Adsorption Equilibrium and Diffusion of CH4, CO2, and N2 in Coal-Based Activated Carbon.

Coal-based activated carbon is an ideal adsorbent for concentrating CH4 from coalbed methane and recovering CO2 from industrial waste gas. In order to upgrade the environmentally protective preparation technology of coal-based activated carbons and clarify the adsorption equilibrium and diffusion rules of CH4, CO2, and N2 in these materials, we prepared granular activated carbon (GAC) via air oxidation, carbonization, and physical activation using anthracite as the raw material. Also, we measured the adsorption isotherms and adsorption kinetic data of GAC by the gravimetric method and characterized its surface chemical properties. According to the results, GAC had abundant micropore structures with a pore size mainly in the range of 5.0-10.0 Å, and its surface was covered with plentiful oxygen-containing functional groups. The specific pore structure and surface chemical properties could effectively improve the separation and purification effects of GAC on CH4 and CO2. In the temperature range of 278-318 K, the equilibrium separation of CH4/N2 by GAC with a coefficient between 3 and 4 could be achieved. Also, the CO2/CH4 separation coefficient decreased with the increase in temperature but remained around 3. The bivariate Langmuir equation could describe the adsorption behaviors of GAC on CH4/N2, CO2/N2, and CH4/CO2. With the increase in the concentrations of CH4 and CO2 in the gas phase, the difference between the adsorption capacity of CH4 or CO2 and that of N2 became greater. The change of the gas ratio did not affect the characteristics of preferential adsorption of CH4 and CO2. At different temperatures (278, 298, and 318 K), the diffusion coefficients of CH4, N2, and CO2 at various pressure points showed predominately a small variation without an obvious trend. These results demonstrated that the separation of CH4/N2, CO2/N2, and CH4/CO2 by the activated carbon could only rely on the equilibrium separation effect rather than the kinetic effect.

Ultra-low-cost fabrication of polymer-based microfluidic devices with diode laser ablation.

In this work, a diode laser ablation approach was used for the fabrication of PMMA-based microfluidic devices. Compared with the conventional CO2 or femtosecond laser fabrication method, the proposed laser ablation method based on diode laser significantly lowered the cost in the fabrication of polymer-based microfluidic devices with comparable resolution and surface quality. PMMA substrate was used for the laser ablation process, due to the transparency of PMMA in the diode laser's working wavelength, a layer of Kraft tape was applied on the surface of PMMA for the absorption of laser energy, and microchannels were then achieved on the surface of PMMA with the proposed low-cost diode laser system. The comparison between the proposed method and the CO2 laser ablation method was also conducted in this study. The profile of the fabricated microchannels was carefully characterized, several microfluidic devices were also fabricated for the demonstration of the proposed fabrication method using a diode laser.

Laser-induced selective wax reflow for paper-based microfluidics.

This study proposes a novel method for the fabrication of paper-based microfluidic devices using laser-induced selective thermal reflow for wax penetration. A layer of wax was evenly deposited on the front side of a filter paper; then a low-cost diode laser was used to scan the designed area from the back side of the filter paper. At the laser irradiated spot, the wax was heated, melted down and penetrated through the whole thickness of the filter paper, and formed hydrophobic barriers on the hydrophilic cellulose fibers. The patterned hydrophobic wax barriers on the filter paper defined the flow path of the fluid for the paper-based microfluidic device. Compared with conventional two-step (deposit and reflow) approaches for paper-based microfluidics using wax barriers, e.g. wax printing, stamping or photolithography, the proposed fabrication protocol achieved wax patterning and reflow simultaneously, conducted during the laser scan process, and without the requirement for any sophisticated instruments or a cleanroom environment. A series of tests were also conducted for the characterization of the proposed paper-based microfluidic device fabrication technique. The fabrication technique used in this approach could have broad application potential in point-of-care diagnosis and testing, especially for applications in the developing world.