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Industrial & Engineering Chemistry Research pubs.acs.org/IECR Article Figure 8. (a) R2 for a linear fit (linearity) during the adsorption stage of the cycle (red) with the relative humidity (blue) over the same time period and (b) the relative humidity (blue) of the effluent during the regeneration portion of the experiment. Figure 9. Breakthrough curves sampled through 2000 cycles of zeolite 4A (a). The uptake capacity of zeolite 4A over 2000 cycles as determined by breakthrough time (red, ○), regeneration region integration (blue, Δ), averaged breakthrough analysis (purple, ■), and TGA (black, ⧫) with a linear fit of the TGA data (black, −) (b). The residual error of the breakthrough time analysis (red, ○), regeneration region integration (blue, Δ), and averaged breakthrough analysis (purple, ■) compared to a linear fit of the TGA data (c). Only every 10th cycle is recorded and reported. 4. RESULTS AND DISCUSSION The objective of this work was to cycle adsorbents over 2000 times. At ca. 15 min per cycle, it took 500 h (∼21 days) to collect 2000 cycles for eight samples. To minimize the time of adsorption and desorption, the size of each sample material was restricted to 20−25 mg. Similarly, the time for heating and cooling of the sample cells needed to be minimized while still being capable of cycling to high temperatures (T ≥ 250 °C). Aluminum was found to be a suitable material for the heating block due to its relatively low volumetric heat capacity (Cv = 2.376 × 106 J m−3 °C−1 at T = 20 °C) and high thermal conductivity (λ = 273 W m−1 K−1 at T = 20 °C).26 For the sample cells, stainless steel and aluminum were investigated. Steel was found to heat slower due to its larger volumetric heat capacity (stainless steel 316, Cv = 3.732 × 106 J m−3 °C−1 at T = 20 °C)23 compared to aluminum. For higher regeneration temperatures (T ≥ 250 °C), the temperature inside the steel cells was found to be up to 10 °C less than the set temperature as a result of stainless steel’s smaller thermal conductivity (λ = 13.9 W m−1 K−1 at T = 20 °C),23 which resulted in a greater temperature gradient. Aluminum was found to be the better material for the sample cells due to its low heat capacity and durability over continuous cycling. The results for the change in the uptake capacity of zeolites 4A and 13X are displayed in Figures 9 and 10, respectively (the average of two sample cells for each material). From these figures, it is shown that the results of the breakthrough curve analyses are in good agreement with the TGA results. The average residual differences for the different breakthrough plot analysis methods are summarized in Table 1. By comparing the residuals with a two-tailed P test (where the null hypothesis was that the difference between the two methods was zero), it was found that for zeolite 4A, there is no significant difference in the two breakthrough plot analysis methods, while for zeolite 13X, the difference was significant. Figure 10 clearly shows that the integration method was more reliable for the zeolite 13X samples. While one might be tempted to conclude that the integration method is more robust, we still feel that both methods should be used because the integration method may have issues with wet gas regeneration. The results of this work were compared to the results from Ruthven17 and Belding et al.’s20 studies (Figure 11). It is clear that the general trend for zeolite 13X within this work and the https://doi.org/10.1021/acs.iecr.1c00469 7491 Ind. Eng. Chem. Res. 2021, 60, 7487−7494PDF Image | Rapid Cycling Thermal Swing Adsorption Apparatus
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