SE506808C2 - Pressurised peroxide pulp bleaching with a safety system - Google Patents

Abstract

Safe pressurised peroxide pulp bleaching at least at 8% pref. 10-16% consistency, in a bleaching vessel designed for at least 0.5 pref. 0.7 MPa overpressure, the pulp fed by a pump pref. with fluidised elements and venting, at a temperature greater than 90, pref. greater than 100 and more pref. greater than 105 deg C. The peroxide is over 5 kg/BDMT pref. in association with a bleaching stage which bleaches the pulp to over 75% ISO brightness. If plugging and/or power failure occurs measures are taken, without using mechanical safety valves, to prevent the pressure exceeding a certain limit.

Description

506 808 in .z bleach vessel (1), dimensioned for at least 0.5 Mpa overpressure, preferably 0.7 Mpa overpressure, the pulp being fed to the vessel (1) by means of a pump (2), preferably a pump with organ oxidizing means and venting, and the pulp fed to the vessel has a temperature exceeding 90 ° C, preferably exceeding 100 ° C, more preferably exceeding 105 ° C, and the pulp is bleached with peroxide in an amount exceeding 5 kg / BDMT, preferably in connection with a bleaching step which bleaches the pulp to a brightness exceeding 75% ISO, after which the pulp is extracted by means of a first connection via a line (4), characterized in that during plugging and / or power outages measures are taken, essentially without the use of mechanical safety valves, which prevent the pressure in said ink vessel or connecting part is allowed to exceed a certain setpoint, whereby a valve (C) which is arranged at the discharge end at adjacent vessels (1) opens a second connection with an outlet line (4) from the vessel (1) when the pressure in the vessel (1) exceeds a certain setpoint, preferably about 0.1 MPa above a first setpoint, which valve (C) preferably closes again when the pressure drops back below said setpoint.

A further aspect according to the invention is that said pump (2) is switched off when the pressure in the bleach vessel exceeds a desired first setpoint, preferably approximately 0.55 MPa overpressure, in 0.05 MPa.

A further aspect according to the invention is a bypass line which connects the pump (2) to the bleach vessel (1) and which is opened by means of a valve (H) when the pump (2) starts.

A further aspect according to the invention is that the mass is heated in a mixer (3) arranged between the pump (2) and the bleach vessel (1) and that the steam supply, by means of a valve (B) and also the supply of any other urn fluid, such as oxygen, to The mixture (3) is interrupted when the pressure in the ink vessel exceeds a desired initial setpoint, preferably 0.55 MPa overpressure in 0.05 MPa. A further aspect according to the invention is that a safety valve (A) opens for connection to a lower pressure, preferably atmospheric pressure, for a pipeline running between the valve (B), at the mixer (3), and the valves (E) and (D ) when the pressure in the reactor exceeds a desired setpoint, preferably about 0.05 MPa higher than said first setpoint.

A further aspect according to the invention is that said vessel (1) is arranged with a discharge scraper (5) and the approached valve (C) is arranged, preferably without a distance directly on the vessel (1), so that the scraper (5) cleans in front of this valve (C), thereby eliminating the risk of the formation of a pulp plug.

A further aspect according to the invention is that the distance between the valve cone and the outer edge of the scraper is less than 300 mm, preferably 200 mm, and more preferably 100 mm.

A further aspect according to the invention is that said vessel (1) is equipped with an bursting plate (9) which opens towards lower pressure at a pressure inside the vessel exceeding the approximate initial setpoint, preferably with 0.15 MPa overpressure.

A further aspect according to the invention is that said drain (4) leads to a "stand pipe" (6) which is arranged with a overflow drain (7) which preferably murmurs in an area which is at least partially surrounded by a non-liquid-permeable wall (8). .

BRIEF DESCRIPTION OF THE DRAWINGS As follows, the invention will be explained in more detail with reference to the accompanying urb guras in which: Figure 1 shows a preferred embodiment for arranging a safety system in connection with pressurized peroxide ink vessels, Figure 2 shows a preferred detailed solution of the discharge end of such a vessel.

Detailed description Figure 1 thus shows a preferred embodiment of a system according to the invention. A pressurized peroxide vessel (1) which is hydraulically filled normally operates at a pressure halfway up in the vessel of about 3-5 bar. The pressure is maintained by means of a medium consistency pump (2) which thus feeds the pulp to the bleach vessel (1). Between the pump (2) and the bleach vessel (I) there is a mixer (3) which, in order to increase the pulp temperature, is fed with steam, preferably intermediate pressure steam, so that the temperature of the pulp in the preferred case exceeds 100 ° C. In some cases (e.g. to increase pressure or to prevent so-called “condensate cracks”), it may be desirable to also supply oxygen to the mixture (3). The peroxide is preferably fed to the pulp before or in connection with the pump (2). Due to the high temperature and the high pressure in the reactor, a very efficient bleaching of the pulp is achieved.

The pulp is discharged by means of a scraper (5) (see fi g 2) in the top of the vessel (1) and is conveyed via a line (4) to a so-called stand 'pipe (6) where the pulp is "degassed". The stand spout (6) is further arranged with a overflow drain (7) which opens into an area which is at least partially surrounded by a non-liquid-permeable wall (8).

In order to safely operate this reactor, a number of valves etc. are arranged, of which the most important functions are described in the following. Between the pump (2) and the mixer (3) there is a shut-off valve (G) which is normally open. In a bypass line past the mixer (3) a valve (H) is arranged which is normally closed. In the main line for the supply of steam or oxygen, one (or two) normally open valve (B) is arranged. The side of the valve (B) which is not in contact with the mixer side can be brought into contact with atmospheric pressure by opening the valve (A) which is normally closed. There are also valves (E) and (D) for regulating the fate of steam and oxygen, respectively. A manually shut-off valve (F) is arranged at the bottom of the reactor. At the top of the reactor there is arranged an extra line 10 which connects the reactor top to the outlet pipe (4) when a valve (C) opens. In addition, two pressure sensors (1, PZ) and (2, PZ) are arranged at the top of the reactor. Where appropriate, when this is desired, the reactor at the top is also provided with a "ran b1eck" (9).

According to the preferred embodiment, the reactor is designed for a maximum pressure of 0.7 MPa overpressure at the top at a temperature of 180 ° C. The preferred safety system operates as follows. At a first boom level, 0.55 MPa overpressure, which is then measured by one of the independent pressure sensors, the MC pump is stopped, the valves for steam and possibly oxygen supply (D) and (E), respectively, are closed as well as the valve (B). As a result, no new oxygen or steam can be added to the mixer (3). The valve (B) is equipped with a spring for closing the valve.

At a second child level, 0.6 MPa overpressure, the valve (A) opens, so that the volume in the pipe between the control valves for oxygen and steam and the valve (B) can be ventilated. The valve (A) is equipped with a spring to open.

At a third pressure level, 0.65 MPa overpressure, the valve (C) at the top of the reactor opens 100% and thus connects this extra line 10 to the outlet pipe (4). The valve (C) is arranged with a spring for the opening function.

In connection with a total power failure and in the absence of a backup system, such as air, for example, the safety valve (C) would open and pulp would flow out uncontrollably if no action was taken. To avoid this, the safety valve (C) can be connected to a priority electrical circuit and / or to an auxiliary system, for example an air system. If there is no such 506 808 auxiliary system, this can be connected to an air tank with a non-return valve. Such a tank must be able to hold the volume to ensure at least 10 actuations of the valve (C). The solenoid acting on the safety valve can be driven by the power backup system for the instrumentation.

It is important that the connection line in which the valve (C) is fixed is made as short as possible, in order to avoid pressure drops.

In some cases, as already mentioned, the reactor is provided with a blasted tin / rupture disk which has a rupture value of 0.7 MPa. Preferably, a temperature sensor is installed in the pipe after the rupture disk, by means of which an indication can be obtained that the disk is broken and a signal that stops the pump (2).

According to a preferred embodiment, there is a position sensor which shields if the hand valve (F) is closed, which then switches off the pump (2).

Figure 2 shows that the various valves (discharge control valve 11, emergency valve C and auxiliary valve 12) are arranged so that the discharge scraper (5) cleans in front of them in connection with it rotating. In order to avoid the possibility of pulp plugs being built up, the valves are arranged directly on the vessel. According to a preferred embodiment, the distance between the valve cone and the scraper end should not exceed 200 mm and the outer end of the scraper blade should be designed so that it sweeps past the entire inlet of each opening leading to a valve or the like.

Claims (4)

506 808 Patent claims A method for safely conducting pressurized peroxide bleaching of pulp, at a consistency exceeding 8%, preferably 10-16%, in a bleach vessel (1), dimensioned for at least 0.5 MPa overprint, preferably 0.7 MPa overpressure, wherein the pulp is fed to the vessel (1) by means of a pump (2), preferably a pump with means and aeration, and the pulp fed to the vessel has a temperature exceeding 90 ° C, preferably exceeding 100 ° C, more preferably exceeding 105 ° C, and the pulp is bleached with peroxide in an amount exceeding 5 kg / BDMT, preferably in connection with a bleaching step which bleaches the pulp to a brightness exceeding 75% ISO, after which the pulp is discharged via a first connection via a line (4), characterized in that when plugging and / or power failure measures are taken, substantially without the use of mechanical safety valves, which prevent the pressure in said bleach vessel or connecting part from being allowed to exceed a certain setpoint, wherein a valve (C) provided at the vent end at said vessel (1) opens a second connection with a drain line (4) from the vessel (1) when the pressure in the vessel (1) exceeds a certain setpoint, preferably about 0.1 MPa above a first setpoint, which valve (C) preferably closes again when the pressure drops back below said setpoint. Method according to claim 1, characterized in that said vessel (1) is arranged with a discharge scraper (5) and said valve (C) is arranged, preferably without a distance directly on the vessel (1), so that the scraper (5) cleans from the front this valve (C), thereby eliminating the risk of the formation of a pulp plug. Method according to claim 3, characterized in that the distance between the valve cone and the outer edge of the scraper is less than 300 mm, preferably 200 mm, and more preferably 100 mm. Method according to claim 1, characterized in that said pump (2) is switched off when the pressure in the ink vessel exceeds said first setpoint.