Production Scheduling for Pulp Mills & Recoveries

Norm Lasslett

Procex Pty Ltd

 

SUMMARY

In a capital-intensive industry such as pulp and paper, maximum utilisation of assets is essential. Breakdowns of equipment, or constraints in other interdependent areas, often reduce the utilisation of key production units. 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. Some of the principles involved in planning to maximise production are discussed, along with description of the systems required to assist mills to achieve this goal.

Optimal storage levels are calculated using the probability of shutdowns upstream and downstream of each storage. 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 supply, requires that priorities for pulp users be set and adhered to.

Issues including the scheduling of downtime for maintenance and cleaning (eg. Evaporator boil-outs), and methods for dealing with constraints are also 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.

 

INTRODUCTION

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.

When the mill is out of balance, it is difficult to restore the balance without losing production. For example, any breakdown or restriction in the recovery circuit results in an increase in black liquor levels, and a consequent reduction in pulp production to ensure that the maximum black liquor storage volume is not exceeded. The pulp mill production rates chosen are often over-conservative and the pulp production lost is never recovered.

Because of the significant amount of capital invested in pulp and paper complexes, maximum utilization of assets is essential. Therefore, it is important to ensure that production units run at the maximum rates required to satisfy pulp demand and to build stocks to optimal levels.

 

BREAKDOWNS - THE DOMINO EFFECT

When there is a breakdown, and a plant area shuts down, domino effects are triggered as other plant areas, upstream and downstream are forced to shut. The only things inhibiting this consequence are the buffer storages between processes. The larger the available capacity in these storages, the more time available to restart the processes that have stopped, before shutting down more plant areas.

Larger storage tanks will provide a greater buffer which will slow the domino effect, but size is usually limited during mill design due to pressures on capital cost and the increased operating cost of carrying larger inventory.

 

OPTIMAL STORAGE LEVELS

When a shutdown does occur, it would be advantageous to maximize the capacity available in storages to cushion the domino effect. However, the disruption may be upstream or downstream of the storage, so the likelihood of shutdowns upstream and downstream must be considered. If there is an unreliable plant upstream, the storage should be kept fairly full, and if there is an unreliable plant downstream, the storage should be kept fairly empty.

The reliability of the plants immediately adjacent to a tank can be used to calculate the optimal level. However, the reliability of plant areas further afield must also be considered if the intermediate storage volumes are small and a breakdown is likely to cascade beyond them within the schedule period. As the calculated time to fill or empty the intermediate storages decreases, the probability that an unplanned shutdown will propagate beyond it increases.

Therefore, optimal storage levels can be calculated using the probabilities of shutdowns upstream and downstream of each storage. Maintaining these optimal levels will make better use of the available capacity to avoid shutting down or reducing production rates. During normal operation, the historical reliability statistics are used to calculate optimal levels. However, if a known shutdown is approaching, the optimal levels should be adjusted to best prepare for the shutdown.

Thus, over time, if the distribution of breakdowns follows the same pattern, the shutdowns of other plant areas will be minimized. Quality will be more consistent by avoiding shutting down or reducing production rates in other plant areas. Similar logic can be applied to stocks of dried pulp and liquor storages.

A frequency distribution of the duration of unplanned shuts due to problems within the process area of interest shows the number of shuts where:
    Shuts shorter than the Time to Empty will avoid shutting down the plant
    Shuts greater than the Time to Empty will allow the plant to run longer before shutting down.

BOTTLENECKS
Theory says that the throughput of any plant is limited by its bottlenecks. Therefore, the throughput of bottlenecks should be maximized as much as possible. Under steady-state conditions, the whole plant should operate at the production rate that is limited by the bottleneck. However, pulp and paper mills are more dynamic than that. Paper machines and pulp mills need to change grade, breakdowns occur, evaporators foul and need to be washed, customer service priorities change, environmental constraints happen, etc. etc.

Hence, the plant rarely runs at its maximum. The correct balance between pulp production, weak black liquor, strong black liquor, green liquor, white liquor and lime is seldom achieved. One of the objectives of a scheduling system is to restore the balance, with all the storages at their optimal levels as quickly and smoothly as possible.


COMPETITION FOR RESOURCES
When pulp mills are competing for a limited resource, such as steam, white liquor supply, or black liquor processing capacity, the available resource must be rationed in a logical manner. Criteria, which must be considered, include:

    Importance of customer orders
    Profitability of alternate grades
    Stock levels

Priorities, based on these criteria, must be set to determine the correct allocation of resources. The “rules” which define how shortages in capacity are to be managed must be agreed between the different mill areas. Agreement between areas on these “business rules” result in more consistent decisions which are directed at maximizing the total mill result, instead of maximizing one area to the detriment of another.

Expert systems are rule-based systems that are appropriate for implementing such systems.


DEALING WITH CONSTRAINTS
The goal is to satisfy the demand for pulp from the end users. This will be attempted despite constraints. For example, in the case of a Weak Black Liquor constraint, pulp production will not be cut back until the WBL storages approach their maximum allowed levels. Of course, this puts the mill in a precarious position whereby an unplanned upset may cause the mill to shut. However, revenue flows from the sale of product, so this should be maximized at all times. Choosing more conservative production rates to allow a greater buffer in WBL storages to deal with an unplanned upset will ultimately result in a lost opportunity to sell product. The maximum allowed levels need to be chosen carefully, to only leave enough unused capacity to deal with any emergency situation which may impact safety or environmental factors.

When there is a mismatch between white liquor demand and the capacity to process black liquor, the aim must be to maximize the supply of white liquor so as not to restrict pulp output. Therefore, in the situation where evaporation was restricted due to a set of evaporators being out of service, the levels of strong black liquor and green liquor would be allowed to fall to maximize the white liquor available. This also increases the throughput of the recovery boilers, which are often bottlenecks in the process. 

There may be some additional strategies for easing the situation, such as reducing pulp washer dilution factors, or introducing pulping additives, such as anthraquinone, or by diverting liquor to holding dams. These strategies may not be the most economical means of operation, but may be justified in the short term to prevent greater losses.

Steam is another resource which is limited, but its availability is also dependent on the liquor firing rate in recovery boilers. Hence, when steam is limited and pulp production is constrained, the production of steam is also constrained. The correct balance between supply and demand must be determined to maximize mill output.


DOWNTIME REQUIREMENT
A major disruption to the steady state operation of a pulp and paper complex is the occurrence of downtime, for example:
 Paper machine breakdowns
 Planned paper machine downtime
 Pulp mill breakdowns
 Planned pulp mill downtime
 Evaporator washes
 Recovery breakdowns
 Planned recovery downtime

Optimal levels during normal operation are based on the probability of downtime due to breakdowns. However, when downtime is planned, the optimal levels need to change as the impending downtime is approached. Therefore, the probability of downtime must change as the planned downtime nears.

 

RECOMMENDED PRODUCTION RATES

Demand from the end users dictates the long-term production rate of a plant. This will be modified in the short to medium term by the need to build or deplete stocks. Constraints or bottlenecks determine the maximum throughput of the plant, but short-term variations in each production area are required to achieve the optimal storage levels. Thus, the production rates for each plant area is determined by the demand for pulp (or liquor), but is modified by the need to drive its downstream storage to its optimal level.

An example of this is the determination of pulp production rate where there is storage for dried pulp. The following table shows the production rate strategies for different rates of downstream demand and stock levels:

TABLE I. PULP STORAGE STRATEGY

Downstream Demand

Stock Level

Action

> Max production

Low

Max. Prodn + reclaim

 

OK

Max. Prodn + reclaim

 

High

Max. Prodn + reclaim

= Max production

Low

Max. Prodn

 

OK

Max. Prodn

 

High

Max. Reclaim + prodn to suit

< Max production

Low

Prodn to demand + stock build

 

OK

Prodn to suit demand

 

High

Max. Reclaim + prodn to suit

= Min production

Low

Prodn to demand + stock build

 

OK

Min. Prodn

 

High

Min. Prodn

< Min production

Low

Prodn to demand + stock build

 

OK

Min. Prodn with excess to stock

 

High

Shut

Pulp mills generally have a minimum production rate. If the demand is less than the minimum production rate and the maximum-constrained inflow of the downstream production unit or storage is greater than the minimum production rate, the pulp mill will be set at minimum production rate. Otherwise, the mill will shut.

 

PLANNING DIFFICULTIES

Planned and unplanned downtime or constraints have a major impact on pulp mill and recovery planning. Other variations, which also have an impact, include:

These variations need to be detected and considered by the planning system. The measurement accuracy of sensors must be adequate for the task.

Mill coordinators may be aware of some constraints not picked up automatically by the planning system. The system must accommodate user input of such conditions.

 

USER DISPLAYS

To be effective, a pulp mill and recovery scheduling system must:

The user displays required include:

Management must highlight the benefits resulting from following the recommendations. By showing the benefits of following the recommended strategy, shift coordinators and area operators will understand the need to use the system.

 

BENEFITS

A study of operation over one week in a pulp mill with three fibre lines showed that use of a production scheduling system would improve operation in the following manner:

Some operating constraints were experienced during this period, but there were no breakdowns in the pulp mill and recovery areas that would cause any process area to shut down.

A study of unplanned shuts over a period of one month showed:

 

CONCLUSION

Production scheduling of pulp mills and recoveries can result in smoother and more profitable operation. Various techniques can be employed to maximize the utilization of assets despite unplanned breakdowns and other operating constraints.

Systems are now available to implement short to medium term production scheduling in complex capital-intensive industries. Installations of such systems have already shown significant benefits.

 

© Copyright 2001 Procex Pty Ltd. All rights reserved.

 

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