Process Water Recycle in Hydrothermal Liquefaction of Microalgae To

Subscriber access provided by EPFL | Scientific Information and Libraries

Article

Process water recycle in Hydrothermal Liquefaction of microalgae to enhance bio-oil yield Elia Armandina Ramos - Tercero, Alberto Bertucco, and Derk W.F. (Wim) Brilman Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef502773w • Publication Date (Web): 16 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

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 free 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 accessible to all readers and 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.

Energy & Fuels 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 33

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

Process water recycle in Hydrothermal Liquefaction of microalgae to enhance bio-oil yield

Elia Armandina Ramos-Tercero a*, Alberto Bertucco a, D.W.F. (Wim) Brilman b*

a

Department of Industrial Engineering DII, University of Padova, Via Marzolo 9, 35131

Padova, Italy b

Sustainable Process Technology Group, Faculty of Science and Technology, University of

Twente, P.O. Box 217, 7500AE Enschede, The Netherlands

*Corresponding author: Elia Armandina Ramos Tercero, e-mail: [email protected] Wim Brilman, e-mail: [email protected]

ACS Paragon Plus Environment

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 33

Abstract In this work the effect of recycling the process water (PW) of Hydrothermal Liquefaction (HTL), to the HTL reactor was investigated, aimed to recover carbon from the organic content of the PW and to develop a solvent-free process. When recycling twice the PW at 220°C, 240°C and 265°C, a significant increase in bio-oil production is observed at all temperatures. An extended series of recycle experiments was performed at 240°C, showing that the bio-oil yield increased up to stationary level, after 6 recycles. To investigate the role of accumulated compounds as acetic acid, additional experiments were carried out with dilution and spiking of recycled PW. PW recycling not only increases bio-oil yield but also enables to reduce operating temperature (and hence costs) of HTL and it is therefore an essential element in developing a solvent-free process for biocrude production from microalgae.

Keywords Hydrothermal Liquefaction, microalgae, oil yields, process water recycle


Page 3 of 33

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 Microalgae seem to be a feasible option as source of high value and environmental friendly fuels. Microalgae offer well-known advantages as CO2 fixation, wastewater depuration capacity and growth rates higher than other terrestrial plants.1 However, algae technology for biofuels production cannot currently compete with respect to traditional fossil fuels for economic reasons.2 To improve this situation many efforts are being directed to increase lipid yields in microalgae. However, the use of high-lipid microalgae species does not automatically result in high bio-oil yields as these species usually do not show high growth rates. Another pending glitch is the water elimination after the microalgae cultivation process, a step which is required for an efficient lipid extraction, but this step is also energetically expensive.3 These problems could be overcome by substituting lipid extraction with a suitable thermochemical biomass conversion process, and by optimizing its operation conditions to recover the lipid part of the biomass with higher yields. The main advantage of such a conversion process is the thermochemical transformation of the non-lipid portion into bio-oil. Hydrothermal liquefaction (HTL) is a biomass conversion process that has generated strong interest in recent years. Through this process, biocrude (or bio-oil) from wet microalgae biomass can be produced with higher yields.4 HTL is carried out at temperatures from 200 °C to 370 °C and at pressures given by the corresponding water vapour pressure. In this process, in addition to the lipid fraction of microalgae, also part of proteins and carbohydrates are transformed into oily components, resulting in higher overall yields of bio-oil.5 Overall, the highest fractions of biocrude is obtained from the conversion of the lipid fraction, followed by proteins and in less amount by carbohydrates.6 In summary, the products of HTL are bio-oil, a gas phase composed in major part by CO2, a solid residue and an process water (PW), rich in soluble organics.


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 4 of 33

So far, temperatures covering the whole HTL range, reaction times varying from 5 to 120 min and screening of

different microalgae species have been investigated to determine their

suitability and the optimum conditions for HTL.7 Marine species like Phaeodactylum tricornutum, Tetraselmis suecica, Nannochloropsis gaditana, Porphyridium purpureum and Dunaliella tertiolecta and fresh water species like Scenedesmus obliquus, Scenedesmus almeriensis, and Chlorella vulgaris have been reported by López Barreiro.8 The use of heterogeneous catalysts, such as Pd/C, Pt/C, Ru/C, Ni/SiO2-Al2O3, CoMo/γ-Al2O3 and zeolite,9 homogeneous catalysts such as alkali salts (K2CO3 or KOH10, Na2CO311) and organic acids like acetic acid of formic acid11 have been studied to promote the HTL process, showing that all the heterogeneous catalyst tested9 improved the oil yields. A higher bio-oil yield was also obtained whit K2CO3 and KOH, in comparison with both no catalyst10 and organic acids situation. When acetic acid was used, a higher heating value bio-oil was obtained.11 More recently, PW, i.e. the aqueous phase from HTL, has received more attention because its high content of C, N and P12,13 makes it suitable to be used in the cultivation step. However, since it is contaminated and expensive to dispose, it needs additional treatment.14 Components of PW from HTL of Chlorella pyrenoidosa were determined by Chao et. al.13 at different operation conditions. These authors reported that the solid ratio in HTL reactor is the predominant parameter affecting the concentration of nutrients in the PW, and observed higher organic acids concentration at optimized conditions for energy recovery. When using PW a nutrient recycling system, for microalgae cultivation and microbial cultivation,15,16 high dilutions are required. García Alba and co-workers17 found that microalgae can be cultivated in HTL PW, but growth rates and biomass concentrations are reduced


Page 5 of 33

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

considerably when PW is diluted with water only, while the addition of micronutrients allows to reach the growth rates and concentrations similar to those with standard growth media. Another recycling study is the one by Biller et. al. 18, where dilution span from 200X to 600X to achieve microalgae growth. However C. vulgaris was able to grow in HTL PW diluted 50X, under sterile conditions.19 Other authors20,21 proposed a catalytic hydrothermal gasification (CHG) of the PW to exploit the significant amount of carbon it contains, resulting in a gas with considerable fractions of CO2 and ammonia.20 Orfield and co-workers16 report a Life Cycle Assessment (LCA) study comparing two pathways for reuse of PW, i.e. CHG and Escherichia coli cultivation, being this last added to HTL of microalgae. They pointed the importance of the aqueous product reuse, that could reduce bio-oil prices (if microbial biomass increases considerably) and enhance the Energy Return on Energy Investment (EROEI) of the CHG process. Few studies have been carried out aimed to recycle the PW into the HTL process itself. Changjun et. al.22 improved hydrochar production via HTL of Salix psammophila, a desert shrub, by reusing the PW. Zhu et. al.23 reported enhanced bio-oil heating values and yields (from 34.9 to 38.4 wt%) with catalyzed HTL of barley straw, when recirculating the PW three times. However, solid residues were shown to increase as well, in fact more than the bio-oil. Since HTL applied to microalgae seems to be a versatile and promising process, various studies7,8,24 around the world are engaged in this problem both at laboratory level and in pilot plants. True commercial operation has not been reported so far, mainly because the industrial production of algal biofuels via HTL has still open challenges, including scale up, bio-oil upgrading, catalysts


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 33

selection and recovery, and other issues such as the accumulation of certain compounds which may affect the growth of algae25. Also the use of organic solvents to recover the bio-oil affects both bio-oil yields and composition,26 increase bio-oil yields but decreases its quality, extracting also some water-soluble organics.27 In this work the effect of recycling PW from HTL of a low-lipid microalgae (C. vulgaris), to the HTL process itself has been investigated, to exploit the high concentration of organic compounds (from 15 to 43 % of carbon from microalgae biomass remain in the PW21). We have evaluated the potential to add this step to the HTL process to enhance bio-oil productivity, as a function of HTL temperatures as well as of consecutive recycling. In order to understand the bio-oil yield change, experiments with addition of acetic acid (earlier identified as catalyst for HTL process)11 and diluted PW are also presented. The compositions of all products were measured, with special attention to PW and bio-oil along the recycles. In particular, elemental composition and several light and mid-boiling compounds in bio-oil are reported. To our knowledge, this potential step in microalgae process has not been investigated in the open literature yet.

2. Materials and Methods 2.1. Microalgae feedstock The microalgae species C. vulgaris used in this study was purchased from a commercial source, in powder form. The proximate and ultimate analyses for C. vulgaris are reported in Table 1, together with their biochemical composition provided by the supplier.


Page 7 of 33

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

For proximate analysis, C. vulgaris was dried at 105 °C for 24 h to quantify the moisture. After moisture elimination, ash content was determined treating the residue at 550 °C for 5 h. The C, H, and N fractions of the dry ash free (d.a.f.) algae were measured in duplicate by the elemental analyzer Thermo Scientific Flash 2000, CHN-S, while Oxygen was obtained by difference, and the HHV was calculated according to Boie’s formula (see section 2.4).

2.2. Experimental Setup and products recovery The experiments were carried out in an in-house made stainless steel autoclave of 45 mL, using essentially the same setup as used by Garcia Alba et al.7 and López Barreiro et al.8, operated in batch mode and mechanically stirred. The temperature inside the reactor was reached by means of a fluidized sand bath in which the autoclave is submerged. The temperature inside the autoclave was measured by a thermocouple located in the bottom plug. The pressure was measured through a gas connection of the top plug to a pressure transmitter. All pressure and temperature data were monitored for the entire duration of the experiments from outside the reaction room for safety reasons. The reactor was charged with 1.85 g (d.a.f.) of dry microalgae and 15 mL of distilled water for the first run, resulting in a concentration of 11 wt%. In subsequent runs the PW was recycled, adding new (dry) microalgae biomass, around 1.85 g (d.a.f.) in each experiment. After each reactor loading, the autoclave was tightly closed by assembling the mechanical mixer device in the lid of the top. Subsequently, a leakage test was performed, flushing three times Nitrogen at 60 bar, to sweep off the air present inside the reactor and to achieve a start reaction atmosphere composed by only Nitrogen, at around 5.5 bar of pressure. The reaction time was 30 min, excluding the heating time (approx. 7 to 10 min). After


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 8 of 33

this 30 min of holding time, the autoclave was fast quenched in a water bath down to room temperature, for products recovery. Three holding temperatures (220 °C, 240 °C and 265 °C) were applied to evaluate the temperature effect on product yields. The experiment with consecutive recycling of the PW was carried out at 240 °C. After the quench and before disassembly the autoclave, the gas produced was collected and measured by a liquid displacement system, using a saline solution in order to avoid the dissolution of methane and carbon dioxide into the liquid. Gas was sampled using a 50 mL syringe for composition analysis. To reduce the use of non-environmental friendly solvents to recover the products, and to obtain a solvent-free PW, the remained content of the reactor was filtered through a 6µm pore size filter, to separate the oily solids and to recover the PW for further recycle. The procedure for products collection is summarized in Figure 1. The mixture containing both the bio-oil produced and the solids (see section 3.1) was collected and subsequently filtered by a vacuum system made up of a Buchner funnel, a side arm flask, and a pump, using a glass microfiber filter Whatman GF/B, 1 µm pore size. The residue still present in the vessel was washed with 70 mL of dichloromethane (DCM), to wash the filter and ensure that the filter cake did not contain bio-oil, but only solid residues. The DCM was evaporated at 45°C in a Rotavapor BÜCHI R-200 equipped with a cold trap system to recover the solvent, and the bio-oil was measured gravimetrically. To remove the remained solvent, the solids were dried at 105 °C for 24 h and quantified. The water soluble organics of the final PW were measured on 2 mL samples, which were heated in the oven at 100 °C for 24 h so to obtain a mixture of organics and ash. Then, the


Page 9 of 33

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

mixture was treated at 550 °C for 5 h to quantify the ash which was subtracted from the previous organics weight, to calculate the organics d.a.f. In order to maximize product recovery and to close mass balances, the reactor was rinsed with 50 mL of DCM after the last run, the resulting mixture was vacuum filtered (glass microfiber filter, Whatman GF/B, 1 µm pore size), and DCM was evaporated at 45°C in a Rotavapor. The bio-oil was measured gravimetrically and the solids were dried at 105 °C for 24 h to remove the solvent completely.

2.3. Analytical procedures The gas samples were analyzed by a gas chromatograph, (Varian Micro GC CP-4900 compound with analytical columns: Molsieve 5A (10 m) and PPQ (10 m), using helium as a carrier gas. For the identification of several light and mid-boiling compounds, and particularly for the quantification of acetic acid in PW, a Gas Chromatography–Mass Spectrometry (GC-MS) analyzer (GC 7890A MS 5975C – Agilent Technologies) equipped with a capillary column (Agilent HP-% MS, HP19091S- 433) was used. Samples were dissolved in acetone (50 mg PW/gr acetone) and filtered (Whatman 0.2 µm filter). Acetic acid concentrations in g/L were calculated using a calibration curve. To identify the elemental composition of bio-oil and solid residue, in terms of nitrogen, carbon and hydrogen (in wt%) an elemental analyzer (EA, Thermo Scientific Flash 2000) was utilized. Oxygen was calculated by difference. All samples were analyzed in duplicate and the average values were taken.


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 10 of 33

In addition, the molecular weight distribution of the bio-oils was obtained by Gel Permeation Chromatography (GPC) using an Agilent 1200 series HPLC system with 3 GPC PLgel 3 µm MIXED-E columns connected in series. The columns temperature was 40°C, with a flow of 1 ml/min, being tetrahydrofuran (THF) the solvent.

2.4. Definitions and calculations The yields of all products, which include gas, bio-oil, solids and water-soluble organics present in PW were determined by (eq 1), based on the total mass of each product ”” over the mass of microalgae (d.a.f.) loaded into the reactor.

%, =

∗ 100

(1)

The higher heating value (HHV) of microalgae (feedstock), biocrude and residual solids were determined according to Boie’s formula28 = 0.3516 ∗ % + 1.16225 ∗ − 0.1109 ∗ * + 0.0628 ∗ ,

(2)

using C, H, N and O data from elemental analysis. The total energy converted in the HTL process from algae to bio-oil was calculated by

-./01 0/234/01% =

∗ ∗ 56


(3)

Page 11 of 33

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

where the recovery is expressed in %, based on the HHV of oil produced and microalgae d.a.f., as well as on their masses. It should be noticed that the external energy input, required to achieve the temperature in the reaction system is not taken into account.

3. Results and discussion 3.1. Water phase recycle at different temperatures The microalgae species C. vulgaris, used in this research, has a lower oil content: in our case only 6.3 % of oil could be extracted by the soxhlet method.29 Although according to literature data values up to 25 % in this species have been reported, it is well known that the amount of lipids varies depending on the culture conditions and strain. Despite its low lipid productivity, C. vulgaris has great advantages such as a fast growth rate and the ability to grow in non-axenic conditions, such as wastewaters.30 As presented in Figure 2, at 220 °C the bio-oil yield increases from 13.3 % in the first run to 21.2 % in the third run (after 2 PW recycles) while the solid residue trend is not clear. The low content of lipids in this microalgae is not a constraint for the production of biocrude but definitely influences the bio-oil yields.31 The low lipid content makes the overall bio-oil yield somewhat lower than in studies at comparable conditions with other algae.5,7 The product yields as a function of temperature are displayed in Figure 2. The yield of bio-oil increases while the one of solid residue decreases with increasing temperature. Our results match with this tendency, that has been already reported in literature4: at 240 °C of reaction temperature


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 12 of 33

the bio-oil yield was 14.0 wt% in the first run, and increased to more than double (31.5 wt%) after 2 recycles. The same trend was observed at a temperature of 265 °C where the minimum conversion of bio-oil moves from 18 wt% to 35.6 wt% after the second recycle. On the other hand, the solid residue yields does not seem to be affected by the PW recycling, but mainly depend upon process temperature. The gas yields increase, especially after the first recycle, at all the three reaction temperature tested (Figure 2), probably due to decomposition of recycled PW organics. The water-soluble organics were measured at the end of the third run, on the basis of the total mass of algae fed (around 5.5 – 5.6 g d.a.f), and values in mass and wt% are reported in Table 2, where a decrease in organics with increasing temperature can be observed. Upon visual inspection of the products, the solids have a ‘wet’ appearance at the temperature of 220 °C (Figure 3A) as well in the first run at 240 °C (Figure 3B). When increasing the number of recycles, the mass of solids increase and its aspect changed from a true solid to a slurry-type texture, and was finally converted to a free-flowing fluid in the last run at 265°C (Figure 3C). This trend can be observed in Figure 3. The yield of the products (gas, bio-oil, PW and solid residues) shows a typical dependence on the temperature, indicating the main reaction mechanisms (hydrolysis, depolymerisation or repolymerisation) for the different temperature ranges.32 In our PW recycling experiments the gas yield and solid yield are essentially independent of the PW composition used. This suggests that the increase in the obtained oil yield is merely due to the saturation of the PW with organics. The elemental composition of oils and their HHV are reported in Table 2. Here, the amount of N increases with temperature and PW recycles, while the C-content decreases a bit as temperature raises and for each subsequent recycle. This affects also the HHV which declines approximately


Page 13 of 33

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 MJ/kg from run 1 to run 3, at all the three temperatures tested, a result which might be explained by the increased content of nitrogenous compounds, originated from the proteins present in the PW. It is likely that large protein molecules have been broken in previous runs, and repolymerized in the following runs with the new biomass fed. The breakdown of protein and the repolymerization effects on the fragments formed strongly depend on the temperature and residence time.10,33

3.2. Consecutive recycling of process water In order to estimate a maximum productivity of oil, a HTL test with consecutive recycle of the PW was performed, at 240 °C of reaction temperature and with a residence time of 30 min. This temperature was selected because it is a good compromise between the average production of oil and effortless filtration. It was found that the yield of bio-oil increased continuously with the number of water recycles. In Figure 4B the ongoing change in texture of the product after the first filtration can be visually detected, and in Figure 4A the bio-oil yield spans from 14.3 wt% in the first run and reaches a maximum of 42.2 wt% after 6 recycles. The water-soluble organics reached 2.6 g after 7 runs, correspondent to 20.1 % of the total mass of algae fed. The pH of the PW after the first run was 6 and continuously decreased to 4 in the last run. In the molecular weight distribution of bio-oils (more information in supplementary material) it can be seen the decrease of molecular weight of the bio-oil with the recycle number. Specifically, the peak at 1000 g/mol decreases, while the one in the range between 200 g/mol to 300 g/mol increases. Similar trends can be seen also in the bio-oils produced at 200 °C and 265 °C.


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 14 of 33

From the recycle experiments it has been shown that a stable and high yield in bio-oil production can be reached. However, the increasing nitrogen content in the bio-oil composition slightly reduced HHV values (Figure 5), indicating a decrease of the biocrude quality with the recycle number. As observed in previous experiments, the gas production increases gradually, and its composition with special emphasis on CO and CO2 provides an idea of the chemical pathway involved. CO in the first run has a value of 2.23x10-3 gCO/galgae(d.a.f.), which progressively decreases to 4.08x10-4 gCO/galgae(d.a.f.) in run 7, whereas CO2 has a slight increase from 3.39x10-2 gCO2/galgae(d.a.f.) up to 4.5x10- 2 gCO2/galgae(d.a.f.). A general view of the reaction pathway based on our results and according to Peterson et al.,34 suggests that both decarbonylation and decarboxylation reactions occur, whereas decarboxylation is clearly dominating. With PW recycling, decreasing pH and increasing organic acid content, the amount of CO decreased significantly and the CO2 content increases, even more than the reduction in CO. The chemical pathways and interactions between the major components of biomass is far from being completely understood. The compounds in oil and PW are numerous and identified and characterized to a limited extent only.35 The complexity even increases when recycling the components in the PW from a previous HTL experiment. Identification of all compounds and reaction pathways is far beyond the scope of this paper.

3.3. Process water composition The molecular identification of the more dominating organic compounds in PW provides some information about the reaction mechanism. PW composition was analyzed, and light and midboiling compounds were detected. The results of HTL PW resulting from experiments at


Page 15 of 33

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

different temperatures were analyzed after the third run, while for the consecutive recycling experiment the PW was analyzed all along the test. At 240°C the concentration of water-soluble organics in the first run was 0.04 g/mL, after the 3th run 0.11 g/mL and after the 7th run 0.19 g/mL. Although the total concentration of organics increased with recycle number, the production of water-soluble organics reporting to the PW in each single run is reduced (and the oil production increases) with increasing recycle number. The components detected in the HTL PW at different temperatures are reported in Table 3 in terms of area percentage (A/Atot). Although probably not all the compounds could be detected by this procedure, as light substances night have been lost in solvent evaporation, the results give an indication of the overall changes in composition. Along the recycles the PW composition was changing as can be observed in Figure 6, which reports the chromatograms of PW of consecutive recycling experiments. Note that the abundance of components (y-axis) are presented at the same scale, showing the increase of the peaks intensity and area, reflecting the accumulation of certain compounds in the PW. It is important to highlight the high fraction of acetic acid in PW identified as the first peak in Figure 6 (RT 7.840 min). The acetic acid concentration in each recycle was measured, ranging from 2.4 g/L after the first run, to concentrations about 8 g/L in run 7. Process water components reported in this work were obtained using microalgae species C. vulgaris, with high protein content of around 50 wt%. As proteins are the main source of nitrogen compounds35, it is important to highlight that results of this study in terms of oil yield and composition of oil and PW are influenced by the gross biochemical composition of the microalgae used both, with and without recycling of the PW.


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 16 of 33

3.4. Acetic acid influence? As mentioned above, acetic acid is one of the main components of HTL PW. Its catalytic effect to enhance bio-oil yield has been investigated by Ross et al.11 for HTL operation at 300 °C and 350 °C for two microalgae species with different lipid content. Comparing the bio-oil yields using organic acids and alkali catalysts, a slight increase was reported when using acetic acid. As the role of acetic acid as catalyst may depend on the reaction conditions applied, a further study on its effect in the case of process water recycling seems relevant, and results of these experiments are reported in this section. The identification of light and mid-boiling compounds of the PW showed accumulation of acetic acid in the consecutive recycling experiments. To identify whether the increasing acetic acid content influenced the liquefaction process, an experiment was performed using a solution of acetic acid as catalyst. The acid concentration applied was 6 g/L and the tests were carried out at 240 °C with a residence time of 30 min. For further qualitative comparison, a control test at the same conditions but without additional acetic acid was performed. The results show a minimal change of the products yields; a slight increase of only 0.62% in oil yield was detected when acetic acid was used, a value with in the experimental error. Furthermore, also the measured chemical properties of bio-oil were not affected by the use of acetic acid, neither significant changes in elemental composition were identified. Elemental composition and bio-oil yields are reported in Table 4. The molecular weight distribution of the bio-oils recovered is a further point to highlight. Figure 7 shows the diagrams of molecular weight distribution of bio-oils obtained in both reactions, the peak of molecular weight around


Page 17 of 33

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

(~100 g/mol) possible corresponds to the degradation products of the GPC-eluent used (tetra hydro furane) no significant changes can be detected when acetic acid was used. On the other hand, although the change in elemental composition is minimum, it causes a variation in HHV values, from 33.6 MJ/kg when acetic acid is present, to 34.1 MJ/kg when it was not . Li et al.22 also reported an improvement in oil yield using Salix psammophila, containing more than 25 wt% of lignin. The role of acetic acid in these experiments was identified as acid catalysis of the hydrolysis reactions of lignin. In our work, on the contrary, C. vulgaris does not contain (significant amounts of) lignin. Based on our results, the oil yields do not seem to be correlated to acetic acid content with PW recycling, as can be observed in Figure 8. Here, the oil yield and the initial concentration of acetic acid in the PW are represented. The black dots were obtained from the consecutive recycling experiments, where a cumulative production of acetic acid and oil yields with PW recycling has been reported; the squares represent the results of the experiment with addition of acetic acid and its equivalent without acetic acid and, finally, the triangles refer to experiments in which PW was diluted with demiwater (for details, see section 3.5) to perform the test with acetic acid concentrations of 2 and 4 g/l. Pointing attention to the PW composition, it is still possible that the organic components based on their concentration change the solvation properties of the PW. Although it is well known that the properties of water changes with the temperature (e.g., when approaches to its critical temperature of 374 °C the behavior changes from non-polar to polar)34, probably the PW could increasingly accept less organics, altering the mechanism to the formation of oils. Besides, the organic molecules present in PW have molecular structures with short hydrocarbon chains and therefore low molecular weight, a fact which would explain the increasing content in compounds of low molecular weight in the oil with the number of recycles (see supplementary material).


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 18 of 33

3.5. Water phase dilution experiments In order to verify how the concentration of organic components in PW affects its properties , and to identify its direct influence on the bio-oil yields, two HTL experiments were conducted at 240 °C with residence time and algae fed concentration as in previous tests. In these experiments PW from run 7 from the consecutive recycle experiment was used; in a first run it was diluted 1:2, while in a second one 1:4, using distilled water in both cases. The biocrude yields and their elemental compositions are presented in Table 5. The highest biooil yield was obtained in the test D-1:2, where the initial concentration of organics was 0.10 gorg/gPW (for D-1:4 it was 0.05 gorg/gPW, the net weight percentages of organics in PW were 26.5 and 33.1, for D-1:2 and D-1:4 respectively). The data presented in Table 5 are in agreement with the consecutive recycling experiments, in which an increment in the total concentration of soluble organics was observed, but with stead decreases (on algae basis) in each single run along the recycles. It was mentioned (section 3.3) that there is accumulation of small organic compounds upon recycling and some of the major were identified (Table 3). This accumulation will however level off, with a kind of steady state. This implies that the PW gets ‘saturated’ with the smaller organic compounds and hence more bio-oil will be recovered in the organic phase when starting with an PW that already contains a significant amount of organics. However, when the PW is more concentrated in organics, these compounds may also show more tendency to polymerize to higher molecular weight compounds reporting directly to the oil phase. This hypothesis could be supported by the N in bio-oil, which is proportional to the oil yields. On the other hand no substantial change was detected in HHV.


Page 19 of 33

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

3.6. Energy recovery ratios The maximum energy recovery was 68%, obtained in the 7th run at 240°C, and this value corresponds to the highest oil yield. Next to the energy recovery into oil, Table 6 and Table 2 also reports the molar O/C and H/C ratios of the bio-oil, in both the series of experiments, at three different temperatures and with increasing water recycle number. As shown in Table 6 and Table 2, the O/C and H/C ratios are close to that of crude oil. In addition a reduction in O/C and H/C ratios of bio-oil with increasing temperature was observed, and when increasing the number of PW recycling. The reduction of these ratios is due to the enhanced content of aromatic compounds. For the 265 °C bio-oil, these ratios do not seem to follow this trend any longer, while N still increases significantly and C slightly decreases. This is probably related to changes in nature and composition of the organic compounds in the oils.

4. Conclusions In this work, process water from hydrothermal liquefaction (HTL) of the low-lipid microalgae C. vulgaris was recycled into the HTL reactor. The solid and bio-oil phases produced were collected together, without using additional solvents. A large increase in oil yields was observed when recycling the process water, at all the temperatures tested. By consecutive recirculation a stationary phase yield was reached, after approximate six recycles. The maximum bio-oil productivity for above algae was 42.2 wt% at 240°C and 30 minutes process time for each run.


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 20 of 33

This yield corresponded also to the highest energy recovery ratio, which was 68 % of the HHV of the algae fed to the system. The increase in bio-oil yield is probably due to saturation of the process water with organics and to the repolymerization of nitrogen-rich organic compounds as shown by the observed corresponding increase of N content in the bio-oil. It was also shown that the concentration of acetic acid has no influence on the bio-oil yield. The possible influence of acetic acid on oil yield was investigated in this work. However it was not possible to confirm any relationship between the bio-oil yields obtained and the acetic acid concentration. Process water recycling is therefore here proposed as a novel step in bio-oil production from microalgae by HTL, aimed to increase bio-oil production at lower temperatures and to reduce the process energy costs.

Acknowledgements E.A. Ramos Tercero acknowledges Mexico's National Council of Science and Technology (CONACyT) for the Ph.D. scholarship. The authors would like to thank the technical staff of the SPT group and HDL (Benno Knaken and Johan Agterhorst) for their excellent technical support and to Stijn Oudenhoven for the GCMS analysis of oils.


Page 21 of 33

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

References (1)

Brennan, L.; Owende, P. Renew. Sustain. Energy Rev. 2010, 14, 557–577.

(2)

Ramos Tercero, E. A.; Domenicali, G.; Bertucco, A. Energy 2014, 76, 807–815.

(3)

Ramos Tercero, E. A.; Sforza, E.; Bertucco, A. Energy 2013, 60, 373–379.

(4)

López Barreiro, D.; Prins, W.; Ronsse, F.; Brilman, D. W. F. (Wim). Biomass and Bioenergy 2013, 53, 113–127.

(5)

Valdez, P. J.; Tocco, V. J.; Savage, P. E. Bioresour. Technol. 2014, 163, 123–127.

(6)

Biller, P.; Ross, A. B. Bioresour. Technol. 2011, 102, 215–225.

(7)

Garcia Alba, L.; Torri, C.; Samorì, C.; van der Spek, J.; Fabbri, D.; Kersten, S. R. A.; Brilman, D. W. F. (Wim). Energy & Fuels 2012, 26, 642–657.

(8)

López Barreiro, D.; Zamalloa, C.; Boon, N.; Vyverman, W.; Ronsse, F.; Brilman, D. W. F. (Wim); Prins, W. Bioresour. Technol. 2013, 146, 463–471.

(9)

Duan, P.; Savage, P. E. Ind. Eng. Chem. Res. 2011, 50, 52–61.

(10)

Singh, R.; Balagurumurthy, B.; Prakash, A.; Bhaskar, T. Bioresour. Technol. 2014.

(11)

Ross, A. B.; Biller, P.; Kubacki, M. L.; Li, H.; Lea-Langton, A.; Jones, J. M. Fuel 2010, 89, 2234–2243.

(12)

Sudasinghe, N.; Dungan, B.; Lammers, P.; Albrecht, K.; Elliott, D.; Hallen, R.; Schaub, T. Fuel 2014, 119, 47–56.

(13)

Gai, C.; Zhang, Y.; Chen, W.-T.; Zhou, Y.; Schideman, L.; Zhang, P.; Tommaso, G.; Kuo, C.-T.; Dong, Y. Bioresour. Technol. 2014.

(14)

Zhu, Y.; Albrecht, K. O.; Elliott, D. C.; Hallen, R. T.; Jones, S. B. Algal Res. 2013, 2, 455–464.

(15)

Nelson, M.; Zhu, L.; Thiel, A.; Wu, Y.; Guan, M.; Minty, J.; Wang, H. Y.; Lin, X. N. Bioresour. Technol. 2013, 136, 522–528.

(16)

Orfield, N. D.; Fang, A. J.; Valdez, P. J.; Nelson, M. C.; Savage, P. E.; Lin, X. N.; Keoleian, G. A. ACS Sustain. Chem. Eng. 2014, 2, 867–874.

(17)

Garcia Alba, L.; Torri, C.; Fabbri, D.; Kersten, S. R. A.; (Wim) Brilman, D. W. F. Chem. Eng. J. 2013, 228, 214–223.


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 22 of 33

(18)

Biller, P.; Ross, A. B.; Skill, S. C.; Lea-Langton, A.; Balasundaram, B.; Hall, C.; Riley, R.; Llewellyn, C. A. Algal Res. 2012, 1, 70–76.

(19)

Du, Z.; Hu, B.; Shi, A.; Ma, X.; Cheng, Y.; Chen, P.; Liu, Y.; Lin, X.; Ruan, R. Bioresour. Technol. 2012, 126, 354–357.

(20)

Davis, R. E.; Fishman, D. B.; Frank, E. D.; Johnson, M. C.; Jones, S. B.; Kinchin, C. M.; Skaggs, R. L.; Venteris, E. R.; Wigmosta, M. S. Environ. Sci. Technol. 2014, 48, 6035– 6042.

(21)

Elliott, D. C.; Hart, T. R.; Schmidt, A. J.; Neuenschwander, G. G.; Rotness, L. J.; Olarte, M. V.; Zacher, A. H.; Albrecht, K. O.; Hallen, R. T.; Holladay, J. E. Algal Res. 2013, 2, 445–454.

(22)

Li, C.; Yang, X.; Zhang, Z.; Zhou, D.; Zhang, L. Bioresources 2013, 8, 2981–2997.

(23)

Zhu, Z.; Rosendahl, L.; Toor, S. S.; Yu, D.; Chen, G. Appl. Energy 2015, 137, 183–192.

(24)

Jazrawi, C.; Biller, P.; Ross, A. B.; Montoya, A.; Maschmeyer, T.; Haynes, B. S. Algal Res. 2013, 2, 268–277.

(25)

Tian, C.; Li, B.; Liu, Z.; Zhang, Y.; Lu, H. Renew. Sustain. Energy Rev. 2014, 38, 933– 950.

(26)

Valdez, P. J.; Dickinson, J. G.; Savage, P. E. Energy & Fuels 2011, 25, 3235–3243.

(27)

Xu, D.; Savage, P. E. Algal Res. 2014, 6, 1–7.

(28)

Boie, W. Energietechnik 1953, 3, 309–316.

(29)

Bligh, E. G.; Dyer, W. J. Can. J. Biochem. Physiol. 1959, 37, 911–917.

(30)

Cabanelas, I. T. D.; Ruiz, J.; Arbib, Z.; Chinalia, F. A.; Garrido-Pérez, C.; Rogalla, F.; Nascimento, I. A.; Perales, J. A. Bioresour. Technol. 2013, 131, 429–436.

(31)

Li, H.; Liu, Z.; Zhang, Y.; Li, B.; Lu, H.; Duan, N.; Liu, M.; Zhu, Z.; Si, B. Bioresour. Technol. 2014, 154, 322–329.

(32)

Yuan, X.; Tong, J.; Zeng, G.; Li, H.; Xie, W. Energy & Fuels 2009, 3262–3267.

(33)

Brand, S.; Hardi, F.; Kim, J.; Suh, D. J. Energy 2014, 68, 420–427.

(34)

Peterson, A. A.; Vogel, F.; Lachance, R. P.; Fröling, M.; Antal, Jr., M. J.; Tester, J. W. Energy Environ. Sci. 2008, 1, 32.


Page 23 of 33

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

(35)

Torri, C.; Garcia Alba, L.; Samorì, C.; Fabbri, D.; Brilman, D. W. F. (Wim). Energy & Fuels 2012, 26, 658–671.

Table 1. Proximate, ultimate and biochemical composition of C.vulgaris used in this work. Microalgae specie Elemental Composition

Lipids (wt%) Carbohydrates (wt%) Proteins (wt%) Ash (wt%)

Chorella vulgaris C (wt%) 46.7 H (wt%) 6.8 N (wt%) 7.4 O* (wt%) 39.1 6.3 33.5 49.5 7.2

Biochemical composition (lipids, carbohydrates and proteins) was provided by the supplier. Oxygen was calculated by difference.

Table 2. Comparison of bio-oil composition, heating values and total water soluble organics at different temperatures. Energy N C H O* HHV molar molar recovery (wt%) (wt%) (wt%) (wt%) (MJ/kg) O/C H/C (%) R1 R2 R3

3.4 4.3 5.7

69.5 69.6 68.6

9.5 9.5 9.0

17.6 16.6 16.7

R1 R2 R3

4.7 6.0 6.9

71.5 69.8 69.5

9.3 8.8 8.7

14.5 15.4 14.9

220 °C 33.8 0.19 33.9 0.18 33.1 0.18 240 °C 34.6 0.15 33.5 0.17 33.3 0.16

13.3 14.0 14.2 0.11.5

265 °C 34.0 0.14 33.4 0.15 33.1 0.15 0.0142 0.04

R1 R2 R3 Crude oil

5.9 6.9 7.4

72.2 70.6 69.8

8.6 8.5 8.6

0.1-2

83-87

10-14

Watersoluble Organics

1.64 1.63 1.58

20 26 33

1.8 g (33%)

1.55 1.52 1.50

24 40 50

1.6 g (29%)

1.44 1.44 1.48

31 48 58 -

1.2 g (21%) -

1.69

* calculated by difference. Water-soluble organics measured after the 3th run, the percentages are in basis of the total algae fed.


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 24 of 33

Table 3. Area percent of compounds identified by GC-MS from second recycle of PW at different temperatures.

R.T.

5.632 7.840 12.340 13.497 17.245 17.702 17.952 30.972 33.219 33.459 63.549 63.771 65.056

Peak area x 10-5/g PW

Compound

2-Butanone Acetic acid 4-methyl-3-Penten-2-one Pyrazine, methyl2-Pentanone, 4-hydroxy-4-methylPyrazine, 2,5-dimethyl1H-Imidazole, 1-methyl-2-vinyl2,2,6,6-Tetramethyl-4-piperidone 2,5-Pyrrolidinedione, 1-methyl4-Piperidinone, 2,2,6,6-tetramethyldl-Alanyl-l-leucine 2-Nonene, 3-methyl-, (E)Pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-

65.8847 2,4(1H,3H)-Pyrimidinedione, dihydro66.230 Phenol, 3,5-dimethoxy5,10-Diethoxy-2,3,7,8-tetrahydro-1H,6H-dipyrrolo[1,2-a;1',2'68.249 d]pyrazine 69.289 Pyrrolo[1,2-a]pyrazine-1,4-dione, hexahydro-3-(2-methylpropyl)-

265°C

240°C

220°C

2.4 13.9 24.7 11.0 23.9 8.0 2.2 13.1 4.0 8.7 6.9 3.7 1.7 4.3 6.0

14.2 7.0 10.2 12.1 7.1 22.9 8.0 5.9 13.6 9.2

15.3 1.4 0.4 0.7

2.9 4.5

8.4

-

Table 4. Yields, composition and heating values of bio-oil produced in: ACE = with initial addition of acetic acid and NAC = control test without additional acetic acid. ID ACE NAC

Bio-oil yield (wt%) 15.51 14.89

N (wt%)

C (wt%)

H (wt%)

O (wt%)

HHV (MJ/kg)

5.2 5.2

70.3 70.8

8.9 9.1

15.6 14.9

33.6 34.1

Table 5. Comparison of bio-oil yield, composition, heating values and energy recovery at different dilution of water soluble organics in initial water medium. Dilution

Bio-oil yield (wt%)

N (wt%)

C (wt%)

H (wt%)

O (wt%)


Energy HHV recovery (MJ/kg) %

Page 25 of 33

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

20.8 25.4

1:4 1:2

7.0 7.3

69.5 69.0

8.6 8.7

14.9 15.0

33.1 33.2

34 41

Table 6. HTL bio-oil elemental composition, O/C and H/C ratios and energy recovery along consecutive PW recycles at 240 °C.

R1 R2 R3 R4 R5 R6 R7

N (wt%)

C (wt%)

H (wt%)

O (wt%)

HHV (MJ/kg)

Molar O/C

Molar H/C

4.7 6.0 7.1 7.7 7.8 8.4 8.9

71.7 69.5 70.5 69.5 67.5 67.5 67.3

9.4 8.8 8.9 8.7 8.4 8.5 8.4

14.2 15.7 13.5 14.1 16.3 15.6 15.4

34.8 33.4 34.1 33.4 32.3 32.2 32.3

0.15 0.17 0.14 0.15 0.18 0.17 0.17

1.57 1.53 1.51 1.50 1.50 1.52 1.50


Energy recovery (%) 25 40 49 59 61 67 68

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

Procedure scheme for product collection. Distilled water was added only in the first run, for the successive rounds process water was used instead. 125x87mm (300 x 300 DPI)


Page 26 of 33

Page 27 of 33

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

HTL Product yields along runs (R1 = run1, R2 = run2 and R3 = run3) in terms of A) bio-oil, B) solids residue and C) gas at different temperatures 220°C, 240°C and 265°C. 260x70mm (300 x 300 DPI)


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

Oily solids after 6µm filtration from recycles (R1 = run1, R2 = run2 and R3 = run3) at different temperatures. A) 220°C (the first vessel correspond to water phase after 3th run), B) 240°C and C) 265°C. 195x48mm (300 x 300 DPI)


Page 28 of 33

Page 29 of 33

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

A) Gas, bio-oil and solid residue yields. B) Oily solids along the consecutive process water recycling at 240°C. 158x152mm (300 x 300 DPI)


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

Yields, nitrogen content and HHV (MJ/kg) of bio-oils at along PW recycle at 240 °C. 288x201mm (300 x 300 DPI)


Page 30 of 33

Page 31 of 33

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

Chromatograms of process water from consecutive recycling test. The abundance of components (y-axis) are presented with the same scale 205x277mm (300 x 300 DPI)


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

Molecular weight distribution by GPC analysis of oil obtained at 240°C with 30 min reaction time. ACE = Acetic acid initial concentration 6.0 g/L, NAC without acetic acid. 126x69mm (300 x 300 DPI)


Page 32 of 33

Page 33 of 33

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

Bio-oil yields as function of initial acetic acid concentration in reaction medium. Gray squares represents the results of the acetic acid experiment, triangles are from dilution of PW experiments and circles are the continuous recycle experiment results. All the results were obtained at 240°C with 30 min reaction time. 288x201mm (300 x 300 DPI)


Process Water Recycle in Hydrothermal Liquefaction of Microalgae To

Recommend Documents