Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With heat recovery
Patent
1997-11-12
1999-10-05
Nguyen, Dean T.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With heat recovery
165113, 165119, 422173, 422185, D21C 1106
Patent
active
059617810
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Within the processing industry, such as the chemical industry and the cellulose industry, it is common practice to deliver the chemicals to the factories in solid form or in the form of highly concentrated aqueous solutions. Prior to use, the concentration of the chemicals usually is lowered by diluting then with water. In for instance pulp mills, large quantities of concentrated sodium hydrate are used and in the industrial plants in such mills large quantities of low-energy vapour are produced, i.e. vapour having a poor heat content. The low-energy vapour usually is utilised to produce hot water. In modern pulp mills, increasingly higher temperatures are requested, particularly in the bleaching plants. No doubt it would be an advantage to be able to utilise low-energy vapour in the preparation of comparatively diluted chemical solutions and at the same time increase their temperature to a level allowing them to be used directly in the production process.
WO 91/18863 discloses a process for the recovery of lower aliphatic acids, such as formic acid, and sugars from the spent cooking liquor from a pulping process carried out with these acids. Spent cooking liquor is evaporated in several steps in a multiple effect evaporator wherein the vapour is returned to previous stages and heats and dilutes the cooking liquor. This publication is silent with ragard to the possibilitty of elevating the boiling points in order to recover maximum thermal energy from the system. Instead, the purpose of the invention described in that publication is to recover acids and sugars.
U.S. Pat. No. 4,755,258 describes multiple effect evaporation of spent liquor, wherein the formed vapour is restored to previous stages and heats the liquor. An advantage is said to be that the dry solids contents of the spent liquor remain nearly unchanged. The plant in accordance with this prior-art publication does not either make use of the boiling-point elevation to recover high-quality energy.
SUMMARY OF THE INVENTION
The primary purpose of the subject invention is to provide a process and a plant for efficient recovery of energy upon dilution of liquids having a considerable elevation of boiling points.
The invention is based on the idea that by condensing vapour in a liquid containing chemicals while simultaneously making use of the condensation heat and the heat of solution generated during the dilution it becomes possible to produce hot water having a higher temperature than the vapour.
Before giving a detail description of the invention some definitions of the terms used herein will be explained.
For instance, the expression "elevation of boiling points" designates the difference in boiling temperatures obtained when boiling an aqueous solution of a chemical and when boiling pure water at identical pressures.
The expression "chemical" is to be given a wide interpretation and relates to water-soluble organic or inorganic compounds and mixtures thereof. As non-limiting preferred examples of such chemicals may be mentioned sodium hydroxide and sulphuric acid.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described in closer detail in the following with reference to the accompanying drawings, wherein:
FIG. 1 is an overall flow chart illustrating one example of application of the invention within the pulp industry, and
FIG. 2 is a schematic view of a preferred plant for performing the process in accordance with the invention, using an absorber of multi-compartment type.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to FIG. 1, vapour is introduced into an absorber 1 comprising a cooler (not illustrated). The vapour has a temperature of approximately 53.degree. C. and is formed by the outflow vapour from an evaporator which is supplied with polluted liquor vapour at a temperature of approximately 58.degree. C. This cleaning step is not necessary if the liquor vapour purity is regarded as acceptable. At the same time, a 50% sodium hydrate solution is introduced into the absorber. The condensate
REFERENCES:
patent: 1003095 (1911-09-01), D'Equevilley-Montjustin
patent: 4343678 (1982-08-01), Franzen
patent: 4363219 (1982-12-01), Koseki et al.
patent: 4431617 (1984-02-01), Farin
patent: 4746437 (1988-05-01), Koseki et al.
patent: 4755258 (1988-07-01), Ryham
patent: 5368096 (1994-11-01), Williams
Tappi Journal, "Reuse of Kraft Mill Secondary Condensates", Jul. 1988, pp. 53-58.
Kirk-Othmer, "Encyclopedia of Chemical Technology", vol. 9, 1980, pp. 85 to 87, John Wiley & Sons.
Nguyen Dean T.
Stora Kopparbergs Bergslags AB
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