Minicomputers and Large Scale Computations

5 A Distributed Minicomputer System for Process Calculations

Downloaded by CORNELL UNIV on September 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/bk-1977-0057.ch005

A.I.JOHNSON, D. N. KIDD, andK.L.ROBERTS University of Western Ontario, London, Canada

During the past decade one of the authors (Johnson) and h i s f a c u l t y colleagues and students have been developing, a p p l y i n g , and evaluating the modular approach to the steady state and dynamic behaviour of process systems. These studies have l e d to two executive systems, GEMCS (1) for steady s t a t e and DYNSYS (2) for dynamic systems studies which have academic and i n d u s t r i a l use. More r e c e n t l y a large timesharing computer, a DECsystem 10, has been used to create an integrated system for process a n a l y s i s and d e s i g n , INSYPS. This system o f f e r s o p t i m i z a t i o n facilities and i n t e r f a c e s to the process designer or analyst through a graphics console (3). With the INSYPS system on the DECsystem 10 the process engineer can call, from one facility, upon the full range of executive programs for handling the various aspects of complex designs and g r e a t l y enhance h i s p r o d u c t i v i t y and creativity. INSYPS on a Large Computer The o r i g i n a l INSYPS deals with two concepts, the use of i n t e r a c t i v e graphics for input and output and automatic linkage between independent packages. The i n t e g r a t i o n of packages i s c a r r i e d out i n a modular fashion to enable one to add new systems or modify e x i s t i n g ones without a f f e c t i n g the e n t i r e s t r u c t u r e . The areas considered for INSYPS are: 1. 2. 3. 4.

Computer Graphics Steady State Simulation Optimization Dynamic Simulation

65 Lykos; Minicomputers and Large Scale Computations ACS Symposium Series; American Chemical Society: Washington, DC, 1977.


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MINICOMPUTERS AND LARGE SCALE COMPUTATIONS

In theory one could make a l l four of these communicate with each other. Figure 1 shows the s y s t e m , i l l u s t r a t i n g b o t h t h e p o s s i b l e l i n k s and those actually implemented. Communication between packages i s e i t h e r a s i n g l e s t e p l i n k a g e o r an iterative link. A single step l i n k a g e t r a n s f e r s i n f o r m a t i o n once per e x e c u t i o n , as i n t h e p l o t t i n g o f the results from a dynamic s i m u l a t o r . I t c a n u s u a l l y be a c c o m p l i s h e d by a data f i l e . T h e i t e r a t i v e l i n k a g e , on t h e other hand, deals with a continuous flow of i n f o r m a t i o n between p a c k a g e s , s u c h a s t h e l i n k b e t w e e n a s i m u l a t o r and the optimization packages. T h i s type of l i n k a g e can o n l y be carried out by interfacing programs specially designed f o r the packages. Some o f t h e c h a r a c t e r i s t i c s of such a program would be: 1. E a s y 'hook-up': t h a t i s , making a subsystem r e a d i l y a v a i l a b l e f o r an a p p l i c a t i o n a r e a w i t h a flexible procedure and associated implementation technique. 2. Efficiency: t h i s i s required i n running time, and is particularly desirable in iterative systems. 3. Automatic operation: once the l i n k has been established the resulting system s h o u l d be automatic in nature. The u s e r s h o u l d not be r e q u i r e d t o know t h e d e t a i l s o f t h e l i n k a g e o f the subsystem.

Figure 1.

Solid line: existing links; dotted line: possible extensions to the system

Lykos; Minicomputers and Large Scale Computations ACS Symposium Series; American Chemical Society: Washington, DC, 1977.

5.

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A Distributed Minicomputer System

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INSYPS was b u i l t w i t h a l l o f t h e a b o v e c r i t e r i a i n mind. It consists of four s e l f - S t a n d i n g subsystems, f o u r l i n k i n g p r o g r a m s , and s i x d a t a files. Figure 2 describes their layout. Since the subsystems are independent o f each o t h e r they a r e n o t loaded t o g e t h e r , e x c e p t when t h e f u n c t i o n t o be o p t i m i z e d r e q u i r e s p l a n t simulation. A p p l i c a t i o n o f INSYPS A chemical process i s u s u a l l y represented by a process flow sheet, which i s made up o f p r o c e s s i n g u n i t s j o i n e d by l i n e s . The l i n e s or streams represent flow of m a t e r i a l or i n f o r m a t i o n . In g e n e r a l t h e flow s h e e t and t h e programs w h i c h s i m u l a t e them deal with the equipment (or p r o c e s s i n g units) and t h e s t r e a m connections. One b u i l d s up the process by putting together t h e d e s i r e d u n i t s , t h e shape and b e h a v i o u r o f which are p r e - d e f i n e d . A set of graphical symbols represents t h e u n i t c o m p u t a t i o n s a n d a d i a g r a m c a n be b u i l t to represent the information flow. Usually there is a one-to-one correspondence between t h e g r a p h i c a l symbol and t h e p r o c e s s i n g u n i t . Each g r a p h i c a l symbol corresponds to a unit computation subprogram i n the l i b r a r y of the simulation system. In g e n e r a l the arrangement of t h e m o d u l e s y m b o l s w i l l be s i m i l a r t o t h o s e on t h e p r o c e s s f l o w d i a g r a m . The designer constructs the flow sheet by s e l e c t i n g p r o c e s s i n g u n i t s f r o m t h e menu. E a c h u n i t he selects i s given a number and parameters. Stream c o n n e c t i o n s a r e a l s o d e f i n e d by p i c k i n g t h e a p p r o p r i a t e stream f u n c t i o n s . For every stream the designer gives i t s number, i t s s o u r c e a n d d e s t i n a t i o n u n i t s , i t s t o t a l flow, temperature, p r e s s u r e , vapour f r a c t i o n , and t h e component concentrations. By convention a material f l o w s t r e a m i s shown b y a s o l i d line, an information flow stream by a dotted l i n e . The o t h e r d r a w i n g o r w r i t i n g o p e r a t i o n s p e r f o r m e d a r e mnemonic a i d s o n l y a n d are not passed t o the s i m u l a t o r . On completion of the diagram the program understands t h e p r o c e s s t o p o l o g y - *-^e i n t e r c o n n e c t i o n s of t h e components nd has a record of the i n p u t / o u t p u t stream parameters f o r each p r o c e s s i n g u n i t in the diagram. T h e e n t i r e p r o c e s s i n f o r m a t i o n c a n be s a v e d on t h e d i s k b y e x e c u t i n g t h e SAVE f u n c t i o n on t h e menu. The o t h e r a s p e c t o f t h e g r a p h i c s s u b s y s t e m i s t h e graphical output. Since a person can e v a l u a t e a graph more q u i c k y t h a n he c a n a l o n g l i s t o f numbers, this aspect of INSYPS greatly facilitates the study of t r a n s i e n t svstems. The d e s i q n e r c a n d i s p l a y a single



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variable o r a g r o u p o f them t o g e t h e r , a n d c a n p o i n t a t any l o c a t i o n on t h e g r a p h a n d h a v e i t s v a l u e t y p e d o u t . He can also h a v e t h e g r a p h f r o m t h e CRT p l o t t e d o n a conventional plotter for better resolution or larger scale. The d e s i g n e r o f t h i s s y s t e m would start o f f by graphically creating the process flowsheet. ( T h e same s t r a t e g y i s used f o r both steady state and dynamic systems.) He i s e x p e c t e d t o know t h e u n i t computations corresponding to h i s process. Any o r a l l information on the process diagram ( i . e . changes i n equipment or streams or their parameters) can be updated graphically. On completion of the input, the designer is provided with an i n f o r m a t i o n f l o w d i a g r a m and a d a t a f i l e to run the d e s i r e d simulator. The results from the dynamic simulator a r e p u t on a d i s k f i l e , and s e l e c t e d i n f o r m a t i o n f r o m t h i s c a n be g r a p h e d . Figure 3 illustrates the i n t e r a c t i o n of the user with the computer programs o u t l i n e d above. C o n t r o l o f a Simple E v a p o r a t o r System The problem presented was to control a one-component total vaporizer with two mode controllers. The study required determination of optimum s e t t i n g s and s t e a d y s t a t e o p e r a t i o n . A diagram o f the p r o c e s s , presented i n f i g u r e 4, shows the control configuration applied. The

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Figure 4 evaporator has a capacity for 800 lbs. of liquid oprerating at normal l e v e l c o n d i t i o n s . The e q u i p m e n t i s d e s i g n e d t o h a n d l e 1700 l b s / h r o f any component at steady state operation. T h r e e main v a r i a b l e s have t o be c o n t r o l l e d i n o r d e r t o c o n t r o l t h e entire process: the level of l i q u i d i n the v a p o r i z e r , the output flow o f t h e v a p o u r , and t h e p r e s s u r e i n t h e vessel. These variables are controlled by manipulating correspondingly: the l i q u i d flow i n t o the vaporizer, the steam through the coil, and t h e f l o w o f v a p o u r l e a v i n g the equipment. Once the unit modules were designed and the process configuration defined the information flow d i a g r a m f o r t h e s i m u l a t i o n was c r e a t e d . F i g u r e 5 shows the diagram a s i t a p p e a r e d on t h e s c r e e n a f t e r i t was drawn, u s i n g the programs o f the graphics subsystem. The i n f o r m a t i o n p r o v i d e d i n the streams r e p r e s e n t s the i n i t i a l c o n d i t i o n s at time z e r o . A l l the given values represent steady s t a t e o p e r a t i o n c o n d i t i o n s , except f o r t h e v a l u e s o f t h e p r e s s u r e i n t h e v e s s e l ( s t r e a m 5) and liquid level (stream 2) which will produce a step change i n the s i m u l a t i o n of those v a r i a b l e s . This can be r e c o g n i z e d by c o m p a r i n g t h e m e n t i o n e d v a l u e s a g a i n s t the s e t p o i n t v a l u e a s s i g n e d to the c o n t r o l they feed. The first simulation r u n was s t a r t e d and t h e r e s u l t s d i s p l a y e d on t h e s c r e e n a s graphs. There were five variables under o b s e r v a t i o n : 1) L i q u i d f l o w , 2) S t e a m f l o w , 3) V a p o u r f l o w , 4) Liquid level, and 5) Vessel pressure. The basic criteria for control settings were: recovery from perturbation to normal steady



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state as fast as possible, minimum amplitude of o s c i l l a t i o n during recovery. Sample comparative results for each of the variables m e n t i o n e d a r e shown i n F i g u r e 6. The g r a p h s d e s c r i b e t h e r e s u l t s o f t h i s e x a m p l e and at the same time illustrate the general procedure already mentioned. I t i s evident that i n every set of curves representing specific v a l u e s t h e r e i s one t h a t c a n be c o n s i d e r e d as t h e b e s t i n p e r f o r m a n c e . A designer has to compromise at this point a s t o w h e t h e r he w a n t s f a s t e r response, s a c r i f i c i n g o s c i l l a t i o n amplitudes and h i g h e r number o f o s c i l l a t i o n s o r s l o w e r r e s p o n s e b u t i n a smoother form. An I n t e g r a t e d P r o c e s s A n a l y s i s and D e s i g n S y n t a x on a D i s t r i b u t e d Computer A s s e m b l y Having demonstrated a potential opportunity to enhance t h e c r e a t i v i t y and p r o d u c t i v i t y o f t h e p r o c e s s and d e s i g n e n g i n e e r w i t h I N S Y P S , a d i s t r i b u t e d c o m p u t e r system i s being developed. The system is tentatively named GRAMPS. The analytical capabilities of this new s y s t e m w i l l be similar to those provided by INSYPS, but several performance b e n e f i t s are expected: 1. Graphic o p e r a t i o n s response speedup. The new system incorporates a programmable g r a p h i c s processor capable of performing a l l flow diagram editing o p e r a t i o n s , graph l a b e l l i n g , and t h e l i k e . 2. Calculâtional response speedup. Process calculations w i l l be p e r f o r m e d on a d e d i c a t e d minicomputer, providing better and more predictable response time than a large timeshared system. 3. Portability. The s y s t e m i s compact, and can be moved o n - s i t e f o r a p e r i o d o f i n t e n s i v e u s e by a chemical company's own engineering personnel. 4. Economy o f O p e r a t i o n . The o p e r a t i n g c o s t s a r e relatively f i x e d , and known i n a d v a n c e ; they are i n s e n s i t i v e to the amount of analytical work d o n e .

C o n f i g u r a t i o n and Operation T h e GRAMPS system (figure 7) consists of two minicomputers and various peripherals and communications l i n k s . G r a p h i c s Computer: This is a 32K minicomputer which is programmed in GRAPPLE (4), a graphics



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p r o g r a m m i n g l a n g u a g e s i m i l a r i n s t y l e t o ALGOL. This system i s manufactured by Systems A p p r o a c h , L t d . i n O t t a w a on l i c e n s e f r o m B e l l N o r t h e r n R e s e a r c h , L t d . I t i n c l u d e s f o u r f l o p p y d i s k d r i v e s f o r l o c a l s t o r a g e , and a T e k t r o n i x 4015 s t o r a g e t u b e graphics terminal with p l o t t e r f o r hard-copy output or d i g i t i z e d input. Process Calculations Computer: This is a 32K PDP-11 '34 running under RT-11 and programmed i n FORTRAN. I t p r e s e n t l y h a s two f l o p p y d i s k drives for l o c a l s t o r a g e and a t e l e p r i n t e r f o r h a r d - c o p y p r i n t o u t . C u r r e n t p l a n s c a l l f o r t h i s s y s t e m t o be expanded to 64K memory w i t h a d d i t i o n o f a f l o a t i n g p o i n t processor and c a r t r i d g e d i s k s t o r a g e . T h e GRAMPS s y s t e m h a r d w a r e c o s t i s about $100,000. The PDP-11 i s t h e n u c l e u s o f t h e s y s t e m ; i t has communication links with t h e GRAPPLE console, the D E C s y s t e m 1 0 , and a t e l e p r i n t e r , and i s r e s p o n s i b l e f o r o v e r a l l s y s t e m c o n t r o l and f i l e r o u t i n g . When the INSYPS capabilities have been fully implemented i n GRAMPS, t h e p r o c e s s f l o w s h e e t e d i t i n g f u n c t i o n w i l l r e s i d e on t h e GRAPPLE c o n s o l e . When the flow sheet and parameters have been s p e c i f i e d , t h e PDP-11 will be passed a file giving the process topology and the associated values. One or more a n a l y s i s o p e r a t i o n s may be c a r r i e d o u t on the PDP-11, with the r e s u l t s being written to f i l e s . Output f i l e s may t h e n be t r a n s f e r r e d t o t h e t e l e p r i n t e r f o r listing a n d / o r t o t h e GRAPPLE c o n s o l e f o r d i s p l a y a s g r a p h s . At present the various analysis packages f o r steady state simulation, optimization, and dynamic s i m u l a t i o n a r e o p e r a t i o n a l on t h e P D P T I I , and current emphasis i s on the communications l i n k between t h e GRAPPLE c o n s o l e and t h e PDP-11. R e s e a r c h and D e v e l o p m e n t Needs The d e v e l o p m e n t o f a m o d u l a r p r o c e s s a n a l y s i s and d e s i g n system with a range of c a p a b i l i t i e s (e.g. steady state simulation, dynamic simulation, optimization, d e s i g n a n d s y n t h e s i s ) on a d i s t r i b u t e d c o m p u t i n g s y s t e m h a s c r e a t e d some e x c i t i n g o p p o r t u n i t i e s f o r r e d e s i g n o f the system executive programs and f o r r e s e a r c h into e f f i c i e n t d a t a and p r o g r a m storage. Low c o s t g r a p h i c s t e r m i n a l s have been shown to greatly e n h a n c e t h e c r e a t i v i t y and p r o d u c t i v i t y o f t h e design engineer. Y e t much n e e d s be l e a r n e d about the quantity a n d q u a l i t y o f i n f o r m a t i o n t o be p r e s e n t e d on t h e s c r e e n , and t h e d i f f e r i n g needs of a range of users. While a l l are familiar with graphical r e p r e s e n t a t i o n o f d a t a , new n e e d s and o p p o r t u n i t i e s f o r the presentation and manipulation o f complex systems await development.



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In the system under development the GRAPPLE console has a significant analysis capability. The best use of this requires further research. It is apparent that easy communication between and among the computers of a distributed system is a key for their effective use. The units should be essentially parallel processors, taking f u l l advantage of the independence and differing strengths of the computer systems. The communication needs are closely tied to the optimum use of the diskette and cartridge disk storage available. The diskettes provide convenient low cost, personal aids to program development and evaluation. Cartridge disks provide long-term, high-volume storage and faster access. The process calculation computer must serve as a communication link to remote computers when these can solve complex problems outside the capabilities of the local system. Distributed minicomputer-based systems open new dimensions for process analysis and design and encourage reconsideration of the existing design methodology. Abstract This paper describes a distributed minicomputer-based system for simulation and optimization studies of chemical process systems. The system provides an integrated analysis environment for the process engineer, including graphical input of flow sheets and display of performance curves. The system was i n i t i a l l y developed on a large timesharing system and is now being re-developed on a distributed minicomputer system. Literature Cited 1

2

3

4

'GEMCS - General Engineering and Management Computation System', (1971), A. I. Johnson and Associates, The University of Western Ontario, Faculty of Engineering Science, London, Canada. 'DYNSYS User's and Systems Manuals', (1976), A. I. Johnson, J . Barney, R. S. Ahluwalia, available from SACDA, The University of Western Ontario, London, Canada. Ahluwalia, R. S., Lopez, J., Johnson, A. I . , Millares, R., 'Integrated Computer Aided Design System for Process Design', Proceedings of the IFAC Symposium on Large Scale Systems Theory and Applications, Udine, Italy, June 1976. Woolsey, L. G . , 'Design for a High Level Graphics Language Machine', Infor, vol. 13 1975), pp. 248-259.


Minicomputers and Large Scale Computations

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