Gas separation: processes – Degasification of liquid – By liquid flow modifying or mechanical agitating
Patent
1994-01-14
1995-10-31
Bushey, C. Scott
Gas separation: processes
Degasification of liquid
By liquid flow modifying or mechanical agitating
55399, 55400, 55457, 95270, 96213, 96214, 96217, 162 57, 2105123, B01D 1900
Patent
active
054625858
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for separating gas from a gaseous material preferably in a closed process. The method in accordance with the present invention is especially suitable for chemical processes in the wood processing industry so as to minimize the environmental disadvantages thereof. The apparatus in accordance with the present invention is applicable, for example, for the separation of residual gases of bleaching processes from fiber suspensions of wood processing industry. While the apparatus in accordance with a preferred embodiment of the invention is mainly designed for the discharge of gas, it may also be used for the discharge of fiber suspension bleach towers. The arrangement in accordance with another embodiment of the present invention is preferably applied in the discharge of gas from low consistency pulps, whereby the consistency of the pulp may be even below 5%.
PRIOR ART
There is a number of known gas discharge apparatuses, which have been used for removal of residual gases of a bleaching stage from fiber suspensions. U.S. Pat. No. 4,209,359 discloses a process for separating residual oxygen from pulp which has been bleached with oxygen. The separation apparatus in accordance with said patent is a relatively large vessel, to which the bleached pulp is discharged from a bleaching stage and in which the pulp is treated at a consistency of about 3%. The pulp is supplied to the vessel tangentially, whereby said pulp is subjected to a centrifugal force, which facilitates the separation of gas in a known manner so that a portion of the gas may be discharged directly from this stage. Thereafter the pulp is allowed to flow to the bottom of the vessel, where pulp is mixed for about 30 seconds to 5 minutes with two mixers of different types, of which the upper is used for pumping the pulp axially downwards and the lower is used for pumping the pulp radially outwards, whereby the pulp is brought into a circulating movement, by means of which residual gas is separated from the pulp.
Disadvantages of the disclosed apparatus are, for example, that it is necessary to dilute the pulp to a low consistency merely for the gas discharge and that the process pressure is not utilized in the form of dynamic pressure whereby, when the vessel and the inlet channel are relatively large, the centrifugal force remains small and the gas separation capacity low. As known, bleaching is carried out preferably at a consistency of about 10 to 12%, whereafter the bleached pulp is led to a washing stage either directly or through a gas separator. If residual gas is not separated from the pulp prior to washing, said gas in the pulp complicates the washing and weakens the washing result considerably. A number of washer types are known in the industry, to which pulp may be supplied at a so called MC consistency (medium consistency), whereby also gas should be removed from the pulp at the MC consistency. Washers operating at the MC range are, for example, diffusers, belt washers and so called DD washers. If it is necessary to dilute the pulp prior to the washing for removal of gas, larger amounts of liquid must be pumped to the washing than if the consistency is maintained original. For example, when the consistency is 3%, there is about 30 kg water in the pulp for each fiber kilogram. When the consistency is about 12% the amount of water has decreased to about 5 kg per a kilogram of fibers. Thus, when the consistency quadruples the amount of the water decreases to one sixth of that of the low consistency. In other words, the dilution of pulp results in that, if MC washers are used, the pulp must be thickened again or alternatively low consistency washers must be used, for example, a suction drum filter, whereby the amount of water--in a way eccessive--to be pumped to the washer is sixfold. Moreover, the arrangement in accordance with the disclosed publication has several portions of the apparatus exposed to the atmosphere, whereby the treatment of pulp does not take
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Niskanen Toivo
Peltonen Kari
A. Ahlstrom Corporation
Bushey C. Scott
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