Rigorous process models were developed in Aspen Plus to track the overall mass and energy balances. These process models were developed according to the process designs shown in Fig. 1. In the MOTU pathway, solubilized carbohydrates and proteins undergo mild oxidative treatment (MOT) and are converted into short-chain carboxylic acids. These acids are then deoxygenated and elongated by a series of catalytic steps to yield hydrocarbon fuels in the naphtha and diesel range. In the MA pathway, the solubilized glucans and proteins are used as substrates for fermentation to a mixed alcohols slate; similar catalytic elongation/deoxygenation steps are considered for the MAU sensitivity case. Material and energy balances generated from the Aspen Plus models are utilized to inform the TEA, which entails a discounted cash flow rate of return analysis for each biorefinery.
Description of biorefineries
Both algal biorefineries share a number of consistent unit operations and herein will be described together. The biorefinery is assumed to be co-located with an algal biomass cultivation facility supporting biomass production and harvesting/dewatering (outside the scope of TEA focus in this work). Both pathways have identical upstream processing operations, with the major variations limited to the fuel upgrading strategy. Each pathway starts with the pretreatment of algal biomass, maintained at a constant rate despite seasonal variation by using a wet anaerobic storage step for peak cultivation seasons (23). Following pretreatment, lipids are extracted from the biomass and separated into unsaturated fatty acids (USFAs), which are upgraded to a polyurethane coproduct, and saturated fatty acids (SFAs), which are upgraded to fuel. The raffinate from extraction is separated into solids and liquids, with solids undergoing further conversion before being dried for sale as a coproduct.
Solubilized protein and carbohydrates present in the liquid are then upgraded to fuels, with the upgrading strategy varying between pathways. The MOTU pathway utilizes mild oxidative treatment to produce carboxylic acids, which are catalytically upgraded to fuels, while the MA pathway produces fusel alcohols via fermentation. Fusel alcohols are subsequently recovered and either used directly as fuels or combined with SFAs to produce fatty acid fusel esters (FAFEs); the alternative FAU case includes catalytic upgrading of the fusel alcohols to hydrocarbons. Both pathways include a nutrient recycle of recovered N, P, and CO2 to the algae cultivation facility and account for all storage and utility needs. Many unit operations are identical to those used in prior analyses, and the details are maintained here as documented in Davis et al. (2020) (6). Detailed technical parameters are presented in the Supplementary Material (Table S1).
Pretreatment. In both the MOTU and MA pathways, high-protein algal biomass with the composition shown in Table 2 is fed as a slurry to flash hydrolysis, following upstream dewatering to 18 wt% ash-free dry weight (AFDW) and seasonal anaerobic storage as required to normalize seasonal flows constantly throughout the year. In flash hydrolysis, the material is subjected to elevated temperatures and pressures (280°C, 1,200 psig) for a short residence time of approximately 10 seconds. These conditions have been shown to convert significant amounts of protein, as well as moderate amounts of carbohydrate, into water-soluble constituents (24, 25). Proteins are converted to both polypeptides, which show more resistance to hydrolysis, as well as free amino acids. Similarly, carbohydrates are converted to monomers (i.e., glucose and mannose) and soluble oligomers. Polar lipids, consisting of polar heads and fatty acid tails, are also assumed to be partially saponified (25). Additional details on the flash hydrolysis operation can be found in Davis et al. (2020) (6).
Table 2
Modeled algal feedstock composition, based on a weighted average of multiple strains across all months of cultivation reflecting year-long outdoor test-bed cultivation campaigns conducted under the DISCOVR consortium (26).
Elemental, ash free dry weight (AFDW) | Average composition (wt %) |
C | 51.5 |
H | 7.6 |
O | 30.2 |
N | 9.3 |
S | 0.2 |
P | 1.2 |
Total | 100.0 |
Component (dry wt%) | |
Ash | 11.0 |
Protein | 40.0 |
Lipidsa | 9.2 |
Non-fuel polar lipid impurities | 5.5 |
Fermentable Carbohydrates | 19.3 |
Other carbohydrates | 3.6 |
Cell mass | 11.4 |
Total | 100.0 |
a Reported as FAME, roughly equivalent to the portion of lipids convertible to fuels |
Solvent Extraction. The pretreated biomass from both pathways proceeds to the lipid extraction section, where it is subjected to multiple agitation and phase separation steps, using ethanol and hexane as co-solvents. Each solvent is recovered and recycled via separate distillation units, recovering hexane from the lipid extract phase and ethanol from the aqueous raffinate phase. The remaining aqueous product, consisting of solubilized proteins and carbohydrates, proceeds to the fuel upgrading train (unique for each pathway), while the extracted lipids proceed to saponification.
Saponification. In previous work, algal lipids have been assumed to undergo a bleaching/degumming operation to remove impurities (6, 27). That work focused mostly on high-carbohydrate biomass, where the lipid fraction contains a significantly lower amount of polar components. In contrast, the majority of lipids present in high-protein biomass are present as polar lipids, consisting of a polar head and a non-polar tail. This larger fraction of polar impurities necessitates the use of a saponification operation, which involves the use of water and caustic to cleave the polar heads from the lipids. A large fraction of these bonds (80%) is assumed to be cleaved in flash hydrolysis, with the remainder cleaved here. The fatty acid tails then form a soap with the cation of the caustic. A strong acid is used to neutralize the fatty acids, which then proceed to cold press separation. The aqueous phase, containing any phosphorous associated with the polar heads, is recycled to the algae ponds for nutrient recovery.
Cold Press. A hydraulic cold press separates SFAs and USFAs by exploiting the difference in the melting point temperatures. This separation is done in a series of five steps over sequentially decreasing temperatures, reaching as low as 9°C. It should be noted that this operation has not been practiced at a commercial scale for the purpose of SFA/USFA separation. Initial experimental results have been promising but do not yield a perfect separation. To represent a future target scenario, the models assume a complete separation of saturated and unsaturated fatty acids. Though there are no losses observed experimentally or in the model (all fatty acids are utilized for PU or fuel), SFA impurities in the USFA-rich phase could have an impact on the final properties of the polyurethane. Alternatively, a pure USFA phase, but with significant losses into the SFA fraction, would negatively impact PU yields.
PU Synthesis. The polyol and subsequent polyurethane synthesis processes are consistent with details published previously (6). Briefly, free fatty acids are reacted with acetic acid and peroxide in a one-pot epoxidation and ring-opening reaction, yielding polyols. Impurities in the polyol stream are removed by a series of distillation steps and the purified polyols are polymerized to flexible polyurethane foam by combination with a diisocyanate. Cutting and handling of the polyurethane foam are done on-site, with a separate storage facility added to account for curing and storage needs.
Raffinate Clarification. Following ethanol solvent recovery, the raffinate from lipid extraction, an aqueous slurry containing solubilized carbohydrates and proteins as well as a portion of the hydrolyzed polar lipid heads, is separated into solid and liquid phases by use of a vacuum belt filter press. Solids for each pathway undergo additional treatment via enzymatic hydrolysis with cellulase. The liquid from the filter press advances to the fuel upgrading section for each pathway.
Solid Treatment. In both MOTU and MA pathways, the unconverted solids consist of proteins, carbohydrates, ash, and “cell mass,” a mixture of chlorophyll, nucleic acids, and other unidentified components in the compositional analysis. These solids may be suitable for sale as a coproduct for a purpose such as animal feed or use as a co-feed for the synthesis of bioplastics, as pursued by the commercial company Algix (discussed later); however, in the latter case, the carbohydrate content (32%) is too high for direct use in the Algix process. Therefore, an additional enzymatic treatment step is used to selectively decrease the carbohydrate content and accordingly further enriches protein to meet Algix specifications. This enzymatic hydrolysis step requires re-dilution to 20% solids prior to reaction. Preliminary experimental work has indicated promising results utilizing cellulase enzyme for this purpose (unpublished data), although an optimum enzyme dosage is not yet established; for this assessment, an enzyme loading target of 10 mg/g carbohydrates is assumed in the TEA model, achieving 50% hydrolysis of residual carbohydrates to sugar monomers.
In either pathway, the product from enzymatic hydrolysis again undergoes solid-liquid separation, with the liquid being diverted to the fuel upgrading train. The solids are dried in a double-drum dryer, yielding a dry solid coproduct suitable for use in the Algix process (or potentially for sale suitable as high-protein animal or fish feed); the boundary of TEA modeling does not include protein processing operations beyond drying, and is configured to solve for the dried solids coproduct value as required to achieve a targeted fuel price metric.
Upgrading: MOTU Pathway. The details of the process design for MOT and catalytic upgrading are largely consistent with those published previously (6). One main difference is that previous modeling had assumed a whole slurry processing configuration through MOT, including insoluble solids. More recent experimental results indicate that solids are more challenging to be processed and converted through MOT at desired high solids content in the slurry, likely stemming from mass transfer limitations between the oxidant and the solids. Conversely, the liquid phase containing solubilized proteins and carbohydrates is more amenable to conversion. Accordingly, in this work, we have assumed only conversion of the soluble liquor fraction, while also reducing the target carbon efficiency to carboxylic acids from 80–60%. Otherwise, the details of the MOT and catalytic upgrading section of the MOTU model remain unchanged with previously documented work. Briefly, oxygen is sparged into a bubble column-type reactor and is consumed along with carbohydrates and protein, yielding mixed carboxylic acids (6). Excess heat from the exothermic reactions is used to generate a portion of the plant’s low-pressure steam requirement, and the CO2 produced is recycled back to the algae ponds.
The aqueous stream from the MOT reactor is first sent to an ion-exchange column to recover the nitrogen and phosphorous present in the stream. Next, the stream is heated and routed to catalytic ketonization, employing a fixed heterogeneous catalysis reactor to upgrade acids to larger chain length ketones in the C3-7 range through coupling reactions. These ketones are recovered via a flash distillation for lighter components and a liquid-liquid separation for heavier components and then sent to the condensation reactor. Here, the ketones are reacted in a batch slurry reactor in the presence of a toluene solvent, producing C6-15 cyclic enones and water. Finally, these cyclic enones are sent to hydrodeoxygenation and are subsequently separated into diesel and naphtha fractions, with the majority of the modeled compounds in the diesel range.
Upgrading: MA Pathway. In this pathway, all unit operations through raffinate solid/liquid separation are consistent with the MOT pathway. Liquid fractions from cellulase hydrolysate clarification and from the solid/liquid separation are combined and routed to fermentation vessels in which Escherichia coli anaerobically converts substrate into five different alcohols: ethanol, isobutanol, 3-methyl-1-butanol (isoamyl alcohol), 2-methyl-1-butanol (active amyl alcohol), and phenylethanol (28). The microorganism preferentially uptakes a number of amino acids while others are untouched and thus become enriched in the fermentation product broth. E. coli seed production is carried out with an external glucose source.
Although full fractionation of the fusel alcohol mixture is feasible with a series of distillation columns and decanters (29), the chosen configuration for this analysis takes a more simplified approach. The fermentation broth is initially routed through a large distillation column for bulk water removal, and the stream containing the mixed alcohols (with around 50 wt% water) is sent to a decanter for phase enrichment. The water-rich phase is further treated in a rectifying column for additional alcohol recovery. The overhead stream of this column is combined with the organic phase issued from the decanter and sent to a molecular sieve for final water removal, based on published literature utilizing molecular sieve dehydration for similar higher-alcohols (21, 22, 30, 31). The majority of this finished product is sent to storage for sale as a blended alcohol fuel product (translated into total GGE fuel yield according to this stream’s lower heating value calculated in Aspen Plus). At the same time, a fraction is routed to the production of FAFEs through the reaction between saturated fatty acids and fusel alcohols. SFAs issued from the cold press are used in this conversion, thus avoiding the need for a hydrotreater in this configuration. The conversion was based on the assumptions by Monroe et al. (2020) (32), in which an Aspergillus oryzae lipase catalyzes the reaction at a stoichiometric alcohol excess of 4.5 for 24h for maximized yields. The process is suitable to low-cost lipids (waste cooking oil, grease) and enables high conversion. Purification of FAFEs and unused alcohol recovery is carried out as an adaptation of conventional biodiesel processes with decanters, washing equipment, centrifuge, a flash vessel for FAFE drying, and a distillation column for alcohol recovery. The unreacted alcohol stream is finally purified with a similar setup as presented above (rectifying column and molecular sieve) and ultimately recycled to the lipase-catalyzed enzymatic conversion. FAFEs are then sold as an additional fuel product, again with resulting fuel yields combined as total GGE based on heating values.
Utilities. In both cases, all required utilities are accounted for in the model, including high/low pressure steam, hot oil (for high-temperature utility heat demands in excess of high-pressure steam allowances), electric power, cooling water, chilled water, plant and instrument air, the clean-in-place (CIP) system, process water, and bulk storage.
Approach to economics
The TEA modeling approach in this work is consistent with that described in prior works (6, 33). The mass and energy balance outputs from the Aspen Plus model were used to determine the number and size of capital equipment items needed. As process conditions and flows change, baseline equipment costs are automatically adjusted using scaling factors. These baseline costs originally were sourced from vendor quotes when available or other means such as (primarily) Aspen Capital Cost Estimator (ACCE) (34) when necessary. The details of these equipment designs have been published in prior reports (6, 27, 35).
Once equipment costs are determined, direct and indirect overhead cost factors are applied to determine a feasibility-level estimate of total capital investment (TCI) in 2016 US dollars (6). Variable operating expenses are calculated based on raw material and utility rates from the Aspen Plus model, while fixed costs (labor, maintenance, insurance, and local taxes) are based on prior works and adjusted based on plant scale (6, 33). The TCI, operating expenses, and fixed costs are used in a discounted cash flow rate of return analysis. A common measurement of process economics employed in prior analyses is the minimum fuel selling price (MFSP) required to obtain a net present value (NPV) of zero for the plant. Here, we also consider an alternate method, namely, setting the fuel selling price to a targeted market value ($2.50/GGE) and solving for the minimum solid coproduct selling price required to support this. Given the wide range of possible solid coproduct values, depending on the ultimate end-use, this is a useful metric for highlighting the sensitivity of the process to this variable. High level financial assumptions used for the TEA are shown in Table S2. These assumptions are based on mature nth-plant operational/economic assumptions, and consistent with prior published work.
High-protein algal biomass is delivered at the conversion plant gate at a price of $575/ton AFDW, reflective of a target cultivation productivity of 25 g/m2/day AFDW and elemental composition outlined in Table 2 (with associated CO2 and fertilizer nutrient costs attributed to the given C/N/P content), combined with dewatering to 18 wt% solids, with all upstream biomass cultivation/dewatering TEA model details consistent with previously published work (2, 36). Enzymes (A. oryzae lipase in the MA pathway and cellulase in both pathways) are costed according to differences in required enzyme rate via scaling from a basis of $6.16/kg of enzyme protein based on previously published work (35) (at considerably smaller scales of enzyme usage here, the cost for enzymes increases to $12.26/kg for lipase and $8.65/kg for cellulase).
Products. Hydrocarbon fuels produced in the MOTU pathway are drop-in fuels in the range of naphtha and diesel and could be sold as such in the market. Fuel outputs from the MA-based biorefinery include mixed alcohols and FAFEs: while the former could be blended with ethanol fuel up to a maximum of 3% (37), the latter could be considered as advanced biodiesel in combination with conventional diesel fuel. Still, it is important to acknowledge the difference in fuel end-products between the two pathways, given that more costs and processing complexity are invested in the MOTU pathway for full upgrading to hydrocarbons; similar upgrading strategies could also be possible with the MA pathway to produce hydrocarbons in place of mixed alcohols as market allowances dictate, considered as an alternative sensitivity case (MAU) at the end of this paper.
Residual solids obtained after solubilization of protein and glucans should be within specification for it to be a suitable precursor to bioplastics able to displace ethylene-vinyl acetate (EVA) elastomers. Based on guidance from the industry, the limits for sale of solids residuals to be leveraged by this technology require > 30% protein, < 20% carbohydrates (preferably < 10%), and < 35% ash.
Finally, algae-based polyurethanes are commercialized at a price of $2.04/lb (2016 dollars), consistent with a 5-year average price for commodity flexible foam (6).