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We have performed a combined quantum mechanical and microkinetic modeling study to understand the nascent decomposition pathways of methane pyrolysis, catalyzed by gas-phase ZnCl

We employ ab initio density functional theory calculations to investigate the mechanisms of phenol (a model compound for lignin-derived bio-oil) alkylation with propylene to 2-isopropylphenol, in the presence and absence of water on H-BEA. We also compute the reaction pathways for phenol alkylation in the uncatalyzed gas phase to understand H-BEA catalysis. In the uncatalyzed gas phase, phenol acts as a self-catalyst, activating propylene by transferring a proton from phenolic oxygen and facilitating the carbon–carbon bond formation reaction. We find that the lowest barrier process proceeds via an intermediate both in the uncatalyzed gas phase and on dry H-BEA. On H-BEA, the intermediate is stabilized by the transfer of a zeolitic proton. Our calculations show that, in comparison to the uncatalyzed gas phase, H-BEA reduces the barrier for converting the reactant complex to the intermediate by ∼40 kJ mol−1, which translates to a four order-of-magnitude increase in the transition-state theory rate coefficient at 350 °C. However, we find that the rate coefficient for the next step, which is also the rate-limiting step, remains unchanged on H-BEA. We further find that the presence of water reduces the activation barrier for this step—i.e., the rate-limiting step—by ∼30 kJ mol−1, which leads to a two order-of-magnitude increase in the transition-state theory rate coefficient at 350 °C.

Hemicelluloses are the earth’s second most abundant structural polymers and are found in lignocellulosic biomass. This work uses a tightly coupled experimental and theoretical analysis to show that the enzymatic hydrolysis of hemicellulose is a two-phase multiscale reactive process, comprising the molecular scale, the pore scale, and the reactor scale. The pore scale is the smallest of these three scales, and the sequence of the length scales is given by reactor scale > molecular scale > pore scale. Enzyme adsorption to the solid xylan particles is the first important step in hemicellulose hydrolysis, followed by the cleaving of β-(1 → 4)-glycosidic bonds by an endoenzyme in solid and liquid phases. Our experiments show that enzyme adsorption and solid-phase hydrolysis primarily occur on the pore surface, with the former, though initially nonequilibrium, attaining equilibrium at 5 h. At the molecular scale, the products (xylose and xylobiose) inhibit the hydrolysis noncompetitively in both liquid- and solid-phase hydrolyses, and the nature of product inhibition remains unaltered by the mixing at the reactor scale. Model–experiment comparisons allow us to quantify the adsorption and desorption rate constants as well as the kinetic constants in the solid- and liquid-phase hydrolyses. The optimum solid loading is obtained as 5 mg/mL, above which substrate inhibition sets in. We develop an “optimal mixing” strategy comprising 55–70 min of initial reactor mixing followed by no mixing for the rest of the hydrolysis, which increases xylose and reducing sugar yields by 6.3–8% and 13–20%, respectively, over continuous mixing at 150 rpm for 1–5 mg/mL xylan, with an energy saving of 94–96% on reactor mixing over 24 h. We quantify the dominant phenomena and the determining timescales at each length scale, and we show that efficient depolymerization of solid carbohydrates results from coupled interscale interactions at various stages of the two-phase hydrolysis process.

This work uses a coupled experimental and modeling approach to explore the effects of macro- and micro-mixing on the kinetics and the dynamics of two-phase enzymatic hydrolysis of hemicellulose. Reactor mixing does not alter the non-competitive nature of product inhibition in hemicellulose hydrolysis by endoxylanase, but produces stronger inhibition that reduces the soluble sugar yield by 8–14.5%, as the mixing speed increases from 0 to 200 rpm. The kinetic constants (Km, Vmax, Kx) assume mass-transfer disguised values at 0–200 rpm. An optimal mixing strategy, comprising of 55–70 min of initial rapid convective macromixing followed by diffusive micromixing (without any macromixing) for the rest of the hydrolysis, increases xylose and reducing sugar yields by 6.3–8% and 13–20%, respectively, over continuous mixing at 200 rpm, for 1–5 mg/ml substrate loading at an optimum enzyme to substrate ratio of 1:20, with an energy saving of 94–96% over 24 h.

We present a coupled experimental and theoretical framework for quantifying the kinetics of two-phase enzymatic hydrolysis of hemicellulose. For xylan loading of 1-5mg/ml, the nature of inhibition by the product xylose (non-competitive), the kinetic constants (Km=3.93 mg/ml, Vmax=0.0252 mg/ml/min) and the xylose inhibition constant (Kx=0.122 mg/ml) are experimentally determined. Our multi-step two-phase kinetic model incorporating enzyme adsorption to the solid substrate and non-competitive product inhibition is simulated using our kinetic data and validated against our experimentally measured temporal dynamics of xylose and reducing sugars. Further experiments show that higher substrate loading reduces the specific adsorption of the endoxylanase to the solid xylan and the enzyme's solid-liquid distribution ratio, which decelerates the solid hydrolysis and accelerates the liquid phase reactions. Thus, the xylose yield increases with substrate loading, which increases product inhibition and decreases reducing sugar yields. An operating cost analysis gives 3mg/ml as the optimal substrate loading.

Understanding the fundamental processes underlying the microbial fuel cells (MFCs) can provide valuable insights in recognizing the key limiting factors, the scope of improvement of the system which in turn helps in the scaling-up the process. The science behind an MFC is complex and it involves a subtle interplay of various fields such as microbiology, physics and electrochemistry (Zhang and Halme 1995). A comprehensive understanding of various parameters involved in the process is essential for the improvement of power generation and to explore further applications. Modelling the system prior to experimentation can provide various perspectives and alternatives saving time and money. The physics of the process can be understood using quantitative predictions using modelling. It also provides valuable information about the dynamics of a process and thus important in reactor design and scale-up. Consequently, efficient monitoring of the process as well as precise control may be achieved through modelling (Marcus et al. 2007). Multi-scale modelling is crucial for a well-defined understanding of the process at both micro and macro scales.

The present study investigates the extraction of lignin from rice straw by bi-directional delignification methodology through minimal energy input towards feedstock preparation for bio-fuel production. In the first stage, the rice straw was treated with sodium chlorite of pH 5.5 followed by sodium hydroxide while in the second stage with sodium hydroxide alone under controlled parameters like, concentration, time and temperature at constant liquor-to-solid ratio. The process was mathematically modeled using central composite design approach and optimized by quadratic regression model with ANOVA (analysis of variance) for showing the relative significance of the factors. The percentages of delignification in two different optimized routes were estimated to be 77.84 and 44.09 respectively. The loss of feedstock in terms of glucose concentration was measured to be 4.23 and 2.51% in the preceding two-stages. Hence, this dual route method well-supported by the impressive experimental results could be promising technique of delignification of rice straw for bio-fuel generation.

Computational and experimental study of a countercurrent direct contact membrane distillation (DCMD) module equipped with commercially available flat sheet hydrophobic polytetrafluoroethylene (PTFE) microporous membrane with polyethylene terephthalate (PET) and polypropylene (PP) support was investigated to remove toxic chromium (VI) from simulated water. PTFE/PET membrane exhibited better performance in terms of normalized flux. Liquid entry pressure (LEP) was used to characterize the thin film membrane. The effects of flowrate, chromium concentration, feed and permeate inlet temperature were studied with significant results on the flux and complete rejection. A two dimensional mathematical model was built up based on mass, momentum and energy balances to explore the effects of operating parameters on flux, flow conduit temperature distributions across entire domain and membrane surface temperatures across module length. The predictions exhibited good agreement with the experimental results using a modified coupled Knudsen and Poiseuille flow models.

Hemicelluloses are the earth’s second most abundant structural polymers, found in lignocellulosic biomass. Efficient enzymatic depolymerization of xylans by cleaving their β-(1 → 4)-glycosidic bonds to produce soluble sugars is instrumental to the cost-effective production of liquid biofuels. Here we show that the multi-scale two-phase process of enzymatic hydrolysis of amorphous hemicelluloses is dominated by its smallest scale–the pores. In the crucial first five hours, two to fourfold swelling of the xylan particles allow the enzymes to enter the pores and undergo rapid non-equilibrium adsorption on the pore surface before they hydrolyze the solid polymers, albeit non-competitively inhibited by the products xylose and xylobiose. Rapid pore-scale reactive dissolution increases the solid carbohydrate’s porosity to 80–90%. This tightly coupled experimental and theoretical study quantifies the complex temporal dynamics of the transport and reaction processes coupled across scales and phases to show that this unique pore-scale phenomenon can be exploited to accelerate the depolymerization of hemicelluloses to monomeric sugars in the first 5–6 h. We find that an ‘optimal substrate loading’ of 5 mg/ml (above which substrate inhibition sets in) accelerates non-equilibrium enzyme adsorption and solid hemicellulose depolymerization at the pore-scale, which contributes three-quarters of the soluble sugars produced for bio-alcohol fermentation.