| Explosion Pulping | Sweden | - | Full scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 57
Background
This case study was submitted on the part of the Working Group on Cleaner Production in Pulp and Paper Industries in the framework of the UNEP IE/PAC Cleaner Production Program with the support of the Technical Research Center of Finland's Non-Waste Technology Research Unit.
Rising electrical power cost has intensified the search for power reduction methods for high-yield pulping. The explosion method of defibering wood, known since the 1920s, has been used mainly for building board manufacture. Renewed interest in this method has been reported for reducing power consumption and enhancing pulp quality.
Cleaner Production Principle
Process modification
Cleaner Production Application
A modification of the explosion process preimpregnates chips with sodium carbonate, whereafter the chips are loaded in the gun and SO2 gas is injected to convert sodium carbonate to sodium sulfite in situ. The released carbon dioxide remains in solution. Upon heating the chips with high pressure steam (50 bar), pressure builds up within the chips due to CO2 formation. The contents of the gun are then blown to a cyclone kept at 5 bar. This sudden expansion of steam and CO2 tears up the chips and produces a coarse fiber. The released steam is condensed in a reboiler to produce 4 bar of fresh steam.
Wood chips were put in an open container and presteamed for 15 min. The container was filled with cold impregnation liquor, and the chips were covered completely. The impregnation time was 10 min and the liquor was a sodium sulfite solution. The chelating agent DTPA was added to the liquor (0,5 % on bone-dry wood). This DTPA (Kelatex 80) contained 1-5 % sodium hydroxide, which resulted in a pH in the impregnation liquor of approximately 11.0.
After impregnation the pressure vessel was filled up with the chips. The amount of the chips in the vessel was 42 kg. The time in the vessel was divided into three sequences: degassing, steaming, and retention time before blowing.
Steam was introduced to heat the raw material and evacuate air. The pressure was raised to 5 bar (160oC) and kept at this temperature for 10 s.
After degassing, the steam pressure was raised to 30 bar instantaneously and maintained at this pressure for 30 s.
In the last sequence the pressure was raised to 50 bar and maintained at this level for 5, 15, or 30 s, depending on the sodium sulfite charges, 3,6%, 8,7%, and 14,8% on bone-dry wood respectively. The valve in the bottom of the vessel was then quickly opened and the chips blown out.
A sample of coarse fiber from each batch was refined in three or four stages to reach different CSF levels. Reference CTMPs were produced from the same raw material (spruce).
To estimate energy consumption, the pulps produced by explosion and refining and a reference pulp were beaten in a PFI-refiner. The pulps were beaten at 10% consistency and then hot disintegrated and tested for freeness (CSF).
Power consumption in a refiner and the PFI-mill revolutions are not exactly comparable. However, in absence of better methods this comparison is made to obtain an idea of the order of magnitude of power consumption. Conventional CTMP requires approximately 1600 kWh/ton when refined to 400 CSF. By explosion pulping, power can be reduced to 300-600 kWh/t.
Pilot-plant tests were conducted. Approximately 300 1 of chips (45 kg) were the raw material to be pulped. These pilot-plant experiments were conducted to verify the feasibility of this process. The process merits further systematic investigation in pilot scale and testing of the pulp in papermaking. Further work is in progress to generate sodium sulfide in situ and more accurately estimate power consumption.
Environmental and Economic Benefits
The sulfite exploded pulp showed superior strength properties at a low sulfur content. Compared with standard CTMP, approximately 25% electrical energy savings are possible.
Constraints
None reported.
Contacts
Review Status
This case study was submitted by the UNEP Working Group on Cleaner Production in the Pulp and Paper Industries, based at the Technical Research Center of Finland (address above) in 1992, as part of a contract for UNEP IE. Before submission, the case studies were reviewed at the Center. They were edited for the ICPIC diskette in June 1997.
Subsequently the case study has undergone another technical review by Dr Prasad Modak at Environmental Management Centre, Mumbai, India, in September 1998.