OXYGEN GAS BLEACHING OF PULP

Abstract: The present invention relates to a method of oxygen gas delignifying lignocelllulosic pulp material in two stages, where a regulation occurs in that in the first reactor, the pressure in the top of the reactor as well as the level of pulp in the reactor are kept constant with the aid of two regulating valves (1 and 2). The first regulating valve (1) is placed in the top of the reactor and the second regulating valve (2) is placed immediately beneath the level of pulp. The first regulating valve (1) is controlled partly by the gas pressure and partly by the pressure difference measured between a first pressure sensor (3) located in the top of the reactor and a second pressure sensor (4) located in the upper part of the reactor and below the level of pulp. The second regulating valve (2) is controlled by the pressure in the top of the reactor such as to maintain the pressure at a desired level. The major part of the gas is discharged through the first regulating valve (1).

Description: OXYGEN GAS BLEACHING OF PULP The present invention relates to a method concerning the oxygen gas de- gnification of hgnocellulose containing pulp material in two stages In two-stage oxygen gas bleaching processes, the deiignification occurs mainly in the first stage In the second stage, deiignification occurs mainly through alkali extraction and alkaline hydrolysis In one known process, see Swedish Patent Specification SE.C2, 505 141 , all oxygen gas and all alkali is charged to a first stage The solubility of the oxygen gas is enhanced by applying a pressure of about 8-10 bar at the top of the reactor The temperature is increased in the second stage in order to improve extraction whilst, at the same time, lowering the pressure to beneath 5 bar No large quantities of oxygen gas are required for the reactions in the second stage The oxygen gas bleaching process has been carried out hitherto in two upstream reactors, which often requires the provision of a pump between the stages An additional mixer is required, to distribute residual gas and avoid channelisation The pulp is heated by supplying steam between the stages, at a relatively high pressure This means that intermediate pressure steam is required These components and associated piping, etc , complicate the plant and result in unnecessarily high investment and operating costs Endeavours have been made to solve these problems, see SE 9601 160- 6 In the process described in this latter publication, the second upstream flow reactor has been replaced by a downstream flow tower The plant has been simplified, by enabling a pump, a mixer, a feeder, a blow tank and part of the piping to be removed One problem with this process is that residual gas is transferred to the se- cond stage When this residual gas is discharged at the top of the downstream flow tower in the second stage, part of the heat delivered between the stages is also discharged Consequently, it would be necessary to discharge the gas before the pulp is heated, i e away from the first reactor This problem is solved by a process according to EP-0 51 1 433 or WO 93/00476 According to this process, the gas is separated from the pulp in a flui- diser at the top of the first reactor This solution, however, does not simplify the plant, since it is necessary to fluidise the entire pulp flow, therewith requiring a higher energy input A number of different variants of two-stage systems for the oxygen gas deiignification of pulp have been developed In many of the systems, only high- grade medium pressure steam can be used to heat the pulp between the two stages This incurs high operating costs The present invention solves these problems by regulating the pressure and the pulp level in the first reactor in such a way that the pressure in the top of the reactor as well as the pulp level will be kept constant during the continuous bleaching process The present invention thus relates to a method for the oxygen gas deiignification of lignocellulose containing pulp material in two stages, where a regulation occurs in that in the first reactor the pressure in the top of the reactor as well as the level of pulp are both kept constant with the aid of two regulating valves A first regulating valve is placed in the top of the reactor and a second regulating valve is placed level with the lower action radius of the discharge scraper beneath the level of pulp in the upper part of the reactor According to one preferred embodiment of the invention, the pulp is heat- ed with low-pressure steam The above embodiments and other preferred embodiments will be apparent from the depending Claims The invention will now be described in more detail by way of reference to the accompanying drawing showing an example of a plant for carrying out the invented method In the illustrated plant, gas is separated from a first upstream flow reactor under a pressure of 5-15 bar, preferably about 8 bar The gas is separated through a regulating valve, so that the largest possible amount of gas will be discharged at the prevailing pressure The dwell time in the first reactor is 10-60 mm and the temperature below 90°C The majority of the gas in the upper part of the first reactor is discharged with the aid of a first regulating valve 1 placed on the top of the reactor An im- proved heat economy is obtained, by transferring a small amount of gas to a second reactor, preferably a downstream flow reactor, together with the pulp, through a second regulating valve 2 The regulating valve 1 is controlled partly by the gas pressure in the reac- tor and partly by the pressure difference measured between two pressure sensors The pressure in the gas phase is registered by a first pressure sensor 3 placed in the top of the reactor A second pressure sensor 4 placed beneath the pulp level registers the pressure on the pressure sensor level The higher the pulp level, the greater the pressure difference The regulating valve 1 is throttled for gas at increasing pulp levels and is opened at decreasing pulp levels The regulating valve 2 is controlled by the pressure in the top of the first reactor, such as to maintain a high pressure level The valve 2 is placed on a level with the lower action radius of the discharge scraper beneath the level of pulp in the upper part of the reactor The cross-sectional area is greater there than at the top of the reactor, which enhances the departure of gas from the pulp surface The pulp is heated after the regulating valve 2 The pressure drop over the regulating valve 2 enables that low pressure steam can be used instead of high- grade medium pressure steam for the heating of the pulp before the second reactor The pressure drop over the regulating valve 2 also results in a certain turbulent pulp flow so that the admixture of steam is more effective The pressure in the reactor 2 is between 0 and 3 bar, preferably atmospheric pressure, and the temperature is above 85°C The temperature difference between the reactor 1 and the reactor 2 is less than 20°C, preferably less than 15°C The average consistency of the pulp is between 8 and 20%

Claims: CLAIMS 1 . A method of oxygen gas deiignification of lignocellulose containing pulp material in two stages, characterized in that in a first reactor, the pressure at the top of the reactor as well as the level of pulp are kept constant by regulation with the aid of two regulating valves, a first regulating valve (1 ) placed in the top of the reactor and a second regulating valve (2) placed beneath the level of pulp in the upper part of the reactor, where the first regulating valve (1 ) is controlled partly by the gas pressure and partly by the pressure difference measured between a first pressure sensor (3) located in the top of the reactor and a second pressure sensor (4) located in the upper part of the reactor beneath the level of pulp, and where the second regulating valve (2) is controlled by the pressure in the top of the reactor in such a way that the pressure is kept at a desired level, whereby the major part of the gas is discharged through the first regulating valve (1 ). 2. A method according to claim 1 , characterized in that oxygen gas and alkali are charged before the first reactor, the pressure in the first reactor is in the range of 5-15 bar and the dwell time is 10-60 min., and the pulp is transferred to a second reactor where the pressure is in the range of 0-3 bar and the dwell time is 45-180 min. 3. A method according to claim 1 or 2, characterized in that the temperature in the first reactor is below 90°C, above 85°C in the second reactor, and the temperature difference between the two reactors is less than 20°C. 4. A method according to any one of claims 1 to 3, characterized in that the pulp consistency is between 8 and 20%. 5. A method according to any one of claims 1 to 4, characterized in that the second regulating valve (2) is placed before the steam inlet and low pressure steam is used for the heating of the pulp after the first reactor. 6. A method according to any one of claims 1 to 5, characterized in that the second regulating valve (2) is placed so as to generate a certain turbulent flow after the first reactor.

Inventors: AASTROEM PER, BOKSTROEM MONICA, LARSSON RUNE, SOEDERSTEN PER-AAKE, AASTROEM, PER, BOKSTROEM, MONICA, LARSSON, RUNE, SOEDERSTEN, PER-AAKE
Applicants: AASTROEM PER, BOKSTROEM MONICA, LARSSON RUNE, SOEDERSTEN PER AAKE, VALMET FIBERTECH AB, AASTROEM, PER, BOKSTROEM, MONICA, LARSSON, RUNE, SOEDERSTEN, PER-AAKE, VALMET FIBERTECH AKTIEBOLAG
Application year: 1999
Main classification: D21C9
IPC codes: D21C9