Chip Quality Index
This is an abstract of the dissertation, Economic and Physical Bases of a Chip Quality Index for Kraft Pulping of Wood by James Robert Ludden, University of Washington, 1977.
Mr Ludden began his study of forestry and economics in April 1975. The research and analysis for this work was completed during 1976 and the first draft written during Lent 1977 and degree conferred in August 1977.
The quality of wood chips used for kraft pulp manufacture is not now well controlled. Variable wood quality causes variations in pulp yield, pulp quality, and production costs. Wood pulp, the desired product, is largely cellulose.
James R Ludden developed a refined model for chemical pulping of wood based on physical and chemical properties of the wood chips that are the primary input to this process. Variables include chemical composition, chip thickness, fiber length, and chip length. Trees are reduced from log to particles by machine processes (often in a sawmill) that controls the particle shape. This varies from saw teeth to planers to dedicated chipping machinery. These raw materials (collectively called ‘chips’) may be kept separate or blended, depending on the pulp mill. The chips are then ‘cooked’ with sodium hydroxide (lye) under high temperature and pressure in a ‘digester’, dissolving the lignin and hemicellulose and reducing the length of the fibers as an adverse side effect. Chemical pulping requires removing the lignin that binds cellulose fibers. Different species of wood, growing under different conditions, contain differing ratios of lignin to cellulose and hemicellulose.
If the chips are too thick, it takes longer for the chemicals to reach the center. Wood chips cook from the outside in towards the center. If the lignin content of the interior of a chip is too high after pulping, the fibers from the center will not separate into pulp.
If the chips are too short, too many of the cellulose fibers are cut and the average fiber length reduced, making the pulp weaker. If the cooking lasts too long, the fibers are shortened by the harsh chemicals. These variables are under the control of the pulp mill.
Different species of tree produce wood of different fiber length (conifers have longer cellulose fibers than hardwoods, for instance). In addition, the ratio of cellulose to lignin and hemicellulose (a form of cellulose that is easily dissolved by the harsh cooking chemicals) varies considerably with tree species and growing conditions. The main stem and branches of the same tree vary widely in composition. Hardwoods differ strongly from softwoods and from other species of hardwood in the chemical composition (and solubility) of their lignin and hemicellulose.
Wood chips usually arrive at the pulp mill in trailers or rail cars, with little or no documentation as to their species composition or growing conditions. To avoid paying rent (‘demurrage’) on the vehicle, the chips are dumped into large piles, further reducing the information about the source. As the chips are stored, they degrade - eventually rotting to the point that they are useless for producing pulp.
Models of the cooking process adequately incorporate the time, chemical concentration, and temperature to determine the amount of lignin removal. But previous to 1977 none of the digester models treated the variable nature of the fiber raw material. The size (and shape) of the chip affects the amount of time, energy, and chemicals required as well as the quality of the resulting pulp.
Insufficiently cooked chips (or interior portions of those chips) do not make good pulp and are removed by screening. Thus long, thin wood chips yield the best quality pulp with the least energy input.