KBMoSS: A process engineering modelling support system

Pergamon

Computers chem. Engng Vol.20, Suppl., pp. $309-$314, 1996 Copyright© 1996ElsevierScienceLid S0098-1354(96)00062.2 Printed in Great Britain. All rights reserved 0098-1354/96 $15.00+0.00

KBMoSS: A PROCESS ENGINEERING MODELLING SUPPORT SYSTEM R. V:~ZQUEZ-ROMAN:J. M. P. KING and R. BAlqARES-ALCANTARA Department of Chemical Engineering University of Edinburgh, Edinburgh EH9 3JL. Scotland (U.K.) A b s t r a c t : Modelling is one of the lnost important activities in process engineering since it provides insight into the behaviour of the system being studied. Modelling is also a complex activity as the formulation of appropriate and consistent models requires help from experts and often results in a knowledge-intensive, time consuming and error-prone task. A knowledge-based modelling support system, KBMoSS, has been developed to foster the exploration of the model space, maintain the model's evolution, enhance the engineer's understanding of the model, and improve cooperation between modellers. Mathematical models can also be automatically generated within this system. The model generation is based on the modelling approach developed by V~,zquez-Rom~,n (1992). Users can either accept the generated model, modify it, or write their own models from scratch. The user interface was designed to allow the description, examination and manipulation of a chemical process with reference to its modelling aspects. It allows flexible navigation through the various parts of the modelling structure and was implemented using the object-oriented paradigm.

INTRODUCTION Modelling is one of the most important activities during process analysis, synthesis, control, simulation, and design. Its goal is to produce a model which contains equations whose numerical evaluation will provide insights into the actuM behaviour of a chemical process. A model is understood here as the mapping of the relationship between physical variables onto mathematical structures such as sets of differential-algebraic equations. A number of refinements to models are normally required to achieve the evolving objectives and goals in a diverse range of chemical engineering activities. This precedes the need to generate appropriate and consistent models. The task is indeed knowledge-intensive, time consuming, and error-prone. It has been claimed that a nmdeller should posses two qualities to write appropriate models: a good background in the subject and an ability to translate the phenomena and structure of a process into a mathematical notation (Himmelblau and Bischoff, 1968). Many procedures have been proposed to formalise and define a methodology to build models. However, all attempts have merely ended in recommendations based on unclear "conunon sense" rules. The difficulty in satis~-ing the requirements of well-defined and consistent models can be efficiently and meaningfully reduced through the use of computers. It is considered here that the use of computers in modelling can be classified as two types: synthesis and support. The use of computers in nmdel synthesis consists of automatic generation of the model based on an appropriate definition of the problem. It has been shown that process representation plays the nmst important role in the simplification of both the problem definition and the model generation (Perkins et M., 1995). In design, process representation is simplified by decomposition of the system at severM degrees of granularity (Douglas, 1988), enabling every part of the system to be modelled separately. Most existing flowsheeting packages provide facilities to automatically generate models based on unit nmdels within their libraries. SpeedUp (Perkins and Sargent, 1982) and gPROMS (Pantelides and Barton, 1992) provide a macro facility for building complex models from simpler ones, and ASCEND (Piela et al., 1991) and DESIGN-KIT (Stephanopoulos et al., 1987) use object-oriented programming to allow the *To whom all correspondenceshould be addressed. On leave from lnati~u~o Tecno169ico de Celaya, MEXICO. $309

$310

European Symposium on ComputerAided Process Engineering---6.Part A

development of models. An extended review of more flowsheeting packages and their main features can be found in (Marquardt, 1991). Unfortunately, the libraries in these systems are either insufficient or they are disorganized with the information dispersed in various manuals. Extending the models library in these systems is always possible but time consuming. Decomposition of naathematical models shows that equations are the basic building blocks representing physico-chemical relations. I-lence the importance of providing aids to assist the building of equations. Two prototypes have been developed in parallel which generate equations of lumped parameter dynamic systems based on a physical description of the system (V£zquez-RomAn, 1992)(Telnes, 1992). A similar approach but for a different type of system has been reported by (Preisig et al., 1990) and (Preisig et al., 1989). Automatic generation of simple one-dimension transport equations can be found in (Dieterich and Eigenberger, 1995). The basic idea in these systems is to build equations from first principles (Meyssami and Asbjornsen, 1989). An object-oriented frame to incorporate more types of equations has been proposed in (Bogusch and Marquardt, 1995). A review of the recent developments in computer aids for process model-building has been presented by Marquardt (1994). In summary, there are two approaches to generate models: via unit descriptions and via physical descriptions. The former approach requires a library of models whereas the latter requires a library of transfer-laws models. Unfortunately, it is difficult and perhaps impossible to cope with all the process engineering modelling requirements because of various reasons such as their number and variety. In fact, an underlined premise in this work is to accept the lack of knowledge of a group of modellers and tlle insufficient capability of computers to generate all required models. As a result, the computer can be used to provide support in the modelling activity. Little work has been carried out in this area. Ill this work, a knowledge-based modelling support system (h'BMoSS) has been developed to foster the exploration of the model space, maintain the model's evolution, enhance the engineer's understanding of tlle model, and improve cooperation between modellers. The following section contains the set of requirements for modelling support, systems based on the exploration-based model of design proposed in (Smithers et al., 1990). Then, the structure of KBMoSS is presented with a brief description of its parts. An example follows the KB:lloSS description to give a flavour of the expected benefits for both experts and novices. Finally, this work concludes with an analysis of the potential benefits that can be expected from this system.

MODELLING

SUPPORT SYSTEMS

KB,1foSS has been developed to foster exploration of the model space, maintain the model's evolution, enhance the understanding of the model, and improve cooperation between several modellers. The purpose of a modelling support system is to improve the modelling activity. Five requirements have been detected for design support systems (Bafiares-Alc£ntara, 1995): exploration, evolution, cooperation, integration, and automation. The same requirements are considered applicable for modelling purposes: • A modelling process can be better understood as a knowledge-based exploration task. A lmmber of alternatives might be explored before goals are achieved. Models may be created by refinement or to cover a different expected phenomena. Support systems should assist in the management of these alternatives, i.e. representation, control, evaluation, and use. Techniques for knowledge representation and reasoning are valuable tools for the improvement of the exploration of alternative models. • Model development is evolutionary in nature. The evolution represents the incremental application of operators to improve the model. Simplified maintenance of consistency, improved record of dependencies among modelling actions, and savings in memory are expected from the representation of evolution. • Cooperation enhances the sharing of data which comes from different viewpoints, objectives, or areas of knowledge. • Integration. The structure of a support system should also allow integration of all types of information and tools which could be required in modelling. • Automation would facilitate the analysis of designers' intention, decisions, and assumptions, in addition to application of methods, and propagation of values among alternatives maintaining consistency and easy navigation on the modelling structures. An important feature of a support system is that saved elements will remain in the environment and can neither be removed nor modified. Preserving this information allows modellers to learn from previous errors and prevents repetitive errors. In addition, persistence of objects supports various functions such as sharing, maintaining, inspecting, and reusing objects.

European Symposium on Computer Aided Process Engineering--6. Part A

$311

In principle, a support system does not take an active part in the solution of the modelling problem which is provided externally by the users, i.e. it is a receptacle for the various solutions proposed by them. However, the system could also contribute as ml expert by proposing a solution. In modelling, this has resulted in a helpful complement to the modelling support system as shown below. User-friendly systems should consist of graphical user interfaces whose development may be guided by the representation principle which states that the problem is almost solved once a problem is described using an appropriate representation (Winston, 1992). The following section presents a global view of a modelling support system developed to demonstrate these ideas.

KBMoSS- T H E E X P E R I M E N T A L

PROTOTYPE

The purpose of KBMoSS is to provide a computer based system to assist a team of cooperating modellers. The prototype has been oriented to ease, speed, aald improve the modelling process. However, the models could be used in other process engineering activities. The graphical user interface consists of six basic windows: the top window, scheme hierarchy, flowsheet, identifiers (i.e. variables), models manager, and models editor. All KBMoSS windows have been designed to operate in a similar way and contain three parts: title pane, interactor pane, and scroll pane. The title pane indicates the purpose of that particular window; the interactor pane gives instructions, sends messages or warnings, and obtains data from users; and the scroll pane shows all the existing objects related to the purpose of the window, providing a means to create, remove, or edit their properties. Menus are activated in this window by clicking the right mouse button anywhere on the pane. The menu shown and the functions available from it depends on the object selected. Figure 1 shows the six basic h'BMoSS windows. The top window allows the creation of a reference to the problem under consideration, i.e. a project. Since KBMoSS is a design oriented system, the project is a receptacle of the concerted effort of a designer or a group of designers in the development of a new plant. The top window in Figure 1 shows that a project under development has been named TEST. A schenae is defined as a collection of units and the connectivity among them. Various schemes may be grouped in families which are called metaschemes. The association is not unique since a scheme may be grouped in more than one metascheme chosen according to any criteria, e.g. schemes created by the same modeller, schemes which are considered the best, schemes created during certain period of time, etc. All schemes related to the same project are shown in the scheme hierarchy window, which in Figure 1 shows the scheme TESTI, a part of the project TEST. Within the scheme hierarchy window, schemes can be created from scratch and its constituent parts edited in the flowsheet scheme window. The flowsheet window provides a meaam to create units and connect them. At present, the modelling approach by V~.zquez-Rom~.n (1992) has been implemented. This approadl defines the process in terms of more elementary operations and mechanisms. Hence a process is decomposed into phases which are interconnected and whose instantaneous states are considered stable. Two or more phases can be grouped in vessels, and the remaining parts of the universe which are infinite sources or sinks are named as veservoirs. Conventional units such as reactors or distillation columns will be considered in future developments. A feature of this system is that saved schemes cannot be deleted or structurally modified. In this case, the flowsheet tool is used to visualise the scheme without modifying the structure of the scheme. However, functional aspects such as physical properties or transfer-laws (turbulent flow, laminar flow, etc.) for connections could be changed. The structural information of a saved scheme can be used in KBMoSS as a basis to create another scheme. This new scheme is considered an evolution of the saved scheme and can be modified provided it is not saved. Saved schemes can also be decomposed into two schemes or two schemes be combined to generate a new scheme. It should be noted that the three windows discussed above are similar to KBDS, a design support system (Bafiares-Alc~,ntara and Lababidi, 1995). The actual modelling activity should start with the activation of the identifiers window from the top window. The identifiers window allows modellers to define the nomenclature of the various variables which may be used within any of the mathematical models. It encourages modellers to use a unique set of variables. It is expected that. standarisation of symbols will promote consistency and clarification when visualising various models simultaneously. New identifiers may be created and added to the knowledge data. Default and bound values, required during the numerical solution of the model, are also defined in this window. Once a nmdel has been generated, symbols contained within it cannot be modified if the model is to remain in the models manager window. As shown in Figure 1, the set of identifiers is the same for all schemes in a project. The models manager window is activated from the scheme hierarchy window. It contains the hierarchy of models related to a particular scheme. Figure 1 shows the models of the scheme TEST1. A model

$312

European Symposium on Computer Aided Process Engineering--6. Part A

Symbol ]~Ot_C~s'CEI~RATION

x

Units z~ollxnol

Default 0.5

Lower-bound 0

IM/~S PRESSURE

a,-3

ii!ili!ii:,iii::iiiilili:i!!iii:,ii!i!ii:i i c-4

~.

l..~ss-ez~z ~ c ~ vz ss-1

~_vzss-1

+ f c - 4 + £ _ c - 3 - f__c-2 = 0;

] u ) m I _ Z N ~ P I - ] ~ E ANCE_VESS- 1 U VESS-I*sTm~,A($n VESS-1) + SI~(xn_ VESS- 1) *$f f _~ESS- 1 + e C-4 + e_C-3 - e_C-9. = O; MOOD]~,1 _ ~

IF P_L-1

Zl~l'-lq, OkT-]~_C-4 > P_R-3 thez~

Figure 1: T h e six basic KB:lloSS windows.

European Symposium on ComputerAided Process Engineering--6. Part A

$313

is automatically created by a simple "click". Then, the model can be edited in a models editor window. If the model has not been saved then equations can be deleted, added to, or modified. The rationale of any of these operations is maintained in the system to allow unexperienced modellers to learn from previous model development, and the eventual improvement of the automatic generation process. At. present, the system is capable of generating the gPROMS model code which demonstrates the feasibility of translating the KB.'lloSS equations into different flowsheeting systems. Once the group of modellers are satisfied with the model, they should then save the model. If some change has to be made to a saved model then it should be evolved into a new model that may be modified. Figure 1 shows that .IlODELI, whose equations are shown in the .models editor window, has been evolved to MODEL2. I(BMoSS has been progranlmed in an object-oriented Common Lisp package and the user interface uses Garnet which provides a powerful set of tools for building sophisticated graphical user interface applications (Myers et al., 1990). Entities within a scheme, models, and also equations are treated as objects according to the object-oriented paradigm.

A MODELLING

EXAMPLE

I N KBMoSS

In this section, a simple example is modelled using the KBMoSS environment ill order to demonstrate the underling support facilities. The case studied is a typical flash tank and an appropriate and consistent model is required as a part of a particular process design. The flash tank contains two phases which flow out of the container whereas another pipe feeds raw material. The description of this problem has to be given in terms of the above representation. The very first step in KBMoSS is to define a project. A simple click on the Top window will guide users through the creation of a project such as the one called TEST in Figure 1. Selecting the project and activating a menu with the right mouse button activates the Identifiers window. Default symbols have been accepted for this example and no changes were made on this window. The Scheme Hierarchy window is then activated from the Top window. If the flash problem were only a. part of the project then more than one scheme should be created and their interdependency would be shown in this window. For this example, however, only the scheme TEST1 has been defined. Selecting the appropriate item in the menu obtained by selecting scheme TEST1 activates the Floatsheet window, see Figure 1. The structural information is then incorporated in this window by activating menus, selecting appropriate items, and following on-line instructions. Almost all incorporated entities and their associated names can be observed in Figure 1:1~1 is the vapour phase, L-1 is the liquid phase, VESS-1 is the container, R.-1, R-2, and R-3 are reservoirs, and C-1, C-3, C-4 and 6:.-2(not shown in the Figure) are the connections. Turbulent flow is the default mechanism assigned to any mass transfer connection. Hence connection C1 should be changed to allow equilibration between both phases, Once the incorporated structural information is satisfactory, the scheme must be saved to generate the mathematical models. Saving the scheme enables the Models Manager window to be activated. The Models Manager window is used to automatically generate models based on the description of the process. ,IlODEL1 has been created by a simple click on the appropriate item. Equations contained in MODEL1 can be shown after activating the Models Editor window by selecting this model and the appropriate item in the popup menu which follows this selection. If this model is satisfactory then it must be saved. The saved model cannot be modified anymore. Now consider the case where a reaction occurs within the flash. Since reaction models have not been developed in the system, the user should modify the equation MODELI_MASS_BALANCE_VESS1 to incorporate the appropriate variable. However, MODEL1 has been already saved and cannot be modified. In this case the user should evolve the model to, say, MODEL2, see Figure 1. Then, activation of the Models Editor window for MODEL2 and selecting the equivalent MODELI_MASS_BALANCE_VESS1 equation would activate a menu associated to the window. Here, the modify item can be selected to perform the required changes. In addition, a new equation to model the reaction should be incorporated by selecting the Create new item in the menu. The rationale of the model provides information about all changes in comparison to the original model. In addition, suppose that users decide that laminar rather than turbulent flow is more acceptable for connection C-4. Crearly, this is not evolution since the description corresponds to different phenomena. Hence a new model should be generated instead of evolving any existing ones. The modelling space is then expanded to cope with all eventualities. This knowledge can be used in the future to improve the automatic generat ion.

$314

European Symposiumon ComputerAided Process Engineering~. Part A

CONCLUSIONS

According to Smithers et al. (1990), there are two ways of enhancing modelling: building artificial systems which replicate human modelling behaviour and building systems which provide support to human modellers. While both approaches have their advantages and disadvantages, it is shown here that they are in fact complementary. A system where both techniques are adopted has been developed here. As expected, a support system requires significant resources and time scales to achieve useful results but tile advantages are great.. h'BMoSS is a modelling support system based on tile exploration-based model of design (Smithers et al., 1990) and tile design support system IfBDS (Bafiares-Alc£ntara and Lababidi, 1995). Potential benefits are the maintenance of consistency during modelling, enhanced documeutation~ reuse of the model and its parts, as well as the provision of easy access to models for their verification, modification, maintenance, and eventual learning by inexperienced users. A particular feature of this system is its contribution as an modeller expert to automatically generate models. TILe work is intended to be applied to tile modelling of chemical process systems but the concept should be equally applic~tble in other areas.

References Bafiares-Alc£nt.ara, R., 1995, Design support systems for process engineeriug-I. Requirements and proposed solutions for a design process representation. Computers [J Chemical Engineering, 19(3):267-277. Bafiares-Alctintara, R. and H.M.S. Lababidi, 1995, Design support systems for process engineering. II. KBDS: An experimental prototype. Computers ~ Chemical Engineering, 19(3):279-301. Bogusch, R., aud W. Marquardt, 1995, A formal representation of process model equations. Computers 6' Ch.emical Engineering, Suppl., 19:$211-$216. Dieterich, E.E. and G. Eigenberger, 1995, Bimap- a tool for computer aided modeling in chemical reaction engineering. Computers ~' Chemical Engineering, Suppl., 19:$773-$778. Douglas, J.M., 1988, Conceptual Design of Ch.emical Processes. McGraw-Hill. Himmelblau, D.M. and K. B. Bischoff, 1968, Process analysis and simulation: deterministic systems. John Wiles & SoiLs. Marquardt, W., 1991, Dynamic process simulation-Recent progress and future challenges. In Proceedings CPC I t , pages 1-24. Texas, USA. Marquardt, W., 1994, Trends in computer-aided process modeling. In En Sup Yoon, editor, Proceedings of the 5th International Symposium on Process Systems Engineering, volume 1, pages 1-24, Kyongju, Korea, May, Seoul National University. Meyssami, B. and O.A. Asbjornsen, 1989, Process modeling from first principles-method and automation. In Summer Simulation Conference, pages 292-299. Myers, B.A., D.A. Giuse, R.B. Daunenberg, B.V. Zanden, D.S. Kosbie, E. Pervin, A. Mickish and P. Marchal, 1990, Garnet: Comprehensive support for graphical, highly-interactive user interfaces. IEEE Computer, November, 23(11):71-85. Pantelides, C.C. and P.I. Barton, 1992, Equation oriented dynamic simulation: Current status and future perspectives. Computers ~ Chemical Engineering (supl.), 17:$263-$285. Perkins, J.D. and R. W. H. Sargent, 1982, Speedup: A computer program for steady state and dynamic simulation and design of chemical processes. AIChE Symposium Series, 78(214):1-11. Perkins, J.D., R. W. H. Sargent, R. V~zquez Roman, and J. H. Cho, 1995, Computer generation of process models. To appear in Computers 6' Chemical Engineering. Piela, P.C., T.G. Epperly, K.M. Westerberg, and A.W. Westerberg, 1991, ASCEND: An object-oriented computer environment for modeling and analysis. Computers ~' Chemical Engineering, 15:53-72. Preisig, H.A., T. Y. Lee, and F. Little, 1990, A prototype computer-aided modelling tool for life-support system models. In 20th International Conference on Environmental Systems, Williamsburg, VA, USA, July. Preisig H.A., T. Y. Lee, F. Little, and B. Wright, 1989, On the representation of life-support system nmdels. In 19th International Conference on Environmental Systems, San Diego, California, USA, July. Smithers, T., A. Conkie, J. Doheny, B. Logan, and K. Millington, 1990, Design as intelligent behaviour: An AI in design research programme. Journal of Artificial Intelligence in Engineering, 5(2):78-109, April. Stephanopoulos, G.,J. Johnston, T. Kriticos, R. Lakshmanan, M.L. Mavrovouniotis, and C. Siletti, 1987, Design-Kit: An object-oriented enviromnent for process engineering. Computers ~.' Chemical Engineering, 11(6):655-674. Telnes, K., 1992, Computer Aided Modeling of Dynamic Processes Based on Elementary Physics. PhD thesis, University of Trondheim. Vgzquez-Rom~n, R., 1992, Computer aids for process model-building. PhD thesis, Imperial College, London SW7 2BY, March. Winston, P.H., 1992, Artificial Intelligence, Third Edition. Addison Wesley.