Subscriber access provided by Nottingham Trent University
Biofuels and Biomass
Hydrothermal liquefaction of concentrated acid hydrolysis lignin in a bench-scale continuous stirred tank reactor Ivan Kristianto, Susan Olivia Limarta, Young-Kwon Park, JeongMyeong Ha, Dong Jin Suh, Youngdo Jeong, and Jungho Jae Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00954 • Publication Date (Web): 04 Jun 2019 Downloaded from http://pubs.acs.org on June 7, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 27 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
Energy & Fuels
Hydrothermal liquefaction of concentrated acid hydrolysis lignin in a bench-scale continuous stirred tank reactor Ivan Kristiantoa, Susan Olivia Limartaa, Young-Kwon Parkb, Jeong-Myeong Haa,c, Dong Jin Suha, Youngdo Jeongd, Jungho Jaee,*
aClean
Energy Research Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
bSchool
of Environmental Engineering, University of Seoul, Seoul 02504, Republic of Korea
cDivision
of Energy and Environment Technology, KIST School, Korea University of Science and Technology, Seoul 02792, Republic of Korea
dCenter
for Biomaterials, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
eSchool
of Chemical and Biomolecular Engineering, Pusan National University, Busan 46241, Republic of Korea
*Corresponding author: Tel.: +82-51-510-2989; E-mail:
[email protected] ACS Paragon Plus Environment
1
Energy & Fuels 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
Page 2 of 27
ABSTRACT
Although numerous studies on liquefaction of lignin for the production of high-yield and highquality bio-oil have been performed in a batch reactor, studies using a continuous flow reactor are very rare. Herein, a bench-scale continuous stirred tank reactor (CSTR) was employed for the liquefaction of lignin for the first time. Lignin obtained using a two-step concentrated acid hydrolysis process from oil palm empty fruit bunch (EFB) was used as a feedstock. The batch reactor experiment was initially conducted to select the best solvent for lignin liquefaction and investigate the effect of a formic acid (FA) additive. Then, a bench-scale experiment was conducted to see how the continuous process conditions can affect the yield and composition of bio-oil. Results showed that more effective depolymerization of lignin to bio-oil is possible in the CSTR due to the fast heating rate. The water/ethanol mixture medium at 350 °C and 28 min space time was found to be the optimum reaction condition to obtain a relatively high yield (51.5 wt%) and low molecular weight (597 g/mol) of bio-oil. Syringol was the most abundant monomer in bio-oils regardless of process conditions, and higher temperature (e.g., 350 °C) was found to promote demethoxylation and alkylation reactions to produce guaiacol and alkyl guaiacol. The addition of FA to the reaction mixture not only increased the bio-oil yield from 51.5% to 60% but also reduced the O/C molar ratio of bio-oil from 0.36 to 0.30, increasing its calorific value to 27.85 MJ kg-1.
KEYWORDS: Lignin depolymerization, Bio-oil, CSTR, Co-solvent, Formic acid
ACS Paragon Plus Environment
2
Page 3 of 27 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
Energy & Fuels
1. INTRODUCTION Lignocellulosic material is considered to be the most feasible and CO2-neutral carbon source to produce sustainable chemicals and fuels.1 It comprises three different beneficial fractions— hemicellulose, cellulose, and lignin. Cellulose and hemicellulose are the sources of versatile chemical intermediates such as organic acids, furfural, and alkyl valerates, while lignin is a source of phenolic species and their derivatives.2 Lignin is a complex phenolic polymer, primarily consisting of three different building blocks—sinapyl alcohol, coniferyl alcohol, and p-coumaryl alcohol. It has a random structure connected with various C-O and C-C linkages, and their distribution are largely influenced by its method of isolation.1 Over the past few decades, various thermochemical processes such as pyrolysis and liquefaction have been explored as potential methods to transform lignin into renewable liquid fuel, called bio-oil. Among them, liquefaction, which is the decomposition of biomass in the presence of a solvent at medium temperature and high pressure, is considered as the most effective method to produce high-yield and high-quality bio-oil from lignin.3 Sub-critical water is the most explored solvent for depolymerization of lignin because water is a green solvent, relatively abundant and cheap, non-toxic, and non-flammable. Sub-critical water is liquid water which is held by pressures at temperatures between its boiling point (100 °C) and critical point (374.1 °C), whereas water becomes supercritical above its critical point (>374.1 °C, >22.064 MPa) where liquid and gas phases cannot be distinguished.4, 5 At high temperatures (> 300 °C), the dielectric constant of water decreases, and it behaves like a non-polar organic solvent rather than ambient liquid water. It also has enhanced acidity (i.e., easier proton donation to molecules), which greatly impacts the decomposition chemistry.6 Short-chain alcohols such as ethanol and isopropanol are also widely investigated as a solvent because of the relatively high
ACS Paragon Plus Environment
3
Energy & Fuels 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
solubility of high molecular weight products from cellulose, hemicellulose, and lignin.7,
Page 4 of 27
8
In
particular, they have lower critical temperatures than water and are in a supercritical state over the temperature range of 300 °C to 400 °C, which is commonly employed for lignin liquefaction. Supercritical alcohols can release active hydrogen species easily during the biomass liquefaction and enhance hydrogenation and hydrodeoxygenation reactions, leading to the effective depolymerization of biomass.9-11 In addition, their excellent transport properties such as high heat and mass transfer rates offer benefits in biomass liquefaction.12 Previous studies have reported the effectiveness of the ethanol/water system in lignin liquefaction owing to the synergistic effects of different solvent properties.13-17 Feng et al. reported that the presence of ethanol within water improves the yield of bio-crude oil and reduces the yield of solid residue during the liquefaction of whole biomass.16 They found that water/ethanol cosolvent (50/50) permeated into the biomass structure much better than water or ethanol alone and increased the solubility of depolymerized heavy oil products. Regarding lignin, Lee et al. studied the effect of the ethanol-to-water solvent ratio on the liquefaction of Kraft lignin and found that a 50/50 water/ethanol mixture is the most effective solvent for obtaining the highest yields of biooil and valuable phenolic monomers.14 They proposed that hot compressed water promotes lignin depolymerization via the hydrolysis of the lignin ether bond, while ethanol acts as a hydrogen donor and alkylating agent to stabilize reactive lignin fragments and free radicals, thereby inhibiting the recondensation/repolymerization reactions. Multiple researchers also highlighted the role of ethanol as a hydrogen donor in enhancing the bio-crude yield.8, 18, 19 In addition, the lower critical point and the dielectric constant of the ethanol/water mixture could lead to higher solubility of lignin and its fragments at relatively low temperatures and, thus, improve the lignin conversion.
ACS Paragon Plus Environment
4
Page 5 of 27 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
Energy & Fuels
To boost the hydrogen donation effect during lignin depolymerization, formic acid (FA) can be added to the reaction mixture as an in-situ hydrogen source. The positive effect of FA on lignin depolymerization is well documented in the literature.20-22 FA can decompose easily to CO2 and H2 under liquefaction conditions and provide additional active hydrogen species to the reaction medium, resulting in the enhancement in the cleavage of the ether bond of the lignin through hydrogenolysis.23 In addition, it was proposed that reactive fragmented species can be stabilized through the hydrogenation of C=C and C=O functional groups which, in turn, leads to the suppression of undesired condensation reactions.20 To date, numerous studies focused on transforming lignin into bio-oil were performed in a batch reactor system. A profound understanding of lignin processing in a continuous reactor system would be beneficial for industrial application, as the continuous reactor system can allow shorter processing time and less energy consumption. Furthermore, the current state of research of lignin transformation in a continuous reactor system is still in its infancy. A few recent studies have successfully transformed biomass into bio-oil in a continuous reactor system. For instances, Abdelaziz et al.24 reported the continuous base-catalyzed depolymerization of lignin into a phenolic-rich bio-oil in a continuous tubular reactor system (50 ml total volume) at temperatures and retention times of 170–250 °C and 1–4 min, respectively. They reported the optimum condition to depolymerize industrial kraft lignin (240 °C, 2 min, 5 wt.% lignin loading, and NaOH/lignin weight ratio = 1). Barreiro et al.25 studied the hydrothermal conversion of Nannochloropsis gaditana and Scenedesmus almeriensis in a continuous stirred-tank reactor (internal volume of 190 mL) and successfully obtained approximately a 55% yield of bio-oil at 350 °C and 15 min of residence time. He et al.26 investigated the effects of organic co-solvents, i.e., n-heptane, toluene, and anisole, on the oil properties. The hydrothermal reaction was
ACS Paragon Plus Environment
5
Energy & Fuels 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
Page 6 of 27
employed in a continuous flow pilot-scale reactor with a capacity up to 90 L h-1. The presence of organic solvent had a minor effect on the total yield of bio-oil, but resulted in the in-situ separation of bio-oil into aqueous and organic phase oil, which is beneficial for the downstream catalytic upgrading process. As the polarity of solvent decreased in the order of anisole > toluene > n– heptane, the degree of phase separation enhanced. Furthermore, the use of non-polar n-heptane also reduced the nitrogen and oxygen content of the bio-oil. Up to now, studies on lignin transformation into bio-oil in a bench-scale continuous system are very rare, although the reaction performance in a batch reactor can be different from those in the continuous reactor. This motivated the present study to investigate the process parameters for the continuous depolymerization of lignin into a high-quality bio-oil. For this purpose, a bench-scale continuous-stirred-tank reactor (CSTR) system was designed to facilitate instant heating and space times in the range of 20–46.7 min to overcome the recalcitrant properties of lignin. The lignin sample used in this study was prepared by the two-step concentrated acid hydrolysis of empty fruit bunch (EFB) from oil palm.27 As a preliminary study, depolymerization of concentrated acid hydrolysis lignin (CAHL) was first performed in a batch reactor to provide quick insight into the effect of solvent and FA addition towards the yield and average molecular weight of bio-oils. The effects of reaction temperature, co-solvent, space time, and addition of FA on the yield and quality of bio-oil (molecular weight and monomer distribution) was then explored using the CSTR to determine how the continuous reaction results differ from those in batch mode.
ACS Paragon Plus Environment
6
Page 7 of 27 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
Energy & Fuels
2. EXPERIMENTAL SECTION 2.1. Material Lignin was obtained from two-step concentrated acid hydrolysis of empty fruit bunch (EFB) from oil palm 27 and was denoted as concentrated acid hydrolysis lignin (CAHL). Ethanol (purity >95%) and isopropanol (IPA) was purchased from Daejung Chemicals & Metals and used without any further purification. The water used in this experiment was purified by using reverse osmosis method (Cascada RO-Water Purification System, Pall Corporation USA). Lignin samples were dried at 105 ºC at reduced pressure for 4 h and sieved to particle size smaller than 60 mesh (
