BackgroundDifferent mechanistic models have been used in the literature to describe the enzymatic hydrolysis of pretreated biomass. Although these different models have been applied to different substrates, most of these mechanistic models fit into two- and three-parameter mechanistic models. The purpose of this study is to compare the models and determine the activation energy and the enthalpy of adsorption of Trichoderma reesei enzymes on ammonia fibre explosion (AFEX)-treated wheat straw. Experimental enzymatic hydrolysis data from AFEX-treated wheat straw were modelled with two- and three-parameter mechanistic models from the literature. In order to discriminate between the models, initial rate data at 49°C were subjected to statistical analysis (analysis of variance and scatter plots).ResultsFor three-parameter models, the HCH-1 model best fitted the experimental data; for two-parameter models Michaelis-Menten (M-M) best fitted the experimental data. All the three-parameter models fitted the data better than the two-parameter models. The best three models at 49°C (HCH-1, Huang and M-M) were compared using initial rate data at three temperatures (35°, 42° and 49°C). The HCH-1 model provided the best fit based on the F values, the scatter plot and the residual sum of squares. Also, its kinetic parameters were linear in Arrhenius/van't Hoff's plots, unlike the other models. The activation energy (Ea) is 47.6 kJ/mol and the enthalpy change of adsorption (ΔH) is -118 kJ/mol for T. reesei enzymes on AFEX-treated wheat straw.ConclusionAmong the two-parameter models, Michaelis-Menten model provided the best fit compared to models proposed by Humphrey and Wald. For the three-parameter models, HCH-1 provided the best fit because the model includes a fractional coverage parameter (ϕ) which accounts for the number of reactive sites covered by the enzymes.
Based on the well-known mechanism describing Michaelis-Menten kinetics, three rate expressions may be developed: the exact solution (Model 1), a rate equation resulting from the pseudo-steady-state assumption (Model 2), and Model 2 with the additional assumption that the amount of free substrate is approximately equal to the total amount of substrate (Model 3). Although Model 1 is the most precise, it must be integrated numerically and it requires three experimentally determined parameters. Models 2 and 3, however, are simpler and require only two parameters. Using dimensionless forms of the three models, we have evaluated the errors in the two simplified models relative to the exact solution using a wide range of parameter values. The choice of model for reactor design depends on the initial substrate to enzyme ratio (alpha(0)), and on the ratio of the Michaelis-Menten constant to the enzyme concentration (sigma). Based on a 2% model error criteria, when alpha(0) > 15 or sigma >or= 100, Model 3 is adequate; if 5 < alpha(0) < 15, or if sigma >or= 10, then Model 2 may be used; and if alpha(0) < 5 and sigma < 10, then the exact solution (Model 1) is required.
The effect of process variables (temperature, pressure, space velocity, and H2/CO feed ratio) on the Fischer‐Tropsch synthesis on a promoted fused iron catalyst was studied in a slurry phase stirred tank reactor (STR). Operating conditions were chosen such that some of the data could be compared with previously reported results, but experiments were also performed at more extreme conditions (temperatures up to 280°C and pressures up to 2.86 MPa) than previously studied in a slurry phase STR. The catalyst activity compares well with previous studies at an H2/CO feed ratio of 1.0 and 1.8, but the activity is lower than previously reported values in the 0.64‐0.72 range of H2/CO feed ratios. Spacetime‐yield increases with pressure, and reactor productivity is best increased by increasing pressure at a constant pressure to space velocity ratio. The water‐gas‐shift reaction is near equilibrium at high conversions, and always proceeds at a slower rate than the Fischer‐Tropsch synthesis in a STR.
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