Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – Gas – vapor or mist contact
Patent
1994-05-02
1996-07-16
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
Gas, vapor or mist contact
162 57, 162 52, 261 86, 366279, 366262, B01F 700, D21C 910, D21C 9153
Patent
active
055363687
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION
The present invention relates to mixing a first medium into a second medium. The present invention especially relates to mixing gas to a medium, but it may as well be applied, for example, for mixing liquids, since mixing of gas has considerably higher requirements than the others both on the mixers and mixing methods. The method and apparatus in accordance with the present invention are especially suitable for mixing gaseous bleaching chemicals, such as oxygen or ozone, used in the bleach plants of the wood processing industry, and for the pulp bleaching process applying the mixing method and apparatus in accordance with the present invention. An excellent application is mixing ozone-containing gas with fiber suspension flowing in a pipe and an ozone bleaching process.
The main objective of the present invention is to develop a method of and an apparatus for mixing large volumes of gas into a medium. Further, since the chemical to be added may be extremely rapidly reacting, such as ozone, said preconditions set great demands on the method and apparatus to be developped.
In most of the modern bleaching plants very often large volumes of gas are desired to be mixed into a medium consistency fiber suspension, which means that the consistency of the fiber suspension is approximately 10-18% and it must be possible to mix a large volume of gas therewith. In other words during the mixing process approximately 40 to 80% of the medium is fiber suspension and approximately 20 to 60% gas, the proportion of the gas most usually being approximately 30 to 50%. It is difficult to have a uniform feed of such a large gas volume and to reach a good mixing result, because gas is separated due to local pressure differences to areas of lower pressure, if possible. The non-uniform mixing results on the increase of chemical loss, which further results in a non-uniform bleaching and in poorer runnability of the process.
The use of above mentioned ozone as a bleaching chemical in bleaching will become more and more popular in the future. There is an ongoing transition from pilot testing to applications in a mill scale, which leads to even higher demands on the apparatus due to the characteristic behavior of ozone. Ozone may be produced and used with the modern technique only in very small proportions, whereby most (usually more than 90%) of the chemical to be mixed with the pulp to be bleached is in fact inert carrier gas compared with the ozone. The result is, of course, that the volume of the gas to be mixed is large. Another significant point is that ozone reacts very rapidly with the material in the fiber suspension. Thus the mixing must be at the same time both very quick, efficient and also uniform in result. Since the ozone immediately reacts with all fibrous material it encounters, the ozone-containing gas may not be allowed to meet only a particular portion of a suspension for a single moment, because it will result in a very uneven bleaching. According to the present technology ozone is not at all a selective chemical and it reacts equally efficiently both with the fibrous material and the lignin to be removed or bleached. In other words, if the ozone dosing for a portion of the suspension is excessive, the ozone quickly causes damage also in the suspension, resulting, of course, in poorer quality of the bleached pulp. Thus the mixing must be very uniform right from the beginning. Due to the non-selectivity the ozone cannot also not be overdosed and also not for the reason that ozone is an expensive chemical.
Ozone may be industrially produced only in relatively dilute mixtures. In other words only 5 to 10% of the gas to be fed for the bleaching, is ozone the rest operating merely as a so called carrier gas. The carrier gas is in most cases either oxygen or nitrogen. Therefore, approximately 10 to 20 times the volume of the ozone carrier gas must be fed and mixed although relatively small volumes of ozone are otherwise sufficient for the bleaching.
Some prior art mixers in the us
REFERENCES:
patent: 2272573 (1939-04-01), Messmore
patent: 2772863 (1956-12-01), Harney et al.
patent: 3048376 (1958-04-01), Howald et al.
patent: 4435085 (1984-03-01), Luthi et al.
patent: 4594152 (1986-06-01), Gullichsen
patent: 5064093 (1978-06-01), Richter
patent: 5279709 (1991-11-01), Niskanen
Makela Mika P.
Niskanen Toivo
Peltonen Kari
A. Ahlstrom Corporation
Czaja Donald E.
Fortuna Jos'e A.
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