High-Rejection Membranes

Equations (11.2)-(11.10) provide a complete model for describing flie performance of an HHIO module. For a given set of qp, Cp, Pp and Pp (typically atmospheric), along with the values of the module physical properties, one can predict 9p> and Cp. A particularly useful solution scheme is in die case of highly rejecting membranes. In this case, most of the solute is retained in the reject. Therefcne, Eq. (11.2) can be simplified to... 

There is also continuing research into higher-performance (high flux and high rejection) membranes to further reduce the size and cost of RO systems. Nanotechnology shows promise for having a role in the development of these high-performance membranes. 

In the case of high rejection membrane, the value of AC is so large that the logarithmic mean concentration does no longer correspond to correct average. To solve this problem, Spiegler and Kedem ( ) divided the membrane into differential elements in the direction of its thickness and applied Eq.(lO) in it as the form of differential equation. The result of integration is expressed... 

Thus, the product water concentration is dependent on the feed concentration, membrane rejection and system recovery. Figure4.14shows the dependence of product concentration on these variables. It is possible to use a lower rejection membrane and a high recovery with a low TDS feedwater. On the other hand, a high TDS feedwater, such as seawater is limited to a low recovery and requires a high rejection membrane. 

The initial studies by Cadotte on interfacially formed composite polyamide membranes indicated that monomeric amines behaved poorly in this membrane fabrication approach. This is illustrated in the data listed in Table 5.2, taken from the first public report on the NS-100 membrane.22 Only the polymeric amine polyethylenimine showed development of high rejection membranes at that time. For several years, it was thought that polymeric amine was required to achieve formation of a film that would span the pores in the surface of the microporous polysulfone sheet and resist blowout under pressure However, in 1976, Cadotte and coworkers reported that a monomeric amiri piperazine, could be interfacially reacted with isophthaloyl chloride to give a polyamide barrier layer with salt rejections of 90 to 98% in simulated seawater tests at 1,500 psi.4s This improved membrane formation was achieved through optimization of the interfacial reaction conditions (reactant concentrations, acid acceptors, surfactants). Improved technique after several years of experience in interfacial membrane formation was probably also a factor. 

A sulfonated polysulfone membrane with commercial potential has been developed in the form of a hollow fiber by workers at Albany International Corporation.86 87 In a 5,000-hour test on 3,500 ppm brackish water at 400 psi, this membrane exhibited 98% salt rejection at 1 gfd. Flux and salt rejection remained constant even with addition of 100 ppm chlorine. In a 12,000-hour test on seawater at the Wrightsville Beach Test Facility, this membrane exhibited 98+% salt rejection and an average flux of 1.5 gfd at 1,000 psi operating pressure. Thus, it is possible to make a high rejection membrane from sulfonated polysulfone. Flux was rather low in this particular case, but suitable for hollow fiber membrane use. 

Recognizing the result (7.2.42) for high rejection membranes as well as tbe expression for from (7.2.37)-(7.2.39), we obtain tbe following result ... 

Solution We will employ equation (7.2.43) under the assumptions of a highly rejecting membrane, constant AP, ku, A,... 

High-Rejection SRWO Membrane Elements High-rejection membrane elements are... 

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