WO1999001606A1

WO1999001606A1 - Method and device for treating steam in a fiber refining process

Info

Publication number: WO1999001606A1
Application number: PCT/FI1998/000568
Authority: WO
Inventors: Pentti Juhola; Arvi Artamo
Original assignee: Rinheat Oy
Current assignee: Rinheat Oy
Priority date: 1997-07-03
Filing date: 1998-07-03
Publication date: 1999-01-14
Anticipated expiration: 2000-01-03
Also published as: FI972847A0

Abstract

According to the disclosed method, steam generated in the defibration of mechanical pulp, which steam is used for transfer of the fibers from the defibrator to a drying process, is separated from the fiber steam whereby its energy content may be utilized. Preferably this heat energy is used to heat the air supplied to the drying process. Simultaneously, the volatile organic compounds contained in the steam are prevented from entering the environment.

Description

METHOD AND DEVICE FOR TREATING STEAM IN A FIBER REFINING PROCESS

Field ofthe invention

The invention relates to a method for the treatment of steam that is generated in the process of defibering mechanically defibered wood pulp, which steam is used for removing the pulp from the defibrator, in order to facilitate the recovery of heat energy in said steam, simultaneously enabling the elimination of environmentally harmful emissions of organic compounds evaporated from the wood during defibration. Particularly, the method is suited for use in the production of pulp for manufacturing medium density fiberboard (MDF), and the recovered energy is used for preheating air for drying. A further objective of the invention is a device for the utilization of said process.

Background of the invention

The wood used as raw material in the production of medium density fiberboard is first defibered into fibers or fiber bundles of desired size. The defibering usually is carried out at a pressure of 5...12 bar and a temperature of 150...180 °C. In the defibering process, the humidity in the wood is vaporized due to the large frictional heat generated, and additionally steam may be fed into the process, on the one hand to soften the lignin and on the other hand for conveying the material in the transfer ducts. Subsequent to the defibrator, a binder resin is added to the fiber stream as well as required additives, whereafter the fiber stream and the steam are blown into the dryer at great velocity. The dryer is usually a so-called pneumatic dryer, wherein the drying takes place by means of hot air, blown through the drying duct and simultaneously carrying the material to be dried through the drying duct. In the dryer, water and other volatile materials are transferred from the fiber to the drying air. The dryer ends in one or several separating cyclones, where the fibers are separated from the drying air, which is directed out of the dryer. The amount of air used for drying fiber is substantial, typically about 20 m³ per kg of dry material. Volatile organic compounds (VOC) evaporating together with the water vapor represent a substantial environmental emission, as they leave the cyclones in their entirety with the exhaust air. Removal of these from air streams of this magnitude would involve investments out of proportion. Disclosure of the invention

General description

A method according to claim 1 has been invented, providing a substantial improvement in the energy economy of a defibering/drying process, simultaneously eliminating the air emis- sions of organic compounds (VOC) originating from the wood raw material. The invention may be practiced using a device according to claim 10. By first directing the stream of fiber/steam from the defibrator to a separation device, which may be a cyclone or another separation device suitable for the purpose, the fibers are separated from the flow of steam. From the separation device, the fibers are directed to the dryer and the steam to a heat exchanging device. The pressure of the separation device is, according to the invention, equal to or preferably somewhat lower than the dryer pressure. According to this definition, the dryer pressure is the pressure in the drying duct essentially at the point of entrance of fibers into the dryer. Preferably, the pressure difference between the separation device and the dryer is adjusted to allow a minor flow of air from the dryer into the separation device, not disturbing the fibers moving by gravity into the drying duct. In a heat exchanging device, the steam is wholly or partly condensed, and the liberated heat is transferred to an appropriate gas or liquid stream, primarily for utilization. Preferably, said heat is used for preheating the flow of drying air fed to the dryer. Together with the steam, acid compounds condense, and are at least partly absorbed into the liquid phase, re- suiting in the condensate being notably acid (pH about 3).

By recycling the condensate and feeding a base, for example sodium hydroxide, into the condensate circuit, the heat exchanging surfaces on the steam side can be washed during operation, simultaneously neutralizing the discharged condensate stream. The noncondensable organic compounds that have entered the heat exchanging device with the water vapor represent a source of VOC emissions. In a method according to the present invention, they may be conveyed from the heat exchanging device to incineration or recovery. Preferably, they are burned in the boiler or hot air generator connected to the dryer. Upstream from the steam-fiber separation device, a split device is preferably installed, providing the option of directing part of the steam/fiber stream from the defibrator directly into the drying duct according to prior art practice. If the entire feed stream is directed in this manner, production may also be carried out without recovery of heat and organic compounds according to the present invention. The separation device may be a cyclone, whereby its efficiency can be controlled by adjusting the back pressure of its upper discharge connection. It may also be another device or apparatus for removal of solids from a gas phase, for example a device based on centrifugal force and provided with rotating parts. Preferably, the separation device is connected to the drying duct without a lock arrangement.

The invention provides for recovery of a very significant amount of the heat of the transfer steam, which would, according to the prior art, leave the process into the surrounding air with the drying gases. In addition, the VOC emissions are eliminated.

Detailed description

The invention is evident from the enclosed drawing, depicting the drying section of a plant producing raw material for medium density fiberboard.

The basic raw material, e.g. wood chips, is fed to the defibering apparatus, in which superheated steam, typically at about 10 bar, is added to the process stream at various stages when required. Defibrator 1 represents the last stage of the defibrating apparatus. Following the defibrator, a split device 2 is provided to enable direction of the fiber/steam flow to startup cyclone 3 during the startup phase, or forward to the drying process. Prior to the dryer, the glue and resin components required for fiberboard production are fed to the process stream as a water emulsion through dosage device 4 into duct 5. Following dosage de- vice 4 is a split device 6, to provide for direction of the fiber/steam flow either directly into the dryer 7, or to a separation device which in the figure is represented by cyclone 8. Cyclone 8 is equipped with a mechanical scraper 9, to ensure that fiber does not stick, clogging the cyclone. The fiber stream separated at the lower part of cyclone 8 is directed into the dryer 7, where the entrance point for the fiber 10 has the form of a venturi to ensure good mixing of the fiber into the drying air.

The steam flow separated from cyclone 8 is directed through duct 11 into heat exchanging equipment 12, in which steam condenses and which is used to preheat the air streams for providing drying air in the dryer 7 and for supporting the combustion in hot air generator 13.

The condensate, which is acid due to raw material components carried over in the steam, exits at the bottom into a wash and neutralization circuit 10, circulated by pump 15. Base is dosed into this circuit from reservoir 16 by means of pump 17, and neutralized condensate leaves the circuit through pipe 18. The solution circulates to rinse the heat transfer surfaces of the heat transfer device. The circuit is further provided with a filter 19. Preferably, sodium hydroxide is used as a base. The noncondensable gases, comprising organic compounds and residual humidity in addition to air, is directed via control valve 20 into the combustion chamber 21 of hot air generator 13. The control valve 20 is used to adjust the steam side pressure of heat recovery device 12 in order to optimize the separation efficiency of cyclone 8. In the optimum state for separation, a minor amount of air flows from the dryer into cyclone 8, counter to the flow of fiber into dryer 7. By thus adjusting the pressure in the separation device relative to the dryer, flow of steam from the separation device into the dryer is inhibited, and no lock arrangement is required between the two said devices.

The flow of preheated air may additionally be controlled by throttling branch duct 22. If the amount of drying air is not sufficient for condensing the whole amount of separated steam, the air flow through the heat exchanger is increased, and the surplus air is expelled through branch 22 according to the figure, or is directed for use as drying air in another drying stage.

The air flow preheated in heat exchanger 8 is directed to hot air generator 13, and the air leaving that unit is used to dry the fiber stream leaving cyclone 8 in drying duct 7. The fibers are separated from the drying air stream in a known manner in product cyclone assembly 23.

Claims

1. A method for the handling of steam generated and added in the mechanical defibration of wood, characterized in that the defibered pulp is directed from the defibrator into a dryer, and the steam used for transferring the fibers from the defibrator is separated from the fibers in a separation device at a pressure essentially equal to or lower than the pressure in the dryer.

2. A method according to claim 1, characterized by that the steam separated from the fi- ber stream is used by means of a heat exchanging device to heat the drying air used to dry the fibers.

3. A method according to claim 1 or 2, characterized by no gas tight lock device being provided between the gas-fiber separation device and the dryer used for drying fiber.

4. A device according to any claim 1 - 3, characterized by that the noncondensable gases contained in the separated steam are directed from the heat exchanging device to incineration or recovery.

5. A device according to any claim 1 - 4, characterized by the separation device being a cyclone.

6. A method according to any claim 1-5, characterized by the function of the separation device being controlled by adjustment of the gas flow leaving the steam side of the heat exchanging device.

7. A method according to any claim I- , characterized by the acid condensate in the heat exchanging device being neutralized with a base.

8. A method according to claim 7, characterized by the that the acid liquor being used for cleansing the heat transfer surfaces.

. A method according to any claim 1-8, characterized by the process being part of a production process for medium density fiberboard (MDF).

10. A device for the handling of steam generated and added in the mechanical defibration of wood, characterized in comprising a fiber dryer and means for separating the steam used for removal of fiber from a defibrator from the fiber stream at a pressure essentially equal to or lower than the pressure in the dryer.

11. A device according to claim 10, characterized by the means for separating the steam from the fiber comprising a cyclone.

12. A device according to claim 10 or 1 1, characterized by comprising a heat exchanging device for heating the drying air used for drying the fibers by means of the steam separated from the fiber stream.

13. A device according to any claim 10-12, characterized by that means have been adapted to the heat exchanging device for neutralizing the acid condensate and for cleansing the heat exchanging surfaces with a base.

14. A device according to any claim 10-13, characterized by that means have been adapted to the heat exchanging device for conveying noncondensable gases to incineration or recovery.