Figure 5 shows a simple batch system consisting of a feed tank, a membrane module and a feed pump which also serves as a recirculation pump. The recirculation pump maintains the desired cross-flow velocity over a certain range of transmembrane pressures depending on the type of pump and its characteristics (centrifugal or positive displacement). The filtration continues until the final concentration or desired permeate recovery is achieved, unless the flux drops to an unacceptable level. For the retention of suspended solids (e.g., bacteria, yeast cells, etc.) the final concentration factor can be anywhere from 2 to 40 (and higher in some applications where a recovery of >98% is required). In order to minimize the concentration effects, a ratio of concentrate flow rate to permeate flow rate of about 10 to 1 is maintained (assuming the density differences are not significant). This ensures that at any given time the concentration of solids in the recirculation loop is only about 10% higher than that in the feed loop. Depending on the operating cross-flow velocity and viscosity of retentate, the pressure drop along the length of the module can vary from 0.5 bar to more than 2 bar. This often necessitates the use of more than one parallel loop and limits the number of modules in series depending on pump characteristics.
The open loop configuration has some advantages in terms of its simplicity, but also has some disadvantages especially when the product is sensitive to heat or shear effects (e.g., intracellular products, some beverages, and enzymes). Furthermore, when higher cross-flow velocities are required (which is typically the case in many applications) the recirculation rates necessary to sustain them may not be achievable in the open loop configuration, especially if it is also desirable to maintain a concentrate to permeate ratio of at least 10.
This problem can be overcome by placing a feed pump between the feed tank and the recirculation pump, as shown in Fig. 6. The discharge pressure of the recirculation pump must be at least greater than the pressure loss along the flow channels in the module or several modules connected in series while maintaining the desired recirculation rate.
Recirculation Pump Figure 6. Schematic of a batch (closed-loop) system.
The pipe sizes for feed and return lines for the closed loop operation are much smaller than that for the open loop system which can also reduce the capital and operating cost. The feed tank size can also be much smaller for the closed loop which then allows shorter residence times for heat or shear sensitive products.
The average flux (Jm) in the batch configuration may be estimated using
Jj-= flux at the final concentration Jj = initial flux
Batch Mode. The closed loop operation shown in Fig. 6 may not be suitable in many situations such as when processing large volumes of product and where high product recoveries (>95%) are required. It is well known that flux decreases with an increase in the concentration of retained solids which may be suspended particles or macrosolutes. When high recoveries are required, high retentate solids must be handled by the cross-flow filtration system. For instance, when a 95% recovery is desired, the concentration of solids in the loop must be 20 times higher than the initial feed concentration (assuming almost quantitative retention by the membrane). If the filtration proceeds beyond the 95% recovery, much higher solids concentration in the retentate loop will result which could adversely affect the flux. Figure 7 shows the schematic of a batch feed and bleed system.
A constant final concentration in the retentate loop can be maintained by bleeding out a small fraction, either out of the system or to some other location in the process. This operation is described as a batch feed and bleed and is commonly used in the processing of many high value biotechnology products such as batch fermentations to recover vitamins, enzymes and common antibiotics. The CFF system will require larger surface area since the system must be designed at the flux obtained at the final concentration factor (e.g., 20 for 95% recovery).
Figure 7. Schematic of a closed loop feed and bleed system.
Continuous Mode. When large volumes are processed the batch feed and bleed system is replaced with a continuous system shown in Fig. 8. The size of the feed tank is much smaller compared to that for the batch system. However, since the concentration of solids changes with time, the permeation rate decreases with time. This requires the adjustment of feed flow to the recirculation loop. This value is obtained by adding total permeation rate to the bleed rate. The concentration buildup in the continuous feed and bleed mode of operation is somewhat faster than the batch mode, which translates into a higher surface area requirement due to lower flux at higher solids concentration. Such an operating configuration, however, serves very well in many large scale fermentation broth clarifications (e.g., common antibiotics such as penicillin and cephalosporin) and is used when long holding times are not a concern.
For continuous processes, the lowest possible system dead volume will enable the operation with low average holding times. This may be important in some applications, especially those involving bacteria-laden liquids. Low system dead volume is also desirable for batch or continuous processes to minimize the volumes of cleaning solutions required during a cleaning cycle.
Diafiltration. The product purification or recovery objective in most UF operations is achievable by concentrating the suspended particles or microsolutes retained by the membrane while allowing almost quantitative permeation of soluble products (such as sugars, salts, low molecular weight antibiotics) into the permeate. This approach to concentration of solids obviously has limitations since recoveries are limited by concentration polarization effects. This limitation can be overcome by the use of diafiltration.The process involves the selective removal of a low molecular weight species through the membrane by the addition and removal of water. For example, in many antibiotics recovery processes, the broth is concentrated two- to fivefold (depending on the extent of flux reduction with concentration). This corresponds to a recovery of 50 to 80%. Higher recoveries are obtainable by adding diafiltration water or solvent in nonaqueous medium. The permeate leaving the system is replaced by adding fresh water, usually through a level controller, at the rate which permeate is removed. Diafiltration efficiency can be varied by the mode of water addition. Figure 9 shows the schematic for a batch and continuous diafiltration process. Diafiltration can be performed at higher temperatures to facilitate higher permeation rates. A possible disadvantage would be the dilution of the product requiring further concentration (e.g., by evaporation).
CF Filtration System
Constant - Volume Batch Diafiltration
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