Distillation: processes – separatory – With chemical reaction – Including step of adding catalyst or reacting material
Patent
1997-07-09
1999-11-23
Manoharan, Virginia
Distillation: processes, separatory
With chemical reaction
Including step of adding catalyst or reacting material
159 163, 159 473, 162DIG8, 203 35, 203 96, 203 97, 203 98, B01D 334
Patent
active
059893940
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a process for purification treatment of condensate from the production of paper or paper pulp. In this process, the condensate, or other contaminated liquid, undergoes stripping in order to remove impurities and malodorous substances. According to the invention, an acidification takes place in conjunction with the stripping, and this acidification is preferably effected above the point of admission of the condensate, which fact facilitates the driving-off of the undesired impurities.
STATE OF THE ART AND PROBLEMS
Owing to the increasing interest in the environment and our understanding of the ecological cycle which exists in nature, there is a great desire, both among consumers and among producers, to decrease the discharges of pollutants arising from human activities. In recent years, very strenuous efforts have been made to decrease the discharges from our pulp and paper mills, and major advances have indeed been made. The demand of the market for paper which has been bleached without using chlorine has led to alternative bleaching chemicals, such as ozone, persulphuric acid, peracetic acid and hydrogen peroxide, coming into use. A goal for pulp producers is to create "the closed mill", in which discharges are minimized and return of liquid flows to the fibre line, and chemical recovery, are maximized. In order to be able to achieve this, it is advantageous for the bleaching process not to contain elemental chlorine or chlorine dioxide, since, among other reasons, chlorine has a corrosive effect on the equipment and can form toxic compounds if it is combusted in the recovery boiler.
In other respects, returning washing liquid in countercurrent to the fibre stream also places great demands on the purity of the liquid. Some washing stages are more sensitive than others to impurities. For this reason, there is often the need for some form of purification of the different condensates which can be utilized as washing liquid. In this context, a distinction is made between different types of condensate since the latter have different contents of impurities.
When the black liquor is extracted from the digester, the pressure is reduced somewhat and the liquor is flashed, resulting in steam being driven off. The steam is accompanied, for example, by terpenes, methanol and reduced sulphur compounds, which, to a large extent, accompany the steam which condenses after cooling has taken place. In this way, a cooking department condensate is formed from which turpentine is normally separated off in a decanter.
The black liquor is normally concentrated by evaporation in several stages. The units, which are termed effects, are numbered in accordance with the route of steam supply within the plant. Thus, fresh steam is supplied to the first effect and the liquor steam which has been obtained in preceding stages is used as a heat source in the subsequent stages. This is possible owing to the gradual decline in pressure. The heat content which remains in the liquor steam from the final effect is condensed in one or more surface condensers. By means of allowing the liquor steam to condense out step-wise on different heat surfaces in the plant, it is possible to segregate, on the one hand, heavily contaminated condensates, to be purified, in a stripper column, and, on the other hand, very pure condensates which can be used directly in the mill without odour treatment or some other form of purification.
Cooking department condensate and heavily contaminated condensate from the black liquor evaporation is normally conveyed to a steam stripper for purification. A steam stripper usually separates off methanol, ethanol, terpenes and malodorous sulphur compounds (hydrogen sulphide, methylmercaptan, dimethyl sulphide and dimethyl disulphide, etc.) very efficiently. The contaminants which have been separated off are obtained in a concentrated stream which has to be dealt with. It is usually conveyed away and combusted. Condensate which has been treated in a steam stripper often has
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Johansson Mikael
Strom Krister
Wernqvist Anders
Fasth Rolf
Kvaerner Pulping AB
Manoharan Virginia
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