SE1650410A1

SE1650410A1 - Screen feed arrangement

Info

Publication number: SE1650410A1
Application number: SE1650410A
Authority: SE
Inventors: Engelfeldt Andreas; Hägglund Magnus
Original assignee: Valmet Oy
Priority date: 2016-03-30
Filing date: 2016-03-30
Publication date: 2017-10-01
Also published as: EP3436634A4; WO2017171608A1; EP3436634A1; US20190127912A1; RU2018138008A; CL2018002693A1; BR112018014577A2

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.(Fig. 1 for publication)

Description

SCREEN FEED ARRANGEMENT TECHNICAL FIELD The present invention relates generally to a system for feeding cellulose pulp to ascreening arrangement and comprises a screening arrangement, a discharge tank connectedto a discharge tank pump, a dilution tank, and a screen feed pump, the outlet of which isconnected to the screening arrangement, wherein an outlet of the discharge tank pump andan outlet of the dilution tank are connected to the screen feed pump; and more particularlyto a system for feeding cellulose pulp to a screening arrangement, wherein there isessentially 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 moredigesters. Altematively, a discharge tank can be filled with pulp which has been processedin other ways, e. g. an oxygen delignification process, wherein the pulp has been exposed toan 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, inwhich the pulp is screened. Most screening arrangement for the screening of cellulose pulprequires a rather low mass concentration of the pulp, and before entering the screeningarrangement, the pulp is therefore diluted with liquid from a dilution tank, from an originalmass concentration of about 8-15 % when coming from the digester, i.e. at the inlet of thedischarge tank, to about 3-5 % at the outlet of the discharge tank. In most pulp productionsystems in use today, the discharge tank, which is also referred to as the blow tank or thestorage tank or storage tower, is a large and high container, and the dilution takes place ina bottom portion thereof Further, to ensure an even mass concentration, agitators aretypically arranged in the bottom portion of the discharge tank. A drawback with thisarrangement is that the energy consumption of the agitators in the bottom portion of thedischarge 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 technicalproblem in a system for the production of cellulose pulp, since regulation based on themass concentration of pulp leaving a discharge tank is inherently a difficult task, mainlybecause of the diff1culties to ensure an efficient agitation and thereby an even and stablemass concentration. Further, both the high energy consumption of the agitators and theregulation problem are becoming more and more severe with the ongoing developmenttrend towards process systems with higher and higher capacity. In today°s systems, it istypically 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 feedarrangement, which is more energy efficient. A further object is to provide a screen feedarrangement which is easier to regulate. A still further object is to provide a screen feedarrangement 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.

A system for producing pulp, typically cellulose pulp, comprises a discharge tank, which isarranged downstream one or more digesters and is used to collect pulp from the digesterand operates as a buffer tank before the pulp is fed to, e.g., a screening arrangement.Altematively, the pulp does not need to come immediately from a digester, instead thepulp can have been further processed, e.g. in an oxygen delignification process, such thatthe pulp, which is feed into a discharge tank, comes from, e.g., an oxygen stage. The pulpcoming from the digester (or from another process step) has typically a mass concentrationof about 8-15 %, whereas the screening arrangement requires a low mass concentration ofabout 3-5 %. In known pulping systems, the necessary dilution of the pulp takes place in abottom portion of the discharge tank; and a discharge tank pump, which is adapted for thepumping 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 createan even and stable mass concentration. Contrary to these kind of systems, in a systemaccording 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 canoptionally be arranged in a bottom portion of the discharge tank. Instead the pulp having amass concentration of about 8-15 % is pumped from the discharge tank by a discharge tankpump, Which is a medium-consistency pump capable of pumping a pulp massconcentration of about 8-15 %. The outlet of the discharge tank pump is connected to theinlet side of a screen feed pump. Furthermore, the inlet side of the screen feed pump is alsoin connection With an outlet of a dilution tank, and in one embodiment of the invention, athree-Way pipe is arranged between a dilution tank and a screen feed arrangement, suchthat the pulp from the discharge tank is introduced into the flow of dilution liquid from thedilution tank to the screen feed pump. The outlet of the screen feed pump is connected to ascreening 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 rejectscoming from one or several screens Which are parts of a screening arrangement. (In knownsystems 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 drawings, Wherein: Fig. l is schematic illustration of a screen feed arrangement according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. l illustrates schematically a screen feed arrangement or system l, 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, therrnomechanical pulp), chemi-mechanicalpulp (e.g. chemi-therrnomechanical pulp) and semi-chemical pulp (e.g. neutral sulphitesemi-chemical pulp). The pulp can also be made from wood raw-materials or one-yearplants. The screen feed arrangement 1 comprises a discharge tank 2, which has an inlet 3and 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 % to13 %. As an altemative, pulp, which is fed into the discharge tank, can also come fromanother type of interrnediate process step, such as from an oxygen step. Pulp enters thedischarge tank 2 via the inlet 3 arranged in a top portion of the discharge tank 2 and leavesthe discharge tank 2 via the outlet 4, which is arranged in a bottom portion of the dischargetank 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 conicalbottom portion 5. As should be noted, there are no means, eg. pipes and valves, providedfor 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 processsystem. To provide a tapered bottom portion of a discharge tank, with an outlet arranged inthe very bottom part thereof, is therefore considered to be a preferred embodiment of thepresent invention. Other shapes of a bottom portion of a discharge tank as well as otherpositions 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 isconnected to a pipe 18, and enters an inlet 6 of a discharge tank pump 7 and leaves thedischarge tank pump 7 via an outlet 8, and is, via a pipe 19, pumped towards a dilutiontank 9, which has an inlet 10 and an outlet 11. According to the invention, the dischargetank pump 7 operates at so-called medium consistency, i.e. discharge tank pump 7 is apump capable of pumping medium-consistency pulp having a medium mass concentrationof about 8-15 %. Here it should be appreciated that the pumping of a relatively smallervolume of medium-consistency pulp requires less energy than the pumping of a relativelylarger volume of low-consistency pulp, eg. 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. Fromanother point of view, it may be appreciated that agitators having a high energyconsumption by the present invention have been replaced with a medium-consistencypump, 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 froma bottom portion of the discharge tank 2 and move the pulp towards the outlet 4. Incomparison with fast-rotating agitators, such slow-moving scrapers have a low energyconsumption, and the total energy consumption with the present invention is still substantially lower than in a conventional system.

Now retuming to Fig. 1, where it is shown that the outlet 11 of the dilution tank 9 is, via apipe 20, connected to an inlet 12 of a screen feed pump 13. The screen feed pump 13 hasan outlet 14, which, via a pipe 21, is connected to a screening arrangement 15. Thescreening arrangement 15 comprises a number of screens 16, of which at least the firstscreen (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 thedischarge tank 2 takes place before the inlet 12 of the screen feed pump 13, and moreparticularly such that the pulp pumped by the discharge tank pump 7 is fed into the dilutionliquid ﬂow from the dilution tank 9. In one embodiment of the invention, the connectionbetween the dilution tank 9 and the screen feed pump 13 comprises a three-way pipe, thethird opening of which is connected to the discharge tank pump 7. In a preferredembodiment 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 screeningarrangement 15 is collected. The reject has typically a low mass concentration, e.g. lessthan 3 %, and can therefore be used to dilute the pulp coming from the discharge tankpump 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 comprisesthe steps of feeding pulp into a discharge tank (2) via an inlet (3) arranged in a top portionof 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 screenfeed pump (13) with ﬂuid from an outlet (11) of a dilution tank (9), feeding the screen feedarrangement (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 leastessentially no, dilution, of the pulp in the discharge tank and no agitators are needed in thedischarge tank, to produce pulp for feeding into a screening arrangement, which has a lowas well as stable mass concentration, which is a difficult technical challenge fromregulation point of view. The problem arises mainly from the diff1culties in achieving anefficient agitation and thereby complete mixing and dilution of the pulp by means ofagitators, and this problem is more pronounced in large discharge tanks. The problem isfurther more pronounced in systems which require relatively less dilution, i.e. it is easier toachieve good mixing and a stable mass concentration when diluting down to a massconcentration of, e.g., 1-2 % than when diluting down to a mass concentration of about5 %. The mass concentration of pulp entering a discharge tank is typically stable, at leastseen in a relatively long time perspective, and since the mass concentration prevails also atthe exit of the discharge tank when employing the present invention, the regulation has inpractice been moved from regulation based on measurement of mass concentration of pulpleaving the discharge tank to regulation based on ﬂow, which is a less challengingtechnical task. As an example, if a certain flow of pulp having a mass concentration of10 % is leaving the discharge tank, it is basically only needed to add the same ﬂow of ﬂuidfrom a dilution tank, to achieve a mass concentration of 5 %. As a comparison, to achievea mass concentration of 5 % by dilution directly in the discharge tank is more demandingbecause of the non-perfect agitation and dilution, which leads to ﬂuctuations in the massconcentration as measured outside of the outlet of the discharge tank, and which therebyleads to difficulties in regulation of the mass concentration. The omission of agitators inthe 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 inventiondirected 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 pulpWhich is to enter the screening arrangement, the main regulation is directed to the ﬂoW ofdilution 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 toaccomplish than regulation of ﬂoW from a discharge tank. Because of the typically smallerdimension of a dilution tank in comparison With a discharge tank, it is further relativelyeasier 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. A screen feed arrangement (1) for a pu1p production process, comprising adischarge tank (2) having an out1et (4), Which is connected to an in1et (6) of a dischargetank pump (7) having an out1et (8), a di1ution tank (9) having an out1et (11), a screen feedpump (13) having an in1et (12) and an out1et (14), and a screening arrangement (15),characterized in that the out1et (8) of the discharge tank (2) and the out1et (11) of thedi1ution tank (9) are connected to the in1et (12) of the screen feed pump (13), the out1et(14) of Which is connected to the screening arrangement (15), and that pu1p Which passesthe out1et (4) of the discharge tank (2) has a mass concentration from about 8 % to about15 %.

2. The screen feed arrangement according to c1aim 1, characterized in that there is no di1ution of the pu1p in the discharge tank (2).

3. The screen feed arrangement according to c1aim 1 or c1aim 2, characterized in that there are no agitators arranged in the discharge tank (2).

4. The screen feed arrangement according to any of the preceding c1aims,characterized in that the discharge tank pump (7) is a pump Which is capab1e of pumping pu1p having a mass concentration of about 8-15 %.

5. The screen feed arrangement according to anyone of the c1aims 1-4, characterized in that the discharge tank (2) has an in1et (3), Which receives pu1p coming from a digester.

6. The screen feed arrangement according to anyone of the c1aims 1-4, characterizedin that the discharge tank (2) has an in1et (3), Which receives pu1p coming from a process step provided between a digester and the screen feed arrangement (15).

7. A method for feeding pu1p to a screen feed arrangement 1), comprising the stepsof: feeding pu1p into a discharge tank (2) via an in1et (3) arranged in a top portion ofthe discharge tank (2), pumping pu1p from the discharge tank (2) Via an Outlet (4) arranged in a bottomportion (5) of the discharge tank (2) by a discharge tank pump (7) to an in1et (12) of ascreen feed pump (13), diluting the pu1p at the in1et (12) of the screen feed pump (13) With ﬂuid from anout1et (11) of a di1ution tank (9), and feeding the screen feed arrangement (1) With di1uted pu1p from an out1et (14) of thescreen feed pump (13), the method being characterized in that the pu1p at the in1et (6) of the discharge tank pump (7) has a mass concentration of about 8 % to 15 %.