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Transmembrane Pressure. The effect of transmembrane pressure on flux is often dependent on the influence of concentration polarization at a specified cross-flow velocity and solids loading. For MF or UF at low solids concentration and high cross-flow velocity, flux may increase linearly with TMP up to a certain threshold value (1 to 3 bar), and then remain constant or even decrease at high TMP values. This is illustrated in Fig. 16.[21] Athigh solids loading, the threshold value may be lower (0.5 to 1.5 bar) and may also require higher cross-flow velocity to offset gel polarization effects. For each application the optimum value may be considerably different and must be empirically determined.

Figure 16. Effect of transmembrane pressure on flux. Yeast concentration, dry-g/L: (O) 8.5; (•) 30.

Transmembrane Pressure, bar

Figure 16. Effect of transmembrane pressure on flux. Yeast concentration, dry-g/L: (O) 8.5; (•) 30.

An optimum TMP value is one which maximizes flux without additional energy costs and helps minimize the effects of membrane fouling. In general, higher the solids concentration, the higher the cross-flow velocity and hence TMP to balance the effects of concentration polarization. In most practical situations, the cross-flow velocity and TMP may be interrelated. One useful approach involves performing pressure excursion studies to determine the optimal flux by varying the TMP at a fixed cross-flow velocity until the threshold TMP is attained and then repeat the tests by selecting a lower or higher cross-flow depending, on the observed trend.[40'

In many biotechnology applications, such as fermentation broth clarifications to produce common antibiotics, optimal values of TMP are in the range of 2 to 3 bar (15 to 30 psi) especially at high cell mass concentrations (> 30 wt.%) and cross-flow velocity range of 4 to 7 m/s.[2][40] In the operation of commercial systems, often several modules (2 to 4) are interconnected to minimize pump costs. This results in significantly higher TMP on the feed end compared to that at the exit (or retentate). Thus, the TMP on the exit end may be closer to the optimal value whereas at the inlet it may be substantially higher (6 or 7 bar), unless the permeate backpressure on each module is controlled independently.

Temperature and Viscosity. The operating temperature can have a: beneficial effect on flux primarily as a result of a decrease in viscosity.f3)[411 There is an additional benefit for shear thinning viscoelastic fluids, where the viscosity reduces with an increase in shear (i.e., cross-flow velocity). Typical examples are clarification of fermentation broths and concentration of protein solutions.[3][42] It must be noted that for most fermentation and biotechnology related applications, temperature control is necessary for microbial survival and/or for product stability (e.g., antibiotics, enzymes, proteins and other colloidal materials).

For mass transfer controlled operations, such as when concentration polarization is dominant, flux enhancement due to temperature increase will depend on the value of mass transfer coefficient. This is related to the cross-flow velocity, diffusion coefficient and viscosity.™ Thus, for example, even though the viscosity may be reduced by a factor of 5, the increase in flux may only be about 50%, due to the nonlinear dependence of flux on viscosity in these situations. For the permeation of a clean liquid (solvent) across a microporous membrane, however, flux increase may be predicted by the Stokes-Einstein relation11^ and will be approximately inversely proportional to the viscosity of the permeate.

pH, Isoelectric Point and Adsorption. In the filtration of proteins and colloidal substances, the solution pH can have a measurable effect on flux, especially around the isoelectric point where they tend to destabilize and precipitate. In addition, the surface charge or isoelectric point of the membrane material must also be considered. For example, most inorganic membranes are made out of materials such as silica, zirconia, titania and alumina which have a charge on the surface with isoelectric pH variation from 2 to 9.131 Similarly polymeric membranes such as cellulose acetate, polyamides and polysulfones also carry surface charges.'71 Surface charge effects may alter the fouling resistance due to changes in the zeta potentials and could have a substantial influence on flux and/or separation performance. For example, proteins with isoelectric point of 8 to 9 will have a positive charge in a neutral or acidic solution and negative charge in alkaline solutions (pH > 9). However, if the above proteins in a neutral medium are filtered through an alumina membrane (isoelectric pH 9) there will be a minimal adsorption on the membrane due to similar charge characteristics. At a solution pH of 8 or 9, which is also the isoelectric pH of protein, however, proteins may precipitate out of solution. This may have a beneficial effect on the flux through a MF or UF membrane. This also illustrates the interactive effects of solution pH, isoelectric pH (of solutes and membrane material) and adsorption.

Feed Pretreatment. In most fermentation and biotechnology related applications, feed pretreatment is not a viable option. This is due to the fact that any alterations in the feed properties, especially through the addition of précipitants or flocculants, will likely contaminate the product and or adversely affect its characteristics.

Prefiltration is recommended when applicable to remove larger particles and other insoluble matter. However, minor pretreatment chemistries may be allowable, such as pH adjustment to precipitate or solubilize impurities or foulants to maximize flux or retention. For example, protein adsorption and fouling can be reduced by adjusting the pH away from its isoelectric point.[6J The selection of a suitable pore diameter or MWCO value is done on the basis of the smallest particle size or smallest macrosolute present in the feed.

6.4 Membrane Cleaning

Likewise, to the inevitable phenomena of membrane fouling, all membrane based filtration processes require periodic cleaning. Without a safe practical, reproducible, cost effective and efficient cleaning procedure, the viability of cross-flow filtration may be highly questionable. Membrane cleaning process must be capable of removing both external and internal deposits. In some special situations, such as strongly adsorbed foulants, recirculation alone may not be adequate and soaking of the membranes in the cleaning solutions for a certain period of time will be necessary. The ultimate success of a membrane process will be largely impacted by the ability of the cleaning procedure to fully regenerate fouled membranes to obtain reproducible initial flux at the start of the next filtration cycle.

The ease of finding an effective cleaning process often depends on the thermal and chemical resistance of the membrane material. In other words, the higher the resistance, the easier it is to develop a suitable cleaning procedure. The choice of a cleaning solution depends on several factors such as the nature of the foulants, and material compatibility of the membrane elements, housing and seals. A few general guidelines are available concerning the removal of foulants or membrane deposits during chemical cleaning.[3][411

Common foulants encountered in biotechnology related applications are inorganic salts, proteins, lipids and polysaccharides. In some food or biochemical applications, fouling due to the presence of citrate, tartrate and gluconates may be encountered. Inorganic foulants (e.g., precipitated salts of Ca, Mg and Fe) can be removed with acidic cleaners whereas, proteina-ceous and other biological debris can be removed with alkaline cleaners with or without bleaching agents or enzyme cleaners. Many acidic and alkaline cleaners also contain small quantities of detergents, which act as complexing or wetting agents to solubilize or remove insoluble particles, colloidal matter and/or to break emulsions. Oxidizing agents such as peroxide or ozone are also sometimes used to deal with certain type of organic foulants.[43] In addition, organic solvents may be required to solubilize organic foulants that are insoluble in aqueous cleaning solutions.

For many polymeric MF/UF membrane modules, material compatibility considerations limit the use of higher cleaning temperatures and strongly acidic/alkaline/oxidizing solutions. Further, with time and repeated cleaning, polymeric filters are susceptible to degradation. The service life of a hydrophobic type is typically a period of 1 to 2 years and up to 4 years for fluoropolymer based membranes.[6] On the other hand, inorganic membranes can be cleaned at elevated temperatures in strongly alkaline or acidic solutions and can withstand oxidizing solutions or organic solvents. The typical useful service life of inorganic membranes exceeds 5 years and may be used for 10 years or longer with proper cleaning, and good operating and maintenance procedures.f3][6]

A careful choice of cleaning solutions and procedures will extend the service life ofthe membrane. In many polymer membrane filtration systems, membrane replacement costs constitute a major component of the total operating cost. Extending the service life of the membrane modules will have a major impact on the return on investment and can be a determining factor for the implementation of a membrane-based filtration technology. Table 10 summarizes the various key parameters that must be considered in developing a cleaning regimen to regenerate fouled membranes.

Product losses during cleaning may be important especially when high recoveries (>95%) are required and the desired product is located in the retentate phase. Additional product loss will occur in the fouled membrane elements. These combined losses may range from 0.5% to 3% which is significant when recovering high value-added product.

6.5 Pilot Scale Data and Scaleup

Scaling up membrane filter systems must proceed in a logical and progressive series of steps. It is practically impossible to extrapolate data from a laboratory scale system to design a production scale system.'441 To ensure commercial success, it is often necessary to supplement laboratory data with pilot system capable of demonstrating the viability of the process. This is typically followed-up with extensive testing using demonstration scale or semi-commercial scale filtration system to obtain long-term flux information and to establish a cleaning procedure to regenerate fouled membrane modules. This exercise is especially important to determine the useful life of membranes. At least a 3 to 6 month testing is recommended regardless of the scale of operation. Pilot scale studies will also allow production of larger quantities of materials for evaluation purposes to ensure that all the separation and purification requirements are adequately met.

It is necessary to ensure that the feed stream characteristics are representative of all essential characteristics, such as age of feed sample, temperature, concentration of all components (suspended and soluble), and pH. The filtration time needed to perform a desired final concentration of retained solids or percent recovery of product passing across the membrane filter (the permeate) must also be consistent with the actual process requirements.

Effects of sample age, duration of exposure to shear and heat, may be very important and must be considered. In the demonstration scale phase, the operating configuration (e.g., batch, feed and bleed, continuous) specified for the production scale system must be used. Careful consideration must be given to the total pressure drop in the flow channels at the desired cross-flow velocity at the final concentration, to ensure proper design of the feed and recirculation pumps.

Table 10. Membrane Cleaning: Key Considerations Type of Foulant Example_Cleaning Solution

Inorganic

Organic

Proteins

Precipitated Ca. Mg, Fe citrate, tartrate gluconate

Enzymes, yeast, pectins

Biological debris E-Coli, bacteria cell walls

Moderate to strongly acidic

Acidic/alkaline solutions

Mild to moderately alkaline

Strongly alkaline preferably with chlorine

Moderately alkaline

Fats/Oils

Stearic acid oleic acid

Strongly alkaline with oxidizing agents or chloride

Polysaccharide Starch, cellulose

Strongly alkaline/acidic or oxidizing solutions

Filter Material Compatibility

Some polymeric (PVDF or PTFE) and most inorganic filters.

Most polymeric or inorganic filters.

Most polymeric and Inorganic filters.

Some polymeric (PVDF or PTFE) and most inorganic filters.

Most polymeric or inorganic filters.

Some polymeric (PVDF or PTFE) and most Inorganic fillers.

Some polymeric (PVDF or PTFE) and most inorganic filters.

It is important to generate the flux data on a continual basis as illustrated in Fig. 17. This type of information is very vital to identify any inconsistencies in the filtration performance and/or to determine if there is any irreversible membrane fouling. Reproducible performance will also be helpful to validate the membrane cleaning regimen for the application.

6.6 Troubleshooting

Filtration equipment must function in a trouble-free manner and perform in accordance with the design basis. Although most carefully designed, engineered and piloted cross-flow filtration systems will perform to design specifications, occasional failures are not uncommon. For proper troubleshooting of CFF systems, the user must be familiar with the principles of membrane separation, operating and cleaning procedures, influence of operating variables on system performance and equipment limitations.

In this chapter, the principles of membrane separations when operating in the cross-flow configuration are discussed in detail along with the influence of operating variables on flux and separation performance. However, proper start-up and shutdown procedures must be followed to maximize the system performance. For instance, the formation or presence of gas or vapor microbubbles can cause severe pore blockage especially for MF and in some UF applications. Therefore, care must be taken to remove air or gas from the feed and recirculation loop at the start of a filtration run to ensure that no air is drawn or retained in the system. This type of operational problem may not only occur during normal filtration but also during backpulsing.

When troubleshooting the cleaning operation, a good understanding of the foulants and process chemistry is highly desirable. A thorough understanding of materials of construction of the seals/gaskets is required for a proper choice of cleaning regimen. The membrane manufacturers guidelines must be properly implemented and combined with the process knowledge and feed characteristics. When working with new or dry membranes, it may be necessary to properly wet the membrane elements. For microporous structures, the use of capillary forces to wet the membrane and fill the porosity is recommended.

6.7 Capital and Operating Cost

The manufacture and purification of many biotechnology products derived from fermentation processes involves several separation steps. Up to 90% of the total manufacturing cost may be attributed to various

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