green engineering - allan

8/9/2019 Green Engineering - Allan

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G r e e nA n d t h e D e s ig n o fF o l lo w t h e s e p r i n c i p l e s

a n d g u i d e l i n e s t o d e s i g ny o u r p r o c e s s p l a n t

t o b e 'g r e e n e r 'David T . AllenUniversity of Texas, AustinC hemical 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 gras p the natu re and m ag-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 t ime, there was sub-stant ial 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" [i l , argued that rapidincreases in population could not besupported by available resources.These ideas had been expressed atleast since the t ime of Thomas Mal-thus, in the 18th century, but rap-idly increasing and unprecedentedworld populations, gave these argu-

C h e m i c a l P r o c e s s e s a n d P r o d u c t s

chel Carson and Barry Commonerargued that i t 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 tha t drive en viron me ntalimpacts. In the early 1970s, Ehrlichand Holdren 14, 5] expressed thisidea simply with what has come tobe called the IPAT equation. Envi-

population, [P , number of peopleaffluence (A, exp resse d in units su cas gross domest ic product |GDper capita), and technology, (T, epressed as impact per unit of GDPI = P (numb er of people) * A ($ GDper capita) * T (impact per $ GDP)

This relatively simple equation hchanged the way many environmetalists view the role of technology. TIPAT equation makes clear that i


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;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. Environ men tal Protection Agency(EPA; Wa shington, D .C), will he use d."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 the

environment. Green engineering em-braces the concept that decisions to

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 m aterials flows.Principle 11. Products, processes, and

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 ou tputs 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, andcul tures.8.Create en gineering solutions beyond

current or dominant technologies;improve, innovate, and invent (tech-nologies) to achieve su sta in 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 metr icsDeveloping guiding principles is thefirst stp 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 National


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

Compound

Acet ic acidAcrylonitriieMaelic anhy-drideSulfurto acid

Sulfuric acidNute : iif.'gative vaals are not inclut:

Process

from methanol by iowpressure carbonylationby ammoxidation ofpfopyienefrom n-butane by par-tial oxidationfrom pyrometailurgicalsulfur dioxidefrom suifur

Materialintensity/lbprod.( Ib/ ib)0,062

0.493

0.565

0.002

0.001

Energy/Ib prod.(103 BTU/Ib)1.825,210.770.073

-0.87DO,; I'iir niat.i^nal list; indicat): tiiat wai^lc niatt'riiils IVuni ulhi'r ptnf'd in the ma ter ial itwe: nega tive values for eiie rg j' use Lndicutp i h

Water/Ib prod,(gal . / lb)1.243.371.660.57

0,7 C H r - l ' ' ^ , 1 r " l- l l - I ' l l i l

Toxics/Ib prod.( Ib/ lb)0.000

0.0150-0.65

0,002

Pollutants/Ib prod.(Ib/ lb)0

0.008

0-0.63

0.002raw mu tt 'ni iLs; l i ir j ind w ater use1 ni't energy gonorat.or

Poilutants+ COj/Ib prod.(Ib/ lb)0.1330.966

2.77-0.04

0.002aw raw m att r i

Institute fnr Chemical Engineers(AIChE), through their Center forWaste Reduction Technologies [70, ii,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 sourceswithin 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 m aterials used d irectlyin the product, minus the mass ofthe product, per unit of manufac-

tured output Water consump tion (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 Em issions of targeted pollu tants(those listed in the Toxic ReleaseInventory) per unit of manufac-tured output Total pollutants (including acidify-

ing emissions, eutrophying emis-sions, salinity, and ozone depleting

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 ind icators, 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 altern ative d esigns are too broad inscope to be fully su mm arized 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 E ngineering Chern is-try Research []6]. While not all of thetools can be discussed in detail h ere, acase study of the design of a group ofpartial oxidation processes, using en-ergy consumption as the green engi-

icals, manufactured through partioxidation processes, were considere(Figure 1). For each product, a bascase process Ilowsheet was identifieTben, heat-integration opportunitieat moderate and aggressive levels integration, were evaluated. Finallprocess redesign was considered, icluding new catalysts, new separtion processes, and other new unoperations. The changes in energefficiency for each process, for thedesign stages, are shown in Figure Improvements in energy efficiency aclearly possible. Some of the efficiencimprovements halved energy use, reative to tbe base case.

In some ways, this case study typical. A stud y sponsored by tbe UDept. of Energy (DOE; WashingtoD.C.) \18\ considered tbe processused to produce commodity chemcals and compared tbe actual energused to the theoretical minimum eergy, where the theoretical minimuis defined as the difference in Gibfree energy between products and ractan ts. Not surprisingly, there wesubstantial differences between theretical minimums and actual energusage. The data for ethylene, as aexample, were striking, but not suprising. Ethylene is manufactureby thermal cracking of ethane anpropane or napthas, a process threquires very high reactor temper


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1 2 0

100Aceticacid Aceticanhydride Mai e i ca n h y d r i d e Terephtha l i cac i d C a p r o i a c t a m

^ 1 Base case ^ 9 Benchmarked heal integrationn^ Optimum heat integration ^M Process redesign

FIGUR E 1 . Energy efficiencies of five partiai oxidation processes in a base caseflow sheet and after multipie tiers of green engineering [77]produc ts typically requ ires 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 i t raisesthe question of whether alternativereaction or separation technologiesmight lead to much more energy ef-ficient processes. Could a catalyticroute for ethylene or propylene m an-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 ofann ual 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 suggest

that once quantifiable sustainabilitymetrics are identified, engineering

can he identified that reduce energyuse, material use, and emissions,however, the design changes shouldnot stop th ere. All of the guiding prin-ciples for green eng ineering str ess 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 examp le 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

of hydrochloric acid is produced foevery mole of vinyl chloride. Considered in isolation, this process mighbe considered wasteful. Half of theoriginal chlorine winds up, not in thedesired product, but in a waste acidBut th e process is not operated in isolation. The waste hydrochloric acidfrom the direct chlorination of ethylene can be used as a raw materiain the oxychlorination of ethylene. Inthis process, hydrochloric acid, ethylene and oxygen are used to manufacture vinyl chloride.rik.^i + r i ' > \ j = ' _ ' r i i j -p TTJWO

By operating both the oxychlorination pathway and the direct chiorination pathway, the waste hydrochloricacid can he used as a raw materiaand essentially all of the moleculachlorine originally reacted with ethylene is incorporated into vinyl chlorideThe two processes operate synergistically and an efficient design for thema nufac ture of vinyl chloride involvesboth processes.Additional efficiencies in the use ochlorine can be obtained by ex pandingthe number of processes included inthe network. In the case involving direct chlorination and oxychlorinationprocesses, both processes incorporatechlorine into the final produc t. Moreextensive chlorine networks haveemerged, linking isocyanate produ cersinto vinyl chloride manufa cturing net-works. In isocyanate manufacturingmolecular chlorine is reacted with car-bon monoxide to produce phosgene:

CO + CI2 -^ COCI2The phosgene is then reacted with anamine to produce an isocyanate andbyproduct hydrochloric acid:RNH2 + COCI2 ^ RNCO + 2HC1The isocyanate is subsequently usedin urethane production, and the hydrochloric acid is recycled. The keyfeature of the isocyanate-processchemistry is that chlorine does notappear in the final product. Thuschlorine can be processed through thesystem without being consumed. I


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Feature Reportpnration 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 m ore u biquitousmaterial, 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 A llen

[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 th e m anufacture 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 overalchallenge th at confronts us improving the ^.Ticiencies of our technologieby an order of magnitude over a generation can be achieved. Edited by Gerald OndreNote: This article waB baited on materials i"Green Engineering: Envimnmpntnlly Consciitus Design ept. of Enei^, Washington, D.C, Jun2001.18. "Ene rgetics, Energy and Environm ental I'rnfile of the U.S. Chemical Industry," repoprepartid for DOE. May 2000, available antt p://w wwl.oere.energy.gov/industry/cheni c H Is/tool s_profile.html19. McCoy, M., Chlorine Link.s Gulf Coast Firm.Chem. and Rn/i. Newa, Sept. 7. pp. 17-21998,


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green engineering - allan

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