Concentrating evaporators – Processes – Involving waste liquid
Reexamination Certificate
1998-08-19
2001-07-10
Manoharan, Virginia (Department: 1764)
Concentrating evaporators
Processes
Involving waste liquid
C159S013200, C159S016300, C159S017100, C159S049000, C159SDIG008, C162S029000, C162S046000, C162S047000, C202S173000, C202S174000, C202S202000, C203S072000, C203S078000, C203S079000
Reexamination Certificate
active
06258206
ABSTRACT:
TECHNICAL FIELD
The present invention relates to the evaporation of spent liquor or other waste liquids, such as black liquor, sulphite liquor or bleaching department filtrate, from cellulose cooking, and the process relates to an evaporation process which yields a purer condensate of the evaporated liquid and a second condensate of the evaporated liquid which is more concentrated with regard to impurities.
STATE OF THE ART
Spent liquor from cellulose cooking, so-called black liquor, has a dry substance content of 14-17%. These substances are cooking chemicals which are to be recovered and organic substances, principally lignin residues, which are to be combusted. Direct combustion of black liquor is not possible, due to this low concentration of solid substances; for this reason, the black liquor has to be concentrated by evaporating off water. This evaporation takes place in various devices and results in a concentrated black liquor whose concentration is of the order of 80%. This concentrated black liquor is then combusted in a recovery boiler, with chemicals and heat energy being recovered.
The evaporation normally takes place in a series of appliances which are in principle constructed as heat exchangers. They consist of a cylindrical container of substantial height, for example 9-13 meters, and are provided internally with heat-exchanger tubes or heat-exchanger plates. In a preferred embodiment, the heat exchanger consists of a tube assembly which is placed vertically between two tube plates through which holes are bored directly opposite the tube ends so that liquid, i.e. black liquor, can be caused to flow vertically down inside the tubes, with steam, which is to bring about the evaporation, being allowed to flow and condense on the outside of the tubes. The tube ends are joined to the tube plates such that there is no connection between the inside and outside, respectively, of the tubes. The black liquor consequently forms a falling film inside the tubes and releases a portion of its water content in the form of steam, so-called liquor steam, which is used as a heating medium in other appliances. Such an appliance is termed a falling film evaporator. Normally, several such appliances are coupled in series. While they can be arranged in such a manner that the liquor flows in countercurrent to the steam, co-current devices are also known. In these latter devices, it is customary to have heat exchangers between the evaporation appliances in order to increase the temperature of the liquor.
The water which is evaporated in an evaporation appliance, a so-called effect, is released in steam form and is used for evaporation in another, subsequent, effect containing weaker liquor. Most of the liquor steam condenses in this latter effect and forms a condensate which is more or less impure. The liquor from the cooking department contains volatile compounds, such as methanol, terpenes and sulphur compounds, which are released from the liquor in each evaporation effect. The impurity which occurs to the greatest extent in the condensate from the black liquor evaporation is methanol. Since methanol is more volatile than water, the liquor steam which is released from the weakest liquor will have the highest content of methanol. Condensates which are formed from liquor steam from stronger liquors contain proportionately much lower contents of methanol. If a liquor steam is allowed to condense out in two stages, a purer condensate and a more contaminated condensate are then formed. This is because the steam which does not condense out in the first stage contains a higher content of methanol and other volatile impurities.
According to the current technique, small constituent flows of the most contaminated evaporation condensates are collected together and conveyed, together with condensate from the cooking department, to a stripper column for a purifying treatment. Evaporation condensate which is purer is collected together separately and either reused in the mill as process water or else discharged. Since the condensates from the different effects possess different degrees of purity, it is also customary, according to the current technique, to mix dirty condensates from one effect which yields relatively pure condensates with a pure condensate from an effect which yields condensates which are more contaminated. This mixed condensate of intermediate purity can also be used in the process or be purified in different ways.
TECHNICAL PROBLEM
The demands for a cleaner environment are nowadays becoming gradually more stringent in most countries. The water which is to be released into rivers and the like must be pure, at least sufficiently pure to ensure that no environmental problems are associated with it. It is no longer acceptable to discharge semi-purified condensate. One way of decreasing the contaminants might be to reuse the condensate in the mill as process water instead of discharging it. However, with such a procedure, the problem then arises that various contaminants accumulate, thereby causing interference in the process and giving rise to problems in the working environment.
For this reason, there has long been a great need within the cellulose industry to be able, on the one hand, to purify the condensates from the black-liquor evaporation aggregates so that they become acceptable for discharge or reuse and, on the other hand, to carry out this purification economically without using several, or very large, so-called stripper columns.
SOLUTION
According to the present invention, a process for evaporating spent liquor or other waste liquids from cellulose cooking, so-called black liquor, sulphite liquor or bleaching department filtrate, has therefore been developed in which the evaporation is performed in several evaporators, so-called effects, where aqueous steam which is driven off from one effect is conveyed onwards into the next effect at a low level, and there, by means of condensation, in its turn drives off further steam from the liquor, and in which the liquor can be conveyed from effect to effect in an arbitrary manner, and in which the aqueous steam contains a variety of relatively low boiling point contaminants, which contaminants are enriched in the steam phase when the steam condenses so that a purer fraction of condensate and a dirtier fraction of condensate, which is more enriched in contaminants, can be taken out separately from each other, which process is characterized in that the purer fraction from one effect is conveyed at a high level into another effect on the steam side where the steam which is arriving is purer than was the steam which was supplied to the effect from which the condensate came.
According to the invention, it is expedient for the dirtier fraction to be supplied, at a high level or at the very top, to another effect on the steam side where the steam which is arriving is more contaminated than was the steam which was supplied to the effect from which the condensate came.
According to the invention, it is expedient for the last effect in the direction of movement of the liquor to be operated using live steam.
According to the invention, it is desirable for the process to be carried out until the pure fraction can either be discharged or returned to the process.
According to the invention, it is expedient for the evaporation process to be performed in a so-called falling film evaporator having one or more vertical partitions between the evaporation tubes.
According to the invention, it is expedient for two or more appliances to be coupled together in series.
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Olausson Lars Gunnar
Pettersson Lars Eric
Wennberg Olof Tryggve
Wernqvist Anders Henrik
Fasth Rolf
Fasth Law Offices
Kvaerner Pulping AB
Manoharan Virginia
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