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Recent Advances in Intensified Ethylene Production – A Review Yunfei Gao, Luke M Neal, Dong Ding, Wei Wu, Chinmoy Baroi, Anne Gaffney, and Fanxing Li ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b02922 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019
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ACS Catalysis
Recent Advances in Intensified Ethylene Production – A Review Yunfei Gao1, Luke Neal1, Dong Ding2, Wei Wu2, Chinmoy Baroi2, Anne M. Gaffney2*, and Fanxing Li1*, 1
Department of Chemical and Biomolecular Engineering, North Carolina State University,
Raleigh, NC 27695-7905 USA. 2
Idaho National Laboratory, P.O. Box 1625, MS 2203, Idaho Falls, ID 83415 USA.
Corresponding authors: *
[email protected] and ⧧
[email protected] Abstract Steam cracking is a well-established commercial technology for ethylene production. Despite decades of optimization efforts, the process is nevertheless highly energy and carbon intensive. This review covers the recent advances in alternative approaches that hold promise in the intensification of ethylene production from hydrocarbon feedstocks ranging from methane to naphtha. Oxidative as well as non-oxidative approaches using conventional, chemical looping, membrane, electrochemical, and plasma assisted systems are discussed. We note that catalysts, electrocatalysts, and/or redox catalysts play critical roles in the performance of these alternative ethylene production technologies. Meanwhile, the complexity to produce polymer-grade ethylene also requires comprehensive considerations of not only (catalytic) reactions for ethylene formation but also feedstock preparation (e.g. air separation for oxidative conversion) and product separations. Although these alternative technologies have yet to be commercially implemented, a number of oxidative approaches have shown promise for close to order-of-magnitude reduction in energy consumption and CO2 emissions when compared to steam cracking. Given the substantial progresses in these research areas and the significant increase in C1 and C2 supplies resulting from the US shale gas revolution, we are excited by the enormous opportunities and potential impacts in the advancement and eventual implementation of significantly intensified ethylene production technologies.
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Keywords: ethylene, process intensification, oxidative coupling of methane; oxidative dehydrogenation; chemical looping; electrochemical ethylene production. 1. Introduction Ethylene is a key building block of the chemical industry. Widely used in the production of various chemical intermediates and polymers, worldwide consumption for ethylene exceeded 150 million tons per year in 2017.1 At present, ethylene is primarily produced through steam cracking of ethane and naphtha. In these processes, the hydrocarbon feedstocks are thermally converted, via gas phase radical reactions, into ethylene and other light olefins as well as hydrogen co-product at high temperatures (> 750 °C).2 The high endothermicity of cracking reactions as well as the complex cryogenic separation schemes make these processes highly energy and carbon intensive, emitting 1 to 2 tons of CO2 for each ton of ethylene produced depending upon the feedstock.2 Meanwhile, conventional steam cracking is near optimal from a thermodynamic first law standpoint, claiming over 90% thermal efficiency but still with significant amount of loss of work (or exergy loss).3 This indicates limited room for further optimization of these conventional technologies without significant process intensification. This review aims to summarize recent progress in ethylene production with the focus on novel approaches for intensifying ethylene production. Many of such approaches have shown promise for process intensification and emission reduction. The topics covered include both oxidative and non-oxidative conversion of methane and C2+ alkanes using direct, chemical-looping, plasma, electrochemical, and membrane-based approaches. We note that catalysis plays a central role in most of the aforementioned approaches and hence topics related to catalyst types, applications, and performance are covered extensively in this article. Nevertheless, emphasis is placed on the various process intensification approaches and their potential advantages compared to state-of-the-art, with the primary aim of presenting the significant process intensification opportunities to catalysis researchers. Readers specifically interested in catalyst design, characterizations, and mechanistic aspects are directed to a few excellent review articles related to these topics.4–6 In addition to the approaches mentioned above, ethylene can be produced from chloromethane7,8 biomass/coal/natural gas derived methanol9–15, ethanol16,17, and syngas18– 21.
Naphtha or paraffinic hydrocarbons, as a feedstock for ethylene production via cracking, can
be derived from biomass or carbonaceous solid wastes via hydrodeoxygenation22–24. These topics are not covered in the present article since direct conversion of alkanes takes the advantage of 2 ACS Paragon Plus Environment
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abundant and low cost alkane feedstock and has the potential to be simpler with reduced number of unit operations.
2. Alternative Approaches for Ethylene Production 2.1 General Considerations Prior to diving into the various potential approaches for ethylene production and the corresponding catalysts, this section aims to summarize the general, thermodynamic considerations for ethylene formation from light alkanes such as methane and ethane (Figure 1 and Figure 2). Within this context, key results from the literature reports are also summarized. Generally speaking, ethylene can be produced from methane, ethane and naphtha from nonoxidative routes, including NCM (non-oxidative conversion of methane) and NDH (non-oxidative dehydrogenation of ethane); and oxidative routes, including OCM (oxidative conversion of methane), ODH (oxidative dehydrogenation of ethane), CO2-ODH and naphtha oxy-cracking. Figure 1 illustrates the relative Gibbs free energy of formation for methane and ethane feedstocks and the various potential products from both non-oxidative routes (Figure 1a), oxidative routes (Figure 1b) and CO2-ODH as a special case (Figure 1c) at two different operating temperatures (600 °C and 1200 °C). Table 1 summarizes the desired reactions and common byproducts in the various ethylene production approaches covered in this manuscript. Although the elementary reaction steps and kinetic considerations for each of these chemical transformation routes can be complicated, the Gibbs free energy diagrams provide very useful information on their potential advantages and limitations. As can be seen from Figure 1a, the non-oxidative routes for ethylene formation, e.g. non-oxidative methane coupling and thermal cracking, are thermodynamically limited, especially in the case of methane. Although such limitations can be alleviated by increasing the operating temperature, the equilibrium constant for ethylene formation from methane is only 0.272 at 1200 °C and 1 atm. This dictates that thermochemical non-oxidative methane coupling needs to be carried out at very high temperatures. In addition, ethylene selectivity in such processes are often limited since further dehydrogenation products such as acetylene, benzene, and coke, are more thermodynamically favored than ethylene at high temperatures. At lower operating temperatures, on the other hand, these byproducts are less favorable thermodynamically. As such, non-thermochemical processes that can directly provide 3 ACS Paragon Plus Environment
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the energy required for methane or ethane transformation at low temperatures have the potential to be significantly more selective than high temperature thermochemical routes. The implications from thermodynamic analysis to catalyst design in non-oxidative alkane conversion are thus two folds: (i) For high temperature thermochemical processes, inhibition of coke and benzene formation is likely to be the most important requirement. Additionally, the predominance of gas phase radical reactions under such operating conditions dictate that reactor design and operational parameters such as temperature, residence time distributions, and mixing can have significant impact on the product selectivity and yield25; (ii) For low temperature, non-oxidative and nonthermochemical approaches, catalysts play a critical role for enhanced activity and olefin selectivity. Examples include electrochemical and plasma enhanced non-oxidative coupling processes. Compared to the non-oxidative routes, oxidative conversion (Figure 1b) offers distinct advantages in terms of thermodynamic driving force for ethylene formation. However, it also faces a different set of challenge in terms of byproduct formation. Although oxidative routes provide favorable ΔG for ethylene formation, the formation of other species such as COx and coke, are more energetically favorable. This trend holds true even for low temperatures, rendering an important challenge on selective ethylene formation that needs to be addressed from catalyst selection and design viewpoints. Use of CO2 as a soft oxidant for ethane ODH can help to improve the selectivity but also has its challenges. As can be seen in Figure 1(c), conversion of CO2-ODH is limited at lower temperatures. Thus, a high temperature is usually required to obtain a high ethane conversion. As will be illustrated in Figure 3, which summarizes the key catalysts in ODH, most of the high ethylene yield results (>40%) are obtained at above 750 °C. It is noted that thermal decomposition of ethane into ethylene and H2 is favored at higher temperatures. The as-formed H2 can react with COx to form methane as a by-product via methanation.26 Other side reactions such as dry reforming and steam reforming of ethane/ethylene can also occur.27,28 Thus, selectivity control is more difficult at high temperatures, as seen by some of the catalysts in Figure 3. As a result, catalyst with higher activity at lower temperatures is a desired property, but by no means a sufficient one in view of the thermodynamic limitations. To overcome these abovementioned thermodynamic equilibrium, N2O and S have been used as “soft oxidant” for OCM29–33 and ODH34–37, but are nonetheless still at their early development stage with the economics still needs to be proved.
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Relative Gibbs free energy of formation (kJ/mol)
Relative Gibbs free energy of formation (kJ/mol)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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Relative Gibbs free energy of formation (kJ/mol)
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200
C2H2
(a)
100
NCM and NDH at 1200 °C
C6H6
C2H4 0
C2H4
C2H6
2CH4
C2H2
2C
C6H6
NCM and NDH at 600 °C
-100
2C
-200 400
(b) ODH at 1200 °C
0
OCM at 1200 °C
C2H4+H2O
C2H6+H2O
C2H6+x2O2
OCM at 600 °C
2CH4+x1O2
C2H6+H2O
C2H4+H2O
C2H4+2H2O
C2H4+2H2O
-400
-800
ODH at 600 °C
2CO+3H2
2CO+2H2
2CO+3H2
2CO+2H2
2C+3H2O
2C+4H2O
2C+4H2O
2CO2+4H2O
2CO2+4H2O
2C+3H2O
-1200 2CO2+3H2O
-1600
100
2CO2+3H2O
(c) CO2-ODH at 600 °C
CO2-ODH at 1200 °C
50
C2H4+CO+H2O
C2H6+CO2
0
-50
Cracking comparison: C2H4+H2+CO2
Cracking comparison: C2H4+H2+CO2
C2H4+CO+H2O
-100
Figure 1. Relative Gibbs free energy of formation in (a) NCM (non-oxidative conversion of methane), NDH (non-oxidative dehydrogenation of ethane), (b) OCM (oxidative conversion of 5 ACS Paragon Plus Environment
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methane), ODH (oxidative dehydrogenation of ethane) and (c) CO2-ODH at 600 °C and 1200 °C. Here, relative Gibbs free energy of formation is calculated by setting the reactant Gibbs free energy to zero (2CH4 in Figure a and b, C2H6 + CO2 in Figure c) and display the relative values of Gibbs free energy of formation for the potential products. Naphtha conversion follows similar trends with OCM and ODH but is not shown in the figure. Table 1. Desired reactions and by-products in oxidative and non-oxidative ethylene production processes
From a process design standpoint, overall heat of reaction (ΔH) can be of critical importance for reactor design, process heat integration/recovery, and energy conversion efficiency. Unlike ΔG, heat of reaction is not a strong function of operating temperature (Figure 2). Therefore, an endothermic (NCM, NDH, CO2-ODH) process operated at lower temperatures that consumes low grade heat, or an exothermic (OCM, ODH) process carried out at relatively high temperatures that generates high grade heat would offer a better opportunity for efficiency improvements based on ease in heat integration and thermodynamic second law analysis. However, the opposite requirement is true from a product selectivity standpoint, as discussed in the previous paragraph and illustrated in Figure 1. This further highlights the intrinsic challenges for intensified ethylene production, which requires comprehensive and balanced considerations of operating conditions, 6 ACS Paragon Plus Environment
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catalyst design, reactor design, and process energy integrations. The above principle, however, does not apply to non-thermally driven processes which make use of electric power, plasma, or microwave energy. These approaches are still at early stage of development with their own sets of challenges, as will be elaborated in later sections.38 300
H kJ/mol ethylene production)
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200
600 °C 1200 °C
CO2-ODH
NCM
NDH/ Cracking
100
0
OCM
ODH
-100
-200
-300
Figure 2. Reaction enthalpy for each mole of ethylene produced in OCM, ODH, CO2-ODH, NCM and NDH/Cracking at 600 °C and 1200 °C (see Table 1 for the reaction stiochiometry) As can be seen, both non-oxidative and oxidative approaches face thermodynamic limitations in selective ethylene formation. As such, catalyst design, process optimization and controlling the kinetics for desirable v.s. non-desirable reactions are of critical importance to achieve optimal product yields. However, this is a challenging task considering the complexity of the reactions: in the case of OCM, surface reactions, surface initiated gas phase reactions, and gas phase radical reactions can all take place. These render significant challenges on catalyst design and impose potential limitations on practical ethylene yields one can anticipate. Figure 3 summarizes the conversion/selectivity/yield and reaction temperature of the key catalysts in direct conversion of methane and ethane, and Tables 2 to 4 summarize the reaction conditions and performances. Detailed analysis with respect to catalyst design and process strategy in methane and ethane conversion will be discussed in Section 2.2 and Section 2.3, respectively.
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Figure 3. Conversion/selectivity/yield and operating temperatures of the key catalysts reported in literature using methane (a) and ethane (b) as the feedstock. Reference numbers are marked along with symbols.
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Table 2. Reaction conditions and performances for OCM, membrane OCM and NCM catalysts arranged in the order of publication year.
Catalyst
Feed gas composition
1980
CaPt/(Cr,MoV)-Al2O3
redox mode N2/CH4/air
1982
Sn,Pb,Sb,Bi,Tl,Cd,MnAl2O3
redox mode N2/CH4/air
500-1000 °C
1 atm
redox mode, 0.5 mins, 1200-4800 h
1985
Li/MgO
CH4/O2=2/1 to 4/1, He balance, P(CH4)0.6atm
(Alkali)-(Rare earth CH4/O2=4/1 to 9/1, N2 balance, oxide)/nano-MgO P(CH4)>0.6atm
2016
800 °C 850 °C
CH4/O2=2/1 to 4/1, N2 balance, 900-1000 °C P(CH4)10
H2, C2H2, BZ and heavies
Fixed-bed/1 g
0.1%
70%
0.07%
2
H2, C2H6, BZ
63
Fixed-bed/0.5 g
1-6%
80-99%
5%
~10
H2, BZ, coke
64
NPD-based plasma, energy cost=1642 kJ/mol C2H4
10-40%
~80%
32%
~2.5
H2, C2H2, BZ
65
Year
Membrane reactor material/Catalyst coating layer
Feed gas composition
Temperature
CH4 conversion
C2+ selectivity
Max. C2+ yield
Ethylene/Ethane
By-product
Ref #
1998
LaSrCoFeO3–δ/None
Diluted CH4 (0.01-0.025 atm)/air
800-900 °C
15-18%
41-76%
13%
Not reported
CO, CO2, C2H6
66
2002
BiYSmO3–δ/None
Diluted CH4 (0.02 atm)/air
900 °C
30-65%
55-67%
37%
Not reported
CO, CO2, C2H6
67
2009
BaCeGdCoFeO3–δ/Mn-Na2WO4-SiO2
Diluted CH4 (0.5 atm)/air
850 °C
51.6%
67.4%
34.8%
1.4
CO, CO2, C2H6
68
2013
SiOC modified alumina/Mn-Na2WO4-SiO2
Diluted CH4 (0.3 atm)/air
750-850 °C
32.5%
57%
18.5%
Not reported
CO, CO2, C2H6
69
Table 3. Reaction parameters and performances for ODH, chemical-looping ODH, CO2-ODH, and membrane ODH catalysts arranged by publication year. Year
Catalyst
Feed gas composition
Temperature
Pressure
GHSV -1
-1
Reactor type/catalyst loading
C2H6 conversion
C2H4selectivity
Max. C2H4 yield
CO2 conversion
Fixed-bed/0.5 g
52-85%
28-73%
55%
By-product
Ref #
N/A
CO, CO2
70 71
halide-LaSrFeO
C2H6/O2/N2=~2/1/4
2001
SrLaNbO
C2H6/O2/N2=2/1/1
860 °C
1 atm
80000 h
Fixed-bed/0.3 g
79%
71%
56%
N/A
CO, CO2, CH4
2002
monolith Pt-Sn
C2H6/O2=2/1
700-800 °C
1 atm
Flow rate= ~10000 sccm
Fixed bed/not specified
60%
88%
52.8%
N/A
CO, CO2, CH4
72
Fixed bed/0.5 g
8-59%
20-83%
38.6%
N/A
CO2
73
Fixed bed/0.2 g
10-44%
10-68%
19%
N/A
CO, CO2, CH4
74
Fixed bed/0.5 to 2 g
87.2%
84.4%
73.6%
N/A
CO, CO2
75 76
2000
2003 2003
O2-cofeed ODH
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
Catalytic and Plasmatic NCM
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NiO/Al2O3 V-Mg-O
C2H6/O2/N2=1/1/8 C2H6/O2/N2=~6/4/90
520-600 °C
350-450 °C 500-600 °C
1 atm
1 atm 1 atm
6000 ml g h -1
-1
-1
3380 ml g h -1
-1
35000 ml g h -1
MoVTeNbO
C2H6/O2/He=9/6/85
2005
MoVTe(Sb)NbO
C2H6/O2/N2=1/1/1.3
440 °C
1atm
600 h
2006
V-Mo-O/Al2O3
C2H6/O2/He=1/2/22
580 °C
1 atm
24-120 l g h
2006
NiO/MgO
C2H6/O2=2/1 to 1/1
600 °C
1atm
30000 ml g h
1 atm
-1
2004
2010
Ni-Nb-O
C2H6/O2/He=10/5/85
340-400 °C
300-400 °C
1atm
2010
MW-CNT
C2H6/O2/He=20/10/70
400 °C
1 atm
2014
Ni-(W,Ti)-O
C2H6/O2/He=10/5/85
330 °C
1atm
2014
Mg/Dy/Li/Cl/O
C2H6/O2/He=5/5/90
450-600 °C
1 atm
238 gcat h mol
C2H6
-1 -1
-1
-1
-1
864 gcat h ml
-1
-1
5000-35000 ml g h -1
1000 gcat h ml
WHSV=0.8 to 2.0 h
-1
Fixed bed/0.5 g
65%
91.9%
59.7%
N/A
CO, CO2
Fixed bed/0.05 to 0.2 g
15-45%
41-77%
24%
N/A
CO, CO2
77
Fixed bed/not specified
68.8%
52.8%
36.3%
N/A
CO, CO2, CH4
78
Fixed-bed/0.4 g
45%
76%
34.2%
N/A
CO, CO2
79
Fixed bed/not specified
5-20%
30-90%
