Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With agitation or forced circulation
Patent
1996-04-01
1998-01-27
Czaja, Donald E.
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With agitation or forced circulation
162 65, 162243, 366279, 3663251, 261 85, D21C 322
Patent
active
057118523
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and a device for mixing fluid into a pulp suspension of cellulose-containing fibre material, such as, for example, so-called MC pulp having a dry matter content of 5-25%. The fluid can, for example, consist of ozone-containing gas, the admixture taking place with the aim of bleaching the pulp suspension with one or more bleaching agents which include ozone, supplied together with a carrier gas.
TECHNICAL BACKGROUND AND PROBLEMS
The mixing of, for example, ozone-containing gas into a pulp suspension, during ozone bleaching, usually takes place at a relatively high pressure since the prospects for the bleaching reaction are appreciably improved as the pressure in the reaction sector increases. Therefore, in order to be able to carry out ozone bleaching effectively, both the pulp suspension and the ozone-containing gas must be pressurized.
The apparatus required for pressurizing ozone constitutes by far the largest part of the investment costs associated with ozone delignification of pulp suspensions. The costs of such apparatus increases progressively in relation to increasing pressurization. When generating the gas, large quantities of energy are supplied to the carrier gas, for example pure oxygen, in ozone generators, in association with which a relatively small quantity of ozone is nevertheless formed since the carrier gas can only contain limited quantities of ozone. The total gas flow, which, as a consequence, is very large, is then compressed in so-called liquid ring compressors which are expensive and susceptible to disturbances.
In this context, the problem with currently known devices for bleaching pulp suspensions with ozone is that the reaction pressure which is possible is limited by the capacity of the compressors. In an example taken from a currently existing device, the compressors operate at an excess pressure of 10 bar. The pressure in the mixer may then, in practice, not exceed 7-8 bar excess pressure if blockage of the compressors at the slightest disturbance is to be avoided.
Nowadays, the admixture usually takes place by the pulp suspension being brought into rotation using a rotor surrounded by a coaxial stator, with the gas being supplied in the periphery of the rotating pulp suspension where the counter pressure for the compressors is greatest.
OBJECT OF THE INVENTION
The object of the present invention is to remove the abovementioned problems by developing a process and a device which make it possible to add the fluid, for example in the form of ozone-containing gas, at a lower pressure than the reaction pressure of the device.
TECHNICAL SOLUTION
The abovementioned object is achieved by the invention making available a process and a device for mixing ozone-containing gas into a pulp suspension of a cellulose-containing fibre material in accordance with subsequent patent claims 1 and 11.
The solution proposed by the invention thus implies, in brief, that the fluid is supplied in the vicinity of the centre of the rotating pulp suspension where, owing to the fact that the centrifugal force increases radially outwards, the local pressure in the pulp suspension is lower than the reaction pressure prevailing at the periphery of the pulp suspension.
The solution makes it possible to increase the reaction pressure, which was previously limited by the compressors, while retaining the same compressor. Alternatively, a smaller, appreciably cheaper compressor can be used while maintaining the same reaction pressure in the device.
The invention therefore offers major advantages as compared with previously known technology.
BRIEF DESCRIPTION OF THE DRAWINGS
In that which follows, the invention will be described in more detail with reference to the accompanying drawings in which:
FIG. 1 shows a longitudinal section view of a first embodiment of the invention,
FIG. 2 shows a section view of a second embodiment of the invention,
FIGS. 3 and 4 show examples of flow-affecting elements on the inside of the stator shell,
FIG. 5 shows a third embodim
REFERENCES:
patent: 4339206 (1982-07-01), Ahs
patent: 4577974 (1986-03-01), Prough et al.
patent: 5405497 (1995-04-01), Torregrossa
Ekholm Rolf
Jansson Ulf
Nystrom Per
Czaja Donald E.
Kvaerner Pulping Technologies Aktiebolag
Leavitt Steven B.
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