Stepan Bazhenov

Gas–liquid membrane contactors for carbon dioxide capture from gaseous streams

Membrane technology is characterized by high efficiency, compatibility and flexibility of various membrane processes in integrated systems, low power consumption, high stability and environmental safety of processes, comparative ease and simplicity of controlling and scaling-up, as well as a unique functional flexibility of the membrane processes. This is why the membrane technology is considered as a promising way to reduce anthropogenic emissions of carbon dioxide into the atmosphere. Gas–liquid membrane contactors are a prime example of high-performance hybrid processes using membrane technologies. Integrating several separation methods in one device (membrane contactor) makes it possible to retain benefits of membrane technology, such as small size and flexibility, complementing them with high separation selectivity typical of CO2 absorption. This review presents the basic principles of operation and design of membrane contactors, and a wide range of materials, membranes, and liquid absorbents for membrane CO2 absorption/stripping are considered. Particular attention has been paid to current studies on CO2 removal from thermal power plant flue gas, natural gas, biogas, and syngas. The examples of pilot-scale and semi-commercial implementation of CO2 absorption/stripping in membrane contactors have been given.

Cross-flow mass transfer in a model hexagonal system of hollow fiber membranes

A method for calculating external mass transfer in a contactor with transverse confined flow of a viscous incompressible liquid (gas) past a hollow fiber membrane at low Reynolds numbers has been proposed. The method is based on the concept of array of hollow fiber membranes as a hexagonal system of parallel fibers to which the Kuwabara cell model with a known flowfield is applicable. Asymmetric membranes with macroporous permeable outer supports have been considered. A solution of the problem on the external Stokes flow past a hollow fiber membrane in the cell, taking into account the hydrodynamic permeability of the support, has been found in terms of the model. To describe the flow inside and outside the support, joining of the general solutions to the Stokes and Brinkman equations has been used. The Saffman slip condition has been set on the surface of the low-permeability membrane. The dependences of the stream function, velocity components, and drag force upon the fiber packing density and support permeability and thickness have been revealed. Efficiency η of solute sorption by the fiber has been calculated for the flowfield found, assuming zero component concentration at the membrane surface (full absorption approximation). The dependences of η on the diffusion Peclet number, support permeability and thickness, and fiber packing density have been calculated. Direct 3D simulation of convective mass transfer in the contactor at low Reynolds numbers has been performed, and the cell model has been shown to be applicable to the calculation of the contactor with a predominantly two-dimensional transverse flow.

Simulation of external mass transfer in hollow-fiber membrane contactors

External transverse laminar flow of a viscous incompressible fluid and convective diffusion of a solute in a model membrane contactor with a regular system of monodisperse fibers have been numerically simulated. A row of equally spaced parallel fibers was taken as the simplest model system, for which the dependences of the drag force and the efficiency of solute absorption by the fiber (Sherwood number of fiber) upon the distance between the axes of adjacent fibers, and the Reynolds and Peclet numbers have been calculated. The influence of the flow inertia has been studied, and it has been shown that the flow field and mass transport in the contactor can be described in terms of the linear Stokes approximation up to as high Reynolds numbers as dense is the fiber array.

Influence of the composition of concentrate solutions on the efficiency of carbon dioxide removal from monoethanolamine aqueous solution by electrodialysis

The feasibility of optimizing the electromembrane removal of carbon dioxide from an aqueous monoethanolamine (MEA) solution with a CO2/MEA molar ratio of 0.2 after the regeneration stage aimed to reduce the loss of MEA with the concentrate has been explored. The influence of the composition of the concentrate solution on the recovery of carbon dioxide and the specific loss of MEA (g MEA/g CO2 recovered) for a process duration of 1 h has been studied. The solution in the concentration cells was either distilled water or aqueous MEA solutions with a different carbon dioxide loadings. It has been found that the CO2 recovery ranges within 56.6–66.5%; the maximum and minimum specific losses of MEA make 4.80 and 1.49 g MEA/g CO2 recovered; and the specific energy consumption is 6.48–9.72 MJ/kg CO2 recovered.

Effect of absorbent vapor on stability of characteristics of a composite PTMSP membrane on nonwoven polyester support during regeneration of diethanolamine solution in membrane contactor

The regeneration of a carbon dioxide-loaded aqueous solution of diethanolamine (DEA) in a membrane contactor-stripper at a temperature of 100°C, an absorbent pressure of 10 atm, and a varying absorbent feed flow rate has been studied. The membranes used were laboratory samples of composite membranes prepared by deposition of thin separation layers of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) on a porous support. The support was MFFK-1 microfiltration membrane (Vladipor) with the filtering porous layer of fluoroplastic F-42 (tetrafluoroethylene-vinylidene fluoride copolymer) deposited on a nonwoven polyethylene terephthalate (PET) support. After the first 10 days of testing, the CO2 flux at the membrane contactor outlet was reduced by a factor of 3 and then stabilized at 2 m3/(m2 h) within the next 80 days. It has been found that along with CO2 transport through the membrane, the vapor of the absorbent solution components is transferred. The concentration of DEA in the condensate was 0.5 wt %, that corresponds to the composition of equilibrium vapor over a 30 wt % DEA aqueous solution at 100°C. Since PTMSP is chemically resistant to the DEA solution at the regeneration temperature, the deterioration of the transport properties of the PTMSP/MFFK(PET) composite membrane with time during the absorbent regeneration is associated with the chemical degradation of the nonwoven PET support by the action of penetrating DEA vapor at a temperature of 100°C. It has been concluded that more chemically and thermally resistant porous supports such as ceramic microfiltration membranes should be used.

Two-Step Electrodialysis Treatment of Monoethanolamine to Remove Heat Stable Salts

Electrodialysis technology was adapted to removal of heat stable salts from aqueous solutions of alkanolamine absorbents, with monoethanolamine as example. Removal of anions of heat stable salts by electrodialysis from a 30 wt % aqueous solution of monoethanolamine with the degree of carbonation of 0.2 mol of CO2 per mole of monoethanolamine was studied. The two-step removal of heat stable salts by electrodialysis allows the monoethanolamine loss to be reduced and the concentration of residual CO2 in the absorbent solution to be decreased. The suggested two-step electrodialysis treatment scheme allows the concentration of heat stable salts to be maintained on the required level from the viewpoint of their corrosion activity, the total volume of the concentrate to be decreased by 50%, and the monoethanolamine loss to be decreased by 30%. The treatment unit with the circulation volume of the monoethanol absorbent of 100 m3 h–1 was calculated for confirming the efficiency of the two-step electrodialysis treatment scheme. As compared to the one-step electrodialysis treatment scheme, the two-step scheme ensures recovery of 50% of monoethanolamine at the same efficiency of the removal of heat stable salts.

Carbon Dioxide Desorption from Amine Solution in a Nonporous Membrane Contactor

Long-term testing of a membrane contactor based on a blend of the nonporous polymers polytrimethylsilylpropyne (PTMSP) and polyvinyltrimethylsilane (PVTMS) has been carried out. The flat-sheet membrane contactor has been tested for CO2 desorption from an aqueous methyldiethanolamine solution at 100°C. It has been found that the mass transfer parameters (CO2 flux and stripping efficiency) of the 95%PTMSP/5%PVTMS membrane stabilize after the 7th day of testing. The CO2 mass transfer coefficient in the membrane contactor has been evaluated, and optimal desorption parameters have been determined.

Modeling of convection–diffusion transport in a hollow-fiber membrane contactor with radial transverse liquid flow

Convection–diffusion processes in a model hollow fiber membrane contactor with transverse radial convergent (divergent) laminar flow with a variable radial velocity have been studied. The model contactor has been an ordered system of monodisperse parallel fibers arranged perpendicular to the feed stream. A formula relating the absorption efficiency of the component in the coaxial fiber layer, depending on the direction of radial flow, to the fiber packing density and the Peclet number has been first derived.