Products and Kinetics for Isothermal Hydrothermal Liquefaction of Soy

Mar 29, 2016 - Soy protein concentrate was hydrothermally treated at isothermal temperatures of 200, 250, 300, and 350 °C for times up to 60 min to p...
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Research Article pubs.acs.org/journal/ascecg

Products and Kinetics for Isothermal Hydrothermal Liquefaction of Soy Protein Concentrate Ligang Luo,†,‡ James D. Sheehan,§ Liyi Dai,‡ and Phillip E. Savage*,†,§ †

Chemical Engineering Department, University of Michigan, Ann Arbor, Michigan 48109-2136, United States Shanghai Key Laboratory of Green Chemistry and Green Process, Department of Chemistry, East China Normal University, No. 500 Dongchuan Road, Shanghai 200241, People’s Republic of China § Chemical Engineering Department, Pennsylvania State University, 160 Fenske Lab, University Park, Pennsylvania 16802, United States ‡

ABSTRACT: Soy protein concentrate was hydrothermally treated at isothermal temperatures of 200, 250, 300, and 350 °C for times up to 60 min to produce a crude bio-oil. Additional product fractions included water-soluble products, gases, and residual solids. We report herein the conversion of protein and gravimetric yields of the different product fractions. The biocrude yield generally increased with both time and temperature as did the yield of gaseous products. The highest biocrude yield was 34%, produced from liquefaction at 350 °C for 60 min. Chemical and physical characterization of the biocrude revealed how its composition and boiling point range changed with reaction time. Finally, we report a reaction network and the parameters for a phenomenological kinetics model that captures the influence of time and temperature on the yields of gas, solid, biocrude, and aqueous-phase products from isothermal hydrothermal liquefaction (HTL) of soy protein concentrate. The reaction network comprised a sole primary path, which converted soy protein concentrate to aqueous-phase products. Secondary reactions of these water-soluble compounds produced biocrude and gases. There was no direct path to biocrude formation from the biomass feedstock. KEYWORDS: Hydrothermal liquefaction, Kinetic, Model, Reaction network, Protein

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INTRODUCTION Combustion of fossil fuels emits ancient carbon into the atmosphere and increases CO2 levels. Combustion of fuels derived from renewable biomass feedstocks, on the other hand, simply recycles atmospheric CO2. Biomass in its natural state has a high moisture content. Processes that can convert wet biomass into a biofuel intermediate can eliminate the energy input otherwise needed for biomass drying. Hydrothermal liquefaction (HTL) is one such process. It converts wet biomass into biocrude via reactions in and with liquid water at elevated temperatures (∼300 °C) and pressures. HTL has been applied to aquatic biomass, terrestrial lignocellulosic biomass, food waste, bacteria and yeast, manure, and sludges.1−6 The biocrudes produced can have a higher heating value of ∼35 MJ/kg, a carbon content of ∼75 wt %, and an abundance of heteroatoms such as oxygen (∼10 wt %) and nitrogen (∼5 wt %). HTL experiments with biomass feedstocks provide important information about the influence of process variables on product yields and quality. The biochemical complexity and diversity of these feedstocks, however, militate against elucidating the HTL reactions of any particular biochemical component when one works with actual feedstocks. Investigations with biomass model compounds, on the other hand, can provide useful © XXXX American Chemical Society

insight into the reaction products, pathways, kinetics, and mechanisms that occur during HTL. The major components of the renewable feedstocks treated by HTL include polysaccharides, proteins, lipids, and lignin. The reactions of each of these materials in and with hot compressed water have received attention previously (e.g., refs 3, 7, and 8). This earlier work focused on molecular reaction products (e.g., amino acid production from protein hydrolysis) and reaction networks but not on results such as biocrude yields, which would be more directly applicable to HTL of biomass. Indeed, prior studies with these model compounds that used the same product recovery protocols used for HTL of actual biomass feedstocks are much more limited in number, and we discuss each below. Biller and co-workers9,10 examined the effects of catalysts on the HTL of model lipid, protein, and carbohydrate components. Teri et al.11 worked with model triglycerides, proteins, and polysaccharides to elucidate the effects of HTL processing conditions on biocrude yields from these materials. Luo et al.12 examined the effect of different catalysts on biocrude produced from HTL of soy protein Received: February 1, 2016 Revised: March 8, 2016

A

DOI: 10.1021/acssuschemeng.6b00226 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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using HACH kits as described previously.15 1H NMR and 13C NMR spectra (Varian, Inova 400 NMR spectrometer) provided information about the C and H-containing functional groups in the biocrudes. FTIR analysis of a thin film of sample was carried out at ambient temperature on a PerkinElmer Spectrum BX instrument. Thermogravimetric analysis (TGA) of biocrude was performed by a TA TGA Q500 instrument. The samples were heated in N2 from 40 to 800 °C at a heating rate of 20 °C/min. Higher heating values (HHVs) of the biocrudes were estimated using the Dulong formula in eq 1, where C, H, O, and S are the wt % of each element.

concentrate. Yang et al.13 studied HTL of crude protein and crude polysaccharide extracted from algal biomass. None of these previous studies, save Yang et al.,13 included characterization of the biocrude from noncatalytic HTL, and none included any analysis of the governing reaction kinetics. Given the presence of protein in many renewable feedstocks suitable for processing by HTL and the lack of information about the biocrudes produced from protein during HTL, we conducted the work reported herein. This article advances the field by reporting on the chemical and physical characterization of biocrude from isothermal HTL of soy protein concentrate, a renewable feedstock, reporting how the yields of different product fractions respond to changes in HTL time and temperature, and reporting a new reaction network and kinetics parameters for the isothermal HTL of this material.

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HHV (MJ/kg) = 0.338C + 1.428(H − O/8) + 0.095S

(1)

RESULTS AND DISCUSSION Reaction Products. Figure 1 shows the yields of the biocrude, aqueous-phase products, gas, and solids product fractions from HTL of soy protein concentrate at the different temperatures and reaction times investigated. Figure 1a shows that biocrude yields of 5−10 wt % formed during the time required for the reactor to reach the sandbath temperature (t = 0 data). For a given reaction time, the yield of biocrude increased with temperature, and for a given temperature, it generally increased with time. The highest yield of biocrude (34 wt %) was achieved at 350 °C and 60 min. Figure 1b shows that the yield of products distributed into the aqueous phase quickly increases to about 35−50 wt % during the initial 10 min of the reaction. After 30 min, the yield of aqueous-phase products changes little, but the yields at 200 °C are always the highest at these longer times. Otherwise, the yields show no consistent trend with respect to the liquefaction temperature. The high yields at short reaction times (including t = 0) suggest that aqueous phase products arose directly from the soy protein concentrate and are primary products. Figure 1c illustrates that the yield of gases increased with both time and temperature. No measurable gas yields occurred during reactor heatup. The highest gas yield was 18 wt % at 350 °C and 60 min. The very low gas yields at the mildest reaction conditions suggest that gases are not primary products that form directly from the soy protein concentrate. Rather, gas formation seems to be from the biocrude and aqueous-phase product fractions. The gas yields in Figure 1c are much lower than the 30−45% yields reported by Yang et al.13 for HTL of crude algal protein. Perhaps this difference is related to a difference between soy and algal proteins. Figure 1d shows that as time progresses, more of the soy protein concentrate is transformed into the other product fractions. The yield of solids decreases rapidly at short reaction times and the rate of this initial decrease increases with temperature. Essentially, complete conversion of the soy protein concentrate was achieved at 350 °C and 60 min. CO2 was always the major gaseous product, as it was in the work of Yang et al.13 Figure 2 shows the amounts (mol %) of the other gases formed from HTL of soy protein concentrate for 60 min at 200, 250, 300, and 350 °C. At the lower temperatures, there were also low yields of H2, CO, and CH4. As the HTL temperature increased, the yields of these minor components increased, as did the yields of ethane and ethylene. The aqueous phase products were analyzed to determine the nitrogen content. It is desirable to have a large percentage of the N atoms in the protein feedstock partition into the aqueous phase rather than the biocrude, where it could cause environmental problems via NOx emissions upon combustion. Table 1 shows the percentage of the N atoms in the protein

EXPERIMENTAL METHODS

Materials. Soy protein concentrate (76% protein) was purchased from a local health food store and used as received. The elemental and biochemical compositions have been reported previously.12 Distilled and deionized water was used in all experiments. All other chemicals were analytical grade and obtained commercially from Fisher Scientific. Helium, hydrogen, and argon were obtained from Cryogenic Gases. Gas standards for instrument calibration were purchased from Air Liquide Specialty Gases. Batch mini-reactors with an internal volume of about 4.3 mL were constructed from a 1/2-in. Swagelok port connector, a matching cap, and a 1/2-in. to 1/8-in. reducing union. Each reactor was fitted with a 9-in. length of 1/8 o.d. tubing, which was connected to a shut-off valve. HTL Procedure. All reactors were loaded with soy protein concentrate and water such that the protein loading was 15 wt %. Each reactor contained enough water for the expanded liquid phase to occupy 95% of the reactor volume at the reaction temperature. For example, reactors were loaded with 0.529 g of soy protein and 3.00 g of water for experiments at 200 °C. Loaded and sealed reactors were placed in a fluidized sandbath preheated to 200, 250, 300, and 350 °C. The reactors reached the sandbath temperature in about 3 min. We take the time at which the reactor reached its isothermal temperature as t = 0. The reactors then remained in the sandbath for an additional 10−60 min, after which they were quickly removed from the sandbath, immersed in room temperature water to cool, and stored at room temperature for at least 12 h to allow the liquid−gas system to reach equilibrium prior to performing the gas-phase analysis. To determine the gas yield, the reactor was weighed before and after releasing the gaseous products into a GC for analysis. We then added 9 mL of dichloromethane (in multiple aliquots) to the reactor and collected the remaining reactor contents. These samples were centrifuged to separate the products into a solid phase and organic and aqueous liquid phases. The separated organic, aqueous, and solid phases were dried in flowing N2 at 35 °C and then weighed. The yields of biocrude, gas, and residual solids were calculated as the mass of each divided by the mass of soy protein concentrate loaded into the reactor. The yield of the aqueous-phase products was determined by difference. To assess experimental variability, independent runs were replicated under nominally identical conditions. Analytical Methods. The methods used here are the same as those used in our earlier work.12,14 The gaseous products were analyzed chromatographically. We tentatively identified and quantified compounds in the biocrude using Agilent 6890 gas chromatographs (GC) with a mass spectrometric and flame ionization detector. These GCs used a capillary column, an inlet temperature of 310 °C, a split ratio of 14.7:1, and an injection volume of 1 μL. The temperature program consisted of an initial oven temperature of 40 °C (isothermal for 3 min) followed by a 4 °C/min ramp to 300 °C, which was then held for 10 min. Helium served as the carrier gas at a flow rate of 1 mL/min. Elemental (C, H, N, S) analysis was performed at Atlantic Microlab, Inc. Analysis of total nitrogen in the aqueous-phase products was done B

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Figure 1. Effect of temperature and time on yields of biocrude (a), aqueous phase products (b), gas (c), and solids (d) from HTL of soy protein concentrate.

Thus, both the biocrude yield and its N-content would begin to increase with time. As time increased further, however, the biocrude compounds could eliminate N- (and O-) containing functional groups via reactions such as deamination and decarboxylation.7,13 Another way to consider the impact of HTL conditions on the elemental composition of biocrude is via atomic ratios. The H/C ratio of the soy protein concentrate is 1.69, but none of the bio-oils have values this high. The N/C ratio in the biocrude produced at 10 min is about half that of the soy protein concentrate. At the most severe HTL conditions examined, the N/C ratio was at its lowest value of 0.07, which is about one-third of that of the original protein feedstock. The O/C ratio in the bio-oil decreases steadily with both the HTL temperature and time. It reaches 0.10 (one-fourth that of the feedstock) at the most severe conditions examined. Table 2 also provides the higher heating value (HHV) and energy recovery (ER) estimated for each biocrude. The heating value of the biocrude increased with both temperature and time, reaching a maximum value of about 37 MJ/kg at 350 °C. This heating value is similar to those reported for biocrude from HTL of protein-rich microalgae.14,16 The energy recovery, defined here as the fraction of the heating value (MJ) in the protein concentrate feedstock that resides in the biocrude recovered after HTL, also increased with HTL time and temperature. The results for entry 25 allow comparison with results reported previously for HTL of soy protein concentrate in the presence of 20 wt % Ru/C catalyst under the same

feedstock that were recovered in the aqueous phase produced at the different HTL times and temperatures. We refer to this percentage as the nitrogen recovery. Heating the protein−water mixture to the sandbath set point temperature, which took about 3 min, transferred only about 10% of the protein N to the aqueous phase (t = 0 data). Holding the reactor at a given set point temperature for the first 10 min gave recoveries of around 50%. After 60 min, the N recovery was ∼65%. At a given temperature, the nitrogen recovery did not change much after 30−40 min. For a given reaction time, HTL at 350 °C always provided a higher N recovery in the aqueous phase than did HTL at the lower temperatures. Elemental Composition of Biocrude. Table 2 provides the elemental compositions of the crude bio-oils produced from liquefaction at each temperature and time. The carbon content of the biocrude always exceeded that of the soy protein (48 wt %). The carbon and hydrogen content in the biocrude increased with both time and temperature, whereas the oxygen content had the opposite trend. The sulfur content was largely insensitive to the HTL conditions. These same trends for C and O have been noted previously for HTL of protein-rich microalgae.14 At all four HTL temperatures, the N content in the biocrude went through a maximum of 10−12 wt % at 20 or 30 min. A scenario consistent with this observation is that the protein concentrate initially decomposes to form primarily watersoluble N-containing compounds. These water-soluble compounds could dimerize and oligomerize in secondary reactions to form larger compounds that would be extracted as biocrude. C

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ACS Sustainable Chemistry & Engineering conditions.12 That catalytic liquefaction experiment produced a biocrude richer in C and H and poorer in O, N, and S than did noncatalytic HTL. The HHV of the biocrude from catalytic HTL was 40.9 MJ/kg, whereas it is 37.0 MJ/kg in the absence of the catalyst. This comparison is fully consistent with these previous results, which demonstrated the efficacy of the Ru/C catalyst for HTL of soy protein concentrate. Boiling Point Distribution of Biocrude. The boiling point distributions of the crude bio-oils produced at 60 min were estimated by measuring weight loss vs temperature via TGA. Table 3 lists the fraction of the total weight loss that occurred within different temperature intervals when biocrude samples produced at different HTL temperatures were heated in N2. The fraction of the volatile biocrude that vaporized between 180 and 250 °C nearly doubled as the HTL temperature increased from 200 to 350 °C. The higher HTL temperatures produced biocrude with more lower-boiling content. Similar trends have also been observed for HTL of protein-rich microalgae.16 The ultimate mass loss from the biocrudes was approximately 82−85% when they were heated to 800 °C. Molecular Analysis of Biocrude. This section presents results from analyses of the biocrude by GC-MS, FTIR, and NMR. These analyses provide information about the types of molecules and functional groups in the biocrude. Figure 3 shows a total ion chromatogram for the biocrudes obtained at 350 °C and holding times of 20 and 60 min. The longer reaction time leads to many products that elute from the GC before 50 min, whereas there were far fewer such products in the chromatogram for the biocrude produced at 20 min. The longer HTL processing time produced smaller, lighter products, presumably because of cracking reactions. Table 4 gives more detail about the chemical identities of the products by categorizing each tentatively identified molecule into the following groups: amides, saturated hydrocarbons, unsaturated hydrocarbons, aromatics, and N-containing, Ocontaining, and N- and O-containing compounds. The sum of the areas of each individual peak in each category gives a qualitative assessment of the relative amounts of each group in the biocrude. Note that some compounds in the biocrude belong to two or more categories (e.g., amides are both N- and O-containing compounds) so the sum for a given row can exceed 100%. Hydrocarbons account for only 7− 10% of the

Table 1. Influence of HTL Time and Temperature on Nitrogen Recovery (%) in Aqueous-Phase Products from HTL of Soy Protein Concentrate time (min)

200 °C

250 °C

300 °C

350 °C

0 10 20 30 40 50 60

9 44 55 60 58 57 60

7 51 54 59 62 64 65

10 47 56 60 58 62 64

13 58 61 66 67 68 67

area in the three biocrudes. About 15% of the area is attributable to amides, and >70% of the total peak area corresponded to compounds containing N and/or O atoms. The relative abundance of O-containing compounds decreased with reaction time from 45% to 34%. The relative abundance of N-containing compounds increased with reaction time. The aromaticity of the biocrude components amenable to GC analysis also increased with time. We note, of course, that GCMS provides information only about the molecules in the biocrude with sufficient volatility to traverse the GC capillary column under the conditions used for analysis. The biocrude also contained components that were too heavy for GC analysis. Figure 4 shows the FTIR spectra of biocrude produced at 350 °C after HTL reaction times of 10 and 60 min. The FT-IR spectra of the crude bio-oils presented strong, broad bands at 3300−3500 cm−1, where the O−H and N−H stretching vibrations in hydroxyl and amino groups appear. There was less absorption in this region for the biocrude produced at 60 min, consistent with this biocrude containing less O and N, as per the elemental analyses in Table 1. The presence of aromatic carbon is consistent with bands at ∼3050 cm−1 and ∼1600 cm−1. The crude bio-oils display bands from 2840 to 3000 cm−1, consistent with the C−H stretching vibrations in methyl and methylene groups. Peaks around 1200 cm−1 may be assigned to the stretching of O−C bonds. The peaks between 1650 and 1750 cm−1 are likely due to the CO groups, perhaps in amides. Figure 5 shows the proton and 13C NMR spectra of the biooils produced by HTL at 350 °C for 10, 20, and 60 min. Figure 5a shows that for all three samples, the strongest resonances reside within the 0.0−1.5 ppm range, where aliphatic methyl and methylene protons appear. This result is consistent with the types of C−H bonds identified by FT-IR. Integrating these peaks leads to the estimates of the overall aliphatic hydrogen content given in Table 5. The aliphatic content of the biocrude increased with reaction time, and HTL for 60 min produced biocrude with the highest percentage (68%) of aliphatic protons. The smaller peaks between 1.5 and 2.8 ppm are consistent with resonances expected from protons on heteroatoms or on a carbon atom α to a heteroatom. These peak areas decreased with increasing HTL treatment time, which is consistent with the N and O content of these biocrudes decreasing with increasing time (see Table 2). The peak at about 5.5 ppm is consistent with the presence of alkenyl protons. Some peaks appear around 7−8 ppm, and these could be due to aromatic protons. The 13C NMR spectra of the crude bio-oils appear in Figure 5b. As expected, there are numerous peaks in the 10−55 ppm region where aliphatic methyl and methylene carbon atoms

Figure 2. Gas composition from HTL of soy protein concentrate at 200, 250, 300, and 350 °C for 60 min. D

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Table 2. Elemental Composition (wt%) and Properties of Biocrudes from HTL of Soy Protein Concentrate at Different Temperatures and Reaction Times entry 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

temperature (°C)

time (min)

soy protein concentrate 200 10 200 20 200 30 200 40 200 50 200 60 250 10 250 20 250 30 250 40 250 50 250 60 300 10 300 20 300 30 300 40 300 50 300 60 350 10 350 20 350 30 350 40 350 50 350 60

C

H

O

N

S

H/C

O/C

N/C

HHV (MJ/kg)

ER (%)

47.9 58.2 59.8 62.5 66.4 68.3 69.8 61.8 63.3 64.8 65.8 68.5 70.6 62.6 64.3 68.3 67.8 70.4 72.2 63.2 65.2 68.1 71.0 72.8 73.7

6.76 7.46 7.69 7.94 8.20 8.41 8.66 7.46 7.87 8.36 8.79 8.68 9.07 8.19 8.30 9.14 9.05 9.41 9.43 8.44 8.39 9.11 9.26 9.45 9.67

26.6 25.9 21.3 17.2 15.4 13.3 12.2 21.5 18.1 16.6 15.4 13.3 11.1 19.9 16.7 12.8 13.6 11.7 10.4 18.5 14.8 13.4 12.0 10.5 9.92

13.6 7.94 10.7 11.9 9.56 9.43 8.84 8.72 10.2 9.78 9.46 9.12 8.56 8.87 10.3 9.24 9.07 8.12 7.56 9.26 11.1 8.95 7.24 6.71 6.35

0.50 0.52 0.51 0.50 0.47 0.56 0.53 0.54 0.55 0.49 0.51 0.43 0.63 0.52 0.47 0.52 0.44 0.39 0.43 0.55 0.53 0.44 0.49 0.48 0.39

1.69 1.54 1.54 1.53 1.48 1.48 1.49 1.45 1.49 1.55 1.60 1.52 1.54 1.57 1.55 1.61 1.60 1.60 1.57 1.60 1.54 1.60 1.57 1.56 1.58

0.42 0.33 0.27 0.21 0.17 0.15 0.13 0.26 0.21 0.19 0.18 0.15 0.12 0.24 0.20 0.14 0.15 0.12 0.11 0.22 0.17 0.15 0.13 0.11 0.10

0.24 0.12 0.15 0.16 0.12 0.12 0.11 0.12 0.14 0.13 0.12 0.11 0.10 0.12 0.14 0.12 0.11 0.10 0.09 0.13 0.12 0.11 0.09 0.08 0.07

21.2 25.8 27.4 29.4 31.4 32.8 33.8 27.8 29.4 30.9 32.1 33.2 34.9 29.4 30.6 33.9 33.5 35.2 36.1 30.2 31.4 33.7 35.1 36.3 37.0

9.01 11.9 17.6 22.8 28.8 34.5 14.2 21.9 24.0 30.1 36.9 43.2 23.5 28.3 36.0 40.6 47.1 51.8 35.8 33.3 39.1 43.5 54.5 60.6

present in the bio-oils. The resonance at around 205 ppm is consistent with the carbonyl carbon in aldehydes or ketones. Finally, the peak at about 180 ppm suggests the existence of carbon atoms in carboxylic and amide groups. Reaction Network and Kinetics Model. In this section, we use the experimental data and HTL literature to determine the reaction network for HTL of soy protein concentrate. We then use this network and the data to determine values for the rate constants and Arrhenius parameters for each pathway in the network. We begin by first applying to soy protein concentrate the reaction network (Figure 6) previously proposed for HTL of the protein in microalgae.17 This network includes parallel primary pathways that convert protein to biocrude and aqueous-phase products and secondary pathways that allow

Table 3. Estimated Boiling Point Distribution of Biocrudes Produced from Soy Protein Concentrate via HTL at 200, 250, 300, and 350 °C for 60 min estimated boiling point range of biocrude (wt %) temperature range (°C)

200 °C

250 °C

300 °C

350 °C

< 180 180−250 250−300 300−350 ≤ 350 350−500 > 500

7.6 14.3 17.5 21.1 60.5 36.3 3.21

7.9 17.2 17.7 20.6 63.4 33.7 2.89

8.2 23.4 17.6 19.7 68.9 28.1 2.94

8.1 26.9 18.5 17.2 70.6 25.1 4.31

appear. Resonances appearing at 110−165 ppm are consistent with alkenyl, aromatic, and heteroaromatic carbon being

Figure 3. Total ion chromatogram for bio-oil obtained from HTL of soy protein concentrate at 350 °C for 20 and 60 min. E

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Table 4. Composition of Biocrude (% of Total Peak Area by GC-MS) from HTL of Soy Protein Concentrate at 350 °C HTL time (min)

amides

sat. hydrocarbons

unsat. hydrocarbons

aromatics

N-comp.

O-comp.

N,O-comp.

10 20 60

16 14 13

1.3 3.3 4.2

5.9 6.1 6.2

22 26 30

32 32 36

46 43 34

28 21 25

Figure 4. FT-IR analysis of biocrude from HTL of soy protein concentrate at 350 °C for 10 and 60 min.

Figure 5. 1H (a) and 13C (b) NMR spectra of biocrude from HTL of soy protein concentrate at 350 °C for 10, 20, and 60 min.

lipids, and polysaccharides. We used their values of k1 and k2 for protein in microalgae, along with their values for k3−k6, to predict an outcome for HTL experiments with protein alone. Figure 7 shows the results of these predictions and compares them to the experimental data reported herein for HTL at 300 °C. This comparison shows that the rate constants for HTL of protein deduced from experiments with microalgae do not describe the product yields from soy protein concentrate. This previous model predicts protein disappearance rates that are faster than those observed experimentally. It also predicts biocrude yields that are too high and gas yields that are too low. Comparisons at other HTL temperatures showed the same trends. Even so, the parameters of Valdez et al.17 correctly predict the general time scale for biocrude production from HTL of protein concentrate being tens of minutes. One reason that the previous model does not provide quantitative prediction of the present biocrude yields could be that the experiments with microalgae included lipids, polysaccharides, and other components in the reactor. That reaction mixture was more complex and had different types of biomolecules present, which could have influenced the kinetics for the different paths.11,13 For example, one could envision water-soluble products formed from HTL of protein alone

for interconversion of aqueous-phase and biocrude compounds and for gas formation. Coupling first-order rate laws for each reaction path with the batch-reactor design equation leads to eqs 2−5. The subscripts on each mass fraction, xi, refer to specific lumped species (i.e., 1 is soy protein concentrate, 2 is aqueous-phase products, 3 is biocrude, and 4 is gases). protein:

dx1 = −k1x1 − k 2x1 dt

aqueous phase products:

(2)

dx 2 = k1x1 + k4x3 − k 3x 2 − k5x 2 dt (3)

biocrude: gas:

dx3 = k 2x1 + k 3x 2 − k4x3 − k6x3 dt

dx4 = k5x 2 + k6x3 dt

(4)

(5)

17

Valdez et al. provide kinetics parameters for each of these pathways, based on experiments with three species of microalgae that contained different relative amounts of protein, F

DOI: 10.1021/acssuschemeng.6b00226 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering reacting differently than water-soluble products formed from HTL of a mixture of protein, carbohydrates, lipids, and other biomolecules in microalgae. Another reason for the difference could simply be that algal protein and soy protein behave differently under HTL conditions. Reaction Network Selection. Since the rate constants for protein HTL deduced from microalgal biomass do not quantitatively predict the HTL kinetics for soy protein concentrate. We next used the network in Figure 6 and the model in eqs 2−5 to fit the experimental data for HTL of soy protein concentrate. This parameter estimation, performed in Mathematica 10.2, led to some rate constants decreasing with increasing temperature, implying a negative activation energy, which is not a chemically meaningful outcome. It appears that the reaction network for HTL of algal biomass presented in Figure 6 does not represent the HTL of soy protein concentrate. Therefore, we proceeded to examine several simpler reaction networks created by systematically eliminating one or more paths from the reaction network in Figure 6. We solved the differential equations describing the networks and estimated the rate constants by minimizing the sum of squared relative error (SSRE) between the experimental and calculated product fraction yields according to eq 6, where n corresponds to an individual experiment and i is the index for each component. ⎛ xi , n(exptl) − xi , n(calcd) ⎞2 ⎟⎟ SSRE = ∑ ∑ ⎜⎜ (x (exptl) + xi(calcd))/2 ⎠ n i ⎝ i,n

Figure 6. Reaction network for HTL of protein (adapted from ref 17).

Figure 7. Experimental (discrete points) and calculated (continuous curves) product fraction yields from HTL of soy protein concentrate at 300 °C. The calculations used rate constants from ref 17.

(6)

Table 6. Statistics for Five Reaction Networks for HTL of Soy Protein Concentratea

Several of the reaction networks considered resulted in rate constants and/or Arrhenius parameters that were unphysical (