to add enzymes and the required heat treatment to deactivate them. Lemon Peel Extract. Two application tests have been run on lemon peel extract. One test objective was simple separation and concentration of the pectin as the desired end product. A "membrane scan" was run using three UF membranes of varying pore sizes and different polymers. This scan showed that UF membranes with pore sizes in the 1000-3000 molecular weight cutoff range (MWCO) performed the best for pectin separation. A qualitative analysis showed that both SEPA-O(CA) and -O(PS) membranes pass a small amount of pectin, viewed as beneficial since the permeating material is the smaller, lower quality pectin molecules and breakdown products. Despite higher pure water flow rates, the membrane scan also showed that smaller pore UF membranes maintained their flux on the process solution better than the larger pore membranes, (a result of less pore-plugginc type fouling). Table I and Figure 5 show the comparative permeate flow rates of the membranes tested, both for pure water and on the process solution. In the second test on a solution from a different extraction process, the objective was to reduce the amount of chemical addition required to precipitate the pectin, a step necessary to allow the reclamation of the essential peel oils. The three UF membranes tested all had MWCO values in the same range, so their polymer characteristics were an important factor in assessing performance. The O(PS) membrane yielded the best flow rate performance, as indicated in Table II. A concentration step increased the pectin level four-fold, according to the processor's analysis. Both tests showed the feasibility of ultrafiltration processing as a means of concentrating and purifying pectin.


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Table I.

Permeate Rates for UF Membrane Spiral Elements on Lemon Peel Waste - Process #1


SEPA Membrane Type: 20K(PS): 20,000 20K(CA): 20,000 O(CA): 1,000

MWCO

Pure Water Q„ 223 209 102

Operating Parameters: Sample Composition:

Table II.

Permeate Rates (lph) Waste Waste Solution Solution Qn - 5 mins. Qp - 25 mins. 23 64 40 72 49 59

50 psig, 25°C 0.1% essential oils 2.7° Brix P H 4.1

Permeate Rates for UF Membrane Spiral Elements on Lemon Peel Waste - Process #2

SEPA Membrane Type: MWCO O(VF): 2,000-3,000 O(PS): 1,000-2,000 O(CA): 1,000

Pure Water Qn 481 * 367 196

Operating Parameters: Sample Composition:

Permeate Rates (lph) Waste Waste Solution Solution Q^ - 45 mins. Q^ - 1 min r 11 * 16 14 25 13 17 100 psig, 25°C 0.5% pectin trace lemon oil 1.1° Brix pH 1.9

Cranberry Juice Clarification. A test was run on cranberry juice to determine performance of large pore UF membranes in the clarification step, to replace the traditional but troublesome DE filtration or the more expensive and inefficient centrifugation process. In DE filtration, filter aid addition and constant operator attention increase operating costs significantly. In addition, filter aid bleed-through during start-up can contaminate product, and the disposal of spent filter aid is an ongoing problem. Both SEPA-50K(PS) and the larger pore HF1(PS) membrane effected good clarification, with the HF1(PS) membrane giving the highest flow rates. Crossflow MF membranes would probably perform even better, providing adequate clarification and yielding higher flow rates at lower operating pressures. This test was revealing in that despite the suspended solids level which was sufficient to blind 25 micron filter cartridges immediately, no prefiltration was performed and spiral-wound elements with the traditional meshspacer were used. Test results showed significant fouling and resulting reduced flux which leveled off at reasonable levels.


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The new corrugated spacer spiral-wound elements should be considered in situations like this where excessive fouling from juice pulp and other suspended solids is expected. BOD/COD Waste Reduction. Successful tests have been run on fruit juice processing waste where the objective was the reduction of BOD and COD for the purpose of waste liquid disposal. In one test, wash water from pear and peach processing lines contained 6,000 mg/1 BOD and 1.5-2.0% fruit sugars (2.0° Brix). Using SEPA-97(CA) RO membrane, the solution was concentrated more than 8-fold to 16.5° Brix. Due to the high osmotic pressure resulting from the concentrated sugars, and some fouling, the expected reduction in permeate occurred. This was easily reversed with a simple cleaning cycle and 100% of the initial pure water rate was recovered. In a similar application, orange waste from an unspecified process contained 0.5% mono- and disaccharide sugars and COD in the range of 5,000 to 6,000 ppm, as well as a small quantity of peel oil. By removing most of the water (processing to high recovery), the conductive solutes were concentrated 8-fold and the refractive solutes 5-fold. Again, all permeate flow lost during processing was completely recovered with a simple cleaning cycle. BOD and COD analyses were not conducted at the time of the test, however, the very low conductivity and refractometer readings of the permeate streams indicated substantial BOD and COD reductions were made. Flavor Enhancement. Flavor enhancement application tests on both wine and juices have shown interesting results, indicating the potential of membrane processing for improving taste. Removal of bitterness and "off flavors" in finished wine, grapefruit juice and orange juice was accomplished using RO membranes with pore sizes controlled within a small range. Flavor control is a difficult application, since the desirable and undesirable flavor and aroma bodies have close molecular size, polar and steric characteristics. However, a processor's taste-testing panel determined that a low quality finished Chenin Blanc had detectable flavor improvement after processing with a SEPA-89(CA) RO membrane. Enhanced Second Press Apple Juice. Another concentration application is increasing the sugar and flavor levels of "second press" apple juice. Second press or "cold diffusion" juice is typically 6° Brix, but can range down to 2° Brix. Early pilot studies conducted by Agriculture Canada using SEPA-97(CA) membrane concluded RO was a viable process for achieving 12% TDS juice with minimal product quality loss and improved economics compared to evaporation (

Reverse Osmosis and Ultrafiltration Applied to the Processing of Fruit

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