Spiral wound membrane modules characteristics

First multi-leaf spiral wound membrane module developed by Don Bray and others at Gulf General Atomic, under US Patent no. 3,417,870, "Reverse Osmosis Purification Apparatus," December, 1968. A multi-leaf spiral configuration improves the flow characteristics of the RO module by minimizing the pressure drop encountered by permeate as it spirals into the central collection tube. 

Characteristics of membrane modules are summarised in Table 1.12. The spiral wound (SW) module shown in Figure 1.18 is used in all RO and NF applications. The RO hollow-fibre (FIF) module, similar to the one shown in Figure 1.19, is now manufactured by only by Toyobo, and is used for seawater desalination. UF HF membrane (see Figure 1.3) was used extensively in the dairy industry, but it has largely been replaced by SW modules. However, cross-flow HF modules are commorJy used in food processing and industrial wastewater treatment [18, 31]. 

Figure 2.27 Membrane flux characteristics of a spiral wound RO module for various feed waters. Source Film-Tec membrane catalogue. For non-pure water solutions the flux reaches a constant, steady state value at higher pressures. For optimal performance, the system should be run below the "critical flux" region, which is the mid-point of the curved part of the curve.

The differences in permselective performance between some irradiated and nonirradiated samples cannot be associated with radiation effects, but with membrane structural dispersion. This is because the membrane samples were taken from a spiral wound module therefore, the differences observed can be due to the lack of homogeneity in the membrane structure and its characteristics along the module. This fact can be illustrated by the dispersion of the experimental results of the nonirradiated samples. 

The membrane shapes described are usually incorporated into compact commercial modules and cartridges. The four more common types of modules are (1) plate-and-frame, (2) spiral-wound, (3) tubular, and (4) hollow-fiber. Table 9.2 is a comparison of the characteristics of these four types of modules. The packing density refers to the surface area per unit volume of module, for which the hollow-fiber modules are clearly superior. However, hollow-fiber modules are highly susceptible to fouling and very difficult to clean. The spiral-wound module is very popular for most applications because of its low cost and reasonable resistance to fouling. 

The Permutit Liberator 1 reverse osmosis unit contains two spiral wound modules of 0.96 m each with Filmtec BW30 membranes, a crosslinked aromatic polyamide. These were selected based on low adsorption and high rejection characteristics. 

The configuration (flat sheet, mbular modules, hoUow fibers, plates units, and spiral wound) also affects membrane performance. Good membrane performance with respect to permeate flow rate and the retention of the desired solute should be balanced with respect to its characteristics such as propensity for fouUng, cost, ease of cleaifing, and substimtion [2,6]. 

Crossflow filtration membranes are available in a variety of configurations. These are hollow fiber, tubular, flat plate, spiral wound, and rotating. Some key characteristics of each module are given in Table 12.2 reproduced here from [6]. 

---