Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With heat recovery
Patent
1984-11-20
1986-06-17
Chin, Peter
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With heat recovery
162 60, D21C 1106
Patent
active
045954569
DESCRIPTION:
BRIEF SUMMARY
This invention relates to a process for the washing of pulp with a maximum recovery of heat energy and chemicals from the digestion liquor.
In the preparation of semi-chemical and chemical pulp, wood chips are digested with a digesting liquor which is selected depending on which type of pulp is intended to be prepared. After digesting, a considerable part of the organic and inorganic components of the digesting liquor remain mixed with the resulting pulp and must be removed therefrom. The pulp and the digesting liquor are separated by washing in a suitable way and the resulting liquor is thereafter mostly evaporated in order to recover the heat values of dissolved organic wood substance and to recover inorganic digesting chemicals for repeated use in the digestion. The washing is also usually carried further than what is justified by an economical recovery of chemicals in order to give the least possible discharges from the pulp mill to the recipient. In most cases, the maximum allowed discharge amounts are stipulated by authorities.
In the washing of pulp, the economical problems in connection with the recovery of chemicals and the environmental problems om minimizing discharge from the plant have so far been given most consideration. The problems of the energy losses arising in the washing process have attracted less attention. With increasing energy costs, however, these problems have become more and more important economically, and it has turned out to be highly desirable to minimize as much as possible the energy losses in the washing process as well as in the other partial processes of the pulp preparation. A total treatment of the whole washing process in this respect has so far been lacking. The present invention eliminates this disadvantage. This invention provides a process for washing of pulp with a maximum recovery of heat. The pulp is washed after the digestion in a plurality of successive washing steps at a superatmospheric pressure in one or more of the steps, and washing filtrate from a following step is used as a liquid for dilution and washing in a given step. The invention is characterized in that
(a) outgoing washing filtrate from a washing step is heat exchanged against diluting and washing liquid going into the subsequent step;
(b) a part of the filtrate from the second washing step is heat exchanged against the filtrate coming from the first step and is then combined with the digestion liquor conveyed to the pulp digestion process;
(c) fresh water is used as the diluting and washing liquid in the last two steps, said water also having been heat exchanged against outgoing washing filtrate from the last and the first washing step;
(d) gases leaving the washing steps are heat exchanged against fresh water which is then used as the diluting and washing liquid in the last step, except the gases from the first step, which are heat exchanged against the cooking liquor to the pulp digestion process, the condensates from the gases resulting from each step being combined with the diluting liquid to the following step except in the steps in which fresh water is used as the diluting liquid;
(e) the uncondensable gaseous fractions resulting from the washing steps are combined after the heat exchange and are finally heat exchanged once more against incoming fresh water, and
(f) the system of pulp coming from the digestion to the first washing step is heat exchanged against the cooking liquor for the pulp digestion process.
By the process of the invention, the heat contents of the pulp and the cooking liquor from the digestion process will be utilized maximally for heating the incoming cooking liquor and the diluting and washing liquids used in each step, inclusive of the supplied fresh water. In this way, diluting and washing liquid of the most suitable liquor strength and temperature will be used in each of the washing steps. Because of this, the efficiency of the washing process will also increase and the dilution factor in washing will be reduced, which further contributes to improving the econ
REFERENCES:
patent: 4154644 (1979-05-01), Ericsson
patent: 4279126 (1981-07-01), Rosenblad
patent: 4310384 (1982-01-01), Meredith et al.
LandOfFree
Pulp washing process does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Pulp washing process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Pulp washing process will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2271961