Economic and Environmental Assessment of Alternatives to the

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Economic and Environmental Assessment of Alternatives to the Extraction of Acetic Acid from Water Norberto García* and Jose A. Caballero Department of Chemical Engineering, University of Alicante, Apto. de Correos 99 - 03080 Alicante, Spain

bS Supporting Information ABSTRACT: A comparison was made between conventional binary distillation and four feasible alternatives flowsheets to separate acetic acid from water attending to both economic and environmental criteria. All extraction steps use diethyl ether as an extractant. Our goals were, first, to estimate the economic and environmental incentives for each of the alternatives proposed in order to understand the trade-offs associated with each system and second, to show how, in some flowsheets, it is possible to rigorously decompose the system and perform detailed optimization without the necessity of simultaneous optimization of all the equipment implied in the flowsheet. To carry out the economic evaluation, two simple criteria were used: Economic Potential and Total Annual Cost. These parameters were chosen because they can be used at various stages in the chemical plant design without the necessity of a complete picture of the industrial process. The environmental impacts are measured through the Ecoindicator-99 methodology, which reflects the advances in the damaged-oriented methodology recently developed for Life Cycle Assessment. For each of the alternatives, detailed optimization was performed to determine the Pareto’s individual curves using the ε-constraint method. With the individual Pareto curves, it is also possible to obtain the compound Pareto’s curve, and to then superimpose the individual Pareto’s curves associated with the five alternatives to identify the trade-offs of this multiobjective optimization and ultimately determine the best alternatives, and even their optimum operational conditions.

1. INTRODUCTION In the 1990s, the usual way to incorporate environmental issues into the design of chemical processes was as constraints of the problem to be optimized (i.e., establish an upper limit for the maximum concentration of pollutants, almost always based on legal compliance) and those designs which fulfilled the specifications were evaluated in economic terms (Economic Potential (EP), Total Annual Cost (TAC), etc). However, this approach has an important drawback because, when environmental issues are included as constraints in terms of concentrations or material flows, it is possible than many potential solutions do not take into account the real environmental problem.1 Although in this paper we deal with the problem of opposite objectives by means of multiobjective optimization, this is not the usual approach in process industries where the manager has to decide which importance each objective has. It is also common to consider environmental issues as constraints by including an additional and new term in the objective function. In these cases, the drawback is the difficulty of finding a good environmental indicator that can be included in the objective function with the economic terms.24 However, decision making involving trade-offs is not considered in many chemical process designs. Cano-Ruiz and McRae in their excellent review5 about the environmentally conscious chemical process design even remarked that books currently used in teaching chemical process design contain little or nothing about environmental issues and how to introduce these aspects in the own design. In this work, we consider the design of the chemical process as a multiobjective problem (both environmental and economic functions are evaluated together). It is important to note that in r 2011 American Chemical Society

multiobjective optimization it is sometimes difficult to improve one objective function without worsening the others.6 In this case, it is necessary to find the noninferior Pareto’s solutions. An inferior solution is that in which there exits at least one solution that is better, and which can be attained without exacerbating the rest of the objective functions. In this paper, the noninferior solutions set is obtained using the ε-constraint method.7 The main objective of this work is to show how multiobjective optimization (MO) could be combined with Life Cycle Assessment (LCA) and, particularly, with Ecoindicator-99 methodology in order to make better decisions in the design of chemical processes, taking in account both economic (benefit) and environmental (minimum impact) performance.8,9 The advantage of using multiobjective in comparison to other methods that also use LCA methodology10,11 is that the generation of optimal solutions does not require establishing any type of subjective preferences to carry out the optimization because all potential solutions are evaluated. That means that the importance of MO is based more on the alternatives set that can be chosen from the noninferior solutions than on the explicit necessity to establish, at the beginning of the optimization process, the priorities that are going to drive the synthesis of the chemical process without previously having considered all the trade-off solutions.12,13

Received: May 18, 2011 Accepted: August 8, 2011 Revised: July 25, 2011 Published: August 08, 2011 10717

dx.doi.org/10.1021/ie201064x | Ind. Eng. Chem. Res. 2011, 50, 10717–10729

Industrial & Engineering Chemistry Research To demonstrate this, a comparison was made among different flowsheets to separate acetic acid from water considering both economic and environmental criteria. Acetic acid is one of the most widely used carboxylic acids. It is used in many reactions as a reaction partner (i.e., during production of acetic-acid esters) and can also be used as a solvent (i.e., production of cellulose acetate and manufacture of pharmaceutical products). As a rule, aqueous acetic acid is obtained during the foregoing processes, and its recovery in most cases is of great economic significance. Separation of the acetic and water mixture by simple rectification is difficult because it has a pinch azeotrope (it is not a real azeotrope but equilibrium concentrations of water and acetic acid are very near to high acid purities) and it therefore requires a column with a large number of stages and a high reflux ratio, thus incurring huge capital and operating costs. In practice, for concentrations below 40% acetic acid, liquidliquid extraction is the most appropriate.7 This study describes the recovery of acetic acid by liquidliquid extraction and the comparison of the results obtained with binary distillation in terms of, on the one hand, the operational cost of the chemical plant using EP and TAC and, on the other hand, the environmental impacts calculated through the Ecoindicator-99 methodology.

2. DESCRIPTION OF ALTERNATIVES This paper presents five process alternatives to recover acetic acid from water. All of them have been simulated and optimized using the chemical process simulator Hysys version 3.215 modifying the number of stages, feed location, and operating conditions, of the columns that take part in the case study.

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The main optimized variables for Alternatives 1, 2, and 3, with respect to the economic criterion, have been taken directly from the literature.16 In the example used, the process requirement is to separate 1109.6 kmol/h of a mixture of acetic acid and water containing 12.7% (molar) acetic acid at 25 °C. The objective is to obtain a minimum of 99.5% (in mols) of acetic acid, also called glacial acetic acid. UNIQUAC equilibrium thermodynamics are used to represent the liquidvapor equilibrium. In addition, the UNIQUAC model implemented in Hysys enables including the particularity of acid acetic to dimerize in the vapor phase. For the alternatives that involve a liquid extraction stage (2, 3, 4, and 5), diethyl ether has been used as a low boiling point extractant (34.6 °C). 2.1. Alternative 1: Binary Distillation. Figure 1 represents the binary distillation of acetic acid from water. In this flowsheet, the distillate is nearly free acid water which may be sent to a treatment facility installation to process trace amounts of acetic acid, and the lower streams are the desired product (glacial acetic acid). Due to the high feed flow, it is necessary to introduce four distillation columns in parallel in order to reduce the column diameters to 4 m (maximum recommended diameter17). The feed is first heated to its bubble temperature to improve separation performance in the columns. The distillation columns are named as Column T-x/y where x is the number of column in the alternative y. Thus, the column T-1/1 is the column 1 of the Alternative 1. 2.2. Alternative 2: Extraction System with Solvent Recovery (Two Distillation Columns). Figure 2 shows the complete flowsheet for Alternative 2. The extracting stream is fed to

Figure 1. Flowsheet for Alternative 1 (Binary distillation).

Figure 2. Flowsheet for Alternative 2 (extraction with solvent recovery and two distillation columns). 10718

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Industrial & Engineering Chemistry Research column T-1/2 and glacial acetic acid is removed as the bottom product. The distillate is a mixture of diethyl ether and water which is decanted into a water-rich phase and an ether-rich phase. The raffinate stream is sent to column T-2/2 to remove small amounts of solvent from the water. The bottom product is nearly solvent-free water which may be sent to a treatment facility to process trace amounts of acetic acid and solvent, or even recycled to other parts of the plant. The distillate is a mixture of diethyl ether and water which is also decanted. The overall feed to the decanter is a mixture of diethyl ether and water which is cooled to the extractor temperature. The ether-rich phase removed from the decanter is recycled to the extractor. 2.3. Alternative 3: Extraction System with Solvent Recovery (Three Distillation Columns). The flowsheet for Alternative 3 is shown in Figure 3. This flowsheet is similar to that of Alternative 2. However, in this case, the extractant is separated in column T-1/3 by removing the etherwater azeotrope as the distillate. This flowsheet therefore requires a third column to separate the acetic acidwater mixture which is removed as the base of column T-1/3. Column T-3/3 has a product of glacial acetic acid and a distillate of an acetic acidwater mixture which is rich in

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water. This flowsheet has an additional recycle stream (the distillate of column T-3/3) which is necessary to avoid the tangent pinch region in the vicinity of pure water when performing the binary acetic acidwater split in column T-3/3. The treatment of raffinate stream is the same as that for Alternative 2. 2.4. Alternative 4: Extraction System with Extractant Recovery for Decantation. In this alternative (Figure 4), the recovery of the solvent is carried out by using only a decantation step. This flowsheet is similar to that of Alternative 2 although it involves recovering the solvent using a decantation stage and not with an additional column as occurs in Alternative 2. The wastewater stream has to be sent to a treatment facility to process trace amounts of acetic acid (45 kg/h) and medium amounts of extractant (1150 kg/h). 2.5. Alternative 5: Extraction System without Solvent Recovery. Although this alternative is, at a glance, not beneficial for either economic and environmental criteria, it has been evaluated in order to assess how detrimental it is in comparison with the others. Figure 5 shows Alternative 5. This flowsheet is similar to that of Alternative 2 although it involves the waste of raffinate and

Figure 3. Flowsheet for Alternative 3 (extraction with solvent recovery and three distillation columns).

Figure 4. Flowsheet for Alternative 4 (extraction with extractant recovery for decantation).

Figure 5. Flowsheet for Alternative 5 (extraction without extractant recovery). 10719


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Table 1. Design Variables for Each Proposed Alternative alternative

2: extractor + 1: binary distillation number of stages for column

2 distillation columns

3 distillation columns

for decantation)

5: extractor + 1 column (without extractant recovery)

col T-1/1: 45

extractor: 45

extractor: 45

col T-2/1: 45

col T-1/2: 24

col T-1/3: 40

extractor: 45

extractor: 45

col T-3/1: 45

col T-2/2: 5

col T-2/3: 10

col T-1/4: 24

col T-1/5: 25

col T-4/1: 45 operating pressure (atm)

4: extractor + 1 column (extractant recovery

3: extractor +

col T-3/3: 60

col T-1/1: 1 col T-2/1: 1

extractor: 1 col T-1/2: 2

extractor: 1 col T-1/3: 2

extractor: 1

extractor: 1

col T-3/1: 1

col T-2/2: 2

col T-2/3: 1

col T-1/4: 2

col T-1/5: 2

col T-4/1: 1

col T-3/3: 2

a given equipment with a specific capacity QB is known, the cost CE of similar equipment with capacity Q can be approximately calculated by M Q CE ¼ CB 3 ð3Þ QB

Figure 6. Technical framework for life cycle assessment (ISO 14040).

distillate streams without solvent recovery. The waste stream has to be sent to a treatment facility to process trace amounts of acetic acid and high amounts of extractant. Table 1 shows the main design variables for each alternative.

3. METHODOLOGY 3.1. Economic Analysis. For the economic analysis two simple criteria have been used (Economic Potential and Total Annual Cost) which are useful in process design because they can be applied at various stages without a complete picture of the chemical plant.18 Therefore, it is usually not possible to account for all the fixed and variable costs until all the operating costs and cash flows are known. Economic Potential (EP).

EP ¼ value of products fixed costs variable costs taxes ð1Þ Total Annual Cost (TAC). TAC ¼ fixed costs þ variable costs þ taxes

ð2Þ

However, the preceding definitions of EP and TAC cost can be simplified if it is accepted that they will be used to compare the relative merits of different structural options. Thus, items that will be common to the options being compared can be neglected. The estimation of the equipment costs has been performed using a costcapacity relationship, which states that if cost CB of

where M is a constant that depends on the equipment type.19 The data used for the estimation of different equipment costs are given in the literature.20,21 In some cases, the published data are old and must be updated with a common basis using cost indexes. All data given in the literature have been brought up to September 2010 values using the Chemical Engineering Process Cost Indexes (CEPCI) published in the Chemical Engineering Journal. The total capital costs of processes, services, and working capital have been obtained by applying multiplying factors to the purchase cost of the individual items of equipment:22 C F ¼ fI 3

∑i CE, i

ð4Þ

where CF is the fixed capital cost for the complete system, CE,i is the cost of equipment i, and fI is the overall installation factor for the complete system and equal to 5.8. To estimate operating costs, the following items have been considered: • Raw material purchase: The cost of the solvent (extractant) used in the extraction systems has been taken into account. The cost of the wateracetic acid mixture has been neglected because it is common to all alternatives. The values of solvent and acetic acid are given in the Chemical Marketing Report.23 • Utility operating costs: Utility operating costs are usually the most significant variable after raw materials. For the economic analysis of each alternative, the costs of electricity, cooling water, and steam have been taken from data given in the literature.24 To express both capital and operating costs on a common basis, the capital cost has been expressed on an annual basis over a fixed time horizon of 30 years with a fixed interest rate equal to 5%. 3.2. Environmental Analysis. Burgess and Brennan25 in their complete review about the application of LCA to chemical processes presented how this objective process for evaluating 10720



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Table 2. Brief Description of the Data Sets Obtained from Ecoinvent Data v.2.0 dataset name diethyl ether, at plant, Europe (kg)

description

processes included

direct hydration of ethylene including materials,

the multi-output-process “ethylene hydration process” delivers the coproducts ethanol 99.7% in H2O and diethyl ether 99.95% in H2O;

energy use, infrastructure and emissions

the allocation is based on mass balance water, completely softened, at plant,

water treatment by ion-exchanger for use as

Europe (kg)

use of chemicals and some emissions for the treatment of water used in

cooling water

electricity, medium voltage, at grid, Spain (kWh)

heat, unspecified, in chemical plant,

power plants

this data set describes the transformation from

average technology used to transmit and distribute electricity; includes

high to medium voltage as well as the trans-

underground and overhead lines, as well as air-vacuum and SF6-

mission of electricity at medium voltage

insulated high-to medium voltage switching stations; electricity production according to related data set

includes the heat production required for the

to be used for heat energy production in an average chemical plant

Europe (MJ)

production of 1 MJ (0.3636 kg) steam from cold water; does not include the water input because steam is often used in closed systems

Table 3. Evaluated Configurations for Each Rectifier Column and the Total Number of Combinations Simulated for Each Alternative configurations evaluated for each rectifier column alternative

design variables

column T-1/y

column T-2/y

1

xAcH, bot = 0.995 Qbot = 35.40 kmol/h

21

2

xAcH, bot = 0.995

16

4

8

6

column T-3/y

total simulated combinations for each alternative 21 64

Qbot = 140.9 kmol/h 3

boilupratio reboiler = 0.17

22

1056

recoveryDiE,top= 100% 4

xAcH, bot = 0.995

16

16

17

17

recoveryAcH,bot = 99.92% 5

xAcH, bot = 0.995 recoveryAcH,bot = 99.92%

the environmental loads associated with a product, process, or activity could be used as a tool for quantitative evaluation of the environmental merits or “cleanliness” of a chemical process and for ranking process alternatives according to their cleanliness. The application of LCA to the design of sustainable chemical processes has two main advantages:26 first, it covers the entire life of the product, process, or activity and, second, LCA aggregates the environmental burdens into a limited set of recognized environmental impact categories (depending on the Life Cycle Assessment methodology used), such as global warming, acidification, ozone depletion, etc. For that reason, LCA is a good framework to provide criteria and quantitative measures that can be used for comparing different process operations and design alternatives. The LCA methodology actually comprises a set of different methods within a common framework while the methodology for impact assessment in particular often differ among users and procedures. According to ISO 1404027 (which covers the principles and framework for conducting LCAs) and ISO 1404428 (which introduces guidelines and requirements), there are four steps to carry out an LCA. Figure 6 shows how these items are interrelated. The first phase, “Goal and scope definition”, serves to define the purpose and extent of the study and it contains a description of the system being studied.29

The functional unit (FU) is established in this step, with the necessary data and information needed for the inventory and impact also being identified. However, our approach focuses on decreasing the environmental impact of the manufacturing stage and, for that reason, the analysis is restricted to this life cycle stage. Thus, the downstream processes such as secondary processing, product use, and disposal are neglected whereas the upstream/input processes are included within the system boundaries. This cycle study can therefore be regarded as a “cradle-to-gate” analysis. The second phase of LCA, “Inventory analysis”, consists of data collection and analysis. Data are often obtained from LCA databases from standard inputs such as energy and materials. Data are then processed to produce an inventory of environmental interventions per functional unit. In these cases, all the environmental burdens are expressed as a function of a fixed amount of glacial acetic acid. Thus, raw materials and energy consumption must be further translated into their corresponding environmental burdens using a specific database that contains emissions inventories associated with a wide range of chemicals. For this work, the inventory data has been obtained from the Ecoinvent database version 2.0.30 The third phase of LCA, “Impact assessment”, serves to evaluate the significance of the environmental interventions contained in a life-cycle inventory (LCI). In practice, an inventory 10721



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Figure 7. Economic Potential and Ecoindicator-99 for all the combinations generated in Alternatives 2 and 3.

will contain a long list of emissions and resource uses. Its purpose is to determine the relative importance of each inventory item and to aggregate interventions into a small set of indicators (or even into a single indicator). This is carried out in order to identify those processes that contribute most to the overall impact or to compare products. In this stage of the process, data are translated into environmental information. In this paper, the Ecoindicator-99 methodology has been used.3133 This model has three different categories. The “human health” damages are specified in Disability Adjusted Life Years (DALYs). A damage of 1 means that 1 life year of 1 individual is lost or 1 person suffers for four years from a disability with a weight of 0.25. On the other hand, “ecosystem quality” damages are specified as PDF 3 m2 3 year. PDF stands for “potentially disappeared fraction” of species. A damage of 1 means that all species disappear from 1 m2 over 1 year, or 10% of all species disappear from 1 m2 over 10 years. With regard to the damage to “resources”, these are specified as MJ surplus energy. A damage of 1 means that due to a certain extraction of resources, further extraction of the same resources in the future will require one additional MJ of energy due to the lower concentration of

resource or other unfavorable characteristics of the remaining reserves. Finally the damage categories are normalized and aggregated into a single impact factor, the Ecoindicator-99. Normalization is based on a damage calculation of all relevant European emissions, extractions, and land-uses and then weighted according to the selected perspective based on Cultural Theory. 3.3. Simplifications and Main Hypothesis Considered. To analyze the economic and environmental incentives for each of the proposed alternatives, some hypotheses have been made. However, they do not have any important effect on the final result as to whether it is accepted, in which case they will be used to compare the relative merits of the different structural options. 3.1.1. Economic Evaluation • All sections in a given column are forced to have the same diameter. • For fixed cost estimation, correlations to chemical equipment design have been used. • The calculated column height has been increased by 6 m to include enough space for vapor separation, liquid held in the 10722



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Table 4. Configuration That Drives to Maximum Economic Potential for Each Evaluated Alternative optimized configuration for maximum alternative 1

economic potential column T-1/1, T-2/1,

EP (105 h/year)

Ecoindicator-99 (108 points/FU)

402.28

2.033

431.69

0.883

432.30

0.887

362.75

1.252

4189.36

31.935

T-3/1 and T-4/1 number of stages: 52 feed stage location: 38 2

column T-1/2 number of stages: 34 feed stage location: 25

Figure 8. Economic Potentials for the combination which drives the economic optimum for each alternative Economic optimization.

base of the column, auxiliary equipment, etc. Plate separation is 0.609 m. • For each distillation column, the best location for the feed is selected which enables reaching the desired separation with minimum energy requirements. • All equipment is made of stainless steel which is not attacked by acetic acid/water mixtures. • Only the expensive equipment is considered: rectifier and liquidliquid extraction columns. The rest of the installation is similar for all alternatives and its contribution to the fixed cost is not significant. • For variable costs, water recovery from heating and cooling systems of 90% is assumed. • It is assumed that all industrial systems operate under ideal conditions, and therefore there are no solvent losses or fugitive emissions in the chemical process. 3.1.2. Environmental Evaluation • A functional unit (FU) of 1.8 106 tons of glacial acetic acid has been selected. As each alternative has a different production rate of acetic acid, the plant working time to reach the FU has been calculated for each alternative. This amount of glacial acetic acid is equivalent to the quantity of acetic acid produced by a standard chemical plant over an operation time of 30 years with an average capacity of 140 kmol/h (with an availability of 80%). • The environmental impact of the feed is not calculated because it is unknown and common to all alternatives. • There are no air emissions due to solvent and component losses. • The environmental impact of sewage waters is not assessed because they are removed to a water treatment stage which is similar in all cases (except Alternative 5). • The environmental impact associated with construction materials is not considered because it is insignificant in comparison with the following items which affect the entire life of the chemical plant (Table 2). These data sets have been chosen from Ecoinvent Data v.2.0 to calculate the Ecoindicator-99 value for each proposed flowsheet. Each data set describes a life cycle inventory at a unit process level which is identified unequivocally by the name, the location, the unit, and a marker for the infrastructure processes. The Ecoinvent database v.2.0 also contains the characterization, damage, or weighting factors for various impact assessment methods. Each topic of an impact assessment method is

column T-2/2 number of stages: 3 feed stage location: 1 3

column T-1/3 number of stages: 10 feed stage location: 8 column T-2/3 number of stages: 3 feed stage location: 1 column T-3/3 number of stages: 35 feed stage location: 25

4


5


described with a category, subcategory, the name, and its unit. The category defines the impact assessment method (such as Ecoindicator-99), the subcategory as either a safeguard subjector or an environmental theme (e.g., “Human Health” in case of Ecoindicator-99). The data set name is used for a further grouping if necessary or where possible (e.g., “climate change” or “carcinogenics” within the “Human Health” subcategory of the Ecoindicator-99 category). • For Ecoindicator-99 value calculations, a hierarchical perspective has been selected because it is backed by scientific and political bodies with sufficient recognition.

4. RESULTS 4.1. Evaluated Configurations for Each Alternative. To determine the best alternative (and its optimal operational conditions) while simultaneously considering both environmental and economic criteria, several configurations have been simulated for all the columns of each alternative, modifying the number of stages and feed location. As a result, a high number of combinations have been simulated and optimized for each flowsheet (Table 3). For each system, EP and Ecoindicator-99 have been calculated. Two specifications have been fixed to converge the complete alternative for each potential column configuration. They are also included in Table 3. 10723



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Table 5. Economic Optimization: Cost Breakdown for Each Alternative Evaluated alternatives configuration which drives to economic optimum

1

2

3

4

5

configuration column T-1: number of stages (feed location)

34 (24)

52 (38)

34 (25)

10 (8)

34 (24)

fixed cost: CTA column T-1 (105 h/year)

21.00

4.25

1.88

4.25

4.25

variable cost: energy costs for column T-1 heater (105 h/year)

29.24

11.11

8.66

11.32

11.32

variable cost: deionized water for column T-1 cooling system (105 h/year)

0.67

0.23

0.17

0.24

0.24

configuration column T-2: number of stages (feed location)

N/A

3 (1)

3 (1)

N/A

N/A

fixed cost: CTA column T-2 (10 h/year) variable cost: energy costs for column T-2 heater (105 h/year)

0.54 1.48

0.54 1.48

variable cost: deionized water for column T-2 cooling

0.00

0.00

N/A

35 (25)

N/A

N/A

5

system (105 h/year) configuration column T-3: number of stages (feed location)

N/A

fixed cost: CTA column T-3 (105 h/year)

2.73

variable cost: energy costs for column T-3 heater (105 h/year)

2.50

variable cost: deionized water for column T-3 cooling system (105 h/year)

0.06

common elements for alternative fixed cost: CTA common equipment (105 h/year) variable cost: energy costs for column common equipment (105 h/year)

0.00 1.51

2.18 0.68

2.19 0.72

2.31 0.51

2.32 0.52

variable cost: deionized water for column common equipment (105 h/year)

0.04

0.07

0.07

0.03

0.04

0.03

0.03

0.02

0.02

variable electric costs associated with column common equipment (105 h/year) variable extractant purchase (105 h/year) CTA alternative (105 h/year)

52.46

0.08

0.08

71.12

4624.45

20.66

21.12

89.59

4643.15

benefits for alternative (105 h/year)

454.73

452.34

453.41

452.34

453.79

EP for alternative (105 h/year)

402.28

431.69

432.30

362.75

-4189.36

Thus, for Alternative 2 for example, 64 potential combinations were evaluated by changing the number of stages of column T-1 (16 simulated configurations) and column T-2 (4 configurations). It is important to note that each distillation column works under sharp-split conditions. For all conventional distillation schemes a “sharp-split” distillation model34 was used, wherein each distillation column is used to make a sharp separation between adjacent components of a homologous series. In a sharp-split distillation sequence, each component leaves the distillation column in a single product stream, either as overheads or bottoms. It is worth observing that the sharp-split operation allows the composition of distillate, and that the base streams do not change although the number of distillation trays, reflux ratios, and feed positions can be very different. In other words, for a given number of trays and a fixed feed position there is a minimum reflux ratio that satisfies the sharp-split constraint. For a single column there is a trade-off between the number of trays and the reflux ratio for both the economic and environmental points of view. What is convenient about sharp-split is that we can isolate those columns from the rest of the process, and perform an individual study of those columns, i.e., obtaining the individual Pareto’s curve, and then obtain the global Pareto’s curve from the individual ones with almost no error. To get the Pareto frontier we used the ε-constraint method.35 The method approximates the Pareto frontier by finding a set of Pareto solutions that belong to it. Each single Pareto solution is found by solving a single objective optimization problem. Such an optimization problem incorporates limits for

the remaining objective functions which are not being directly optimized. Therefore, the original multiobjective problem Pi(εj) can be decomposed into a series of single-objective problems. Original problem:

minff i ðxÞ, f j ðxÞg

ð5Þ

where f i ðxÞ Economical objective function f j ðxÞ Environmental objective function ε-constraint method:

min f i ðxÞ

ð6Þ

subject to x∈X f j ðxÞ e εj ,

i 6¼ j

ð7Þ

In the case we are dealing with, in which we only have two objectives, we first solve two problems. In the first we minimize the TAC (or the EP). In the second we minimize the environmental impact. With these two points we have the extremes of the Pareto curve (sometimes we extend the curves for illustrative purposes). Then we solve a set of single objective problems, i.e., by minimizing TAC in which the environmental impact cannot be greater than an upper bound plus a small quantity (the upper bound is the best value obtained from previous problems). In our case these problems take the form of Mixed Integer Linear 10724



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Table 6. Configuration That Leads to the Minimum Environmental Impact (Minimum Ecoindicator-99 value) for Each Alternative optimized configuration for minimum environmental alternative 1

impact (Ecoindicator-99)

Ecoindicator-99 (108points/FU)

column T-1/1, T-1/2,

EP (105 h/year)

1.780

392.10

0.842

430.57

T-1/3 and T-1/4 number of stages: 100 feed stage location: 77 2

column T-1/2 number of stages: 60

Figure 9. Ecoindicator-99 for the combination which drives the environmental optimum for each alternative Environmental optimization.

feed stage location: 45 column T-2/2 number of stages: 4 feed stage location: 1 3

column T-1/3 number of stages: 20

0.844

429.95

1.211

361.69

feed stage location: 16 column T-2/3 number of stages: 4 feed stage location: 1 column T-3/3 number of stages: 70 4

feed stage location: 46 column T-1/4 number of stages: 60 feed stage location: 45

5

column T-1/5

31.890

4192.08

number of stages: 80 feed stage location: 61

Problems (MILP), because we have integer variables associated with the number of trays and feed tray position in distillation columns. 4.2. Best Alternatives Relating to Economic Criterion. Economic Potential (PE) can be calculated from the TAC for each column as follows: EPi, j ¼ Benefiti TACaux, i

Nk

∑ TACk-n K ¼1

ð8Þ

where • Nk is the number of columns which forms the alternative i (Nk = 1 for Alternatives 1, 4, and 5, Nk = 2 for Alternative 2, and Nk= 3 for Alternative 3). • n is the selected configuration for column k in alternative i. • EPi,j is the Economic Potential (h/year) associated with Alternative i and combination j. Thus, we can express the PE as the difference between the benefit of alternative i (which is independent of the simulated configuration because product recovery is fixed), and the TAC of each column which forms alternative i and its auxiliary equipment (which are independent of column configurations n.). • Benefiti is the benefit (h/year) from selling the final product (glacial acetic acid). This term is independent of the

simulated configurations because recovery percentage and product flow rate are fixed. • TACaux,i is TAC (h/year) associated with auxiliary elements of alternative i. This term includes fixed and variable costs (heating and cooling system, liquidliquid extraction column, pumps, and extractant needs). • TACkn is the TAC (h/year) associated with column k with a specific configuration n which represents a fixed number of stages and specific feed stage location. Each configuration has a different TAC because they have different numbers of stages and feed stage locations (and therefore different operating conditions to get the sharp-split specification). Consequently the energy requirement for each configuration changes. Once TAC is calculated for each alternative combination j, EP can be plotted versus the Ecoindicator-99. In Figure 7, the solutions for the Alternatives 2 and 3 have been included as examples. From this figure the best configuration for each alternative considering economic or environmental criteria can be easily obtained. Note that the best economic configuration is indicated in red and the best environmental configuration is shown in green. The Pareto’s individual curve is indicated as a dashed line and shows the set of noninferior solutions for which no solution can give an improvement of one objective (EP) without detriment to another (Ecoindicator-99). In any case, there is no configuration that simultaneously reaches the economic and environmental best objective function values (utopia solution). As expected, Alternative 5 (where there is no type of extractant recovery) involves negative EP in all combinations (with different column configurations) that were simulated by Hysys 3.2. Regarding the environmental criterion, this alternative presents the worst environmental impact (the highest Ecoindicator99 value) in comparison to the rest of the cases studied. Figure 8 depicts the economic optima for each alternative evaluated. Alternative 5 is a nonprofitable option. Alternative 4, where the solvent is recovered by a single decantation step, presents poor results in comparison with Alternatives 2 and 3. This fact is evidence that solvent recovery is an important key to considering whether we want to manage profitable flowsheets. With regard only to the economic criterion, Alternative 3 is the best option, maximum benefit or EP, along with the Alternative 2 for the extraction of acetic acid from water. 10725



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Table 7. Contribution of the Impact Categories for the Minimum Environmental Solution (a) and for the Minimum Cost Solution (b) for Each Alternative (a) Ecoindicator-99 for the best solutions attending to the environmental criterion Ecoindicator-99 (108 points/FU) alternative damage category human health

ecosystem quality

resources

impact category

1

2

3

4

5

ionizing radiation

0.001

0.000

0.000

0.001

0.006

respiratory effects caused by organic and inorganic substances

0.205

0.097

0.097

0.119

2.341

climate change

0.169

0.080

0.080

0.089

1.323

ozone layer depletion carcinogenic effects

0.000 0.018

0.000 0.009

0.000 0.009

0.000 0.009

0.000 0.143

ecotoxic emissions

0.027

0.013

0.013

0.014

0.212

land occupation and land conversion

0.013

0.006

0.006

0.007

0.079

acidification and eutrophication

0.019

0.009

0.009

0.011

0.210

extraction of minerals

0.001

0.001

0.001

0.003

0.145

extraction of fossil fuels

1.326

0.627

0.628

0.959

27.433

total

1.780

0.842

0.844

1.211

31.890

(b) Ecoindicator-99 for the best solutions attending to the economic criterion Ecoindicator-99 (108 points/FU) alternative damage category human health

ecosystem quality

resources

impact category

1

2

3

4

5

ionizing radiation

0.001

0.001

0.001

0.001

0.006

respiratory effects caused by organic and inorganic substances

0.234

0.102

0.102

0.124

2.346 1.327

climate change

0.193

0.084

0.084

0.093

ozone layer depletion

0.000

0.000

0.000

0.000

0.000

carcinogenic effects ecotoxic emissions

0.021 0.031

0.009 0.014

0.009 0.014

0.010 0.015

0.143 0.212

land occupation and land conversion

0.015

0.007

0.007

0.007

0.079

acidification and eutrophication

0.022

0.010

0.010

0.012

0.211

extraction of minerals

0.002

0.001

0.001

0.003

0.145

extraction of fossil fuels

1.514

0.657

0.661

0.989

27.466

total

2.033

0.883

0.887

1.252

31.935

Table 4 shows the configuration for each alternative which drives maximum economic potential and its Ecoindicator-99 value. It is important to note that the item which most affects the TAC is the energy required for heating equipment and in particular column boilers (Table 5). As is expected, in Alternative 5 the variable costs are higher in comparison with the remaining alternatives due to high demand of the extractant. 4.3. Best Alternatives Relating to Environmental Criterion. The Ecoindicator-99 for an alternative i could be estimated as Eco-99i, j ¼ Eco-99aux, i þ Eco-99Extractant, i þ

Nk

∑ Eco-99k-n K ¼1 ð9Þ

where • Nk is the number of columns which forms the alternative i (Nk = 1 for Alternatives 1, 4, and 5, Nk = 2 for Alternative 2, and Nk = 3 for Alternative 3). • n is the selected configuration for column k in alternative i.

• Eco-99i is the Ecoindicator-99 (Points/FU) associated with Alternative i. • Eco-99aux,i is the Ecoindicator-99 (Points/FU) associated with auxiliary elements for alternative i. This item includes the environmental burdens from centrifugal pumps, heaters and coolers. • Eco-99Extractant,i is the Ecoindicator-99 (Points/FU) associated with the extractant fabrication required for Alternative i. • Eco-99k-n is the Ecoindicator-99 (Points/FU) associated with column k (with a specific configuration n). Each configuration evaluates the environmental burdens from energy requirements and the deionized water used in the heating and cooling system in column k. Table 6 shows the best configuration which drives the environmental optima for each alternative. Figure 9 depicts the value of the Ecoindicator-99 for the minimum environmental solution for each alternative. Table 7 shows the contribution of the environmental impacts that are included in Ecoindicator-99 for the minimum 10726



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Figure 10. Combined representation of the individual Pareto’s curves for Alternatives 1, 2, 3, and 4.

Figure 11. Compound Pareto’s curve obtained from combinations of configurations of Alternatives 2 and 3.

environmental solution (part a) and for the minimum cost solution (part b) for each alternative. As shown in Table 7a), Alternative 2 has a lower energy requirement than Alternative 1 (binary distillation) so it entails a lower Ecoindicator-99 value. Alternatives 2 and 3, although they are more complex systems, have lower Ecoindicator-99 values than Alternatives 1, 4, and 5, and permit a reduction in energy consumption. In fact these alternatives sufficiently reduce the Ecoindicator-99 value to compensate for the use of extractant (diethylether) in the installation. For the environmental criterion, Alternative 2 is the best option for extracting acid acetic from water.

4.4. Combined Analysis Regarding Both Environmental and Economic Criteria. In the set of Pareto’s noninferior

solutions for each alternative (Figure 10), the curves of Alternatives 2 and 3 intersect. That means that there are some potential combinations for Alternative 2 that will enable reaching higher EPs (and possibly lower environmental impacts) than some combinations of Alternative 3. As Pareto’s individual curves for Alternatives 2 and 3 intersect, it is possible, while maintaining same environmental damage (Ecoindicator-99 value), to reach better economic potential with Alternative 3 than 2 (points of Alternative 3 located to the right of 10727


Industrial & Engineering Chemistry Research the intersection point). Nevertheless, shifting to the left from the intersection point, better results for both EP and Ecoindicator-99 can be obtained with Alternative 2. Thus, the compound Pareto’s curve for Alternatives 2 and 3 (Figure 11), could be obtained optimum configurations which contain the set of noninferior solutions. From the compound Pareto’s curve, calculated by applying the ε-constraint method to the set of noninferior solutions obtained in Alternatives 2 and 3, the optimum configurations for each alternative which involves the best trade-off solution can be identified. In this sense, it is important to highlight the fact that there is no optimum alternative (or configuration for operating conditions) pertaining to both criteria (Economic Potential and Ecoindicator-99). Thus, there are some configurations for Alternative 2 that have lower environmental impact but, at the same time, lower EPs in comparison with the EPs obtained in certain configurations of Alternative 3. Alternative 2 presents some of the best configurations concerning the environmental criterion (Ecoindicator-99) although with a low EP so it will be the process designer who will have to assess the “admissible” level of environmental damage and consequently which alternative (and configuration) allows reaching this tolerable impact with the maximum economic potential (maximum benefit).

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economic potential. In these conditions, the designer must rely on his or her experience and ability to express their preferences throughout the optimization cycle. • Configurations with a high number of stages involve lower environmental impact. This fact is evidence that an increase in the number of stages involves a reduction in energy demand and, as consequence, lower Ecoindicator-99 values. • The identification of Pareto’s noninferior solutions provides a valuable insight into the design problem and is intended to guide the decision-maker toward the choice of a sustainable process alternative, which leads to the best profit for a fixed environmental impact (any noninferior solution is locally optimum).

’ ASSOCIATED CONTENT

bS

Supporting Information. The contribution of the data sets to the Ecoindicator-99 value for the minimum cost solution for each alternative, showing where the environmental impacts come from attending to the Ecoindicator-99 methodology. This information is available free of charge via the Internet at http:// pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

5. CONCLUSIONS From the evaluation of the set of solutions obtained for each alternative, the following can be concluded: • Alternative 1 (Binary rectification) represents an alternative to extract acid acetic from water which could be considerably improved if it is combined with a liquidliquid extraction stage because it allows an increase in EP and reduces the environmental burden of the system (mainly by decreasing energy consumption in the system). • To reach better EPs, in Alternative 2, column T-1/2 should have a number of stages between 34 and 60. Columns with an inferior number of stages do not allow reaching a maximum EP. For column T-2/2 it is highly recommended that it is designed with a small number of stages. • For Alternative 3, column T-1/3 has to be designed with between 10 and 20 number of stages. Columns with more stages produce worse EPs. For column T-2/3, results similar to Alternative 2 are obtained. It is recommended that column T-3/3 have a number of stages between 20 to 70. • Alternative 4, although it represents improvements in comparison with Alternative 2, also leads to poor results in comparison with Alternatives 2 and 3. This fact is evidence that solvent recovery is an important aspect to be considered. • Alternative 5 (in any of its configurations evaluated) cannot be a viable solution due to high variable costs of using large amounts of solvent. This high consumption of solvent also involves important environmental burdens and, as a consequence, a high Ecoindicator-99 value. • Alternatives 2 and 3 present the best economic potential. However, if the compound Pareto’s curve is analyzed it can be seen that there is no optimum configuration for either alternative. That means that any configuration cannot reach both minimum environmental damage and maximum

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Economic and Environmental Assessment of Alternatives to the

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