Desalting with Hydrates

A . PROCESS DESCRIPTION

The process of desalting sea water by hydrate formation can be looked upon as a freezing process in which water solidifies and pre­

cipitates at higher temperatures and pressures than does pure ice. This solid contains no salt but does have from 5 to 15 mole % hydrating agent incorporated in it. The latter can be separated by melting, since the agent is chosen to be insoluble in water.

A simple flow sheet for this process is shown in Fig. 7.27. Liquid hydrating agent (labeled M ) is contacted with salt water under such conditions that hydrate forms. The heat of formation is removed by vaporizing M . The slurry of brine and hydrate goes to a wash column, where entrained salt is removed by countercurrent washing with water.

Only about 5 % of the net fresh-water product has to be used for this purpose. Vaporized Μ is compressed and condensed on the pure hydrate, thus yielding fresh water and liquid M. The latter is returned to the freezer and the fresh water is stripped of traces of M .

The heat exchangers precool the feed and heat the product water and brine. The auxiliary compressor rejects heat that is introduced by leakage from the atmosphere and by inefficiencies in pumps and com­

pressors, etc. In order that the condensing rate on the hydrate not be impeded by inert gases, it is necessary to remove dissolved air with a deaerator. A recycle stream of brine is used to regulate the total solids content of the slurry.

The description of this plant is very similar to that described elsewhere for a freezing plant. The principal difference between the two process is in the hydrate formation and separation tanks and in the nature of the solid separated out; but the reaction vessel, washer, heat-exchange strippers, deaerator, and compressors would be similar in design and use.

B . ADVANTAGES OF THE HYDRATE PROCESS

Comparison of either the hydrate or the freezing process with the various distillation and membrane processes shows the former to have several advantages. They have low energy consumption, they have no

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F I G . 7 . 2 7 . Flow diagram for desalting process using hydrates.

scale-formation problems, and they have no delicate or expensive membranes to contend with. Comparison of the hydrate and freezing processes shows that the former has the advantage of being able to operate at temperature levels about 5 to 30°F higher. Higher operating temperatures mean less heat leakage into the system, about 5 % higher

coefficient of performance for the main refrigeration cycle, and over 50 % better performance for the auxiliary refrigeration cycle.

If agents can be found which will form hydrates at moderate pressures but at temperatures above ambient, it is possible that an auxilliary refrigeration cycle could be eliminated completely and heat could then be removed with cooling water.

C. APPLICATION OF THE HYDRATE PROCESS

The feasibility of the method was investigated in 1959 by the Koppers Co., Inc., using propane in accordance with an invention by W . E.

Donath as disclosed in U.S. Patent 2,904,511. Although the Donath patent cites other hydrating substances, the use of propane is especially attractive because of its low cost, its rate of hydrate formation, its favorable reaction conditions, and its effectiveness as a direct heat-exchange medium, which eliminates the need for expensive heat exchangers to remove the heat of formation.

In the Koppers' process, sea water is precooled by heat exchange with product water and reject brine. Hydrate is formed by putting precooled sea water in contact with propane in a reactor. Excess propane is vaporized to remove the heat of formation. The slurry, containing hydrate crystals suspended in brine, is filtered and the mother liquor is washed from the crystals with a part of the product water. The washed hydrate crystals are then decomposed to water and propane, which are separated by decantation. Decomposition is effected by condensing compressed propane vapor on the crystals. Dissolved hydrating agent is removed from the product water and from the waste brine to minimize losses. The flow diagram of the process is illustrated in Fig. 7.28.

One advantage of the Koppers' hydrate process is flexibility: a number of different hydrate-forming agents may be employed. In addition to propane, the use of certain fluorinated hydrocarbons may be economically attractive. For example, dichlorodifluoromethane (refrigerant 12) has been used.

The Sweet Water Development Company, Dallas, Texas, has also evaluated a 700-gpd propane hydrate process pilot plant based on U.S.

Patent 2,974,102. This plant design consists of a reactor vessel, where precooled sea water and propane are brought together at appropriate conditions of temperature and pressure to form hydrate; a series of cyclone separators, where hydrate crystals are washed on the basis of liquid-solid differential densities; and a melter, in which compressed propane gas is condensed on the hydrate crystals to produce fresh water

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F I G . 7 . 2 8 . Flow diagram for K o p p e r s ' hydrate process.

and liquid propane. Product water and propane are separated by decantation. Part of the liquid propane is recycled to the reactor and part to the washing system for cocurrent displacing. Because of thermal inefficiencies, a portion of gaseous propane is further compressed in a secondary compressor and condensed against cooling water.

ACKNOWLEDGMENTS

T h e author wishes to thank the following organizations for permission to reproduce some of the illustrations: Applied Science Laboratories, Inc., College Park, Pa. (Fig. 7 . 5 ) ; Scientific American, New Y o r k (Figs. 7.6 to 7.9); and Colt Industries, Inc., N e w York (Figs. 7.11 to 7.26).

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