Improved Carbon Dioxide Capture Using Nanostructured Ceramic Membranes

A nanoporous-structured tubular hybrid inorganic membrane capable of stripping carbon dioxide from flue gas stream was designed and tested at laboratory scale to improve compliance with various environmental regulations to cushion the effect of global warming. Single gas separation experiments using silica modified ceramic membrane were carried out to investigate individual gas permeation behaviors at different pressures and membrane efficiency after a dip coating method. Four gases: Nitrogen (N2), Carbon dioxide (CO2), Oxygen (O2) and Methane (CH4) were used. Plots of flow rate versus pressure were generated. Results show that the gas flow rate increases with pressure drop. However at above a pressure of 4 bars, the flow rate of CO2 was much higher than the other gases, indicating dominance of a more selective absorptive type transport mechanism.


Introduction
About one third of the overall CO 2 emission globally comes from energy creation.Its decrease and management are vitally critical and they are important factors to ease global warming [1].The position of CO 2 in global warming is a present-day ecological concern and needs urgent attention in the provision of technologies that will curtail the emission of CO 2 [2].CO 2 removal from flue gas stream recorded the success with conventional technology like absorption using glycol, amine and methanol under low temperature, the process with hot potassium carbonate, the reaction with calcium oxide and the use of polymer membrane.These processes employ low temperature resulting in energy losses due to high temperature recuperation as well as cooling of the gas stream [3].Owing to the burning desire to abate global warming especially at the rate of CO 2 emission and its concentration in the atmosphere through flue gases today, inorganic ceramic membrane with distinct characteristics should be a key issue.Cost effective, energy-saving, high chemical resistance, cheap materials are needed in the development of membrane module for manufacturing purposes.Porous inorganic membrane conquers some rather than all of the in-built limitations.They can tolerate higher temperature and basically limit the connection between selectivity and permeability.If properly designing the pore size and its distribution decides its selectivity while the volume fraction porosity regulates and establishes permeability [3].Porous inorganic membrane also shows exceptional evidence of physical and chemical properties, including unresponsiveness to sarcastic environment, stability under high temperature, homogeneous pore structure and reasonable fluxes [4].Some membranes today have been used for decades in CO 2 capture, but, because the membrane is used for natural gas at very high pressure, they are unsuitable for CO 2 capture from flue gases.Design and fabrication of porous ceramic membrane consist of several layers of different materials namely: Aluminium oxide (Al 2 O 3 ), Titanium Oxide (TiO 2 ), Zirconium Oxide (ZrO 2 ), Silicon dioxide (SiO 2 ), Silicon carbide, Zeolite or a mixture of two materials applied on an underlying porous stainless steel, αalumina, γ-alumina, zirconium, zeolite supports [5].The manner in which gas molecules flow across the membrane referred to permeation mechanism is a significant fact in membrane technology.This flow is generally influenced by three factors: the gas properties, morphology of the membrane and the material used for membrane design.Consequently a hybrid material made from ceramics membrane has been well thought out by means of Nanotechnology.The technology is eco-friendly, eco-nomical and very efficient.It's applicable in all forms of CO 2 elimination from other gases and its effectiveness increases comparatively to the CO 2 concentration in the flue gas stream feed.Nanotechnology is among the ranges of technologies paying attention to exploring the carboncapturing technology [6].Membrane separation of gases is a highly complex process and therefore the material used for its preparation should exhibit a long-lasting characteristic, stability and modify in an advanced manner to be adapted to separate specific gases.
However, the innovation is that a so called agent is immobilized in the membrane porous network, thus assisting to attract the CO 2 , and enhancing its transport across the membrane.This is in complete contrast to the more common and older method of using a filter that separates directly between CO 2 and other gases.

Experimental
The gas separation experiment was performed using a membrane support and a nano-structured membrane deposited on a macro porous tubular filter employing a repeat dip-coating technique [6].The gases used for the gas transport tests included Nitrogen (N 2 ), Carbon dioxide (CO 2 ), Oxygen (O 2 ) and Methane (CH 4 ) respectively.
The experiment was carried out to investigate the single gas permeation behaviors of the gases listed above with the permeation done separately and individually.
In Figure 1, the picture at the top is a tubular ceramic support before modification while the one at the bottom is the nano-structured composite which was modified after a dip coating technique.

Morphology of Support
The scanning electron microscopy was used to verify the morphology of the support.The image is shown in

Applicable Theory
The manner in which gas molecules flow across composite membrane comprising of the membrane and the support with respect to the pressure will be determined.In addition, a distinction between viscous and Knudsen flow mechanisms in porous membrane to determine flaws in the membrane before and after coating will be illustrated.A schematic illustration of the transport porous is shown in Figure 5 involving interactions between gas molecules and pore surface to determine flows in the membrane before and after coating will be illustrated.A schematic illustration of the transport process is    shown in Figure 5.
Values of gas permeation rate of the top layer from Figure 5 were calculated using the formula [7]   where F (l/min) is the flow rate through the membrane and S (m 2 ) is the membrane surface area of the flow pathway P f (bar) and P b (bar) are absolute feed pressure and borderline pressure between the support and the toplayer.P b can be calculated from Equation (2) below where C and D are constants relative to membrane support characteristics and Z given below for gas permeation through the composite membrane.
 

Results and Discussion
Accordingly, Figure 6 presents a plot of flow rate against pressure and the trend explains the proportionality for both parameters.
Figure 7 shows the plots of the ratio of flow rate of CO 2 to that of N 2 in relationship to the ratio of the molecular weight of CO 2 to that of N 2 .The trend confirms that at pressure of 2 bar and above more CO 2 was obtained from the separation.
Figure 8 also demonstrated that at pressure of 3 bars and above more of CO 2 was separated from O 2 while Figure 9 illustrates that from 2.4 bars and above more CO 2 will be removed from CH 4 .
Figure 10 shows that gas flow through the support (sup) increased as the pressure was increased.For the  top layer, the effect of pressure on the gas permeation was negligible even after a pressure build up.
As expected, the layer showed only minor dependence of permeance with average pressure suggesting the absence of defects.

Conclusion
Using the combination of Knudsen flow and viscous flow, it has been possible to extract the transport of the thin membrane layer in a composite hybrid system.For the support overall results have shown that both viscous and Knudsen flow can have a significant effect on the single gas transport.The dip coating technique had a significant effect toward achieving a zero level membrane deficiency.However, real flue gas will consist of mixtures and work is currently ongoing to obtain mixed gas transport and selectivity.

Figure 7 .
Figure 7. Ratio of flow rates and molecular weights of CO 2 and N 2 at different pressures.

Figure 8 .
Figure 8. Ratio of flow rates and molecular weights of CO 2 and N 2 at different pressures.