placement of equipment

8/12/2019 Placement of Equipment

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PlacementofHeatEngine,HeatpumpandReactors Module06 Lecture42

Module06:Integrationandplacementofequipment

Lecture42:PlacementofHeatEngine,HeatpumpandReactors

Keyword:HeatEngine,HeatPump,Reactor

Heat rejected by a heat engine can be used as a hot utility in the process. Similarly, heat

rejectedin

the

condenser

and

heat

removed

from

evaporator

by

aHeat

pump

can

also

be

used

asahotandcoldutilityrespectively.Thus,whileintegratingHeatEnginesandHeatpumpsorin

that matter any system offering utility the principles of pinch outlined in earlier lectures on

placementofutilityapplieshere.Basedonthisprincipletheappropriateplacement(i.e.above,

below or across pinch) can be decided. To arrive at the appropriate placement, symbolic

diagramofthesystem(Heatengine,heatpumpandreactor)andthatoftheprocessisused.

Fig.42.1(a)Showsthecompositecurveofaprocessandpart(c)ofthefigureshowsthesymbolic

representationoftheprocesswhichisdividedintotwodistinctareasaboveandbelowpinchby

pinchtemperature.Theregionabovethepinchregion isaheatsinkandrequiresheatingQhuandtheregionbelowthepinch(heatsource)requirescoolingQcu. Thebasisforselectionwill

bebased

on

the

fact

that

whether,

the

integration

of

any

system

is

increasing

,decreasing

or

keeping the utility demand of the process as it was. As in Fig.42.1 the process has been

convertedtoasymbolicform,inFigs.42.2and42.3thesymbolicformsofheatengineandheat

pumpsareshownrespectively.

(a) (b) (c)

T

H

Hotutilitytemp.

Above pinch regionHeat sink (needs heat)

Below pinch regionHeat source(needs cooling)

Pinch

Temperature

Qhu

Qcu

Coldutilitytemp.

PinchPinch

Sink

Source

Qhu

Qcu

Fig.42.1(a)Compositecurveofprocess,(b)Physicalrepresentationofprocessand(c)Symbolicprocess

Fig.42.2(a)Steamturbineas heatengine,(b)Thermodynamicconceptbehindit(c)Symbolicheatengine

(a) (b)(c)

Q2

Q1 T1

T2

W

Heat

Engine

Heat,Q1

Heat,Q2

(Source)

Temperature,T1

Temperature,T2

(sink)

T1>T2

Work,W

HPSteam

LPSteam

Power


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INTEGRATION OF HEAT ENGINE

The synthesis of heat and power systems involves appropriately combining power producing and

power consuming systems with heat recovery networks so as to minimize costs and maximize

efficiency. Some of important heat engines are steam turbine Rankin cycles, open-cycle gas

turbines, closed-cycle gas turbines, organic Rankin cycles, and Diesels engines.

HEAT ENGINES

A heat engine, shown in Fig.42.2, is a device which accepts heat Q1from a source at temperature

T1, rejects heat Q2 to a sink at a lower temperature T2, and generate work W. From

thermodynamics,

W = Q1 Q2 first law (42.1)

W / Q1c second law (42.2)

And c= 1-T2/ T1 Carnot efficiency (42.3)

Because real heat engines are irreversible, an equation introducing machine efficiency, mech, for

the heat engines may be written as,

W = mechcQ1 0 e < 1. (42.4)

Pinch

Sink

Source

Qhu

Qcu

IfHeatEngine(HE)is notintegrated

withprocess

TotalHeatinput=Qhu+Q1

TotalHeatout=Qcu+Q2

Fig.42.4

Heat

engine

without

integration

with

process

HE

Q2

Q1 T1

T2

W

Condenser,T1

Q1

Evaporator,T2

Q2

W

Compressor

ThrottleValve

Fig.42.3(a)Closedcycleheatpump,(b)Thermodynamicconceptbehindit(c)Symbolicformofheatpump

T1>T2Heat

pump

Heat,Q1

Heat,Q2

(Source)

Temperature,T1

Temperature,T2

(sink)

T1>T2

Work,W

(a) (b) (c)

Q2

Q1 T1

T2

W


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PLACEMENT OF HEAT ENGINE RELATIVE TO PINCH

It is a known fact that, the process acts as a heat sink above the pinch and as a heat source below

the pinch and the heat engine takes heat from a higher temperature level and rejects heat at a

lower temperature level. While doing so it produces work. The appropriate placement of heat

engine is discussed below:

Placement of Heat Engine Above pinch

When the engine exhaust(Q2) is hotter than pinch temperature and above the pinch (Fig. 42.5a),

the engine takes heat Q1and rejects heat (Q2= Q1-W) into the sink. Thus, the integrated system

uses W units of additional heat in excess of the process requirements. However, it produces an

equal amount of work also at the same time. In other words, heat is converted to shaft work at

almost a efficiency of 100 % because of the integration. More over, the heat Q2rejected by the

engine substitutes an equal amount of hot utility and the hot utility requirement of process

reduces to, Qhu-Q2. Thus, the placement is proper.

Placement of Heat Engine across pinch

When the engine exhaust is colder than pinch temperature and the engine is across the pinch (Fig.

42.5b), the engine rejects heat (Q2= Q1-W) into a process source. This heat simply cascades

through the below-pinch region and increases the cold utility requirement for its satisfaction.

Thus, it is improper placement.

Placement of Heat Engine below pinch

Below the pinch (Fig. 42.5c), the engine is placed such that it absorbs heat Q 1from the process

source and thereby reduces the cold utility demand; again, the engine is properly placed and has a

marginal efficiency of 100 % as it converts the excess process heat into work rather than waste

heat.

Fig.42.5 shows that the exhaust heat Q2from an engine integrated above the pinch can at best

replace all the hot utility requirement of the process (Qhu), and the maximum heat Q1absorbed by

an engine integrated below the pinch can equal the cold utility requirement (Qcu).


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HE

Q2

Q1 T1

T2

W

Pinch

Sink

Source

Qhu Q2

Qcu

HeatEngineintegrated

aboveprocesspinch

TotalHeatin =QhuQ2+Q1=Qhu+W

TotalHeatout =Qcu

Fig.42.5a.Heatengineintegratedabovethepinch

HE

Q2

Q1 T1

T2

WPinch

Sink

Source

Qhu

Qcu+Q2


acrossprocesspinch

TotalHeatin=Qhu+Q1

TotalHeatout=Qcu+Q2

Fig.42.5b.Heatengineintegratedacrossthepinch

HE

Q2

Q1 T1

T2

W

Pinch

Sink

Source

Qhu

Qcu Q1


belowprocesspinch

Heatin=Qhu

Heatout=QcuQ1+Q2=Qcu W

Fig.42.5c.Heatengineintegratedbelowthepinch


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The conclusion of heat engine integration with process is given below:

Total Hot Utility Total Cold utility Recommendations

Without Integration Qhu+Q1 Qcu+Q2

Above process pinch QhuQ2+Q1=Qhu+W Qcu PROPER PLACEMENT

Across process pinch Qhu+Q1

Qcu

+Q2

IMPROPER PLACEMENT

Below process pinch Qhu QcuQ1+Q2=Qcu W PROPER PLACEMENT

HEAT PUMPS

A heat pump, shown in Fig. 42.3, is a heat engine operating in reverse. It accepts heat Q2 at

temperature T2, rejects heat Q1 at a higher temperature T1, and therefore consumes work W.

Form thermodynamics,

W = Q1 Q2 first law (42.5)

W / Q1c second law (42.6)

And c= 1-T2/ T1 Carnot efficiency (42.7)

Equation for real (irreversible) heat pumps as,

W = cQ1/ mech= Q1/ COP 0 mech < 1. (42.8)

Where COP and mechdenote the coefficient of performance and machine efficiency respectively.

Pinch

Sink

Source

Qhu

Qcu

Heat Pumpandprocess

withoutintegration

Heatavailable=Q1;Heatin=QhuHeatrequired=Q2;Heatout=Qcu

Fig.42.6Heatpump andprocesswithoutintegration

Q2

Q1 T1

T2

WHP


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PLACEMENT OF HEAT PUMP RELATIVE TO PINCH

It is a fact that, the process behaves as a heat sink above the pinch and as a heat source below the

pinch. The heat pump picks up heat from a lower temperature level and rejects heat at a higher

temperature level. For this service work has to be invested. The appropriate placement of heat

pump w.r.t the process is discussed below:

Placement of Heat Pump across the pinch

If heat pump placed across the pinch (Fig. 42.7a), it takes heat Q2from the process source below

the pinch and rejects heat (Q1= Q2+ W) to the process sink above the pinch. Thus, the work

input, W, leads to a reduction of hot utility as well as the cold utility. After integration hot utility

demand drops to Qhu-Q2-W and cold utility demand drops to QcuQ2. As it decreases both the

hot and cold utility demand it is appropriately placed.

Placement of Heat Pump above the pinch

If heat pump is placed above the pinch (Fig. 42.7b), then the hot utility gets reduced by W units;

however, W units of work are also added and so on net energy savings result.

When placed below the pinch (Fig.42.7b), Q2units of heat are circulated in a loop; also, work,

W, is added to the heat source and so cold utilities are increased by W units, leading to improper

placement and wastage.

Q2

Q1 T1

T2

WHP Pinch

Sink

Source

QhuQ1

Qcu Q2

HeatPumpintegrated

acrossprocesspinch

Heatin=Qhu Q1=QhuQ2W

Heatout=Qcu Q2

Fig.42.7a.Heatpumpintegratedacrossthepinch


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The conclusion of heat pump integration with process is given below:

Total Hot Utility Total Cold utility Recommendations

Without Integration Heatavailable withHP =Q1;ProcessHeatin=QhuHeatrequiredforHP=Q2;ProcessHeatout=Qcu

Above process pinch Qhu Q1=Qhu Q2W Qcu- Q2 IMPROPER PLACEMENT

Across process pinch Qhu

W

Qcu

PROPER PLACEMENT

Below process pinch Qhu Qcu+W IMPROPER PLACEMENT

Q2

Q1 T1

T2

WHP

Pinch

Sink

Source

QhuW

Qcu

HeatPumpintegrated

aboveprocesspinch

Heatin=QhuW

Heatout=Qcu

Fig.42.7b.Heatpumpintegratedabovethepinch

Temperature

intervals

Fig.42.7c.Heatengineintegratedbelowthepinch

Pinch

Sink

Source

Qhu

Qcu+W

HeatPumpintegrated

belowprocesspinch

Heatin=Qhu

Heatout=Qcu+W

Q2

Q1 T1

T2

WHP


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HeatIntegrationCharacteristicsofReactors

The reactor is the heart of a process and its operating conditions are fixed during the

development of process flowsheet, based on maximum yield, selectivity, catalyst life and

market driven product quality. Thus, designers and operators are often unwilling to make

majorchanges in thereactionconditions.Becauseof this ,directheat integrationof reactors

with

process

stream

is

hardly

done.

Under

this

backdrop

,

pinch

analysis

may

propose

acceptablerefinementswhichmayallowtheprocesstointegratebetter.

Heatintegrationofreactorsinvolvesthefollowingthreesteps:

Decomposition Modeling Thermodynamicanalysis

Inthedecompositionofreactors,itsenergyfunctions(enthalpychangeofthereaction,transferofthis

enthalpy

from/to

the

fluid,

interchange

of

energy

between

fluids

entering

the

reactor

at

different

temperatures and exchange with the surrounding) are considered separately. Then, the reaction

enthalpy isrepresentedbyareactorsprofileandtheotherenergyfunctionsbyaprocessprofile;then,

thereactormaybeinvestigatedapartfromtherestoftheprocessandHEN.

Modelingofreactors isnecessarywhentherearetwoormore inletstreamsand/orseveralcontacting

patterns.Careful modelingof areactorprovidesabetterrepresentation forstructural and parametric

optimizationwhichisthefocusduringthethermodynamicanalysis.

Thermodynamic analysis of the reactors includes proper placement of reactors relative to the

pinch and matching of reactor and process profiles.

Properplacementofreactordependsontypesofreactors.Generallyreactorsareoftwotypes:

Exothermic and Endothermic

Theheatdutyontheheating/coolingmediumofareactorisgivenbyQREACT= - (HSTREAMS+ HREACT)

Where, QREACT= reactor heating or cooling required.

HSTREAMS= enthalpy change between feed and product streams.

HREACT= reaction enthalpy (negative in the case of exothermic reactions).

INTEGRATIONOFREACTORSWITHBACKGROUNDPROCESS

ExothermicReactor


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IntegrationwithbackgroundprocessabovethePinch

Fig. 42.8 shows a process represented simply as a heat sink and heat source divided by the

pinch.Fig.42.8ashowstheprocesswithanexothermicreactorintegratedabovethepinch.The

minimumhotutilitycanbereducedbytheheatreleasedbyreaction,QREACT. Thusisacceptable

Integrationwith

back

ground

process

below

the

Pinch

Fig.42.8bshowsanexothermicreactorintegratedbelowthepinch.Thehotutilityrequirement

cannot be reduced because the process above the pinch needs at least QHmin to satisfy its

enthalpyimbalance.Moreover,thecoldutilityrequirementoftheprocessisincreasedbyQREACT

units.

Therefore, there is no benefit from integrating an exothermic reactor below the pinch. The

appropriatelyplacementforexothermicreactorsisabovethepinch.

EndothermicReactor


Fig.42.9ashowsanendothermicreactor integratedabovethepinch.Theendothermic

reactorremovesQREACTfromtheprocessabovethepinch,whichneedsatleastQHmintosatisfy

QHmin

Exothermic

reactor

QREACT

Pinch

QCmin+QREACT

T

(b)

Fig.42.8Appropriateplacementofanexothermicreactor

QHmin

QREACT

Exothermic

reactor

QREACT

Pinch

QCmin

T

(a)


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its enthalpy imbalance. Thus an extra QREACT must be imported from the hot utility to

compensate, and thus, there is no benefit by integrating an endothermic reactor above the

pinch.


Fig.

42.9b

shows

an

endothermic

reactor

integrated

below

the

pinch.

The

reactor

importsQREACTfrompartoftheprocessthatneedstorejectheatanddecreasesthecoldutility

amount.ThusintegrationofreactorreducesthecoldutilityconsumptionbyQREACT.

There isnoobviousbenefit from integrating anendothermic reactor above thepinch.

Theappropriateplacementforendothermicreactorsisbelowthepinch.

QHmin

Endothermic

reactor

QREACT

Pinch

QCminQREACT

T

(b)

Fig.42.9Appropriateplacementofanendothermicreactor

QHmin+QREACT

Endothermic

reactor

QREACT

Pinch

QCmin

T

(a)


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References:

1. RobinSmith,ChemicalProcessDesign,McGrawHill,1995.2. B.Lynhoffetal.,ProcessIntegrationfortheEfficientUseofEnergy,Institutionof

ChemicalEngineers,1983.

3. R.BENSTEADandF.W.SHARMAN,(1990),HEATPUMPSANDPINCHTECHNOLOGY,HeatRecoverySystems&CliP,10,4,387398.

4. SAIDASM.RANADE,(1988),NEWINSIGHTSONOPTIMALINTEGRATIONOFHEATPUMPSININDUSTRIALSITES,HeatRecoverySystems&ClIPVol.8,No.3,255263.

5. ChiI Tuan, YiLung Yeh, ChiJen Chen, TingChien Chen,(2011)Performanceassessment with Pinch technology and integrated heat pumps forvaporized

concentration processing, Journal of the Taiwan Institute of Chemical Engineers.

placement of equipment

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