Adsorption and desorption kinetics of hydrocarbons in FCC catalysts studied using a tapered element oscillating microbalance (TEOM). Part 1: experimental measurements

An experimental procedure to measure the adsorption and desorption kinetics of hydrocarbons in fluid catalytic cracking (FCC) catalysts using a tapered element oscillating microbalance is described. It enables adsorption rates to be measured on a timescale of about . Experiments using n-hexane, n-heptane, n-octane, toluene and p-xylene were performed on both a commercial FCC catalyst and a pure rare-earth exchanged zeolite Y sample under non-reacting conditions (temperatures of 373-). Heats of adsorption for these hydrocarbons are reported. The overall adsorption and desorption kinetics are found to depend on carrier gas flowrate in cases when adsorption is strong indicating that the length of the catalyst bed can have a significant influence on the observed kinetics. However, at high carrier gas flowrates the overall adsorption and desorption kinetics do not depend on the amount of catalyst present. It is found that the rates of adsorption and desorption of the hydrocarbons studied are the same for the FCC catalyst as for the pure zeolite Y sample. This means that mass transport in the matrix component of the FCC catalyst is rapid and not a limiting step in the adsorption process for the hydrocarbons studied in this work.     

Temperature oscillation calorimetry for the determination of the heat capacity in a small-scale reactor

This contribution describes a method for heat capacity determination in a small-scale reaction calorimeter under quasi-isothermal conditions ( to ). The heat capacity of the reactor content is calculated from the amplitudes and the phase shifts of the reactor temperature, of the electrical heater and of the cooling rate when forced temperature oscillations are applied. The heat capacities of eight solvents (water, ethanol, methanol, acetone, 1-octanol, diethylenglycol, toluene, and 1-butanol) covering a wide range of viscosity were calculated for various experimental conditions, including reactor volume and stirrer speed. Systematic deviations were detected when compared to the corresponding literature values. Straight line calibration with the total heat transfer coefficient and two modern multivariate calibration techniques (partial least squares and neural network) were applied to correct for these deviations. The different calibrations show similar precision and allow for an online determination of the heat capacity with an accuracy comparable to other published methods. Successful applications include the determination of the heat capacities for n-heptane, for various homogenous ethanol/water mixtures, and during the course of the hydrolysis of concentrated sulfuric acid.     

Lipase-catalysed hydrolysis of blackcurrant oil in supercritical carbon dioxide

The effect of reaction conditions on the extent of conversion in hydrolysis of blackcurrant oil was investigated. The enzyme used was Lipozyme, a lipase from Mucor miehei immobilised on macroporous anionic resin. The reaction was carried out in a continuous flow reactor at 10- and 30-50°C with carbon dioxide saturated with oil and water (55-100%) flowing up through the enzyme bed. Analysis of product composition indicated unfavourable hydrodynamics with significant mixing in the reactor when solvent interstitial velocity was lower than , while above this velocity value the flow pattern was near to plug flow. Lipase stability was very good with no activity reduction observed during a long-term experiment. The reaction rate was a function of the ratio of enzyme load to solvent volumetric flow rate. A complete hydrolysis of oil was achieved in the experiments carried out with the enzyme load of and CO₂ flow rate of 0.4-. The effects of pressure (10-) and temperature (30-40°C) on the reaction rate were small, and the effects of CO₂ saturation with water and of enzyme distribution in the reactor were negligible. Lipozyme displayed specificity towards linolenic acids; the release of α-linolenic acid was faster and that of γ-linolenic acid slower than the release of other constituent acids present in blackcurrant oil.     

Conformational end effects in unperturbed chains and star-branched polymers

End effects in unperturbed chains with a specified number of bonds, n, have previously been characterized using the conformations at individual bonds and the mean square unperturbed dimensions of subchains situated at various positions along the chain. Both criteria have found that the end effects are extremely weak in typical flexible chains, such as unperturbed polyethylene. Of course, these criteria detect no end effect at all in a freely jointed chain. This conclusion requires modification if the end effects are defined differently, using the mean square radius of gyration of the n+1 chain atoms about chain atom i, denoted . The values of depend on i in all chains, including the freely jointed chain. The range for the volume specified by suggests that the average density of chain segments about a terminal atom is roughly 1/3 of the average density of chain segments about the atom midway along the contour of any long unperturbed chain. The severity of these end effects in finite chains depends on the detailed local structure, becoming more severe for more extended chains. The end effect detected by in regular freely jointed star-branched polymers becomes more severe as the number of branches increases.     

Kinetics of absorption of carbon dioxide in aqueous piperazine solutions

In the present work the absorption of carbon dioxide into aqueous piperazine (PZ) solutions has been studied in a stirred cell, at low to moderate temperatures, piperazine concentrations ranging from 0.6 to , and carbon dioxide pressures up to 500 mbar, respectively. The obtained experimental results were interpreted using the DeCoursey equation [DeCoursey, W., 1974. Absorption with chemical reaction: development of a new relation for the Danckwerts model. Chemical Engineering Science 29, 1867-1872] to extract the kinetics of the main reaction, 2PZ+CO₂→PZCOO^-+PZH⁺, which was assumed to be first order in both CO₂ and PZ. The second-order kinetic rate constant was found to be at a temperature of , with an activation temperature of . Also, the absorption rate of CO₂ into partially protonated piperazine solutions was experimentally investigated to identify the kinetics of the reaction . The results were interpreted using the Hogendoorn approach [Hogendoorn, J., Vas Bhat, R., Kuipers, J., Van Swaaij, W., Versteeg, G., 1997. Approximation for the enhancement factor applicable to reversible reactions of finite rate in chemically loaded solutions. Chemical Engineering Science 52, 4547-4559], which uses the explicit DeCoursey equation with an infinite enhancement factor which is corrected for reversibility. Also, this reaction was assumed to be first order in both reactants and the second-order rate constant for this reaction was found to be at 298.15 K.     

Validation of a population balance model for olivine dissolution

The dissolution of silicate minerals, among them olivine, in water enables its subsequent reaction with carbon dioxide to form magnesium carbonate, a process called aqueous mineral carbonation. A general model for the dissolution of olivine, based on a population balance approach, has been developed. For this purpose, the dissolution rate of olivine has been measured as a function of varying particle size and pH at . Three separate particle populations in three different size ranges were used: a sub 90, a 90-180, and a 180- size fraction. The pH was varied between 2 and 4.75 using HCl. Experiments were carried out in a flow-through reactor under a nitrogen atmosphere of 20 bar. The dissolution extent varied from 12% up to complete dissolution, depending on experimental conditions. Particle size distributions of the different size fractions were measured with a Coulter Multisizer^®. Using the assumption of a surface controlled reaction, the solution to the population balance equation was coupled with a reactor model. Data were fitted to the model to obtain a shape modulated dissolution rate, , combining the volume shape factor k_v with the dissolution rate D, which has the dimensions of a velocity. Including earlier published dissolution experiments an overall correlation for the dissolution rate was regressed. Using the general model, the limitations of the simplified model employed in an earlier publication are illustrated.     

Kinetics and equilibrium of dissolved oxygen adsorption on activated carbon

The macroscopic adsorption behavior of dissolved oxygen on a coconut shell-derived granular activated carbon has been studied in batch mode at 301 and 313 K for initial dissolved oxygen concentrations of 10-30 mg/l and oxygen/carbon ratios of 2-180 mg/g. BET (Brunauer, Emmett, and Teller) surface area, micropore volume, and pore size distribution were determined from N₂ isotherm data for fresh and used samples of carbon. The surface groups were characterized using Boehm titrations, potentiometric titrations, and FTIR study. The material is characterized by its high specific surface area , microporocity (micropore volume ), its basic character ( total basic groups) and its high iron content (15,480 ppm Fe). BET n-layer isotherm describes adsorption equilibrium suggesting cooperative adsorption and important adsorbate-adsorbate interactions. Kinetic data suggest a process dependent on surface coverage. At low coverage a Fickian, intraparticle diffusion rate model assuming a local equilibrium isotherm (oxygen dissociation reaction) adequately describes the process. The calculated diffusion coefficients (D) vary between and for initial oxygen concentration of 10 and 20 mg/l, respectively. Sensitivity analysis shows that the oxygen dissociation equilibrium constant determines the equilibrium concentration, whereas the diffusion coefficient controls the kinetic rate of the adsorption process having no effect at the final equilibrium concentration. A combined kinetic mass transfer model with concentration-dependent diffusion (parabolic form) has been developed and successfully applied on the dissolved oxygen adsorption system at high surface coverage. For equilibrium uptake of the estimated mean mass transfer coefficient and adsorption rate constant are and , respectively.     

Morphology of conducting polymeric coatings: quantitative comparison between theory and experiment

The density profile of electrosynthesized polymeric layers on carbon fibers is obtained from gravimetric and scanning electron microscopy (SEM) data. The coating density ρ is found to decrease with increasing radial distance, r, subject to the power law . Based on a modified diffusion-limited aggregation (DLA) model, it is shown that lattice Monte Carlo simulation of the coating process predicts the experimentally observed power law exponent.     

Laser desorption-ionization time of flight mass spectrometry of various carbon materials

The possibilities of commercially available MALDI-TOF mass spectrometric instrumentation equipped with 337 nm nitrogen laser in carbon materials analysis have been examined. Laser desorption-ionization of synthetic diamond, graphite, glassy carbon, carbon nano-tubes, and diamond-like thin layer results in the formation of two positively charged ( and odd-numbered 44-346) and one negatively charged carbon cluster set (). Fullerene C₆₀ was analyzed for comparison. From the clusters mass spectra, several similarities, concerning “magic number” ions with high signal intensities, can be observed. It is concluded that differing carbon materials are subjected by UV-laser pulses to similar physico-chemical changes. Heavy carbon clusters of low stability and characteristic loss of neutral C₃ particle in case of the smaller clusters during post-source decay (PSD) implies that the species observed do not possess stable fullerene structure, which might have been expected for C_n species with n ? 20.     

Catalyst deactivation and engineering control for steam reforming of higher hydrocarbons in a novel membrane reformer

The catalyst deactivation and reformer performance in a novel circulating fluidized bed membrane reformer (CFBMR) for steam reforming of higher hydrocarbons are investigated using mathematical models. A catalyst deactivation model is developed based on a random carbon deposition mechanism over nickel reforming catalyst. The results show that the reformer has a strong tendency for carbon formation and catalyst deactivation at low steam to carbon feed ratios for high reaction temperatures and high pressures . The trend is similar for the cases without and with hydrogen selective membranes. Based on this preliminary investigation, an engineering control approach, i.e., in-site control with a concept of critical/minimum steam to carbon feed ratio, is proposed and used to determine the carbon deposition free regions for both cases without and with hydrogen membranes. The comparison between the reported data and model simulation shows that the critical steam to carbon feed ratio predicted by the model agrees well with the reported industrial/experimental operating data.     

Study of supercritical three-phase methanol synthesis with n-hexane at supercritical state

A process of supercritical three-phase methanol synthesis on a Cu-based catalyst C302-2, which has high activity at low temperature and low pressure, has been carried out in a mechanically agitated slurry reactor with paraffin as the inert liquid medium and n-hexane as the supercritical medium. The reaction conditions are as follows: pressure ranging from 6.0 to , temperature ranging from 235 to and mass space velocity from 450 to . The influences of these conditions on the conversion of CO and the outlet methanol mole fraction have been investigated in detail. The results show that both the conversion of CO and outlet methanol mole fraction decreased when the mass space velocity and the temperature were increased under the condition of supercritical n-hexane. In addition, we compared the three-phase slurry bed methanol synthesis with and without supercritical medium. The results show that the conversion of CO, CO₂ and H₂ as well as outlet methanol mole fraction of supercritical three-phase methanol synthesis are obviously higher than those chemical equilibrium values of gas-solid reaction under the corresponding experimental condition. That is to say, the process of supercritical three-phase methanol synthesis with n-hexane at supercritical state can remove the limitation of chemical reaction balance of the reversible exothermic methanol synthesis reaction on the conversion of reactants by introducing a supercritical medium that plays an important role in the reaction-separation coupling process in methanol synthesis, by which the conversion of reactants and outlet methanol mole fraction at supercritical condition are increased greatly. Therefore, they are higher than those of three-phase methanol synthesis without supercritical n-hexane. The advantage of supercritical three-phase methanol synthesis is self-evident. Our present study provides an experimental foundation for further engineering exploitation research on the three-phase methanol synthesis process with supercritical medium in three-phase slurry reactors.     

Sorption effect on kinetics of etherification of tert-amyl alcohol and ethanol

The kinetics of the etherification of tert-amyl alcohol with ethanol, catalysed by a strong ion-exchange resin, Amberlyst 15, has been studied in a batch reactor at temperatures between 323 and and at a pressure of . The kinetics was described by a coupled sorption-reaction model. Flory-Huggins (F-H) model was used to predict the solubility of reaction components in the resin phase at (non-reactive conditions). Both sorption interaction parameters of F-H model and kinetic constants were determined by non-linear least-squares method. Langmuir-Hinshelwood (L-H) and power law (PL) kinetic models were compared with the experimental data. Although both the models were found to be equally successful, L-H model has shown a slightly better representation than PL model.     

Oxidation kinetics of Iron(II) complexes of trans-1,2-diaminocyclohexanetetraacetate (cdta) with dissolved oxygen: Reaction mechanism, parameters of activation and kinetic salt effects

The present investigation takes concern about a spiny environmental problem afflicting the pulp mill industry exploiting the Kraft sulfate-pulp process where dilute total reduced sulfur contaminants are co-mixed with oxygen in large-volume gas effluents. A potential Redox process for removing the total reduced sulfurs consists in oxidizing them by means of iron(III) organometallic complexes while the co-mixed oxygen mediates the oxidative regeneration of iron(II) into iron(III) complexes. In this work, the oxidation kinetics of iron(II) trans-1,2,-diaminocyclohexanetetraacetate (cdta) complexes with molecular oxygen (O₂) as the source oxidant was investigated for a wide pH range (1.75<pH<12) in a 3.2 dm³ single-phase stirred cell reactor within the [281-323 K] temperature range. Simultaneous measurements of iron(II)-cdta (50-) and O₂ (0.5-) were used to clarify the reaction mechanism which has been interpreted differently in previous works. The observed kinetic data in alkaline solutions could be accounted for in terms of three forward [Fe²⁺cdta^4-+O₂ (rate-limiting, k_1,app), , 2Fe²⁺cdta^4-+H₂O₂] and one reverse [ (k_-1,app,n=0 or 1)] elementary steps. Assessment of the rate-limiting apparent rate constant led to the following results ( at and , , ). Fe³⁺OH^-cdta^4-, being the dominating iron(III) product at pH>10, was found to be less reactive than Fe³⁺cdta^4- with the superoxide intermediate , thus reducing the effect of the reverse step at higher pH. A study on the effect of electrolytes on the reaction rate led to the conclusion that salts increase the rate constant k_1,app. Finally, kinetic results in acidic conditions leading to the formation of other iron(II)-cdta complexes (i.e., Fe²⁺cdta^4-H⁺) and another superoxide intermediates are reported and discussed.     

A kinetic study on anaerobic reduction of sulphate, part II: incorporation of temperature effects in the kinetic model

The effects of temperature on the kinetics of anaerobic sulphate reduction were studied in continuous bioreactors using acetate as an electron donor. Across the range of temperatures applied from 20 to , the increasing of volumetric loading rate up to 0.08 to resulted in a linear increase in reduction rate of sulphate. The increasing reaction rate showed a lower dependence on volumetric loading rate in the range 0.1-. Further increase in volumetric loading rate above was accompanied by wash out of bacterial cells and a sharp decrease in reaction rate. Despite a similar pattern for dependency of reaction rate on volumetric loading at all temperatures tested, the magnitude of reaction rate was influenced by temperature, with a maximum rate of observed at . The effect of temperature on maximum specific growth rate (μ_max) and bacterial yield was insignificant. The values of maximum specific growth rate and yield were and 0.56-0.60 kg bacteria (), respectively. The decay coefficient (k_d) and apparent saturation constant () were both temperature dependent. The increase of temperature resulted in decreased values of , and higher values for k_d. Using the experimental data effect of temperature was incorporated in a kinetic model previously developed for anaerobic reduction of sulphate.