Determination of mass transfer rates in wetted and non-wetted microporous membranes

The mass transfer resistance of microporous membranes placed between a gas and a liquid phase was studied for both wetted and non-wetted membranes. It was found that the mass transfer coefficient can be described according to the film model in which the porosity of the membrane and the tortuosity of the pores is incorporated. For the non-wetted membranes (mean pore diameter of 0.1 um), the Knudsen and continuum diffusion must be taken into account. No difference was observed in the values of the liquid-phase mass transfer coefficients between systems with and without a membrance at the gas¿liquid interface, despite the different hydrodynamic situation at the interface. The influence of a chemical equilibrium reaction on the mass transfer through a wetted membrane was analysed mathematically (two-film concept). With this model the tortuosity factor of the membrane was calculated from experimentally determined fluxes

Microporous hollow fibre membrane modules as gas-liquid contactors. Part 2: Mass transfer with chemical reaction

Absorption determined by mass transfer in the liquid is described well with the Graetz-Lévèque equation adapted from heat transfer. The influence of a chemical reaction on the mass transfer was simulated with a numerical model and tested on the absorption of CO2 in a hydroxide solution. Absorption determined by mass transfer in the gas phase and the pores of the membrane was also analysed experimentally and numerically. It was found that the gas phase concentration profile is established at a very short distance from the entrance of the fibre. This results in a constant Sherwood number along the fibre. A module coated with a very thin silicone rubber layer showed absorption rates comparable to the uncoated membranes. If absorption liquids are used which wet the membranes, resulting in leaky membranes, such a coating can enlarge the application of microporous hollow fibre membrane module

Selective removal of H2S from sour gases with microporous membranes. Part II. A liquid membrane of water-free tertiary amines

In the present study the application of a liquid membrane for selective removal of H2S from gases also containing CO2 was investigated. The liquid membrane was filled with pure methyl-di-ethanol-amine (MDEA). A theoretical model was developed to describe: (a) the chemical equilibrium between the dissolved gas and MDEA in the membrane and (b) the physical equilibrium between the solute (CO2 and H2S) in the gas and the liquid phase. Experimentally H2S and CO2 fluxes were determined in a setup consisting of two well mixed gas phase compartments separated by a flat liquid membrane. The fluxes were interpreted with the theoretical model and separately measured physical constants (solubility, diffusivity and the porosity/tortuosity factor of the membrane material). No reaction of CO2 with MDEA was observed which is attributed to the absence of water. A weak acid/base interaction of H2S and MDEA was found to increase the H2S transport through the membrane which includes higher selectivity. This effect is more pronounced at lower partial pressures of H2S

Microporous hollow fibre membrane modules as gas-liquid contactors. Part 1: Physical mass transfer processes. A specific application: mass transfer in highly viscous liquids

Gas-liquid mass transfer has been studied in a membrane module with non-wetted microporous fibres in the laminar flow regime. This new type of gas/liquid contactor can be operated stabily over a large range of gas and liquid flows because gas and liquid phase do not influence each other directly. Therefore foam is not formed in the module, gas bubbles are not entrained in the liquid flowing out of the reactor and the separation of both phases can be achieved very easily. These phenomena often limit the applicability of conventional contactors, e.g. a bubble column which was also studied in the present work. The large mass transfer area of a bundle of small fibres offers the possibility of creating a compact gas/liquid mass exchanger. However, owing to the small channels in and around the fibres the flow of either gas or liquid becomes laminar which reduces the mass transfer capacity of the module. Therefore the mass transfer coefficients in the laminar flow regime were determined experimentally. For mass transfer determined by the transport in the liquid phase it was found that the active mass transfer area is equal to the total membrane area, regardless the porosity of the fibre. For processes with liquid flowing through the fibres, the influence of fibre diameter, diffusivity in the liquid, liquid viscosity and liquid velocity on mass transfer can be correlated extremely well with the Graetz-Lévèque solution derived for the analogous case of heat transfer. For liquid flowing around regularly packed fibres mass transfer was described satisfactory with a correlation derived from a numerical solution for the similar heat transfer problem [Miyatake and Iwashita, Int. J. Heat Mass Transf., 33 (1990) 416]. Correlating mass transfer in liquid flowing around irregularly packed fibres was not possible because of the undefined dimensions of the different channels between the fibres

Selective removal of H2S from sour gases with microporous membranes. Part II. A liquid membrane of water-free tertiary amines

In the present study the application of a liquid membrane for selective removal of H2S from gases also containing CO2 was investigated. The liquid membrane was filled with pure methyl-di-ethanol-amine (MDEA). A theoretical model was developed to describe: (a) the chemical equilibrium between the dissolved gas and MDEA in the membrane and (b) the physical equilibrium between the solute (CO2 and H2S) in the gas and the liquid phase. Experimentally H2S and CO2 fluxes were determined in a setup consisting of two well mixed gas phase compartments separated by a flat liquid membrane. The fluxes were interpreted with the theoretical model and separately measured physical constants (solubility, diffusivity and the porosity/tortuosity factor of the membrane material). No reaction of CO2 with MDEA was observed which is attributed to the absence of water. A weak acid/base interaction of H2S and MDEA was found to increase the H2S transport through the membrane which includes higher selectivity. This effect is more pronounced at lower partial pressures of H2S

Selective removal of H2S from sour gas with microporous membranes. Part I. Application in a gas-liquid system

The selective removal of H2S from gases containing several acidic components by absorption in aqueous alkanolamines is determined by the ratio of the partial mass transfer resistances in the gas and liquid phase and the solubility (physical and chemical) of these gases in the absorption liquid. The influence of the mass transfer resistances is experimentally studied in the present study. The simultaneous absorption of H2S and CO2 in aqueous solutions of methyl-di-ethanol amine (MDEA) was studied in a stirred cell with flat, horizontal microporous wetted or non-wetted membranes which increase the partial mass transfer resistances in the liquid or gas phase, respectively. It was found that non-wetted membranes do not increase the H2S flux because the absorption rate determining mass transfer step which is located in the gas phase, is reduced, while the CO2 transport is not affected significantly. Wetted membranes reduce the transport of the amine to the gas-liquid interface which introduces an additional transport limitation of the amine. Therefore the H2S and CO2 flux are both determined by the mass transfer in the liquid phase which is generally not the case for H2S with a gas-liquid interface without a membrane. The introduction of a non-wetted membrane in the gas-liquid interface has no effect on the values of the liquid phase mass transfer coefficients, despite the different hydrodynamic situation at the interface. In this case however, a considerable difference was observed for the gas phase mass transfer coefficients. The influence of physical and chemical solubility is studied in the second part of the present paper in which a liquid membrane of pure MDEA is investigated

Microporous hollow fibre membrane modules as gas-liquid contactors. Part 2. Mass transfer with chemical reaction

Absorption determined by mass transfer in the liquid is described well with the Graetz-Lévèque equation adapted from heat transfer. The influence of a chemical reaction on the mass transfer was simulated with a numerical model and tested on the absorption of CO2 in a hydroxide solution. Absorption determined by mass transfer in the gas phase and the pores of the membrane was also analysed experimentally and numerically. It was found that the gas phase concentration profile is established at a very short distance from the entrance of the fibre. This results in a constant Sherwood number along the fibre. A module coated with a very thin silicone rubber layer showed absorption rates comparable to the uncoated membranes. If absorption liquids are used which wet the membranes, resulting in leaky membranes, such a coating can enlarge the application of microporous hollow fibre membrane module

Microporous hollow fibre membrane modules as gas-liquid contactors. Part 1. Physical mass transfer processes:A specific application

Gas-liquid mass transfer has been studied in a membrane module with non-wetted microporous fibres in the laminar flow regime. This new type of gas/liquid contactor can be operated stabily over a large range of gas and liquid flows because gas and liquid phase do not influence each other directly. Therefore foam is not formed in the module, gas bubbles are not entrained in the liquid flowing out of the reactor and the separation of both phases can be achieved very easily. These phenomena often limit the applicability of conventional contactors, e.g. a bubble column which was also studied in the present work. The large mass transfer area of a bundle of small fibres offers the possibility of creating a compact gas/liquid mass exchanger. However, owing to the small channels in and around the fibres the flow of either gas or liquid becomes laminar which reduces the mass transfer capacity of the module. Therefore the mass transfer coefficients in the laminar flow regime were determined experimentally. For mass transfer determined by the transport in the liquid phase it was found that the active mass transfer area is equal to the total membrane area, regardless the porosity of the fibre. For processes with liquid flowing through the fibres, the influence of fibre diameter, diffusivity in the liquid, liquid viscosity and liquid velocity on mass transfer can be correlated extremely well with the Graetz-Lévèque solution derived for the analogous case of heat transfer. For liquid flowing around regularly packed fibres mass transfer was described satisfactory with a correlation derived from a numerical solution for the similar heat transfer problem. Correlating mass transfer in liquid flowing around irregularly packed fibres was not possible because of the undefined dimensions of the different channels between the fibres

Musculoskeletal Ultrasound For the Orthopedic Surgeon: Foot and Ankle

Ultrasound of the foot and ankle offers low cost, in-office real-time dynamic imaging without radiation. Given the complex and relatively smaller size of the many joints of the foot, ultrasound guidance can increase the accuracy of intra-articular aspirations and injections. Whether helping to establish a diagnosis, aid in nonoperative treatment or localize anatomy for postoperative pain control, ultrasound is an invaluable imaging technique for a foot and ankle surgeon