Production of Renewable Light Olefins from Fatty Acid Methyl Esters

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Production of Renewable Light Olefins from Fatty Acid Methyl Esters by Hydroprocessing and Sequential Steam Cracking Peiyong Sun, Sen Liu, Yupeng Zhou, Shenghong Zhang, and Zhilong Yao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03889 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 9, 2018

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ACS Sustainable Chemistry & Engineering

Production of Renewable Light Olefins from Fatty Acid Methyl Esters by Hydroprocessing and Sequential Steam Cracking Peiyong Sun,† Sen Liu,‡ Yupeng Zhou,† Shenghong Zhang,*,† and Zhilong Yao† †

Beijing Key Laboratory of Enze Biomass Fine Chemicals, Beijing Institute of Petrochemical

Technology, No. 19 Qingyuanbei Road, Daxing District, Beijing 102617, China ‡

College of Chemistry and Environmental Engineering, Anyang Institute of Technology, No. 73

Huanghe Street, Anyang 455000, Henan, China

Corresponding author: Tel: 86-10-81292304; Fax: 86-10-81292124. E-mail: [email protected] (Shenghong Zhang)

Keywords: light olefin, bio-paraffin, fatty acid methyl ester, hydrodeoxygenation, steaming cracking

Abstract: A route to produce renewable light olefins from fatty acid methyl esters (FAMEs), which are originally derived from waste cooking oils, has been demonstrated on a laboratory

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scale. Pre-saturation of FAMEs prior to their hydrodeoxygenation over sulfided NiMo/Al2O3 catalysts in a fixed bed reactor avoided an undesirable guard bed fouling, which would appear inevitably when hydrotreating unsaturated FAMEs under the identical conditions, leading to a smooth operation during a period of 1000 h. Cracking performance of the resulting bio-paraffins with straight chains were then evaluated in a micropilot furnace and compared with naphtha cracking. Under a coil outlet pressure of 0.10 MPa, a coil outlet temperature of 820 oC, a residence time of 0.23 s and a steam dilution of 0.75, the overall yield of C2 ~ C4 olefins when cracking bio-paraffins was 68.4%, much higher than that of naphtha cracking (52.9%). In particular, the yields of valuable ethylene (36.3%), propylene (18.1%) and 1,3-butadiene (7.5%) added up to 61.9%, 30% higher than that from naphtha cracking, indicating the viable potential of the route to produce renewable light olefins in the existing cracking units.

INTRODUCTION C2-C4 olefins are the key building blocks used widely to manufacture polymers, solvents, additives, adhesives, and other useful chemicals.1 These light olefins, in particular ethylene and propylene, are primarily produced at present via steam cracking of fossil feedstocks.2 Steam cracking is, however, a highly energy-intensive process, consuming 40% of the total energy in the entire petrochemical industry and releasing the similar amount of CO2 to the valuable end products.3 With the increasing environmental concerns and the declining petroleum resources, there have been more public and economic pressures to produce light olefins from renewable feedstocks to diminish their environmental footprints, as well as the cost of production. Triglyceride-rich biomass as a kind of renewable, inexpensive and abundantly available resource shows a high potential to meet the challenge of sustainable production of green light


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olefins.4-6 These feedstocks have similar skeleton structures and physical properties (density, boiling point, hydrogen/carbon ratio, etc.) to those of the petroleum-derived hydrocarbons,5 rendering them easy to handle in the conventional refinery processes like fluid catalytic cracking (FCC).6 Cracking of triglycerides and their derivatives over zeolites (e.g., HZSM-5) and commercial FCC catalysts, thus, has been intensively investigated in the past few decades.6-12 The yields of C2-C4 olefins vary differently, but usually below 30%, with the employed catalysts and reaction conditions. Gasoline-range components formed via olefin oligomerization, cyclization and aromatization are always the dominant products, accounting for more than a half of the converted triglycerides.13-14 Catalytic cracking of triglyceride-rich biomass is, therefore, apparently suitable to produce liquid fuels rather than light olefins. Apart from catalytic cracking, steam cracking offers another way to produce light olefins from triglyceride-based biomass. Steam cracking of vegetable oils at temperature above 800 oC and a short residence time of 0.1 ~ 0.5 s yields the similar products as naphtha cracking.15-17 However, a considerable amount of carbon oxides are observable due to the high oxygen contents in the feeds, which brings additional complication to the sequential separation sections.18 Moreover, oxygenates such as formaldehyde and methanol, cannot be avoided in the steam cracking of glycerides.16 Presence of these oxygenates, even at a trace amount, would cause serious fouling in the following separation operation.19 To guard the safety of the downstream separation, triglycerides and their derivatives have to be deoxygenated before insertion into the production schemes of current steam cracking.20 The oxygen content in the feeds can be removed by hydroprocessing, yielding mainly linear paraffinic hydrocarbons.21-27 These straight-run bio-paraffins with identical structures to those derived from crude oil can be easily handled in the existing infrastructure of conventional 3 Environment ACS Paragon Plus


refinery. Subsequent cracking of bio-paraffins, compared with the pyrolysis of triglycerides and their derivatives, reduces significantly both COx and oxygenates in the effluent due to the absence of oxygen in the feedstock.19-20 The yields of ethylene, propylene and 1,3-butadiene when cracking the deoxygenated hydrocarbons from vegetable oils, have been reported to be 37.9%, 19.5% and 7.5%, respectively, at a coil outlet pressure (COP) of 1.7 bar, a coil outlet temperature (COT) of 835 oC, and a steam dilution of 0.45.5 These results suggest a high potential of producing light olefins from the renewable and carbon neutral triglyceride biomass (e.g., vegetable oils). The use of edible vegetable oils to produce olefins, however, seems much less viable in China because of the expanding gap between supply and demand caused by the increasing population. Nevertheless, waste cooking oils (WCOs), generated at an amount of about five million tons every year in China,28 offer an opportunity to produce olefins from these low cost and widely available triglycerides. WCOs composed of used cooking oils, waste fats and greases, contain mainly solubilized alkali/alkaline earth metals, phosphorus, water, free fatty acids, and solid impurities,29 which prevents the direct use of WCOs as raw materials for catalytic processes. Transesterification of WCOs with excess methanol provides versatile fatty acid methyl esters (FAMEs) that can be potentially used as renewable feedstocks to produce oxygen-free fuels and valuable chemicals. To develop technology for the sustainable production of olefins from WCOs-derived FAMEs, an integrated process combining saturation and hydroprocessing of FAMEs, and the following steam cracking, as illustrated in Scheme 1, has been proposed and examined in the present work. Mono- and polyunsaturated FAMEs are first saturated in a continuous stirred-tank reactor (CSTR) in the presence of Ni-based catalysts; The produced saturated FAMEs are then deoxygenated to 4 Environment ACS Paragon Plus

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oxygen-free paraffinic liquids by hydrotreating over the sulfided NiMo/Al2O3 (denoted as NiMoS/Al2O3) catalysts; Finally, the bio-derived paraffins are cracked into light olefins in a micropilot cracking plant. Reaction conditions for each stage have been roughly optimized to maximize the final olefin yields, which are further used to address the technical feasibility of FAMEs-derived bio-paraffins being complements to fossil naphtha in the production of light olefins.

Scheme 1. A simplified process flow diagram for the production of light olefins from fatty acid methyl esters EXPERIMENTAL SECTION Catalyst Preparation. NiMo/Al2O3 catalysts were prepared with a wet impregnation method. A NiO-Al2O3 paste was first prepared by mixing NiO and γ-Al2O3 powders (at a mass ratio of 3:7) enough in a 10 wt% HNO3 aqueous solution, followed by extrusion, drying at 120 oC overnight and the final calcination at 540 oC for 4 h. The obtained Ni-Al mixed oxides were then cut into cylinders of 2~3 mm in length and 2 mm in diameter before impregnation in an aqueous solution of nickel nitrite and ammonium heptamolybdate. After separation and drying at 80 oC,



the obtained solid was calcined again at 540 oC for 4 h to produce NiMo/Al2O3 catalyst precursors. Catalyst Characterization. Nitrogen adsorption and desorption isotherms were measured at 196 oC using an Autosorb-iQ analyzer (Quantachrome). The specific surface area was estimated from nitrogen adsorption data in the relative pressure (p/p0) range from 0.05 to 0.30 using the Brunauer-Emmett-Teller (BET) method, and the total volume was calculated from the adsorbed amount of N2 at p/p0=0.99. The composition of NiMo/Al2O3 catalysts was determined by an Xray fluorescence (XRF) spectrometer (S4 Explorer, Bruker). Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-7000 X-ray diffractometer (Cu Kα, λ = 1.5418 Å, 40 kV, 40 mA) at a scan rate of 2o·min-1. Amounts of the organic deposits on the used catalysts were quantified with thermogravimetric analysis (TGA) on a SDT 650 simultaneous thermal analyzer (TA instruments) from room temperature to 800 oC in a dry air flow (30 mL·min-1). Saturation of FAMEs. Hydrogenation of FAMEs, which contained about 66.8 wt% monoand polyunsaturated methyl esters (purchased from Jiangxi Xufeng Chemical Co., Ltd), was carried out in a Parr 5500 stainless steel vessel (600 mL, Φ70 mm × 160 mm). Typically, 250 g FAMEs and 7.5 g Ni3Al alloy catalysts (75 μm, Figure S1 in the Supporting Information, Jiangsu Tailida Co., Ltd.) were added into the reactor, followed by replacing the residue air in the vessel with nitrogen for three times. The reactor was then heated at a rate of 10 oC·min-1 to a target temperature (e.g., 190 oC) with a stirring rate of 1000 r·min-1. Upon the temperature, the inner nitrogen was replaced by hydrogen immediately before starting to record reaction time. Catalytic Hydroprocessing. Hydrodeoxygenation of the saturated FAMEs over NiMo/Al2O3 catalysts was performed in a fixed-bed reactor (Φ20 mm × 600 mm). 20 g catalysts were placed


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in the center of the reactor with a thermometer inserted into the catalyst bed to monitor the reaction temperature. A pre-bed and a post-bed, both of which consisted of 40 mL inert glass beads (O.D. = 0.5 mm), were employed to establish trickle-flow conditions (with an estimated tube-to-particle-diameter ratio of 9.6) and support the catalyst bed, respectively. Prior to reaction, a hydrogen flow (200 mL·min-1) and a solution of 2 wt% CS2 in hexane (0.5 mL·min-1) were continuously fed into the reactor to reduce and sulfide the catalyst, as the temperature ramped up to 320 oC at a rate of 0.5 oC·min-1 and further remained at this temperature for 4 h. Then, the catalyst bed was heated to the target temperature and pressurized with H2 to the desired pressure before feeding FAMEs containing 1 wt% CS2. The liquid and gas supplies were controlled, respectively, by Laballience Series II pumps and SevenStar D07-11C mass flow controllers. The stream exiting from the reactor was cooled to 0 oC before passing through a gas-liquid separator, and the condensed liquid phase was collected for quantitative analysis. Steam Cracking. Cracking of the FAMEs-derived hydrocarbons was conducted using a micropilot plant unit in Beijing Research Institute of Chemical Industry (BRICI). The cracking furnace developed by BRICI is divided into seven separate sections, each of which is electronically heated independently, so that any given temperature file can be readily set in the reaction zone to maintain an approximately fixed residence time (e.g., 0.23 ±0.003 s) at different coil outlet temperatures (COTs). Inside the furnace is a tubular reactor, which is made of thermal stable NiCr stainless steel (20 wt% Ni and 25 wt% Cr). The reactor has an internal diameter of 8 mm and a length of 8 m, with fourteen thermometers along it to measure the process gas temperature at different positions. The feedstock containing about 120 ppm sulfur contents and water were fed separately by metric pumps and preheated before passing through the reactor, as illustrated in Scheme 2. Steam cracking of bio-paraffins was generally operated at a steam



dilution (δ) of 0.75 kgsteam/kgoil, a coil inlet temperature (CIT) of 596 oC, a residence time (τ) of 0.23 or 0.38 s, a coil outlet pressure (COP) of 0.10 MPa and COTs from 780 to 820 oC. For simplification, effect of reactor wall on heat transfer and the radial distribution of process gas temperature, as well as the coke deposition on the inner surface of the cracking coil, was not taken into account since more attention was focused on the cracking performance of bioparaffins to produce valuable light olefins.

Scheme 2. Schematic diagram of the cracking apparatus Product Analysis. Hydrogenation of unsaturated FAMEs over the Ni3Al alloy catalyst led to the products containing saturated methyl esters only. The reaction mixture was first diluted with hexane at a volume ratio of 1/9, and then quantitatively analyzed using a Shimadzu gas chromatography 2010 (GC-2010) equipped with a capillary column (HP-5ms, 30 m × 0.25 mm × 0.25 μm) and a flame ionization detector (FID). The conversion of unsaturated FAMEs was expressed as the converted moles of unsaturated methyl esters divided by the total moles of FAMEs in the products. As to the hydropocessing of FAMEs over NiMoS/Al2O3 catalysts, the products after being quenched at 0 oC were divided into gas and liquid phases. Gaseous products containing mainly


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CH4, COx and few C2 ~ C5 light alkanes were not considered when calculating the product selectivities, as CH4, COx, and the other insignificant gaseous organics were out of focus in this work. The liquid-phase composed of unconverted FAMEs, deoxygenated hydrocarbons, and oxygenates such as fatty acids and fatty alcohols, was analyzed by the GC mentioned above and a gas chromatograph-mass spectrometer (GC-MS, Shimadzu GCMS-QP2010 Ultra). The straight-run paraffins in the products were identified by comparing their individual characteristic retention time in GC-FID profile with those of the standard n-alkanes (C6 ~ C24, Sinopec). GCMS was then employed to confirm the n-paraffins, as well as to identify the iso-paraffins in the products. The composition of the liquid phase was also quantified with the GC using a modified area normalization method. The conversion of FAMEs (X(FAMEs)) was defined as (∑C2n-1 + ∑C2n) / ∑C2n:0 × 100% (n = 5 ~ 12), where C2n:0 represents the moles of saturated methyl esters in the effluent, C2n-1 and C2n stand, respectively, for the moles of the detectable products (including paraffins, fatty acids and alcohols) with odd and even number of carbon atoms.22 While the selectivity (S) of a kind of products (e.g., n-paraffins with the number of carbon atoms between 8 and 24) was expressed as their mole fraction, the moles of these homologues divided by the total moles of products in the liquid phase. During the operation of steam cracking, the cracker effluent was carefully quenched before separation into gas and liquid phases (Scheme 2). The gaseous products were sampled and transferred to an on-line Agilent 6890 GC equipped with a HP-PLOT Q capillary column (15 m × 0.53 mm × 50 μm) and a thermal conductivity detector (TCD) to separate and quantify permanent gases. The liquid products were weighed by an electronic balance and then quantified by another Agilent 6890 GC equipped with a FID and a HP-PONA column (50 m × 0.20 mm × 0.50 μm) to separate light alkanes and alkenes, pyrolysis gasoline and fuel oils.



RESULTS AND DISCUSSION Catalyst Characterization. NiMo/Al2O3 catalyst precursor after calcination at 540 oC has a specific surface area of 93 m2·g-1, and a total pore volume of 0.29 mL·g-1. Contents of NiO, MoO3 and Al2O3 determined by XRF are 29.4%, 8.1% and 62.5%, respectively, corresponding to a molar ratio of Ni/Mo/Al at 5.85/1.00/27.1. XRD patterns of the fresh NiMoS/Al2O3 catalyst (Figure 1) show the sharp diffraction peaks at 2θ of 21.8o, 31.1o, 37.8o, 44.3o, 49.7o and 55.2o, attributed to crystalline Ni3S2 (JCPDS 44-418). Additional peaks with low intensity are also visible at 2θ of 14.4o, 32.7o and 39.5o, which are assigned to MoS2 (JCPDS 37-1492). These characteristics demonstrate clearly that Ni and Mo exist in crystalline Ni3S2 and MoS2, respectively, for the studied catalyst.

Figure 1. XRD patterns of the fresh and used NiMoS/Al2O3 catalysts. Hydroprocessing of Unsaturated FAMEs. Direct hydroprocessing of FAMEs containing a significant amount of unsaturated methyl esters over the NiMoS/Al2O3 catalyst in a fixed-bed reactor resulted in a serious guard bed fouling, as shown in Figure 2. The feeding pressure required for the pump to deliver methyl esters increased with reaction time due to the guard bed 10 Environment ACS Paragon Plus

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fouling, and caused finally the pump to stop working after reaction for 350 h. A brown jelly-like substance covering the glass beads of the pre-bed was observed and further identified to be composed totally of combustible organics by TG-DTA. The jelly was probably formed by the polymerization of unsaturated FAMEs over glass bead surface at high temperatures, as reported previously in the hydroprocessing of vegetable oils.5

Figure 2. Variation of the catalytic performance of NiMoS/Al2O3 with reaction time in the hydrodeoxygenation of unsaturated FAMEs. Reaction conditions: 380 oC, 2 MPa, H2/FAMEs = 1000/1 (V/V), WHSV = 1 h-1. FAMEs Saturation. To avoid the undesirable guard bed fouling and the consequent periodic replace of the guard bed media in the hydroprocessing of unsaturated FAMEs, hydrogenation of unsaturated FAMEs prior to hydroprocessing was carried out in a batch reactor in the presence of commercial Ni3Al alloy catalysts. Hydrogenation of FAMEs over Ni-based catalysts at moderate temperatures results in the saturation of C=C double bonds instead of C=O bonds.30 Under the optimized conditions containing a catalyst dose of 2 wt%, a reaction temperature (T) of 190 oC, and a hydrogen pressure (p) of 2.0 MPa, more than 99.5% of FAMEs were saturated in 30



minutes (Figure S2). The resulting saturated FAMEs consisted mainly of methyl esters of fatty acids with even carbon atoms from C10 to C24, among which methyl stearate and methyl palmitate accounted for about two thirds of the total weight, as listed in Table 1. Table 1. Composition of the saturated FAMEs. Composition C10:0 FAME C12:0 FAME C14:0 FAME C16:0 FAME C18:0 FAME C20:0 FAME C22:0 FAME C24:0 FAME

Contents (wt %) 5.89 7.48 9.40 26.26 41.88 5.91 2.40 0.79

Hydrodeoxygenation of Saturated FAMEs. Hydroprocessing of glycerides and their derivatives to long chain hydrocarbons has been critically reviewed in terms of reaction mechanism and kinetics in the recently published literatures.22, 31-33 In general, glycerides are first hydrolyzed to the corresponding fatty acids adsorbing on the catalyst surface as the dominant reaction intermediates. Sequential hydrogenation of these intermediates produces aldehydes and/or alcohols, which then undergo hydrodeoxygenation (HDO), a process combining direct hydrogenation of a keto intermediate and dehydration, to produce hydrocarbons with the same carbon atoms as fatty acids.22 This process is, namely, the HDO reaction path, in which oxygen is removed as water. While those n-alkanes with odd carbon atoms are mainly derived from decarbonylation (DCO, CO removal) or decarboxylation (DCO2, CO2 removal) processes of reaction intermediates.33 To explore the reaction network of the FAME hydroprocessing over NiMoS/Al2O3 catalysts, the reaction was performed at a relatively low temperature (320 oC) to avoid a full conversion of 12 Environment ACS Paragon Plus

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FAMEs. The resulted product distribution against the weight hourly space velocity (WHSV) of FAMEs is plotted in Figure 3. The selectivity of fatty acids increases monotonously from 1.98% to 18.01%, as the WHSV increases from 1 to 6 h-1. The observed increasing selectivity of fatty acid with the declining residence time suggests that fatty acids are the primary products with a relatively slow rate to other intermediates or products. Apart from the fatty acids, fatty alcohols instead of aldehydes were identified at high space velocities, implying that aldehydes derived probably from the hydrogenation of fatty acids are the unstable reaction intermediates with high reactivity to form other molecules (e.g., rapid hydrogenation to fatty alcohols).33 Indeed, the selectivity of fatty alcohols rises from zero to 7.32% as the WHSV increases from 1 to 4 h-1, and declines slightly to 5.36% when increasing WHSV further to 6 h-1. These observations indicate that alcohol intermediates observed in the reaction can be fully converted to paraffins through the sequential dehydration-hydrogenation reactions under low space velocities.



Figure 3. Dependences of FAME conversions and the corresponding product distributions on the weight hourly space velocity (WHSV) in the hydroprocessing of FAMEs over the NiMoS/Al2O3 catalyst, FAMEs (●), paraffins with odd () and oven () number of carbon atoms, iso-paraffins (○), fatty acids (◊), and fatty alcohols (■). Reaction conditions: 320 oC, p(H2) = 2 MPa, H2/FAMEs = 1000/1 (V/V). At the same time, a declining selectivity of paraffins with even number of carbon atoms (C2n) from 76.38% to 50.37%, and a reversed trend from 19.48% to 24.92% for the paraffinic hydrocarbons with the odd number of carbon atoms (C2n-1), are observed with the increase in space velocity from 1 to 6 h-1. The much higher selectivity of C2n than that of C2n-1 suggests the dominant HDO pathway in the hydroprocessing of FAMEs over the NiMoS/Al2O3 catalyst, as the C2n and C2n-1 paraffins are respectively derived from HDO and DCO/DCO2 processes.31-33 In addition, the much lower selectivities of iso-paraffins (< 2%) observed at various space velocities imply the negligible isomerization of n-paraffins over NiMoS/Al2O3 catalysts under the employed reaction conditions. According to the above observations, HDO and DCO/DCO2 products coexist in the hydroprocessing of FAMEs over the NiMoS/Al2O3 catalyst, and fatty acids/alcohols are the main reaction intermediates. Thus, a simplified reaction network involving HDO, DCO and DCO2 is proposed in Scheme 3 for the deoxygenation of FAMEs. FAMEs adsorbed on the catalyst surface proceed through hydrolysis first to produce fatty acids in the presence of steam, followed by either consecutive hydrogenation to fatty alcoholic intermediates via aldehydes or direct decarboxylation to C2n-1 paraffins (DCO2 pathway). The produced alcohols undergo dehydration and further hydrogenation, leading to the final C2n paraffins (HDO pathway). The unstable aldehyde intermediates may also undergo sequential decarbonylation and hydrogenation to 14 Environment ACS Paragon Plus

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produce C2n-1 paraffinic hydrocarbons (DCO pathway). Hydroprocessing of FAMEs over the NiMoS/Al2O3 catalyst proceeds obviously through a similar reaction network to that of glyceride deoxygenation.32-34 More attention is, thus, put on how to maximize the yield of hydrocarbon products in the following work.

Scheme 3. Proposed reaction pathway for the hydroprocessing of FAMEs over the NiMoS/Al2O3 catalyst. Influences of reaction conditions, including temperature, pressure, volume ratio of hydrogen to FAMEs (H2/FAMEs), and the WHSV of FAMEs, on the catalytic performance of NiMoS/Al2O3 have been studied using a single-factor-at-a-time method, with the results displayed in Figure 4. FAME conversions and paraffin selectivities increase from 87.4% and 98.0% to 99.5% and 99.7%, respectively, with the rising temperature from 320 to 380 oC. Relatively high temperature favors the hydrogenation of reaction intermediates like fatty acids, improving the overall paraffin selectivity. While hydrogen pressures above 2 MPa have little effect on the performance of the NiMoS/Al2O3 catalyst, because the FAME conversions and paraffin selectivities are both higher than 99%, close to the thermal equilibrium of reaction. Different from pressure, the ratio of H2/FAMEs affects significantly the product distribution, although the FAME conversions remain almost constant at 99.5%. The paraffin selectivities go up from 95.1% to 99.7% as the H2/FAMEs ratios increase from 500 to 1000, and then change slightly with increasing the ratio



further to 2000. Meanwhile, the selectivities of oxygenates (e.g., fatty acids and alcohols) drop from 4.9% to 0.1%, reflecting the facilitated hydrogentaion of oxygenate intermediates at high H2/FAMEs ratios,35 consistent with the proposed reaction pathway in Scheme 3. Considering the space velocity of FAMEs, its increase from 1 to 6 h-1 causes a continuous decline in the FAME conversion from 99.5% to 80.9%, as well as a decrease in the paraffin selectivity from 99.7% to 95.4%. Therefore, the optimal reaction conditions for the hydrodeoxygenation of FAMEs over the NiMoS/Al2O3 catalyst contain a temperature of 380 oC, a H2 pressure of 2 MPa, a volume ratio of H2/FAMEs at 1000 and a space velocity of FAMEs at 1 h-1. Under such conditions, the overall paraffin selectivity is higher than 99%.


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Figure 4. Effects of reaction temperature, H2 pressure, volume ratio of H2 to FAMEs and space velocity of FAMEs on the catalytic performance of NiMoS/Al2O3 in the hydrodeoxygenation of FAMEs to bio-paraffins. Reaction conditions: (a) 2 MPa, H2/FAMEs = 1000/1, WHSV = 1h-1, (b) 380 oC, H2/FAMEs = 1000/1, WHSV = 1h-1, (c) 380 oC, 2 MPa, WHSV = 1h-1, and (d) 380 oC, 2 MPa, H2/FAMEs = 1000/1. Figure 5 shows the stability of the NiMoS/Al2O3 catalyst in the hydrodeoxygenation of FAMEs during a long reaction period of 1000 h. The FAME conversion remains above 99.8% for the first 200 h, and then steps slowly down to 98.3% in the next 250 h. After that, an in-situ treatment of the NiMoS/Al2O3 catalyst with hydrogen and sulfurizing reagent (1wt % CS2 in hexane) recovered the conversion up to 99.8% again, despite followed by a similar trend in the next 500 h. However, the selectivity of n-paraffins seems insensitive to the reaction time, fluctuating between 97% and 98% during the testing period. The stable paraffin selectivity as well as the recoverable catalytic activity leads to an enough durability of the NiMoS/Al2O3 catalyst in the hydroprocessing of FAMEs. This can be partly explained by the structural stability of the NiMoS/Al2O3 catalyst, which was reflected by the identical XRD patterns of the catalyst before and after reaction, as shown in Figure 1. The observed slight decline in the FAME conversion after reaction for 250 h was probably caused by the carbonaceous deposits on the catalyst surface due to the polymerization of active intermediates,36 as suggested by TG curves of the used catalyst in the supplementary Figure S3. The amount of the deposits on the used catalyst was about 3.18 wt%, which could be further reduced to 1.46 wt% by an in-situ hydrogenation treatment, accounting for, at least to some extent, the recovered activity of NiMoS/Al2O3 after the in-situ regeneration.



Figure 5. Long-term stability of NiMoS/Al2O3 catalysts in the hydrodeoxygenation of FAMEs. Conditions: 380 oC, 2 MPa, H2/FAMEs = 1000/1 (V/V), WHSV = 1 h-1. Table 2. Composition (wt %) of bio-paraffins used for steam cracking. Carbon number 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 sum

n-paraffins 1.84 2.26 2.37 2.51 2.95 3.24 10.35 12.06 18.05 20.18 2.41 2.15 1.29 1.19 0.41 0.41 83.67

iso-paraffins 0.43 0.96 1.07 1.24 1.61 1.44 2.04 1.81 2.81 1.42 1.12 0.38 0 0 0 0 16.33

Steam Cracking of Bio-paraffins. Because of the limited paraffin production from the hydrodeoxygenation of FAMEs with the lab-scale apparatus, bio-paraffins (400 kg) derived from the hydroprocessing of FAMEs in a cooperated pilot plant were used as feedstock for steam


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cracking. The composition of the employed bio-paraffins was a little different from that of lab products, as listed in Table 2. Alkanes in the carbon range of C15 ~ C18 account for about 68.7 wt% of the paraffin feedstock, and the overall content of straight-run hydrocarbons is 83.7 wt%, less than that of the lab counterpart (about 97%, Figure 5). Nevertheless, aromatics and oxygenates are not detectable in the above feedstock used for steam cracking. Other physical properties including density, bureau of mines cracking index (BMCI), initial and end boiling points are listed in Table 3 and compared with those of a typical straight-run naphtha. Table 3. Physical properties of fossil naphtha and bio-paraffins.

Density (20 oC, g·cm-3) BMCI Composition (%) n-paraffins iso-paraffins Olefins Naphthenes Aromatics Distillation temperature (oC) Initial boiling point 10% 30% 50% 70% 90% End boiling point

Naphtha 0.7041 9.78

Bio-paraffins 0.7771 0.557

37.83 35.23 0 16.16 10.33

83.67 16.33 0 0 0

41.2 64.6 80.4 97.5 114.3 132.8 150.1

162.6 255.4 284.6 291.1 295.8 301.5 313.0

To assess the viability of the above bio-paraffins as renewable complements to fossil naphtha in the production of light olefins, their cracking performances are compared with each other in Table 4. Both cracking effluents contain various products catalogued roughly into permanent gases, light alkanes, light alkenes, pyrolysis gasoline, and pyrolysis fuel oil, similar to the results reported by Pyl et al..19-20 However, bio-paraffins produce a much higher light olefin yield



(68.5%) than naphtha (52.9%) in the cracking reaction due to their high n-alkane content (83.7%). Steam cracking of hydrocarbons molecules proceeds through successively splitting of ethylene molecules via a free radical mechanism,37 and n-paraffins have been reported to yield much more ethylene than iso-paraffins and naphthenes with the same number of carbon atoms.38 Consequently, naphtha results in a lower olefin yield than the renewable feedstock, due to its high contents of both iso-paraffins (35.2%) and naphthenes (16.2%). Table 4. Product distributions (wt %) for steam cracking of fossil naphtha and bio-paraffinsa

Permanent gases (C0-C1) H2 CO CO2 Light alkanes (C1-C4) Methane Ethane Propane n-Butane i-Butane Light alkenes (C2-C4) Ethylene Propylene n-Butene i-Butene 2-Butene 1,3-Butadiene Others Pyrolysis gasoline (C5-C9) Benzene Toluene Xylenes Others Pyrolysis fuel oil (C+10) Diesels Kerosene a

Naphtha 0.95 0.91 0.03 0.01 18.09 13.45 3.87 0.54 0.14 0.09 52.91 27.45 14.74 1.56 1.96 0.77 5.31 1.12 19.61 5.01 4.55 3.28 6.77 8.48 5.04 3.44

Bio-paraffins 0.88 0.49 0.29 0.10 13.22 8.92 3.66 0.45 0.18 0.01 68.47 36.30 18.14 4.26 0.44 0.88 7.46 0.99 16.35 3.81 1.01 0.76 10.77 1.18 0.53 0.65

Process conditions: CIT = 596 oC, CIT = 820 oC, COP = 0.1 MPa, τ = 0.23 s, δ = 0.75.

In addition, cracking of bio-paraffins produces a much smaller BTX (i.e., benzene, toluene, and xylenes) yield (5.6%) than naphtha cracking (12.8%), despite the comparable gasoline yields 20 Environment ACS Paragon Plus

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(16.4% vs. 19.6%). This is also closely related to the compositions of both feedstocks for cracking. Naphtha contains a significant amount of iso-paraffins (35.2%), naphthenes (16.2%) and aromatics (10.3%), with a BMCI value of 9.78; while the renewable feedstock consists mainly of n-paraffins (83.7%) and iso-paraffins (16.3%), yielding a BMCI value of 0.56. The smaller BMCI value of bio-paraffins implies a low potential to produce aromatics,38 in line with their quite low BTX yield measured when cracking. To maximize production of the valuable light olefins over pyrolysis gasoline and fuel oil, effect of COTs on the product distribution for the steam cracking of bio-paraffins is plotted in Figure 6(A). Under a COP of 0.10 MPa, a CIP of 596 oC, a δ of 0.75 and a τ of 0.23 s, the yield of C1 ~ C4 alkanes increases monotonously from 10.5% to 13.2% as the COT increases from 780 to 820 oC; whereas the yield of C2 ~ C4 olefins increases initially from 61.0% with the rising COTs, gets a maximum of 69.2% at 810 oC, and then falls slightly to 68.4% at a COT of 820 oC. The yield of pyrolysis gasoline, different from light olefins, exhibits the reversed trend within the same COT range, declining from the initial 23.1% to 15.9% at 810 oC and then rising to 16.3% at a higher temperature. A similar trend to the pyrolysis gasoline is also observed for pyrolysis fuel oil, with a minimum yield of 1.5% at 810 oC. Pyrolysis fuel oil consists mainly of unconverted long-chain paraffins and their primary decomposition products. Thermal conversion of these heavy paraffins produces either pyrolysis gasoline through rapid dehydrogenation into aromatics39 or light olefins by the successive decomposition of larger olefins through retro-ene reactions40-41. Both processes involve a succession of hydrogen abstractions and β–scission reactions via radical pathways, and aromatics can be further cracked into small olefins in a similar way.37, 42 Indeed, formation of light olefins is substantially preferred at high temperatures, reflected by the increasing light olefin yields as the COT rises. However, too high temperature



(e.g., 820 oC) reduces the total olefin yield due to the increasing secondary condensation of olefins to gasoline and fuel oil components.19

Figure 6. Effect of COTs on the product distribution for the steam cracking of bio-paraffins. Reaction conditions: CIT = 596 oC, COP = 0.1 MPa, τ = 0.23 s, δ = 0.75. Following a similar trend to that of light olefins, the sum of valuable ethylene, propylene and 1,3-butadiene increases from 52.4% to 61.8% when the COT rises from 780 to 810 oC, and changes slightly with a further increase of COT to 820 oC, as presented in Figure 6(B). This suggests that a COT between 810 and 820 oC is optimal to maximize the overall yield of


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ethylene, propylene and 1,3-butadiene for the studied COP (0.10 MPa), dilution (0.75) and residence time (0.23 s). Higher COTs will favor the formation of less valuable gasoline components due to a series of recombination reactions (e.g., addition of allylic radicals on dienes) and intramolecular additions.37 Under the optimized conditions discussed above, the yields of ethylene, propylene and 1,3-butadiene are 36.3%, 18.1% and 7.5%, respectively. Furthermore, steam cracking of the bio-paraffins was also performed at a residence time of 0.38 s in the same reactor, as shown in Figure S4. Similar results to those of 0.23 s were obtained and the desirable yields of ethylene, propylene and 1,3-butadiene yields was 37.0%, 17.7% and 6.9%, respectively, under the process conditions containing a COP of 0.10 MPa, a CIT of 496 oC, a COT of 810 oC, and a dilution of 0.75. Compared with naphtha cracking, the achieved much higher overall yield of the valuable olefins implies that the renewable bio-paraffins derived from FAMEs can act as viable complements to the declining petroleum-derived naphtha to produce light olefins in the existing steam cracking units. CONCLUSIONS To produce renewable light olefins, a three-step process involving (i) hydrogenation of unsaturated FAMEs over Ni3Al alloy catalysts, (ii) hydrodeoxygenation of the saturated FAMEs over NiMoS/Al2O3 catalysts, and (iii) the following steam cracking of the resulting bio-paraffins has been demonstrated on a laboratory scale. Saturation of FAMEs prior to hydrodeoxygenation led to a smooth hydroprocessing operation during a reaction period of 1000 h. Sequential cracking of the deoxygenated bio-paraffins in a micropilot plant gave an overall yield of valuable ethylene, propylene and 1,3-butadiene as high as 61.9%, a gain of 30% compared with fossil naphtha cracking under the identical process conditions. Hence, the bio-paraffins derived



originally from waste cooking oils provide a promising supply for the sustainable production of light olefins in the existing cracking units. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxx. XRD patterns of the employed Ni3Al alloy catalysts in the hydrogenation of unsaturated FAMEs; Effects of temperature on the conversion of unsaturated FAMEs; TG curves of the used NiMoS/Al2O3 catalysts; and the product distributions for the steam cracking of bio-paraffins at a residence time of 0.38 s (PDF) AUTHOR INFORMATION Corresponding Author *Tel.:+86-10-81292304. E-mail:[email protected]. ORCID Shenghong Zhang: 0000-0003-2051-2006 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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Financial supports from the National Natural Science Foundation of China (21703012), and the Scientific Research Project of Beijing Municipal Education Commission (KZ 201010017001, KM 201610017003) are gratefully acknowledged. This work was supported in part by the Beijing Research Institute of Chemical Industry (BRICI). The authors would like to thank Dr. Yonggang Zhang for his help on the analysis of cracking products of naphtha and bio-paraffins. REFERENCES (1) Sadrameli, S. M. Thermal/catalytic cracking of hydrocarbons for the production of olefins: A state-of-the-art review I: Thermal cracking review. Fuel 2015, 140, 102-115. DOI: 10.1016/j.fuel.2014.09.034. (2) Corma, A.; Corresa, E.; Mathieu, Y.; Sauvanaud, L.; Al-Bogami, S.; Al-Ghrami, M. S.; Bourane, A. Crude oil to chemicals: light olefins from crude oil. Catal. Sci. Technol. 2017, 7 (1), 12-46. DOI: 10.1039/c6cy01886f. (3) Rahimi, N.; Karimzadeh, R. Catalytic cracking of hydrocarbons over modified ZSM-5 zeolites to produce light olefins: A review. Appl. Catal., A 2011, 398 (1-2), 1-17. DOI: 10.1016/j.apcata.2011.03.009. (4) Zhang, H.; Cheng, Y. T.; Vispute, T. P.; Xiao, R.; Huber, G. W. Catalytic conversion of biomass-derived feedstocks into olefins and aromatics with ZSM-5: the hydrogen to carbon effective ratio. Energy Environ. Sci. 2011, 4 (6), 2297-2307. DOI: 10.1039/C1EE01230D. (5) Dijkmans, T.; Pyl, S. P.; Reyniers, M. F.; Abhari, R.; Van Geem, K. M.; Marin, G. B. Production of bio-ethene and propene: Alternatives for bulk chemicals and polymers. Green Chem. 2013, 15 (11), 3064-3076. DOI: 10.1039/c3gc41097h. (6) Melero, J. A.; Iglesias, J.; Garcia, A. Biomass as renewable feedstock in standard refinery units. Feasibility, opportunities and challenges. Energy Environ. Sci. 2012, 5 (6), 7393-7420.



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SYNOPSIS Bio-paraffins derived from fatty acid methyl esters provide viable complements to naphtha for the sustainable production of light olefins in the conventional cracking units.


Production of Renewable Light Olefins from Fatty Acid Methyl Esters

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