The present invention relates to refrigeration, and to separation, and more particularly to separating fluid mixtures into dilute components and concentrated components by freezing and then melting the frozen fluid. Fluid mixtures include suspensions, solutions, emulsions and the like.
Freeze-melt techniques in the present field of art may be exemplified by several prior art applications of these techniques. For instance, Hadzeriga, in U.S. Pat. No. 3,681,931, describes the treatment of phosphate rock slimes by freezing. Upon melting, the mixture separates into a top layer of clear liquid substantially free of suspended matter and a bottom layer of concentrated suspension.
Phosphate slime is a colloidal, aqueous suspension which is normally difficult to separate and is left in large settling ponds appurtenant to the mining operation which produced it. In the freeze-melt process, the slime is "statically frozen" by freezing a substantial portion of the liquid in a quiescent, non-agitated state to temperatures preferably between -20.degree. C. and -80.degree. C. Slow freezing produces better separation by producing larger ice crystals to compact suspended solids. After freezing, static thawing is carried out at ambient temperature.
Prior art processes and equipment for the treatment of phosphate slimes have, insofar as is known, heretofore been of the "bulk freezing" type wherein a container of slime is filled, frozen, melted and decanted. After the separated components have been removed, the container is refilled, and the cycle is begun again.
Freeze-melt techniques have also been applied to treatment of sewage sludge. Sewage sludge is an aqueous organic colloidal suspension which is difficult to separate by filtration or settling. Sludge which has been frozen and melted can be filtered quite easily, however. Sludge freezing process efficiency has been found to be independent of freezing temperature, length of time the sludge was kept frozen, and the thickness of the sludge. Process efficiency does, however, depend on the slow and full freezing of the sludge.
A sludge freezing plant typically employs a vapor compression refrigeration type system. Ammonia may be used as the refrigerant. The sludge is contained in a first large tank and cold, vaporizing ammonia is passed through an evaporator comprising vertical pipes passing through the tank. The hot, vaporized ammonia is then passed through pipes in a second tank which holds previously frozen sludge which is to be melted by the warm vapor. When a tank is fully frozen, the flow of ammonia is reversed, and melting begins in the first tank, and freezing begins in the second. After a "batch" has been melted, the tank contents are emptied through the bottom of the tank and passed to a filter for separation.
Freeze-melt techniques have also been applied to the desalinization of sea water. Separation is based on the fact that when salt water is partially frozen, the ice crystals that form are free of salt. In a desalinization plant described by G. Karnofsky in Chemical Engineering Progress, Volume 57, No. 1, January 1961, p. 42, butane is used as the refrigerant in a vapor compression refrigeration system. The seawater is frozen by a direct contact with boiling butane at slightly less than 1 atm. pressure. The butane vapor from the freezer is then compressed and condensed by cold from previously formed ice. The vapor in turn warms and melts the ice. Melted water is passed out through a heat exchanger against entering seawater. The condenser and heat exchanger arrangement is designed to improve the thermal efficiency of the system.
Briefly, in a vapor compression system, the refrigerant in a vapor phase is first compressed by a compressor, then cooled in a condenser to liquid form. This is the high pressure side of the system. The liquid refrigerant then passes through an expansion valve, which maintains pressure on the high side, to the low pressure side of the system. There, the refrigerant passes through an evaporator, where it is vaporized and expands, thus absorbing heat and causing refrigeration of the material surrounding the evaporator. The vaporized refrigerant is passed to the compressor to complete the cycle. Just as heat is absorbed by the refrigerant due to vaporization in the evaporator, heat is liberated in the condenser due to the condensation there.
A principal problem to be overcome in a freeze-melt process is thermal efficiency. The thermal efficiency of a vapor compression refrigeration system is related to the condenser and evaporator temperatures. The thermal efficiency of a refrigeration system may be expressed as: ##EQU1## where: T.sub.1 =evaporator temperature, absolute;
T.sub.2 =condensing temperature, absolute; and PA1 C.O.P.=the coefficient of performance of the refrigeration cycle. PA1 h.sub.d =enthalpy of vapor leaving the compressor, B.t.u./lb; and PA1 hg=enthalpy of vapor entering the compressor, B.t.u./lb.
The heat of compression is expressed as: EQU Heat of compression=h.sub.d -h.sub.g ( 2)
where:
Enthalpy refers to the heat content of the refrigerant. The net refrigeration effect refers to enthalpy lost in the evaporator. It can be seen from equation (1) above that the smaller the difference between the evaporator temperature and the condensing temperature, the more efficient the process will be. Since enthalpy of the vapor is related to temperature, equation (2) demonstrates that lowering the necessary compressor temperature and pressure differences will also increase refrigeration efficiency.