S.P. Kaldis

Reduction of CO2 emissions by a membrane contacting process

A membrane-based gas–liquid contacting process was evaluated in this work for CO2 removal from flue gases. The absorption of CO2 from a CO2–N2 mixture was investigated using a commercial hollow fiber membrane contactor and water or diethanolamine as absorbing solvents. Significant CO2 removal (up to 75%) was achieved even with the use of pure water as absorbent. By using aqueous amine solutions and chemical absorption, mass transfer improved, and CO2 removal was nearly complete (∼99%). A mathematical model was developed to simulate the process and it was validated with experimental data. Results show that membrane contactors are significantly more efficient and compact than conventional absorption towers for acid gas removal.

Gas permeation through PSF-PI miscible blend membranes

The permeation rates of He, H2, CO2, N2 and O2, are reported for a series of miscible polysulfone-polyimide (PSF-PI) blend membranes synthesized in our laboratory. For gases which do not interact with the polymer matrix (such as He, H2, N2 and O2), gas permeabilities in the miscible blends vary monotonically between those of the pure polymers and can be described by simple mixture equations. In the case of CO2, which interacts with PI, blend permeabilities decrease somewhat, compared to pure PSF and PI. This, however, is accompanied by a two-fold improvement in the critical pressures of plasticization vs. polyimide. Permselectivities of CO2N2 and H2CO2 in the blends deviate from mixing theory predictions, in contrast to selectivities of gas pairs which do not interact with PI. Differential scanning calorimetry measurements of pure and PSF/PI blend membranes show one unique glass transition temperature, supporting the miscible character of the PSF/PI mixture. Optical micrographs of the blend membranes clearly indicate perfect homogenization and no phase separation. Frequency shifts and absorption intensity changes in the FTIR spectra of the blends, as compared with those of the pure polymers, indicate mixing at the molecular level. This compatibility in mixing PSF and PI, results essentially in a new blend polymer material, suitable for the preparation of gas separation membranes. Such membranes combine satisfactory gas permeation properties, reduced cost, advanced resistance to harsh chemical and temperature environments, and improved tolerance to plasticizing gases.

SIMULATION OF MULTICOMPONENT GAS SEPARATION IN A HOLLOW FIBER MEMBRANE BY ORTHOGONAL COLLOCATION — HYDROGEN RECOVERY FROM REFINERY GASES

Abstract Modeling of hollow fiber asymmetric membranes can provide useful guidelines to achieve desirable separations of multicomponent gas mixtures. Especially in cases of high commercial interest, such as hydrogen recovery from refinery streams, the accurate prediction of membrane separation performance is important. In this work, the appropriate model equations are solved by orthogonal collocation to approximate differential equations, and to solve the resulting system of non-linear algebraic equations by the Brown method. This technique is applied for the first time in a multicomponent gas separation by hollow fiber membranes and offers minimum computational time and effort, and improved solution stability. The predictions of the mathematical model are compared with experimental results for the separation and recovery of hydrogen from a typical gas oil desulfurization unit for various feed pressures, temperatures and stage cuts. In general, there is a very good agreement between simulation and experimental results. Further application of the developed mathematical model to various refinery gas streams of interest reveals that high permeate purity (99.95+), and high recovery (0.6–0.9), can be achieved even in a one-stage membrane unit. The reported experimental results and the theoretical analysis demonstrate the potential which polymer membrane technology has for the separation of hydrogen from refinery gas streams.

Interrelation Between Phase State and Gas Permeation in Polysulfone/Polyimide Blend Membranes

The phase state of polysulfone/polyimide (PSF/PI) blends has been studied by differential scanning calorimetry, rheology, and X-ray scattering. The blends rich in PSF form miscible blends when prepared by solution casting from a common solvent. In these PSF-rich blends, the single dynamic process in rheology shifts and broadens, with composition reflecting the change in local friction and the enhancement of concentra- tion fluctuations, respectively. Heating to temperatures above the glass transition temperature results in phase separation into PSF- and PI-rich domains. An apparent phase diagram has been constructed, and helium permeability has been measured in different regimes corresponding to miscible, partially miscible, and completely phase- separated states. We find that one component (PI) controls the permeability values and activation energies for helium permeation in the blends. Gas permeation is found to be very sensitive to local concentration fluctuations and thus can be used as a probe of the phase state in polymer blends.

Gas Separation Properties of Polyimide-Zeolite Mixed Matrix Membranes

Mixed matrix membranes (MMMs) of polyimide (PI) and zeolite 13X, ZSM-5 and 4A were prepared by a solution-casting procedure. The effect of zeolite loading, pore size, and hydrophilicity/hydrophobicity of zeolite on the gas separation properties of these mixed matrix membranes were studied. Experimental results indicate that permeability of He, H2, CO2, and N2 increased with zeolite loadings. Selectivity of H2/N2 shows a slight improvement for low loadings of zeolites 13X and ZSM-5 but has a decreasing trend for zeolite 4A and high loadings of zeolites 13X and ZSM-5. In addition, selectivity of H2/CO2 remains low (1–3) while selectivity of CO2/N2 is significantly improved with the incorporation of the three zeolites in the polyimide membrane. Experimental permeabilities are higher than those predicted by the Maxwell model except for H2 and N2 permeabilities of the PI-4A system which are consistent with the predicted permeabilities. The proposed modified Maxwell model is capable of predicting the permeabilities of polyimide-zeolite 4A MMMs, but fails to simulate the permeability increase induced by interface voids in the polyimide-zeolite 13X and ZSM-5 systems.

Pore Size Reduction and Performance Upgrade of Silica Membranes with an Ambient Temperature C-ALD Post-Treatment Method

An ambient temperature C-ALD (with NH3 as catalyst) method was employed to modify sol-gel silica membranes, in order to reduce their pore size and upgrade their gas separation performance. Membranes were characterized before and after modification in terms of permeance and permselectivity with the use of He and N2, as gas separation indicators. The application of the proposed method significantly improved the permselectivity of the membranes accompanied with a minor permeance reduction. The process temperature renders it ideal for application in commercial membrane modules which utilize economic, current available, sealing technologies. The performance of the C-ALD modified silica membranes in various gas separation applications was further examined, indicating that they are potential candidates for H2 separation in various chemical processes. Finally, the stability of the C-ALD modified silica membranes in H2O was studied, indicating some performance loss during the first hours of the membranes exposure to a H2O atmosphere, which nevertheless remained high and steady then on.

Techno-Economic Assessment of Polymeric, Ceramic and Metallic Membranes Integration in an Advanced IGCC Process for H2 Production and CO2 Capture

In the present study, the integration of membrane technology in an Integrated Gasification Combined Cycle (IGCC) system has been considered, in order to reduce the power plant’s CO2 emissions. In this respect three different membrane materials was examined (polymeric, ceramic and metallic) taking into account the latest advances in membranes’ development. The simulation of membranes separation performance was conducted in a Visual Fortran code and this was incorporated in an Aspen Plus flow diagram for the overall performance assessment. The energy analysis of the alternative cases show that CO2 capture in this hybrid IGCC scheme is technically feasible but with an accompanying energy penalty in the power plant’s output. Taking into account both technical and economic issues the most promising scenario seems to be the integration of a 2-staged ceramic membranes system for H2 separation and CO2 capture.

Simulation of a molten bath gasifier by using a CFD code

In this work the simulation of a foaming molten slag gasification reactor was performed with a commercially available Computational Fluid Dynamics code. In the standard body of the code appropriate User-Defined Subroutines were devised and incorporated for the complete description of the process. The flow field and heat transfer equations were solved together with the simulation of the coal gasification phenomena. The model results are in good agreement with the respective experimental data, indicating the validity of the proposed model and thus providing a useful tool for the analysis of the molten slag gasifier operation.

Polymer membrane conditioning and design for enhanced CO2-N2 Separation

Publisher Summary Polymer membranes are used for the separation of carbon dioxide from nitrogen in coal-fired power plant gases. Two different types of polymer materials are examined: polysulfone and polyimide. Polyimide-based membranes exhibit better selectivity and temperature resistance than polysulfone ones. Carbon dioxide (CO 2 ) sorption and subsequent plasticization can affect adversely polyimide membrane properties. CO 2 conditioning is also used to alter CO 2 /N 2 permselectivity of polyimide membranes. Conditioning near and above the critical partial pressure of plasticization improved permselectivity by up to an order of magnitude, for over 4–18 hours due to membrane tightening and/or strong antiplasticization. For the performance evaluation of large-scale membrane units, two mathematical models are used, one for long hollow fiber modules and one for complete gas mixing. Simulations show that although permselectivity is important up to moderate values, at high values, module design and operating parameters affect the performance and process economics equally.

ANALYTICAL AND NUMERICAL SOLUTIONS OF THE MASS CONTINUITY EQUATION IN THE LUMEN SIDE OF A HOLLOW-FIBER MEMBRANE CONTACTOR WITH LINEAR OR NONLINEAR BOUNDARY CONDITIONS

Mass transfer in fully developed laminar flow in hollow-fiber membrane contactors is encountered in a variety of many important applications, such as supported gas and liquid membranes, reverse osmosis, pervaporation, membrane reactors, and biological processes. In this article the complexity of the partial differential equation that describes the concentration profile in the lumen with the associated linear or nonlinear boundary conditions at the fiber wall is simplified by means of analytical and numerical methods using current computational tools. A comparison between the numerical and analytical solution for the linear case reveals the inadequacy of the latter for the evaluation of the lumen Sherwood numbers in the entrance region. For a nonconstant concentration of the diffusing component in the shell side an integro-differential boundary condition at the fiber wall arises, which was approximated by the Gauss–Jacobi orthogonal collocation method.