Process computer network in a chemical plant

Digi tal Computer Applications to Process Con trol, Van © IFAC and orth-Holland Publishing Company (1977)

auta Lemke ,ed.

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PROCESS by

F. A.

V~

HARMELE·

E. J.

Akzo Research Laboratories Co rp 0 rat eRe s a r h rnhem, The ether lands

1.

BSTRACT

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BRIEF DESCRIPTIO

These sections are:

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The vinyl chloride plant produce s 1000 tons of vinyl chloride per day. The production of vinyl chloride is based on ethylene and chlorine via a cracking proc ss of ethylene dichloride. The plant essentially consists of fi 'e sections (Figure 1).

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fter corr ction and calculation of more complex variables these data are stored into data files of the tim sharing computer. All th authorised time-sharing users hav access to the process data files from their own programs written in BASIC. The project was realised in about la months and th system has been operational since October, 1974.

During a p riod of two years a numb r of pro ess studi s w r carried out in a \'inyl chluride plant with the aid of a mobile process cornputer. As a result of these experiments a motivation could be given for the installation of a process information system. This system had to meet the demands of all plant disciplines involved, viz. process operators, production management, and engineering. Therefore, a modular computer network has been designed on th basis of an in-house time sharing system and four front-end processors. The communication between th comput rs is controlled by means of a real-time supervisory program in the central time sharing computer. Every five minutes a compl te set of actual process data is collected.

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Akzo Zout Chemie Henoelo (0) The T therlands

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FIGURE 1: Vinyl chloride p1.ant (schematic).

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two sub-plants for the pruQuction of ethylene dichloride: a direct chlorination and an ox -chlorination process; - a distillation section for eth lene dichlo ride; a crackin section; - a distillation section for hydrochloric acid and vin I chloride.

VAPOUR

The complete plant is controlled by one control room. The 1200 process ariables are measured and 250 ariables are controled by means of con entional controllers. Additional process information for the operators is obtained from on-line quality analysers and from off-line laboratory analyses of samples of process streams.

3.

STEAM PRODUCT

MOTIVATIO FOR THE I STALL OF A COMPUTER SYSTEM

During a period of two years three experiments were carried out with a mobile process computer on arious sections of the process. The results indicated -among other things- the possibility of more efficient process control by using complex ariables calculated from on-line measurements. Complex variables are fQr example: heat exchange coefficients, chemical conversions, ratios, efficiencies, etc. An example of a complex va riable is p re s ented in Figure 2. Valuable light components have to be stripped from a tarry product. This separation is realised as a batch process in a heated vessel. When a certain maximum viscosity has been reached, the residue has to be pumped out. As direct measurement of the viscosit is too complicated, the heat exchan e coefficient from steam to product is calculated and used as a measure of the \"iscosity. Quantification of the improvements to be obtained by using complex variables showed that the installation of a computer system would be economically attracti 'e. Besides, the s stern was expected to be er useful for three other reasons: - it can offer actual process information to production management, who will thus get a better 0 erall picture of the plant performance; - it will be a ood tool for he on-line n v est at ion 0 th e pro c e s s to f· n d ne\.\.- useful complex process \-ariables and optimal process settin s; - it c an a Iso be use d e r y well for engineering and maintenance purposes of the process equipment.

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FIGURE 2: Example of complex variable: calculation or a heat exchange coefficient.

4.

SYSTE

TS

From the demands of the future s stern users and from the experience gained during the experiments with the mobile conputer the following sys tern requi rements could be deri -ed: - Process data ha e to be collected from on-line measurements and off-line from the process operators and the laboratory. - Complex 'ariables have to be calculated. - The 'arious disciplines related to the process must ha"·e easy access to the collected process data files. These disciplines are: process operators, production mana ement, process de ·elopmen en neerin and maintenance. - Selected process informat·on must be presented on-line to the process operators. - In view 0 expected expansions in the future the computer s stern has to be modula r in ha rdwa re and soft a re. o di r ec t ala rm funct· on s 0 r on -lin e clo sed loop control ha e to be performed b the s stern. A maximum time dela of 10 minutes between actual measurement and presentation to the operator is permitted.

PROCESS CO 1PUTER

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ETWORK I

Some figures about the principal system information flow are presented in Figure 3.

5.

5.1

SYSTEM DESrG

Various technical solutions are possible to meet the system requirements as mentioned above. A design was chosen on the basis of an in-house time sharing system (Digital Equipment RSTS/E) and four front-end microprocessors. The decision to use this configuration was made for the following reasons: - A distribution of functions by using comparati ely simple modules minimizes the effect of errors and provides an easily expandable system. The microprocessors are independently connected to the time sharing computer, as if they we re te rminal s. - When using a time sha ring compute r, the users will have easy access to the process data. - It is possible to use recently developed, low-cost microcomputers. The computers have been set up in accordan c e with th e m a s t e r - si a v e s y s t em, th e t i m e sharing computer functioning as master. The functions assigned to the computer types are as follows:

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Functions of the front-end microcomputer s

The front-end microcomputers have to run independently on a real time base. They have to provide the following function s: Con t r 01 0 f th e pro c e s sin t e r f ace. Sampling and scaling of process ariables. Digital filtering and conversion to standard units (for instar.ce, the root extraction from flow differential pressure signals). Execution of received commands from the central time sharing computer. Generation of analog values for trend recording. Local facilities for on-line troubleshooting. 5.2

Functions of the central time sharing computer

Apart from its time sharing task, the central computer has to provide the following function s: Control of the front-end computers by means of a set of commands for, among other things, the collection of pro c e s s da ta . - Analysis of errors in the front-end computers and in the communication.

DATA FILES (INTERNALLY STORED): every 5 minutes, , hour, 8 hours and 3 x 24 hours

ON- LINE MEASUlUMENTS: every minute 128 pneumatic signals

96 thermocouples 30 electric signals 5 quality analysers

COMPU~

MANUAL INPUT: random from

FEEDBACK TC PROCESS OPERATORS: every C; minutes, , hour and 8 hours.

-process operators -the laboratory

REPORTS TO PRODUCTION

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TIME-SHARING FACILITIES

FIGURE 3:

Principal system information.

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- VINYL CHLORIDE PLANT

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4 FRONT-END COMPUTERS & INTERFACE

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FIGURE 4: Computer configuration vinyl chloride plant. - Processin of additional process information r c i\-ed \'ia terminals in the process control room and the laboratory, - Corr ction of process \'ariables and calculation of complex variables. - Gene ration o' proce ss data file on a t'me base of ~ Hlinut s, and 1, and 3x24 hours. - F edback of process in ormation to the process operator and repor in o to prod ction mana ement.

6.

H.-\RD\\ .--\RE .-\. -D SOFT' ARE

.--\ men ion d in chapte r 5, the e1e c e d compu er system consi ts o' four front-end comp te rs and a central compute r, For manual inp t o' add't'onal pruce s 'n'ormation from h proce s control room and the labora ory, t\,\·o e rnlinal wi 11 be u ed. Sy tem ou p t or the operator \\,'11 be ' \'en on a hi rd ernl'nal. For produc 'un mana ement and en ineer'n p rpo a term'nal w;ll be a\-ailable 'n the comp ter room. The complete con 0 ra ion's 'n F' re 4. 6, 1

Front-end conlp ter

As no itable microcomputer 'wa comm rc'ally a\'ailable (end 19(3), a confi ration

was built by the Appli d Physics Dept. of kzo th Intel 800 integrated circuit being used as the central processor. The front-end computers op rate on a basic frequency of 2 Hz from their own real time clock. On every pulse o· the lock one proc ss \'ariable is sampled, filtered and stored (accumulati\'ely). The idle time until the next p u Is e can be use d for co m m un ' cat ion (p re parin data to be transmitted to the central computer) and to answer requests from a locally connected terminal for troubleshootin purpo e . Commerc'al instruments such as pneumatic scanners and electronic con\'erters for thermoco pI connect'on, are u ed a an interface for the process sionals. To meet th demand for modu1arity the frontend computers h \'e been standardized in thlee models ('ncludin o their sof vv'are and 'nterface. - .--\ nlodel for the connection of pne nlatic and h' h 1 vel electric ' nals. Th's model also pro\'ides analo 0 tput si nals (trend record'n ), - A model for the connect' on 0 thermoco ple . - A model for the connec ion 0 qual'ty anal, ers or example a chromatoraphs. For on-line trouble hoot'n e\'ery model has been provided with a facility for the connect~

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ion of a local t To fa ilitat ov an analo r fer on input of ve

6.2

ST ORK I.

rminal (asynchronous ASCII). rall s st m error det ction nce si nal is conn cted to r r model. sharing sy stem

Th ntral in-house ime sharin computer consists of a 'RSTS lE" s stem from Di ital Equipm nt with a B_ SIC-PLUS lan ua processor. 0 hardware or software modification had to b mad for this application. For the communication with th front-end computers a real time supervisory pro ram was written in BASIC. This pro ram is run every fiv minut s and provides the following func tion s: - Communication ith th front- nd computers: asking for and rec i ing of process data and transmission of filt r coeffi ci en ts and \'al u s for tr nd recording. - Error checking: th proc ss data received from th front-end computers are checked for format and the value of a refer nce input. A status number also re c e i v e d i san a 1~JTS e d and, if ne c e s sa r y , actions will be taken. These actions can be to repeat a command, to restart th front-end computer, and to print alarm me s sage s. - Data file creation. One primary data file of all process data obtained on-line is created

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including in ormation about the quality of the data. The up rv' sor pro ram could only b real. sed by usino spec' al statem n rom B_ SICPLUS uch as: TI 1E, \ AIT and 0_ ERROR GOTO. B th sup r isory pro ram anoth r packa e of pro rams i acti\:ated. This packa forms a se ondary data fil from the primar fil and from man ual input of the pro e s op rators and the laborator . It also reat s fil s over periods of tim : 1 hour, hours and 3x2 hours, and nera es Yeports. - 11 secondary files consist of corre t d proess data and omplex variables. Th s st m prog ram sand fil s a re shown in Fi ur 5.

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11 the t rminals used for man al input of proc ss data and for output of reports are no ::-mal tim sha rin t rminal s and can be used as su h. hen process information has to b supplied to the system, an authoris d user can write the data on a sp ial file. - 11 authorised users from th \'arious discipline can se the proces data of th s condar files from th ir own pro rams written in B SIC-PLUS.

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PROCESS DAmA FILES OVER 5 MINUTES t 1 t 8 AND 3x24 HOURS

FILE FOR PRIMARY PROCESS DATA

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FIGURE 5: Real time programmiug of the time sharing

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Hardware performance: the time between failures of the network appears to be about 6 months. - Soft are performance: so far no traceable software errors occurred in the front-end computers. However, about twice a month a format error occurs in the data recei ed from a front-end computer. The source of these errors can be attributed to induced electric pulse s in the microcomputers or on th communication lin s. - Acceptance: in gen ral, the system has been well accepted, although so far not all the discipline s have made the expected intensive use of it. Programs have been developed by the process engineers for the process operators relating to: . Optimal settings for the reboiler steam fl ow s an d re fl ux r a ti 0 s 0 f des till at ion columns as functions of the production rat e , and a 1so a s a fun c t ion 0 f a critical quantity of an important product component . . Optimization of the efficiency of the oxychlorination sub-plant: calculation of the product composition of a fluid bed reactor from process input flows. Process settings obtained from this calculation lead to a better reactor efficiency than those from the measurements of an on-line quality analyser. - Profits: one year after installation of the system a saving on energy and raw materials of Hfl. 600,000. - was reached. This result was fa ourably influenced by the drastic increase of the ene rgy and ethylene cos ts.

Front-end computers and terminals are connected to the time sharing system b means of four-wire full duplex asynchronous ASCII lines. The transmission speed used for the front-end computers amounts to 27 characters pe r second. If desired, public telephone lines can be used for the data transport. All communication is initiated by means of commands from the central computer. For these commands a set of characters are chosen which are recognised by the front-end compute r s. Error checking during transmission is based on sof t war e form at t e s ti n g . In cas e 0 fer r 0 r an action can be repeated. 0 parity check is used.

8.

PROJECT REALISATIO

The project started in January 1974, when a decision was reached about the in estment and the system configuration. The computer system was operational in Tovember 1974. The critical path in the project planning turned out to be the long delivery time of the process interface for thermocouple connection (delivery time 1 year). To overcome this difficulty a provisional technical solution was made for the most important thermocouples. o unexpected difficultie s appeared during the execution of the project. As no detailed documentation on some characteristics of the time sharing software was a ·ailable, the· had to be in\:estigated b trial and error methods. The system realisation costs are (Dutch guilders, 1974): Di ital Equipment RSTS/E Hfl. 216,000. 71,000. 4 Front-end microcomputers Process interface 96,000. 71,000. Installation System desi n Superv'sory pro ram Soft are ront-end compute r s En ineerin Tra'n'n ~.

2 man-months 2 man -months man-months

5 man-months 1 man -m on th

B. ~ot included are the costs o' the terminals (already ava'lable) and the pro ramm'n of the application software (4 man-months).

9.

PERFOR 1A. TeE, PROFITS

Durin the past two ears s stern practice has been obta'ned in he f" eld of computer performance, acceptance b the users and economical profits.

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10.

SYSTE

At the end of 1975 the computer network has been extended by connecting it to a butanol plant. This has been realised by means of three front-end computers and three term'nals. Two term'nals are located 'n the process control room and one in a laborator . The modular s ·stem design pro 'ed to be er' successful for the implementat·on. For only one front-end cornputer had ne soft are to be r·tten. 'thin the time sharing system the major part of the programming could be copied from the v'n 1 chloride plant appl·cation. Because of the distance between tre butanol plant and the t'me sharing system (12 km), time division multiplexers are used to facil'tate the transm' ssion of all data \,'a one leased telephone line. The expans'on costs amount to Hfl. 1,000. - per connected process variable. In the near uture an automat' c samplin 'report'n as chromatograph of a laboratory w'll be connected to the s stern. The installation of computer net orks of the

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described desi n in oth r plants is bein discussed.

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