Ramon L. Cerro

Dispersion during bubble-train flow in capillaries

Abstract The name bubble-train flow is used to identify the cocurrent flow of gas bubbles and liquid slugs through a capillary. A model was developed to analyze mass transfer between bubbles and liquid slugs during bubble train flow. This mass transfer model, derived from the equations of transport, is based on the fluid flow profiles and has no adjustable parameters. A comprehensive account of the experiments designed to test the model is presented. The single capillary model was used to analyze mass transfer of a passive saline solution introduced as a pulse signal. Theoretical results were used to determine the concentration vs time distribution of the saline solution inside the capillary channels. These results were compared with experimental residence times determined using a conductimetric technique. Very good agreement between theory and experiments was found for single capillaries. The mass transfer model, however, must be modified to take into account macroscopic flow distribution effects before it can be used for the analysis of mass transfer and chemical reaction inside the monolith froth reactor.

Mass transfer from oscillating bubbles in bioreactors

In bioreactors, the purpose of aeration is to transfer oxygen from air bubbles into the liquid phase where the biological reactions occur. In some cases, aeration is also used as a mixing tool. In all aeration devices there is a continuous liquid phase and, generally, a dispersed gas phase in the form of bubbles. The most common aeration devices are stirred tank reactors, bubble columns and sieve plate reactor-towers. Bubble shape, bubble volume, and associated liquid flow patterns are key aspects of bubble dynamics in sieve plates. The most prevalent bubble shapes are spherical, oscillating (wobbling), ellipsoidal, spherical-cap, and skirted. Bubble volume and bubble shapes determine the surface to volume ratios, a very important parameter in determining overall mass transfer rates. Mass transfer in sieve plate reactors takes place, predominantly, within the oscillating bubble regime. Mass transfer rates from oscillating bubbles can be orders of magnitude larger than mass transfer rates from spherical bubbles. A fundamental correlation for mass transfer from single, oscillating bubbles was developed based on a solution of the mass transfer equations following the domain perturbation technique first outlined by Joseph (1973) and the hydrodynamics results of Tsamopoulos and Brown (1983). The correlation derived here entirely from hydrodynamics and mass transfer concepts, introduces the effect of interfacial tension in bubble mass transfer from fundamental concepts, has no adjustable parameters, and agrees very well with experimental data.

Hysteresis during contact angles measurement.

A theory, based on the presence of an adsorbed film in the vicinity of the triple contact line, provides a molecular interpretation of intrinsic hysteresis during the measurement of static contact angles. Static contact angles are measured by placing a sessile drop on top of a flat solid surface. If the solid surface has not been previously in contact with a vapor phase saturated with the molecules of the liquid phase, the solid surface is free of adsorbed liquid molecules. In the absence of an adsorbed film, molecular forces configure an advancing contact angle larger than the static contact angle. After some time, due to an evaporation/adsorption process, the interface of the drop coexists with an adsorbed film of liquid molecules as part of the equilibrium configuration, denoted as the static contact angle. This equilibrium configuration is metastable because the droplet has a larger vapor pressure than the surrounding flat film. As the drop evaporates, the vapor/liquid interface contracts and the apparent contact line moves towards the center of the drop. During this process, the film left behind is thicker than the adsorbed film and molecular attraction results in a receding contact angle, smaller than the equilibrium contact angle.

Moving contact lines and Langmuir-Blodgett film deposition.

The objective of this paper is to point out the close relationship between contact line dynamics and LB film depositions, and it is designed to serve as a blueprint for future analysis of the LB technique. Moving contact lines and contact angles play a major role in Langmuir-Blodgett ultrathin film depositions. Although the effect of contact angles has been recognized for many years, a fundamental and comprehensive explanation of the phenomena taking place at the contact line has not been formulated before. Our understanding of contact line dynamics has improved thanks to careful experiments and new theoretical developments. Flow patterns depend on dynamic contact angle and the ratio of viscosities of the gas and liquid phases. More recently dynamic contact angles-and flow patterns-have been linked to forces of molecular and double-layer origin. The dynamic relationship of flow patterns to interfacial and transport properties can be used to explain seemingly contradictory experimental results reported by researchers during more than 60 years of experience with the L-B technique. Windows of operability can be defined for X-type and Z-type depositions that are useful in the design of experimental and industrial L-B deposition equipment.

Production of chemicals from cellulose and biomass-derived compounds through catalytic sub-critical water oxidation in a monolith reactor

Abstract Selective conversion of cellulose to small organic molecules, including carboxylic acids, represents a potential route for upgrading biomass resources to value-added chemical precursors. However, since biomass resources such as cellulose are not soluble in water, the use of a catalyzed system requires a novel reactor design that facilitates slurry flow. As demonstrated herein, the monolith froth reactor is uniquely suited for the conversion of solid materials in a four-phase (solid catalyst, solid reactant, gaseous reactant, and aqueous solution) reaction system. Reactions were performed using a palladium catalyst and 1000 ppm ( w ) cellulose, and results were compared with previous experiments conducted over platinum. At 150°C, nearly 100% conversion of the cellulose was achieved in approximately 5 h , which compares favorably with results obtained using a platinum catalyst. The palladium catalyst gave a different distribution of intermediate products compared to platinum. For example, acetic acid and malic acid achieved yields of 40 and 80 ppm ( w ) , respectively, using the palladium catalyst. The product selectivity was evaluated under pH control, through the addition of acetic or carbonic acid, and shown to have only a minor effect on the performance of the system.

Use of Ceramic Monoliths as Stationary Phase in Affinity Chromatography

The use of coated ceramic monoliths as support for affinity chromatography is described. Ceramic monoliths are robust active matrix supports and present a very small pressure drop. Monoliths are coated with a very thin agarose gel layer and activated using a standard activation process for agarose beads. Experiments demonstrate that enzyme adsorption occurs exclusively on the outside surface of the agarose coating since enzyme molecules are too large to fit into the porous matrix. Adsorption and desorption rates are large and production of enzyme per unit monolith volume justifies further exploring this separation process for large throughput operation.

Pressure drop in monolith reactors

Abstract A theoretical model for the computation of pressure drop in bubble-train flow inside capillaries of square cross-section was developed. The model is based on three contributions: hydrostatics, viscous pressure drop, and capillary pressure drop. Capillary pressure drop is related to the shape of the fronts and ends of the bubbles. The model does not include entrance or exit effects, has no adjustable parameters, and agrees very well with available experimental data. For a given set of flow parameters, bubble velocity and liquid slug average velocity are computed as a function of gas and liquid superficial velocities. The length of the unit cell determines the number of bubbles inside the capillary for a given flow situation. The model requires experimental information of average bubble lengths to compute the length of a unit cell consisting of a bubble and a liquid slug. The three pressure contributions for a unit capillary length are linear functions of the number of bubbles inside the capillary. The length of the bubbles in bubble-train flows is a critical parameter in the computation of pressure drop.

An analytical solution for a partially wetting puddle and the location of the static contact angle

A model is formulated for a static puddle on a horizontal substrate taking account of capillarity, gravity and disjoining pressure arising from molecular interactions. There are three regions of interest--the molecular, transition and capillary regions with characteristic film thickness, hm, ht and hc. An analytical solution is presented for the shape of the vapour-liquid interface outside the molecular region where interfacial tension can be assumed constant. This solution is used to shed new light on the static contact angle and, specifically, it is shown that. (i) There is no point in the vapour-liquid interface where the angle of inclination, theta, is identically equal to the static contact angle, theta(o), but the angle at the point of null curvature is the closest with the difference of O(epsilon2) where epsilon2 = ht/hc is a small parameter. (ii) The liquid film is to O(epsilon) a wedge of angle theta(o) extending from a few nanometers to a few micrometers of the contact line. A second analytical solution for the shape of interface within the molecular region reveals that cos theta has a logarithmic variation with film thickness, cos theta=cos theta-ln[1-h2(m)/2h2]. The case, hm = 0, is of special significance since it refers to a unique configuration in which the effect of molecular interactions vanishes, disjoining pressure is everywhere zero and the vapour-liquid interface is now described exactly by the Young-Laplace equation and includes a wedge of angle, theta(o), extending down to the solid substrate.

Investigation of transmembrane protein fused in lipid bilayer membranes supported on porous silicon

This article investigates a device made from a porous silicon structure supporting a lipid bilayer membrane (LBM)fused with Epithelial Sodium Channel protein. The electrochemically-fabricated porous silicon template had pore diameters in the range 0.2~2 µm. Membranes were composed of two synthetic phospholipids: 1,2-diphytanoyl-sn-glycero-3-phosphoserine and 1,2-diphytanoyl-sn-glycero-3-phosphoethanolamine. The LBMwas formed by means of the Langmuir-Blodgett and Langmuir-Schaefer techniques, at a monolayer surface tension of 26 m Nm−1 in room temperature and on a deionized water subphase, which resulted in an average molecular area of 0.68–0.73 nm2. Fusion of transmembrane protein was investigated using Atomic Force Microscopy. Initial atomic force microscopy results demonstrate the ability to support lipid bilayers fused with transmembrane proteins across a porous silicon substrate. However, more control of the membrane’s surface tension using traditional membrane fusion techniques is required to optimize protein incorporation.

Enhanced CO2 diffusion in wedges

A porous media is a collection of particles of different shapes and sizes. Particles can be deformed such that they rarely contact at a point, but on extended surfaces. As a consequence of these contacts between particles, the resulting voids can present a large number of cusp-like sections resulting in a low-angle intersection of solid surfaces. Depending on the angle at which surfaces interact and on the contact angle between a liquid and the solid surface, a meniscus will rise on a wedge in a similar way a liquid meniscus rises inside a capillary. If the sum of the half-angle of the wedge and the contact angle is less than π/2, there is no possible equilibrium shape and a liquid filament will rise to fill the entire length of the wedge. It was found that filaments have an active role in the enhanced diffusion process between carbon dioxide and liquid hydrocarbons. The presence of filament not only increases the mass transfer area but also plays an important role in the convective mixing process that occurs inside the pore.