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www.ingentaselect.com=titles=09575820.htm Trans IChemE, Vol 81, Part B, September 2003

FUNCTIONAL MODELLING FOR A SUSTAINABLEPETROCHEMICAL INDUSTRY

G. P. J. DIJKEMA1, J. GRIEVINK2 and M. P. C. WEIJNEN3

1,3Faculty of Technology, Policy and Management, Delft University of Technology, Delft, The Netherlands2Faculty of Applied Sciences, Delft University of Technology, Delft, The Netherlands

I n a transition towards a sustainable petrochemical industry, its systemic structure andfunctions must be considered degrees-of-freedom in the search for and the speci� cation ofdevelopment options that reach beyond traditional research and development (R&D) and

beyond the optimization of proven technology, proven system concepts and established processnetworks. Functional modelling allows technology-free speci� cation of systems, generalizationof system concepts, and recalibration of the use or value of substances in a petrochemicalnetwork. It facilitates moving up or down the ladder of system aggregation levels, which allowsone to ‘step out of the box’ to the plant or chemical complex level, or alternatively to connect tothe unit-operation level. Application to the aromatics industry serves to illustrate that therebythe abstraction from present system content and from proven technology is achieved, whichallows the speci� cation of the desired content of technological inventions that enable or relyupon changes in system structure: process system innovations. The realization of trigeneratefunctions through the petrochemical industry involves a paradigm-shift. Rather than consider-ing the industry a threatening source of pollution, safety and health risks, it is an enabler ofglobal CO2 emission reduction.

Keywords: sustainability; petrochemical industry; process synthesis; process systeminnovation; trigeneration.

INTRODUCTION

Over the past decades, the petrochemical industry hasreduced its harmful emissions signi� cantly, amongst othersvia environmental management and technological develop-ment. Meanwhile, the industry has realized that it mustcontinuously renew its license-to-operate obtained fromlocal communities and other stakeholders. Through theResponsible Care programme, therefore, the industry hasbeen developing its communications and image with localcommunities and the general public. While the depletion offossil resources and the threat of global-warming imply thatdramatic change is required in industry to achieve sustain-ability, only a few companies have acknowledged thatsustainability is crucial for their long-term prosperity(Dijkema and Mayer, 2001). The Dutch Scienti� c Councilfor Government Policy recently has categorized CO2 emis-sion, climate change, biodiversity and continuously growingenergy-use as wicked problems (Wetenschappelijke Raadvoor het Regeringsbeleid, 2003), because to date these havelargely escaped (inter)national policy and regulation, largelybecause of lack of public awareness and political consensus.These are dif� cult problems to communicate to the generalpublic, because there is no direct exposure to the associatedadverse effects that develop gradually over decades in aglobal context. Meanwhile, many tough environmentalproblem have been turned into manageable problems in

the past 30 years, through end-of-pipe measures, technologydevelopment, process improvements, pollution preventionetc. It is, however, the wicked problems that must beaddressed to enable sustainable development ‘ . . . thatmeets the needs of the present without compromising theability of future generations to ful� l their own needs’ (WorldCommission on Economic Development, 1987).

A sustainable petrochemical industry is characterized byappropriate resource selection, effective resource utilization,the avoidance of waste and emissions and a product slatethat fosters a sustainable society. Drastic reduction ofresource consumption and CO2 emission, however, is awicked problem for the petrochemical industry, as it islinked to its present use of fossil feedstock and becausemost companies compete in global markets where opinionson CO2 and global-warming vary. Thus, despite ef� ciencygains, the industry’s CO2 emission has remained highlargely because of the continuous growth of productionvolume, which is illustrated by the doubling of ethyleneproduction in the past 25 years.

The petrochemical industry has been classi� ed a science-based industry (Pavitt, 1984). It has an impressive record oftechnological innovation at or below the level of singleplants, where fundamental and applied research have fuelledcontinuous improvement of, for example, catalysts, reactors,separation technology. Considerable ef� ciency improve-ments have been realized, as ‘innovative design and clever

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process modi� cations are essential to the economic health ofthe producer of commodity chemicals’ (Wei et al., 1979). Atransition towards a sustainable petrochemical industry,however, requires innovation at all system levels, fromreaction path to process system, from chemical plant topetrochemical complex or industry. These innovations mustreach beyond traditional R&D and beyond the optimizationof proven technology, proven system concepts and estab-lished process networks.

The question arises therefore what innovations arerequired to meet the challenge of a sustainable petrochemicalindustry and how the required technological or systemiccontent can be speci� ed? Addressing the second part ofthis question, in this paper we report some of our work on thedevelopment of a systemic approach to specify the desiredcontent of process system innovations in the petrochemicalindustry. These are de� ned as changes in the systemstructure or system design of the petrochemical industry,its industrial complexes, or individual plants (Table 1). Thesecan be enabled by technological inventions or vice versa.

Firstly, innovation, technological development andprocess synthesis are addressed and the research questionis elaborated. Secondly, it is shown that functional model-ling provides a suitable mechanism for abstraction fromthe present system content and from proven technology.Employing the procedure thus developed, conceptualprocess system innovations for the case of industrialaromatics are developed. Thirdly, a case of rethinking thefunctions of petrochemical complexes is presented: therealization of a trigenerate industrial complex.

SYSTEMS IN THE PETROCHEMICAL INDUSTRY

Petrochemicals are produced in bulk quantities in large-scale out-door facilities. Whilst these are complex installa-tions in appearance, they can all be represented as a systemwhere a number of physical inputs are transformed to anumber of outputs, for example a steam cracker transformsnaphtha to ole� ns and aromatics. As far as the representa-tion of the systems design is concerned, each petrochemicalplant can be completely characterized by a single con-� guration of a limited number of process sections orfunctional units (Zevnik and Buchanan, 1963) such asreaction, separation, process � ows and process recycles.Since operating conditions and system design can beselected to meet a range of objectives, and a variety ofapparatus can be selected to implement each functional unit,the design of any commercial petrochemical plant representsonly one out of a large set of feasible con� gurations. Thus, a

variety of process systems for, for example, propylene oxideor methanol production therefore are currently in use.

The entire petrochemical industry and parts thereof,such as the Rotterdam Chemical Cluster, can be representedas a structured con� guration of individual plants thatare connected by some kind of � ow. These are systemswhere oil products are converted to suitable feedstock forthe polymer industry and other sectors, utilizing natural gasbased products where feasible. Analogous to the design ofindividual plants, the present industry’s con� guration isonly one instance of a very large set of possible systemcon� gurations (Table 1).

‘Process synthesis is the invention of conceptual chemicalprocess designs’ (Siirola, 1996: 10), and the questionwhether ‘the invention of chemical process designs can beorganized, systematized, or even automated’ is at the focusof process synthesis research (Siirola, 1996: 3). Referring toindustrial practice, however, Siirola hints at the relativelyscarcely addressed problem of having to generate � owsheetsfrom scratch, as often � owsheets from previously realizedplants are available. Genuinely novel process systemconcepts, such as distributed chemical manufacturing(Rowe et al., 1997) are scarcely reported. As of lately, processsynthesis techniques have been used for the development ofnew chemicals (e.g., Marcoulaki and Kokossis, 2000) and theimportance of the relation between product and processresearch has been emphasized (Charpentier, 2002).

Despite the research agenda suggested by Grossmann andWesterberg (2000), another largely unexplored ‘white spot’in process synthesis R&D is the conceptual design ofpetrochemical complexes comprising multiple plants, let-alone true multiscale approaches that link the complex,single plant and unit operation level. A notable exceptionbeyond the vast body-of-work on the design and optimiza-tion of energy-exchange or utility networks is the work onthe synthesis of reactor and reactor=separation networks(e.g. Papalexandri and Pistikopoulous, 1996; Glasser andHildebrandt, 1997; Sargent, 1998) and work on exploringsome solution-space beyond existing system concepts whereprocess alternatives have not been pre-postulated (Ismailet al., 2001). Contributionsthat focus on sustainabledevelop-ment are relatively few in number. In the past decade, theinclusion of environmental impact minimization intoprocess synthesis was addressed (Buxton et al., 1997;Yang and Shi, 2000). The inclusion of environmental criteriainto the design process was elucidated (Herder, 1999;Sharratt, 1999). The inherent multicriteria design problemof pollution prevention and waste management wasaddressed (Steffens et al., 1999; Mellor et al., 2002; Linkeand Kokossis, 2003) as well as the incorporation of

Table 1. Overview of system levels in the petrochemical industry.

System System elements Intra-connectivity Degree-of-freedom

Petrochemical industry Complexes plants Mass � ow, infrastructures Selection & con� guration of elements;Feedstock=product state and composition

Complex Chemical plants Mass & energy � ows; utility infrastructure Selection & con� guration of elements;Feedstock=product state and composition

Chemical plant Unit operations Mass & energy � ows; utility network Selection & con� guration of elements;Conditions & � ow composition

Unit operation Apparatus Direct transfer of mass, heat, power, impulse; Selection & con� guration of elements;Operating conditions

Apparatus Internals Mechanical, electrical, chemical Principle & design

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life-cycle assessment into optimal chemical process systemdesign (Azapagic, 1999; Burgess and Brennan, 2001; Khanet al., 2001). Others incorporated engineering thermody-namics into process design and optimization and suggestedthorough consideration of the available energy resources andtheir conversion to useful work (Kalitventzeff et al., 2001;Bakshi, 2002).

In process synthesis research, however, conscious re-designof the industry system structure beyond single plants hashardly attracted attention. Meanwhile, in the scienti� ccommunity at large the system design of the industry hasbeen considered � xed. With the maturation of the industry(Stobaugh, 1988), changes or extensions of the industry’sstructure have become incidental consequences of success-ful fundamental R&D. Thus it may be seen that one cannotbe sure that the largely ‘bottom-up’ approach employed inthe scienti� c community will provide suf� cient and timelysolutions to support a transition to a sustainable petrochem-ical industry. Systemic innovations do require a long timeperiod for development and implementation irrespective ofthe mechanism by which they have been identi� ed. Thefocus of our work, therefore, is on early conceptual speci-� cation or invention of the desired content of process systeminnovations for a sustainable petrochemical industry.

METHODOLOGY

Conjecture

How to achieve sustainability is an open problem thatcannot be solved by planning and optimization or traditionalR&D alone. We conjecture that in order to prepare for asustainable future of the petrochemical industry, integratedinnovation at all system levels (Table 1) is required, frommaterial to apparatus to plant to petrochemical complex andindustry con� guration. Furthermore, the industry systemstructure must be considered a degree-of-freedom in thesearch and speci� cation of yet unknown technologicaldevelopments, as well as in the adoption of innovationsthat originate from outside the sector.

Objective and Research Question

Our objective was the development of a systemicapproach to specify the desired content of process systeminnovations in the petrochemical industry. This preferablyincludes and links together analysis and subsequent synth-esis. The research question addressed is whether it is at allpossible and worthwhile to devise some mechanism tosystematize the search for technological innovations. Isthere a ‘top-down’systems approach that allows assessmentfollowed by the conceptual speci� cation of R&D options forprocess system innovation in the petrochemical industry?

Development and Testing

The method for conceptualization of process systeminnovation has been based on the body-of-knowledge ofprocess system engineering, thermodynamics and functionalmodelling. The method has been speci� ed and re� ned in thecourse of a number of case studies. Particularly, its applica-tion to the use of fuel cells in the chemical industry has ledto a number of patents (Dijkema, 1997; Hemmes and

Dijkema, 1998), which is an illustration of their applicabilityalbeit not a formal proof of the methods. Application of themethods to industrial aromatics led to a number of newsystem concepts for resource utilization improvement. Weused functional modelling to develop trigeneration concepts.Since also their early evaluation would help to fostersustainable development, a straightforward linear program-ming (LP) model was used to assess the extent of CO2reduction of a trigenerate industrial complex.

FUNCTIONAL MODELLING

Traditional Approach

Initially, our objective was the development and testing ofa systemic approach to specify the desired content ofinnovation of the petrochemical industry. In a traditionalsearch for improvement options, a ranking of relativelypoorly performing chemical processes is the logical starting-point. Firstly, possible causes for the losses calculated mustbe established. Secondly, improvement opportunities withineach of these chemical processes are investigated. Thirdly,opportunities that involve system change outside the chemi-cal plant system boundary are investigated with the explicitcondition that technology and structure of subsystemswithin each chemical plant are a degree-of-freedom.Fourthly, the opportunities thus formulated for each singleplant must be tested for applicability to other chemicalplants. Those that pass the test can be considered promisingopportunities that can have wider application, possibly to allweak elements of the petrochemical industry identi� ed.Finally, opportunities identi� ed can be subjected to expertreview.

Example: Industrial Aromatics

In the Netherlands, benzene (B), toluene (T) and xylenes(ortho-xylene, oX and para-xylene, pX respectively) areproduced from pyrolysis gasoline and reformed naphtharespectively. These aromatics serve as intermediate for anumber of chemical product chains that lead to, amongstothers, polystyrene, styrene copolymers such as ABS, nylonand polyethyleneterephthalate(PET). At the time, the Dutchindustrial aromatics system comprised some 40 plants. Anoverview is given in Figure 1.

The results of an assessment of the situation of the Dutchindustrial aromatics system in 1990 using process informa-tion available in open-literature data (Chauvel and Lefebvre,1989), indicate that the energy-ef� ciency in this part of theDutch petrochemical industry is high, some 77%. Ranking,however does yield a number of relatively weakly perform-ing chemical processes, notably caprolactam, phenol andphthalic anhydride production (Dijkema and Grievink,2000). The annual losses from the processes employed inthe Dutch system together amount to some 110 PJ. Appar-ently, there is scope for improvement of these processes, andbecause of their large production capacities world-wide thetotal industry improvement potential is large.

Relatively straightforward opportunities can be identi� edby looking beyond the system boundary of the particularprocess studied. The most obvious one is to reconsider thefate of unwanted by-products. In many plants utilization ofsuch by-products within the plant system-boundary is by

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FUNCTIONAL MODELLING FOR A SUSTAINABLE PETROCHEMICAL INDUSTRY 333

recovering the fuel-value only. To upgrade their utilizationrequires fuel replacementand identi� cation of some destinationof the by-products. Nevertheless, to upgrade by-productsand to reap the associated bene� ts is a longstanding traditionin the petrochemical industry which in many cases has led tonew business development. In modern styrene units, forexample, the hydrogen present in recycle off-gas is capturedfor pure hydrogen production. Also in older styrene plantsor in the hydro-dealkylation of toluene other utilization ofsuch gases than as fuel may be contemplated. In caprolac-tam, the large exergy loss and ammonium sulphate produc-tion were an incentive for the development of a newgeneration of caprolactam processes.

Another opportunity is in catalyst research, where, tradi-tionally, much research and development is devoted to� nding the ultimate selective catalyst. Looking beyond theboundary of a single plant, however, suggests the develop-ment of effective catalysts for ‘high conversion, high yield’that are not necessarily selective to a single product. Suchcatalysts must be designed to be employed in processsystems where the combination of unselective reaction andproduct separation can be effectuated at operating condi-tions that lead to limited exergy loss only. Concurrently, thepossibilities to develop some utilization of by-products andthe associated business must be considered.

System Elements and System Decomposition

In studies of the chemical industry, the single chemicalplant is usually selected as the central system level foranalysis. Indeed, this is a reasonably identi� able systemelement in the petrochemical industry, and information on‘typical’ chemical processes for almost every commerciallyproduced commodity chemical can be found in the openliterature. This selection, however, is hardly addressedexplicitly in the literature (Dijkema and Reuter, 1999).From a number of case studies we concluded that modellingthe petrochemical industry as a network of single plantssuf� ces for a � rst assessment. However, on critical re� ec-tion, and in spite of the opportunities for innovation identi-� ed, we had some concern about the methodologicalunderpinning of the identi� cation procedure developed,because the selected decomposition of the industry intoindividual cases and process plants can be characterized astrivial and heuristic. This is unsatisfactory, because it may be

argued that the opportunities and research themes identi� edare largely a function of the decomposition of the industryapplied. Therefore we investigated the possibilities of func-tional modelling (Baylin, 1990) to model petrochemicalsystems for the purpose of speci� cation of the content ofprocess system innovations.

Functional Modelling, a Useful Concept?

In functional modelling a function is concerned with whatshould be achieved, not how. The key feature of functionalmodelling is that the speci� cation of systems, subsystems orsystem elements is completely technology-free (Asbjørnsen,1992). As in system engineering and input=output analysisthe particular means of action of any of these items is notdescribed nor prescribed.

At a particular system level functional modelling therebyprovides a mechanism for abstraction of present systemcontent in the search for new system con� gurations. One ofthe functions within the subsystem where aromatics areconsumed, for example, is the production of styrene.Presently, in the aromatics production system this functionrequires the production of essentially pure B. When,however, some direct conversion of 2 C4’s (e.g., butadiene)to styrene becomes technically and economically feasible,the link between the ‘production of B’ and ‘production ofstyrene’ will be removed. Meanwhile the functional modelsof both the aromatics consumption and production systemdo not change because of such development (Figure 2).

Functional modelling enables the search for opportunitiesfor R&D, because the concept of functions allows:(a) technology-free speci� cation of systems and alternatives.The technology content has thus effectively become adegree-of-freedom in petrochemical system design for intra-process innovations—improvement of currently employedprocesses; (b) suitable generalization of a system concept,because the abstraction provided by functional modellingallows expansion of an initial set of functions identi� ed.Rather than limiting the function of a chemical process tothe production of one or a few petrochemicals, a plant canbe classi� ed a multifunctional unit. Thereby, a perspectiveon new opportunities is opened for utilization and changeof process system con� gurations; and (c) optionallyrecalibration of the use and=or value of substances or� ows in a petrochemical network. This opens the way to

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inter-process innovations, as a chemical process system’swaste products change into co-products, thereby increasingits net resource utilization.

When confronted with the task of modelling an existingcomplex system, such as the petrochemical industry or anypart thereof, the abstraction provided by functional modellingallows one to: (1) generalize at some top system level: forexample the primary function of ‘the petrochemical industry’is to produce suitable feedstock for polymer production;(2) visualize the link of this subsystem of our industrialeconomy to other industrial sectors or parts thereof, therebyeventually allowing some linkage with needs of societycatered for by ‘industry’, for example the need for ‘hygienicpackaging of food’ by the supply of suitable and affordablepolymer � lm; (3) arrive at a suitable functional decompositionof the petrochemical industry that � nally allows the speci� -cation of technological or system content of innovations forsustainable development. In the industrial aromatics system,for example, at the top-level of abstraction, we can distinguisha production system for essentially pure B, T, oX and pX,and a system where their use as intermediate feedstock orconsumption of these aromatics is a common denominator(Dijkema and Grievink, 2000); and (4) Adapt systemboundaries. Because the decomposition is based on systemand subsystem boundary selection it opens the way forconsideration of alternative systems by expanding the setof functions in the model at a particular system level, or toview the system as a part of a larger system.

Example: Functional Model of the Industri alAromatics System

Functional modelling of the aromatics system illustratesthese items when modelling an existing system: (1) gene-ralize: the general primary function of this subsystem of thepetrochemical industry is similar to the primary function ofthe industry, ‘produce a number of suitable feedstocksfor polymer production’; (2) visualize: the links of thissubsystem with the petrochemical industry and beyond are

visualized in Figure 3. This indicates that the subsystem islargely dependent on the functions of the re� ning sector andthe associated technological implementations, as well as onthe subsystem for the production of ole� ns. The demandfor automotive fuels that are void of volatile carcinogeniccompounds has resulted in a reduction in allowablearomatics levels in gasoline (Auto Oil I and II). This canhave a dramatic effect on the links between these sectors, asthe modi� cations in re� ning change the supply and demandbalance of aromatics world-wide, which will initiate processsystem change and development in all sectors; and(3) decompose. The aromatics system can suitably bedecomposed in a production subsystem (BTXProd) and aconsumption subsystem (BTXCon). Further decompositionof BTXCon is done by de� ning system elements thatproduce the intermediates and polymers respectively.Further decomposition of the BTX production system,however, is not straightforward. Usually, aromatics are � rstisolated from non-aromatics. Subsequently, somewhere inthe production system the mixture (BTX) is transformed toessentially pure B, T, oX and pX, while mixed-xylene (mX)is usually converted and recycled to extinction. In ‘presentstate-of-the-art’ there exist a variety of system con� gurationsthat depend on feedstock selection and required product-mix. Functional modelling, however, allows abstraction to asingle functional model that encompasses all possible reali-zations of the conversion of complex hydrocarbon mixturesto the required product-mix of B, T, mX, oX and pX. Thede� nition of the functions listed in Table 2 suf� ces.

FUNCTIONAL MODELLING IN A STRATEGY FORPROCESS SYSTEM INNOVATION

The aromatics example above illustrates that functionalmodelling provides a mechanism to arrive at some techno-logy-free superstructure for the petrochemical industry orparts thereof. To really contribute to the effectuation ofsustainable development in system concepts selected forindustrial implementation, production losses must be

Figure 2. Change in functions.

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FUNCTIONAL MODELLING FOR A SUSTAINABLE PETROCHEMICAL INDUSTRY 335

minimized or even eliminated at acceptable costs. The ulti-mate goal is to convert the feedstock into desired productsonly with minimum energy loss from the production system.

A strategy to arrive at a functional speci� cation oftechnological or system content for sustainable developmentin the petrochemical industry can comprise the followingsteps:

(1) development of a functional model of the existingsystem (technology-free!)

(2) assessment of the existing system implementation andidenti� cation of weak elements

(3) development of alternative functional system conceptsby using functional superstructure

(4) evaluation of the scope of improvement achievable bythe process system innovations developed

Steps 1 and 2 have been addressed above. Step 3 is wherethe process system innovations must be conceptualized andspeci� ed. As stated, based on the results of the aromaticsystem assessment, an R&D portfolio is preferred thatincludes options for the change of individual processes,process technology, the structure of the aromatics system,and the development of new process routes or products thatreplace existing functional links between this system and‘the industry’. Step 4 can be completed when the innovativeprocess system concepts have been developed into a con-ceptual process design, where again one employs somesystematic series of steps to complete the design. For a plantthat must produce a speci� ed amount of a speci� c product,these included assessment of building blocks, synthesis ofalternatives, analysis of dynamic behaviour and evaluationof expected performance (Biegler et al., 1997).

In the search for R&D directions, it is common to look at(incremental) improvement of currently employed processes.Using functional modelling, the same can be done. In theprocess network for the production of mixed-xylenes, forexample, the ef� ciency of the stabilization of pyrolysisgasoline can be increased by replacement of the selectivehydrogenation of ‘impurities’ in the feedstock. It appears thatthese impurities can be recovered by enhanced extraction asvaluable products. Whilst this involves a change in therealization of function 1a in Table 2, pre-treatment, actuallya new functional element is introduced: ‘recover impurities Z

Table 2. Functions in the aromatics system (after Dijkema and Grievink,2000).

1. Produce BTX-rich stream 5. Produce pure pXa) Pretreat and precondition 6. Produce pure oXb) Separate aromatics=non-aromatics 7. Produce pure mXc) Polish to required speci� cation 8. Produce BX-mixture

2. Produce pure B 9. Produce TX mixture3. Produce pure T 10. Produce BT mixture4. Produce mixed-X

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as products’, which will have an effect beyond the particularprocess studied.

In addition to intra-process innovations, we can search forprocess system innovations using the abstraction by func-tional modelling. One might argue, for example, that whilstproduction of B can be done via hydrodealkylation, theprimary function of that process really is to upgrade T thatwould otherwise be sent to the gasoline-pool. Since this is aproduct destination that is limited today and may be blockedunder the new Auto Oil regulations in the not too distantfuture, and because the hydrodealkylationprocess involves arelatively high energy-loss, would it be possible to concep-tualize an aromatics system where this function is notemployed? This can involve, for example, the developmentof applications of T and X that replace the use of B, which infact implies the introduction of new intermediate functionsin BTXCon. The conversion of T into para-methyl-styrene(PMS) represents such a new function (Keading et al.,1982). The polymer, poly-para-methyl-styrene (PPMS) canreplace ordinary polystyrene in a range of applications, i.e.,its functions. As a consequence, the actual realization ofBTXProd will be affected, because the operation of thedealkylation process can be avoided, as well as its associatedsystem inef� ciencies. Its functional model, however,remains the same because dealkylation is only one optionfor realization of function 4, produce pure B.

TOWARDS TRIGENERATION COMPLEXES?

Introducti on

The transformation of chemical plants to trigenerationsystems for the simultaneous production of chemicals,electric power and heat is an example of generalization byfunctional modelling.

In petrochemical plants, traditionally, heat is supplied inindustrial furnaces and reaction furnaces, for example, thenaphthacrackers or EDC cracking furnaces. In a study onthe heat demand of the Dutch industry it was revealed thatthe chemical industry largely requires heat at a temperaturelevel between 300¯C and 800¯C (Boot and Van Wees,1982), which led the authors to remark that ample opportu-nity existed for cogeneration in this industry. Indeed mostpetrochemical sites in the Netherlands either alreadyincluded cogeneration at the time or such facilities havebeen erected since. Most of these appear to have beendesigned for industrial process–heat demand, the surpluspower being transported to the grid.

Functional Modelling for the Formulation ofSystem Concepts

Where the combination of chemical reaction and energyconversion is fundamental to chemical engineering andcrucial to achieve proper plant design, surprisingly trigenera-tion is not. Trigenerate unit operations, however, haveemerged recently under the label of ‘multifunctionalreactors’,notably fuel cell reactors that incorporate some membrane.In contrast with such science-based innovation of unit opera-tions within chemical plants, the concept of trigenerationsuggests innovation beyond the boundary of individualplants, complexes or even the industry by expanding its setof functions (Dijkema, Grievink and Weijnen, 2001).

Thus, trigeneration represents a novel, functional view ofthe large-scale petrochemical industry: it not only serves toprovide petrochemicals, but also acts as a large heat-sinkthat enables a net global CO2 emission reduction by trigen-eration. The realization of trigeneration in the (petro)chem-ical industry thus involves a paradigm-shift. Rather thanconsidering the industry a possible and threatening source ofpollution, safety and health risks, it must be considered anenabler of a reduction in global CO2 emission by increasedfossil resource utilization by viewing it a co-producer ofchemicals, electric power and heat.

In system design, the next step is to consider otheroptions to bring about the required functions, in this casepower generation from natural gas. Trigeneration systemdevelopment implies that there must be an initial focus onwhat possible alternative realization of a particular systemmay exist in case the system performs trigenerate functions.Thus, such a procedure must be extended with (1) determi-nation of the aggregate system level used to start theanalysis; (2) assessment of the (possible) system environ-ment; and (3) selection of the most appropriate systemboundary and system products.

In the example we have added to a hypothetical petro-chemical complex the possibility of inclusion of additionalsystem elements:

stand-alone power generation (power plant)cogeneration type 3, heat-to-power ratio 3:1cogeneration type 4, heat-to-power ratio 4:1steam turbine operating between 600 C and 200 Csteam generator, 600 Csteam generator, 200 C

These suf� ce to give a � rst impression of trigeneratepetrochemical complexes. The list can be expanded,however, for a more sophisticated analysis, for example,more temperature levels. Notably the 600¯C level producedby cogen-3 or cogen-4 can also be perceived as thetemperature level of the heat-sink presented by industrialfurnaces or industrial reaction furnaces. Thus, the analysisalso leads to insight into the merits of trigenerate petro-chemical plants by coupling of gas turbines to industrialfurnaces.

Selection of Trigeneration System Concept

Our objective was to evaluate the impact of converting apetrochemical complex to a trigenerate petrochemicalcomplex on total CO2 emission. Thus, we compared thebase case, optimal system design to meet internal power andheat exactly, with alternatives where a net exporter ofelectric power to the grid is anticipated. A straightforwardlinear programming (LP) model was constructed to selectthe optimal combination of system elements for heat andpower generation. Since this illustration comprises a small-scale problem, it could easily be implemented in MS Excel.The optimal solutions were calculated using MS ExcelSolver functionality.

The model representation is (after Stadtherr and Rudd,1976, 1978):

Acc ‡ r ¡ p ˆ 0

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FUNCTIONAL MODELLING FOR A SUSTAINABLE PETROCHEMICAL INDUSTRY 337

The elements of matrix Ac are the system elements’ input-output coef� cients;

c is the vector of capacities; c ¶ 0;

r is the vector of feedstocks; r µ s (upply);

p is the vector of products; p ¶ d (emand);

An LP-model in standard form (Ax ˆ 0) is obtained byusing

A ˆ [AcjI j ¡ I ] x ˆ [c r p]T

The optimal network design variables x0 are found byoptimization.

minx F subject to Ax ˆ 0; F(x) ˆ fT x

F is a vector of the coef� cients of the linear objectivefunction.

The coef� cients of the A-matrix are listed in Table 3.These have been expressed in megawatts (MW). A negativesign indicates an input, a positive sign an output. In thiscase, F is set such that r, which represents total fuel input isminimized. Thus, under the assumption of fossil fuelcombustion to drive the energy conversion, in this optimiza-tion the net CO2 emission will also be minimized. Since theactual emission depends on the fuel-mix employed, a � rstindication is obtained considering natural gas (CH4) as thefuel of choice.

Figures 4 and 5 illustrate the optimization results for apetrochemical complex that is characterized by an export ofelectric power (E) and heat demand at 600¯C (HT) and200¯C (LT) respectively in a ratio of HT:LT:E ˆ 4:8:E.

These results demonstrate that trigeneration using apetrochemical complex as high-level heat-sink indeedleads to the minimum fuel consumption (MW):

at no export of electricity HT and LT steam are producedas stand-alone. Any internal power demand can besupplied by the grid, or, if substantial, can be suppliedby a Cogen-4.at limited electricity export, � rst Cogen-4 capacity isrealized or expanded, subsequently steam turbine capa-city is addedincreasing the allowed electricity export to the gridbrings Cogen-3 into being, thereby providing the opti-mal match between electric power and heat required� nally, above 6 MW E-export (per 12 MW of heatHT:LT 4:8), only Cogen-3, a steam turbine andstand-alone power plant capacity are selected for optimaltrigeneration

The results show the optimal selection of system elementspicked to satisfy internal heat demand with the amount ofelectric power that can be exported from an industrialcomplex. Secondly, the minimum fuel consumption calcu-lated for trigeneration demonstrates that an industrialcomplex can be used as a heat-sink for electric powerproduction. Finally, the analysis shows that the transforma-tion to trigeneration leads to avoidance of CO2 emissioncompared with disjoint chemicals and electric power produc-tion. This has been illustrated in Table 4, where, based onthe results of system element selected (Figures 4 and 5) thenet avoided amount of CO2 has been calculated for eachextra MW of electricity exported on top of internally pro-duced 4:8 HT:LT heat. The numbers are valid assuming fuelis natural gas (methane) with a lower heating valve (LHV) of50 (MJ kg¡1).

Figure 4. Total trigenerate power generation and power generation distribu-tion as a function of power export to the grid.

Figure 5. Total trigenerate fuel consumption and fuel distribution as afunction of electricity export to the grid.

Table 3. Coef� cients used in the A-matrix. Includes net loss per element (MW).

System elements

Cogen-3 Cogen-4 Power plant StmGen-1 StmGen-2 Stmtrbine

Energy carriersFuel 1 1 1 1 1 0Electricity 0.24 0.19 0.5 0.02 0.02 0.14Heat—600 C 0.76 0.76 0 0.85 0 1Heat—200 C 0 0 0 0 0.85 0.76

Energy loss 0.05 0.05 0.5 0.17 0.17 0.1

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DISCUSSION AND CONCLUSIONS

The need for sustainable development provides a majorincentive to elucidate the question of process system inno-vation. The demand for sustainability has emerged, however,at a time where the petrochemical industry is largely in aphase of maturation. Combined with the demands of share-holders, this has lead to a wave of restructuring of theindustry to increase economic performance. Measuresemployed include mergers and acquisitions, product-portfolio shift, and restructuring of organizations. Whitehead,however, signals a lack of strategic initiatives to improve theeconomic performance of the industry (Whitehead, 2000).We believe that process system innovation and the methodsdeveloped may contribute to a sound strategy for sustain-ability that includes improved ecological, economic andsocietal performance.

Sustainable development requires innovation at all systemlevels, from reaction path to process system, from chemicalplant to petrochemical complex or industry. Any newapproach to innovation in the chemical industry however,must have sound technological and system content in orderto have a chance to achieve some impact in the industry. Webelieve that functional modelling combined with evaluationas presented in this paper does meet this criterion, andtherefore may help industry in its struggle with the conceptof sustainability, and its perceived lack-of-content.

The examples given for industrial aromatics productionand trigenerate industrial complexes demonstrate the useful-ness of functional modelling for the identi� cation of novelopportunities for technological and system change. Thearomatics example emphasizes the link between productdevelopment and process system innovation. The structureof trigenerate petrochemical complex varies subject to thetechnologies available and the amount of electric powerthat can be exported to the grid. By employing a straight-forward LP-model, it was demonstrated that the large-scalepetrochemical industry can enable a net reduction of globalCO2 emission by trigeneration, where its production systemsact as a large heat-sink. In bringing trigeneration systemsinto being, one must face uncertainties with respect toinitial capital expenditure and the margins that depend onelectricity, heat and chemicals revenue. Trigeneration,however, appears to be a promising option for ‘base-load’,which matches the petrochemical industry’s requirement ofcontinuous capacity utilization. Moreover, in a liberalizedelectricity market, trigeneration may represent one of the‘hidden’ or emergent innovation options. The market willsimply set a price for the base-load electric power from thetrigenerate petrochemical industry, as well as for any alleged

CO2 credits. Thus, a paradigm-shift in perception of thepetrochemical industry may result: instead of a threateningsource of pollution, safety and health risks, it becomes anenabler of global CO2 emission reduction.

Functional modelling provides a structured method forabstraction, that allows one to freely consider novel realiza-tions of existing functions of the chemical industry at everysystem level of interest, and thereby address the openproblem of process system innovation in a systematic, top-down approach. Only after new concepts have developed,must the hard work of assessment of their technological andeconomic impact begin.

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ACKNOWLEDGEMENT

This paper has been based on material originally presented at the6th World Congress of Chemical Engineering, Melbourne, Australia,September 2001 (Dijkema, Grievink and Weijnen, 2001).

ADDRESS

Correspondence concerning this paper should be addressed toIr. G.P.J. Dijkema, Faculty of Technology, Policy and Management, DelftUniversity of Technology, Delft, The Netherlands.E-mail: g.p.j.dijkema@tbm.tudelft.nl

The manuscript was received 1 April 2003 and accepted for publicationafter revision 3 September 2003.

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