Hollow fiber membrane based separation costs

As in the case of all membrane separation processes, the choice of an appropriate module type is based on feed type (viscosity, suspended solids content, and particle size), required membrane packing density (based on flux, total throughput, and available floor space), good flow hydrodynamics (for minimization of concentration polarization, effective cleaning and sanitation), and module cost. The various types of membrane modules and their fabrication have been reviewed by Strathmann.f Hollow fiber modules, which have the highest membrane packing density of all module types, are the most suitable for use in OD because of the inherently low flux of this process. However, the membranes that have provided the best fluxes and volatiles retention because of their relatively large pore diameters and porosities, that is those fabricated from PTFE, have not yet become available in hollow fibers with an acceptably low wall thickness. 

Commercial membrane separation processes include reverse osmosis, gas permeation, dialysis, electrodialysis, pervaporation, ultrafiltration, and microfiltration. Membranes are mainly synthetic or natural polymers in the form of sheets that are spiral wound or hollow fibers that are bundled together. Reverse osmosis, operating at a feed pressure of 1,000 psia, produces water of 99.95% purity from seawater (3.5 wt% dissolved salts) at a 45% recovery, or with a feed pressure of 250 psia from brackish water (less than 0.5 wt% dissolved salts). Bare-module costs of reverse osmosis plants based on purified water rate in gallons per day are included in Table 16.32. Other membrane separation costs in Table 16.32 are f.o.b. purchase costs. 

Diffusion across thin membranes can sometimes produce chemical and physical separations at low cost. These low costs have spurred rapid development of membrane separations, especially during the last 20 years. This rapid development has sought both high fluxes and high selectivities. It has included the separation of gases, of sea water, and of azeotropic mixtures. It has used hollow fibers and spiral wound modules it has centered on asymmetric membranes with selective layers as thin as 10 nm. This rapid development is a sharp contrast to other diffusion-based separations like absorption, where the basic ideas have been well established for 50 years. 

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