Comparative Techno-Economic and Environmental Analysis of

Article pubs.acs.org/IECR

Comparative Techno-Economic and Environmental Analysis of Ethylene and Propylene Manufacturing from Wet Shale Gas and Naphtha Minbo Yang† and Fengqi You*,‡ †

Department of Chemical and Biological Engineering, Northwestern University, Evanston, Illinois 60208, United States Robert Frederick Smith School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14853, United States

‡

S Supporting Information *

ABSTRACT: In this work, we perform a comparative techno-economic and environmental analysis for manufacturing ethylene and propylene from naphtha and from shale gas with rich natural gas liquids (NGLs). We first propose two novel process designs for producing ethylene and propylene from NGLs-rich shale gas. These two designs employ steam cocracking of an ethane−propane mixture and an integration of ethane steam cracking and propane dehydrogenation, respectively. For benchmarking, we also consider a conventional process design in which ethylene and propylene are produced via steam cracking of naphtha. Detailed process models are developed for all the three designs to obtain the mass and energy balances of each unit operation. On this basis, techno-economic analysis and life cycle analysis are conducted for each of the three designs in order to systematically compare the production costs and life cycle environmental impacts of ethylene and propylene manufactured from shale gas and naphtha based on the same conditions. The economic analysis indicates that manufacturing ethylene and propylene from NGLs-rich shale gas is more attractive than from naphtha. The environmental impact analysis shows that manufacturing ethylene and propylene from NGLs-rich shale gas results in lower life cycle water consumption but higher life cycle greenhouse gas emissions.

1. INTRODUCTION Ethylene and propylene are important building blocks for manufacturing plastics, fibers, and other chemicals. In the United States, ethylene is primarily manufactured via steam cracking of hydrocarbons, including natural gas liquids (NGLs, ethane, propane, butanes, etc.), naphtha, and gas oil.1 In addition to ethylene, propylene is obtained as a byproduct in steam crackers, which accounts for roughly half of the total propylene production.2,3 In recent years, the successful application of advanced extraction technologies resulted in a boom of shale gas production in the United States and provides extra NGLs at low costs for the chemical manufacturing industry.4−8 As a result, manufacturing ethylene and propylene from shale gas-based feedstocks (e.g., ethane and propane), instead of from naphtha, is of growing interest.9 With different feedstocks, ethylene and propylene may require different production costs and result in different environmental impacts. It is of great significance to investigate the economic and environmental implications of ethylene and propylene manufacturing from different feedstocks.10,11 Therefore, the objective of this study is to compare the production of ethylene and propylene from NGL-rich shale gas and naphtha in terms of economics and life cycle environmental impacts. There are some recent publications regarding the process design and synthesis for shale gas processing and upgrading.12 Shale gas is considered as the feedstock to produce syngas,13 methanol,14 liquid fuels, and light olefins. For the production of © XXXX American Chemical Society

ethylene, He and You developed three process designs by integrating shale gas processing with ethane steam cracking, which could significantly increase the overall profitability.15 Later, an integrated process design combining the conversion of methane, ethane, and propane and the dehydration of bioethanol was proposed to improve the economics and environmental impacts for ethylene production.16 Salkuyeh and Adams proposed a polygeneration process design to produce ethylene and electricity from shale gas via methane oxidative coupling, which showed the ethylene production costs could be close to those of steam cracking methods.17 However, these publications only focus on the production of ethylene from shale gas and do not explore the economics and environmental implications of manufacturing propylene from shale gas. In addition, some contributions were made to addressing the water management in shale gas production,18optimal design of shale gas supply chain,19 and uncertainty in shale gas supply chain design and optimization.20 Environmental impacts of manufacturing chemicals from shale gas are of particular interest. A recent concern associated with shale gas production is the fugitive methane emissions,21 which may result in high greenhouse gas (GHG) emissions.22 Received: Revised: Accepted: Published: A

January 25, 2017 March 19, 2017 March 22, 2017 March 22, 2017 DOI: 10.1021/acs.iecr.7b00354 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Block flow diagrams for the manufacturing of ethylene and propylene from shale gas and naphtha.

consumption of ethylene and propylene manufactured from shale gas and naphtha are investigated and compared. The key contributions of this work are summarized as follows: (1) two novel process designs for manufacturing ethylene and propylene from NGLs-rich shale gas and (2) systematic comparison of manufacturing ethylene and propylene from NGLs-rich shale gas and naphtha based on the same conditions from economics and environmental perspectives. The rest of the paper is organized as follows. Section 2 presents the description of each process design. Results of process simulation, economic analysis, and environmental analysis are provided and compared in Section 3. The conclusion of this work is given in Section 4.

Several studies have examined the life cycle GHG emissions of shale gas. Laurenzi and Jersey examined the life cycle GHG emissions of Marcellus shale gas for power generation.23 Dale et al. performed a process-based life cycle assessment (LCA) for Marcellus shale gas.24 The upstream carbon footprints of shale gas and conventional natural gas were also compared.25 Another concern of shale gas production is the considerable water usage associated with horizontal drilling and hydraulic fracturing.26 Shale gas production is reported to consume notably more water than conventional natural gas.27 For olefins production, the steam cracking of hydrocarbons (e.g., ethane, propane, naphtha, etc.) is highly energy intensive and thus produces significant amounts of GHGs.28 Besides, waste heat needs to be discharged to the ambient environment, and this is commonly performed by cooling water systems that consume a considerable amount of water.1 To the best of our knowledge, there are very limited studies focusing on environmental impacts of manufacturing olefins from shale gas, except a recent study on ethylene production from shale gas.29 However, it remains inconclusive whether shale gas can be regarded as a low-carbon feedstock for the production of ethylene and propylene compared with naphtha. In this work, we first develop two novel process designs for producing ethylene and propylene from NGLs-rich shale gas. In the first design, the ethane−propane mixture is sourced from shale gas at first and then stream co-cracked to produce ethylene and propylene. In the second design, ethane and propane are derived from shale gas separately. Ethylene is mainly produced via the steam cracking of ethane, and propylene is mostly manufactured via the dehydrogenation of propane. For the purpose of a systematic comparison, we also model a conventional naphtha cracking process that produces ethylene and propylene from naphtha. We develop detailed process models for all three designs to obtain the mass and energy balances. Next, we conduct a techno-economic analysis and life cycle analysis for each of the three designs. Finally, production costs, life cycle GHG emissions, and life cycle water

2. PROCESS DESCRIPTION In this study, shale gas from the Marcellus region in the United States is analyzed for the production of ethylene and propylene because there is an increasing attention for producing olefins in this region from the locally produced shale gas.30 Table S1 in the Supporting Information gives the composition of the raw shale gas considered in this study. Depending on the raw shale gas composition, two process designs for manufacturing ethylene and propylene from shale gas are proposed as depicted in Figure 1(a) and (b), respectively. Each design consists of two stages: shale gas processing and olefins production. In the co-cracking design shown in Figure 1(a), raw shale gas is first fed into an acid gas removal unit to remove the acid component. Then, the sweet gas is introduced into a dehydration unit to reduce the water content. NGLs are further recovered from the dry gas by a cryogenic separation unit. The resulting methane-rich gas is pressurized and sent out as pipeline gas, and the mixture of ethane and propane is sent to produce ethylene and propylene. At the olefins production stage, the ethane−propane mixture derived from shale gas and the unreacted ethane and propane are steam co-cracked in the steam cracking unit. After being cooled, pressurized, and B

DOI: 10.1021/acs.iecr.7b00354 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article


Figure 2. Process flowsheets for the processing of Marcellus shale gas.

rich MEA solution is depressurized via another valve (V-102) and preheated in a heat exchanger (E-103), and then, it is fed to a stripper (T-102). In stripper T-102, acid gas in the rich MEA solution is stripped off, and the MEA solution is regenerated. The lean MEA solution from stripper T-102 bottom is first cooled in a heat exchanger (E-103) and then pumped to the top of absorber T-101 along with the makeup MEA and water. The gas effluent from the top of stripper T-102 enters a flash drum (V-102) after being cooled. The liquid from flash drum V-102 is pumped to the top of stripper T-102, and the gas from flash drum V-102 is acid gas. Since acid gas is removed in an amine-based unit, the sweet gas leaving absorber T-101 is water saturated. Water must be reduced to prevent hydrate formation in the cryogenic separation process.31 Triethylene glycol (TEG) is the typical choice in most instances for water removal from natural gas.31 A TEG-based dehydration unit is depicted in Figure 2(b). The sweet gas enters an absorber (T-201) from the bottom and contacts with a lean TEG stream. The rich TEG stream from the bottom of absorber T-201 is depressurized via a valve (VLV-201) and flashed in a flash tank (V-201) to remove most hydrocarbons. Then, the rich TEG stream is fed into a distillation tower (T-202) for TEG regeneration after being preheated in a heat exchanger (E-203). Water and hydrocarbons from the overhead condenser of tower T-202 are purged, and the lean TEG stream from the tower bottom is sent to a stripper (T-203) to further reduce the water content in the TEG stream. A small portion of dry gas from the top of absorber T-201 is used as strpping gas in stripper T-203. The regenerated TEG stream mixed with the makeup TEG is pumped to the top of absorber T-201 after being cooled in a cooler (E-302). After dehydration, shale gas is sent to a cryogenic separation process for NGLs recovery. As shown in Figure 2(c), the dry gas is first cooled in a heat exchanger (E-301), and then, it enters a two-phase separator (V-301). The gas product from separator V-301 flows through an expander (EX-301) to further reduce the temperature before it is fed into another separator (V-302). Liquid products from separators V-301 and V-302 are introduced into a demethanizer (T-301). The gas product from separator V-302 and that from the top of demethanizer T-301

purified, the cracking gas is then sent to an olefin separation unit and spilt into ethylene, propylene, hydrogen, methane, ethane, propane, and other products. In the technology-integrated design shown in Figure 1(b), the shale gas processing stage is similar to that in the cocracking design. The difference between these two shale gas processing stages is the NGLs fractionation unit for separating the ethane−propane mixture into ethane and propane. The olefins production stage considers the integration of ethane steam cracking and propane dehydrogenation technologies. Ethane is fed into the ethane steam cracking unit, while propane is taken as the feedstock for the propane dehydrogenation unit. The cracking gas and the effluent from the propane dehydrogenation unit are sent to an olefins separation unit, where they are separated into ethylene, propylene, and other products. In addition to the two novel process designs that we propose, a conventional process design for manufacturing ethylene and propylene from naphtha is modeled and analyzed for comparing economic and environmental performances of the products. As shown in Figure 1(c), naphtha is first steam cracked in a stream cracking unit. Similarly, the cracking gas is cooled, pressurized, and purified. After that, the cracking gas is separated into ethylene, propylene, and other products in an olefins separation unit. Details of each unit are given in the following subsections. 2.1. Shale Gas Processing. Figure 2 shows the flowsheets for the processing of Marcellus shale gas with the composition given in Table S1. It comprises five units, namely, acid gas removal, dehydration, NGLs recovery, compression, and NGLs fractionation. Since CO2 in shale gas could lead to solids formation in the cryogenic separation process for NGLs recovery, the CO2 concentration should be reduced to less than 50 ppmv.31 As show in Figure 2(a), a monoethanolamine (MEA)-based absorption unit is employed for acid gas removal. Raw shale gas is fed into the bottom of an absorber (T-101) and contacts with a lean MEA solution from the top of absorber T-101. The rich MEA solution from absorber T-101 bottom passes through a pressure relief valve (VLV-101) followed by a flash tank (V101) to remove dissolved light hydrocarbons. After that, the C


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Figure 3. Process flowsheets of olefins production by steam co-cracking of ethane−propane mixture: (a) ethane−propane steam co-cracking and (b) olefins separation.

reactor (PFR) model.29 In this work, a mass-based hydrocarbons-to-steam ratio of 1:0.4 is applied.32 The reaction scheme for the steam co-cracking of an ethane−propane mixture is given in Table S2 in the Supporting Information. The cracking gas from the cracking furnace flows through a heat exchanger (E-501) to generate high pressure (HP) steam at 11 MPa, which is subsequently superheated in the cracking furnace.1,33 The cracking gas is further cooled by another heat exchanger (E-502) to preheat dilution steam and ultimately cooled to 40 °C in a quench tower by contacting with cooling water. After that, a five-stage compressor is employed to pressurize the cracking gas to 3.7 MPa. The compression power is provided by steam turbines driven by the superheated HP steam. During compression, the temperature of the cracking gas is controlled below 100 °C by intermediate coolers to prevent olefin polymerization and subsequent equipment fouling.1 Acid components in the cracking gas are removed in the caustic tower after the third stage compression, according to reactions 1 and 2. The compressed cracking gas is introduced into the olefin separation unit after the water content is reduced in a molecular sieve dryer.

are methane rich and mixed. The composition of the mixed product has met the corresponding specifications of pipeline gas, as shown from Table S1. After heat recovery in heat exchanger E-301, a small portion of methane-rich gas is consumed for steam generation, and the rest is introduced into a gas compression unit as shown in Figure 2(d). The methanerich gas is pressurized to 6 MPa and sent out as pipeline gas. Power generated by Expander EX-301 provides part of the power for gas compression; additional power is provided by steam turbines driven by the steam generated via the combustion of methane-rich gas. In the co-cracking design, the NGLs product (i.e., the ethane−propane mixture as shown in Table S1) from the bottom of demethanizer T-301 is sent to the olefins production stage. In the technology-integrated design, the NGLs product is introduced into a deethanizer (T401), where the mixture is split into ethane and propane, as illustrated in Figure 2(e). Then, ethane and propane are sent to the olefins production stage. 2.2. Co-Cracking of Ethane−Propane. In the co-cracking design, the mixture of ethane and propane coming from the shale gas processing stage is steam co-cracked. Figure 3 shows the process flowsheets of the olefins production stage in the cocracking design. Typically, a commercial steam cracking furnace consists of convection and radiant sections.1 In the convection section, hydrocarbons, dilution steam, and boiler feedwater are heated. In the radiant section, hydrocarbons are cracked into small molecules, such as ethylene, propylene, hydrogen, methane, etc. For the sake of convenience, the steam cracking furnace is simulated using a heater combined with the plug flow

CO2 + 2NaOH → 2Na 2CO3 + H 2O

(1)

H 2S + 2NaOH → 2Na 2S + 2H 2O

(2)

In the olefins separation unit shown in Figure 3(b), the cracking gas is first cooled to about −37 °C through two heat exchangers (E-510 and E-504) and a cooler (E-505). Then, the cracking gas enters a separator (V-501). The gas product from D


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Figure 4. Process flowsheets of olefins production by the integration of ethane steam cracking and propane dehydrogenation: (a) ethane steam cracking, (b) propane dehydrogenation, and (c) olefins separation. The steam cracking unit is similar to that in Figure 3, and the rest of the olefins separation section is similar to that in Figure 3 excluding a debutanizer.

in heat exchangers E-504 and E-511. A liquid stream is extracted from the 99th tray of C2 splitter T-509 as a coolant in heat exchanger E-510 and returned to the 101st tray. The liquid product from the bottom of deethanizer T-504 is fed into a depropanizer (T-505). The overhead product from depropanizer T-505 contains propane and propylene that are separated in a C3 splitter comprising two distillation columns (T-506 and T-507). Polymer-grade propylene with a purity of 99.5% is obtained from the top of column T-506, and propane from the bottom of column T-507 is fed into cracking furnaces. Last, a debutanizer (T-508) is used to separate the bottom product from depropanizer T-505 into a C4 mixture and C5+ mixture. 2.3. Integration of Ethane Steam Cracking and Propane Dehydrogenation. The technology-integrated design considers the integration of ethane steam cracking and propane dehydrogenation. Figure 4 presents the process flowsheets of the olefins production stage in the technologyintegrated design. Ethane sourced from shale gas along with the recycled ethane is sent to the ethane steam cracking unit, which is similar to the one described in Figure 3. The reaction scheme used for ethane steam cracking is given in Table S3 in the Supporting Information. Propane derived from shale gas and the recycled propane are merged as the feedstock for the propane dehydrogenation unit, as shown in Figure 4(b). Meanwhile, hydrogen is injected into the propane feed to reduce the coke formation on the catalyst with a mole-based hydrogen-to-hydrocarbons ratio of 0.6:1.35 The reaction section consists of four reactors (R-601−R-604) with fired heaters (FH-601−FH-604).36 The mixture of propane and hydrogen is introduced into the reactors after being heated to over 630 °C.35 In this work, reactors are modeled using the PFR model, and the kinetic model for propane dehydrogenation is given in

separator V-501 is introduced into a series of cold boxes. Liquid products from separators V-501 to V-504 are fed into a demethanizer (T-503) where methane and hydrogen in the liquid phase are removed. The gas phase from separator V-504 is ultimately cooled to −165 °C in a heat exchanger (E-515) and fed into separator V-505. The gas product from separator V-505 is hydrogen rich, and the liquid product is methane rich. These two streams serve as coolants for heat exchanger E-515 and for heat exchangers E-511−E514 along with the gas product from demethanizer T-503. The bottom product of demethanizer T-503 is nearly methane free and is introduced into a deethanizer (T-504). After being preheated and mixed with some hydrogen-rich gas, the gas product from deethanizer T-504 is sent to a hydrogenator (R-501), where acetylene is reacted with hydrogen and converted into ethylene or ethane, according to reactions 3 and 4. C2H 2 + H 2 → C2H4

(3)

C2H 2 + 2H 2 → C2H6

(4)

The separation of ethylene and ethane often requires large distillation columns with 120−180 trays.34 The gas product from hydrogenator R-501 is fed into a C2 splitter (T-509) with 120 trays, after being cooled in a heat exchanger (E-506). The ethylene product with a purity of 99.9% is drawn from the ninth tray of C2 splitter T-509. The gas product from the top of C2 splitter T-509 is ethylene rich and is further cooled in a cooler (E-509) to reduce the amount of gas recycled to be recompressed. The liquid product from separator V-506 is routed to the first tray of C2 splitter T-509. Ethane from the bottom of C2 splitter T-509 is depressurized and finally recycled as a feedstock of cracking furnaces, after being heated E


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Figure 5. Process flowsheets of olefins production by the steam cracking of naphtha: (a) naphtha steam cracking and (b) olefins separation. The olefins separation unit is similar to that in Figure 3.

Table S4 in the Supporting Information.35 The effluent from reactor R-604 is cooled to 40 °C and compressed to 1.2 MPa, and then, it is sent to an olefins separation unit, as shown in Figure 4(c). In the separation unit, the product from the propane dehydrogenation unit is further cooled and preseparated in separators (V-601−V-603). Liquid products from separators V-601−V-603 are introduced into a deethanizer (T601) to remove C2 and lighter components. The gas product of the deethanizer T-601 is ethane rich and treated as a feed for steam cracking. The liquid product containing C3 and heavier components is introduced into a depropanizer. Note that the rest of the olefins separation unit is similar to that described in Figure 3 excluding a debutanizer. The gas product from separator V-603 is hydrogen rich and is purified in a pressure swing adsorption (PSA) unit. As aforementioned, a portion of the purified hydrogen is injected into the propane feed. 2.4. Naphtha Steam Cracking. For the purpose of a fair comparison, a conventional naphtha cracking design is investigated in this work as well. Figure 5 presents the process flowsheets of the naphtha cracking design for the production of ethylene and propylene. Similar to the co-cracking of the ethane−propane mixture, naphtha is preheated and steam cracked in cracking furnaces. A steam-to-naphtha ratio of 0.5:1 is applied in this work.1 The reaction scheme used for naphtha steam cracking is given in Table S5 in the Supporting Information. After heat recovery in a heat exchanger (E-701), the cracking gas is further cooled by contacting with cooling oil in a quench oil tower. The resulting mixture is introduced into a gasoline fractionator (T-701), where the cracking gas is further cooled and the heavy fraction is condensed. Most of the condensed fraction is split and recycled to the quench oil tower and gasoline fractionator T-701 after being cooled, while the rest is stripped by low pressure (LP) steam in a stripper (T702). The gas product from the top of stripper T-702 is routed

to fractionator T-701, and fuel oil is obtained from the bottom. Parallel to naphtha cracking furnaces, the recycled ethane and propane are co-cracked in a separate furnace.37 The cracking gas is cooled in heat exchangers E-707 and E-706. After that, these two cracking gas streams merge into one and enter a quench water tower (T-703), where the cracking gas is ultimately quenched to 40 °C by contacting with cooling water. The effluent from the bottom of tower T-703 consists of heavy hydrocarbons and water, and it is separated by a threephase separator (V-701). The water stream along with the makeup water is split and recycled to the quench water tower (T-703) as cooling water and to generate dilution steam. The hydrocarbon stream is spilt and introduced into fractionator T701 and another stripper (T-704). In stripper T-704, the hydrocarbon stream is boiled to strip off the dissolved gas. Gas products from quench water tower T-703, stripper T-704, and separator V-701 are mixed and pressurized to 3.7 MPa via a five-stage compressor. The condensation of water and hydrocarbons from the former three stages is routed into separator V701, while the condensate from the last two stages is fed into a condensate stripper (T-706). The gas product from stripper T706 is fed to the fourth stage of the compressor for recompression, and the bottom effluent is introduced into a depropanizer in the olefins separation unit for further fractionation. After being dried by a molecular sieve, the cracking gas is sent to the olefins separation unit, which is similar to that described in Figure 3. 2.5. Power and Refrigeration Generation. Power generation via steam turbines is operated on the Rankine cycle.38 As shown in Figure 6, water is first pressurized and heated to generate HP steam. At shale gas processing stages, HP steam is generated via the combustion of methane-rich gas; at olefins production stages, HP steam is generated by heat recovery from the cracking furnaces and fired heaters. The HP F


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modeled in detail to analyze and compare the economic and environmental profiles of the products manufactured through different feedstocks and routes.

3. RESULTS AND DISCUSSION Following the Aspen HYSYS Customization Guide,41 all the three process designs discussed in the previous section are modeled and simulated in Aspen HYSYS V7.2. Detailed operating parameters of important columns including distillation towers, strippers, and absorbers are listed in Table S6 in the Supporting Information. Heat integration of each process design is extracted and investigated using Aspen Energy Analyzer V7.2, according to the Aspen Energy Analyzer User Guide.42 3.1. Mass and Energy Balances. In this study, we consider a production scale of 1 Mt/yr ethylene for the three designs following a recent report.30 The operating time is 8000 h/yr.29 The mass and energy information on each unit operation in the three designs is determined by HYSYS simulation, and the results are summarized in Table 1. To meet the given ethylene production rate, the co-cracking design consumes 969.5 million standard cubic feet per day (MMSCFD) of raw shale gas. At the stage of shale gas processing, pipeline gas is the main product. About 77.0% of the energy content of raw shale gas remains in the pipeline gas product, and 20.0% of that is taken by the mixture of ethane and propane. Also, the methane-rich gas used for utility generation occupies 2.7% of the energy content of raw shale gas. At the olefins production stage, the steam cracking of the ethane−propane mixture results in an ethylene yield of 63.4 wt % of the feed and a propylene yield of 14.4 wt % of the feed. It means that propylene and ethylene are produced with a ratio of 0.23. Typically, process energy use is expressed in terms of specific energy consumption.34 The specific energy consumption of the olefins production stage in the co-cracking design is determined as 23.9 GJ/t ethylene, which is slightly higher than the specific energy consumption for ethane steam cracking reported in the existing literature.34 The technology-integrated design consumes 1242.0 MMSCFD of raw shale gas to satisfy the given ethylene production rate. Similarly, at the shale gas processing stage, pipeline gas is the main product, which takes 76.6% of the energy content of raw shale gas. Ethane and propane totally occupy 20.0% of the energy content of raw shale gas, and the utility generation consumes 3.1% of the energy content of raw

Figure 6. Process flowsheet of power generation.38

steam is then expanded in steam turbines, where steam energy is converted to mechanical power. Next, LP steam exiting steam turbines is used to satisfy on-site thermal requirements or condensed by cooling water. Finally, the condensed water is recycled to the pump. Refrigeration is used for NGLs recovery, NGLs fractionation, and olefins separation. Figure 7 shows refrigeration cycles of propylene, ethylene, and methane. In the propylene cycle, propylene is first pressurized to 1.6 MPa via a multistage compressor (K-1) driven by steam turbines.39 Propylene is then condensed by cooling water and/or process streams. Depending on the process requirements, the condensed propylene is depressurized to provide cold utilities with temperatures of −25 and −40 °C. Lastly, propylene gases are gathered and recirculated to compressor K-1 for recompression. The ethylene cycle is similar to the propylene cycle. Ethylene is compressed to 2.0 MPa via compressor K-2 and then condensed using cooling water and propylene refrigerant.39 Depending on the process requirements, ethylene is used to provide cold utilities with temperatures of −75 and −102 °C. In addition, methane is used to provide the cold utility with a temperature of −135 °C.1 In the methane cycle, methane is pressurized to about 2.8 MPa and condensed by the ethylene refrigerant. In this section, we present two novel process designs for manufacturing ethylene and propylene from NGLs-rich shale gas. The major difference between the two designs is the technologies employed for converting ethane and propane into ethylene and propylene. Also, a conventional process design for the steam cracking of naphtha is introduced for the purpose of a fair comparison in systems analysis.40 Therefore, three process designs for the production of ethylene and propylene are

Figure 7. Process flowsheets of refrigeration generation: (a) propylene cycle, (b) ethylene cycle, and (c) methane cycle. G


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Industrial & Engineering Chemistry Research Table 1. Mass and Energy Balances for Manufacturing Ethylene and Propylene from Shale Gas and Naphtha shale gas processing stage input shale gas (MMSCFD) shale gas (MMBTU/h)a MEA (kg/h) TEG (kg/h) output pipeline gas (MMSCFD) pipeline gas (MMBTU/h)a ethane−propane (t/h) ethane (t/h) propane (t/h) direct emissions CO2 (t/h) utilities power (MW)b LP steam (125 °C) (GJ/h)b MP steam (175 °C) (GJ/h)b HP steam (250 °C) (GJ/h)b recycling cooling water (kt/h)c make up water (kt/h) olefins production stage input ethane−propane (t/h) ethane (t/h) propane (t/h) naphtha (t/h) NaOH (kg/h) output ethylene (t/h) propylene (t/h) C4s (t/h) C5+ (t/h) hydrogen (t/h) utilities power (MW)b LP steam (125 °C) (GJ/h)b MP steam (175 °C) (GJ/h)b total fuel consumption (GJ/h) external fuel demand (GJ/h) recycling cooling water (kt/h)c make up water (kt/h) direct emissions CO2 (t/h) a

co-cracking design

technology-integrated design

naphtha cracking design

969.5 47628 26.0 26.2

1242.0 61012 33.4 33.6

− − − −

820.2 36673 197.3 − −

1045.7 46754 − 150.9 101.8

− − − − −

78.0

111.8

−

97.0 58.7 13.3 13.8 59.6 1.2

139.7 165.2 17.1 17.6 89.4 1.8

− − − − − −

197.3 − − − 60.6

− 150.9 101.8 − 56.0

− − − 368.0 84.5

125.0 28.4 8.4 1.6 10.8

125.0 87.1 13.1 0 17.7

125.0 51.3 7.2 134.2 8.3

88.9 414 0 2992 1643 54.5 1.1

142.8 1509 0 3977 3397 122.9 2.5

93.0 447 5.9 3341 −351 90.6 1.8

148.3

169.8

b

164.0 c

High heating value (HHV) basis. Power and steam are generated on-site. MP steam: medium pressure steam. In cooling water systems, evaporation loss rate = 0.00153 × cooling range; blow down loss = 1.5 × evaporation loss.43 Cooling range is equal to 5 °C in this analysis.

shale gas. At the olefins production stage, ethane and propane are consumed with a total mass flow rate of 252.7 t/h. In terms of mass flow rate, ethylene and propylene account for 49.5 and 34.5 wt % of the feed, respectively. It is found that the technology-integrated design results in a propylene-to-ethylene ratio of 0.70. The corresponding specific energy consumption is determined as 31.8 GJ/t ethylene. In the naphtha cracking design, naphtha is consumed with a mass flow rate of 368.0 t/h to meet the given ethylene production rate. The steam cracking of naphtha shows an ethylene yield of 34.0 wt % of the feed and a propylene yield of 13.9 wt % of the feed. It means that propylene and ethylene are produced with a ratio of 0.41. The corresponding specific

energy consumption is identified as 26.7 GJ/t ethylene, which meets the value reported in literature.34 Comparing shale gas processing stages in the two proposed designs, the main difference is the NGLs fractionation unit for the separation of the ethane−propane mixture. To separate the ethane−propane mixture into ethane and propane, more refrigeration and LP steam are needed. As a result, in the technology-integrated design, slightly more methane-rich gas is combusted and less pipeline gas is produced based on per unit of raw shale gas. The three olefins production stages rely on different technologies and feedstocks. From Table 1, we can see that more naphtha is consumed in the naphtha cracking design than H


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Industrial & Engineering Chemistry Research Table 2. Economic Evaluation Results of Shale Gas Processing Stages designs

co-cracking design


raw shale gas cost (M$/yr) other feeds cost (M$/yr) plant overhead (M$/yr) labor cost (M$/yr) maintenance (M$/yr) total capital cost (M$) capital depreciation (M$/yr) property taxes and insurance (M$/yr) general expenses (M$/yr) total production cost (M$/yr)

1050.3 3.4 7.2 11.5 3.0 369.4 18.5 7.4 108.9 1210.2

1345.5 3.9 9.1 14.6 3.5 440.5 22.0 8.8 139.2 1546.6

Table 3. Economic Evaluation Results of Olefins Production Stages designs

co-cracking design


naphtha cracking design

hydrocarbon feed cost (M$/yr) other feeds cost (M$/yr) plant overhead (M$/yr) labor cost (M$/yr) maintenance (M$/yr) total capital cost (M$) capital depreciation (M$/yr) property taxes and insurance (M$/yr) general expenses (M$/yr) total production cost (M$/yr)

350.1 38.3 3.9 2.9 5.0 912.6 45.6 18.3 45.9 510.0

451.2 89.8 5.2 3.6 6.8 1167.0 58.4 23.3 63.1 701.4

1265.9 3.2 4.3 3.2 5.5 1042.8 52.1 20.9 134.0 1489.1

consumption quantity multiplied by the corresponding price. Economic parameters for estimating costs of feedstocks and utilities are listed in Table S7 in the Supporting Information. Aspen Process Economic Analyzer V7.2 is used to estimate the costs of labor, maintenance, and plant overhead.45 Capital depreciation is estimated based on the total capital cost using a straight-line method over the plant life, as given by eq 5.46 The total capital cost of a process is also estimated using Aspen Process Economic Analyzer V7.2 and based on existing sources as given in Table S8 in the Supporting Information. The cost of process equipment, units, or plants can be converted to current value using the Chemical Engineering Plant Cost Index according to eq 6.47 The property taxes and insurance are estimated as 2% of the total project cost.46 General expenses including selling expenses, research and development costs, and administrative expenses are estimated as 9% of the total production cost.48

ethane and propane consumed in the other two designs in terms of mass flows. This is because ethane and propane can achieve higher ethylene yields compared with naphtha in steam cracking. Besides, the technology-integrated design requires more ethane and propane than the co-cracking design. The reason is that ethylene is mainly produced via the steam cracking of ethane in the technology-integrated design. From the viewpoint of the production of ethylene and propylene, the three designs could coproduce ethylene and propylene. However, the co-cracking design results in a propylene-toethylene ratio lower than the naphtha cracking design, meaning that there exists a propylene gap if shifting feedstocks from naphtha to an ethane−propane mixture for steam crackers. The technology-integrated design could result in a propylene-toethylene ratio higher than the naphtha cracking design. This indicates that the propylene gap can be eliminated by the integration of ethane steam cracking and propane dehydrogenation. In terms of energy consumption, the olefins production stage in the technology-integrated design consumes more energy than those in the other two designs. The first reason is that the propane dehydrogenation reaction is energy intensive and requires fired heat. The second reason is that the dehydrogenation product not only requires power for compression but also needs refrigeration and steam for separation. Note that the naphtha cracking design produces excess fuel, as the stream cracking of naphtha results in high methane yields. 3.2. Economic Analysis. The total annual production cost can be estimated as the sum of direct manufacturing costs (feedstocks, utilities, labor-related operations, and maintenance), plant overhead, fixed costs (property taxes, insurance, and capital depreciation), and general expenses.44 On the basis of the obtained mass and energy balances, the total cost of feedstocks and utilities can be determined by summing the cost of each feedstock and utility, which is equal to the annual

DP =

TCC ls

(5)

⎛ CEPCI 2016 ⎞⎛ S ⎞ β C2016 = C base⎜ ⎟⎜ ⎟ ⎝ CEPCIbase ⎠⎝ S base ⎠

(6)

where DP is the capital depreciation; TCC is the total capital cost; ls is the life span; C2016 is the cost in 2016; Cbase is the cost in the base case; CEPCI2016 is the cost index in 2016; CEPCIbase is the cost index in the base case; S is the actual capacity; Sbase is the capacity in the base case; β is the cost scale factor. To evaluate production costs of manufacturing ethylene and propylene from the raw shale gas given in Table S1, costs for producing ethane and propane are evaluated at first. The results of the economic evaluation of shale gas processing stages are given in Table 2. We find that raw shale gas cost dominates the total production cost. The other notable contributions come from the fixed costs and general expenses. At shale gas I


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associated with the co-cracking design and 2.1 times of those associated with the technology-integration design. Such results indicate that manufacturing ethylene and propylene from the considered shale gas is more cost effective than from naphtha. 3.3. Environmental Analysis. The environmental impacts of manufacturing ethylene and propylene from NGLs-rich shale gas and naphtha are systematically analyzed following the LCA approach.50,51 The primary goal of this LCA study is to assess and compare the life cycle environmental impacts of ethylene manufactured from NGLs-rich shale gas and naphtha. The functional unit is defined as 1 kg of ethylene produced at the gate of plant. Because the use and end-of-life phases of the ethylene products could vary significantly, a cradle-to-gate LCA is considered in this study.52 This LCA encompasses the environmental impacts during the acquisition of feedstocks and the processing of shale gas (for manufacturing ethylene and propylene from shale gas), as well as the manufacturing of ethylene and propylene. The investigated life cycle impact categories include GHG emissions and water consumption. The life cycle inventory is calculated based on the mass and energy balances provided in the previous subsections. Data used to model GHG emissions and water consumption during feedstocks acquisition are collected from publications and the Ecoinvent database,53 as listed in Table S9 in the Supporting Information. Because shale gas processing and olefins production stages produce multiple products, the corresponding mass and energy flows as well as the associated environmental burdens must be allocated to each of the products to accurately reflect their individual contributions to the environmental impacts.54 In this study, coproduct allocation is applied based on the mass allocation method and the economic value allocation method.54 On the basis of the coproduct allocation, the environmental impacts for the production of propylene can be identified as well. 3.3.1. Life Cycle GHG Emissions. Figure 9 shows the breakdown of life cycle GHG emissions of ethylene produced from NGLs-rich shale gas and naphtha. For manufacturing ethylene from shale gas, shale gas production and olefins production stages are major contributors for GHG emissions, while shale gas processing stages contribute less than 10% of the total value. These observations are consistent with the recent studies on shale gas environmental impact analyses.55−57 For manufacturing ethylene from naphtha, the production of naphtha causes GHG emissions close to the downstream olefins production stage. From Figure 9, it is shown that ethylene manufactured by the steam cracking of naphtha shows the least life cycle GHG emissions. Compared with ethylene manufactured from naphtha, ethylene produced through the technology-integrated design results in higher life cycle GHG emissions by 19% using the economic value allocation method or by 25% using the mass allocation method. Ethylene manufactured via the co-cracking design causes the highest life cycle GHG emissions, which are nearly 1.40 times of the life cycle GHG emissions associated with the steam cracking of naphtha. Such results imply that manufacturing ethylene from the considered shale gas is not competitive in terms of GHG emissions. By employing coproduction allocation, we can also identify the life cycle GHG emissions for manufacturing propylene from NGLs-rich shale gas and naphtha, as shown in Figure 10. On the basis of the economic value allocation, propylene produced via the naphtha steam cracking design has the lowest GHG emissions, which are 71.4% and 83.8% of the GHG emissions

processing stages, multiple products including pipeline gas, ethane, and propane (or ethane−propane mixture) are produced. Thus, cost allocation is employed to estimate costs for producing ethane and propane from raw shale gas. In general, methods for cost allocation include the physical measure method, economic value method, and zero cost method.49 The physical measure method is reasonable for cases that the product market values are about the same, and the zero cost method assigns all costs to the main product.49 Because economic values of pipeline gas, ethane, and propane differ greatly and pipeline gas is the main product at shale gas processing stages, the physical measure method and the zero cost method are not suitable. Therefore, the economic value method is selected in this study. In the co-cracking design, allocation factors for the pipeline gas and ethane−propane mixture are determined as 71.0% and 29.0%, respectively. In the technology-integration design, allocation factors for pipeline gas, ethane, and propane are determined as 70.9%, 11.3%, and 17.8%, respectively. As a result, the production cost for the ethane−propane mixture is estimated as $221.8/t, and production costs for ethane and propane are estimated as $145.8/t and $337.3/t, respectively. Subsequently, costs for manufacturing ethylene and propylene via the three designs are systematically evaluated. Table 3 shows the economic evaluation results of olefins production stages in the three designs. The olefins production stage in the technology-integrated design requires the highest capital cost because of investments in the propane dehydrogenation unit and the expanded olefins separation unit. The steam co-cracking of the ethane−propane mixture results in a lower total capital cost than the steam cracking of naphtha. As shown from Table 3, the naphtha cracking design requires the highest total production cost. This is because the consumption quantity and price of naphtha are higher than those of ethane and propane. Similarly, the economic value allocation method is employed to evaluate the production costs of ethylene and propylene. For the co-cracking design, the technologyintegrated design, and the naphtha cracking design, allocation factors of ethylene are determined as 62.3%, 41.9%, and 41.3%, respectively, and those of propylene are determined as 18.5%, 38.0%, and 22.1%, respectively. On this basis, the production costs of ethylene and propylene are determined and compared as shown in Figure 8. It is shown that the production costs of ethylene and propylene are dominated by the direct manufacturing costs for all the three designs. The technologyintegrated design shows the best economic performance for manufacturing ethylene and propylene. Manufacturing ethylene and propylene via the naphtha steam cracking design results in the highest production costs, which are about 1.9 times of those

Figure 8. Breakdown of production costs for the manufacturing of ethylene and propylene via the three designs (2016 dollar value). J


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Figure 9. Breakdown of life cycle GHG emissions of ethylene produced via the three designs.

life cycle water consumption. Compared with ethylene manufactured via the co-cracking design, ethylene manufactured via the technology-integrated design causes higher life cycle water consumption by 29% based on the economic value allocation or by 42% based on the mass allocation. The steam cracking of naphtha results in notably higher life cycle water consumption than the steam co-cracking of the ethane− propane mixture by over 200%. Considering life cycle water consumption, manufacturing ethylene from the considered shale gas via the co-cracking design is more attractive than from naphtha. Similarly, the life cycle water consumption of propylene manufactured via the three designs can be determined based on coproduction allocation, as given in Figure 12. If coproduction

Figure 10. Breakdown of life cycle GHG emissions of propylene produced via the three designs.

of propylene produced via the co-cracking design and the technology-integrated design, respectively. On the basis of the mass allocation, propylene produced via the naphtha steam cracking design shows lower GHG emissions than propylene produced via the co-cracking design and the technologyintegrated design by 27.3% and 20.1%, respectively. Regarding life cycle GHG emissions, naphtha is more attractive than the considered shale gas for the manufacturing of propylene. 3.3.2. Life Cycle Water Consumption. Life cycle water consumptions of ethylene manufactured from NGLs-rich shale gas and naphtha are compared in Figure 11. For manufacturing ethylene from shale gas, water consumed at the olefins production stage dominates life cycle water consumption of ethylene. However, for manufacturing ethylene from naphtha, water consumption associated with the production of naphtha is more important. As shown from Figure 11, ethylene manufactured via the co-cracking design leads to the lowest

Figure 12. Breakdown of life cycle water consumption of propylene produced via the three designs.

allocation is based on economic values, propylene produced via the naphtha steam cracking design leads to significantly higher water consumption than propylene produced via the cocracking design and the technology-integrated design by 207.5% and 137.6%, respectively. If coproduction allocation is performed based on mass flows, propylene produced via the naphtha steam cracking design still shows the highest water consumption, which is 302.6% and 213.0% of the water consumption of propylene produced via the co-cracking design and the technology-integrated design, respectively. In terms of life cycle water consumption, manufacturing propylene from the considered shale gas is more competitive than from naphtha. 3.4. Systematic Comparison. Figure 13 provides a comprehensive comparison of production costs, life cycle GHG emissions, and life cycle water consumption for manufacturing ethylene and propylene from NGLs-rich shale

Figure 11. Breakdown of life cycle water consumption of ethylene produced via the three designs. K


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ethane and propane do not affect the ethylene production cost because ethane and propane are neither feedstocks nor final products. Also, we find that a high propylene price could result in a low ethylene production cost, which is different from the other prices analyzed in Figure 14. In the technology-integrated design, the ethylene production cost is more sensitive to the propylene price than those in the other two designs. This is because the technology-integrated design produces more propylene than the other two designs. It is noteworthy that the naphtha cracking design still results in higher ethylene production cost than the other two designs even the naphtha price decreases by 40%. Such results indicate that NGLs-rich shale gas is more attractive than naphtha for the production of ethylene from the standpoint of economics. As for environmental performance, economic parameters could affect allocation factors when economic value allocation is considered. Herein, we present effects of economic parameters on life cycle GHG emissions of ethylene for illustration. From Figure 15, we see that the ethylene price is most important for life cycle GHG emissions of ethylene. The propylene price is the second important factor after the ethylene price. Note that the increase in propylene price decreases life cycle GHG emissions of ethylene because both ethylene and propylene are the products to which the GHG emissions are allocated. Ethane and propane prices also affect life cycle GHG emissions of ethylene in the co-cracking design and the technologyintegrated design, but they have little effect on the naphtha cracking design. Additionally, we find that life cycle GHG emissions of ethylene produced via the naphtha cracking design are always lower than those of ethylene produced via the other two designs under various changes.

Figure 13. Relative cost and environmental impacts of ethylene and propylene produced from shale gas and naphtha: (a) environmental impacts based on economic value allocation and (b) environmental impacts based on mass allocation.

gas via the two proposed designs and from naphtha via the conventional naphtha cracking design. For each category, the largest value is normalized to 1. Thus, each point represents the relative value. Although the economic value allocation method results in different life cycle environmental impacts for ethylene and propylene, relative values considering ethylene and propylene are the same. In each figure, a point closer to the center indicates the better corresponding performance. We find that the economic value allocation method and the mass allocation method show similar results: (1) Ethylene and propylene produced via the technology-integrated design have the lowest production costs. (2) Ethylene and propylene produced through the co-cracking design result in the least water consumption. (3) Ethylene and propylene produced via the naphtha cracking design lead to the lowest GHG emissions. From Figure 13, we also find that manufacturing ethylene and propylene from NGLs-rich shale gas is more attractive than from naphtha considering production costs and water consumption. However, manufacturing ethylene and propylene from naphtha is more competitive than from NGLs-rich shale gas in terms of GHG emissions. 3.5. Sensitivity Analysis. In order to better understand the impacts of altering economic parameters on the economic and environmental performance of manufacturing ethylene and propylene from NGLs-rich shale gas and naphtha, a sensitivity analysis is conducted. For illustration, Figure 14 presents the sensitivity analysis result of ethylene production cost based on economic value changes of raw shale gas, ethane, propane, naphtha, ethylene, and propylene. It is shown that the feedstock price and the ethylene price have great impacts on the ethylene production cost. In the co-cracking design and the technologyintegrated design, changes in ethane and propane prices also lead to notable effects on the ethylene production cost. However, in the naphtha cracking design, price changes of

4. CONCLUSION This work addressed the comparison of manufacturing ethylene and propylene from NGLs-rich shale gas and naphtha regarding economic and environmental performance. For manufacturing ethylene and propylene from shale gas, we proposed two novel process designs. We also investigated a conventional design considering the steam cracking of naphtha for the production of ethylene and propylene. On the basis of process modeling and HYSYS simulation, we conducted techno-economic analysis and life cycle analysis for the three designs. The economic analysis showed that manufacturing ethylene and propylene from NGLs-rich shale gas via the co-cracking design and the technology-integrated design resulted in lower production costs than from naphtha by 48.2% and 52.3%, respectively. As for

Figure 14. Sensitivity analysis of ethylene production cost: (a) co-cracking design, (b) technology-integrated design, and (c) maphtha cracking design. L


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Figure 15. Sensitivity analysis of life cycle GHG emissions of ethylene: (a) co-cracking design, (b) technology-integrated design, and (c) naphtha cracking design. (3) Allen, K. The Changing Dynamics and New Challenges Facing the North American Petrochemical industry. http://www.slideshare. net/14Balmoral/the-changing-dynamics-and-new-challenges-facingthe-north-american-petrochemical-industry (accessed October 1, 2016). (4) DeRosa, S. E.; Allen, D. T. Impact of natural gas and natural gas liquids supplies on the United States chemical manufacturing industry: production cost effects and identification of bottleneck intermediates. ACS Sustainable Chem. Eng. 2015, 3, 451−459. (5) Siirola, J. J. The impact of shale gas in the chemical industry. AIChE J. 2014, 60, 810−819. (6) Gao, J.; You, F. Design and optimization of shale gas energy systems: Overview, research challenges, and future directions. Comput. Chem. Eng. 2017, DOI: 10.1016/j.compchemeng.2017.01.032. (7) Hughes, J. D. A reality check on the shale revolution. Nature 2013, 494, 307−308. (8) Kerr, R. A. Natural Gas From Shale Bursts Onto the Scene. Science 2010, 328, 1624−1626. (9) Jenkins, S. Propylene production via propane dehydrogenation. Chem. Eng. 2014, 121, 27−28. (10) Foster, J. Can Shale Gale Save the Naphtha Crackers? https:// www.platts.com/IM.Platts.Content/InsightAnalysis/ IndustrySolutionPapers/ShaleGasReport13.pdf (accessed October 28, 2016). (11) Gong, J.; You, F. Sustainable design and synthesis of energy systems. Curr. Opin. Chem. Eng. 2015, 10, 77−86. (12) Gong, J.; Yang, M.; You, F. A systematic simulation-based process intensification method for shale gas processing and NGLs recovery process systems under uncertain feedstock compositions. Comp ut. Chem. Eng. 201 6, DOI: 10.1016/j.compchemeng.2016.11.010. (13) Martinez-Gomez, J.; Nápoles-Rivera, F.; Ponce-Ortega, J. M.; ElHalwagi, M. M. Optimization of the production of syngas from shale gas with economic and safety considerations. Appl. Therm. Eng. 2017, 110, 678−685. (14) Julián-Durán, L. M.; Ortiz-Espinoza, A. P.; El-Halwagi, M. M.; Jiménez-Gutiérrez, A. Techno-economic assessment and environmental impact of shale gas alternatives to methanol. ACS Sustainable Chem. Eng. 2014, 2, 2338−2344. (15) He, C.; You, F. Shale Gas Processing Integrated with Ethylene Production: Novel Process Designs, Exergy Analysis, and TechnoEconomic Analysis. Ind. Eng. Chem. Res. 2014, 53, 11442−11459. (16) He, C.; You, F. Toward more cost-effective and greener chemicals production from shale gas by integrating with bioethanol dehydration: Novel process design and simulation-based optimization. AIChE J. 2015, 61, 1209−1232. (17) Khojasteh Salkuyeh, Y.; Adams, T. A., II A novel polygeneration process to co-produce ethylene and electricity from shale gas with zero CO2 emissions via methane oxidative coupling. Energy Convers. Manage. 2015, 92, 406−420. (18) Gao, J.; You, F. Optimal design and operations of supply chain networks for water management in shale gas production: MILFP

environmental impacts, we assessed the life cycle GHG emissions and life cycle water consumption for the manufacturing of ethylene and propylene. On the basis of the economic value allocation, ethylene and propylene manufactured from NGLs-rich shale gas via the co-cracking design and the technology-integrated design led to higher GHG emissions than those produced from naphtha by 40.0% and 19.4%, respectively. This indicated that naphtha could be regarded as a low-carbon feedstock for the production of ethylene and propylene compared with NGLs-rich shale gas. However, manufacturing ethylene and propylene from naphtha caused significantly higher water consumption than from NGLs-rich shale gas via the co-cracking design and the technologyintegrated design by 207.5% and 137.6%, respectively. We also obtained similar results by investigating the environmental impacts based on the mass allocation method.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b00354. Input data, reaction kinetics, and model parameters. (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*Phone: (607) 255-1162. Fax: (607) 255-9166. E-mail: fengqi. [email protected]. ORCID

Fengqi You: 0000-0001-9609-4299 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from National Science Foundation (NSF) CAREER Award (CBET-1643244). This invited contribution is part of the I&EC Research special issue for the 2017 Class of Influential Researchers.

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REFERENCES

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