Experiences in Designing Solvents for the Environment

Experiences in Designing Solvents for the Environment

Meirong Li,† Paul F. Harten,* and Heriberto Cabezas

U.S. Environmental Protection Agency, National Risk Management Research Laboratory,Sustainable Technology Division, Industrial Multimedia Branch, 26 West Martin Luther King Drive,Cincinnati, Ohio 45268

Solvents used throughout industry are chosen to meet specific technological requirements suchas solute solubility, cleaning and degreasing ability, or utility as a medium for paints andcoatings. With the increasing awareness of the human health effects and environmental risksof solvent use, the replacement of current solvents with more benign alternatives has become anecessity. In this paper, we initially outline the elements of a solvent design theory, and on thebasis of this theory, we then discuss the PARIS II (Program for Assisting the Replacement ofIndustrial Solvents) solvent design algorithms and software. The PARIS II software designstechnologically effective and environmentally benign substitute solvents. We finally present anddiscuss several cases of solvent substitutes developed using PARIS II. These cases includesubstitutes for pure chemicals and mixtures involving components from a wide range of chemicalfamilies, including normal hydrocarbons, ketones, alcohols, aromatics, and organic and aqueousmixtures. These case studies show that replacement solvents can be found with consistentlylower potential environmental impacts while maintaining similar levels of technical performance.

1. Introduction

Solvents are widely used in modern life activities,such as painting, extracting, and cleaning. With theincreasing awareness of the health and environmentalconsequences of using solvents, efforts have been un-dertaken to reduce or eliminate the use of harmfulsolvents. These efforts have, in many cases, been drivenby new regulations and international treaties. The 1987Montreal Protocol is the first international declarationaimed at reducing and eliminating the use of somechemicals and solvents to protect all living beings onthe earth. Two main kinds of techniques for reducingor eliminating harmful solvents have been pursued. Oneis to find alternative technologies that do not usesolvents; the other is to find alternative solvents, alsocalled solvent substitution. Unfortunately, the newsolvent-free technologies developed according to the firsttechnique always require the design and use of newprocesses, new equipment, and new operations. Solventsubstitution is preferable because it can often useexisting processes with minor or no modifications.However, the solvent substitution technique presentsa significant technical difficulty: the design of substitutesolvents that are as efficient as the current solvents butless risky to human health and the environment.

Within solvent substitution methodologies, are thethree following categories: searching solvent databasesfor single-chemical substitutes, designing new chemi-cals, and designing mixture substitutes. In the firstcategory of searching for single-chemical replacements,the basic requirement is a solvent database. Then, usingspecified desired values and allowed ranges for eachproperty, one can screen the database. Only chemicalsmeeting all property requirements are considered to be

candidate replacement solvents. The representativework in this category is that of Joback.1 However,database solvent searches do not always find single-chemical substitutes because solvent databases containonly limited numbers of chemicals and might notcontain a chemical that can meet the specified propertyrequirements.

In the second category, the user designs new chemi-cals to meet specified requirements. The basic work inthis category is molecular design, determining thestructure of feasible chemical compounds that exhibitthe specified properties. This approach is very usefulwhen no single-chemical replacements can be found.The representative research groups in this category arethose of Gani2-5 and Joback.6 The third category isdesigning mixture replacements if no single-chemicalreplacement can be found. The basic problem in thiscategory is to select adequate components and todetermine the mixture composition. Because of theproblem complexity, computers and software tools areneeded in this category. The representative efforts inthis category are those of Klein7 and Cabezas.8-10

The PARIS II methodology encompasses the first andthird categories. This means that PARIS II can searchfor single-chemical replacements and design mixturesolvents. In this paper, the authors briefly introduce thesolvent design theory and computer algorithm used inPARIS II. Then, several cases involving several chemicalfamilies and pure components and mixtures are dis-cussed to illustrate the use of PARIS II. The chemicalfamilies include normal hydrocarbons, ketones, alcohols,aromatics, and organic and aqueous mixtures. As ascreening tool, PARSI II can narrow the list of solventreplacement candidates, which significantly reduces thetime and cost of finding and designing replacementsolvents. The suggested replacement solvents in thesecases might or might not work as replacements forspecific applications. Experimental verification is neededbefore their actual implementation.

* Corresponding author. Fax: 513-569-7471. E-mail:[email protected].

† Research Associate, National Research Council.

5867Ind. Eng. Chem. Res. 2002, 41, 5867-5877

10.1021/ie010574s CCC: $22.00 © 2002 American Chemical SocietyPublished on Web 10/22/2002

2. Solvent Design Theory

Solvent substitution is an attractive technology be-cause of the economic advantages of not having toreplace equipment or processes and the ease of imple-mentation as discussed below. However, these advan-tages of solvent substitution create at least threesignificant technical challenges: (1) the need to findsubstitutes as effective as currently used solvents; (2)the need to use existing processes, equipment, andoperations; (3) the simultaneous need to have lessimpact on human health and the environment. To meetthese challenges, substitute solvents should: (1) matchthe performance and operational properties of currentsolvents for existing processes and equipment, (2) matchthe functional properties of current solvents to be aseffective as current solvents, and (3) improve on theenvironmental properties of current solvents to satisfythe need for better environmental behavior. In addition,there needs to be some way to compare possible solventreplacements to choose one solvent over another.

2.1. Performance and Operational Properties.For any solvent, the behavior and effectiveness aredetermined by many properties rather than any singleproperty. Therefore, multiple physical properties ratherthan any one single physical property should be con-sidered in designing substitute solvents. The perfor-mance and operational properties used in PARIS IIinclude molecular mass, liquid density, boiling temper-ature, vapor pressure, surface tension, viscosity, ther-mal conductivity, and flash point.

Molecular Mass. Molecular mass is the sum of themolecular weight of all atoms in a molecule. Molecularmass is not directly related to solvent performance oroperation, but a variety of properties are known to bepartly correlated with molecular mass. Therefore, thisproperty can be useful in the design of substitutesolvents.

Liquid Density. Matching density is necessary toavoid changes in capacity and/or transport in processesthat use solvents. In addition, when solvents are usedfor extraction operations, the density difference betweenliquid phases is necessary for both stage-wise andcontinuous-contact equipment operation. The density ofsolvent mixtures is approximately estimated by a mole-fraction-weighted sum of the pure-component densities.8

Boiling Temperature. The boiling point of a sub-stance is the temperature at which its vapor pressureis equal to the external pressure. For a pure liquidsubstance, the boiling point is a single temperature. Fora mixture, the boiling process begins at the mixturebubble point and ends at its dew point, so the mixtureboils in a temperature range rather than at a singletemperature. In the design of substitute solvents, themixture bubble point should be treated as the boilingtemperature of the mixture. In our work, we use theboiling point of pure components calculated from DIP-PR11 correlations and data, and we compute the bubbletemperature of mixtures according to standard thermo-dynamic approaches8 using the UNIFAC method3,12 toestimate activity coefficients.

Vapor Pressure. The vapor pressure is the pressureexerted by a saturated vapor in equilibrium with thesaturated liquid. Solvents with higher vapor pressuresevaporate more easily. The vapor pressure of a solventmixture depends on the components, composition, andtemperature. We compute the vapor pressures of pure

components from DIPPR11 correlations and data andthose of mixtures according to standard thermodynamicapproaches8 using the UNIFAC method3,12 to estimateactivity coefficients.

Surface Tension. The cohesive force at a liquidsurface is called the surface tension. Matching surfacetension is important in terms of matching wettingcharacteristics, and settling times, as well as for otherreasons. For example, a liquid with a high surfacetension will not wet a surface or readily mix withanother liquid. The surface tension of a solvent mixturedepends on the components, composition, and temper-ature. We estimate the surface tensions of pure com-ponents from DIPPR11 correlations and data and thoseof mixtures from the MacLeod-Sudgen correlation.8,13,14

Viscosity. All fluids exhibit a definite resistance tochange of form. This property, a sort of internal friction,is called viscosity. Matching viscosity is necessary toavoid changes in transport power in the processes orfunction requirements. For example, a lower viscosityis preferred in handling because less power is requiredto pump liquids. However, in applications requiringadhesive properties, a higher viscosity might be prefer-able. Liquid mixture viscosities depend on temperature,pressure, and composition. We estimate pure-componentviscosities from DIPPR11 correlations and data andpredict liquid mixture viscosities from the group con-tribution method of Cao et al.15

Thermal Conductivity. Thermal conductivity is thetime rate of transfer of heat by conduction through aunit thickness across a unit area for a unit differencein temperature. A high thermal conductivity is desirablein applications where rapid heat transfer through thesolvent is important. On the other hand, if the solventis part of an insulator, then a low thermal conductivityis desirable. We estimate pure-component thermalconductivities from DIPPR11 correlations and data andmixture thermal conductivities from the method of Li.16

Flash Point. The flash point is the lowest tempera-ture at which a liquid gives off sufficient vapor to forman ignitable mixture with air near the surface of theliquid. The flash point is one of the most important firesafety characteristics of solvents. We determine theflash points of pure components from DIPPR11 correla-tions and data, and we compute the flash points ofmixtures using a modification of the method of Wal-sham.17

2.2. Functional Properties. Matching the physicalproperties of solvents alone might not always yieldsubstitutes that are as effective as the solvents theyreplace. This is true because these physical propertiesdo not directly represent the molecular interactionsbetween solvent and solute. These interactions arecaptured by the infinite-dilution activity coefficients ofdifferent representative solutes in the solvents.

Infinite-Dilution Activity Coefficient. The infinite-dilution activity coefficient is the limit of the activitycoefficient as the concentration of a solute approaches0. We use 10 representative solutes chosen from differ-ent chemical families: ethanol, diethyl ether, acetone,water, benzene, cis-2-heptene, n-propyl chloride, n-heptadecane, n-propylamine, and dimethyl disulfide.Therefore, the infinite-dilution activity coefficient rep-resents the molecular interactions between one moleculeof each of the specified 10 representative solutes andthe molecules in the solvent. The selected solvents are

5868 Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002

representative of 10 different types of molecular struc-tures and interactions. This set of 10 chemicals is merelyrepresentative and convenient, and a different set ofrepresentative chemicals would likely work equally well.The infinite-dilution activity coefficient is defined by

where γi∞ is the activity coefficient of component i at

infinite dilution, γi is the activity coefficient of compo-nent i, and xi is the mole fraction of component i. Wecompute infinite-dilution activity coefficients by extrapo-lation using the UNIFAC method3,12 to estimate activitycoefficients.

2.3. Environmental Properties. A main objectiveof solvent substitution is to reduce the environmentalimpact of current solvents. A necessary condition toachieve this objective is the ability to compare differentsolvent formulations in term of their potential environ-mental impacts. For each solvent considered, includingany solvent being replaced, two indexes, an environ-mental index and an air index, are used. To calculatethese indexes, a database with human- and environ-mental-impact data across eight categories for each ofthe 1635 chemicals in the DIPPR database is used. Thechemical environmental impact database is based on thework of Young and Cabezas.18,19

Environmental Index. The environmental index(Ψi) is a composite measure of the relative potentialimpact of a chemical or mixture on human health andthe environment. This includes eight environmentalimpact categories: human toxicity by ingestion, humantoxicity by dermal/inhalation exposure, ecological aquatictoxicity, ecological terrestrial toxicity, photochemicaloxidation, acidification, ozone depletion, and globalwarming. For a pure chemical i, Ψi is calculatedaccording to

where Ψi is the environmental index of chemical i, thesummation is performed over the eight categories ofimpacts, Rj is the user-assigned weight of impact jindependent of chemical i, and æj ij is the normalizedimpact score of chemical i for impact category j.

For a mixture, a weight-fraction-averaged index iscalculated from

where Ψm is the mixture environmental index formixture m, Wi is the weight fraction of chemical i inthe mixture, the summation is performed over allchemicals present in the mixture, and all other symbolshave their previously assigned meanings.

Air Index. The air index is the overall relativemeasure of the potential impact of a chemical or mixtureof chemicals mediated through the air on human healthand the environment. The air index is calculated bymultiplying the environmental index of each chemicalby its fugacity in the liquid phase. The fugacity givesan estimate of the tendency of a chemical to vaporize.For a pure chemical, the air index is given by

where Piv is the vapor pressure of component i, Ψi is

the environmental index of component chemical i asgiven by eq 2, and P is the pressure at which the solventis being used.

For a solvent consisting of a mixture of componentchemicals, the mixture air index is given by

where xi is the mole fraction of chemical component i,γi is the activity coefficient of chemical component i, Pi

v

is the vapor pressure of chemical component i, Ψi is theenvironmental index of component chemical i as givenby eq 2, Mi is the molecular mass of component chemicali, and P is the pressure at which the solvent is beingused.

2.4. Ranking Solvent Replacements. The previ-ously discussed properties of a solvent can be viewedas a 19-dimensional property. Seven dimensions areperformance and operational properties, 10 dimensionsare functional properties, and 2 dimensions are envi-ronmental properties. These 19 characters represent allof the properties discussed in the preceding subsectionsof Solvent Design Theory. Any existing solvent can thenbe represented as a point in this 19-dimensional prop-erty space. In our efforts at finding a replacement for asolvent, we search for solvents (or points) that are asclose as possible to the original solvent (or point), buthave zero environmental impact values in the environ-mental dimensions. Distances between points in this 19-dimensional property space can be calculated using anormalized Euclidian metric. In PARIS II, we have used

where doi is the difference between the desired solvento and possible replacement solvent i, and Sj

o and Sji are

the solvent’s jth coordinate in property space. In eachdimension of property space, the difference is normal-ized by the tolerance range. Using this measure, we cancompare and rank all possible solvent replacements. Ina similar way, this same measure can be extended todescribe distances between solvent mixtures in propertyspace.

3. PARIS II Computer Algorithm

The PARIS II computer algorithm consists of ninemajor steps embodied in nine different screens in theuser interface. However, this is a complex program andthe nine screens correspond roughly but not exactly tothe nine steps. Figure 1 briefly shows the computeralgorithm, and we describe each step of the algorithmas below.

Step 1. Define the Current Solvent System. Theuser inputs the current solvent composition and theoperating temperature and pressure at which solvent

γi∞ ) lim

xif0γi(t,x) (1)

Ψi ) ∑j)1

8

Rjæj ij (2)

Ψm ) ∑i

WiΨi ) ∑i

Wi∑j)1

8

Rj æj ij (3)

Ψiair )

PivΨi

P(4)

Ψmair )

∑i

xiγiPivΨiMi

P∑j

xjMj

(5)

doi ) ∑j)1

19

|Sjo - Sj

i| (6)

Ind. Eng. Chem. Res., Vol. 41, No. 23, 2002 5869

is used. The operating conditions are used to ensure thatonly liquid chemicals are used as replacement solventsand to adjust physical properties. The user can chooseup to nine chemicals as components of the currentsolvent by searching family, name, or CAS number.

Step 2. Assign Weights to Environmental ImpactCategories. The user assigns a weight to each of theeight environmental impact categories to suit the par-ticular application. A higher weight is assigned to moreimportant categories, and a lower weight is assigned toless important categories. PARIS II assigns a defaultweight of 5 to all of the categories as a starting point.

Step 3. Calculate the Properties of the CurrentSolvent. Using the information entered at steps 1 and2, PARIS II calculates the physical, environmental andfunctional properties of the solvent to be replaced.

Step 4. Assign the Target and Tolerance Rangesfor the Replacement Solvent’s Properties. The userassigns a target and a tolerance range (a desired value,an upper bound, and a lower bound) for each property.The user can assign only a desired value, an upperbound, and a lower bound for environmental propertiesbut not the tolerance. The user can assign only a lowerbound for the flash point. A default set of targets based

Figure 1. Flowchart for PARIS II computer algorithm.


on the properties of the solvent being replaced isassigned using PARIS II. Also, a default set of tolerancesbased on the distribution of each property in thedatabase is assigned as a start.

Step 5. Search for Single-Chemical Replace-ments. PARIS II first searches for single-chemicalreplacements. If several single-chemical replacementsare found, then these replacements are ranked. Userscan view the properties of the suggested replacementsand compare these properties to those of the currentsolvent. At this step, the user can accept one of thereplacements or go back to step 4 to revise the currentsearch by modifying the tolerance ranges, possiblyfinding different single-chemical replacements. Whensingle-chemical replacements are found, PARIS II willnot go further to design mixture replacements.

Step 6. View Candidate Single-Chemical Re-placements. If no single-chemical replacements arefound, a ranked list of candidates is displayed. Theseare chemicals that do not meet all of the requirementsspecified in step 4. The user can compare the propertiesof each candidate, view the property violations, anddecide to accept a replacement on the basis of his/herknowledge and experience. The user can also go backto step 4 to widen the tolerance ranges, or the user cango to the next step to continue with the design of amixture replacements for the current solvent.

Step 7. Select Candidates for Mixture Replace-ments. The user can select the primary component fora mixture replacement. PARIS II is designed to try tofind a mixture replacement with the least number ofcomponents, so the user can add only one chemical at atime. After the primary component is chosen, PARIS IIreranks all of the chemicals to complement the alreadychosen primary component chemical. The user can selectany chemical from the ranked list of candidates to themixture.

Step 8. Search for Mixture Replacements. PARISII attempts to find the best proportions for the selectedmixture. If a successful match is found, then the usercan review the successful replacements. If there areseveral successful replacements, these replacements areranked, and up to five replacements are shown. Thereplacement properties are also displayed. At this step,the user can exit the program to begin a new search orrefine the search by changing the tolerance rangesspecified in step 4. If no successful replacement is found,the ranked unsuccessful replacement mixtures areshown. The user can review the properties of theunsuccessful replacements and check the property toler-ance ranges that were violated. The user can againdecide to accept a replacement on the basis of his/herknowledge and experience.

Step 9. Select New Candidates for MixtureReplacement. If no successful mixture is found, it isnecessary to return to step 7 to either change thecandidate chemicals or add additional chemicals to themixture. Steps 7-9 are repeated until a successfulreplacement is found.

4. Solvent Design

To illustrate the use of the solvent design theory andthe PARIS II computer algorithm discussed above,several examples for different cases are discussed in thissection. We use these examples as illustrations of theuse of solvent design theory and the PARIS II algorithm

for the case of general applications using default toler-ances and property targets. The resulting substitutesolvent designs might or might not work as replace-ments for specific applications, but they would, ingeneral, mimic the behavior of the original solvent. Wealso advise that experimental verification of any com-puter-generated results is needed before actual imple-mentation.

Six examples that cover single-component and mix-ture solvents are discussed in this section. The resultsare given in Tables 1-6. For single-component solvents,n-nonane, methyl ethyl ketone, 1-butanol, and benzeneare the representative solvents for the four differentchemical families normal hydrocarbon, ketones, alco-hols, and aromatics, respectively. For mixture solvents,a mixture of 50 mol % methyl ethyl ketone and 50 mol% toluene is used to represent organic mixtures, and amixture of 50 mol % water and 50 mol % 1-butanol isused to represent aqueous mixtures. The pure-compo-nent and mixture solvents used here are chosen to berepresentative of molecules with different types ofdominant molecular interactions, e.g., dispersion forcesin normal hydrocarbons, polar forces and hydrogenbonding in alcohols, π-electron forces in aromatics,hydrogen bonding in water, and double-bond dipoles.The objective here is to illustrate how it is possible tofind or design replacement solvents for different typesof solvents.

Tables 1-6 list the tolerance ranges, physical proper-ties, environmental properties, and infinite-dilutionactivity coefficients for the solvents being replaced orthe current solvent and candidate substitute solvents.The set of default tolerance ranges and property targetsare used for all examples, and the exact values for eachproperty are shown in Tables 1-6. The desired propertyvalues for solvent substitutes are set to be the propertyvalues of the current solvent. Because a higher flashpoint is generally safer, the flash point value of thecurrent solvent is considered to be the lower bound fora replacement. No tolerance range is given for flashpoints. Because lower air and lower environmentalindexes are generally better, the air index and environ-mental index of the current solvent are separatelyconsidered to be the upper bounds for the air andenvironmental indexes for the replacement solvent. Zerovalues are the desired values for the air and environ-mental indexes because lower indexes are generallypreferable. No tolerance ranges are given for the airindex and the environmental index. For particularapplications, all of these values might need to be resetfor certain requirements. In Tables 1-6, N/A means notapplicable for this requirement property, N means notviolated, and V means violated.

4.1. Replacements for Pure Solvents. NormalHydrocarbons. n-Nonane is a normal hydrocarbon andnonpolar solvent with a high environmental index. Fivesingle replacements were successfully found by PARISII. The two single replacements of 2,7-dimethyloctaneand n-butylcyclopentane, which are separately rankednumber 1 and number 5, respectively, are listed inTable 1.

Ketones. Methyl ethyl ketone (MEK) is an excellentexample of a ketone solvent that is widely used andincluded in the regulatory listing of the U.S. Environ-mental Protection Agency under the Toxics ReleaseInventory and Clean Air Act. No single replacement


could be found for MEK by PARIS II, but a mixturereplacement solvent of 75 mol % 2-pentanone and 25mol % acetone was found, which is shown in Table 2.

Alcohols. 1-Butanol was used as the example of analcohol and a polar solvent. No single replacement couldbe successfully found for 1-butanol by PARIS II. Amixture replacement solvent of 75 mol % 2-methyl-1-propanol, 15 mol % 1-pentanol, and 10 mol % 3-methyl-2-butanol was found, as shown in Table 3.

Aromatics. Aromatic solvents have at least onebenzene ring in their structures, which makes theirbehavior relatively complicated when compared withthat of normal hydrocarbons. Benzene is the simplestexample of an aromatic solvent. No single or mixturereplacement was found for benzene by PARIS II, but amixture of 70 mol % fluorobenzene, 20 mol % cyclohex-

ane, and 10 mol % 1,4-dioxane came closest, with onlythree property violations, as shown in Table 4.

4.2. Replacements for Solvent Mixtures. OrganicMixtures. A mixture of 50 mol % methyl ethyl ketoneand 50 mol % toluene was used as an example of acommon organic mixture used as a solvent. This par-ticular mixture is often used to remove coatings. Nosingle replacement could be found by PARIS II for thismixture. However, a mixture replacement of 65 mol %3-pentanone, 20 mol % ethyl acetate, and 15 mol %methyl isobutyrate was successfully found, as shown inTable 5.

Aqueous Mixtures. A mixture of 50 mol % waterand 50 mol % 1-butanol was used as an example of anaqueous mixture. No single replacement could be foundby PARIS II for this mixture, but a mixture replacement

Table 1. Replacement for n-Nonane: Tolerance Ranges, Physical Properties, Environmental Properties, andInfinite-Dilution Activity Coefficients of n-Nonane and Replacements 2,7-Dimethyloctane and n-Butylcyclopentanea

single replacements

propertytolerancerange (%) n-nonane 2,7-dimethyloctane n-butylcyclopentane

molecular mass (kg/kmol) 11 128 142 126liquid density (kg/m3) 14 714 720 781boiling temperature (K) 10 424 433 430vapor pressure (kPa) 30 0.58 0.408 0.507surface tension (kg/s2) 16 0.0224 0.0219 0.0254viscosity × 104 (kg/m‚s) 30 6.73 7.81 8.31thermal conductivity (J/m‚s‚K) 30 0.13 0.102 0.113flash point (K) N/A 304 306 305air index (impact/kg) N/A 0.158 0.0186 0.0145environmental index (impact/kg) N/A 27.6 4.62 2.91

infinite-dilution activity coefficientethanol 25 26.3 25.1 30.7diethyl ether 30 1.08 1.03 1.17acetone 28 4.81 4.55 5.29water 30 168 157 208benzene 30 1.40 1.32 1.38cis-2-heptene 30 0.97 0.946 1.01n-propyl chloride 30 1.15 1.10 1.24n-heptadecane 30 0.808 0.864 0.763n-propylamine 30 2.22 2.12 2.43dimethyl disulfide 30 5.54 5.27 5.87

a N/A means not applicable, N means not violated, and V means violated.

Table 2. Replacement for Methyl Ethyl Ketone: Tolerance Range, Physical Properties, Environmental Properties, andInfinite-Dilution Activity Coefficients of Methyl Ethyl Ketone (MEK), 2-Pentanone and a Mixture of 75 mol %2-Pentanone and 25 mol % Acetonea

propertytolerancerange (%) MEK 2-pentanone

75 mol % 2-pentanone/25 mol % acetone

molecular mass (kg/kmol) 11 72.1 86.1 V 79.1 Nliquid density (kg/m3) 14 800 803 N 800 Nboiling temperature (K) 10 353 375 N 356 Nvapor pressure (kPa) 30 12.3 4.74 V 11.5 Nsurface tension (kg/s2) 16 0.024 0.0239 N 0.0237 Nviscosity × 104 (kg/m‚s) 30 3.96 4.70 N 4.30 Nthermal conductivity (J/m‚s‚K) 30 0.145 0.142 N 0.145 Nflash point (K) N/A 267 280 N 267 Nair index (impact/kg) N/A 0.752 0.0458 N 0.177 Nenvironmental index (impact/kg) N/A 6.19 0.981 N 1.25 N

infinite-dilution activity coefficientethanol 25 2.24 2.31 N 2.27 Ndiethyl ether 30 1.46 1.27 N 1.35 Nacetone 28 1.01 1.03 N N/A N/Awater 30 7.38 8.71 N 8.06 Nbenzene 30 1.32 1.16 N 1.23 Ncis-2-heptene 30 3.36 2.63 N 2.95 Nn-propyl chloride 30 1.07 0.95 N 1.01 Nn-heptadecane 30 11.3 7.4 V 8.98 Nn-propylamine 30 1.00 0.932 N 0.962 Ndimethyl disulfide 30 0.725 0.759 N 0.741 N



of 15 mol % 2-butanol, 55 mol % water, and 35 mol %1-pentanol was found, as shown in Table 6.

5. Discussion

As shown in the above examples, it is possible to findor design replacements for both single-chemical andmixture solvents using either single chemicals or mix-tures as replacements. Typically, the simpler andweaker the molecular interactions are, e.g., dispersionforces in normal hydrocarbons, the easier it is to find areplacement, and the higher the likelihood that a single-chemical replacement can be found. Conversely, the

more complex and stronger the molecular interactions,e.g., hydrogen bonding in alcohols or water, the harderit is to find a replacement, and the lower the likelihoodthat a single-chemical replacement will be found.

5.1. Pure Solvents. n-Nonane is a nonpolar chemicalthat belongs to the n-alkane family that contains onlysingle carbon bonds. n-Alkane liquids have a relativelysimple structure, a straight long chain, among organicsolvents. Five single replacements were successfullyfound for n-nonane. As examples, two replacements arelisted in Table 1, 2,7-dimethyloctane and n-butylcyclo-pentane. They are separately ranked number 1 andnumber 5, respectively,the best and worst of the suc-

Table 3. Replacement for 1-Butanol: Tolerance Range, Physical Properties, Environmental Properties, andInfinite-Dilution Activity Coefficients of 1-Butanol; 2-Methyl-1-propanol; a Mixture of 80 mol % 2-Methyl-1-propanol and20 mol % 1-Pentanol; and a Mixture of 75 mol % 2-Methyl-1-propanol, 15 mol % 1-Pentanol, and 10 mol %3-Methyl-2-butanola

propertytolerancerange (%) 1-butanol

2-methyl-1-propanol

80 mol % 2-methyl-1-propanol/20 mol %

1-pentanol

75 mol % 2-methyl-1-propanol/15 mol % 1-pentanol/

10 mol % 3-methyl-2-butanol

molecular mass (kg/kmol) 11 74.1 74.1 N 76.9 N 77.6 Nliquid density (kg/m3) 14 806 798 N 802 N 803 Nboiling temperature (K) 10 391 381 N 385 N 385 Nvapor pressure (kPa) 30 0.93 1.39 V 1.18 N 1.21 Nsurface tension (kg/s2) 16 0.0244 0.0226 N 0.0232 N 0.0232 Nviscosity × 104 (kg/m‚s) 30 25.3 33.3 V 33.3 V 32.8 Nthermal conductivity (J/m‚s‚K) 30 0.153 0.132 N 0.137 N 0.136 Nflash point (K) N/A 302 301 V 303 N 303 Nair index (impact/kg) N/A 0.0422 0.0204 N 0.0166 N 0.0169 Nenvironmental index (impact/kg) N/A 4.59 1.48 N 1.42 N 1.40 N

infinite-dilution activity coefficientethanol 25 1.04 1.04 N 1.04 N 1.04 Ndiethyl ether 30 1.80 1.80 N 1.75 N 1.74 Nacetone 28 1.91 1.91 N 1.90 N 1.89 Nwater 30 5.34 5.35 N 5.38 N 5.39 Nbenzene 30 2.97 2.97 N 2.85 N 2.82 Ncis-2-heptene 30 4.15 4.15 N 3.95 N 3.90 Nn-propyl chloride 30 2.24 2.24 N 2.18 N 2.16 Nn-heptadecane 30 16.0 16.0 N 14.9 N 14.6 Nn-propylamine 30 0.188 0.188 N 0.187 N 0.186 Ndimethyl disulfide 30 0.395 0.395 N 0.393 N 0.393 N


Table 4. Replacement for Benzene: Tolerance Range, Physical Properties, Environmental Properties, andInfinite-Dilution Activity Coefficients of Benzene; Fluorobenzene; a Mixture of 80 mol % Fluorobenzene and 20 mol %Cyclohexane; and a Mixture of 70 mol % Fluorobenzene, 20 mol % Cyclohexane, and 10 mol % 1,4-Dioxanea

propertytolerancerange (%) benzene fluorobenzene

80 mol % fluorobenzene/20 mol % cyclohexane

70 mol % fluorobenzene/20 mol % cyclohexane/10 mol %1,4-dioxane

molecular mass (kg/kmol) 11 78.1 96.1 V 93.7 V 92.9 Vliquid density (kg/m3) 14 873 1010 V 963 N 964 Nboiling temperature (K) 10 353 357 N 355 N 356 Nvapor pressure (kPa) 30 12.6 10.3 N 12.2 N 11.6 Nsurface tension (kg/s2) 16 0.0282 0.0269 N 0.0264 N 0.027 Nviscosity × 104 (kg/m‚s) 30 6.00 5.59 N 6.03 N 6.49 Nthermal conductivity (J/m‚s‚K) 30 0.143 0.126 N 0.126 N 0.129 Nflash point (K) N/A 262 259 V 260 V 260 Vair index (impact/kg) N/A 0.489 0.0928 N 0.0894 N 0.0801 Nenvironmental index (impact/kg) N/A 3.92 0.914 N 0.816 N 0.777 N

infinite-dilution activity coefficientethanol 25 10.1 8.30 N 10.4 N 9.35 Ndiethyl ether 30 0.978 0.588 V 0.713 N 0.874 Nacetone 28 1.55 1.06 V 1.43 N 1.56 Nwater 30 1780 3.93 V 8.21 V 7.58 Vbenzene 30 N/A N/A N/A N/A N/A N/A N/Acis-2-heptene 30 1.80 1.35 N 1.20 V 1.39 Nn-propyl chloride 30 0.998 0.874 N 0.958 N 1.08 Nn-heptadecane 30 1.56 1.37 N 0.989 V 1.51 Nn-propylamine 30 1.66 1.15 V 1.26 N 1.39 Ndimethyl disulfide 30 1.10 0.792 N 1.12 N 1.25 N



cessful replacements. 2,7-Dimethyloctane belongs to thedimethylalkane family that also contains only singlecarbon bonds but has two side chains. All of the physicalproperties and infinite-dilution activity coefficients of2,7-dimethyloctane matched those of n-nonane verywell, as listed in Table 1. The air index of 2,7-dimethyl-octane is 0.0186 impact/kg as compared to 0.158 impact/kg for n-nonane, which is about an 8-to-1 reduction. Theenvironmental index was also reduced from 27.6 to 4.62impact/kg, which is about a 6-to-1 reduction. n-Butyl-cyclopentane belongs to alkylcyclopentane family that

contains rings of single carbon bonds. All of the physicalproperties and infinite-dilution activity coefficients ofn-butylcyclopentane closely matched those of n-nonane.The air index was reduced from 0.158 to 0.0145 impact/kg, which is about an 11-to-1 reduction, and theenvironmental index was reduced from 27.7 to 2.91impact/kg, which is about a 10-to-1 decrease. Note thatthe chemical ranked number 5 has better air andenvironmental indexes than the chemical ranked num-ber 1. The ranking is based not only on the environ-mental properties, but also on how close the physical

Table 5. Replacement for a Mixture of 50 mol % Methyl Ethyl Ketone and 50 mol % Toluene: Tolerance Range, PhysicalProperties, Environmental Properties, and Infinite-Dilution Activity Coefficients of a Mixture of 50 mol % Methyl EthylKetone and 50 mol % Toluene; 3-Pentanone; a Mixture of 65 mol % 3-Pentanone and 35 mol % Ethyl Acetate; and aMixture of 65 mol % 3-Pentanone, 20 mol % Ethyl Acetate, and 15 mol % Methyl Isobutyratea

propertytolerancerange (%)

50 mol % methylethyl ketone/

50 mol % toluene 3-pentanone65 mol % 3-pentanone/35 mol % ethyl acetate

65 mol % 3-pentanone/20 mol % ethyl acetate/

15 mol % methyl isobutyrate

molecular mass (kg/kmol) 11 82.1 86.1 N 86.8 N 88.9 Nliquid density (kg/m3) 14 834 810 N 837 N 837 Nboiling temperature (K) 10 361 375 N 362 N 365 Nvapor pressure (kPa) 30 9.22 4.98 V 7.75 N 6.76 Nsurface tension (kg/s2) 16 0.0259 0.0248 N 0.0242 N 0.0243 Nviscosity × 104 (kg/m‚s) 30 4.54 4.44 N 4.32 N 4.33 Nthermal conductivity

(J/m‚s‚K)30 0.138 0.143 N 0.143 N 0.142 N

flash point (K) N/A 264 286 N 273 N 274 Nair index (impact/kg) N/A 0.542 0.0834 N 0.185 N 0.129 Nenvironmental index

(impact/kg)N/A 6.54 1.70 N 2.13 N 1.73 N

infinite-dilution activity coefficientethanol 5 3.66 2.64 V 2.78 N 2.82 Ndiethyl ether 30 1.12 1.12 N 1.05 N 1.06 Nacetone 28 1.22 1.17 N 1.13 N 1.14 Nwater 30 11.8 12.3 N 12.4 N 12.5 Nbenzene 30 1.01 1.10 N 1.05 N 1.02 Ncis-2-heptene 30 2.05 1.93 N 2.05 N 2.05 Nn-propyl chloride 30 0.967 0.885 N 0.866 N 0.864 Nn-heptadecane 30 4.37 3.22 N 4.07 N 4.12 Nn-propylamine 30 1.28 0.936 N 0.935 N 0.896 Ndimethyl disulfide 30 0.607 0.881 V 0.877 V 0.785 N


Table 6. Replacement for a Mixture of 50 mol % Water and 50 mol % 1-Butanol: Tolerance Range, Physical Properties,Environmental Properties, and Infinite-Dilution Activity Coefficients of a Mixture of 50 mol % Water and 50% 1-Butanol;2-Butanol, a Mixture of 50 mol % Water and 50 mol % 2-Butanol; and a Mixture of 15 mol % 2-Butanol, 55 mol % Water,and 35 mol % 1-Pentanola

propertytolerancerange (%)

50 mol % water/50 mol % 1-butanol 2-butanol

50 mol % 2-butanol/50 mol % water

15 mol % 2-butanol/55 mol % water/

30 mol % 1-pentanol

molecular mass (kg/kmol) 11 46.1 74.1 V 46.1 N 47.4 Nliquid density (kg/m3) 14 837 806 N 837 N 842 Nboiling temperature (K) 10 367 372 N 361 N 366 Nvapor pressure (kPa) 30 3.80 2.45 V 4.78 N 4.14 Nsurface tension (kg/s2) 16 0.0486 0.0231 V 0.048 N 0.0510 Nviscosity × 104 (kg/m‚s) 30 17.0 31.7 V 20.3 N 19.5 Nthermal conductivity (J/m‚s‚K) 30 0.190 0.135 N 0.171 N 0.188 Nflash point (K) N/A 311 297 V 300 V 313 Nair index (impact/kg) N/A 0.0443 0.0136 N 0.0143 N 0.00736 Nenvironmental index (impact/kg) N/A 3.70 0.564 N 0.454 N 0.808 N

infinite-dilution activity coefficientethanol 25 1.09 1.05 N 1.09 N 1.09 Ndiethyl ether 30 2.52 1.81 N 2.52 N 2.48 Nacetone 28 1.67 1.91 N 1.67 N 1.63 Nwater 30 N/A N/A N/A N/A N/A N/A N/Abenzene 30 2.04 2.96 V 2.04 N 1.85 Ncis-2-heptene 30 14.6 4.15 V 14.6 N 14.5 Nn-propyl chloride 30 5.44 2.25 V 5.44 N 5.47 Nn-heptadecane 30 169 16.1 V 169 N 169 Nn-propylamine 30 0.0143 0.188 V 0.0143 N 0.0142 Ndimethyl disulfide 30 0.273 0.394 V 0.273 V 0.279 N



properties and infinite-dilution activity coefficients com-pare to those of the original solvent. Last, the fact thatboth a branched and a cyclic molecule appear to beadequate replacements for a straight-chain alkane isremarkable but not entirely surprising given the factthat n-nonane does not have strong intermolecularforces.

Methyl ethyl ketone (MEK) contains a carbonyl groupwith a weak dipole. The polarization of the carbonylgroup creates dipole-dipole attractions between MEKmolecules. However, ketones do not have O-H or N-Hbonds, so their molecules cannot form strong hydrogenbonds with each other. According to the author’s experi-ences, at the default tolerance ranges, PARIS II cannotfind any single replacement that meets all of theproperty requirements for methyl ethyl ketone. Fromthe single-chemical candidate listing that was found, wechose 2-pentanone as the first component of the replace-ment mixture. As shown in Table 2, there were threeproperty violations between 2-pentanone and methylethyl ketone. 2-Pentanone has a higher molecular mass(19% higher), a much lower vapor pressure (62% lower),and a lower infinite-dilution activity coefficient (35%lower) for n-heptadecane. For 2-pentanone, the air indexis 0.0458 impact/kg as compared to 0.752 impact/kg forMEK, and the environmental index is 0.981 impact/kgas compared to 6.19 impact/kg for MEK. To correct theseproperty requirement violations, the second componentshould have a lower molecular mass, higher vaporpressure, and higher infinite-dilution activity coefficientfor n-heptadecane without too much variation in theother properties. Using the first component of 2-pen-tanone, PARIS II reranked the candidate listing, andwe chose acetone as the second component. PARIS IIthen showed that a mixture of 75 mol % 2-pentanoneand 25 mol % acetone (both 2-pentanone and acetonebelong to the ketone family) meets all requirementsspecified for a replacement. For this mixture substitutesolvent, the air index was reduced from 0.752 to 0.177impact/kg, which is about a 4-to-1 reduction, and theenvironmental index was reduced from 6.19 to 1.25impact/kg, which is about a 5-to-1 reduction. As Table2 shows, 2-pentanone has a lower air index and envi-ronmental index than the mixture of 75 mol % 2-pen-tanone and 25 mol % acetone. This tells us that, tomatch the other properties, we have to accept the costof slightly higher environmental impacts. It is importantto note, however, that the solvent design algorithmlogically chose a mixture of two other ketones as areplacement.

1-Butanol belongs to the alcohol family with -OHfunctional groups that exhibits strong polarization andhydrogen-bonding capabilities. As Table 3 shows, PARISII could not find single replacements for 1-butanol, buta mixture replacement of 75 mol % 2-methyl-1-propanol,15 mol % 1-pentanol, and 10 mol % 3-methyl-2-butanolwas successfully found as a replacement for 1-butanol.For the first component, 2-methyl-1-propanol, threeproperty violations occur: higher vapor pressure, higherviscosity, and 1 K lower flash point. After the secondcomponent, 1-pentanol, was added into the mixture, thevapor pressure and flash point problems were corrected.The third component, 3-methyl-2-butanol, was added tocorrect the viscosity problem. For this mixture substi-tute solvent, the air index was reduced from 0.0422 to0.0169 impact/kg, which is about a 3-to-1 reduction, andthe environmental index was reduced from 4.59 to 1.40

impact/kg, which is about a 3-to-1 reduction. All of thechemicals of this mixture, 2-methyl-1-propanol, 1-pen-tanol, and 3-methyl-2-butanol, belong to alcohol family.It is also important to note that the solvent designalgorithm systematically corrected all of the propertyrequirement violations.

Compared to the solvents mentioned above, benzeneis the simplest aromatic solvent and has, perhaps, morecomplicated behavior because of its cyclic and conju-gated structure. As shown in Table 4, the first compo-nent, fluorobenzene, had seven violations of the propertyrequirements. There were three physical property viola-tions: higher molecular mass, higher liquid density, andlower flash point. There were four infinite-dilutionactivity coefficients violations: diethyl ether, acetone,water, and n-propylamine. The second component, cy-clohexane, corrected the low liquid density violation andtwo activity coefficient violations, but it added one newactivity coefficient violation. The second component alsoreduced the air index and the environmental index. Thethird component, 1,4-dioxane, corrected two infinite-dilution activity coefficients violations. The best formu-lation of these three chemicals was 70 mol % fluoroben-zene, 20 mol % cyclohexane, and 10 mol % 1,4-dioxanewith three property requirement violations remaining.Of these violations, the physical property violations arenot very large. The molecular mass is 19% higher thanthe desired value, and the flash point is 2 K lower thanthe desired value. However, the infinite-dilution activitycoefficient for water is too small. In fact, we have notyet successfully found a completely satisfactory replace-ment for benzene partly because of this particularproblem. Fluorobenzene belongs to C, H, F compounds,cyclohexane belongs to the cycloalkanes, and 1,4-dixanebelongs to the epoxides.

5.2. Solvent Mixtures. The PARIS II solvent designalgorithm can find replacements for solvent mixturesas well. Two examples are illustrated in Tables 5 and6. For an organic mixture of 50 mol % methyl ethylketone and 50 mol % toluene, as shown in Table 5, amixture of 65 mol % 3-pentanone, 20 mol % ethylacetate, and 15 mol % methyl isobutyrate was found tobe an acceptable replacement. For the first component,3-pentanone, there are three property violations: lowvapor pressure, low infinite-dilution activity coefficientfor ethanol, and high infinite-dilution activity coefficientfor dimethyl disulfide. The second component, ethylacetate, corrected the first two violations. The thirdcomponent methyl isobutyrate corrected the last viola-tion. For the replacement solvents, the air index wasreduced from 1.28 to 0.896 impact/kg, which is about a1.4-to-1 reduction, and the environmental index wasreduced from 6.54 to 1.73 impact/kg, which is about a4-to-1 reduction.

For an aqueous mixture of 50 mol % water and 50mol % 1-butanol, as shown in Table 6, a mixturereplacement of 15 mol % 2-butanol, 55 mol % water, and30 mol % 1-pentanol was successfully found. The firstcomponent, 2-butanol, came with 11 violations. Thesecond component, water, corrected nine of these viola-tions. The third component, 1-pentanol, corrected twoviolations. For the replacement solvents, the air indexwas reduced from 0.542 to 0.129 impact/kg, which isabout a 4-to-1 reduction, and the environmental indexwas reduced from 6.54 to 1.73 impact/kg, which is abouta 4-to-1 reduction. During the process of searching forreplacements, we found that water cannot easily be


replaced by any other component. The reason is thatwater is quite unique in its highly structured hydrogen-bonded network, and it is generally the best solvent. Inaddition, it is the basis of life so it has no environmentalimpacts, with the possible exception of floods.

It should be noted that the computations required foreach of these examples were finished in minutes, or atworst hours, depending on the number of componentsin the mixture. This should be contrasted to the daysor weeks or months that can be required to perform thesame searches experimentally on the laboratory bench.Therefore, computer-based solvent design algorithmscan effectively reduce the time and cost of finding anddesigning replacement solvents by orders of magnitude.Judging from our previous experiences in designingsubstitute solvents, candidate chemicals can usually befound among the top 20 ranked chemicals in the listingdeveloped by the PARIS II algorithm. Occasionally,however, it might be necessary to try more candidates.As these examples showed, good replacements are likelyto be found from chemicals in the same family as theoriginal solvent. This occurs because the chemicalfamilies are grouped with similar structures and prop-erties. In addition, with increasing complexity of themolecular structures, e.g., from single bonds to doublebonds, the difficulty of successfully finding replacementsalso increases. As in the case with benzene, at times, itmight not be possible to find entirely satisfactoryreplacements by the criteria outline here and embodiedin the PARIS II algorithm. However, one should notethat the criteria presented here involve 20 differentproperties (10 activity coefficients and 10 operational,safety, and environmental properties). This is well abovethe requirements for most successful solvent substitu-tions carried out in industrial practice for particularapplications. Typically solvents are replaced on the basisof solubility parameters and a few other properties. Thesolvent design theory and the PARIS II algorithm,therefore, apply requirements that tend to capturenearly all of the potential behavior of a solvent coveringnearly all its potential uses. For any one particularapplication, though, some of these properties are notcritical. It is, therefore, possible for a potential user todisregard some of the property requirements and stillobtain a successful replacement solvent. Thus, the notentirely satisfactory replacement for benzene might stillbe perfectly adequate for a great many uses. This broadrange of property requirements provides the flexibilityof tailoring property requirements to particular uses,i.e., to custom design solvents.

6. Conclusions

A solvent design theory that includes a reasonablycomprehensive list of necessary operational, functional,safety, and environmental solvent properties and thePARIS II solvent design algorithm based on this theoryhave been discussed. To illustrate the use of the theoryand the algorithm in designing substitute solvents,several case studies for pure solvents and solventmixtures from several chemical families were discussed.These cases cover a broad range from relatively simplysolvents such as n-nonane to more complex ones suchas benzene and aqueous and organic mixtures. Thecomputer program PARIS II makes the task of solventdesign relatively easy. The case studies showed thatPARIS II could find solvent replacements from the samefamily as the original solvents and from completely

different families. In general, as the complexity of asolvent’s molecular structure increases, the difficulty offinding a successful replacement also increases. Thecase studies also showed that, in every case, substitutesolvents consistently have lower potential environmen-tal impacts while maintaining similar levels of technicalperformance. It, therefore, seems possible that thePARIS II algorithm can design substitute solvents thathave lower associated human health and environmentalrisks without necessarily sacrificing technical perfor-mance. This can be counterintuitive because it is gener-ally thought that environmental improvement exacts atechnical performance toll, especially for complex fluidsand mixtures.

Acknowledgment

This work was performed while Meirong Li was inresidence at the National Risk Management ResearchLaboratory under a Research Associateship through theNational Research Council.

Nomenclature

Rj ) user-assigned weight of impact j independent ofchemical i

γi∞ ) activity coefficient of component i at infinite dilution

γi ) activity coefficient of component iæj ij ) normalized impact score of chemical i for impact jΨi ) environmental index of component iΨm ) mixture environmental index for mixture mΨi

air ) air index of component chemical iΨm

air ) mixture air index for mixture mMi ) molecular mass of component chemical iPi

v ) vapor pressure of component iP ) pressure at which the solvent is being usedT ) temperatureWi ) weight fraction of chemical ixi ) mole fraction of chemical component idoi ) distance between the original solvent o and possible

replacement solvent i in property spaceSj

o ) desired solvent’s jth coordinate in property space

Sji ) replacement solvent’s jth coordinate in propertyspace

Literature Cited

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(8) Cabezas, H.; Zhao, R.; Bare, J. C.; Nishtala, S. R. DesigningEnvironmentally Benign Solvent Substitutes. In Tools and Meth-ods for Pollution Prevention; Kluwer Academic Publishers: Dor-drecht, The Netherlands, 1999; p 317.

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Received for review July 5, 2001Revised manuscript received August 18, 2002

Accepted August 28, 2002

IE010574S


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Experiences in Designing Solvents for the Environment