The Pennsylvania State University. The Graduate School. College of Engineering CONTINOUS SEPARATION OF COMMODITY CHEMICALS FROM PARTIALLY

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1 The Pennsylvania State University The Graduate School College of Engineering CONTINOUS SEPARATION OF COMMODITY CHEMICALS FROM PARTIALLY DEOXYGENATED PYROLYSIS OILS VIA FLASH DISTILLATION A Thesis in Agricultural and Biological Engineering by Thomas Matthew McVey 2018 Thomas Matthew McVey Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science May 2018

2 The thesis of Thomas Matthew McVey was reviewed and approved* by the following: Daniel Ciolkosz Assistant Professor of Agricultural and Biological Engineering Thesis Adviser Ali Demirci Professor of Agricultural and Biological Engineering Manish Kumar Associate Professor of Chemical Engineering Paul Heinemann Professor of Agricultural and Biological Engineering Department Head of Agricultural and Biological Engineering *Signatures are on file in the Graduate School. ii

3 Abstract This study investigated the potential for continuous flash distillation for fractionating partially deoxygenated bio-oils such as tail gas reactive pyrolysis (TGRP) bio-oil to extract the component chemicals of benzene, toluene, and xylene (BTX), which are critical feedstocks for producing many refinery products. A model chemical mixture was used to mimic the lowoxygen nature of TGRP bio-oil, as TGRP bio-oil was available in limited quantities. The process conditions investigated with respect to separation efficiency were temperature (120, 130, and 140 C) and input flow rate (2 and 3 ml min -1 ). Also investigated was steady-state operation, comparison of results between batch and continuous modes of distillation, and the use of a model bio-oil mixture as representative of low oxygen bio-oil, such as TGRP oil. Steadystate operation was found to be achieved in 9 of 13 runs according to a self-defined criterion. Mean BTX extraction percentage (gout/gin) for a flow rate of 2 ml min -1 was found to be 16.21% for 120 C, 34.31% for 130 C, and 50.81% for 140 C. while for 3 ml min -1 it was 15.14% for 120 C, 36.83% for 130 C, and 39.07% for 140 C. While the mass of BTX extracted per minute was found to be larger for modeled results compared to experimental results, the trends with respect to operating temperature were positively correlated, verifying the theoretical effects of operating conditions on BTX separation. Limited availability and crossover of relative data for comparable batch (amount of BTX distilled via tops = 7 wt%) and continuous (BTX = 1.83 wt% average per fraction) distillation runs allowed for only limited observations and comparisons between distillation modes. These results suggest that continuous, atmospheric distillation of low-oxygen bio-oils is a reasonable means for separating commodity, refinery-grade chemicals from biomass pyrolysis-derived bio-oils. iii

4 Table of Contents List of Tables... viii List of Figures... x Acknowledgements... xiii Chapter 1 - Introduction... 1 Chapter 2 - Literature Review Introduction Biorefinery concept Conversion of biomass to bio-oil Hydrothermal liquefaction (HTL) Conversion of biomass to bio-oil pyrolysis Properties of bio-oil Post-pyrolysis upgrading techniques for bio-oil and future needs Alternative pyrolysis methods Pyrolysis using alternative carrier gases Pressurized batch pyrolysis Tail-gas reactive pyrolysis Pyrolysis reactors Bio-oil distillation iv

5 2.9 Batch distillation of deoxygenated bio-oil Flash vaporization Chapter 3 - Goal, Objectives, Research Questions Research Objectives Research Questions Chapter 4 - Methodology Construction and commissioning of the flash drum Preparation of model bio-oil Model bio-oil and TGRP oil testing Analytical techniques Sample weighing Organic vs. aqueous fractions of tops GC/MS of bottoms and organic fraction of tops Moisture content of all 3 fractions in selected samples Summary of all runs Process modeling Data analysis Preliminary testing procedure Measures of performance v

6 4.6.1 Data analysis for research question 1: Is steady-state, i.e., thermodynamic equilibrium within the flash drum, achieved? Data analysis for research question 2: Do theoretically modeled results correlate with experimental results? Data analysis for research question 3: Do batch distillation results for separation of BTX from TGRP bio-oil translate well to a continuous mode of distillation? Data analysis for research question 4: what is the effect of temperature and input flow rate on the mass of BTX in the organics fractions of the tops? Data analysis for research question 5: what is the effect of temperature and/or flow rate on the mass of the organic phase of the tops fraction? Data analysis for research question 6: do the results from continuous flash distillation of model bio-oil correlate and translate to the results obtained from flash distillation of experimentally produced TGRP oil? Data analysis for research question 7: what is the mass balance of water for the distillation procedure? Chapter 5 - Results and Discussion Preliminary results: distillation of water/acetone mixture Determination of steady-state Correlation between theoretically modeled results and experimental results Correlation between batch results and continuous distillation results Effect of temperature and flow rate on mass of BTX extracted in tops vi

7 5.5 Effect of temperature and flow rate on the mass of the organic phase in the tops fraction Correlation between results from distillation of TGRP and model bio-oil Water mass balance Chapter 6 Additional findings Chapter 7 Conclusions and Recommendations for Future Research References Appendix vii

8 List of Tables Table 1. Summary of pyrolysis methods with operating conditions and major products... 7 Table 2. Comparison of two typical thermochemical processes for bio-oil production Table 3. Typical chemical composition of fast pyrolysis liquid Table 4. Brief description, treatment condition, and technical feasibility of the current techniques used for upgrading bio-oil Table 5. Composition of traditional versus TGRP bio-oils produced from oak and switchgrass. 15 Table 6. Comparison of selected properties of bio-oils produced by hydrothermal liquefaction of swine manure and pyrolysis of wood and heavy fuel oil Table 7. Chemical composition of model bio-oil mixture Table 8. Experimental plan for model bio-oil runs Table 9. Summary of model bio-oil runs Table 10. Summary of data collected for distillation of model bio-oil Table 11. Calculated percent differences of tops mass between fractions for each run Table 12. Summary of 95% confidence intervals and mean values for BTX extraction as a function of temperature and flow rate Table 13. Summary of 95% confidence intervals and mean values of BTX extraction percentage as a function of temperature and flow rate Table 14. Summary of confidence intervals and mean values of mass of tops organic fractions as a function of temperature and flow rate Table 15. Summary of model oil and TGRP oil results at T = 130 C, F = 2 ml min Table 16. Summary of water mass yield calculations Table 17. Mass percentage of chemicals in organic fraction of tops (benzene indene) viii

9 Table 18. Mass percentage of chemicals in organic fraction of tops (phenol m-cresol) Table 19. Mass (g) of chemicals in tops (benzene phenol) Table 20. Mass (g) of chemicals in tops (benzene phenol) Table 21. Mass percentage of chemicals in bottoms (benzene phenol) Table 22. Mass percentage of chemicals in bottoms (naphthalene m-cresol) Table 23. Mass (g) of chemicals in bottoms (benzene phenol) Table 24. Mass (g) of chemicals in bottoms (naphthalene m-cresol) Table 25. Aspen HYSYS results at T = 120 C (mass percentage basis) Table 26. Aspen HYSYS results at T = 130 C (mass percentage basis) Table 27. Aspen HYSYS results at T = 140 C (mass percentage basis) Table 28. Mass percentage of chemicals in distilled TGRP bio-oil Table 29. Moisture content determination of fractions from water/acetone distillation runs ix

10 List of Figures Figure 1. Reaction pathway for hydrothermal liquefaction of biomass (Xiu & Shahbazi, 2012).. 5 Figure 2. Reaction pathways for flash pyrolysis of biomass... 6 Figure 3. Relative content of different groups of chemicals in liquid products obtained under N2, CO2, CO, CH4, and H2 atmospheres (Zhang et al., 2011) Figure 4. Example profile of changing concentration of N2 and recycle gas in reaction atmosphere (Mullen et al., 2013) Figure 5. Yield distribution of pyrolysis products for switchgrass under varying concentrations of recycled product gas (Mullen et al., 2013) Figure 6. Fluidized bed pyrolysis reactor (Bridgwater, 2011) Figure 7. Distillation-reactive procedures of bio-oil upgrading (Wang et al., 2013) Figure 8. Schematic of atmospheric distillation coupled with co-pyrolysis(zhang et al., 2013). 21 Figure 9. Distribution of organic compounds in distillate and raw bio-oil and their separation efficiency in distillation (Zhang et al., 2013) Figure 10. GC/MS compositions of each collected fraction from distillation of (a) TGRP and (b) regular bio-oils created from switchgrass (Elkasabi et al., 2014) Figure 11. Overall yields of each compound type from the distillation process (Elkasabi et al., 2014) Figure 12. Moisture content of distilled fractions (Elkasabi et al., 2014) Figure 13. Schematic of a flash drum (Iggland & Mazzotti, 2015) Figure 14. Methodology flow chart Figure 15. Schematic drawing of flash drum system x

11 Figure 16. Startup period trend for mass of organics and BTX in the tops at a temperature of 120 C and flow rate of 2 ml min Figure 17. Experimental versus theoretical BTX extraction per minute at a flow rate of 2 ml min Figure 18. Experimental versus theoretical BTX extraction per minute at a flow rate of 3 ml min Figure 19. BTX extraction (g out/g in) from both continuous and batch modes of distillation of TGRP bio-oil with associated trend witnessed from continuous distillation of model bio-oil Figure 20. Mass of BTX extracted per minute in the tops as function of temperature at a flow rate of 2 ml min Figure 21. Mass of BTX extracted per minute in the tops as function of temperature at a flow rate of 3 ml min Figure 22. Extraction rate of BTX (gout/gin) as a function of temperature at a flow rate of 2 ml min Figure 23. Extraction rate of BTX (gout/gin) as a function of temperature at a flow rate of 3 ml min Figure 24. Theoretical effect of flow rate on mass flow of BTX via the tops as a function of temperature Figure 25. Theoretical effect of expanded operating temperature range on mass flow of BTX from tops at a flow rate of 2 ml min Figure 26. Linear region for theoretical effect of expanded operating temperature range on mass flow of BTX from tops at a flow rate of 2 ml min Figure 27. Residual versus fitted values for Model xi

12 Figure 28. Normal Q-Q plot for Model Figure 29. Residuals versus leverage for Model Figure 30. Residuals versus fitted values for final model Figure 31. Normal Q-Q plot for final model Figure 32. Standardized residuals versus leverage for final model Figure 33. Mass of TOPS organic fraction as a function of temperature for a 20-minute interval at a flow rate of 2 ml min Figure 34. Mass of TOPS organic fraction as a function of temperature for a 20-minute interval at a flow rate of 3 ml min Figure 35. Concentration increase of BTX from input oil to organic phase of tops fraction as a function of temperature and input flow rate Figure 36. Ratio of mass of BTX in tops to tops + bottoms at a flow of 2 ml min Figure 37. Ratio of mass of BTX in tops to tops + bottoms at a flow of 3 ml min xii

13 Acknowledgements I would like to thank my graduate advisor, Dr. Daniel Ciolkosz, for all of his assistance and guidance throughout the duration of my Master s research project and and the process of writing my Master s thesis. I would also like to thank my committee members, Dr. Ali Demirci and Dr. Manish Kumar, who helped in defining aspects of the project and providing revisions to this document. Special thanks is given to Dr. Yaseen Elkasabi and all of the staff and researchers in the Sustainable Biofuels and Co-products Research division at the USDA Eastern Regional Research Center in Wyndmoor, PA. Dr. Elkasabi was a critical aspect in the development of this research project. Other critical personell towards the advancement of this project were Thomas Coleman and Craig Einfeldt. I would also like to thank my fiancee, Shelby Owens, who provided encouragement and understanding throughout the entirety of my time as a Master s student at Penn State. xiii

14 Chapter 1 - Introduction The objective of a petroleum refinery is to process and refine the input crude oil into more useful products such as naphtha, diesel fuel, kerosene and liquefied petroleum gas (Schaschke, 2014). Originally designed for crude petroleum oil processing, refineries have the potential to be adapted to renewable bio-feedstocks by substituting biomass pyrolysis oil for crude petroleum oil. One of the most critical processes within the unit operations of a petroleum refinery is that of fractional distillation. Fractional distillation involves the separation of crude oil into distinct fractions or cuts, according to the range of boiling points of the various constituents. These fractional cuts that are separated by the distillation process are then available for further downstream processing within the refinery. Some common post-distillation processing methods used within a petroleum refinery include hydrotreating, catalytic reforming, and catalytic cracking. Traditional distillation techniques have often been considered impractical for fractionation of biomass pyrolysis oils due to chemical and thermal instability related to the high oxygen content and acidity of the oil. One means of addressing this problem is the use of tailgas reactive pyrolysis (TGRP), which produces a less oxygenated and less acidic bio-oil (Mullen, Boateng, & Goldberg, 2013). TGRP is carried out by recycling the produced tail-gas, which consists of CO, CO2, H2, and some light hydrocarbons, as the fluidizing agent within the bed of the pyrolysis reactor. The introduction of tail-gas leads to a reductive atmosphere within the reaction vessel which subsequently leads to altered chemical pathways and deoxygenation of the pyrolysis oil produced. The reduction in acidity and oxygen content of the pyrolysis oil results in a more thermally and chemically stable bio-oil. With a more stable bio-oil, traditional 1

15 distillation techniques may be usable to separate the oil into fractions that can then be further processed and upgraded downstream (Elkasabi, Mullen, & Boateng, 2014). Traditional distillation procedures may be possible for bio-oil produced from TGRP due to reduced reactivity and higher energy content of the bio-oil. If separation and refining of TGRP bio-oils proves to be compatible with refinery technology, industrial-scale refining of pyrolysis oils could become an achievable and feasible reality. Thus, there is a need to investigate the effectiveness of distillation as a means for continuously fractionating bio-oil produced via TGRP. Elkasabi et al., (2014) reports successful demonstration of traditional batch distillation techniques on pyrolysis oils, illustrating the correlation between reduced reactivity of TGRP bio-oils and increased quality and quantity of desired chemicals within fractional cuts. However, continuous distillation processes such as flash distillation are a more likely candidate for commercial applications. Flash distillation involves the pumping of a feed stream through a spray nozzle (i.e., across a pressure drop) to induce fractional vaporization of the feed, which separates into liquid and vapor fractions according to volatility of components within the mixture. Components with boiling points greater than the operating temperature within the flash drum enter the vapor phase and exit the tank as gas, while less volatile components remain in the liquid phase and fall to the bottom of the reaction vessel as bottoms. With simple batch distillation of TGRP oils successfully demonstrated (Elkasabi et al., 2014), the next logical step is to characterize the performance of TGRP bio-oil when subjected to continuous distillation in order to determine its potential for industrial-scale applications of bio-oil upgrading. If single-stage continuous separation of TGRP pyrolysis oils via flash distillation proves to be successful, it could become a key component in the adaptation of petroleum refining systems to the use of bio-based feedstocks. 2

16 Chapter 2 - Literature Review 2.1 Introduction This literature review begins with a brief summary regarding the current state of biomass conversion technologies, specifically dealing with pyrolysis and current methods for upgrading pyrolysis oils. The concept of a biorefinery is reviewed in the context of the existing petroleum refining model. The review then segues into an overview of current distillation techniques, followed by a discussion of the recent discovery of a modified pyrolysis method which results in a more stable bio-oil product. The next focus is on the technologies now available for upgrading this new form of pyrolysis oil, specifically on the topic of batch and continuous distillation of the oil. The literature review concludes with a discussion on the knowledge gaps existing within this field of research, which includes a brief introduction to the potential for continuous separation of chemicals from tail-gas reactive pyrolysis oils. 2.2 Biorefinery concept For the successful production of energy and commodity chemicals from biomass feedstocks to replace petroleum-based products, a viable biorefinery concept must be developed. Biorefining is defined as the sustainable processing of biomass into a spectrum of marketable products and energy (IEA, 2007). The key to success in biorefining will be to model the process after that which has already been proven to work, i.e., a petroleum refinery. However, one of the most immediately recognizable differences of a biorefinery from a petroleum refinery is the idea that the biorefining feedstock will come from a variety of sources, and will not be one steady stream of feedstock like that in a petroleum refinery. Simply, the feedstock for a 3

17 biorefinery will be much more heterogeneous and not nearly as consistent in composition and physical properties as petroleum. An advantage to the heterogeneity of biomass is the potential for a much greater variety of producible chemicals and value-added products compared to petroleum, but this heterogeneity will also lead to the necessity for a much wider range of processing and refining technologies. While only 4-5% of petroleum worldwide is used for the production of chemicals and plastics, the economic return on these value-added products is comparable to that of petroleum-derived energy products (Bridgwater, 2011). This is why fractionating and refining bio-oil into value-added products such as commodity chemicals and bio-plastics is critical to the success of a bio-based renewable energy economy. The functional purpose of refining is to separate the feedstock into fractions that can be further processed and upgraded to create value-added products. The usual first step in the refining process is to fractionate the crude product into its more basic components. The main technology used to separate crude oil into compositionally distinct fractions is distillation. In comparison to petroleum, bio-oil contains a higher mass percentage of oxygen, less hydrogen, and less carbon, which results in a thermally unstable oil unsuitable for traditional distillation techniques (Cherubini, 2010). Therefore, a deoxygenation pre-processing step may be needed that would improve characteristics of the oil such as miscibility with hydrocarbons and thermal stability. Oxygen can be removed via catalytic cracking or hydrotreating. However, this is not an economically feasible solution, and the upgrading methods often lead to the formation of undesired products such as coke or acid (Huber & Corma, 2007). Therefore, the most prominent roadblocks to developing a successful and economically viable biorefining procedure are the thermal instability of bio-oil and its immiscibility with hydrocarbon feedstocks, which both are directly related to the high oxygen content associated with biomass pyrolysis oil. 4

18 2.3 Conversion of biomass to bio-oil Biomass does not naturally decompose into bio-oil, but instead must be subjected to a processing step such as hydrothermal liquefaction (HTL) or pyrolysis to create bio-oil Hydrothermal liquefaction (HTL) HTL involves the reaction of biomass in water at an elevated temperature and pressure with or without the presence of a catalyst (Xiu & Shahbazi, 2012). Experimental oil yields of 31.2% in non-catalytic liquefaction and 63.7% in catalytic liquefaction were achieved by Demirbas et al., 2000, with lignin being the most critical factor affecting oil yield and yield of char (Demirbas, 2000). One reaction pathway for HTL, in this case using a nickel alkali catalyst, is illustrated in Figure 1. The figure illustrates specifically how cellulosic biomass is converted into liquid and gaseous products using HTL and catalysts. HTL is characterized by temperatures around C, long residence times (0.2-1 h), and relatively high operating pressures (5-20 MPa), with typical oil yields ranging from 7-70%, depending on factors such as substrate type, temperature and pressure, residence time, and solvent and catalyst choice (Xiu & Shahbazi, 2012). A key advantage of oils produced via HTL is lower oxygen and moisture contents compared to pyrolysis oils. However, HTL is an expensive process compared to other thermochemical conversion techniques due to high pressures and hydrogen requirements. Figure 1. Reaction pathway for hydrothermal liquefaction of biomass (Xiu & Shahbazi, 2012). 5

19 An advantage of HTL over dry processes is that it can be used to convert high moisture waste materials into liquid fuel products without first drying the feedstock. However, HTL is early in its research and development stages, and therefore not a likely choice at present for industrial scale processing of biomass. Also, the majority of research conducted on liquefaction is done in batch form, which does not always translate into the continuous processing format of a typical refinery Conversion of biomass to bio-oil pyrolysis Pyrolysis is the process of thermally degrading biomass in an oxygen starved gaseous environment at moderate temperatures ( C). Pyrolysis results in the production of three major products in varying degree and composition depending upon operating conditions: liquid (known as bio-oil), gas, and char. The reaction pathway for pyrolysis is illustrated in Figure 2. Figure 2. Reaction pathways for flash pyrolysis of biomass (Xiu & Shahbazi, 2012). 6

20 Pyrolysis can be categorized as either slow, fast, or flash pyrolysis, dependent upon operating conditions such as temperature, biomass heating rate, particle size, and residence time. Slow pyrolysis is characterized by slow heating rates (5-7 K min -1 ) with high char yields, and subsequent low yields of liquid and gaseous products (Goyal, Seal, & Saxena, 2008). Fast pyrolysis is characterized by rapid heating of biomass in a matter of seconds with heating rates around 100 C s -1 (Balat, Balat, Kırtay, & Balat, 2009). Flash pyrolysis is characterized by very high heating rates with reaction/residence times of less than a second (Bridgwater, 2003). Fast and flash pyrolysis show preferential production towards liquid products. An overview of the distinctions between pyrolysis methods is summarized in Table 1. Table 1. Summary of pyrolysis methods with operating conditions and major products. Pyrolysis type Temperature ( C) Residence time Heating rate ( C/s) Major products Liquid (30%) Conventional/ Med-high Long Low Char (35%) slow pyrolysis min 10 Gas (35%) Liquid (50%) Med-high Short High Fast pyrolysis Char (20%) s 100 Gas (30%) Liquid (75%) Flash High Very short Very high Char (12%) pyrolysis < 0.5 s >500 Gas (13%) (Balat, Balat, Kirtay, & Balat, 2009; Boyt, 2003) During pyrolysis, biomass follows a complex series of reactions as it decomposes into varying amounts of liquid, gas, and solid products. A simplified two-step mechanism that 7

21 describes the major processes of biomass pyrolysis was proposed by Demirbas, 2017, in which biomass is converted to char, volatiles, and gases, and the char is further degraded to more char, volatiles, and gases. The order of thermal degradation of macromolecules within biomass during pyrolysis proceeds as follows: hemicelluloses ( K), cellulose ( K) and lignin ( K) (Demirbas, 2017). Chemical composition of the biomass feedstock has a strong influence on product distribution, with lignin associated with char formation and cellulose and hemicelluloses associated with bio-oil production. The yield of solid products decreases with increasing temperature, while liquid production sees a maximum between K (Demirbas, 2017). Lower temperatures and longer vapor residence times favor char production, while moderate to high temperatures and short residence times favor liquid production; hence fast and flash pyrolysis are the preferred methods for bio-oil production with typical liquid yields of around 75%. Some of the features of fast pyrolysis that are associated with maximum liquid yields are high heating rates (achieved in part by finely grinding the biomass), careful control of pyrolysis reaction temperature, short vapor residence times of typically less than 2 seconds, and rapid cooling of the volatile gas vapors (Bridgwater, 2003). A summary of the comparisons between the liquid products of pyrolysis and HTL is provided in Table 2. With liquefaction and pyrolysis as two prominent means for converting biomass into a liquid energy product, pyrolysis arguably holds the greatest potential for commercial production of liquid fuel products from biomass and replacement of non-renewable, fossil-derived petrol products. The reasoning for this is mostly due to the considerably lower capital cost for pyrolysis. One study found capital costs for pyrolysis and hydrothermal liquefaction (at a biomass feedstock rate of 65 kilotons per year) to be about $35 million and $47 million, respectively (Hayward et al., 2015). 8

22 Table 2. Comparison of two typical thermochemical processes for bio-oil production. (Xiu & Shahbazi, 2012) 2.4 Properties of bio-oil Several terms are used to denote pyrolysis liquids including pyrolysis oil, bio-crude, biofuel oil, wood liquid, wood oil, liquid smoke, wood distillates, pyroligneous tar, and pyroligneous acid, with the most common term (and the term used in this document) being biooil (Balat, Balat, Kirtay, et al., 2009). The liquid fraction from biomass pyrolysis consists of an oxygenated organic mixture containing both an aqueous phase (primarily water) and the nonaqueous bio-oil phase (primarily high molecular weight aromatics). Bio-oil contains over 400 chemicals, with major organic groups being acids, esters, alcohols, ketones, aldehydes, phenols, alkenes, aromatics, nitrogen compounds, furans, guaiacols, syringols, and sugars (Goyal et al., 2008). Pyrolysis derived bio-oil typically contains between 15-30% water, which is not easily separated, and has a typical density of around 1200 kg m -3 (Bridgwater, 2003). Crude bio-oil from pyrolysis is characterized by a dark brown, viscous appearance, with a distinctive acrid and smoky odor. It is immiscible in petroleum-derived fuels, but is miscible in polar solvents such as methanol. The oil is very high in oxygen, which is believed to be the primary cause of stability issues, non-miscibility with hydrocarbons, and temperature sensitivity (Bridgwater, 2011). One 9

23 of the major concerns limiting commercialization of pyrolysis for the production of biomass derived liquid fuels is the many undesirable properties associated with pyrolysis oils. These properties include high water content, high viscosity, high ash content, high oxygen content, and acidity (Bridgwater, 2011). The overall effect of these qualities of pyrolysis oil is that it is thermally unstable and is not compatible with integration into existing oil upgrading and refining technologies. The general chemical composition of pyrolysis oils is illustrated in Table 3. Table 3. Typical chemical composition of fast pyrolysis liquid. (Bridgwater, et al., 2001) 2.5 Post-pyrolysis upgrading techniques for bio-oil and future needs Bio-oil can be upgraded through various means, including physical, catalytic, and thermochemical techniques. Physical upgrading processes include filtration, solvent addition, and emulsification with diesel fuel. Catalytic upgrading options include hydrotreating and zeolite cracking. Chemical upgrading consists of gasification for the production of syngas and synfuels (A V Bridgwater, 2011). Table 4 provides a concise overview of the various upgrading methods employed for the improvement of bio-oil quality and stability. 10

24 Table 4. Brief description, treatment condition, and technical feasibility of the current techniques used for upgrading bio-oil. (Xiu & Shahbazi, 2012) All of these upgrading methods have yet to demonstrate the combination of economic viability and practical functionality that is needed for their commercial adoption. Thus, novel upgrading processes are needed in order to easily integrate bio-oil into the existing refining infrastructure, allowing for feasible industrial-scale production of fuels and value-added co-products with comparable chemical and physical properties to traditional petroleum-based products. One such novel approach could be the use of innovative pyrolysis methods to produce a higher quality biooil. 11

25 2.6 Alternative pyrolysis methods Rather than upgrading bio-oil after pyrolysis, it may be preferable to employ an alternative pyrolysis process that results in a bio-oil that needs little or no upgrading. Three methods that have been investigated include use of alternative carrier gases, pressurized batch pyrolysis, and tail-gas reactive pyrolysis Pyrolysis using alternative carrier gases Pyrolysis is typically carried out in an inert gas atmosphere, most often N2. Zhang et al. studied the impact of alternative gas atmospheres on the composition of pyrolysis products. Fast pyrolysis was performed on corn cobs in a fluidized bed reactor using N2, CO, CO2, CH4, and H2 atmospheres; the latter four being the most abundant components of the gas produced during the pyrolysis process. A summary of the results of different atmospheres on bio-oil composition is shown in Figure 3. Fast pyrolysis of corn cobs under various gas atmospheres resulted in total liquid yields of 49.6% for CO, 55.3% for CO2, 56.4% for H2, 57.1% for N2, and 58.7% for CH4 with moisture content ranging from ~20-30% (H. Zhang et al., 2011). The CO atmosphere resulted in oxygen within the oil being converted into CO2, while the H2 atmosphere resulted in considerably more water and lower amounts of other products within the organic fraction than the other gas atmospheres. 12

26 Figure 3. Relative content of different groups of chemicals in liquid products obtained under N2, CO2, CO, CH4, and H2 atmospheres (Zhang et al., 2011) Pressurized batch pyrolysis Capunitan and Capareda, (2011), performed pyrolysis of corn stover in a batch reactor at a pressure of 100 psi and varying temperatures of 400, 500, and 600 C. Biomass was placed in the reactor followed by nitrogen gas purging to displace oxygen from the vessel. The reactor was heated until it reached the desired operating temperature, and held at that temperature for 20 min. All products remained inside the reactor during the 20-minute reaction time, including the gaseous pyrolysis products. The liquid yield ranged from 28-31% depending on the operating temperature. The most significant results from this study were the oxygen and moisture contents of the bio-oil. Oxygen content of the bio-oil was found to be only 10.6%, and the moisture content was 12.8%, compared to values typically seen of around ~20% oxygen and ~30% moisture for pyrolysis oils (Capunitan & Capareda, 2012). The low oxygen content is a 13

27 desirable result, and may be related to the presence of pyrolysis gas in the reaction vessel during pyrolysis. The presence of the char in the vessel during the pyrolysis reaction procedure, which acts as a vapor cracking catalyst, is a likely explanation of the relatively low yield of oil Tail-gas reactive pyrolysis Tail Gas Reactive Pyrolysis (TGRP), the recycling of the gaseous effluent from the pyrolysis process into the fast pyrolysis reactor, has been proposed as an alternative approach to pyrolysis reactor design (Mullen et al., 2013). In this process, the inert nitrogen atmosphere is replaced over time with a specified amount of tail gas, consisting of CO, CO2, H2, and light hydrocarbons, which leads to a reductive atmosphere (Figure 4). The oil produced via TGRP is considerably lower in oxygen and less acidic when compared to traditional pyrolysis oils produced in inert gas atmospheres over zeolite catalysts (Table 5). Figure 4. Example profile of changing concentration of N2 and recycle gas in reaction atmosphere (Mullen et al., 2013). 14

28 Table 5. Composition of traditional versus TGRP bio-oils produced from switchgrass. Traditional (no catalyst) Catalyzed TGRP Water (wt %) Carbon (wt %, db) Hydrogen (wt %, 6.03 db) Nitrogen (wt %, 0.92 db) Oxygen (wt %, db) C/O H/C HHV (MJ/kg, db) TAN (mg KOH/g) (Mullen et al., 2013) The improvement in bio-oil quality as a result of the TGRP process is accompanied by a reduction in mass yield of bio-oil and corresponding increase in non-condensable gas production and water yield (Figure 5). This appears to be caused by the release of oxygenated compounds and moisture from the bio-oil, resulting in a more organic-concentrated and thus higher quality bio-oil product. This trend is similar to the difference observed between non-catalyzed and catalyzed pyrolysis. The path of oxygen rejection in both catalytic pyrolysis and TGRP is believed to be through water formation. The major catalytic difference between TGRP and zeolite catalyzed pyrolysis is the selectivity ratio for single ring aromatics such as benzene, toluene, and xylene, with pyrolyzed switchgrass oil showing a BTX concentration of ~4% compared to BTX concentration of ~1.5% for zeolite catalyzed pyrolysis (Mullen et al., 2013). 15

29 Yield (wt%) Amount of recycled product gas in reaction atmosphere (vol%, steady-state) Bio-oil organics Water Bio-char Non-condensable gases Figure 5. Yield distribution of pyrolysis products for switchgrass under varying concentrations of recycled product gas (Mullen et al., 2013). The bio-oil produced from TGRP, catalytic pyrolysis, or other deoxygenation methods can make traditional distillation procedures a more feasible process for upgrading the oil, due to reduced reactivity and higher energy content of the bio-oil. If separation and refining of lowoxygen bio-oils proves to be compatible with refinery technology, industrial-scale refining of pyrolysis oils could become an achievable and feasible reality. 2.7 Pyrolysis reactors The most common biomass pyrolysis reactor configuration is that of a fluidized bed reactor (Figure 6), due to the need for high heating rates and short residence times. This also requires the use of very small biomass feedstock particles. Char must be removed from the 16

30 reactor during pyrolysis as it acts as a vapor cracking catalyst and can accelerate the aging and instability of bio-oil. Char removal is most typically achieved using cyclone separators. The condensable gaseous products of pyrolysis require rapid cooling to minimize secondary reactions, while aerosols require additional coalescence, typically achieved through electrostatic precipitation (ESP) (Bridgwater, 2011). Figure 6. Fluidized bed pyrolysis reactor (Bridgwater, 2011). 2.8 Bio-oil distillation Distillation of bio-oil, a key step in the refining process, has proven difficult, most likely because of its relatively high water content, high viscosity, high ash content, high oxygen content, and high corrosiveness (Xiu & Shahbazi, 2012). These properties of bio-oil are believed to make it incompatible with traditional refining technologies such as simple, atmospheric 17

31 distillation. High water content can lead to undesirable properties such as instability of the oil and low heating value. The water in bio-oil is also not readily separable and can negatively affect catalysts. Char can accelerate aging of the oil and lead to undesirable cracking reactions, as well as cause blockages within filters. High oxygen content causes thermal instability and non-miscibility with hydrocarbons. Thermal instability of bio-oil leads to undesirable changes of the physical properties of the oil when exposed to heat during distillation. These properties include reactivity, decomposition into multiple phases, and irreversible increases in viscosity (Bridgwater, 2011). Table 6 provides a comparison of some important properties between biooil produced from both HTL and pyrolysis, and compares these oils to the benchmark of heavy petroleum fuel oil. Table 6. Comparison of selected properties of bio-oils produced by hydrothermal liquefaction of swine manure and pyrolysis of wood and heavy fuel oil. (Xiu & Shahbazi, 2012) Due to the reactive nature of bio-oil, the maximum distillability in a simple distillation column is ~50%, with liquids beginning to react and polymerize below 100 C to produce a solid 18

32 residue that cannot be distilled. (Mohan, Pittman, & Steele, 2006). Therefore, either the bio-oil must be upgraded prior to use (as mentioned in previous sections) or else more complex distillation technologies must be used to successfully distill bio-oil. Alternative techniques researched for fractionation of pyrolysis oils include reactive distillation (Wang, et al., 2013), molecular distillation (Guo et al., 2010; Wang et al., 2009), and atmospheric distillation of biooil followed by pyrolysis of the atmospheric distillation residue (Zhang, et al., 2013). Reactive distillation of bio-oil has been carried out using p-toluene over a sulfonic acid catalyst loaded on biomass activated carbon. The schematic for the reactive distillation process can be seen in Figure 7. After distillation, the physical and chemical properties of the oil were successfully upgraded to engine quality standards. The heating value of the oil was increased from 25 to MJ/kg, with a final ph value of 6 (Wang et al., 2013). Figure 7. Distillation-reactive procedures of bio-oil upgrading (Wang et al., 2013). Molecular distillation involves the separation of bio-oil into light, middle, and heavy fractions. The mechanism for molecular distillation is described as follows: The process is distinguished by the following features: short residence time in the zone of the molecular 19

33 evaporator exposed to heat; low operating temperature due to high vacuum in the space of distillation; and a characteristic mechanism of mass transfer in the gap between the evaporating and condensing surfaces. The separation principle of molecular distillation is based on the difference of molecular mean free path. The passage of the molecules through the distillation space should be collision free (Ramaswamy, 2013) Molecular distillation does not require very high temperatures (~80 C), although it must carried out in a vacuum. The light fraction of molecular distilled bio-oil contains mostly water (~70%) and low boiling point acids, while the middle and heavy fractions contain mostly phenols. Wang et al. (2009) also studied molecular distillation of bio-oil. Molecular distillation was carried out at temperatures of 70, 100, and 130 C to study the effect of temperature on compound separation into light, middle, and heavy fractions. An evaporation temperature of 70 C led to a light and middle fraction yield of 57%, while a temperature of 130 C led to a yield of 83% with no coking; light fraction yield was 26.4% higher from 70 to 130 C, and middle fraction yield was doubled from 70 to 130 C (Wang et al., 2009). Another alternative method of fractionation involves atmospheric distillation of bio-oils followed by pyrolysis of the atmospheric distillation residue. A schematic for the process is shown in Figure 8. Figure 9 shows separation efficiencies ranging from ~25 to ~95% for a range of organic compounds. The aim of this process is to find a use for the polymerized product produced during distillation of bio-oil. While this process does allow for atmospheric distillation of bio-oil, the distillate yield was only 52 wt% of the original oil (Zhang et al., 2013) 20

34 Figure 8. Schematic of atmospheric distillation coupled with co-pyrolysis(zhang et al., 2013). Figure 9. Distribution of organic compounds in distillate and raw bio-oil and their separation efficiency in distillation (Zhang et al., 2013) 21

35 While these three distillation techniques show some promising results, they are not ideal for large-scale production of pyrolysis oils and chemicals derived from bio-oil. The first technique requires costly vacuum pressures and catalysts. The second technique requires even more extreme and costly vacuum conditions. The third and final technique results in only ~50% mass recovery of liquid distillates. 2.9 Batch distillation of deoxygenated bio-oil While traditional bio-oil is not amenable to distillation without advanced distillation techniques, de-oxygenated bio-oil such as TGRP oil may be more suitable. Atmospheric batch distillation of TGRP bio-oil was successfully demonstrated by Elkasabi et al. (2014). TGRP bio-oils exhibit greater concentrations of valuable chemicals like phenols and naphthalenes, along with greater thermal stability allowing for prolonged exposure to high temperatures (Elkasabi et al., 2014). Figure 10 shows the compositions of the various collected fractions from distillation according to their final cut temperature. Figure 11 shows the overall yields of each component separated via distillation, i.e., the separation efficiency. Another important note from the distillation procedure is the observation of condensation reactions between alcohols and acids, thus resulting in created water. Another potential cause for the observation of water in fractions well above its boiling point is the fact that water and phenol are known to form an azeotrope. An azeotrope is defined as a mixture of two liquids that boils at constant composition; i.e. the composition of the vapour is the same as that of the liquid (Rennie, 2016). However, traditional bio-oils show a considerably larger influence of condensation reactions, resulting in greater amounts of water synthesis and product loss 22

36 (Elkasabi et al., 2014) Figure 12 illustrates the moisture content of the fractions collected throughout the distillation procedure. Figure 10. GC/MS compositions of each collected fraction from distillation of (a) TGRP and (b) regular bio-oils created from switchgrass (Elkasabi et al., 2014). 23

37 Figure 11. Overall yields of each compound type from the distillation process (Elkasabi et al., 2014). Figure 12. Moisture content of distilled fractions (Elkasabi et al., 2014). 24

38 2.10 Flash vaporization While batch distillation is useful at the benchtop scale, continuous distillation processes are needed for industrial-scale operations. One of the most common methods for continuous separation of oil into varying fractions is flash vaporization. Flash vaporization, or flash distillation, involves the pumping of a multi-component mixture through a spray nozzle (i.e., across a pressure drop) to induce partial vaporization of the feed, which separates according to volatility of components within the mixture. More volatile components enter the vapor phase and rise to the top of the tank, while less volatile components remain in the liquid phase and fall to the bottom of the reaction vessel as bottoms. A simple schematic illustrating single-stage flash vaporization of a binary feed can be seen in Figure 13. Figure 13. Schematic of a flash drum (Iggland & Mazzotti, 2015). 25

39 The liquid phase is then drained from the bottom, while the vapor phase passes out of the device and is condensed, yielding a more concentrated form of desirable and valuable compounds. A flash drum is scaled-up as a function of the gas flow rate. The main criterion to be scaled is the drum diameter. The following equation is used for scaling a flash drum (Gerber, 2014): = where, dj = flash drum diameter Aj = gas volumetric flow rate Ideally, the liquid and vapor phases exist in thermodynamic equilibrium with one another within the flash chamber. Upon reaching this equilibrium state, the flash drum is considered to be in steady state operation and should theoretically be producing a steady output of tops and bottoms fractions with constant compositions. During flash distillation, a mixture is pumped through a nozzle or throttling valve into a flash chamber in which it is partially vaporized and separated according to the range of volatility of chemicals within the mixture. The mixture to be distilled, in this case bio-oil, is placed in an infeed reservoir from which it is pumped. The bio-oil in this reservoir is stirred and heated to ensure uniformity and bring the oil temperature closer to the target setpoint within the drum. The pump provides a pressure increase, with exact values depending on mass flow settings and pump performance characteristics. The pressurized bio-oil then passes through a nozzle. As it enters the drum, the nozzle induces a pressure drop that in turn causes the stream to atomize, or spray out in the form of droplets. Upon entering the drum, the stream is partially vaporized. The 26

40 more volatile components become vapor and are drawn from the top of the drum. The less volatile components remain as liquid and fall to the bottom of the drum, from which they are drained using a valve. Flash distillation is a separation process which can be considered as an equilibrium stage process. At the entrance to the flash chamber, the stream is separated into two phases, which are in intimate contact with another, facilitating constant mass transfer between the phases (i.e., vapor is always condensing and liquid is always vaporizing). At equilibrium, the two phases exiting the drum have temporally constant temperature, pressure, and composition. When the rate of mass transfer between the two phases is equal, this is defined as equilibrium. Mass transfer rate is defined as (Wankat, 2007): = ( ) ( ) ( ) The area is defined as the area across which the mass transfer occurs. The driving force is defined as the force facilitating mass transfer as caused by differences in concentration. Calculations for characterizing performance of the equilibrium between the vapor and liquid phases of a multicomponent mixture are based on K-values, which are indicative of the distribution of chemicals throughout the two phases. The K-value can be simply defined as the ratio of a chemical species in the vapor to its concentration in liquid ( = ). K-values are dependent on the temperature and pressure within the drum. From Raoult s law, K-values of 27

41 individual components can be calculated using the vapor pressure of the component divided by the total pressure of the mixture (Wankat, 2007): = ( ) When analyzing vapor liquid equilibrium conditions, K-values are selected and calculated based on several potential thermodynamic models (Wilson, NRTL, Peng-Robinson, etc.) depending on the characteristics of the component(s) being distilled. All of this leads to the equation used for analyzing and defining performance of a flash distillation system: the Rachford-Rice equation. The Rachford-Rice equation is solved iteratively to determine the fraction of feed vaporized as a function of the operating conditions within the drum. Using the Rachford-Rice equation, the ratio of vapor to feed mass flow is estimated and input into the equation, with all other values being known. The known values are defined by choosing the operating conditions of the drum and the composition of the feed. Successful estimation of the vapor to mass feed ratio is indicated by convergence of the equation to zero. ( 1) = ( 1) where, = " " = = 28

42 = The primary examples of industrial-scale flash vaporization are for desalination (Muthunayagam, Ramamurthi, & Paden, 2005) and crude oil fractionation. Flash vaporization has been proven a successful means of desalinating sea water. An advantage of flash vaporization over other separation techniques (such as reverse osmosis) is that it does not require considerable pressure within the reaction vessel. Crude oil flash distillation is often achieved utilizing a temperature gradient within the tank and multiple condensation trays to collect different compounds that condense at different temperatures. The most critical component for efficiently separating desired chemicals from crude oil in a distillation column is the number of trays necessary for desired removal rates. Testing procedures for determining the efficiency of a distillation column are given in ASTM D2892 (Drews, 1998). Existing literature does not appear to include information on the performance of continuous distillation of low-oxygen pyrolysis oils. Therefore, there exists a necessity for research to characterize the effectiveness of continuous distillation, specifically in the form of flash vaporization, of TGRP bio-oils. If proven successful, single-stage continuous flash distillation of TGRP bio-oil will be a step toward developing a viable bio-refining process. 29