WO2007013851A1

WO2007013851A1 - System and method for treating cellulose pulp in connection with the emptying of a batch digester.

Info

Publication number: WO2007013851A1
Application number: PCT/SE2006/050238
Authority: WO
Inventors: Rickard Andersson; Lars SJÖDIN; Bertil Ernerfeldt; Johan Carlsson
Original assignee: Metso Paper, Inc.
Priority date: 2005-07-29
Filing date: 2006-07-03
Publication date: 2007-02-01
Also published as: SE0501758L; SE528833C2

Abstract

A method and a system for treating a cellulose pulp, where fibers are separated at digestion in one or a plurality of digesters (1), which are emptied after performed digestion stages, and where at least one partial flow of the pulp emptied from the digester (1) passes through a thickener (5) before it moves on to subsequent process stages, and the filtrate valve of the thickener is adjusted so that the desired pulp concentration after the thickener is achieved.

Description

System and method for treating cellulose pulp in connection with the emptying of a batch digester .

This invention relates to a method and a system for treating a cellulose pulp. More precisely, it relates to a method for treating a cellulose pulp after a digestion stage, and a system for carrying out the treatment method. The method and the system are intended to be used as a link in the pulp manufacture of a lignocellulosic material.

At chemical pulp manufacture wood chips are digested in a digestion liquid, which consists of chemicals, which dissolve lignin, in order to separate the cellulose fibers from the lignin. The digestion can be carried out in batches or as a continuous process. At batch digestion the digester is filled with wood chips and digestion liquid and heated. After a certain digestion time the digester is emptied of pulp and waste liquor and is filled again with wood chips and digestion liquid, whereafter a new digestion cycle starts. The emptying of the digester can take place, for example, by means of a pump, which pumps the pulp out of the digester. After the emptying, the pulp often is moved to a drainage tank, which acts as a storage tank between the digesters and subsequent process stages such as, for example, screening and washing stages.

In certain cases one does not manage to empty the digester completely by the pumping method. As a first step one tries to flush out the remaining pulp by the addition of a small amount of liquid, often dilution liquid from later stages in the process. At such an addition of dilution liquid, the pulp concentration drops somewhat. When one can find out that there still remains some pulp (for example by weighing the digester), further measures must be taken. One can carry out still another emptying of the digester, which implies that the digester, at least partially, is filled with new liquid. As a result of this addition of more liquid, the concentration of the pulp suspension is reduced still more. It is well known that all types of variations (in flows, pulp concentration, etc.) should be avoided in order to achieve maximum production capacity in a fiber line. The reason that such variations still occur may often be that the process conditions for some reason change or in some cases that there are known, but unavoidable variations.

One way to avoid problems, which can arise later in the fiber line due to variations in concentration after the digester/ digesters can be to place a balancing tank in connection to the drainage tank. If the concentration gets higher than normal, one can by dilution to the balancing tank correct the variation so that the outgoing concentration from the balancing tank and further in the fiber line always is the same. The problem of this solution, however, is that there is no possibility of solving problems arising due to too low concentrations.

The present invention has the object to solve the problems of concentration variations in both directions, i.e. to have the possibility of both increasing too low a concentration and reducing too high a concentration. A special object of the invention is to solve the problem that too low a concentration develops.

A further object of the invention is to provide a system and a method to handle occasional concentration reductions, which may occur after the digester.

A system according to the invention comprises at least one digester adapted for batch digestion, at least one drainage tank for receiving pulp emptied from the digester after a performed digestion stage and at least one thickener. The thickener is arranged in a position after the digester such that at least a partial flow of the pulp emptied from the digester passes through the thickener before it moves on to one or more subsequent process stages. The thickener is adapted for leveling out variations in concentration in connection with (in association with) emptying of the digester. The thickener is preferably arranged in connection with the drainage tank, that is before or after the drainage tank, and is controlled such that a desired pulp concentration is achieved after the thickener.

According to an exemplary embodiment, the thickener is placed after the digester, so that the pulp suspension is conveyed through the thickener before the pulp arrives at the drainage tank. The thickener may for example be arranged directly after the digester in the draining line from the digester. Alternatively, the thickener can be placed in a branched line, which runs in parallel with the draining line, so that only a partial flow of the pulp emptied from the digester passes through the thickener. At another embodiment the thickener is placed after the drainage tank, but before the screening room.

The thickener can be a combined thickener and screen which in addition of increasing the concentration of the pulp suspension also acts as a knot separator. A suitable type of such a combined thickener is described in the Swedish patent application 0302530- 1. The skilled person recognizes, however, that other types of thickeners, including separate ("non-combined") thickeners possibly preceded by a separate screen, may also be used. The thickener is preferably pressurized.

When the thickener is a combined knot screen and thickener, the knots and large impurities, which could cause problems, are separated. In the cases when a combined thickener/ knot screen is used, the screen room can be designed with fewer stages, because the need of separate knot screens can, at least partially, be eliminated. At high production capacities, however, it can be necessary to design the system with a combination of separate knot screens and combined thickener/ knot screens. Alternatively, the system can be designed with separate knot screens placed before the thickeners which ensure the concentration level after the digester. In cases when the concentration is too high after the digester, dilution as earlier can take place, for example, to the drainage tank, so that subsequent process stages can be run at optimum and uniform concentration. In cases when the concentration is at a normal level, the pulp or a partial flow of the pulp is allowed to pass through the thickener without being thickened. The degree of thickening is preferably controlled by means of one or more valves in the thickener, e.g. by adjusting the filtrate valve. The filtrate from the thickener can be recycled to a tank where displacement liquid to the digester is collected. The filtrate can also directly be recycled to the digester.

With a system according to the invention both the situation with emptying the digester and lower concentration at the end of the emptying, due to the addition of dilution liquid, and the case with an extra emptying and, thus, further addition of liquid be handled. The system, furthermore, can also handle situations where too high concentration occurs after the digester. A further advantage of the system according to the invention is that concentration variations can be dealt with at an early stage.

The invention is described in the following in greater detail with reference to the Figures.

In Figure 1 is shown a process layout of a system according to known art. From a digester 1 a cellulose pulp is moved to a drainage tank 2. Between the drainage tank 2 and subsequent knot screens 3 there is a balancing tank 4 where concentration variations can be dealt with by dilution. The dilution liquid can be fetched, for example, from screening stages located later in the process line.

Figure 2 shows a process layout of a system according to an embodiment of the invention. At the embodiment shown a thickener 5 is placed in direct connection to the digester 1. The illustrated thickener is a combined knot screen/ thickener but embodiments with separate thickeners are also possible. The pulp passing through the thickener is moved to a subsequent drainage tank 2.

The thickener 5 is adapted for levelling out variations in concentration (peaks as well as valleys) in connection with emptying of the digester 1. The thickener 5 is controlled such that a desired pulp concentration is achieved after the thickener. The desired concentration can be determined by adjusting e.g. the filtrate valve of the thickener 5. In this way, the concentration of the pulp suspension from the drainage tank 2 may be kept at a smooth or uniform level. The filtrate from the thickener 5 can possibly be recycled as dilution liquid to the digester 1, as illustrated by stretched line in the Figure.

Figure 3 shows a process layout according to another embodiment of the invention. At the embodiment shown a thickener 5 is arranged after the drainage tank 2. A knot screen 3 is arranged after the digester, before the drainage tank 2. The knot screen ensures that the thickener, which is not necessarily a combined thickener/ knot screen, can act without problems caused by large impurities.

Figure 4 shows an embodiment where a partial flow of the pulp passes through a thickener 5 before the drainage tank 2. Depending on the capacity of the thickener and the desired degree of thickening, several such partial flows with several thickeners may be necessary.

The invention, of course, is not restricted to the embodiments shown, but can be varied within the scope of the claims.

Claims

1. A system for treating a cellulose pulp, comprising at least one digester (1) for batch digestion and at least one drainage tank (2) for receiving pulp emptied from the digester (1) after a performed digestion stage, characterized by a thickener (5) arranged in a position after the digester such that at least a partial flow of the pulp emptied from the digester (1) passes through the thickener (5) before it moves on to a subsequent process stage, the thickener being adapted for levelling out variations in concentration in connection with emptying of the digester (1).

2. A system as defined in claim 1, characterized in that the thickener (5) is arranged in connection with the drainage tank (2), that is before or after the drainage tank (2), and is controlled such that a desired pulp concentration is achieved after the thickener.

3. A system as defined in claim 1 or 2, characterized in that the thickener (5) is arranged in a line running in parallel with the digester draining line, so that only a partial flow of the pulp passes through the thickener.

4. A system as defined in any one of the previous claims, characterized in that the thickener (5) is arranged in a position directly after the digester (1), so that the pulp passes through the thickener before it moves on to the drainage tank (2).

5. A system as defined in any one of claims 1 to 3, characterized in that the thickener (5) is arranged after the drainage tank (2).

6. A system as defined in any one of the previous claims, characterized in that the thickener (5) is pressurized.

7. A system as defined in any one of the previous claims, characterized in that at least one knot screen (3) is placed before the thickener (5) .

8. A system as defined in any one of the previous claims, characterized in that the thickener (5) is a combined thickener and knot screen.

9. A method for treating a cellulose pulp, where fibers are separated at batch digestion in at least one digester (1) and pulp emptied from the digester (1) after a performed digestion stage is received at a drainage tank (2), characterized in that at least a partial flow of the pulp emptied from the digester (1) is conveyed through a thickener (5), before it moves on to a subsequent process stage, and that the thickener is controlled such that variations in concentration in connection with emptying of the digester (1) are levelled out.

10. A method as defined in claim 9, characterized in that the pulp passes the thickener (5) in connection with being brought to or from the drainage tank (2) and that the thickener (5) is controlled such that a desired pulp concentration is achieved after the thickener.

1 1. A method as defined in claim 9 or 10, characterized in that the pulp passes through the thickener (5) directly after the digester (1) without first passing through some other stage.

12. A method as defined in claim 9 or 10, characterized in that the pulp passes through the thickener (5) after passing through the drainage tank (2).