Intrinsic rejection

A = 4.05 X lO " cm/(s-kPa)(4.1 X 10 cm/(s-atm)) and = 1.3 x 10 cm/s (4)//= 1 mPa-s(=cP), NaCl diffusivity in water = 1.6 x 10 cm /s, and solution density = 1 g/cm . Figure 4 shows typical results of this type of simulation of salt water permeation through an RO membrane. Increasing the Reynolds number in Figure 4a decreases the effect of concentration polarization. The effect of feed flow rate on NaCl rejection is shown in Figure 4b. Because the intrinsic rejection, R = 1 — Cp / defined in terms of the wall concentration, theoretically R should be independent of the Reynolds... 

The intrinsic rejection and maximum obtainable water flux of different membranes can be easily evaluated in a stirred batch system. A typical batch unit (42) is shown in Figure 5. A continuous system is needed for full-scale system design and to determine the effects of hydrodynamic variables and fouling in different module configurations. A typical laboratory/pilot-scale continuous unit using computer control and on-line data acquisition is shown in Figure 6. 

Some confusion can occur over the rejection coefficients quoted by membrane module manufacturers. The intrinsic rejection of good quality membranes measured in a laboratory test system might be in the range 99.5 to 99.7 %, whereas... 

The observed rejection [Eq. (8a)j is clearly the important one for a practical separation operations, but it includes the confounding effects of concentration polarization. Since C k < Cj, the observed rejection is less than the intrinsic rejection and can be determined by estimating the solute wall concentration with Eq. (7). 

A relation is needed between these measurable quantities and R, the intrinsic rejection coefficient defined by Equation 4. First, is defined as the local axial solute flux averaged over the fiber cross-section with axial diffusion negligible. 

Values calculated for the intrinsic rejection coefficient at each value for J were used to find a and P in Equation 7, according to a Newton-Rhaphson iterative least-squares procedure previously described (7 ). 

Rejection A measure of how well a membrane retains or allows passage of a solute. When based on concentrations in the bulk of the permeate and retentate streams, it is the apparent rejection. The intrinsic rejection is based on concentrations at the membrane surfece. Sometimes used interchangeably with retention factor. 

The performance equation of a membrane can be derived by introducing intrinsic rejection (R = 1 — c lcf) and observed rejection (Robs = 1 — 0 /02) with c5 being the bulk concentration at the permeate side ... 

Adesalinadon experiment with laackish water (1000 ppm NaQ) with cellulose acetate hollow fibers gives a flux of2000 Lmz.day. The intrinsic rejection is 94 % and the mass transfer coefficient is k = 5.4 10 m/s. 

Similarly, the permeate concentration of the unionized solute Qp can be obtained using the definition of intrinsic rejection Ri of the unionized species 1,... 

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