Dynamic adsorption of Xe on a fixed bed adsorber at 77 K

Dynamic adsorption of Xe on a fixed bed adsorber at 77 K 1 2 first-author 1 corresp Corresponding Email:lijunli@mail.tsinghua.edu.cn Corresponding Email:lijunli@mail.tsinghua.edu.cn lijunli@mail.tsinghua.edu.cn 2 2 2 2 2 2 Key Laboratory of Particle and Radiation Imaging of Ministry of Education, Department of Engineering Physics, Tsinghua University, Beijing 100084, China Key Laboratory of Particle and Radiation Imaging of Ministry of Education, Department of Engineering Physics, Tsinghua University, Beijing 100084, China Northwest Institute of Nuclear Technology, Xi’an 710024, China Northwest Institute of Nuclear Technology, Xi’an 710024, China In designing a fixed bed adsorber, it is vital to understand dynamic adsorption properties of the unit. Temperature is an important effect on adsorbent performance, as the dynamic adsorption coefficients tend to increase with decreasing temperature. To minimize the volume of the fixed bed adsorber, the dynamic adsorption characteristics of Xe were studied at 77 K by employing a variety of adsorbents under different operational conditions. The carbon molecular sieve performed better than that of activated carbon. Both operational conditions and the presence of gaseous impurities were found to affect the adsorption properties. Dynamic adsorption Xe 77 K Carbon molecular sieve Fixed bed adsorber 1. Introduction 1 Because of their high fission yields, suitable half-lives and low environmental background concentrations, xenon isotopes are important for detection of clandestine nuclear tests and various aspects of radiation safety [ 1 1 ]. Due to their very low environmental concentrations, xenon isotopes are very difficult to detect unless they are concentrated by separating them from other gaseous atmospheric components. A common method of xenon enrichment and separation is physical adsorption, which is used in international monitoring systems (IMS) and on-site inspection (OSI) equipment of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), e.g., ARSA [ 2 2 - 4 4 2-4 ], SPALAX [ 5 5 ], ARIX [ 6 6 ], SAUNA [ 4 4 , 7 7 ], SAUNA-OSI, ARIX 3F [ 8 8 ], and XESPM-II techniques [ 1 1 ]. The adsorption materials used in these systems include carbon-based molecular sieves, granular-activated carbons, and zeolite molecular sieves. Adsorption is an exothermal process, and dynamic adsorption capacity of a fixed bed adsorber is inversely proportional to its temperature [ 9 9 , 10 10 ]. The dynamic adsorption coefficient of activated carbon for xenon was 2.56 L/g at 276 K and 0.86 L/g at 328 K [ 9 9 ]. Therefore, it is essential to study the properties of Xe on various adsorbents using a cryostat designed to hold the lower bed temperature and minimize the Xe adsorber dimensions. Conveniently obtainable liquid nitrogen (LN 2 ), at 77K, is used to refrigerate gases in the adsorber efficiently and rapidly. In this work, a fixed bed adsorber at 77 K was used to study the dynamic adsorption coefficients of different adsorbents and the effects of various operation conditions. 2. Theory 1 According to the adsorption equilibrium theory, dynamic adsorption coefficients can be calculated from the breakthrough curves [ 10 10 - 14 14 10-14 ]: <math display="block" id="M1"><mrow><msub><mi>k</mi><mrow><mi>d</mi><mi>B</mi></mrow></msub><mo>=</mo><mi>F</mi><msub><mi>t</mi><mrow><mn>0.05</mn></mrow></msub><mo>/</mo><mi>m</mi><mo>,</mo></mrow></math> <math display="block" id="M2"><mrow><msub><mi>k</mi><mi>d</mi></msub><mo>=</mo><mi>F</mi><msub><mi>t</mi><mrow><mn>0.5</mn></mrow></msub><mo>/</mo><mi>m</mi><mo>.</mo></mrow></math> where, k dB is the breakthrough dynamic adsorption coefficient, k d is the dynamic adsorption coefficient, m is the adsorbent mass, t 0.05 is the breakthrough time, t 0.5 is the equilibrium time, and F is flow rate of outlet gas. The adsorption quantity can be calculated by <math display="block" id="M3"><mrow><mi>q</mi><mo>=</mo><mi>F</mi><msub><mi>t</mi><mrow><mn>0.5</mn></mrow></msub><msub><mi>C</mi><mn>0</mn></msub><mo>/</mo><mi>m</mi><mo>=</mo><msub><mi>k</mi><mtext>d</mtext></msub><msub><mi>C</mi><mn>0</mn></msub><mo>,</mo></mrow></math> where C 0 is the equilibrium concentration of Xe in the inlet gas. The length of unused bed ( LUB ) to optimize the packed bed length can be approximated by <math display="block" id="M4"><mrow><mi>L</mi><mi>U</mi><mi>B</mi><mo>=</mo><mtext> </mtext><mrow><mo>(</mo><mrow><mn>1</mn><mo>−</mo><msub><mi>t</mi><mrow><mn>0.05</mn></mrow></msub><mo>/</mo><msub><mi>t</mi><mrow><mn>0.5</mn></mrow></msub></mrow><mo>)</mo></mrow><mi>h</mi><mo>,</mo></mrow></math> where h is the actual length of the packed bed. The specific adsorption volume V x can be calculated by Eq. (5), where V c is the inner volume of the adsorber. <math display="block" id="M5"><mrow><msub><mi>V</mi><mtext>x</mtext></msub><mo>=</mo><mi>F</mi><msub><mi>t</mi><mtext>x</mtext></msub><mo>/</mo><msub><mi>V</mi><mi>c</mi></msub><mo>,</mo></mrow></math> Zhou Chongyang [ 15 15 ] improved the Wheeler–Jonas equation [ 11 11 ] for convenient calculation of the overall adsorption rate constant ( k v ) for analysis of the dynamic effect. <math display="block" id="M6"><mrow><mfrac><mrow><msub><mi>t</mi><mrow><mn>0.05</mn></mrow></msub></mrow><mrow><msub><mi>t</mi><mrow><mn>0.5</mn></mrow></msub></mrow></mfrac><mo>=</mo><mn>1</mn><mo>−</mo><mfrac><mi>F</mi><mrow><msub><mi>V</mi><mtext>c</mtext></msub><mo>⋅</mo><msub><mi>k</mi><mtext>v</mtext></msub></mrow></mfrac><mi>ln</mi><mo stretchy="false">(</mo><mfrac><mrow><msub><mi>C</mi><mn>0</mn></msub><mo>−</mo><mn>0.05</mn><msub><mi>C</mi><mn>0</mn></msub></mrow><mrow><mn>0.05</mn><msub><mi>C</mi><mn>0</mn></msub></mrow></mfrac><mo stretchy="false">)</mo><mo>=</mo><mn>1</mn><mo>−</mo><mn>2.944</mn><mo>⋅</mo><mfrac><mi>F</mi><mrow><msub><mi>V</mi><mtext>c</mtext></msub><mo>⋅</mo><msub><mi>k</mi><mtext>v</mtext></msub></mrow></mfrac><mo>,</mo></mrow></math> 3. Experimental 1 3.1. Experimental apparatus 2 The test apparatus for dynamic adsorption experiments is shown schematically in Fig. 1 Fig. 1 . N 2 and Xe were used as the carrier gas and adsorbate, respectively, while the impurity gas included CO 2 , CO and water vapor. The pressure sensor, dew point meter and flow meter were used to measure inlet and outlet pressures, dew point and flow rate, respectively. A gas chromatograph (GC) was used to measure the concentrations of Xe, CO 2 and CO. The experiment procedures are as follows. The N 2 and Xe gases were pressurized initially at 0.4 MPa using their respective regulators. The mass flow controllers were adjusted to obtain desired Xe concentration and gas flow rates so that the gases could be mixed adequately and homogeneously. The gas pressure was then adjusted using the adjustable valves at the bed inlet, after which Xe concentration of the inlet gas was determined using the GC (the gas circuit denoted by the dotted line in Fig. 1 Fig. 1 ), employing a 13XHP molecular sieve column to remove water vapor and CO 2 before these species condensed inside the packed bed at low temperature. The pre-cooling column and the fixed bed adsorber were placed in a thermostat filled with LN 2 at 77 K. Various fixed bed adsorbers were used, which were made of cylindrical copper tubes and filled with different adsorbents (hereinafter referred to as the adsorption columns). After the effluent gas from the buffer tank achieved a pressure sufficiently stable for the GC analysis, the Xe concentration in the effluent was monitored at 5-min intervals. At the end of gas circuit, the flow rate was determined with the soap-film flow meter, and dew point was monitored by dew point meters near Valve 4 and the buffer tank. The effects of gaseous impurities on Xe adsorption were determined by switching on the pressurized containers of CO 2 and CO or the water vapor generator. This was accomplished by closing Valves 3, 4 and 6 and opening Valve 5. 3.2. Adsorbents 2 The adsorbents included carbon molecular sieves and granular coconut shell-activated carbon. The 01-CMS, 501CMS and 601CMS were produced by Shanghai Institute of Fine Chemicals, China. TJ-CMS was manufactured by Tianjin Chemical Reagent Co., Ltd., China. NM-GAC was produced by Zhongsen Activated Carbon Co., Ltd., Inner Mongolia, China. HN-GAC was procured from Shuangxinlong Industrial and Trading Co., Ltd., Hainan Province, China. Properties of the adsorption columns and adsorbents are summarized in Table 1 Table 1 , where S BET is the BET specific surface area; V t is the total pore volume; S micro and V micro are the specific surface area and the pore volume for micropores calculated with the t-Plot method, respectively; L micr o is the diameter of micropores calculated with the D-R model; and E 0 is the characteristic energy of adsorption. * Adsorption column, Φ 3.175 mm×200 mm, inner diameter, 1.59 mm; Adsorbent mesh, 0.25–0.38 mm 1 1 Types 1 1 Index 1 1 Mass (g) 1 1 S BET (m 2 /g) 1 1 V t (cm 3 /g) 1 1 S micro (m 2 /g) 1 1 V micro (cm 3 /g) 1 1 L micro (nm) 1 1 E 0 (kJ/mol) 1 1 Carbon molecular sieve 1 1 01-CMS 1 1 0.20 1 1 1049 1 1 1.04 1 1 942 1 1 0.43 1 1 1.18 1 1 20.6 1 1   1 1 TJ-CMS 1 1 0.26 1 1 1129 1 1 0.50 1 1 1047 1 1 0.41 1 1 0.68 1 1 27.3 1 1   1 1 501CMS 1 1 0.20 1 1 1023 1 1 0.99 1 1 890 1 1 0.41 1 1 1.17 1 1 20.6 1 1   1 1 601CMS 1 1 0.19 1 1 1002 1 1 0.97 1 1 861 1 1 0.40 1 1 1.20 1 1 20.4 1 1 Activated carbon 1 1 NM-GAC 1 1 0.18 1 1 1038 1 1 0.65 1 1 784 1 1 0.34 1 1 0.93 1 1 23.0 1 1   1 1 HN-GAC 1 1 0.22 1 1 874 1 1 0.47 1 1 887 1 1 0.42 1 1 1.19 1 1 20.5 3.3. Gas chromatography 2 A HP6890 GC equipped with a thermal conductivity detector (TCD) was used to analyze the gas concentrations. The operational conditions are presented in Table 2 Table 2 . * Detector temperature, 205℃; column size, Φ 1/8 in×2 m; column mesh, 60–80; makeup flow, 2 mL/min. 1 1 Analysis objects 1 1 Oven temperature (℃) 1 1 Types of column 1 1 Column flow (mL/min) 1 1 Ref flow (mL/min) 1 1 CO 2 , Xe 1 1 80 1 1 Porapak Q 1 1 10 1 1 26 1 1 CO, Xe 1 1 100 1 1 5A 1 1 12 1 1 33 4. Results and Discussion 1 4.1. Dynamic adsorption of Xe on different adsorbents 2 Characteristics of dynamic Xe adsorption on different adsorbents were determined using the gas circuit in Fig. 1 Fig. 1 . The results are given in Table 3 Table 3 . Based on the specific Xe desorption volume of 8 for activated carbon or a carbon molecular sieve [ 15 15 ], and purities of Xe adsorbed by different adsorbents, the overall adsorption rate constant ( k v ) could be calculated by Eq. (6). 1 1 Adsorbents 1 1 C 0 (×10 −6 V/V ) 1 1 F (mL/min) 1 1 t 0.05 (min) 1 1 t 0.5 (min) 1 1 k dB (L/g) 1 1 k d (L/g) 1 1 q (mL/g) 1 1 LUB (cm) 1 1 Purity (%) 1 1 k v (min −1 ) 1 1 01-CMS 1 1 39.07 1 1 515 1 1 142 1 1 482 1 1 365.8 1 1 1242 1 1 48.51 1 1 14.1 1 1 75.4 1 1 5424 1 1 TJ-CMS 1 1 39.07 1 1 491 1 1 157 1 1 577 1 1 385.5 1 1 1417 1 1 55.35 1 1 14.6 1 1 82.0 1 1 5010 1 1 501CMS 1 1 38.64 1 1 510 1 1 146 1 1 551 1 1 372.2 1 1 1405 1 1 54.27 1 1 14.7 1 1 77.4 1 1 5151 1 1 601CMS 1 1 43.04 1 1 508 1 1 135 1 1 275 1 1 360.6 1 1 735 1 1 31.62 1 1 10.2 1 1 65.5 1 1 7411 1 1 NM-GAC 1 1 38.64 1 1 547 1 1 110 1 1 200 1 1 301.1 1 1 547 1 1 21.15 1 1 9.0 1 1 54.6 1 1 9034 1 1 HN-GAC 1 1 38.64 1 1 529 1 1 62 1 1 247 1 1 164.1 1 1 654 1 1 25.26 1 1 15.0 1 1 63.7 1 1 5249 Typical breakthrough curve of TJ-CMS at 77 K is shown in Fig. 2(a) Fig. 2(a) . The curve is S-shaped, and multilayer adsorption can be seen clearly. The Xe boiling point is 165 K, while LN 2 is at 77 K, xenon may be liquefied. The kinetic energy of xenon molecules then decreases, and the inner pressure of the adsorber increases. This increases the contact time between the adsorbent pore surfaces and xenon molecules, and xenon in liquid state is captured by carbon adsorbents. Meanwhile, because of the large specific surface area of carbon adsorbents, more adsorption sites can be provided for xenon. These factors cause advantageous conditions for larger adsorption capacity. From Table 3 Table 3 , k dB and k d values of the carbon molecular sieves are higher than those of the activated carbon. For carbon molecular sieves, the maximum k dB and k d values for the TJ-CMS are 385.5 and 1417 L/g, respectively, and the purity of Xe is 82%, which is 2.1×10 4 times the inlet concentration of Xe. For activated carbons, the k dB of the NM-GAC is greater than that of the HN-GAC, though its k d is lower. The LUB of 01-CMS, TJ-CMS, and 501CMS are approximately 14 cm. The k d of NM- GAC is 547.4 L/g at 77 K, which is over 200 times the k d at 276 K [ 9 9 ]. It is apparent that multilayer adsorption occurs on the CMS and activated carbons at 77 K, leading to the much higher adsorption capacities of the materials. From Table 1 Table 1 , TJ-CMS has a larger BET area and micropores surface area, with narrower micropore diameter than those of other adsorbents, which means larger adsorption capacity and better adsorption selectivity for Xe. The k dB values are important, and they are commonly used to design the first stage of an adsorption column to ensure high recovery of Xe. The results indicate that the minimum k v is 5010 min −1 (TJ-CMS), and the maximum k v is 9034 min −1 (NM-GAC). The k v varies with the gas diffusion rate, which depends on the volume proportion of macropores and medium pores. For confirming the multilayer adsorption of Xe on carbon adsorbents, the adsorption equilibrium isotherm of TJ-CMS was measured at 77 K with nitrogen as the analysis gas by autosorb iQ Station (Quantachrome Instruments, USA). The isotherm of TJ-CMS is shown in Fig. 2(b) Fig. 2(b) . According to the classification of IUPAC [ 16 16 ], the adsorption isotherm of TJ-CMS at 77 K is Ⅰ-type, which agrees well with the multisite Langmuir model. Micropore filling occurs at smaller P/P 0 ratios. Due to the existence of macropores and medium pores, multilayer adsorption takes place as the relative pressure P/P 0 increases; while as P/P 0 increases, the phenomenon of capillary condensation appears. Both multilayer adsorption and capillary condensation are useful for enhancing the adsorption. However, when the degree of capillary condensation becomes too large, the adsorbed gas may be frozen, and it may even jam the fixed bed adsorber. 4.2. The Xe concentration effects 2 The Xe concentration affects the dynamic adsorption performance. The coefficients of Xe dynamic adsorption on TJ-CMS tested at different Xe concentrations are shown in Fig.3(a) Fig.3(a) . The k dB and k d decrease with increasing Xe concentration until k d =117×10 −6 V / V , where they begin to increase. Polynomial fittings were performed between ln k dB or ln k d and ln C 0 , and the results are shown in Eqs.(7) and (8). <math display="block" id="M7"><mrow><mtext>ln</mtext><msub><mi>k</mi><mrow><mtext>dB</mtext></mrow></msub><mo>=</mo><mtext> </mtext><mn>0.545</mn><msup><mrow><mrow><mo>(</mo><mrow><mtext>ln</mtext><msub><mi>C</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow></mrow><mn>2</mn></msup><mo>−</mo><mn>4.845</mn><mtext>ln</mtext><msub><mi>C</mi><mn>0</mn></msub><mo>+</mo><mn>16.17</mn><mo>,</mo><mtext>   </mtext><msup><mi>R</mi><mn>2</mn></msup><mo>=</mo><mn>0.930</mn></mrow></math> <math display="block" id="M8"><mrow><mtext>ln</mtext><msub><mi>k</mi><mtext>d</mtext></msub><mo>=</mo><mtext> </mtext><mn>0.226</mn><msup><mrow><mrow><mo>(</mo><mrow><mtext>ln</mtext><msub><mi>C</mi><mn>0</mn></msub></mrow><mo>)</mo></mrow></mrow><mn>2</mn></msup><mo>−</mo><mn>2.199</mn><mtext>ln</mtext><msub><mi>C</mi><mn>0</mn></msub><mo>+</mo><mn>12.30</mn><mo>,</mo><mtext>   </mtext><msup><mi>R</mi><mn>2</mn></msup><mo>=</mo><mn>0.989</mn></mrow></math> In Fig. 3(b) Fig. 3(b) , the adsorbed quantity increases linearly with the Xe concentration. As the concentration increases, the gases in the adsorption column tend to liquefy at 77 K, and capillary condensation occurs. Also, because of the large specific area of TJ-CMS, sufficient adsorption sites can be provided, and the liquefaction of Xe at 77 K can be captured easily by micropores of the TJ-CMS adsorbent. According to the gas-solid adsorption theory, the equilibrium adsorption amount of gas adsorbent increases as the partial pressure of the key gas component increases. The increasing Xe concentration is equivalent to the increasing partial pressure of Xe. Therefore, the above factors result in the decreased adsorption coefficient and the increased adsorption amount. The k v values listed in the Table 4 Table 4 indicate that k v varied slightly with the Xe concentration. 1 1 C 0 (×10 -6 V/V ) 1 1 F (mL/min) 1 1 t 0.05 (min) 1 1 t 0.5 (min) 1 1 k v (min −1 ) 1 1 20 1 1 519 1 1 332 1 1 1192 1 1 5346 1 1 37 1 1 509 1 1 194 1 1 739 1 1 5124 1 1 117 1 1 515 1 1 105 1 1 560 1 1 4704 1 1 235 1 1 492 1 1 222 1 1 602 1 1 5783 4.3. The gas flow rate effects 2 Using the TJ-CMS carbon molecular sieves, the effects of gas flow rate on the dynamic adsorption of Xe at 77 K were studied under flow rate of 509–1348 mL/min and Xe concentrations of 37×10 −6 – 45×10 −6 V/V . As shown in Fig.4 Fig.4 , the k dB and k d decrease with increasing flow rate, from k dB =380 L/g and k d =1500 L/g at the gas flow rate of 509 mL/min, to k dB =150 L/g and k d =450 L/g at the gas flow rate of 1348 mL/min. Because the linear velocity of gas increases with the flow rate, the gas flowing through the adsorption column may be not adequately cooled, hence the increase in thermal motion of the gas molecules. Under these conditions, flow rate makes a great difference in the dynamic adsorption performance. As shown in Fig. 4(b) Fig. 4(b) , k v increases with the gas flow rate. This agrees with the results in Ref. [ 15 15 , 17 17 ]. As the Xe diffusion rate increases with the gas flow rate, the contact between Xe molecules and the adsorption interfaces of adsorbents becomes easier, and contact time shortens. Therefore, k v increases with the flow rate, but the adsorption capacity decreases. 4.4. The inlet pressure effects 2 The inlet pressure effects on the dynamic adsorption of Xe at 77 K were assessed at the gas flow rates of 565–573 mL/min and Xe concentration of 33×10 −6 – 45×10 −6 V / V . As shown in Fig. 5 Fig. 5 , the k dB and k d increase with the inlet pressure, agreeing with reports in Ref.[ 10 10 ]. The inlet pressure is an most important factor for designing adsorption columns and gas circuits. The k v data in Table 5 Table 5 indicate that k v changes slightly with the inlet pressure. 1 1 P (kPa  ) 1 1 F (mL/min) 1 1 t 0.05 (min) 1 1 t 0.5 (min) 1 1 k v (min −1 ) 1 1 108 1 1 565 1 1 130 1 1 440 1 1 5960 1 1 150 1 1 565 1 1 136 1 1 486 1 1 5823 1 1 200 1 1 550 1 1 144 1 1 539 1 1 5578 1 1 301 1 1 573 1 1 145 1 1 525 1 1 5883 4.5. The inner diameter effects 2 The adsorption capacity of adsorbents is known to decrease with increasing linear velocity of the gas. A common way of reducing the effects of a high linear velocity is to increase the pre-column pressure and inner diameter of the adsorption column. Therefore, we studied inner diameter effects of the adsorption column on Xe dynamic adsorption over the carbon molecular sieve TJ-CMS at 77 K. The results are summarized in Table 6 Table 6 . The linear velocities in each test were about the same. While the k dB data obtained at the inner diameters of 1.6 and 4.2 mm were similar (283 and 257 L/g), the k dB became 1215 L/g at the inner diameter of 6.8 mm, when the inlet pressure increased to 401 kPa from a few hundred kPa. 1 1 Flow rate (mL/min) 1 1 Linear velocity (cm/s) 1 1 Inner diameter of adsorption column(mm) 1 1 Inlet pressure (kPa) 1 1 k dB (L/g) 1 1 565 1 1 476 1 1 1.6 1 1 108 1 1 283 1 1 3755 1 1 452 1 1 4.2 1 1 193 1 1 257 1 1 10804 1 1 496 1 1 6.8 1 1 401 1 1 1215 4.6. The CO 2 concentration effects 2 The CO 2 concentrations effects on the dynamic adsorption of Xe were examined on TJ-CMS at 77 K, under the gas flow rates of 518–576 mL/min and Xe concentration of 37×10 −6 – 40×10 −6 V / V .As shown in Fig. 6(a) Fig. 6(a) , the k dB value on TJ-CMS decreases with the CO 2 concentration and then increases. Because of the competitive adsorption, the existence of CO 2 results in the decrease of xenon adsorption capacity. The sublimation temperatures of CO 2 and Xe are 194 K and 165 K, respectively, so CO 2 can be liquefied easier than xenon, and the extent of gas liquefaction inside the adsorption column increases with the CO 2 concentration. The extent of capillary condensation is very high during the adsorption process of CO 2 , and the micropores used to adsorb Xe are occupied by CO 2 molecules so that the micropores volumes which are fit for the adsorption of xenon become smaller. The inlet pressure versus the ventilation duration at 2722×10 −6 V / V of CO 2 is plotted in Fig. 6(b) Fig. 6(b) . It shows that the inlet pressure would increase to some degree due to CO 2 liquefaction. As the CO 2 concentration increases, the flow rate reduces owing to CO 2 condensation. So, CO 2 must be removed prior to Xe adsorption. 4.7. The CO concentration effects 2 The CO concentration effects on the dynamic adsorption of Xe were assessed with TJ-CMS at 77 K, under the gas flow rate of 509–554 mL/min, Xe concentration of 35×10 −6 –37×10 −6 V / V and inlet pressure of 114–129 kPa. The data are summarized in Table 7 Table 7 . The k dB and k d decrease with increasing CO concentration. Therefore, CO must also be removed before Xe adsorption. * Flow rate, 509–554 mL/min; Xe concentration, 35×10 −6 –37×10 −6 V / V ; inlet pressure, 114–129 kPa 1 1 CO concentration (×10 −6 V/V ) 1 1 t 0.05 (min) 1 1 t 0.5 (min) 1 1 k dB (L/g) 1 1 k d (L/g) 1 1 0 1 1 157 1 1 577 1 1 385.5 1 1 1417 1 1 100 1 1 145 1 1 560 1 1 302 1 1 1168 1 1 200 1 1 130 1 1 500 1 1 274 1 1 1054 1 1 1000 1 1 128 1 1 438 1 1 273 1 1 953 4.8. The water vapor concentration effects 2 According to Ref.[ 1 1 ], water vapor tends to reduce the adsorption of Xe, and it may also form ice plugs within the adsorption column. The water vapor effects on the dynamic adsorption of Xe were therefore analyzed using the carbon molecular sieve TJ-CMS at 77 K. The k dB values are 415.8, 256.5 and 245.0 L/g at moisture contents of 1×10 −6 , 76×10 −6 and 553×10 −6 V/V , respectively, at Xe concentrations of 36.6–38.5×10 −6 V/V and flow rates of 533–575 mL/min. The k dB values over TJ-CMS decrease with increasing water vapor concentration. Moreover, at higher concentrations, the water vapor readily condenses and freezes in the adsorption column, creating a blockage. Then, water vapor must be removed before Xe adsorption. 4.9. Uncertainty estimation 2 The sources of uncertainties of the dynamic adsorption coefficients of Xe include the uncertainty of the concentration of standard gases (Xe, CO 2 , and CO; < 2%), the uncertainty of the values of flow rates with the flow rate controller ( < 2%), the uncertainty of the test values of the pressure sensor ( < 2%), and the uncertainty of the peak area of GC ( < 3%). The above entries are independent from each other, and the combined standard uncertainty of the dynamic adsorption coefficient is less than 10% ( k =2). 5. Conclusion 1 It is necessary to study the dynamic adsorption properties of a gas before designing a fixed bed adsorber. In this paper, the dynamic adsorption characteristics of Xe on various fixed bed adsorbers were studied at 77 K, including the dynamic adsorption coefficients of different adsorbents and the effects of a variety of operational conditions. The dynamic adsorption performances of carbon molecular sieves are very attractive because they allow us to minimize the adsorption volume at ultra-low temperatures. The TJ-CMS generated the highest k dB and k d values among the adsorbents assessed. And increasing the concentrations of CO 2 , CO, and water vapor reduced the adsorption of Xe because these gases tended to liquefy and even freeze in the adsorption column, resulting in pipeline blockage. 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Adsorpt Sci Technol 29 ( 1 ), 71 - 82 ( 2011 ). doi: 10.1260/0263-6174.29.1.1.71 http://doi.org/10.1260/0263-6174.29.1.1.71 A simple method for calculating the overall adsorption rate constant in the Wheeler- Jonas equation The authors are wishing to acknowledge Dr. Chong- Yang Zhou for his creative suggestions. 10.1007/s41365-017-0264-1 **This work was supported by the National Natural Science Foundation of China (No. 11405134) 2016-04-26 2016-10-29 2016-11-02 2017-07-26 2017-08-01 2021-04-11 © China Science Publishing & Media Ltd. (Science Press), Shanghai Institute of Applied Physics, the Chinese Academy of Sciences, ChineseNuclear Society and Springer Nature Singapore Pte Ltd. 2017 This is a post-peer-review, pre-copyedit version of an article published in Nuclear Science and Techniques. The final authenticated version is available online at: http://dx.doi.org/10.1007/s41365-017-0264-1 http://dx.doi.org/10.1007/s41365-017-0264-1 . 2017 Nuclear Science and Techniques 08 28