The present invention relates to a method for pressurized peroxide bleaching and, more specifically, to a method for carrying out pressurized peroxide bleaching safely, i.e. to a method in association with pressurized peroxide bleaching which is intended to eliminate pos¬ sible risks of injury to personnel or of damage of a mechanical nature. Our own patent SE-C-500616 has previously dis¬ closed a method for carrying out pressurized peroxide bleaching of pulp at a consistency exceeding 8 %, in a bleaching vessel designed for overpressure, with the pulp being fed to the vessel by means of a pump and heated to a temperature exceeding 90°C and being bleached with peroxide using a quantity exceeding 5 kg/BDHT.

As the peroxide decomposes, oxygen gas is formed. If the discharge from an above-described bleaching vessel suddenly stops, the pressure in the reactor will increase gradually due to decomposition of the peroxide and the formation of oxygen gas. The risk therefore exists that a bleaching vessel of this type, or surrounding equipment, could be exposed, once the stoppage has been ongoing for a period of time, to a pressure which exceeds its permitted pressure limit.

An object of the present invention is to create a safety system which eliminates the risk of the above- mentioned forbidden pressure limit being reached within the vessel or any part of its surrounding equipment. Due to the nature of the milieu, a fibre-containing suspen¬ sion, such a system cannot be secured using mechanical safety valves since, once such a valve has been used for the first time, fibres will inevitably have become located between the cone and the seat, leading to malfunction.

The object of the present invention is achieved using a method according to Patent Claim 1.

A further aspect according to the invention is that the said pump (2) is shut off when the pressure in

the bleaching vessel exceeds a desired first set point, preferably approximately 0.55 MPa overpressure, ± 0.05 MPa.

A further aspect according to the invention is a bypass conduit which links the pump (2) to the bleaching vessel (1) and which is opened by means of a valve (H) when the pump (2) stops.

A further aspect according to the invention is that the pulp is heated in a mixer (3) arranged between the pump (2) and the bleaching vessel (1) and that the supply of steam, by means of a valve (B) , and also the supply of other possible fluid, such as oxygen gas, to the mixer (3) is interrupted when the pressure in the bleaching vessel exceeds a desired first set point, preferably 0.55 MPa overpressure ± 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 pipe conduit which runs between the valve (B) , at the mixer (3) , and the valves (E) and (D) when the pressure in the reactor exceeds a desired set point, preferably approxi¬ mately 0.05 MPa higher than the said first set point.

A further aspect according to the invention is that a valve (C) , which is arranged at the discharge end of the said vessel (1) , opens a second connection to an outlet pipe (4) from the vessel (1) when the pressure in the vessel (1) exceeds a certain third set point, prefer¬ ably about 0.1 MPa greater than the said first set point, which valve (C) preferably shuts again when the pressure falls back below the said set point.

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

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 the said vessel (1) is equipped with a rupture disc (9) which opens towards lower pressure at a pressure inside the vessel which exceeds the said first set point, preferably by 0.15 MPa overpressure.

A further aspect according to the invention is that the said outlet conduit (4) leads to a standpipe (6) which is arranged with a spillway (7) which preferably opens out in an area which is at least in part enclosed by a wall (8) which is impervious to liquid.

Brief description of the figures The invention will be elucidated in more detail below with reference to the attached figures in which: Figure 1 shows a preferred embodiment for arranging a safety system in association with a pressurized peroxide bleaching vessel, and Figure 2 shows a preferred detailed solution for the discharge end of such a vessel.

Detailed description

Figure 1 accordingly shows a preferred embodiment of a system according to the invention. A pressurized peroxide vessel (1) , which is filled hydraulically, normally operates at a pressure, half-way up the vessel, of about 3-5 bar. The pressure is maintained with the aid of a medium-consistency pump (2) which thus feeds the pulp to the bleaching vessel (1) . Between the pump (2) and the bleaching vessel (1) there is a mixer (3) which, in order to raise the temperature of the pulp, is fed with steam, preferably medium-pressure steam, so that the temperature of the pulp in the preferred case exceeds 100°C. In certain cases (for example, in order to increase the pressure or to prevent so-called "condensate bangs"), it can be desirable also to supply oxygen gas to the mixer (3) . The peroxide is preferably supplied to the pulp either prior to or at the pump (2) . Very effective

bleaching of the pulp is achieved due to the high temperature and the high pressure in the reactor.

The pulp is discharged, using a scraper (5) (see Fig. 2), from the top of the vessel (1) and is conveyed via a conduit (4) to a so-called standpipe (6) in which the pulp is "degassed". The standpipe (6) is additionally arranged with a spillway (7) which opens in an area which is at least in part enclosed by a wall (8) which is impervious to liquid. In order to be able to operate this reactor safely, there are arranged a number of valves etc., the most important functions of which are given below. Between the pump (2) and the mixer (3) there is a shut- off valve (G) which is normally open. A valve (H) , which is normally closed, is arranged in a bypass conduit which circumvents the mixer (3). One (or two) valve(s) (B) , which is/are normally open, is/are arranged in the main conduit for supplying steam and oxygen gas. That side of the valve (B) which is not in contact with the mixer side can be brought into contact with atmospheric pressure by opening valve (A) , which is normally closed. In addition, valves (E) and (D) are present for regulating the flow of steam and of oxygen gas, respectively. A valve (F) , which can be shut off manually, is arranged at the bottom of the reactor. An additional conduit (10) is arranged at the top of the reactor, which conduit links the top of the reactor with 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. In cases where it is desired, a "rupture disc" (9) is also arranged at the top of the reactor.

According to the preferred embodiment, the reactor is constructed for a maximum pressure of 0.7 MPa overpressure at the top at a temperature of 180°C. The preferred safety system functions as follows. At a first set point, 0.55 MPa overpressure, which is thus then measured by one of the independent pressure sensors, the MC pump is stopped, and the valves for the supply of steam and, where appropriate, oxygen gas, (E) and (D) ,

respectively, are closed, as is the valve (B) as well. This therefore ensures that no fresh oxygen or any fresh steam can be supplied to the mixer (3) . The valve (B) is equipped with a spring for closing the valve. At a second set point, 0.6 MPa overpressure, the valve (A) opens so that the volume in the pipe between the regulating valves for oxygen gas and steam and the valve (B) can be ventilated. The valve (A) is equipped with a spring in order to open. At a third pressure level, 0.65 MPa overpressure, the valve (C) at the top of the reactor opens fully, thereby connecting this additional conduit (10) to the outlet pipe (4) . The valve (C) is arranged with a spring for the opening function. If the electricity supply were completely cut off, and if there were no reserve system, such as, for example, air, the safety valve (C) would open and pulp would flow out in an unregulated manner if no preventive measures were taken. In order to avoid this happening, the safety valve (C) can be connected to a prioritized electrical circuit and/or to an auxiliary system, for example an air system. If there is no such auxiliary system, the valve can be connected to an air tank having a nonreturn valve. This tank must be able to accommodate the volume which is required for ensuring at least ten actuations of the valve (C) . The solenoid which acts on the safety valve can be operated by the power back-up system for the instrumentation.

It is important that the connecting conduit in which the valve (C) is located is made as short as possible in order to avoid a drop in pressure.

In certain cases, as has already been mentioned, the reactor is arranged with a rupture disc, which expediently has a rupture value of 0.7 MPa. A temperature sensor is preferably installed in the pipe downstream of the rupture disc, which sensor can be used to provide an indication that the disc is ruptured and a signal which stops the pump (2) .

According to a preferred embodiment, a position sensor is present which senses whether the manual valve (F) is being shut and which then shuts off the pump (2) .

Figure 2 shows that the different valves (the outflow control valve 11, the emergency valve C and the additional flange 12) are arranged so that the discharge scraper (5) cleans in front of these valves as it rotates. With a view to avoiding the possibility, of pulp plugs building up, the valves are arranged directly on the vessel. According to a preferred embodiment, the distance between valve cone and scraper end must not exceed 200 mm and the outer edge of the scraper blade should be shaped so that it sweeps past the whole of the inlet to each opening which leads to a valve or the like.