Anurag Mehra

Gas absorption in the presence of particles showing interfacial affinity : case of fine sulfur precipitates

Abstract Enhancements in the absorption rate of acetylene into aqueous solutions of iron chelates of HEDTA, caused by the presence of fine, freshly precipitated sulfur particles, have been investigated in a stirred cell reactor with flat interface. The acetylene gas is essentially inert towards the iron chelate, whose presence is required solely for in situ precipitation of sulfur from hydrogen sulfide, i.e. for introducing sulfur into the system prior to acetylene absorption. The sulfur particle size distributions were determined using Coulter Counter measurements. The rate data were interpreted by developing, and using, a heterogeneous, unsteady state mass transfer model based on Danckwerts’ surface renewal theory and particle-to-interface adhesion. The enhancement factor is shown to depend upon the fraction of the interface ‘covered’ by the solid particles, the average particle size, the distribution coefficient of the solute partitioning between the solid and the liquid phases and the physical gas–liquid (liquid side) mass transfer coefficient. The model predicts the trends in the enhancements correctly, and has been shown to interpret the experimental data reasonably well. As a reference case, the absorption of acetylene into slurries of fine carbon particles in similar liquid media was also studied. The sulfur and carbon particles indeed behave similarly and the additional data on carbon particle systems also points to the qualitative validity of the proposed model. Nevertheless, the new model seems to be too simple for predicting quantitative agreement.

Intensification of multiphase reactions through the use of a microphase—I. theoretical

Abstract A comprehensive theory of mass transfer with chemical reaction in the presence of a microphase has been developed using Higbies penetration theory as well as the surface renewal theory due to Danckwerts. The conditions under which the various special cases hold are derived especially with regard to steady state approximations where the simpler film model for mass transfer may be used. Analytical solutions have been given wherever possible. The general applicability as well as the flexibility of the theory is demonstrated by analysing a variety of physical situations like the case of reversible reaction of the diffusing solute with the microphase and that of instantaneous reaction between the solute and a dissolved liquid (continuous) phase reactant. The effect of the rate constants in either phase, the mass transfer coefficient, microphase constituent size and its Brownian diffusivity, as well as the distribution coefficient of the diffusing solute between the continuous and the microphases, on the specific reaction rate are investigated. A comparison with some of the earlier models in the literature has been given.

Intensification of multiphase reactions through the use of a microphase—II. experimental

Abstract The specific rates of alkaline hydrolysis of solid esters—2,4-dichlorophenyl benzoate, p -chlorophenyl benzoate and phenyl benzoate—in the presence of a second emulsified liquid phase were measured in a 67 mm i.d. mechanically agitated contactor. For 2,4-dichlorophenyl benzoate the above study was also carried out in microemulsions and allied micellar media. Significant enhancements in the specific rates of hydrolysis, as high as 29, were observed. The above data on solid-liquid systems, along with those of absorption of gases, namely, isobutylene, but-1-ene and propylene into emulsions of an additional liquid phase in aqueous sulfuric acid as well as microemulsions and various micellar solutions, published earlier, have been analysed with the unsteady state theory developed in Part I of this study. In some of the cases, where bulk concentration of the sparingly soluble reactant was finite, it was necessary to modify the original theory. The predicted values of the enhancement factor, for systems where macroemulsions were used, were found to be in good agreement with the experimental values of the enhancement in the specific rate. For the other cases, where a priori prediction was not possible the model equations provided a reasonable basis for correlation of data. The analysis of experimental data brings out clearly the importance of time dependent phenomenon like accumulation within the microphase and demonstrates the necessity of models based on unsteady state formulations for such cases.

Gas absorption in reactive slurries: Particle dissolution near gas-liquid interface

Abstract The rates of gas absorption into reactive slurries constituted by “fine” particles of a sparingly soluble reactant are known to be enhanced when the particle size is smaller than the characteristic diffusional lengths of the reactive species. This study examines the process of particle dissolution and the consequent change in particle size(s) near the gas-liquid interface , in the presence of diffusional gradients, using Higbies extended theory of mass transfer with chemical (instantaneous) reaction. The effect of changing particle size (including complete dissolution of the particles in this “film” zone) on the mechanism and extent of enhancement in the specific rate of absorption has been assessed using a population balance approach to track the interaction of the dissolution process with the evolving particle size distributions. It has been shown that the rates predicted from the proposed theory differ considerably from those computed using models available in the literature, for particles which are “small” enough. A variety of initial particle size distributions of different spreads have been used to show that for a given mean particle size, wide distributions produce lower enhancements in the specific rate than narrow ones. The specific rate-batch time trajectories for a typical batch slurry reactor have been generated along with the evolution of the particle size distributions in the bulk slurry phase in order to track the solid conversion as a function of batch time. Such conversions computed from theories available in the literature are likely to be gross overestimates in relation to the actual scenario. Some of the reported experimental absorption data have been reinterpreted in the light of the models developed here.

Pulmonary targeting of nanoparticle drug matrices.

BACKGROUND Nanoparticle drug matrices using lipids or liposomes, with diameters of 40-300 nm, have recently been developed to encapsulate drugs like Insulin, Budesonide, and Rifampicin for pulmonary delivery raising interest in their regional lung deposition. METHODS Lung deposition has so far been modeled using a one-dimensional transport equation, with or without moving airway boundaries, and a lumped deposition term for particle diffusion, sedimentation, and impaction. Here, a two-dimensional transport model has been developed with an explicit treatment of radial diffusion, the primary mechanism for nanoparticle deposition. Regional lung deposition was calculated using Weibels whole lung model geometry during normal breathing and medical inhalation cycles. CONCLUSIONS Model predictions agree well with measurements of total and pulmonary lung deposition for particles of 10 nm to 10 μm, with earlier models incorporating moving boundaries and aerosol dynamics, and with the reported regional lung deposition of inhaled dry powder insulin. To simulate medical inhalation, the model was run with inhalation times from 2-6 sec and breath hold from 0-10 sec. A high and relatively invariant pulmonary deposition fraction between 70 and 95% was predicted for a broad nanoparticle size range (50-200 nm) for inhalation cycles with breathing rate between 500 and 2000 cm(3) sec(-1) and breath hold of 5-10 sec. Thus, nanoparticles may be able to deliver consistent lung doses, over modest breath hold periods, even with intrapatient variability in breathing rate. A linearized nomogram was provided as a heuristic for design of nanoparticle drug matrices to target the pulmonary lung.

Absorption of hydrogen sulfide in aqueous solutions of iodides containing dissolved iodine: Enhancements in rates due to precipitated sulfur

The specific rates of absorption of hydrogen sulfide in aqueous solutions of iodides, containing dissolved iodine, were measured in a 55 mm i.d. stirred cell, with batch and semi-batch modes of operation. The specific rates of absorption exceeded the rates that may be realised in the instantaneous reaction regime by factors as high as 8. This effect appears to be due to the precipitating (product) colloidal sulfur transporting dissolved hydrogen sulfide, through strong interaction, from near the gas-liquid interface into the bulk liquid. The relative values of the enhancement factor due to precipitated sulfur decrease with an increase in the concentration of dissolved iodine. A two-parameter model, incorporating the uptake of hydrogen sulfide by sulfur and the agglomeration of sulfur particles, has been developed which provides a reasonable framework to correlate the experimental data.

Modeling of oxygen transport in blood-perfluorocarbon emulsion mixtures: Part II: tissue oxygenation.

The models developed in the accompanying article in this issue for oxygen transport in uniform (pseudo-homogeneous model) and non-uniform (core annulus model) dispersions of erythrocytes and perfluorocarbons (PFCs) have been analyzed with a fixed wall flux boundary condition. Such a situation arises in the case of oxygen transport from a capillary to the surrounding tissue. The results reveal that PFCs are extremely effective in increasing the tissue oxygen tension during enriched air breathing. The increased oxygen capacity of blood on the addition of PFC emulsion is the main contributor toward this increase. If the contribution of the accompanying increase in the arterial oxygen tension also is considered, the resulting increase in the tissue oxygen tension was even larger than that observed by Braun et al., and hence other factors, such as an increase in the cardiac output or service of erythrocyte free capillaries, need not be invoked. A near wall excess of PFC droplets, if it occurs, has been shown to have a negligible effect on the tissue oxygen tension during normal rates of tissue oxygen consumption when the intracapillary gradients are small. In these conditions, the capillary phase may be taken to be uniform. A criterion has been developed to assess the magnitude of the gradients within the capillary. Accordingly, in cases of elevated rates of tissue oxygen consumption or low arterial oxygen tensions, the internal gradients become important and the capillary phase can no longer be taken to be uniform. ASAIO Journal 1998; 44:157–165.

Modeling of oxygen transport in blood-perfluorocarbon emulsion mixtures. Part I : Oxygen uptake in tubular vessels

Perfluorocarbon (PFC) emulsions are usually used as mixtures with blood to enhance its capacity for oxygen, because stand alone PFC emulsions cannot perform the normal regulatory functions of blood. These mixtures have been found to be very effective in increasing the tissue oxygen tension, especially at high oxygen partial pressures, and many experimental observations exist in the literature in support of this fact. The explanations for these observations are still speculative and un-quantified, however. In this work, models have been developed to describe oxygen transport in uniform and non-uniform mixtures of blood and PFCs. For the latter case, the extreme situation of central migration of erythrocytes is considered, wherein the erythrocytes occupy the central core region of the vessel surrounded by a plasma annulus. The predictions of the proposed models have been examined using a fixed wall oxygen tension, and the oxygen transport characteristics of mixtures have been presented with reference to blood alone. It was found that at high oxygen tensions the addition of PFCs significantly increases the oxygen wall flux into the tube. This increased flux, coupled with effects of competing oxygen sinks (erythrocytes and PFC droplets), leads to an anomalous increase in the average oxygen tension for short distances from the tube entrance. It has been shown that a near wall excess of PFC droplets is not necessary to cause this increase, as mentioned by Vaslef and Goldstick. For longer distances, however, the addition of PFCs leads to a decrease in the average oxygen tension. ASAIO Journal 1998; 44:144–156.

modeling of Oxygen Uptake in Perfluorocarbon Emulsions : some Comparisons With Uptake By Blood

&NA; The use of perfluorocarbons emulsified in water as blood substitutes (artificial blood), is well known. Although considerable research has been devoted to the study of stability, toxicity, and gas solubility properties of these emulsions, there is no quantitative guide to the oxygen transport behavior in such emulsions, especially with reference to this transport process in actual blood. This paper describes a mathematical model from which the oxygen flux into a straight, cylindrical tube carrying a perfluorocarbon emulsion may be computed. The solutions to the proposed model can be adapted to that for a capillary or for a single tube in a blood oxygenator. The rates of oxygen transfer, so obtained, have been compared with analogous transfer rates that can be achieved in natural blood flowing under identical conditions. Therefore, the minimal solubilization capacity for oxygen required of a perfluorocarbon emulsion can be estimated on a quantitative basis. The modeling approach used in this study is based on the well tested theory of mass transfer in microheterogeneous media reported in the chemical engineering literature. ASAIO Journal 1996;42:181‐189.

Gas absorption in slurries of finite-capacity microphases

Abstract Absorption of a gas into slurries constituted by “fine” particles microdispersed in a liquid phase, which remove the diffusing solute gas dissolved in the liquid phase, from near the gas—liquid interface by reaction, complexation, solubilization (including surface adsorption) or catalytic action, is a relatively novel means for intensifying the specific rates of gas absorption and even manipulating selectivity in the case of multiple gaseous solutes. It has been shown in this paper that it is possible to develop analytical models for describing the absorption behaviour in the presence of microphase constituents which have a finite, exhaustible capacity to “store” the diffusing solute and/or the concentration of the solute within the microphase vs that in its vicinity in the original liquid phase shows a non-linear equilibrium relationship. The non-linearities may be approximated by segmental (piecewise) linearization. Mass transfer models of this type are conceptualized in terms of different zones of “varying” capacity within a liquid surface element. A theory has been developed for such exhaustible microphases including the conditions of validity for the various rate expressions and structured classifying regimes. Data on the absorption of hydrogen into slurries of lanthanum—nickel alloys in oil, reported in the literature, have been analyzed. The predicted values of the specific rate are found to be in reasonable agreement with the experimentally observed values.