Production Scheduling for Pulp Mills

Norm Laslett; presented at PulPaper2004 in Helsinki, June 2004.

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ABSTRACT

In capital-intensive industries such as pulp and paper, maximum utilisation of assets is essential. The design capacities of production units in a pulp mill/recovery complex are often matched such that under steady state conditions, the whole mill can run at maximum. However, because of the dynamic nature of breakdowns, restrictions and varying demands, the steady state scenario is rarely achieved.

Breakdowns of equipment, or constraints in other interdependent areas, often reduce the utilisation of key production units. When there is a breakdown, and a plant area shuts down, domino effects follow as other plant areas, upstream and downstream are forced to shut.

Smart selection of targets for stock levels, tank levels and unit production rates can maximise the utilisation of key assets despite unplanned breakdowns and restrictions.

Constraints or bottlenecks determine the maximum throughput of the mill, but short-term variations in each production area are required to achieve the optimal storage levels. The demand for each grade of pulp depends on paper machine making plans, market pulp demands and desired stock levels.

Competition for finite resources, such as white liquor supply, black liquor processing capacity and steam availability, requires that priorities for pulp users be set and adhered to.

Some of the principles involved in planning to maximise production are discussed, along with description of the systems available to assist mills to achieve this goal. Issues including the scheduling of downtime for maintenance and cleaning (eg. Evaporator boil-outs), washer dilution factor optimisation and methods for dealing with constraints are discussed.

Simulations highlighting the improvements resulting from following the recommended strategies demonstrate the importance of having a system in place to assist planning and scheduling of pulp mills.