Instrumentation andControl of

Methanol Plants

P.B. FranklandG.W. Griffiths

Davy McKee(Oil andChemicals) Ltd.

This paper describes the instrumentation and control of methanol plants and relates to past

experience and future developments. Brief descriptions of the reforming and methanol synthesis

processes are given. Past experience relates to large throughput plants, designed for severe

ambient conditions and remote locations.

INTRODUCTION

Methanol, which is primarily used to make formalde­hyde and acetic acid and likely to assume a more impor­tant role in the future as a fuel, fuel additive, or forprotein synthesis, has previously been supplied in plantsrated around the 1000 MTD. Early in 1977 Davy signed acontract with the USSR to design and supply equipmentfor construction of the world's two largest methanolplants in one of the world's most inhospitable climates.This was to call upon the combined expertise of alldisciplines.

Instrument manufacturers were undergoing a periodof change in the United Kingdom with the development ofmicroprocessors and VDU systems; therefore, the selec-tion of instrumentation was one of the major factors ofthe contract.

We were concerned at that time with obsolescence,both technological and functional; therefore, the objec­tive of this paper is to give some background on what hasbeen achieved compared with what can be achieved in the1980s regarding process control.

Presented at a session ofthe Process Measurement and Control Division. ISA/ 81.St. Louis.

0019-0578/82/02/0001/13/$2.50 ©ISA 1982

PROCESS DESCRIPTION(1)

The following description is based on a 2500 MTD,predominately two-stream plant suitable for use in alow-temperature environment. It conforms generally tothe methanol plants that were engineered by DavyMcKee (Oil and Chemicals) Ltd. for the USS R importingagency Techmashimport.

The plant is based on the ICI low-pressure methanolprocess and incorporates the Davy "low carbon" and the"reduced energy" concepts. The process route wasarranged mainly in two streams because of equipmentsize and problems with transportation.

With the "low carbon" concept, methanol is producedfrom a natural gas feedstock without the addition of C02to the synthesis gas. Excess hydrogen is purged from thesynthesis loop to maintain the desired reaction condi­tions. The "reduced energy" concept relates to the wayheat recovery is integrated with the main process in ahighly efficient manner and also to the way waste pro­ducts are burned in the reformer to produce useful heat.

ISA Transactions. Vol. 21, No.2 1

A simplified process flow diagram is given in Figure 1.

*Synthesis gas can also be obtained by "partial oxidationof crude" or by "coal gasification."

Extensive utilities are included in the overall project,i.e.,

• Water treatment• Auxiliary boiler plant• Bulk storage• Rail loading• Workshops / laboratories• Electrical reticulation• Instrument air / N 2

• Fire fightingHowever.. they will not be discussed here except wherenecessary for clarity.

The route to methanol from natural gas can beapproximated by:

Natural Gas Compression and Pretreatment

A battery limits su pply of natural gas is received at apressure of approximately 7 bars and at a normaltemperature of 15°C, which can theoretically drop to-55°C. It then passes through a separator to removeliquids and solids and splits into a feedstock stream andburner fuel streams for the reformers, auxiliary boiler,feedstock preheater, and pilots for the flare.

The feedstock is then compressed to 30 bars by atwo-stage centrifugal compressor, which runs at approx­imately 8000 rpm, absorbs about 8 MWand is driven by asteam turbine fed by medium-pressure superheated steam(28 bars, 340°C). The warm compressed gas then passesto the feedstock preheater where its temperature is raisedto 400°C prior to desulphurization. This is carried out byfirst passing the gas through a bed of cobalt molybdatecatalyst and then through two parallel beds of zinc oxidewhere the sulphur content is reduced to 0.5 ppm. Thefeedstock then splits into two streams prior to the reform­ing section. The two streams are identical and rejoin justbefore the synthesis gas compressor inlet.

2 [SA Transactions. Vol. 21, No. 2

Reforming and Heat Recovery

The object of the reforming process is to transform agas that consists mainly of CH4 to a synthesis gas thatcontains H2, CO, and <;::02. This is achieved by addingsteam to the feedstock in a fixed ratio, further preheat­ing the mixture in the reformer flue gas duct to 500°C,and passing it through the reformer. In the presence ofnickel catalyst, steam will react with gaseous hydrocar­bon at elevated temperatures and pressures to givereformed gas. The ratio of feedstock to steam is criticalfor satisfactory catalyst operation; too little causes car­bon build-up on the catalyst. Loss of steam for even ashort period can permanently damage the catalyst.

The reformer consists of a box heated by gas burnersthrough which pass 496, 13 m long, 100 mm diametervertical tubes filled with catalyst. Manifolds and pigtailsare provided at the top and bottom so that all the tubesare paralleled. It is equipped with 153 downward-firingburners that provide the necessary heat of reaction forreforming. All burners are fed with combustion air thathas been preheated by the flue gas and is supplied underslight pressure from a steam turbine-driven combustionair fan. The furnace itself operates at a slight negativepressure.

The feedstock/ steam mixture enters through the topmanifold, passes through the catalyst, and leaves fromthe bottom manifold as reformed gas at a temperature of860°C and a pressure of 20 bars. Following heat recov­ery, this reformed gas, which is also known as synthesisgas, is then used for methanol synthesis.

The flue gases are collected in channels in the floor ofthe furnace. These channels feed into a common collectorwhere the flue gases are extracted by a steam turbine­driven ID fan and discharged to atmosphere. Thetemperature is controlled by 10 auxiliary burners.

Heat is recovered from the reformer flue gases to raisehigh-pressure superheated steam (175 tonnes per hour at105 bars and 490°C from each reformer) and preheatingof feedstock/ steam and combustion air. Steam raisingand feedstock! steam preheating are carried out in coilslocated within a refractory-lined steel duct connected tothe furnace through which the flue gases pass. Combus­tion air preheating is achieved using two separate "rotat-·ing heat sink" exchangers operating in parallel.

Heat from the reformed gas is recovered in two boilersoperating in parallel and then by HP boiler FW heating,refining column reboiling, topping column reboiling, anddeaerator FW heating. It is then reduced to 40°C by airand water coolers. Steam is condensed out from therefining column reboiler onwards and is removed inseparators. Dissolved gases are then extracted from thecondensate in a stripper prior to cooling and return to thewater treatment plant.

Water level in the H P steam is controlled using 3­element control in the normal way. Superheat tempera­ture is controlled by spray attemperation at a rate based

= CH4 + S! Desulphurization

= CH4 + H20

~ Reforming*= H2 + CO + C02 + CH 4

I Converting

rCH4 + H2 (Purge)= CHaOH + H20! Distillation= CHaOH + «0.05% H20)

Feedstock

Refined methanol

Feedstock + steam

Crude methanol

Synthesis gas

on inlet and outlet temperatures of the high-temperaturesections of the superheater.

Gas Compression and Methanol Synthesis

The two synthesis gas streams are combined followingheat recovery and condensate removal. This combinedstream then feeds the steam turbine-driven, 2-stage~ syn­gas compressor, which runs at approximately 7000 rpmand absorbs 31 MW at normal conditions. The pressureis raised from about 15 bars to 80 bars in two stages andeach stage is followed by an air cooler. Condensed wateris removed in a separator prior to the gas passing to thecirculator.

The turbine receives steam at 105 bars and 490°C andoperates near the limit of power that can be supplied froma single machine under these conditions. The speed of theturbine is governed to maintain a constant pressure at thesuction to the compressor by throttling "its" steamsupply. However, the M P steam system is predominantlysupplied from the passout of the syn-gas compressorturbine. This means that the M P steam system demandwill affect turbine speed (and, hence, compressor suctionpressure), and turbine speed will affect MP steam supplypressure. To reduce this interaction and ensure smoothoperation a special computing function was installed. It isessential to maintain MP steam for the reformer even ifthe syn-gas compressor trips out. This is achieved byincluding a desuperheating bypass valve in parallel withthe compressor, which will restore MP steam pressureand flow to pre-trip values within a half-second.

The compressed syn-gas is then fed to a circulator withsuction at approximately 77 bars and delivers gas to themethanol loop at about 87 bars. The circulator runs atapproximately 6000 rpm and normally absorbs 12.0M W. It is driven by a stream turbine operating on steamat 100 bars at 490°C.

The gas from the circulator splits into two identicalstreams, each consisting of a synthesis reactor (methanolconverter) and associated heat exchangers. Each streamsplits into a further two parts: quench gas and reactorfeed. The feed gas passes through two interchangerswhere it is heated up by effluent gas from the reactor.

The synthesis reactor operates between 240° C and270°C at 80 bars under normal conditions. As the feedgas passes through the reactor catalyst an exothermicreaction takes place. To moderate the temperature rise inthe reactor, it is necessary to inject cold "quench" gas atthree levels in the reactor. The ratio offeed-to-quench gashas to be maintained to within a close tolerance.

The performance of the process is dominated by thesynthesis catalyst performance and the mechanical designof the reactor vessel.

The gas leaving the reactor is used for heating BFWand the reactor feed gas. Following this, methanol andwater start to condense out. It then passes on to a mainair-cooled condenser followed by a water-cooled trim

condenser. The gas-liquid mixture then passes to thecrude methanol separator, after which the separated gasportion is supplemented by synthesis gas at the suctionside of the circulator. The liquid portion" which is princi­pally an 80:20 mixture of methanol and water (i.e., crudemethanol), passes through a pressure let-down stageprior to being transferred to storage. The flash gases fromeach of these stages are burned in the reformer.

Residual methane and excess hydrogen are purgedfrom the synthesis loop prior to the addition of freshsyn-gas to maintain an acceptable level of inerts and thedesired chemical equilibrium in the loop. This purge gasis let down approximately 40 bars when part is recycled tothe desulphurization section, the remainder being burnedin the reformer.

Methanol Purification

Purification is achieved by two stages of distillation:topping to remove permanent gases and light ends, fol­lowed by refining to remove the water and heavierorganic materials. A single topping column is used toprocess the total methanol production, but two columnsoperating in parallel are required for the refining stage.Both columns operate at less th..an 2 bars under normalconditions.

The topping column receives crude methanol fromstorage, which has been preheated by an exchanger in therefining column bottom line. To avoid substantially sub­cooling the reflux, the overheads are condensed in twostages. An air-cooled condenser condenses the bulk of theoverheads while the remaining vapor passes to a secon­dary water-cooled condenser that operates at a lowertemperature. This maximizes the amount of methanolcondensed while at the same time minimizing the reboilerduty. It also enables the small amount of impuritiesremaining (other than water) to be removed convenientlyby a secondary methanol purge, which is burned in thereformer. The main and secondary condensate streamsboth feed into a common reflux drum and the non­condensibles are passed to the reformer where they areburned. The topping column is reboiled by two gas­heated reboilers, one in each reformed gas stream.

Topped methanol is pumped from the base of thetopping column and is fed directly to the refining column,where water concentrates at the bottom and methanol atthe top. A small side-stream of heavy organic materials isdrawn off around the 10th tray where the concentration isat its highest. These fuseloils are subsequently burned inthe reformer. Each refining column is reboiled using agas-heated reboiler that provides approximately two­thirds of the required heat. The balance of the heat isprovided by a reboiler, which is supplied with LP steamat a rate that maintains the correct column temperatureprofile. Water is withdrawn from the base of the columnsand used to preheat the topping column feed. It is thenstored in an underground sump while the organic concentra­tion is checked before proceeding to biological treatment.

[SA Transactions. Vol. 21, No.2 3

PRODUCT METHANOL

AUXILIARYBOILER

WATERTREATMENT

RAW WATER

FLARE

COOLING WATERSYSTEM

• 1ELECTRICPOWER

NATURALGAS

Figure 1. 2500 tonne per day methanol plant

The overhead from each refining column passes to anair-cooled condenser and then to a reflux drum. The totalreflux is returned to the top tray while the product isremoved at about 3 trays down to ensure it is free of liglitends. The refined methanol then passes through a water­cooled exchanger and on to one of the two 2500 m'capacity test tanks in the storage area. The refinedmethanol main storage consists of two 20~000m ' capacitytanks. It is then distri buted by rail tanker cars to the user.

PAST EXPERIENCE

Low-Temperature Aspects

As can be seen from Table I, only the months of Julyand August can offer minimum temperatures abovefreezing point. Therefore, it was important to plan andcoordinate the delivery of materials that could be dam­aged by prolonged exposures to minus temperatures intothe "weather window," as we called it.

Material selected and testing formed a very importantpart of the project due to the extreme weather conditionsthat would prevail in the area.

Most stainless steels will of course be unaffected bycold temperatures, but on large diameter lines and fit­tings these tend to be very expensive. Therefore, subjectto suitability of process requirements, low-temperature

carbon steel was used, which was satisfactory for operat­ing in the range of -55/ +1500 C. It was also a suitablematerial for control valves and relief valves.

Table 1

Temperature

Average Averagedaily monthly Absolute

Max Min Max Min Max Min

degrees CelsiusJanuary -12.9 -20.3 -2.5 -35.9 0.9 -44.1

February -9.3 -17.7 0.2 -30.5 4.6 -40.7

March -2.3 -11.2 5.5 -25.5 9.9 -34.3

April 8.5 -1.3 18.3 -13.2 24.8 -21.6

May 16.5 5.3 25.9 -3.5 29.4 -10.4

June 20.0 8.8 28.9 0.0 34.6 -2.7

July 23.7 12.9 30.7 5.7 34.6 3.2

August 20.0 10.1 27.5 2.1 29.8 0.2

September 14.1 5.7 24.5 -2.1 26.7 -3.7

October 4.4 -1.3 14.4 -12.9 17.3 -19.2

November -2.1 -7.5 5.5 -22.4 9.7 -29.4

December -9.1 -16.0 0.3 -29.6 4.2 -40.8

Year average 6.0 -2.7 31.7* -38.0* 34.6 -44.1

*Average of highest / lowest each year.

4 ISA Transactions. Vol. 21, No.2

The selection of such material still had to be provenwhen cast. In order to do this, impact testing was carriedout using standard (10 mm X 10 mm) Charpy V-notchspecimens and apparatus described in ASTM A370 1975and carried out at a maximum temperature of minus55°C. In no case was the average impact value allowed tobe less than 2.6 kgrn, cm-.

If the specified average impact value was not attained,a further three specimens had t? be selected from a posi­tion similar to that from which the failed set had beentaken.

The results of these impact tests were included in theequipment records to form part of the overall documen­tation requirements.. All this added to the burden of the equipment supplier

in trying to meet his deadlines.Manufacturing control valves and relief valves from

low-temperature carbon steel was a solution to theextreme climatic conditions, but for field transmittersand indicators this was not practical.

To be able to use standard manufacturers' equipment,transmitters and local controllers were housed in double­skin fiberglass boxes, with a layer of insulation betweeneach skin and the temperature maintained by means ofsteam heating to ensure that the equipment could con­tinue to operate under the severest of conditions.

Instrument cabling was supplied at a cost of $2 X 106

with an overall sheath manufactured from CSP, whichallowed the cable to be installed in temperatures down to-20°C and suitable for operation down to -55°C.

Transportation

Mention has already been made of the large size ofcertain of the items of equipment and also of a tightengineering schedule. This schedule of equipment deliv­ery was further complicated by the route the equipmenthad to follow to reach the two sites and the naturalhazards of these routes ..F or equipment able to be carriedby the railway system the route is fairly simple: ship toLeningrad, then by railway direct to site. The only prob­lem concerns items that deteriorate if subjected to verycold temperatures.

for items unable to fit on the railway gauge, the routeto Gubaha is simpler than that to Tomsk. Such items aretransferred from ship to large canal barge at Leningradand progress through the canal system to a point some120 km from the site. At this point on the canal bank alanding stage has yet to be built. The journey so far musttake place during the summer when the canals are ice­free. However, the next stage is across a marshy tract ofland passable only during the winter when the marshfreezes and a rough roadway can be cut. Hence the over­size equipment must wait until it can be ferried a piece at atime over the frozen marsh on a low loader. This lowloader with a capacity of 5000 tonnes was part of Davy'ssupply and is sent in parts to be assembled prior to use.

Even this route has its size limitations and the refiningcolumns must be transported in two pieces and welded upon site prior to erection.

For oversize equipment going to Tomsk, the routerequires the use of a seagoing barge, which was loaded upwith particular oversize items at UK ports. Then, underthe supervision of a Russian tug master, it was towedaround the north ofN orway and the USSR to the estuaryof the Ob River, up the Ob to its tributary the Tom, andso to a landing point 20 km from the plant site. As forGubaha, the route is subject to a weather windowthrough which the oversize consignment must pass, as theArctic Ocean is passable only for a limited season and theriver Ob is frozen for about 6 months of the year. To missthe weather window in this case would have meant adelay of almost a year.

The seagoing barge, which was part of Davy's supply,had an overall length of 101 m, overall breadth of 17 m,moulded depth of 6.5 m, and was designed and con­structed to meet the full requirements of the USSR Regis­ter of S hipping at a cost of $2 X 106• Its cargo was worth$3 X 106•

One benefit of this method of transportation was thatin this case the refining columns were transported in onepiece.

Instrumentation and Control

General Philosophy

Conventional electronic analog instrumentation wasused on the plant with a centralized control room consist­ing of 28 m of control panel length and a senior operator'sconsole.

The panel contained all instrumentation required forprocess control and safety of the plant, including' a mimicdiagram containing indicating lights and alarms for levelflow and pressure parameters as well as position of planttri p valves.

The operator's console contained indication and trendrecording of the major plant parameters but no overrid­ing control facilities.

Control of MP steam supply, which is so critical topreserving the life of the catalyst, was achieved by ahydraulic-operated, minicomputer control desuperheat­ingj pressure reduction valve that was driven to a prede­termined position on the failure of the compressor tur­bine. This total system was supplied at a cost of$250,000per plant.

The reason for using this type of instrumentation wasthat in 1977 when the job was negotiated, distributedcontrol systems were new in the market place in Europeand relatively untried and tested, and although the con­tract called for latest technology to be used, that technol­ogy had to be proven. The fact was acknowledged that inthe design the use of local control systems had to be keptto a minimum because of the severe climatic conditions towhich the plant and its operators would be subjected.

[SA Transactions. Vol. 21_ No.2 5

L,--.. ___

There is no doubt that with today's technology, thistype of plant and its associated supporting facilities, suchas the demin plant, rail loading, auxiliary boilers, etc.,lends itself to the use of distributed control systems.

Installation in Hazardous Areas

Because of the nature of the feedstock and the pro­duct, the plant was classified as a Class 1, Division Ihazard. In order that the equipment could be maintainedand checked in the "live" condition it was decided that anintrinsically safe system be used. This was achieved byusing shunt-diode safety barriers. In addition we had tosolve the problems of fitting this intrinsically safe, elec­tronic system into a plant containing a substantial net­work of power cables without compromising the integrityof the milliamp signals by interference from the powersystems.

In order to achieve this, intrinsic cables were physi­cally separated from all forms of power cable in accor­dance with the following:

12V DC 600 mm minimum distance24V DC 600 mm minimum distance

IIOV AC I Ph 50 Hz 600 mm minimum distance220V AC I Ph 50 Hz 600 mm minimum distance220V DC 600 mm minimum distance380V AC 3 Ph 50 Hz 1000 mm minimum distanceAll voltages above380 V 1200 mm minimum distance

It was also a requirement to physically separate allcables from process piping and equipment containingcombustible liquids or gases to an extent of 500 mmminimum clear of the "heat zone."

On-Line Analyzers

In the production of methanol, on-line analyzers playan important part in the monitoring of product purity.

For this methanol plant the product is 99.95 percentpure and the impurity to be detected is 2 ppm ethanol.This is achieved by means of process chromatographstaking samples over 5-minute intervals.

Apart from the analysis of oxygen in the reformerfurnace flue gas, it was also a requirement to analyze theoxygen content in all the relief valve headers to ensurethat the gas mixture did not reach the explosive levelprior to being burned at the flare tip. This was achievedby a multi-stream paramagnetic oxygen analyzer with apump producing a sample flow of 10 liters/ minute and aresponse time of 10 seconds.

Infrared analyzers were used to check the carbonmonoxide (CO) and hydrogen (H2) in the synthesis gas tocheck the efficiency and monitor the life of the catalyst.

These analyzers were skid-mounted and located in aheated I air conditioned analyzer house delivered to sitecompletely piped and wired.

6 [SA Transactions. Vol. 21, No.2

Safety Systems

The safety of the plant and its personnel was achievedby monitoring both the process and the environment inwhich the operators had to work.

The plant and equipment was protected in the conven­tional way by means of pressure/vacuum relief valvesand by a shutdown system operated by electromechanicalrelays.

One interesting feature of the safety valve system wasthat each relief valve was fitted with one or more skinthermocouples to ensure that the winterization protec­tion was working and the valve body and associateddischarge pipework were not subject to thermal shockduring operation.

In order to comply with the statutory regulations,personnel had to be protected from toxic concentrationin the buildings. This was achieved by the location ofsensitive detectors in such places as the foam storage areafor ammonia detection and in various compressor andpump houses for the detection of hydrogen and methane.

Upon detection of these gases being present, the ana­lyzers would operate not only an alarm system but startemergency air conditioning systems to clear the buildingsof any such hazard.

The shutdown system was operated by flow, pressure,level, and temperature sensors located in various parts ofthe process where any serious deviation from the normaloperating level would be dangerous. The action of anyone of these sensors would cause the appropriate controlvalve to close or open or any pump to stop or start,depending on the requirement to maintain safety.

An annunciator light located in the mimic diagramand mounted above the main control panel gave not onlya visual and audible indication that a trip function hadoperated, but also that appropriate valves and/ or motorshad in fact operated.

Table 2Summary of statistics per site

Area (boundary limits) 126 X 103 m-

Low climatic temperature -55°C

High climatic temperature +36°C

Snow cover 800 mm

Frost penetration 2 m

P & I diagrams 70

Process flow diagrams 16

Instrument drawings 2000

Instrument specifications 300

Instrument cable 150 km

Control valves 400

Relief valves 220

Control panel size (total) 40 m

Instrument order valve $6 X 106

FUTURE DEVELOPMENTS

General

The position until the mid 1970s was such that low costcrude oil and natural gas tended to inhibit capital expen­diture on energy conservation, particularly by petro­chemical contractors. This situation has changed to suchan extent that the viability of more and more plants isbeing influenced by energy considerations. This situationis also complicated by the present turndown in the plantconstruction market, which means that contractors haveto become more competitive to survive.

Cutting costs can only be taken so far: in the end theweakest - though not necessarily the smallest - will goto the wall. This means that contractors have to enhanceboth their technical and cost competitiveness. Plantdesigners of all disciplines can therefore assume a moreimportant role by improving their designs, which, hope­fully, will result in additional business as a consequenceof the better product.

Energy efficient plants are now highly desirable, and itis expected that this state of affairs will continue. It is notinconceivable that legislation may be introduced to pro­hibit or penalize energy waste and that Government"Energy Inspectors" may be appointed to work alongside "Health and Safety Inspectors".

Process engineers are already working on this problemby including better thermal insulation, adding morewaste heat recovery systems and designing low-energyprocesses. However, a large proportion of these gains canbe lost if the plant is not operated with the objective ofminimizing the net energy consumption. This is where thecontrol engineer can use his expertise to include moreeffective control strategies. His objective in this respectshould be to design a scheme that will enable the productspecification to be achieved as economically as possiblewith a minimum of down time.

Two techniques that are beginning to be used morefrequently in process plant control strategies are optimalcontrol and multivariable control. While the main theo­retical work was carried out a long time ago, applicationin the process industry has been slow and has usuallybeen implemented by dedicated computers installed by,or under the direction of, the end user.

With the advent of the latest generation ofVDU-basedsystems, the situation is now such that contractors canuse these advanced techniques. By demonstrating exper­tise in this area their reputations will improve and hope­fully so will their business chances.

The situation is further complicated by new plantconcepts such as:

• Floating plants• Barge-mounted plants• Undersea plants• Larger onshore integrated multiplant complexes• Modularized plants

The above all require minimum operator activity inthe plant. This is because of space restrictions and possi­bly the environment; hence, centralized control is essen­tial. In addition, the highly integrated nature of theseplants, where hold-up capacity will be reduced to a min­imum, requires greater knowledge of the processdynamic behaviour, including any interactions. Modu­larized plants based on the principle of "maximum pre­assembly" (MPA) also place a premium on space. In thiscase, skid-mounted control rooms for any sizable plantcan only realistically be achieved using VDU-based sys­tems. It is in these areas where the major developments ininstrumentation and control of methanol plants are likelyto take place. There will of course be other developmentsin valves, transducers, etc., but these will not affect theoverall plant to such an extent.

Apart from complicating control strategies, some ofthe a bove concepts also require us to re-think our tradi­tional measurement techniques because of plant motion.

Centralized Display Systems

Probably the most significant changes in instrumenta­tion and control of process plants to occur in recent timescan be attributed to application of solid-state electronics.With the advent of microprocessors, even recent solid­state developments are being overshadowed. Their signal­processing capabilities are such that by using multiplex­ing techiques one microprocessor can perform numeroustasks simultaneously. In addition, the smallness of thesedevices means that equipment sizes can be greatlyreduced.

In the near future (if not already) microprocessor­based systems will become entrenched and enter morediverse fields of application such as intelligent transduc­ers. There will be hardly any area of measurement andcontrol left unaffected by this revolution. The main effectwill be the almost total acceptance of centralized displaysystems with distributed control, where appropriate.With the additional power available from these systemsmore sophisticated control strategies can be implementedand "real" automatic control of continuous processesachieved economically.

There will of course be areas where microprocessortechnology remains unaccepted because of the nature ofthe application or because of the environment. An exam­ple of this may be well-head control, particularlyoffshore.

Pressures resulting in a greater utilization of VDU­based systems are coming from three main directionsi.e., end users, contractors, and equipment manu­facturers.

The end users appear to be moving in this direction forvarious reasons, but mainly because of operationaladvantages. Plants are becoming larger and more com­plex, which makes the supervisory control available from

[SA Transactions. Vol. 21, No.2 7

digital systems very attractive. For example, data log­ging, process calculation, and advance control facilitiesare readily available. In addition, maintenance should besimpler as all systems have built-in fault diagnostic rou­tines, which in most cases will indicate which printedcircuit board, if any, is faulty. Digital systems are also notsusceptible to drift, unlike analog systems. This, coupledwith the very impressive reliability figures being quoted,should lead to less down time. However, actual operatingdata is needed to confirm or otherwise match the manu­facturer's optimism.

From the contractor's point of view the advantages aredifferent. The market situation is now such that a digitalsystem with automatic backup on fault can be cheaperthan a conventional electronic or pneumatic system (seeAppendix I). This is based strictly on hardware consider­ations; if construction and commissioning costs are takeninto account the comparison is even more heavily weigh­ted toward digital systems. To remain cost-competitive,therefore, it is advantageous for contractors to offer dig­ital systems where appropriate. In addition, the size andweight advantages of digital systems mean smaller con­trol rooms. Also it allows plant modularization usingmaximum pre-assembly (MPA) techniques to be takenmuch further. This can also reduce the overall plant costs.Another aspect is that contractors have to be seen to beusing the latest technology and also have to demonstratetheir competence in this field.

The main functional differences between the earliercomputer-based digital systems used from the 1950sonwards and the present systems lie mainly in the soft­ware. The earlier systems used purpose written softwareand any changes required the services of a specialistprogrammer. The work involved could be considerableand time consuming as modularized software was gener­ally not used. The systems currently being marketed havebeen designed on a firmware basis for basic systems withmodularized software available for the more complextasks. This is a good selling point because systems cannow be set up and reconfigured by engineers who needvery little, if any, programming expertise. M~nufacturersrightly push this aspect because programming has hither­to been a major disadvantage of digital systems. Theyhave also designed systems to minimize the requiredspares holding, which has very real advantages to the enduser. The technical side is also being stressed as the powerof these systems appears to be limited only by the inge­nuity of the applications engineer. The effect of this to alarge extent is that the capabilities of these machines farexceed the current capacity of most engineers to utilizethem fully.

This is likely to change in the near future becauseengineers now leaving universities have been brought up

8 ISA Transactions. Vol. 21, No.2

on computers and are no longer inhibited by digital sys­tems. In addition, process changes will probably beforthcoming from chemical engineers as they come torealize the full potential of these systems. It may alsoresult in a different approach to process plant design asdynamic simulation becomes more widespread.

A further advantage of the digital system concept isthat distributed control/ intellegence becomes a practicalproposition. Potentially this approach could reduce thecost of cabling to such an extent that the associatedsavings cover the cost of a digital system. As an example,cabling for one of the 2500 MTD methanol plants des­cri bed earlier costs $2 x 106 • If the ladder racking andstructural steel costs are included, this figure rises to $2.4x 106• Clearly a relatively modest saving in cabling wouldoffset a considerable part, if not all, of the cost of a digitalsystem. Distributed control systems have not yet beenwidely used, and information from actual installationsregarding real cost savings that have been achieved andalso on any practical difficulties will be extremely useful.

A possible distributed control arrangement is given inFigure 2. Each local control room is likely to be equippedwith perhaps 2 minimum specification VDUs, a printer,and some conventional chart recording facilities. Themain control room would have a suite of perhaps fourVOUs, main plus standby printers, VOU copier, colorgraphics (if not standard), conventional chart recordingfacilities, and supervisory control facilities. The distrib­uted control would be located at the appropriate mea­surement/ actuation centers of gravity. Automatic back­up on fault would be provided for critical control loops.

The introduction of optic-fiber transmission links inindustrial enviroments is also likely to have an impact ondigital systems. The very wide bandwidth and immunityto electromagnetic interference of light transmissionmeans that very fast data rates can be achieved. This willenable much larger systems to be accommodated on asingle data highway than is possible with conventionalco-axial cable.

Also with the introduction of optic-fiber strain gagesand other sensors, the way is clear for a large range oftransducers to be interrogated by optical signals. Withthe falling cost of optic-fiber transmission, its suitabilityfor use with digital systems, and being intrinsically safefor use in a hazardous environment, its use is bound toincrease. It may be that in due course optic-fibers willvirtually replace signal cables as presently used.

Finally, it is anticipated that safety shutdown systemswill also be dominated by microprocessors and willinclude majority voting logic. However, it is not envis­aged that they will be fully integrated with the maindigital control system.

WATERTREATMENTPLANT

\-- -----,\ ,

RAILUNLOADINGPLANT

AUXILIARY

BOILER

PLANT

___ MAIN t STANDBYDATA HIGHWAY

LCR LOCAL CONTROl ROOM

MCR: MAIN CONTROl ROOM

D· DISTRIBUTED CONTRX.CENTRE AS REOUIRED

LCR

I

I

/ \/ \

@---ffi

~

-- -------- -- ---

--

-----

--------

METHANOL

PLANT

M

C

R

§--JDl" / L::JLCR

Figure 2. Possible distributed control system for methanol plant plus utilities

Process Control

The current trend, which is likely to continue, istowards more and more sophistication, one objectivebeing to minimize operator intervention. This releasesthe operator from mundane tasks and enables him toconcentrate on supervisory aspects.

With regard to optimal control, even a single-variablelinear search can be highly effective. As a simple example..the warm-to-cold shot ratio of synthesis gas feed to themethanol converter can be optimized. The objectivewould be to minimize purge gas flow and the H P steamrequirements of the syngas circulator (see Figure 3). Thecost function could be:

As mentioned above, problems encountered in min­imizing energy consumption and controlling plants withsignificant interaction are likely to increase. These prob­lems require the application of optimal and multivariablecontrol techniques. In addition, logical control systemswill probably increase in complexity. They will be calledupon to influence analog control strategies as necessaryso that plants can be started up automatically and also sothat throughputs can be changed automatically. This iseasier said than done.

J= k, W + k2 WP s

where:

J = objective function, $/ h

(1)

kl = syngas cost, $ / kg

k2 = H P steam cost, $ / kg

W = purge gas flow tate, kg/ hp

W = HP steam flow rate, kg/ hs

Note: k, and k2 have increased rapidly over the lastfew years.

The ratio would be set initially to some estimatedvalue, then incremented or decremented in small stepsuntil the minimum value of J is found. This is known as alinear search in one dimension. A simple algorithm thatwill carry out the above is shown in Figure 4. This caneasily be implemented on most centralized digital controlsystems currently available. The time delay betweenchanging a variable and calculating a new value for J isincluded to allow the system to settle; it will depend upon .the particular process. The amount by which a variablemay be incremented would probably be between 1 per­cent and 5 percent of the optimizer output span. Morecomplex multivariate search routines'f could also be'implemented on currently available VDU-based controlsystems, but at the moment really powerful routinesrequire online computers.

ISA Transactions. Vol. 21. No.2 9

W w WARM SHOT FLOW

W c COlD SHOT FLOW

METHANOL

CONVERTER

Ww

We

HP STEAM, Ws

~-----.----- -------4. REFORM ED GAS

PC

PURGE GA S, WP

L------~~IC ...--_. CRUDE METHANOL

Figure 3. Simplified methanol synthesis loop

INCREMENT

:x: BY (} :x:

NO

ISJ

SMALLER

YES

DECREMENT

:x: BY 6 :J:

NO ISJ

SMALLER

YES

x = WARM TO COLD SHOT RATIO. SEE FIG 3

Figure 4. Flow chart for single variable, linear search optimization routine

10 [SA Transactions - Vol. 21, No.2

(3); (w =0 ~ 00)

n

Then a system is diagonal dominant by column if

where

G = plant transfer function matrix

Kb

= decoupling controller transfer function matrix

qji = element of Q, (i,j =1,2... ,n)

n = number of plant inputs/ outputs

",' = frequency, radians/ second

Each element of Kb

consists of either a simple gainconstant or one or more lead/ lag functions. For a 2­input/2-output system there would be 4 such elements.

This multivariable technique is currently being con­sidered for improving control of the exit gas temperaturefrom the reformer furnace.

It is extremely powerful and robust and also results ina system of high integrity, which means that if a controlloop is set to manual or is accidentally open-circuited theremaining part of the system remains stable and undercontrol. This is not the case with some alternative multi­variable techniques.

There are a lot of other advanced control techniquesavailable but they are not yet candidates for use by non­specialists and, hence, are rarely used on process plants.

Y t

OECOUPLING PLANT

CONTROLLER(J)

....::>

~b(5) ~ (s)o,....::>0

Yn

(2)Q(/w) = G Uw). Kb Uw)

PIO CONT~LLERS~a (s),

rI

+

(J)

t-Z0o,

....UJV)

r n +

The objective of multivariable control is to reduce theeffects of interaction to a tolerable level. This can some­times be achieved by use of feedforward or cascade con­trol. However, when more complex situations ariseadvance techniques are called for. One of the problemswith the deterministic approach to multivariable controlis that an adequate mathematical model is required. Theusefulness of the multivariable control technique is there­fore dependent upon the type and accuracy of modelrequired. It must also be possible for a competent, non­specialist engineer to understand, implement, and main­tain the system. Strategies that depend upon an exactstate-space model are therefore looked upon with suspi­cion by most practicing engineers. However, control sys­tems that can be designed using a lumped parametermodel, possibly based on plant measurements, are muchmore attractive and have a higher chance of beingimplemented.

One particularly good multivariable approach is makethe system diagonally dominant. (3,4,5) This method seeksto reduce interaction to an acceptable level by introduc­ing a decoupling controller so that each loop can beconsidered independently. The decoupled loops can thenuse PID controllers, tuned in the normal way (see Figure5). The objective of the decoupling controller is to makethe forward path gain modulus, from the ith input to theith output, greater than the sum of the gain moduli fromthe ith input to all other outputs, for all frequencies.From Figure 5 it is seen that

y(s): G(~). Kb(')' Ka(,>'a(,)

~(5) z die 9 {-kat (5)-:... , ka n (~)}

Figure 5. Multivariable control system

[SA Transactions. Vol. 21, No.2 11

If process plants are to be controlled more effectivelyby use of advanced control techniques, then dynamicinformation must be made available. There is great scopefor lumped parameter models to be correlated with oper­.ating plants so that realistic simulation can take place onrelatively small computer systems. If advances in thisarea take place then it should be possible to approach theideal control system shown in Figure 6, i.e, product speci­fication set points are adjusted and the plant then func­tions completely automatically.

CONCLUSION

The past philosophy and future developments of thecontrol of methanol plants have been discussed. Thesedevelopments can of course be applied to other types ofprocess plants, but in doing so the contractor will have tolook closely at his present methods of operation and hisapproach to process control.

DISTURBANCES

PRODUCT QUALITY

MEASURE MENTS ~

INFERENTIAL OR

ANALYSIS

PRODUCT

SPECIFICATIONS --....../

MULTI VAR IABLE CONTROL

SYSTEM INCLUDING:

ANALOGUE CONTROL

SEQUENCE LOGIC

TRIP SYSTEM

PHYSICAL CONSTRAINTS

OPTIMISATION PROGRAMS

ETC

PLANT PRODUCTS

Figure 6. Ideal process plant control system

12 [SA Transactions. Vol. 21, No.2

APPENDIX I1250 MTPD Battery Limits Methanol Plant Instrument Systems - FOB Cost Comparison

Electronic Pneumatic DigitalItem system system system

Panel 528,000(2) 456~000 360,000

Control room 192,000 192,000 108,000

Field instruments'" 432,000 408,000 432,000

Valves 720,000 720~000 720,000

Analyzers 106,000 106~000 106,000

Air compressors 180,000 218,000 180,000

Cable (dist. control not used) 144,000 7,000 144,000

Multicore signal tubing (Cu) 0 348,000 0

Installation mat. (1) 480,000 521,000 480,000

Elec. power equip. 19,000 0 19,000

Trip system 10,000 10,000 10,000

Totals(4) $2,811,000 $2,986~OOO $2,559,000

Notes:

1. Installation materials include cable racks, instrumentsupports, impulse piping, and all instrument air supplytubing. Signal tubing is not included.

2. Estimates around $456K have been received for thispanel, but the above price has been used because it wasprovided by the same source as the pneumatic price.

3. Field instrument costs are approximately equal becausesavings on pneumatic d/ p cells are offset by the extracost of pneumatic temperature transmitters, etc. Manu­facturers indicate that for conventional panel plusfield instruments, electronic equipment is approxi­mately 10 percent more expensive. However, in overallterms this is not the case, see above.

4. Shipping and site installation costs will also come outin favor of the digital system.

REFERENCES

I. Humphreys, G. C. and Dunster, M., 1979. Engineering of theworld's largest methanol plants. Technical paper.

2. Latour, P.R., 1979. Online computer optimization I: Whatit is and where we do it. Hydrocarbon Processing June, pp.73-82.

3. Rosenbrock, H.H., 1969. Design of multivariable systemsusing the inverse Nyquist array. PROC lEE 116, pp.1929/1936.

4. Griffiths, G.W., 1977. Design of multivariable control sys­tems in the direct Nyquist plane. M. Sc. Thesis, The CityUniversity, London.

5. Griffiths, G.W., 1981. Multivariable control system designusing Bode techniques. In Proceedings, ISA/8}, St. LouisConference.

[SA Transactions. Vol. 21, No.2 13