Concentrating evaporators – Processes – Involving waste liquid
Reexamination Certificate
2001-01-29
2004-11-23
Manoharan, Virginia (Department: 1764)
Concentrating evaporators
Processes
Involving waste liquid
C159S029000, C159SDIG008, C159SDIG001, C159S017100, C202S205000, C202S234000, C202S174000, C202S186000, C203S014000, C203S073000, C203S078000, C203S079000, C203S087000, C162S016000, C162S047000, C162S036000
Reexamination Certificate
active
06821382
ABSTRACT:
TECHNICAL FIELD
The invention relates to a method and a plant for cleaning contaminated condensate emanating from pulp production of semichemical and chemical cellulose pulp, including evaporation of spent liquor.
BACKGROUND OF THE INVENTION
When producing chemical cellulose pulps a lignocellulose material, for instance wood in the form of chips, is dissolved employing either an acid or alkaline process. During a cooking step lignin which forms the middle lamella between wood fibers goes into solution in a cooking liquid, and the fibers after digestion are separated from each other and form a cellulose pulp. Aside from the lignin content of the wood, a considerable part of the hemicellulose in the wood also goes into solution. How much is determined by the pulping degree, which in general is represented as cooking yield in percent. Also a minor part of the wood cellulose content goes into solution. An example of an acid cooking process is the sulphite process and an example of the alkaline cooking process is the sulphate process.
Other known alkaline cooking processes are the polysulfide pulping process and soda type (sodium hydroxide) process, in which catalysts such as quinone compounds can be used. Included within the sulphate process is utilization of high sulfidity pulping to counter current cooking, in which white liquor (primarily a mixture of sodium hydroxide and sodium sulphide) is added during the cooking phase and utilization of a chemical treatment of lignocellulose material, prior to the sulphate pulping process.
The sulphite method can be classified according to the base used in the cooking liquor, as calcium, magnesium, ammonium and sodium. There are sodium and magnesium cooking liquors, which are usually recovered.
After pulping the lignocellulose material, the cooking liquor is separated from the fibers. The spent cooking liquor known as black liquor is referred to as thin liquor in connection with recovery, the main part of thin liquor being water. The dry solid content in thin liquor (lignin, hemicellulose, cellulose, residual chemicals, etc.) is for instance within the range 15-20%. Before combustion of the thin liquor in a recover boiler with combustion of the organic compounds to mainly carbon dioxide and water with recovery of energy and permitting the inorganic compounds to form a residual in the form of a smelt along with recover of the liquor for production of new cooking liquor, the dry solid content must be increased to at least 55%. This liquor with the increased dry solid content is referred to as black liquor or thick liquor. The thick liquor is created by evaporation of thin liquor in five to seven steps. Each step is referred to as a stage.
Modern mills for production of chemical cellulose pulp attempt to reduce fresh water consumption as much as possible and also attempt to reduce the discharge of waste liquor. This may be achieved by closure of the liquid system to a higher or lower degree. This means that spent liquor from bleaching stages must be reused and may be mixed with spent cooking liquor. Thin liquor is therefore sometimes a mixture of spent cooking liquor and spent liquor from different types of bleaching.
During evaporation of thin liquor, condensate is generated. Condensate at some locations may be relatively clean, and can therefore be used at one of several positions in the pulp mill. At other locations contaminated condensate is generated. According to the present invention, it is imperative that heavily contaminated condensate be cleaned. Production of chemical cellulose pulp also generates other types of unclean condensate. Digesting of lignocellulose material is done under pressure such that after cooking a gas mixture is released from the digester and this mixture contains steam as well as organic- and inorganic compounds. During conventional batchwise cooking spent liquor leaves the digester together with generated cellulose pulp. The gas mixture may be condensed to a so-called cooking condensate. Gas mixtures are also collected at other places in the cooking plant and those are condensed, and are also known as cooking condensate and not blow steam condensate. Cooking condensate is generated during batch wise and continuous cooking of lignocellulose material. The contaminated cooking condensate is mixed with unclean evaporation condensate and that mixture may be cleaned according to the present invention. According to the present invention, it is possible to clean the condensate separately, but that is not preferred.
There are several semichemical pulping processes. An example is the neutral sulphite semichemical process, (NSSC). The pulping degree is very low for that type of process; mechanical defibration is therefore necessary for liberation of fibres. In some cases, the pre-treatment liquor or the cooking liquor is recovered. If the recovered liquor is evaporated, the present invention can be used.
For cleaning of unclean condensate according to conventional techniques, at least one stage called stripping is used. Stripping means that the unclean condensate is blown with steam such that volatile compounds in the contaminated condensate follow the steam flow and leave the condensate. According to conventional technique, separate or detached stripper plants are used. The steams used in the stripper plants are admission steams generated in the recovery boiler, or steam from any evaporator stage in the evaporation plant. Further, the stripper plants operate at atmospheric- or over pressure.
In Swedish letters patent 7704352-9 (423915) a method is described for recovery of sulphur compounds, volatile alcohols such as methanol and by-products such as turpentine or similar compounds from contaminated condensate. According to the described method, stripping is performed in two positions, one at the top of a rebuilt evaporation stage resulting in a more expensive evaporation plant and the other in a detached stripper. In the detached stripper, steam from the last evaporation stage is used for stripping of contaminated condensate. The use of the steam for the last evaporation stage provides an economic advantage. However the utilization of steam is limited to 20-25% of the total amount steam from the last evaporation stage. The surplus steam from the last evaporation stage is condensed in a conventional manner by using condensers and heat exchangers. As coolant, normally fresh water is used, resulting in production of warm water with low value.
DETAILED DESCRIPTION OF THE INVENTION
The known technique for cleaning of unclean condensate, as described above, is uneconomical. The reason for that is that stripping is performed in separate or detached plants with the use of high value steam, mainly admission steam.
The present invention solves this problem and relates to a method for cleaning of unclean condensate arising from pulp production of chemical or semichemical pulp. The invention involves evaporation of spent liquor using a cleaning plant having in series several coupled condensers, characterized in that vapour generated at the last evaporation stage, i.e. process steam from the last evaporation stage which has the lowest steam pressure, and unclean condensate, feed a combined stripper and condenser, in which process steam and the unclean condensate flow counter current so that heat exchanging take place between the process steam and unclean condensate. This results in volatile compounds in the condensate being separated from the condensate and following the flow of process steam at the same time as indirect cooling occurs, resulting in condensation of the main part of the process steam. The rest or uncondensed portion of the process steam flows to another condenser, in which the remaining process steam successfully cools indirectly, resulting in water and turpentine condensing and collecting together. Methanol thereafter condenses. The first mentioned collected condensate is separated from the main part of the turpentine content, and is removed from the plant. Residual condensate is fed back to the combined stripper/con
Manoharan Virginia
Papsea AB
Ware Fressola Van der Sluys & Adolphson LLP
LandOfFree
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