Products, Pathways, and Kinetics for the Fast Hydrothermal

Research Article pubs.acs.org/journal/ascecg

Products, Pathways, and Kinetics for the Fast Hydrothermal Liquefaction of Soy Protein Isolate James D. Sheehan† and Phillip E. Savage*,‡ †

Department of Chemical Engineering, College of Engineering, The Pennsylvania State University, 3058 Research Drive, State College, Pennsylvania 16801, United States ‡ Department of Chemical Engineering, The Pennsylvania State University, 119 Greenberg, University Park, Pennsylvania 16802, United States S Supporting Information *

ABSTRACT: We conducted nonisothermal fast hydrothermal liquefaction (HTL) of soy protein isolate (SPI) for batch holding times of 10−300 s and at temperatures up to 500 °C. The SPI solids rapidly formed water-soluble products, some of which subsequently formed biocrude. The highest biocrude yields (38−40 wt %) were obtained within 45−120 s, at which times the reactor temperatures had reached 375−435 °C. The highest recovery of nitrogen in the aqueous-phase products (80% of that present in SPI) occurred prior to formation of high biocrude yields. Ammonia formation was significant when the hydrothermal reaction medium reached supercritical conditions; over 50% of the atomic N appeared as ammonia under such conditions. We deduced the reaction pathways and developed a kinetic model from the experimental data. The reaction network includes two types of aqueous-phase products formed during HTL of protein. The first type arises directly from the SPI and the second arise from biocrude. The model accurately correlated the product yields under the fast HTL experimental conditions studied, and it accurately predicted the yields of product fractions from isothermal HTL of SPI at 300 and 350 °C. KEYWORDS: Sub- and supercritical water, Nonisothermal kinetic modeling, Biocrude, Nutrient recovery

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INTRODUCTION

Algal biocrude is a mixture of thousands of different molecules that include monoaromatic compounds, aliphatic compounds, oxygenated compounds, N-heterocycles, and polycyclic aromatic compounds.5 It can possess 70−95% of the energy content of petroleum crude oil and has higher heating values (HHV) of up to 39 MJ/kg.1,5−8 Biocrude has the potential to supplement petroleum crude oil as a liquid fuel source. One of the limitations of microalgae-derived HTL biocrude is that it contains large amounts (4−8 wt %) of nitrogen, much higher than petroleum crude oil.6−8 Nitrogen in biocrude is undesirable because it lowers the heating value, produces NOx upon combustion, and decreases the stability of biocrudederived fuels during storage.9 The nitrogen content of microalgal biocrude originates primarily from proteins and amino acids.1,8 Depending on the strain of microalgae, the protein composition can range 6 to 71 wt %.10 Peptide bonds in proteins hydrolyze quickly during HTL and release constituent amino acids, which can undergo decarboxylation to produce alkylamines and carbonic acid or deamination to form ammonia

Biomass feedstocks can be utilized as renewable resources for producing liquid fuels and chemicals. Microalgal biomass, in particular, has been regarded as a renewable feedstock with high potential. Microalgae are single-cell, aquatic organisms that have higher photosynthetic efficiencies, faster growth rates, and higher area-specific yields than terrestrial biomass.1 The production of liquid fuel precursors and intermediates from microalgae using the thermochemical process of hydrothermal liquefaction (HTL) has been studied extensively. HTL can process wet biomass so it obviates drying of the wet feedstock, which can be energy intensive.1,2 During HTL, water is typically at subcritical temperatures (200−370 °C) and pressures (10−22 MPa). The cell membranes of microalgae rupture and leave the constituent biomacromolecules of the cell exposed.3 Subsequently, the biomacromolecules depolymerize into smaller organic molecules. The solubility of organic compounds and promotion of acid- and base-catalyzed reactions in subcritical water facilitates the breakdown of microalgae.4 Ultimately, the HTL of microalgae produces an energy-dense crude bio-oil, a nutrient-rich aqueous-phase coproduct, a CO2-rich gas phase, and solid phase residue. © XXXX American Chemical Society

Received: August 4, 2016 Revised: September 8, 2016

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DOI: 10.1021/acssuschemeng.6b01857 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering Table 1. Amino Acid Profiles (wt %) of Soy Protein Isolate and Various Microalgae Strainsa amino acid

soy protein isolate

Spirulina maxima

Scenedesmus obliquus

Chlorella ellipsiodae

Chlorella pyrenoidosa

Chlorella vulgaris

Dunaliella bardawil

Ala Arg Asp Cys Glu Gly His Ile Leu Lys Met Phe Pro Ser Thr Try Tyr Val

4.3 7.6 11.5 1.2 19.0 4.2 2.6 4.8 8.1 6.2 1.4 5.2 5.1 5.2 3.7 1.4 3.7 5.0

7.5 7.2 9.5 0.4 13.9 5.3 2.0 6.6 8.8 5.1 1.5 5.4 4.3 4.6 5.1 1.5 4.3 7.2

9.9 7.8 9.2 0.7 11.8 7.8 2.3 4.0 8.0 6.2 1.7 5.3 4.3 4.2 5.6 0.3 3.5 6.6

13.4 6.4 9.7 0.8 11.6 11.4 1.9 5.0 10.2 6.5 0.7 4.6 5.5 5.7 5.4 0.0 1.9 8.7

6.5 6.2 6.5 0.0 10.2 5.3 1.5 3.7 4.4 8.7 2.0 5.0 4.4 2.4 3.5 1.5 3.0 5.6

10.3 7.6 10.2 0.0 15.1 6.9 2.2 3.5 10.5 7.0 1.4 6.1 5.5 6.4 5.8 0.0 3.1 7.7

8.0 8.0 11.4 1.3 14.0 6.1 2.0 4.6 12.1 7.7 2.5 6.4 3.6 5.1 5.9 0.8 4.1 6.4

a

Table adapted from ref 20.

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and organic acids.1,11−13 These products can recombine into aromatic ring structures and N-containing heterocyclics.1 Understanding the formation of N-containing compounds during HTL and reducing the nitrogen content in biocrude is paramount to its potential as a source for liquid fuels. The HTL of proteins and their conversion into biocrude has been previously studied.6,14−16 These studies have focused on the isothermal HTL of proteins under subcritical conditions (Tc = 374 °C for water) and holding times ranging 10 to 90 min. Generally, under isothermal conditions, biocrude yields increase with increasing temperature and holding time. However, due to the long holding times used for isothermal HTL, the initial events that lead to the incorporation of undesirable N-containing compounds derived from protein into biocrude have not been observed. No work has yet been dedicated to studying the “fast” HTL of proteins under rapid heating with short holding times of tens of seconds. Yet “fast” HTL of proteinaceous biomass (microalgae and bacteria) produced biocrude yields exceeding those achieved under conventional, isothermal HTL conditions.8,17,18 During fast HTL, the conversion of biomass into biocrude is rapid with maximum biocrude yields at times occurring within 60 s.8,17 Moreover, given the shorter holding times (tens of seconds), fast HTL allows one to probe primary pathways and investigate the initial conversion of protein and the conversion of protein-derived compounds into biocrude. The abundance of protein-rich potential HTL feedstocks (e.g., microalgae, food waste, bacteria), the promise of fast HTL for producing higher biocrude yields, and the complete absence of any prior work on fast HTL of protein motivated the work reported herein. In this study, we quantified product yields, the elemental content of biocrude, and the recovery of ammonia and organic N in the aqueous-phase products produced from the fast HTL of protein. We then developed a reaction network and quantitative model for the kinetics of the HTL of protein under nonisothermal conditions. This information collectively provides insight on the production of biocrude from protein.

MATERIALS AND METHODS

Materials. Soy protein isolate (Fitness Laboratories) served as the feedstock for fast HTL. Its amino acid profile (as provided by the supplier) is similar to those of several microalgae strains as shown in Table 1. Deionized water prepared in house was used for the hydrothermal medium. We used dichloromethane (99.9%, HR-GC grade EMD Millipore) for recovering biocrude from the reactors and nitrogen (99.999%, Praxair) for evaporating solvent from the biocrude samples. Dichloromethane was chosen as the recovery due to its standard use in previous studies;6,8,15,16,18 however, the choice of solvent can affect the overall recovery and composition of biocrude.19 Reactors and Fast HTL Procedure. We constructed batch minireactors from 316 stainless-steel Swagelok tube fittings, and the reactors had internal volumes of approximately 1.67 mL. We loaded carefully weighed amounts of soy protein isolate (31.3−92.2 mg) and deionized water (282−830 mg) into the reactors so that the soy protein isolate would make up 10 wt % of the total loaded mass for each experimental trial. We loaded deionized water into the reactors in amounts such that a pressure no higher than 400 bar would be attained should the reactors reach the sand bath set point temperatures of 400, 450, and 500 °C. We also constructed proxy reactors to estimate the internal temperatures experienced during HTL. The proxy reactors were identical to the reactors used for HTL of SPI except they included a reducing union that sealed within an Omega TJ-36 series thermocouple. We loaded the proxy reactors with deionized water such that a pressure of approximately 400 bar would be reached at the set point temperature. The temperature profiles of the proxy reactors were recorded using an Omega UWBT series wireless transmitter and measurements of the reactor temperatures were taken every 0.1 s. We did separate trials with the thermocouple at two different radial positions (center and nearer the wall) within the proxy reactor and used the mean value as the representative reactor temperature at a given time point. Figure 1 shows the reactor temperature profiles for the sand bath set point temperatures of 400, 450, and 500 °C. After the reactors were loaded and sealed, we placed them into a Techne IFB 151 fluidized sand bath set at and preheated to 400, 450, or 500 °C, where they underwent rapid heating. After the reactors had been in the sand bath for a desired amount of time (t ≤ 300 s), we removed them and placed them in an ice bath to quench the reaction. As shown in Figure S1, the quenching of reactors to room temperature was rapid and completed in the span of seconds. After removing the cooled reactors from the ice bath, we dried the exterior and allowed them to equilibrate at room temperature for several hours. B


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and unity. Each experiment was done in triplicate and the reported product yields are the means of the three trials. SPI powder and biocrude were sent to Atlantic Microlab, Inc. for the analyses of C, H, N, and S. The higher heating values of the biocrude were estimated with the Dulong formula (eq 1) and the energy recovered in the form of biocrude was calculated using eq 2.

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

(1)

energy recovery = (HHV of biocrude × biocrude yield) /(HHV of SPI)

(2)

The ammonia and total nitrogen contents of the aqueous-phase products were determined using HACH TNT 828 Total Nitrogen and TNT 832 Ammonia kits and a Shimadzu UV-3600 UV−vis-NIR spectrophotometer. We compared the absorbance of samples against known ammonia and nitrate standards. The reported values are the means of triplicate runs.

Figure 1. Temperature profiles of batch reactors at sand bath set point temperatures 400 °C (blue), 450 °C (red), and 500 °C (green). Product Recovery and Analyses. We weighed the dried reactors before and after opening them. The difference between these two measurements is taken as the mass of the gas products. We next poured the contents of each reactor into its own test tube. We then extracted additional material from each reactor by adding 9 mL of dichloromethane and transferring the dichloromethane-soluble products into its respective test tube. The products soluble in dichloromethane are classified as biocrude. The product fractions (aqueousphase products, biocrude, and residual solids) were separated by the procedure of Valdez et al.7 This procedure involves a series of mixing (with vortex mixer) and separation (via centrifugation) steps. Dichloromethane was evaporated from the biocrude with a Labconco RapidEvap Vertex Evaporator. The solid residue products were dried overnight in an oven at approximately 70 °C. The dried products were weighed to quantify their respective gravimetric yields (mass of the dried product divided by the mass of the SPI powder loaded into the reactor). We determined the aqueous-phase product yields as the difference between the sum of the other products yields

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RESULTS AND DISCUSSION This section provides information about the yields of the four major production fractions and how their yields varied with holding time and the sand bath set point temperature. Following sections discuss the composition of the biocrude and recovery of nitrogen atoms in the aqueous-phase coproduct. The final section provides a reaction network for HTL of SPI along with a quantitative kinetics model that is demonstrated to have predictive capabilities. Product Yields. Table 2 shows the yields of the product fractions formed from fast HTL of SPI. The first row shows results from a control experiment that implemented the reactor loading and product recovery procedure but kept the reactor at room temperature. 55 wt % of the SPI remained a solid powder

Table 2. Yields (wt %) of Solids, Aqueous-Phase Products, Biocrude, and Gas from the Fast HTL of SPI at Different Sand Bath Set Point Temperatures set point temperature (°C)

holding time (s)

reactor temperature (°C)

N/A 400

N/A 30 45 60 90 120 180 300 10 20 30 45 60 90 120 180 300 20 30 45 60 90 120 180 300

room temp. 262 308 340 375 389 396 398 162 249 310 370 399 433 443 448 449 254 319 377 410 461 484 498 500

450

500

solids 55 33 5 3 2 1 0 1 50 34 8 4 1 1 0 0 0 35 1 3 2 1 1 1 4

C

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

4 0 2 0 0 0 0 5 3 3 7 1 0 0 0 1 7 1 1 2 1 1 0 4

aqueous-phase products 43 58 72 63 51 53 60 59 46 60 68 61 51 48 54 61 65 56 74 53 55 53 57 62 63

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

3 2 5 3 4 4 4 2 1 4 5 2 3 5 3 2 3 8 9 8 6 5 5 5

biocrude 2 6 18 30 39 39 32 28 1 3 19 29 39 40 34 24 19 5 20 38 33 34 27 19 11

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2 3 7 1 2 3 0 1 2 3 5 3 3 3 5 3 4 3 1 3 3 1 1 4

gas 0 4 4 5 8 7 8 13 3 4 5 6 9 11 12 15 16 4 5 6 10 12 15 18 22

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

2 1 1 4 2 2 3 2 2 7 2 1 2 2 4 2 1 4 4 7 4 5 5 4


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Table 3. Atomic Ratios, Higher Heating Values (HHV), and Energy Recovery of SPI and Biocrude from HTL at Different Conditions set point temperature (°C) 400

450

500

time (s)

reactor temperature (°C)

SPI powder feedstock 60 90 120 180 300 30 45 60 90 120 180 300 30 45 60 90 120 180 300

H/C

O/C

N/C

S/C

HHV (MJ/kg)

1.8 1.6 1.6 1.6 1.6 1.5 1.6 1.6 1.7 1.6 1.6 1.6 1.5 1.6 1.7 1.6 1.6 1.5 1.3 1.3

0.54 0.21 0.15 0.16 0.16 0.15 0.21 0.21 0.24 0.18 0.15 0.14 0.13 0.21 0.28 0.18 0.17 0.13 0.17

0.24 0.14 0.13 0.12 0.11 0.10 0.12 0.14 0.14 0.13 0.11 0.11 0.10 0.13 0.15 0.13 0.13 0.11 0.12 0.12

0.0043 0.0065 0.0057 0.0059 0.0051 0.0050 0.0056 0.0053 0.0058 0.0047 0.0100 0.0069 0.0069 0.0064 0.0069 0.0057 0.0045 0.0043 0.0093

20 30 33 33 33 34 31 30 30 32 33 34 35 30 28 31 32 34 31

340 375 389 396 398 310 370 399 433 443 448 449 319 377 410 461 484 498 500

energy recovery (%) 46 65 65 54 48 30 45 59 65 58 41 33 31 54 53 56 47 30

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

11 2 3 5 0 5 8 5 5 5 9 5 5 1 5 5 2 2

examined.15 Biller et al. studied HTL of soya protein under the same conditions and observed a maximum biocrude yield of 18 wt %.6 Yang et al. studied HTL of crude, microalgal protein and reported a maximum biocrude yield of 16.29 wt % at 280 °C and 20 min.16 The highest biocrude yields in the present study appeared at conditions where the reactor temperature had exceeded the critical temperature of water. The previous studies, on the other hand, explored only subcritical conditions. This difference suggests that fast HTL, especially if it momentarily includes supercritical temperatures, may lead to higher biocrude yields during HTL. The gas yields were 60 s), SPI solids are depleted and biocrude formation is most likely driven by aqueous phase compounds undergoing decarboxylation, deamination, or dehydration, leading to simultaneous increase in biocrude yield and reduction in heteroatom content. The decrease in heteroatom content could be further rationalized by compounds in the biocrude undergoing deamination, decarboxylation, and dehydration reactions concurrently as less polar molecules from the aqueous phase are forming biocrude. This trend in the atomic ratios of biocrude is also observed for conventional HTL of soy protein concentrate; however, the time scale is over tens of minutes at subcritical temperatures of 200−350 °C.15 Similar interactions between the aqueous-phase products and biocrude were observed for the HTL of Nannochloropsis sp.17 For a given set point temperature, the largest HHVs for biocrude were generally obtained at the most severe experimental conditions. For the 400 °C set point condition, at 300 s and 398 °C, biocrude had HHV of 35 MJ/kg. Similarly, for 450 °C set point condition, at 300 s and 449 °C, biocrude had a HHV of 35 MJ/kg. At the 500 °C set point condition, however, the highest HHV was 34 MJ/kg at 120 s and 484 °C and not at 180 s. This reduction in heating value at the most severe conditions is due to the reduction of H/C and increase of O/C, which we attribute to the possible aromatization and cyclization of compounds under these severe conditions. The HHVs of biocrude produced from fast HTL of SPI are comparable to those from isothermal HTL of soy protein6,15 and microalgal, crude protein16 for holding times for tens of minutes. The HHV values for these other biocrudes generally ranged from 30 to 37 MJ/kg.6,15,16 As has been demonstrated for algal biomass, bacteria, and yeast,8,18 fast HTL of SPI provides in a minute or two biocrude yields and energy densities comparable to or exceeding those available from isothermal HTL for tens of minutes. The energy recovery from HTL of SPI was determined by the heating value (heteroatom and H content) and yield of the biocrude. For the 400 °C set point condition, the highest energy recovery was 65% at 90 s and 375 °C and also at 120 s and 389 °C. Comparably, for the 450 °C set point condition, the highest energy recovery was 65% at 90 s and 433 °C. The highest energy recovery for the 500 °C set point condition was only 56%, occurring at 90 s and 461 °C. The highest energy recovery for each set point temperature occurred for biocrude with HHV over 32 MJ/kg and yields over 34 wt %. The energy recovery for biocrude produced via fast HTL of SPI was higher than in previous studies on isothermal HTL of proteins. Yang et al. performed HTL of microalgal, crude protein under isothermal conditions at 300 °C for 20 min and achieved an energy recovery of just 23%.16 Biller et al. produced biocrude with an energy recovery 30.5% under isothermal conditions at 350 °C for 60 min.6 Similarly, Luo et al. produced biocrude via HTL of soy protein concentrate at 350 °C for 60 min however they achieved an energy recovery of 60.6%.15 The high energy

Figure 2. Percent of N atoms in SPI appearing as ammonia (blue) and total N (yellow plus blue) in aqueous-phase products from HTL at a set point temperature 450 °C. Reactor temperatures at each holding time are given by red diamonds.

atoms that appears as ammonia and in total in the aqueousphase products produced at the 450 °C set point temperature. The transfer of N atoms from SPI into the aqueous phase is rapid. The total N reached 80% in the aqueous-phase products at only 30 s. As holding time and the reactor temperature approached 60 s and 400 °C, the atomic N in the aqueous phase decreases to 59%. The decrease of N in the aqueous phase is due to its transfer to biocrude as shown in Table 3. As the holding time and temperature increase further, the total N in the aqueous phase increases to 72%, likely due to the decomposition of molecules in the biocrude. The nitrogen recoveries reported herein for the aqueous phase are comparable to those reported in previous studies.22,15 The ammonia content of the aqueous-phase products increased steadily with holding time and temperature. At a holding time of 30 s and temperature of 310 °C, 9% of biomass N is in the form of ammonia. After reaching 180 s and roughly 450 °C, approximately 50% of biomass N is ammonia. The increase of N in the aqueous-phase products with increasing holding time is consistent with the decline in the N/C ratio of biocrude in Table 3 under the same experimental conditions. Kinetic Modeling of Fast HTL of SPI. We modified a reaction network recently advanced for the fast HTL of a protein-rich Nannochloropsis sp. to model the kinetics for fast HTL of SPI.17 The earlier network comprises the irreversible formation of aqueous-phase products, biocrude, and gas from biomass solids, the irreversible formation of gas and volatiles from aqueous-phase products, and the reversible formation of biocrude from aqueous-phase products. Our modifications include lumping together volatiles and aqueous-phase products for simplicity and adding a pathway for the formation of gas from biocrude. Additionally, we posit the existence of two types of aqueous-phase product molecules. The data in Table 2 show two regions where the yield of aqueous-phase products increases. We believe the first of these two increases is due to their primary formation from SPI solids and the second to their secondary formation from biocrude. The total yield of aqueousE


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ACS Sustainable Chemistry & Engineering phase products is the sum of the yields of the primary and secondary aqueous-phase products. Figure 3 shows the reaction network for the fast HTL of SPI.

We used Mathematica 10.2 to solve the system of ordinary differential equations and simultaneously estimate the Arrhenius pre-exponential factors, Ai, and activation energies, Ei, by minimizing the sum of squared residuals (SSR). The SSR, as shown in eq 9, is the sum of the squared differences between the experimental yield, xi(t), and the calculated yield, xi,m(t), at a given holding time, t. SSR =

∑ ∑ (xi(t) − xi,m(t ))2 i

(9)

t

Table 4 shows the optimized Arrhenius parameters for the fast HTL of SPI. The activation energies for the conversion of Table 4. Arrhenius Parameters for the Fast HTL of SPI rate constant

Figure 3. Reaction network for fast HTL of SPI.

kSA1 kSB kSG kA1B

The reaction network in Figure 3 differs from that offered recently15 for isothermal HTL of soy protein concentrate (not the SPI used in the present study) at subcritical temperatures. This earlier network excluded the primary formation of biocrude from protein (i.e., kSB = 0) because this step was not needed to model the data adequately. As we will show shortly, the network above does a very good job of predicting the product yields from isothermal HTL at subcritical temperatures. Moreover, including primary formation of biocrude from protein did lead to a better model for fast HTL of SPI. The difference in the HTL networks for the two different soy protein products is consistent with differences in the HTL rates and yields for the product fractions. These differences are probably due to the differences in the starting feedstocks. Though both are mostly soy protein, the SPI used in the present study is a more highly processed material that is >90 wt % protein. The soy protein concentrate used in the previous study is about 70 wt % protein and it can also contain fiber. We described the kinetics for each pathway using first order rate laws and the Arrhenius equation, and then coupled them with the batch reactor design equation, as shown below in eqs 3−7. The xi entities are yields of each product fraction and the kij are the rate constants for pathways that connect reactant i to product j. The temperature profiles of the proxy reactors for each set point temperature were fitted using power series models and incorporated directly into the model to give the reactor temperature as a function of time. Additionally, we have assumed that all reactions take place in a single fluid phase. dx S = −(k SA1 + k SB + k SG)xS dt

(3)

dxA1 = k SA1xA1 − (kA1B + kA1G)xA1 + kBA1x B dt

(4)

dxA2 = kBA2x B dt

(5)

dx B = k SBxS + kA1BxA1 − (kBA2 + kBG)x B dt

(6)

dxG = k SGxS + kA1GxA1 + kBGx B dt

(7)

xA = xA1 + xA2

(8)

kA1G kBA2 kBG

pathway solids to aqueous-phase products (1) solids to biocrude solids to gas aqueous phase-products (1) to biocrude aqueous phase-products (1) to gas biocrude to aqueous-phase products (2) biocrude to gas

log[Ai] (log[1/s])

Ei (kJ/mol)

4.01

56.0

5.34 5.17 4.08

68.1 72.1 85.1

5.33 4.26

125 87.2

5.28

111

solids (S) to primary aqueous-phase products (A1) and solids to biocrude (B) are similar to the activation energy (43 kJ/mol) calculated for the hydrolysis of microalgal proteins23 in subcritical water. These activation energies all being in the same range is reasonable because one expects hydrolysis of protein to be the dominant chemical process. The activation energy for the conversion of primary aqueous-phase products to biocrude is within the range (46−191 kJ/mol) determined previously for decomposition of amino acids.12 This similarity is consistent with the idea that water-soluble products undergo reactions such as decarboxylation or deamination to form less polar molecules more soluble in the dichloromethane used for biocrude recovery. The activation energies for the gasification of the primary aqueous-phase products and biocrude are also comparable to activation energies reported by Li and Brill11 for the decarboxylation of numerous amino acids (111−191 kJ/ mol). The activation energy for the formation of aqueous-phase products (A2) from biocrude is also comparable to those determined for the HTL of Nannochloropsis sp.17 Figure 4 shows the model calculated yields for the fast HTL of SPI along with the experimental data. Overall, the proposed reaction network and optimized Arrhenius parameters capture the trends in the observed product yields for all three set point temperatures. To test the predictive ability of the model, we conducted additional experiments with SPI, but under isothermal HTL conditions. Specifically, we conducted HTL experiments at 300 and 350 °C for times ranging from 5 to 60 min. These conditions test the model’s ability to predict results at both lower temperatures and longer times than those used to determine the model parameters. We measured the yield of each product fraction from each experiment and used the model to predict the yields for each experiment as well. Figure 5 displays a plot that compares the experimental and predicted product yields. Circles represent yields of product fractions from HTL of SPI at 300 °C whereas triangles represent data at F


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Figure 4. Experimental (discrete points) and model calculated yields (continuous curves) for fast HTL of SPI at set point temperatures 400 °C (a), 450 °C (b), and 500 °C (c).

Having shown that the model can correlate experimental data for fast HTL of SPI and that it can predict outcomes for isothermal HTL, we now use the model to calculate product fraction yields from isothermal HTL over a wide range of temperatures, but at short times. Figure 6 displays contour plots of the calculated yields for each product fraction as a function of temperature and time. Figure 6a shows the effects of reactor temperature and holding time on the yield of SPI solids. SPI solids decompose readily during HTL and conversion is complete by 120 s for all temperatures considered. At temperatures >300 °C, conversion of SPI solids is complete within 10 s. Figure 6b shows the calculated yields for the aqueous-phase products. The highest yields for the aqueous-phase products occur in two regions, the first under mild conditions (200 °C < T < 225 °C, 150 s < t < 300 s) whereas the second takes place under more severe temperatures (450 °C < T < 500 °C, 150 s < t < 300 s). The formation of aqueous-phase products under the mild conditions corresponds with the decomposition of SPI solids. Under the severe temperature conditions, the formation of aqueous-phase products is due to the decomposition of biocrude. Figure 6c shows the model calculated yields for biocrude. High biocrude yields are available under a large variety of operating conditions. The conditions that give high yields of biocrude are essentially identical with the conditions for the lowest yields of aqueous-phase products. The strong interplay between the aqueous-phase products and biocrude indicate that optimum conditions exist for recovering protein-derived

Figure 5. Parity plot for product fraction yields from isothermal HTL of SPI. Circles (solids, light blue; aqueous-phase products, orange; biocrude, red; gas, yellow) represent data from HTL at 300 °C. Triangles (solids, purple; aqueous-phase products, green; biocrude, blue; gas, brown) represent data from HTL at 350 °C.

350 °C. Perfect predictions from the model would lead to all points residing on the diagonal line. Though perfection was not achieved, the model does do a very good job of predicting the product yields from isothermal HTL. We also point out that in addition to this test being a true prediction (none of the experimental data were used to determine the model parameters), it is also an extrapolation to a reaction regime (isothermal HTL for longer times) that was not included in the parametrization. The experimental and model predicted product fraction yields, as well as the reactor temperature profiles under isothermal conditions have been provided in the Supporting Information section. G


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Figure 6. Yields of solids (a), aqueous-phase products (b), biocrude (c), and gas (d) from HTL of SPI as functions of isothermal temperature and time.

treatment of related compounds. This modeling work led to a new insight regarding HTL, namely the need to account separately for primary and secondary aqueous-phase products. All previous modeling work had lumped the aqueous-phase products from biocrude together with the primary aqueousphase products.

products in the aqueous-phase vs incorporating them into biocrude. Figure 6d shows the model calculated yields for gas. For a given holding time, the yield of gas increases gradually with increasing temperature and increases slowly with time at each temperature. The highest gas yields occur at the most severe operating conditions (T > 480 °C and t > 180 s).

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CONCLUSIONS This work provides the first study of fast HTL for protein, and thereby broadens the applicability of this process for valorizing biomass. Fast HTL can produce biocrude with a high heating value in yields of nearly 40 wt % and aqueous-phase products with ∼80% of the biomass nitrogen in just minutes. These results show that the protein fraction in protein-rich materials such as microalgae and wastes from food processing and distribution can be processed by fast HTL to recover both energy (biocrude) and nutrients (aqueous phase N) for fertilizer. Note, however, that there is interplay between the components in the biocrude and aqueous-phase product fractions. The formation of biocrude influences the recovery of N in the aqueous phase as the lowest N recovery in the aqueous-phase products occurred when the highest biocrude yields were attained. The reaction network and kinetic model for the fast HTL of SPI correlated product yields within the experimental conditions tested and accurately predicted product fraction yields for HTL of SPI under isothermal conditions, which were outside those used to estimate the model parameters. The activation energies in the model are chemically reasonable and comparable to values observed previously for hydrothermal

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01857. Figure S1: Temperature profile of a reactor immersed in a sand bath at a set point temperature of 450 °C. The reactor is removed from the sand bath and quenched at 300 s. Table S1: Eemental composition of biocrude produced from the fast HTL of SPI. Table S2: Experimentally observed and model predicted product fraction yields resulting from HTL of SPI under isothermal conditions. Figure S2: Temperature profiles of proxy reactors at set point temperatures of 300 and 350 °C PDF)

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AUTHOR INFORMATION

Corresponding Author

*P. E. Savage. E-mail address: [email protected]. Notes

The authors declare no competing financial interest. H


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(20) Becker, E. Microalgae Biotechnology and Microbiology; Cambridge University Press: New York, 1994. (21) Shen, J. L. Soy Protein Solubility: The Effect of Experimental Conditions on the Solubility of Soy Protein Isolates. Cereal Chem. 1976, 53, 902−909. (22) Garcia-Moscoso, J. L.; Obeid, W.; Kumar, S.; Hatcher, P. G. Flash Hydrolysis of Microalgae (Scenedesmus Sp.) for Protein Extraction and Production of Biofuels Intermediates. J. Supercrit. Fluids 2013, 82, 183−190. (23) Garcia-Moscoso, J. L.; Teymouri, A.; Kumar, S. Kinetics of Peptides and Arginine Production from Microalgae (Scenedesmus Sp.) by Flash Hydrolysis. Ind. Eng. Chem. Res. 2015, 54, 2048−2058. (24) Changi, S.; Faeth, J. L.; Mo, N.; Savage, P. E. Hydrothermal Reactions of Biomolecules Relevent for Microalgae Liquefaction. Ind. Eng. Chem. Res. 2015, 54, 11733−11758.

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Products, Pathways, and Kinetics for the Fast Hydrothermal

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