Article pubs.acs.org/EF
Hydrothermal Treatment of Protein, Polysaccharide, and Lipids Alone and in Mixtures Gule Teri, Ligang Luo,† and Phillip E. Savage*,‡ Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States ABSTRACT: We subjected a set of model compounds (cornstarch and cellulose as model polysaccharides, soy protein and albumin as model proteins, sunflower oil and castor oil as model lipids) to the processing conditions and product recovery protocol commonly used for hydrothermal liquefaction (HTL) of algal biomass to make crude bio-oil. The model compounds were treated individually and in mixtures at 300 and 350 °C for batch holding time ranging from 10 min to 90 min. The model lipids produced the highest yield (>90 wt %) of biocrude (operationally defined as material soluble in dichloromethane), followed by the model proteins (∼30−35 wt %) and then the model polysaccharides (∼10−15 wt %). The production of biocrude at 350 °C occurred fully within the first 10 min of treatment, and the biocrude yield changed very little at longer times. Liquefaction at 350 °C and 60 min nearly doubled the biocrude yields from polysaccharides, relative to those obtained at 300 °C and 20 min. Otherwise, the yields from the different model compounds at the milder and the more-severe conditions were comparable. In most instances, the biocrude yield from hydrothermal treatment of mixtures was very similar to the mass-averaged yield calculated from the individual compound results. The chief exceptions were binary combinations of polysaccharide and protein under the more-severe conditions. For these mixtures, the biocrude yield exceeded the mass-average yield calculated from the pure compound results, thereby providing evidence that interactions influencing the biocrude yield can occur during hydrothermal treatment of mixtures of the biomolecules. Even so, a quantitative model built on the assumption that the lipids, polysaccharides, and proteins react independently during HTL predicted biocrude yields for ternary mixtures more accurately than did a model with three additional parameters that allowed for the possibility of interactions between the different model compounds.
1. INTRODUCTION Biomass can be converted by several processes to different solid, liquid, or gaseous products that can then be used as or further converted to biofuels. Algae are a promising biomass feedstock for many reasons. For example, they can be grown to have a high oil content and nonarable land can be used for their cultivation. The main biochemical components of algae are polysaccharides, proteins, and lipids. Traditional production of biofuel from algae requires biomass drying and then extraction of the lipids. These steps require time and energy, and they add to the processing cost.1 Hydrothermal liquefaction (HTL), on the other hand, is a more effective method to produce biocrude from a wet biomass feedstock. The process uses elevated temperature (e.g., 280− 370 °C) and pressure (e.g., 10−25 MPa), and sometimes a catalyst to improve biocrude yield.2,3 Under these conditions, water is in a subcritical liquid phase that has enhanced solubility for organic compounds, and the water molecules can act as both a reactant and catalyst.4,5 Hydrothermal liquefaction can reduce the energy demand (relative to drying) and generate biofuel intermediates from the polysaccharide, protein, and lipid components of the algal biomass. During HTL, the biomass macromolecules decompose into smaller molecules that, upon cooling, partition into the biocrude or the aqueous, gas, or solid phases. Many of these smaller molecules are reactive under HTL conditions, and some can oligomerize to form compounds with a wide range of molecular-weight distributions.6 There is a rich amount of literature on the reactions of proteins, polysaccharides, and lipids in water at elevated © XXXX American Chemical Society
temperatures and pressures (see, for example, refs 7−10). These prior studies have examined the reaction products and afforded reaction networks and kinetics for different compounds. Though providing important information about the chemistry, this prior work does not provide information about how much lipid, polysaccharide, or protein gets converted to the biocrude, gas, aqueous-phase, and solid-phase product fractions during HTL. This information would be very useful for the development of process engineering models for algae HTL. We are aware of only two prior studies that subjected individual biomacromolecular algae model compounds to both conventional HTL conditions and the subsequent product work-up procedure.11,12 Biller and co-workers11,12 reported biocrude yields from HTL of several types of model compounds at 350 °C, 200 bar for 1 h. They used albumin and soya protein as model proteins, starch as a model polysaccharide, and sunflower oil as a model lipid. The biocrude yields ranked as follows: lipids > proteins > carbohydrates. However, this work is limited, in that it considered just a single HTL condition, a single model polysaccharide and model lipid, and no binary mixtures. The limited prior work on the HTL of model biomacromolecules that are relevant for algal biomass, and the absence of work dealing with their binary mixtures motivated the research Received: August 6, 2014 Revised: November 19, 2014
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dx.doi.org/10.1021/ef501760d | Energy Fuels XXXX, XXX, XXX−XXX
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Figure 1. Temporal variation of yield of solids (wt %) from hydrothermal treatment of model compounds at 350 °C. were taken out of the sand bath and placed in a water bath to cool to room temperature. 2.3. Gas-Phase Analysis. The gas phase was analyzed with a gas chromatography (GC) device equipped with a thermal conductivity detector as described previously.3 The yield of each gas species was calculated as the number of moles of the species divided by the mass of the model compounds loaded into the reactor. 2.4. Biocrude Recovery and Analysis. After analyzing the gas phase, we opened the cool reactors and added aliquots of dichloromethane to wash out the reactor. No additional water was added. The reactor material was collected, vortexed, and then centrifuged at 500 rcf for 1 min. This treatment separated the organic, aqueous, and solid phases. We transferred the dichloromethane phase (biocrude-containing phase) into a separate glass tube, and the remaining aqueous and solid phases were again vortexed and centrifuged at 1500 rcf for 3 min to better separate those two phases. We then transferred the aqueous phase into a preweighed plastic centrifuge tube and determined its mass. The dichloromethane and solid phases were dried over flowing nitrogen. After 90 min, the dichloromethane had evaporated completely, and the mass of biocrude was recorded. We calculate the product yields as the mass of a product fraction recovered divided by the mass of the model compound(s) loaded into the reactor. The biocrude-phase products were identified using a mass spectrometer (MS) detector with an Agilent Model 6890 GC gas chromatograph that was equipped with a Model HP-5MS capillary column (50 m × 0.2 mm × 0.33 μm). The injection port temperature was 310 °C. The temperature program consisted of a 4 min soak at 50 °C, followed by ramps to 110 °C at 2 °C min−1, to 160 °C at 10 °C min−1, to 300 °C at 25 °C min−1, and a final hold of 3 min. The C, H, O, N, and S content of the model compounds and the biocrudes were determined by Atlantic Microlab, Inc. We also fractionated the biocrude into its light (hexane-soluble) and heavy (hexane-insoluble, dichloromethane-soluble) components. We added 4 mL of hexane to the biocrude and separated it into two different phases using a centrifuge at 1500 rcf for 3 min. The hexane solubles were recovered, the hexane evaporated, and the mass of light biocrude then determined. Each experiment was done in duplicate or triplicate. We report mean values and use the standard deviation as the reported experimental uncertainty.
reported herein. We report on the HTL of six different model compounds alone and in binary and ternary mixtures. We also report on the influence of the HTL conditions (time and temperature) and potential interactions between the different components on the biocrude yields. These results provide new insight into how the various main biochemical components of the microalgae cell get converted into the different product fractions that are typically isolated from HTL of algal biomass.
2. EXPERIMENTAL SECTION This section describes all materials used in this research along with the procedures used for the hydrothermal treatment experiments. 2.1. Materials. Albumin (egg white protein), soy protein, cornstarch, sunflower oil, and castor oil were obtained in high purity from local stores as food and nutrition supplements. Nannochloropsis sp. (57% protein, 8% polysaccharide, 32% lipid, 3% others) was purchased from Reed Mariculture, Inc., as a slurry (32.5% solids). Cellulose in high purity (99.31%), dichloromethane (>99%), and hexane (>99%) were all obtained from Fisher Scientific and used as received. Distilled and deionized water, prepared in house, was used throughout the experiments. We carried out most experiments in SS316 stainless-steel batch reactors assembled from 1/2-in. Swagelok tube fittings (one port connector and two caps). Each reactor had an internal volume of ∼4.1 mL. Experiments designated for gas analysis were conducted in similar reactors equipped with a union, an 8-in. length of 1/4-in. outer-diameter (OD) stainless-steel tubing, and a high-pressure shut-off valve. 2.2. Procedure. Reactors were seasoned prior to first use by being filled with water and placed in a fluidized sand bath at 350 °C for an hour. This treatment removed any residual organic material that might have been on the reactor walls. Upon cooling, the interior of each reactor was rinsed thoroughly with dichloromethane. Conditioned and cleaned reactors were loaded with 0.416 g of model compound(s) and 2.357 g of water for reactions at 300 °C, and with 0.3356 g of model compound(s) and 1.902 g of water for reactions at 350 °C. In both cases, the model compounds contribute 15 wt % to the total mass in the reactor, and the liquid phase expands to fill 95% of the total reactor volume under reaction conditions. When performing experiments that included gas analysis, we added 10 bar of helium to the reactor after loading it with the model compound and water. This helium later served as a standard to quantify gas yields. The loaded reactors were sealed and placed in a preheated, fluidized sand bath (Techne, Model SBL-2D) and maintained at 300 or 350 °C using a temperature controller (Techne, Model TC-8D). The reactors reached the setpoint temperature within 2−3 min. According to the steam tables, the pressure due to saturated water is 86 and 165 bar at 300 and 350 °C, respectively. When reaching the desired holding time, the reactors
3. RESULTS AND DISCUSSION The product fractions from each experiment included gases, water-soluble products, dichloromethane-soluble products (biocrude), and solid products insoluble in both water and dichloromethane. In this article, we focus on the biocrude yield, B
dx.doi.org/10.1021/ef501760d | Energy Fuels XXXX, XXX, XXX−XXX
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Figure 2. Temporal variation of yield of biocrude (wt %) from hydrothermal treatment of model compounds at 350 °C.
Figure 3. Biocrude yields (wt %) from hydrothermal treatment of different model compounds under two different conditions.
compounds. The biocrude yield from sunflower oil is >90% at all times investigated. The sunflower oil is fully soluble in dichloromethane in the absence of any hydrothermal treatment. In contrast, neither soy protein nor cornstarch is soluble in dichloromethane, but after hydrothermal treatment for 10 min, the protein produces a biocrude yield of ∼30 wt % and the cornstarch produces a yield of ∼15 wt %. These initial yields are largely time-invariant, which again suggests that the key HTL reactions are complete within the first 10 min at this temperature. The biocrude yields, being in the order of lipids > protein > carbohydrate, are consistent with the result given by Biller and Ross11,12 for the HTL of model compounds at 350 °C and 60 min. However, the biocrude yields reported here are higher than those of Biller and Ross for all three types of model compounds. This difference in the quantitative results is likely due to differences in the experimental procedures employed (e.g., reactor heating rates, model compound loading, solvent amounts). Figures 1 and 2 show that the batch holding time at 350 °C has little influence on the yield of solids or biocrude. To better determine the influence of process conditions on the product yields, we conducted a set of experiments under much milder hydrothermal conditions (300 °C, 20 min). Figure 3 compares biocrude yields from hydrothermal treatment at 300 °C and 20 min with those at 350 °C and 60 min. We also expand the model compounds in view to consider a second polysaccharide (cellulose), a second protein (albumin), and a second lipid (castor oil).
since it is the product fraction of primary interest. However, we do present some data for the other fractions in a few occasions. This section first discusses the reaction of each model compound alone and then the reactions of binary and ternary mixtures. We compare hydrothermal treatment under mild conditions (300 °C, 20 min) with that under more-severe conditions in regard to both temperature and time (350 °C, 60 min), investigate the influence of batch holding time on product yields at 350 °C, and determine the influence of interactions from different compounds on the biocrude yields. 3.1. Model Compounds Alone. Figure 1 shows the temporal variations of the yields (in terms of wt %) of solid products from hydrothermal treatment of cornstarch, soy protein, and sunflower oil at 350 °C. Sunflower oil generates no solids at all, and the yields from soy protein were typically ∼4 wt % after 30 min. The measured yields from cornstarch are typically higher than those from the other materials, but the true yield of solids from cornstarch was even higher than the values shown. The solid particles formed from cornstarch adhered to the reactor wall and were difficult to recover fully. Based on the masses of aqueous phase, biocrude, and gas recovered, we estimate that the solids yield from cornstarch could be as high as 30 wt %. Figure 1 also shows that, other than the data for soy protein at 10 and 20 min, the reaction time does not have any significant influence on the yield of solids. This result means that, for cornstarch, the solid-forming reactions are essentially complete within 10 min at 350 °C. Figure 2 shows the biocrude yields from hydrothermal treatment of the lipid, protein, and polysaccharide model C
dx.doi.org/10.1021/ef501760d | Energy Fuels XXXX, XXX, XXX−XXX
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Figure 3 shows that both proteins and the sunflower oil produce essentially the same biocrude yields under both the milder conditions and the more-severe conditions. In addition, both the animal and the vegetable protein give almost the same biocrude yields under both experimental conditions. In contrast, the yields of biocrude from cornstarch and cellulose are nearly twice as high under the more-severe conditions than under the milder conditions. Under given conditions, however, both polysaccharides produce essentially the same biocrude yields. This result, along with the similarity in yields from the two proteins, suggests that the structural differences between different proteins or different polysaccharides do not lead to significantly different biocrude yields when these materials react individually. Table 1 gives the elemental composition of the model compounds, an alga,3 and the biocrudes produced from each at
Figure 4. Van Krevelen plot for model compounds and biocrudes from HTL at 350 °C for 60 min.
Table 1. Elemental Composition and Higher Heating Value (HHV) of Model Compounds and Biocrudes (350 °C, 60 min)
from 0.10 to 0.97, these ratios in the biocrude range only from 0.08 to 0.15. This similarity in the ultimate values for the O/C ratios suggests that there might be some common end point for deoxygenation of proteins, polysaccharides, and lipids by HTL. Since the deoxygenation was always accompanied by a reduction in the H/C ratios, the model compounds that experienced the greatest deoxygenation (i.e., polysaccharides) also experienced the greatest reduction in the H/C ratio. Figure 4 also shows data for HTL of a high-protein microalga for comparison. As with the model compounds, HTL of the alga reduced the O/C ratio to a value of ∼0.1. Figure 5 shows the relative amounts of light and heavy biocrude in each sample. Biocrude from lipids has the highest light proportion, and biocrude from carbohydrates has the highest heavy proportion. These results can be rationalized in terms of the results shown in Figure 1 for solids formation, if the same types of reactions form both solids and heavy biocrude. The carbohydrates produced the highest solids (DCM-insoluble and water-insoluble) yield, so one might expect that they would also produce the highest heavy biocrude (DCM-soluble, hexane-insoluble) proportion. In all of the experiments at 350 °C and 60 min, we recovered at least 90% of the total mass initially loaded into the reactors. The recovered aqueous phase accounts for roughly 85% of the total mass recovered, which corresponds to the initial water loading being 85% of the total mass. To show how the mass was distributed among the nonwatersoluble materials post-reaction, we offer Figure 6. Soy protein, albumin, and sunflower oil yield
