Figure 9. Batch and continuous diafiltration process modes.
Single stage continuous configuration may not be economical for many applications since it operates at the highest concentration factor or lowest flux over most of the process duration. Multistage continuous systems on the other hand, can approximate the flux obtained in the true batch mode, depending on the number of stages. The concentrate from each stage becomes the feed to the next stage. The number of stages required will depend on the final recovery or retentate concentration. Figure 10 shows the schematic of a three-stage continuous system.
The optimum number of stages will depend on the application, but typically lie between 2 and 4, with the greatest benefit resulting from a single stage to a two stage continuous system. The biggest advantage in using multistage continuous configuration, especially in fermentation and biotechnology applications, is the minimization of residence time, which may be crucial in preventing excessive bacterial growth or to handle heat labile materials.'151 The other advantage of a continuous system is the use of a single concentrate flow control valve. As membrane fouling and/or concentration polarization effect begins to increase over the batch time, flux decreases. This requires the continuous or periodic adjustment of concentrate flow which may be accomplished with the ratio controller.
One disadvantage with a multistage system is the high capital cost. It is necessary to have one recirculation pump per loop which drives the power requirements and operating costs much higher compared with the batch feed and bleed configuration.
6.0 PROCESS DESIGN ASPECTS
6.1 Minimization of Flux Decline With Backpulse or Backwash
Almost all cross-flow filtration processes are inherently susceptible to flux decline due to membrane fouling (a time-dependent phenomenon) and concentration polarization effects which reflect concentration buildup on the membrane surface. This means lower flux (i.e., product output) which could drive the capital costs higher due to the requirement of a larger surface area to realize the desired production rate. In some situations, the lower flux could also result in lower selectivity which means reduced recoveries and/or incomplete removal of impurities from the filtrate. For example, removal of inhibitory metabolites such as lactic acid bacteria1161 or separation of cells from broth while maximizing recovery of soluble products.121
Concentrate Flow Control Valve
Flow Control Valve
Figure 10(b). Continuous multistage cross-flow filtration system.
Figure 10(b). Continuous multistage cross-flow filtration system.
Backwashing or backpulsing with permeate can help remove excessive membrane deposits and hence minimize flux decline.*35 Cross-flow micro-and ultrafilters typically operate as surface filtration devices with insignificant pore plugging. If severe pore plugging occurs, backpulse will most likely be ineffective in preventing precipitous flux decline. This type of irreversible fouling may only be corrected by cleaning by chemical and/or thermal heat treatment.
An essential difference between a backpulse and a backwash is the speed and force utilized to dislodge accumulated matter on the membrane surface. In backpulsing, periodic counter pressure is applied, typically in a fraction of a second (0.1-0.5 seconds), while generating high permeate backpressure (up to 10 bar). Backwash on the other hand is relatively gentle where permeate backpressure values may increase up to 3 bar over a few second duration. Backwash is commonly used with polymer MF/UF filters due to their lower pressure limitations14''171 compared with inorganic MF/UF filters where backpulsing is used. The maximum benefit of backpulse or backwash is obtained when the retentate pressure during instantaneous reverse filtration is lowest and the applied permeate backpressure is highest.
Depending on the operating configuration, a periodic backpulse may be applied on the entire filtration system or when several modules are operating in series, subsequent application will produce more effective results. In the latter case, the retentate pressure may be higher as a result of pressure loss through the interconnected feed channels. It is recommended that when a backpulse or backwash is used, it is applied for the shortest duration possible (to minimize the loss of productivity), it uses minimum permeate volume, and begins simultaneously with the filtration process.
Backpulsing is less effective for some smaller pore diameter UF membranes (MWCO <30,000 or pore diameter less than 0.02 |im) and where dense layers are formed or gelatinous products are filtered. It is important to bear in mind that, although backpulsing has the ability to minimize the concentration polarization effects and produce a higher average flux, a certain portion of the permeate is consumed (1 to 3 % by volume). If permeate is the product of interest, then the net realized flux will be average flux minus permeate volume used during backpulsing.
In the conventional cross-flow filtration described in previous section, the transmembrane pressure (TMP) along the feed flow channels varies substantially from the feed end of the module to the exit or retentate end. This occurs due to the pressure loss in the feed channels to maintain the desired flow rate (and hence cross-flow velocity). The shell side or the permeate side is held at a constant pressure. There may be several important consequences which can contribute to a relatively lower flux or loss in separation efficiency. A major consequence is the formation of a nonuniform layer of suspended solids, colloidal matter, and/or gel-forming microsolutes retained on the membrane. It is not uncommon to experience a TMP value up to 5 0% higher at the module inlet compared to that at the outlet, especially at high shear rate or cross-flow velocity. This could result in a substantially lower average flux. In some applications (e.g., milk or cheese concentration, whey concentration and fermentation broth clarification for product recovery) significant differences in the retention characteristics have also been observed. In many biotechnology related applications, where MF or UF membranes are used, the primary objective is to retain particles (e.g., whole cells or lysed cells, yeast, colloidal matter, and/or macrosolutes such as enzymes, pyrogens, proteins, and in some situations oily emulsions). In order to accommodate the wide variations in particle size distributions, a pore diameter is selected that is small enough to retain all the particles or macrosolutes, but large enough to allow the permeation of smaller molecular weight soluble products such as common antibiotics, mono- and disaccharides, organic acids and soluble inorganic salts.
Nonuniform TMP values over the filtration surface area may cause substantial (up to 50%) reduction in the product recovery in the permeate. A novel approach to improving the flux and/or product recovery utilizes the concept of a uniform transmembrane pressure. This is achieved by varying the permeate side pressure with an independent recirculation pump to adjust the TMP to a constant value. A schematic of the UTP and conventional cross-flow configuration is shown in Figs. 11 and 12, respectively. The TMP profiles for the two operational modes are shown in Fig. 13. Flux improvements up to 500% have been achieved compared with the conventional cross-flow mode in many important food, beverage and biotechnology applications.
An additional benefit is reduced fouling which means longer duration of operation for batch processes and easier cleaning of membrane modules for repeated usage. The only major requirement is the ability of the membrane structure to withstand backpressures up to 5 bar on shell side when filtering high viscosity products such as gelatins, or feed streams with high dissolved solids (20 to 70 wt.%).
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