External Mass Transfer Limitations Research Articles

Binding kinetics of antigen by immobilized antibody: influence of reaction order and external diffusional limitations

The influence of reaction order and external mass transfer limitations on the binding kinetics of antigen in solution to antibody covalently or non-covalently attached to a cylindrical fiber-optic biosensor is presented. Both single-step and dual-step binding of antigen to antibody is considered. The rate of attachment of antigen to antibody is linear for all reaction orders in the time frame (100 min) considered. The rate of attainment of saturation levels of antigen in solution close to the surface is very rapid (within 20 min). An increase in the presence of mass transfer (denoted by a Damkohler number increase) decreases the saturation level of the antigen close to the surface, and the rate of antigen attachment to the antibody covalently or non-covalently bound on the surface for one-half, one, one and a half, and second-order reaction. As intuitively expected, an increase in the initial antigen concentration in solution increases the saturation level of the antigen close to the surface, and the rate of antigen attachment to the antibody covalently bound on the surface for all the reaction orders considered. Non-dimensional plots presented in the analysis help extend the analysis to different antigen-antibody systems. A decrease in the external diffusional limitations has an effect of decreasing the effect of reaction order on the saturation levels of antigen close to the surface and the rate of attachment of the antigen in solution to the antibody on the surface.

Camet oxidation catalyst for cogeneration applications

Environmental pollution control regulations have required the use of oxidation catalysts to reduce emissions of carbon monoxide and unburned hydrocarbons from cogeneration plants. Oxidation catalyst performance requirements include low pressure drop and high conversion efficiency. The conversion limit is controlled by external mass-transfer limitations determined by the properties of the monolith substrate. Conventional monoliths contain identical channels that pass straight through the monolith and are parallel to the flow direction. This paper discusses the conversion and pressure drop performance of a new metal monolith substrate based on a folded metal foil that is corrugated in a herringbone pattern. The external mass- transfer pressure drop performance of this type of metal monolith catalyst is compared with conventional ceramic substrate based catalysts.

An investigation of heparinase immobilization.

A systematic investigation of the parameters that affect the efficiency of immobilizing heparinase onto cyanogen bromide activated crosslinked 8% agarose beads was conducted. Two experimental measures, the "fraction bound" and the "fraction retained," were used to monitor the coupling efficiency. The fraction bound is the portion of the total initial enzyme that is bound to the agarose gel. The fraction retained is the fraction of bound enzyme that is active. The product of the two measures indicates the coupling efficiency. The activity of the immobilized heparinase was measured under conditions free of both internal and external mass transfer limitations, and thus, the fraction retained represents the true immobilized enzyme activity. Increasing the degree of activation of the beads results in an increase in the fraction bound, the fraction retained, and consequently, the coupling efficiency. As the ratio of enzyme solution to gel volume increases from 1.5 to 2.2, the fraction bound remains constant but the fraction retained decreases (heparinase concentration; 0.15 mg/mL and degree of activation; 9.5 mumol of cyanate esters/g of gel). At volume ratios greater than 2.2, both the fraction bound and the fraction retained decline continuously. Changing the heparinase concentration in the coupling solution changes the coupling efficiency in a manner similar to that of the volume ratio change. When heparin is added during the coupling process, the fraction bound declines as the heparin concentration increases, whereas the fraction retained increases up to a heparin concentration of 12 mg/mL and decreases thereafter. When arginine, lysine, and glycine are used to block the unreacted cyanate ester groups after the coupling process, the immobilized heparinase shows different pH optima of 6.5, 6.9, and 7.2, respectively. Based upon these findings, a protocol to optimize heparinase immobilization is developed.

ON THE INFLUENCE OF EXTERNAL DIFFUSIONAL LIMITATIONS ON OPTIMUM TEMPERATURE OPERATIONS POLICIES IN DEACTIVATING FIXED-BED REACTORS

The influence of external mass transfer limitations on optimum temperature operations policies, and on the reaction and deactivation parameters is determined for a first-order primary reaction and first-and d th-order reactant-independent deactivation kinetics occurring in a fixed-bed reactor. This is now done using the correct optimum temperature operations criterion. The rate of temperature rise is lower when external mass transfer limitations are present compared to the diffusion-free condition. The apparent activation energy for reaction is not only different from its intrinsic value but it is also time-variant along the optimum temperature operations policy.

ETHYLENE OXIDATION ON SILVER CATALYSTS: EFFECT OF ETHYLENE OXIDE AND OF EXTERNAL TRANSFER LIMITATIONS

Ethylene oxidation to ethylene oxide and to carbon dioxide over silver catalysts was studied in a CSTR. The effects of two factors on the catalyst performance were examined. The first was the presence of excess ethylene oxide in the feed. A kinetic model was introduced which assumed that ethylene and ethylene oxide compete for the same sites on the catalyst surface. This model provided reasonable quantitative agreement with kinetic and potentiometric measurements. The second factor that was studied was the presence of external heat and mass transfer limitations. It was found that such limitations cause a significant decrease of the selectivity to ethylene oxide. This decrease is a result of the temperature difference between the catalyst surface and the bulk of the gas phase and of the fact that the activation energy of ethylene combustion is greater than that of ethylene epoxidation. The contribution of other factors such as inhibition by CO2 or possible incomplete mixing in the reactor is shown to be in...

A multispecies biofilm model

Using a continuum approach and observing conservation principles, an analytical mathematical model of microbial interaction in biofilms was developed. The model predicts changes in biofilm thickness and describes the dynamics and spatial distribution of microbial species and substrates in the film. It allows for biomass detachment due to shear stress and sloughing, external mass transfer limitations, as well as variations in substrate concentrations in the bulk liquid. A computer implementation of the model is provided using an example of heterotrophicautotrophic competition to illustrate how the observed phenomena can be numerically reproduced and indicating how they might affect overall biofilm performance.

Dynamics of Nitrification in a Biological Fluidized Bed Reactor

Dynamics of nitrification in a biological fluidized bed reactor was investigated with numerical simulations and experimentally in a small scale reactor. A mathematical model was developed describing reactor dynamics, substrate removal and biomass development. Variable microbial activity was used as a quantitative representation of the state of the microorganisms. External and internal mass transfer limitations were considered in substrate removal modelling. The effects of biofilm growth on reactor hydrodynamics and on substrate removal kinetics were incorporated into the model. In the experimental part, the performance of the nitrifying fluidized bed was monitored as the influent concentration was varied following an irregular square wave pattern. The observed changes of the effluent concentration depended not only on actual conditions but also on the past history of the system. Further analysis was carried with the help of the developed model. The concept of variable microbial activity was instrumental in explanation of the microbial dynamics. Predictive capacities of the properly calibrated model were demonstrated.

Mass transfer phenomena during potassium carbonate catalysed carbon steam gasification reactions in a microbalance setup

A description is given of the external mass transfer properties in a Cahn microbalance setup by means of naphthalene sublimation experiments. The functional relationship found can be expressed by: Sh = 1.2 + 1.2 Re 0.25 Sc 0.6 From studies of the potassium carbonate catalysed reaction of activated carbon with low pressure steam this relationship appears to be applicable up to temperatures of 1200 K. It is shown that both the reactor geometry and free convection have little influence on measured mass transfer coefficients at Reynolds numbers commonly applied in a microbalance. “Classical” flow rate variation tests in order to reveal external mass transfer limitations can lead to an incorrect interpretation of the mass transfer behaviour in a microbalance. It appears that carrier gas variations provide more reliable information. Experimentally determined effective diffusion coefficients within the carbon bed agree satisfactorily with theoretical values. It is finally shown that for carbon gasification reactions studied in a microbalance, the region of inter particle and external mass transfer limitations can be entered simultaneously.

A packed-bed balance reactor for gas adsorption and gas-solid reactions under elevated pressures

A packed-bed balance reactor (PBBR) is presented as a novel and relatively low-cost alternative to a microbalance for use in adsorption experiments and gas-solid reactions under elevated pressures. The apparatus consists of a packed-bed flow reactor placed, together with the inlet and outlet connections, on a commercially available electronic balance. This construction combines the advantages of a microbalance (direct weight measurement) with those of a packed-bed flow reactor (elevated pressure conditions, larger sample size and less severe external mass transfer limitations under flow conditions). The gas flow rate through the reactor does not affect the weight read-out, whereas the pressure dependency is caused by the amount of gas present in the reactor. The operating principle is demonstrated by CO2 adsorption on activated charcoal under flow conditions and by gasification of activated charcoal with CO2. Specific surface areas, calculated from adsorption isotherms, correspond well with those obtained from a conventional volumetric method. Gasification reaction rates based on the weight decrease equal those calculated from the gas flow rate and the product gas analysis.