APPARATUS FOR CONTINUOUS PROCESSING OF PULP

Abstract: An apparatus for continuous processing of pulp with an activation chemical in gas phase, preferably nitrogen dioxide, and a gas containing oxygen. The apparatus consists of a gas-tight vessel (1) with gas-tight infeed device (2) at the top and gas-tight discharge device (5) at the bottom. A conduit (13) for the supply of nitrogen dioxide is connected to the top of said vessel, which is divided into an upper portion (8) and a lower portion (9). A conduit (14) with a fan (15) for the recycling of gas from the discharge device (5) is connected to the transition between the upper portion (8) and lower portion (9), where the fan is capable to produce a gradient in the downward flow rate between gas and pulp. A conduit (16) for the supply of oxygen gas is connected to said recycling conduit (14).

Description: Apparatus for continuous processing of pulp This invention relates to the processing of pulp with an activating chemical in gas phase, preferably nitrogen dioxide, and a gas containing oxygen, and it also relates to an apparatus for carrying out said processing. Pulp is processed with nitrogen dioxide and oxygen gas as a pre-treatment prior to alkaline oxygen gas deligni- fication. It was thereby found that the supply of nitrogen dioxide and oxygen gives rise to several different chem¬ ical reactions in the pulp. In order to obtain a positive result from these reactions, the treatment must be divid¬ ed into at least two phases. In the first phase, the nitrogen dioxide shall react with the pulp without the presence of oxygen gas. The second reaction phase, how¬ ever, shall take place in the presence of oxygen gas, which can be supplied as pure oxygen or as a gas contain¬ ing oxygen. This type of treatment is described, for example, in SE-PS with publication numbers 421 38 and 451 023. The conditions required at the treatment are such, that the equipment must meet very great requirements. It is known to use a reactor vessel, to which nitrogen dioxide is supplied at the end where the pulp is supplied, and oxygen gas is supplied at the discharge end. This implies, however, that a common gas space is formed, and the treatment will not be at optimum, see above. In order to separate the two phases, it was necessary to use two separate vessels with a conveying device for the pulp therebetween. It is hereby certainly possible to carry out the treatment in an optimum manner, but the apparatus equipment is complicated and requires much space. The present invention implies that optimum conditions for the processing of pulp can be obtained in a single gas-tight vessel with nitrogen dioxide and oxygen gas. The characterizing features of the invention are defined in the attached claims. The invention is described in greater detail in the following, with reference to the accompanying Figure showing a preferred embodiment of the invention. The apparatus shown comprises a gas-tight vessel 1, which upwardly is provided with a feed device 2 for gas-tight infeed of pulp. The device 2 preferably is a screw 3, through which the pulp is supplied in the form of a gas-tight plug. For breaking and disintegrating the pulp plug at its entrance to the vessel 1, a special device 4 is provided. At the bottom of the vessel 1 a discharge device 5 for gas-tight discharge of the pulp is located. Said device preferably comprises a tank 6 in direct connection with the vessel 1, in which tank 6 a dilut¬ ion zone is maintained with an outlet 7 below the liquid level. The vessel 1 further is divided into an upper portion 8 and a lower portion 9- These two portions may have the same diameter, or the diameter of the lower portion may be greater. The diameter of the lower portion preferably is 10-40.2 and suitably 20-30$ greater than the diameter of the upper portion. The upper portion 8 comprises a mixing zone 10 and a reaction zone 11. The pulp level in the reaction zone is indicated by a level transmitter 12. A conduit 13 for the supply of nitrogen dioxide is connected to the mixing zone 10 at the top of the vessel 1. A conduit 14 for the recycling of gas is connected between the discharge device 5 and vessel 1 in the transition between the upper portion 8 and lower portion 9- Gas is supplied preferably symmetrically about the periphery of the vessel. In the conduit 14 a fan 15 is located for ensuring the gas flow. A conduit 16 for the supply of oxygen gas is connected to said recycling conduit 14. The oxygen gas can be supplied as pure oxygen or as a gas containing oxagen. The vessel 1 extends downward in the tank 6 of the discharge device 5 so that a space 17 is formed above the dilution zone between the outer surface of the wall of vessel 1 and the tank wall 6. The conduit 14 for the recycling of gas is connected to said space 17. At the bottom of vessel 1 an opening 18 extending all about is located, through which the processed pulp is intended to be discharged by means of a rotary scraping device 19 to the surrounding tank 6. Alternatively, the entire bottom can be designed as a rotary conic disc, on which the entire pulp bed is resting. The discharge there takes place at the periphery of the disc. Also other discharge types, of course, can be imagined, for example screw discharge. The tank 6 is arranged to receive the pulp from the vessel 1 and is provided with dilution nozzles 20, which are arranged tangentially in order to cause the pulp to rotate simultaneously with the dilution. The tank can also be formed with a screw-shape passageway direct¬ ed downward to the outlet 7 . A separate after-reaction vessel 21 may possibly be provided after the vessel 1, and a pump 22 be provided to pump the pulp directly from the vessel to said separate vessel 21. The pulp processed in the apparatus according to the invention shall be manufactured chemically, preferably sulphate pulp, but also other chemical pulps can be used. Pulp with a concentration of 20-35$ is supplied to the vessel 1 through the feed device 2 in such a manner, that gas is prevented to leak out. Air at the same time is prevented from following along with the pulp into the vessel 1. The pulp upon its entering the vessel 1 is disinte¬ grated by the device 4 and allowed to fall down through the mixing zone 10 in the upper portion 8 of the vessel 1. There nitrogen dioxide is admixed whic is supplied through the conduit 13 in an amount of 10-20 kg/ton pulp. The level for the pulp bed in the vessel 1 is determined by means of the level transmitter 12. In the reaction zone 11 the reaction between the nitrogen dioxide and pulp starts. In this zone the pulp and gas are moved continuously down at substantially the same speed, i.e. the nitrogen dioxide included in the pulp does not move in relation to the pulp. After the reaction zone 11 in the upper portion 8 of the vessel 1 the pulp enters the lower portion 9 of the vessel. There gas is supplied through the conduit 1 . This gas consists of recycled residue gas, which was separ¬ ated from the pulp in the discharge device 5 , mixed with oxygen gas SΪ which is supplied through the conduit 16 in an amount of 5-10 kg (A per ton pulp. The gas supply through the conduit 14 and the distribution of the gas flow in the entire pulp bedare facilitated when the diameter of the vessel 1 increases in the transition between the upper portion 8 and the lower portion 9. Owing to the gas recycling in the conduit 14, the gas low through the lower portion 9 of the vessel 1 can be controlled. The fan 15 3 thus, shall ensure a gradient in the downward flow rate between gas and pulp, in such a manner, that the gas speed shall exceed the pulp speed through this portion of the vessel 1. The speed gradient shall be sufficiently great to cause such a pressure drop over the pulp bed in the lower vessel portion 9, that a good gas flow distribution in the entire pulp bed is obtained. The pulp is discharged from the bottom of the vessel 1 by means of the rotary scraping device 19 to the surr¬ ounding tank 6. The pulp is there diluted to low con¬ centration, preferably 3-5% t by means of the dilution nozzles 20, which at the same time cause the pulp to rotate along the screw-shaped passageway to the outlet 7. The flow through the outlet is controlled so that a liquid level is maintained in the tank 6 for ensuring a gas-tight discharge. The main part of the gas, which has not reacted with the pulp, goes off in the tank 6. This gas is discharged through the conduit 14 via the space 17, which is so dimensioned that pulp and liquid are not taken along with the gas flow. The processed pulp is thereafter pumped to continued processing in subsequent process steps. An after- reaction however, may possibly be required, which takes place in the separate vessel 21, through which the pulp with the remaining gas content is passed continuously, being given the stay-time required for completing the reaction. The invention, of course, is not restricted to the embodiment shown, but can be varied within the scope of the invention idea.

Claims: Claims 1. An apparatus comprising a gas-tight vessel (1) for continuous processing of pulp with an activation chem¬ ical in gas phase, preferably nitrogen dioxide, and oxygen gas, c h a r a c t e r i z e d b y the combination comprising a) an infeed device (2) at the top of the vessel (1) for gas-tight infeed of the pulp and at the bottom a dis¬ charge device (5) for gas-tight discharge of the proc¬ essed pulp; b) a disintegration device (4) for the pulp at the feed-in; c) a conduit (13) for the supply of nitrogen dioxide to the top of the vessel (1); d) an upper portion (8) of the vessel (1) with a mixing zone (10) and a reaction zone (11) for the pulp and nitrogen dioxide; e) a lower portion (9) of the vessel (1), the diameter of which is equal with or greater than the diameter of the upper portion (8), suitably 10-40$, preferably 20-30$ greater; f) a conduit (14) with a fan (15) for recycling gas from the discharge device (5) to the transition between the upper and lower vessel portion (8 and, respectively,9) _. which fan (15) is arranged to produce a gradient in the downward flow rate between gas and pulp: g) a conduit (16) for the supply of oxygen gas to the lower vessel portion __9)_ . which conduit (16) is conn¬ ected to the As conduit (14) for recycling of gas from the discharge device (5). 2. An apparatus as defined in claim 1, c h a r a c t - e r i z e d i n that the discharge device (5) com¬ prises a tank (6) in direct connection with the lower portion (9) of the vessel (1), which tank (6) has a dilution zone with an outlet (7) below the liquid level. 3. An apparatus as defined in claim 2, c h a r a c t ¬ e r i z e d i n that the tank (6) comprises a space (17) above the dilution zone, which space is subst¬ antially free of pulp, and the conduit (14) for gas recycling is connected to said space (17). 4. An apparatus as defined in claim 2 or 3, c h a r ¬ a c t e r i z e d i n that a scraping device is located at the bottom of the vessel (1) for radial transfer of the pulp to said tank (6). 5. An apparatus as defined in the claims 2 and 3, c h a r a c t e r i z e d i n that a separate after- reaction vessel (21) is located after the vessel (l), and a pump (22) is provided to pump the pulp directly from the vessel (1) to said separate vessel (21).

Inventors: BERG E BERTIL, LINDBLAD PER-OLOV, LINDSTROEM ALF I, SIMONSON OLOF, BERG, E., BERTIL, LINDBLAD, PER-OLOV, LINDSTROEM, ALF I, SIMONSON, OLOF
Applicants: SUNDS DEFIBRATOR IND AB, SUNDS DEFIBRATOR INDUSTRIES AKTIEBOLAG
Application year: 1989
Main classification: D21C9
IPC codes: D21C9