Cross-flow Membrane Contactor Research Articles

A conceptual evaluation of cross-flow membrane contactor for desalination process

A conceptual evaluation of cross-flow membrane contactor for desalination process

Control of Sequential MTO Reactions through an MFI-Type Zeolite Membrane Contactor.

A membrane for controlling methanol-to-olefin (MTO) reactions was developed, which featured an MFI-type zeolite membrane (Si/Al = 25) that was synthesized on a porous α-alumina substrate using a secondary growth method. Here, the H2/SF6 permeance ratios were between 150 and 450. The methanol conversion rate was 70% with 38% ethylene selectivity and 28% propylene selectivity as determined using a cross-flow membrane contactor. In order to improve the olefin selectivity of the membrane, the MFI zeolite layer (Si/Al = ∞) was coated on an MFI-type zeolite membrane (Si/Al = 25). Using this two-layered membrane system, the olefin selectivity value increased to 85%; this was 19% higher than the value obtained during the single-layer membrane system.

Post-combustion CO2 capture and recovery by pure activated methyldiethanolamine in crossflow membrane contactors having coated hollow fibers

Gas absorption and stripping using membrane contactors is one of a number of carbon capture and storage technologies being investigated for separating CO2 from flue gas. Energy consumption in CO2 absorption-stripping employing amine-containing aqueous absorbent solutions is strongly influenced by demanding stripping conditions involving higher temperatures. The utility of using pure methyldiethanolamine (MDEA) activated by piperazine as a reactive absorbent was studied using a compact hollow fiber device for the absorber as well as the stripper. Water needed for reactive absorption of CO2 in tertiary amine MDEA was obtained from simulated humidified flue gas stream which is saturated with moisture in actual practice. This study investigated also the absorption and stripping performances of aqueous absorbent solutions containing 80% and 90% activated MDEA (aMDEA). Considerable CO2 stripping from CO2-loaded pure aMDEA absorbent was achieved at 92 °C while absorption was carried out at 25–46 °C. Further, the CO2 stripping rate was far higher for pure MDEA compared to the other two aMDEA solutions with water. Results are reported for the performance of the absorption-stripping process as a function of humidified simulated flue gas flow rate and the absorbent flow rate. High values of the overall mass transfer coefficients (MTCs) have also been reported for absorption. The highest volumetric gas phase based overall MTC obtained was 0.504 sec−1. This system has a much lower absorbent circulation load, eliminates the energy needed to heat and evaporate water present in aqueous absorbent solutions and benefits from the absence of excess water during stripping.

Gas-liquid absorption in industrial cross-flow membrane contactors: Experimental and numerical investigation of the influence of transmembrane pressure on partial wetting

Membrane wetting is a limitation to mass transfer in industrial Hollow-Fiber Membrane Modules (HFMM). The influence of transmembrane pressure on wetting is investigated through both experimental and numerical works. On the one hand, experiments for CO2 absorption in softened water are carried out at pilot scale with Liqui-Cel Extra-Flow modules operating in a cross-flow mode. After several hours of operation (50-300 min), the membrane partially wetted mode is established at steady state and decreases the absorbed CO2 flowrates up to 10.5% compared to membrane dry mode. Interestingly, CO2 absorption is found to be independent on different transmembrane pressure (0.5 and 2.5 bar), both in the dry and partially wetted mode. On the other hand, a 2D predictive tool is developed to help in the design of HFMM gas absorption systems. The model depicts the complex hydrodynamics in commercial cross-flow modules. The liquid in the shellside is flowing through the fiber bundle considered as a porous medium. The liquid mass-transfer coefficient is non-uniform over the membrane module and is estimated at a local scale. The parameters from the empirical Chilton-Colburn correlation are optimized with experimental results in the dry mode through a Particle Swarm Optimization (PSO) algorithm. The simulation results are in good agreement with the experiments (-131+17%). In the partially wetted mode at steady state, wetting is then described by considering a log-normal pore size distribution and the Laplace-Young equation. The transmembrane pressure decreases the simulated absorbed CO2 flowrate. By comparing to experimental data, the numerical relative error is increasing with transmembrane pressure. Therefore, a suggestion is made to adapt the description of wetting in HFMM gas -liquid models.

Analysis of the effect of module design on gas absorption in cross flow hollow membrane contactors via computational fluid dynamics (CFD) analysis

Abstract The performance of cross flow hollow fiber membrane contactors (HFMCs) used for gas absorption is greatly influenced by module characteristics. In this study, a comprehensive investigation has been conducted on the module design and its impact on the performance of cross flow HFMCs using computational fluid dynamics (CFD) analysis. For modules operated with liquid solvent flowing through the shell-side, the overall mass transfer coefficient is a strong function of shell-side flow behavior which in turn is determined by shell geometries. Shell-side liquid flow behavior is difficult to predict and therefore often overlooked. In this work, a CFD simulation of a lab scale cross hollow fiber membrane contactor was developed to estimate shell-side flow and overall mass transfer performance. The CFD model was validated with experimental data from the lab scale module for CO 2 absorption. The validated model was then used to investigate the geometric effect of module design on the shell-side flow and consequent absorption performance. A few design factors were examined such as the shell configurations, inlet geometries, fiber bed height and packing density. This work shows that a properly designed CFD simulation can be used as a useful tool to optimize module design for cross flow membrane contactors and improve absorption performance.

Performance characteristics of cross-flow membrane contactors for liquid desiccant systems

• Different types of flat plate membrane contactors developed to eliminate carryover in liquid desiccant systems. • Two-dimensional steady-state model developed to predict performance of contactors. • The simulated results are found to be in good agreement with experimental findings. • Performance of the contactors depends significantly on the membrane characteristics. • Parametric analysis carried out to select best operating ranges of design parameters. Membrane based indirect contact liquid desiccant dehumidification technology subsides the serious concern of liquid desiccant droplet carryover observed in conventional direct contact liquid desiccant systems. In the membrane contactor the air and liquid desiccant streams flow in alternate channels in cross-flow arrangement, separated by micro-porous semi-permeable hydrophobic membranes. Only water vapor can pass through the membranes but liquid desiccant cannot permeate. A two-dimensional steady-state mathematical model for semipermeable membrane based indirect contactors as dehumidifiers for liquid desiccant dehumidification applications has been developed. The model can predict the air and desiccant parameters inside the dehumidifier and the outlet parameters for a given input parameters. Five different membrane contactors have been fabricated and series of experiments have been conducted to validate the mathematical model. Aqueous solution of lithium chloride has been used as desiccant. The maximum deviations between experimental and predicted values are within ±10% for outlet specific humidity and outlet enthalpy of air, ±15% deviation in dehumidification effectiveness and ±20% deviation in enthalpy effectiveness. The distributions of major parameters viz. temperature, humidity, concentration, etc., within the contactor have been presented. Parametric analysis has been carried out to study the effects of membrane characteristics, contactor design, fluid flow rates, ambient conditions and desiccant concentration on the performance of the contactors.

Carbon Dioxide Capture from Flue Gases Using a Cross-Flow Membrane Contactor and the Ionic Liquid 1-Ethyl-3-methylimidazolium Ethylsulfate

Carbon dioxide (CO2) emissions have to be controlled and reduced in order to avoid environmental risks. Membrane processes in combination with the use of ionic liquids are recently under research and development in order to demonstrate a zero solvent emission process for CO2 capture. In this work, the application of a cross-flow membrane contactor is studied for CO2 absorption when the ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate is used as solvent. A mathematical model considering a parallel flow configuration is applied for a cross-flow system in order to describe the mass transfer rate. At a macroscopic level, Koveralla is calculated considering different mixing models corresponding to plug flow and continuous stirred models and a first order mass transfer rate. A microscopic model based on laminar flow has been applied, obtaining a membrane mass transfer coefficient of km = 3.78 × 10−6 m·s−1, which is about five times higher than that obtained in the macroscopic model. The interfacial area, a, allows the comparison of efficiencies between cross-flow and parallel membrane contactor systems in terms of the product (Koveralla).

Numerical mass balances for cross flow membrane contactors and their approximations

Membrane contactor processes for stripping of sparingly soluble components such as oxygen are well understood. For readily soluble components in cross flow hollow fiber membrane modules, however, mass balances are less well established. In the present study, numerical and analytical solutions for cross flow modules are presented and compared with hitherto used solutions. Based on the comparison the best approximate solution is proposed.

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.

Modelling of cross-flow membrane contactors: physical mass transfer processes

Traditionally, hollow fiber membrane contactors used for gas–liquid contacting were designed in a shell and tube configuration with shell-side fluid flowing parallel to the fiber-side fluid, either in co-current or counter-current pattern. The primary limitations of these so-called ‘parallel flow’ contactors are the shell-side flow channeling or mal-distribution due to non-uniform packing of the hollow fibers, higher shell-side pressure drop and relatively lower mass transfer coefficients. These limitations can be eliminated or reduced substantially by placing hollow fibers perpendicular to the flow direction. In these cross-flow membrane contactors the concentrations of both fluids vary in both directions i.e. in the direction of the flow as well as in the direction perpendicular to the flow. Hence, unlike parallel flow contactors, simple logarithmic averaging of the concentration driving force cannot be used to predict performance of the cross-flow membrane contactors. Similar changes in the driving force are also found in the cross-flow shell and tube heat exchanger. An analytical expression based on heat transfer analogy is derived in this work to describe the mass transfer in these hollow fiber cross-flow contactors. However, it was found that when the change in the volumetric flow of the compressible fluid is significant heat transfer analogy cannot be used to predict the performance of the cross-flow gas–liquid membrane contactor. Therefore, a detailed numerical model is developed to analyze the performance of the cross-flow membrane contactor in such cases. The model takes into account the shell-side mixing, change in concentration driving force in all direction as well as cascading two or more cross-flow modules to give overall co- or counter-current flow arrangement. To validate the model and developed analytical expression, carbon dioxide absorption experiments were carried out in cross-flow membrane contactor using water as a solvent. The predictions of the developed numerical model were found to be in good agreement with the experimental results.