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Keywords:

  • cement kiln;
  • distillation;
  • hazardous waste-solvent incinerator;
  • industrial ecology;
  • life-cycle assessment (LCA);
  • waste-solvent management

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information

A comparison of various waste-solvent treatment technologies, such as distillation (rectification) and incineration in hazardous-waste-solvent incinerators and cement kilns, is presented for 45 solvents with respect to the environmental life-cycle impact. The environmental impact was calculated with the ecosolvent tool that was previously described in Part I of this work. A comprehensive sensitivity analysis was performed, and uncertainties were quantified by stochastic modeling in which various scenarios were considered. The results show that no single treatment technology is generally environmentally superior to any other but that, depending on the solvent mixture and the process conditions, each option may be optimal in certain cases. Nevertheless, various rules of thumb could be derived, and a results table is presented for the 45 solvents showing under which process conditions and amount of solvent recovery distillation is environmentally superior to incineration. On the basis of these results and the ecosolvent tool, an easily usable framework was developed that helps decision makers in chemical industries reduce environmental burdens throughout the solvent life cycle. With clear recommendations on the environmentally optimized waste-solvent treatment technology, the use of this framework contributes to more environmentally sustainable solvent management and thus represents a practical application of industrial ecology.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information

Organic solvents are among the most important chemical resources used in the pharmaceutical and specialty chemicals industries in terms of quantity. Once the solvents cannot be reused in a process and thus become waste solvents, there are only a few technologies available for their treatment. The main choices are distillation or thermal treatment in either hazardous-waste-solvent incinerators or cement kilns (Seyler et al. 2006). From an environmental perspective, it is not known what treatment technology is superior. All technologies are associated with impacts on the environment (see Capello et al. 2007). A systematic evaluation of these technologies provides industry with the knowledge needed to reduce environmental burdens throughout the life cycle of organic solvents. It is therefore highly relevant in the context of industrial ecology.

A suitable method for comprehensively quantifying the environmental impact of these technologies is the life-cycle assessment (LCA) method (EN ISO 14040 1997). It considers all impacts to humans and the environment during the whole life cycle of solvents, including impacts from raw material extraction, solvent production, use of energy and ancillaries, and waste-solvent treatment (Hofstetter et al. 2003). However, one major drawback is that full LCA studies are data and time intensive. To overcome these limitations, we used the ecosolvent tool presented in part I of this two-article series (Capello et al. 2007) for a systematic environmental assessment of three common technologies to determine general conclusions and rules of thumb about environmentally superior waste-solvent treatment options. Such results may provide simple and quick answers for decision makers and thus contribute to the adoption of environmentally preferable solvent management.

In an initial step, we determined whether one of the three technologies implemented in the ecosolvent tool is generally the environmentally superior treatment option. To this end, we performed a sensitivity analysis to determine process parameters of particular importance with regard to environmental performance. The analysis thus enabled a determination of the range of environmental impacts for each technology.

Next, we obtained solvent-specific results by comparatively calculating the environmental impacts of 45 commonly used solvents treated with the three technologies. To this end, various scenarios were taken into account. For example, various energy production chains and fractions of secondary components were considered for distillation. Additionally, several recovery rates were taken into account to determine rules of thumb that specify under which process conditions one treatment technology is generally environmentally superior to another.

Methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information

The Ecosolvent Tool

The ecosolvent tool is a generic life-cycle assessment tool that allows for the environmental comparison of distillation (rectification) and thermal treatment in hazardous waste incinerators 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 14041 1998) 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 emissions flows, ancillary uses, and generation of coproducts, are calculated (table 1).

Table 1.  Inventory parameters of the three life-cycle inventory (LCI) models implemented in the ecosolvent tool
LCI models (abbreviation)Distillation model (dist)Hazardous waste incinerator model (ws inc)Cement kiln model (cement)
  1. aIn the LCI model of the cement kiln, changes in the emissions as a consequence of substituting fossil fuels with waste solvent are calculated and therefore expressed as differences (Δ).

Inventory parameters (abbreviation)Use of steam (st)Use of fuel oil (oil)ΔCO2 emissions (ΔCO2)a
Use of electricity (el)Use of ancillaries (anc)ΔNOX emissions (ΔNOx)a
Use of nitrogen (N2)Energy production (energy)ΔMetal emissions (Δmetals)a
Use of ancillaries (anc)CO2 emissions (CO2)Fossil fuel substitution (fuel)
Outlet air treatment (air)Other emissions (em) 
Residue incineration (res) 
 Waste-water treatment (ww) 
Solvent recovery (solv) 

In the ecosolvent tool, the calculated inventory data are linked to background inventory data (production of ancillaries, fuels, and energy) taken from the ecoinvent database (ecoinvent Centre 2004). Inventory data of 32 of the solvents that were used in this work will be published in the Version 2.0 update of the ecoinvent database. The full LCI can be assessed with various life-cycle impact assessment methods. In the present article, we applied eco-indicator 99 (H/A; Goedkoop and Spriensma 2000), cumulative energy demand (Jungbluth and Frischknecht 2004), the method of ecological scarcity (UBP'97; Hischier 2004), and the global warming potential (IPCC 2001).

Determining the Relevance of Inventory Parameters

All inventory parameters (table 1) contribute to the total environmental impact of a waste-solvent treatment technology to varying degrees. To identify relevant parameters, we performed a sensitivity analysis. The variability of the environmental impacts originates from two sources. First, variability between sources and objects influences the LCA outcome (Huijbregts 1998) due to different solvent properties, differences in technologies that produce the same product (e.g., steam or electricity production), and different technologies (e.g., batch and continuous distillation). This variability can be accounted for through defining scenarios. Therefore, a best-case and a worst-case scenario were considered for each inventory parameter. The best-case scenario reflects the situation of minimal environmental burdens and maximal environmental credits. The worst-case scenario comprises maximal environmental burdens and minimal environmental credits. The detailed description of the scenarios is presented in Supplementary Table S1 on the Web.

The second source is the parameter uncertainty (Huijbregts 1998). To quantify the parameter uncertainty, we applied quantitative stochastic modeling (Monte Carlo simulation [Vose 2000], as implemented in the ecosolvent tool). To this end, probability distributions were used for all model parameters. Probability distributions of the environmental impact scores were determined for all inventory parameters as output. The detailed calculation of the single inventory parameters is presented in Supplementary Tables S2 and S3 on the Web.

Technology-Specific Assessment

The total environmental impact of a technology (Itech) is defined as the sum of the environmental impacts of the single inventory parameters (Iip). Thus, on the basis of the sensitivity analysis, the total environmental impact of the three technologies was calculated according to equations (1), (2), and (3) (see table 1 for an explanation of the abbreviations used):

Waste-solvent incinerator:

  • image(1)

Cement kiln:

  • image(2)

Distillation:

  • image(3)

Solvent-Specific Assessment

To derive solvent-specific results, we comparatively assessed the treatment of 45 waste-solvent mixtures for both distillation and incineration using the ecosolvent tool. Each waste-solvent mixture was assumed to be a binary mixture that contained one of the most commonly used solvents in the pharmaceutical and specialty chemical industries (Seyler et al. 2006) as the main component. Three scenarios were considered, representing a minimum, average, and maximum environmental impact. In these scenarios, the parameter uncertainty of the inventory flows was taken into account for both technologies, as described in the section above. With regard to the distillation, the scenarios included additional assumptions concerning the steam production; the use of ancillaries, electricity, and nitrogen; and the treatment of outlet air (table 2).

Table 2.  General scenario definition used for the solvent-specific assessment
Distillation model
ScenarioSteam productionUse of ancillariesOutlet air treatmentUse of electricity and nitrogenParameter uncertainty
Minimum impact distillationWaste-solvent incinerationNoOutlet air incinerationGeneric data of continuous distillationBest case: 2.5th percentile
Average impact distillationWaste-solvent incinerationpH adjustment and equipment cleaningOutlet air incinerationGeneric data of continuous distillationAverage: 50th percentile
Maximum impact distillationIncineration of fossil fuelsEntrainerDirect emission of outlet air as nonmethane volatile organic carbon (NMVOC)Generic data of batch distillationWorst case: 97.5th percentile
Incineration models
Minimum impact incinerationBest case: 2.5th percentile
Average impact incinerationAverage: 50th percentile
Maximum impact incinerationWorst case: 97.5th percentile

In addition to these scenarios, it was assumed that the distillation residuals were treated in a hazardous waste incinerator. The solvent recovery was considered separately, as it proved to be a key parameter in prior studies (see Capello et al. 2007 or Hofstetter et al. 2003).

An initial assessment seeks to determine the solvents and the process conditions for which distillation is the environmentally superior treatment option to incineration. To this end, we assumed that each of the 45 solvents would be present as the main component in the binary mixture. The secondary component was chosen to show environmentally optimal results in the incineration models. The shares of the main component and the secondary component were varied continuously, from a minimum of 0.34 kg/kg waste solvent to a maximum of 0.98 kg/kg waste solvent. The recovery rate of the distillation, which defines the amount of the recovered main component from the total amount of main component present in the mixture, was set to 90% for shares of the main component below 0.9 kg/kg waste solvent. Above 0.9 kg/kg waste solvent, it was increased linearly up to 99% according to experts' opinion (Expert Panel 2003–2005). The amount of recovered solvent varied, therefore, from 0.31 kg/kg waste solvent to 0.97 kg/kg waste solvent, which is the range we found in industry (see Capello et al. 2005). Thus, the environmental impact was calculated for both technologies as a function of the shares of the components. With regard to distillation, worst-case conditions, average conditions, and best-case conditions were considered for all 45 solvents and compared to environmental best-case conditions for incineration (see table 2). In many cases, a threshold amount of recovered solvent could be calculated beyond which distillation became the superior technology. The values of these threshold recoveries were determined for the 45 solvents.

A second assessment attempted to determine the solvents for which incineration is generally environmentally superior to distillation. To this end, worst-case conditions were considered for incineration (table 2). With regard to distillation, optimal conditions were assumed (table 2), as well as a maximum solvent recovery of 0.97 kg recovered solvent per kilogram waste solvent (Capello et al. 2005). Additionally, the secondary component was assumed to be the solvent with the worst environmental results in the incineration models.

Note that these assessments are based on several assumptions. For example, no pretreatment of the waste solvent is required before distillation, the distillation residue is not treated in sewage plants, and the recovery rate is 90%. In some cases, these assumptions may not reflect the actual process properly, but they represent the general conditions in chemical industries (Expert Panel 2003–2005).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information

Relevance of the Single Inventory Parameters

The results of the sensitivity analysis are depicted in figure 1 for incineration and figure 2 for distillation. Further results from the sensitivity analysis are given in Supplementary Table S2 on the Web.

image

Figure 1. Potential environmental impact of the inventory parameters in the hazardous waste-solvent incinerator and cement kilns models.

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image

Figure 2. Potential environmental impact of the inventory parameters in the distillation model. The organic residue is treated in either a waste-solvent incinerator or cement kilns. The results of both alternatives are shown.

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With respect to the hazardous-waste-solvent incinerator (figure 1), the credits of energy production (Ienergy) and carbon dioxide (CO2) emissions (ICO2) contributed most to the environmental impact in the best-case scenario. Important environmental impacts from direct emissions (Iem), from the use of ancillaries (Ianc), and from supplemental fuel oil (Ioil) only arose in the worst-case scenario, due to the low net calorific value of the waste solvent or due to heteroatoms. Heteroatoms, such as sulphur, nitrogen, or chlorine, require the use of ancillaries, especially sodium hydroxide (Seyler et al. 2005). High nitrogen content may also cause environmental burdens due to nitrogen oxide (NOx) emissions. In the absence of heteroatoms, the net calorific value is the most important waste-solvent property, because the energy credits as well as the amount of supplemental fuel, if necessary, are a function of net calorific value. Additionally, in the case of organic solvents, the net calorific value positively correlates with the carbon content, unless functional groups are present (Wypych 2001). Therefore, the CO2 emissions are mostly related to the net calorific value.

With the eco-indicator 99 and UBP'97 methods, all parameters could be assessed on a fully aggregated level. The results of both methods show a good correlation of the relative importance of the single inventory parameters. The global warming potential deviated from the results of the former methods with regard to the environmental impact arising from direct emissions (Iem), as the only assessable direct emission was carbon monoxide (CO2 emissions are considered separately in ICO2). As the emissions of carbon monoxide were allocated to the waste-solvent mass, the best- and worst-case scenarios resulted in the same impact score. Nevertheless, the global warming potential was a suitable indicator of the environmental impact if the waste solvent did not contain nitrogen. Otherwise, impact assessment methods that take into account NOx emissions were more appropriate. Similarly, the results calculated with the cumulative energy demand also correlated very well with the results of other methods, but no emissions could be assessed.

In the cement-kiln model, changes in CO2 and the NOx emissions (IΔCO2, IΔNOx) were due to the carbon and the nitrogen content of the waste solvent, respectively, as well as to the net calorific value of the waste solvent. The latter is important, as the substituted fossil fuel also contained nitrogen and carbon. The substitution of coal, in particular, led to a decrease of CO2 emissions, as coal shows a high carbon content per net calorific value (Seyler et al. 2004). In the best-case scenario, the change in NOx emissions was also primarily related to the net calorific value. This affected the NOx emissions (worst-case scenario) only if the nitrogen content of the waste solvents was high.

Figure 1 shows that all inventory parameters may be of environmental relevance in the case of cement kilns. In the worst-case scenario, IΔNOx could become the most important parameter. Because the formation of NOx in cement kilns is incompletely understood (van Oss and Padovani 2003), the NOx emissions were calculated with uncertainty with regard to the conversion rate of fuel nitrogen to NOx (52%–92%; Baumbach 1993) as well as the efficiency of NOx reduction facility (0% in case this equipment is missing to 85%; see Capello et al. 2007). Therefore, the results of the NOx emissions show high uncertainty. Also, IΔmetals could be of high importance, but only in the case of high heavy metal content in the waste solvent. In the absence of high metal and nitrogen content, the net calorific value of the waste solvent determined the amount of substituted fossil fuels and also the changes in other emissions.

The results according to the methods eco-indicator 99, UBP'97, and the global warming potential (with the exception of NOx and metal emissions) were consistent in terms of the relative importance of the single inventory parameters (figure 1). By contrast, cumulative energy demand as a stand-alone indicator was not appropriate, as only the credits of the fossil-fuel substitution were assessed.

With respect to the distillation model, the solvent recovery contributed the most to the total environmental impact (Isolv), followed by residue treatment (Ires), use of ancillaries (Ianc), and use of steam (Isteam) in the best-case scenario (Figure 2). In the worst-case scenario, environmental impacts arising from the residual treatment (Ires) might become the most important inventory parameter, depending on the impact assessment method chosen. The environmental impact arising from the use of electricity (Iel), nitrogen (IN2), the waste-water treatment (Iww), and the emissions of outlet air (Iair) were of minor importance, except for in the worst-case scenario of outlet air assessed with UBP'97. In this scenario, it was assumed that the outlet air, containing nonmethane volatile organic carbon (NMVOC), was directly emitted to the atmosphere.

The results according to eco-indicator 99 and UBP'97 correlated well (figure 2). The global warming potential and the cumulative energy demand were suitable indicators of environmental impact, under the conditions that outlet air was treated thermally and, with respect to the cumulative energy demand, that no nitrogen-containing distillation residuals were incinerated in cement kilns. All results shown in figure 1 and figure 2 are in accordance with the relative importance of the single inventory parameters presented in Supplementary Table S2 on the Web.

Technology-Specific Assessment

On the basis of the results of the sensitivity analysis, the total environmental impacts of the hazardous waste incinerator (Iws inc), the cement kiln (Icement), and the distillation (Idist) were calculated according to equations (1), (2), and (3) (figure 3). The comparison of the three technologies shows that with all impact assessment methods, the ranges of potential environmental impacts overlapped for the three technologies. Therefore, no single waste-solvent treatment technology was, in general, environmentally superior to any other. This finding implies that conclusions about optimal treatment options must be drawn on a more detailed level: for instance, as a function of solvent properties or technology specifications.

image

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

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Solvent-Specific Assessment

An initial assessment attempted to determine for which of the 45 solvents distillation was, in general, environmentally superior to incineration. To this end, the environmental impact of distillation and incineration was calculated as a function of solvent recovery. With increasing content and, therefore, recovery of the main component, distillation improved environmentally. The environmental impact of incineration, by contrast, increased because the amount of the second component, which showed optimal results in the incineration models, decreased. With this procedure, we determined solvent recoveries at which distillation and incineration had the same environmental impact. Hereby, the scenarios of minimum, average, and maximum environmental impact of distillation (table 2) were compared to the scenario of minimal environmental impact of incineration.

Figure 4 illustrates the comparison of distillation and hazardous waste incineration considering the mixture of monochlorobenzene (main component) and pentane as an example. In the best-case scenario, distillation was generally the superior treatment option. Also, with regard to the average distillation scenario, distillation became environmentally superior at a low solvent recovery (0.48 kg/kg waste solvent; point B in figure 4). When the monochlorobenzene recovery exceeded 0.85 kg/kg waste solvent, distillation was generally the environmentally superior treatment option, even in the worst-case scenario (point A in figure 4).

image

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

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The procedure shown in figure 4 was performed for all 45 waste-solvent mixtures and the four impact assessment methods: eco-indicator 99, cumulative energy demand, UBP'97, and global warming potential. Table 3 shows the results that are in accordance with the outcome of all of the four impact assessment methods. For instance, the value of 0.67 of acetic anhydride in table 3 indicates that distillation was better than incineration under all circumstances, for all impact assessment methods and all secondary components, if the recovery was higher than or equal to 0.67 kg/kg waste solvent. Detailed results are presented in Supplementary Tables S4, S5, S6, and S7 on the Web for all impact assessment methods specifically.

Table 3.  Amounts of recovered solvent beyond which distillation is generally the environmentally superior treatment technology to incineration
Solvent (kg/kg waste solvent)CAS-no.Distillation is superior to hazardous waste incinerator at a solvent recovery ofDistillation is superior to cement kiln at a solvent recovery of
Worst-case distillationAverage distillationBest-case distillationWorst-case distillationAverage distillationBest-case distillation
  1. Note: CAS-no. = CAS registry number, a unique numeric identifier for chemical substances; MTBE = Methyl tert-butyl ether.

  2. aDistillation is environmentally superior also with a minimal solvent recovery of 0.31 kg/kg waste solvent (Capello et al. 2005).

Acetic acid64-19-7No0.48AlwaysaNoNo0.87
Acetic anhydride108-24-70.67AlwaysaAlwaysaNo0.780.59
Acetone67-64-1No0.61AlwaysaNoNoNo
Acetonitrile75-05-80.800.45AlwaysaNoNoNo
Benzaldehyde100-52-70.840.32AlwaysaNoNo0.73
Benzyl alcohol100-51-60.880.41AlwaysaNoNoNo
Butanol (1-)71-36-3No0.950.58NoNoNo
Butanol (2-)78-92-2No0.740.44NoNoNo
Butanol (Iso)78-83-1No0.810.46NoNoNo
Butyl acetate123-86-40.770.44AlwaysaNoNo0.79
Butylen glycol110-63-40.51AlwaysaAlwaysaNo0.700.45
Cyclohexane110-82-7NoNo0.85NoNoNo
Cyclohexanone108-94-10.820.43AlwaysaNoNo0.77
Dichloromethane75-08-20.87AlwaysaAlwaysaNo0.940.78
Diethyl ether60-29-7NoNoNoNoNoNo
Dimethylformamide68-12-20.830.40AlwaysaNoNo0.83
Dioxane123-91-10.890.45AlwaysaNoNo0.86
Ethyl acetate141-78-60.770.40AlwaysaNoNo0.83
Ethanol64-17-5NoNo0.54NoNoNo
Ethyl benzene100-41-4NoNo0.69NoNoNo
Formaldehyde50-00-0NoNoNoNoNo0.94
Formic acid64-18-60.690.32Alwaysa0.890.720.53
Heptane142-82-5NoNoNoNoNoNo
Hexane (n-)110-54-3NoNoNoNoNoNo
Hexane (Iso)96-14-0NoNo0.57NoNoNo
Isoamyl acetate628-63-70.780.44AlwaysaNoNo0.74
Isobutyl acetate110-19-00.750.43AlwaysaNoNo0.80
Isopropyl acetate108-21-40.840.42AlwaysaNoNo0.89
Methanol67-56-1NoNo0.64NoNoNo
Methyl acetate79-20-9No0.760.40NoNoNo
Methyl cyclohexane108-87-2No0.650.36NoNoNo
Methyl ethyl ketone78-93-3No0.880.54NoNoNo
Methyl formate592-84-70.940.47AlwaysaNoNo0.82
Methyl isobutyl ketone108-10-10.39AlwaysaAlwaysaNo0.700.53
Monochlorobenzene108-90-70.850.48AlwaysaNoNoNo
MTBE1634-04-4NoNoNoNoNoNo
Pentane109-66-0NoNoNoNoNoNo
Pentanol71-41-00.970.600.36NoNoNo
Propanol (1-)71-23-80.740.38AlwaysaNoNo0.92
Propanol (Iso)67-63-0No0.810.40NoNoNo
Propionaldehyde123-38-60.820.48AlwaysaNoNo0.96
Tert-amyl alcohol75-85-4No0.870.52NoNoNo
Tetrahydrofuran109-99-90.41AlwaysaAlwaysa0.840.530.32
Toluene108-88-3NoNo0.46NoNoNo
Xylene1330-20-7NoNo0.39NoNoNo

The second assessment attempted to determine the solvents for which incineration was generally the environmentally superior treatment option, for all scenarios and choices of the secondary component. In contrast to distillation, no solvents were found for which this was the case, although incineration may be better under specific circumstances.

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information

The aim of the environmental assessment presented in this work is to facilitate decision making in chemical industries to reduce environmental burdens that are associated with the use of organic solvents. To implement environmental improvements, decision makers need quick and easily applicable decision support, which is, in many cases, difficult to obtain with a complex method, such as LCA (Lifset 2006). In the present work, however, such convenient rules of thumb and recommendations were derived (see figures 1, 2, and 3 as well as table 3). These results help to implement industrial ecology in practice.

The technology-specific assessment showed that with all impact assessment methods the ranges of potential environmental impact overlapped. Thus, no single waste-solvent treatment option was generally environmentally superior to any other. However, two general tendencies can be identified.

First, the waste-solvent treatment in cement kilns was generally environmentally superior to the hazardous waste incinerators, because the substituted fossil fuels in the cement kilns also contain impurities that cause emissions (sulphur, nitrogen, metals) and because the substitution of coal reduces CO2 emissions, as the ratio of carbon content and heating value is higher for coal (Seyler et al. 2004) than for organic solvents. The hazardous waste incinerator, conversely, is subject to stricter regulations in terms of emission limits, and, therefore, more ancillaries are needed to fulfill these regulations, such as sodium hydroxide in the scrubber (Seyler et al. 2005).

Second, distillation tended to be the environmentally superior treatment option for the majority of the investigated solvents: When we considered average distillation conditions (see table 3) and average solvent recovery of 0.71 kg/kg waste solvent (Capello et al. 2005), distillation turned out to be environmentally superior to incineration in hazardous waste incinerators for 25 out of the 45 solvents. Under best-case conditions and a good solvent recovery rate of 0.84 kg/kg waste solvent (Capello et al. 2005), this was even the case for 38 solvents (table 3). In Switzerland, most of the incinerated waste solvent is treated in hazardous waste incinerators or in similar incinerators (approximately 120,000 tonnes per year estimated for 2002 [Seyler et al. 2006], compared to 30,000 tonnes per year in cement kilns in 2002 [Cemsuisse 2003]) due to more restrictive legislation for cement kilns and due to transport outside chemical production sites. The comparison of distillation and hazardous waste incineration is therefore of higher practical relevance. Thus, the finding that in many cases distillation was environmentally superior to the hazardous waste incineration confirms the general waste treatment policy of many chemical companies, namely that recycling is preferable to incineration (see Safety & Environment reports, e.g., Ciba Specialty Chemicals AG 2004; Hoffmann-La Roche AG 2002).

More specific rules of thumb can be determined for subsets of solvents with similar properties and certain technological conditions. Incineration is a good treatment option for waste solvents with high net calorific value due to high environmental credits in the incineration models (Ienergy or Ifuel; see figures 1 and 2). Incineration should not be chosen, from an environmental perspective, if the waste solvent contains heteroatoms, such as nitrogen, sulphur, or halogens, or if it shows large fractions of heavy metals due to the higher need of ancillaries (Ianc; see Figures 1 and 2) and the emissions to air (INOx, Iem; see figures 1 and 2).

With regard to distillation, the same solvent properties also turned out to be important, because the residue treatment in incineration influenced the environmental impact (Ires; see figures 1 and 2). In contrast to incineration, the environmental impact of distillation is mainly determined by the credits of solvent recovery (Isolv; see figures 1 and 2). Therefore, distillation is the environmentally optimal treatment technology when the distillation process is conducted with high solvent recovery and when highly elaborated solvents are recovered that show high environmental credits for the avoidance of virgin solvent production. Both contributions affect the environmental assessment substantially: With the increase from an average solvent recovery of 0.71 kg/kg waste solvent to a good solvent recovery of 0.85 kg/kg waste solvent (Capello et al. 2005), the number of scenarios in which distillation is the environmentally superior treatment technology increases by 16 (hazardous waste incinerator) and 12 (cement kiln), respectively (table 3). These results show that the solvent recovery is a key parameter in the environmental assessment. This finding is also in accordance with the results of the sensitivity analysis. With regard to the number of environmental credits for the avoidance of virgin solvent production, even single production steps in petrochemical manufacturing can influence the outcome of the environmental assessment drastically. For example, due to the additional environmental impact of the esterification of isobutanol to isobutyl acetate and of isopropanol to isopropyl acetate, distillation under worst-case conditions (hazardous waste incinerator) and best-case conditions (cement kiln) becomes the environmentally superior treatment options at high solvent recovery. Similarly, the environmental impact of the isomerization of n-hexane to isohexane also makes distillation under best-case conditions environmentally superior to the hazardous waste incinerator at a solvent recovery of 0.57 kg/kg waste solvent (table 3).

Specific recommendations can be made with regard to the solvents acetic anhydride, butylene glycol, dichloromethane, formic acid, methyl isobutyl ketone, and tetrahydrofuran. For these solvents, distillation turned out to be the environmentally favorable treatment technology in most cases: Compared to the hazardous waste incinerator, distillation was the superior treatment technology at almost minimal solvent recoveries, even if the average scenario was considered. Also, an average distillation at a good solvent recovery was environmentally superior to incineration in cement kilns. Either these solvents receive high credits for the solvent recovery because of elaborate production processes (acetic anhydride, butylene glycol, methyl isobutyl ketone, tetrahydrofuran [Stoye 2000]) or, in the case of formic acid, the low net calorific value (4.6 MJ/kg [Yaws 1999]) only leads to minimal environmental credits in the incineration models. In the case of dichloromethane, the combination of both is determinant.

The solvent-specific assessment revealed that there is no specific solvent for which incineration is generally the environmentally superior treatment technology. For the solvents cyclohexane, ethanol, ethyl benzene, formaldehyde, iso-hexane, methanol, toluene, and xylene, distillation was only generally superior when we assumed the best-case scenario and high solvent recovery. Furthermore, with regard to heptane, hexane, methyl tert-butyl ether (MTBE), and pentane, incineration showed a comparable environmental impact to distillation under best-case conditions. In cases in which the distillation is conducted under nonfavorable conditions—especially if the solvent recovery is low and only associated with small environmental credits (e.g., methanol) or if entrainer is required to separate azeotropic mixtures—distillation is not necessarily superior to incineration, in particular not compared to treatment in cement kilns. With regard to the aromatic and aliphatic solvents, this finding is, on the one hand, related to the fact that these solvents lead to high environmental credits in the incineration due to their high net calorific values (>40 MJ/kg; Yaws 1999). The recovery of ethanol, methanol, and formaldehyde, on the other hand, accounts for low environmental credits in the distillation because their petrochemical manufacturing requires only a few production steps (Stoye 2000). The case of MTBE is another one in which the combination of both is determinant. Therefore, researchers should investigate in detail the treatment of waste-solvent mixtures containing aliphatic or aromatic solvents for the specific mixture (e.g., with the ecosolvent tool) to determine the environmentally superior treatment option.

In addition to these recommendations, the results presented in table 3 are of general interest to practitioners in the chemical industry because they can be used for a quick checkup. On the one hand, they can be used to analyze already operating distillation processes when process conditions and the solvent recovery are known to determine whether distillation was the environmentally preferable choice. On the other hand, these results can also be used in the stage of product development at which little information about solvents is available. These results are particularly useful for experts in the field of distillation, because these individuals have the experience to estimate the amount of recovered solvent to be expected without much effort.

Finally, to provide an easily usable instrument for decision makers in chemical industries, we present all the results of this article, combined with the ecosolvent tool presented in part I (Capello et al. 2007), structured in a clearly arranged framework (figure 5). With the framework presented in figure 5, precise recommendations on the environmentally superior treatment technology can be made in many cases. If the waste solvent contains as the main component acetic anhydride, butylene glycol, dichloromethane, formic acid, methyl isobutyl ketone (MIK), or tetrahydrofuran (THF), distillation is environmentally superior than incineration even at very low solvent recoveries (see table 3). When information on the amount of recovered solvent in a distillation process is known, the threshold values presented in table 3 may be sufficient for a precise recommendation. Otherwise, the ecosolvent tool should be used. In case no significant result is obtained with the ecosolvent tool, more information on the amount of recovered solvent, energy and ancillaries, and use of the distillation and incineration technology should be gathered to reduce the uncertainty. Nevertheless, in some cases, the differences between the treatment technologies will not become significant. In such cases, or if the mixture is composed of solvents other than the 45 solvents we investigated, the general rules of thumb help to identify the treatment technology that tends to be environmentally favorable.

image

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

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Acknowledgments

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information

We gratefully acknowledge the Swiss Federal Office of Energy (Project No. 100065), Ciba Specialty Chemicals AG, Ems-Dottikon AG, Lonza Group Ltd., Novartis Pharma AG, Hoffmannn-La Roche AG, and Siegfried Ltd. for their funding of this project.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information
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About the Authors

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information

Christian Capello was a PhD student in environmental sciences at the time the article was written. Currently, he works as a researcher in the Safety and Environmental Technology Group, ETH Zurich, Zurich, Switzerland. Stefanie Hellweg was a senior researcher in the Safety and Environmental Technology Group, ETH Zurich, and is now professor of ecological systems design at the Institute of Environmental Engineering, ETH Zurich. Konrad Hungerbühler is a professor at the Safety and Environmental Technology Group, ETH Zurich.

Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Acknowledgments
  8. References
  9. About the Authors
  10. Supporting Information
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