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Energy 77 (2014) 382e390

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Energy

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A novel application of exergy analysis: Lean manufacturing tool toimprove energy efficiency and flexibility of hydrocarbon processing

M�at�e Haragovics a, *, P�eter Mizsey a, b

a Dept. of Chemical and Environmental Process Engineering, Budapest University of Technology and Economics, Budafoki út 8. F II, H-1111 Budapest,Hungaryb Research Institute of Chemical and Process Engineering, University of Pannonia, Egyetem u. 10, P.O. Box 125, H-8200 Veszpr�em, Hungary

a r t i c l e i n f o

Article history:Received 8 October 2013Received in revised form12 August 2014Accepted 4 September 2014Available online 3 October 2014

Keywords:Exergy analysisDistillation energy efficiencyLean manufacturingFlexible site

* Corresponding author.E-mail address: harmat@kkft.bme.hu (M. Haragov

http://dx.doi.org/10.1016/j.energy.2014.09.0110360-5442/© 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

This work investigates the techniques used in evaluating distillation structures from lean manufacturingpoint of view. Oil & gas industry has already started adopting lean manufacturing principles in differenttypes of processes from information flow to processing technologies. Generally, energy costs are the mostimportant factors in processing hydrocarbons. Introducing flexibility desired by lean principles to thesystem may conflict energy efficiency of the system. However, this does not mean that the economicoptimum is the energetic optimum. Therefore all possible changes due to temporarily stopped or notfully utilised plants have to be investigated, resulting in a large amount of cases that have to be evaluated.For evaluation exergy analysis can be used as it involves all energy types, and evaluation is straight-forward. In this paper plain distillation structures are investigated, and the boundaries of the systems areset up according to the status of the site. Four component case studies are presented that show that thevery same distillation structure can be more or less efficient depending on the status of the industrialsite. It is also shown that exergy analysis used with different boundaries on the same system can showflexibility of the system and reveals potentials.

© 2014 Elsevier Ltd. All rights reserved.

1. Introduction

As the global economic environment challenges refineries inEurope, it has become vital to force changes in the processes.Several oil companies have started to adopt the lean manufacturingphilosophywhich gives a new framework to their business. Some ofthese companies apply lean in certain fields, but for example theHungarian oil & gas company, MOL Group has started full leantransformation of its whole business.

Lean manufacturing has its roots in automotive industry. It isbased on and derived from the Toyota Production System, themanagement philosophy that had been developed through decadesby Toyota after the SecondWorldWar. The idea has become famousafter the book of Womack et al. [1] was published in 1991.

Lean focuses on different aspects of production in order toensure the flow of value. It is rather a set of techniques and phi-losophies than a very strict discipline. Despite the differences of theindustries principles can be fitted on several processes.

ics).

According to the lean philosophy one of the main inhibitorsof value flow is inflexibility. It is important to reduce inflexibilityin order to shorten lead time, to lower stocks, to quickly reactto changes in demand, and to achieve just in time production.This approach also implies Stop & Go operation of plants anddevices.

Basically, refineries have not been built flexible neither in termsof stopping and restarting nor considering feedstock. Usually thereis only a slight variance in feedstock and products. This fact shows apossible direction for future improvements, thus it is important toconcentrate on the design of plants that have better energy effi-ciency, while they are much more flexible than existing ones.

Flexibility might be a new direction of development, but energyefficiency is always important. Due to the large quantities ofseparated materials, the energy consumption of distillation in theworld is very high. As large amount of energy is used in refineriesand petrochemical plants, every small improvement yields in greatsavings. It is not unusual that modifications of processes have a rateof return measured in a few months. Flexibility, however, chal-lenges present technologies and equipment. It has to be imple-mented early, in the design phase, and it makes the design morecomplex due to the large number of possibilities in operating

Nomenclature

SymbolGreek letterD differenceh thermodynamic efficiency [%]

Subscript0 ambientC condenserirr irreversibleloss lossR reboilersep separationB bottoms productD distillate producte molar exergy [kJ/kmol]_E exergy flow [kW]

F feed stream_H enthalpy flow [kW]_n mole flow [kmol/s]_Q heat flow [kW]s molar entropy [kJ/(kmol K)]_S entropy flow [kW/K]T temperature [K]_W work flow [kW]

AbbreviationsALK alkaneARO aromaticBHI backwards heat integratedDS direct sequenceEP elevated pressureHP higher purityLP lower purityRR reflux rate [-]TSA total site analysis

M. Haragovics, P. Mizsey / Energy 77 (2014) 382e390 383

modes. Several operating modes are already applied in continuousplants and the number of modes shall be increased for a trulyflexible industrial site. The more plants the site has, the morepossible combinations of operating and stopped plants are possible.Moreover, these plants can have many modes for producingdifferent product qualities resulting in even higher number ofpossibilities. In such a flexible scenario it is possible that two plantsare running heat integrated to each other, but one of the units hasto be stopped. In this case the efficiency of the whole site maydecrease. Depending on the structure of the systems this drop canbe smaller or larger depending on flexibility. This is something thathas to be evaluated in order to be able to choose which solution touse.

Reducing energy consumption is important both from economicand environmental points of view, as most of the energy con-sumption are still covered by fossil energy sources. Consumingenergy also means emission of CO2 and other flue gas compoundsinto the atmosphere. Many of these gases are considered to causegreenhouse effect and, at elevated concentrations, global warming.

Two ways of possible improvements have: use more efficienttechnologies or improve efficiency of existing equipment. Severalrecent papers cover the improvement of heat integrated distillationsystems like internally heat integrated distillation column [2],intensified heat integrated column [3], and totally heat integrateddistillation column [4].

Kencse and Mizsey compared different energy integratedschemes to by assessing several factors [5].

Heat integration, or more broadly: energy integration is alsofeasible between different processes like feed preheat, not neces-sary within distillation. The two different approaches whether tosolve heat integration in the same process or to integrate differentprocesses might be equally efficient in certain cases, but theirflexibility can be different.

The other way is to improve the mode of operation. Despiteevery efforts to develop more efficient distillation technologieseconomic environment can have bigger effect on the overall effi-ciency of a plant. Most of the design calculations assume thatconditions are those of that the scheme was designed for. In realityplants are built for decades, during which period demand for theproducts fluctuate. Under the designed load efficiency of plantsusually decline, and in some cases stop and go operation of theplant can be more efficient. The decline However, stopping a plant

has effects on connected plants, heat integrations may be sepa-rated. This changes the behaviour and the efficiency of the runningplant. This effect has to be examined.

2. Evaluation and improvement of energy efficiency

During design engineers have several tools to evaluate andimprove distillation structures from energy point of view. The mostrenowned tools are Pinch Analysis and Exergy Analysis.

2.1. Pinch Analysis

The idea of Pinch Analysis was first introduced by Linnhoff [6]for heat integration. Later the idea was further developed, andpublished as a handbook for engineers [7]. The basic idea in PinchAnalysis is to reduce the external heating and cooling utility usageby preferring heat exchange between process streams. The methodis aided by visualisation tools like CC (composite curve) and theGCC (grand composite curve). This technique aids optimal uti-lisation of different quality heats.

Since its initial publication, Pinch Analysis has received greatattention, and several contributions have been made. The idea ofintegration networks has already involved mass transfer networks[8], and is called Mass Pinch. This approach can help to reduce freshwater intake for example [9], or to optimise carbon emission [10].Pinch Analysis has been successfully applied also for properties[11], for power [12] and lately for hybrid power systems [13].

The full history of Pinch Analysis is very accurately summarisedin the work of Kleme�s and Kravanja [14].

Pinch technique is a method that collects available heat supplyand demand and visualises it on the so called composites curvesand grand composite curves, that show where heat integrationpossibilities are. It can be applied for plants or even complete sites.The main drawback of Pinch Analysis for our causes is that it doesnot include all energy types, moreover, for every case new com-posite curves need to be created. In the case of a larger site it wouldgenerate a vast number of charts that have to be evaluated. A so-lution to extend Pinch Analysis to include different forms of en-ergies is presented in the work of Feng and Zhu [15], however, themethod still uses charts that are difficult to evaluate when a vastamount of possibilities are analysed.

M. Haragovics, P. Mizsey / Energy 77 (2014) 382e390384

Suphanit et al. [16] also used Pinch Analysis to detect heattransfer potential in a divided wall column, but used exergy anal-ysis alongside with Pinch to gather more information to choose theexact plate where heat should be added or not.

2.2. Exergy analysis

The other well-known option that has to be mentioned here isthe Exergy Analysis. Exergy has the advantage of handling all formsof energies together. Therefore, we used exergy as the base of ourinvestigation.

Exergy analysis can be applied in several forms. Researchersinvestigating exergy have discussed the development of exergyanalysis methods and models that quantify exergy loss and ther-modynamic efficiency of distillation columns like Fony�o's two partarticle of reversible distillation [17] and of finite cascade models[18]. Bandyopadhyay [19] has based the exergy calculations on IRS(Invariant Rectifying and Stripping) curves. Zhu [20] developed amethod to allocate Cumulative Exergy Consumption betweenuseful products.

On the other hand the basic equations of thermodynamicsrelated to exergy are used for calculations to obtain thermodynamicefficiency of distillation systems.

Kencse and Mizsey use exergy analysis in parallel with green-house gas emission [21] and later combined with both greenhousegas emission and cost assessment [5].

Exergy analysis can also be applied for the examination ofcomplex distillation plants to locate process sections where exergylosses are high and improvements are possible. Rivero et al. haveused exergy analysis to detect critical point where improvementscould be made [22].

2.3. Total site integration/analysis

The more systems or plants are involved in the integration, themore integration possibilities are. That also means more tempera-ture levels by which a better utilisation of exergy may be possiblewith better distributed heat transfers. Thus, another expression hasto be mentioned here is the TSA (total site analysis). TSA aims toutilise heat at more temperature levels, by including several plantsand sites in the integration, providing even more possibilities andavailability temperatures than a smaller scale integration. The termwas first introduced by Dhole and Linnhoff [23]. They extended thePinch technique to a larger scale; Total Site Integration is energyintegration between different plants and factories. The method wasfurther developed by Kleme�s et al. [12].

Since the first application of the idea, many publications havebeen born investigating possibilities of integration betweenneighbouring plants of many types. Hackl et al. have investigatedchemical companies [24], Matsuda et al. identified large savingpotential in a steel plant, which was considered well integratedbefore the investigations [25]. Manesh et al. have investigated adesalination plant, first by identifying potential for integration,then, in the next step, by a targeting model [26]. Bandyopadhyayet al. presented a method, that does not remove pockets from in-dividual GCCs in order to increase overall integration of the site[27].

Modifications of the original concept have also been developed,for example the work of Varbanov et al. [28]. Total site analysis canalso be combined with exergy analysis [29], as these methods areclosely related.

Topics of Total Site Integration, and exergy usually involve manytypes of energy and equipment; however, in our workwe show thatthe idea of Total Site Integration can also ensure greater possibil-ities in the exergy analysis of distillation systems.

In this article an ideal background heat cascade is assumed, thatensures zero exergy loss outside the boundaries of a process that isintegrated to this heat cascade. A large site can be such a heatcascade: several processes providing several integration possibil-ities at several temperatures.

3. Thermodynamic efficiency of distillation

3.1. Exergy analysis: energy and efficiency

Since the energy related crises, processes were optimised forenergy consumption, but usually the quality of the energy was notconsidered. However, different energy types and even energies ofthe same type generally have different qualities. The difference canbe originated from the conversion of one into the other. Conversionbetween different forms of energy is not possible without losses.

Regarding heat, the same amount of heat can be of higherquality if it is available on higher temperatures. To investigate heattransfer ability Guo et al. [30] introduced a new physical quantity:entransy. The conversion of higher temperature heat to lowertemperature heat is flawless, but in the reverse direction it is notpossible without using additional work.

Exergy, however, makes a difference between higher and lowerquality energy. In the calculation of exergy not only the first, but thesecond law of thermodynamics is also taken into account. Bydefinition, exergy is the maximal possible useful work during aprocess that brings the system into thermodynamic equilibriumwith a heat reservoir. The heat reservoir usually is the environment.Exergy of heat transfer can be expressed in the form of Eq. (1). It isclearly visible that the Carnot-efficiency shows the quality of heat.

E ¼ _Q ��1� T0

T

�(1)

Physical exergy of material streams is Eq. (2).

E ¼�_H � _H0

�� T0 �

�_S� _S0

�(2)

Summarising the advantages of using exergy analysis: it canhandle all types of energies together, it takes irreversibilities intoconsideration and expresses that heat energy and work are notequivalents. Moreover, it also shows that heat degrades fromhighertemperatures to lower temperatures. A simple enthalpy balancefails to indicate these differences between different forms of en-ergy. For this reason exergy analysis shall be in focus during design.

The second law of thermodynamics states that entropy in aclosed system can only increase. In the distillation process the heatis introduced into the system through the reboiler, then throughthe columnwhere the useful work is done, and finally it is extractedfrom the system in the condenser at a lower temperature than itwas introduced to the reboiler. This process makes it possible toseparate the liquid mixture, and during this process heat degrades.

The ideal distillation column, considering exergy, would be thereversible distillation column (Fig. 1) introduced by Fony�o [17]. It iscalled reversible, because irreversible change does not occur in it;thus, there is no entropy generation. To achieve this, infinitesimalmass transfer gradient is needed in every point of the column, thatalso means an infinite number of plates, as well as negligible pres-sure drop. Heat transfer has to be distributed along the height of thecolumn at each tray, and as a consequence, the number of heattransfers is also infinite, and the quantity of each heat transfer isinfinitesimal. Below the feed only heat inputs and above the feedonly heat outputs are applied. This implies that the material flowalso changes along theheight, resulting in anunusual columnshape.

Fig. 1. Reversible distillation column.

M. Haragovics, P. Mizsey / Energy 77 (2014) 382e390 385

These very strict criteria of the reversible column render it atheoretical, infeasible solution. However, many of its features can beimplemented during design of distillation columns. Thermallycoupled distillation systems with side-stripper, side-rectifier, or heatintegration are all ways pointing towards this column, to improveenergyefficiency; that is, efficientheatutilisation.Multiple feed inletsandmultiple side-draws are also desirable. Internal heat transfer andevendistribution of heat inputs and outputs are also required to builda thermodynamically efficient system. Optimisation of feed stagesand feed thermal state is also necessary. In certain cases heat pumpscan also be applied. The more the distillation system resembles theideal column, the more effective the distillation will be.

4. Calculation method

The method we use to calculate the exergy loss and thermo-dynamic efficiency is described by Seader and Henley [31].Suphanit et al. [16] and de Koeijer and Rivero [32] also used thismethod, and recent publication by Sun et al. [33] includes the sameprinciple for exergy analysis of distillation systems. First the irre-versible entropy loss (Eq. (3)) needs to be calculated.

D _Sirr ¼Xout

_n� sþ

_QC

TC

!�Xin

_n� sþ

_QR

TR

!(3)

Having the entropy loss calculated and knowing the tempera-ture of the heat reservoir, it is possible to calculate the exergy loss(Eq. (4)).

_Eloss ¼ T0 � D _Sirr (4)

Exergy loss (lost work/anergy) can also be calculated directlyfrom the availability (exergy) balance (Eq. (5)).

_W loss ¼Xin

�_n� eþ _Q

�1� T0

TR

���Xout

�_n� eþ _Q

�1� T0

TC

��

(5)

The work of separation is also needed; that is, the difference ofthe exergy flowing in and flowing out with material streams (Eq.(6)).

_Wsep ¼Xout

ð _n� eÞ �Xin

ð _n� eÞ (6)

Finally, the efficiency is calculated by dividing the minimaluseful work by the overall energy input.

h ¼_Wsep

_Wsep þ _Eloss(7)

Exergy analysis means several methods, it can focus on exergyloss per plates to optimise a column, others use properties ofstreams connecting different units to evaluate exergy loss. In thelatter case, it is very important to properly define the systemboundaries, i.e., to define the system that is to be investigated andthe streams entering and leaving this system. Proper use of exergyanalysis, however, can fulfil the desired role of being a more usefulindicator of energy performance of distillation than the enthalpybalance, and can add even more than a Pinch Analysis in somesituations detailed in the following.

5. Exergy analysis and flexibility

For large number of calculations and possibilities it is importantto have a simple and fast calculation method. In this study theexergy analysis is based on the method described by Seader andHenley [31]. The main advantage of this method is its simplicity:only the input and output streams of units have to be used incalculations.

Considering flexibility, it is important to separate the core pro-cess, that has the same parameters in all conditions where thegiven process maywork. The core process is of which the propertiesdo not change regardless of the state of integration. The parametersof the core process have to be constant to ensure the desired sep-aration. The exergy loss in the core process is necessary to achievethe desired separation with the chosen system. All other exergylosses depend on the technological environment. As two extremecases this core process can be integrated into an ideal heat cascadeor used independently, applying universal cooling and heatingutilities. These cases are the most and least ideal cases of a flexibleindustrial site that a process can face. In the first scenario, all energyleaving the process is utilised optimally, as the ideal backgroundheat cascade needs heat at exactly the same temperatures as thecore process can provide, and it also provides heat at sufficientlevel. While in the latter case the heat is provided at higher levelthan needed and carried away by cooling water without using anyportion of it. Such case can happenwhen the processes that providethe background heat cascade are stopped, and the heating andcooling tasks are accomplished with universal utilities. In theaforementioned cases the energy efficiency of the core processremains the same but the overall energy efficiency decreases as agreat exergy loss occurs outside the boundaries of the core processwith universal utilities. Universal utilities are those that can be used

Fig. 2. Boundaries of different cases.

M. Haragovics, P. Mizsey / Energy 77 (2014) 382e390386

in most applications, like steam networks or cooling water takenfrom the environment.

In practice the difference between the cases means differentboundaries for the analysis that are the boundaries of the processes,i.e., the heat cascade or the environment. In this sense the realimportance of boundary is the presumption, that all exergy leavingthe system is fully utilised. Otherwise the exergy would be 0.Optimal usage means that heat is fully utilised elsewhere in thebackground cascade, at the temperature at which it leaves thedistillation system. In other words: the exergy of the stream, that iscrossing the boundary, is useful; no portion of it is lost due totemperature differences until it reaches the next process in linewhere it is utilised. Otherwise the boundary does not represent thesystem, and needs to be redefined.

The most theoretical approach is when the system boundary isright around the core process. Thatmeans, that the heat exchangingtemperatures are exactly those of the materials involved in the coreprocess, liquids and vapours of the distillation (see Fig. 2, dashed)entering the heat exchanger. This approach presumes infinitepossibilities of applicable utilities. In this case even the usualtemperature differences needed for heat transfer are not taken intoaccount, and this case then most strictly describes the distillationprocess, the separation of materials without auxiliary unit opera-tions. This case is analogue to the ideal case of a distillation systemintegrated into a large site with infinite possibilities in integration,and infinite possibilities of temperature levels of heat exchange.

In a less theoretic case a problem with this approach arises. Theexergy that is leaving the system is regarded useful; however,placing the boundary one step further we encounter the nexttemperature decrease without associated useful work. In heat ex-changers heat exchange surface is finite, thus, there must be atemperature difference. Moreover, a certain temperature differenceis defined in order to optimise costs. This means, that the heatcrosses the boundary at a lower temperature, than in the case of thecore process. In these cases some exergy is lost: higher temperatureheat enters the system, than the process really needs; and lowertemperature heat exits, than the process provides. Furthermore,this is still an idealistic approach. It still assumes that heat leavingthe system is used in another process exactly at its temperature ofleaving the heat exchanger. If the temperature of utilisation in thenext unit is lower than the exit temperature of the stream from thepreceding unit, the difference in exergy is lost.

Considering the widest boundaries (Fig. 2, dotted dash) thestream crossing the boundary has 0 physical exergy, as no other

process uses the extracted heat. This happens in the case of a singleprocess or when the temperature is so low, that its heat contentcannot be utilised any more, and it is released in the environmentwith cooling water or in other forms. In this case the heat providedby the last process is not used at all.

Regarding a single distillation plant as the whole site, the exergyloss in the core distillation system is the same as in the integratedcase, but thewhole process with all auxiliary processes and streamshave a greater exergy loss. Without any integration, the exergycontent of all exiting streams are wasted.

In a flexible site, the integration possibilities vary between thesetwo extremities. When several plants are operating at the sametime, it provides a large heat cascade, but when all other plants arestopped, the integration will become impossible.

Thus, we investigate these two cases of boundaries. In the firstcase, the boundaries are around the core process of distillation. Inthe second case, the individual site is considered, with no inte-gration possibilities outside of distillation. The models are simpli-fied as much as possible: there is no temperature difference neededfor heat transfer, meaning zero driving force in all cases.

Case #1. This is the case that investigates the core process. Theexergy analysis is carried out under the assumptions that:

� the driving force of heat transfer is zero for the sake of simplicity(DT ¼ 0),

� there is a background process/heat cascade, that can deliver andaccept (sourceesink) heat at every temperature level (total siteproblem),

� the heating in the reboilers and the cooling in the condenserstakes place with streams obtained from the background processat zero driving force,

� there is no anergy (destroyed exergy) during the operationwhere the products of rectification cool down to the ambienttemperature, since their exergy is fully utilised in the back-ground process also at zero driving force.

This case corresponds to the ideal case of Total Site Integration,thus, it has the maximal efficiency possible. In other words, thedistillation process is totally and optimally integrated to a largebackground process (ideal total site). Efficiency of an operatingplant or site can only be lower than this. This boundary assumesthat only the exergy loss of distillation is important, withoutauxiliary equipment or processes.

M. Haragovics, P. Mizsey / Energy 77 (2014) 382e390 387

Case #2. This case is fully based on a single distillation systemwithout a background process. That means, that there is only oneunified heat source: a steam system; and there is only one unifiedheat sink: the cooling water taken from, and sent back to theenvironment. Steam systems guarantee heat transfer betweendistinct plants far from each other. Some steam recovery may bepossible, but it is not usual. In these rare cases the recovered steamis fed into a system with lower pressure than the original heatsource. In other cases either higher pressure steam or shaft workmay be used to regenerate it.

The heat that leaves distillationmay be used in the plant or closeto the plant, but in several cases it is simply released away. It may betaken away by the cooling water, or the hot product may cool downduring storage or transportation. Whichever way, the final state ofthe products will be the equilibrium with the environment,releasing heat without doing work. Ultimately, all of the exitingexergy will become lost, as at the cold side boundary of the systemthe temperatures will be equal to the ambient temperature.

The exergy analysis is carried out under the assumptions that:

� uniform cooling water is applied at the condensers, and productcooling operations to ambient temperature (final bottomproduct also),

� the driving force of heat transfer is zero, again. Utilised coolingwater is not used anywhere else, and its exergy content iscalculated as anergy (exergy loss),

� uniform heating medium is applied at the reboilers.

6. Four component case studies

In the following, case studies are presented on two differentquaternary mixtures. Different distillation schemes, that completethe same separation task, are compared according to thermody-namic efficiency. These cases represent a flexible plant in differentconditions.

Due to the importance of distillation in hydrocarbon processingindustry the two quaternary mixtures are the following:

� alkane hydrocarbons,� aromatic hydrocarbons.

Alkane hydrocarbon mixture feed is an equimolar mixture of

� n-pentane,� n-hexane,� n-heptane,� n-octane.

While the aromatic mixture feed is an equimolar mixture of

� benzene,� toluene,

Table 1Compositions of alkane hydrocarbon product streams (mole fraction).

Distillate 1 Distillate 2 Distillate 3 Bottoms 3

Sloppy

n-Pentane 0.82 0.18 0.00 0.00n-Hexane 0.15 0.70 0.15 0.00n-Heptane 0.03 0.12 0.63 0.23n-Octane 0.00 0.00 0.22 0.77

Sharp

n-Pentane 0.96 0.04 0.00 0.00n-Hexane 0.04 0.93 0.04 0.00n-Heptane 0.00 0.03 0.90 0.06n-Octane 0.00 0.00 0.06 0.94

� ethylbenzene,� cumene.

Additionally we examine two cases of product purity with bothmixtures:

� sloppy separation with lower purities (ALK-LP (alkane-lowerpurity), ARO-LP (aromatic-lower purity)),

� sharp separation with relatively high purities (ALK-HP (alkane-higher purity), ARO-HP (aromatic-higher purity)).

Alkane product purities can be seen in Table 1, and aromaticproduct purities are listed in Table 2.

The base case is the conventional DS (direct sequence, Fig. 3).Additionally, a backwards heat integrated alternative of the directsequence is also among the alternative structures (DS-BHI (directsequence-backwards heat integrated), Fig. 4). Forward integrationproved uneconomic according to a previous study by Emtir et al.[34], therefore, it is not investigated in this study.

A modified version of the direct sequence is also examined (DS-EP (direct sequence-elevated pressure), Fig. 3): the pressures in Col.2 and Col. 3 are equivalent to those of the backwards heat inte-grated scheme (DS-BHI). This system helps to make a distinctionbetween the effects of integration, and the effects of modificationsneeded for integration. Backwards heat integration needs elevatedpressures in some of the columns to be able to achieve tempera-tures high enough to realise the necessary heat transfer betweenthe source and sink columns.

In order to have the same product streams and work of sepa-ration all the product streams have the same pressure. In the case ofDS-EP and DS-BHI some of the products leaving the columns havehigher pressure than in the base case, where all columns operate onatmospheric pressure. In these cases an additional heat exchangeris included, in order to reduce pressure to atmospheric, whilekeeping the mixture in liquid phase. In this case, the heat duties ofproduct heat exchangers are considered as recovered heat in Case#1.

For simulations the Aspen Plus software is used. The chosenthermodynamic property is PengeRobinson, as in the very begin-ning of the work anomalies have been experienced with SRK(SoaveeRedlicheKwong) property model.

Pressures of distillation columns can be seen in Table 3, refluxrates are shown in Table 4 for alkane systems and in Table 5 foraromatic systems.

7. Results and discussion

7.1. Exergy analysis of the core distillation process (Case #1)

As an indicator of performance the thermal efficiency is chosen(Eq. (7)). Taking a look at the results, Fig. 5a shows an unusualtendency in the case of direct sequence structures. The most simpledirect sequence (DS) proves to be the most efficient in all cases.Moreover, the heat integrated direct sequence is worse in bothcases than its sibling structure without heat integration (DS-EP).

These results may be surprising at first sight, but after taking asecond look at it all these phenomena can be explained. The firstthing to mention is the higher efficiency of lower pressure systems.At higher pressures, the boiling point rises. In terms of exergyhigher temperature energy is more valuable, hence, a more valu-able energy is used for the same work of separation, consequentlylowering efficiency at higher pressures.

During the design of the DS-BHI, the pressures were set to levelsthat ensure proper temperature difference between the two sidesof the common heat exchangers of neighbouring columns. This

Table 2Compositions of aromatic hydrocarbon product streams (mole fraction).

Distillate 1 Distillate 2 Distillate 3 Bottoms 3

Sloppy

Benzene 0.78 0.22 0.00 0.00Toluene 0.15 0.75 0.10 0.00Ethylbenzene 0.05 0.03 0.63 0.29Cumene 0.02 0.00 0.27 0.71

Sharp

Benzene 0.98 0.02 0.00 0.00Toluene 0.02 0.98 0.00 0.00Ethylbenzene 0.00 0.00 0.93 0.07Cumene 0.00 0.00 0.07 0.93

Fig. 3. Conventional direct sequence (DS, DS-EP).

Fig. 4. Direct sequence with backwards heat integration (DS-BHI).

Table 4Reflux rates and temperatures for alkane systems.

DS DS-EP DS-BHI

LP

RR 0.2 0.2 0.2Col. 1 TC 40.0 40.0 40.0

TR 85.4 85.4 82.3RR 1.1 1.2 1.2

Col. 2 TC 62.1 95.0 82.3TR 105.5 141.2 139.1RR 0.4 0.8 0.8

Col. 3 TC 96.3 154.6 139.1TR 117.5 177.8 177.8

HP

RR 1.2 1.2 1.2Col. 1 TC 36.7 36.7 36.7

TR 89.9 89.9 93.7RR 3.0 5.0 5.1

Col. 2 TC 67.3 99.8 93.7TR 108.5 144.3 144.2RR 2.4 6.0 5.9

Col. 3 TC 97.8 155.5 144.2TR 123.2 183.9 183.8

Table 5Reflux rates and temperatures for aromatic systems.

DS DS-EP DS-BHI

LP

RR 0.2 0.2 0.2Col. 1 TC 85.1 85.1 85.3

TR 123.9 123.9 123.7RR 2.2 2.5 2.5

Col. 2 TC 102.6 138.5 123.7TR 141.1 179.6 178.8RR 1.0 1.5 1.5

Col. 3 TC 136.2 199.7 178.8TR 146.5 211.4 211.4

M. Haragovics, P. Mizsey / Energy 77 (2014) 382e390388

temperature difference is chosen as a rule of a thumb, and its aim isto ensure finite area surfaces. Due to the assumption that outsidethe system no driving force is needed, the designed temperaturedifference is disadvantageous in this case of boundaries. DS-EPstructure operates at the same temperatures and on same pres-sures as DS-BHI, and produces the same products, has the verysame inner flows. But, in the DS-EP case, heat is not transferredbetween the columns, instead it is exchanged with the idealbackground heat cascade, where the heat makes useful work whileit is cooled down to the temperature, at which it enters theinvestigated distillation system again.

The most interesting result is the difference between the resultsof DS and DS-BHI configurations. Heat integration is a popular wayto achieve lower energy consumption. In spite of the fact that heatconsumption can be significantly reduced, these results show thatdespite sparing a lot of heat, this kind of heat integration is not

Table 3Pressures of distillation columns (bar).

DS DS-EP DS-BHI

Col. 1 1 1 1Col. 2 1 2.5 2.5Col. 3 1 4.2 4.2

necessarily the best solution. The reasons for the worse perfor-mance of the heat integrated scheme have been mentioned before:firstly, this kind of integration needs elevated pressures, whichrequires more exergy; secondly, heat transfer is better distributedwithout heat integration. In other words, DS structure uses moreenergy, but at the same time it also provides more energy for otherprocesses, and these energies are of higher quality when they passthe boundary.

7.2. Exergy analysis of standalone system (Case #2)

In the previous case of boundary the possibilities of heat ex-change temperatures were unlimited, meaning that all heat leavingthe system is utilised somewhere else.

In a non-ideal situation this is usually not possible. In the worstscenario the system has no integration possibilities. Uniform heatexchange temperatures are used, that usually are steam supplysystems or unified cooling utilities, preferably cooling water (atambient temperature). In this case, this situation is considered.Thus, all cooling temperatures are T0, there is no further integrationpossibility.

In the previous case, it was assumed that no higher temperatureheat was used than needed. That assumption is only valid if an

HP

RR 1.2 1.2 1.2Col. 1 TC 80.1 80.1 80.0

TR 129.3 129.3 131.1RR 3.3 5.2 5.0

Col. 2 TC 110.9 146.9 131.1TR 142.9 181.6 181.4RR 3.5 5.0 5.1

Col. 3 TC 136.5 199.7 181.4TR 150.6 215.9 215.9

M. Haragovics, P. Mizsey / Energy 77 (2014) 382e390 389

integration with an ideal heat cascade is possible. In that case allheat leaving the system will be utilised at the temperature of itssource as well. In a standalone environment plants usually use theirown steam and cooling water system for heating and cooling,where it is possible. Hence, all cooling temperatures are fixed atlower temperature than needed, and heating temperatures arehigher than necessary.

To represent the worst case calculations include only two tem-peratures: one temperature for the cooling water and one tem-perature for the heating steam. This situation is quite realistic for

Fig. 5. Results of four component case studies.

even amedium sized plant that has no integration possibilities withother processes, or other processes are stopped. In this case, how-ever, exergy calculation loses most of its meaning as differences intemperatures disappear. As the utilised heat is from the samesource in all cases, the difference in exergy loss only comes from thedifference in quantities of the utilised heat. In other words, exergybalance is simplified practically to an enthalpy balance. Ranksbased on exergy analysis will then be like ranks based on simpleenthalpy analysis.

Comparing efficiencies (Fig. 5b) with enthalpy consumption(Fig. 5c) shows that the ranks are the same. The similarities areobvious. In all cases the DS-BHI scheme has the highest efficiencyand the lowest energy consumption. In the case of utilities withsuch limited temperature possibilities, exergy analysis does notdelivermore information than a simple enthalpy balance. However,for comparison of the standalone mode and integrated mode it isnecessary.

8. Comparison of operating modes

As the results show, the very same distillation schemes can beevaluated as efficient and less efficient, depending on the integra-tion level with the surrounding site that means different systemboundaries. As a part of a flexible site, the possibilities can varybetween the ideal heat cascade integrated mode and standalonemode. Theoretically the integration level can vary from day to dayby changing operation modes of connected plants according torolling plans.

Case #1 shows that the heat integration in the same distillationprocess lowers exergetic efficiency compared to the distributedheat exchange of the simple direct scheme integrated to the idealheat cascade. However, from flexibility point of view, heat inte-grated distillation scheme provides a steadier performance. Instandalone mode, that is if there was no heat cascade, just thedistillation system, heat integration saves exergy by using lessenthalpy than the direct scheme while the more provided enthalpyof the direct scheme cannot be used. It is important to rememberthat the same schemes and parameters are considered.

The ambivalent behaviour of heat integration can be explainedin other words: a DS scheme consumes more heat than DS-BHI, butat the same time it also provides more heat at the cold ends. Andwhat is more important: majority of this heat is higher quality heat,at higher temperatures than those exiting heat integrated systemsat the last column. This behaviour is recognised by exergy analysis,but not by enthalpy analysis. However, one crucial question re-mains: can another process provide and use this heat or not. If yes,the question of flexibility remains.

The previous cases are simple cases, however it already means12 different cases, that would mean the same number of CCs andGCC-s for a Pinch Analysis. Generally sites are lotmore complicated.With exergy analysis it is simpler to test this large number ofconditions and cases, moreover, it can easily combine differentenergy types.

It could be seen that heat integrationwithin the same process isnot necessarily a good solution as it may require modifications thatlower thermodynamic efficiency. On the other hand, in a flexibleenvironment its performance is steadier.

The optimal processes highly depend on thewhole site, whetherit is used often as a standalone system with fixed and constantutility temperatures, or there is a site that provides possibility forintegration.

Apart from site flexibility questions, the comparison of thecurrent state and ideally integrated processes provides an oppor-tunity to discover the energy reserves in the examined process. This

M. Haragovics, P. Mizsey / Energy 77 (2014) 382e390390

reserve reveals the potential that can be targeted with betterintegration.

9. Conclusion

Hydrocarbon industry andmany other industries have started toadopt lean seriously since the last great recession. It becameobvious thatmind-set has to be changed, processes have to bemoreflexible and inventories have to be reduced due to negative effectsof fluctuations in demand and prices.

Plants that were designed several years and often decades agoexperience problems while following fluctuating rolling plans.Underutilisation has greater negative effects on plants specific en-ergy consumptions than any technological improvement cancounterweight, thus Stop & Go operation or more flexible tech-nologies would be desirable if it is possible to solve.

Leanprinciplesmayalso appear indesign, and industrial sites canbe less prone to economic instability in terms of energy efficiency.

From lean manufacturing point of view flexibility of processes isimportant, and by using exergy analysis energy efficiency can beeasily evaluated to find the optimal alternative from the set ofpossible solutions. This solution is as flexible as needed and is en-ergy efficient at the same time. This new method of using exergyanalysis grants access to new information that was neglectedbefore, moreover with it is possible to handle evaluation partlyautomatically.

Exergy analysis combined with simulations of plants anddifferent scenarios of average downtimes can yield in a site that hasgood and steady specific energy consumption regardless ofconditions.

Acknowledgements

This workwas supported by T�AMOPe 4.2.2.A-11/1/KONV-2012-0072 project of Hungary and the European Union and KMR e 12-1-2012-0066 project of Hungary.

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