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Competitive Adsorption Equilibrium Isotherms of CO, CO2, CH4, and H2 on Activated Carbon and Zeolite 5A for Hydrogen Purification Milad Yavary, Habib Ale Ebrahim,* and Cavus Falamaki Chemical Engineering Department, Petrochemical Center of Excellency, Amirkabir University, Tehran 15875-4413, Iran ABSTRACT: Adsorptive separation techniques such as pressure swing adsorption can be employed on a large scale to separate and purify hydrogen. The streams of hydrogen are multicomponent mixtures of H2, CO2, CO, and CH4. Competitive isotherms of adsorption of a multicomponent gas mixture (CO2, CO, CH4, and H2) on two commercial sorbents (activated carbon and 5A zeolite granules) are presented in this work. Adsorption equilibrium is investigated in a wide range of pressure (1 to 18 bar) at three temperatures (25, 35, and 45 °C). A study has been performed on the loading ratio correlation and Langmuir models for the adsorption. The adsorption capacity of different components of the gas mixture on activated carbon follows the order CO2 > CH4 > CO > H2, and the adsorption capacity order for the zeolite is CO2 > CO > CH4 > H2. The loading amount of carbon dioxide on zeolite 5A increases quickly at low pressure, and disruption at these pressures is hard. Hence, carbon dioxide should not reach into the zeolite layer. For activated carbon, the Langmuir model fit the data with fairly good accuracy within the operating conditions studied. Both models were able to fit the experimental data for zeolite 5A with good accuracy.

1. INTRODUCTION Hydrogen is necessary for hydrocracking and hydrotreating refinery units, and it will also be a clean fuel for society in the future. Steam reforming of natural gas combined with the water gas shift reaction is the main method of hydrogen production. Additionally, the separation of hydrogen from other components remaining in the reforming process is required. The pressure swing adsorption (PSA) process is usually used to purify streams with large amounts of hydrogen.1−6 Nevertheless, streams of hydrogen are generally multicomponent mixtures. In the simplest case, the outlet of steam methane reforming and the high-temperature water−gas shift reaction undergoing separation contain four components (H2, CO2, CO, and CH4).7−9 Using multilayer columns with different adsorbents is the usual method in industrial PSA units. A layered bed comprising various sorbents for cyclic adsorption is not a new idea.10−13 In fact, it has become reality on the industrial scale for gas separation from multicomponent mixtures by PSA.14,15 The strongly adsorptive components can be adsorbed by the first layer of sorbents, and the second layer can be used to remove other ingredients.9 Reliable gas−solid equilibrium data is an essential part of the design of an adsorptive separation process. In industrial PSA processes, two layers of activated carbon and zeolite 5A are used for hydrogen purification.1,2,15 On the other hand, the competitive experimental determination of isotherms is a relatively tedious process. It requires equilibrium experiments at an extensive variety of pressures for two adsorbents and a gas mixture at three or more temperatures. There are many studies on layered beds including various sorbents for cyclic adsorption and ion exchange processes.10−14,16 The main approach of layered bed design is using an adsorbent with a low capacity in the first part of the column and a stronger sorbent in the second part in order to © XXXX American Chemical Society

maximize the adsorption and subsequently diminish the column size. By optimization, the best ratio of two adsorbents in a column can be determined.13,17,18 The best design of a layered bed of different sorbents is based on modeling the PSA process, and these two layers undergo simultaneous breakthroughs of the various components. Also, other operational parameters such as the feed composition and feed velocity are important in optimal layered bed designs. In the present work, the loading ratio correlation (LRC) and the Langmuir model have been used as two models for gas− solid multicomponent adsorption for a hydrogen PSA purification feed. The experimental validation of the two models has been presented, and the competitive adsorption of a quaternary mixture of H2, CO2, CO, and CH4 over activated carbon and zeolite 5A adsorbents has been considered individually. Adsorption equilibrium isotherms of a quaternary gas mixture on activated carbon and zeolite 5A were determined between 1 to 18 bar and at various temperatures (25, 35, and 45 °C). The reported isotherms in this work are an important part of the design requirements of a two-layer PSA process to purify H2. The results of experiments cover the appropriate conditions of a PSA process for pure hydrogen production. It should be noted that compared to pure component adsorption the experimental data for competitive adsorption in gas mixtures is very scarce. As a consequence, most of these experimental works have been investigated with simplified conditions for the number of components that are adsorbed. Therefore, the expected accuracy of computations based on these results in a real situation of a particular multicomponent Received: April 15, 2016 Accepted: September 2, 2016

A

DOI: 10.1021/acs.jced.6b00312 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

carefully and were added to the vessel volume. To check the measured total system volume, a test was accomplished without adsorbent, and the total volume was calculated by the difference between the loading cell pressure and the final equilibrium pressure of the system.The total amount of inserted gas and the gas remaining after the adsorption step were determined by the volumetric method with P−V−T measurements. The two cells and connections were placed in a water bath with a controlled constant temperature for the adsorption test. The gas mixture at constant pressure was connected to the loading cell while the desired cell pressure was achieved. After the stabilization of the pressure and temperature in the cell, upon opening the valve connecting the two cells, the adsorption step was carried out. This occurs within 20 min, and thus the equilibrium pressure is measured about 20 min after the commencement of the adsorption step. Some of the preliminary experiments were done in more than 1.5 h to determine the equilibrium condition, and the decrease in total pressure (equilibrium pressure) and temperature fluctuations versus time were recorded. Actually, the pressure and temperature changes after 20 min were very small. Each vessel and connection was placed in a water bath in which the high convective coefficient of water allows the system to reach thermal stability quickly. After equilibrium is attained, the valve between the two vessels is closed and sampling from the loading cell for molar concentration analysis may take place. The mass spectroscopy (MS) from Leda Mass is used for the continuous analysis of the multicomponent gas sample. As a result, the apparent partial pressures (PPs) of the gas mixture components versus time are obtained. However, these PPs are not true partial pressures. The cracking pattern parameters, ionization sensitivities, and also baseline subtraction for each component should be used to calculate the real partial pressures and mole fractions.19 On the basis of the obtained data, the masses of adsorbed components (H2, CO2, CO, and CH4) can be determined. The mass balance for each component gives

mixture adsorbed at a given pressure and temperature on an adsorbent is not very high. In most studies on the multicomponent adsorption of a gas mixture consisting of H2, CO2, CO, and CH4 for the simulation of PSA systems for hydrogen purification, individual (single) component adsorption isotherms have been used. Consequently, the novelty of the present study is the investigation of the competitive adsorption of a multicomponent gas mixture using a common feed composition for a PSA unit for hydrogen purification. The coefficients of mathematical relations for the multicomponent adsorption of H2, CO2, CO, and CH4 on activated carbon and zeolite 5A have been presented in this work.

2. EXPERIMENTAL SECTION 2.1. Adsorbents and Gaseous Mixture. The adsorbents employed in this work are activated carbon and zeolite 5A. The activated carbon is an industrial-type granule used in natural gas sweetening plants with amine solutions, with about a 1120 m2/ g specific surface area. Zeolite 5A is from Srici Co., China, with the trade name LQ-MS-05 molecular sieves in the shape of spheres with a 4 mm diameter. The BET surface area for zeolite 5A is about 800 m2/g. The gaseous mixture that is used is composed of H2 (75.5 mol %), CO2 (16.5 mol %), CO (4.5 mol %), and CH4 (3.5 mol %). It simulates a PSA hydrogen-purification feed, obtained from a catalytic steam reformer where the high-temperature water−gas shift reaction also takes place. 2.2. Equipment and Procedure. The static volumetric method was considered to design the experimental apparatus, whose diagram is presented in Figure 1. The apparatus consist

mi1 = mi2 + miads

(1)

where m1i is the mass of component i of the gas mixture supplied to the loading cell prior to adsorption, m2i is the mass of component i of gas remaining in the loading cell after the adsorption equilibrium state, and mads i is the mass of component i being adsorbed on a certain mass of the adsorbent. The initial mass of the components can be calculated from the relation mi1 = Miyi1n t

(2)

where Mi is the molecular weight, y1i is the initial mole fraction of component i, and nt is the total number of moles of the gas originally supplied. nt is calculated by the following equation of state (ideal gas law)

nt = Figure 1. Schematic diagram of the adsorption apparatus.

P1V LC RTZ1

(3)

where Z1 is the compressibility factor of the gas mixture and VLC is the volume of the loading cell. The numerical value of Z1 can be calculated from an analytical EOS, based on values from pressure and temperature. Calculated compressibility factors were between 0.996 and 0.999. Similar to eqs 2 and 3, the final mass of component i in the gas sample taken from the loading cell after the equilibrium state is as follows

of two cells, a pressure gauge, a pressure transmitter, a constanttemperature water bath, a vacuum pump, and valves. All parts are stainless steel, and all lines are 1/4 in. nominal pipe size. The loading cell and adsorption cell volumes are 260 and 245 cm3, respectively. The cell volumes were measured using the volume of water required to fill them. Moreover, to define the volume of connections, their inner diameter and length were measured B



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Table 1. Equilibrium Data for Activated Carbon (Adsorbed Amount of Gas Components and Molar Fraction of Remaining Gas) final molar fraction of the gas phase after equilibrium

adsorbed amount (mmol/g) temperature (K)

equilibrium pressure (bar)

H2

CH4

CO2

CO

total

H2 (%)

CH4 (%)

CO2 (%)

CO (%)

298 298 298 298 298 298 308 308 308 308 308 308 318 318 318 318 318 318

2.08 4.82 9.24 12.295 15.445 18.56 2.1 4.85 9.32 12.43 15.532 18.76 2.11 5.02 9.38 12.48 15.62 18.77

0.0315 0.0877 0.1434 0.1889 0.2234 0.2516 0.0382 0.0743 0.1367 0.1626 0.1906 0.2202 0.0326 0.0713 0.1111 0.1335 0.1522 0.1749

0.0459 0.1109 0.1885 0.2263 0.2676 0.2989 0.0396 0.0843 0.1441 0.1812 0.2074 0.2308 0.0289 0.0696 0.1106 0.1437 0.1583 0.1838

0.7298 1.5360 2.4552 3.1422 3.8625 4.1674 0.6325 1.3382 2.3665 2.7404 3.4165 3.7774 0.6051 1.1936 1.9972 2.4468 3.0472 3.3721

0.0207 0.0593 0.1058 0.1419 0.1562 0.1832 0.0197 0.0437 0.0801 0.1065 0.1267 0.1373 0.0178 0.0450 0.0655 0.0809 0.0999 0.1127

0.8279 1.7939 2.8928 3.6993 4.5097 4.9012 0.7300 1.5405 2.7274 3.1908 3.9411 4.3658 0.6844 1.3795 2.2845 2.8049 3.4577 3.8435

83.58 83.42 81.61 81.42 81.00 80.86 82.86 82.48 81.52 80.79 80.70 80.26 82.62 81.83 80.68 80.22 80.24 79.79

3.27 3.20 3.24 3.28 3.31 3.30 3.30 3.31 3.35 3.34 3.37 3.38 3.43 3.36 3.40 3.39 3.43 3.42

8.42 8.75 10.58 10.74 11.11 11.28 9.15 9.55 10.50 11.29 11.34 11.76 9.26 10.20 11.29 11.78 11.72 12.19

4.73 4.63 4.57 4.56 4.58 4.56 4.69 4.66 4.63 4.59 4.59 4.59 4.69 4.61 4.62 4.61 4.61 4.59

Table 2. Equilibrium Data for Zeolite 5A (Adsorbed Amount of Gas Components and Molar Fraction of Remaining Gas) final molar fraction of gas phase after equilibrium

adsorbed amount (mmol/g) temperature (K)

equilibrium pressure (bar)

H2

CH4

CO2

CO

total

H2(%)

CH4 (%)

CO2 (%)

CO (%)

298 298 298 298 298 298 308 308 308 308 308 308 318 318 318 318 318 318

1.995 4.54 9.532 12.633 16.69 18.59 1.98 4.565 9.48 12.59 15.92 19.26 2.04 4.65 9.45 12.8 15.93 19.37

0.0275 0.0453 0.0552 0.0636 0.0680 0.0714 0.0203 0.0365 0.0480 0.0554 0.0582 0.0653 0.0161 0.0250 0.0410 0.0482 0.0515 0.0576

0.0412 0.0676 0.1046 0.1158 0.1301 0.1328 0.0338 0.0613 0.0964 0.1118 0.1216 0.1366 0.0276 0.0527 0.0862 0.1029 0.1174 0.1256

0.6959 1.0475 1.4032 1.5325 1.6621 1.7178 0.5472 0.8579 1.1842 1.3134 1.4274 1.5134 0.4388 0.7026 0.9956 1.1243 1.2297 1.3177

0.1489 0.2292 0.2960 0.3256 0.3474 0.3659 0.1468 0.2131 0.2911 0.3120 0.3385 0.3543 0.1298 0.1932 0.2628 0.2865 0.3077 0.3273

0.9135 1.3897 1.8589 2.0375 2.2077 2.2879 0.7481 1.1689 1.6197 1.7926 1.9457 2.0697 0.6122 0.9736 1.3856 1.5618 1.7063 1.8283

84.63 81.62 79.42 78.73 78.16 77.97 83.34 80.80 79.06 78.46 78.04 77.74 81.99 80.01 78.64 78.11 77.81 77.52

3.37 3.38 3.39 3.40 3.41 3.42 3.39 3.37 3.38 3.39 3.40 3.40 3.41 3.38 3.38 3.39 3.39 3.41

8.99 11.51 13.32 13.88 14.34 14.50 10.40 12.33 13.72 14.18 14.51 14.75 11.58 13.06 14.11 14.50 14.73 14.95

3.01 3.48 3.88 3.99 4.09 4.11 2.87 3.49 3.84 3.97 4.05 4.11 3.01 3.55 3.87 4.00 4.07 4.13

mi2 = Miyi2 n2 n2 =

P 2(V LC + V AC − V ads) RTZ2

gas mixture were measured at 25, 35, and 45 °C and between 1 and 18 bar. All data obtained at equilibrium including the molar fraction of the remaining gas and the adsorbed amount of gases are given in Tables 1 and 2. The data of adsorption equilibrium are shown in Figures 2 to 7 for various gas components and adsorbents studied in this work. The adsorbed amounts of the different ingredients of the gas mixture on activated carbon follow the order of CO2 > CH4 > CO > H2, whereas the order is CO2 > CO > CH4 > H2 for zeolite 5A. The adsorption capacity of H2, CO2, and CH4 on activated carbon is more than that on zeolite 5A. For the polar CO molecule, the adsorbed amount on zeolite 5A is more than for activated carbon. For activated carbon, the adsorbed CO2 at maximum pressure and minimum temperature is 4.167 mmol/ g.

(4)

(5)

where VAC and Vads are the volumes of the adsorption cell and adsorbent, respectively. Equilibrium experiments were performed for activated carbon and zeolite 5A at temperatures between 25 and 45 °C and pressures between 1 and 18 bar. The whole test was done by using helium with and without adsorbent to define the adsorbent volume based on the difference in final pressures.

3. RESULTS AND DISCUSSION Adsorption equilibrium isotherms of a gas mixture containing carbon dioxide, methane, carbon monoxide, and hydrogen in a C



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Figure 2. Equilibrium isotherms and amount of gas adsorbed on activated carbon at 25 °C.


The adsorbed amount of CO on zeolite 5A at 18.6 bar and 25 °C is 99.7% more than on activated carbon under the same conditions. By increasing the temperature from 25 to 45 °C at the maximum operating pressure, the adsorption capacities of CO decrease to 62.6 and 11.8% for activated carbon and zeolite 5A, respectively. For CH4, the maximum amount of adsorption on activated carbon is 125% more than on zeolite A. From 2 to 18.6 bar at minimum temperature, the adsorption capacity of CH4 on activated carbon increases to about 300% in comparison to the

For zeolite 5A, the amount of CO2 adsorbed increases with high steepness from the minimum pressure to the middle pressures. Therefore, CO2 should be adsorbed in first adsorbent layer (activated carbon). Adsorbed amounts of CO2 at 25 °C from 2 to 18.6 bar increase 471 and 146% in the cases of activated carbon and zeolite 5A, respectively. At maximum pressure and from 45 to 25 °C, the adsorption capacity of CO2 on activated carbon increases about 23.6%, whereas for zeolite 5A this value amounts to 30.3%. D



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Figure 5. Equilibrium isotherms and amount of gas adsorbed on zeolite 5A at 25 °C.

for a PSA system, considering the higher density of zeolite, more adsorbent may be packed in a fixed column volume, which improves the process productivity. One of the models utilized for the description of experimental adsorption equilibria in gas−solid systems is the Langmuir model. This model is frequently employed in analyzing the experimental data of adsorption for a gas mixture of H2, CO2, CO, and CH4, on both activated carbons and zeolites.20,21 Also, the multicomponent Langmuir−Freundlich isotherm or loading ratio correlation is employed. In this study, experimental adsorption isotherms for H2, CO2, CO, and CH4

lower value of 222% for zeolite 5A. Also, by decreasing the temperature of the adsorption system from 45 to 25 °C at maximum pressure, the adsorbed amount of CH4 increases to 62.6 and 5.8% for activated carbon and zeolite adsorbents, respectively. From the experimental data shown in Figures 2 to 7, it can be observed that activated carbon should be utilized as the primary layer in a PSA arrangement. On this first layer, CO2, CH4, and CO can be adsorbed. The second layer (zeolite 5A) will adsorbe the rest of the methane and carbon monoxide. By employing a second layer of zeolite in the adsorption column E



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over the activated carbon and zeolite 5A were determined over a wide range of pressure (1 to 18 bar) at three temperatures (25, 35, and 45 °C). The measured data were correlated by the loading ratio correlation (LRC) as22−24 q* = i

qsibipini (6)

Generally, all of its parameters can be considered to be a function of temperature: qsi = a1i + a 2iT

(8)

⎛a ⎞ ni = a5i exp⎜ 6i ⎟ ⎝T ⎠

(9)

Another approach used in analyzing multicomponent equilibria is the Langmuir model, as follows:

N

1 + ∑ j = 1 bjpjnj

⎛a ⎞ bi = a3i exp⎜ 4i ⎟ ⎝T ⎠

qi* =

(7)

qsiBi Pyi N

1 + ∑ j = 1 Bj Pyj

(10)

The parameters of this equation can be determined as F



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Table 3. LRC Coefficients for Activated Carbon and Zeolite 5A adsorbent

a1 (mol/kg)

gases

activated carbon

zeolite 5A

−19.514 22.531 81.525 18.05 4.9763 2.7446 10.447 12.165

H2 CH4 CO2 CO H2 CH4 CO2 CO

a3 (bar−n)

a2 (mol/kg K) 8.320 −5.310 −2.210 −3.640 −5.700 −1.600 −2.250 −2.830

× × × × × × × ×

−2

10 10−2 10−1 10−2 10−3 10−3 10−2 10−2

9.681 1.813 6.416 2.049 3.851 4.370 1.683 1.106

× × × × × × × ×

10−2 10−4 10−4 10−6 10−8 10−2 10−3 10−1

a4 (K)

a5

a6 (K)

−836.6 1944.4 1660.2 3068.3 3722.7 415.29 1729.1 285.95

−1.2371 2.9471 2.308 2.2639 3.7912 2.1478 1.9711 1.2278

630.31 −588.49 −368.67 −320.03 −949.36 −468.58 −378.92 −166.04

Table 4. Langmuir Coefficients for Activated Carbon and Zeolite 5A adsorbent

gases

a1 (mol/kg)

a2 (mol/kg K)

activated carbon

H2 CH4 CO2 CO H2 CH4 CO2 CO

−23.131 −9.5592 7.567 −16.668 −28.68 −4.024 −3.3088 −21.23

8069.1 4638.5 2204.6 7548 10 161 2201 1982.5 10 183

zeolite 5A

qsi = a1i +

⎛ a 2i ⎞ ⎜ ⎟ ⎝T ⎠

⎛b ⎞ Bi = b1i exp⎜ 2i ⎟ ⎝ RT ⎠

b1 (bar) 2.248 3.725 2.051 5.534 8.619 1.868 1.607 9.001

× × × × × × × ×

100 10−4 10−3 10−4 10−4 10−3 10−4 10−4

b2 (J/mol) −1.435 1.443 1.069 1.065 3.34 1.15 2.01 1.18

× × × × × × × ×

104 104 104 104 103 104 104 104

the order is CO2 > CO > CH4 > H2 for zeolite 5A. The adsorbed amount of CO2 on zeolite 5A increased quickly at low pressures, and disruption at these pressures is hard. Hence, CO2 should not reach into the zeolite layer. The adsorption equilibrium data of all components for both adsorbents were correlated using the Langmuir and LRC models. Both models were able to fit the experimental data for the zeolite with good accuracy. It was observed that the adsorption amount for CO and CO2 is increased from minimum to middle pressures, rapidly. At higher pressures, the steepness of adsorbed amount is decreased. Thus, because of exponent n in the LRC relationship, this model could fit the experimental data more accurately. It can be concluded that the LRC model well predicted the adsorbed amount of components on zeolite 5A, and the Langmuir model showed better results to fit the adsorption data for activated carbon.

(11)

(12)

The adsorption data of all components on both adsorbents (Figures 2 to 7) were fitted using the loading ratio correlation and Langmuir models. The parameters obtained for all gases are presented in Tables 3 and 4. Mean errors in the Langmuir model are 1.5 and 5.1% for activated carbon and zeolite 5A, respectively. Also, average error values of the LRC model for all components are 9 and 0.8% for activated carbon and zeolite 5A, respectively. The Langmuir model is able to fit the data with acceptable precision under temperature and pressure ranges for the mixture of four gas components on activated carbon. However, the LRC model shows some incompatibilities in the prediction of CO and CO2 adsorption isotherms on activated carbon and could not fit all of the experimental data with acceptable accuracy. The obtained results for activated carbon of the LRC model at lower pressures and 45 °C show more errors in comparison with other process conditions. Both the Langmuir and LRC models are able to fit the experimental data for the zeolite with a good accuracy. The LRC model can cover all data better than the Langmuir model can. It is observed that the extent of adsorption for CO and CO2 increases quite rapidly from minimum to middle pressures. At higher pressures, the steepness of the adsorption rate decreases. Consequently, because of exponent n in the LRC relationship, this model could fit the experimental data more accurately.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS The multicomponent competitive adsorption of a gas mixture was measured in the temperature span of 25−45 °C and the pressure range of 1−18 bar on activated carbon and zeolite 5A, individually. The adsorbed capacity of the components on activated carbon follows the order CO2 > CH4 > CO > H2, and G



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