Adesoji A. Adesina

Hydrocarbon synthesis via Fischer-Tropsch reaction: travails and triumphs

Abstract The present paper provides a synopsis of the Fischer-Tropsch (FT) synthesis for hydrocarbons. Emphasis is placed on issues related to catalyst development and design strategies, contemporary views on reaction kinetics and mechanism and the significant advancement in industrial reactor technology. Activity and product selectivity control still remain major incentives for continued research activities and the recent application of multimetallic catalysts appears promising. The article also highlights the usefulness of periodic operation of FT reactors for product distribution, time-average catalyst longevity and synthesis rates.

Selective adsorption of uranium(VI) from aqueous solutions using the ion-imprinted magnetic chitosan resins

The ion-imprinted magnetic chitosan resins (IMCR) prepared using U(VI) as a template and glutaraldehyde as a cross-linker showed higher adsorption capacity and selectivity for the U(VI) ions compared with the non-imprinted magnetic chitosan resins (NIMCR) without a template. The results showed that the adsorption of U(VI) on the magnetic chitosan resins was affected by the initial pH value, the initial U(VI) concentration, as well as the temperature. Both kinetics and thermodynamic parameters of the adsorption process were estimated. These data indicated an exothermic spontaneous adsorption process that kinetically followed the second-order adsorption process. Equilibrium experiments were fitted in Langmuir, Freundlich, and Dubinin-Radushkevich adsorption isotherm models to show very good fits with the Langmuir isotherm equation for the monolayer adsorption process. The monolayer adsorption capacity values of 187.26 mg/g for IMCR and 160.77 mg/g for NIMCR were very close to the maximum capacity values obtained at pH 5.0, temperature 298 K, adsorbent dose 50 mg, and contact time 3 h. The selectivity coefficient of uranyl ions and other metal ions on IMCR indicated an overall preference for uranyl ions. Furthermore, the IMCR could be regenerated through the desorption of the U(VI) ions using 0.5 M HNO(3) solution and could be reused to adsorb again.

Computational fluid dynamic (CFD) simulation of a pilot-scale annular bubble column photocatalytic reactor

The behavior of an 18-l pilot-scale photocatalytic reactor has been investigated using a computational fluid dynamic (CFD) approach. The granular Eulerian model was used to describe the multiphase flow system. Solid recirculation was predicted while liquid velocity vectors were influenced by the gas flow. The companion radiation transport equation was iteratively solved using a finite-volume-based discrete ordinate method. The first-order photodegradation kinetics of spent Bayer liquor previously studied in the same reactor was used to evaluate the CFD simulation. A Pearson correlation coefficient 0.974 between simulated and experimental data is indicative of model adequacy.

Improved alkene selectivity in carbon monoxide hydrogenation over silica supported cobalt-molybdenum catalyst

Abstract The Fischer-Tropsch synthesis performance of a Co-Mo bimetallic catalyst containing 6Co:lMo:4K:100SiO 2 has been studied and compared with a similar monometallic cobalt catalyst. The bimetallic system showed nearly 100% improvement in alkene-alkane ratio at the experimental conditions of 101 kPa and 280°C (553 K) and CO-H 2 ratio of 1:19 to 19:1. Ethylene-to-methane ratio (EMR) over the Co-Mo catalyst was about 70-100% better than the cobalt catalyst over the wide composition range studied. The kinetics over the Co-Mo catalyst also followed conventional Anderson-Schulz-Flory (ASF) polymerisation kinetics suggesting that there is no fundamental change in the reaction mechanism. The increase in chain growth probability and alkene content was attributed to substantial methane suppression caused by the introduction of molybdenum to the catalyst. Activation energy values for the light hydrocarbons over the bimetallic catalyst were in the range 85–120kJ/mol and about 10-15% lower than the corresponding estimates over the monometallic system.

Thermolysis of hydrogen sulphide in an open tubular reactor

Abstract This paper addresses the kinetic aspects of the thermal decomposition of H2S in a flow reactor. Experiments conducted in a quartz tube with argon/H2S feed over a wide composition spectrum (20–100% H2S) at four temperatures (1030–1070 K) show that the reaction is essentially first order in H2S partial pressure. Theoretical models based on a free radical mechanism involving the abstraction of hydrogen from H2S as the rate-determining step show the linear dependency of thermolysis rate on reactant composition. Hydrogen yield also increases monotonically with feed composition at all temperatures. Interestingly, the activation energy for H2 production of 200 kJ mol−1 is lower than that for the global decomposition reaction of about 241 kJ mol−1, consistent with the view that the initiation step requiring the breaking of the HS bond is the rate-controlling step rather than the termination involving the combination of two hydrogen radicals.

Three-phase catalytic hydrogenation of α-methylstyrene in supercritical carbon dioxide

Abstract The three-phase catalytic hydrogenation (TPCH) of α-methylstyrene using supercritical carbon dioxide (scCO 2 ) in a slurry reactor is reported. Kinetic data are presented for the reaction at 323 K over the range of pressure from 7.0 to 13.0 MPa using a carbon-supported palladium catalyst. The experimental data are fitted to a first-order power-law model. A detailed explanation of the methodology used to isolate the effect of CO 2 on the rate of reaction is presented. Particular attention is given to the phase behaviour of the reaction system and the volumetric expansion of the liquid phase with CO 2 . It is shown that scCO 2 significantly enhances the rate of reaction. This effect is attributed to the enhancement of the solubility of hydrogen in the liquid phase.

CFD Analysis of the Radiation Distribution in a New Immobilized Catalyst Bubble Column Externally Illuminated Photoreactor

A new externally irradiated photoreactor configuration combining the excellent mass transfer characteristics of a bubble column operation with the separation power of an immobilized catalyst on quartz plates has been investigated using computational fluid dynamics (CFD) simulation. The radiative transport equation (RTE) in conjunction with the Navier-Stokes equations were solved to obtain the light incident radiative flux and the light absorbed by the immobilized titania as a function of the gas superficial velocity, the angle of inclination, and the separating distance between the plates. The model employed water and air as the fluid phases and the results indicated that gas bubbling considerably increased the incident radiation in the gas-liquid mixture enhancing the radiative flux and the absorbed radiation on the titania-coated plates. The CFD results pave the way for the optimization of a solar photocatalytic reactor for the degradation of organic pollutants.

Fischer-Tropsch synthesis under periodic operation

Abstract The published literature is reviewed on the performance of the Fischer-Tropsch Synthesis (FTS) under forced cyclical changes in feed composition. Six catalysts have been investigated by two separate research teams. Most studies have employed a strategy using periodic exposure of the catalyst to pure H2. Experimental evidence is that H2 pulsing provides a significant increase in the time-average rate of formation of the lower carbon number paraffins for all the catalysts considered. Cobalt is the only catalyst for which olefin production also increases. For the lower paraffins, formation rates exceed the maximum rates attainable under steady-state operation at a specified temperature and pressure. Product distribution is also modified by H2 pulsing, but the modification depends on the catalyst. For Ru and Co catalysts, there is a decrease in the mean carbon number and a shift towards paraffinic products. The Mo catalysts investigated showed an increase in the mean carbon number under composition forcing, but at cycle periods that depress rates of hydrocarbon formation. With Fe, CH4 formation is strongly stimulated, but the product distribution of the other hydrocarbon products is unchanged. FTS mechanisms proposed in the recent literature seem adequate to explain qualitatively the composition forcing experiments. There is opportunity for further investigation and suggestions for such are given.

Particle residence time distribution (RTD) in three-phase annular bubble column reactor

Abstract The residence time distribution of solids in an 18-l pilot scale three-phase annular reactor has been obtained as a function of gas and liquid flow rates in order to characterize the influence of fluid dynamics on solids mixing. Both co-current and countercurrent operations were investigated. Gas velocity in the range 0.05– 0.5 cm s −1 was employed, while the liquid velocity was varied between 0.03 and 0.13 cm s −1 . Commercial titania powder was used as a tracer. The RTD curves for the co-current operation showed a bimodal behavior whose peaks decreased with decreasing gas flow rate. As a result, the solid flow dynamics was modeled by a series of CSTRs in parallel with a plug flow reactor. The model also admitted interphase solid transport between the wake and bulk liquid phase and yielded predictions with a mean squared error between 3% and 6%. The RTD behavior for countercurrent operation was, however, unimodal and therefore modeled as a series of stirred tanks with a recycle stream. The associated mean squared error was in the range 1–4%. Although fluid flow rates were held constant during tracer experiments, there seemed to be an evolution of solids flow pattern within the reactor as evidenced by the time-dependency of the intensity function, λ .

Isobutene hydration over Amberlyst-15 in a slurry reactor

Abstract The synthesis of tertiary butyl alcohol (TBA) via isobutene (iB) hydration was studied over Amberlyst-15 sulfonic acid catalyst particles using pure water and aqueous TBA solutions in a bubbling slurry reactor. Preliminary studies to investigate mass transfer effects showed that pore diffusion was present for catalyst particles greater than 165 μm in diameter. Therefore, intrinsic kinetic measurements were made using 90.5 μm catalyst particles and a catalyst loading of 10 kg m −3 . The kinetic measurements revealed that iB hydration is a pseudo-first-order reaction with an activation energy of 69 kJ mol −1 . Isobutene hydration experiments using TBA concentrations in water revealed a hindering effect of TBA, which indicates that separation of TBA formed by iB hydration in three-phase reactors using catalytic distillation is promising from a process design perspective.