George W. Roberts

Hydrodynamics of gas-lift reactors with a fast, liquid-phase reaction

Abstract The reactive absorption of CO 2 into concentrated KOH solutions was studied in an external-loop, gas-lift reactor. Three different inlet gas compositions were used: air, 50–50 vol% air–CO 2 , and pure CO 2 . The downcomer liquid velocity and the axial profile of the cross-sectionally averaged gas holdup in the riser were measured. The reaction is so fast that the CO 2 is consumed appreciably along the riser, and this causes a significant reduction in the liquid circulation relative to a system with no reaction. A one-dimensional, pseudo-steady-state model has been developed to describe the interactions of hydrodynamics, mass transfer, and chemical reaction for the bubbly flow regime in the riser. The model considers mass transfer from the gas to the liquid phase and its enhancement due to the chemical reaction, and is based on the spatially averaged equations of continuity, momentum, and macroscopic mechanical energy. The rate of liquid circulation, and the axial variation of gas holdup, gas composition, pressure, and gas and liquid velocity, are predicted. The gas/liquid mass transfer coefficient and the bubble radius at the sparger, neither of which was known a priori, were used to minimize the error of the data with respect to the model.

Kinetics of Enzymatic Depolymerization of Guar Galactomannan

A new mathematical model based on Michaelis Menten (MM) kinetics is developed to predict the changes in molecular weight distribution (MWD) during the enzymatic depolymerization of guar galactomannan. The model accounts for the effect of branching by considering the guar molecule as a substrate having three types of bonds with different MM kinetic parameters. The overall kinetics of the enzymatic reactions then can be represented in terms of composite kinetic parameters that are functions of the MM parameters for the individual bonds. The depolymerization is assumed to follow a random scission mechanism, in which an enzyme randomly attacks the substrate molecule at any one of the three types of bonds, and leaves the substrate on cleavage of the bond. Expressions for the variation in molecular weights during depolymerization are developed by applying moment generating techniques to the kinetic model. The model is evaluated against the complete MWD obtained using gel permeation chromatography. During the initial stages of depolymerization, the enzymatic reaction is in the zero-order regime of MM kinetics and the polydispersity index (PDI) increases with time. Subsequently, the PDI decreases as the depolymerization tends to follow first order kinetics. We also show that for a zero-order, random or nonrandom scission, the variation of PDI with time can exhibit a maximum. These analyses confirm that an increase in PDI during the depolymerization is not necessarily due to nonrandom scission, as previously concluded.

Decomposition of polymerisation initiators in supercritical CO2: a novel approach to reaction kinetics using a CSTR

Abstract A novel method for simultaneous determination of the decomposition rate constant, k D , and the initiator efficiency, f , of a free-radical polymerisation initiator has been developed. The decomposition is carried out in a continuous-stirred-tank reactor (CSTR) in the presence of a radical scavenger. Mass balances for the initiator and the scavenger show that both f and k D can be determined from a set of constant-temperature experiments with a varying reactor mean residence time. Experiments were conducted in supercritical carbon dioxide (scCO 2 ) using diethyl peroxydicarbonate (DEPDC) as the free-radical initiator and galvinoxyl as the free-radical scavenger. The decomposition of DEPDC was first-order between 338 and 358 K. The activation energy, E A , was 132 kJ/mol, the pre-exponential factor, A D , was 6.3×10 16 s −1 , and the initiator efficiency, f , was 0.6. These values are similar to those reported in the literature for DEPDC decomposition in traditional organic solvents. Varying the carbon dioxide pressure between 240 and 310 bar at 348 K had no effect on the value of k D . Transition state theory predicted no significant effect of pressure or solvent composition on the initiator decomposition rate constant.

Alcohol synthesis in a high-temperature slurry reactor

Abstract A family of hydrocarbon liquids has been discovered that is stable at temperatures up to about 400°C in the presence of synthesis gas. The performance of a commercial, ‘zinc chromite’, ‘high pressure’ methanol synthesis catalyst was evaluated in a slurry reactor using two of these liquids, decahydronaphthalene and tetrahydronaphthalene, to suspend the catalyst. The evaluation covered a range of temperatures from 275° to 425°C, total pressures from 6.9 to 17.2 MPa, H2/CO ratios from 0.5 to 2.0 and space velocities from 1500 to 10 000 sl kg(catalyst)−1 h−1. Methanol was the only significant product at the lower end of this temperature range. The methanol synthesis reaction was close to equilibrium at the highest temperature, and there were significant quantities of dimethyl ether, olefins, methane and carbon dioxide in the product. Catalyst performance was sensitive to the composition of the slurry liquid, but was relatively stable in decahydronaphthalene over a long period of time.

Broadening of molecular-weight distribution in solid-state polymerization resulting from condensate diffusion

A kinetic model for the solid-state polymerization of poly(bisphenol A carbonate) in a single particle has been developed and used to investigate the broadening of molecular-weight distribution as a result of slow condensate diffusion. The model is based on melt-phase transesterification kinetics and Fickian diffusion of phenol, the condensate, in the amorphous regions of the semicrystalline particle. Model predictions compare favorably to experimental data. When diffusion is slow compared to reaction, a condensate concentration gradient is established. This gradient induces a molecular-weight gradient, which results in a broadened overall molecular-weight distribution with an overall polydispersity above the theoretical limit for homogenous step-growth polymerization. As the mass transfer resistance inside the particle is decreased, the average molecular weight increases faster with time, and the overall polydispersity decreases. A stoichiometric imbalance of end groups decreases the obtainable molecular weight but mitigates the deleterious effects of slow condensate diffusion.

Catalytic destruction of methyl tertiary butyl ether (MTBE) with a Pt/Rh monolithic automotive exhaust catalyst

Abstract The destruction of methyl tertiary butyl ether (MTBE) was studied using a Pt/Rh monolithic automotive catalyst. Inlet temperatures to the catalyst ranged from about 450 to about 775 K, and the amount of oxygen in the feed ranged from 0 to about 120% of stoichiometric. The presence of carbon dioxide, water and i -octane (2,2,4-trimethyl pentane) in the feed was also investigated. The decomposition of MTBE into methanol and i -butene (2-methyl-1-propene) was kinetically significant at these conditions. Both methanol and i -butene were present in the effluent from the catalyst at all operating conditions. Carbon dioxide and water had no significant effect on the kinetics of MTBE disappearance. The rate of MTBE destruction was faster than the oxidation of i -octane.

Manipulation of hydrophobic interactions in associative polymers using cyclodextrin and enzyme

We examine a new approach to reversibly modulate hydrophobic interactions in associative polymers using cyclodextrins (CD) and enzymes that cause scission of the α-1, 4 linkages in cyclodextrins. The associative polymers have a comb-like structure with pendant hydrophobic groups randomly attached to the polymer backbone. The intermolecular interaction between hydrophobic groups forms a transient network resulting in thickening of solutions containing the polymer. The CDs, doughnut-shaped cyclic polysaccharides, encapsulate the hydrophobes within their hydrophobic cavity and eliminate hydrophobic interactions. This results in several orders of magnitude reduction in solution viscosity and other viscoelastic properties. Subsequent degradation of the CDs by enzymes restores the hydrophobic interactions and the original rheological properties. A rheokinetic model is developed to describe the kinetics of the enzymatic reactions. The model accounts for equilibrium between the CD bound to the hydrophobes and free CD in solution and assumes the enzyme hydrolyzes only the free CD in the solution, which causes the release of the bound CDs in order to maintain equilibrium. The reaction is assumed to follow Michaelis Menten kinetics and the kinetic parameters are determined by tracking the changes in the viscoelastic properties of the polymer solution during the enzymatic scission of CD. The effects of enzyme concentration, polymer concentration and temperature on the rate of recovery of the original rheological properties are experimentally determined, and used to validate the trends of the rheokinetic model.

Cyclodextrin–hydrophobe complexation in associative polymers

We develop a new rheology-based method to study the complexation of cyclodextrins with hydrophobes in hydrophobically modified associative polymer solutions. The associative polymers have comb-like structure with hydrophobic groups randomly attached to the polymer backbone. Intermolecular interactions between the hydrophobic groups form a transient network resulting in thickening of the polymer solutions. On addition of cyclodextrins (CD) to the solution, the hydrophobes are encapsulated within the hydrophobic cavity of the cyclodextrins. This reduces viscoelastic properties of the polymer solution by several orders of magnitude. We exploit the existence of a dynamic equilibrium between CD adsorbed to the hydrophobes and free CD in the solution, to develop a rheology-based Langmuir-type adsorption isotherm for estimating the binding constant for molecular complexation. The model is based on the assumption that the amount of CD adsorbed is proportional to the reduction in elastic modulus of the polymer solution due to the encapsulation of the network junctions by CD. The effects of temperature on binding constant are studied to estimate the enthalpy and entropy of complexation. Experiments are conducted with both α-and β-CD at different polymer concentrations and temperatures to estimate the relative strength of binding of the CDs. At a given temperature and a polymer concentration, α-CD has a lower binding constant compared to that of β-CD, indicating higher affinity of α-CD to adsorb onto the hydrophobes. Since α-CDs have a smaller ring size, they can snugly fit to the hydrophobes and the association leads to higher enthalpy and entropy change.

Continuous precipitation polymerization of vinylidene fluoride in supercritical carbon dioxide: Molecular weight distribution

The surfactant-free precipitation polymerization of vinylidene fluoride (VF2) in supercritical carbon dioxide was studied in a continuous stirred autoclave. The polymerization temperature ranged from 65 to 85°C, the average residence time in the reactor varied from 10 to 50 min., and the pressure was between 210 and 305 bar. Diethyl peroxydicarbonate was used as the initiator. The fractional conversion of monomer varied from 7 to 26%, the number-average molecular weight of the polymer was between about 14,000 and 79,000, and the weight-average molecular weight was between about 21,000 and 700,000. In many cases, the polymer exhibited a bimodal molecular-weight distribution, especially at high monomer concentrations.

Deactivation of Pt/alumina catalysts for the hydrodechlorination of 1,1,1-trichloroethane

Vapor phase catalytic hydrodechlorination of 1,1,1-trichloroethane (TCA) has been studied using various supported platinum catalysts in a plug microflow reactor. The reactor was operated at temperatures ranging from 250 to 350°C, a H 2 :TCA:He ratio of 10:1:89, a space velocity of 24 L/g cat-h, and atmospheric pressure. To study the deactivation process, tests were carried out by dividing the catalyst bed into three segments (inlet, middle, outlet) separated by glass wool plugs. Although the catalysts showed high initial activity, rapid deactivation was also observed. For example, when using a Pt/η-alumina catalyst at 250°C, essentially complete TCA conversion was observed initially; however, after 15 h TCA conversion had declined to