Advanced control and optimisation applications at MOL

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Pergamon

Computers and Chemical Engineering Supplement (1999) S775-S778 (I 1999 Elsevier Science Ltd. All rights reserved PlI: 5009 8·135 4199/00065-4

Advanced Control and Optimisation applications at MOL Laszlo BODAl, Dr. Alms JOZSEF, Antal KATONA

Rendell BYRD

MOL, Szazhalombatta, Hungary

Aspentech, Houston, TX, USA

ABSTRACT

The profit efficiency and competitiveness on the market ofhydrocarbon products are a must in nowadays Hungary. The new economical environment forces the former local companies to think global and to be innovative and proactive in acquiring and introducing worldwide new techniques and technologies. All this effects the recently privatised MOL Hungarian Oil and Gas Companyl. The activity ofMOL Hungarian Oil and Gas Company comprises mining, processing and trading in all kind of hydrocarbons in Hungary and in the region. Danube Refinery is the most complex refinery by its production structure where the crude processing is concentrated in the city of Szazhalombatta. Since itsfoundation in 1963 the refinery has been the major player in production and subsequent supply of a dominant part of the motor fuels marketed in Hungary. The Danube Refinery is currently capable ofprocessing 7 MMTPA crude and issuing 'a couple ofhundreds ofdifferent products as waxes, lube oils, specific solvents. fuels. maleic anhydride and motor fu els as the mainstay ofits production. The quality development policy at the Danube Refinery has always been following the most advanced trends in environment protection and performance enhancement and economy. This mission has lead to a challenge and commitment to pursuit of excellence in both products' qualities and the technologies involved in their manufacturing. Most refineries have already recognised that their overall utilisation ratio is capped by market limitations and that in order to widen their margins they should find some other ways like reaching higher throughputs at conversion facilities by debottlenecking, using sophisticated control tools alleviating some vital constrains, maintaining tighter product specifications closer to the quality constraints, maintenance cost reduction, energy efficiency improvement and environment pollution control. Most ofthe above involve a higher level ofprocess control techniques than those implemented in the past. The use ofthe highly sophisticated Advanced Process Control (APC) and Optimisation technologies, developed by Aspentech and,customised to the actual processes ofthe Refinery by local specialists is not only a newf eature ofthe control system of production faciliti es but a significant enhancement of the technical skills of the workforce at Danube Refinery. Based on the software applications the refinery expects to stabilise quality production, increase valuable product yields and to reduce maintenance costs to get a higher profit and a stronger competitive position. This paper outlines an overview and history ofAPC technology at MOL and highlights the benefits reaped from the SMCA applications and the current move into the DMCPlus technology. 1. INTRODUCTION MOL Hungarian Oil & Gas Co. and Aspen Technology Inc., (Aspentech) signed a long-term service agreement in 1998. Under the terms of the agreement Aspentech will support MOL with tile installation of the latest software products of Aspentech and will implement or upgrade advanced controls on all major production facilities at Danube Refinery. Some of them were applied first at MOL as an industrial application. MOL and AspenTech (former Set point) have enjoyed a successful relationship during the past five years. 2. REFINERY-WIDE ADVANCED CONTROL APPLICATIONS AND OPTIMISATION AT MOL In 1993 a systematic development started at MOL with participation of Setpoint to implement latest APC technologies on the major units of the refinery.

In order to ensure a good return on investment, tile first successful project was completed on tile FCC unit. Later, successful APC applications were completed at three crude-vacuum distillation units. The commissioning phase of the HF-Alkylation and MTBE units is currently in progress. The pretest and design phase of APC technologies of several more units will be started within a year. Based on the profitable applications of the APC technology MOL has decided to implement optimisation on tile units where APe' applications are already running smoothly and further benefits are potentially achievable. The systematic development of APC technologies and optimisation requires a reliable technical , instrumentation background and process control systems. Before any APC or optimisation project was started, a reinstrumentation and development of new measurement points had to be completed. Since most

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of the processing units at tile Danube refinery have DCS (Distributed Control System) already in place, the realisation of any higher level APC and Optimisation system can be realized without significant refurbishment. 3. ADVANCED CONTROL Before the achieved benefits at MOL are summarised a brief theoretical background is given in this chapter about tile APC technology. 3.1 OVERVIEW OF SOFTWARE HIERARCHY In this section tile structure of tile APC and optimisation technology and the relevant functions of a typical oil refinery is described . All of these functions are already available in a software supported and integrated enviroriment. The usage of these applications has became extremely popular worldwide in tile recent years because of their effectiveness. The purpose of these software applications is the powerful and effective information management of each area to get an on-line tool for tile rapid and business oriented decisions. The DCS systems represent the lower level of the integrated applications, which replaced the former mechanical and pneumatic control systems in the 70s. These have a continuous supervision and smallscale correction of the operational parameters like pressure, temperature, flows, etc. The APC systems represent the second level. This application took its start in tile early 90s at MOL. Current APC technology utilises muItivariable, constrained and model predictive controllers. These are to control tile product qualities and yields based on physical-chemical calculations and model equations and give setpoints to the lower level PID controllers with a 15-60 sec calculation frequency. The Optimisation systems use hourly averages of tile unit operational data and control the operation of the connected units taking into account the coherent refinery optimum conditions and requirements based on economical information. Optimization is to coordinate the operation of the connected units online way to provide maximum profit based on the product, feedstock and utility prices . The Scheduling allows a 1-2 weeks scheduling offline, taking into account the equipment and storage tank capacities at tile refinery , TIle Planning can be used for giving directions and decisions on planning of the possible product structure based on different crude oil slate. Planning can cover a year or a continuous 3 months period. TIle SAP system covers and shows information about tile operation of the whole company in finance, human resources , economics and oilier areas. The use of SAP beyond global financial information has a lot at pitfalls and inconveniences.

3.2 OVERVIEW OF APC THEORY Based on tile APC software applications the continuous on-line product quality control and yield structure optimisation becomes feasible . TIle use of the APC systems further helps to improve the energy efficiency, reduce tile environmental pollution and operate the units closer both to the safety and operational margins. These margins are: product quality limits (endpoints. 95 % points, viscosity, etc.) temperatures, product yields , pressures, equipment capacities, etc. The APC systems help to maximise the valuable product yields with a controlled constant quality, because it changes the PID controllers setpoints to satisfy tile quality and yield requirements and tile units can be operated closer to the constraint parameters. The APC software supported environment provides the necessary tools for tile above and the resulting general advantages in the operation are as follows : l. A very close approximation of any targeted value of any primary or computed, derived variable . Earlier only certain main constraints were taken into account effectively and tile rest of them were kept much further from tile limits to ensure a safe operation of tile unit. 2. The APC controllers handle all of the constraints at the same time. 3. Based on frequently repeated calculations on-line modifications are possible in the system operation. TIle controller continuously eliminates the disturbance effects. It increases tile utilisation of the reserves . The direct and indirect advantages of the use of APC systems can be summarised as follows: Direct advantages: ease of reproduction of constant product qualities, closer operation to the quality limits ensures a cost reduction, elimination of unit bottlenecks, increased amount of the most valuable product, energy efficiency improvement. Indirect advantages: standardised unit operation in a specified way, operators can operate larger number of controllers, better understanding of the unit behaviour. The characteristics of tile controller can be summarised according to the following terms: MULTIVARIABLE: In any process there is an interaction among the independent (manipulated , disturbance, feedforward) and dependent (controlled) variables. One change of any variable has an effect on the rest. Based on tile model between the independent and dependent variables, tile controller can handle the interactions, find tile best solution for the necessary changes and control the process. MODEL PREDICTIVE: Based on tile step responses the controller uses a linear dynamic model of the process. This allows predicting tile open loop behaviour of the controlled variables for a period of time into the future knowing the changes of the

Computers and Chemical Engineering Supplement (1999) S775-S778

manipulated variables, which would allow any potential violation of the process. The controller uses linear, empirical and dynamic models of the process. These are empirical, because derived from observed process measurements rather than theoretical principles. Dynamic, because describes how the process changes as time passes. CONSTRATINED: In the overall control plan all constraints arc considered. There are controlled and manipulated variables in the control hierarchy, which are operated between upper and lower limits. These limits are taken into account by the controller and the operation will be as close as possible to these limits to allow the maximum performance. OPTIMISING: TIle controller uses cost information of raw materials, products and utilities, the current values of the manipulated variables and the predicted values of the controlled variables for the optimisation purposes. TIle controller tries to reach the most economic operating point of the process at every execution. TIle dynamic optimisation problem minimises controlled variable error away from the LP calculated steady state operating point, taking into account the manipulated variable limits. RIGOROUS: There is no assumption made about the form of the model and the system can be described or approximated by a system of linear differential equations 4. BENEFITS OF THE APPLICATIONS AT MOL DANUBE REFINERY There is a systematic data collection and result monitoring at our refinery. All tIle measured LAB data with the actual system parameters relevant to sampling time are collected for later analysis. Separately, all the LAB data included the full distillation curves, and the daily average yields are also collected in EXCEL files. The Infoplus system, in place since 1993 (which were upgraded to Infoplus.21 during the summer of 1998), provides a large amount of historic process data. TIle huge amount of data ensures the possibility of a later evaluation of the system behaviour, model analysis and upgrade, and furthermore a performance analysis. All the routine calculations are connected on-line to the real system. Macro subroutines help in the analysis in getting a faster and a standardised form of assesment. TIle on-line tools, which either use on-line values or hourly averages, help and give directions for the operators to make changes in their daily operation. TIle daily averages like product yields and LAB measurements give indications for the unit managers about the operation. TIle monthly averages of the product yields are good indicators of the system performance for the control, process engineers and unit managers, how the system behaviour changes in a longer period compared with the earlier results. Based on the longer-term monthly averages the control engineer can make decisions, if a

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modification of a controller structure or a model for the quality prediction is necessary. The performance analysis of the atmospheric/vacuum distillation units and the reactor/regenerator and distillation columns of the FCC unit showed an efficiency improvement after the commissioning period of the SMCA controllers. As a result of the continuos monitoring of these systems MOL has been able to maintain the increased yield of the most valuable products and even to improve them beyond the benefits provided by the original controller. Beside the increased yields, the product qualities became predictable and the standard deviation of the controlled qualities became smaller with the controller application. This results in a tighter product specification closer to the product quality limits. A general observation is that me number of failures of different mechanical parts declined as a result of a more stable operation. As a result of me Multivariable Controllers operation MOL has been able to increase the yields of the most valuable products in a distillation column at least by 1-3 %. In that way the most valuable product is deprived from the neighbouring materials, while the qualities of all me products can be kept within their specified boiling range. It means, that the multi variable controller can reduce the overlapping of different petroleum fractions. TIHi overall increase of all the light valuable product yields in a distillation unit can be at least 5-6 % higher and this can be maintained in a long time horizon. TIle valuable heavy products of the vacuum towers can be increased by at least 0.5-1 % and deprived from the vacuum residue while increasing the yield of the vacuum gasoiI. TIle paraffinic products of one of the vacuum towers, where the Hungarian crude oil is being processed, have excellent quality for lube oil production. The increased production yield ensures a higher profit in me lube oil area. The paraffinic products of the other vacuum tower, where Russian crude oil is being processed arc the feedstocks of the FCC unit. The increased amount of paraffinic products is profitable from point of view of the FCC unit. If we take into account, that the increased amount offeedstock contains less vacuum gasoil than before, the capacity of the FCC reactor is even higher, because light vacuum gas oil has a very low conversion in me FCC reactor. Earlier, it caused an unused and absorbed capacity of the reactor with higher energy consumption. The use of multivariable controllers at the FCC unit brought about a 3-4 % capacity improvement of the reactor and a 3-4 % additional propylene production. These facts with the above mentioned increased distillation efficiency resulted in a less than a year return of investment for the FCC and other units. The major benefits we highlighted show the effectiveness and profitability of the commissioned multi variable applications at MOL. Based on this experiment MOL expects additional profit at other

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units and other applications, as well. Based on the practice of MOL, these benefits can be maintained with a systematic and continuous performance monitoring. These are the facts to be considered and while MOL chooses further Aspen applications in their units.

5. RECOMMENDA nON The benefits offered by APC applications are clear and obvious in any standalone unit. TIle economical benefits can be drawn directly. if the increased product yield is a final marketable product of the unit. For example, if the increased propylene yield at the FCC unit or the special naphtha production at a crude oil distillation unit are final products and these can be sold directly. TIle analysis and appreciation of the benefits is more difficult in such a complex refinery, where a product of any units is being refined and processed in additional units. Can the benefits and advantages be maintained in' the whole process after tile first successful step? TIle analysis of this is quite complex and shows further necessary steps into the directions of optimisation. 'For example, if the amount of the increased gas oil product contains naphtha. which could be potential for valuable gasoline production. In the later gas oil stabilisation steps this naphtha is removed and probably will be a component of the feedstock of the ethylene plan. Which operation method can make more profit for the whole refinery? It is also difficult to express directly in financial terms , if the increased amount of 'F CC feedstock from a distillation unit contains less gas oil, which has a low conversion in the reactor. In this case what is reduction in the energy consumption beside the additionally increased reactor capacity? To get the answer the product prices, amounts and energy consumption should be expressed in financial terms. To get a successful comprehensive analysis of the refining and blending processes of the refinery further analysis and cooperation is necessary among the different groups of the organisation of the refinery . In this cooperation the plaruting and scheduling departments should also be involved to get an overall picture of the proposed and final results . Further improvements on a complex refinery level can be achieved with a strong cooperation between these departments, because achievable overall benefits can be maintained and realised only in teamwork. To get a successful cooperation certain structural developments of the organisation are also necessary between the traditional planning and the newly formed conirol, optimisation and scheduling departments. Aspen Technology now has a compatible software structure for all of these functions and most of them are already available at MOL. These facts ensure the possibility of a successful and good cooperation between the departments in the analyzation of

process data and operation of the units in order to realise their best performance. The multivariable predictive control systems have an additional advantage for a refinery. Once a good model is developed for the product quality prediction the refinery has an on-line tool to monitor it in contrast with the traditional and 'periodical LAB analysis. Beside the fact, that the product qualities are continuously monitored and kept at their setpoints by the APC systems , a certain reduction is also possible in the LAB analysis frequency and consequently in the labor demand. TIle calculated product property gives a new tool for the Quality Insurance ambitions in the refinery. The Danube Refinery has an ISO 9002 Quality Insurance, in which the usage of the .predictive control applications should also be mentioned.

CONCLUSION Multivariable predictive control and optimisation strengthen the business strategy of MOL in achieving benefits. With a long term application of these tools the company can increase its business position in the new economical envirorunent and face with the challenges, what comes into the front in the com ing years. These are the production of high quality products on an effective and environmental friendly way, improved business performance of the company, the need for a continuous improvement and development and the internationalisation in the European Union . MOL believes that tile usage and a continuous improvement of tile latest teclmological developments, in tile fields of APC and Optimisation. can increase its financial results and competitive position in a longer term. TIllS ensures tile results in tile regional Central-Eastern European market and in tIIC worldwide-accepted perfonnancc analysis. as well, Due to the calculations of MOL all of the completed projects have shO\"11 a return of investment within a year. In order to keep and improve the benefits a continuous system performance monitoring is necessary. Not only the technical factors, but also the human factors and the structural construction of tile organisation are greatly responsible for tile success of tile applications. APC has a direct advantage for simple units and low complexity refineries, while refineries, which have a higher complexity factor additional tools, like optimisation, are necessary to ach ieve and maintain tile maximum potential benefits refinery wide. TIle achievable software packages offer suitable tools to achieve and maintain tile maximum potential benefits in a complex processing Oil Company. TIle application of tile APC and upper level software teclmologies can support an effective performance improvement for MOL in the international business environment. what can help in achieving and sustaining the highlighted benefits.