21 Heat Integration of Distillation Columns

21.1 THE HEAT INTEGRATIONCHARACTERISTICS OFDISTILLATION

The dominant heating and cooling duties associated with adistillation column are the reboiler and condenser duties. Ingeneral, however, there will be other duties associated withheating and cooling of feed and product streams. Thesesensible heat duties usually will be small in comparisonwith the latent heat changes in reboilers and condensers.

Both the reboiling and condensing processes normallytake place over a range of temperature. Practical consider-ations, however, usually dictate that the heat to the reboilermust be supplied at a temperature above the dew point ofthe vapor leaving the reboiler and that the heat removed inthe condenser must be removed at a temperature lower thanthe bubble point of the liquid. Hence, in preliminary designat least, both reboiling and condensing can be assumed totake place at constant temperatures.

21.2 THE APPROPRIATE PLACEMENTOF DISTILLATION

Consider now the consequences of placing simple distil-lation columns (i.e. one feed, two products, one reboilerand one condenser) in different locations relative to theheat recovery pinch. The separator takes heat QREB intothe reboiler at temperature TREB and rejects heat QCOND ata lower temperature TCOND . There are two possible waysin which the column can be heat integrated with the rest ofthe process. The reboiler and condenser can be integratedeither across, or not across, the heat recovery pinch.

1. Distillation across the pinch. This arrangement is shownin Figure 21.1a. The background process (which does notinclude the reboiler and condenser) is represented simplyas a heat sink and heat source divided by the pinch. HeatQREB is taken into the reboiler above pinch temperatureand heat QCOND rejected from the condenser below pinchtemperature. Because the process sink above the pinchrequires at least QHmin to satisfy its enthalpy balance, theQREB removed by the reboiler must be compensated for byintroducing an extra QREB from hot utility. Below the pinch,the process needs to reject QCmin anyway, and an extra heatload QCOND from the condenser has been introduced.

Chemical Process Design and Integration R. Smith 2005 John Wiley & Sons, Ltd ISBNs: 0-471-48680-9 (HB); 0-471-48681-7 (PB)

By heat integrating the distillation column with theprocess and by considering only the reboiler, it might beconcluded that energy has been saved. The reboiler hasits heat requirements provided by heat recovery. However,the overall situation is that heat is being transferred acrossthe heat recovery pinch through the distillation column andthat the consumption of hot and cold utility in the processmust increase correspondingly. There are fundamentally nosavings available from the integration of a separator acrossthe pinch1,2.

2. Distillation not across the pinch. Here the situationis somewhat different. Figure 21.1b shows a distillationcolumn entirely above the pinch. The distillation col-umn takes heat QREB from the process and returnsQCOND at a temperature above the pinch. The hot util-ity consumption changes by (QREB QCOND ). The cold

Figure 21.1 The Appropriate placement of distillation columns.(From Smith R and Linnhoff B, 1988. Trans IChemE ChERD,66: 195, reproduced by permission of the Institution of Chemi-cal Engineers.)

446 Heat Integration of Distillation Columns

utility consumption is unchanged. Usually, QREB andQCOND have a similar magnitude. If QREB QCOND ,then the hot utility consumption is QHmin , and thereis no additional hot utility required to run the col-umn. It takes a free ride from the process. Heat inte-gration below the pinch is illustrated in Figure 21.1c.Now the hot utility is unchanged, but the cold util-ity consumption changes by (QCOND QREB ). Again,given that QREB and QCOND usually have similar mag-nitudes, the result is similar to heat integration abovethe pinch.

All these arguments can be summarized by a simplestatement: the appropriate placement for separators is notacross the pinch1,2. Although the principle was originallystated with regard to distillation columns, it clearly appliesto any separator that takes in heat at higher temperature andrejects heat at lower temperature.

If both the reboiler and condenser are integrated withthe process, this can make the column difcult to start upand control. However, when the integration is consideredmore closely, it becomes clear that both the reboiler andcondenser do not need to be integrated. Above the pinch,the reboiler can be serviced directly from the hot utilitywith the condenser integrated above the pinch. In thiscase, the overall utility consumption will be the sameas that shown in Figure 21.1b. Below the pinch, thecondenser can be serviced directly by cold utility withthe reboiler integrated below the pinch. Now the overallutility consumption will be the same as that shown inFigure 21.1c.

21.3 USE OF THE GRAND COMPOSITECURVE FOR HEAT INTEGRATIONOF DISTILLATION

The appropriate placement principle can only be appliedif the process has the capacity to provide or accept therequired heat duties. A quantitative tool is needed to assessthe source and sink capacities of any given backgroundprocess. For this purpose, the grand composite curve canbe used. Given that the dominant heating and cooling dutiesassociated with the distillation column are the reboiler andcondenser duties, a convenient representation of the columnis therefore a simple box representing the reboiler andcondenser loads2. This box can be matched with thegrand composite representing the remainder of the process.The grand composite curve would include all heating andcooling duties for the process, including those associatedwith separator feed and product heating and cooling, butexcluding reboiler and condenser loads.

Consider now a few examples of the use of thissimple representation. A grand composite curve is shownin Figure 21.2a. The distillation column reboiler andcondenser duties are shown separately and are matchedagainst it. The reboiler and condenser duties are on oppositesides of the heat recovery pinch and the column does not t.In Figure 21.2b, although the reboiler and condenser dutiesare both above the pinch, the heat duties prevent a t. Partof the duties can be accommodated, and if heat integrated,

(a) An inappropriately placed distillation column. (b) A distillation column that can only bepartially integrated.

Condenser

Reboiler

T*

H

Condenser

Reboiler

T*

H

Figure 21.2 Distillation columns that do not t against the grand composite curve. (From Smith R and Linnhoff B, 1988, TransIChemE ChERD, 66: 195, reproduced by permission of the Institution of Chemical Engineers.)

Evolving the Design of Simple Distillation Columns to Improve Heat Integration 447

(a) A column appropriately placedabove the pinch.

(b) A column appropriately placed belowthe pinch.

T*

H

T*

H

Figure 21.3 Distillation columns that t against the grand composite curve. (From Smith R and Linnhoff B, 1988, Trans IChemEChERD, 66: 195, reproduced by permission of the Institution of Chemical Engineers.)

that would be a saving, but less than the full reboiler andcondenser duties.

The distillation columns shown in Figure 21.3 both t.Figure 21.3a shows a case in which the reboiler duty canbe supplied by hot utility. The condenser duty must beintegrated with the rest of the process. Another example isshown in Figure 21.3b. This distillation column also ts.The reboiler duty must be supplied by integration with theprocess. Part of the condenser duty in Figure 21.3b mustalso be integrated, while the remainder of the condenserduty can be rejected to cold utility.

21.4 EVOLVING THE DESIGN OFSIMPLE DISTILLATION COLUMNSTO IMPROVE HEAT INTEGRATION

If an inappropriately placed distillation column is shiftedabove the heat recovery pinch by changing its pressure, thecondensing stream, which is a hot stream, is shifted frombelow to above the pinch. The reboiling stream, which isa cold stream, stays above the pinch. If the inappropriatelyplaced distillation column is shifted below the pinch, thenthe reboiling stream, which is a cold stream, is shifted fromabove to below the pinch. The condensing stream stays belowthe pinch. Thus appropriate placement is a particular case ofshifting streams across the pinch, which in turn is a particularcase of the plusminus principle (see Chapter 19).

If a distillation column is inappropriately placed acrossthe pinch, it may be possible to change its pressure to

achieve appropriate placement. Of course, as the pressureis changed, the shape of the box also changes, sincenot only do the reboiler and condenser temperatureschange but also the difference between them. The relativevolatility will also be affected, generally decreasing withincreasing pressure. Thus, both the height and the widthof the box will change as the pressure changes. Changesin pressure also affect the heating and cooling dutiesfor column feed and products. These streams normallywould be included in the background process. Hence, theshape of the grand composite curve will also change tosome extent as the column pressure changes. However,as pointed out previously, it is likely that these effectswill not be signicant in most processes, since thesensible heat loads involved will usually be small bycomparison with the latent heat changes in condensersand reboilers.

If the distillation column will not t either above or belowthe pinch, then other design options can be considered.One possibility is double-effect distillation as shown inFigure 21.4a3. The column feed is split and fed to twoseparate parallel columns. The classical application ofdouble-effect distillation is to choose the relative pressuresof the columns such that the heat from the condenserof the high-pressure column can be used to provide thereboiler heat to the low-pressure column. In isolation,the scheme would save energy, approximately halving theenergy consumption by using the same energy twice indifferent temperature ranges. The energy reduction will

448 Heat Integration of Distillation Columns

T*

H

P

P

C2

C1

C1

C2

Figure 21.4 Double-effect distillation. (From Smith R and Linnhoff B, 1988, Trans IChemE ChERD, 66: 195, reproduced by permissionof the Institution of Chemical Engineers.)

be at the expense of increased capital cost. However,used on a stand-alone basis in this way, in reducing theheat load on the system, the temperature difference overthe distillation system increases. If an attempt is madeto heat integrate this double-effect distillation with therest of the process, the increased temperature differenceacross the system might create problems. The increasedtemperature difference might prevent integration above thepinch (perhaps because the required high temperature inthe reboiler creates fouling) or below the pinch (perhapsbecause the required low temperature in the condenserwould require expensive refrigeration).

However, there is no fundamental reason why these twocolumns must be linked together thermally. Figure 21.4bshows two columns that are not linked thermally and, asa result, each can be individually appropriately placed.Obviously, the capital cost of such a scheme will be higherthan that of a single column, but it may be justied byfavorable energy savings.

Another design option that can be considered if a columnwill not t into the grand composite curve is the use ofan intermediate condenser, as illustrated in Figure 21.5.The shape of the box is now altered, because theintermediate condenser changes the heat ow through thecolumn with some of the heat being rejected at a highertemperature in the intermediate condenser. The particulardesign shown in Figure 21.5, the match with the grandcomposite curve would require that at least part of theheat rejected from the intermediate condenser should bepassed to the process. An analogous approach can be usedto evaluate the possibilities for the use of intermediatereboilers. For intermediate reboilers, part of the reboilerheat is supplied at an intermediate point in the column,

Condenser

IntermediateCondenser

B

A

R

C

C

A + B

Reboiler

H

T*

Figure 21.5 Distillation of column with intermediate condenser.The prole can be designed to t the background process.(From Smith R and Linnhoff B, 1988, Trans IChemE ChERD,66: 195, reproduced by permission of the Institution of Chemi-cal Engineers.)

Capital Cost Considerations 449

at a temperature lower than the reboiler temperature.Flower and Jackson4, Kayihan5 and Dhole and Linnhoff6have presented procedures for the location of intermediatereboilers and condensers.

21.5 HEAT PUMPING IN DISTILLATION

Various heat-pumping schemes have been proposed as ameans of saving energy in distillation. Of these schemes,use of the column overhead vapor as the heat pumping uidis usually the most economically attractive. This scheme,known as vapor recompression, is illustrated in Figure 21.6.

For heat pumping to be economic on a stand-alone basis,it must operate across a small temperature difference, whichfor distillation means close boiling mixtures. In addition,the use of the scheme is only going to make sense if thecolumn is constrained to operate either on a stand-alonebasis or at a pressure that would mean it would be across thepinch. Otherwise, heat integration with the process mightbe a much better option. Vapor recompression schemes fordistillation therefore only make sense for the distillation ofclose boiling mixtures in constrained situations3.

21.6 CAPITAL COST CONSIDERATIONS

The design changes suggested so far for distillation columnshave been motivated by the incentive to reduce energycosts by more effective integration between the distillationcolumn and the rest of the process. There are, however,

capital cost implications when the distillation design andthe heat integration scheme are changed. These implicationsfall into two broad categories: changes in distillation capitalcost and changes in heat exchanger network capital cost.Obviously, these capital cost changes should be consideredtogether, along with the energy cost changes, in orderto achieve an optimum trade-off between capital andenergy costs.

1. Distillation capital costs. The classical optimization indistillation, as discussed in Chapter 9, is to trade-off capitalcost of the column against energy cost for the distillation bychanging the reux ratio. In Chapter 9, this was discussedfrom the situation of distillation columns operating onutilities and not integrated with the rest of the process.Experience gained with this traditional optimization has ledto rules of thumb for the selection of reux ratios. Typically,the optimum ratio of actual to minimum reux ratio is usuallyaround 1.1. Practical considerations often prevent a ratio ofless than 1.1 being used, as discussed in Chapter 9.

If the column is inappropriately placed with the process,then an increase in reux ratio causes a correspondingoverall increase in energy, and the trade-off rules apply. If,however, the column is appropriately placed, then the reuxratio can often be increased without changing the overallenergy consumption as shown in Figure 21.7. Increasing theheat ow through the column decreases the requirement fordistillation stages but increases the vapor rate. In designsinitialized by traditional rules of thumb, this would havethe effect of decreasing the capital cost of the column.However, the corresponding decrease in heat ow through

OVERHEADS

BOTTOMS

FEED

Liquid

W

TrimCooling

Reflux

Vapor

Figure 21.6 Heat pumping in distillation. A vapor recompression scheme. (From Smith R. and Linnhoff B, 1988, Trans IChemEChERD, 66: 195, reproduced by permission of the Institution of Chemical Engineers.)

450 Heat Integration of Distillation Columns

Q

Q Q Q

QHminQHmin

Q

REBREB

CONCOND

Less Heat flowavailable in process

Column Capital

HEN CapitalTotal

T* T Cost

H RROPT

Figure 21.7 The capital/capital trade-off for an appropriately integrated distillation column. (From Smith R and Linnhoff B, 1988,Trans IChemE ChERD, 66: 195, reproduced by permission of the Institution of Chemical Engineers.)

the process will have the effect of decreasing temperaturedifferences and increasing the capital cost of the heatexchanger network, as shown in Figure 21.7. Thus, thetrade-off for an appropriately integrated distillation columnbecomes one between the capital cost of the column and thecapital cost of the heat exchanger network3, Figure 21.7.

Consequently, the optimum reux ratio for an appropri-ately integrated distillation column will be problem specicand is likely to be quite different from that of a stand-alonecolumn operated from utilities.

2. Heat exchanger network capital costs. It is easy for thedesigner to become carried away with the elegance ofpacking boxes into space around the grand compositecurve. However, the full implications of integration areonly clear when the corresponding composite curves ofthe process with the distillation column are considered.Temperature differences become smaller throughout theprocess as a result of the integration. This means that thecapital-energy trade-off should be readjusted, and a largerTmin might be required. The optimization of the capital-energy trade-off might undo part of the savings achievedby appropriate integration.

Unfortunately, the overall design problem is even morecomplex in practice. Large temperature differences in theprocess (i.e. space in the grand composite curve) couldequally well be exploited to allow the use of moderatetemperature utilities or the integration of heat engines,heat pumps, and so on, in preference to integration ofdistillation columns. There is thus a three-way trade-offbetween distillation design and integration, utility selectionand the capital-energy trade-off (Tmin optimization).

21.7 HEAT INTEGRATIONCHARACTERISTICS OFDISTILLATION SEQUENCES

The problem of distillation sequencing was discussed inChapter 11, where the distillation columns in the sequencewere operated on a stand-alone basis using utilities forthe reboilers and condensers. Following the approachin Chapter 11, the best few nonintegrated distillationsequences would be found. These sequences would then beheat integrated as discussed above. Figure 21.8 shows howheat integration can be applied within a two-column direct

ABC

ABC

A

B

C

BC

Heat Integration?

(a) Forward heat integration. (b) Backward heat integration.

ABC

ABC

A

B

C

B

C

Heat Integration?

Figure 21.8 Heat integration of a sequence of two simple distillation columns. (From Smith R and Linnhoff B, 1988, Trans IChemE,CHERD, 66: 195, reproduced by permission of the Institution of Chemical Engineers.)

Heat Integration Characteristics of Distillation Sequences 451

distillation sequence for the separation of three products. InFigure 21.8a, the rst distillation column has been increasedin pressure such that the condenser of the rst column canprovide the heat for the reboiler of the second column,sometimes known as forward integration. In Figure 21.8b,the pressure of the second column has been increased suchthat the condenser of the second column can provide theheat for the reboiler of the rst, sometimes known asbackward integration. Both schemes will bring about asignicant reduction in the energy requirement.

This approach to the overall problem breaks down thedesign procedure into two steps of rst determining thebest nonintegrated sequence and then heat integrating. Thisassumes that the two problems of distillation sequencing

NonintegratedSequences

IntegratedSequences

Solutions

Cost

Figure 21.9 For unconstrained sequences of simple columns,the best few nonintegrated sequences tend to turn into the bestfew integrated sequences. (From Smith R and Linnhoff B, 1988,Trans IChemE ChERD, 66:195 reproduced by permission of theInstitution of Chemical Engineers.)

and heat integration can be decoupled. Studies have beencarried out where sequences of simple distillation columnswere rst developed without heat integration and all reboil-ers and condensers serviced by utilities7,8. These sequenceswere then heat integrated. Comparison of the nonintegratedand integrated sequences showed that the congurationsthat achieved the greatest energy saving by integrationwere already amongst those with the lowest energy require-ment prior to integration7,8. The result is illustrated inFigure 21.9. The best few nonintegrated sequences tend toturn out to be the best few integrated sequences in termsof total cost (capital and operating). It must be emphasizedthat this decoupling depends on the absence of signicantconstraints limiting the heat integration potential within thedistillation sequence. If there are signicant constraints, forexample, limitations on the pressure of some of the columnsdue to reboiler fouling that limit the heat integration poten-tial, then the result might not apply.

It is important to emphasize that, when consideringdistillation sequences, the focus should not be exclusivelyon the single sequence with the lowest overall cost. Rather,because there is often little difference in cost between thebest few sequences, and because of the uncertainties inthe calculations and the fact that other factors need tobe considered in a more detailed evaluation, the best fewshould be evaluated in more detail rather than just one.

If the design problem in the absence of signicant con-straints can be decoupled in this way, there must be somemechanism behind this. Take two different sequences forthe separation of a four-component mixture, Figure 21.103.Summing the feed owrates of the key components (seeChapter 9) to each column in the sequence, the totalowrate is the same in both cases, Figure 21.10. However,the ow of nonkey components is different, Figure 21.10.

ABCD

ABCD

A

D

BCD

BC

B

C

m = mA + 2(mB + mC) + mDkeys

m = mB + mC + mDnon-keys

ABCD

ABC

BC

A

B

C

ABCD

D

m = mA + 2(mB + mC) + mDkeys

m = mA + mB + mCnon-keys

Figure 21.10 The mass owrate of key components are the same for all sequences, but the mass owrate of nonkey components varies.(From Smith R and Linnhoff B, 1988, Trans IChemE ChERD, 66:195 reproduced by permission of the Institution of Chemical Engineers.)

452 Heat Integration of Distillation Columns

Solutions

Keys

Nonkeys

m

Figure 21.11 The overall owrate decomposed into key andnonkey components. (From Smith R and Linnhoff B, 1988,Trans IChemE ChERD, 66:195 reproduced by permission of theInstitution of Chemical Engineers.)

In general, the ow of key components is constant andindependent of the sequence while the ow of nonkeysvaries according to the choice of sequence, as illustratedin Figure 21.113.

It thus appears that the owrate of the nonkeys may accountfor the differences between sequences. Essentially, nonkeycomponents have two effects on a separation. They cause8:

1. an unnecessary load on the separation, leading to higherheat loads and vapor rates;

2. a widening of the temperature differences across columnssince, light nonkey components cause a decrease incondenser temperature and heavy nonkey componentscause an increase in the reboiler temperature.

These effects can be expressed quantitatively in temper-ature-heat proles as illustrated in Figure 21.12. A high

LowmNonkeys

HighmNonkeys

Condensers

Reboilers

Temperature

Enthalpy

Figure 21.12 Temperature-heat proles for sequences of simplecolumns with no constraints having a low owrate of nonkeycomponents tend to be favorable. (From Smith R and Linnhoff B,1988, Trans IChemE ChERD, 66:195 reproduced by permissionof the Institution of Chemical Engineers.)

owrate of nonkey components leads simultaneously tohigher loads and more extreme levels, Figure 21.12.

Whether heat integration is restricted to the separationsystem or allowed with the rest of the process, integrationalways benets from colder reboiler streams and hottercondenser streams. This point has been dealt with inmore general terms in Chapter 19. In addition, whencolumn pressures are allowed to vary, columns withsmaller temperature differences are easier to integrate, sincesmaller changes in pressure are required to achieve suitableintegration. However, is there any conict with capital cost?A column sequence that handles a large amount of heatmust have a high capital cost for two reasons:

1. Large heat loads to be transferred result in large reboilersand condensers.

2. Large heat loads will cause high vapor rates and theserequire large column diameters.

It is thus unlikely that a distillation sequence will havea small capital cost and a large operating cost or viceversa. A nonintegrated sequence with a large heat loadhas high vapor rates, large heat transfer areas and largecolumn diameters. Even if the heat requirements of thissequence are substantially reduced by integration, large heattransfer areas and large diameters must still be provided.Any savings in utility costs achieved by heat integrationmust compensate for these high capital costs before thissequence can become competitive with a nonintegratedsequence starting with a smaller heat load. Thus, capitalcost considerations reinforce the argument that the best fewnonintegrated sequences with the lowest heat load are thosewith the lowest total cost.

It is interesting to reect on the heuristics for sequencingof distillation columns in Chapter 11. Heuristics 2, 3 and 4from Section 11.1 tend to minimize the owrate of nonkeycomponents. Heuristic 1 relates to special circumstanceswhen there is a particularly difcult separation8.

The mechanism by which nonkey components affecta given separation is more complex in practice thanthe broad arguments presented here. There are complexinterrelationships between the volatility of the key andnonkey components, and so on. Also, it is often the case thatthe distillations system has constraints to prevent certainheat integration opportunities. Such constraints will oftenpresent themselves as constraints over which the pressureof the distillation columns will operate. For example,it is often the case that the maximum pressure of adistillation column is restricted to avoid decompositionof material in the reboiler. This is especially the casewhen reboiling high molar mass material. Distillation ofhigh molar mass material is often constrained to operateunder vacuum conditions. Clearly, if the pressure of thedistillation column is constrained, then this restricts theheat integration opportunities. Another factor that can create

Heat Integration Characteristics of Distillation Sequences 453

problems is that, as the pressure of each column in thesequence is varied, this has implications for the downstreamcolumn in terms of the feed condition for the downstreamcolumn. If two columns are at the same pressure, then asaturated liquid leaving one column will be saturated as itenters the second column. However, if the pressure of therst column becomes higher than the second as a result ofa pressure change, then the saturated liquid from the rstcolumn will become a subcooled feed to the second. If thepressure of the rst column becomes lower than the secondas a result of a pressure change, then the saturated liquidfrom the rst column will become a partially vaporized orsuperheated feed to the second. The feed condition can havea signicant inuence on the column design.

Constraints might be applied for the sake of reducingthe capital costs (e.g. to avoid long pipe runs). Inaddition, constraints might be applied to avoid complexheat integration arrangements for the sake of operabilityand control (e.g. to have heat recovery to a reboiler froma single source of heat, rather than two or three sourcesof heat).

In addition to these issues regarding constraints forsimple columns, there is also the issue of the introductionof complex columns into the sequence. Figure 21.13aillustrates the thermal characteristics of a direct sequence oftwo simple columns. Once the two columns are thermallycoupled, as illustrated in Figure 21.13b, the overall heatload is reduced. However, all of the heat must be suppliedat the highest temperature for the system. Thus there isa trade-off in which the load is reduced, but the levelsrequired to supply the heat become more extreme. Thecorresponding case for the indirect sequence is shown inFigure 21.14. As the indirect sequence is thermally coupled,the heat load is reduced, but now all of the heat mustbe rejected at the lowest temperature. Thus, there is abenet of reduced load but a disadvantage of heat rejectionat more extreme levels. The same problem occurs with

1

2

12

Enthalpy(b) Thermally complied direct sequence.

Tem

pera

ture

1

2

12

Enthalpy(a) The direct sequence.

Tem

pera

ture

Figure 21.13 Thermally coupling the direct sequence changesboth the loads and levels.

1

2

ReboilerReboiler

CondenserCondenser

12

Tem

pera

ture

(a) The indirect sequence.Enthalpy

1

2

12

Tem

pera

ture

(b) Thermally coupled indirect sequence.Enthalpy

Figure 21.14 Thermally coupling the indirect sequence changesboth the loads and levels.

thermally coupled prefractionators. However, this time bothheat supply and rejection must be carried out at the mostextreme temperatures.

All of these arguments demand a more sophisticatedapproach to the heat integration of distillation sequences,such that the sequence and heat integration (includingcomplex columns) are explored simultaneously.

Example 21.1 Two distillation columns have been sequencedto be in the direct sequence (see Figure 21.8). Opportunities forheat integration between the two columns are to be explored.The operating pressures of the two columns need to be chosen toallow heat recovery. Data for Column 1 and Column 2 at variouspressures are given in Tables 21.1 and 21.2.

Medium-pressure (MP) steam is available for reboiler heating at200C. Cooling water is available for condensation, to be returned

Table 21.1 Data for Column 1.

P

(bar)TCOND(C)

TREB(C)

QCOND(kW)

QREB(kW)

1 90 120 3000 30002 130 160 3600 36003 140 170 4000 40004 160 190 4300 4300

Table 21.2 Data for Column 2.

P

(bar)TCOND(C)

TREB(C)

QCOND(kW)

QREB(kW)

1 110 130 5500 55002 130 153 6000 60003 150 175 6300 63004 163 190 6500 65005 170 200 6600 6600

454 Heat Integration of Distillation Columns

to the cooling tower at 30C. Assume a minimum permissibletemperature difference for heat transfer of 10C. Determine theminimum utility requirements for:

a. both columns operating at 1 bar,b. forward heat integration,c. backward heat integration.

Solution

a. Heat integration between condenser and reboiler is feasible if

TCOND TREB + TminFrom Tables 21.1 and 21.2, when both columns operate at 1bar, no heat integration is possible. Thus,

QHmin = 3000 + 5500= 8500 kW

QCmin = 3000 + 5500= 8500 kW

b. Consider forward heat integration by increasing the pressure ofColumn 1. Because heat duties will increase with increasingpressure, low operating pressures are preferred. Thus, Column2 is kept at 1 bar with a reboiler temperature of 130C. Thismeans that the minimum condensing temperature of Column1 must be 140C, which corresponds with a pressure of 3 bar.From Tables 21.1 and 21.2:

QHmin = 4000 + (5500 4000)= 5500 kW

QCmin = 0 + 5500= 5500 kW

c. For backward heat integration, the appropriate operatingpressures are 1 bar for Column 1 and 2 bar for Column 2.Thus, from Tables 21.1 and 21.2:

QHmin = 0 + 6000= 6000 kW

QCmin = 3000 + (6000 3000)= 6000 kW

Thus, to minimize utility costs, forward heat integration shouldbe used with Column 1 at 3 bar and Column 2 at 1 bar.

21.8 HEAT-INTEGRATED DISTILLATIONSEQUENCES BASED ON THEOPTIMIZATION OF ASUPERSTRUCTURE

In order to explore distillation sequencing and heatintegration simultaneously, a more automated approach to

the problem is required. First, the variation in reboilerduty and temperature and condenser duty and temperaturewith pressure needs to be modeled. If the basic separationtask is xed, then a model can be developed for theseparation task and its pressure varied. This will provide therelationship between the duties, temperatures and pressures.The variation of the heat duties and temperatures withpressure will be nonlinear. However, the nonlinear behaviorcan be approximated using piecewise linearization. Suchtechniques were discussed in Chapter 3 for modelingnonlinear behavior and turning a nonlinear model intoa linear model for use in optimization. This establishesquantitative information on the variation of heat duties andtemperatures with pressure, as required for exploring heatintegration. All the separation tasks for the sequence needto be modeled in this way. In Chapter 11, an approach wasdeveloped for the sequencing of nonintegrated distillationcolumns based on the optimization of a superstructureof the distillation tasks9. In principle, this could beextended to include also the heat integration opportunities.Binary variables can be introduced into the optimizationto represent the existence of possible heat integrationmatches. Each possible match between a condenser and areboiler, a condenser and a cold steam and a reboiler anda hot stream would require a binary variable to representits existence. Binary variables would also be neededto represent the relative temperatures of condensers andreboilers (i.e. whether heat transfer would be feasible). Thisintroduces a signicant number of new binary variablesinto the optimization and makes it very difcult to solvefor large problems. Thus, a different representation for thesuperstructure for the heat-integrated columns is required10.

The minimum number of simple columns required toseparate NC end products is (NC 1). An equipment-based superstructure can be developed, rather than a task-based superstructure. As an example, Figure 21.15a showssequences of three columns for the separation of a four-product mixture. The tasks that can be carried out bycolumns can be grouped according to the key components.

A/B separation tasks A/BCD, A/BC and A/B B/C separation tasks AB/CD, B/CD, AB/C and B/C C/D separation tasks ABC/D, BC/D and C/D

An equipment-based superstructure can be developedcontaining the minimum number of columns. This isillustrated in Figure 21.15b where three Columns X, Y andZ can carry out one of a group of tasks. The connectionsbetween the columns depend on the task selection. Thesynthesis problem becomes one of determining which taskis to be carried out in which column. The advantageof the equipment-based superstructure for heat integrationconsiderations is that heat-integrated options are related tothe number of columns and not to the number of separationtasks. Thus, in Figure 21.15b, only the condenser/reboiler

Heat Integration of Distillation Columns Summary 455

X Y Z

ABC/D BC/D C/D

AB/CD AB/C B/CD B/C

A/BCD A/BC A/B

(b) Different tasks can be carried out in the same column.

(a) Sequences of columns for a four product separation.

A

B

C

D

ABCD

ABC

AB

A B

C

D

ABCD

BC

BCD

ABCD

B

C

A

D

BCD C

D

Figure 21.15 An equipment-based superstructure.

connections between Columns X, Y and Z need to beconsidered for heat integration, rather than each separationtask. Binary variables can be used to represent the matchesbetween condensers and reboilers, relative temperatures,and so on. Using an equipment-based superstructuredramatically reduces the number of variables. Binaryvariables are associated with assigning a Task i to Column jand also to indicate whether a given heat integration matchis active11.

If nonlinear capital costs considerations are left out ofthe formulation, then the problem can be formulated asan MILP problem (see Chapter 3). Once nonlinear costsare included, the problem becomes an MINLP, with allof the problems associated with nonlinear optimizations.As with many such problems, if the required formulationturns out to be an MINLP, then an MILP can be used toprovide a good initialization for the MINLP, simplifyingits solution.

21.9 HEAT INTEGRATION OFDISTILLATIONCOLUMNS SUMMARY

The appropriate placement of distillation columns whenheat integrated is not across the heat recovery pinch. Thegrand composite curve can be used as a quantitative tool toassess integration opportunities. The scope for integratingconventional distillation columns into an overall processis often limited. Practical constraints often prevent inte-gration of columns with the rest of the process. If the

column cannot be integrated with the rest of the process,or if the potential for integration is limited by the heatows in the background process, then attention must beturned back to the distillation operation itself and complexarrangements considered.

Once the distillation is integrated, the driving forcesbetween the composite curves become smaller. This inturn means that the capital-energy trade-off for the heatexchanger network should be readjusted accordingly.

When heat integrating nonintegrated simple distillationsequences in the absence of signicant constraints, thebest few nonintegrated distillation sequences are usuallyamongst the best few integrated sequences. This resultsfrom excessive owrates of nonkey components increasingreboiler and condenser duties and, at the same time,widening the temperature differences across the sequence asa whole. Larger heat duties and more extreme temperaturestend to reduce the heat integration opportunities. However,this tends to happen only in the absence of signicantconstraints.

The use of complex columns (side-strippers, side-rectiers and thermally coupled prefractionators) reducesthe overall heat duties for the separation at the expenseof more extreme temperatures for reboiling and con-densing. Heat integration benets from smaller duties,but more extreme temperatures make the heat integrationmore difcult).

Thus, the introduction of constraints and complexcolumns demands a simultaneous solution of the sequencingand heat recovery problems. This can be carried out on thebasis of the optimization of a superstructure.

456 Heat Integration of Distillation Columns

21.10 EXERCISES1. Table 21.3 represents a problem table cascade (Tmin = 20C).

Table 21.3 Heat ow cascade forExercise 1.

Interval temperature(C)

Heat ow(kW)

160 1000150 0130 1100110 1400100 900

80 130040 140010 1800

10 190030 2200

The utilities available are given in Table 21.4. Both lowpressure (LP) and medium pressure (MP) steam are available.

Table 21.4 Utility data for Exercise 1.

Utility Cost

Cooling water (20 to 40C) Cost = 0.3 [$kw1y1]Refrigeration at 0C

and 40CCost of electricity =

7.5 [$kW1y1]LP steam at 107C raised from

boiler feed water at 60CCredit for LP

steam = 1.8 [$kW1y1]MP steam at 200C Cost = 2.25 [$kW1y1]

a. By drawing the process grand composite curve, set refrig-eration and other utility loads below the pinch around thefull process steam raising capacity. Calculate the duties ofall these. Data for boiler feed water are:

Specic heat capacity = 4.2 (kJkg1K1)Latent heat of vaporization = 2238 (kJkg1)

b. The power required by the refrigeration levels is given by

W = QC0.6

(TH TC

TC

)

where W = power required for the refrigeration cycleQC = the cooling dutyTC = temperature at which heat is taken into the

refrigeration cycle (K)TH = temperature at which heat is rejected from

the refrigeration cycle (K)Assume heat rejection from refrigeration is to cooling water.Calculate the total energy cost of the utilities.

c. A distillation column presently using MP steam and coolingwater is to be integrated into this process. If the reboiler

temperature is 120C, the condenser is 90C and each hasa duty of 1100 kW, redraw the process grand compositeincluding the integrated distillation column. Show how theabove utilities would now be best placed below the pinch.Determine whether the integration of the column has led toan overall energy cost saving.

2. A problem table analysis for a given process produces the heatow cascade in Table 21.5 for Tmin = 10C.

Table 21.5 Problem table cascade forExercise 2.

Interval temperature(C)

Cascade heatow (MW)

295 18.3285 19.8185 4.8145 085 10.845 12.035 14.3

a. A distillation column, which separates a mixture of tolueneand diphenyl into relatively pure products, is to be integratedwith the process. The operating pressure of the columnhas been xed initially to atmospheric (1.013 bara). Atthis pressure, the toluene condenses overhead at a constanttemperature of 111C and diphenyl is reboiled at a constanttemperature of 255C. What would be the consequence ofintegrating the distillation column with the process at apressure of 1.013 bara?

b. Can you suggest a more appropriate operating pressure forthe distillation column if it is to be integrated with theprocess? The reboiler and condenser loads are both 4.0MW and can be assumed not to change signicantly withpressure. The vapor pressures of toluene and diphenyl canbe represented by:

ln Pi = Ai BiT + Ci

where Pi is the vapor pressure (bar), T is the absolutetemperature (K) and Ai, Bi and Ci are constants, which aregiven in Table 21.6.

Table 21.6 Vapor pressure constants.

Component Ai Bi Ci

Toluene 9.3935 3096.52 53.67Diphenyl 10.0630 4602.23 70.42

Vacuum operation should be avoided and the reboilertemperature kept as low as possible to minimize fouling.

c. Sketch the shape of the grand composite curve after thedistillation column has been integrated.

References 457

3. In Example 21.1, consider introducing a double-effect columnto replace Column 2 for the backward heat integrationarrangement. Assume an equal split of owrate into the double-effect column. Is there any benet from the arrangement?

4. A direct sequence of two distillation columns produces threeproducts A,B and C. The feed condition and operating pres-sures are to be chosen to maximize heat recovery opportunities.To simplify the calculations, assume that condenser duties donot change when changing from saturated liquid to saturatedvapor feed. This will not be true in practice, but simpliesthe exercise. Assume also that the reboiler duty for saturatedliquid feed is the sum of the reboiler duty for saturated vaporfeed plus the heat duty to vaporize the feed. Data for the twocolumns are given in Tables 21.7 and 21.8.

Table 21.7 Exercise 4 data for Column 1.

P

(bar)TCOND(C)

TREB(C)

Saturatedliquid feed

Saturatedvapor feed

QCOND(kW)

QREB(kW)

TFEED(C)

QFEED(kW)

1 90 130 3000 3000 110 20002 110 152 4000 4000 130 18003 130 173 5000 5000 150 16004 150 195 6000 6000 170 1500

Table 21.8 Exercise 4 data for Column 2.

P

(bar)TCOND(C)

TREB(C)

Saturatedliquid feed

Saturatedvapor feed

QCOND(kW)

QREB(kW)

TFEED(C)

QFEED(kW)

1 120 140 3000 3000 130 15002 140 165 4000 4000 150 13002.5 150 178 4500 4500 160 1200

Cooling water is available with a return temperature of 30Cand a cost of $4.5 kW1y1. Low-pressure steam is availableat a temperature of 140C and a cost of $90 kW1y1.Medium-pressure steam is available at a temperature of 200Cand a cost of $135 kW1y1.The minimum temperaturedifference allowed is 10C.a. List the possible heat integration opportunities (include

steam generation opportunities and feed heating opportu-nities).

b. Calculate the minimum cost for backward heat integrationby optimizing the column pressures.

c. Calculate the minimum cost for backward heat integrationby optimizing the column pressures, but disallow heatrecovery between condensers and reboilers. Keep both feedsto be saturated liquids.

d. If the feeds to both columns are saturated vapor, calculatethe minimum utility cost, but disallow heat recoverybetween condensers and reboilers. Use the pressures fromPart b above.

e. Repeat Part b, but keeping the column pressures to 1 bar.

5. Consider the use of a side-rectier or side-stripper for theseparation of a three-product mixture. Assume that thermallycoupled columns operate at the same pressure. Also, assumethe feed to be saturated liquid. Data for the operation of thetwo arrangements are given in Tables 21.9 and 21.10.

Table 21.9 Side-rectier data for Exercise 5.

P

(bar)TCOND1

(C)TREB1(C)

QCOND1(kW)

QREB1(kW)

TCOND2(C)

QCOND2(kW)

1 90 140 2000 4500 120 25002 110 165 2500 5500 140 30002.5 120 178 3000 6500 150 3500

Table 21.10 Side-stripper data for Exercise 5.

P

(bar)TCOND1

(C)TREB1(C)

QCOND1(kW)

QREB1(kW)

TREB2(C)

QREB2(kW)

1 90 140 5000 3000 120 25002 110 165 6000 3500 140 25002.5 120 178 7000 4000 150 3000

Using the utility data from Exercise 4:a. When both columns operate at 1 bar, which of the two

complex column arrangements will have lower utilitycosts?

b. Compare the results with the direct sequence of heat-integrated simple columns operating at 1 bar.

c. Optimize column pressure in both complex column arrange-ments to minimize utility costs.

REFERENCES

1. Umeda T, Niida K and Shiroko K (1979) A ThermodynamicApproach to Heat Integration in Distillation Systems, AIChEJ, 25: 423.

2. Linnhoff B, Dunford H and Smith R (1983) Heat Integrationof Distillation Columns into Overall Processes, Chem EngSci, 38: 1175.

3. Smith R and Linnhoff B (1988) The Design of Separators inthe Context of Overall Processes, Trans IChemE ChERD, 66:195.

4. Flower JR and Jackson MA (1964) Energy Requirements inthe Separation of Mixture by Distillation, Trans IChemE, 42:T249.

5. Kayihan F (1980) Optimum Distribution of Heat Load inDistillation Columns Using Intermediate Condensers andReboilers, AIChE Symp Ser, 192(76): 1.

6. Dhole VR and Linnhoff B (1993) Distillation Column Tar-gets, Comp Chem Eng, 17: 549.

7. Freshwater DC and Ziogou E (1976) Reducing EnergyRequirements in Unit Operations, Chem Eng J, 11: 215.

458 Heat Integration of Distillation Columns

8. Stephanopoulos G, Linnhoff B and Sophos A (1982) Synthe-sis of Heat Integrated Distillation Sequences, IChemE SympSer, 74: 111.

9. Shah PB and Kokossis AC (2002) New Synthesis Frameworkfor the Optimization of Complex Distillation Systems, AIChEJ, 48: 527.

10. Smith EMB and Pantelides CC (1995) Design of Reac-tion/Separation Networks Using Detailed Models, CompChem Eng, 19: S83.

11. Samanta A (2001) Modelling and Optimisation for Synthesisof Heat-Integrated Distillation Sequences in the Context ofOverall Processes , PhD Thesis, UMIST, UK.

Chemical Process Design and IntegrationContentsPrefaceAcknowledgementsNomenclatureChapter 1 The Nature of Chemical Process Design and Integration1.1 Chemical Products1.2 Formulation of the Design Problem1.3 Chemical Process Design and Integration1.4 The Hierarchy of Chemical Process Design and Integration1.5 Continuous and Batch Processes1.6 New Design and Retrofit1.7 Approaches to Chemical Process Design and Integration1.8 Process Control1.9 The Nature of Chemical Process Design and Integration SummaryReferences

Chapter 2 Process Economics2.1 The Role of Process Economics2.2 Capital Cost for New Design2.3 Capital Cost for Retrofit2.4 Annualized Capital Cost2.5 Operating Cost2.6 Simple Economic Criteria2.7 Project Cash Flow and Economic Evaluation2.8 Investment Criteria2.9 Process Economics Summary2.10 ExercisesReferences

Chapter 3 Optimization3.1 Objective Functions3.2 Single-variable Optimization3.3 Multivariable Optimization3.4 Constrained Optimization3.5 Linear Programming3.6 Nonlinear Programming3.7 Profile Optimization3.8 Structural Optimization3.9 Solution of Equations using Optimization3.10 The Search for Global Optimality3.11 Summary Optimization3.12 ExercisesReferences

Chapter 4 Thermodynamic Properties and Phase Equilibrium4.1 Equations of State4.2 Phase Equilibrium for Single Components4.3 Fugacity and Phase Equilibrium4.4 VaporLiquid Equilibrium4.5 VaporLiquid Equilibrium Based on Activity Coefficient Models4.6 VaporLiquid Equilibrium Based on Equations of State4.7 Calculation of VaporLiquid Equilibrium4.8 LiquidLiquid Equilibrium4.9 LiquidLiquid Equilibrium Activity Coefficient Models4.10 Calculation of LiquidLiquid Equilibrium4.11 Calculation of Enthalpy4.12 Calculation of Entropy4.13 Phase Equilibrium and Thermodynamic Properties Summary4.14 ExercisesReferences

Chapter 5 Choice of Reactor I Reactor Performance5.1 Reaction Path5.2 Types of Reaction Systems5.3 Reactor Performance5.4 Rate of Reaction5.5 Idealized Reactor Models5.6 Choice of Idealized Reactor Model5.7 Choice of Reactor Performance5.8 Choice of Reactor Performance Summary5.9 ExercisesReferences

Chapter 6 Choice of Reactor II - Reactor Conditions6.1 Reaction Equilibrium6.2 Reactor Temperature6.3 Reactor Pressure6.4 Reactor Phase6.5 Reactor Concentration6.6 Biochemical Reactions6.7 Catalysts6.8 Choice of Reactor Conditions Summary6.9 ExercisesReferences

Chapter 7 Choice of Reactor III Reactor Configuration7.1 Temperature Control7.2 Catalyst Degradation7.3 GasLiquid and LiquidLiquid Reactors7.4 Reactor Configuration7.5 Reactor Configuration for Heterogeneous Solid-Catalyzed Reactions7.6 Reactor Configuration from Optimization of a Superstructure7.7 Choice of Reactor Configuration Summary7.8 ExercisesReferences

Chapter 8 Choice of Separator for Heterogeneous Mixtures8.1 Homogeneous and Heterogeneous Separation8.2 Settling and Sedimentation8.3 Inertial and Centrifugal Separation8.4 Electrostatic Precipitation8.5 Filtration8.6 Scrubbing8.7 Flotation8.8 Drying8.9 Separation of Heterogeneous Mixtures Summary8.10 ExercisesReferences

Chapter 9 Choice of Separator for Homogeneous Fluid Mixtures I Distillation9.1 Single-Stage Separation9.2 Distillation9.3 Binary Distillation9.4 Total and Minimum Reflux Conditions for Multicomponent Mixtures9.5 Finite Reflux Conditions for Multicomponent Mixtures9.6 Choice of Operating Conditions9.7 Limitations of Distillation9.8 Separation of Homogeneous Fluid Mixtures by Distillation Summary9.9 ExercisesReferences

Chapter 10 Choice of Separator for Homogeneous Fluid Mixtures II Other Methods10.1 Absorption and Stripping10.2 LiquidLiquid Extraction10.3 Adsorption10.4 Membranes10.5 Crystallization10.6 Evaporation10.7 Separation of Homogeneous Fluid Mixtures by Other Methods Summary10.8 ExercisesReferences

Chapter 11 Distillation Sequencing11.1 Distillation Sequencing Using Simple Columns11.2 Practical Constraints Restricting Options11.3 Choice of Sequence for Simple Nonintegrated Distillation Columns11.4 Distillation Sequencing Using Columns With More Than Two Products11.5 Distillation Sequencing Using Thermal Coupling11.6 Retrofit of Distillation Sequences11.7 Crude Oil Distillation11.8 Distillation Sequencing Using Optimization of a Superstructure11.9 Distillation Sequencing Summary11.10 ExercisesReferences

Chapter 12 Distillation Sequencing for Azeotropic Distillation12.1 Azeotropic Systems12.2 Change in Pressure12.3 Representation of Azeotropic Distillation12.4 Distillation at Total Reflux Conditions12.5 Distillation at Minimum Reflux Conditions12.6 Distillation at Finite Reflux Conditions12.7 Distillation Sequencing Using an Entrainer12.8 Heterogeneous Azeotropic Distillation12.9 Entrainer Selection12.10 Trade-offs in Azeotropic Distillation12.11 Multicomponent Systems12.12 Membrane Separation12.13 Distillation Sequencing for Azeotropic Distillation Summary12.14 ExercisesReferences

Chapter 13 Reaction, Separation and Recycle Systems for Continuous Processes13.1 The Function of Process Recycles13.2 Recycles with Purges13.3 Pumping and Compression13.4 Simulation of Recycles13.5 The Process Yield13.6 Optimization of Reactor Conversion13.7 Optimization of Processes Involving a Purge13.8 Hybrid Reaction and Separation13.9 Feed, Product and Intermediate Storage13.10 Reaction, Separation and Recycle Systems for Continuous Processes Summary13.11 ExercisesReferences

Chapter 14 Reaction, Separation and Recycle Systems for Batch Processes14.1 Batch Processes14.2 Batch Reactors14.3 Batch Separation Processes14.4 Gantt Charts14.5 Production Schedules for Single Products14.6 Production Schedules for Multiple Products14.7 Equipment Cleaning and Material Transfer14.8 Synthesis of Reaction and Separation Systems for Batch Processes14.9 Optimization of Batch Processes14.10 Storage in Batch Processes14.11 Reaction and Separation Systems for Batch Processes Summary14.12 ExercisesReferences

Chapter 15 Heat Exchanger Networks I Heat Transfer Equipment15.1 Overall Heat Transfer Coefficients15.2 Heat Transfer Coefficients and Pressure Drops for Shell-and-Tube Heat Exchangers15.3 Temperature Differences in Shell-and-Tube Heat Exchangers15.4 Allocation of Fluids in Shell-and-Tube Heat Exchangers15.5 Extended Surface Tubes15.6 Retrofit of Heat Exchangers15.7 Condensers15.8 Reboilers and Vaporizers15.9 Other Types of Heat Exchange Equipment15.10 Fired Heaters15.11 Heat Transfer Equipment Summary15.12 ExercisesReferences

Chapter 16 Heat Exchanger Networks II Energy Targets16.1 Composite Curves16.2 The Heat Recovery Pinch16.3 Threshold Problems16.4 The Problem Table Algorithm16.5 Nonglobal Minimum Temperature Differences16.6 Process Constraints16.7 Utility Selection16.8 Furnaces16.9 Cogeneration (Combined Heat and Power Generation)16.10 Integration Of Heat Pumps16.11 Heat Exchanger Network Energy Targets Summary16.12 ExercisesReferences

Chapter 17 Heat Exchanger Networks III Capital and Total Cost Targets17.1 Number of Heat Exchange Units17.2 Heat Exchange Area Targets17.3 Number-of-shells Target17.4 Capital Cost Targets17.5 Total Cost Targets17.6 Heat Exchanger Network and Utilities Capital and Total Costs Summary17.7 ExercisesReferences

Chapter 18 Heat Exchanger Networks IV Network Design18.1 The Pinch Design Method18.2 Design for Threshold Problems18.3 Stream Splitting18.4 Design for Multiple Pinches18.5 Remaining Problem Analysis18.6 Network Optimization18.7 The Superstructure Approach to Heat Exchanger Network Design18.8 Retrofit of Heat Exchanger Networks18.9 Addition of New Heat Transfer Area in Retrofit18.10 Heat Exchanger Network Design Summary18.11 ExercisesReferences

Chapter 19 Heat Exchanger Networks V Stream Data19.1 Process Changes for Heat Integration19.2 The Trade-Offs Between Process Changes, Utility Selection, Energy Cost and Capital Cost19.3 Data Extraction19.4 Heat Exchanger Network Stream Data Summary19.5 ExercisesReferences

Chapter 20 Heat Integration of Reactors20.1 The Heat Integration Characteristics of Reactors20.2 Appropriate Placement of Reactors20.3 Use of the Grand Composite Curve for Heat Integration of Reactors20.4 Evolving Reactor Design to Improve Heat Integration20.5 Heat Integration of Reactors SummaryReference

Chapter 21 Heat Integration of Distillation Columns21.1 The Heat Integration Characteristics of Distillation21.2 The Appropriate Placement of Distillation21.3 Use of the Grand Composite Curve for Heat Integration of Distillation21.4 Evolving the Design of Simple Distillation Columns to Improve Heat Integration21.5 Heat Pumping in Distillation21.6 Capital Cost Considerations21.7 Heat Integration Characteristics of Distillation Sequences21.8 Heat-integrated Distillation Sequences Based on the Optimization of a Superstructure21.9 Heat Integration of Distillation Columns Summary21.10 ExercisesReferences

Chapter 22 Heat Integration of Evaporators and Dryers22.1 The Heat Integration Characteristics of Evaporators22.2 Appropriate Placement of Evaporators22.3 Evolving Evaporator Design to Improve Heat Integration22.4 The Heat Integration Characteristics of Dryers22.5 Evolving Dryer Design to Improve Heat Integration22.6 Heat Integration of Evaporators and Dryers Summary22.7 ExercisesReferences

Chapter 23 Steam Systems and Cogeneration23.1 Boiler Feedwater Treatment23.2 Steam Boilers23.3 Steam Turbines23.4 Gas Turbines23.5 Steam System Configuration23.6 Steam and Power Balances23.7 Site Composite Curves23.8 Cogeneration Targets23.9 Optimization of Steam Levels23.10 Site Power-to-heat Ratio23.11 Optimizing Steam Systems23.12 Steam Costs23.13 Choice of Driver23.14 Steam Systems and Cogeneration Summary23.15 ExercisesReferences

Chapter 24 Cooling and Refrigeration Systems24.1 Cooling Systems24.2 Recirculating Cooling Water Systems24.3 Targeting Minimum Cooling Water Flowrate24.4 Design of Cooling Water Networks24.5 Retrofit of Cooling Water Systems24.6 Refrigeration Cycles24.7 Process Expanders24.8 Choice of Refrigerant for Compression Refrigeration24.9 Targeting Refrigeration Power for Compression Refrigeration24.10 Heat Integration of Compression Refrigeration Processes24.11 Mixed Refrigerants for Compression Refrigeration24.12 Absorption Refrigeration24.13 Indirect Refrigeration24.14 Cooling Water and Refrigeration Systems Summary24.15 ExercisesReferences

Chapter 25 Environmental Design for Atmospheric Emissions25.1 Atmospheric Pollution25.2 Sources of Atmospheric Pollution25.3 Control of Solid Particulate Emissions to Atmosphere25.4 Control of VOC Emissions to Atmosphere25.5 Control of Sulfur Emissions25.6 Control of Oxides of Nitrogen Emissions25.7 Control of Combustion Emissions25.8 Atmospheric Dispersion25.9 Environmental Design for Atmospheric Emissions Summary25.10 ExercisesReferences

Chapter 26 Water System Design26.1 Aqueous Contamination26.2 Primary Treatment Processes26.3 Biological Treatment Processes26.4 Tertiary Treatment Processes26.5 Water Use26.6 Targeting Maximum Water Reuse for Single Contaminants26.7 Design for Maximum Water Reuse for Single Contaminants26.8 Targeting and Design for Maximum Water Reuse Based on Optimization of a Superstructure26.9 Process Changes for Reduced Water Consumption26.10 Targeting Minimum Wastewater Treatment Flowrate for Single Contaminants26.11 Design for Minimum Wastewater Treatment Flowrate for Single Contaminants26.12 Regeneration of Wastewater26.13 Targeting and Design for Effluent Treatment and Regeneration Based on Optimization of a Superstructure26.14 Data Extraction26.15 Water System Design Summary26.16 ExercisesReferences

Chapter 27 Inherent Safety27.1 Fire27.2 Explosion27.3 Toxic Release27.4 Intensification of Hazardous Materials27.5 Attenuation of Hazardous Materials27.6 Quantitative Measures of Inherent Safety27.7 Inherent Safety Summary27.8 ExercisesReferences

Chapter 28 Clean Process Technology28.1 Sources of Waste from Chemical Production28.2 Clean Process Technology for Chemical Reactors28.3 Clean Process Technology for Separation and Recycle Systems28.4 Clean Process Technology for Process Operations28.5 Clean Process Technology for Utility Systems28.6 Trading off Clean Process Technology Options28.7 Life Cycle Analysis28.8 Clean Process Technology Summary28.9 ExercisesReferences

Chapter 29 Overall Strategy for Chemical Process Design and Integration29.1 Objectives29.2 The Hierarchy29.3 The Final Design

Appendix A Annualization of Capital CostAppendix B Gas CompressionB.1 Reciprocating CompressorsB.2 Centrifugal CompressorsB.3 Staged Compression

Appendix C Heat Transfer Coefficients and Pressure Drop in Shell-and-tube Heat ExchangersC.1 Pressure Drop and Heat Transfer Correlations for the Tube-SideC.2 Pressure Drop and Heat Transfer Correlations for the Shell-SideReferences

Appendix D The Maximum Thermal Effectiveness for 12 Shell-and-tube Heat ExchangersAppendix E Expression for the Minimum Number of 12 Shell-and-tube Heat Exchangers for a Given UnitAppendix F Algorithm for the Heat Exchanger Network Area TargetAppendix G Algorithm for the Heat Exchanger Network Number of Shells TargetG.1 Minimum Area Target for Networks of 12 ShellsReferences

Appendix H Algorithm for Heat Exchanger Network Capital Cost TargetsIndex