| The RDH Process - An Extended Delignification Process | Finland | 1985 | Full scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 32
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.
Cleaner Production Principle
Process modification
Cleaner Production Application
In the operating cycle of the RDH system, the batch digester is charged with chips and packed with liquor or steam. The technique increases packing density by up to 10%, thereby increasing pulp production per digester. The digester is then filled with warm liquor of high sulfidity (lox active alkali) at 100oC. The elevated pressure in the digester serves to uniformly impregnate the chips. The warm liquor is displaced with hot, white and black cooking liquors. The digester is heated, and the cook continues to the desired H-factor. At the end of the cook, displacement continues with washer filtrate until the pulp temperature is below boiling point. The displaced liquor is collected in an accumulator. The digester is discharged either with compressed air or by using pumping machine. Special heat exchangers are employed to preheat the white liquor for the next cook to about 155oC.
The pulp made via extended delignification was bleached to 90% ISO using 7.5% active chlorine (11.7% for conventional kraft pulp). Caustic soda consumption decreased from 4.3% to 2.7%. Pulp strength properties were equal to or superior to the conventional kraft pulp.
Unbleached and bleached pulp strength increased 10% at Joutseno. Displacement of the hot spent liquor with washer filtrate is equivalent to a washing stage.
The capacity of the Joutseno Pulp mill is 800 ADt/day of bleached pulp.
The RDH process has been developed by Beloit Corp. and it is in a commercial stage of development. It was introduced to the industry in 1985.
RDH is now on stream at Joutseno, Finland; S.D. Warren's Westbrook mill, Bowater Southern at Calhoun, Tennessee; Fletcher Challenge Canada at Crofton, British Columbia; at Willamette in Bennettsville, South Carolina, and at Chung Hwa in Taiwan, while Potlatch will go on stream with RDH at their mill in Cloquet, Minnesota, in about two years.
Environmental and Economic Benefits
The RDH (Rapid Displacement Heating) process makes possible extended delignification. The RDH process has proven itself not only by cooking stronger, more uniform pulp than conventional systems, but by significantly reducing energy and chemical costs at the same time. The RDH installation reduces the digester steam consumption by 75-85%. Because the RDH cooking process removes more lignin from wood chips than do conventional systems, less chlorine based chemicals are needed in the bleaching phase (when cooking to lower kappa number) to bleach the pulps to 90% ISO brightness than is needed in conventional kraft pulping. Caustic soda consumption is also decreased. The fossil fuel consumption and the blow tank sulfur TRS emissions have been reduced.
| Material Category | QTY Before | QTY After |
| Waste Generation : | N/A | N/A |
| Feedstock Use: | N/A | N/A |
| Water Use: | N/A | N/A |
| Energy Use: | 4 GJ/t | 2 GJ/t |
The rebuilding of an existing cooking department implies a significant investment.
The operational and maintenance costs are slightly higher than in conventional cooking.
There is a payback time of over ten years.
Constraints
When rebuilding existing digesters for the modified cooking process, new screens have to be mounted in the digesters and a number of large pressurized accululators installed.
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 submissions 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.