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An experimental adsorbent screening study for CO2 removal from N2
Microporous and Mesoporous Materials 76 (2004) 71–79 www.elsevier.com/locate/micromeso
An experimental adsorbent screening study for CO2 removal from N2 Peter J.E. Harlick, F. Handan Tezel
*
Department of Chemical Engineering, University of Ottawa, 161 Louis-Pasteur, Ottawa, Ont., Canada K1N 6N5 Received 9 February 2004; received in revised form 27 July 2004; accepted 27 July 2004 Available online 6 October 2004
Abstract The selection of a suitable adsorbent for CO2 removal from flue gas (mixture of CO2 and N2) has been carried out. The limiting heats of adsorption, and HenryÕs Law constants for CO2 with a N2 carrier, were determined for a group of 13 zeolite based adsorbents, including 5A, 13X, NaY, NaY-10, H-Y-5, H-Y-30, H-Y-80, HiSiv 1000, H-ZSM-5-30, H-ZSM-5-50, H-ZSM-5-80, H-ZSM5-280, and HiSiv 3000. The CO2 pure component adsorption isotherms and expected working capacity curves for pressure swing adsorption (PSA) application were determined for a selected promising subgroup of these adsorbents. The results show that the most promising adsorbent characteristics are a near linear CO2 isotherm and a low SiO2/Al2O3 ratio with cations in the zeolite structure which exhibit strong electrostatic interactions with carbon dioxide. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Carbon dioxide removal from nitrogen; Flue gas; Adsorption of nitrogen; Adsorption isotherms; Zeolites; HenryÕs Law constant; Heat of adsorption; Working capacity
1. Introduction With an increasing worldwide population and an increased per capita demand for energy, concern is now being raised over the CO2 emissions resulting from the gaseous by-products of combustion processes. In order to meet the present and future constraints placed on the allowable emissions of CO2, the solution lies with reduction and recovery. Pressure swing adsorption (PSA) is a very promising separation and recovery process for this type of application. In the design of an adsorption based separation process, the choice of the adsorbent is the most crucial design consideration [20,21,25]. In this study, several adsorbent–adsorbate characteristics have been considered.
*
Corresponding author. Tel.: +1 613 562 5800x6099; fax: +1 613 562 5172. E-mail address: [email protected] (F.H. Tezel). 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.07.035
These include the initial loading effects (HenryÕs Law constant, initial heat of adsorption), pure component adsorption capacity and isotherm shape, and the expected working capacities for PSA applications. For this study, an extensive literature review and experimental adsorbent screening with CO2 and N2 gases have been completed. Thirteen adsorbents, described in Table 1, have been examined and the most promising ones were selected for further study. Pure component CO2 adsorption isotherms were determined at 22 °C and pressures ranging from 1 to 1900 Torr. The concentration pulse method (CPM) was used for determining the HenryÕs Law constants and the associated heat of adsorption parameters [26,24,18,1,2,11]. In this method, a column (dimensions are given in Table 2) was packed with the adsorbent to be studied. The adsorbent was regenerated under helium gas purge at 200 °C for 12 h to get rid of all the moisture and other gases that may be adsorbed by the adsorbent. The same regeneration procedure was used for all the adsorbents studied. After the regeneration, the column temperature
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P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79
Nomenclature b DH EWC K0 Kp P
Toth isotherm parameter, (1/atm)t limiting heat of adsorption, kJ/mol expected working capacity, mol/kg pre-exponential factor, mol/kg/atm HenryÕs Law constant, mol/kg/atm pressure, Torr, atm
q qm R T t
was reduced to the experiment temperature. The helium gas was used as the carrier gas and a very small amount of adsorbate was injected into the carrier gas as an impulse. The response of the column was monitored by measuring the concentration of the adsorbate at the end of the column. From the first moment of the response peak, HenryÕs Law constant, Kp, can be determined. More details of the experimental set-up can be found elsewhere [11,12,14]. The pure component isotherms were determined with a modified constant volume apparatus (Micrometetics Accusorb 2400). The quickest and easiest method for the initial loading study is by using the concentration pulse method for determining HenryÕs Law constants (Kp, mol/kg/atm). The Kp values are proportional to the initial slope of the adsorption isotherm as a large Kp value corresponds to a steep initial slope of the adsorption isotherm. When Kp values are determined at different temperatures (as P[CO2] ! 0), a VanÕt Hoff plot can be constructed and the limiting heat of adsorption is defined by Eq. (1). DH K p ¼ K 0 exp ð1Þ RT
amount adsorbed, mol/kg Limiting amount adsorbed, mol/kg gas constant, kJ/mol/K temperature, K Toth isotherm parameter, Dimensionless
form. The experimental conditions used with the concentration pulse method are given in Table 2. When the Kp and DH values are considered at the same time, an initial adsorbent screening can be performed. This is accomplished by removing all adsorbents that exhibit very high heats of adsorption, as these tend to require higher energy costs for the regeneration cycle. Since the adsorption process is exothermic, a high DH value will produce large quantities of heat within the adsorbent bed during an adsorption process cycle. This will cause an increase in the local column temperature. This increase in the column temperature will have an adverse effect on the adsorption capacities of the components. The result is a loss of capacity, and thus decreased process throughput. Similarly, in desorption a large temperature drop may also occur. This drop may cause certain components to freeze out on the adsorbent (such as moisture).
2. Results and discussion 2.1. Initial loading effects
The heat of adsorption is a function of the strength of adsorption. This quantity is defined by the slope found by regression of the experimental data in dimensional
There are several sources of initial loading data available for CO2 and N2 with several adsorbents (i.e.,
Table 1 Description of adsorbents studied Adsorbent type
Supplier
Pellet form
SiO2/Al2O3 ratio
Cation form
Pore size (nm)
Binder
Particle size
Type ZSM-5-30 ZSM-5-50 ZSM-5-80 ZSM-5-280 HSiv 3000 (Type ZSM5) H-Y-5.1 H-Y-30 H-Y-80 HSiv 1000 (Type Y) NaY-10 NaY 13X 5A
+
% wt
CV
CPM
Zeolyst Zeolyst Zeolyst Zeolyst UOP
Powder Powder Powder Powder 00 1/16
30 50 80 280 >1000
H H+ H+ H+ Na+
0.6 0.6 0.6 0.6 0.6
Kaolin Kaolin Kaolin Kaolin Provided as
15 15 15 15 pellet
20–60 20–60 20–60 20–60 00 1/16
20–60 20–60 20–60 20–60 20–60
Zeolyst Zeolyst Zeolyst UOP
Powder Powder Powder 00 1/16
5.1 30 80 >20
H+ H+ H+ Na+
0.8 0.8 0.8 0.8
Kaolin Kaolin Kaolin Provided as
15 15 15 pellet
20–60 20–60 20–60 00 1/16
20–60 20–60 20–60 20–60
UOP UOP UOP Linde
1/16 00 1/16 00 1/16 00 1/8
>10 5.1 2.2 3–5
Na+ Na+ Na+ Na+
0.8 0.8 0.8 0.5
Provided Provided Provided Provided
pellet pellet pellet pellet
1/16 00 1/16 00 1/16 00 1/8
00
CV represents the constant volume method, and CPM represents the concentration pulse method.
as as as as
00
20–60 20–60 20–60 20–60
P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79 Table 2 Experimental and column specifications used with the CPM Carrier gas Particle size Average particle diameter Bed porosity Column length Column inner diameter Total flowrate Total pressure Regeneration temperature Regeneration pressure Regeneration time
UHP nitrogen 20–60 550 0.382 24.75 0.45 30.0 1.0 200 1.0 12
Mesh lm – cm cm cc/min at 40 °C Atm °C Atm Hours
[27,9,7,8]). However, the availability of HenryÕs Law constants at the required design conditions is limited. A comparison of the available literature data was difficult since the studies were performed under varied thermodynamic conditions; temperature and pressure. Selected results of a literature survey based on the heat of adsorption of the type of adsorbents used in this study are given in Table 3. The experimentally determined HenryÕs Law constants are given in Table 4, and plotted in Figs. 1 and 2. The change in internal energy and heat of adsorption (CO2 in N2) for each of the adsorbents are given in Table 5. Some adsorbents did not exhibit sufficient CO2 adsorption in order to be accurately characterized across all the temperatures implied on the concentration pulse method used in this study. The heat of adsorption data obtained with the chosen adsorbents agree with the trends given by the literature data. For example, from the experimental data in Table 5, the order of magnitude of the heat of adsorption was as follows: 5A > 13X > NaY > ZSM 5 From the literature data, the same trend was observed (Table 3). However, the magnitude of the heat of adsorption shown in Table 3 varies widely for some adsorbents from different research groups, therefore, no inference can be drawn against the presently determined heat of adsorption values. Many insights can be realized with the data obtained. First, the magnitude of the Kp values at specific temperatures are known. These values relate to the shape of the adsorption isotherm in the HenryÕs Law region. In general, a large Kp value results with a steep, rectangular isotherm. For CO2, these types of isotherms are not favourable for industrial adsorption systems where the feed mole fraction of more adsorbed component is high (for example, >5%). The data also showed that a high heat of adsorption does not always translate from high Kp values. Within the zeolite-Y family of adsorbents, HiSiv-1000 exhibited this behaviour for CO2. Although the Kp values at the low range of temperatures studied is not very high, the slope of the VanÕt Hoff plot
73
Table 3 Literature review of CO2 and N2 heats of adsorption with various adsorbents Adsorbent
Heat of adsorption CO2 (kcal/mol)
Silicalite Silicalite Silicalite Silicalite H-ZSM-5-30 H-ZSM-5-30 H-ZSM-5-30 Na-ZSM-5-30 Na-ZSM-5-30 Na-ZSM-5-30 Cs-ZSM-5-30 Rb-ZSM-5-30 K-ZSM-5-30 Li-ZSM-5-30 Na-ZSM-5-47 H-ZSM-5-54 H-ZSM-8-30 Na-ZSM-8-30 Na-ZSM-20-30 H-ZSM-20-30 Li-X X Na-X Na-X Na-X Na-X Na-X Na-X K-X Mg-X Ca-X Ba-X Ag-X 13X Na-Y H-Y Na-Y Ce(46)Na-Y Ce(72)Na-Y H-Y K-L HK-L Na-M H-M
21.7 24.1 23.4 27.2 26.1 28.8 38.0 46.3 42.0 50.0 33.0 34.9 36.0 58.9 46.3 28.8 27.0 35.9 27.0 20.0 35.0
15.1 17.6
27.2 25.0 18.4
49.1 48.1 47.3 47.5 50.0
36.0 33.0 25.0 36.5 35.7 40.8 27.0 24.3 30.1 38.2 44.5
Source references
N2 (kcal/mol)
18.4 34.7 31.4 20.1 31.0 25.0
[5] [9] [27] [7] [5] [27] [8] [27] [5] [8] [27] [27] [27] [27] [8] [8] [5] [5] [22] [22] [19] [5] [10] [10] [8] [8] [8] [8] [8] [10] [10] [10] [10] [10] [6] [22] [22] [4] [4] [4] [4] [4] [4] [4]
for this adsorbent is high, (Fig. 1). This is due to the low Kp values at high temperatures; this behaviour is promising. High Kp values translate into a smaller mass of adsorbent for a set cycle time, since the capacity is much higher. However, adsorbents with relatively high capacities at 1–2 atm pressure usually provide rectangular shaped isotherms and they often result with energy intensive isothermal regeneration in order to realize a high cyclic capacity for PSA applications. The smallest pore size adsorbent that was studied was 5A. This adsorbent contained the Ca+2 cation, had one of the lowest SiO2/Al2O3 ratio among the adsorbents
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Table 4 HenryÕs Law constants determined by the CPM for CO2 with a N2 carrier Temperature (°C)
0 10 15 20 25 30 40 60 80 100 120 125 130 140 150 170 175 200
HenryÕs Law constant (mol/kg/atm) ZSM-5
HiSiv
HY
NaY
30
50
80
280
3000
1000
5
30
80
5
10
– – – 27.7 – – 11.0 5.06 2.44 1.32
– – – – – 9.42 6.23 2.86 1.54 0.784
– – – – – 7.47 4.98 2.31 1.19 0.666
– – – – – 6.16 4.05 1.99 0.986 0.560
– – – – – 12.1 7.89 3.55 1.77 1.03
– – – – – – –
– – – – – – –
– – – – – – –
– – – – – – –
– – – – – – –
– – – – – 25.4 16.6 7.80 3.67 2.00 0.590 – – – – – – –
– 11.7 – – 5.60 – 3.02 1.22 0.688 0.409 – – – – – – – –
2.42 – 1.30 – – 0.781 0.556 0.324 0.199 0.116 – – – – – – – –
1.58 1.11 – 0.760 – 0.570 0.414 0.238 0.158 0.0933 – – – – – – – –
– – – – – – 37.9 17.4 8.21 4.08 – – 1.70 – 1.01 – – 0.367
– – – – – 41.9 26.9 12.18 5.45 2.97 – –
NaY
1000
5A
– – – – – – – – 18.2 7.99 4.05 – – 2.03 – 0.92 – 0.465
– – – – – – – – – 12.8 – 5.14 – – 2.04 – 1.01 0.529
1000
NaY-10
13X
– – – – –
13X
ZSM-5-30
ZSM-5-30 HiSiv-3000
HiSiv-3000 ZSM-5-50 ZSM-5-80 ZSM-5-280
} 100
ZSM-5-50
K p (Dimensionless)
K p (Dimensionless)
5A
ZSM-5-80 ZSM-5-280
100
HY-30
HiSiv-1000 HY-5.1
HY-80
10
2.0
2.5
3.0
3.5
4.0
1000/T ( K-1 )
2.6
2.8
3.0
3.2
3.4
3.6
1000/T ( K-1 )
Fig. 1. VanÕt Hoff plots of the HenryÕs Law constants for CO2 with a N2 carrier for various zeolites.
Fig. 2. VanÕt Hoff plots of the HenryÕs Law constants for CO2 with a N2 carrier for various pentasil based adsorbents.
used in this study. Therefore, this material exhibited the strongest net cation-quadrupole interaction with about 8 sites per unit cell [23], and the lowest CO2 mobility within the pores. As shown in Fig. 1 and Table 4, the HenryÕs Law constants were the highest. The 13X and NaY adsorbents provided high heats of adsorption for CO2. These high heats of adsorption are largely due to the quadrupole interaction of CO2 with the cationic nature of the adsorbent surface [17]. Both
of these adsorbents have low SiO2/Al2O3 ratios. Lower ratios require more cations to balance the charge distribution within the structure and result in high degree of heterogeneity. The 13X structure is more heterogeneous than NaY and thus exhibits a higher heat of adsorption. In both adsorbents, the Na+ cation was used to balance the charge distribution. This cation has a very strong electrostatic field with which to interact with the quadrupole moment of both adsorbates. Therefore, a method
P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79 Table 5 Experimental heat of adsorption and internal energy data for CO2 in N2 with various adsorbents DU (kJ/mol)
DH (kJ/mol)
H-Y-5.1 H-Y-30 H-Y-80 HiSiv-1000 NaY-10 NaY 13X 5A HiSiv-3000 ZMS-5-30 H-ZSM-5-50 H-ZSM-5-80 H-ZSM-5-280
10–100 0–100 0–100 30–120 30–100 40–200 80–200 100–200 30–100 20–100 30–100 30–100 30–100
30.5 21.0 22.7 32.5 33.1 33.0 39.2 43.6 30.6 31.9 30.3 29.8 29.5
33.1 23.7 25.3 34.9 35.9 36.1 42.5 47.1 33.4 34.6 33.1 32.6 32.3
of interaction control may be used by incorporating a cation with a smaller electrostatic interaction potential, such as H+. The strong adsorbate-cation interaction also occurred with the HiSiv-1000 (type Y) and HiSiv-3000 (silicalite) adsorbents. However, since the HiSiv-1000 adsorbent has a larger SiO2/Al2O3 ratio than NaY, the amount of active (accessible) cationic adsorption sites is less and therefore the experimental amount adsorbed was less. The ZSM-5 (pentasil) family of adsorbents used in this study included SiO2/Al2O3 ratios of 30, 50, 80, and 280 with H+ as the cation, and HiSiv-3000. The HiSiv 3000 adsorbent is based on the ZSM-5 structure but with a high SiO2/Al2O3 > 1000 where Na+ is the dominant cation. For these adsorbents the magnitude of the HenryÕs Law constants followed the SiO2/Al2O3 ratio for HZSM-5, shown in Fig. 2. However, the HiSiv 3000 adsorbent exhibited stronger HenryÕs region adsorption than the H-ZSM-5 adsorbents with SiO2/Al2O3 ratioÕs above 50. Therefore, although there are very few cations within the HiSiv 3000 structure, the cation-quadrupole interaction appears to be dominant for this adsorbent. These results also follow the magnitude of the adsorption isotherms given by Harlick and Tezel [13,15].
Amount Adsorbed (mol/kg)
Temperature range (°C)
Filled Symbols Pass 1 Open Symbols - Pass 2 Lines - Toth Isotherm
HZSM-5-30
1
HY-5 13X
NaY
0.1
HiSiv 3000
0.01 0.001
0.01
(A)
0.1
1
Pressure (atm.)
Filled Symbols Pass 1 Open Symbols - Pass 2 Lines - Toth Isotherm
5
13X NaY
4
Amount Adsorbed (mol/kg)
Adsorbent
75
3 HZSM-5-30 HiSiv 3000
2 HY-5
1
(B) 0 0.0
0.5
1.0
1.5
2.0
2.5
Pressure (atm.) Fig. 3. Carbon dioxide isotherms shown in log–log scale in (A), and linear scale in (B).
2.2. Pure component isotherms Adsorbent screening study based on CO2 adsorption capacities was performed by evaluating the adsorbents with the highest Kp and DH quantities, in each adsorbent family. Further adsorbents in the same group were evaluated until the capacities were deemed too low. The following adsorbents were studied for CO2 capacity: 13X; NaY; HiSiv1000; H Y 5; ZSM 5 30; HiSiv3000 The isotherms obtained at 22 °C are shown in Fig. 3A in log scale and in Fig. 3B in linear scale. The points repre-
sent the experimental data, and the curves represent the regressed form of the Toth isotherm given by Eq. (2). q¼
qm bP t 1=t
ð1 þ ðbP Þ Þ
ð2Þ
The filled symbols were obtained by regenerating the fresh adsorbent at 200 °C for 12 h followed by the adsorption study (pass 1). The open symbols were obtained as a repeat of pass 1 (200 °C regeneration for 12 h followed by adsorption) without changing the adsorbent sample (pass 2). From this data it can be seen
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that adsorption capacities of each of the adsorbents increased in the following order (across all pressures)
5.0 Reg. Pressure = 0.0013 atm.
4.5
13X > NaY > H ZSM 5 30 > HiSiv3000
4.0
13X
>HY5
13Xð2Þ < NaYð5Þ ¼ H Yð5Þ Based on these properties, the effects of the cation and SiO2/Al2O3 ratio were investigated. The H-Y-5 adsorbent is similar to the NaY adsorbent where the Na+ cation was replaced with the H+ cation. Based on this information, the H-Y-5 adsorbent was expected to behave similar to NaY except that it would exhibit lower capacities. A similar study was performed by Calleja et al. [3] for the ZSM-5 adsorbent. They showed that the Na-ZSM-5 capacity was higher than the H+ form of the same adsorbent. However, the difference that they found was no more than 25%. In this study with the Y-type zeolite, the effect of the cation was much more noticeable. As shown in Fig. 3, the H-Y-5 adsorbent exhibited adsorption capacities that were less than half the capacities exhibited by the Na+ form (NaY). The large difference is directly attributed to the strength of the quadrupole-cation interaction between CO2 and the adsorbent material. Therefore, it is evident that the Na+ should be the preferred cation. The effect of the SiO2/Al2O3 ratio resulted with an increase in the amount adsorbed with a decrease in the SiO2/Al2O3 ratio in faujasite type zeolites. The ZSM-5-30 adsorbent is based on the pentasil structure and has a SiO2/Al2O3 ratio of 30. This adsorbent exhibited the next highest capacity compared to the 13X and NaY adsorbents. More importantly, the isotherm shape is not as rectangular as the 13X or NaY isotherms. Due to this behaviour, the ZSM-5 adsorbent with a high (>1000) SiO2/Al2O3 ratio was investigated: HiSiv3000. This adsorbent exhibited a relatively linear isotherm above 0.5 atm. 2.3. Adsorption capacity shape dependency Using these CO2 isotherms, the data were further scrutinized by examining some possible expected (ideal) pressure swing cycles. These expected cycles were quantified by using the Toth isotherm. The loading (feed) pressure was varied from a regeneration pressure to the limit of the experimental data (1900 Torr). From the difference in capacities, the expected working capacity (EWC) was determined. Using this procedure, all the
Working Capacity (mmol/g)
The 13X, NaY and H-Y adsorbents are based on the faujasite unit cell. Within this group of adsorbents, the SiO2/Al2O3 ratio is different. The lower this ratio, the more cations are required to balance the charge structure of the materials. The following describes the SiO2/Al2O3 ratio relationship between the adsorbents.
NaY
3.5 3.0 HZSM-5-30
2.5 HiSiv 3000
2.0
HY-5
1.5 1.0 0.5 0.0 0.001
0.01
0.1
1
10
Pressure CO2 (atm.) Fig. 4. Expected working capacities for CO2 with various adsorbents under equilibrium conditions at 22 °C for a regeneration pressure of 0.0013 atm (1.0 Torr).
adsorbents were evaluated. Selected results are shown in Figs. 4 through 6. The data shown in Fig. 4 represent the expected working capacities with a regeneration pressure of 0.0013 atm (1 Torr). It can be seen from this plot that the EWC follows the magnitude of the isotherm: high EWC with high adsorption capacity. However, when the regeneration pressure was increased to 0.026 atm (20 Torr), as shown in Fig. 5, the NaY adsorbent exhibited higher EWC than 13X for CO2 feed pressures greater than 0.37 atm (280 Torr). Also, the difference between the EWC curves for the H-ZSM-5-30 and HiSiv 3000 adsorbents relative to the 13X and NaY adsorbents were reduced, relative to the low pressure regeneration cycles. When the regeneration pressure was further increased to 0.40 atm (300 Torr), the adsorbent choice for high expected working capacities is clear, NaY (Fig. 6). Further, the H-ZSM-5-30 adsorbent exhibited capacities higher than 13X for CO2 feed pressures greater than 2.0 atm. Following this trend it can be expected that the ZSM-5-30 and HiSiv 3000 adsorbents would outperform the 13X adsorbent when the regeneration pressure is high. This behaviour is solely due to the shape of the adsorption isotherm: rectangular isotherm requires low pressure regeneration to realize a high expected working capacity. Based on these data, the EWC values all decreased with increasing regeneration pressure, as expected. 13X and NaY adsorbents were directly compared for expected working capacities and the results are shown as
P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79 4.0
Solid Lines - NaY Dashed Lines - 13X
Reg. Pressure = 0.026 atm.
13X
3.0 2.5
HZSM-5-30 HiSiv 3000
2.0
HY-5
1.5 1.0
Regeneration Pressure CO2 (atm.)
Na Y
3.5
Working Capacity (mmol/g)
77
0.0
0.1
0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.01
0.5 0.0
4.0
0.1
0.1
1
Pressure CO2 (atm.) Fig. 5. Expected working capacities for CO2 with various adsorbents under equilibrium conditions at 22 °C for a regeneration pressure of 0.026 atm (20 Torr).
Fig. 7. Expected working capacity contours (mol/kg) for CO2 with NaY and 13X under equilibrium conditions at 22 °C. The solid line represents the cross-over of the EWC of the adsorbents.
Table 6 Recommended pressure conditions to use with NaY and 13X
1.8 Reg. Pressure = 0.40 atm.
1.6 NaY
Working Capacity (mmol/g)
1
Feed Pressure CO2 (atm.)
HZSM-5-30
1.4 1.2
HiSiv 3000
1.0 13X
Feed CO2 pressure
Regeneration CO2 pressure
Adsorbent choice
Low High Low High
Low Low High High
13X 13X NaY NaY
HY-5
0.8 0.6 0.4 0.2 0.0 1
Feed Pressure CO2 (atm.) Fig. 6. Expected working capacities for CO2 with various adsorbents under equilibrium conditions at 22 °C for a regeneration pressure of 0.4 atm (300 Torr).
contours (mol/kg) in Fig. 7. From these data, the optimal equilibrium adsorbent performance is clear, 13X with low regeneration pressure, NaY for high regeneration pressure. This behaviour is summarized in Table 6, for the pressure range used in this study.
For example, the adsorbents follow a linear relationship with respect to feed and regeneration pressures where the EWC for both 13X and NaY intersect, as shown in Fig. 7 as the solid thick line. To the right of this line, the NaY adsorbent exhibited higher EWC, whereas to the left, the 13X adsorbent exhibited higher EWC. This behaviour is indicative of industrial practice; 13X is often the adsorbent choice for dilute CO2 removal from process streams. Further details of the NaY adsorption behaviour as a function of temperature along with detailed examination of expected working capacities for PSA, TSA and PTSA applications were completed in our other work, [16]. The expected working capacities for H-ZSM-5-30 and HiSiv 3000 adsorbents were also compared and are shown in Fig. 8 as contours (mol/kg). For these materials the EWC curves do not cross in the pressure range studied. This is due to the similar shape of each adsorption isotherm. It is only in the high feed and regeneration pressure region where the contours approach each other.
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P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79
Regeneration Pressure CO 2 (atm.)
Solid Lines - H-ZSM-5-30 Dashed Lines - HiSiv 3000
0.0
0.1 0.5 0.5
1.0
1.0 1.5 1.5 2.0
2.0
0.01
2.5
0.1
1
Feed Pressure CO2 (atm.) Fig. 8. Expected working capacity contours (mol/kg) for CO2 with H-ZSM-5-30 and HiSiv 3000 under equilibrium conditions at 22 °C.
3. Conclusions This study enabled a detailed examination of how the temperature and isotherm shape can greatly affect the working capacity of the adsorbent. Also, since the adsorption process is exothermic, the heat released during adsorption will tend to increase the column temperature as the heat transient moves through the column. Another important property is that the net heat effects of adsorption decrease as temperature increases due to lower adsorption potentials. Therefore, the importance of the heat effects and operating temperature on the adsorption capacities cannot be overlooked. The optimal design of a PSA separation unit is a complex task as the separation and recovery effectiveness are highly sensitive to the chosen operating conditions and adsorbent type. In this study, an extensive adsorbent screening was completed. The results have provided a few promising adsorbents. However, an optimal process design will depend on the combination of appropriate adsorbent choice and operating conditions. If a low pressure CO2 feed and very low regeneration pressure is used then the NaY and 13X adsorbents should be used. As the CO2 feed and regeneration pressure increases, the adsorbent with the linear isotherm (across the operating pressure range) should be used. References [1] B.A. Buffman, M. Mason, G.D. Yadav, Retention volumes and retention times in binary chromatography, Journal of Chemical Soceity—Faraday Transactions I 81 (1985) 161–173.
[2] B.A. Buffman, G. Mason, M.J. Heslop, Binary adsorption isotherms from chromatographic retention times, Industrial and Engineering Chemistry Research 38 (3) (1999) 1114–1124. [3] G. Calleja, J. Pau, J.A. Calles, Pure and multicomponent adsorption equilibrium of carbon dioxide, ethylene and propane on ZSM-5 zeolites with different Si/Al ratios, Journal of Chemical and Engineering Data 43 (6) (1998) 994–1003. [4] V.R. Choudhary, S. Mayadevi, Adsorption of methane, ethane, ethylene, and carbon dioxide on X, Y, L, and M zeolites using a gas chromatography pulse technique, Separation Science and Technology 28 (8) (1993) 1595–1607. [5] V.R. Choudhary, S. Mayadevi, Adsorption of methane, ethane, ethylene, and carbon dioxide on high silica pentasil zeolites and zeolite-like materials using gas chromatography pulse technique, Separation Science and Technology 28 (13–14) (1993) 2197–2209. [6] K.T. Chue, J.N. Kim, Y.J. Yoo, S.H. Cho, Comparison of activated carbon and zeolite 13X for CO2 recovery from flue gas by pressure swing adsorption, Industrial and Engineering Chemistry Research 34 (2) (1995) 591–598. [7] J.A. Dunne, R. Mariwala, M. Rao, S. Sircar, R.J. Gorte, A.L. Myers, Calorimetric heats of adsorption and adsorption isotherms. 1. O2, N2, Ar, CO2, CH4, C2H6, and SF6 on silicalite, Langmuir 12 (24) (1996) 5888–5895. [8] J.A. Dunne, M. Rao, S. Sircar, R.J. Gorte, A.L. Myers, Calorimetric heats of adsorption and adsorption isotherms. 2. O2, N2, Ar, CO2, CH4, C2H6, and SF6 on NaX, H-ZSM-5, and Na-ZSM-5 zeolites, Langmuir 12 (24) (1996) 5896–5904. [9] T.C. Golden, S. Sircar, Gas adsorption on silicalite, Journal of Colloid and Interface Science 162 (1) (1994) 182–188. [10] H.W. Habgood, Adsorptive and gas chromatographic properties of various cationic forms of zeolite X, Canadian Journal of Chemistry 42 (1964) 2340–2350. [11] P.J.E. Harlick, F.H. Tezel, A novel solution method for interpreting binary adsorption isotherms from concentration pulse chromatography data, Adsorption 4 (6) (2000) 293–309. [12] P.J.E. Harlick, F.H. Tezel, The importance of experimental data regression for interpreting binary adsorption isotherms from concentration pulse chromatography data: a novel functional set, Canadian Journal of Chemical Engineering 79 (2) (2001) 236–245. [13] P.J.E. Harlick, F.H. Tezel, Adsorption of carbon dioxide, methane and nitrogen: pure and binary mixture adsorption by ZSM-5 with SiO2/Al2O3 ratio of 30, Separation Science and Technology 37 (1) (2002) 33–60. [14] P.J.E. Harlick, F.H. Tezel, Use of concentration pulse chromatography for determining binary isotherms: comparison with statically determined binary isotherms, Adsorption 9 (3) (2003) 275–286. [15] P.J.E. Harlick, F.H. Tezel, Adsorption of carbon dioxide, methane and nitrogen: pure and binary mixture adsorption for ZSM-5 with SiO2/Al2O3 ratio of 280, Separation and Purification Technology 33 (2003) 199–210. [16] P.J.E. Harlick, F.H. Tezel, Equilibrium analysis of cyclic adsorption processes: expected CO2 working capacities with NaY, Separation Science and Technology, submitted for publication. [17] Y.Y. Huang, Quadrupole interaction of carbon dioxide on silicaalumina surface, Journal of Physical Chemistry 77 (1) (1973). [18] S.H. Hyun, R.P. Danner, Gas adsorption isotherms by use of perturbation chromatography, Industrial Engineering Chemistry Fundamentals 24 (1985) 95–101. [19] J.N. Kim, K.T. Chue, K.I. Kim, S.H. Cho, J.D. Kim, Nonisothermal adsorption of nitrogen–carbon dioxide mixture in a fixed bed of zeolite-X, Journal of Chemical Engineering of Japan 27 (1) (1994). [20] R. Kumar, Pressure swing adsorption process: performance optimum and adsorbent selection, Industrial and Engineering Chemistry Research 33 (6) (1994) 1600–1605.
P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79 [21] R. Kumar, Effect of variable feed concentration on the performance of a pressure swing adsorption process, Adsorption 1 (3) (1995) 203–211. [22] J. Pires, M. Brotas de Carvalho, F. Ramoa Ribeiro, E.G. Derouane, Carbon dioxide in Y and ZSM-20 zeolites: adsorption and infrared studies, Journal of Molecular Catalysis (1993) 85. [23] D.M. Ruthven, Principles of Adsorption and Asorption Processes, John Wiley and Sons, Inc., New York, NY, 1984. [24] D.M. Ruthven, R. Kumar, An experimental study of singlecomponent and binary adsorption equilibria by a chromato-
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graphic method, Industrial Engineering Chemistry Fundamentals 19 (1980) 27–32. [25] D.M. Ruthven, S. Farooq, K. Knaebel, Pressure Swing Adsorption, VCH Publishers, New York, NY, 1994. [26] E. Van der Vlist, J. Van der Meijden, Determination of the adsorption isotherms of the components of binary gas mixtures by gas chromatography, Journal of Chromatography 79 (1973) 1–13. [27] T. Yamazaki, M. Katoh, S. Ozawa, Y. Ogino, Adsorption of CO2 over univalent cation-exchanged ZSM-5 zeolites, Molecular Physics 80 (2) (1993) 313–324.
An experimental adsorbent screening study for CO2 removal from N2 Peter J.E. Harlick, F. Handan Tezel
*
Department of Chemical Engineering, University of Ottawa, 161 Louis-Pasteur, Ottawa, Ont., Canada K1N 6N5 Received 9 February 2004; received in revised form 27 July 2004; accepted 27 July 2004 Available online 6 October 2004
Abstract The selection of a suitable adsorbent for CO2 removal from flue gas (mixture of CO2 and N2) has been carried out. The limiting heats of adsorption, and HenryÕs Law constants for CO2 with a N2 carrier, were determined for a group of 13 zeolite based adsorbents, including 5A, 13X, NaY, NaY-10, H-Y-5, H-Y-30, H-Y-80, HiSiv 1000, H-ZSM-5-30, H-ZSM-5-50, H-ZSM-5-80, H-ZSM5-280, and HiSiv 3000. The CO2 pure component adsorption isotherms and expected working capacity curves for pressure swing adsorption (PSA) application were determined for a selected promising subgroup of these adsorbents. The results show that the most promising adsorbent characteristics are a near linear CO2 isotherm and a low SiO2/Al2O3 ratio with cations in the zeolite structure which exhibit strong electrostatic interactions with carbon dioxide. Ó 2004 Elsevier Inc. All rights reserved. Keywords: Carbon dioxide removal from nitrogen; Flue gas; Adsorption of nitrogen; Adsorption isotherms; Zeolites; HenryÕs Law constant; Heat of adsorption; Working capacity
1. Introduction With an increasing worldwide population and an increased per capita demand for energy, concern is now being raised over the CO2 emissions resulting from the gaseous by-products of combustion processes. In order to meet the present and future constraints placed on the allowable emissions of CO2, the solution lies with reduction and recovery. Pressure swing adsorption (PSA) is a very promising separation and recovery process for this type of application. In the design of an adsorption based separation process, the choice of the adsorbent is the most crucial design consideration [20,21,25]. In this study, several adsorbent–adsorbate characteristics have been considered.
*
Corresponding author. Tel.: +1 613 562 5800x6099; fax: +1 613 562 5172. E-mail address: [email protected] (F.H. Tezel). 1387-1811/$ - see front matter Ó 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2004.07.035
These include the initial loading effects (HenryÕs Law constant, initial heat of adsorption), pure component adsorption capacity and isotherm shape, and the expected working capacities for PSA applications. For this study, an extensive literature review and experimental adsorbent screening with CO2 and N2 gases have been completed. Thirteen adsorbents, described in Table 1, have been examined and the most promising ones were selected for further study. Pure component CO2 adsorption isotherms were determined at 22 °C and pressures ranging from 1 to 1900 Torr. The concentration pulse method (CPM) was used for determining the HenryÕs Law constants and the associated heat of adsorption parameters [26,24,18,1,2,11]. In this method, a column (dimensions are given in Table 2) was packed with the adsorbent to be studied. The adsorbent was regenerated under helium gas purge at 200 °C for 12 h to get rid of all the moisture and other gases that may be adsorbed by the adsorbent. The same regeneration procedure was used for all the adsorbents studied. After the regeneration, the column temperature
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Nomenclature b DH EWC K0 Kp P
Toth isotherm parameter, (1/atm)t limiting heat of adsorption, kJ/mol expected working capacity, mol/kg pre-exponential factor, mol/kg/atm HenryÕs Law constant, mol/kg/atm pressure, Torr, atm
q qm R T t
was reduced to the experiment temperature. The helium gas was used as the carrier gas and a very small amount of adsorbate was injected into the carrier gas as an impulse. The response of the column was monitored by measuring the concentration of the adsorbate at the end of the column. From the first moment of the response peak, HenryÕs Law constant, Kp, can be determined. More details of the experimental set-up can be found elsewhere [11,12,14]. The pure component isotherms were determined with a modified constant volume apparatus (Micrometetics Accusorb 2400). The quickest and easiest method for the initial loading study is by using the concentration pulse method for determining HenryÕs Law constants (Kp, mol/kg/atm). The Kp values are proportional to the initial slope of the adsorption isotherm as a large Kp value corresponds to a steep initial slope of the adsorption isotherm. When Kp values are determined at different temperatures (as P[CO2] ! 0), a VanÕt Hoff plot can be constructed and the limiting heat of adsorption is defined by Eq. (1). DH K p ¼ K 0 exp ð1Þ RT
amount adsorbed, mol/kg Limiting amount adsorbed, mol/kg gas constant, kJ/mol/K temperature, K Toth isotherm parameter, Dimensionless
form. The experimental conditions used with the concentration pulse method are given in Table 2. When the Kp and DH values are considered at the same time, an initial adsorbent screening can be performed. This is accomplished by removing all adsorbents that exhibit very high heats of adsorption, as these tend to require higher energy costs for the regeneration cycle. Since the adsorption process is exothermic, a high DH value will produce large quantities of heat within the adsorbent bed during an adsorption process cycle. This will cause an increase in the local column temperature. This increase in the column temperature will have an adverse effect on the adsorption capacities of the components. The result is a loss of capacity, and thus decreased process throughput. Similarly, in desorption a large temperature drop may also occur. This drop may cause certain components to freeze out on the adsorbent (such as moisture).
2. Results and discussion 2.1. Initial loading effects
The heat of adsorption is a function of the strength of adsorption. This quantity is defined by the slope found by regression of the experimental data in dimensional
There are several sources of initial loading data available for CO2 and N2 with several adsorbents (i.e.,
Table 1 Description of adsorbents studied Adsorbent type
Supplier
Pellet form
SiO2/Al2O3 ratio
Cation form
Pore size (nm)
Binder
Particle size
Type ZSM-5-30 ZSM-5-50 ZSM-5-80 ZSM-5-280 HSiv 3000 (Type ZSM5) H-Y-5.1 H-Y-30 H-Y-80 HSiv 1000 (Type Y) NaY-10 NaY 13X 5A
+
% wt
CV
CPM
Zeolyst Zeolyst Zeolyst Zeolyst UOP
Powder Powder Powder Powder 00 1/16
30 50 80 280 >1000
H H+ H+ H+ Na+
0.6 0.6 0.6 0.6 0.6
Kaolin Kaolin Kaolin Kaolin Provided as
15 15 15 15 pellet
20–60 20–60 20–60 20–60 00 1/16
20–60 20–60 20–60 20–60 20–60
Zeolyst Zeolyst Zeolyst UOP
Powder Powder Powder 00 1/16
5.1 30 80 >20
H+ H+ H+ Na+
0.8 0.8 0.8 0.8
Kaolin Kaolin Kaolin Provided as
15 15 15 pellet
20–60 20–60 20–60 00 1/16
20–60 20–60 20–60 20–60
UOP UOP UOP Linde
1/16 00 1/16 00 1/16 00 1/8
>10 5.1 2.2 3–5
Na+ Na+ Na+ Na+
0.8 0.8 0.8 0.5
Provided Provided Provided Provided
pellet pellet pellet pellet
1/16 00 1/16 00 1/16 00 1/8
00
CV represents the constant volume method, and CPM represents the concentration pulse method.
as as as as
00
20–60 20–60 20–60 20–60
P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79 Table 2 Experimental and column specifications used with the CPM Carrier gas Particle size Average particle diameter Bed porosity Column length Column inner diameter Total flowrate Total pressure Regeneration temperature Regeneration pressure Regeneration time
UHP nitrogen 20–60 550 0.382 24.75 0.45 30.0 1.0 200 1.0 12
Mesh lm – cm cm cc/min at 40 °C Atm °C Atm Hours
[27,9,7,8]). However, the availability of HenryÕs Law constants at the required design conditions is limited. A comparison of the available literature data was difficult since the studies were performed under varied thermodynamic conditions; temperature and pressure. Selected results of a literature survey based on the heat of adsorption of the type of adsorbents used in this study are given in Table 3. The experimentally determined HenryÕs Law constants are given in Table 4, and plotted in Figs. 1 and 2. The change in internal energy and heat of adsorption (CO2 in N2) for each of the adsorbents are given in Table 5. Some adsorbents did not exhibit sufficient CO2 adsorption in order to be accurately characterized across all the temperatures implied on the concentration pulse method used in this study. The heat of adsorption data obtained with the chosen adsorbents agree with the trends given by the literature data. For example, from the experimental data in Table 5, the order of magnitude of the heat of adsorption was as follows: 5A > 13X > NaY > ZSM 5 From the literature data, the same trend was observed (Table 3). However, the magnitude of the heat of adsorption shown in Table 3 varies widely for some adsorbents from different research groups, therefore, no inference can be drawn against the presently determined heat of adsorption values. Many insights can be realized with the data obtained. First, the magnitude of the Kp values at specific temperatures are known. These values relate to the shape of the adsorption isotherm in the HenryÕs Law region. In general, a large Kp value results with a steep, rectangular isotherm. For CO2, these types of isotherms are not favourable for industrial adsorption systems where the feed mole fraction of more adsorbed component is high (for example, >5%). The data also showed that a high heat of adsorption does not always translate from high Kp values. Within the zeolite-Y family of adsorbents, HiSiv-1000 exhibited this behaviour for CO2. Although the Kp values at the low range of temperatures studied is not very high, the slope of the VanÕt Hoff plot
73
Table 3 Literature review of CO2 and N2 heats of adsorption with various adsorbents Adsorbent
Heat of adsorption CO2 (kcal/mol)
Silicalite Silicalite Silicalite Silicalite H-ZSM-5-30 H-ZSM-5-30 H-ZSM-5-30 Na-ZSM-5-30 Na-ZSM-5-30 Na-ZSM-5-30 Cs-ZSM-5-30 Rb-ZSM-5-30 K-ZSM-5-30 Li-ZSM-5-30 Na-ZSM-5-47 H-ZSM-5-54 H-ZSM-8-30 Na-ZSM-8-30 Na-ZSM-20-30 H-ZSM-20-30 Li-X X Na-X Na-X Na-X Na-X Na-X Na-X K-X Mg-X Ca-X Ba-X Ag-X 13X Na-Y H-Y Na-Y Ce(46)Na-Y Ce(72)Na-Y H-Y K-L HK-L Na-M H-M
21.7 24.1 23.4 27.2 26.1 28.8 38.0 46.3 42.0 50.0 33.0 34.9 36.0 58.9 46.3 28.8 27.0 35.9 27.0 20.0 35.0
15.1 17.6
27.2 25.0 18.4
49.1 48.1 47.3 47.5 50.0
36.0 33.0 25.0 36.5 35.7 40.8 27.0 24.3 30.1 38.2 44.5
Source references
N2 (kcal/mol)
18.4 34.7 31.4 20.1 31.0 25.0
[5] [9] [27] [7] [5] [27] [8] [27] [5] [8] [27] [27] [27] [27] [8] [8] [5] [5] [22] [22] [19] [5] [10] [10] [8] [8] [8] [8] [8] [10] [10] [10] [10] [10] [6] [22] [22] [4] [4] [4] [4] [4] [4] [4]
for this adsorbent is high, (Fig. 1). This is due to the low Kp values at high temperatures; this behaviour is promising. High Kp values translate into a smaller mass of adsorbent for a set cycle time, since the capacity is much higher. However, adsorbents with relatively high capacities at 1–2 atm pressure usually provide rectangular shaped isotherms and they often result with energy intensive isothermal regeneration in order to realize a high cyclic capacity for PSA applications. The smallest pore size adsorbent that was studied was 5A. This adsorbent contained the Ca+2 cation, had one of the lowest SiO2/Al2O3 ratio among the adsorbents
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Table 4 HenryÕs Law constants determined by the CPM for CO2 with a N2 carrier Temperature (°C)
0 10 15 20 25 30 40 60 80 100 120 125 130 140 150 170 175 200
HenryÕs Law constant (mol/kg/atm) ZSM-5
HiSiv
HY
NaY
30
50
80
280
3000
1000
5
30
80
5
10
– – – 27.7 – – 11.0 5.06 2.44 1.32
– – – – – 9.42 6.23 2.86 1.54 0.784
– – – – – 7.47 4.98 2.31 1.19 0.666
– – – – – 6.16 4.05 1.99 0.986 0.560
– – – – – 12.1 7.89 3.55 1.77 1.03
– – – – – – –
– – – – – – –
– – – – – – –
– – – – – – –
– – – – – – –
– – – – – 25.4 16.6 7.80 3.67 2.00 0.590 – – – – – – –
– 11.7 – – 5.60 – 3.02 1.22 0.688 0.409 – – – – – – – –
2.42 – 1.30 – – 0.781 0.556 0.324 0.199 0.116 – – – – – – – –
1.58 1.11 – 0.760 – 0.570 0.414 0.238 0.158 0.0933 – – – – – – – –
– – – – – – 37.9 17.4 8.21 4.08 – – 1.70 – 1.01 – – 0.367
– – – – – 41.9 26.9 12.18 5.45 2.97 – –
NaY
1000
5A
– – – – – – – – 18.2 7.99 4.05 – – 2.03 – 0.92 – 0.465
– – – – – – – – – 12.8 – 5.14 – – 2.04 – 1.01 0.529
1000
NaY-10
13X
– – – – –
13X
ZSM-5-30
ZSM-5-30 HiSiv-3000
HiSiv-3000 ZSM-5-50 ZSM-5-80 ZSM-5-280
} 100
ZSM-5-50
K p (Dimensionless)
K p (Dimensionless)
5A
ZSM-5-80 ZSM-5-280
100
HY-30
HiSiv-1000 HY-5.1
HY-80
10
2.0
2.5
3.0
3.5
4.0
1000/T ( K-1 )
2.6
2.8
3.0
3.2
3.4
3.6
1000/T ( K-1 )
Fig. 1. VanÕt Hoff plots of the HenryÕs Law constants for CO2 with a N2 carrier for various zeolites.
Fig. 2. VanÕt Hoff plots of the HenryÕs Law constants for CO2 with a N2 carrier for various pentasil based adsorbents.
used in this study. Therefore, this material exhibited the strongest net cation-quadrupole interaction with about 8 sites per unit cell [23], and the lowest CO2 mobility within the pores. As shown in Fig. 1 and Table 4, the HenryÕs Law constants were the highest. The 13X and NaY adsorbents provided high heats of adsorption for CO2. These high heats of adsorption are largely due to the quadrupole interaction of CO2 with the cationic nature of the adsorbent surface [17]. Both
of these adsorbents have low SiO2/Al2O3 ratios. Lower ratios require more cations to balance the charge distribution within the structure and result in high degree of heterogeneity. The 13X structure is more heterogeneous than NaY and thus exhibits a higher heat of adsorption. In both adsorbents, the Na+ cation was used to balance the charge distribution. This cation has a very strong electrostatic field with which to interact with the quadrupole moment of both adsorbates. Therefore, a method
P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79 Table 5 Experimental heat of adsorption and internal energy data for CO2 in N2 with various adsorbents DU (kJ/mol)
DH (kJ/mol)
H-Y-5.1 H-Y-30 H-Y-80 HiSiv-1000 NaY-10 NaY 13X 5A HiSiv-3000 ZMS-5-30 H-ZSM-5-50 H-ZSM-5-80 H-ZSM-5-280
10–100 0–100 0–100 30–120 30–100 40–200 80–200 100–200 30–100 20–100 30–100 30–100 30–100
30.5 21.0 22.7 32.5 33.1 33.0 39.2 43.6 30.6 31.9 30.3 29.8 29.5
33.1 23.7 25.3 34.9 35.9 36.1 42.5 47.1 33.4 34.6 33.1 32.6 32.3
of interaction control may be used by incorporating a cation with a smaller electrostatic interaction potential, such as H+. The strong adsorbate-cation interaction also occurred with the HiSiv-1000 (type Y) and HiSiv-3000 (silicalite) adsorbents. However, since the HiSiv-1000 adsorbent has a larger SiO2/Al2O3 ratio than NaY, the amount of active (accessible) cationic adsorption sites is less and therefore the experimental amount adsorbed was less. The ZSM-5 (pentasil) family of adsorbents used in this study included SiO2/Al2O3 ratios of 30, 50, 80, and 280 with H+ as the cation, and HiSiv-3000. The HiSiv 3000 adsorbent is based on the ZSM-5 structure but with a high SiO2/Al2O3 > 1000 where Na+ is the dominant cation. For these adsorbents the magnitude of the HenryÕs Law constants followed the SiO2/Al2O3 ratio for HZSM-5, shown in Fig. 2. However, the HiSiv 3000 adsorbent exhibited stronger HenryÕs region adsorption than the H-ZSM-5 adsorbents with SiO2/Al2O3 ratioÕs above 50. Therefore, although there are very few cations within the HiSiv 3000 structure, the cation-quadrupole interaction appears to be dominant for this adsorbent. These results also follow the magnitude of the adsorption isotherms given by Harlick and Tezel [13,15].
Amount Adsorbed (mol/kg)
Temperature range (°C)
Filled Symbols Pass 1 Open Symbols - Pass 2 Lines - Toth Isotherm
HZSM-5-30
1
HY-5 13X
NaY
0.1
HiSiv 3000
0.01 0.001
0.01
(A)
0.1
1
Pressure (atm.)
Filled Symbols Pass 1 Open Symbols - Pass 2 Lines - Toth Isotherm
5
13X NaY
4
Amount Adsorbed (mol/kg)
Adsorbent
75
3 HZSM-5-30 HiSiv 3000
2 HY-5
1
(B) 0 0.0
0.5
1.0
1.5
2.0
2.5
Pressure (atm.) Fig. 3. Carbon dioxide isotherms shown in log–log scale in (A), and linear scale in (B).
2.2. Pure component isotherms Adsorbent screening study based on CO2 adsorption capacities was performed by evaluating the adsorbents with the highest Kp and DH quantities, in each adsorbent family. Further adsorbents in the same group were evaluated until the capacities were deemed too low. The following adsorbents were studied for CO2 capacity: 13X; NaY; HiSiv1000; H Y 5; ZSM 5 30; HiSiv3000 The isotherms obtained at 22 °C are shown in Fig. 3A in log scale and in Fig. 3B in linear scale. The points repre-
sent the experimental data, and the curves represent the regressed form of the Toth isotherm given by Eq. (2). q¼
qm bP t 1=t
ð1 þ ðbP Þ Þ
ð2Þ
The filled symbols were obtained by regenerating the fresh adsorbent at 200 °C for 12 h followed by the adsorption study (pass 1). The open symbols were obtained as a repeat of pass 1 (200 °C regeneration for 12 h followed by adsorption) without changing the adsorbent sample (pass 2). From this data it can be seen
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P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79
that adsorption capacities of each of the adsorbents increased in the following order (across all pressures)
5.0 Reg. Pressure = 0.0013 atm.
4.5
13X > NaY > H ZSM 5 30 > HiSiv3000
4.0
13X
>HY5
13Xð2Þ < NaYð5Þ ¼ H Yð5Þ Based on these properties, the effects of the cation and SiO2/Al2O3 ratio were investigated. The H-Y-5 adsorbent is similar to the NaY adsorbent where the Na+ cation was replaced with the H+ cation. Based on this information, the H-Y-5 adsorbent was expected to behave similar to NaY except that it would exhibit lower capacities. A similar study was performed by Calleja et al. [3] for the ZSM-5 adsorbent. They showed that the Na-ZSM-5 capacity was higher than the H+ form of the same adsorbent. However, the difference that they found was no more than 25%. In this study with the Y-type zeolite, the effect of the cation was much more noticeable. As shown in Fig. 3, the H-Y-5 adsorbent exhibited adsorption capacities that were less than half the capacities exhibited by the Na+ form (NaY). The large difference is directly attributed to the strength of the quadrupole-cation interaction between CO2 and the adsorbent material. Therefore, it is evident that the Na+ should be the preferred cation. The effect of the SiO2/Al2O3 ratio resulted with an increase in the amount adsorbed with a decrease in the SiO2/Al2O3 ratio in faujasite type zeolites. The ZSM-5-30 adsorbent is based on the pentasil structure and has a SiO2/Al2O3 ratio of 30. This adsorbent exhibited the next highest capacity compared to the 13X and NaY adsorbents. More importantly, the isotherm shape is not as rectangular as the 13X or NaY isotherms. Due to this behaviour, the ZSM-5 adsorbent with a high (>1000) SiO2/Al2O3 ratio was investigated: HiSiv3000. This adsorbent exhibited a relatively linear isotherm above 0.5 atm. 2.3. Adsorption capacity shape dependency Using these CO2 isotherms, the data were further scrutinized by examining some possible expected (ideal) pressure swing cycles. These expected cycles were quantified by using the Toth isotherm. The loading (feed) pressure was varied from a regeneration pressure to the limit of the experimental data (1900 Torr). From the difference in capacities, the expected working capacity (EWC) was determined. Using this procedure, all the
Working Capacity (mmol/g)
The 13X, NaY and H-Y adsorbents are based on the faujasite unit cell. Within this group of adsorbents, the SiO2/Al2O3 ratio is different. The lower this ratio, the more cations are required to balance the charge structure of the materials. The following describes the SiO2/Al2O3 ratio relationship between the adsorbents.
NaY
3.5 3.0 HZSM-5-30
2.5 HiSiv 3000
2.0
HY-5
1.5 1.0 0.5 0.0 0.001
0.01
0.1
1
10
Pressure CO2 (atm.) Fig. 4. Expected working capacities for CO2 with various adsorbents under equilibrium conditions at 22 °C for a regeneration pressure of 0.0013 atm (1.0 Torr).
adsorbents were evaluated. Selected results are shown in Figs. 4 through 6. The data shown in Fig. 4 represent the expected working capacities with a regeneration pressure of 0.0013 atm (1 Torr). It can be seen from this plot that the EWC follows the magnitude of the isotherm: high EWC with high adsorption capacity. However, when the regeneration pressure was increased to 0.026 atm (20 Torr), as shown in Fig. 5, the NaY adsorbent exhibited higher EWC than 13X for CO2 feed pressures greater than 0.37 atm (280 Torr). Also, the difference between the EWC curves for the H-ZSM-5-30 and HiSiv 3000 adsorbents relative to the 13X and NaY adsorbents were reduced, relative to the low pressure regeneration cycles. When the regeneration pressure was further increased to 0.40 atm (300 Torr), the adsorbent choice for high expected working capacities is clear, NaY (Fig. 6). Further, the H-ZSM-5-30 adsorbent exhibited capacities higher than 13X for CO2 feed pressures greater than 2.0 atm. Following this trend it can be expected that the ZSM-5-30 and HiSiv 3000 adsorbents would outperform the 13X adsorbent when the regeneration pressure is high. This behaviour is solely due to the shape of the adsorption isotherm: rectangular isotherm requires low pressure regeneration to realize a high expected working capacity. Based on these data, the EWC values all decreased with increasing regeneration pressure, as expected. 13X and NaY adsorbents were directly compared for expected working capacities and the results are shown as
P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79 4.0
Solid Lines - NaY Dashed Lines - 13X
Reg. Pressure = 0.026 atm.
13X
3.0 2.5
HZSM-5-30 HiSiv 3000
2.0
HY-5
1.5 1.0
Regeneration Pressure CO2 (atm.)
Na Y
3.5
Working Capacity (mmol/g)
77
0.0
0.1
0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.01
0.5 0.0
4.0
0.1
0.1
1
Pressure CO2 (atm.) Fig. 5. Expected working capacities for CO2 with various adsorbents under equilibrium conditions at 22 °C for a regeneration pressure of 0.026 atm (20 Torr).
Fig. 7. Expected working capacity contours (mol/kg) for CO2 with NaY and 13X under equilibrium conditions at 22 °C. The solid line represents the cross-over of the EWC of the adsorbents.
Table 6 Recommended pressure conditions to use with NaY and 13X
1.8 Reg. Pressure = 0.40 atm.
1.6 NaY
Working Capacity (mmol/g)
1
Feed Pressure CO2 (atm.)
HZSM-5-30
1.4 1.2
HiSiv 3000
1.0 13X
Feed CO2 pressure
Regeneration CO2 pressure
Adsorbent choice
Low High Low High
Low Low High High
13X 13X NaY NaY
HY-5
0.8 0.6 0.4 0.2 0.0 1
Feed Pressure CO2 (atm.) Fig. 6. Expected working capacities for CO2 with various adsorbents under equilibrium conditions at 22 °C for a regeneration pressure of 0.4 atm (300 Torr).
contours (mol/kg) in Fig. 7. From these data, the optimal equilibrium adsorbent performance is clear, 13X with low regeneration pressure, NaY for high regeneration pressure. This behaviour is summarized in Table 6, for the pressure range used in this study.
For example, the adsorbents follow a linear relationship with respect to feed and regeneration pressures where the EWC for both 13X and NaY intersect, as shown in Fig. 7 as the solid thick line. To the right of this line, the NaY adsorbent exhibited higher EWC, whereas to the left, the 13X adsorbent exhibited higher EWC. This behaviour is indicative of industrial practice; 13X is often the adsorbent choice for dilute CO2 removal from process streams. Further details of the NaY adsorption behaviour as a function of temperature along with detailed examination of expected working capacities for PSA, TSA and PTSA applications were completed in our other work, [16]. The expected working capacities for H-ZSM-5-30 and HiSiv 3000 adsorbents were also compared and are shown in Fig. 8 as contours (mol/kg). For these materials the EWC curves do not cross in the pressure range studied. This is due to the similar shape of each adsorption isotherm. It is only in the high feed and regeneration pressure region where the contours approach each other.
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P.J.E. Harlick, F.H. Tezel / Microporous and Mesoporous Materials 76 (2004) 71–79
Regeneration Pressure CO 2 (atm.)
Solid Lines - H-ZSM-5-30 Dashed Lines - HiSiv 3000
0.0
0.1 0.5 0.5
1.0
1.0 1.5 1.5 2.0
2.0
0.01
2.5
0.1
1
Feed Pressure CO2 (atm.) Fig. 8. Expected working capacity contours (mol/kg) for CO2 with H-ZSM-5-30 and HiSiv 3000 under equilibrium conditions at 22 °C.
3. Conclusions This study enabled a detailed examination of how the temperature and isotherm shape can greatly affect the working capacity of the adsorbent. Also, since the adsorption process is exothermic, the heat released during adsorption will tend to increase the column temperature as the heat transient moves through the column. Another important property is that the net heat effects of adsorption decrease as temperature increases due to lower adsorption potentials. Therefore, the importance of the heat effects and operating temperature on the adsorption capacities cannot be overlooked. The optimal design of a PSA separation unit is a complex task as the separation and recovery effectiveness are highly sensitive to the chosen operating conditions and adsorbent type. In this study, an extensive adsorbent screening was completed. The results have provided a few promising adsorbents. However, an optimal process design will depend on the combination of appropriate adsorbent choice and operating conditions. If a low pressure CO2 feed and very low regeneration pressure is used then the NaY and 13X adsorbents should be used. As the CO2 feed and regeneration pressure increases, the adsorbent with the linear isotherm (across the operating pressure range) should be used. References [1] B.A. Buffman, M. Mason, G.D. Yadav, Retention volumes and retention times in binary chromatography, Journal of Chemical Soceity—Faraday Transactions I 81 (1985) 161–173.
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