UF Plant Design

In most cases, the flux is too low to operate in the single-pass mode the recovery of permeate in a single pass is a small percentage of the feed (very little concentration of retained species). Recirculation of the process stream across the membrane is necessary to obtain the desired concentration or recovery. Typically the ratio of recirculation rate to permeate flux is 10 to 100-fold. Recirculation can be accomplished with either a batch concentration or with a feed and bleed operating mode. 

In a batch operating mode, the retentate is circulated back to the feed reservoir. As the permeate is removed, the volume in the reservoir goes down and the concentration of retained species goes up. Freely permeable species (e.g., salts) remain at the same concentration in the reservoir and in the permeate stream. Eventually, the volume in the reservoir becomes too small to pump and the run is over. A new batch must be charged to the reservoir to continue. 

The feed and bleed mode permits continuous filtration the feed stream is fed into a recirculating loop. The concentration of retained species will continue to increase with ever decreasing flux, unless a purge stream is taken from the loop. 

Often a ratio controller is used to keep the feed to bleed ratio constant-equal to the volumetric concentration ratio required. This means that even with flux decay, the concentration ratio and recovery will remain constant. 

It is unwise to use the feed and bleed mode to concentrate a batch of solution. Once steady state is achieved, the membrane filter at the final concentration for the duration of the run. In a batch process, a higher average flux is achieved due to the gradually increasing concentration. Thermodynamically, the feed and bleed mode is less efficient due to increase in entropy upon mixing the low concentration feed with the high concentration recirculating retentate. 

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Table 2.12 Tubular UF plant design parameters Item description... 

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