Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With agitation or forced circulation
Patent
1991-11-06
1993-11-30
Jones, W. Gary
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With agitation or forced circulation
96217, 95261, B01D 1900
Patent
active
052661601
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method of and an apparatus for treating pulp, preferably in a closed process. The method according to the invention is particularly well applicable in chemical processes of the wood processing industry for reducing the environmental damages thereof. More specifically, the apparatus according to the present invention is suitable for separation of residual gases remaining in the fiber suspensions of the wood processing industry after a bleaching process. In addition to its main use, which is degassing, a preferred embodiment of the invention can further be employed in the discharge of fiber suspension from a bleaching tower.
PRIOR ART
A number of degassing devices are known for removing residual gases remaining in the fiber suspension after a bleaching stage. U.S. Pat. No. 4,209,359, of 1980, discloses a process of separating residual oxygen from a pulp bleached with oxygen. The separation device according to the patent is quite a large vessel into which the pulp is discharged from a bleaching stage and in which the pulp is treated at the consistency of approx. 3%. The pulp is introduced into the vessel tangentially which subjects the pulp to a centrifugal force promoting separation of gas in a way know per se in such a way that part of the gas can be removed directly from this stage. After that the pulp is allowed to flow to the bottom of the vessel where it is agitated for times of about 30 seconds to 5 minutes with two mixers of different types, the upper one of which is employed to pump the pulp axially downwards and the lower one radially outwards which creates a vortex flow in the pulp which separates residual gas from the pulp.
The drawback of the above apparatus is that the pulp must be diluted to a low consistency only in order to separate the gas. It is a known fact that the most advantageous consistency of pulp for the bleaching is in the range from about 10% to about 12%. After this the bleached pulp is taken either directly or via degassing to a washing plant. If residual gas is not separated from the bleached pulp prior to washing the gas in the pulp will impede the washing and will substantially impair the washing result. If the pulp must be diluted for the degassing process prior to washing, remarkably larger amounts of liquid must be used in the washing than at the original consistency. For example, if the consistency is 3% there is approx. 30 kg free water per 1 kg fiber in the pulp. When the consistency is 12% the amount of free water is only about 5 kg per 1 kg fiber. Thus, if the consistency is quadrupled the amount of free water is one sixth, only, of the amount of free water present in the low consistency. Diluting the pulp thus means that six times the amount of water required by undiluted pulp must unnecessarily be pumped to the washer. Further, the solution of the presented specification comprises several spaces open to the surrounding atmosphere which means that the pulp is not treated in a pressurized closed hydraulic space. FIG. 6 illustrating the process of the patent specification discloses that a bleaching tower 36, a gas separator 10 and a filter 46 are open pressureless devices. These involve contact between air and the pulp and thus problems with foam and smell. The object of the present invention is to eliminate the problems of the apparatus according to the U.S. Pat. No. 4,209,359. In the apparatus of the present invention, the pulp is treated in an airless closed space.
U.S. Pat. No. 4,362,536 discloses a device for removing gas from a pulp flowing in a pipe before the pulp freely drops to a pulp vessel. Gas is separated by introducing the pulp tangentially to a separator in which a rotating rotor increases the rotating speed of the pulp and the centrifugal force separates the gas to the center of the device wherefrom it is removed. Barrier plates are used to prevent the pulp from flowing out with the gas. The rotor has not been designed to raise the pressure of the pulp to be treated as an increase of pressur
REFERENCES:
patent: 3832276 (1974-08-01), Roymoulik et al.
patent: 4209359 (1980-06-01), Sethy
patent: 4410337 (1983-10-01), Gullichsen et al.
patent: 4435193 (1984-03-01), Gullichsen
patent: 4826398 (1989-05-01), Gullichsen
patent: 4886577 (1989-12-01), Wiley
Kurtz, K. D., 1978 TAPPI Engineering Conference (Sep. 1978) pp. 349-357.
Henricson Kaj O.
Niskanen Toivo
Jones W. Gary
Kamyr Inc.
Lamb Brenda
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