Date of Graduation

5-2012

Document Type

Thesis

Degree Name

Bachelor of Science in Chemical Engineering

Degree Level

Undergraduate

Department

Chemical Engineering

Advisor

Penney, William Roy

Abstract

The use of sea water desalination to produce potable water for maritime vessels is essential for eliminating storage of potable water. Reverse osmosis (RO) is the technology of choice for desalination purposes; however, raw seawater must undergo pretreatment to avoid rapid fouling and plugging of RO membranes and modules. Current pretreatment technology is effective in the open ocean but in coastal regions involves frequent replacement of filter cartridges and increased maintenance costs. The standard pretreatment process consists of 20 μm cartridge filters and 3 μm cartridge filters in series. Several technologies were considered as options for improving the current pretreatment process. Multimedia filtration followed by hollow-fiber ultrafiltration was considered, but it was rejected because of the high required membrane surface area, high capital cost, and high pressure drop requirements. Mechanically-enhanced self-cleaning filters, followed by either microfiltration (MF) or ultrafiltration (UF), were also considered. The selfcleaning filters were rejected because of mechanical complexity and high initial investment. The technology selected by The Salty Hogs is a sand filter followed by a 0.1 μm hollow-fiber microfiltration (HMF) unit. The sand filter is less expensive and less complex than a multimedia filter, and the HMF unit requires much less area and has lower capital costs than a UF unit. The sand filter/HMF combination can handle feeds with various turbidity levels and meet the task requirements to feed 21 gpm of filtered water to the RO unit, which is the flow rate required for the RO unit to produce 12,000 gallons/day of potable water with 40% recovery. Although the project focused on oceanic applications, inland water desalination plants can integrate The Salty Hogs’ technology into existing RO desalination processes. This report presents the design for a full-scale, shipboard system that incorporates sand filtration and hollowfiber microfiltration. The shipboard unit is designed for optimal power usage and minimal space requirements. The water provided by the recommended process is filtered to 0.1 μm, an improvement that will lengthen the lifetime of the current spiral-wound RO membranes. The system is automated to back-wash the sand filters with water and back-flush the HMF membranes with air. Periodic chemical cleaning is required and involves the use of a sodium hydroxide wash followed by an acetic acid rinse. The spent sodium hydroxide and acetic acid solutions will be combined and neutralized before being discharged to the ocean. Task #5 4 University of Arkansas The economic parameters for the system have been determined. Purchased equipment cost is $21,000, and the total capital investment, based on factored estimates, is $133,000. Over the projected three year lifetime of the RO membranes, the proposed process provides an incremental savings, including reduced maintenance and filter usage, of $92,000, over the three year project lifetime when compared to the current cartridge filter system. The WERC task statement specified a goal for energy usage of 1.2 HP, which is about 10% of the total power required for the RO operation. The current full-scale design uses ~3 HP.

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