| The ALCELL Process for Eliminating Sulfur from Pulping | Canada | 1991 | Full scale |
MANUFACTURE OF PAPER AND PAPER PRODUCTS # 16
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
New technology
Cleaner Production Application
Repap Enterprises started up a mill at Newcastle to demonstrate and test a proprietary organosolv pulping method known as Alcell process. The pulping process consists of the following steps. The key to the process is to sequence pulping in three stages. Preheated chips are packed in the digester using steam, and the steaming is continued to displace air from the chips. Preheated solvent, 50% (weight/weight) denatured ethanol/water mixture, that has been previously used as a wash liquor for two earlier batches is pulped in and rapidly brought up to pulping temperatures 190-200oC, corresponding to an operational pressure of 400-500 psig. The liquor is continuously circulated during the pulping. At the end of this period, the liquor is displaced by wash liquor that was used in the second washing stage of the previous batch. The displaced liquor flows to the lignin and sugar recovery system. The third-stage wash liquor is displaced by fresh liquor and flows into an storage tank for use in the second stage. The second stage-wash liquor is drained into another tank for use in the digester.
At this point, the digester contains soaked, delignified chips and alcohol-water vapor at pulping temperature and pressure. It is depressurized and the departing vapor is condensed for reuse as fresh pulping liquor. Finally, the alcohol remaining in the digester is driven off by steaming. The pulp is diluted with water and pulped out of the digester for cleaning and bleaching treatments. The first-stage spent liquor entering the by-product recovery area is first flashed and then diluted with process water to precipitate the dissolved lignin. After settling, the solid lignin is separated by centrifugation, washed and dried. The filtrate enters a distillation tower, where alcohols and some acetic acid and furfural are recovered. Finally, the remaining aqueous liquor is further evaporated to a sugar syrup.
The facility has produced pulp from individual species as well as mixtures of northern hardwoods, aspen, birch and maple. The pulp has been bleached in the adjacent kraft mill's bleach plant, mostly in combination with softwood kraft, using conventional oxygen bleaching procedures and further bleaching by DED sequences. In general the pulps are more easily bleached than kraft. The physical properties of bleached Alcell and kraft pulps were comparable. The specific bending stiffness of Alcell pulps is improved over kraft pulps. It is an advantage in alkaline papermaking, where significant filler is used in the sheet.
The product lignin can amount to about 18% by weight of the dry wood charge. it is highly hydrophobic; contains no measurable sulfur and very low ash, and as a consequence is quite distinct from either lignosulfonates or kraft thiolignin. It is a uniform product with a number average molecular weight of about 1000 daltons, a softening point of about 145oC, and a glass transition temperature of about 100oC. The dried and bagged product has a moisture content of less than 3%, and a median particle size of 20-40 um.
The by-product furfural, which is recovered as an impure side draw, is upgraded and sold.
Environmental and Economic Benefits
The facility in Newcastle has cost in excess of 95 million CAD (1989 or 1991) to build and operate.
Without a need for a recovery furnace, lime kiln, causticizers, and brownstock washers, the Alcell process has a substantial capital cost advantage over a comparably scaled kraft mill.
Without the recovery furnace and other high-maintenance equipment, an Alcell mill should have lower maintenance and operating labor costs. On the other hand, the recovery and sale of lignin and other by-products creates additional demand for externally generated energy. So the utility and cooking chemical cost is higher.
The process is odorless, is free from TRS (total reduced sulfur) and methyl mercaptans, is free from liquor dregs and lime grits, and produces minimal amounts of knots and rejects. The major effluent, apart from the bleach-plant effluent, is the stillage evaporator condensate, which contains only easily treated BOD (mostly acetic acid, ethanol, and a few other minor organic volatiles). The bleach-plant effluent, as currently anticipated for the 300-metric-ton/day mill, will contain no dioxins or furans, and AOX levels should be below 1 kg/metric ton of pulp. An Alcell mill requires far less water than a kraft mill.
Constraints
Due to volatility of pulping chemicals no leaks and spills can be tolerated, both for environmental and economic reasons.
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.