Gas-liquid mass transfer with parallel reversible reactions—I. Absorption of CO2 into solutions of sterically hindered amines

A numerical method developed by Verteeg (1989, Chem. Engng Sci.44, 2295–2310; 1990, Chem Engng Sci.45, in press) is applied to some specific problems in gas—liquid mass transfer. The experimental results of Chakraborty (1986, Chem. Engng Sci.41, 997–1003) and Zioudas and Dadach (1986, Chem. Engng Sci.41, 405–408) on the absorption of Co2 into aqueous solutions of sterically hindered amines are evaluated with the numerical model. It is shown that studying the absorption of CO2 into aqueous solutions of sterically hindered amines requires a rigorous numerical solution of the differential equations describing the mass transfer instead of analytical and numerical approximations based on a reduction of the number of reactions by neglecting or lumping reactions. It is demonstrated that the absorption rates of CO2 into sterically hindered amine solutions can be explained in terms of the established reactions rates of CO2 in amine solutions alone, and no new reaction paths are necessary to explain the observed behaviour

Gas-liquid mass transfer with parallel reversible reactions—II. Absorption of CO2 into amine-promoted carbonate solutions

A numerical method developed by Verteeg (1989, Chem. Engng Sci.44, 2295–2310; 1990, Chem. Engng Sci.45, in press) is applied to the absorption of CO2 into amine-promoted carbonate solutions. The experimental results of Savage (1984, Faraday Discuss. Chem. Soc.77, 17–31) are evaluated with the numerical model. It is shown that a rigorous numerical solutions of the differential equations describing the mass transfer gives more insight into the actual process than analytical and numerical approximations based on a reduction of the number of reactions by neglecting or lumping reactions

Gas-liquid mass transfer with parallel reversible reactions—III. Absorption of CO2 into solutions of blends of amines

A numerical method developed by Versteeg (1989, Chem. Engng Sci.44, 2295–2310; 1990, Chem. Engng Sci.45, in press) is applied to the absorption of Co2 into solutions of blends amines. The experimental results of Critchfield and Rochelle (1987) are evaluated with the numerical model. It is shown that a rigorous numerical solution of the differential equations describing the mass transfer gives more insight into the actual process than analytical approximations based on a reduction of the number of reactions by neglecting or lumping reactions

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

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 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

A thermodynamic framework to predict thermophysical properties that control pMDI aerosol generation

Activity coefficient models are introduced to provide a thermodynamic framework for simultaneously predicting multiple thermophysical properties of relevance to pressurized metered dose inhaler (pMDI) aerosol formation. The UNIFAC and UNIQUAC models are discussed in the context of calculation of saturated vapor pressure, surface tension and liquid viscosity using molecule and functional group interaction parameters. New interaction parameters are generated and presented for HFA134a/ethanol mixtures using experimental data for saturated vapor pressure, surface tension and viscosity. The UNIFAC model is shown to give adequate predictivity and can be used when no experimental data is available. Better predictions were obtained with the UNIQUAC model, which is most useful when high-quality measurement data are obtained. The use of these models for flexible thermophysical property prediction of low-global warming potential (GWP) formulations is discussed, with potential developments to improve model fits and better utilize the experimental data

Coupling of Active Motion and Advection Shapes Intracellular Cargo Transport

Intracellular cargo transport can arise from passive diffusion, active motor-driven transport along cytoskeletal filament networks, and passive advection by fluid flows entrained by such motor/cargo motion. Active and advective transport are thus intrinsically coupled as related, yet different representations of the same underlying network structure. A reaction-advection-diffusion system is used here to show that this coupling affects the transport and localization of a passive tracer in a confined geometry. For sufficiently low diffusion, cargo localization to a target zone is optimized either by low reaction kinetics and decoupling of bound and unbound states, or by a mostly disordered cytoskeletal network with only weak directional bias. These generic results may help to rationalize subtle features of cytoskeletal networks, for example as observed for microtubules in fly oocytes.Comment: revtex, 5 pages, 5 figures, to appear in PRL (http://prl.aps.org/