Liquid purification or separation – Plural distinct separators – Filters
Patent
1996-08-14
1998-06-23
Popovics, Robert
Liquid purification or separation
Plural distinct separators
Filters
2103331, 210340, 162233, 162251, 137607, 137876, 137545, 137266, B01D 2900
Patent
active
057700729
DESCRIPTION:
BRIEF SUMMARY
PRIOR APPLICATION
This is a PCT/SE95/00062 filed on 24 Jan., 1995.
TECHNICAL FIELD
The present invention relates to a valve arrangement for use preferably in a removal system which includes digester screens for digester liquids in cellulose digesters. The valve arrangement can of course also be used in other connections where it is desired intermittently to change the flow direction in the inflow lines to the valve.
STATE OF THE ART
Valves of different types are arranged in all flow systems for liquids. The valves may be intended for throttling the liquid flow, shutting off the same or for changing the flow direction. In connection with, for example, cellulose digesting, there is a sophisticated pipe system including valves for removing liquid from the digester and for supplying the same or new liquid to the digester at another place. This pipe system is extensive and a large number of valves is required in this system for adequate regulation of the liquid flows. The result of digesting itself is greatly dependent on these liquid flows being correct and having the correct temperature. The liquid which is removed and brought back to the digester therefore usually passes through a heat exchanger which gives the correct temperature.
The cellulose digester, which preferably consists of an upright closed cylinder, is designed for a high pressure, for example 9 bar. It is very tall, today in most cases approximately 60 meters, and is at given levels, at least two, provided with screen arrangements for removing digesting liquid. The screens in each screen arrangement are arranged as a girdle around the entire digester and may, for example, number 24 in one girdle. Such screens and removal systems for digesting liquid are described in greater detail in, for example, the American patents 2 695 232 and 4 693 785 and also PCT SE9300311.
TECHNICAL PROBLEM
When a liquid containing solid particles of a given size passes through a screen, the solid particles remain on the front side of the screen while the liquid goes through. Gradually, a great quantity of such particles, for example cellulose in a cellulose digester, can be accumulated on the screen and there has a tendency to stop up the screen so that the liquid flow through the latter decreases or stops. It is therefore necessary to remove this solid material so that the screen is not stopped up or plugged, which normally, in a continuous pulp digester, takes place with the aid of the continuous movement of the chip/pulp plug down through the digester. According to the two above-mentioned American patents, the screens were therefore shut off at one time so that the pulp which lies on top of them is guided further down into the digester, by means of the plug which is formed in the digester. According to U.S. Pat. No. 4,693,785, which reveals a valve with a rotor which determines from which of the many inlet openings removal is to take place, the removal can be made to take place asymmetrically. The incoming digesting liquid is in this case supplied centrally into the cylindrical digester and the flow from the central mouth therefore remains unsymmetrical and can be changed throughout the course of events, which is intended to keep the screen surfaces clean.
Subsequently, the trend was towards using back-flushing in order to keep the screens clean. Another trend is to digest the pulp to a much lower kappa number in the digester, which means that more sizing agents are digested out of the chips, which in turn means that the fibre plug down in the digester becomes more homogeneous and suspension-like. A suspension-like fibre pulp has a. stronger tendency than chips to stop up a screen. This problem is especially accentuated if use is made of the ITC.RTM. digesting method developed by Kvaerner Pulping, in which the pulp is digested to a very low kappa number at: essentially the same temperature level in all the digesting zones of the digester. This method is described in greater detail in our own application SE 9203462 which also indicates that use must be made
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Eriksson Stig
Gronvold-Hansen Christen
Fasth Rolf
Kvaerner Pulping AB
Popovics Robert
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