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Novel Differential Column Method for Measuring Multicomponent Gas Adsorption Isotherms in NaY Zeolite Babak Shirani and Mladen Eic* Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 S Supporting Information *

ABSTRACT: Separation and purification of gas mixtures using selective adsorbents is widely used in different industries such as gas drying, air separation, and H2 purification. Equilibrium analysis involving adsorption of binary gas mixtures provides important information related to the adsorbent performance in the separation of gases. In this study, a novel technique termed “differential column technique” was developed for binary isotherm measurement employing streams containing carbon dioxide, carbon monoxide, and ethylene at different compositions. This technique is based on measuring the gas desorption by changing equilibrium pressure conditions. The isotherm curve was generated by summing desorption amounts desorbed at each pressure step. Through the application of this technique, the single-component isotherms of CO2, CO, and ethylene on zeolite NaY were measured, and the isotherms were compared to the results obtained by a standard gravimetric technique. (The average relative deviation is less than 6%.) The main advantage of the technique is the significant time savings, e.g., one experimental run is required to generate an isotherm compared to multirun experiments using a standard breakthrough technique, in addition to using a simpler experimental setup and generally smaller amount of sample (agglomerated or in a powder form). Another important feature of this technique is the relatively simple extension that allows measurements of gas mixture equilibria. As such, the proposed technique has the potential to be used as a fast screening technique for adsorbent selection based on single-component or mixture analysis. To investigate the consistency of the proposed technique, the binary isotherms of competitive, CO2−C2H4, and noncompetitive, CO2−CO, mixtures were investigated at different gas compositions. In addition, the effects of sorbate concentrations in the gas phase and interactions with the NaY zeolite active surface were investigated in relation to the adsorption selectivity and capacity, i.e., strong interaction of both CO2 and ethylene with NaY site resulted in close adsorption selectivity 0.8 ≤ SCO2/C2H4 ≤ 1.7, while CO2 adsorbed more selectively compared to CO, 14 ≤ SCO2/CO ≤ 30, as a result of weak CO interaction with the adsorbent sites. Finally, the binary adsorption isotherms and selectivity were predicted by the multisite Langmuir model using the single component’s isotherm parameters. Modest agreements (error ≤ 28%) were obtained between the predicted and experimental results.

1. INTRODUCTION Separation techniques based on adsorption have been widely used in industries for purification of feed and product streams. In addition to the high efficiency and high loading capacity, adsorption processes provide low cost and low energy consumption, which make them excellent candidates for separation of gases and liquids. The performance of an adsorption process is mainly dependent on the capacity and selectivity of porous solids called adsorbents.1 To determine the performance of the adsorbent for separation of gas mixtures, intensive investigations have to be carried out on adsorption equilibrium and kinetics of the gas components.2−6 Different analytical techniques, such as gravimetric,7,8 volumetric,9,10 inverse pulse chromatography,11,12 breakthrough measurements,13−15 zero length chromatography (ZLC),16−19 total desorption method,20,21 and isotope exchange technique,22 have been utilized to determine the equilibrium capacity and selectivity of adsorbents for gas (vapor) adsorption and separation. These techniques have been used for singlecomponent gas adsorption and some, such as breakthrough, chromatography, ZLC, and isotope exchange techniques, were © XXXX American Chemical Society

extended to measure multicomponent adsorption isotherms. Compared to single-component adsorption isotherm measurements, multicomponent adsorption isotherm measurements and analysis are much more challenging. Determination of the multicomponent isotherms, i.e., equilibrium analysis, provides the necessary information for designing adsorption systems for separation and purification of the mixtures. Because measuring the multicomponent isotherms presents experimental challenges and is time-consuming, the goal of this study was to investigate a novel methodology to measure the multicomponent isotherms accurately and time effectively utilizing a simple experimental setup. In that regard, this method can be applied for fast screening of a large number of adsorbent candidates by generation of multicomponent isotherms, and based on them, selection of the best adsorbent for a mixture separation of interest can be made. As with the Received: Revised: Accepted: Published: A

September 11, 2016 November 28, 2016 January 6, 2017 January 6, 2017 DOI: 10.1021/acs.iecr.6b03525 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Textural and Chemical Properties of NaY Zeolite adsorbent NaY zeolite

pellet diameter (cm)

pellet length (cm)

particle density (g/mL)

SBET (m2/g)

total pore volume, vt (cm3/g)

micropore volume (cm3/g)

micropore diameter (nm)

Si/Al

∼0.17

0.27−0.43

1.11

567.4

0.439

0.401

0.74

1.22

Figure 1. Differential column experimental setup. MFC, mass flow controller; P, pressure transducer; BPR, back-pressure regulator.

isotope exchange technique,22 the new method is based on multistep desorption and associated effluent concentration measurements using mass spectrometry. The single and binary adsorption of carbon dioxide, carbon monoxide, and ethylene were investigated on a commercial NaY zeolite. On the basis of the single-component isotherms, NaY zeolite exhibited higher selectivity for separation of carbon dioxide from carbon monoxide compared to carbon dioxide separation from ethylene. The consistency of the novel technique was checked by measuring the single-component adsorption isotherms of carbon dioxide, carbon monoxide, and ethylene using the proposed differential column technique and comparing with the results obtained by standard gravimetric techniques. The novel differential column technique was further extended to measure the binary isotherms of competitive, e.g., carbon dioxide−ethylene, and noncompetitive, e.g., carbon dioxide−carbon monoxide, mixtures in NaY zeolite. Using this technique, the adsorption isotherms of mixtures with different compositions of CO2−CO and CO2− C2H4 were measured on NaY zeolite at 25 °C. The results indicated that despite the higher CO concentration in the gas phase compared to CO2, NaY zeolite showed higher affinity to CO2 molecules. As a result, even a small CO2 concentration in the gas phase prevented the CO adsorption. In comparison to CO, ethylene molecules create stronger bonds to the adsorption sites, thus exhibiting a more comparable adsorption affinity to carbon dioxide. Therefore, the adsorption of ethylene and CO2 gas mixture is more competitive compared to the CO2−CO mixture. The multisite Langmuir model was successfully applied to predict the binary adsorption isotherms applying the parameters estimated from single-component isotherms. Compared to the conventional multicomponent adsorption measurement techniques, e.g., chromatographic and breakthrough techniques, the proposed method has better accuracy and requires a simpler experimental setup; in addition, it is more time-effective. The main drawback of this technique is its requirement of a relatively large amount of adsorbent samples.

bp =

(q/qs) (1 − q/qs)a

(1)

Parameter a is an empirical constant representing the number of active sites occupied by each sorbate molecule (qsi·ai for all sorbate in the mixture should be a constant value to satisfy mixture thermodynamic consistency24). If a number of active sites approaches unity, the model reduces to the regular (one-site) Langmuir model.23 Equation 1 is an approximation of Nitta’s equation by neglecting interactions between the adsorbed species due to stronger bond formation between sorbate molecules and adsorbent active sites. The above model can be extended for predicting multicomponents isotherms: bipi =

(qi /qsi) (1 − ∑i qi /qsi)ai

(2)

3. EXPERIMENTAL SECTION 3.1. Materials. Chemically pure (≥99%) carbon dioxide, ethylene, carbon monoxide, and alphagaz helium (purity ≥ 99.999%) were supplied by Air Liquid Canada Inc. The NaY zeolite used in this study was provided by Xebec Adsorption Co. (Blainville, Quebec, Canada) as a proprietary sample. The adsorbents were in the form of cylindrical pellets. The scanning electron microscopy (SEM), transmission electron micrsocopy (TEM), and chemical elemental analyses were performed on the samples and the SEM and TEM micrographs are shown in Figure S1 of the Supporting Information. According to the TEM micrographs, NaY zeolite pores are ordered and parallel. Elemental analysis was also performed on the NaY sample, and the results are displayed in Figure S2. 3.2. Textural Characterization. Pore diameter, pore volume, and surface area of the NaY zeolite were determined from N2 adsorption isotherm at 77 K. The isotherm was measured by volumetric method using a Belsorp-max (Bell Japan Inc.) instrument. In this measurement, around 100 mg sample of zeolite NaY was pretreated overnight at the temperature of 130 °C and under vacuum. The N2 isotherm was measured from ∼3 × 10−4 kPa up to atmospheric pressure and is shown in Figure S3. Textural properties determined from the isotherm are summarized in Table 1. The total pore and

2. EQUILIBRIUM ISOTHERM MODEL The multisite Langmuir was utilized to predict the singlecomponent isotherms as follows:23 B

DOI: 10.1021/acs.iecr.6b03525 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 2. Pressure differential procedure for 30% CO2−70% C2H4 mixture: exit concentration comparisons for blank column and with NaY adsorbent. (a) Carbon dioxide: step 1, ΔPCO2 = 28.9−13.9 kPa; step 2, ΔPCO2 = 13.9−6.4 kPa; step 3, ΔPCO2 = 6.4−0 kPa. (b) Ethylene: step 1, ΔPC2H4= 67.5−32.5 kPa; step 2, ΔPC2H4 = 32.5−15 kPa; step 3, ΔPC2H4 = 15−0 kPa.

micropore volumes were determined from N2 isotherm at P/ Psat = 0.99 and αs-plot analysis performed at P/Psat = 0.4, respectively. The mean micropore diameter was determined using the Horvath−Kawazoe method. 3.3. Gravimetric Method. An Intelligent Gravimetric Analyzer (IGA-003) by Hiden-Isochema Ltd. was used to measure single-component adsorption isotherms. The instrument contains a highly sensitive microbalance with a weighing resolution of ±0.1 μg inside a stainless steel chamber that is connected to a vacuum pumping station. The system temperature was kept constant by surrounding the adsorption chamber with a water jacket connected to recycling water passing through a thermo-bath. The adsorption isotherms and the corresponding uptakes, at each pressure step, were measured, and for each pressure step, sufficient time (∼5 h) was allowed to reach equilibrium (less than 1% change in the uptake amount). During this time, the changes of the sample weight were recorded and processed by IGA software. Before the adsorption step, the sample pretreatment was carried out overnight at 130 °C and under vacuum. The isotherm measurements were repeated at least two times, and the results were close to each other, e.g., less than 2% difference between the uptake amount at the same pressure step. 3.4. Experimental Setup and Analysis of Binary Isotherms. The binary adsorption isotherms were measured by the differential column technique using the experimental setup displayed in Figure 1. As shown in the figure, three mass flow controllers (MFC) were used to adjust the flow rates of carbon dioxide−carbon monoxide or carbon dioxide−ethylene binary mixtures with helium as a carrier gas. The pure gases exiting the mass flow controllers were mixed and flowed to the adsorption column. The diameter and length of the column were 2.54 and 3 cm, respectively (this column holds about 7 g of NaY adsorbent). The column was placed in a GC oven to keep it at constant temperature. The flow rate of the gas mixture flowing to the column was set to a high rate (∼350 mL/min) to keep temperature changes due to the adsorption heat effects negligible. According to Sircar,25 although the flow of feed gas facilitates heat removal from adsorbents, small temperature increases are expected even at high flow rates.

While negligible temperature changes of adsorbent result in considerable changes of the adsorption kinetic rate constants,25 in this study, small temperature effects on the adsorption equilibria, e.g., uptake amounts, were considered negligible. To increase the column pressure above atmospheric pressure (up to 3 bar), a back-pressure regulator was installed at the column’s exit. The concentrations of the sorbates in the mixture exiting the column were measured continuously using a residual gas analyzer (QMS 300 Gas Analyzer, Stanford Research System). Using the setup shown in Figure 1, a known composition of mixture of CO2−CO and CO2−C2H4 at high concentrations (partial pressures) was fed to the regenerated adsorbent in the column at constant temperature and total pressure until the adsorbent was saturated, i.e., the exiting concentrations of gas sorbates, measured by residual gas analysis (RGA), were constant. After the equilibrium between the feed stream and adsorbent was established, the concentrations of the sorbates were lowered proportionally by introducing a known flow rate of helium stream to the feed and keeping the total flow rate constant, thus effectively reducing the flow rates of sorbates to the mixture with He. This mode of operation caused the change in the equilibrium condition in the column resulting in desorption of the adsorbed species until a new equilibrium between the feed and adsorbent was established. The desorption steps were repeated until only pure helium was running through the column, thus creating conditions for the total desorption of adsorbed species at the end of the experimental step. To confirm the complete desorption of the adsorbed sorbates, heat was also applied to regenerate the adsorbent in the last step. At the end of the last step, i.e., pure helium in the feed stream, the temperature of the column was increased to 130 °C to regenerate the sample more effectively. After being cooled, the column was weighed to check if the weight of regenerated (unloaded) adsorbent stayed constant after the experiment, confirming that the regeneration was complete. Sample graphs demonstrating stepwise concentration reductions of binary feed stream, consisting of carbon dioxide (30%) and ethylene (70%) with time are displayed in Figure 2. The partial pressures of carbon dioxide and ethylene were C


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Figure 3. Gravimetric single-component adsorption isotherms: (a) carbon dioxide, (b) carbon monoxide, and (c) ethylene on NaY zeolite. Dashed lines are multisite Langmuir model predictions (symbols represent experimental data).

⎛ P ⎞ nstep i = Q ⎜ t ⎟ ⎝ R·T ⎠

reduced from 28.9 and 67.5 kPa, respectively, to 0 kPa (corresponding to a pure He flow in the last step). To measure the adsorbed amounts of sorbate gases, a blank test was also performed using an empty column at similar operating conditions, i.e., total pressure, temperature, feed composition, and flow rate, as for the column’s operation containing adsorbent. The blank measurement results for 30% CO2− 70% C2H4 feed stream are also shown in Figure 2. When desorption graphs from the blank and adsorbent packed columns are compared, it is clear that the steady-state concentrations, graph plateaus, are similar for both columns (the changes in the gas phase composition are similar for both columns at each pressure step). Changes in the exiting sorbates’ concentrations, after reduction in the feed concentrations, were followed simultaneously by the blank measurements (shown as sharp declines in CO2−C2H4 desorption diagrams of Figure 2a,b), thus confirming that the steady-state conditions were reached in a short period of time. The amounts of species desorbed by concentration (partial pressure) reduction were determined by comparing the sorbate desorption curve from the adsorbent-loaded column with the blank curve at each pressure step. The areas under the sample desorption and blank curves (Figure 2a,b) were evaluated, and the desorbed amounts of adsorbed species were calculated according to the following equation:

∫0

∞

(ySample − yBlank )dt

(3)

In eq 3, nstep i is the CO2, C2H4, or CO amount desorbed at each pressure step (mmol/g adsorbent). The integral term in eq 3 is defined by eq 4 as follows: nstep i =

Q ·Pt (ASample − ABlank ) R·T

(4)

The units of A in eq 4 are given in mole fraction·sec. To determine yCO2 and yCO from the RGA signals, calibration curves were generated for the mixtures. A sample calibration curve is presented in section S2 of the Supporting Information. Keeping the total flow rate (Q) and total pressure (Pt) constant for all pressure steps, the number of moles of species desorbed at each step by pressure reduction was determined from high-pressure CO2−C2H4 mixture to 0, pure helium. Eventually, by summing the desorbed quantities from 0 up to the desired pressure and dividing the resulting value by the mass of the regenerated adsorbent, the adsorption amount at each pressure step was determined, as follows: n Total, i = D

nstep i + nstep i + 1 + nstep i + 2 + ... + nstep N Adsorbent mass

(5)


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Industrial & Engineering Chemistry Research Table 2. Multisite Langmuir Isotherm Model Parameters at T = 25 °C adsorbate

qs (mmol/g)

b (1/kPa)

a

avg. rel. error (%)

Ko (mmol/g·kPa)

−ΔH0 (kJ/mol)

carbon dioxide carbon monoxide ethylene

5.922 4.967 4.036

0.0365 0.0016 0.1308

1.848 1.945 2.707

5.70 4.04 6.12

0.1915 0.0193 0.2337

25.47 4.23 14.01

Figure 4. Comparison of single-component isotherms obtained by gravimetric (IGA) and differential column techniques for (a) carbon dioxide, (b) carbon monoxide, and (c) ethylene on zeolite NaY at T = 25 °C. (Symbols represent experimental data.)

where nTotal i is the total amount of CO2/CO/C2H4 sorbate adsorbed at partial pressure i (Pi) and nstep N is the amount desorbed at the last step by purging with pure helium. To minimize the effect of dead volume, a small diameter, 1/8 in. tubing, was used from the inlet valve to the RGA detector. In addition, the length of the gas lines after the inlet valve was minimized.

The multisite Langmuir model for single-component adsorption (eq 1) was fitted against isotherms’ experimental data for all temperatures. Because the binary isotherm measurements were performed at T = 25 °C, only the isotherm model parameters for this temperature are summarized in Table 2. For the other temperatures, the parameters’ data are provided in the Supporting Information (Table S1). The average relative errors displayed in Table 2 represent the isotherm model’s mean deviation from the experimental data and are calculated as follows:

4. RESULTS AND DISCUSSION 4.1. Single-Component Adsorption Isotherms. The single-component adsorption isotherms of carbon dioxide, carbon monoxide, and ethylene at four different temperatures (10, 20, 25, and 30 °C) were measured using gravimetric techniques (for comparison with the proposed technique), and the results are shown in Figure 3a−c. All isotherms follow type I behavior according to the Burnauer classification.26 The multisite Langmuir model was applied for modeling the experimental data. In contrast to the standard (single-site) Langmuir model, in which a molecule is assumed to occupy only one active site, this model is based on a possibility of multisite occupation by a single molecule.23 Consequently, the effect of adsorption of a molecule on the neighboring adsorption sites is considered in this model.23

Average RError =

n ⎛ ∑i = 1 ⎜1 − ⎝

n

qi ,model ⎞ qi ,exptl

⎟ ⎠ (6)

where n is the number of experimental points. The errors, between the experimental and model’s results, are performance indicators of the isotherm model predictions of the experimental equilibrium results. From the isotherms’ gradients at pressures close to zero (0− 5 kPa), the values of Henry’s constants (K0 = ∂q∂p at p → 0) can be calculated and are shown in Table 2. The heat of adsorption at zero coverage (θ = 0) was calculated using the Clausius−Clapeyron equation:27 E


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Industrial & Engineering Chemistry Research ΔH0 (7) RT The values of −ΔH0, shown in Table 2, increase in the order of CO (lowest), ethylene, and CO2 (highest). This proves stronger bonds and greater interactions between carbon dioxide and NaY zeolite active sites compared to carbon monoxide and ethylene. Single-component isotherms at the same conditions (T = 25 °C and P = 300 kPa) were repeated using the proposed differential column technique, described in section 3.4. The total flow rate of gas feed stream (adsorbate and carrier gas) for all isotherm measurements was adjusted to 350 mL/min. Through calculation of the area difference between the desorption curves of the blank and NaY-loaded columns and subsequent application of eq 4, the adsorption capacity of CO2, CO, and C2H4 sorbates was calculated for each pressure step (nstep). Consequently, by summing up the nstep’s from 0 kPa to the specified pressure, using eq 5, the isotherm diagrams for single-component adsorption of carbon dioxide, carbon monoxide, and ethylene were generated and compared to the gravimetric isotherms as shown in panels a, b, and c of Figure 4, respectively. The average relative deviations (eq 6) between the two techniques were calculated and are summarized in Table 3.

simplified form of the multisite model that excludes sorbate− sorbate interactions. The average relative deviations of the extended multisite Langmuir model from the binary experimental data are summarized in Table 4. The deviations from experimental data are about 12−29%, which shows the multisite Langmuir model can be used to estimate the binary CO2−C2H4 isotherms with a reasonable accuracy. The selectivity of carbon dioxide with respect to ethylene was calculated using the following expression:

ln K 0 = ln K 0′ −

SCO2 /C2H4 =

adsorbate

carbon dioxide

carbon monoxide

ethylene

4.7

6.1

5.5

(q/p)C2H4

(8)

CO2/C2H4 selectivity values for the three binary mixtures are presented in Figure 6. According to this figure, NaY zeolite is more selective to carbon dioxide in comparison to ethylene, except at total pressures less than 35 kPa. The greater quadrupole moment of carbon dioxide (4.3 × 10 −26 esu cm2)28,29 compared to that of ethylene (3.41 × 10−26 esu cm2)30 results in creation of stronger bonds between CO2 molecules and NaY active sites compared to ethylene (also greater ΔHCO2 compared to ethylene, as shown in Table 2). However, the difference between quadrupole moments of these two sorbates is modest, resulting in the selectivity values close to 1. This clearly indicates competitive adsorption of carbon dioxide and ethylene. At the total pressure lower than atmospheric pressure, the selectivity is dependent on the total pressure, whereas at Pt > 100 kPa, the selectivity values do not change considerably with increasing the binary mixture total pressure. Consequently, the NaY zeolite surface acts more heterogeneously to the CO2/ C2H4 mixture at total pressure less than atmospheric, which causes nonideality of the adsorbed phase in this pressure range.31 When eq 2 is rewritten in terms of q/p for both carbon dioxide and ethylene and the corresponding equations for each species are divided, the following equation for estimation of selectivity using the multisite Langmuir model is developed:

Table 3. Deviation of Differential Column Technique from Gravimetric Method avg. rel. dev. (%)

(q/p)CO2

Based on Table 3, the deviations of the differential column technique isotherm data from the standard (considered very accurate) gravimetric technique are within ∼6%, which proves the reliability of the novel technique developed in this study. 4.2. Binary Mixture Isotherms. The differential column technique was extended to measure the binary adsorption isotherms of competitive binary sorbates with close adsorption capacity and selectivity, e.g., carbon dioxide and ethylene. To fully evaluate the effectiveness of the proposed technique, the binary isotherms of carbon dioxide and carbon monoxide, which form a noncompetitive binary mixture for adsorption on NaY, were also investigated. The multisite Langmuir model for mixtures (eq 2) was applied to predict the binary isotherm experimental data using single-component isotherm parameters (Table 2), and the results are presented in the following subsections. 4.2.1. Carbon Dioxide−Ethylene Binary Isotherms. The binary mixture isotherms of carbon dioxide and ethylene on NaY zeolite at three different compositions, i.e., 70% CO2−30% C2H4, 50% CO2−50% C2H4, and 30% CO2−70% C2H4, were measured at 25 °C. The total pressure of the binary gas mixture was set at 350 kPa, and the obtained isotherms are shown in Figure 5a−c. Multisite Langmuir model parameters obtained from the single-component experimental isotherm data of carbon dioxide and ethylene (Table 2) were applied to the extended multisite Langmuir model (eq 2), and the results are shown in Figure 5. According to Figure 5, the extended form of the multisite Langmuir model predicts the binary isotherms of CO2−C2H4 at all compositions satisfactorily. This is likely due to weak CO2− C2H4 interactions in the adsorbed phase, thus justifying the

(SCO2 /C2H4)model (aCO − aC H ) qC H ⎞ 2 2 4 qs,CO bCO ⎛ qCO 2 4 ⎟ 2 2 ⎜ 2 1− = − qs,C H bC2H4 ⎜⎝ qs,CO qs,C H ⎟⎠ 2 4 2 2 4

(9)

The selectivity is reduced to the simple Langmuir extended model if aCO2 = aC2H4. The selectivity predictions using the extended multisite Langmuir model (eq 9) are also shown in Figure 6. The model is obviously not successful in predicting selectivity, especially at low pressures. The multisite Langmuir model assumption is based on ideal adsorbed phase on homogeneous surfaces.23 Because faujasite zeolites are considered heterogeneous, which results in the adsorbed phase nonideality (γadsorbed phase < 1), deviations of the model from experimental results are expected, especially at low pressures (see Figures 5 and 6). 4.2.2. Carbon Dioxide−Carbon Monoxide Binary Mixture Isotherms. Carbon dioxide−carbon monoxide binary isotherms on NaY zeolite were measured at compositions of 40% CO2− 60% CO, 20% CO2−80% CO, and 10% CO2−90% CO. Because the adsorption capacity of carbon monoxide is not comparable to that of carbon dioxide binary mixtures, the higher concentrations of carbon monoxide were examined. The F


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Figure 5. Binary adsorption isotherms of carbon dioxide−ethylene on zeolite NaY at T = 25 °C: (a) 70% CO2−30% C2H4, (b) 50% CO2−50% C2H4, and (c) 30% CO2−70% C2H4. Dashed lines represent extended multisite Langmuir model. (Symbols represent experimental data.)

create stronger electrostatic interactions with the zeolite active sites. These bonds are much stronger than the electrostatic bonds created by carbon monoxide with the zeolite active sites.32 As a result, the adsorbed CO molecules are easily displaced by CO2 molecules regardless of their concentration in the gas phase. The CO2−CO binary isotherms for all compositions were also predicted using the extended multisite Langmuir model (Figure 7). As presented in Table 2, unlike for CO2−C2H4 mixture, the (qs × a)s for carbon dioxide and carbon monoxide are not exactly equal. According to Rao and Sircar,24 this results in thermodynamic inconsistency of the model predicting binary experimental isotherms. However, the (qs × a)s for the CO2− CO mixture are fairly close, i.e., (qs × a)CO2 = 10.9 and (qs × a)CO = 9.7, which is reasoned that this mixture obey thermodynamic consistency constraint to a satisfactory degree. The difference in (qs × a)s for CO2 and CO in the mixture is attributed to slight nonideality of the mixture adsorbing in NaY zeolite. According to Bai et al.,33 multisite Langmuir model is not a suitable model for nonideal mixtures and large differences in the adsorption capacity of components and their adsorption strengths result in different values of qs and a. Consequently, forcing the model to have equal qs × a for all components

Table 4. Average Relative Deviations (%) of the Multisite Langmuir Model Fittings from the Experimental Carbon Dioxide−Ethylene Isotherms avg. rel. deviation (RError %) mixture

CO2

C2H4

30% CO2−70% C2H4 50% CO2−50% C2H4 70% CO2−30% C2H4

19.4 17.4 12.3

25.2 28.1 19.4

binary isotherms were measured at 25 °C and the total pressure of 300 kPa, and the results are shown in Figure 7a. The results showed that CO adsorption was negligible compared to CO2 even at the high concentrations of carbon monoxide. To provide better clarity, the CO isotherms are shown in Figure 7b separately. Increasing the CO concentration from 60% to 90% in the gas phase results in a slight increase in the adsorption uptake of carbon monoxide, e.g., at p = 180 kPa, q60%CO ∼ 0.36 mmol/g and q90%CO ∼ 0.5 mmol/g. According to the information provided in section 4.2.1, the carbon dioxide quadrupole moment is 4.3 × 10−26 esu cm2 compared to a much smaller moment for carbon monoxide at 2.5 × 10−26 esu cm2. This clearly indicates that CO2 molecules G


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Figure 6. Dependence of CO2/C2H4 selectivity on the total pressure of the binary mixtures of 70% CO2−30% C2H4, 50% CO2−50% C2H4, and 30% CO2−70% C2H4 at the temperature of 25 °C. (Symbols represent experimental data; dashed lines represent extended multisite Langmuir model prediction.)

Figure 7. Binary adsorption isotherms of carbon dioxide and carbon monoxide in their respective mixtures on zeolite NaY: (a) CO2 and CO binary isotherms of 40% CO2−60% CO, 20% CO2−80% CO, and 10% CO2−90% CO (■, CO2 experimental isotherm points; ▲, CO experimental isotherm points); (b) carbon monoxide isotherms of 60, 80, and 90%. Dashed lines represent extended multisite Langmuir model.

causes inaccuracy in the multicomponent isotherms prediction. Also, the effect of surface heterogeneity, which causes additional interactions between adsorption sites and sorbate molecules, has to be also considered in thermodynamic inconsistency of the MSL model for nonideal systems.33 In contrast to considerable differences in adsorption strengths and capacities of CO2 and CO, their thermo-physical properties are not very different, which results in close (qs × a)s. The average deviations between the isotherm model and the experimental data are shown in Table 5. In comparison to CO2−C2H4 binary mixtures, the extended multisite Langmuir model predicted the CO2−CO isotherms with a better accuracy (9 < RError < 23.5), and given that, the model can be reliably applied to predict the CO2−CO binary isotherms. The selectivity of carbon dioxide/carbon monoxide was calculated using eq 8 and the results for the three CO2−CO mixtures are shown in Figure 8. As expected, the selectivity values are large (SCO2/CO ∼ 15−30) and gradually decrease by increasing the total pressure. The reduction of the selectivity by

Table 5. Average Relative Deviation (%) of the Multisite Langmuir Model Fittings to the Experimental Carbon Dioxide−Carbon Monoxide Isotherms avg. rel. deviation (RError %) mixture

CO2

CO

40% CO2−60% CO 20% CO2−80% CO 10% CO2−90% CO

15.7 18.6 23.3

14.5 9.1 8.6

increasing the pressure is an indication of nonideal behavior of the adsorbed phase. On the basis of Figure 8, by increasing the carbon dioxide concentration in the gas phase (from 10% to 40%), the selectivity changes over pressure were increased significantly, e.g., for 10% CO2−90% CO and 30 < Pt < 300, selectivity decreases from ∼28 to ∼22, whereas for 40% CO2− 60% CO and similar total pressure change, selectivity decreases from ∼30 to ∼14. This indicates that an increase in the carbon dioxide concentration enhances the interaction between CO2 H


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Figure 8. Dependence of CO2/CO selectivity on the total pressure of the binary mixtures of 40% CO2−60% CO, 20% CO2−80% CO, and 10% CO2−90% CO at 25 °C. (Symbols represent experimental data; dashed lines represent extended multisite Langmuir model prediction.)

single-component adsorption isotherms of carbon dioxide, ethylene, and carbon monoxide were obtained using the differential column technique, and the results were compared to those determined by the gravimetric technique. The small deviations of the single-gas isotherms between these two techniques, i.e., error ≤ 6%, indicated the reliability of the new method for measuring adsorption isotherms. According to binary isotherm results, the adsorption capacities of carbon dioxide and ethylene on NaY zeolite were comparable, i.e., 0.8 ≤ SCO2/C2H4 ≤ 1.7, because these molecules have close quadrupole moments (QC2H4/CO2 = 0.8), making a case for the competitive adsorption. In contrast, carbon dioxide selectivity over carbon monoxide on the same adsorbent is significantly higher because of a noncompetitive adsorption of these species (14 ≤ SCO2/CO ≤ 30). The carbon monoxide quadrupole moment is significantly smaller than that of carbon dioxide, i.e., QCO/CO2= 0.58, and thus has a negligible effect on creating strong bonds with the adsorption sites. The multisite Langmuir model was used to predict the binary adsorption isotherms. Modest agreements were obtained between binary experimental results and the model predictions (with an average relative deviation ≤ 28%). However, the model failed to estimate the selectivity changes with total pressure. The most significant deviations from experimental selectivity were at low pressures. At low pressures below 100 kPa, the interactions between the adsorbed molecules are even more negligible because the molecules are further apart, but on the other hand, the interactions with adsorption sites are the strongest. Consequently, the effects of different molecular interactions with the adsorbent sites and the heterogeneity of the NaY surface were considered to be the main reasons for the adsorbed phase deviation from ideality.

and CO and reduces CO−CO interactions in the gas phase. As a result, the higher CO2 concentration in the gas phase prevents CO diffusion into the pores, thus reducing CO adsorption in NaY zeolite. Using isotherm parameters for CO2 and CO from Table 2 to eq 9, the selectivity values by the multisite Langmuir model were calculated, and the results are shown in Figure 8. Similar to CO2−C2H4 binary mixtures, the model poorly predicts the selectivity and correctly indicates only the decreasing trend of the selectivity with total pressure. As mentioned earlier, one possible reason for the model’s prediction failure is due to heterogeneity of the adsorbent surface, which results in nonideality of the adsorbed phase. Furthermore, different interactions between different sorbates and the pore’s surface area also enhance the adsorbed phase nonideality. Consequently, because the model is based on assuming a homogeneous surface for the adsorbent, it can be used only to estimate a trend of the selectivity values with pressure.

5. CONCLUSION In this study, adsorption isotherms of binary gas mixtures on NaY zeolite were obtained using a novel approach called the differential column technique. In comparison to classical methods for measuring binary adsorption isotherms, this technique is faster, requires simpler experimental setup, and is easily extended to reliably measure the multicomponent isotherms for the broad range of pressures. In this study, a relatively large amount of sample was used, e.g., ∼7 g, based on the flow rate of the feed gas that was used in the experiments. This limits the application of this technique to relatively large amounts of commercial or synthesized adsorbents. However, smaller amounts of sample, e.g., down to 1 g, can also be employed if much lower flow rates are used in the measurements, e.g., ≤50 mL/min (not used in this study because of the limitations of mass flow controllers available for the low measurement ranges). To determine the accuracy margin of the technique, the adsorption of the competitive binary sorbates of CO2 and C2H4, and noncompetitive CO2− CO sorbates, were investigated at different composition of sorbates in the gas mixture. Prior to binary measurements, the

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03525. I


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NaY zeolites surface and chemical analysis; RGA calibration curves; multisite Langmuir isotherm model parameters (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: +1 506 453-468. Fax: +1 506 453-3591. E-mail: [email protected]. ORCID

Mladen Eic: 0000-0001-9292-0231 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from Mitacs Accelerate program and Xebec Adsorption Company. Also special thanks go to Aiden Joseph Schenkels for his help with the experiments.

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NOMENCLATURE a = Multisite Langmuir model empirical constant b = Adsorption equilibrium constant (1/kPa) Ko = Henry’s constant (mmol/(g kPa)) nstep,i = Amount of sorbate adsorbed at each pressure step (mmol/g) Pi = Partial pressure of sorbate (kPa) Pt = Total pressure (kPa) Q = Total flow rate of the feed (mL/min) q = Adsorbed (uptake) concentration (mmol/g) qs = Monolayer saturation capacity (mmol/g) R = Universal gas constant (m3 pa/(K mol)) SBET = BET surface area (m2/g) Sm/n = Selectivity of component m over n T = Temperature (K) ySample, yBlank = Exiting mole fraction of the sorbates for the sample loaded and empty column −ΔH0 = Heat of adsorption at zero coverage (kJ/mol)

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