Feed and bleed

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). 

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. 

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Process Flow The schematic in Fig. 22-56 may imply that the feed rates to the concentrate and diluate compartments are equal. If they are, and the diluate is essentially desalted, the concentrate would leave the process with twice the salt concentration of the feed. A higher ratio is usually desired, so the flow rates of feed for concentrate and feed for diluate can be independently controlled. Since sharply differing flow rates lead to pressure imbalances within the stack, the usual procedure is to recirculate the brine stream using a feed-and-bleed technique This is usually true for ED reversal plants. Some nonreversal plants use slow flow on the brine side avoiding the recirculating pumps.. Diluate production rates are often 10X brine-production rates. 

Thermal-Hydraulic Analysis must be extended to at least 24 hours into the accident. This study used calculations for feed-and-bleed operation with a charging pump, and with gravity teed from the refueling water storage tank (RWST),... 

The CE plants use U-tube steam generators with mixed capability to establish feed-and-bleed cooling. . Several CE plants are designed without pressurizer power-operated valves. The RCPs are a By WesI ise... 

Membrane modules can be configured in various ways to produce a plant of the required separation capability. A simple batch recirculation system has already been described in cross-flow filtration. Such an arrangement is most suitable for small-scale batch operation, but larger scale plants will operate as feed and bleed or continuous single pass operation (Figure 16.20). 

Fig. 16.20. Schematic flow diagrams of (a) single-stage feed and bleed , (b) multiple-stage feed and bleed and (c) continuous single-pass membrane plants.

Estimate the minimum number of membrane modules required for the operation of this process (a) as a single feed and bleed stage and (b) as two feed and bleed stages in series. Assume operation for 20 hours each day. 

Continuous Operation Continuous operation (also called feed-and-bleed) involves the partial recycle of the retentate. The residence time and number of pump passes are in between those of single-pass and batch operation, depending on the fraction of retentate recycled. Several continuous units can be plumbed with the retentate flow feeding the next consecutive system. This configuration is commonly used for large-scale membrane systems. 

It is also possible to run an ED process in the continuous mode (Figure 7B) by letting simultaneously a fresh feed enter the ED unit and the final product flow out of it. This operation can be performed in one or more than one turn. The latter is generally referred to as the feed-and-bleed mode (Figure 7C) and needs a portion of the product to be blended with the raw inlet feed solution. This procedure is applied when the feed solute concentration fluctuates and a continuous flow of product is required or when the demineralization degree is low. To minimize the overall membrane... 

FIG. 7 Flow diagrams of the different operating modes for an ED unit (A) batch process (B) continuous single-passage process and (C) feed-and-bleed process. 

Figure 6.25 Flow diagram of a feed-and-bleed ultrafiltration unit used to concentrate a...

Figure 6.26 Flow schematic of a three-stage feed-and-bleed ultrafiltration system used to recover poly(vinyl alcohol) (PVA) sizing agents used in the production of cotton/synthetic blend fabrics [28]...

For treating water containing VOCs with separation factors of more than 500, for which concentration polarization is a serious problem, feed-and-bleed systems similar to those described in the chapter on ultrafiltration can be used. For small feed volumes a batch process as illustrated in Figure 9.16 is more suitable. In a batch system, feed solution is accumulated in a surge tank. A portion of this solution is then transferred to the feed tank and circulated at high velocity through the pervaporation modules until the VOC concentration reaches the desired level. At this time, the treated water is removed from the feed tank, the tank is loaded with a new batch of untreated solution, and the cycle is repeated. 

Figure 11.12 Schematic of a three-stage feed-and-bleed hollow fiber coupled transport concentrator [49]...

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