Waste-Solvent Management as an Element of ... - ACS Publications

Oct 4, 2006 - Lonza AG, Postfach, CH-3930 Visp, Switzerland. The treatment of organic waste solvents is an important issue in the chemical industry as...
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Ind. Eng. Chem. Res. 2006, 45, 7700-7709

Waste-Solvent Management as an Element of Green Chemistry: A Comprehensive Study on the Swiss Chemical Industry Christina Seyler,*,† Christian Capello,† Stefanie Hellweg,† Christian Bruder,† David Bayne,‡ Alfred Huwiler,§ and Konrad Hungerbu1 hler† Department of Chemistry and Applied Biosciences, ETH Zurich, CH-8093 Zurich, Switzerland, Ciba Specialty Chemicals AG, Postfach, CH-4002 Basel, Switzerland, and Lonza AG, Postfach, CH-3930 Visp, Switzerland

The treatment of organic waste solvents is an important issue in the chemical industry as large amounts accrue annually. As all treatment options are associated with specific environmental impacts, a sound management of waste solvents can therefore lead to large ecological benefits. The goal of this work was therefore to identify opportunities and requirements for an environmentally sound waste-solvent management. To this end, a survey was carried out together with the Swiss chemical industry. The results show that only a few technologies are used on a grand scale, such as incineration and distillation for solvent recovery. But waste-solvent management is strongly influenced by boundary conditions such as costs, logistics, legislation and guidelines, storage capacity, safety considerations, and the existing technologies on the production site. Thus, we identified that the best opportunity to optimize waste-solvent management would be in an early stage of process design, for instance, before scale-up for the launching of the production or when production campaigns change. Two actual case studies from the chemical industry show how environmentally sound waste-solvent management can be integrated in the planning of a chemical company and lead to ecological benefits. In the first case study a company investigates different routes of waste-solvent treatment for waste streams which are treated outside the plant by third parties. In the second case study different options for the treatment of a dimethoxy ethane mixture are assessed, taking into account environmental impacts by a lifecycle perspective. 1. Introduction Organic solvents are used by a wide variety of industrial sectors especially by the chemical industry. On one hand, they are used in large quantities; on the other hand, they can show hazardous properties such as high volatility or toxicity. Therefore, organic solvents are of environmental concern and should be taken into special consideration when designing chemical products and processes. Green chemistry or green engineering looks at “the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances” and “identifies methods of using renewable feedstocks, minimizing energy usage and decreasing negative impacts on human health and environment”.1,2 Waste-solvent management concepts which have the potential to minimize hazardous waste, to reduce raw material input, and to lower emission of toxic substances should therefore be part of environmentally friendly chemical product and process design. Not always is it possible to make the right decisions at an early design stage already. Therefore, it is important to optimize waste-solvent management in daily operation. Often the different treatment options become clear at this stage only. This is especially true for chemical processes run as batch processes in multipurpose plants. However, to make the right decisions at the right time and to support effectively environmental decision-making, there is a need to understand the (waste-) solvent management process. Therefore, the goals of this paper are (a) to investigate and * Corresponding author. Tel.: +41 1 633 63 28. Fax: +41 1 633 12 79. E-mail: [email protected]. † Swiss Federal Institute of Technology. ‡ Ciba Specialty Chemicals AG. § Lonza AG.

illustrate the decision-making processes involved in wastesolvent management in chemical production, (b) to draw conclusions about solvent flows that can be influenced by environmental decision-making, and (c) to identify potentials for an effective waste-solvent management. For this purpose we carried out a survey in the Swiss chemical industry. The results, which were obtained between 2000 and 2004, are presented in this paper. First, we discuss facts and figures about solvent use and disposal in Switzerland and particularly in the Swiss chemical industry (section 2). Those figures show the importance of organic (waste-) solvents in this industrial sector. Second, in section 3, we describe the waste-solvent management situation in the chemical industry with a focus on batch production processes. We present the constraints limiting the degrees of freedom for waste-solvent management in daily operation and discuss the opportunities to integrate waste-solvent management during product and process design. In section 4 two industrial case studies are presented to show how the optimization of waste-solvent management can lead to ecological benefits. Finally, we draw conclusions in section 5. 2. Organic Solvents in the Swiss Chemical Industry: Facts and Figures 2.1. Solvent Mass Flows. To give an overview and identify priority solvent streams in Switzerland, we established a solvent mass flow balance for Switzerland and the Swiss chemical industry based on the year 2001 (Figure 1). Only small amounts of solvents are produced in Switzerland and, thus, almost all fresh solvents are imported. Of the 440000 t imported annually,3 more than half are consumed by the chemical industry.4 From Figure 1 it is apparent that important solvent mass flows are fresh solvent imports, waste-solvent recycling, waste-solvent incineration, and emissions to air.

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Figure 1. Solvent flow balance for Switzerland and the Swiss chemical industry (2001). Numbers in black denote flows within the chemical industry; numbers in gray are the totals for Switzerland (the latter are only shown if significantly different from the chemical industry flows). Data sources for solvent flows were (a) Swiss Society of Chemical Industries,4 (b) Swiss external trade statistics,3 (c) Swiss Agency for the Environment, Forests and Landscape,5 (d) literature,6 (e) expert’s judgment,7 and (f) Swiss hazardous-waste statistics.8

Figure 1 illustrates that the chemical industry is a major player with regard to the use, regeneration, and disposal of solvents. This result highlights the importance of sound concepts for solvent management in the chemical industry, in particular, with regard to waste-solvent management. The two major treatment options for organic solvents are recycling (distillation) and incineration. Both of these treatment options cause, on one hand, environmental impacts, for instance, from emissions and the use of steam, in the case of incineration and distillation, respectively. On the other hand, distillation produces secondary solvent and incineration generates energy. Both of these coproducts lead to a savings in environmental impact. Capello et al. have shown that either option may be superior to the other, depending on the composition of solvent as well as on process parameters, such as the recovery rate.9 Because of the large amounts of waste solvents to be treated (Figure 1), environmentally optimized waste-solvent management may lead to considerable environmental benefits. To illustrate the importance of solvents in chemical production facilities, we investigated the mass flows of all chemicals used and produced in a typical Swiss chemical plant for the year 1998 (Figure 2). The plant presented mainly produces pharmaceuticals. The production is exclusively carried out in multipurpose facilities (about 300 facilities) in batch production. Based on the mass flows presented in Figure 2, the ratio of waste solvent to end product is quantified to be about 30 (by assuming 30% water and 70% waste solvent in waste for incineration). For comparison, the ratio of waste solvent to product for less high-value chemicals is approximately 1.5.11 One reason for the high ratio is the complexity of the chemistry in multipurpose plants producing specialty chemicals. Complex and multistage purification processes lead to an additional increase of the waste-solvent amount.12 It is to be expected that this ratio will even increase in the future because specialty chemicals tend to become more complex (i.e., have more product steps) and standards for product quality will become more restrictive. Thus, the large solvent-to-product ratio of specialty-chemical production illustrates the high importance of solvents in the Swiss chemical industry. 2.2. Solvents Used. The Swiss chemical industry is characterized by products creating a high added value such as pharmaceuticals and special chemicals (denoted as “specialty chemi-

Figure 2. Mass flows in the production site Basel of the Roche company, 1998.10 This mass flow analysis does not include waste solvents which are regenerated inside the process facility (process-integrated recycling). Note that the presented numbers also include a water fraction that was not revealed explicitly in this study. This is the reason the mass balance does not come out even. Presumably, a large fraction of water ends up in the waste-solvent mixture for incineration.

cals”). In 2004, 61% of the worldwide sale of the top-ten Swiss chemical companies resulted from pharmaceuticals and diagnostics. Special chemicals such as plastic additives or reactive dyestuffs accounted for another 24%.4 The spectrum of products in the Swiss chemical industry is very broad and is estimated to cover more than 30000 single products. However, the quantities of manufactured products can often be measured in kilogram rather than ton.4 The structure of the Swiss chemical industry is also reflected in its specific manufacturing plants. In general, these plants have a broad spectrum of products, but produce only small amounts of each. The production of such specialty chemicals is usually carried out batchwise in multipurpose plants. This means that in the beginning of a so-called “production campaign”, the

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Figure 4. Shares of technologies for the externally treated waste solvent in the six chemical plants (2002).

Figure 3. Quantity-based rankings of the 50 organic solvents used in the six chemical plants under study in 2002.

production equipment is put together according to the needs of the respective production step. At the end of the campaign, which usually lasts only weeks, the equipment is rearranged for the next production step or for the next product. Some of these plants are used mainly as pilot plants to test the scale-up until a product is launched on the market. Afterward, the production is transferred outside of Switzerland for economic reasons. Organic solvents are used for different purposes in the chemical industry, namely, as reaction media for chemical synthesis, for product purification, as raw material, as formulation media, and for rinsing of the reaction vessels before using them in the next production step. To get an overview on the types and amounts of solvents used in the Swiss chemical industry, a survey was carried out that included six of the largest chemical production plants in Switzerland.7 These plants had an overall consumption of about 120000 t of fresh solvent, out of 250000 t totally used in the Swiss chemical industry.4 The survey showed that, in the year 2002, 50 solvents were used in quantities of over 10 t/year. The detailed list of the 50 solvents used in 2002 is presented in the Supporting Information. Figure 3 shows a quantity-based ranking of these solvents using the data of the six chemical plants under study (black bars in Figure 3). These results can be strongly influenced by plants having a high overall solvent consumption. Therefore, we also investigated the solvent use in the six plants individually. We assigned points from 10 to 1 for the 10 solvents showing the highest consumptions in the individual plants. The points were then summed up for all solvents. The ranking based on the individual solvent consumptions in the six plants under study are also shown in Figure 3 (gray bars). The first three solvents of the quantity-based ranking accounted for about 65% of the total consumption in 2002 (tonnage-wise). Acetone contributed nearly 30% (34000 t), followed by methanol with 20% (23000 t) and toluene with about 15% (18000 t). The result of the plant-specific rankings confirms that methanol, toluene, and acetone were the most important solvents in terms of quantity used. Additionally, 2-propanol, ethyl acetate, and ethanol showed a high relevance in most of the plants, although they were used in smaller total amounts (5700, 8000, and 7200 t, respectively). All other solvents were consumed in fewer amounts in the plants under study. The large variety of solvents used in the Swiss chemical industry is due to the broad product spectrum and the complexity of the manufactured products that often require several produc-

tion steps. Usually, every synthesis step uses its own solvent. For pharmaceuticals, on average 0.9 types of solvents are necessary per production stage.13 Thus, when a product has 10 synthesis steps, this leads to a consumption of nine different solvents for only one end product. It can be summarized that, in the Swiss chemical industry, a very broad spectrum of solvents is used. This variability, of course, has to be dealt with in subsequent waste-solvent management. Another characteristic of the used solvents is that the amount of a single solvent can be very small. 2.3. Waste-Solvent Treatment. After a solvent has been used in a chemical process it cannot, as a rule, be reused without prior treatment because it may (1) be mixed with water or other solvents, (2) contain particulate matters or salts, or (3) be contaminated with educts, products, byproducts, catalysts, or any other auxiliaries. Most of the solvents used in a chemical process are processed internally in the process where they accrue. Such processintegrated recycling allows for the use of various recycling technologies, such as distillation (flash distillation, rectification), extraction, stripping, decantation, adsorption, or membrane technologies because the waste solvent has a constant composition.14,15 But part of the used solvents also leaves the production facility and has to be treated externally in special treatment plants. In the following we will only deal with waste solvents that are externally treated. Such a treatment may be done within the same chemical production site where the waste solvent occurs as well as off-site. Three different treatment options for the external treatment of waste solvents are applied in the Swiss chemical industry. These are recycling, thermal treatment, and disposal. The externally regenerated waste solvent is often treated in centrally managed solvent recycling centers using distillation. Thermal treatment of waste solvents can be done in wastesolvent treatment facilities, which are run by the companies themselves. Usually steam and electricity is produced which is used directly on the production site. Solvents that are only slightly contaminated may also be used to substitute fossil fuels in company internal power stations or waste-gas treatment facilities. Another purchaser of relatively pure waste solvents is the cement industry, which uses it to substitute coal and heavy fuel oil. Disposal is of minor importance in waste-solvent management. The only type that still plays a role in Switzerland is the drainage in wastewater treatment plants. Such a discharge is done only for waste solvents that are highly degradable and have high water contents (e.g., water/methanol mixtures).16 The shares of these technologies are very different according to the circumstances of the specific plants. Figure 4 shows the average share (mass-based) of the six plants under study for the year 2002.7

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Twenty-five percent of the waste solvents was recovered mainly using distillation (rectification). Of these, 18% was reused in the production processes inside the chemical plant (recycling). Seven percent was used outside the plants for applications with lower quality requirements than the original chemical process (down cycling). Typical purchasers for such low-grade solvents are, for example, the coloring and varnish industry or the automotive industry, which uses 2-propanol from chemical synthesis as antifreezes in windscreen washer systems.12 An additional 3% of the waste solvents was treated in the regeneration facilities but just to prepare them for use as alternative fuels. In total, 71% of the waste solvents underwent thermal treatment. About one-fourth of this amount met the requirements (e.g., high net calorific value, low water content, and low chloride concentration) for use as alternative fuels in different company internal or external facilities. The remaining threefourths was strongly contaminated and was therefore incinerated in special waste-solvent treatment plants or in a hazardous-waste treatment plant (a rotary kiln, originally designed for solid wastes, which was operated by one of the chemical plants under study). In all incineration technologies, the waste solvent substitutes fossil fuels and contributes to the energy production, mostly in the form of steam and electricity. Finally, 4% of the waste solvents under study was sent to a wastewater treatment plant. Assuming these findings to be representative of the Swiss chemical industry, the total waste-solvent streams per treatment technology were extrapolated from the 120000 t/year used in the six plants under study to the total of 250000 t/year of solvent used in Switzerland (Figure 1). Thus, the regenerated waste solvent-without process-integrated recycling-amounts to approximately 60000 t/year, the incinerated waste solvent to approximately 180000 t/year, and the waste solvent disposed of in wastewater treatment plants to approximately 10000 t/year. The latter two results correspond well with the figures in Figure 1. From an environmental perspective, distillation as well as incineration can result in environmental benefits. Distillation of waste solvents saves raw materials and incineration of waste solvent produces energy (steam and electricity), which otherwise would have been produced with, e.g., fossil fuels. To get an impression of the energy-saving potentials due to optimized waste-solvent management, the following estimation was made. Assuming a similar efficiency of waste-solvent incineration plants and other types of thermal power plants, and assuming an average net calorific value of approximately 22 MJ/kg for waste solvents in Switzerland,17 the incineration of the annual 250000 t total waste solvent would save 5.5 × 106 GJ of feedstock energy from other sources (e.g., fossil fuels such as natural gas, oil, or coal) that amounts to a primary energy demand of 7.2 × 106 GJ-equiv (cumulative energy demand),18 calculated for the average European energy supply mix. This accounts for approximately 0.6% of the primary energy demand of the total feedstock energy used in Switzerland in 2005 (total 1.16 × 106 TJ-equiv, calculated based on Swiss energy statistics19). On the other hand, if the 250000 t total waste solvent was distilled, approximately 1.4 × 106 GJ-equiv would be needed for the distillation assuming average energy requirements of 1.4 kg of steam/kg of waste solvent.20 But if the avoided petrochemical solvent production is taken into account, approximately 12 × 106 GJ-equiv is saved (approximately 0.9% of the primary energy demand of the total feedstock energy used in Switzerland in 200519) based on the assumption of an average

solvent recovery of 0.71 kg/kg of waste solvent20 and an average energy demand of 67 MJ-equiv/kg of fresh solvent (cumulative energy demand, calculated for the solvent mix shown in Figure 3).18 Although distillation shows a higher savings potential than incineration, there are waste-solvent mixtures for which incineration leads to higher energy savings, e.g., in cases where separation is energy-intensive due to small differences in boiling points for the compounds being separated or strong intermolecular interactions. Therefore, the total annual energy-savings potential of environmentally meaningful waste-solvent management may be even higher than the 12 × 106 GJ-equiv estimated exclusively for distillation. These figures suggest that choosing the optimal treatment option for solvents may lead to very significant energy savings, even in a country as small as Switzerland. 3. Waste-Solvent Management in the Swiss Chemical Industry 3.1. Boundary Conditions and Their Influence on WasteSolvent Management in Daily Operation. To study by what aspects decisions on the waste-solvent treatment are influenced in daily operation, we made expert interviews. These interviews showed that the waste-solvent management situation was rather different in the six plants under study. No “general” solutions or strategies applicable to all the plants were identified. Nevertheless, some general statements on waste-solvent management can be made which are valid for all the plants: (1) Regeneration and thermal treatment of waste solvent was usually carried out by the chemical plants themselves. If incineration facilities were lacking, the waste solvent was sent to other companies for treatment at market price conditions. (2) The chemical plants studied did not contain a very diverse set of regeneration technologies. Mainly multipurpose distillation columns, and in particular, batch columns, were used for wastesolvent regeneration. No sophisticated technologies such as membrane technology were operated. (3) The overall waste-management situation seemed rather suboptimized at first sight. We found, for example, that a lot of solvents were incinerated, which in principle, could have been distilled. (4) Decisions on how a waste solvent should be treated were usually made “just in time” and seldom planned prospectively. Furthermore, we found that various boundary conditions strongly influenced waste-solvent management and thus limited the degrees of freedom for decisions. The most important boundary conditions and their main decision factors are summarized in Table 1. The largest influence on waste-solvent management was found to come from the existing technologies on the site. If a plant had a large regeneration facility, distillation was favored, and if a plant had its own incineration kiln, thermal treatment of waste solvent dominated. Amortization of technical devices and a high utilization rate were key factors in waste-solvent management. To give an example, in one of the plants a hazardous-waste kiln was operated. This kiln had been designed for incineration of plant internal and external hazardous waste, as a rule, solid waste. In times of low utilization of the kiln, waste solvents were also incinerated to use the free capacity, even if burning liquids in such a kiln is not the best option from a technical point of view. Another large influence came from the current free capacity of the treatment devices. If the distillation columns were coincidentally occupied by other processes, the solvent was burnt instead. If the incineration device was occupied by, e.g., a large external order, distillation

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Table 1. Boundary Conditions and Decision Factors for Waste-Solvent Management boundary conditions

decision factors

existing technologies on the site

(a) plant has own regeneration or incineration devices (b) free capacity in the on-site devices (c) treatment facilities are organized as centrally managed profit centers

storage capacity and costs

(a) limited storage capacity (b) costs for storage and safety measures

legislation and guidelines

(a) regulation of use of nonvirgin solvent in production processes (b) regulation of use of waste solvent as alternative fuel (c) regulation of waste-solvent disposal in wastewater treatment plants

logistics and handling

(a) consumer within the plant (b) total amount and continuity of the accrued waste solvent (c) no market for noncontinuous down cycling products (d) costs and safety measures for transports

economy

(a) actual price of solvent/availability on the market (b) actual price of waste-solvent treatment (energy, labor costs, disposal of residue)

was favored. In one of the plants under study, a facility for thermal treatment of waste solvents was also responsible for the energy production of the whole chemical production site. Besides the treatment of waste solvents, the incineration plant therefore also had to provide a continuous amount of steam and electricity. This led to the situation that a waste solvent could only be incinerated if it “fit” into the fuel program of the plant. A third aspect which influenced the waste-solvent management was the organizing of treatment facilities as profit centers. In the late 1990s there was a general trend in the Swiss chemical industry to outsource basic services and to concentrate on the core business, in this case, the production of chemicals. Therefore, in some chemical plants the regeneration and incineration facilities were organized as profit centers. The facilities were allowed to accept external waste solvents and to ask for market prices. This opening of the treatment services to an external market strongly influenced the company’s internal waste-solvent management, by binding capacities and by increasing costs due to market-related prices for waste-solvent treatment. Another large influence on waste-solvent management arises from storage capacity of the production sites. In principle, waste solvents can be kept until there is free capacity in the regeneration facility. Also, storage of regenerated solvents for the next production campaign is possible. However, it was found that most of the plants had quite limited storage capacity. Therefore, incineration was often favored even if, theoretically, regeneration of the waste solvent was possible. To give an example, one of the plants under study had a production site located in the middle of a city. As the area was completely overbuilt, there was no place for additional storage tanks. The site had a maximum storage capacity for 2 days of waste-solvent receipts. The waste-solvent management in this plant was therefore strongly influenced by the needs of getting rid of the waste, as an accumulation of waste solvents would lead to a production stop. Another aspect that influences the waste-solvent management strategies is storage costs. As solvents are inflammable and often toxic, rigorous safety precautions have to be taken when storing solvents. This is particular the case if solvents have to be stored in tank wagons for a short time in case all fix installed storage tanks are full. As all this is costly, chemical plants usually tend to choose “fast” solutions, which often is incineration. Further limitations arise from legislation and implementation guidelines. Especially when solvents are used to produce pharmaceuticals, they have to fulfill rigorous quality standards such as regulations imposed by current good manufacturing practice (GMP) of the U.S. Food and Drug Administration.21 Therefore, companies often hesitated to use regenerated solvents

because the quality controls needed for GMP standards would be too costly. In such cases, waste solvents were incinerated. Whereas almost all types of waste solvents can be treated in special waste-solvent incinerators, the use of waste solvent as alternative fuel is limited in terms of waste-solvent composition (in particular, metals and organic halogens) as well as origin (e.g., waste solvent from the synthesis of biologically active products must not be treated in cement kilns).22,23 The wastesolvent treatment in wastewater treatment plants is also strictly regulated. As well as general standards of wastewater treatment that have to be followed,24 the authorities define additional specific requirements for the treatment of industrial wastewater.25 Other aspects influencing waste-solvent management lie in the area of logistics. It was found that waste solvents were preferentially regenerated when a consumer within the plant could be found. For instance, very often production facilities that had already used a specific solvent before wanted to reuse it for their next campaign. One of the chemical plants distinguished between “customer solvents” and “pool solvents”. Customer solvents were regenerated separately and the customer defined the standard of the distilled product. By contrast, for pool solvents all solvents of the same type were mixed after regeneration and stored in a large tank until customers ordered it. The quality demands of pooled solvent were usually lower than those for customer solvents. The advantage of “pooling” was a better utilization of the tank capacities and a lower incineration rate. A further limitation due to logistical aspects was the amount and continuity of the waste solvent. Solvent recycling in batch columns requires at least 3-5 m3 and recycling in continuous columns even 20-40 m3, at minimum.7 If the amount of waste solvent was lower than the required amount for distillation, the waste solvent was incinerated instead. Another finding was that few solvents were used outside the chemical plants. A use of waste solvents in other industries with lower quality demands (down cycling) is possible, but uncommon. The problem with waste solvents from the Swiss chemical industry seemed to be the lack of continuity in amount and quality, caused by the short production campaigns. Finally, it was found that transportation of waste solvents outside the plant is avoided if possible. As waste solvents are hazardous wastes, transport is associated with high risks. Moreover, it is costly and needs official authorization.26 Finally, waste-solvent management was strongly influenced by economic factors. Expensive solvents such as tetrahydrofurane (price: 3410 US$/t in Feb 2005)27 were recycled more, consequently, than cheap solvents such as acetone (price: 1320 US$/t in Feb 2005).27 The price of most solvents is coupled with the price of crude oil and can fluctuate by (50% during

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7705 Table 2. Stages in Chemical Processes Development, Their Decision Degrees, and Grades of Knowledge on the Waste-Solvent Treatment Optionsa

stages in process development research on chemical synthesis process design campaign change scale-up for launching on other production site operation

decision degree in terms of waste-solvent management

knowledge of waste-solvent properties

knowledge of waste-solvent treatment equipment

example for environmental decisions concerning waste-solvent treatment or optimization

high medium medium medium

low medium high high

low high high high

choice of simpler synthesis route optimizing operation conditions choice of waste-solvent treatment option optimizing operation conditions

low

high

high

change to other available waste-solvent treatment technologies

a “Campaign change” and “scale-up for launching on other production site” are stages which do not fit directly into the scheme of classical process design.29 They were added as they are important stages for Swiss chemical production.

1 year. Also, availability on the market played a role. In times when a specific solvent was scarce, distillation gained in importance. Therefore, the decision on which treatment option would be chosen for a certain solvent could turn out to be different at a particular time. Also, the cost for treatment of waste solvents had an influence on the waste management strategies. The main costs for operation of the regeneration facilities are for energy consumption, labor, and the disposal of distillation residues. Costs for operation of the incineration facility mainly originate from labor costs and consumption of auxiliary chemicals. Limiting conditions for recycling was, e.g., distillation residues, which would become so contaminated that their thermal treatment would be more expensive than the thermal treatment of the original waste solvent. Also, cases were found where regeneration would be too expensive to reach the required quality standard for regenerates and cases where the amount of waste solvent was too low to cover the cost of batch distillation. In such situations incineration of waste solvents was preferred. 3.2. Why Do Environmental Criteria Play a Minor Role in Current Waste-Solvent Management? All companies under study made efforts to improve their environmental performance, also in waste-solvent management. But measures for environmental improvement generally focus on local problems and not on life-cycle thinking. Usually, simple guidelines concerning waste in general are established. For example, the priorities for waste management are defined as (1) waste avoidance, (2) waste reduction, (3) waste recycling, and (4) waste disposal. This “waste hierarchy” is very rough and it is based on the assumption that recycling is always environmentally preferable to disposal. This assumption, however, is questionable. For instance, Capello et al. have shown that either treatment option, incineration or distillation, may be environmentally superior to the other, depending on the composition of the solvent as well as on process parameters, such as the recovery rate.9 Besides such waste-hierarchy guidelines, indicators from the “responsible care program” were also used as measures of the environmental performance of waste management.28 Such indicators are, e.g., the total amount of hazardous and nonhazardous waste. One major shortcoming of these indicators is that they do not differentiate between different types of waste and that they are not specific to waste-solvent treatment processes. Apart from these rather crude environmental rules of thumb, environmental aspects are apparently of minor importance in current waste-solvent management, as can be seen in Table 1. We identified three reasons for this: first, the boundary conditions met in the Swiss chemical plants set narrow limits for management options. This is, on one hand, due to the specific features of the accrued waste solvents, they show a large variation in type and contamination, occur in rather small

quantities, and are normally produced noncontinuously. On the other hand, the existing technologies on the production site, their capacities, and utilization ratios are limiting from a technical point of view. Second, since environmental aspects are already subject to regulations and guidelines, mostly with respect to emission limits, there is no mandatory need for further environmental optimization. Third, we found no instruments or tools that were used specifically to quantify the environmental impact of waste-solvent treatment processes. The lack of adequate and reliable tools to assess the environmental impact of waste-solvent treatment is an important reason for the low importance of environmental criteria in waste-solvent decisionmaking. 3.3. Integrating Waste-Solvent Management Concepts in Product and Process Design. The results of the survey showed that only a few technologies are important for externally treated waste solvents in daily operation. This is an advantage for developing environmentally sound waste-solvent management concepts because mainly the variations in distillation and incineration technologies have to be taken into account. An additional advantage is that waste solvents are captured and transported from the place of use to the treatment facilities and, thus, the treatment process or technology can be changed independently from the production process. This theoretical flexibility, of course, is limited by the constraints mentioned in Table 1. However, boundary conditions such as logistical aspects, storage capacity, and capacity of a plant depend on the planning interval. A change of the waste-solvent treatment process or even of the technology during the operating phase of production processes may sometimes be difficult. But in the stage of product or process design these boundary conditions are less restricting and various decisions may be made that influence the external waste-solvent treatment. Even if the grades of knowledge about the waste-solvent properties or the equipment available differ during the planning process, environmental aspects may theoretically be integrated at every stage. Table 2 gives an overview of the different stages during the development of a chemical process and examples of environmental decisions which can be made during those stages. Theoretically, an optimization of the use of solvents can be done already in the research stage. Decisions on synthesis routes, reaction steps, and purification methods influence the type and amount of used solvents and thus the type and amount of accruing waste solvents.29 We found that none of the plants under study systematically integrated (waste-) solvent management at this stage. One reason might be the high pressure for market introduction of specialty chemicals. A delay of market introduction for a special chemical (e.g., a reactive dyestuff) can lead to a loss of one million US$ per month or even to a

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Figure 5. Solvent flows in the Swiss sites of Ciba Specialty Chemicals in 2002.

loss of one million US$ per day as far as pharmaceuticals are concerned.13 Another reason might be that costs for use of solvents and for treatment of waste solvents are still very low compared to other costs of a chemical plant. Therefore, the pressure to integrate solvent management aspects during the design stage is still very low. Waste-solvent management can furthermore be influenced during the process-design stage. There are less degrees of freedom for decision-making than in the research stage but knowledge of the waste-solvent properties has grown and there might already be information on the production site, thus also existing treatment technologies. One possibility for obtaining specific recommendations on environmentally optimal wastesolvent management is the use of decision-support tools. These tools provide quantitative results on the environmental impact of the various waste-solvent treatment strategies. Decision support tools, which are applied in the research or process-design stage, have to be capable of dealing with missing or imprecise information because, usually, no precise data on the wastesolvent treatment processes (e.g., measured energy demands or recycling rates) are available. Such tools have been developed by, e.g., Cavin or Capello et al.30,31 Another opportunity for optimizing waste-solvent management was identified in the stage of change of production campaigns on existing multipurpose equipment. In this situation the waste-solvent treatment also has to be specified or adapted to a new campaign and can therefore be influenced effectively. In this stage data on the amount and properties of the occurring waste solvent is available as well as data on the treatment technologies. Management concepts that are applied at this stage have to deal with less uncertainty than in the process design stage, as process data are now available. An assessment based on the specific features of a waste solvent (e.g., chemical properties, pollutants, and net calorific value), including all technical treatment alternatives available on the site, is possible. To allow a holistic view of the problem, such concepts should always adapt a life-cycle perspective. A similar decision situation as for a product campaign change can be found when a chemical process undergoes a scale-up for transfer from the launch site to the final production site. Information on waste and waste-treatment technologies are available and the decision degree is still high compared to the operation stage. Again, environmental decision-support tools may be used in these stages to quantify the environmental impact of the various wastesolvent treatment options. Finally, waste management concepts may also be applied to already operating processes, although decisions with regard to the waste-solvent treatment are restricted the most. When capacity is available on several treatment options at the same time, as may be the case if a set of multipurpose equipment is used for the waste-solvent treatment, alternatives might be possible. In particular, if the waste-solvent treatment is outsourced to specialized companies, such a situation might occur. These companies have the option to use various technologies

for waste-solvent treatment (incineration capacity and multipurpose distillation columns). Therefore, they participate in the decision-making process in collaboration with the contractor, or they do the decision-making independently, if they sell regenerated solvents to third-party companies. 4. Decision Situations for Environmental Waste-Solvent Management in Practice: Two Case Studies 4.1. Case Study 1: Evaluating Various Routes for Outsourced Waste-Solvent Treatment at Ciba Specialty Chemicals AG. After the merger of Ciba-Geigy and Sandoz in 1996 and the spin-off of Ciba Specialty Chemicals AG (Ciba), the regeneration, disposal, and recycling of some 150000 t/year of solvents (out of approximately 200000 t/year, see Figure 5) from the Swiss sites, either used in the production of specialty chemical products or generated as byproducts, were regulated in a number of long-term contracts with joint ventures, service companies, and former affiliates. Some of the contract partners also provide energy to both Ciba and third parties, and a major source of this energy is the incineration of waste solvents. The contractors therefore have an incentive both to maximize the volumes of waste solvent incinerated and to minimize the proportion of fuel purchased to generate steam and electricity, leading to overproduction of steam when waste-solvent volumes exceed energy demand. The decision to regenerate or incinerate lies in most cases with Ciba and is based on volume requirements plus a simple comparison of cost of new solvent with the contractor’s standard cost of regeneration. For surplus volumes, the decision to regenerate or incinerate lies with the contractor. In all of these situations, a clear environmental justification for the chosen route is rarely available. These long-term contracts are now coming to an end, and Ciba has the opportunity to open these contractual “black boxes” and analyze the economics and environmental impact of each existing route for each significant solvent or solvent mixture as well as evaluate possible new routes. In the last 10 years, additional capacity and routes have become available, and extension of in-house regeneration capacity is possible if supported by environmental requirements or a sound economic case. Ciba Specialty Chemicals believes that the integration of environmental management and product stewardship into all relevant business processes are essential elements in business leadership and is committed to exploring all reasonable opportunities to improve performance in these areas. Line management is responsible for identifying and implementing such improvement potential and is supported in this by specialists and the environmental audit function of the global Ciba group. For some distillation residues, halogenated solvents, and highly contaminated wastes, there is clearly no alternative to high-temperature incineration followed by multistage purification of off-gases, such as is provided in the local hazardous-

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Figure 6. Technically possible treatment options for the dimethoxy ethane (DME) mixture.

waste incinerator. Other solvents are used in such volumes that logistics demand regeneration as close as possible to the producing site. However, for a significant volume (ca. 30000 t/a) lightly contaminated solvents and solvent mixtures such as ethanol, methanol, 2-propanol, methyl ethyl ketone, and toluene, a number of routes are open, ranging from regeneration on- and off- site and use as fuel in standard steam boilers through combustion in specially-designed steam boilers with waste gas cleaning to co-incineration with effluent sludge. For all waste solvents, precise information is available with respect to the composition and, for in-house treated waste solvents, also with respect to the treatment process. But with consideration of the waste-solvent treatment that is conducted in service companies or former affiliates, detailed information on the process conditions or even technology is sometimes scarce. Concepts that allow for the evaluation of environmental impacts of the various routes must therefore be able to deal with missing or imprecise information. A potential tool that may be used in such a situation has been developed recently (see Capello et al.).31 This tool enables for both line management and group auditors to evaluate existing and possible new routes for used solvents, in preparation for the end of the existing contract periods. Although the sustainability of waste disposal and recycling practices is not subject to approval by environmental authorities in Switzerland, the results of analysis will provide a record for future management and demonstrate that environmental impacts have been taken into account. 4.2. Case Study 2: Choosing the Best Ecological Solution for the Treatment of DME at the Lonza Production Site in Visp. Lonza Group Ltd. (Lonza) is an international chemical and biotechnology company operating mainly as custom manufacturer of chemical intermediates and active ingredients as well as biopharmaceuticals for the pharmaceutical and agrochemical industries. The waste-solvent mixture we discuss in this case study originates from the first synthesis step of the production of a pharmaceutical ingredient. The waste-solvent mixture contains dimethoxy ethane (DME) as the main component (21.3 wt %) and water (35.3 wt %), toluene (41.3 wt %), 1-ethoxy1-methoxy ethane (EME) (1.3 wt %), and impurities (0.7 wt %) as secondary components. This DME mixture accrues in a quantity of 135 t annually at the Lonza production site in Visp, Switzerland. Currently, from the 135 t of DME mixture, 25 t of pure DME (purity of >98%) is recovered using distillation. The regenerated DME is reused in the same synthesis step (scenario DME distillation). The distillation of the DME mixture is rather

complex. First, the organic phase of the DME mixture (90 t/a) is separated from the aqueous phase (water/ethanol, 45 t/a) using phase separation. After that, the DME is recovered batchwise with two distillation steps (Figure 6). In the first distillation step, DME and EME are partly separated from the toluene (35 t/a). The residues (55 t/a) are incinerated in the local boiler house to produce steam. In case no steam is required, these residues may also be treated in cement kilns. In the second distillation step, which is conducted on the same distillation column, the DME is separated from the EME. To obtain the required DME quality and reduce the content of EME below 0.3 wt %, the distillation process has to be conducted with very high reflux ratios up to 10:1. Therefore, this distillation is very energyintensive. Approximately 2.3 kg of steam/kg of DME/EME mixture is needed, which is almost 1 kg more steam than in an average distillation process.18 The residues of the second distillation are incinerated in a special waste incineration plant. From a technical point of view, two other treatment options for the DME mixture are possible (Figure 6). The first option is that, after the phase separation, the DME mixture is processed batchwise in the first distillation column to fuel substitute (80 t/a). Such fuel substitute shows high quality (high net calorific value and low contamination) and it may therefore be used for steam production in the local boiler house (scenario DME fuel). In the second option, the entire DME mixture after phase separation (90 t/a) is incinerated in a special waste incineration plant (scenario DME incineration). All three treatment scenarios (DME distillation, DME fuel, and DME incineration) have their advantages and disadvantages that are summarized in the following: (1) Scenario DME distillation: (+) The recovered DME is an expensive solvent (approximately 4 US$/kg).27 Therefore, distillation is economically comparable to incineration. (+) The DME mixture accrues in large amounts and continuously (10 t/d, over 200 d/a). (+) The purified DME is reused in the first synthesis step. This is in accordance with the GMP guidelines.21 (-) The separation of DME and EME requires a lot of energy due to the high reflux ratio of 10:1. (2) Scenario DME fuel: (+) The fuel substitute of high quality can be used directly instead of fossil fuels in the boiler house for steam production. (-) When no steam is needed in the production site, the fuel substitute has to be incinerated in cement kilns, which is costly. (-) The distillation column is occupied, although no solvent is regenerated.

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(3) Scenario DME incineration: (+) Free capacity in the distillation column for more easily separated waste-solvent mixtures. (+) Energy recovery for steam production. (-) The incineration of waste solvent in the special waste incinerator is expensive due to the strict regulations in terms of emission limits and, thus, the ancillaries needed to fulfill these regulations. The three scenarios are all technically possible. But the scenario “DME fuel” is rather unlikely because when the fuel substitute cannot be used in the boiler house, it is an expensive option and the distillation equipment is blocked. Additionally, the current situation at the production site in Visp is that the in-house regeneration facility is operating at full capacity and regeneration capacity is currently in great demand. The other scenarios “DME distillation” and “DME incineration” are economically equal. Therefore, Lonza would like to include environmental aspects for the decision of whether the current DME treatment should be continued, although it includes a very energy-intensive distillation process, or whether it would be better that the DME be incinerated directly and the free capacity of the distillation column be used instead to regenerate other waste-solvent mixtures that are more easily separated. In the case of the DME mixture, primary data are available since the distillation processes are already operating. This simplifies the environmental assessment. Also the technology of a potential incineration of the DME mixture is known (special waste incinerator). Based on this precise information, the environmental assessment tool developed by Capello et al. was used to calculate the environmental impact of both treatment options.31 This generic life-cycle assessment tool allows for the environmental comparison of distillation (rectification) and thermal treatment in special waste-solvent incinerators and cement kilns for specific, user-defined waste-solvent mixtures. These technologies are represented by so-called life-cycle inventory (LCI) models. With these models, waste-solvent specific life-cycle inventory data, such as emissions flows, ancillary uses, and generation of coproducts, are calculated and linked to background inventory data (production of ancillaries, fuels, and energy). Finally, the full life-cycle inventory can be assessed with various life-cycle impact assessment methods. With regard to the DME mixture, the environmental impact was expressed as cumulative energy demand (Figure 7). The distillation turned out not to be environmentally superior to incineration because energy for two distillation steps is required (Figure 7). In particular, the second distillation step is energy-intensive. Therefore, from an environmental perspective it could be favorable to use free distillation capacity for other waste-solvent mixtures, which are more easily separated (lower reflux ratio) than the DME/EME mixture and which can be distilled in one step and to incinerate the DME mixture instead. 5. Conclusions Organic solvents and, thus, the resulting waste solvents play an important role in the Swiss chemical industry. Waste solvents are classified as hazardous wastes as they are volatile, inflammable, and as a rule toxic. Also from an energetic point of view, it is important to make sound management decisions. As waste solvents generate large mass streams and as they can either be recycled to save fresh solvents or be thermally treated recovering the chemically bound energy, sound waste-solvent management can lead to large savings of raw materials and energy. Choosing the right treatment option is therefore of high importance from an environmental point of view and more emphasis should be

Figure 7. Cumulative energy demand of the petrochemical production and the treatment of the DME mixture using distillation and incineration. The results were calculated using the environmental assessment tool developed by Capello et al.31

placed on this. This is true not only for daily operation but also for product and process design. Looking at the different stages in chemical product and process design, we found that environmental aspects might theoretically be integrated at every stage. The stage with the highest potential for optimizing waste-solvent management in the Swiss chemical industry was found to be the campaign change or the scale-up for the launching of a chemical on another production site. In both situations the knowledge of occurring waste solvent as well as of existing technologies on the site is high and there are still more degrees of freedom than in the operating stage. Concerning the operation stage in Swiss chemical plants, we found that the large potential for environmental optimization in waste-solvent management is partly offset by economical and logistic boundary conditions limiting the degrees of freedom of waste management in daily operation. Such conditions arise from long-term investment decisions such as in-plant infrastructure. Sometimes, the need to cover fixed costs by using free capacities leads to restrictions in theoretically possible treatment options. Another aspect, which we identified in the chemical companies, was a general tendency for “fast” solutions. As storage capacities are scarce and costly, many solutions, which could have been optimal from an environmental point of view, were out of the question. Nevertheless, as the two case studies showed, there is room and willingness for improvements. With these case studies two different situations are presented: In the first case study, incineration as well as distillation might be applied since waste solvent is treated in large part by service companies that have the choice between both technologies. In the second case study, it is the goal of the company to optimize the existing in-house regeneration facility with regard to the currently operating processes because regeneration capacity is in great demand. Both cases illustrate the large potential and relevance of implementing environmental life-cycle thinking into practice. One possibility to support waste-solvent management is the use of specialized tools which have recently become available (see, e.g., Cavin or Capello et al.).30,31 With these tools, waste-

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solvent treatment processes can be analyzed based on a lifecycle perspective. Acknowledgment 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, Valorec Services AG, and Siegfried Ltd. for funding of this project and also for providing data. Supporting Information Available: Tabular listing of 50 solvents used in a 2002 survey of several of the largest chemical production plants in Switzerland (PDF). This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Allen, D. T.; Shonnard, D. R. Green Enginnering - EnVironmentally Conscious Design of Chemical Processes; Prentice Hall: Upper Saddle River, 2002. (2) Anastas, P. T.; Heine, L. G.; Williamson, T. C. Green Engineering; American Chemical Society: Washington, 2001. (3) Eidgeno¨ssische Oberzolldirektion. Schweizer Aussenhandel 2002: Statistik nach Waren und La¨nder (Statistics of Swiss External Trade 2002); Eidgeno¨ssische Oberzolldirektion: Bern, 2003. (4) SGCI, Swiss Society of Chemical Industries. The Swiss Chemical and Pharmaceutical Industry; Schweizerische Gesellschaft fu¨r Chemische Industrie: Zurich, 2004. (5) BUWAL, Swiss Federal Office for Environment. Anthropogene VOC-Emissionen Schweiz 1998 und 2001; Bundesamt fu¨r Umwelt, Wald und Landschaft: Bern, 2003. (6) Whim, B. P.; Johnson P. G. Directory of SolVents; Blackie Academic & Professional: London, 1996. (7) Expert Panel of the project “Waste-Solvent Management in Chemical Industry” consisting of Ciba Speciality Chemicals AG, Ems-Dottikon AG, Lonza Group Ltd., Novartis Pharma AG, Hoffmann-La Roche AG, Siegfried Ltd., and Valorec Services AG, 2003-2005, http://www.sust-chem.ethz.ch/ research/lifecycle/solvents.html. (8) BUWAL, Swiss Federal Office for Environment. Sonderabfallstatistik (VVS Statistik) 2002; Bundesamt fu¨r Umwelt, Wald und Landschaft: Bern, 2003. (9) Capello, C.; Hellweg, S.; Hungerbu¨hler K. Environmental Assessment of Waste-Solvent Treatment in the Pharmaceutical and Specialty Chemicals Industry. Part 2. General Rules of Thumb and Specific Recommendations. Submitted for publication to the J. Ind. Ecol. 2005. (10) F. Hoffmann-La Roche AG. Internal report on the mass balance of the Basel plant; Roche AG: Basel, 1998. (11) Utiger, L. Internal study on the mass flows for the production of fertilizer, basic intermediates and exclusiVe products; Lonza AG: Visp, 1997. (12) Bruder, C. Abfall-Lo¨sungsmittelmanagement in der chemischpharmazeutischen Industrie; Diploma thesis at the Swiss Federal Institute of Technology Zu¨rich, Gruppe Umwelt- und Sicherheitstechnologie: Zurich, 2000. (13) Seyler, C. Ein inputabha¨ngiges O ¨ koinVentar-Modell fu¨r die thermische Verwertung Von Abfall-Lo¨sungsmittel in der chemisch-pharmazeutis-

chen Industrie; Ph.D. Thesis no. 15089 at the Swiss Federal Institute of Technology Zu¨rich: Zurich, 2003. (14) Smallwood, I. SolVent RecoVery Handbook; McGraw-Hill: New York, 1993. (15) Cheremisinoff, N. Handbook of Chemical Process Equipment; Butterworth-Heinemann: Boston, 2000. (16) Aegerter, D. O ¨ koeffizienz der Entsorgung Von Abfall-Lo¨sungsmitteln in der Abwasserreinigungsanlage, Abfall-Lo¨sungsmittel-Verwertungsanlage, Zementwerken und Regeneration; Novartis Services AG: Schweizerhalle, 1998 (17) Seyler, C.; Hofstetter, T. B.; Hungerbu¨hler, K. Life cycle inventory for thermal treatment of waste solvent from chemical industry: a multiinput allocation model. J. Clean. Product. 2005, 13, 1211-1224. (18) Jungbluth, N.; Frischknecht, R. CumulatiVe Energy Demand. LCIA Implementation. Final Report ecoinVent 2000 No. 3; Swiss Centre for Life Cycle Inventories: Du¨bendorf, 2004. (19) Swiss Federal Office of Energy. Statistics of Swiss Energy Consumption 2005; http://www.bfe.admin.ch/energie, 2005. (20) Capello, C.; Hellweg, S.; Badertscher, B.; Hungerbu¨hler, K. LifeCycle Inventory of Waste-Solvent Distillation: Statistical Analysis of Empirical Data. EnViron. Sci. Technol. 2005, 39 (15), 5885-5892. (21) Food and Drug Administration. Current Good Manufacturing Practice (CGMP). Final Rule. 21 CFR Parts 808, 812, and 820 Medical DeVices; http://www.fda.gov/cdrh/comp/gmp.html, 1996. (22) Verordnung u¨ber die Luftreinhaltung, SR 814.318.142.1; Der Schweizerische Bundesrat: Bern, 1985. (23) BUWAL, Swiss Federal Office for Environment. Richtline zur Entsorgung Von Abfa¨llen in Zementwerken; Bundesamt fu¨r Umwelt, Wald und Landschaft: Bern, 1998. (24) Verordnung u¨ber den Gewa¨sserschutz, SR 814.600; Der Schweizerische Bundesrat: Bern, 1998. (25) BUWAL, Swiss Federal Office for Environment. Einleitung Von Abwa¨ssern der chemischen Industrie in Gewa¨sser und in die o¨ffentliche Kanalisation. Mitteilungen zum Gewa¨sserschutz Nr. 40; Bundesamt fu¨r Umwelt, Wald und Landschaft: Bern, 2001. (26) Verordnung u¨ber den Verkehr mit Sonderabfa¨llen, SR 814.610; Der Schweizerische Bundesrat: Bern, 1986. (27) Chemical Market Reporter Prices & People. Chem. Mark. Rep. 2005, 267 (7). (28) International Council of Chemical Associations. The Responsible Care Program; http://www.responsiblecare.org, 1992. (29) Hungerbu¨hler, K.; Ranke, J.; Mettier, T. Chemische Produkte und Prozesse, Grundkonzepte zum umweltorientierten Design; Springer-Verlag: Berlin, 1999. (30) Cavin, L. A systematic approach for multi-objectiVe process design in multi-purpose batch plants; Ph.D. Thesis no. 14898 at the Swiss Federal Institute of Technology Zu¨rich: Zurich, 2002. (31) Capello, C.; Hellweg, S.; Badertscher, B.; Betschart, H.; Hungerbu¨hler, K. Environmental Assessment of Waste-Solvent Treatment in the Pharmaceutical and Specialty Chemicals Industry. Part 1. The ECOSOLVENT Tool. Submitted for publication to the J. Ind. Ecol. 2005. (32) Stoye, D. Ullmann’s Encyclopedia of Industrial Chemistry; WileyVHC: Weinheim, 2000.

ReceiVed for reView April 25, 2006 ReVised manuscript receiVed August 15, 2006 Accepted August 22, 2006 IE060525L