Microporous Hollow Fibre Membrane Modules Research Articles

Mathematical modeling of CO2 separation from gaseous-mixture using a Hollow-Fiber Membrane Module: Physical mechanism and influence of partial-wetting

The present study describes a steady-state phenomenological model for CO2 separation via reactive absorption into aqueous Diethanolamine (DEA) solution using a micro-porous Poly-propylene (PP) Hollow-Fiber Membrane Module (HFMM). The developed model is based on the fundamental mechanisms of molecular diffusion, bulk convection and liquid-phase chemical reaction, and simultaneously accounts for the consequences of ‘partial-wetting’ phenomenon. Furthermore, a physically-consistent wetting mechanism has been formulated assuming that the membrane pores may be modeled as a bundle of straight cylindrical capillaries with distinct radii (characterized by the membrane pore-size distribution) and equal lengths, while keeping in mind the various pore-scale micro-physical phenomena. Under the simplifying parameterizations of the Finite-Volume Method (FVM), the source-code for discretized equations was compiled and implemented using C++ Language for a co-current module operation with aqueous DEA solution flowing inside the fiber-lumen and CO2–N2 gaseous mixture passing through the shell-side. A Benchmarking Analysis revealed an excellent agreement between the model predictions and the experimental data reported in open-literature, thereby validating the current model formulation, and rendering it fundamentally relevant with respect to the wetting-phenomenon. In addition, the module performance in terms of CO2 flux, Overall Mass-Transfer Coefficient (MTC), and Removal-Efficiency, has been systematically analyzed pertaining to the physical influence of other operating variables such as absorbent concentration, hydrodynamics, pressure, temperature, and membrane characteristics. From a modeling standpoint, it may be concluded that the present model successfully captures various observations vis-à-vis the process of CO2 separation using micro-porous HFMMs, reported previously in the literature. Moreover, for a given gas-phase hydrodynamics, the current set of results suggest the existence of a unique liquid-phase hydrodynamic regime, bounded by a minimum and a maximum permissible pressure, under which the module can be effectively operated without any dispersive losses. Besides, the currently developed model has been demonstrated to explain the reduction in CO2 flux over time by allowing for morphological changes, including an enlargement in the average pore-size and a broadening of the pore-size distribution.

Theoretical and experimental studies of membrane wetting in the membrane gas–liquid contacting process for CO 2 absorption

A systematic simulation has been carried out in membrane contactors to study CO 2 capture by water and diethanolamine (DEA) aqueous solutions from a CO 2/N 2 mixture under the wetted and the non-wetted operation modes. Two types of microporous hollow fiber membrane modules made of polypropylene (PP) and polyvinylidene fluoride (PVDF) hollow fibers were used to conduct CO 2 absorption experiments. The corresponding experimental data were used to verify simulated results. Experimental study showed that the membrane wetting could not be completely avoided especially in the case of chemical absorption. Both experimental and theoretical study disclosed that the membrane wetting would result in a significant drop of CO 2 flux. The simulation results further revealed that for the physical absorption of CO 2 by water, the proportion of membrane phase resistance in the overall mass transfer resistance increased from less than 5 to about 90% when the operation mode was shifted from non-wetted to wetted. As for the chemical absorption, analysis on the mass transfer resistance revealed that the ratio of the membrane resistance increased sharply from 10 to 70% when only 10% membrane length was wetted. With the introduction of an extra resistance caused by the membrane wetting, the mass transfer in the wetted membrane phase finally became the rate-controlling step. It was not as effective to enhance the CO 2 flux by increasing the inlet gas velocity or the liquid velocity as in the non-wetted mode of operation.

Modelling of cross-flow membrane contactors: Mass transfer with chemical reactions

Conventionally, carbon dioxide and hydrogen sulphide are absorbed using aqueous alkanolamines or carbonate solution in column type of equipment. However, in view of the unparallel advantages offered, the use of microporous hollow fiber membrane modules is an attractive alternative. In the present study application of cross-flow membrane contactors for absorption of these gases using aqueous potassium carbonate as a solvent is explored. The carbon dioxide and hydrogen sulphide absorption into aqueous solutions carbonate involves complex chemical reactions. The effect of these chemical reactions on the absorption into a liquid flowing though a hollow fiber membrane may not be described using conventional mass transfer models like e.g. the penetration or surface renewal model due to the lack of a well defined liquid phase bulk and the presence of a laminar velocity profile in the mass transfer zone. Moreover, in the case of cross-flow membrane contactors, the concentrations of both fluids, inside and outside the fibers, vary in both directions i.e. in the direction of flow and in the direction normal to the flow. Hence the theoretical analysis of the cross-flow membrane contactor is more complicated. To describe the effect of the chemical reaction on the absorption rate in the cross-flow hollow fiber contactor, a detailed mathematical model was developed using first principles. A complete scheme of the reversible ionic reactions and equilibria involved was implemented in the model to describe the solute uptake. The experiments were carried out study the effects various parameters such as gas and liquid velocities, bulk concentrations of solute gas and liquid phase reactant. The theoretical predictions were compared with experimental results. An excellent match between experimental results and model predictions was obtained.

A Comparison of Porous and Non-Porous Gas-Liquid Membrane Contactors for Gas Separation

Measurements have been made on the separation of CO 2 from gas phase by aqueous absorbents (water and diethanolamine solution) using non-porous and microporous hollow fibre membrane modules. The use of an absorbent liquid in the permeate side of the permeator enhances CO 2 separation and reduces the requirements for a high pressure ratio across the membrane. A mathematical model based on the effective permeability of the gaseous mixtures has been used to assess the performance of both hollow fibre membranes, and a correlation for the shell side mass transfer has been developed. Overall mass transfer in the microporous and non-porous membranes is dominated by the liquid film coefficient and a combination of liquid film and membrane resistances, respectively.

Modeling of Microporous Hollow Fiber Membrane Modules Operated under Partially Wetted Conditions

Gas−liquid mass transfer has been studied theoretically in a microprous hollow fiber membrane module operated under partially wetted conditions in the laminar flow regime. Dissolved oxygen removal has been used as an example for the simulation study. The mathematical model developed consists of nonlinear partial differential equations and is solved using the orthogonal collocation technique. The effect of membrane wetting pressures on the overall mass transfer performances of the module has been examined. The results indicate that under partially wetted operating mode, a maximum overall mass transfer coefficient is attainable with respect to the water velocity, which is completely different from the results obtained under both wetted and nonwetted conditions where the overall mass transfer coefficient is generally increased with water velocity. The phenomena of partial wetting of the membrane may provide an explanation to the observation that the overall mass transfer coefficient is increased to a maximum value and then is decreased with water velocity in a gas−liquid contactor.

A preliminary evaluation of microporous hollow fiber membrane modules for the liquid-liquid extraction of actinides

Microporous hollow fiber (MHF) membrane modules provide an inexpensive, low-maintenance, and dispersion-free approach to continuous-flow liquid-liquid extraction processes. This work involves analyzing the potential for using these modules for process-scale metal separations such as radioactive waste stream cleanup and environmental remediation. In this work, MHF modules are used to remove neodymium (a surrogate for americium) from 2 M nitric acid using DHDECMP [dihexyl N, N-diethylcarbamoylmethylphosphonate] and CMPO [ n-octyl(phenyl)- N, N-diisobutylcarbamoylmethylphosphine oxide] as extractants in diisopropylbenzene. The modules are also used to concentrate the neodymium from the organic by back-extracting it into 0.01 M nitric acid. The performance of this technology is discussed in terms of the number of theoretical equilibrium extraction stages provided by each module, as well as how the module's extraction and back-extraction capabilities are affected by different aqueous and organic flow rates, and by the different extractants.

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 CO 2 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: 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.

Use of sealed end hollow fibers for bubbleless membrane aeration: experimental studies

A novel microporous hollow fiber membrane module was evaluated for the oxygenation of water. Pure oxygen is maintained inside a bundle of sealed-end polypropylene fibers at a pressure below the bubble point. The water to be aerated is pumped over the outside of the fibers. The high oxygen concentration gradient encourages the oxygen to diffuse across the porous membrane wall, and to dissolve directly into the water without the formation of bubbles. The membrane is hydrophobic and the pores small enough that they stay dry and gas filled so that the transport across the membrane is by gaseous diffusion. The membrane itself therefore provides little resistance to transfer, and the mass transfer rate is controlled by diffusion across the liquid film on the outside of the fibers. Modules of varying size were tested and transfer correlations were developed for module design. The process gave 100% oxygen transfer efficiency at a reasonable power input. For water quality management and waste treatment, oxygenation by membranes appears to offer many advantages over conventional aeration devices.