Environ. Sci. Technol. 2002, 36, 5517-5520
How Green Is a Chemical Reaction? Application of LCA to Green Chemistry X A V I E R D O M EÅ N E C H , * J O S EÄ A . A Y L L O Ä N , A N D J O S EÄ P E R A L Departament de Quı´mica, Universitat Auto`noma de Barcelona, Bellaterra, Spain JOAN RIERADEVALL Departament d’Enginyeria Quı´mica, Universitat Auto`noma de Barcelona, Bellaterra, Spain
In the present work Life Cycle Assessment (LCA) is used in order to evaluate a chemical reaction from an environmental point of view. The objective is to assess the usefulness of this methodology as an environmental tool to be applied to green chemistry. As an example, two routes of obtaining maleic anhydride are compared using LCA, to ascertain which one is the most environmentally friendly. From the results obtained in this work it can be concluded that LCA seems to be a valuable tool for the environmental assessment of a chemical reaction, because it takes into account all the life cycle stages of the process and discusses the impact of the environmental burdens inventoried according to a diversity of impact categories.
in the chemical process, as well as the production and treatment of all wastes generated, must be considered (see Figure 1). Thus, although green chemistry focuses on pollution prevention, a complete LCA must also consider the wastes produced and their management. In the case of comparing two or more chemical processes of to obtain the same product, the stages corresponding to the distribution, use, and final waste management of such product can be taken out of the system under analysis. As an example, in the present paper the LCA conceptual tool is used for the comparison, from an environmental point of view, of two routes of maleic anhydride (MA) synthesis. A possible way to produce MA is the oxidation of benzene with oxygen gas over a V2O5 catalyst, at 3 to 5 bar of pressure and at a temperature of 350 to 450 °C (6, 7):
The use of butene as feedstock is an alternative way of obtaining MA. This process is also catalyzed by V2O5 under the same experimental conditions used for reaction [1] (6, 7):
Introduction From its appearance in the early 1990s, green chemistry has been a subject of increasing interest. Green chemistry has been defined as the use of chemistry for pollution prevention by means of a proper design of products and processes that reduces or eliminates the use and generation of hazardous substances (1). As a consequence, green chemistry focuses on the following: a) the process design, maximizing the amount of reactant that ends up in the product, and decreasing the amount of waste products (the atom economy principle (2)); b) the use of environmentally benign auxiliary materials (solvents, catalysts, etc.); and c) energy savings by means of the use of energy efficient systems and the design of suitable processes that minimize the energy consumption. Considering the scope of the green chemistry, it is clear that attention must be focused on all the stages of the life cycle of the chemical reaction: from the production of the reactants and auxiliary substances from raw resources to the synthesis of the desired reaction product and to the possible waste products, which must be treated properly prior to their disposal (3). Recently, some efforts in green chemistry research are focused on the assessment of the environmental benefits of a clean reaction (4). In this way, Life Cycle Assessment (LCA), used to evaluate the environmental burdens associated to the entire life cycle of a product, process, or activity (5), can be a valuable conceptual tool for assisting to this task. Then, for a chemical reaction, all the environmental burdens for the production of reagents and auxiliary products involved * Corresponding author phone: 34935811702; fax: 34935812920; e-mail:
[email protected]. 10.1021/es020001m CCC: $22.00 Published on Web 11/14/2002
2002 American Chemical Society
The past and present industrial use of these reactions has been strongly influenced by the cost of benzene and butene (7). From the atom economic point of view, the second procedure is more environmentally friendly, because all C atoms of butene end up as MA, while for reaction [1] only 67% of C atoms of benzene are used to produce MA. Also for reaction [2], the oxygen efficiency is greater than in reaction [1] (50% vs 33%); only in terms hydrogen consumption reaction [1] shows an atom efficiency greater than reaction [2] (33% vs 25%). To carry out a more global comparative analysis between both reactions, the environmental burdens associated with i) the waste products formed in both reactions, ii) the oxygen consumed in the oxidation process, and iii) the organics used as starting compounds, must be considered. Because the experimental conditions under which both reactions occur are the same, the other parameters that affect the reaction (catalyst, solvents, and energy consumption) are not considered, as they do not alter the results of the comparative assessment.
Discussion To apply the LCA conceptual tool to the environmental comparison between the routes [1] and [2] of the MA production, the following stages must be considered: i) VOL. 36, NO. 24, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. System-product and flow diagram for a chemical reaction. definition of the goal and scope of the study, ii) inventory analysis, iii) impact assessment, and iv) interpretation of the results. Goal and Scope of the Study. The study is intended to show the applicability of this conceptual tool for the environmental assessment of a chemical reaction. Concretely, the objective of this work is to decide which of the two routes, [1] and [2], for MA production is “greener”. This comparison is intended to be illustrative and for this reason a very simple case has been chosen. The selected functional unit to which the environmental burdens are addressed is one mole of MA produced by chemical reaction. In the life cycle of MA production, the following steps must be considered: 1) production of the reactive compounds from raw materials, 2) the production of energy from the MA synthesis, and 3) treatment of wastes. The environmental loads associated with the chemical reaction for MA production have not been considered because this chemical process is performed at the same experimental conditions for both routes (see Introduction). It has been assumed that the reactants are quantitatively transformed to products. Inventory Analysis. To perform the present LCA study, average data have been used, and they were obtained from the Plastic Waste Management Institute (PWMI) (8) for benzene and butene production, and from Bundesamt Fu ¨r Umwelt, Wald Und Landschaft (BUWAL) (9) for oxygen production from air; both data reports are available through the SIMAPRO 4.0 program (10). The waste treatment step corresponds to the treatment of the water produced in the chemical reaction; in this way, it has been assumed that the treatment consists of a primary physicochemical stage where a phase separation occurs, followed by a biological stage (aerobic plus anaerobic degradation) and an anaerobic sludge treatment. The Inventory data were obtained from a wastewater treatment plant in Girona (Spain) (11). Table 1 shows the values of the air and water emissions, the solid waste generated, and the energy consumed for the production of 1 mol of benzene, butene, and oxygen and for the treatment of 1 mol of water produced in the chemical reaction. To estimate the energy released in the chemical production of MA, it has been considered that the process occurs at 700 K. The enthalpies of 1 mol MA production from benzene and butene have been deduced from the standard enthalpies of formation and from the corresponding heat capacities at constant pressure of all gas-phase chemicals that take part in both reactions (12). The enthalpy of formation of gas-phase MA at 298 K has been estimated from the corresponding bond enthalpies (13). The difference between the enthalpies at standard conditions (1 bar) and at 3 to 5 bar, the pressure actually needed to obtain MA in both reactions, has been assumed negligible (14). The values obtained for the reaction enthalpies of 1 mol MA production from benzene and butene at 700 K are -1.58 MJ mol-1 and -0.957 MJ mol-1, respectively. This chemical energy can be reused in the same process or it can be exported 5518
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TABLE 1. Emissions to Air (in g) and to Water (in mg), Solid Waste Generated (in g), and Energy Consumed (in MJ), from the Production of 1 mol of Benzene, Butene, and Oxygen, Respectively, and for the Treatment of 1 mol of Watera emissions to air (g)
benzene
butene
oxygen
water treatment
CO2 CO CxHy H2S HCl NOx SOx
42 0.047 0.65 0.39 × 10-3 1.2 × 10-3 0.53 0.23
28 0.022 0.39 0.056 × 10-3 0.56 × 10-3 0.34 0.17
6.8 0.0012 1.2 × 10-3 n 1.1 × 10-3 0.015 0.063
n n n n n n n
emissions to water (mg)
benzene
butene
oxygen
water treatment
mineral acid COD NH4+ CxHy NO3PO43solid waste (g) energy (MJ)
3.1 15.6 4.0 6.2 0.08 n 0.76 5.8
2.2 11.2 0.56 5.0 n n 0.46 3.8
n 0.032 0.056 0.022 0.096 n n 0.062
n 0.076 0.045 n 0.072 0.086 0.18 n
a
n: negligible. COD: Chemical Oxygen Demand.
for other external uses, thus decreasing the environmental loads corresponding to the generation of an equivalent amount of energy from electricity or other sources. It has been assumed that the obtained chemical energy is converted to electricity with an efficiency of 31% (8). The avoided environmental burdens associated with the use of this energy are summarized in Table 2; these values were estimated from environmental impact data of electricity production based on the UCPTE (Union for the Connection, Production and Transport of Electricity) model, for an average European scenario (42.9% thermal, 36.9%, nuclear and 20.2% hydroelectrical).The environmental burdens considered in the analysis include the construction, use, dismantling and disposal of the corresponding energy delivering plants (8). Impact Assessment and Interpretation. The inventory data have been classified and characterized according to the following global impact categories (15, 16): Global Warming Potential (GWP), Acidification Potential (AP), Eutrophication Potential (EuP), Ozone Formation Potential (OFP), Energy Consumption (EC), and Solid Waste Production (SWP). The toxicity related impact categories (human toxicity and ecotoxicity potentials) have not been taken into account, because no international consensus has yet been achieved for the assignment of characterization factors to these environmental impacts. In Tables 3 and 4, the contributions to the different impact categories of the corresponding stages of the life cycle of 1 mol MA production from benzene (Table 3) and butene (Table 4), excluding the chemical reaction stage, are summarized. As the functional unit considered is
TABLE 2. Avoided Environmental Loads Coming from the Use of the Energy Obtained in the Chemical Production of 1 mol of MA from Benzene and from Butenea
a
emissions to air (g)
from benzene
from butene
CO2 CO CxHy HCl SOx
58 0.011 0.040 0.0096 0.31
35 0.0065 0.0022 0.0058 0.19
emissions to water (mg)
from benzene
from butene
COD NH4+ CxHy NO3PO43solid waste energy/MJ
0.27 0.48 0.19 0.72 1.45 n 0.49
0.17 0.29 0.11 0.44 0.88 n 0.30
n: negligible.
TABLE 3. Contributions to the Environmental Impact Categories of the Different Stages of the Life Cycle of the MA Production from Benzene (Excluding the Chemical Reaction Stage)a GWP/ gCO2 benzene production oxygen production waste production and treatment energy production a
42 31 88 67
AP/ gSOx
EuP/ OFP/ gPO43- gC2H2
0.60 0.069 0.33 0.008 0.0012 n 0.31
n
EC/ MJ
SWP/ g
0.26 5.8 0.76 n 0.28 n n 0.06 0.36 n
0.49 n
n: negligible.
TABLE 4. Contributions to the Environmental Impact Categories of the Different Stages of the Life Cycle of the MA Production from Butene (Excluding the Chemical Reaction Stage)a GWP/ gCO2 butene production oxygen production waste production and treatment energy production a
AP/ gSOx
EuP/ OFP/ gPO43- gC2H2
EC/ MJ
SWP/ g
28 0.41 0.044 20 0.22 0.006 0.0010 0.018 0.001
0.16 3.8 0.46 n 0.19 n n 0.09 0.54
40.5
n
0.19
n
0.30 n
n: negligible.
the production of 1 mol of MA, the values in Table 3 are referred to 1 mole of benzene and 4.5 mols of oxygen as reactants and to 2 mols of CO2 and 2 mols of water (with the corresponding treatment) as waste products. Similarly, the values in Table 4 are referred to the production of 1 mol of butene and 3 mols of water (with the corresponding treatment). In Figure 2a,b the relative contributions to the different impact categories of the stages considered for the life cycle of the MA production from benzene and butene, respectively, are depicted. As can be seen, considering the MA production from benzene (Figure 2a), the avoided burdens due to the energy production significantly contribute to decrease the environmental impacts associated with global warming and acidification. Of the three stages that exert environmental impacts, production of the organic compound, oxygen production, and waste generation and treatment, the main contribution in all impact categories analyzed comes from
FIGURE 2. Relative contributions to the corresponding impact categories of the different stages of the life cycle of MA production from (a) benzene (b) and butene as a feedstocks. GWP: Global Warming Potential; AP: Acidification Potential; EuP: Eutrophication Potential; OFP: Ozone Formation Potential; EC: Energy Consumption; and SWP: Solid Waste Potential.
TABLE 5. Total Contributions (Sum for the Three Stages Considered) of the Different Impact Category Potentials for the Production of 1 mol of MA from Benzene and Butene MA production
GWP/ gCO2
AP/ gSOx
EuP/ gPO43-
OFP/ gC2H2
EC/ MJ
SWP/ g
from benzene from butene
94 7.5
00.63 0.46
0.077 0.051
0.26 0.16
5.6 3.8
1.1 1.0
the production of benzene, except for GWP, for which the main environmental contribution corresponds to the waste production and treatment. Also, this last stage significantly contributes (about 30%) to SWP. On the other hand, it must be noted that 40% of the acidification potential corresponds to the oxygen production stage. With respect to MA obtention from butene (Figure 2b), the avoided loads coming from energy production, practically counterbalance the environmental impacts corresponding to global warming and appreciably reduces the acidification potential. The production of this last organic is also the main environmental contribution of the three stages considered, except for SWP category, for which the 54% of the impact is due to waste production and treatment. The stage corresponding to oxygen production significantly contributes to GWP and AP. In view of the results depicted in Figure 2a,b, it seems clear that the main environmental impacts of the life cycle of MA production come from the burdens associated with the production of benzene and butene. Consequently, the most significant environmental improvements will come from enhancing the corresponding processes using these chemicals. On the other hand, it must be emphasized the appreciable reduction of environmental impacts produced by the energy generation from the heat released by the chemical production of MA. In this way, it must be noted that the important reduction in the Global Warming Potential observed in the life cycle of MA production by the two routes (see Figure 2a,b) is a result of the high ratio of thermal energy in the energy mix for electrical production considered. Table 5 summarizes the total impact values for the different category potentials for the production of MA from VOL. 36, NO. 24, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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benzene and butene. From these data, it can be concluded that the production of MA from butene is “greener” than its production from benzene as a feedstock. In all the impact categories considered, the environmental impacts of the MA production from benzene are more than 30% greater than the corresponding impacts from using butene as feedstock, except for the solid waste production category, for which the environmental impact is similar for both routes of MA production. A hypothesis assumed in the present analysis that can affect the results of the environmental comparison is the consideration that MA production from benzene and butene, respectively, is quantitative in both cases. In fact, it is known that during the MA production from benzene, benzoquinone is obtained as a byproduct, with a maximum yield of benzene conversion to MA of 75% (17). However, the obtention of MA from butene can be complete if excess oxygen is used (18). Thus, these last evidences points and reinforce the previous findings about the preference of the butene route for MA production from an environmental point of view. On the other hand, site specific data for water treatment have been used; however, because the environmental impact weight of this stage respect to all life cycle of the MA production is negligible, the use of data coming from other treatment plants that use the same technology does not practically alter the results. For butene and benzene production, that are relevant stages from environmental point of view (see Tables 3 and 4), average data coming from the same sourse have been used. The present work shows an easy example of LCA application intended to ascertain the most environmental friendly way to obtain a substance between two chemical processes. For studies dealing with the greeness of a single chemical, also the stages downstream of its production, such
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as distribution, storage, use of the chemical and waste management after its use, must also be considered.
Literature Cited (1) Anastas P. T.; Warner, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (2) Trost B. M. Angew. Chem. Int. Ed. Engl. 1995, 34, 259. (3) Anastas P. T.; Lankey, R. L. Green Chemistry 2000, 2, 289. (4) Curzons A. D.; Constable, D. J.; Mortimer, D. N.; Cunningham, V. L. Green Chemistry 2001, 3, 1. (5) Guidelines for LCA; a Code of Practice; Consoli, F.; et al. SETAC: Sesimbra, 1993. (6) Lancaster M. Ed. Chem., March 2000, 40. (7) Encyclopedia of Chemical Technology; Kirk-Othmer, Eds.; John Wiley and Sons: New York, 1978; Vol. 4, p 368. (8) PWMI. Eco-profiles of the European Plastics Industry; Brussels, 1998. (9) BUWAL. O ¨ koinventare fu ¨r Verpackungen; Schriftenreihe Umwelt 250, Bern, 1996. (10) PreConsultants B. V. Amersforst. The Netherlands. 1997. (11) Vidal, N.; Poch, M. Tecnologı´a del Agua 1999, 189, 41. (12) Standard Reference Database 42; National Institute of Standards and Technology (NIST): 2002. (13) Levine, I. N. Physical Chemistry; McGraw-Hill: New York, 2001. (14) Moore, W. J. Physical Chemistry; Longman: London, UK, 1976. (15) Hauschild M.; Wenzel, H. Environmental Assessment of Products; Chapman and Hall: London, UK, 1998; Vol. 1. (16) Nordic Guidelines on LCA; Nordic Council of Ministers: Coppenhagen, 1995. (17) Encyclopedia of Chemical Technology; Kirk-Othmer, Eds.; John Wiley and Sons: New York, 1978; Vol. 14, p 780. (18) Encyclopedia of Chemical Technology; Kirk-Othmer, Eds.; John Wiley and Sons: New York, 1978; Vol. 1, p 136.
Received for review January 3, 2002. Revised manuscript received September 26, 2002. Accepted October 7, 2002. ES020001M
