Concentrating evaporators – Processes – Involving waste liquid
Patent
1997-06-27
1999-08-31
Manoharan, Virginia
Concentrating evaporators
Processes
Involving waste liquid
159 132, 159 172, 159 201, 159 241, 159 243, 159 274, 159 49, 159DIG8, 162 3011, 162 47, 202174, 203 71, 203 89, 203DIG14, B01D 126, D21C 1110
Patent
active
059449500
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an improved final evaporation of black liquor from cellulose digesting in several, at least two, evaporation appliances of a type which is known per se.
STATE OF THE ART
Spent liquor from cellulose digesting, so-called black liquor, has a dry substance content of 14-17%. These substances are digesting chemicals which are to be recovered and organic substances, principally lignin residues, which are to be combusted. As a result of this low concentration of solid substances, the black liquor cannot be combusted directly and has, therefore, to be concentrated by evaporating off water. This evaporation takes place in different devices and results in a concentrated black liquor with a concentration in the region of 80%. This concentrated black liquor is then combusted in a recovery boiler while recovering chemicals and heat energy.
The evaporation usually takes place in a series of appliances which, in principle, are constructed as heat exchangers. They consist of a cylindrical container of substantial height, for example 8-10 meters, and are provided internally with heat-exchanging tubes or heat-exchanging plates. In a preferred embodiment, the heat exchanger consists of a tube assembly which is vertically placed between two tube plates which are drilled directly in front of the ends of the tubes so that liquid, black liquor, can be brought to flow vertically downwards inside the tubes while steam, which is to bring about the evaporation, is allowed to flow and condense on the outside of the tubes. The ends of the tubes are joined to the tube plates so that there is no communication between the inside and the outside, respectively, of the tubes. The black liquor consequently forms a falling film inside the tubes and gives off some of its water content in the form of steam, which is used for evaporation in other appliances. An appliance of this nature is termed a falling film evaporator. Normally, several such appliances are coupled in series. While they can be arranged so that the liquor flows countercurrent to the steam, concurrent devices are also known. In these latter devices, it is usual to have heat exchangers located between the evaporation appliances in order to increase the temperature of the liquor.
At atmospheric pressure, thin liquor having a dry substance content of 14-17% boils slightly above the boiling point of water (100.degree. C.). However, the boiling point of the black liquor will increase gradually as the liquor is concentrated, and the elevation of the boiling point is, for example, approximately 8.degree. C. for a liquor having a dry substance content of approximately 50%, approximately 12.degree. C. for a liquor having a dry substance content of approximately 60%, and approximately 25.degree. C. for a very concentrated liquor having a dry substance content of approximately 80%. This means that the steam which is fed on the outside of the tubes, and which is to bring about the evaporation, must have a condensation temperature which exceeds the boiling point of the most concentrated liquor, i.e. when the counterpressure of the evaporated steam on the liquor side is, for example, 1 bar (excess pressure) corresponding to a condensation temperature of 120.degree. C., a temperature in the region of 145.degree. C. must be imparted to the liquor film. In those appliances with a lower concentration of dry substance, for example 70% and 60%, the liquor film boils at 136 and 132.degree. C., respectively.
It is previously known to feed steam, primary steam, which is at the same pressure and temperature, to all the appliances involved in the final evaporation effect and to bring together the evaporated steam from all the appliances into one and the same line under the same pressure. In such a case, therefore, the differences in temperature between the primary steam and the boiling points in the different appliances will be different.
In order to minimize the heat surface and to make it possible to wash the final evaporation effect, the latter is
REFERENCES:
patent: 2651356 (1953-08-01), Sadtler
patent: 3763020 (1973-10-01), Drew et al.
patent: 4530737 (1985-07-01), Ostman
patent: 4857146 (1989-08-01), Andersson
patent: 4909899 (1990-03-01), Kiiskila159 2.1
Chemical Recovery In the Alkaline Pulping Process, 1985, Evaporation and Concentration of Black Liquor by Tappi Press, pp. 26-27.
Olausson Lars
Wennberg Olle
Fasth Rolf
Kvaerner Pulping AB
Manoharan Virginia
LandOfFree
Process for the final evaporation of black liquor does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for the final evaporation of black liquor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for the final evaporation of black liquor will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2425676