Evaluating Spiral-wound Separator Performance

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The recent evolution of spiral-wound membrane element (sepralator) design brings an increased need to understand the intricacies of testing, operating and predicting performance of this most popular crossflow membrane design. After over a decade of stagnant development, the last five years have produced several innovations, which diversify and expand the applications as well as improve the performance of sepralators.

These innovations include improved materials of construction for specialized applications such as ultrapure water reclamation (1). Another innovation is the Full-Fit™ design, which eliminates the outer cover/brine seal combination. This design reduces microbial growth and provides better chemical rinse-out for cleaning/sanitizing in medical/pharmaceutical applications. Also by improving fluid dynamics the Full-Fit design reduces fouling when operating on particle laden and/or gel-forming solutions. Clear channel 'tubular spiral' sepralators reduce axial pressure drop on viscous solutions, and are able to handle high 'plugging' streams (2). Permeate channel design innovations allow increased temperature capability even at high pressures (3). Tapered shape spirals conform exactly to standard cartridge filter sumps (4).

Sepralator designs are now expanding and improving to complement the membrane improvements, which are regularly commercialized now as crossflow membrane technology comes of age. To fully utilize these design improvements, sepralator operation and performance knowledge must also improve to keep pace.

Some operational and test technique nuances are not widely known. 'Standard' test methods alone are inadequate, or in some cases inaccurate, and improved understanding must be more widely communicated. This paper presents some guidelines and illustrations for testing sepralators and predicting performance in three areas:

1. Separation
2. Production rate (flux)
3. New characterization concerns

Separation Measurement

The 'industry accepted standard' test for reverse osmosis (RO) sepralators is fairly simple, but one often overlooked detail (the feedwater) can make a significant performance rating difference. There is no 'industry accepted standard' separation test for ultrafiltration (UF) sepralators, but polyethylene glycols (PEGs) and dextrans of varying molecular weights are commonly used as marker molecules. Different operating conditions, especially crossflow velocities and solute concentrations, can yield radically different separation results. No accepted test method to judge microfiltration (MF) sepralators exists.

Most RO sepralator manufacturers state similar test conditions to achieve their solute separation specification; 225 or 400 psig (15.5 or 27.6 bar), 77°F (25°C), 10-15% recovery and a solution of 2000 ppm NaC1 (5,6,7). However, the more specific nature of that test solution is a significant factor in the separation outcome. If tap water is used as the NaCl solvent, and that tap water has high levels of divalent salts, higher rejection will be seen since conductivity is the normal measurement method. For example, Colorado River Water (CRW) is the basis for the municipal water supply in much of southern California - where RO 'grew up' and where many spiral RO manufacturers are currently located.
To quantify this effect, we tested a sepralator containing medium rejection, cellulose-acetate based membrane, on 0.2% NaC1 solution based both on simulated CRW and RO treated tap water (Table 1). While the difference between 94.8 and 95.9% rejection may not appear significant at a casual glance, the passage value of 5.2% on the RO treated tap water solution represents 27% more salt in the permeate than the 4.1% CRW-based test solution yields.

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