overall site optimisation by pinch technology
TRANSCRIPT
IChemE 0263-8762/97/$10.00+0.00© Institution of Chemical Engineers
OVERALL SITE OPTIMISATION BY PINCHTECHNOLOGY
By *B. LINNHOFF and **A. R. EASTWOOD (MEMBER)
*Department of Chemical Engineering, University of Manchester Institute of Science and Technology**LinnhoffJ March Ltd, Manchester
The importance of integration in process plant design has long been recognised. This is evideoced for example, by the existenceof complex heat and power networks in modern prodoction sites. These networks represent site-wide integration. They usuallyallow process heating duties in several processes to be effected at low cost by using turbine pass-out steam after obtaininga credit for power generated in a central turbine. When looking for energy saving in retrofit projects, an installed heat andpower network of this type usually introdoces a counter-prodoctive element. A redoction in steam heating duties leads to areduction in turbine pass-out steam which, in turn, leads to a reduction in power generation. This makes optimisation of theprocess alone and optimisation of the process in the site context two rather different tasks. More complications are giventhrough other more general reflections across a site. A modem processing plant soch as ethylene, for instaoce, is usually linkedinto the overall site and interacts with other plants through several utilities, not just back pressure steam. This makes itDetessary to not only understand the fundamental process demands but also to analyse these demands in the site context.Process licensors will usually prefer to consider standard designs for individual processes. The best project, on the other hand,will almost invariably be site dependent. This artide highlights the distiDttion between the application of process integrationtechniques-in particular pioch technology-within an individual process and their wider application in the overall site context.The "counter prodoctive" aspect of heat savings in the context of isntalled heat and power networks is given attention. Inaddition to energy, the artide discusses yields, flexibility and capital cost savings.
INTRODUCTION
Over the last four years Pinch Technology has beenwidely used for the integration of process plants. Virtually every sector of the process industries has benefitted.This experience clearly teaches that integration must beundertaken in a total site context.
This paper considers how process plants interface withutility systems. It describes the development of an optimum retrofit scheme in the context of an overall site.Individual process improvements are not taken in isolation. Rather, their total impact is considered in agenuinely site wide context.
The paper takes as its example an ethylene process.This is because ethylene processes are not only complexbut interact with the site utility system in a complicatedway. The process employs a large number of separateutilities. These utilities are closely coupled with theoverall factory site.
In terms of ethylene technology the paper adds toprevious advances1,2.
Figure 1 is a simplified schematic of a "typical"ethylene plane showing the following, not uncommon,process features:
• Pyrolysis furnace and quench steam raising• Principal mechanical drives
-raw gas compressor-propylene refrigeration set, Fl--ethylene refrigeration set, F2-boiler feed water pump
• "Cold box" with steam and fridge demand (severallevels)
• Other process steam demands (several levels)
• Import steam from central power station (severallevels)
Figure 2 is an overall site plan in which the ethyleneplant is shown in relation to other plants in the factoryas well as to the central boiler house and power station.Again, this type of situation is typical of many sites ina wide cross-section of industries. Generating pressuresteam at 120 bar (VHP) is raised in the main boilers andall of this steam is fed to the turbo-generators in thepower station. Steam is extracted at three levels (HP, MPand LP) and distributed around the factory; there is nocondensing of steam in the power station. Site electricitydemand is satisfied by import from the grid in additionto self-generation.
Figure 3 represents a more detailed look at theethylene plant steam system. We can see that condensingsteam turbines are employed on both the raw gascompressor and the propylene refrigeration compressordrives. Total steam for heating duties around the siteamounts to 270 tons/hour distributed as shown. Thecentral boiler house supplies 210 t/h of steam to theturbo generators to raise 16MW of electrical power.Quench steam raised in the ethylene plant amounts to150 t/h.
Taking energy savings as the first objective in ourretrofit, we must try to:
• Reduce hot utility requirements in all plants wherecost-effective
• Increase power generation
Achieving either or both of these objectives shouldresult in reduced operating costs. There is, however, a
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OVERALL SITE OPTIMISATION BY PINCH TECHNOLOGY
FRIDGE SETS
ETHYLENEt-o'--+-<I"PRODUCT
III
I I
I iL ...J
RAW GASCOMPRESSOR
BFWPUMP
HP STEAM
:MP STEAM nLP STEAM I I I
QUENCHSTEAM
FUEL
Figure I. Ethylene plant schematic.
basic conflict between these objectives in that a reductionin steam heating duty will result in a reduction in turbineextraction steam and consequently a reduction in backpressure power generation.
(This is representative of many European sites. In theUSA, the ranking between options (2) and (3) is becoming less clear-cut. None of the methodology to bediscussed in the present article depends on this ranking.
HP USERS(TOTAL SITE I
MP USERS!TOTAL SITE I
LP USERSllOTAl SITE)
270 t/h TOTAl
t1PORTED
'n"'"16HW
WENCH STEAM
FUEL
lP I----......:.-----"-=---L---'--....I..l
M.P I--__--=-_---JL...-..--_---JL.-.;-_-:-....I..l
CONDENSATE.
II t1PORTED{7 ELECTRICITY
'" ELECTRICALPOWER
PRELIMINARY ASSESSMENTFirst we set out the economic criteria for possible
retrofit projects. This often includes ranking of thevarious means of obtaining shaftwork.
The marginal cost of VHP steam generation in ourexample, which is based on European conditions, is£7.50 per ton and the cost of imported electricity is £28per MWh. The condensing turbines in the ethylene plantcan generate 220 kWH per ton of VHP steam whichmeans that the marginal cost of "condensing shaftwork"is £34/MWh. In producing shaftwork, then, we have thefollowing ranking in terms of cost:
(I) Back pressure steam turbine (least expensive)(2) Imported electricity from the grid(3) Condensing steam turbine (most expensive)
UEL
Figure 2. Overall site plan. Figure 3. Ethylene steam system.
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LINNHOFF and EASTWOOD
Figure 4. Analysis of separate plants.
LP USERS
M.P USERS
240 t/h TOTAL
35
FURTHER IMPROVEMENTThe system shown in Figure 5, although offering
reasonable savings at good payback, still includessignificant amounts of shaftwork derived from condensing steam. We would ideally like to replace the propylenecompressor (FI) turbine with an electric motor. Wecannot, however, treat this turbine in isolation as closerinspection of the steam system reveals. Figure 6 showsthe steam from the ethylene compressor (F2) turbine isfixed by the duty of the compressor at 80 t/h. Processrequirements for LP steam are given as 80 t/h and theimport of LP steam from the power station turbine is ata minimum value of 20 t/h determined by the characteristics of the power station turbo-alternator. To replaceF I condensing turbine by an electric motor would meanthat 20 t/h of LP steam would be surplus to demandwould have to be vented if not being condensed in theturbine. Further savings would therefore appear to bestrictly limited because we have run up against thisoperability constraint.
At this point we need a thermodynamic tool whichwill address the complex interaction of heat and powerinherent in the overall plant complex in a rigorous andsystematic way.
QUENCHSTEAM
USE OF THE GRAND COMPOSITE CURVEThe ideal tool with which to analyse the process/utility
interface and interactions of heat and power networks isthe grand composite curve3 which is now well accepted.
~, ,, ,<'RJEL ""-24"--
',-'
Figure 5. Steam system after implementation of heat recovery projects.
HP I---........~~r---- ......~-:--,.....-.........
!'I.P 1-----;--......-r------''--:---;-...I.o1
tLP 1-----'----........-7---L--..L-...I.o1
~,CONOENSATE ~~~/
-10%
A
-5%-12%
....and each process
is made to achieve target
Rather, it is part of the approach to establish the rankingup front, whatever it may be.)
Given the case above, our objectives are to reduce oreliminate the use of condensing turbines and introduceelectric motors and/or back pressure turbines wherepossible.
Let us now assume that careful studies have beenperformed on the separate process units to minimiseenergy consumption by process improvements includingbetter heat recovery, improved housekeeping and whatever else may be appropriate. In pinch technology termslet us assume that all projects are known to bring theindividual plants "on target" (Figure 4). The result ofsuch a thorough, and site-wide, campaign of process-byprocess improvements may be that the total steamdemand for process heating could be reduced from 270t/h to 240 t/h, a saving of 14% at the MP level and 11 %at the LP level (Figure 5). However, we wish to retainas much as possible of the existing steam system, reducing the condensing steam component in preference toback-pressure, in line with our economic criteria.
In the revised scheme shown in Figure 5 we havereduced the condensing steam to its minimum value inthe raw gas compressor drive and introduced an electrichelper motor to make up the deficit in shaftwork. Fuelto the boiler house is reduced by 24% (assuming constant boiler house efficiency) but imported electricity isincreased by 7MW to cater for the helper motor (4 MW)and the reduction in output of the turbo-generator (3MW).
The reduction in main boiler house steam raising isworth £375 per hour and the increase in import electricity amounts to £197 per hour. The net saving withthis combination of projects is therefore £178 Per houror £1.44 million p.a. The associated capital cost of £2.2million means that the overall project payback period isapproximately 18 months. This could be consideredquite typical for overall benefits to be achieved from aprocess-by-process campaign of improvements.
After studying
each process carefully....
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OVERALL SITE OPTIMISATION BY PINCH TECHNOLOGY
300200
HEAT IHWI
100
////
//
/
//
//
//
//
//
//
/HP __
PIP ,/,..-----"
_~J--'
1600
1400
1200
1000
TEMP.(OC)
800
600
400
~' -..EXCESS 0HEAT I
Figure 8. More accurately.
for process heating. Total shaftwork of 44 MW produced by the steam system is also indicated.
This diagram is thermodynamically incorrect since itdoes not show the correct interaction of process andutilities but it does give a clear picture of the powercycles. The derivation of a more accurate utility line isnot discussed in this article but is reproduced as Figure8. We can now see more clearly just what is happeningat the existing process/utility interface.
Now Figure 8 indicates sub-optimality in two ways:
1. The excessive use of heat caused by the lack of processintergration (i.e. the overshoot of the utility profile onthe grand composite curve). This is equivalent (interms of heat duty) to the 30 t/h of steam which wesaw earlier could be saved by improved process heatrecovery within the process plants plus the unrecovered heat in the flue gases below the existingstack temperatures.
2. The poor fit of steam levels against the grand composite curve. The curve indicates an opportunity touse more LP steam in preference of HP and MPsteam. The process actually requires 20 t/h HP, 100t/h MP and 120 t/h LP-the same total of 240 t/h butin a different distribution.
Now, the revised steam distribution will allow us toproduce more shaftwork from the same total amount ofsteam and the increased LP demand will avoid theproblem of minimum flow in the back end of the powerstation turbine. This will give us more flexibility in ourrevised steam system design. Figure 9 illustrates such abetter matched process/utility interface. Note that theflue gas line is now slightly steeper than before becauseof the reduced steam raising at the boiler house.
300
~-,,--,--aOtfh
200
HEAT IMW)
F/01POWER
STATION
100
LP
//
//
//
//
/
//
//
__""-SITE GRANDVHP STEAM RAISING __ COMPOSITE
F===9==~;;';";;~~HP5====-t·'
I---,Mc.;;.P_-::-::t../ SHAFT-.. ------. WORK,
_r'" 44MW
<~' PROcESS PINCH
400
600
800
1000
TEMP.rOc)
1400
Figure 6. L.P. Steam constraints.
Cannot replace F1 by IMPORT
without venting LP steam.
1200
1600
20 fh
L.P. HEADER
Figure 7 illustrates the grand composite curve for theoverall site above the process "pinch" which occurs atabout 80°C in this example. This grand composite curveis shown by the broken line upon which we can overlaythe primary utility for the site, namely flue gas from thecombustion of hydrocarbon fuels. The figure shows theexisting flue gas line which combines the flue gases of thecentral boiler house and ,he ethylene pyrolysis furnace.
The diagram further shows that at high process temperatures flue gas is required for process heating (i.e. inthe pyrolysis furnace), whereas at lower temperaturesVHP steam raising can be employed, with turbine passout steam being used at successively lower temperatures
Figure 7. Existing heat and power profile against grand compositecurve.
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LINNHOFF and EASTWOOD
1600
11.00
1200
moTEI'1PI"C)
800
600
400
200
,,;oJ'"
/,/',
,//
,/,,,
/H ,-
MP ,/LP , .1
/ J-'-..J
.....\
COOLINGWATER
~--r--20.C
o'C
F1
F2
SUB-AMBIENT/ GRAND COMPOSITE
FORETHYLENE PLANT
100 200
HfAT IMWIFigure 10. Interaction of fridge systems.
Figure II. Sub-ambient grand composite curve for ethylene.
2. Temperature levels not properly matched with pro-cess requirement.
By optimising heat recovery we can save about 15% oncompressor shaftwork, producing a 1.5 MW saving intotal shaftwork.
Fl FRIDGELEVELS
F2 FRIDGELEVELS_
~------------~-Fl HEATI REJECTION
II
Fl LOAD F2 LOAD tEXISTING 4-0 MW 7-0 MW 11-0
POSSIBILITY A 3-5 MW 6-5 MW 100
POSSIBILITY B 4-0 MW 6-0 MW 10-0
POSSIBILITY C 3-0 MW 7-0 MW 10-0
.. :..::=.=j=./--==--------------'=--- F2 HEAT~-- -- ----1---w' REJECTION
O·
+20·
_80·
-20·
-100
Figure 9. Improved matching to process.
SUB-AMBIENT CONSIDERAnONSThe sub-ambient region of an ethylene plant is ex
tremely complex involving perhaps six levels of refrigeration in addition to the primary cold utility of coolingwater and/or air cooling. The six levels of refrigerationare achieved by using two separate but interlinkedsystems.
• Fl, propylene refrigeration system(10 to -40°C)
• F2, ethylene refrigeration system( - 60 to - 100°C)
Each system may extract heat from the process atthree levels and, in addition, there will be a temperaturelevel at which the propylene system extracts waste heatfrom the ethylene system. These interactions are illustrated in Figure lOin which the refrigeration systems areplaced against the sub-ambient grand composite curve ofthe ethylene process. The "pinch" at -40°C is, technically, a cooling water pinch since it is formed aftermaximising the use of cooling water in the pinch analysis(cooling water being the least expensive cold utility). Weshall see that it is the interaction of the two refrigerationsystems which allows increased flexibility in design.
Figure 11 shows the existing refrigeration levels on thegrand composite curve (chilling duties represented bysolid lines, waste heat rejection by broken lines). Theexisting compressor duties are 4 MW and 7 MW forpropylene and ethylene respectively. Again we see suboptimality caused by:
I. Larger than necessary chilling duties shown by theovershoot of each refrigeration level on the grandcomposite. These can be reduced by improved processheat (or cold) recovery.
Chern Eng Res Des, Vol. 65, September 1987
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OVERALL SITE OPTIMISATION BY PINCH TECHNOLOGY
~o
-IMW+1] MW
Difference fromexisting design
Figure 5 Figure 12-3 MW 0-4 MW -12 MW
o+7MW
The raw gas compressor turbine arrangement is unchanged since we are already at minimum condensingflow rate but the remaining 35 tlh of condensing steamfrom the propylene refrigeration system are eliminated.VHP steam raising is reduced by a further 35 tlh fromthe level in Figure 5 (85 tlh reduction on the existingsteam raising) but import electricity is increased by 4MW over our earlier design to cater for the new motorand the slight reduction in turbo alternator output (anoverall increase in imported electricity of 11 MW overthe existing operation). Total project savings are therefore £330 per hour or £2.6 million annually. With anestimated capital expenditure of £3.3 million, projectpayback is in the order of 15 months. This compareswith the apparent maximum savings identified earlier of£1.44 million at an 18 month payback (Figure 13).
In terms of power generation, compare the twoschemes in Figure 5 and Figure 12 to the existing design:
• Back pressure generation• Condensing generation• Saving in refrigeration
systems• Import
2~0 t Ih TOTAL
(UNC HANGED I
IMPORTI IPOWERV ·~MWFUEL
LP ~--.....:....----------lL...--.L.-..L...t
,"'...." ~'"
HP I-----L......:....---,,-------L---,r---..L...j
MP I--- --'-_~I
80
Figure 12. Improved steam system.
On the other hand, increased product yield is oftenmore important to us than energy saving and a trade-offis often possible between the two. In this example, wecan increase yield by 2%, the energy penalty in refrigeration being a saving of only 10% rather than the 15%calculated above. If this 10% saving (1.0 MW) is distributed uniformly between the two refrigeration systems,we arrive at Possibility A in Figure II with the shaftworkof each compressor reduced by 10%.
By also induding the FI/F2 interaction in our analysiswe can obtain the same overall saving of 1 MW butarrange the total saving to be on either the ethylenemachine alone (Possibility B) or on the propylene machine alone (Possibility C).
Alternatively, we have increased the back-pressurepower generation per ton of VHP steam at the boilerhouse. Total back-pressure power generation is now 24MW from 125 tlh at the boiler house compared with 24MW from 210 tlh in the existing design and 21 MW from160 tlh in the earlier project which involved in-plantoptimisation only.
Also, we must not overlook the fact that we haveimproved product yield by 2%, a feature which we havenot included in our economic analysis but which is likelyto be of considerable value.
5SAVINGS$10' p.•.
4
Figure 13. Comparative economics.
PROJECT B OFFERS LARGER SAVINGS
AT A SHORTER PAYBACK!
FINAL DESIGNFigure 12 shows our improved design (Possibility C in
Figure 11) following the overall site analysis and thesub-ambient consideration described above. These factors have allowed us to introduce two elements offlexibility which avoid the earlier constraints in ourretrofit projects.
The increased LP steam demand has increased thepotential for power generation (per ton of HP steam)and allowed us to eliminate the condensing turbine onthe propylene compressor without penalty. Theflexibility within the two refrigeration systems has further allowed us to minimise the capital cost of the newelectric motor introduced on the propylene compressor.
,./' '"//L"'/
/ '"'" ///
''''// 18 MONTH"';'" AVERAGE,v PAYBACK
PROJECT A BYINOIVIDUAL PLANTOPTIMISATION
5INVESTMENT$ 10'
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LINNHOFF and EASTWOOD
ADDRESSCorrespondence on this paper should be addressed to Professor B.Linnhoff, Department of Chemical Engineering, UMIST, PO Box 88,Manchester M60 IQD.
REFERENCESI. Linnhoff B. and Vredeveld D. R., 1984, Chern Eng Prog, July.2. Linnhoff B. and Witherell W. D., 1986, Oil & Gas J, April 7.3. Linnhoff B. et al. 1982, User Guide on Process Integration for the
Efficient Use of Energy. (I, Chern E)
work availability. Also, process steam savings may addunwelcome constraints to central steam system improvement.
To obtain best overall site strategies we recommendthe use of grand composite curves of the whole site inorder to determine the optimum levels of utility supply.This applies to both hot and cold utilities. Multi-levelsystems (such as ethylene refrigeration) often introducean element of flexibility which we can exploit to goodeffect.
Figure 14 is a logic diagram of the design philosophy.Such a systematic approach is vital to optimum
retrofit design. There is no substitute known to us for thegrand composite curve. It combines all the elements ofprocess heating and cooling, the process/utility interfaceconstruction and allows us to assess the opportunitiesfor co-generation within the overall factory. At the sametime we can systematically address the trade-offs inherent in the system between capital and energy, betweenyield and energy, and between flexibility and energy. Noother approach allows us to investigate all these tradeoffs both separately and in combination. Table 1 contains just a few of the many examples of the total siteintegration studies that have been carried out over thepast few years and are known to us. The table demonstrates the scope revealed by such studies.
Table I. Experience of total sites
Project payback
12-24 months2-3 years
9-16 months15 months
2-2! yearsup to 3 years
15 months2 years
2 years
Cost Savings
Petrochemicals 30% of imported fuelImproved co-generation30% of total energy30% of total energy20-40% of total energy25% + debottlenecking15% total energy50% increase in powergeneration
Foodstuffs 25% of total energy
Industry
(Courtesy: Linnhoff March)
InorganicsChemicalsPharmaceuticalsResinsPigmentsSteelworks
Improve profitability (reduced utilityconsumption, increased yield, flexibility)on each process by any legitimate means.
IDetermine optimum levels of site Jservices from grand composite.
IExploit interaction between differentsystems (e. g. propylene and ethylenerefrigeration I.
IModel cogeneration and site servicesystems into process consumers (notbased on design detail but based ontarget curves l.
IDesign of projects (treating process andservice systems simultaneously l.
A COMMENT ABOUT EXPANSION PROJECTSThis case study relates to the retrofit design of an
existing chemical plant complex. In this example, thetotal site approach has reduced boiler house steamraising from 210 to 125 t/h whereas the plant-by-plantapproach only reduced steam raising from 210 t/h to 160t/h. This could have a tremendous impact on factoryexpansion plans in a case where the existing boilers werealready at full capacity and further increase in steamdemand would necessitate the purchase of additionalboiler plant.
The significant improvement in energy and yield described earlier could well be accompanied by a substantial capital cost saving if factory expansion plans wouldhave involved the purchase of additional boiler plant.
CONCLUSIONSRetrofit projects of individual process plants aimed at
improved yield, energy savings, expansion, etc. willincrease profits in an overall complex but there could bebetter profit improvement still if interactions are properly understood. For example, the reduced steam demand inherent in an energy saving project can becounter-productive since it often leads to reduced shaft-
Figure 14. The manuscript was received 26 June 1987 and acceptedfor publication.
Chern Eng Res Des, Vol. 65, September 1987