P. Uronen

Heuristic Algorithm for Production Control of an Integrated Pulp and Paper Mill

The work reported here is a part of a larger development project going on in the Division of Control Engineering, University of Oulu in cooperation with some Finnish pulp and paper industry. In this project the development of the hierarchical dynamic production control system for an integrated pulp and paper mill is studied. The whole system will include a paper mill, pulp lines of the mill, a chemical recovery cycle and the whole energy system of the mill. n nBased on the mathematical models a general simulation program package for the production control was developed. The scheduling is based on repeated simulations and on some simple logical rules for the calculation of the production schedules. The scheduling proceeds iteratively from a given production schedule to a production schedule that satisfies the system contstraints.

Different Approaches for the Production Control of a Pulp Mill

Abstract Three approaches to carry out the pulp mill production control calculations are discussed. These approaches are a simulation method, a hierarchical optimization algorithm and a network flow algorithm. In the simulation approach a general program package for the production control calculations was developed. The scheduling is based on a simulation model and some simple logical rules for production scheduling. It proceeds iteratively from a given production schedule to a production schedule that satisfies all the system constraints. The production schedules of both the pulp lines of the mill and the chemical recovery cycle can be calculated using Tamura’s time delay algorithm. This algorithm was modified so that the specific features of the problem can be taken into account. These include the compensation of planned shut-downs, identification of infeasible situations, etc. The use of standard linear programming algorithms to production control calculations is restricted by the fact that they need a considerable amount of core memory. However, for some special cases network flow algorithms that can solve LP-problems have been developed. In this connection Ford-Fulkerson out-of-kilter algorithm is considered. The applicability of these approaches was compared using simulations with UNIVAC 1100/20-computer of the University of Oulu. In the simulations the preselected simulation period of 48 hours was divided into intervals of 8 hours. Several kinds of test runs that correspond problems in the real plant environment were carried out.

Hierarchical Simulation of Ferroalloy Processes

Abstract An extensive, hierarchical simulation system is developed for ferroalloy processes. It has been applied to hot charge submerged arc smelters and both preheated as well as cold charge can be used. Thus the simulation system is also a useful tool for designing and comparing process and production alternatives. The structure of the simulation system is hierarchical, comprising four modelling and calculation levels. On the first level the process is decomposed into subsystems e.g. preheating, the feeding ring and the electric furnace. The second level further subdivides these into zones. On the third level, spatial dependence of the variables is taken into account by using a rectangular grid and on the fourth level the detailed simulation models are calculated for a volume element. At the initial state of alternative specification, input data contains considerable uncertainties. In order to gradually reduce the number of uncertainties an expert system is coupled with the lower levels of the system. Fuzziness is taken into account in the input data of the simulation system by using discretized values. A simplified fuzzy linear model is developed on the basis of simulation experiments. User interface is very important in these kinds of multilayer systems with a variety of properties and user requirements. The same user interface is used for the simulation system and the fuzzy simulation model. This system can easily be connected to digital automation systems. The modelling and simulation is done in FORTRAN 77 and various hardware has been used in development and application. This simulation system can be used in many different ways: in production and operation planning, process design, process research and in multilevel optimization and process control. The present applications of the lower levels of the system include feasibility studies for the smelter, tools to assist pilot plant runs and realtime assistance for operators in smelters. These parts of the system are in everyday use in industry today. The higher levels of the system are targeted mainly for engineering design, dimensioning and scale-up purposes.

Modelling and Optimization of Mud Filter and Lime Kiln

Abstract The possibilities to improve the operation of the lime cycle of a sulfate pulp mill through optimizing the controls of the mud filter and the lime kiln are discussed. By simulations, where real operational data is used, it is proved that there are great potentials to achieve considerable energy and chemical savings in the chemical recovery system. The problems and their solution methods are studied in three levels: viz. in 1) operational, 2) computational and 3) instrumental levels. Special attention is paid to modelling and optimization procedures, and a few new pragmatic methods are presented and proposed for future tests and real time use for ex. to optimize the set points and blowing interval of the mud filter. The studies show that in order to be able to optimize the operation of the calcining system, an adaptive control scheme is required. Some simple adaptive estimation and disturbance compensation methods are presented to fulfill the needs of real-time control. This is due to many kinds of process disturbances as well as variation in the filter performance between blowing intervals. In the end the applicability of the proposed methods is discussed in the light of mill experiences.

A Practical Approach to the Control of Recovery Boilers

Abstract The recovery boiler is an expensive and critical subprocess in the sulphate pulp mill and it will quite often cause a bottleneck in production. The burning process of black liquor is also quite sensitive to various disturbances (quality and analysis of BL). The recovery boiler has a dual function: on the one hand, to convert the used digesting chemicals to a reusable form, and on the other hand to utilize the heat produced by the burning process for steam generation as effectively as possible. The operation of a recovery boiler also has a substantial effect on environmental pollution. It is very understandable, then, that in the last few years much research and development work has been carried out in order to improve the control of the recovery boiler. It is also evident that digital computers can be used effectively to control recovery boilers: some systems are already in operation. This paper deals with the possible ways of tackling this problem. Some references are also made to experiences in Finland with a computer control system for recovery boilers developed in close cooperation with the user, the vendor and research workers and in continuous use at the Kaukopaa Mill of the Enso-Gutzeit Company since winter 1976. The economical results achieved with this system will also be briefly presented.

CONTROL OF AN INDUSTRIAL DRUM FILTER BY USING ADAPTIVE MODELS

Abstract Vacuum filters are often used to dry and wash lime mud before calcining it in the lime kiln of a sulphate pulp mill. The control of the kiln itself is quite problematic because of long time delays and complicated burning reactions. To avoid many kilns control problems and to achieve energy savings in the kiln, the mud filter should operate steadily and produce dry mud. Here, based on a systematic study of the process, a self-tuning type two-variable control scheme is proposed for the filter. First, adaptive process models are specified based on mill experiments. The self-tuning control scheme is then developed for the drum filter. The proposed control method consists of a twovariable parameter adaptive control algorithm which includes compensation for filter efficiency and some other disturbances. Optimization of the blowing and scraping intervals is also possible. The controller is directly applicable to mud filters provided that they are controllable and the process model is identified. Energy savings of about 10 % can be achieved using this method. The method provides a general approach for drum filter control. In the end the general applicability of the methods and the results are also discussed.

A Comparison of Different Control Strategies of a Rotary Drier

Abstract This paper presents control results for a rotary drier given by a conventional feedback control, control based on material and energy balances and by a state control. The results are presented separately and a comparison of these different control strategies is made. The comparison is based on the simulations with an Eclipse S/140 computer and on the experiments carried out with a pilot-plant rotary drier.

Trends in Digital Control Applications in Pulp and Paper Industry

Abstract The first applications of real-time digital control to pulp and paper industry stem from early 60’s. Then a centralized hardware was applied to quite large and poorly defined problems. These early projects were not very successful; one of the main reasons being the unreliable and limited hardware. During 70’s the so-called packaged systems entered into the market. These were built by using the existing minicomputers and those systems were dedicated to smaller and well-defined problem areas; for example dry end controls of paper machine, batch-digesting, Kamyr digester, bleach plant etc. Today we are ahead a third generation of these systems in the Pulp and Paper Industry. The microprocessor technology has created the powerful microcomputers and the distributed digital instrumentation systems. This has lead to the concept: decentralized control and centralized reporting and supervision. This means that the lowest level of the control hierarchy will be handled by a digital instrumentation system and with a local datanetwork (data highway, bus) a hierarchical structure including area control and coordination, production planning and scheduling and management information system at the higher levels a total mill-wide hierarchy will be built. There is also clearly a trend towards more integration between process control functions and other EDP activities inside the organization. These existing systems and future needs and trends are discussed in the paper.

Improving Automation in the Causticizing Plant and Lime Kiln

Abstract This paper deals with a research project concerning with the improvment of automation in the causticizing plant and the lime kiln of the sulphate pulp mill. The project is still continuing at the University of Oulu in close co-operation with two Finnish pulp mills and a system vendor.

Mill Wide Systems - The Way Ahead?

Abstract During the last 10 years there has been an ever increasing interest among the managers and engineers in the Pulp and Paper Industry towards the so-called mi 11-wide systems. Even if the term mi 11-wide system is not well-defined, there are several such systems available in the market. About 50 mills have such integrated computer systems in operation, which most specialists could call millwide systems. These existingsystems, however, differ largely in their mode of implementation and in their operations and functions. Some of these systems are using centralized HW, some others decentralized, distributed HK. Some are based on direct process connections via separate interfaces (dataloggers), others are hierarchically integrated with process control systems and are effectively utilizingdata-highway and local network technology. However, the main emphasis must be on the SW-side - what does the system do and what kind of mill-wide tools are really needed for the management at different levels of organization ? It is to believe that interest towards mi 11 wide systems is still naininn momentum and many new installations will be nut into operation in the very near future especially if the economic situation of the Worlds Puln and Paper Industry will improve. There are some commercial systems and solutions already available in the market and anain there exists a danner that the suppliers will too much dictate the HI-bound solutions and the mills i.e. the users are not in the position to tailor or specify these systems accordinn to their own needs and views. On the other hand this situation needs much more attention and work also from the users side. It is not any more so much the Question about know how but know what.