Recycling product/permeate

In the top product (permeate) recycle additional energy is required to recompress the recycled portion of the permeate stream. As expected the ratio of the recycle flow rate to that of the feed rate (called the compressor load) affects the conversion. There is an... 

P reactant product(s) high 8. low Pp high Da feed location at Z O total permeate recycle independent of flow mode... 

This reactor can be operated in two ways. When the substrate is fed to the shell side of the hollow fiber module ("back flush mode"), the substrate comes in contact with the enzyme in the fiber wall and product passes into the lumen of the fiber from which it exits the module. When the substrate is fed to the lumen of the fibers (with all permeate ports closed), it will pass from the lumen to the shell side where it contacts the enzyme and products will recycle back to the lumen ("recycle mode") (see Figure 3.72). The "recycle mode" has the advantage over the "back flush mode" in that the substrate does not have to be free of suspended matter. In the "back flush mode", particles in the substrate would plug the sponge wall. 

The important thing is that using recycle (or reflux) of the more-permeable product and recycle (or reflux) of the less-permeable product can induce sharper separations, just as in distillation. Furthermore, as far as the total number of stages is concerned, it makes very little difference as to whether the feedstream is introduced at the reject or permeate side of the cell at the feed location and whether or not it is partitioned. Not only this, it makes very little difference whether the feedstream composition coincides with the reject or permeate compositions. 

Taylor [33], Morgan [34], Collins [35], Friesen et al. [36] all describe different variations of a hollow fiber module design for gas dehydration in which a fraction of the permeate product is recycled as a sweep gas in the shell side of the module however none of them report permeation rate ratios as high as those reported by Kikukawa and Sakai [38]. 

A flow diagram of a simple cross-flow system is shown in Figure 16.12. This is the system likely to be used for batch processing or development rigs it is in essence a basic pump recirculation loop. The process feed is concentrated by pumping it from the tank and across the membrane in the module at an appropriate velocity. The partly concentrated retentate is recycled into the tank for further processing while the permeate is stored or discarded as required. In cross-flow filtration applications, product washing is frequently necessary and... 

Computers permeate all areas of the plastics industry from the concept of a product design, to raw material, to processing, to marketing, to sales, to recycling, to government and industry regulations, and so on. Computers have their place, but most important is... 

Wastewater treatment and water purification applications employ UF in a TFF or NFF mode to produce permeate product with reduced colloids, pyrogens, and viruses. Oil droplets in wastewater are retained by UF for recycle or disposal at a significantly reduced vol-... 

Rules of Thumb With a few notable exceptions such as H2 through Pd membranes, membrane separations are not favored when a component is required at high purity. Often, membranes serve these needs by providing a moderate purity product which may be inexpensively upgraded by a subsequent process. Increasing the purity of N2 by the introduction of H2 or CH4 to react with unwanted O2 is a good example. Unless permeates are recycled, high product purity is accompanied by lower product recovery. 

The complex formed when a mordant dyeing is aftertreated in a dichromate solution is retained by the wool in preference to the unmetallised mordant dye, which may desorb to some extent during the treatment. The latter is rather unstable in an oxidising solution and quinonoid by-products are often formed. If the chromium complex of the dye is formed from the desorbed dye in solution, this will further complicate the composition of the aftertreatment liquor. Thus reuse of mordant dyeing and aftertreatment baths is not an option. Furthermore, 100% rejection of dichromate ions would be required if the permeate of a membrane process treating the effluent was to be recycled [42]. 

The final intriguing use of CLAs is in the immobilization of enzymes in the soapy shell in order to carry out an enzymatic reaction. Thus the hydrolysis of -nitrophenyl acetate to / -nitrophenol has been demonstrated by immobilizing a lipase into the shell of a CLA. The CLAs were then pumped through a cross-fiow membrane, where they were separated and recycled, with the product appearing in the permeate [70]. 

Boyaval, P. and Corre, C. 1987. Continuous fermentation of sweet whey permeate for propionic acid production in a CSTR with UF recycle. Biotechnol. Lett. 9(11), 801-806. 

Propionic acid and its derivatives are used in food, perfume and plastic applications. Traditional processes for making this compound, however, have limited productivity due to the low growth rate of the propionic bacteria and the inhibitory effect of the acid on the fermentation. The cheese whey permeate can be an inexpensive source of propionic acid. Propionic acids can be produced by fermentation of sweet whey permeate in a stirred tank reactor with cells separated from the medium and recycled back to the reactor by an ultrafiltration Z1O2 membrane on a carbon support [Boyaval and Corre, 1987]. This arrangement reduces the propionic acid concentration and increases the... 

---