Toyobo RO Module for Seawater Desalination

Seawater desalination by reverse osmosis is the most effective method for the production of freshwater among various desalination technologies. HoUow-fiber RO membranes and flat-sheet membranes have been developed for brackish water and seawater desahnation by a two-pass process since 1976 (Ohya, 1976). In spite of a satisfactory result of two-pass seawater desalination processes, the one-pass process has the advantages of simple and compact plant, simple operation, easy maintenance, and the lowest energy consumption. Although several one-pass seawater desalination systems by reverse osmosis have been 

In biologicaUy achve seawaters sterilizahon by chlorine is considered a very effective soluhon to prevent biological fouhng from occurring in the RO process. Material properties 

The bundle of several tens of thousands of hollow fibers is fabricated into an element and installed in a module. HoUow fibers in an element are arranged in a mutually crossed configuration without any kinds of supporting materials between hollow-fiber layers. 

Conventional hollow-fiber RO module consisted of a single element inserted in one pressure vessel. However, economical and technical considerations demanded modules with a number of elements inserted in one pressure vessel. Toyobo successfully produced double-element modules on a commercial basis for the first time in 1979 and was followed by various types of double-element modules based on their flow path design. 

Feed water enters the feed center pipe of the feed-side element, feed water flows radially from the center pipe, past the hollow fibers in the bundle, and away from the element as concentrated brine of the feed-side element. Feed flow is uniformly distributed in the RO module throughout the cross-wound hoUow-fiber bundle. The brine water of feed-side element flows on the periphery of the element, passes through the narrow space between the pressure vessel and element, and flows to the periphery of the brine-side element and 

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This provides mechanical stability to the fiber array in the RO element. The fibers at one end of the element are precisely cut so that product water can be discharged from the bore of the fibers. Toyobo RO modules for seawater desalination have configurations in the doubleelement type as shown above. Because the permeate water of each element in a pressure vessel can be obtained directly, the quality of each permeate water can also be measured directly. This feamre allows easier maintenance of an RO plant (Kannari, 1995). 

ACTUAL PERFORMANCE OF TOYOBO RO MODULE FOR SEAWATER DESALINATION... 

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]. 

This summary describes the history of cellulose triacetate RO membrane, description of cellulose triacetate hollow-fiber RO modules of Toyobo for seawater desalination, actual operation results, and recent RO modules of cellulose triacetate including most recently developed advanced modules. 

Research and development of the hollow-fiber-type module using a cellulose acetate membrane was conducted by Monsanto, Toyobo, and others, in addition to Dow Chemical. Toyobo announced an RO module for one-pass desalination of seawater that used the cellulose triacetate hollow-fiber membrane module in 1979 (Orofino, 1970 Ukai et al., 1980). 

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