Recirculation rate filtration

An alternative method of reducing the resistance to filtration is to recirculate the slurry and thereby maintain a high velocity of flow parallel to the surface of the filter medium. Typical recirculation rates may be 10-20 times the filtration rate. By this means the cake is prevented from forming during the early stages of filtration. This can be particularly beneficial when the slurry is flocculated and exhibits shear-thinning non-Newtonian properties. This method of operation is discussed by Mackley and Sherman(21) and by Holdich, Cumming and Ismail(22). 

This particular fermentation is performed in 1+00 liter batches. Tangential flow filtration was carried out with a Pe licon cassette (Millipore Corp.). The filtration area was 25 ft using a 100,000 molecular weight cut off ultrafiltration membrane. The inlet pressure was maintained at 1+0 psig while the outlet pressure was 25 psig and recirculation rates of the cell suspension were 3 5 GPM tangential to the upstream membrane surface. Pumping of the cell suspension was performed with a Moyno screw pump. 

Thus, the XMF-RO integrated system recovers 75-85% of wastewater for reuse. The SEC of XMF is, however, much higher than semi dead-end membrane filtration processes (Table 5.2) because of very high cross-flow recirculation rates. The total energy consumption is 65 kW based on SEC of 3.0 kWh/m for the XMF unit and 0.3 kWh/m for the RO unit (Table 5.3). The integrated system SEC is 3.61 kWh/m. ... 

To continuously separate FT wax products from ultrafine iron catalyst particles in an SBCR employed for FTS, a modified cross-flow filtration technique can be developed using the cross-flow filter element placed in a down-comer slurry recirculation line of the SBCR. Counter to the traditional cross-flow filtration technique described earlier, this system would use a bulk slurry flow rate below the critical velocity, thereby forcing a filter cake of solids to form between the filter media and the bulk slurry flow, as depicted in Figure 15.2b. In this mode, multiple layers of catalyst particles that deposit upon the filter medium would act as a prefilter layer.10 Both the inertial and filter cake mechanisms can be effective however, the latter can be unstable if the filter cake depth is allowed to grow indefinitely. In the context of the SBCR operation, the filter cake could potentially occlude the slurry recirculation flow path if allowed to grow uncontrollably. 

There are two principal modes under which deep bed filtration may be carried out. In the first, dead-end filtration which is illustrated in Figure 7.1, the slurry is filtered in such a way that it is fed perpendicularly to the filter medium and there is little flow parallel to the surface of the medium. In the second, termed cross-flow filtration which is discussed in Section 7.3.5. and which is used particularly for very dilute suspensions, the slurry is continuously recirculated so that it flows essentially across the surface of the filter medium at a rate considerably in excess of the flowrate through the filter cake. 

A flow diagram of a simple cross-flow system is shown in Figure 8.5. This is the system likely to be used for batch processing or development rigs and 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 partially 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 is achieved by a process known as diafiltration in which wash water is added to the tank at a rate equal to the permeation rate. 

Typically, for cooling water makeup, the filtration rates are 4 to 6 gpm/sq ft of media surface area, with backwash rates of 10 to 12 gpm/sq ft for a period of 5 to 8 minutes. When the filter is used for removing suspended solids from recirculating cooling water, the filtration rate may be as high as 10 to 15 gpm/sq ft and the backwash rate 15 to 20 gpm/sq ft. 

Filtration tests for recirculated silica-M slurry from Test 1, using 1 and 3 pm rating depth filters resulted in immediate plugging, whereas tests with 5 and 9 pm nominal filters (Planargard CMP5 and CMP9, respectively) resulted in Ap... 

Feed and bleed A continuous mode of CFF where the feed is pumped into the recirculation loop at a rate which equals the summation of the permeation rate and the flow rate of retentate. It allows for increased control over the effect of concentration on the filtration performance. 

---