use process integration to set material reuse and

JURUTERA, April 20068

C O V E R S T O RY

IntroductionDue to stringent environmental

regulations and rising costs of rawmaterials, the industry is now makingan effort to shift from the conventionalpractice of treating waste after it hasbeen produced (termed as end-of-pipetreatment) to a more proactiveapproach of minimising waste at theroot cause (termed as waste minimisationor pollution prevention). The merit ofpractising waste minimisation is of twofold. Beside the reduction of wastetreatment cost, the company also savesfrom reduced raw material costs, whichmay include purchase cost andtreatment cost.

Concurrently, the development ofsystematic design techniques for wastereduction, material reuse and recyclingwithin a process plant has seenextensive progress. In particular,process integration has been one of themost promising techniques for materialreuse and recycle over the last decade.Conventionally, process integration ormore specifically, pinch analysis (thegraphical approach of process integra-tion) is known to engineers as an energy

saving tool. However, with the rise oftwo new elements, i.e. mass and propertyintegration, material reuse and recyclecan now be handled efficiently withprocess integration techniques. In thecontext of process integration, reusemeans that the effluent from one unit isused in another unit and does not re-enter the unit where it has beenpreviously used. On the other hand,recycle allows the effluent to re-enter theunit where it has been previously used.This article discusses the author’srecent works on process integrationbased on pinch analysis techniques forthe design of liquid, gas and solidreuse/recycle system.

Mass integration for water andhydrogen reuse/recycle

Mass integration was introducedbased on the analogy of heatintegration [1]. Water minimisation waslater introduced [2] as a special case ofmass integration, aiming for themaximum reuse/recycle of waterwithin the various water sinks (unitsthat require feed water) and watersources (units that produce water).

Much later, another special case of massintegration that has gained muchattention is the hydrogen pinchanalysis [3]. Both special cases will beillustrated in details in the followingsection.

A typical pinch analysis studyconsists of two stages, i.e. settingminimum fresh resource and wastedischarge flowrates (often termed astargeting), followed by network designto achieve the targeted flowrates. It isworth mentioning that the focus onpinch analysis is the targeting step.With targeting, the engineer sets abaseline target to determine how well areuse/recycle system can actuallyperform based on first principle.Knowing the targets ahead willeliminate the query of “will there be abetter design?” during any designexercise. Once the minimum materialtargets are established, a reuse/recyclenetwork can be designed using anynetwork design tools. The compositecurve [4] and cascade analysistechnique [5] are by far the mostpromising techniques in locating theminimum targets in a material

Use Process Integration to Set Material Reuseand Recycling Targets.....................................................................................................................................................................................................................

By: Sdr. Dominic Foo Chwan Yee, University of Nottingham (Malaysia Campus)

Figure 1: Process flow diagram of AN production

(a) without water reuse/recycle (b) with water reuse/recycle

008•010•012•014•cover story 3/16/06 10:57 AM Page 8


C O V E R S T O RY

reuse/recycle network. The use of thesetools will be shown in the followingcase studies.

Water Pinch AnalysisFigure 1(a) shows the process flow

diagram of a Acrylonitrile (AN)production [6]. Products from thereactor is cooled and partiallycondensed. The reactor off-gas is sent toa scrubber that uses fresh water as thescrubbing agent. The bottom productfrom the scrubber is separated into theaqueous layer and an organic layer in adecanter. The organic layer is laterfractionated in a slightly vacuumeddistillation column that is induced by asteam-jet ejector. There are two watersinks in this process, i.e. boiler feedwater (BFW) and feed stream to thescrubber; and four water sources whichinclude the off-gas condensate, aqueouslayer from decanter, distillation columnbottoms and steam-jet ejectorcondensate. Ammonia (NH3) is the mainimpurity in this process. The limitingwater data (maximum impuritycomposition and minimum flowrate) foreach process unit is given in Table 1.

To locate the minimum waterflowrates in this process, the engineercan either plot the composite curve(Figure 2a) or carry out the numericalcalculation using the algebraicapproach of cascade analysis (Figure2b), which can be automated in aspreadsheet e.g. MS Excel. Figure 2shows that the fresh water (FF) andwastewater discharge (FD) flowrates ofthe AN process have been reducedsignificantly to 2.06 kg/s and 8.16 kg/s.

This corresponds to a reduction of 70%and 38% respectively (original flowratesare obtained from the sum of individualflowrates in Table 1). A pinchcomposition is found at the compositionof 14 ppm, which represents the mostconstrained area in the reuse/recyclenetwork where maximum recovery maybe achieved. Due to space limitation,readers are advised to refer to theoriginal sources for the detailedexplanation of these methodologies [4, 5].

In order to achieve the minimumflowrates, the following guidelines canbe used to design the network manuallyor by any linear programmingtechnique [6, 7]:(a) Flowrate for sinks:

(1)

where Fi, j is the flowrate between sourcei and sink j.Material content:

(2)

(b) Flowrate for sources:

(3)

Any flowrate from a source that isnot fed to a sink will leave as a discharge(waste) stream.

The resulting reuse/recycle schemeis shown in Figure 1(b). Many otherprocess changes such as the use ofwater purifying units (e.g. filtration,adsorption, etc.) and the elimination ofwater-consuming units can also beassessed [5, 7].

Hydrogen Pinch AnalysisSince hydrogen is a valuable

resource in the crude oil refinery, itsreuse/recycle will lead to big saving inoperation cost. Figure 3 shows asimplified refinery process that consistsof two hydrogen consumers A and Band a hydrogen plant [8]. Currently,200 million standard cubic feet per day(MMscfd – omitted in Figure 3) aresupplied from the hydrogen plant.

One should note that the hydrogensinks and sources data needed forhydrogen pinch analysis is not directlytaken from the flowsheet. Instead theyare calculated by the followingequations:

Sink: FSK = FM + FR (4)

(5)

Source: FSR = FP + FR (6)ySR = yP = yR (7)

where FSK, FSR, FM, FR and FP are thesink, source, make-up, recycle andpurge flowrates respectively; whileySK, ySR, yM, yR and yP referring to theirhydrogen compositions respectively.

Previously, the analysis of impurityreuse/recycle system has always beencarried out by graphical approaches[3, 4, 9]. However, with the latestadvancement in the algebraictechnique [10], the targeting can nowbe carried out easily, accurately andpromptly using manual calculation orvia a spreadsheet programme. Resultfor the refinery example is shown inthe cascade table in Table 2, which is atabulated representation of cascade

Figure 2: Targeting the minimum fresh water and wastewater flowrates for AN production

(a) with composite curve [4] (b) with cascade analysis technique [5]

Water sinks (SK) Flowrate NH3

composition

j Stream FSK (kg/s) ySK (ppm)

1 BFW 1.2 0

2 Scrubber 5.8 10

Water sources (SR) Flowrate NH3

composition

i Stream FSR (kg/s) ySR (ppm)

1 Distillation bottoms 0.8 0

2 Off-gas condensate 5 14

3 Aqueous layer 5.9 25

4 Ejector condensate 1.4 34

Table 1: Limiting water data for AN production



C O V E R S T O RY

analysis of Figure 2(b). Following thedesign guideline in Equations 1 to 3,one of the possible reuse/recyclenetworks is shown in Figure 3(b).

Table 2 shows the fresh hydrogen feed(FF) has been reduced from 200 to 183MMscfd, which represent a significantlysaving of 8.5%. Note that, for a completerefinery process which typically has morethan two hydrogen-consuming units,greater saving is encountered [8, 9]. Notealso that 34% reduction of hydrogendischarge as fuel (FD) has been achieved,i.e. from 50 ro 33 MMscfd.

Property IntegrationDespite the importance of mass

integration techniques for materialrecycle/reuse, these are limited toaddress problems that are governed bythe composition of process streams(e.g. NH3 composition for ANproduction; mol% impurity forhydrogen reuse/recycle). However,composition is only one of the manymaterial chemical and physicalproperties. Other commonly encoun-tered properties include pH, density,viscosity, turbidity, solubility, etc.

Material reuse/recycle associated withthese properties does not fall into theconventional mass integrationproblems. This is the subject ofproperty integration. In propertyintegration, a general mixing rule todefine all possible mixing patternsamong the individual properties isgiven by [11]:

(8)

Fibre reuse/recycling in a paper makingprocess

Figure 4 shows a papermakingprocess taken from [12, 13]. Woodchips are digested and chemicallytreated in the Kraft digester before theproduced pulp is sent to the bleachingsection. The product from this section,i.e. bleached fibre is then sent to twopaper machines (Paper Machine I andII), where they are converted into finalpaper products. Rejected productsfrom Machine I are further treated inHydro Pulper and Hydro Sieve beforethe waste and waste fibre streams(broke) are finally discharged.However, due to environmentalconcerns, the broke is to be recycled tothe two paper machines. An externalfresh fibre source is currently fed topaper machine II to supplement itsfibre need. Thus, by recycling thebroke, resource usage is maximisedand fresh fibre consumption can bereduced.

To evaluate the quality of the broketo be reused or recycled, we focus onreflectivity R∞, a dimensionlessproperty that is defined as thereflectance of an infinitely thick

Figure 4: A papermaking process

Figure 3: A refinery process

(a) without hydrogen reuse/recycle (b) with hydrogen reuse

(a) without fibre recycle (b) with fibre recycle

y ∑FSK ∑FSR ∑FSR –∑FSK FC ∆m Cum. ∆mFF = 183

1.00183 11.35

7.20 400 -400 11.35-217 -3.91

9.00 350 350 7.44133 4.56

12.43 600 -600 12.00-467 -12.00

15.00 500 500 0FD = 33 28.05 (PINCH)

100.00 28.05

Table 2: Cascade table for targeting hydrogen reuse/recycling in refinery process



C O V E R S T O RY

material compared to an absolutestandard, i.e. magnesium oxide. Themixing rule for reflectivity R∞ is givenas:

(9)

In order to carry out a cascadeanalysis for this problem, one requiresthe operator value of reflectivity.Comparing Equation 8 and 9, we canexpress the operator for R∞, (R∞), asfollows:

(10)

Flowrate, reflectivity and itsassociated operator value of each sinkand source is shown in Table 3.

The minimum fresh fibre feed (FF)and discharged (FD) flowrates are easily

identified from the cascade table inTable 4 as 14.95 ton/h and 24.95 ton/hrespectively [13]. These targets agreewith those obtained by propertycomposite curve [12]. The final networkdesign is shown in Figure 4(b).

ConclusionThis paper summaries some of the

author ’s recent works on materialreuse/recycle system design based ofprocess integration, or morespecifically pinch analysis techniques.Via mass and property integration,material reuse and recycle can now becarried out for systems involvingliquid, gas and solid. By setting thereuse/recycle targets, the engineeridentifies the maximum possible savingthat can be achieved in a reuse/recyclesystem prior to detailed networkdesign. ■

recycle/reuse networks. Industrial& Engineering Chemistry Research.42: 4319-4328.

[5] Manan, Z. A., Tan, Y. L. and Foo,D. C. Y. (2004). Targeting theminimum water flowrate usingwater cascade analysis techniqueAIChE Journal. 50(12): 3169-3183.

[6] El-Halwagi, M. M. (1997).Pollution prevention through processintegration: systematic design tools.San Diego: Academic Press.

[7] Hallale, N. (2002). A new graphicaltargeting method for waterminimisation. Advances inEnvironmental Research. 6(3): 377-390.

[8] Hallale, N. and Liu, F. (2001).Refinery hydrogen managementfor clean fuels production.Advances in Environmental Research.6: 81-98.

[9] Alves, J. J. and Towler, G. P. (2002).Analysis of Refinery HydrogenDistribution Systems. Industrial &Engineering Chemistry Research. 41:5759-5769.

[10] Manan, Z. A. and Foo, D. C. Y.(2003). Setting Targets for Waterand Hydrogen Networks UsingCascade Analysis, paper presentedin AIChE Annual Meeting, 2003San Francisco.

[11] Shelley, M. D., El-Halwagi, M. M.(2000). Componentless design ofrecovery and allocation systems: afunctionality-based clusteringapproach. Computers and ChemicalEngineering. 24: 2081-2091.

[12] Kazantzi, V., El-Halwagi, M. M.(2005). Targeting material reuse viaproperty integration. ChemicalEngineering Progress. 101(8): 28-37.

[13] Foo, D. C. Y., Kazantzi, V., El-Halwagi, M. M. and Manan, Z. A.(2006). Cascade analysis fortargeting property-based materialreuse networks. ChemicalEngineering Science. 61: 2626-2642.

REFERENCES

[1] El-Halwagi, M. M. andManousiouthakis, V. (1989).Synthesis of mass exchangenetworks. AIChE Journal. 35(8):1233-1244.

[2] Wang, Y. P. and Smith, R. (1994).Wastewater minimisation. ChemicalEngineering Science. 49: 981-1006

[3] Towler, G. P., Mann, R., Serriere, A.J-L. and Gabaude, C. M. D. (1996).Refinery hydrogen management:cost analysis of chemicallyintegrated facilities. Industrial &Engineering Chemistry Research.35(7): 2378-2388.

[4] El-Halwagi, M. M., Gabriel, F. andHarell, D. (2003). Rigorousgraphical targeting for resourceconservation via material

Process Flowrate Reflectivity, R∞ ψ(ton/h) (dimensionless) (dimensionless)

(Sinks) Paper Machine I 100 0.85 0.382Paper Machine II 40 0.90 0.536

(Sources) Process Fibre 90 0.88 0.469Broke 60 0.75 0.182Fresh fibre Unknown 0.95 0.738

Table 3: Limiting data for papermaking process

Table 4: Cascade table for targeting fibre reuser/recycle in the papermaking process

ψK ∑FSK ∑FSR ∑FSR –∑FSK FC ∆m Cum. ∆mFF = 14.95

0.73814.95 3.02

0.536 -40 -40 3.02--25.05 -1.67

0.469 90 90 1.3564.95 5.66

0.382 -100 -100 7.01--35.05 -7.01

0.182 60 60 0FD = 24.95 4.54 (PINCH)

0.000 4.54


use process integration to set material reuse and

Documents