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Int J Energy Environ Eng (2014) 5:349–356 351 Fig. 1 Schematic of the experimental set-up for fixed bed. MC mixing chamber, M mass flow meter, P pressure gage, FGA flue gas analyzer Table 3 Experimental conditions Bed weight Reactor length Reactor diameter Influent CO2 concentration Inlet flow rate Bed porosity Superficial velocity Adsorption temperature Adsorption total pressure Methods 20 g 200 mm 30 mm 13.8 vol. % 15 LPM 0.5 0.13 m/s 25–60 °C 1 bar the require rate. Flue Gas Analyzer (KM9106) was used to measure the inlet sample gas and the outlet gas properties. The apparatus was tested for leak absence and for accuracy through calibrations with an empty tank. Table 3 give details of the operating parameters. The process employed a single-bed PSA unit. For the adsorption process, first the air was allowed to flow through the reactor and then followed by carbon dioxide. To maintain equal flow rate and pressure of both the flows, the gas mixture was allowed to pass through the mixing chamber. The flow of air was then stopped and allowed only the pure CO2 to flow through the reactor for mea- surement of the mass of CO2 adsorbed per the mass of the adsorbent. To know the effective adsorbed mass on the adsorbent materials, Qeff, two quantities were defined: Qt that is the mass containing the reactor including the adsorbents and Qd which is the mass of the empty reactor or dead volume. The Qeff was then calculated with the equation as follows: Qeff = Qt - Qd. Results and discussions Adsorption breakthrough curves Adsorption is a transient process and the amounts of material adsorbed within a bed depend both on position and time. The CO2 breakthrough curves for the three adsor- bents are shown through Figs. 2, 3, 4 and the experimental conditions are given in Table 3. The breakthrough curves show ratio of the outlet concentration and the influent concentration against the contact time at an atmospheric pressure, and at a temperatures of 25, 35, 45 and 60 °C. The general pattern of the breakthrough curves were achieved as expected for all adsorbents. For zeolite 13X, the adsorption breakthrough occurs at 20 min and for zeolite 4A, the adsorption breakthrough occurs at 16 min. This shows that the pore diameter of zeolite 13X (10 A ̊ ) and zeolite 4A (4 A ̊ ) is sufficient for the CO2 to enter into the zeolites channels. The major cations of zeolites are Na and K and this major cation appears to play a main role in the adsorption of CO2. Also, sodium appears to be the favorable cation for the adsorption of CO2. The saturation time for zeolite 13X is longer than that of zeolite 4A which is due to the larger pore volume. For AC, the breakthrough time is 8 min at 25 °C. The breakthrough time with AC was shorter as compared to zeolites, indicating that the CO2 adsorption capacity of AC is lower than that of zeolites. The differences in adsorption observed with zeolites and AC should be related to the differences in the chemical nature at the surface and porosity. Both the zeolites have higher surface area than AC and that may have contributed to the higher adsorption capacity of zeolites. The experimental set-up for the adsorption test consists of a CO2 cylinder and Gas Compressor interconnected through a pipe to the system as shown in Fig. 1 based on [25]. The Gas Compressor could provide sufficient pressure up to 10 kg/cm2 with a discharge capacity of 84 LPM. The gas mixing chamber was made up of a GI pipe of 20 mm diameter and 80 mm in length to achieve the required mixture of air and CO2 (13.8 vol. %) at a constant rate. Ceramic wool was wrapped around the reactor above the heater coil for insulation. Thermocouples (K-type) were inserted in the bed to measure the bed temperatures with accuracy of ±0.5 %. A Glass Tube Rotameter of the range 0–35 LPM was used to control the mass flow rate and measure the mass flow rate of the incoming gas. The reactor was made up of GI pipe with 30 mm diameter with 1.5 mm wall thickness and 200 mm in height. A pressure monitoring system was attached to know the incoming pressure and experiments were carried at varying pressure to see the effect of pressure in the adsorption process at a constant temperature. A number of needle valves were attached in the system to control the system operation at 123PDF Image | Carbon dioxide adsorption on zeolites and activated carbon by pressure swing adsorption
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