Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Continuous chemical treatment or continuous charging or...
Reexamination Certificate
1997-04-03
2001-07-17
Nguyen, Dean T. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Continuous chemical treatment or continuous charging or...
C162S038000, C162S042000, C162S045000
Reexamination Certificate
active
06261413
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for continuously guiding liquids when digesting pulp in a digester.
2. Discussion of the Background
In a modern pulp digestion process, several steps are required, thus, among others, an impregnation under particular process conditions, e.g. at a low temperature, and digestion under different conditions, e.g. at a higher temperature, as well as, if desired, one or several washing steps, which are effected by using hot and/or cold washing liquid or washing filtrate, respectively. For the production of pulp, the solvents used in the individual process steps for the dissolving out of certain components must have certain properties, such as, e.g., the type of solvent, whether aqueous or organic, concentrations of acids, alkali and salts, temperatures, etc.
For reasons of costs and last but not least for environmental reasons it is desirable to keep the amount of chemicals and the liquid required in the individual steps as low as possible and to re-use the solvents utilized to the greatest possible extent.
According to WO 91/05103, preconditioned wood chips are preheated in a first chamber by means of circulating black liquor at a low temperature and are continuously fed into a second chamber at high pressure and temperature. In the second chamber, the wood chips are advanced to a cooking temperature by circulating hot black liquor, whereupon they are transferred into the digester together with white liquor.
U.S. Pat. No. 4,693,785 relates to a digester for continuous processing. Supply and discharge of the liquors used are effected via a plurality of annular screen systems.
In U.S. Pat. No. 3,752,319, a strainer for separating liquid from a wood chips-liquid-mixture is described.
SUMMARY OF THE INVENTION
It is the object of the present invention to utilize in a process of the initially defined kind the structural equipment already present and to carry out the digesting procedure as economically as possible, with a minimum of newly added chemicals and fresh water.
According to the invention, this object is achieved in that liquid used in a particular process step can circulate between containers or tanks arranged at the inflow and outflow side of the digester and when this particular process step has been reached, the digester is connected into the circulation between the containers or tanks associated with this particular process step. To carry out the present invention, one pair of containers or tanks is assigned to each desired process step; e.g., to impregnation using an impregnating liquor (IL) an IL
1
and an IL
2
tank, to digestion using a cooking liquor (CL) a CL
1
and a CL
2
tank. The respective solvent used in that process step is then continuously circulated between the respective associated tanks
1
and
2
(or
2
and
1
, respectively). In other words, this corresponds to an internal tank-to-tank circulation. If now, for instance, the solvent present in a tank
1
is needed in a process step in a pulp cooking device (digester), this digester is simply connected into the circulation from tank
1
to tank
2
. Thus, solvent is provided in the respective tank
1
for direct use in this process step, and in tank
2
the spent solvent arriving from the process step is collected. It is also possible that the first container or tank arranged at the inflow side is formed by piping between the second container or tank and the digester. In this case the continuous feeding of chemicals takes place directly into the piping leading into the digester, the liquid flowing in the piping providing a sufficient mixing.
According to a preferred embodiment of the present invention, the used liquid is continuously discharged from the container or tank arranged at the outflow side of the digester and, by means of continuously added doses of chemicals and subsequent continuous temperature adjustment by means of heat exchangers, its original properties are restored, and it is recycled in circulation into the container or tank arranged at the inflow side of the digester. Thus, the demand for chemicals to be newly added can be kept low, since only the amount of chemicals actually spent must be replaced. Moreover, substantial amounts of energy can be saved, since merely an amount of energy corresponding to the thermal loss occurring in the digester has to be supplied. When the respective process step has been finished, the internal tank-to-tank circulation again becomes effective.
According to a preferred embodiment of the present invention a portion of the spent liquid is continuously branched off, at the outflow side, and continuously fed into a second circulation between containers or tanks arranged at the inflow and outflow side of the same and/or a different digester. Thereby it is possible to adjust the volume as well as the content of chemicals in the different circulation with minimal efforts and energy requirement.
Furthermore, it is preferable in the above mentioned case if the missing volume is replaced by continuously feeding fresh chemicals and/or washing solution and/or liquid from a different circulation. In this maimer a cascade-like connection of the different circulations provides for additional savings.
It is also advantageous if the second circulation is associated with the same particular process step as the first circulation. Only the amount of chemicals spent during the particular process step has to be added and the original temperature has to be restored before the liquid can be reused.
However, preferably the second circulation is associated with a different particular process step than the first circulation. In this case it is sometimes not even necessary to add chemicals or to heat the liquid.
Furthermore it is preferred it at the outflow side of the circulation(s) between the containers or tanks a portion of the spent liquid is continuously branched off and continuously fed into a chemical processing plant for the recovery of chemicals, the missing amount of volume being replaced in the container or tank by the addition of washing solution and/or excessive liquid from a different circulation. This prevents an excessive concentration of organic and inorganic components in the circulatory system.
With the present invention, inter alia the following objects are achieved:
irrespective of the characteristics of the filling (such as, e.g., humidity, density, temperature) occurring in the respective process step, the contends of the digester (wood chips) are always subjected to constant solvent conditions;
the solvent conditions can easily be adapted to new requirements;
the energy content of the solvents is not created by heating up in a digester and storing in a tank after that process step, and instead the energy content of the solvents is created tank-internally (by tank-to-tank circulation) and is provided from the tank to the digester contents to heat up the same;
no circulation means whatsoever associated with the digester are required;
all the comsumption values are continuous;
a smaller tank volume is needed;
a shorter digestion period is required;
a uniform pulp quality is attained and can be altered easily; and
a very simple and transparent management control for the modules of the tank farm, quality optimation, energy and sequence can be realized.
Moreover, it is possible with the process of the present invention to provide at the outflow side of the digester for a concentration profile of chemicals increasing with the reaction time. By this unique feature of the process according to the present invention it is possible to have uniform treatment of the content of the digester during the whole reaction time, whereas according to conventional processes the concentration profile of chemicals at the outflow side of the digester is always decreasing. Clearly a concentration profile increasing with the reaction time means that the pulp can be more uniformly treated, impregnated, cooked, etc, since at any time during the reaction a sufficient amount of chemicals is pre
Impco-Voest-Alpine Pulping Technologies GmbH
Nguyen Dean T.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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