Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX
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Adsorbent Characteristic Regulation and Performance Optimization for Pressure Swing Adsorption via Temperature Elevation Peixuan Hao, Yixiang Shi,* Shuang Li, Xuancan Zhu, and Ningsheng Cai
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Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: Physical adsorbents are conventionally supposed to work at a normal temperature (298 K). However, elevating the working temperature can also restrain the adsorption of effective gas and may improve the adsorbent performance. Temperature variation is demonstrated to be a way to regulate the adsorbent performance. In this study, a coal-based activated carbon was synthesized and characterized. The CO2 and H2 adsorption capacities are inversely associated with the adsorption temperature, and a larger adsorption heat does not mean a higher adsorption capacity. Although the CO2 adsorption capacity is much higher than that of H2, sometimes the H2 adsorption capacity is more sensitive to the temperature. The adsorption selectivity for CO2 over H2 improves when the temperature rises from 298 to 353 K. A four-bed pressure swing adsorption (PSA) model was developed on the basis of an on-site pilot-scale PSA apparatus to determine the practical separation performance of adsorbents under different working conditions. The simulation results showed that, when the product gas purity is the same, the recovery rate at 353 K is approximately 2% higher than that at 298 K, indicating that the improvement in adsorption selectivity can make up for the declining adsorption capacity. More importantly, the improvement in gas recovery implies a higher energy efficiency or higher productivity. This phenomenon also exists in some other adsorbents reported before, including chemical adsorbents and composite adsorbents.
1. INTRODUCTION Pressure swing adsorption (PSA) is considered a promising technique for gas separation because of its flexibility. The working temperature and pressure range can be easily changed as required. Various adsorbents, such as activated carbon, carbon molecular sieves, zeolite, metal−organic frameworks (MOFs), and metal oxide adsorbents, have been used in PSA. With the development of new adsorbents over time, PSA has found wide applications, including syngas purification, air separation, H2 production, CO2 capture, and toxic gas removal.1−13 Adsorbent performance is sensitive to the working temperature. Chemisorption adsorbents usually work at a high temperature. At lower temperatures, the adsorption capacity may be low or the reaction between the adsorbent and gas would not occur. Li et al.5 reported a type of hydrotalcite. Its CO2 adsorption capacity reaches 1.93 mmol/g at 573 K and 1 atm. At the same working condition, the CO2 adsorption capacity of alkali nitrates, such as molten-salt-modified commercial MgO, is higher than 16 mmol/g.14 For activated carbon, MOFs, and zeolite, however, a lower temperature means higher adsorption capacity. Hence, most experiments involving physical adsorbents are conducted at 273−323 K.1,4,15−18 Prasetyo et al.19 evaluated adsorption isotherms of CH4 and CO2 on carbon molecular sieves at different temperatures. The adsorption capacity at 323 K is only approximately 45% of that at 293 K. Yuan et al.20 also characterized mesoporous carbon at 278, 298, and 318 K. The variation trends of adsorption capacity are the same regardless of the gas type. Considering that physisorption capacity declines with increasing the temperature, some adsorbents are also used in temperature swing adsorption (TSA). Mason et al.21 evaluated two representative MOFs in detail for their use in post-combustion CO2 capture via TSA. Ntiamoah et al.22 used a hot CO2-rich gas to regenerate zeolite. © XXXX American Chemical Society
With a regeneration temperature of 523 K, the purity of CO2 obtained is higher than 91% and the recovery rate is 84%, indicating that the adsorption capacity is highly dependent upon the temperature and the desorption effect is excellent at 523 K. It is conventionally accepted that adsorption capacity should be as high as possible. For example, physical adsorbents usually work at a normal temperature. Nevertheless, the effectiveness of gas separation in PSA depends upon the adsorption capacity, selectivity, adsorption/desorption kinetics of adsorbents, and PSA operation procedure used. These factors have been analyzed in our previous works.23 The characteristics of 5A zeolite reported by Merel et al.24 demonstrate that adsorption selectivity improves when the temperature varies from 298 to 323 K. The adsorption rate may also increase at higher temperatures in some situations.20,25 Thus, increasing the adsorption temperature may not mean deterioration in the purification performance, which is comprehensively decided by the factors mentioned above. In other words, elevating the temperature may instead improve the separation performance of some adsorbents in practical utilization, which challenges the conventional opinion. When it comes to chemical and composite adsorbents, the relation between adsorbent performance and temperature is more sophisticated. However, the influence of the working temperature on PSA effectiveness was rarely discussed in the previous studies. Whether temperature Special Issue: Carbon Dioxide Capture and Utilization - Closing the Carbon Cycle Received: August 14, 2018 Revised: October 16, 2018 Published: October 17, 2018 A
DOI: 10.1021/acs.energyfuels.8b02829 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels elevation can improve adsorbent performance has not been confirmed. In this study, CO2 and H2 adsorption isotherms of coalbased activated carbon were evaluated. The variation in its performance at different temperatures was analyzed. On the basis of an on-site pilot-scale PSA system, a four-bed PSA model, which uses the 4−2−1 technique (four beds in the PSA unit, two pressure equalization steps per cycle, and one bed operational at the adsorption step), was developed and validated to simulate the working of various adsorbents. The relationships between the working temperature, adsorption heat, capacity, selectivity, and separation performance were analyzed, and a new thinking to develop adsorbents was proposed.
2. CHARACTERISTICS Coal-based activated carbon was produced via the following steps: Anthracite (Taixi, Shenning Group) was ground in a tube ball mill (21 revolutions/min) for 10 min. Approximately 30 wt % tar and extra 6 wt % urea were added to the coal powder. The mixture was kneaded in a biaxial carbon mixer heated by circulating conduction oil at 80 °C for 15 min, forming a paste. The paste was extruded into granules whose length was 2−3 cm and diameter was 6 mm. The semi-finished adsorbent was carbonized in air at 673 K for 20 min and activated in steam at 1173 K for 90 min, at a heating rate of 20 °C/min. After activation, the adsorbent was cooled in the furnace. The selfsynthesized samples were named 80C. The adsorption capacities of H2 and CO2 and adsorption/ desorption rates were evaluated on a high-pressure adsorption apparatus, 3H-2000PH (Beishide Instrument Co., Ltd., China). Specific surface area analyzer ASAP-2020 (Micromeritics, Norcross, GA, U.S.A.) was used to characterize the surface area and pore diameter distribution. Further, the structure of the sample was examined via X-ray diffraction (D8 Advance, Bruker). The surface area of 80C was calculated to be 1164 m2/g by the Brunauer−Emmett−Teller (BET) theory. The pore volume was 0.603 mL/g, and the micropore volume was 0.500 mL/g. The activation time can be prolonged up to 120 min, and the surface area can reach 1300 m2/g. However, it cannot improve adsorption capacity distinctly and will cause the deterioration of strength and productivity. The pore size distribution, which was determined by the density functional theory (DFT), is shown in Figure 1. The sample shows a
Figure 2. XRD pattern of the 80C adsorbent. carbons. The broad peaks at 23.5° and 45° correspond to reflections from 002 and 100 planes of amorphous carbon, respectively. The H2 and CO2 isotherms on 80C are shown in Figure 3. The dots represent the experimental data, and the lines represent the
Figure 3. CO2 and H2 adsorption isotherms at different temperatures. fitting results with the Langmuir model. The CO2 and H2 adsorption capacities are sensitive to the temperature. Nevertheless, the decline rate of H2 and CO2 adsorption is not exactly the same, and the rate is also related to the temperature range. From 298 to 353 K, the CO2 adsorption capacity decreases by only approximately 15%, while it is 35% for H2. Between 353 and 413 K, the variation trend becomes different. The CO2 adsorption capacity declines faster than that for H2. The isotherms demonstrate that the selection of CO2 over H2, which is defined as follows, is a function of the adsorption pressure and temperature: qCO 2 selectivity = (at the same partial pressure) qH (1) 2 Because 80C is a type of physisorption adsorbent, CO2 adsorption on its surface is close to ideal adsorption, especially when the pressure is not high. Its isotherm fits the Langmuir model.
Figure 1. Pore size distributions calculated from N2 adsorption isotherms.
qe − CO =
narrow pore size distribution centered at approximately 0.65 nm, which is twice the dynamic diameter of a CO2 molecule (0.33 nm). Figure 2 is the X-ray diffraction (XRD) pattern of the adsorbent. The activated carbon sample is amorphous, and the degree of graphitization is low. Therefore, the XRD pattern does not show any sharp peak in the range of 5−90°, which is the case in most activated
2
qmbPCO2 1 + bPCO2
(2)
In this pressure range, the H2 isotherm can be approximately described using a linear function. qe − H = kPH2 2
B
(3) DOI: 10.1021/acs.energyfuels.8b02829 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. Values of Parameters in the Adsorption Isotherm Models temperature (K)
qm (mmol g−1)
b (MPa−1)
k (mmol g−1 MPa−1)
r2
298 323 353 413
2.64 2.64 2.64 2.64
6.05 4.85 3.64 1.04
0.122 0.097 0.072 0.063
0.9944 0.9962 0.9980 0.9975
The values of the parameters are listed in Table 1, where r is the coefficient of determination. The adsorption heat ΔH (>0) can be calculated from the isotherms using the Clausius−Clapeyron equation.
Table 2. CO2 and H2 Adsorption Heats Calculated from Adsorption Isotherms temperature range (K)
CO2 adsorption heat (kJ/mol)
H2 adsorption heat (kJ/mol)
298−323 323−353 353−413
7.08 9.07 25.25
7.34 9.42 2.70
1 zy ji P zy ΔH jij 1 lnjjj 2 zzz = jj − zzz j P1 z z j T1 R T 2{ k { k
(5)
A higher adsorption heat means that the adsorption capacity decreases faster when the temperature rises. As shown in Table 2, the temperature, adsorption heat, and adsorption capacity show uncertain relationships. For the same adsorbent, a higher adsorption heat does not mean a higher adsorption capacity. When the temperature is lower than 353 K, the average H2 adsorption heat is higher than that of CO2. However, when the temperature increases to 413 K, the H2 adsorption heat decreases to only 2.70 kJ/mol, while the CO2 adsorption heat reaches 25.25 kJ/mol. Thus, the temperature no longer significantly influenced the H2 adsorption capacity, whereas the CO2 adsorption capacity sharply decreases with increasing the temperature. The adsorption/desorption kinetics are shown in Figure 4. With the rising temperature, the adsorption and desorption rates were always high. The adsorption/desorption process lasted only for several seconds, and the influence of the temperature was not obvious. In the following simulation, the adsorption/desorption process is described using a linear driven force model. The working performance of the adsorbent is related to its adsorption capacity, selectivity, and adsorption/desorption rate. As discussed above, when the adsorption temperature increases, the adsorption capacity monotonically decreases. The selectivity shows a complex relationship with the temperature. Hence, the comprehensive performance is also uncertain. At 0.8 MPa, for example, when the temperature increases from 298 to 353 K, the adsorption capacity decreases but the selectivity improves from 23 to 30. Further, the adsorption/desorption rate is always high. In practical applications, the improvement in selectivity may make up for the lower adsorption capacity. This has been verified in the following simulation. It indicates that controlling the working temperature is an instrument to regulate the adsorbent performance.
3. SIMULATION AND DISCUSSION A test-scale (6 Nm3/h) PSA unit was developed in the ammonia plant of Quanji Energy Co., Ltd., Shanxi province, China. The apparatus contained four adsorption beds packed with 207C. The syngas (489 K) originating from the coal gasifier reacted in the water−gas shift furnaces and then served as the feed gas for the PSA system. The cyclic operation of the PSA apparatus and working conditions are given in Tables 3 and 4, respectively. N2 was used as the purge gas, and at the rinse step, N2 was fed to the apparatus to drive the gas
Figure 4. Adsorption and desorption rates of (a) CO2 and (b) H2 at different temperatures. selectivity =
qmb k(1 + bP)
(4)
Table 3. 4−2−1 PSA Cyclic Operationa t0/2 bed bed bed bed
1 2 3 4
t0/2
2t0 AD ER1
I BD R
t0/2
t0/2
R Pu
ED1
3t0 FP ER2 ED2
2t0 ED1 AD ER1
I BD
3t0
t0/2
ED2 R Pu
FP ER2
t0/2 BD
I
ED1 AD ER1
2t0
3t0
t0/2
Pu
ER2 ED2
I
2t0
3t0
Pu
FP ER2 ED2
ER1 BD
R FP
t0/2
ED1 AD
a
AD, adsorption; ED1, pressure equalization drop step 1; ED2, pressure equalization drop step 2; BD, blow down; Pu, purge; ER1, pressure equalization rise step 1; ER2, pressure equalization rise step 2; FP, final pressurization; R, rinse; and I, idle. C
DOI: 10.1021/acs.energyfuels.8b02829 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 4. PSA Experimental Working Conditions temperature (°C) adsorption pressure (MPa) t0 (s) feed gas flow rate (Nm3/h) component of feed gas
Table 5. Boundary Conditions in the Model
100−200 1.3−2.4 20−27 3.0−7.0 55% H2 and 45% CO2
process
( adsorption
remaining in the adsorption bed, whose main component was H2, to the product tank. The rinse step is useful to improve the recovery rate.26−30 Each step lasted for t0 (s), and there were 24 steps in a cycle. A model based on the pilot-scale PSA unit for the CO2/H2 separation system for the multi-bed PSA process was developed using a commercial simulation platform gPROMS. Some inconsequential factors were excluded by introducing the following assumptions: (1) The bed temperature is uniform and fixed. (2) Adsorption beds are one-dimensional. Variations along the radial direction can be neglected in comparison to those along the axial direction. (3) The gas is a mixture of H2, N2, and CO2. It is ideal and obeys the ideal gas equation. Mass balance and momentum balance were considered in this model. Mass balance for N2, CO2, and H2 is as follows: ∂Ci = ∂t
∂ 2Xi
− ρp
1 − eb ∂qi eb ∂t
∂Ci |z = L = 0 ∂z pressure equalization drop
v|z = 0 = 0 v|z = L = vout ∂Ci |z = L = 0 ∂z blow down
v|z = L = 0
(∑ Ci)D ∂zi |z=L = −vin(Ci|z=L − Cin−i)
2
= 0.04357T1.5
purge
v|z = L = vin
∂X
(∑ Ci)D ∂zi |z=L = −vin(Ci|z=L − Cin−i)
2
∑ ρi 1 − eb ∂Pi (1 − eb) = 150μv + 1.75 v |v | 2 3 ∂z d p eb 3 d p eb
pressure equalization rise
(8)
purity =
v|z = L = vin
∂X
(∑ Ci)D ∂zi |z=L = −vin(Ci|z=L − Cin−i)
(9) final pressurization
2 ∑ Pin − i
RT H2 purity and recovery are defined as follows: mass flow rate of outlet H 2 mass flow rate of outlet gas
mass flow rate of outlet H 2 recovery = mass flow rate of inlet H 2
∂Ci |z = 0 = 0 ∂z v|z = 0 = 0
The relationships between Vin, Vout, Fin, and Fout are given by eqs 9 and 10. ∑ Pin − i RT
∂Ci |z = 0 = 0 ∂z ∂v |z = 0 = 0 ∂z
(7)
The pressure variations inside the bed can be captured using Ergun’s equation.
Fout = 0.0224voutπR bed
∂Ci |z = 0 = 0 ∂z ∂v |z = 0 = 0 ∂z
(6)
−0.5
Fin = 0.0224vinπR bed 2
∂Ci |z = 0 = 0 ∂z
∂X
(∑ Pi)(∑ 1/Mi) (∑ VD1/3−i)
−∑
∂Ci |z = L = 0 ∂z
v|z = 0 = vin
where the diffusion coefficient can be calculated as follows: D
)
∂v |z = L = 0 ∂z
∂(∑ Ci) ∂Xi ∂(vCi) +D − ∂z ∂z ∂z
(∑ Ci)D ∂z 2
equation
∂X ∑ Ci D i |z= 0 = vin(Ci|z= 0 − Cin− i) ∂z
(10)
∂Ci |z = 0 = 0 ∂z v|z = 0 = 0 v|z = L = vin ∂X
(∑ Ci)D ∂zi |z=0 = vin(Ci|z=0 − Cin−i)
(11) rinse
∂Ci |z = L = 0 ∂z
∂v |z = L = 0 ∂z
(12)
Boundary conditions corresponding to each step of the PSA cycle are given in Table 5. In the simulation model, the kinetic parameters, CO2 and H2 adsorption capacities, adsorption pressure, feed gas flow rate, and cycle time were the same as those in the practical PSA apparatus. More details about the model and values of parameters are provided elsewhere.23 The main evaluation parameter for analyzing the performance of a PSA is the purity and recovery rate of the product
v|z = 0 = vin ∂Ci |z = L = 0 ∂z idle
∂Ci |z = 0 = 0 ∂z v|z = 0 = 0 v|z = L = 0
D
DOI: 10.1021/acs.energyfuels.8b02829 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 6. Comparison of Experimental and Simulation Data experiment simulation experiment simulation
outlet H2 purity (%)
H2 recovery rate (%)
cycle time (s)
flow rate (Nm3/s)
adsorption pressure (MPa)
temperature (K)
>99.9 >99.9 99.8 99.9
41 43.4 66−73 72.0
336 336 648 648
6.9 6.9 4.7 4.7
2.2 2.2 1.4 1.4
393 393 443 443
adsorption/desorption process is completed in 30 s in this 4−2−1 technique process, the further improvement in the kinetic performance will not effectively improve the working performance. On the other hand, if the adsorption/desorption rate is low, the kinetic performance will be much more important.23 For physical adsorption, there is a negative correlation between adsorption capacity and temperature. The kinetic performance is typically superior. With increasing the working temperature, the variation trend of selectivity influences the comprehensive working performance. If the improvement of the selectivity is large enough to compensate for the negative effect, temperature elevation will be a method to optimize the separation performance. Chemical adsorption offers excellent selectivity. Different from physical adsorption, when the temperature is elevated, the chemical adsorption capacity does not show a fixed change rule. On the one hand, most adsorption processes are exothermic reactions, and the chemical equilibrium constant usually decreases at an elevated temperature, which leads to the deterioration of adsorption capacity. On the other hand, the chemisorption capacity may rise in some situations for the sake that the higher temperature endows the molecules more activation energy, by which the adsorption process can be conducted more favorably. Moreover, chemisorption leads to large adsorption heat; therefore, the difference in kinetic performance is more distinct when the temperature varies. All of these factors should be taken into consideration when we evaluated the adsorbent, and the elevated temperature is more likely to be advantageous to chemical adsorbents. Some adsorption processes are the combination of physical and chemical adsorption, and chemical adsorption can be artificially added to physical adsorption, such as nitrogendoped activated carbon and an activated-carbon-supported chemical adsorbent.38−42 At elevated temperatures, physical adsorption of effective gases is restrained and selectivity improves.39,41
Figure 5. Product gas purity and recovery rate at different temperatures.
gas. As observed in Table 6, the simulation results match with the experimental results well. To a significant extent, this model is reliable for estimating the performance of adsorbents. The practical working performance of 80C at different temperatures in PSA was analyzed using the model. The results are presented in Figure 5. For a fixed cycle procedure and working condition, when the cycle time increases, the purity of the production gas drops and recovery improves. At different temperatures, when the purity is the same, the recovery rate at 353 K is much higher than that at 298 and 413 K. The difference can reach over 2%. In an integrated gasification combined cycle (IGCC) system, the recovery rate of fuel dominates the system power generation efficiency. If the recovery increases by 2%, the efficiency can rise by approximately 1%.28 When it comes to other applications, such as air separation and synthesis of ammonia, the higher recovery rate also means lower energy consumption in the production process. It demonstrates that, for adsorbent 80C, a lower working temperature does not mean a better separation performance. This phenomenon also occurs in some other adsorbents.19,21,24,31−37 For these adsorbents, elevating the working temperature is a method to improve the separation performance in PSA. However, for the commercial activated carbon (207C, Calgon Co., Ltd.), when the adsorption temperature rises from 298 to 473 K, the adsorption capacity and selectivity deteriorate simultaneously. The adsorbents mentioned in this study are only some examples. The general laws and correlations among the adsorption capacity, selectivity, kinetic performance (presented as the adsorption/desorption rate), temperature, and working performance should be discussed. A previous study and the simulation results in this study have shown that adsorption capacity and selectivity are of similar importance for PSA.23 Besides, kinetic performance is also a key factor. If the adsorption/desorption rate is high, which means that the
4. CONCLUSION A coal-based activated carbon adsorbent was synthesized and characterized. The pore distribution, CO2 and H2 adsorption capacities, adsorption heats, and adsorption/desorption rates were analyzed. Appropriate models were used to describe the adsorption and desorption processes. Although the CO2 adsorption capacity is far higher than that for H2, the adsorption heat for H2 is similar to that for CO2 between 298 and 353 K, and when the temperature increased, the adsorption selectivity, selecting CO2 over H2, improved. According to simulation results, the improvement in selectivity makes up for the declining adsorption capacity. The separation performance at 353 K is much higher than that at 298 and 413 K. Optimizing the temperature is demonstrated to be a method to improve the PSA performance. However, it should also be noted that this improvement only occurs in some adsorbents. For other adsorbents, such as 207C, elevating the temperature will have the opposite effect. E
DOI: 10.1021/acs.energyfuels.8b02829 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels
(2) Khunpolgrang, J.; Yosantea, S.; Kongnoo, A.; Phalakornkule, C. Alternative PSA process cycle with combined vacuum regeneration and nitrogen purging for CH4/CO2 separation. Fuel 2015, 140, 171. (3) Sircar, S.; Golden, T. C. Purification of hydrogen by pressure swing adsorption. Sep. Sci. Technol. 2000, 35, 667. (4) Delgado, J. A.; Á gueda, V. I.; Uguina, M. A.; Sotelo, J. L.; Brea, P.; Grande, C. A. Adsorption and diffusion of H2, CO, CH4, and CO2 in BPL activated carbon and 13X zeolite: Evaluation of performance in pressure swing adsorption hydrogen purification by simulation. Ind. Eng. Chem. Res. 2014, 53, 15414. (5) Li, S.; Shi, Y.; Yang, Y.; Zheng, Y.; Cai, N. High-performance CO2 adsorbent from interlayer potassium-promoted stearate-pillared hydrotalcite precursors. Energy Fuels 2013, 27, 5352. (6) Belmabkhout, Y.; Sayari, A. Isothermal versus non-isothermal adsorption− desorption cycling of triamine-grafted pore-expanded MCM-41 mesoporous silica for CO2 capture from flue gas. Energy Fuels 2010, 24, 5273. (7) Yin, J.; Qin, C.; An, H.; Liu, W.; Feng, B. High-temperature pressure swing adsorption process for CO2 separation. Energy Fuels 2012, 26, 169. (8) Tomadakis, M. M.; Heck, H. H.; Jubran, M. E.; Al-Harthi, K. Pressure-swing adsorption separation of H2S from CO2 with molecular sieves 4A, 5A, and 13X. Sep. Sci. Technol. 2011, 46, 428. (9) Hao, P.; Shi, Y.; Li, S.; Liang, S. Oxygen sorption/desorption kinetics of SrCo0. 8Fe0. 2O3-δ perovskite adsorbent for high temperature air separation. Adsorption 2018, 24, 65. (10) Wu, H. C.; Lin, Y. S. Effects of Oxygen Vacancy Order− Disorder Phase Transition on Air Separation by Perovskite Sorbents. Ind. Eng. Chem. Res. 2017, 56, 6057. (11) Ribeiro, A. M.; Grande, C. A.; Lopes, F. V. S.; Loureiro, J. M.; Rodrigues, A. E. Four beds pressure swing adsorption for hydrogen purification: Case of humid feed and activated carbon beds. AIChE J. 2009, 55, 2292. (12) Ribeiro, A. M.; Grande, C. A.; Lopes, F. V. S.; Loureiro, J. M.; Rodrigues, A. E. A parametric study of layered bed PSA for hydrogen purification. Chem. Eng. Sci. 2008, 63, 5258. (13) Ribeiro, A. M.; Santos, J. C.; Rodrigues, A. E. PSA design for stoichiometric adjustment of bio-syngas for methanol production and co-capture of carbon dioxide. Chem. Eng. J. 2010, 163, 355. (14) Qiao, Y.; Wang, J.; Zhang, Y.; Gao, W.; Harada, T.; Huang, L.; Hatton, T. A.; Wang, Q. Alkali nitrates molten salt modified commercial MgO for intermediate-temperature CO2 capture: Optimization of the Li/Na/K ratio. Ind. Eng. Chem. Res. 2017, 56, 1509. (15) Arami-Niya, A.; Rufford, T. E.; Zhu, Z. Activated carbon monoliths with hierarchical pore structure from tar pitch and coal powder for the adsorption of CO2, CH4 and N2. Carbon 2016, 103, 115. (16) Jalilov, A. S.; Li, Y.; Tian, J.; Tour, J. M. Ultra-High Surface Area Activated Porous Asphalt for CO2 Capture through Competitive Adsorption at High Pressures. Adv. Energy. Mater. 2017, 7, 1600693. (17) Xing, W.; Liu, C.; Zhou, Z.; Zhang, L.; Zhou, J.; Zhuo, S.; Yan, Z.; Gao, H.; Wang, G.; Qiao, S. Superior CO2 uptake of N-doped activated carbon through hydrogen-bonding interaction. Energy Environ. Sci. 2012, 5, 7323. (18) Zhang, Z.; Wang, B.; Zhu, C.; Gao, P.; Tang, Z.; Sun, N.; Wei, W.; Sun, Y. Facile one-pot synthesis of mesoporous carbon and Ndoped carbon for CO2 capture by a novel melting-assisted solvent-free method. J. Mater. Chem. A 2015, 3, 23990. (19) Prasetyo, I.; Rochmadi, R.; Wahyono, E.; Ariyanto, T. Controlling Synthesis of Polymer-Derived Carbon Molecular Sieve and Its Performance for CO2/CH4 Separation. Eng. J. 2017, 21, 83. (20) Yuan, B.; Wu, X.; Chen, Y.; Huang, J.; Luo, H.; Deng, S. Adsorption of CO2, CH4, and N2 on ordered mesoporous carbon: Approach for greenhouse gases capture and biogas upgrading. Environ. Sci. Technol. 2013, 47, 5474. (21) Mason, J. A.; Sumida, K.; Herm, Z. R.; Krishna, R.; Long, J. R. Evaluating metal−organic frameworks for post-combustion carbon
The existence of chemical adsorption may enhance the effect of temperature variation for the sake that the chemical adsorption capacity does not show a fixed relationship with the temperature. The elevated temperature may not lead to an obvious deterioration of the adsorption capacity but can improve the selectivity and kinetic performance dramatically. Introducing chemical adsorption in physical adsorption and working at higher temperatures are methods to develop new adsorbents.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yixiang Shi: 0000-0001-8720-9699 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was financed by the Shanxi Province Science and Technology Major Projects (MH2015-06), the National Key R&D Program of China (2017YFB0601900), and the China Postdoctoral Science Foundation (2017M610890).
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NOMENCLATURE t = time (s) Mi = molecular mass of i (g mol−1), where i = H2, CO2, or N2 R = ideal constant (J mol−1 K−1) T = temperature of the adsorption bed (K) Pi = partial pressure of i (Pa), where i = H2, CO2, or N2 Ci = molar concentration of i (mol m−3), where i = H2, CO2, or N2 Xi = mole fraction of i, where i = H2, CO2, or N2 ρi = density of i (kg m−3), where i = H2, CO2, or N2 ρp = adsorbent density (kg m−3) eb = void fraction of the adsorption bed (MPa−1) dp = adsorbent diameter in the adsorption bed (m) z = height in the bed (m) v = flow speed of gas in the bed (m s−1) μ = gas dynamic viscosity (Pa s) D = diffusion coefficient (m2 s−1) VD−i = liquid state molar volume of i (cm3 g−1 mol−1), where i = H2, CO2, and N2 Rbed = adsorption bed radius (m) qe−i = adsorption capacity of i at a given pressure (mmol g−1), where i = H2 or CO2 qm = saturated adsorption capacity of CO2 b = Langmuir model adsorption parameter (MPa−1) k = Linear adsorption isotherm model parameter (mmol g−1 MPa−1) Fin = inlet flow rate (Nm3 h−1) Fout = outlet flow rate (Nm3 h−1) vin = inlet flow velocity (m s−1) vout = outlet flow velocity (m s−1) L = bed height (m)
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REFERENCES
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DOI: 10.1021/acs.energyfuels.8b02829 Energy Fuels XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.energyfuels.8b02829 Energy Fuels XXXX, XXX, XXX−XXX
