268273 Process Synthesis and Technoeconomic Evaluation of a Catalytic Biomass-to-Fuels Strategy
Lignocellulosic biomass, an alternative for petroleum, is a sustainable source of carbon for producing chemicals and fuels in an environmentally friendly way. Conversion of lignocellulosic biomass to valuable intermediates is a critical step to develop effective biomass-to-biofuel strategies. Levulinic acid (LA) is one of these platform chemicals that can be produced from lignocellulosic biomass and transformed into liquid fuels, fuel additives and even other specialty chemicals. In this respect, we developed an LA-based catalytic strategy, in which an alkylphenol solvent is used for extraction of intermediates from the sulfuric acid solution, to convert lignocellulosic biomass into liquid hydrocarbon fuels. Then, we compared the alkylphenol strategy with a previously reported butyl acetate strategy in terms of cost-effectiveness.
In the butyl acetate strategy, first, the hemicellulose fraction of the biomass is solubilized and removed using dilute acid pretreatment. Pretreated biomass, which contains cellulose and lignin, is sent to a cellulose deconstruction unit, in which cellulose is converted to levulinic acid and formic acid with 55% yield in the presence of sulfuric acid, whereas the remaining cellulose is degraded to humins as a result of undesired polymerization reactions. After cellulose deconstruction, solid residues (lignin and humins) are filtered from the product stream, then mixed with the hemicellulose fraction that was removed by dilute acid pretreatment, and sent to a boiler/turbogenerator to produce heat and electricity covering the utility requirements of the process. Excess electricity is sold to the grid. Then, LA is converted to γ-valerolactone (GVL) over a RuRe/C catalyst in the presence of sulfuric acid. GVL is extracted from the sulfuric acid and GVL aqueous solution using butyl acetate solvent and sulfuric acid is recycled back to the cellulose deconstruction reactor. Finally, GVL is separated from butyl acetate by distillation and purified GVL is converted to butene and to butene oligomers.
The potential shortcoming of the butyl acetate strategy is the use of an expensive RuRe/C catalyst to prevent the negative effects of sulfuric acid in the catalytic processing. To address this shortcoming, an alkylphenol (2-sec-butyl phenol (SBP)) solvent is used for extraction of LA, which is produced in low concentration, from sulfuric acid solution. This strategy also allows for the aqueous phase containing sulfuric acid to be recycled back to the cellulose deconstruction reactor efficiently. Another advantage of alkylphenols is their high boiling points, so that extraction at elevated temperatures, i.e. at cellulose deconstruction temperature, is possible and leads to potential energy and equipment cost savings. The LA obtained from pretreated biomass is selectively reduced to GVL (while the FA is decomposed to H2 and CO2) over a RuSn/C catalyst in the presence of SBP solvent without hydrogenating the solvent. After the GVL production, SBP and GVL can be separated by distillation recovering GVL as a top product. The GVL would then be processed to liquid fuels. This sulfuric acid management strategy provides downstream catalytic processing at high yields in the absence of sulfuric acid and with a cheaper catalyst to convert LA into GVL.
After developing a model for the alkylphenol-based separation strategy, we investigated the economic impact of integrating this strategy with the original process. To determine the economic potential of these strategies, we carried out detailed process simulation studies (based on experimental results for all associated reactions) and capital/operational cost calculations. After optimizing the process configurations of the alkylphenol strategy, the minimum selling price (MSP) for butene oligomers is around $4.70 per gallon of gasoline equivalents (GGE). Also, we identified that the MSP is sensitive to changes in the catalyst lifetime. For a catalyst lifetime of less than 6 months, the alkylphenol strategy results in better MSP than that can be obtained from the butyl acetate strategy. Finally, a comparison of the alkylphenol strategy with a lignocellulosic ethanol production facility, which has an MSP equal to $5.13/GGE, reveals the attractiveness of this LA-based catalytic approach.