incorporation of safety and sustainability in conceptual design via a

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Incorporation of Safety and Sustainability in Conceptual Design via a Return on Investment Metric Karen Guillen-Cuevas,† Andrea P. Ortiz-Espinoza,† Ecem Ozinan,‡ Arturo Jiménez-Gutiérrez,† Nikolaos K. Kazantzis,§ and Mahmoud M. El-Halwagi*,‡ †

Departamento de Ingeniería Química, Instituto Tecnológico de Celaya, Celaya, Gto 38010, México Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843, United States § Department of Chemical Engineering and Center for Resource Recovery and Recycling, Worcester Polytechnic Institute, Worcester, Massachusetts 01609-2280, United States ‡

ABSTRACT: Traditionally, comprehensive process design has relied on a sound and insightful techno-economic analysis framework. In particular, once a base-case design is selected, further analysis is carried out to assess the environmental impact as well as possible process safety implications. Furthermore, limited modifications to the base-case design are typically carried out to concurrently address environmental and safety concerns. This sequential approach can thus lead to suboptimal process system designs when multiple objectives of profitability, sustainability, and safety are considered. Indeed, the simultaneous consideration of these objectives during the conceptual design phase of a project should lead to superior performance profiles. In the present work, a new approach for the incorporation of safety and sustainability considerations early enough during the conceptual design of the process is presented. This approach is based on extending the conventional return on investment (ROI) analysis to include positive or negative impacts of potential design changes that emerge when sustainability and safety issues are simultaneously considered. Within the proposed context, this becomes possible through a meaningful and potentially insightful extension of the traditional economic performance metric to account for safety and sustainability-relevant performance criteria. In particular, a safety and sustainability weighted return on investment metric (SASWROIM) is introduced to systematically integrate the aforementioned multiple objectives and therefore inform the conceptual design stage in a methodologically sound and insightful manner. To illustrate the usefulness of the proposed approach, two case studies on the production of butadiene and methanol are considered and analyzed within the above context. It is demonstrated that this new approach can be used by decision makers to reliably evaluate the economic viability of a project as well as the assorted impact on the environment and process safety. KEYWORDS: Design, Sustainability, Safety, Profitability, Integration, Multiobjective optimization

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are provided in literature.3−5 Moreover, several studies have been undertaken to include safety and/or sustainability in process design through multiobjective optimization methods.6−13 Recently, El-Halwagi14 introduced a new metric referred to as the Sustainability Weighted Return on Investment Metric (SWROIM) for use in process integration and improvement projects. The basic idea is to extend the conventional return on investment (ROI) concept by incorporating process integration targeting (benchmarking) and relevant sustainability metrics. For instance, let us consider the case when process integration is used to generate a number of project alternatives indexed as p = 1,2,..., NProjects. For the pth project, a new term called the Annual Sustainability Prof it “ASP” is defined as follows:

INTRODUCTION

There is a growing awareness of the importance of including safety and sustainability issues in the design of industrial processes. While there is general consensus on the economic criteria to assess the performance of a design alternative, there is much less agreement on how to quantify the safety and sustainability-relevant performance objectives/specifications of a process. There is even less consensus on how to reconcile the safety and sustainability objectives with the profitability criteria of the process. Several metrics have been proposed for assessing safety and sustainability. Roy et al.1 and Hassim2 provided recent reviews of metrics for safety and occupational health hazards that can be used for assessing the design of industrial processes. These metrics address various safety issues pertaining to the process and the chemicals involved in the process. They also correspond to different objectives and their use depends on the design stage. Comprehensive reviews of the various approaches of evaluating sustainability metrics in engineering systems and for including sustainability in design © XXXX American Chemical Society

Received: October 18, 2017 Revised: November 28, 2017 Published: December 5, 2017 A

DOI: 10.1021/acssuschemeng.7b03802 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering ⎡ ASPp = AEPP ⎢1 + ⎢⎣

NIndicators

∑ i=1

⎛ Indicatorp , i ⎞⎤ ⎟⎥ wi⎜ Target ⎝ Indicatori ⎠⎥⎦

ROIBase = (1)

Indicatorp , i IndicatoriTarget

captures the relative contribution of project p toward meeting the targeted performance for the ith sustainability metric. Consequently, the term ASPp represents a generalized functional form (metric) of profit that extends beyond the traditional one used for the quantification of economic profit to include possible sustainability gains or losses of the project. The Sustainability Weighted Return on Investment Metric “SWROIM” of project p is defined as follows: SWROIM p =

ASPp TCIp

(3)

where AEPBase and TCIBase are, respectively, the annual after-tax economic profit and the total capital investment of the basecase design. A set of sustainability and safety indicators {i|i = 1,2,...,NIndicators} is considered. Furthermore, a comprehensive assessment of sustainability and safety indicators yields values indexed as IndicatorBase,i. The associated target-values for these indicators are designated as IndicatorTarget,i. It should be pointed out that these target-values are determined through benchmarking techniques, desired company (strategic or operational) objectives or best achievable practices.15,18,19 As a result of process synthesis, analysis, and engineering activities, a number of design alternatives can be generated. Systematic techniques such as process integration tools15,19−21 may be used to develop these alternatives. They may involve a combination of changes in process configuration, types, number, and sizes of units as well as operating conditions. Each of these design alternatives is represented by an index p with p = 1,2,..., NProjects. For the pth project, a new term called the Annual Safety and Sustainability Prof it “ASSP” is introduced as follows:

where i is an index for the different sustainability indicators and the weighing factor wi is a ratio representing the relative importance of the ith sustainability indicator compared to the annual net economic profit. The term Indicatorp,i represents the value of the ith sustainability indicator associated with the pth project, and the term IndicatorTarget corresponds to the target i value of the ith sustainability indicator (obtained from process integration benchmarking). Therefore, the term

AEPBase TCIBase

(2)

where TCIP is the total capital investment of the pth project. The use of an ROI-based metric to account for sustainability during conceptual design has several merits. It offers a framework that is familiar to the process engineers. It also methodically represents the extent of process improvement measured versus desired benchmarks. Finally, it enables the trade-off of the multiple objectives associated with economic profitability and sustainability to emerge and be analyzed in a transparent and insightful manner. The objective of this paper is to introduce an ROI-based metric that incorporates safety and sustainability and can be used during the conceptual design stage. Compared to SWROIM, several challenges must be overcome. While mass and energy integration targets for sustainability can be readily determined for process integration projects, safety targets are not absolute. There is no such thing as an absolutely safe process. Instead, a process can always be made safer. Furthermore, while mass and energy metrics associated with sustainability are conserved,15 safety metrics are not.16,17 In light of the above realizations and responding to the needs delineated previously, the present research work aims at introducing an augmented ROI-based metric that includes safety and sustainability, as well as illustrating its potential usefulness in conceptual process design option development. The present paper is organized as follows: First, the Safety and Sustainability Weighted Return on Investment metric (SASWROIM) is introduced and its main structural characteristics identified and discussed. Furthermore, a comprehensive case study is included to illustrate the merits of the aforementioned metric and evaluate the performance of the proposed approach. Finally, some concluding remarks are presented in the paper’s last section.

ASSPp ⎡ = AEPP ⎢1 + ⎢⎣

NIndicators

∑ i=1

⎛ IndicatorBase , i − Indicatorp , i ⎞⎤ ⎟⎟⎥ wi⎜⎜ ⎝ IndicatorBase , i − IndicatorTarget,i ⎠⎥⎦ (4)

where wi is a weighting factor in the form of a ratio representing the relative importance of the ith safety or sustainability indicator compared to the annual net economic profit. These weights are selected based on the core values of the company and the relative importance of the sustainability and safety factors compared to economic profit. Indicatorp,i is the value of the ith safety or sustainability indicator associated with the pth design. The denominator IndicatorBase,i − IndicatorTarget,i represents the maximum desired improvement in the ith indicator. The numerator IndicatorBase,i − Indicatorp,i is the improvement (when the difference is positive) or deterioration (when the difference is negative) associated with the pth design ⎛ IndicatorBase ,i − Indicatorp,i ⎞ ⎟ represents option. Therefore, the ratio ⎜ Indicator − Indicator ⎝ Base , i Target,i ⎠ the fractional contribution of the pth design option toward meeting the target performance associated with the ith safety or sustainability metric. The term ASSPp is a generalized form for the quantification of the overall profit gain that includes the traditional form of economic profit as well as possible safety and sustainability benefits of the project. This generalized profit term is enhanced when there is an improvement in safety or sustainability compared to the base-case design, and its value is reduced when the specific alternate design option results in a deterioration of the safety or sustainability-relevant performance indicators when compared to those associated with the base-case project. The Safety and Sustainability Weighted Return on Investment “SASWROIM” of design p is expressed as follows:

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SAFETY AND SUSTAINABILITY WEIGHTED RETURN ON INVESTMENT METRIC Consider a base-case design project for which the conventional economic return on investment “ROI” is calculated through the following expression:

SASWROIM p = B

ASSPP TCIP

(5a) DOI: 10.1021/acssuschemeng.7b03802 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Block flow diagram for the shale gas-to-ethylene portion of the process22

Figure 2. Flowsheet for the ethylene-to-butadiene portion of the process22

=

⎡ ⎛ IndicatorBase ,i − Indicatorp,i ⎞⎤ N ⎟ AEPP ⎢1 + ∑i =Indicators wi⎜ Indicator − Indicator ⎥ 1 ⎝ Base , i Target,i ⎠⎦ ⎣ TCIP

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BUTADIENE CASE STUDY Consider the butadiene process described by Ozinan and ElHalwagi.22 Figures 1 and 2 show the base-case design for the two sections converting shale gas to ethylene followed by the dimerization of ethylene to butadiene according to the following reactions: Methane cracking to acetylene:

SASWROIM p

(5b)

SASWROIM provides an economic basis for the “real” cost/ value of a project based on the directly tangible financial performance as well as the indirectly tangible impact on the environment and safety using return on investment as a unifying basis of the multiple objectives. An improvement in the safety or sustainability indicators of a proposed design leads to an enhancement of SASWROIM. Conversely, a project leading to the deterioration of safety or sustainability indicators causes the value of SASWROIM to go down. The value of SASWROIM is then compared to the minimum acceptable (or threshold) ROI for a project. Consequently, a project with an attractive ROI but poor SASWROIM should not be selected. On the other hand, a project with superior sustainability and safety performance can bring the value of SASWORIM over the threshold value and thus warrant its implementation. The main features and practical usefulness of the new metric are evaluated and discussed in the two ensuing case studies.

2CH4 → C2H 2 + 3H 2

Hydrogenation of acetylene to ethylene: C2H 2 + H 2 → C2H4

Dimerization of ethylene to butadiene: 2C2H4 → C4 H6 + H 2

The dimerization reaction temperature is one of the most important design variables because of its impact on yield, cost, safety, and sustainability. For the base-case design processing 10 MM standard cubic feet (SCF) per hour of shale gas, a dimerization temperature of 1773 K is selected. Simulation studies using ASPEN Plus followed by techno-economic analysis for the base case were performed and the key results are shown in Table 1. C

DOI: 10.1021/acssuschemeng.7b03802 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Summary of Economic Evaluation Results for the Base Case Design with a Dimerization Temperature of 1773 K Item

Results

Annual Operating Cost (MM$/yr) Fixed Capital Investment (MM$) Working Capital Investment (MM$) Total Capital Investment (MM$) Annual Sales of Products (MM/yr) Annual After-Tax Profit (MM$/yr) ROI (yr−1%)

585.2 504.06 88.95 593.01 680.43 81.78 13.79

Table 3. Impact of Dimerization Temperature on Profitability, Carbon Footprint, and Hazard

FEDI > 500 500 > FEDI > 400 400 > FEDI > 200 200> FEDI > 100

Extremely hazardous Highly hazardous Hazardous Moderately Hazardous

CO2 emissions (MTPA)

FEDI reactor

1273 1523 1773 2023 2273 2523 2773

5.48% 11.02% 13.79% 16.29% 18.22% 19.77% 20.37%

148,987 196,923 256,545 295,646 358,289 421,826 497,721

306.4 490.9 593.4 670.8 734.2 788.6 836.6

Figure 3. ROI and SASWORIM for various dimerization temperatures.

When a minimum threshold value of 10 yr−1% is used, the lowest carbon footprint and safest design at 1273 K is not selected because the value of SASWROIM is well below the minimum 10%. Nonetheless, the most economically profitable design at 2773 K does not offer the highest value of SASWROIM because of its excessive carbon footprints and fire and explosion hazards. It is also worth noting that there is a temperature range (from about 2000 to 2500 K) where the value of SASWROIM is around 15 yr−1%, and the values of the ROI are attractive (from 16 to 19 yr−1%). Within this region, the design efforts can focus on viable operating temperatures that balance the economic profitability, sustainability, and safety performance requirements. While the ROI values represent the directly tangible profitability of the design, the values of SASWROIM augment the indirectly tangible impacts on the environment and safety with the conventional ROI. As mentioned earlier, the weighting factors for sustainability and safety should reflect the company’s core values relative to profit. If there is difficulty in picking these values, a sensitivity analysis may be carried out to assess the impact on the augmented metric. Figure 4 shows the SASWROIM values versus the dimerization temperature for different combinations of wCO2 and wSafety. As can be inferred from the figure, the operating temperature range from 2000 to 2200 K offers attractive values of ROI and SASWROIM. Therefore, the process engineers can reliably focus their attention on this specific range regardless of the particular “sensitivity” of the sustainability and safety weights.

Table 2. Fire and Explosion Hazard Ranking Based on the FEDI24 Fire and Explosion Hazard Characterization

ROI (yr−1%)

Next, the SASWROIM method is applied to the case study. The target values for the carbon footprint and FEDI were taken as 148,987 MTPA and 306.4, respectively, which correspond to the lowest values within the investigated temperature range. The weights wCO2 and wSafety are taken as 0.1 and 0.1, respectively. The results of the analysis are shown in Figure 3.

The company uses a minimum ROI value of 10 yr−1%. The base-case design is an economically attractive option because it offers an ROI value of 13.79 yr−1%. Next, the carbon footprint is evaluated for the process using the EPA’s method for stationary combustion sources.23 The key data and assumptions used in the evaluating the carbon footprint include the use of natural gas as a fuel for the heating utilities, a heat content of the fuel of 1029 Btu/SCF, a carbon content of 14.47 kg C/ MMBtu, and an oxidized fraction of 1.0. The result is the emission of 53.06 CO2 kg/MMBtu. For electric-power generation, the emissions factor was calculated to be 0.73 tonne of CO2/MWh. Therefore, the estimated total emissions associated with the base-case process (direct emissions as well as emissions resulting from energy consumption) on a life-cycle basis was calculated to be 256,545 t per annum (MTPA) of CO2-eq. Different metrics may be used to assess the safety of the system.1 In this case study, safety is assessed through the Fire, Explosion and Damage Index (FEDI) proposed by Khan and Abbasi24 as a variation of the Dow Fire and Explosion Index.25 Detailed stream data, equipment data, NPFA rankings, chemical thermodynamic data, process unit layout, and chemical reaction data are also required. The FEDI analysis is carried out through several steps including the categorization of the type of unit, calculation of energy factors, assignment of penalty factors, and the evaluation of the potential for damage. The hazard ranking proposed by Khan and Abbasi24 is summarized by Table 2. For the base-case dimerization

FEDI

Temperature (K)

temperature of 1773 K, the value of FEDI for the reactor is 593.4, which places it in the extremely hazardous category. To explore the effect of the reactor temperature on the individual values of ROI, carbon footprint, and FEDI, different temperatures between 1273 and 2273 K were considered, and the results are shown in Table 3. One can observe that although the lowest dimerization temperature offers the lowest carbon footprint as well as fire and explosion hazards, the corresponding ROI is unacceptable. Therefore, it is important to endeavor to reduce the hazard and carbon footprint while maintaining an acceptable ROI. D

DOI: 10.1021/acssuschemeng.7b03802 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 4. Sensitivity analysis for ROI and SASWORIM with different weights.

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METHANOL CASE STUDY For the second case study, the production of methanol from shale gas reported by Ortiz-Espinoza et al.26 was considered. The process consists of three primary stages: shale gas reforming via partial oxidation, methanol synthesis, and purification. Methanol synthesis is carried out under high pressure. Ortiz-Espinoza et al.26 evaluated the impact of methanol-synthesis pressure on several indicators including the ROI, environmental, and safety indicators. For this case, sustainability was evaluated through greenhouse gas emissions and reported on a life cycle basis as tonnes of CO2-equivalents (CO2-e) per tonne of product. For safety assessment purposes, the process route index (PRI) was used. In particular, the PRI27 is used to evaluate the inherent safety of a process with focus on level of explosiveness. The index is calculated based on key stream parameters of the process such as, density, pressure, temperature, energy, and combustibility and lumps them in a single value to rank the process according to its inherent safety level. Within this context, lower numbers correspond to safer options. The results are shown in Table 4.

Figure 5. ROI and SASWORIM for various pressures of methanol synthesis.

objectives related to profitability, safety, and sustainability. An integrated metric (SASWROIM) has been proposed to combine the multiple objectives by extending the conventional definition of the return on investment (ROI). In particular, the proposed approach is based on extending the conventional ROI analysis to include positive or negative impacts of a process modification project on safety and sustainability performance profiles for a base case scenario. This integration of safety and sustainability-relevant performance criteria/objectives can be enabled through the inclusion of weighted factors for each of the safety and sustainability indicators considered. Additionally, targeted improvements for safety and sustainability are used to assess the relative performance of the process design modification project compared to desired/target overall performance profiles. Indeed, the modified process indicators are compared to the base case design indicators as well as the aforementioned target values. Two case studies on butadiene and methanol production were considered in order to evaluate the potential merits of the proposed approach through detailed simulations. For butadience manufacture, the results show that while the economic ROI favors the highest reaction temperature, the value of SASWROIM shows a nonmonotonic behavior favoring an intermediate temperature for which the objectives of profitability, sustainability, and safety are reconciled. For methanol synthesis, higher pressure enhances the economic performance and reduces the carbon footprint but increases the explosiveness potential. The application of SASWROIM balances these factors and identifies an optimal pressure for which the three objectives are simultaneously considered. Therefore, this new approach can be potentially used by decision makers to show the economic viability of the project as well as characterize its impact on the environment and process safety.

Table 4. Impact of Methanol-Synthesis Pressure on Economic, Environmental, and Safety Indicatorsa

a

Pressure (bar)

ROI (yr−1/%)

CO2-e (T/T methanol)

PRI

83 70 60 50

46.90 43.06 38.97 33.03

14.90 15.89 17.07 19.02

17.56 13.51 10.72 8.11

Adapted from ref 26.

As can be seen from Table 4, the lower the pressure is, the lower the ROI is (because of lower yield). Also, lower pressure leads to an increase in CO2 emission and a safer operation. The targets set for this case study are 14.9 tonne CO2-eq/tonne methanol for the sustainability indicator and 8.11 for the PRI safety indicator. The base case was selected for a pressure of 60 bar, where the values of the indicators are 17.07 tonne CO2-eq/ tonne methanol and 10.72 for the PRI. The weighted factors are 0.1 for CO2-e and 0.1 for PRI. The results for the SASWROIM are shown in Figure 5. It can be observed that while the conventional ROI has a monotonic behavior the SASWROIM has a maximum at 70 bar where the objectives of safety, sustainability and profitability are reconciled.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Arturo Jiménez-Gutiérrez: 0000-0001-7994-7122 Mahmoud M. El-Halwagi: 0000-0002-0020-2281

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Notes

CONCLUSIONS The present research work has introduced a new metric to enable the inclusion of safety and sustainability early enough in the conceptual design stage, while further enhancing the quality of the base-case design option based on multiple performance

The authors declare no competing financial interest.

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DOI: 10.1021/acssuschemeng.7b03802 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acssuschemeng.7b03802 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX