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Desalination by Direct-Contact Distillation
Initiated: June 2001
Team: R. W. Bradshaw, S. K. Showalter, K. T. Welch (student intern)
Existing desalination technologies produce only a small fraction of all freshwater supplies because of the relatively high cost. The high cost is due to complex process equipment, intensive energy requirements, and excessive maintenance due to salt fouling. This project developed a conceptual design of a novel desalination process based on distillation of saline water by direct-contact heat exchange with a non-volatile working fluid that provides process heat. There are two primary advantages of a direct-contact desalination compared to other thermal distillation methods. First, the heat transfer coefficients for direct-contact boiling are much, much larger than heat transfer coefficients when a tubing wall separates the fluids. Thus, the size and cost of a direct-contact boiler can be drastically reduced compared to conventional tubular heat exchangers used in existing thermal distillation plants. The proposed direct-contact process also enables the dissolved solids in the saline water source to be collected by an appropriate working fluid, avoiding the discharge of a concentrated brine stream, if desired. direct-contact distillation unit
Conceptual sketch of a direct-contact distillation unit.
The direct-contact distillation technique can eliminate fouling problems that arise from dissolved constituents in the saline feedwater, compared to multi-stage flash (MSF) distillation, and addition of feedwater conditioning chemicals that are needed by both MSF and reverse osmosis. During this project, a process flowsheet was prepared for direct-contact desalination and calculations of direct-contact heat transfer coefficients were used to estimate the size of the distillation stage. The result was that a direct-contact boiler is drastically smaller than conventional tubular heat exchangers in MSF plants, and could be as small as a single MSF stage. A molten salt was identified as potentially an optimal working fluid because it enables the dissolved solids in the saline water to be collected and removed as a solid by-product. The capacity of the molten salt to collect dissolved solids typical of saline water was evaluated by laboratory experiments. These tests showed that sodium chloride can be precipitated from the molten salt by simply lowering the temperature of the melt, which should facilitate separation from the bulk solution using, for example, a hot sedimentation-filtration process.

The goal of this project was to develop a conceptual design of a novel thermal desalination process based on direct-contact distillation and evaluate key physical properties of the proposed working fluid.

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