Externally wound membranes

The externally wound membranes were developed by Universal Water Corp., San Diego, California. In this design, the membrane element consists of a porous supporting tube on which are simultaneously wound, in helical fashion, a strip of permeable fabric overlaid with a helical wound strip of semlpermeable membrane film. Adjacent turns of the membrane overlap in winding, and these overlaps are sealed by a bonding solvent so that, the membrane Itself... 

A module just described is a fixed module in which the membrane elements are stationary. A radical departure from the conventional mode of R.O. systems operations is the rotary system wherein externally wound membrane elements clustered around a central shaft are rotated in a stationary pressure vessel filled with pressurized feed stock. Here the separated permeate is replaced by an equivalent volume of fresh feed stock under a constant head. The basic rotary concept is depicted in Fig. 3. 

The most important features of the externally wound membranes... 

Externally Wound Tubular Membrane Elements in Modular Assemblies Production and Application... 

The externally wound tubular membrane system and conventional tubular designs have the necessary design, production and functional features to meet more demanding task of processing fluids of high particulate content and, in rotary assemblies, treatment of delicate and structurally sensitive fluids and chemical mixtures. 

Membranes for externally wound elements were either obtained as finished product from commercial sources, or produced in-house from raw materials. 

As noted above, externally wound tubular membrane elements are fabricated primarily of extruded ABS tubing (natural), cellu-loslc or blend membranes and dacron paper. The cartridge assembly designed to house the membrane elements and provide for the uniform dlstrlbutlon/flow of feed stock is fabricated of extruded thin walled polystyrene headers. Pressure vessels were fabricated either by using epoxy coated steel pipe or fiberglass vessels. 

Fabrlcatlon/assembly of externally wound tubular membrane elements is accomplished by specially designed equipment that simultaneously and continuously winds, in helical fashion. Infinite lengths of membrane and backing material strips onto prefabricated tubular support structures. Membrane strip winding overlaps are solvent bonded during the winding process. 

A modular assembly housing externally wound tubular membrane elements may be defined as a pressure vessel within which are assembled a multiplicity of individual elements, means for connecting and sealing said elements to common headers and a feed stock distribution system that provides for adequate feed flow across membrane surfaces. 

In the development and design of modular assemblies housing a multiplicity of externally wound tubular membrane elements, the following factors were considered effectiveness of element packing arrangements and density, feed stock distribution patterns for controlled and uniform feed flow across membrane surfaces, sealing reliability of modular sub-assemblies, overall system reliability, module serviceability and ease of maintenance, system productibil-ity, and economic viability. 

In utilization of externally wound tubular membrane elements in modular assemblies for conventional systems operations, that is, selective separation via the circulation of pressurized feed across membrane surfaces, flow guide tubes were found to be essential for the proper distribution and control of feed flow. This was achieved singularly by the use of individual element shrouds or flow tubes creating an annular gap through which feed stock was circulated across membrane surfaces. 

R.O. systems utilizing externally wound tubular membrane element in modular assemblies have been used in the desalination of brackish and sea waters, the treatment and/or concentration of industrial waste waters, the separation/concentration of fluid food, pharmaceuticals and chemical solutions, and the manufacture of water purifiers for domestic use. Generally, externally wound tubular membrane systems have been found to be highly suitable for ultrafiltration applications in the processing Industry and in water pollution control applications. 

The mentioned characteristics of BNC, as well as other properties such as, for example, its capability of maintaining a moist environment at the wound surface while preventing external bacteria entry, the possibility of designing the shape of the membranes obtained, its ease of sterilization, good permeability, and nontoxicity, makes BNC a very promising biomaterial to be used as woimd-dressing material. Details on the physical and mechanical properties of microbial cellulose which promote its use as wound-dressing material are summarized in Table 2.3. 

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