j.1530-9290.2008.00009.x

A P P L I C AT I O N S A N D I M P L E M E N TAT I O N

Environmental Assessmentof Waste-Solvent TreatmentOptionsPart II: General Rules of Thumb and SpecificRecommendations

Christian Capello, Stefanie Hellweg, and Konrad Hungerbuhler

Keywords:

cement kilndistillationhazardous waste-solvent incineratorindustrial ecologylife-cycle assessment (LCA)waste-solvent management

Supplementary material is available onthe JIE Web site

Summary

A comparison of various waste-solvent treatment technolo-gies, such as distillation (rectification) and incineration inhazardous-waste-solvent incinerators and cement kilns, is pre-sented for 45 solvents with respect to the environmental life-cycle impact. The environmental impact was calculated withthe ecosolvent tool that was previously described in Part I ofthis work. A comprehensive sensitivity analysis was performed,and uncertainties were quantified by stochastic modeling inwhich various scenarios were considered. The results showthat no single treatment technology is generally environmen-tally superior to any other but that, depending on the solventmixture and the process conditions, each option may be op-timal in certain cases. Nevertheless, various rules of thumbcould be derived, and a results table is presented for the 45solvents showing under which process conditions and amountof solvent recovery distillation is environmentally superior toincineration. On the basis of these results and the ecosolventtool, an easily usable framework was developed that helpsdecision makers in chemical industries reduce environmentalburdens throughout the solvent life cycle. With clear recom-mendations on the environmentally optimized waste-solventtreatment technology, the use of this framework contributesto more environmentally sustainable solvent management andthus represents a practical application of industrial ecology.

Address correspondence to:Christian CapelloSafety and Environmental Technology GroupETH Zurich, Wolfgang-Pauli Str. 10, HCICH-8093 Zurich, [email protected]://www.sust-chem.ethz.ch

c© 2008 by Yale UniversityDOI: 10.1111/j.1530-9290.2008.00009.x

Volume 12, Number 1

www.blackwellpublishing.com/jie Journal of Industrial Ecology 111


Introduction

Organic solvents are among the most impor-tant chemical resources used in the pharmaceu-tical and specialty chemicals industries in termsof quantity. Once the solvents cannot be reusedin a process and thus become waste solvents,there are only a few technologies available fortheir treatment. The main choices are distillationor thermal treatment in either hazardous-waste-solvent incinerators or cement kilns (Seyler et al.2006). From an environmental perspective, it isnot known what treatment technology is supe-rior. All technologies are associated with impactson the environment (see Capello et al. 2007).A systematic evaluation of these technologiesprovides industry with the knowledge neededto reduce environmental burdens throughoutthe life cycle of organic solvents. It is there-fore highly relevant in the context of industrialecology.

A suitable method for comprehensively quan-tifying the environmental impact of these tech-nologies is the life-cycle assessment (LCA)method (EN ISO 14040 1997). It considers allimpacts to humans and the environment duringthe whole life cycle of solvents, including impactsfrom raw material extraction, solvent production,use of energy and ancillaries, and waste-solventtreatment (Hofstetter et al. 2003). However, onemajor drawback is that full LCA studies are dataand time intensive. To overcome these limita-tions, we used the ecosolvent tool presented in partI of this two-article series (Capello et al. 2007) fora systematic environmental assessment of threecommon technologies to determine general con-clusions and rules of thumb about environmen-tally superior waste-solvent treatment options.Such results may provide simple and quick an-swers for decision makers and thus contribute tothe adoption of environmentally preferable sol-vent management.

In an initial step, we determined whetherone of the three technologies implemented inthe ecosolvent tool is generally the environmen-tally superior treatment option. To this end,we performed a sensitivity analysis to deter-mine process parameters of particular impor-tance with regard to environmental performance.

The analysis thus enabled a determination ofthe range of environmental impacts for eachtechnology.

Next, we obtained solvent-specific results bycomparatively calculating the environmental im-pacts of 45 commonly used solvents treated withthe three technologies. To this end, various sce-narios were taken into account. For example, var-ious energy production chains and fractions ofsecondary components were considered for distil-lation. Additionally, several recovery rates weretaken into account to determine rules of thumbthat specify under which process conditions onetreatment technology is generally environmen-tally superior to another.

Methods

The Ecosolvent Tool

The ecosolvent tool is a generic life-cycleassessment tool that allows for the environ-mental comparison of distillation (rectification)and thermal treatment in hazardous waste in-cinerators and cement kilns for specific, user-defined waste-solvent mixtures (see Capello et al.2007). These technologies are represented by so-called life-cycle inventory (LCI; EN ISO 140411998) models that are all based on industry data(Capello et al. 2005; Seyler et al. 2004, 2005).With these models, waste-solvent-specific life-cycle inventory parameters, such as emissionsflows, ancillary uses, and generation of coprod-ucts, are calculated (table 1).

In the ecosolvent tool, the calculated inven-tory data are linked to background inventorydata (production of ancillaries, fuels, and energy)taken from the ecoinvent database (ecoinventCentre 2004). Inventory data of 32 of the sol-vents that were used in this work will be pub-lished in the Version 2.0 update of the ecoinventdatabase. The full LCI can be assessed with vari-ous life-cycle impact assessment methods. In thepresent article, we applied eco-indicator 99 (H/A;Goedkoop and Spriensma 2000), cumulative en-ergy demand (Jungbluth and Frischknecht 2004),the method of ecological scarcity (UBP’97;Hischier 2004), and the global warming potential(IPCC 2001).

112 Journal of Industrial Ecology


Tabl

e1

Inve

ntor

ypa

ram

eter

sof

the

thre

elif

e-cy

cle

inve

ntor

y(L

CI)

mod

els

impl

emen

ted

inth

eec

osol

vent

tool

LCI

mod

els

(abb

revi

atio

n)D

istill

atio

nm

odel

(dist

)H

azar

dous

was

tein

cine

rato

rm

odel

(wsi

nc)

Cem

entk

ilnm

odel

(cem

ent)

Inve

ntor

ypa

ram

eter

s(ab

brev

iati

on)

Use

ofst

eam

(st)

Use

offu

eloi

l(oi

l)�

CO

2em

issi

ons(

�C

O2)

a

Use

ofel

ectr

icit

y(e

l)U

seof

anci

llari

es(a

nc)

�N

OX

emis

sion

s(�

NO

x)a

Use

ofni

trog

en(N

2)En

ergy

prod

ucti

on(e

nerg

y)�

Met

alem

issi

ons(

�m

etal

s)a

Use

ofan

cilla

ries

(anc

)C

O2

emis

sion

s(C

O2)

Foss

ilfu

elsu

bsti

tuti

on(f

uel)

Out

leta

irtr

eatm

ent(

air)

Oth

erem

issi

ons(

em)

Res

idue

inci

nera

tion

(res

)W

aste

-wat

ertr

eatm

ent(

ww

)So

lven

trec

over

y(s

olv)

a Inth

eLC

Imod

elof

the

cem

entk

iln,c

hang

esin

the

emis

sion

sasa

cons

eque

nce

ofsu

bsti

tuti

ngfo

ssil

fuel

swit

hw

aste

solv

enta

reca

lcul

ated

and

ther

efor

eex

pres

sed

asdi

ffere

nces

(�). Determining the Relevance

of Inventory Parameters

All inventory parameters (table 1) contributeto the total environmental impact of a waste-solvent treatment technology to varying degrees.To identify relevant parameters, we performed asensitivity analysis. The variability of the envi-ronmental impacts originates from two sources.First, variability between sources and objects in-fluences the LCA outcome (Huijbregts 1998)due to different solvent properties, differencesin technologies that produce the same product(e.g., steam or electricity production), and dif-ferent technologies (e.g., batch and continuousdistillation). This variability can be accountedfor through defining scenarios. Therefore, a best-case and a worst-case scenario were considered foreach inventory parameter. The best-case scenarioreflects the situation of minimal environmen-tal burdens and maximal environmental credits.The worst-case scenario comprises maximal envi-ronmental burdens and minimal environmentalcredits. The detailed description of the scenariosis presented in Supplementary Table S1 on theWeb.

The second source is the parameter uncertainty(Huijbregts 1998). To quantify the parameteruncertainty, we applied quantitative stochasticmodeling (Monte Carlo simulation [Vose 2000],as implemented in the ecosolvent tool). To thisend, probability distributions were used for allmodel parameters. Probability distributions of theenvironmental impact scores were determined forall inventory parameters as output. The detailedcalculation of the single inventory parameters ispresented in Supplementary Tables S2 and S3 onthe Web.

Technology-Specific Assessment

The total environmental impact of a technol-ogy (Itech) is defined as the sum of the environ-mental impacts of the single inventory param-eters (Iip). Thus, on the basis of the sensitivityanalysis, the total environmental impact of thethree technologies was calculated according toequations (1), (2), and (3) (see table 1 for an ex-planation of the abbreviations used):

Capello et al., Environmental Assessment of Waste-Solvent Treatment Options 113


Table 2 General scenario definition used for the solvent-specific assessment

Distillation model

Steam Outlet air Use of electricity ParameterScenario production Use of ancillaries treatment and nitrogen uncertainty

Minimumimpactdistillation

Waste-solventincineration

No Outlet airincineration

Generic data ofcontinuousdistillation

Best case: 2.5thpercentile

Average impactdistillation

Waste-solventincineration

pH adjustmentandequipmentcleaning

Outlet airincineration

Generic data ofcontinuousdistillation

Average: 50thpercentile

Maximumimpactdistillation

Incineration offossil fuels

Entrainer Direct emissionof outlet air asnonmethanevolatileorganiccarbon(NMVOC)

Generic data ofbatchdistillation

Worst case:97.5thpercentile

Incineration modelsMinimum

impactincineration

– – – – Best case: 2.5thpercentile

Average impactincineration

– – – – Average: 50thpercentile

Maximumimpactincineration

– – – – Worst case:97.5thpercentile

Waste-solvent incinerator:

Iwsinc = Ioil + Ianc + ICO2 + Iem+I energy (1)

Cement kiln:

Icement = I�CO2 + I�NOx + I�metals+I f uel (2)

Distillation:

Idist = Ist + Iel + IN2+I anc+I air+I res+I ww+I solv

(3)

Solvent-Specific Assessment

To derive solvent-specific results, we compar-atively assessed the treatment of 45 waste-solventmixtures for both distillation and incineration us-ing the ecosolvent tool. Each waste-solvent mix-ture was assumed to be a binary mixture that con-tained one of the most commonly used solvents in

the pharmaceutical and specialty chemical indus-tries (Seyler et al. 2006) as the main component.Three scenarios were considered, representing aminimum, average, and maximum environmen-tal impact. In these scenarios, the parameter un-certainty of the inventory flows was taken intoaccount for both technologies, as described inthe section above. With regard to the distilla-tion, the scenarios included additional assump-tions concerning the steam production; the useof ancillaries, electricity, and nitrogen; and thetreatment of outlet air (table 2).

In addition to these scenarios, it was assumedthat the distillation residuals were treated in ahazardous waste incinerator. The solvent recov-ery was considered separately, as it proved to be akey parameter in prior studies (see Capello et al.2007 or Hofstetter et al. 2003).

An initial assessment seeks to determinethe solvents and the process conditions forwhich distillation is the environmentally superior



treatment option to incineration. To this end,we assumed that each of the 45 solvents wouldbe present as the main component in the binarymixture. The secondary component was chosento show environmentally optimal results in theincineration models. The shares of the main com-ponent and the secondary component were var-ied continuously, from a minimum of 0.34 kg/kgwaste solvent to a maximum of 0.98 kg/kg wastesolvent. The recovery rate of the distillation,which defines the amount of the recovered maincomponent from the total amount of main com-ponent present in the mixture, was set to 90% forshares of the main component below 0.9 kg/kgwaste solvent. Above 0.9 kg/kg waste solvent,it was increased linearly up to 99% accordingto experts’ opinion (Expert Panel 2003–2005).The amount of recovered solvent varied, there-fore, from 0.31 kg/kg waste solvent to 0.97 kg/kgwaste solvent, which is the range we found inindustry (see Capello et al. 2005). Thus, theenvironmental impact was calculated for bothtechnologies as a function of the shares of thecomponents. With regard to distillation, worst-case conditions, average conditions, and best-case conditions were considered for all 45 sol-vents and compared to environmental best-caseconditions for incineration (see table 2). In manycases, a threshold amount of recovered solventcould be calculated beyond which distillation be-came the superior technology. The values of thesethreshold recoveries were determined for the 45solvents.

A second assessment attempted to deter-mine the solvents for which incineration isgenerally environmentally superior to distil-lation. To this end, worst-case conditionswere considered for incineration (table 2).With regard to distillation, optimal conditionswere assumed (table 2), as well as a maximumsolvent recovery of 0.97 kg recovered solvent perkilogram waste solvent (Capello et al. 2005).Additionally, the secondary component was as-sumed to be the solvent with the worst environ-mental results in the incineration models.

Note that these assessments are based on sev-eral assumptions. For example, no pretreatmentof the waste solvent is required before distillation,the distillation residue is not treated in sewageplants, and the recovery rate is 90%. In some

cases, these assumptions may not reflect the ac-tual process properly, but they represent the gen-eral conditions in chemical industries (ExpertPanel 2003–2005).

Results

Relevance of the SingleInventory Parameters

The results of the sensitivity analysis are de-picted in figure 1 for incineration and figure 2 fordistillation. Further results from the sensitivityanalysis are given in Supplementary Table S2 onthe Web.

With respect to the hazardous-waste-solvent in-cinerator (figure 1), the credits of energy produc-tion (Ienergy) and carbon dioxide (CO2) emissions(ICO2) contributed most to the environmentalimpact in the best-case scenario. Important en-vironmental impacts from direct emissions (Iem),from the use of ancillaries (Ianc), and from supple-mental fuel oil (Ioil) only arose in the worst-casescenario, due to the low net calorific value ofthe waste solvent or due to heteroatoms. Het-eroatoms, such as sulphur, nitrogen, or chlorine,require the use of ancillaries, especially sodiumhydroxide (Seyler et al. 2005). High nitrogencontent may also cause environmental burdensdue to nitrogen oxide (NOx) emissions. In theabsence of heteroatoms, the net calorific value isthe most important waste-solvent property, be-cause the energy credits as well as the amountof supplemental fuel, if necessary, are a functionof net calorific value. Additionally, in the caseof organic solvents, the net calorific value posi-tively correlates with the carbon content, unlessfunctional groups are present (Wypych 2001).Therefore, the CO2 emissions are mostly relatedto the net calorific value.

With the eco-indicator 99 and UBP’97 meth-ods, all parameters could be assessed on a fullyaggregated level. The results of both methodsshow a good correlation of the relative impor-tance of the single inventory parameters. Theglobal warming potential deviated from the re-sults of the former methods with regard to theenvironmental impact arising from direct emis-sions (Iem), as the only assessable direct emis-sion was carbon monoxide (CO2 emissions are



Fig

ure

1Po

tent

iale

nviro

nmen

tali

mpa

ctof

the

inve

ntor

ypa

ram

eter

sin

the

haza

rdou

sw

aste

-sol

vent

inci

nera

tor

and

cem

ent

kiln

sm

odel

s.



Fig

ure

2Po

tent

iale

nviro

nmen

tali

mpa

ctof

the

inve

ntor

ypa

ram

eter

sin

the

dist

illatio

nm

odel

.The

orga

nic

resid

ueis

trea

ted

inei

ther

aw

aste

-sol

vent

inci

nera

tor

orce

men

tki

lns.

The

resu

ltsof

both

alte

rnat

ives

are

show

n.



considered separately in ICO2). As the emissionsof carbon monoxide were allocated to the waste-solvent mass, the best- and worst-case scenariosresulted in the same impact score. Nevertheless,the global warming potential was a suitable in-dicator of the environmental impact if the wastesolvent did not contain nitrogen. Otherwise, im-pact assessment methods that take into accountNOx emissions were more appropriate. Similarly,the results calculated with the cumulative energydemand also correlated very well with the resultsof other methods, but no emissions could be as-sessed.

In the cement-kiln model, changes in CO2 andthe NOx emissions (I�CO2, I�NOx) were due tothe carbon and the nitrogen content of the wastesolvent, respectively, as well as to the net calorificvalue of the waste solvent. The latter is impor-tant, as the substituted fossil fuel also containednitrogen and carbon. The substitution of coal, inparticular, led to a decrease of CO2 emissions, ascoal shows a high carbon content per net calorificvalue (Seyler et al. 2004). In the best-case sce-nario, the change in NOx emissions was also pri-marily related to the net calorific value. This af-fected the NOx emissions (worst-case scenario)only if the nitrogen content of the waste solventswas high.

Figure 1 shows that all inventory parametersmay be of environmental relevance in the case ofcement kilns. In the worst-case scenario, I�NOx

could become the most important parameter. Be-cause the formation of NOx in cement kilns isincompletely understood (van Oss and Padovani2003), the NOx emissions were calculated withuncertainty with regard to the conversion rateof fuel nitrogen to NOx (52%–92%; Baumbach1993) as well as the efficiency of NOx reductionfacility (0% in case this equipment is missingto 85%; see Capello et al. 2007). Therefore, theresults of the NOx emissions show high uncer-tainty. Also, I�metals could be of high importance,but only in the case of high heavy metal contentin the waste solvent. In the absence of high metaland nitrogen content, the net calorific value ofthe waste solvent determined the amount of sub-stituted fossil fuels and also the changes in otheremissions.

The results according to the methods eco-indicator 99, UBP’97, and the global warming

potential (with the exception of NOx and metalemissions) were consistent in terms of the rela-tive importance of the single inventory param-eters (figure 1). By contrast, cumulative energydemand as a stand-alone indicator was not ap-propriate, as only the credits of the fossil-fuelsubstitution were assessed.

With respect to the distillation model, the sol-vent recovery contributed the most to the totalenvironmental impact (Isolv), followed by residuetreatment (Ires), use of ancillaries (Ianc), anduse of steam (Isteam) in the best-case scenario(figure 2). In the worst-case scenario, environ-mental impacts arising from the residual treat-ment (Ires) might become the most importantinventory parameter, depending on the impactassessment method chosen. The environmentalimpact arising from the use of electricity (Iel), ni-trogen (IN2), the waste-water treatment (Iww),and the emissions of outlet air (Iair) were ofminor importance, except for in the worst-casescenario of outlet air assessed with UBP’97. Inthis scenario, it was assumed that the outlet air,containing nonmethane volatile organic carbon(NMVOC), was directly emitted to the atmo-sphere.

The results according to eco-indicator 99 andUBP’97 correlated well (figure 2). The globalwarming potential and the cumulative energydemand were suitable indicators of environmen-tal impact, under the conditions that outlet airwas treated thermally and, with respect to thecumulative energy demand, that no nitrogen-containing distillation residuals were incineratedin cement kilns. All results shown in figure 1 andfigure 2 are in accordance with the relative im-portance of the single inventory parameters pre-sented in Supplementary Table S2 on the Web.

Technology-Specific Assessment

On the basis of the results of the sensitiv-ity analysis, the total environmental impacts ofthe hazardous waste incinerator (Iwsinc), the ce-ment kiln (Icement), and the distillation (Idist)were calculated according to equations (1), (2),and (3) (figure 3). The comparison of the threetechnologies shows that with all impact as-sessment methods, the ranges of potential en-vironmental impacts overlapped for the three



Figure 3 Potential total environmental impact of the three technologies. With respect to distillation, resultsare shown for the organic residue treatment in both the hazardous waste incinerator (incinerator) andcement kilns (cement).

technologies. Therefore, no single waste-solventtreatment technology was, in general, environ-mentally superior to any other. This finding im-plies that conclusions about optimal treatmentoptions must be drawn on a more detailed level:for instance, as a function of solvent properties ortechnology specifications.

Solvent-Specific Assessment

An initial assessment attempted to determinefor which of the 45 solvents distillation was,in general, environmentally superior to inciner-ation. To this end, the environmental impactof distillation and incineration was calculated



Figure 4 Environmental impact as a function of solvent recovery considering the mixture ofmonochlorobenzene and pentane as an example.

as a function of solvent recovery. With increas-ing content and, therefore, recovery of the maincomponent, distillation improved environmen-tally. The environmental impact of incineration,by contrast, increased because the amount of thesecond component, which showed optimal resultsin the incineration models, decreased. With thisprocedure, we determined solvent recoveries atwhich distillation and incineration had the sameenvironmental impact. Hereby, the scenarios ofminimum, average, and maximum environmen-tal impact of distillation (table 2) were comparedto the scenario of minimal environmental impactof incineration.

Figure 4 illustrates the comparison of distil-lation and hazardous waste incineration consid-ering the mixture of monochlorobenzene (maincomponent) and pentane as an example. In thebest-case scenario, distillation was generally thesuperior treatment option. Also, with regard tothe average distillation scenario, distillation be-came environmentally superior at a low solventrecovery (0.48 kg/kg waste solvent; point B infigure 4). When the monochlorobenzene recov-

ery exceeded 0.85 kg/kg waste solvent, distilla-tion was generally the environmentally superiortreatment option, even in the worst-case scenario(point A in figure 4).

The procedure shown in figure 4 was per-formed for all 45 waste-solvent mixtures and thefour impact assessment methods: eco-indicator99, cumulative energy demand, UBP’97, andglobal warming potential. Table 3 shows the re-sults that are in accordance with the outcome ofall of the four impact assessment methods. Forinstance, the value of 0.67 of acetic anhydride intable 3 indicates that distillation was better thanincineration under all circumstances, for all im-pact assessment methods and all secondary com-ponents, if the recovery was higher than or equalto 0.67 kg/kg waste solvent. Detailed results arepresented in Supplementary Tables S4, S5, S6,and S7 on the Web for all impact assessmentmethods specifically.

The second assessment attempted to deter-mine the solvents for which incineration wasgenerally the environmentally superior treat-ment option, for all scenarios and choices of the



secondary component. In contrast to distillation,no solvents were found for which this was thecase, although incineration may be better underspecific circumstances.

DiscussionThe aim of the environmental assessment pre-

sented in this work is to facilitate decision makingin chemical industries to reduce environmentalburdens that are associated with the use of organicsolvents. To implement environmental improve-ments, decision makers need quick and easily ap-plicable decision support, which is, in many cases,difficult to obtain with a complex method, such asLCA (Lifset 2006). In the present work, however,such convenient rules of thumb and recommen-dations were derived (see figures 1, 2, and 3 aswell as table 3). These results help to implementindustrial ecology in practice.

The technology-specific assessment showedthat with all impact assessment methods theranges of potential environmental impact over-lapped. Thus, no single waste-solvent treatmentoption was generally environmentally superior toany other. However, two general tendencies canbe identified.

First, the waste-solvent treatment in cementkilns was generally environmentally superior tothe hazardous waste incinerators, because thesubstituted fossil fuels in the cement kilns alsocontain impurities that cause emissions (sulphur,nitrogen, metals) and because the substitutionof coal reduces CO2 emissions, as the ratio ofcarbon content and heating value is higher forcoal (Seyler et al. 2004) than for organic sol-vents. The hazardous waste incinerator, con-versely, is subject to stricter regulations in termsof emission limits, and, therefore, more ancillar-ies are needed to fulfill these regulations, such assodium hydroxide in the scrubber (Seyler et al.2005).

Second, distillation tended to be the environ-mentally superior treatment option for the major-ity of the investigated solvents: When we consid-ered average distillation conditions (see table 3)and average solvent recovery of 0.71 kg/kg wastesolvent (Capello et al. 2005), distillation turnedout to be environmentally superior to incinera-tion in hazardous waste incinerators for 25 outof the 45 solvents. Under best-case conditions

and a good solvent recovery rate of 0.84 kg/kgwaste solvent (Capello et al. 2005), this was eventhe case for 38 solvents (table 3). In Switzerland,most of the incinerated waste solvent is treated inhazardous waste incinerators or in similar incin-erators (approximately 120,000 tonnes per yearestimated for 2002 [Seyler et al. 2006], comparedto 30,000 tonnes per year in cement kilns in 2002[Cemsuisse 2003]) due to more restrictive leg-islation for cement kilns and due to transportoutside chemical production sites. The compar-ison of distillation and hazardous waste inciner-ation is therefore of higher practical relevance.Thus, the finding that in many cases distillationwas environmentally superior to the hazardouswaste incineration confirms the general wastetreatment policy of many chemical companies,namely that recycling is preferable to inciner-ation (see Safety & Environment reports, e.g.,Ciba Specialty Chemicals AG 2004; Hoffmann-La Roche AG 2002).

More specific rules of thumb can be deter-mined for subsets of solvents with similar prop-erties and certain technological conditions. In-cineration is a good treatment option for wastesolvents with high net calorific value due to highenvironmental credits in the incineration models(Ienergy or Ifuel; see figures 1 and 2). Incinerationshould not be chosen, from an environmentalperspective, if the waste solvent contains het-eroatoms, such as nitrogen, sulphur, or halogens,or if it shows large fractions of heavy metalsdue to the higher need of ancillaries (Ianc; seeFigures 1 and 2) and the emissions to air (INOx,Iem; see figures 1 and 2).

With regard to distillation, the same sol-vent properties also turned out to be important,because the residue treatment in incinerationinfluenced the environmental impact (Ires; seefigures 1 and 2). In contrast to incineration, theenvironmental impact of distillation is mainlydetermined by the credits of solvent recovery(Isolv; see figures 1 and 2). Therefore, distilla-tion is the environmentally optimal treatmenttechnology when the distillation process is con-ducted with high solvent recovery and whenhighly elaborated solvents are recovered thatshow high environmental credits for the avoid-ance of virgin solvent production. Both con-tributions affect the environmental assessment



Tabl

e3

Am

ount

sof

reco

vere

dso

lven

tbe

yond

whi

chdi

stilla

tion

isge

nera

llyth

een

viro

nmen

tally

supe

rior

trea

tmen

tte

chno

logy

toin

cine

ratio

n

Dist

illat

ion

issu

perio

rto

haza

rdou

sD

istill

atio

nis

supe

rior

tow

aste

inci

nera

tor

ata

solv

entr

ecov

ery

ofce

men

tkiln

ata

solv

entr

ecov

ery

ofSo

lven

t(kg

/kg

was

teso

lven

t)C

AS-

no.

Wor

st-c

ase

dist

illat

ion

Ave

rage

dist

illat

ion

Bes

t-ca

sedi

still

atio

nW

orst

-cas

edi

still

atio

nA

vera

gedi

still

atio

nB

est-

case

dist

illat

ion

Ace

tic

acid

64-1

9-7

No

0.48

Alw

aysa

No

No

0.87

Ace

tic

anhy

drid

e10

8-24

-70.

67A

lway

saA

lway

saN

o0.

780.

59A

ceto

ne67

-64-

1N

o0.

61A

lway

saN

oN

oN

oA

ceto

nitr

ile75

-05-

80.

800.

45A

lway

saN

oN

oN

oB

enza

ldeh

yde

100-

52-7

0.84

0.32

Alw

aysa

No

No

0.73

Ben

zyla

lcoh

ol10

0-51

-60.

880.

41A

lway

saN

oN

oN

oB

utan

ol(1

-)71

-36-

3N

o0.

950.

58N

oN

oN

oB

utan

ol(2

-)78

-92-

2N

o0.

740.

44N

oN

oN

oB

utan

ol(I

so)

78-8

3-1

No

0.81

0.46

No

No

No

But

ylac

etat

e12

3-86

-40.

770.

44A

lway

saN

oN

o0.

79B

utyl

engl

ycol

110-

63-4

0.51

Alw

aysa

Alw

aysa

No

0.70

0.45

Cyc

lohe

xane

110-

82-7

No

No

0.85

No

No

No

Cyc

lohe

xano

ne10

8-94

-10.

820.

43A

lway

saN

oN

o0.

77D

ichl

orom

etha

ne75

-08-

20.

87A

lway

saA

lway

saN

o0.

940.

78D

ieth

ylet

her

60-2

9-7

No

No

No

No

No

No

Dim

ethy

lform

amid

e68

-12-

20.

830.

40A

lway

saN

oN

o0.

83D

ioxa

ne12

3-91

-10.

890.

45A

lway

saN

oN

o0.

86Et

hyla

ceta

te14

1-78

-60.

770.

40A

lway

saN

oN

o0.

83Et

hano

l64

-17-

5N

oN

o0.

54N

oN

oN

oEt

hylb

enze

ne10

0-41

-4N

oN

o0.

69N

oN

oN

oFo

rmal

dehy

de50

-00-

0N

oN

oN

oN

oN

o0.

94Fo

rmic

acid

64-1

8-6

0.69

0.32

Alw

aysa

0.89

0.72

0.53

Hep

tane

142-

82-5

No

No

No

No

No

No

Hex

ane

(n-)

110-

54-3

No

No

No

No

No

No

Hex

ane

(Iso

)96

-14-

0N

oN

o0.

57N

oN

oN

oIs

oam

ylac

etat

e62

8-63

-70.

780.

44A

lway

saN

oN

o0.

74



Tabl

e3

Con

tinue

d

Dist

illat

ion

issu

perio

rto

haza

rdou

sD

istill

atio

nis

supe

rior

tow

aste

inci

nera

tor

ata

solv

entr

ecov

ery

ofce

men

tkiln

ata

solv

entr

ecov

ery

ofSo

lven

t(kg

/kg

was

teso

lven

t)C

AS-

no.

Wor

st-c

ase

dist

illat

ion

Ave

rage

dist

illat

ion

Bes

t-ca

sedi

still

atio

nW

orst

-cas

edi

still

atio

nA

vera

gedi

still

atio

nB

est-

case

dist

illat

ion

Isob

utyl

acet

ate

110-

19-0

0.75

0.43

Alw

aysa

No

No

0.80

Isop

ropy

lace

tate

108-

21-4

0.84

0.42

Alw

aysa

No

No

0.89

Met

hano

l67

-56-

1N

oN

o0.

64N

oN

oN

oM

ethy

lace

tate

79-2

0-9

No

0.76

0.40

No

No

No

Met

hylc

yclo

hexa

ne10

8-87

-2N

o0.

650.

36N

oN

oN

oM

ethy

leth

ylke

tone

78-9

3-3

No

0.88

0.54

No

No

No

Met

hylf

orm

ate

592-

84-7

0.94

0.47

Alw

aysa

No

No

0.82

Met

hyli

sobu

tylk

eton

e10

8-10

-10.

39A

lway

saA

lway

saN

o0.

700.

53M

onoc

hlor

oben

zene

108-

90-7

0.85

0.48

Alw

aysa

No

No

No

MT

BE

1634

-04-

4N

oN

oN

oN

oN

oN

oPe

ntan

e10

9-66

-0N

oN

oN

oN

oN

oN

oPe

ntan

ol71

-41-

00.

970.

600.

36N

oN

oN

oPr

opan

ol(1

-)71

-23-

80.

740.

38A

lway

saN

oN

o0.

92Pr

opan

ol(I

so)

67-6

3-0

No

0.81

0.40

No

No

No

Prop

iona

ldeh

yde

123-

38-6

0.82

0.48

Alw

aysa

No

No

0.96

Ter

t-am

ylal

coho

l75

-85-

4N

o0.

870.

52N

oN

oN

oT

etra

hydr

ofur

an10

9-99

-90.

41A

lway

saA

lway

sa0.

840.

530.

32T

olue

ne10

8-88

-3N

oN

o0.

46N

oN

oN

oX

ylen

e13

30-2

0-7

No

No

0.39

No

No

No

Not

e:C

AS-

no.=

CA

Sre

gist

rynu

mbe

r,a

uniq

uenu

mer

icid

enti

fierf

orch

emic

alsu

bsta

nces

;MT

BE

=M

ethy

lter

t-bu

tyle

ther

.a D

isti

llati

onis

envi

ronm

enta

llysu

peri

oral

sow

ith

am

inim

also

lven

trec

over

yof

0.31

kg/k

gw

aste

solv

ent(

Cap

ello

etal

.200

5).



substantially: With the increase from an aver-age solvent recovery of 0.71 kg/kg waste solventto a good solvent recovery of 0.85 kg/kg wastesolvent (Capello et al. 2005), the number of sce-narios in which distillation is the environmen-tally superior treatment technology increases by16 (hazardous waste incinerator) and 12 (cementkiln), respectively (table 3). These results showthat the solvent recovery is a key parameter in theenvironmental assessment. This finding is alsoin accordance with the results of the sensitivityanalysis. With regard to the number of environ-mental credits for the avoidance of virgin sol-vent production, even single production steps inpetrochemical manufacturing can influence theoutcome of the environmental assessment drasti-cally. For example, due to the additional environ-mental impact of the esterification of isobutanolto isobutyl acetate and of isopropanol to isopropylacetate, distillation under worst-case conditions(hazardous waste incinerator) and best-case con-ditions (cement kiln) becomes the environmen-tally superior treatment options at high solventrecovery. Similarly, the environmental impact ofthe isomerization of n-hexane to isohexane alsomakes distillation under best-case conditions en-vironmentally superior to the hazardous waste in-cinerator at a solvent recovery of 0.57 kg/kg wastesolvent (table 3).

Specific recommendations can be made withregard to the solvents acetic anhydride, butyleneglycol, dichloromethane, formic acid, methylisobutyl ketone, and tetrahydrofuran. For thesesolvents, distillation turned out to be the envi-ronmentally favorable treatment technology inmost cases: Compared to the hazardous wasteincinerator, distillation was the superior treat-ment technology at almost minimal solventrecoveries, even if the average scenario wasconsidered. Also, an average distillation at agood solvent recovery was environmentally su-perior to incineration in cement kilns. Ei-ther these solvents receive high credits forthe solvent recovery because of elaborate pro-duction processes (acetic anhydride, butyleneglycol, methyl isobutyl ketone, tetrahydrofuran[Stoye 2000]) or, in the case of formic acid,the low net calorific value (4.6 MJ/kg [Yaws1999]) only leads to minimal environmentalcredits in the incineration models. In the case

of dichloromethane, the combination of both isdeterminant.

The solvent-specific assessment revealed thatthere is no specific solvent for which inciner-ation is generally the environmentally superiortreatment technology. For the solvents cyclo-hexane, ethanol, ethyl benzene, formaldehyde,iso-hexane, methanol, toluene, and xylene, dis-tillation was only generally superior when we as-sumed the best-case scenario and high solventrecovery. Furthermore, with regard to heptane,hexane, methyl tert-butyl ether (MTBE), andpentane, incineration showed a comparable en-vironmental impact to distillation under best-case conditions. In cases in which the distillationis conducted under nonfavorable conditions—especially if the solvent recovery is low and onlyassociated with small environmental credits (e.g.,methanol) or if entrainer is required to separateazeotropic mixtures—distillation is not necessar-ily superior to incineration, in particular not com-pared to treatment in cement kilns. With regardto the aromatic and aliphatic solvents, this find-ing is, on the one hand, related to the fact thatthese solvents lead to high environmental creditsin the incineration due to their high net calorificvalues (>40 MJ/kg; Yaws 1999). The recoveryof ethanol, methanol, and formaldehyde, on theother hand, accounts for low environmental cred-its in the distillation because their petrochemi-cal manufacturing requires only a few productionsteps (Stoye 2000). The case of MTBE is anotherone in which the combination of both is deter-minant. Therefore, researchers should investigatein detail the treatment of waste-solvent mixturescontaining aliphatic or aromatic solvents for thespecific mixture (e.g., with the ecosolvent tool)to determine the environmentally superior treat-ment option.

In addition to these recommendations, the re-sults presented in table 3 are of general interestto practitioners in the chemical industry becausethey can be used for a quick checkup. On the onehand, they can be used to analyze already operat-ing distillation processes when process conditionsand the solvent recovery are known to deter-mine whether distillation was the environmen-tally preferable choice. On the other hand, theseresults can also be used in the stage of productdevelopment at which little information about



Figure 5 Methodological framework to obtain recommendations on the environmentally optimizedwaste-solvent treatment in the chemical industry. (a)methyl isobutyl ketone, (b)tetrahydrofuran, (c)net calorificvalue, (d)distillation should be conducted in a continuous mode, if possible.

solvents is available. These results are particu-larly useful for experts in the field of distillation,because these individuals have the experience toestimate the amount of recovered solvent to beexpected without much effort.

Finally, to provide an easily usable instru-ment for decision makers in chemical industries,we present all the results of this article, com-bined with the ecosolvent tool presented in part I(Capello et al. 2007), structured in a clearly ar-ranged framework (figure 5). With the frameworkpresented in figure 5, precise recommendationson the environmentally superior treatment tech-nology can be made in many cases. If the wastesolvent contains as the main component aceticanhydride, butylene glycol, dichloromethane,formic acid, methyl isobutyl ketone (MIK), ortetrahydrofuran (THF), distillation is environ-

mentally superior than incineration even at verylow solvent recoveries (see table 3). When infor-mation on the amount of recovered solvent in adistillation process is known, the threshold valuespresented in table 3 may be sufficient for a preciserecommendation. Otherwise, the ecosolvent toolshould be used. In case no significant result is ob-tained with the ecosolvent tool, more informationon the amount of recovered solvent, energy andancillaries, and use of the distillation and incin-eration technology should be gathered to reducethe uncertainty. Nevertheless, in some cases, thedifferences between the treatment technologieswill not become significant. In such cases, or if themixture is composed of solvents other than the45 solvents we investigated, the general rules ofthumb help to identify the treatment technologythat tends to be environmentally favorable.



Acknowledgments

We gratefully acknowledge the Swiss FederalOffice of Energy (Project No. 100065), Ciba Spe-cialty Chemicals AG, Ems-Dottikon AG, LonzaGroup Ltd., Novartis Pharma AG, Hoffmannn-La Roche AG, and Siegfried Ltd. for their fundingof this project.

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About the Authors

Christian Capello was a PhD student in en-vironmental sciences at the time the article was

written. Currently, he works as a researcher in theSafety and Environmental Technology Group,ETH Zurich, Zurich, Switzerland. StefanieHellweg was a senior researcher in the Safety andEnvironmental Technology Group, ETH Zurich,and is now professor of ecological systems designat the Institute of Environmental Engineering,ETH Zurich. Konrad Hungerbuhler is a profes-sor at the Safety and Environmental TechnologyGroup, ETH Zurich.


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