Membrane processes flow channel spacers

The performance of a membrane process is a function of the intrinsic properties of the membrane, the imposed operating and hydrodynamic conditions, and the namre of the feed. This chapter describes methods available to enhance performance by various techniques, mainly hydrodynamic but also chemical and physical. The focus is on the liquid-based membrane processes where performance is characterized by attainable flux, fouling control, and separation capabilities. The techniques discussed include secondary flows, flow channel spacers, pulsed flow, two-phase flow, high shear devices, electromagnetic effects, and ultrasound. 

The membranes in electrodialysis stacks are kept apart by spacers which define the flow channels for the process feed. There are two basic types(3), (a) tortuous path, causing the solution to flow in long narrow channels making several 180° bends between entrance and exit, and typically operating with a channel length-to-width ratio of 100 1 with a cross-flow velocity of 0.3-1.0 m/s (b) sheet flow, with a straight path from entrance to exit ports and a cross-flow velocity of 0.05-0.15 m/s. In both cases the spacer screens are... 

Hollow-fibre membrane modules are similar to the capillary type described above, but with fibres of outside diameters ranging from 80 to 500 pm. It is usual to pack a hollow-fibre module with many hundreds or thousands of these fibres, thus membrane area per unit volume is extremely hi. It should be apparent that filtration using hollow-fibre modules is only realistic with process fluids prefiltered to prevent fibre blockage fins limits the technology and it is applied mainly in UF. Also used in uhrafiltration is a spiral-wound membrane module which is often compared to a Swiss roD. The membrane and a spacer are wound round a former, with an appropriate permeate spacer flow is introduced and removed from the ends. This module design is not appropriate for solid-liquid separation, even when filtering colloids, because of the possibility of flow channel blockage and so it will not be discussed any finther. 

Membrane gas-separation systems have found their first applications in the recovery of organics from process vents and effluent air [5]. More than a hundred systems have been installed in the past few years. The technique itself therefore has a solid commercial background. Membranes are assembled typically in spiral-wound modules, as shown in Fig. 7.3. Sheets of membrane interlayered with spacers are wound around a perforated central pipe. The gas mixture to be processed is fed into the annulus between the module housing and the pipe, which becomes a collector for the permeate. The spacers serve to create channels for the gas flow. The membranes separate the feed side from the permeate side. 

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