Refrigeration – Processes – Fractionally solidifying a constituent and separating the same
Patent
1991-03-15
1993-01-26
Capossela, Ronald C.
Refrigeration
Processes
Fractionally solidifying a constituent and separating the same
62123, 62542, 203 11, B01D 904, C02F 122
Patent
active
051813963
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method of freezing and melting a solution to enable the ingredients of the solution to be separated from each other and, more specifically, to such a method of freezing and melting by heat energy at a relatively low temperature.
The method is especially suitable for the process industry, for cleaning waste waters of a process by using the waste heat of the very process as a source of energy. Such an application is, for example, concentration of waste waters of a pulp bleaching plant by the waste heat released from the process at a temperature level of 50.degree. C. The method is also applicable to other processes of the chemical industry in which components of a solution are to be separated from each other.
Crystals produced in freezing of some solutions such as, for example, brine, are pure solvent i.e. water, and the dissolved salts remain in the solution By separating the ice crystals from the solution and by washing and melting them, pure solvent can be separated from the concentrated solution.
The solution can be frozen by evaporating it at the triple point, where the solid, liquid and vapor phases are all in equilibrium with one another. In practice, when, for example, an aqueous solution is concentrated, this results in the temperature falling below the freezing point of pure water.
The vapor temperature is, in other words, below zero, causing the vapor to condense to ice onto heat transfer surfaces This is one of the weaknesses of the prior freezing methods. It has been avoided in some methods by compressing the vapor to a temperature higher than the freezing point by means of a compressor. The specific volume of the vapor, being huge at such temperatures, causes problems in the construction of a satisfactorily-operating compressor. Finnish patent No 73818 discloses compression of vapor by a steam jet ejector suitable for dilute solutions such as brine, which is used as a source of heat for a heat pump, the freezing point of the brine being not very much below the freezing point of pure water.
In the present invention, both condensation and re-evaporation of cold vapor is effected by an absorption heat pump, whereby the vapor is condensed into an absorbing liquid and does not form ice. In this case, the process is not very sensitive to the temperature of the vapor, which may be several degrees below zero.
Due to the low operating temperature, it i s possible to use waste heat for the heating of the vapor generator of the absorption heat pump, the temperature of such waste heat not exceeding 50.degree. C. The absorber of the absorption heat pump can thus be cooled with cooling water, the temperature of which may exceed 20.degree. C. The liquid to be frozen can be cooled near to the freezing point in a multistage flash evaporation plant, which is cooled by the discharge liquid melted from ice.
Considering the environmental protection, the process according to the present invention has several advantages; troublesome emissions in dilute solutions can be cleaned by waste energy, whereby not even heat pumps using freon are needed.
The invention is characterized by a method of freezing, in which the ice developed from the treated solution at the triple point is separated from the concentrated solution, which is discharged from the process; the vapor released from the solution is condensed into an absorption medium, which is cooled by a coolant warmer than the condensing vapor; the liquid absorbed from the vapor is re-evaporated at a higher temperature; part of such vapor is further taken to melt the ice separated from the solution and another part is absorbed in an absorber of a second absorption pump, and the pure liquid melted from the ice is taken to cool a multi-stage flash distillation process, where, by means of evaporation, the solution to be treated cools near to the freezing point.
An embodiment of the invention is described more in detail below, by way of example and with reference to the accompanying drawings, in which
FIG. 1 is a flow sheet schematically ill
REFERENCES:
patent: 3344616 (1967-10-01), Owen
patent: 4236382 (1980-12-01), Cheng
patent: 4314455 (1982-02-01), Engdahl
patent: 4735641 (1988-04-01), Engdahl
A. Ahlstrom Corporation
Capossela Ronald C.
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