Article pubs.acs.org/IECR
Incorporating Sustainability into the Conceptual Design of Chemical Process-Reaction Routes Selection Kailiang Zheng, Helen H. Lou,* Preeti Gangadharan, and Krishna Kanchi Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, Texas 77710, United States ABSTRACT: Limited availability of natural resources and rising raw material cost, accompanied by growing societal and environmental concerns, urge the engineers to incorporate sustainability issues into the design of new chemical process and the retrofit of traditional process. Yet due to the multidimensional nature of sustainability, as economic, societal, and environmental issues need to be considered together, a structured sustainability assessment tool is needed to serve as the basis for any process design, analysis, improvement, and decision making. This paper presents a methodology to assist reaction pathway selection in light of sustainability. At this conceptual design stage, the sustainability performance of different potential reaction pathways is evaluated, which can not only help the designers improve the screening efficiency by eliminating inferior reaction alternatives systematically, but also identify the key areas for further improvement in future design, thus reducing the complexity and labor in the following basic engineering design stage. The sustainability of each reaction pathway is assessed in terms of profit potential, driving force of the pathway (Gibbs free energy), inherent safety index, potential environmental index, and atom economy. The efficacy of this approach is demonstrated by several case studies of reaction routes selection, including the propylene oxide (PO) production process, carbon dioxide reduction technology, and cellulosic ethanol production technology.
1. INTRODUCTION The volatile change of feedstock price, and arising societal and environmental concerns, together with ever-advancing technology, have rendered it possible to manufacture a single or set of chemical compounds from various precursors by different alternative reaction routes. This brings in opportunities for new chemical process design and the retrofit of traditional process. Process development usually takes a long time period. Conceptual design plays a very critical role in the process development. Many routes, which may require different starting reactants, are evaluated, and only several routes which are worth further consideration are picked to save time and effort. In the current intensive economic competition, there is an increasing pressure to reduce the development time for chemical process design. This requires an efficient method in the conceptual design stage for reaction routes selection. Meanwhile, the value of sustainable development has been well recognized by the industries and societies. The chemical industry has been striving for sustainability through inventing, and implementing cleaner technologies, recycling and reuse, eliminating waste products, reducing emission of greenhouse gases, avoiding the use of toxic substances, and reducing energy intensity of processes.1 Over the past decades, many methodologies and indicators have been proposed to assess sustainability issue in chemical process design.2−11 These methodologies include sustainability aspects as an integral part of process design and have paved a way for systematic sustainable design method. However, many of them require detailed information about the process flowsheet which are not available in the conceptual design stage. This paper proposes a methodology to evaluate the sustainability performance of each alternative reaction route in terms of driving force of the pathway (Gibbs free energy), profit potential, inherent safety index, potential environmental index, © 2012 American Chemical Society
and atom efficiency respectively. It can not only help screen out inferior routes systematically and efficiently prior to initiating basic engineering design, but also identify the key areas for improvement in the future. The earlier the potential problems are indentified, the less expense will be needed to handle them. For example, waste treatment problems for a particular route can be avoided as early as possible by looking into its potential environmental effect in the conceptual design stage. Then in the basic engineering design, appropriate effort should be made on this route to handle the waste treatment problem identified in the conceptual design stage.
2. REACTION PATHWAY SELECTION METHODOLOGY Two approaches, which are differentiated by whether the thermodynamic analysis and economic analysis are conducted parallelly or sequentially, are proposed to facilitate routes selection. Figure 1a shows the sequential flowchart where the thermodynamic feasibility of each route is assessed first. Any alternative route which fails this criterion is terminated and cannot proceed to the next step. In the second step, any alternative route surviving the scrutiny of the first step is assessed for its economic potential. Obviously, those routes which cannot generate profits should not be considered further unless the market prices undergo dramatic changes. The market price has substantial impact on the economic assessment result which may change from time to time. The thermodynamic analysis result reflects the intrinsic nature of reaction routes and does not change with market situation. Therefore, usually the thermodynamic assessment should be conducted first and followed by Received: Revised: Accepted: Published: 9300
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Figure 1. (a) Sequential reaction routes selection flowchart; (b) parallel reaction routes selection flowchart.
economic evaluation. In the third step, those routes which passed the first two selection criteria are subject to parallel assessments of societal impact, environmental impact, and atom economy. Societal and environmental criteria are harder to quantify, since there is no unanimously accepted method of quantification. Meanwhile, their evaluation is relatively subjective and influenced by the decision maker’s own experience and judgment. Thus the analysis of these three categories are set in
a parallel relationship and left for the decision maker’s own judgment. Those alternative routes surviving the scrutiny of the previous three steps are considered good candidates and worth more time and effort in the following design stage. Figure 1b shows a flowchart where thermodynamic analysis and economic analysis are set in parallel and an analyzer block is added after these two analyses. The introduction of this parallel flowchart can speed up routes selection. Although thermody9301
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namic assessment is intrinsic and does not fluctuate with market change, economic and thermodynamic analysis both have vital impact on the routes selection, which is, if any of them fails, the corresponding route will be infeasible and should be eliminated. For example, in a routes selection, if route i has an infeasible economic assessment, then this route can be terminated directly and the thermodynamic assessment of this route need not to be conducted. In general, the sequential approach is recommended for most cases and the parallel approach is suitable for the cases when both the economic and thermodynamic data are available. The sustainability indicators adopted in the approach regarding the thermodynamic, economic, societal, and environmental analysis and atom economy are presented below. Then several case studies will be provided to demonstrate the efficacy of this method. It is worth noting that because of the multidimensional nature of sustainability, to maximize the sustainability of a design is actually a multiobjective optimization problem. As mentioned above, people may have a different subjective weighting factor for each index. Then, the multiobjective problem can be converted to a single objective problem by normalizing the index of different aspects and assigning different subjective weighting factors to each normalized index. 2.1. Indicator on Thermodynamic Analysis. Gibbs free energy of reaction is used as the evaluation tool for the thermodynamic feasibility analysis. It can tell the driving force of each alternative reaction route, the spontaneous direction of reaction and indicate whether the desired reaction is thermodynamically favorable or not. It is well-defined and can be calculated by the following two equations at standard state: Δ r G° = Δ r H ° − T Δ r S ° Δ r G° =
∑ νiΔf G°product,i − ∑ νjΔf G°reactant,j
group’s paper12 where the sustainability of two biodiesel processes using different catalysts was quantified. 2.2. Indicator on Economic Analysis. For the economic evaluation, the profit-potential estimation method, originally conceived by Rudd and his colleagues,13 is estimated under the most ideal conditions, where the reactants are totally converted into the reaction products. For each alternative reaction route, its profit-potential (ΔP) is estimated as the difference between the prices of the final reaction products (∑νiPproduct,i) and those of the starting reactants (∑νjPreactant,j) for manufacturing a unit quantity of desired product as shown in eq 3. ΔP =
∑ νiPproduct, i − ∑ νjPreactant,j
(3)
where ΔP is the profit-potential; ν is the stoichiometric coefficient for chemical component; Pproduct,i and Preactant,j is the price of product i and reactant j. If the estimated profit-potential is less than zero, which means the raw material cost for a particular route is higher than the product price, then the development of this route can be terminated immediately. There is no way for this route to be profitable after considering other equipment and operation costs in the further design. However, even if the profit-potential for a particular route is exceedingly higher than the profit-potentials of other routes, it does not mean the other route should be eliminated, as the evaluation of other aspects need to be considered together. In addition, the economic advantage of relatively cheaper raw materials may be diminished if considering the expensive equipments and operating costs 2.3. Indicator on Societal Analysis. Societal indicators of sustainability involve many criteria which are normally influenced subjectively by the decision maker’s knowledge and experience. It is hard to quantify these criteria in numeric equations like the profit and Gibbs free energy, but it can be scaled based on specific and distinctive measurement, even though the scaled values of social indicators are more nebulous than those of profit and Gibbs free energy. Since some societal criteria, such as environment related issues, have been included in the environmental assessment, safety analysis is mainly viewed as the societal indicator for each alternative reaction route. Safety is the second nature of chemical industry. Many researchers have contributed a number of safety analysis methods, such as Dow Fire and Explosion Hazard Index,14 Mond Index,15 and HAZOP (Hazard and Operability Analysis). However, many of those safety analysis methods require detailed information about equipment and process condition, which are suitable for a later basic engineering design stage rather than this conceptual design stage when lots of information is unknown or incomplete. Heikkila16 proposed a very good safety analysis method, namely inherent safety index, which requires relatively less information when compared to other methods while still covers many safety aspects. Lou etc.12 extends Heikkila’s work by considering both the severity and quantity of chemicals and process equipments together and proposes an enhanced inherent safety index (EISI) method which is adopted herein. This method has two subindices: chemical inherent safety index and process inherent safety index. Chemical inherent safety index reflects the hazards presented by chemicals, and process inherent safety index deals with hazards due to equipment and inventory in the plant. For this conceptual design, the detailed information about the type and quantity of the equipments, and also the inventory, is not available, thus only the process temperature and pressure are used to assess the process safety index.
(1) (2)
Where ΔrG°, ΔrH°, ΔrS° are the change of gibbs free energy, enthalpy, entropy for the reaction at standard state (298K, 1 atm), respectively; ν is the stoichiometric coefficient for chemical component, and ΔfG°reactant,j and ΔfG°product,i denote the gibbs energy of formation at standard state for the reactant chemical j and product chemical i, respectively. If the value of the Gibbs free energy for a specific reaction, Δ rG°, is less than zero, then the reactants will react spontaneously, and this reaction is thermodynamically favorable. If the value of ΔrG° is slightly higher than zero, then this reaction is possibly thermodynamically favorable. By adjusting the conditions like reacting temperature, the Gibbs reaction energy for this reaction may become less than zero at a different condition and this reaction becomes thermodynamically favorable. If the value of ΔrG° is significantly higher than zero, and it is almost impossible or need extreme reacting condition to make ΔrG° less than zero, then this alternative reaction route is thermodynamically unfavorable and cannot actually happen in the normal manufacturing condition. Thus, this alternative route should be eliminated. Please note that, at this reaction pathway selection stage, the thermodynamic feasibility is checked to root out those thermodynamically infeasible pathways. Certainly, catalyst affects reaction rate and selectivity, but the choice of catalyst also depends on many factors, including the cost of the catalyst, the cost of the corresponding vessel, the reactor design, etc. The influence of catalyst is counted at the design stage after the reaction pathway is chosen. A detailed example was given in the 9302
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As shown in Table 1,16 the chemical inherent safety index (ICI) comprises four subindices, which are flammability, explosiveness,
reduction) algorithm was adopted. This explicit and specific methodology was first introduced by Young and Cabezas (US EPA) in 199918 to assess the environmental impact of a chemical process. It uses a basic concept, PEI index (potential environmental impact), to represent the relative environmental friendliness or unfriendliness of a specific chemical process. The PEI index is based on the traditional mass and energy balance, which affects the environmental impact across the process boundaries. When evaluating the environmental impact of a chemical process in the WAR algorithm, both the mass and energy part of the process can be assessed. A database of relative environmental impact scores has been created and embedded into the WAR software to provide the basic information of potential environmental impact for each chemical component. Table 2 shows the eight different categories, which can be further reduced to four categories, used by the WAR algorithm to
Table 1. Structure of Total Inherent Safety Index name
symbol
score
chemical inherent safety index flammability explosiveness toxic exposure corrosiveness process inherent safety index temperature pressure
ICI IFL IEX ITOX ICOR IPI ITEMP IPRES
ICI = IFL + IEX + ITOX + ICOR 0−4 0−4 0−4 0−2 IPI = ITEMP + IPRES 0−4 0−4
toxic exposure, and corrosiveness. For each individual component, a score will be given to each of the four subindices on the basis of the properties of that component, and the safety index of this component is the sum of the scores of each subindex. To evaluate the chemical inherent safety index for each reaction route, the safety index of each component k is multiplied by its mass flow rate to consider both the hazard and quantity of chemical components as shown in eq 4. ICI (route) =
∑ (ICI,k × Mk)/unit product
Table 2. Impact Categories in WAR Algorithm25 general impact category human toxicity
ingestion inhalation/dermal aquatic toxicity
ecological toxicity
(4)
where ICI (route) is the chemical inherent safety index for the route; ICI,k is the chemical inherent safety index for component k; Mk is the mass flow rate of chemical k calculated by the stoichiometric coefficients to produce a unit quantity of product in each route. It is worth noting that a lower score indicates a safer feature. Thus the reaction route with a lower inherent safety index is a safer and preferable route regarding its societal performance. As in the conceptual design stage, the detailed type and quantity of process equipments and also the inventory information are not available, thus, the process inherent safety index (IPI) is the sum of the safety index quantified by the temperature (ITEMP) and pressure (IPRES) of the process to produce the unit product. Note that a company may put a limit on the temperature and pressure conditions based on their judgment. One synthesis route that has extremely high temperature or pressure may not be worth an engineering pursuit because of the safety and economic concerns. It is worth pointing out that hazard and risk are two factors both contributing to the safety issue. An inherent safe route is not necessarily lower in risk; it simply requires fewer independent protection layers. However, at the reaction pathway selection stage, too little information is available to quantify the cost of different potential protection layers and the inventory information. The group’s paper12,17 described at the further design stage, how the inventory information is counted and how a root cause analysis is conducted to figure out the most important factors affecting safety. Since the chemical and process inherent safety indices represent different categories of hazards, the values of these two indices may not be at the same order of magnitude, and also they may subjectively have different weighting factors for practitioners, thus it is recommended to consider these two indices separately during the routes selection. 2.4. Indicator on Environmental Analysis. Environmental impacts are also subjectively influenced by the decision maker. Many quantification methods are proposed by researchers in this field. In the approach provided herein, the WAR (waste
impact category
global atmospheric impacts regional atmospheric impacts
terrestrial toxicity global warming potential ozone depletion potential acidification potential photochemical oxidation potential
measure of impact category LD50 OSHA PEL Fathead minnow LC50 LC50 GWP ODP AP PCOP
quantify the environmental index. Similar to the chemical inherent safety index, the lower PEI indexes stands for more environmental friendliness. Thus a reaction route with lower PEI index is more environmentally friendly and favorable with regard to environmental concern. 2.5. Indicator on Atom Economy Analysis. Most chemists have traditionally measured the efficiency of a reaction by the concept of percentage yield, however this indicator only tells part of the story. If a reaction produces much more waste than the desired product in terms of mass, then no matter what the yield is, only a small part of the mass of reactants will actually end up in the desired product. While the percentage yield is often considered as a measure of how efficiently reactants are used in making a final product, it neglects to measure what fraction of reactant atoms ends up in the desired product versus how many end up in products that are considered waste. In an effort to foster awareness of the atoms of reactants that are incorporated into the desired product and those that are wasted (incorporated in the undesired products), Barry Trost developed the concept of atom economy in 199119 and was awarded a Presidential Green Chemistry Challenge Award for this concept in 1998. For a reaction, a×A+b×B→c×C+d×D
where a, b, c, d are stoichiometric coefficients for components A, B, C, and D. Component C is the desired product and D is the waste product. Then, the atom economy of this reaction can be calculated by the following simple equation: 9303
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c × M w of product C a × M w of A + b × M w of B
Thus, this route should be terminated due to its infeasible thermodynamic assessment. Similar to the first reaction route, the other two routes have the same thermodynamic feasibility issues and thus should be deleted and not worth further consideration.
(5)
Although atom economy does not account for solvents, reaction yield and reactant molar excess, it is a simple representation of the “green-ness” of a reaction as it can be calculated without the need of experimental results. It is useful as the reaction route with low atom economy can be identified at the chemical route selection stage, prior to entering the laboratory experiment part. In the proposed reaction routes selection method, atom economy of different chemical routes are calculated and compared. The larger the atom economy, the greener the reaction route is and the higher the percentage of reactants ends up in the desired product while the fewer waste generated.
4. CASE STUDY: ASSESSMENT OF A CELLULOSIC ETHANOL PRODUCTION ROUTE Cellulosic ethanol is one of the second generation biofuels aimed at reducing fossil fuel dependence and green house gas emission. Thus there are many researches at this field. One patent describes a reaction route to produce cellulosic ethanol and 2-propanol from wood waste. First wood waste is gasified to syngas. Then the proposed route consists of three steps, which are (I) converting syngas to methanol using a methanol synthesis catalyst, (II) converting methanol into ethylene and propylene using a methanol-to-olefins catalyst, (III) hydrating ethylene into ethanol and propylene into 2-propanol. The economic price of ethylene and propylene is higher than the price of ethanol. The third step of this proposed route converts high-value feedstock (1.52 $/kg ethylene converted by ICIS Pricing spot price at May 2011) into low-value product (0.81 $/kg ethanol converted by ICIS Pricing spot price at May 2011) and thus is not feasible in the economic point of view. According to the parallel routes selection method provided before, the economic infeasibility can directly screen out this patented cellulosic ethanol production route and there is no need to conduct the thermodynamic feasibility assessment. Actually, Dow Chemical is progressing a project to produce ethylene and polyethelene from sugar cane ethanol in Brazil.20,21 And Braskem, a Brazilian producer, already started up a sugar cane ethanol-based ethylene plant for the production of so-called green PE (polyethylene) at its site in Triunfo, southern Brazil, in September 2010.21 Braskem has been successfully selling its green PE at a premium and has concluded supply agreements with major companies, including consumer goods manufacturer Procter & Gamble, Swedish food packaging company Tetra Pak and Japanese cosmetics company Shiseido.21 This also demonstrates the economic infeasibility of the three-step cellulosic ethanol product route proposed above.
3. CASE STUDY: ASSESSMENT OF THREE CARBON DIOXIDE REDUCTION ROUTES Many technologies are under development to convert carbon dioxide in order to reduce its effect on global warming. Three reaction routes, which are proposed for reducing carbon dioxide emission, are listed below. Each of the reaction routes consists of two reaction steps and the three routes are shown in eqs 6−11. (1) 3CO2 + 4Al → 2Al 2O3 + 3C
(6)
2Al 2O3 → 4Al + 3O2
(7)
CO2 + 2Zn → 2ZnO + C
(8)
2ZnO → 2Zn + O2
(9)
(2)
(3) 3CO2 + 4Fe → 2Fe2O3 + 3C
(10)
2Fe2O3 → 4Fe + 3O2
(11)
On the basis of the sequential reaction routes selection approach proposed above, the thermodynamic assessments for each route are conducted, and the ΔrG° calculations for each reaction step at standard conditions (298 K and 1 atm) are listed in Table 3.
5. CASE STUDY: ASSESSMENT OF PROPYLENE OXIDE (PO) PRODUCTION ROUTE In this part, to further illustrate the reaction routes selection approach proposed above, a case study on the sustainability assessment of the PO/SM (styrene monomer), PO/MTBE (methyl tert-butyl ether), and HPPO (hydrogen peroxide
Table 3. ΔrG° Calculation for eqs 6−11 at Standard Conditions reaction
ΔrG° (kJ/mol)
ΔrH° (kJ/mol)
ΔrS° (kJ/mol/k)
eq 6 eq 7 eq 8 eq 9 eq 10 eq 11
−1981.49 3164.69 −246.62 641.02 −303.93 1487.13
−2170.90 3351.40 −307.50 701.00 −470.50 1651.00
−0.53 −0.11 −0.13 −0.09 −0.47 −0.17
Table 4. Gibbs Free Energy of Reaction for the Three Routes reaction
ΔrG° at 298 K, 1 atm (kJ/mol) PO/MTBE Route −68.02 −177.19 −14.24
eq 13 eq 14 eq 15
Take the first reaction route (eq 6 and 7) for example. From Table 3, ΔrG° for eq 6 is negative, which means this reaction occurs spontaneously. Since the ΔrG° for eq 7 is positive, this reaction cannot occur at standard condition. According to eq 1, since ΔrH° is positive and ΔrS° is negative for reaction eq 7, the ΔrG° is always positive and this means the second step of this route cannot occur spontaneously at any temperature condition.
PO/SM Route −58.49 −180.44 5.06
eq 17 eq 18 eq 19 HPPO Route eq 20 9304
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propylene oxide) reaction routes to produce propylene oxide (PO) is provided. Propylene oxide (CAS no. 75-56-9) (methyloxirane, 1,2epoxypropane) is an important organic chemical product used primarily as a reaction intermediate for the production of polyether polyols, propylene glycol, alkanolamines, glycol ethers, and many other useful products. It has the second largest production capacity in propylene derivatives, ranking only after polypropylene.22 A continuing trend in the propylene oxide industry is the drive to develop and commercialize process routes that do not produce sizable quantities of coproducts and do not use chlorine-based chemistry. PO/SM (LyondellBasell, Shell, etc.) and PO/MTBE (Huntsman, etc.) routes account for the majority of the current global production of PO, and coproduce styrene monomer and tert-butyl alcohol, respectively.23 However, they require relatively large capital investments and present difficulties in balancing the markets for propylene oxide and the coproducts, leading to considerable volatility in the economic performance over time. Although significant propylene oxide capacity is based on chlorohydrins process (CHPO), this route suffers from environmental liabilities and large investment requirements.24 Recently, much attention has been directed to the HPPO route (BASF and DOW, Evonik and Uhde, etc.24), which is based on epoxidation of propylene using hydrogen peroxide as the oxidant. There are also other alternative reaction routes. However, in this case study, only the PO/SM, PO/MTBE, and HPPO routes are studied for reaction routes selection. 5.1. Thermodynamic Feasibility Assessment Result. The Gibbs free energy of reaction is the driving force of the chemical reaction, which can tell the spontaneous direction of the reaction. The following shows the calculation results of Gibbs free energy of reaction for the three routes. PO/MTBE route and PO/SM route both undergo three detailed reaction steps, while HPPO route is a one-step reaction.
Table 5. Chemical Price Information converted price ($/kg)
chemical
source
time
C3H6 (propylene)
1.251
ICIS pricing
C4H10 (isobutane)
0.7178
O2
0.0932
CH3OH (methanol)
0.3605
Barnes & Click Energy price data industrial gases/ combustion ICIS Pricing
C5H12O (MTBE)
0.821
ICIS pricing
C3H6O (propylene oxide) C8H10 (ethylbezene)
2.0393
ICIS pricing
0.948
ICIS pricing
C8H8 (styrene monomer) H2O2 (hydrogen peroxide)
1.1464
ICIS pricing
0.7606
US Peroxide
Aug, 2010 Sep, 2010 2005 Sep, 2010 Sep, 2010 Aug, 2010 Aug, 2010 Aug, 2010 Oct, 2009
Table 6. Profit-Potential Calculations for Each Route route
profit-potential ($/kg PO)
PO/MTBE route PO/SM route HPPO route
1.4108 1.405 0.6875
Table 7. Process Temperature and Pressure for Each Route reaction
temperature range (°C)
eq 7 eq 8 eq 9
95−150 100−130 5−250
eq 11 eq 12 eq 13
140−150 95−130 250−280
eq 14
55−90
average T (°C)
pressure range (bar)
PO/MTBE Route 122.5 20.75−55.35 115 14.8−35.5 127.5 0.25−25 PO/SM Route 145 2.06−2.75 112.5 25−40 265 1 HPPO Route 72.5 30
average P (bar)
ref
38.05 25.15 12.625
23 23 26
2.405 32.5 1
23 23
30
27
23
Table 8. PO/MTBE Route: ICI = 50.39, IPI = 9 chemical inherent safety index (safety index severity/ton PO) chemical
flammability, IFL
CH3OH (methanol) C4H10 (isobutane) C4H10O2 (tert-butyl hydroperoxide) O2 C3H6 (propylene) H2O C4H10O (TBA) C3H6O (propylene oxide) C5H12O (MTBE) chemical inherent index, ICI
1.65 4.00 3.10 0.00 2.90 0.00 5.10 4.00 6.07 26.83
explosiveness, IEX
toxic limit, ITOX
1.10 1.10 1.00 2.00 0.00 0.00 0.00 0.00 0.72 1.45 0.00 0.00 1.28 3.83 2.00 3.00 1.52 4.55 7.62 15.94 process inherent safety index (safety index severity/ton PO)
corrosiveness, ICOR
total
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
3.86 7.00 3.10 0.00 5.07 0.00 10.21 9.00 12.14 50.39
process
temperature, ITEMP
pressure, IPRES
total
step 1 step 2 step 3 process inherent index, IPI
1.00 1.00 2.00 4.00
2.00 2.00 1.00 5.00
3.00 3.00 3.00 9.00
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Table 9. PO/SM Route: ICI = 39.73, IPI = 8 chemical inherent safety index (safety index severity/ton PO) chemical
flammability, IFL
C8H10 (ethylbenzene) C8H10O2 (EBHP) C8H10O (1-phenyl ethanol) O2 C6H6 (propylene) H2O C8H8 (styrene monomer) C3H6O (propylene oxide) chemical inherent index, ICI
5.48 0.00 2.10 0.00 2.90 0.00 3.59 4.00 18.07
explosiveness, IEX
toxic limit, ITOX
1.83 5.48 0.00 0.00 0.00 0.00 0.00 0.00 0.72 1.45 0.00 0.00 1.79 5.38 2.00 3.00 6.35 15.31 process inherent safety index (safety index severity/ton PO)
corrosiveness, ICOR
total
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
12.80 0.00 2.10 0.00 5.07 0.00 10.76 9.00 39.73
process
temperature, ITEMP
pressure, IPRES
total
Step 1 Step 2 Step 3 process inherent index, IPI
1.00 1.00 2.00 4.00
1.00 2.00 1.00 4.00
2.00 3.00 3.00 8.00
Table 10. HPPO Route: ICI = 17.58, IPI = 3 chemical inherent safety index (safety index severity/ton PO) chemical
flammability, IFL
C3H6 (propylene) H2O H2O2 (hydrogen peroxide) C3H6O (propylene oxide) chemical inherent index, ICI
2.89 0.00 0.00 4.00 6.89
explosiveness, IEX
toxic limit, ITOX
0.72 1.45 0.00 0.00 0.00 2.93 2.00 3.00 2.72 7.37 process inherent safety index (safety index severity/ton PO)
corrosiveness, ICOR
total
0.00 0.00 0.59 0.00 0.59
5.06 0.00 3.51 9.00 17.58
process
temperature, ITEMP
pressure, IPRES
total
process inherent index, IPI
1.00
2.00
3.00
Table 11. PEI Index for the Three Routes PEI output rate (PEI/ h)
routes PO/ MTBE PO/SM HPPO
PEI output/mass PEI of product (PEI/ generation kg) rate (PEI/h)
PEI generation/ mass of product (PEI/kg)
2.802
2.802
−1.774
−1.774
3.509 2.069
3.509 2.069
−1.423 −1.983
−1.423 −1.983
(1) PO/MTBE route. Overall reaction: C3H6 (propylene) + C4H10 (isobutane) + C H3OH (methanol) + O2 → C3H6O (propylene → C3H6O (propylene oxide) + C5H12O (MTBE) + H 2O
(12)
PO/MTBE route. Detailed reaction steps: i.
C4 H10 (isobutan e) + O2 → C4 H10O2 (tert‐butyl hydroperoxide)
ii.
(13)
C4 H10O2 (tert‐butyl hydroperoxide) + C3H6 (propylene) → C4H10O (tert‐butyl alcohol/TBA) + C3H6O (propylene oxide)
Figure 2. PEI indexes per time. (14) 9306
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HPPO Route. One-step reaction: C3H6(propylene) + H 2O2 → C3H6O(propylene oxide) + H 2O
The Gibbs free energy of reaction for each step of all the three routes are calculated and listed in Table 4. Although eq 19, the third step of PO/SM route, has a small positive ΔrG° value, this issue can be easily solved by adjusting the reaction temperature condition. It can be concluded that all the three routes are thermodynamically favorable and have the comparable values of Gibbs reaction energy in the standard condition at 298 K and 1 atm. 5.2. Economic Potential Assessment Result. After the gibbs reaction energy calculation and analysis, the profitpotential of each reaction route is calculated. On the basis of the overall reaction of the three routes and the corresponding chemical price data obtained, the profit-potential of each route is calculated based on the unit of $/kg PO. The chemical price data information is shown in Table 5. The economic evaluation results are shown in Table 6. From the results, it seems that the HPPO route is not as profitable as the other two routes. However, it should be emphasized again that this is just a preliminary analysis at the conceptual design stage, and the purpose of this assessment is to see under the most ideal condition, whether the routes can make profit or not. The evaluated profit-potential mainly considers the price difference between product and raw material cost. As mentioned in the methodology description part, the economic advantage of relatively cheaper raw materials may diminish if it encounters much more expensive equipments and operating costs. Determining the exact profitability of reaction routes is the task of the further more detailed process design, which is conducted after the selection of reaction routes. Each of the PO/ MTBE, PO/SM routes takes three reaction steps to produce PO. Those two routes are more complicated, and need many more equipments, control work and other efforts to operate well. Those issues will be considered in the following basic engineering design stage, and they will greatly increase the cost of PO/MTBE, PO/SM routes. 5.3. Societal impact Assessment Result. A chemical inherent safety analysis for the three routes is conducted by focusing on the four categories: flammability, explosiveness, toxicity, and corrosion. To produce 1 ton of PO product, the mass amount of the required reactants and the produced byproduct are calculated on the basis of the stoichiometric ratio. Then the chemical inherent safety index for each route to produce 1 ton of PO is calculated (shown in Tables 8, 9, and 10). In addition, the process temperature and pressure information for each route is listed in Table 7. Average values of temperature and pressure are adopted for the assessment of process inherent safety index IPI, whose results are also provided in Tables 8−10. For the inherent safety index, a smaller value means a safer reaction route. From this safety analysis, the HPPO route has a chemical inherent safety index of 17.58 and process inherent safety index of 3, which is less than those indices of the PO/SM route (39.73 and 8) and the PO/MTBE route (50.39 and 9). It is very obvious that the HPPO route is much safer and better than the other two routes whether in chemical safety or process safety category. This is due to the relatively mild process temperature and pressure, simpler reaction step (one step) and less dangerous chemicals involved in the HPPO route.
Figure 3. PEI indexes per mass of product.
Table 12. Atom Economy for the Three Routes atom economy reaction routes
PO
PO and byproduct
PO/MTBE route PO/SM route HPPO route
0.35 0.32 0.76
0.89 0.9 0.76
iii.
C4 H10O (TBA) + CH3OH (methanol) → C5H12O (MTBE) + H 2O
(15)
PO/SM Route. Overall reaction: C3H6 (propylene) + C8H10 (ethylbenzene) + O2 → C3H6O (propylene oxide) + C8H8 (styrene monomer) + H 2O
(16)
PO/SM Route. Detailed steps: i.
C8H10 (ethyl benzene) + O2 → C8H10O2 (ethylbenzene hydroperoxide/EBHP)
ii.
(17)
C8H10O2 (ethylbenzene hydroperoxide/EBHP) + C3H6 (propylene) → C3H6O (propylene oxide) + C8H10O (1‐phenyl ethanol)
(18)
iii. C8H10O (1‐phenyl ethanol) → C8H8 (styrene monomer) + H 2O
(20)
(19) 9307
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Industrial & Engineering Chemistry Research 5.4. Environmental Impact Assessment Result. The environmental impacts of the reaction routes are assessed by WAR algorithm. Again, because this is an early stage analysis, the stoichiometric mass of reactants and byproduct needed to produce one kg/h of PO product are used in the calculation of WAR software. The environmental assessment results using WAR software are shown in Table 11 and corresponding Figures 2 and 3) are also shown. Because the mass flow rate 1 kg/h is used as the product flow rate, the PEI output rate (PEI/h) and PEI output/mass of product (PEI/kg), the PEI generation rate (PEI/h) and PEI generation/mass of product (PEI/kg) have the same values. For the PEI output/mass of product (PEI/kg), the HPPO route has the lowest environmental impact. For the PEI generation rate, all the values are negative. This shows the reaction routes convert chemicals with higher environmental impacts to chemicals with lower environmental impacts. Because the lower the PEI index is, the better the environmental performance is, and the HPPO route is the most favorable route in this environmental analysis part. 5.5. Atom Economy Assessment Result. SM (styrene monomer) and MTBE are the byproduct for the PO/SM route and PO/MTBE route, respectively. Table 12 shows the calculation results of atom economy for the three routes. The case where only PO is considered as the desired product and the case where both PO and byproduct are considered as desired product are calculated separately. Through the assessment of different aspects, the PO/SM, PO/ MTBE, and HPPO routes are comparable in Gibbs free energy of reaction, profit-potential, and atom efficiency at this early reaction routes selection stage. The HPPO route has better performance in the safety and environment aspects than the other two routes, which meanwhile identifies that the PO/SM and PO/MTBE routes need to improve their safety and environment performance in the further design. All of these three routes deserve further consideration.
ACKNOWLEDGMENTS
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REFERENCES
This work is supported in part by the National Science Foundation under Grant Nos. 0737104, 0731066 and the Texas Waste Research Center.
(1) Tanzil, D., Beaver, E., Designing for Sustainability: Overview. In Transforming Sustainability Strategy into Action: The Chemical Industry; Beloff, B., et al., Eds.; Wiley Interscience: Hoboken, NJ, 2005. (2) IChemE. The Sustainability Metrics, Sustainable Development Progress Metrics Recommended for Use in the Process Industries; Institute of Chemical Engineers: Rugby, U.K., 2002. (3) Anastas, P. T., Warn, J. C. Green Chemistry: Theory and Practice; Oxford University Press: New York, 1998. (4) Allen, D. T., D. R. Shonnard Green Engineering: Environmentally Conscious Design of Chemical Processes and Products. AIChE J. 2001, 47 (9), 1906−1910. (5) Clift, R. Clean TechnologyAn Introduction. J. Chem. Technol. Biotechnol. 1995, 62, 321−326. (6) Hendershot, D. C. A New Spin on Safety. Chem. Process. 2004, 67 (5), 16−23. (7) AIChE Center for Waste Reduction and Technology (CWRT), Collaborative ProjectsFocus Area: Sustainable Development; AIChE: New York, NY, 2000. (8) Azapagic, A.; Perdan, S. Indicators of Sustainable Development for Industry: A General Framework. Trans IChemE 2000, 78 (Part B), 243− 261. (9) Azapagic, A.; Millington, A.; Collett, A. A Methodology for Integrating Sustainability Considerations into Process Design. Chem. Eng. Res. Des. 2006, 84 (A6), 439−452. (10) Carvalho, A.; Gania, R.; Matos, H. Design of Sustainable Chemical Processes: Systematic Retrofit Analysis Generation and Evaluation of Alternatives. Process Saf. Environ. Prot. 2008, 86 (5), 328− 346. (11) Othman, M. R.; Repke, J. U.; Wozny, G.; Huang, Y. L. A Modular Approach to Sustainability Assessment and Decision Support in Chemical Process Design. Ind. Eng. Chem. Res. 2010, 49 (17), 7870− 7881. (12) Li, Xiang; Zanwar, Anand; Jayswal, Abhishek; Lou, Helen H.; Huang, Y. Incorporating Exergy Analysis and Inherent Safety Analysis for Sustainability Assessment of Biofuels. Ind. Eng. Chem. Res. 2011, 50, 2981−2993. (13) Rudd, D. F.; Powers, G. J.; Siirola, J. J. Process Synthesis; Prentice Hall: Englewood Cliffs, NJ, 1973. (14) Dow Chemical Company. Dow’s Fire & Explosion Index Hazard Classification Guide, 6th ed.; American Institue of Chemical Engineers: New York, 1987. (15) ICI Mond Division. The Mond Index, 2nd ed.; PLC Publications: Winnington, U.K., 1985. (16) Heikkila, A. M. Inherent Safety in Process Plant Design An IndexBased Approach. Helsinki University of Technology: Helsinki, 1999. (17) Jayswal, A.; Li, X.; Zanwar, A.; Lou, H. H.; Huang, Y. A sustainability root cause analysis methodology and its application. Comput. Chem. Eng. 2011, 35 (12), 2786−2798. (18) Young, D. M.; Cabezas, H. Designing Sustainable Processes with Simulation: The Waste Reduction (WAR) Algorithm. Comput. Chem. Eng. 1999, 23 (10), 1477−1491. (19) Trost, B. M. The Atom EconomyA Search for Synthetic Efficiency. Science 1991, 254, 1471−1477. (20) Guzman, D. d. Dow, Mitsui JV in Sugar Polymers. http://www. icis.com/blogs/green-chemicals/2011/07/dow-mitsui-jv-in-sugarpolymer.html (accessed April, 2012). (21) ICIS Dow studies bio-based propylene routes. http://www.icis. com/Articles/2011/02/28/9438198/dow-studies-bio-basedpropylene-routes.html (accessed April, 2012). (22) Cavani, F.; Gaffney, A. M., Synthesis of Propylene Oxide: A Successful Example of Sustainable Industrial Chemistry. In Sustainable
6. CONCLUSION This paper provides a methodology to assist the reaction routes selection in the conceptual process design in light of sustainability. It assesses each route in the aspects of thermodynamic, economic, societal, and environmental analysis and atom efficiency. The several case studies strongly demonstrate that the proposed sequential and parallel routes selection approach can be readily applied to the conceptual design of chemical processes, efficiently and systematically eliminating inferior alternative routes. Then an approach proposed by Lou12 can be applied to incorporate sustainability assessment into the following basic engineering design where more information is available. A root cause analysis method can be integrated with other process design principles to identify the root cause of sustainability issues arising in the process design and solve those issues efficiently and timely.17
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AUTHOR INFORMATION
Corresponding Author
*Tel.: 409-880-8207. Fax: 409-880-2197. E-mail: Helen.lou@ lamar.edu. Notes
The authors declare no competing financial interest. 9308
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Industrial Chemistry, 1 ed.; Cavani, F., Centi, G., Perathoner, S., Trifiró, F., Eds.; Wiley-VCH: Weinheim, Germany, 2009; p 621. (23) Dow About Propylene Oxide Technology Document- Kirk Othmer. http://www.dow.com/propyleneoxide/info/index.htm. (24) ChemSystems PERP Program. Propylene Oxide; Nexant, Inc.: San Francisco, CA, 2009. (25) Pintaric, Z. N.; Kravanja, Z. Selection of the Economic Objective Function for the Optimization of Process Flow Sheets. Ind. Eng. Chem. Res. 2006, 45 (12), 4222−4232. (26) Nelson, E. A. Preparation of MTBE from TBA and methanol. Patent 4918244, 1990. (27) Propylene Oxide The Evonik−Uhde HPPO Technology; Uhde: Dortmund, Germany, 2011.
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