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Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article command center so that when the water alarm of the flood sensors is triggered, the safety circuit will turn off the apparatus and close the valve for the water supply. Whenever any of the three components of the safety circuit is activated, a manual reset of the circuit is required before the operation of the apparatus can continue. 2.4. Rapid TSA Cycling Apparatus Procedure. Over a full adsorption cycle (adsorption, regeneration, and cooling) both the composition of the gas and the temperature of the cells were controlled. To effectively cycle the adsorbents, a LabVIEW program was implemented where three relays were used (cooling water, fire rod heating, and wet/dry gas) to control five different steps of the cycle. Each step for the cycle is marked in Figure 6. A full cycle on this instrument began by Figure 6. Breakthrough (blue) curve for zeolite 4A with the corresponding temperature (red) profile for the full cycle conducted during an experiment. Dashed lines indicate what the conditions are during the experiment with regards to temperature control and gas composition. flowing a wet gas (N2/CO2/H2O) through the sample cells for a set amount of time (∼330 s). Once the set time was complete, the wet gas flow was switched to dry gas (N2/CO2) and the fire rods were turned on to heat the aluminum block at 80 °C per minute, marking the start of the regeneration process. The regeneration temperature was set to T = 280 °C under the dry CO2. At T = 280 °C, the fire rods were cycled on and off to maintain this temperature, i.e., proportional control. The cycling of the fire rods resulted in a fluctuation in the temperature of less than ± 3 °C. When the water content reading approached the baseline value of the dry gas (relative humidity ≈ 4%), the fire rods were switched off and cold water flowed through the coolant channels in the block until the blockreachedT=33°C.AtT=80°C,thewetgaswas switched on and the next adsorption cycle was initialized to save on cycling time. For the experiments reported in this work, each cell was loaded with 20−25 mg of an adsorbent. Each overall adsorption and regeneration cycle took less than 15 min. 2.5. Thermogravimetric Analysis. External water ad- sorption analyses were conducted using a thermogravimetric analysis (TGA) instrument (TGA 550 Auto apparatus, TA Instruments). This was done before, after, and midway through the total test time. During activation in the TGA, dry He (T = 400 °C, p = 0.8905 bar, at 16 mL min−1) was passed over the adsorbent. The temperature increased at a rate of 25 °C min−1 and the sample was held at T = 400 °C for 120 min. During the adsorption process, 1.4 mol % H2O in He (T = 35 °C, p = 0.8905 bar, at 16 mL min−1) was passed over the adsorbent and these conditions were held for 240 min. The TGA operated with a He flow through the balance and furnace. For the adsorption experiments, the He gas flowing over the adsorbent was first passed through a water saturator and then diluted by dry He before passing over the adsorbent. The flow rate of He passing through the water saturator was controlled using a Brooks SLA5800 mass flow controller (MFC) and the saturator temperature was controlled using a Polyscience chilling circulating bath. 3. DATA PROCESSING 3.1. Breakthrough Plot Analysis. To analyze the raw data of the cycling apparatus, a program designed through Microsoft Visual Basic for Excel was used following the flow chart shown in Figure 7. Two methods were used for the Figure 7. Logic of the analysis program for analyzing the breakthrough curves. analysis of the breakthrough plots to determine the uptake of water. The first method was to identify the breakthrough times and then track their changes through the cycles. To find the breakthrough time, the inflection point of the slope of the relative humidity (RH) versus time was used. Tracking of the R2 values of linear regression (least-squares) for a linear fit over 25 s of the RH was chosen as the method to identify the breakthrough time as it was observed that the minimum value of R2 coincided with the breakthrough time as shown in Figure 8a, i.e., the inflection point for the onset of breakthrough is the least linear portion of the data. The second method was to integrate relative humidity during regeneration to assess the water, which was removed from the material surface. This was done through summation between the time when dry gas was initiated and the time when wet gas was reintroduced, i.e., the complete regeneration portion of the cycle. To externally determine the change in water adsorption capacity (n, mmol g−1) from the TGA data, the difference in mass between the mass of the dried sample and the final mass of the sample was calculated. 3.2. Water Uptake from Breakthrough Plot Analyses. Relative adsorbent performance change was evaluated by the ratio of the breakthrough time (tb/tb,0) or relative regeneration humidity integration (I/I0) for each measured cycle. The averaged breakthrough time and integration for the first 30 cycles were assumed to be the initial breakthrough time (tb,0) and integration (I0). These relative values were then used to scale the water uptake capacity, as determined using the initial TGA measurement, i.e., the ratios for each cycle were multiplied by the n0 obtained from the TGA. https://doi.org/10.1021/acs.iecr.1c00469 7490 Ind. Eng. Chem. Res. 2021, 60, 7487−7494PDF Image | Rapid Cycling Thermal Swing Adsorption Apparatus
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