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. 

A variation of the basic plate-and-frame concept is the spiral-wound module, which is widely used today in reverse osmosis, ultrafiltration, and gas separation. Its basic design is illustrated in Figure 1.33 (c). The feed flow channel spacer, the membrane, and the porous membrane support are rolled up and inserted into an outer tubular pressure shell. The filtrate is collected in a tube in the center of the roll. 

Plate and frame units are also used for cross-flow filtration. Some units take sheet stock MF membranes while others work best with a preassembled membrane cassette-a sandwich of two outer layers of membrane sealed to an inner filtrate collection screen (see Figure 2.47). A cross-flow spacer is placed between the filter packets and stacked in a plate and frame arrangement (see Figure 2.48). In some systems, the cross-flow spacer is a screen, but the "flow-channel" spacer shown in Figure 2.47 is less prone to fouling. 

Figure 2.47 Cross-flow membrane cassette (two layers of membrane enclosing inner collection screen) and flow-channel spacer.

Of the various CFF modules available, plate and frame devices which utilize screen spacers between membranes are prone to accumulation of cells on the cross-members of the screen resulting in flow blockage. "Flow-channel spacers" like those shown in Figure 2.47 are less prone to fouling. 

For tubular and flat sheet membrane modules, one of the commonly used hydrodynamic techniques for the concentration polarization control is turbulence promoters, such as spacers used in spiral wound membrane modules, helical insert used in tubular membrane modules, and the corrugated membrane for the flat sheet membrane. Research has been conducted to assess the effect of the membrane flow channel spacers and inserts on the membrane filtration and to optimize their design. 

Different from the secondary flows and flow channel spacer techniques, the pulsed flow method is to generate a pressure fluctuation wave in either the feed or permeate flow channel using certain oscillators. The fluctuating pressure wave can enhance the membrane filtration through reducing the boundary layer or induced instant local backflushing flow as discussed in Section 10.4.1. 

In the spiral-wound configuration membranes are sandwiched together with feed flow channel spacers and the porous membrane support around a central permeate collecting tube (Fig. 2.3b). Commercial systems are about 1 meter long with diameters between 10 and 60 cm. Membrane areas can be in the range of 3-60 m. Spiral-wound membranes offer a good membrane surface/volume and low capitaEoperating cost ratios. Nevertheless, they cannot be mechanically cleaned and a feed pretreatment is required. 

Two basic flow schemes are used tortuous path flow and sheet flow (Fig. 22-59). Tortuous path spacers are cut to provide a long path between inlet and outlet, providing a relatively long residence bme and high velocity past the membrane. The flow channel is open. Sheet flow units have a net spacer separating the membranes. Mass transfer is enhanced either by the spacer or by higher velocity. 

Feed flow across feed channel spacer... 

The area required for processing A = Qo — QVJ, where Qo Q is the perrneate volumetric flow, can be estimated by using the approximation / = 0.33/initiai + 0.67/finai (Cheryan, Ultrafiltration Handbook, Technomic, Lancaster, Pa., 1986) and a suitable flux model. An appropriate model relating flux to crossflow, concentration, and pressure is then applied. Pressure profiles along the retentate channel are empirically correlated with flow for spacer-filled channels to obtain A = APfQ/AT. 

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... 

Figure 19.4. The spiral wound membrane module for reverse osmosis, (a) Cutaway view of a spiral wound membrane permeator, consisting of two membranes sealed at the edges and enclosing a porous structure that serves as a passage for the permeate flow, and with mesh spacers outside each membrane for passage of feed solution, then wound into a spiral. A spiral 4 in. dia by 3 ft long has about 60 sqft of membrane surface, (b) Detail, showing particularly the sealing of the permeate flow channel, (c) Thickness of membranes and depths of channels for flows of permeate and feed solutions.

Spiral-wound elements, as shown in Figure 2, consist primarily of one or more membrane "leaves, each leaf containing two membrane layers separated by a rigid, porous, fluid-conductive material known as the "permeate channel spacer." The permeate channel spacer facilitates the flow of the "permeate", an end product of the separation. Another channel spacer known as the "high pressure channel spacer" separates one membrane leaf from another and facilitates the flow of the high pressure stream through the element. The membrane leaves are wound around a perforated hollow tube, known as the "permeate tube", through which the permeate is removed. The membrane leaves are sealed with an adhesive on three sides to separate the feed gas from the permeate gas, while the fourth side is open to the permeate tube. 

The high pressure "residual gas" mixture remains in the high pressure channel spacer, losing more and more of its acid gas and being enriched in hydrocarbon gas as it flows through the element, and exits at the opposite end of the element. 

In tortuous-path stacks there is no need for spacer screens as thicker membranes, narrow channels, and plenty of cross-straps are used. On the contrary, in sheet-flow stacks spacers of different geometry and thickness are necessary to prevent membrane contact (that would result in burning through), as well as to induce turbulence in the flowing solution (Kuroda et al., 1983). Spacers generally consist of a sealing frame and a net in the... 

Figure 9.5. Diagram showing FFF flow channel cut from a thin spacer and sandwiched between two walls. The field is applied perpendicular to the flow.

A number of factors can lead to high pressure drop, including membrane scaling, colloidal fouling, and microbial fouling. These three factors all involve deposition of material onto the surface of the membrane as well as onto components of the membrane module, such as the feed channel spacer. This causes a disruption in the flow pattern through the membrane module, which, in turn, leads to frictional pressure losses or an increase in pressure drop. 

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