GreenAnd the Design of

Follow these principlesand guidelines to design

your process plantto be 'greener'

David T. AllenUniversity of Texas, Austin

Chemical products and processesmake modern life possible. Thesystems that provide housing,transportation, health care,

and food fbr billions of people rely onchemical products, but as demand forthese essential materials grows, theenvironmental impacts of the prod-ucts and the processes that createthem are becoming a greater concern.As this concern about the magnitudeof the associated environmental foot-prints increases, engineers, and par-ticularly chemical engineers, will facenew challenges.

To grasp the nature and mag-nitude of the challenges that willbe faced by engineers, it is usefulto invoke a simple equation thatemerged from the environmentalmovement in the U.S. in the early1970s. At that time, there was sub-stantial debate concerning whetherthe environmental challenges facedby the U.S. were largely driven bypopulation growth, or by the natureof technology. Books like "The Popu-lation Bomb" [il, argued that rapidincreases in population could not besupported by available resources.These ideas had been expressed atleast since the time of Thomas Mal-thus, in the 18th century, but rap-idly increasing and unprecedentedworld populations, gave these argu-ments new life. In contrast, promi-nent environmentalists such as Ra-

Chemical Processes and Products

chel Carson and Barry Commonerargued that it was the nature oftechnology that was the source ofenvironmental problems [2, 3\. Ofcourse, neither population nor thenature of technology is exclusivelytho cause of the environmental chal-lenges we face. It is a combinationof factors that drive environmentalimpacts. In the early 1970s, Ehrlichand Holdren 14, 5] expressed thisidea simply with what has come tobe called the IPAT equation. Envi-ronmental impacts (/), Ehrlich andHoldren argued, are the product of

population, [P, number of people);affluence (A, expressed in units suchas gross domestic product |GDP|per capita), and technology, (T, ex-pressed as impact per unit of GDP):

I = P (number of people) * A ($ GDPper capita) * T (impact per $ GDP)

This relatively simple equation haschanged the way many environmen-talists view the role of technology. TheIPAT equation makes clear that in-stead of being a cause of the problem,better technologies, providing lowerimpacts per dollar of GDP, are viewed

3 6 CHEMICAL ENGINEERING WWW.CHE.COM DECEMBER 2007

;is the enabler of improvedworld-wide affluence.

The engineeringchallengeflow much better will ourtechnologies need to be?World populations are cur-rently growing at rates ofl-2''> /̂yr. Worldwide eco-nomic output is increasinghy 3-5%/yr, with largerincreases in some rapidlydeveloping countries. As-suming that the product ofpopulation and affluencer)('r capita is increasingaL .5'?'i/yi; using the simplelogic of the IPAT equa-tion, the product of popu-lation and affluence (P*A)will increase by 60% in 10years, hy 250% in 25 years,.md by more than a fac-tor of 10 in 50 years. Justto keep impacts the same,our technologies will needto improve by a factor of2-3 in 25 years and 10 in50 years.

Can engineers, particu-larly cheniical engineers,reduce the environmentalimpacts of their designs bya factor of 10? Engineeringdirected at the problem ofreducing the environmen-tal footprints of processesand products is referred toby a variety of terms, in-cluding green engineering,cleaner production, andeco-efficiency. While all ofthese terms are in com-

mon use, and can have subtly difTer-ent meanings, in this article the term"green engineering", as deiined by theU.S. Environmental Protection Agency(EPA; Washington, D.C), will he used.

"Green engineering is the design,commercialization, and use of pro-cesses and products, which are feasi-ble and economical while minimizing1) generation of pollution at the sourceand 2) risk to human health and theenvironment. Green engineering em-braces the concept that decisions toprotect human health and the envi-ronment can have the greatest impact

and cost effectiveness when appliedearly to the design and developmentphase of a process or product" [6].

Green-engineering principlesThe general approach that has beenused in green engineering, and com-plementary efforts in green chemistry,is to defme a broad set of principlesthat can guide designs, then to de-velop metrics and design tools thatsupport these objectives. Anastas andWarner [7] proposed guiding prin-ciples for green chemistry that havebeen widely accepted, and a parallelset of 12 green engineering principleshave been defined by McDonough andothers [8]:

Principle 1. Designers need to striveto ensure that all material and energyinputs and outputs are as inherentlynonhazardous as possible.Principle 2. It is better to preventwaste than to treat or clean up wasteafter it is formed.Principle 3. Separation and purifi-cation operations should be designedto minimize energy consumption andmaterials use.Principle 4. Products, processes,and systems should be designed tomaximize mass, energy, space, andtime efficiency.Principle 5. Products, processes, andsystems should be "output pulled"rather than "input pushed" throughthe use of energy and materials.Principle 6. Embedded entropyand complexity must be viewed asan investment when making designchoices on recycle, reuse, or benefi-cial disposition.Principle 7. Targeted durability, notimmortality, should be a design goal.Principle 8. Design for unnecessarycapacity or capability (for example,"one size fits all") solutions should beconsidered a design flaw.Principle 9. Material diversity inmulticomponent products should heminimized to promote disassemblyand value retention.Principle 10. Design of products,processes, and systems must includeintegration and interconnectivity withavailable energy and materials flows.Principle 11. Products, processes, andsystems should be designed for perfor-mance in a commercial "afterlife".

Principle 12. Material and energyinputs should be renewable ratherthan depleting.

An alternative set of nine guid-ing principles has been defined by65 scientists and engineers partici-pating in a green-engineering work-shop. These principles are posted onEPA's website [61:1. Engineer processes and products ho-

listically, use systems analysis, andintegrate environmental impact as-sessment tools.

2.Conserve and improve natural eco-systems while protecting humanhealth and well-being.

3.Use life-cycle thinking in all engi-neering activities.

4. Ensure that all material and energyinputs and outputs are as inherentlysafe and benign as possible.

5.Minimize depletion of natural re-sources.

6.Strive to prevent waste.7.Develop and apply engineering so-

lutions, while being cognizant oflocal geography, aspirations, andcultures.

8. Create engineering solutions beyondcurrent or dominant technologies;improve, innovate, and invent (tech-nologies) to achieve sustain ability.

9.Actively engage communities andstakeholders in development of en-gineering solutions.

These two separate Hsts of guidingprinciples show that while there isnot universal agreement about theprecise objectives of green engineer-ing, guiding principles generally sug-gest reducing energy use, reducingmaterial use, reducing emissions, andthinking about entire supply chains(life cycles).

Green-engineering metricsDeveloping guiding principles is thefirst st«p in the process of green en-gineering, but, the principles provideonly general guidance, not specificgoals. To be put into practice, spe-cific, measurahle objectives (metrics)must be established. For the designof chemical processes and products,among the most widely recognizedset of sustainability metrics are thosedeveloped by the Canadian NationalRoundtable on the Environment andthe Economy \9\ and the American

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TABLE 1. REPRESENTATIVE ENVIRONMENTAL PERFORMANCE METRICSFOR CHEMICAL MANUFACTURING PROCESSES (BRIDGESTO SUSTAINABILITY,2000)

Compound

Acetic acid

Acrylonitriie

Maelic anhy-dride

Sulfurto acid

Sulfuric acid

Nute: iif.'gative vaals are not inclut:

Process

from methanol by iowpressure carbonylation

by ammoxidation ofpfopyiene

from n-butane by par-tial oxidation

from pyrometailurgicalsulfur dioxide

from suifur

Materialintensity/lbprod.(Ib/ib)

0,062

0.493

0.565

0.002

0.001

Energy/Ib prod.(103 BTU/Ib)

1.82

5,21

0.77

0.073

-0.87

DO,'; I'iir niat.i^nal list; indicat): tiiat wai^lc niatt'riiils IVuni ulhi'r ptnf'd in the material itwe: negative values for eiiergj' use Lndicutp ih

Water/Ib prod,(gal./lb)

1.24

3.37

1.66

0.57

0,7

•CHr- l ' ' ^ ,1 r"l- l l - I ' l l i l

Toxics/Ib prod.(Ib/lb)

0.000

0.015

0

-0.65

0,002

Pollutants/Ib prod.(Ib/lb)

0

0.008

0

-0.63

0.002

raw mutt'niiLs; liir jind water use1 ni't energy gonorat.or

Poilutants+ COj/Ib prod.(Ib/lb)

0.133

0.966

2.77

-0.04

0.002

aw raw mattr i-

Institute fnr Chemical Engineers(AIChE), through their Center forWaste Reduction Technologies [70, i i ,see also Chapter 8 of Reference 121.The team of engineers and scientistsassembled by the AIChE identifiedfive core sustainability metrics forchemical processes fas summarizedin Allen and Shonnard I72|:• Energy consumed from all sources

within the manufacturing or deliv-ery process per unit of manufacturedoutput (with electricity consump-tion converted to equivalent fueluse, based on an average efficiencyof converting energy to electricity inpower piants)

• Total mass of materials used directlyin the product, minus the mass ofthe product, per unit of manufac-tured output

• Water consumption (includingwater present in waste streams,contact cooling water, water ventedto the atmosphere and the fractionof non-contact cooling water lost toevaporation) per unit of manufac-tured output

• Emissions of targeted pollutants(those listed in the Toxic ReleaseInventory) per unit of manufac-tured output

• Total pollutants (including acidify-ing emissions, eutrophying emis-sions, salinity, and ozone depletingsubstances) per unit of manufac-tured output

These metrics match well with the

general guidelines identified in thegi'een-engineering principles: useless energy, use less raw materials,generate less waste. What makes theAiChE measures particularly valu-able for chemical manufacturing,however, is that benchmarks havebeen developed. For many commoditychemicals, the values of indices havebeen calculated for industry standardflowsheets. A few examples are shownin Table 1(73,141.

These data provide benchmarksagainst wbich engineers can comparetbeir designs. With a set of measur-able performance indicators, the thirdstep in the process of green engineer-ing, evaluating alternative designs,can be performed.

Green-engineering practicesThe tools that can he used in develop-ing alternative designs are too broad inscope to be fully summarized here. In-terested readers can refer to the text-book on green engineering \12], soft-ware tools on the U.S. EPA's website[61, and other sources, such as specialissues of the journals. Environmen-tal Science and Technology [15] andIndustrial and Engineering Chernis-try Research []6]. While not all of thetools can be discussed in detail here, acase study of the design of a group ofpartial oxidation processes, using en-ergy consumption as the green engi-neering metric, is illustrative. In thisevaluation \17], five commodity chem-

icals, manufactured through partialoxidation processes, were considered(Figure 1). For each product, a base-case process Ilowsheet was identified.Tben, heat-integration opportunities,at moderate and aggressive levels ofintegration, were evaluated. Finally,process redesign was considered, in-cluding new catalysts, new separa-tion processes, and other new unitoperations. The changes in energyefficiency for each process, for thesedesign stages, are shown in Figure 1.Improvements in energy efficiency arcclearly possible. Some of the efficiencyimprovements halved energy use, rel-ative to tbe base case.

In some ways, this case study istypical. A study sponsored by tbe U.S.Dept. of Energy (DOE; Washington,D.C.) \18\ considered tbe processesused to produce commodity chemi-cals and compared tbe actual energ>'used to the theoretical minimum en-ergy, where the theoretical minimumis defined as the difference in Gibbsfree energy between products and re-actants. Not surprisingly, there weresubstantial differences between theo-retical minimums and actual energyusage. The data for ethylene, as anexample, were striking, but not sur-prising. Ethylene is manufacturedby thermal cracking of ethane andpropane or napthas, a process thatrequires very high reactor tempera-tures. After the ethylene is produced,separating ethylene and propylene

3 8 CHEMICAL ENGINEERiNG WWW,CHE.COM DECEMBER 2007

120

100

Aceticacid

Aceticanhydride

Maieicanhydride

Terephthalicacid

Caproiactam

^ 1 Base case ^ 9 Benchmarked heal integration

n^ Optimum heat integration ^M Process redesign

FIGURE 1. Energy efficiencies of five partiai oxidation processes in a base caseflow sheet and after multipie tiers of green engineering [77]

products typically requires cryogenicoperations. This combination of highantl low temperature processing re-quirements makes the actual energyconsumption much greater than thenet internal energy differences be-tween feedstocks and products. Thetheoretical minimum energy analy-sis is a great simplification of actualprocess requirements, hut it raisesthe question of whether alternativereaction or separation technologiesmight lead to much more energy ef-ficient processes. Could a catalyticroute for ethylene or propylene man-ufacturing be employed? Could non-cryogenic separations in ethylenemanufacture be used?

The DOE report [18] identifies avariety of processes where improve-ments in catalysis, for example, couldlead to improvements in energy effi-ciency. More than 800 trillion Btu ofannual energy savings associated withimprovements in catalysts were iden-tified in the analy-sis. With oil valuedat $90/bbl (roughly $0.40-0,50/lh, with20,000 Btu/lb), these potential energysavings have a value of tens of billionsof dollars per year.

These simple case studies suggestthat once quantifiable sustainabilitymetrics are identified, engineeringtools can he used to identify substan-tial improvements in chemical processdesigns. Changes within processes

can he identified that reduce energyuse, material use, and emissions,however, the design changes shouldnot stop there. All of the guiding prin-ciples for green engineering stress theimportance of life cycles and supplychains. So. in addition to looking forimprovements in single processes andfacilities, systems and supply chainsof chemical processes should be exam-ined. Practitioners of green engineer-ing should examine whether chemi-cal manufacturing systems can bedesigned that use waste energy andwaste materials from other processes.This is not a new idea in chemical en-gineering. For decades, chemical engi-neers have practiced the art of usingwaste materials and waste heat fromone process in other processes. Con-sider a classic example — the manu-facture of vinyl chloride.

Billions of pounds of vinyl chlo-ride are produced annually. Approxi-mately half of this production occursthrough the direct chlorination ofethylene. Ethylene reacts with mo-lecular chlorine to produce ethylenedichloride (EDO. The EDC is thenpyrolyzed, producing vinyl chlorideand hydrochloric acid.

CI2 CIH2C-CH2CI

CIH2C-CH2CI -> H2C=CHC1 -t- HCI

In this synthesis route, one mole

of hydrochloric acid is produced forevery mole of vinyl chloride. Consid-ered in isolation, this process mightbe considered wasteful. Half of theoriginal chlorine winds up, not in thedesired product, but in a waste acid.But the process is not operated in iso-lation. The waste hydrochloric acidfrom the direct chlorination of eth-ylene can be used as a raw materialin the oxychlorination of ethylene. Inthis process, hydrochloric acid, ethyl-ene and oxygen are used to manufac-ture vinyl chloride.

rik.^i + ri '>\j='_'riij -p TTJWO —?

By operating both the oxychlorina-tion pathway and the direct chiorina-tion pathway, the waste hydrochloricacid can he used as a raw materialand essentially all of the molecularchlorine originally reacted with ethyl-ene is incorporated into vinyl chloride.The two processes operate synergisti-cally and an efficient design for themanufacture of vinyl chloride involvesboth processes.

Additional efficiencies in the use ofchlorine can be obtained by expandingthe number of processes included inthe network. In the case involving di-rect chlorination and oxychlorinationprocesses, both processes incorporatechlorine into the final product. Moreextensive chlorine networks haveemerged, linking isocyanate producersinto vinyl chloride manufacturing net-works. In isocyanate manufacturing,molecular chlorine is reacted with car-bon monoxide to produce phosgene:

CO + CI2 -^ COCI2

The phosgene is then reacted with anamine to produce an isocyanate andbyproduct hydrochloric acid:

RNH2 + COCI2 ^ RNCO + 2HC1

The isocyanate is subsequently usedin urethane production, and the hy-drochloric acid is recycled. The keyfeature of the isocyanate-processchemistry is that chlorine does notappear in the final product. Thus,chlorine can be processed through thesystem without being consumed. Itmay be transformed from molecularchlorine to hydrochloric acid, but thechlorine is still available for incor-

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pnration into final products, such asvinyl chloride, that contain chlorine.A chlorine-hydrogen chloride net-work incorporating both isocyanateand vinyl chloride has developed inthe Guff Coast of the US. \19]. Themolecular chlorine is sent to hothdirect chlorination processes and toisocyanate manufacturing. The by-product hydrochloric acid is sent tooxychlorination processes or calciumchloride manufacturing. The networkhas redundancy in chlorine flows.such that most processes could relyon either molecular chlorine or hy-drogen chloride.

Chlorine is not the only materialfor which such material cycles couldbe identified. A far more ubiquitousmaterial, water, can also be effec-tively cycled through multiple pro-cesses. Water is used in virtually allindustrial processes and major oppor-tunities exist for reuse since, in gen-eral, only a small amount of water isconsumed; most water in industrialapplications is used for cooling, heat-ing or processing of materials, not asa reactant. Further, different indus-trial processes and industrial sectorshave widely varying demands forwater quality. Water exchanges andreuse provide a significant opportu-nity. An example of such opportuni-ties is described by Keckler and Allen

[301, (For more on water reuse, seeCE, October 2006, pp. 50-54).

Identifying which processes couldbe most efficiently integrated is notsimple and the design of tbe idealnetwork depends on available mar-kets, what supphers and markets formaterials are nearby, and other fac-tors. What is clear, however, is thatthe chemical process designers mustunderstand not only their process,but also processes that could supplymaterials, and processes that coulduse their byproducts. And, tbe analy-sis should not be limited to chemicalmanufacturing. Continuing with ourexample of waste hydrochloric acidand the manufacture of vinyl chloride,hyproduct hydrochloric acid could beused in steel making, or byproducthydrochloric acid from semiconduc-tor manufacturing might be used inmanufacturing chemicals.

Final remarksThis papej- has outlined the generalsteps in promoting green engineering— defining guiding principles, estab-lishing metrics and using engineer-ing tools to meet design objectives.The metrics and design tools that arepart of green engineering should beemployed not only within chemicalprocesses but also between processes.If these methods become a part of all

engineering designs, then the overallchallenge that confronts us — improv-ing the ^.Ticiencies of our technologiesby an order of magnitude over a gen-eration — can be achieved. •

Edited by Gerald Ondrey

Note: This article waB baited on materials in"Green Engineering: Envimnmpntnlly Con-sciitus Design <if Chemical PnicesHi'si" [12\.More details on the types of quantitative de-sign tools that arp beniminK avnilahlc forchfmical unginetrs can bu found there.

AuthorDavid T. Allen is the Melviii H. Gertz ResKntsChair in Chemical Enginel^^i^g and the nirec-tor of th(! Center fur Energy and EnvironmentalRe.iources at the UniverHJly of Texas at Austin(1 University Station C0400. Austin. TX 7H712.Phone: 512-471-0049; Fax: 512-471-1720; Kmail:8llen@che.iitexas.edu). His research interestslie in air quality and pollulion prevention. He isthe author of four iKKiks and over 180 papers inthese areas. The qiiahtv cf his rctficarth luia ln"<"nrecognized by the National Science foundationIthrough the Presidential Young InvestigatorAward), the AT&T Foundation (through an In-dustrial Ecology P'ellowship), thf American In-stitute of Chemical F^ngineers (through tht? I'ecilAward for contributions to environmental en-gineering), and lhe State of Texas ahrough theGovernor'H Environmental Excellence' AwarcH.In addition. Dr. Allen i.-* actively involved in de-veloping green engineering educational materi-als for the chemical engineering curriculum.His most recent effort iw a textV>ook nn design ofchemical proi-eswes and prcKhicts, jointly devel-oped wilh the U.S. RPA. Dr. Allen received hisB.S. ChE, with diwtinctiun. from Cornell Uni-versity in 197y. Hi.s M.S. and Ph.D. degrees inChE were awarded by tbe Ciilifcirnia In.-ttituteof Technology in 1981 and 19H;i. He has beldreftylar faculty appointments at UCLA and theUniversity of Texas, and visiting UHpointinentaat the California ln.stitute of TeclinolDgy andthe University of California, Santa Barbara; hejoined the University of Texas in 1995.

References1. Ebrlich. P.R./The IVpulati.m Bomh." BalIan-

tine, New York, N.Y., 1968.

2. Carson, R., "Silent Sping," Houghton-Miffiin.Boston, Ma.'is., 1962.

3. Commoner. IJ.. The Environmental Cost ofEconomic Growth, in "Population. Resourcesand the Envinmment,' edited by R. G. Rid-ker, U.S. (iovernment Printing OITic(\ Wash-ington. DC. pp. 339-363,1972.

4. Ehrlich, P. and Huldren, J., Impact of Popula-tion Growth, Science 171,1212-1217.1971.

ft. Ebrlicb, P. find Holdren. .1., Impact, of Popula-tion (jrowlh, in "Population. Resources andthe Environment," edited by R. G. Ridker.U.S. Government Printing Ofiice. WaHhing-ton, D.C., pp. 365-;i77,1972.

6. Weh site of the US. Environmental Protec-tion Agency, Washington, D.C. (www.epa.gov/oi)pt/greenengineering). accessed November2007.

7. Anastas. P.T. and Warner,.I.e.."Green Chemis-try: Theory and Practice," Oxford UniversityPreas. New York. N.Y. 1998.

8. McDonough. W.. Braungart, M., Anastas, P.T,and Zimmerman, J.B., Applying the Prin-ciples of Orpon Engineering to Cradlo-to-cradle Design. Eni'. Sci. and 7lTft., 37 (231.pp. 434A-S41A,a003,

9. National Roundtable on the Enviriinment andthe Economy iNRTEE), "Mea.suring Eco-efR-ciency in Busint'.'̂ .s: Keasibility of a Core Setof Indicators". Renouf" Publishing, Ottowa.Canada, 1999 (ISBN 1-895643-98-8).

10. American Institute of Chemical Engineers(AIChE). Center for Waste Reduction Tech-nologies (CWRT), "Sustainability Metrics In-terim Report # r , AIChE CWRT, New York.N.Y,1998.

U. AIChE CWRT, "Sustainahility Metrics In-terim Report #2", AIChE CWRT New York,N.Y,1999.

12. Allen, DT and Shonnard. D.R.. -Green Engi-neering: Environmentally Conscious Designof Chemicai Processes," Prentice Hall, Engle-wood Cliffs, N.J., 2001.

13. Bridges to Sustainabiiity, "^ustainahilityMetrics: Making Decisions fur Major Chemi-cal Products and FaciliLie.s", Houston, Tex.,2000.

14. Schwarz, J.,Beloff,B..and Beaver. E.. Use Sus-tainability Metrics to Guide Decision-making",Chcm.Enf'.Prri)i.,9fini, pp. 5S-fi'J. 2002.

15. Euv. Sci.and Tech., Special Issue on GreenEngineering,37 (23) pp. 5,269-5470. 2003.

16. Ind.and Eng.Ckem. Hes., Special Istsue onGreen Chemistry and Engineering 41 (18),pp. 4,43&-4,688, 2002.

17. Bridges to Su.iitainahility. "A Pilot Study ofEnergy Performance Levels for the II.S.('h<'mical Industry," reptirt prepared for U.S.I>ept. of Enei^, Washington, D.C, June,2001.

18. "Energetics, Energy and Environmental I'rn-file of the U.S. Chemical Industry," reportprepartid for DOE. May 2000, available at:ntt p://w wwl.oere.energy.gov/industry/cheni-i c H Is/tool s_profile.html

19. McCoy, M., Chlorine Link.s Gulf Coast Firm.s,Chem. and Rn/i. Newa, Sept. 7. pp. 17-20,1998,

20. Keckler, S.E. and Allen, D.T. Material ReuseModeling; A Network Flow ProgrammingApproacb, J.of lnd.Ecol., 2(4) 79-92 (19981.

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