SCREEN FEED ARRANGEMENT

Abstract

A screen feed arrangement for a pulp production process, comprising a discharge tank having an outlet, which is connected to an inlet of a discharge tank pump having an outlet, a dilution tank having an outlet, a screen feed pump having an inlet and an outlet, and a screening arrangement, wherein the outlet of the discharge tank and the outlet of the dilution tank are connected to the inlet of the screen feed pump, the outlet of which is connected to the screening arrangement, and that pulp which passes the outlet of the discharge tank has a mass concentration from about 8% to about 15%. A corresponding method for feeding pulp to a screen feed arrangement is also presented.

Description

TECHNICAL FIELD

The present invention relates generally to a system for feeding cellulose pulp to a screening arrangement and comprises a screening arrangement, a discharge tank connected to a discharge tank pump, a dilution tank, and a screen feed pump, the outlet of which is connected to the screening arrangement, wherein an outlet of the discharge tank pump and an outlet of the dilution tank are connected to the screen feed pump; and more particularly to a system for feeding cellulose pulp to a screening arrangement, wherein there is essentially no dilution of the pulp in the discharge tank and the pulp coming from the discharge tank has a mass concentration of about 8-15%.

BACKGROUND

A process system for the production of cellulose pulp comprises typically a discharge tank, which during operation is being filled with cellulose pulp coming from one or more digesters. Alternatively, a discharge tank can be filled with pulp which has been processed in other ways, e.g. an oxygen delignification process, wherein the pulp has been exposed to an oxygen stage. The outlet of the discharge tank is connected to a discharge tank pump, which feeds the cellulose pulp from the discharge tank to a screening arrangement, in which the pulp is screened. Most screening arrangements for the screening of cellulose pulp requires a rather low mass concentration of the pulp, and before entering the screening arrangement, the pulp is therefore diluted with liquid from a dilution tank, from an original mass concentration of about 8-15% when coming from the digester, i.e. at the inlet of the discharge tank, to about 3-5% at the outlet of the discharge tank. In most pulp production systems in use today, the discharge tank, which is also referred to as the blow tank or the storage tank or the storage tower, is a large and high container, and the dilution takes place in a bottom portion thereof. Further, to ensure an even and steady mass concentration, agitators are typically arranged in the bottom portion of the discharge tank. A drawback with this arrangement is that the energy consumption of the agitators in the bottom portion of the discharge tank is high. To regulate the mass concentration of the pulp in the discharge tank, from a rather high concentration of about 8-15% to a rather low concentration of about 3-5%, or even lower, with a high degree of accuracy is also a challenging technical problem in a system for the production of cellulose pulp, since regulation based on the mass concentration of pulp leaving a discharge tank is inherently a difficult task, mainly because of the difficulties to ensure an efficient agitation and thereby an even and stable mass concentration. Further, both the high energy consumption of the agitators and the regulation problem are becoming more and more severe with the ongoing development trend towards process systems with higher and higher capacity. In today's systems, it is typically also difficult to design and dimension the capacity of a screen feed pump to the requirements of a screening arrangement.

An object of the present invention is therefore to provide an improved screen feed arrangement, which is more energy efficient. A further object is to provide a screen feed arrangement which is easier to regulate. A still further object is to provide a screen feed arrangement by which it is easier to dimension and match the capacity of a screen feed pump to the requirements of a screening arrangement.

SUMMARY OF THE INVENTION

The above-mentioned objects are achieved with a screen feed arrangement according to the independent claims. Preferred embodiments are set forth in the dependent claims.

1. A system for producing pulp, typically cellulose pulp, comprises a discharge tank, which is arranged downstream one or more digesters and is used to collect pulp from the digester and operates as a buffer tank before the pulp is fed to, e.g., a screening arrangement. Alternatively, the pulp does not need to come immediately from a digester, instead the pulp can have been further processed, e.g. in an oxygen delignification process, such that the pulp, which is fed into a discharge tank, comes from, e.g., an oxygen stage. The pulp coming from the digester (or from another process step) has typically a mass concentration of about 8-15%, whereas the screening arrangement requires a low mass concentration of about 3-5%. In known pulping systems, the necessary dilution of the pulp takes place in a bottom portion of the discharge tank; and a discharge tank pump, which is adapted for the pumping of pulp having a mass concentration of about 3-5%, is arranged at an outlet disposed in the bottom portion of the discharge tank. Typically, there are also agitators provided in this bottom portion of the discharge tank, to agitate the pulp and thereby create an even and stable mass concentration. Contrary to these kind of systems, in a system according to the present invention, there is no dilution of the pulp in the discharge tank, and preferably no agitators are provided in the discharge tank—although scrapers can optionally be arranged in a bottom portion of the discharge tank. Instead the pulp having a mass concentration of about 8-15% is pumped from the discharge tank by a discharge tank pump, which is a medium-consistency pump capable of pumping a pulp mass concentration of about 8-15%. Thus, the discharge tank and in particular the outlet of the discharge tank are configured for discharge of pulp having a mass concentration from about 8% to about 15%. Furthermore, the inlet side of the screen feed pump is also in connection with an outlet of a dilution tank, and in one embodiment of the invention, a three-way pipe is arranged between a dilution tank and a screen feed arrangement, such that the pulp from the discharge tank is introduced into the flow of dilution liquid from the dilution tank to the screen feed pump. The outlet of the screen feed pump is connected to a screening arrangement and feeds diluted pulp, which has a mass concentration of about 3-5% (or even lower), to this screening arrangement.

In one embodiment, the dilution tank is a reject tank, which is used to collect rejects coming from one or several screens which are parts of a screening arrangement. (In known systems used today, such a reject tank is, via a reject tank pump, instead connected to the discharge tank, to dilute the pulp contained therein.)

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be further explained hereinafter by means of non-limiting examples and with reference to the appended drawing, wherein:

FIG. 1 is schematic illustration of a screen feed arrangement according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates schematically a screen feed arrangement or system 1, which is part of a pulping process for the production of pulp, typically cellulose pulp. Further, the screen feed arrangement 1 can be used on many types of pulp; for example, mechanical pulp (e.g. groundwood pulp, stone groundwood pulp, thermomechanical pulp), chemi-mechanical pulp (e.g. chemi-thermomechanical pulp) and semi-chemical pulp (e.g. neutral sulphite semi-chemical pulp). The pulp can also be made from wood raw-materials or one-year plants. The screen feed arrangement 1 comprises a discharge tank 2, which has an inlet 3 and an outlet 4. The inlet 3 of the discharge tank 2 is via a pipe 17 connected to a digester (not shown in the FIGURE), which produces pulp having a so-called medium consistency, which implies a mass concentration in the interval of about 8% to 15%, or about 8% to 13%. As an alternative, pulp, which is fed into the discharge tank, can also come from another type of intermediate process step, such as from an oxygen step. Pulp enters the discharge tank 2 via the inlet 3 arranged in a top portion of the discharge tank 2 and leaves the discharge tank 2 via the outlet 4, which is arranged in a bottom portion of the discharge tank 2. In the embodiment illustrated in FIG. 1, the discharge tank 2 has a tapered, i.e. conical, bottom portion 5, and the outlet 4 is located at the bottom part of the conical bottom portion 5. As should be noted, there are no means, e.g. pipes and valves, provided for dilution of the pulp contained in the discharge tank 2, and no agitators are arranged, which is contrast to the typical design of a discharge tank being part of a pulping process system. To provide a tapered bottom portion of a discharge tank, with an outlet arranged in the very bottom part thereof, is therefore considered to be a preferred embodiment of the present invention. Other shapes of a bottom portion of a discharge tank as well as other positions for an outlet therefrom are, however, within the scope of the present invention. FIG. 1 shows further that pulp leaves the discharge tank 2 via the outlet 4, which is connected to a pipe 18, and enters an inlet 6 of a discharge tank pump 7 and leaves the discharge tank pump 7 via an outlet 8, and is, via a pipe 19, pumped towards a dilution tank 9, which has an inlet 10 and an outlet 11. According to the invention, the discharge tank pump 7 operates at so-called medium consistency, i.e. discharge tank pump 7 is a pump capable of pumping medium-consistency pulp having a medium mass concentration of about 8-15%. Here it should be appreciated that the pumping of a relatively smaller volume of medium-consistency pulp requires less energy than the pumping of a relatively larger volume of low-consistency pulp, e.g. pulp with a mass concentration of about 3-5%. Pumping of low-consistency pulp from a discharge tank is the typical situation when pulp is diluted in the discharge tank and thereafter is pumped to a screening arrangement, i.e. in screen feed arrangements that are not within the scope of the present invention. From another point of view, it may be appreciated that agitators having a high energy consumption by the present invention have been replaced with a medium-consistency pump, which has a relatively low energy consumption. Although not shown in FIG. 1, scrapers can be arranged in the bottom portion of the discharge tank 2, to scrape pulp from a bottom portion of the discharge tank 2 and move the pulp towards the outlet 4. In comparison with fast-rotating agitators, such slow-moving scrapers have a low energy consumption, and the total energy consumption with the present invention is still substantially lower than in a conventional system.

Now returning to FIG. 1, where it is shown that the outlet 11 of the dilution tank 9 is, via a pipe 20, connected to an inlet 12 of a screen feed pump 13. Here, it can be noted that in a preferred embodiment, the dilution tank 9 is directly connected to the inlet 12 of the screen feed pump 13, i.e. that there are no other components or devices arranged in the pipe 20, or at least no active devices, such as a separate pump, arranged in the pipe 20. The screen feed pump 13 has an outlet 14, which, via a pipe 21, is connected to a screening arrangement 15. The screening arrangement 15 comprises a number of screens 16, of which at least the first screen (seen in a downstream direction) typically requires a mass concentration of about 3-5% (or even lower). According to the invention, the dilution of the pulp coming from the discharge tank 2 takes place before the inlet 12 of the screen feed pump 13, and more particularly such that the pulp pumped by the discharge tank pump 7 is fed into the dilution liquid flow from the dilution tank 9. In one embodiment of the invention, the connection between the dilution tank 9 and the screen feed pump 13 comprises a three-way pipe, the third opening of which is connected to the discharge tank pump 7. In a preferred embodiment of the invention, also shown in FIG. 1, the dilution tank 9 is also a reject tank, in which reject, via a pipe 22, coming from one or more of the screens 16 of the screening arrangement 15 is collected. The reject has typically a low mass concentration, e.g. less than 3%, and can therefore be used to dilute the pulp coming from the discharge tank pump 7. It is, however, within the scope of the present invention that the dilution tank is separate tank, or a tank which is connected to other parts of a pulping production system.

A corresponding method for feeding the screen feed arrangement (1) with pulp comprises the steps of feeding pulp into a discharge tank (2) via an inlet (3) arranged in a top portion of the discharge tank (2), pumping pulp from the discharge tank (2) via an outlet (4) arranged in a bottom portion (5) of the discharge tank (2) by a discharge tank pump (7) to an inlet (12) of a screen feed pump (13), diluting the pulp at the inlet (12) of the screen feed pump (13) with fluid from an outlet (11) of a dilution tank (9), feeding the screen feed arrangement (1) with diluted pulp from an outlet (14) of the screen feed pump (13), wherein the method is characterized in that the pulp at the inlet (6) of the discharge tank pump (7) has a mass concentration of about 8% to 15%.

From the description above, it should have been appreciated that there is no, or at least essentially no, dilution, of the pulp in the discharge tank and no agitators are needed in the discharge tank, to produce pulp for feeding into a screening arrangement, which has a low as well as stable mass concentration, which is a difficult technical challenge from regulation point of view. The problem arises mainly from the difficulties in achieving an efficient agitation and thereby complete mixing and dilution of the pulp by means of agitators, and this problem is more pronounced in large discharge tanks. The problem is further more pronounced in systems which require relatively less dilution, i.e. it is easier to achieve good mixing and a stable mass concentration when diluting down to a mass concentration of, e.g., 1-2% than when diluting down to a mass concentration of about 5%. The mass concentration of pulp entering a discharge tank is typically stable, at least seen in a relatively long time perspective, and since the mass concentration prevails also at the exit of the discharge tank when employing the present invention, the regulation has in practice by the present invention been moved or transformed from regulation of mass concentration of pulp leaving the discharge tank to regulation based on flow, which is a less challenging technical task. It should, however, be appreciated that the ultimate goal also for the present invention is a regulation of mass concentration, i.e. regulation from a relatively high mass concentration of pulp in a discharge tank to a relatively low mass concentration before the pulp enters a screening arrangement. As an example, if a certain flow of pulp having a mass concentration of 10% is leaving the discharge tank, it is basically only needed to add the same flow of fluid from a dilution tank, to achieve a mass concentration of 5%. As a comparison, to achieve a mass concentration of 5% by dilution directly in the discharge tank is more demanding because of the non-perfect agitation and dilution, which leads to fluctuations in the mass concentration as measured outside of the outlet of the discharge tank, and which thereby leads to difficulties in regulation of the mass concentration. The omission of agitators in the discharge tank also provides for a relatively lower energy consumption. To summarize, regulation of the amount of pulp from the discharge tank is in the present invention directed to regulation of pulp flow, rather than regulation of mass concentration, which is a favorable situation since the regulation of flow is relatively easier to accomplish than regulation of mass concentration. To achieve the desired mass concentration of the pulp which is to enter the screening arrangement, the main regulation is directed to the flow of dilution liquid coming from the dilution tank. Since the dilution tank typically is smaller, than a discharge tank, the regulation of flow from the dilution tank is typically easier to accomplish than regulation of flow from a discharge tank. Because of the typically smaller dimension of a dilution tank in comparison with a discharge tank, it is further relatively easier to dimension a screen feed pump with respect to the requirements of a screening arrangement, including its pressure classes and tubing between individual screens.

Claims

1.-7. (canceled)

8. A screen feed arrangement for a pulp production process, comprising a discharge tank having an outlet, which is connected to an inlet of a discharge tank pump having an outlet, a screen feed pump having an inlet and an outlet, and a screening arrangement, which outlet of the screen feed pump is connected to the screening arrangement, the outlet of the discharge tank being configured for discharge of pulp having a mass concentration from about 8% to about 15%, characterized by a dilution tank having an outlet, and in that the outlet of the discharge pump and the outlet of the dilution tank are via a three-way pipe connected to the inlet of the screen feed pump.

9. The screen feed arrangement according to claim 8, characterized in that there is no dilution of the pulp in the discharge tank.

10. The screen feed arrangement according to claim 8, characterized in that there are no agitators arranged in the discharge tank.

11. The screen feed arrangement according to claim 8, characterized in that the discharge tank pump is a pump which is capable of pumping pulp having a mass concentration of about 8-15%.

12. The screen feed arrangement according to claim 8, characterized in that the discharge tank has an inlet, which is connected to a digester.

13. The screen feed arrangement according to claim 8, characterized in that the discharge tank has an inlet, which receives pulp coming from a process step provided between a digester and the screen feed arrangement.

14. A method for feeding pulp to a screen feed arrangement, comprising the steps of: feeding pulp into a discharge tank via an inlet arranged in a top portion of the discharge tank, andpumping pulp from the discharge tank via an outlet arranged in a bottom portion of the discharge tank by a discharge tank pump, the pulp at the inlet of 10 the discharge tank pump having a mass concentration of about 8% to 15%, the method being characterized bydiluting the pulp from the discharge tank pump with fluid from an outlet of a dilution tank in a three-way pipe connected to the inlet of a screen feed pump, andfeeding the screen feed arrangement with diluted pulp from an outlet of the screen feed pump.