Adsorption of Water Vapor on Carbon Molecular Sieve - American

ARTICLE pubs.acs.org/IECR

Adsorption of Water Vapor on Carbon Molecular Sieve: Thermal and Electrothermal Regeneration Study Rui P. P. L. Ribeiro, Carlos A. Grande,* and Alírio E. Rodrigues Laboratory of Separation and Reaction Engineering (LSRE), Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, s/n, 4200-465 Porto, Portugal ABSTRACT: In this work, we studied the safety issues related to the application of electric swing adsorption (ESA) processes to streams with high relative humidity. Carbon molecular sieve 3K was chosen as the selective adsorbent because of its good electricityconducting properties. We also measured breakthrough curves of water vapor at different temperatures (296-347 K) using different partial pressures of water vapor. Additionally, a comparison of the performance of electric swing adsorption (ESA) and temperature swing adsorption (TSA) processes was also performed. The ESA experiments performed confirm that it is possible to employ the ESA technique in separations in which the feed streams are saturated with water.

’ INTRODUCTION Adsorption processes are cyclic, and the adsorbents must be periodically regenerated to be used in such processes.1,2 Regeneration can be carried out by reducing the pressure of the system (pressure swing adsorption, PSA), by using a desorbent (simulated moving bed, SMB), or by increasing the temperature (temperature swing adsorption, TSA). When the temperature is increased, the capacity of the adsorbent is reduced, so that the preadsorbed compounds are released and the adsorbent is regenerated.3 TSA processes are normally employed in dilute streams because the heating/cooling steps take several hours to complete regeneration.4 Heating of the adsorbent is carried out by passing a hot gas through the column (normally water vapor). The main applications of TSA include volatile organic compound (VOC) removal and gas drying.3,5,6 Carbon dioxide capture from flue gases employing the TSA technique has also been suggested.7 When the concentration of contaminant to be removed increases, the feed time is reduced, and the regeneration step must be more efficient or a large number of columns will have to be employed. In an adsorption process for CO2 capture from flue gases, it is essential to have small units to process large amounts of feed continuously.8 In previous works, we have suggested the use of electric swing adsorption (ESA) as an alternative process to capture CO2 from flue gases.9,10 This technology presents two advantages when compared to classical TSA processes: the heating time can be dramatically reduced because heat is produced in situ, and the concentration of preadsorbed CO2 can be facilitated.11 The only disadvantage of the ESA process is the consumption of electric energy, a primary source of energy and the product of a power plant. Another issue for this process is its applicability to saturated streams of water: Is the process safe, or are massive short circuits possible? The main purpose of this work was to demonstrate the feasibility of using ESA in streams with high relative humidity without any safety problems. The applicability of ESA processes in humid conditions was tested using a feed stream comprising r 2011 American Chemical Society

water vapor balanced with helium. The ESA experiments were performed with an adsorbent able to conduct electricity and with a relatively small loading of water (to reduce the time of the experiments). For this purpose, we selected carbon molecular sieve 3K (Takeda, Tokyo, Japan). Carbon molecular sieves (CMSs) are carbonaceous materials produced from several different precursors such as coal, biomass, petroleum, or polymeric precursors.12 CMSs are characterized by having a relatively narrow micropore size distributions and are generally employed in kinetic gaseous separations.13-16 CMS adsorbents are employed in several gaseous separations like oxygen purification from air, propane-propylene separation and natural gas purification.12,17-20 To have a theoretical understanding of the process and of the effect of operating variables, we have started by measuring breakthrough curves of water vapor in the adsorbent at different H2O partial pressures and different temperatures (296, 301, 321, 323, and 347 K). These experiments gave information related to adsorption equilibrium and kinetics of water in this adsorbent. After measuring adsorption properties, we studied two different regeneration procedures: external heating the column (resembling TSA operation) and direct ESA heating.

’ EXPERIMENTAL SECTION Material. The adsorbent material studied in this work was a carbon molecular sieve, namely, CMS 3K, supplied by Takeda Co. (Tokyo, Japan). The extrudates of carbon molecular sieve employed had an average diameter of 1.8 10-3 m and a macropore diameter of 3.12 10-7 m. The physical properties of the CMS-3K were previously determined21 and are presented in Table 1. Received: May 20, 2010 Accepted: December 13, 2010 Revised: December 13, 2010 Published: January 12, 2011 2144

dx.doi.org/10.1021/ie101133g | Ind. Eng. Chem. Res. 2011, 50, 2144–2156

Industrial & Engineering Chemistry Research

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Table 1. Adsorbent Properties and Experimental Conditions Properties of CMS 3K Adsorbent property

value 1.8 10

particle diameter (m) 3

pellet density (kg/m )

-3

1060

property

value

macropore diameter (m)

3.12 10-7

pellet porosity

0.46

Column Properties property

value

property

value

mass of adsorbent (g)

74.0

εcolumn

0.33

bed length (cm)

9.0

Fcolumn (kg/m3)

711.6

bed diameter (cm)

3.96

run

T (K)

PH2O (bar)

Ptotal (bar)

Experimental Conditions Qfeed (L/min)

Qpurge(L/min)

qads(mol/kg)

qdes(mol/kg)

1

301

0.034

1.03

1.16

1.14

7.7

7.5

2

301

0.030

1.01

1.36

1.32

7.2

6.9

3

301

0.026

1.02

1.49

1.46

6.7

4

301

0.020

1.02

1.98

1.93

5

301

0.018

1.02

2.12

2.07

5.1

5.2

6

296

0.011

1.04

2.99

2.98

2.3

2.4

7 8

321 323

0.020 0.028

1.00 1.00

1.59 1.31

1.56 1.265

0.51 0.56

0.52 0.58

9

323

0.031

1.01

1.32

10

347

0.021

1.01

1.57

Adsorption Equilibrium and Kinetics. The adsorption equilibrium and kinetics of water vapor in CMS-3K were determined by measuring several breakthrough curves with different partial pressure of water in the feed stream. A schematic representation of the setup employed in breakthrough experiments is shown in Figure 1. The breakthrough experiments were performed using a column with a length of 0.090 m and a diameter of 0.0396 m, filled with the selected adsorbent. The experimental conditions of the breakthrough experiments are listed in Table 1. Breakthrough experiments (adsorption and desorption) were performed at 296, 301, 321, 323, and 347 K at partial pressures of water vapor between 0.011 and 0.034 bar. Before starting the breakthrough experiments, the adsorbent was degassed at 433 K for 3 h using a helium stream of 0.26 L/min (at 296 K and 1 bar). Nearly saturated streams of water vapor were obtained by bubbling helium in a closed tank (bubbler). This stream can be mixed with a dry helium stream before entering the column to obtain different partial pressures of water vapor, as shown in Figure 1. To avoid water condensation in the tubes (before and after the column), the tubes were covered with several heating ropes, and the temperature was adjusted using a temperature controller. Relative humidity and temperature were measured at the inlet and outlet of the column using two Hygrotest 650 humidity/temperature sensors (Testo, Lenzkirch, Germany). The pressure at the column outlet was measured with a pressure transducer from Swagelok (Solon, OH). Flow rates of the two helium streams were controlled by two Bronkhorst (Netherlands) mass flow controllers. After passing through the column, the outlet flow rate was measured with a FVL-1607A flowmeter (Omega Engineering, Stamford, CT). Helium employed in the experiments was supplied by Air Liquide (Alges, Portugal) with purity higher than 99.999%.

6.6 5.8

0.67 1.538

0.16

0.17

Figure 1. Experimental setup for the breakthrough experiments: FM, flow meter; BPR, back-pressure regulator; HS, humidity sensor; MFC, mass flow controller.

External Heating Desorption (TSA-like Mode). One of the possible techniques for regenerating columns filled with adsorbents is to increase the temperature of the bed. As temperature 2145

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Table 2. ESA Column Properties and Experimental/Simulation Conditions Column Properties value property

property

value

bed length (cm)

23.0

mass of adsorbent (g)

110.7

bed diameter (cm)

2.99

εcolumn

0.35

Experimental/Simulation Conditions run tfeed (s)

Figure 2. (a) TSA cycle: 1, feeding step; 2, heating step; 3, cooling step. (b) ESA cycle: 1, feeding step; 2, electrification step; 3, cooling step.

increases, the bed capacity decreases significantly, and the adsorbed compounds are released.1-5 After exiting the heated column, if the stream of water vapor comes into contact with cold tubing, it can partially condense. To avoid condensation at the column exit, the tubes were heated in the same way as in the breakthrough curve experiments. In this work, we tested two different heating modes. The first one was to heat the column externally, using an electrical resistance wrapped around the column filled with the adsorbent. The resistance employed was a rope with a length of 3 m and a total resistance of 365.5 Ω. A potential of 220 V ac was applied to the resistance during the heating step, and the current intensity delivered was controlled by means of a rheostat (from 40 to 400 W). A model 1705 multimeter (TTi, U.K.) was employed to measure the power delivered during the heating step. The initial cyclic experiments consisted of a single cycle composed of three steps: feeding, heating, and cooling. During heating and cooling, a purge with dry carrier gas (helium) was performed countercurrently to feed stream. The scheme of the cycle employed is shown in Figure 2. Electrothermal Desorption (ESA Mode). The second desorption technique was direct electrothermal heating using the Joule effect. However, these experiments could not be performed in the same column as the previous studies, as the adsorbent should be electrically isolated from the rest of the unit. A column was designed and built for use in this study. The physical properties of the column are listed in Table 2, together with the operating conditions employed in the experiments. The column was made up of two different parts: an external stainless steel jacket in which a Teflon column was placed. The adsorbent was placed inside the Teflon column, where two electrodes were placed at each of the column end. Antifiring material (Interam, 3M, St. Paul, MN) was placed between the Teflon column and the stainless steel jacket to accommodate different expansion rates and avoid column bypassing. Single-cycle experiments (feeding, heating, and cooling) were performed by changing the heating source to direct electric heating. For this reason, we termed this step as electrification. In these experiments, during the electrification and cooling steps, a purge with dry carrier gas (helium) was fed to the column in countercurrent mode, as shown in Figure 2. The voltage applied to the conducting adsorbent bed was controlled by means of a power supply unit, model TSX 1820P (TTi).

’ THEORETICAL SECTION Adsorption Equilibrium . Virial Model. It is known that the

adsorption equilibrium of water vapor in activated carbon and

simulationa 2000

experiment 2000

theating (s)

600

600

tcooling (s) PH2O (bar)

1050

8400 0.027

Qfeed (L/min)

1.61

Qheating/cooling (L/min)

0.44

Pfeed (bar)

1.00

Pelectrification (bar)

1.03

Pcooling (bar)

1.03

T (K)

299

voltage (V) Cps (J/kg 3 K)

14 880

Cpw (J/kg 3 K)

500

Fw (kg/m3)

8238

Fp (kg/m3)

1060

U (W/m2 3 K)

9

hw (W/m2 3 K)

80

productivity (mol/h 3 kg) energy consumption (kJ/mol)

0.12b

0.17

687.9

405.5

a

Simulation performed considering a homogeneous electric resistance of 5 Ω. b Productivity obtained for a recovery of 80%.

carbon molecular sieves follows a type V isotherm according to the IUPAC classification.1,4 For this reason, it cannot be described by Langmuir-type models. The adsorption equilibrium data of water vapor were therefore described using the virial isotherm model.22,23 This model is very flexible, allowing for the description of several types of isotherms.24 The Virial model is thermodynamically correct for low and high coverages and allows the prediction of multicomponent behavior through analytical expressions.22 The virial isotherm can be obtained by applying the bidimensional virial state equation to the Gibbs isotherm.25,26 The virial isotherm, truncated at the second virial coefficient, is represented by q 3 2 P ¼ exp 2Aq þ Bq ð1Þ KH 2 where q is the amount adsorbed, P is the adsorbate partial pressure, KH is the Henry constant, and A and B are the virial coefficients. The dependence of the Henry constant on temperature is obtained through the van’t Hoff equation ! - ΔH KH ¼ K¥ exp ð2Þ Rg T where K¥ is the adsorption constant at infinite temperature, -ΔH is the isosteric heat of adsorption, Rg is the ideal gas constant, and T is the temperature. 2146



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The virial coefficients have a temperature dependence represented by ¥ X Am A ¼ ð3Þ m T m¼0 B ¼

¥ X Bm m¼0

Tm

ð4Þ

and are truncated after the second term. The parameter optimization was done by minimizing the difference between simulated and experimental results, using the square of residuals (SOR) function.27 The parameter fitting was done with MatLab software, version 7.4.0.287(R2007a), from The MathWorks, Inc. (Natick, MA) Equilibrium Theory. Adsorption equilibrium experimental data were obtained for water partial pressures higher than 0.011 bar. Equilibrium theory provides a tool to predict the adsorption equilibrium behavior at lower partial pressures. Equilibrium theory is a simple and straightforward way to describe adsorption, considering instantaneous equilibrium between the fluid and adsorbed phases.28-30 According to equilibrium theory (considering constant velocity, isothermal conditions, and no axial dispersion), the propagation velocity of concentration C can be obtained by Dz u i ¼ ð5Þ uc ¼ Fb dq Dt C 1þ εb dC where uc is the velocity of propagation of concentration C, ui is the interstitial velocity, Fb is the density of the bed, εb is the bed void, and dq*/dC represents the derivative of the adsorption isotherm. Polanyi Potential Theory. Polanyi’s potential theory2,31-33 proposes an interpretation of the adsorption phenomenon based on the concept of a surface force field that originates equipotential surfaces that correspond to a specific volume of an adsorbed specie. The potential theory considers the volume adsorbed, W, as a function of the potential, A. This dependence gives rise to a characteristic curve for each system that is independent of the temperature. This property of the potential theory is of great interest because it allows prediction of the behavior of a gas-solid system for any temperature. The adsorption potential is equivalent to the work required for the compression of a gas from pressure P to its saturation pressure Ps. The adsorption potential can be defined, for 1 mol of an ideal gas, as Z Ps Ps ð6Þ v dP ¼ Rg T ln A ¼ P P The adsorbed space volume is given by W ¼ qVm

within the micropores is a combination of classical micropore resistance together with the an additional barrier-type resistance at the mouth of the micropore.14-20,34 The model employed in this work considers a single resistance at the micropore (combining micropore and barrier resistance at the mouth of the micropore).35 The mathematical model employed to describe the fixed-bed behavior is described in Table 3. The model employed assumes ideal gas behavior and uniform bed porosity along the column. The model considers only variations in the axial coordinate. This model has already been validated for different gases.36 External Heating Desorption (TSA Mode). The description of desorption with the external heating device was done employing the same mathematical model as described above. The only difference in the model employed in this section is in the wall energy balance, where a heat generation term was included in the heating step. The wall energy balance presented in Table 3 should be replaced (in the heating step) by ~ pw DTw ¼ Rw hw ðTg - Tw Þ - Rwl UðTw - T¥ Þ þ VI θ Fw C πRc 2 Lc Dt ð8Þ where V is the electric potential employed, I is the electric current passing through the resistance, Rc is the column radius, Lc is the column length, and θ is the coefficient of effective energy employed in the adsorbent heating. This coefficient was experimentally determined and reflects the amount of energy that was really employed to heat the adsorbent. Note that, in using this heating method, a large portion of heat will be lost to the surroundings. Electrothermal Desorption (ESA Mode). Electrothermal adsorbent regeneration is based on heating the adsorbent by the Joule effect that occurs when an electric current passes through a conductor that is placed within the column. The electric conductor material can be the adsorbent material itself or some conducting material located inside the column for the effect. This process is normally termed electric swing adsorption (ESA). The electrothermal desorption was also described by the mathematical model presented in Table 3. However, the solid energy balance should be changed to include the heat generation term due to the Joule effect.9,37 For the ESA experiments, the energy balance in the solid phase listed in Table 3 should be replaced by (in the electrification step) " # n n X X ~ v;ads; i þ Fp C ~ vi þ Fp ~ ps DTs ci C Æqi æC ð1 - εc Þ εp Dt i¼1 i¼1 ¼ ð1 - εc Þεp Rg Ts

þ ð1 - εc Þa0 hf ðTg - Ts Þ þ

ð7Þ

The potential theory was the basis for the development of several models to describe the adsorption equilibrium phenomenon. In this work, we used Polanyi’s theory to verify that all of the points measured at different temperatures fell along a single characteristic curve. Adsorption Kinetics. Carbon molecular sieves (CMSs) are bidisperse adsorbents.12 The control mechanism for diffusion is generally within the micropores. In CMS materials, the resistance

n X Dci DÆqi æ þ Fb ð- ΔHi Þ Dt DT i¼1

VI πRc 2 Lc

ð9Þ

’ RESULTS AND DISCUSSION Adsorption Equilibrium. Adsorption equilibrium data of water vapor on CMS 3K were obtained at 296, 301, 321, 323, and 347 K. Only one breakthrough experiment was performed at 296 K (for a feed partial pressure of 0.011 bar). 2147



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Table 3. Mathematical Model for Fixed-Bed Experiments of H2O on CMS 3K component mass balance

εc

DCi D Dyi Dðui Ci Þ a0 kfi Dax Ct - ð1 - εc Þ ¼ εc ðCi - ci Þ Dz Dz Dt Dz Bii þ 1

Ergun equation 2

150μg ð1 - εc Þ 1:75ð1 - εc ÞFg DP ¼ ui þ jui jui Dz εc 3 dp εc 3 dp 2

LDF equation for the macropores

εp

DÆqi æ Bii Dci ¼ εp Kp, i ðCi - ci Þ þ Fp Dt Bii þ 1 Dt

Kp, i ¼

15Dp, i Rp 2

LDF equation for the micropores

DÆqi æ ¼ Kμ, i ðqi - Æqi æÞ Dt

Kμ, i ¼

15Dμ, i rμ 2

Dμ, i ¼ Dμ, i expð - Ea, i =Rg Tg Þ}

gas-phase energy balance

solid-phase energy balance

~v εc Ct C

" ð1 - εc Þ εp

n X i¼1

DTg DTg DT D ~ p g þ εc Rg Tg DCt - ð1 - εc Þa0 hf ðTg - Ts Þ - 2hw ðTg - Tw Þ λ ¼ - ui Ct C Dz Dt Dz Dz Dt Rc

~ vi þ Fp ci C

n X i¼1

# ~ v;ads, i þ Fp C ~ ps Æqi æC

n X DTs Dci DÆqi æ þ ð1 - εc Þa0 hf ðTg - Ts Þ ¼ ð1 - εc Þεp Rg Ts þ Fb ð - ΔHi Þ Dt Dt DT i¼1

wall energy balance ~ pw Fw C

DTw ¼ Rw hw ðTg - Tw Þ - Rwl UðTw - T¥ Þ Dt

Rw ¼

Dc eðDc þ eÞ

Rwl ¼

1 Dc þ e ðDc þ eÞ ln Dc

Bosanquet equation 1 1 1 ¼ τp þ Dp, i Dm, i Dk, i

!

axial dispersion Dax, i ¼ ð0:45 þ 0:55εc ÞDm, i

The equilibrium theory was employed to analyze the data obtained in this experiment because this was the breakthrough experiment that showed lower-temperature peaks and can be considered isothermal. In the experimental breakthrough experiments with higher-temperature peaks, isothermal conditions

cannot be considered, and the equilibrium theory cannot be employed. The equilibrium theory allowed a prediction of the adsorption trend at lower partial pressures of water for a temperature of 296 K. The adsorption equilibrium data are shown in Figure 3. 2148



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Figure 3. Amount of H2O adsorbed on CMS 3K: experimental points at (b) 296, (2) 301, (Δ) 321, (0) 323, and(9) 347 K. The line represents fitting with the virial model.

Figure 4. Polanyi’s characteristic curve. The plot shows results for (b) 296, (2) 301, (Δ) 321, (0) 323, and (9) 347 K.

Table 4. Virial Adsorption Isotherm Fitting Parameters for H2O on CMS 3K parameter

value

parameter

value

K¥ (mol/kg 3 bar) -ΔH (kJ/mol)

5.70 10-6 40.000

A1 (kg 3 K/mol) B0 (kg/mol)

-7.50 0.0414

A0 (kg/mol)

-0.307

B1 (kg 3 K/mol)

0.000195

The adsorption equilibrium data were fitted with the virial isotherm model. As can be seen in Figure 3, the model fits all of the data with good accuracy. The parameters employed in the isotherms fitting are listed in Table 4. Water adsorption on carbonaceous adsorbents has been studied by several researchers, and the results obtained point to a low affinity between the surface of the carbon and the water molecules at low partial pressures.38 The molecules of adsorbed water seem to act as secondary adsorption sites, significantly enhancing the adsorption capacity at higher partial pressures also followed by capillary condensation.39 For these reasons, water adsorption isotherms on activated carbon surfaces are normally type IV or V. The virial model employed allowed a good fitting of the experimental data, confirming the expected isotherm shape. The data obtained in this study are in agreement with previous data obtained in carbon molecular sieve.40 The adsorption data obtained for water vapor on the studied carbon molecular sieve for the different temperatures studied were analyzed with Polanyi’s theory equation, and the resulting characteristic curve is shown in Figure 4. The characteristic curve obtained (Figure 4) shows that, independently of the temperature, the characteristic curve can describe the water-CMS 3K system. Polanyi’s theory is a useful theoretical tool that is capable of predicting the equilibrium trend independently of temperature. Adsorption Breakthrough Curves. The adsorption kinetics of water vapor on CMS 3K was investigated by analysis of breakthrough experiments performed using different partial pressures of water vapor at five different temperatures. Figure 5a shows the experimental (points) and simulated (solid lines) results for the partial pressure of water vapor at the column exit, and Figure 5b shows the temperature histories at two different positions within the column (0.015 and 0.073 m from feed inlet).

Figure 5. (a) Breakthrough curve for an inlet partial pressure of 0.034 bar at 301 K (O, experimental data; ;, simulation). (b) Temperature history of the CMS 3K bed at (O) 1.5 and (b) 7.3 cm. Lines represent simulation data.

The parameters employed in the simulations are presented in Table 5. The exit water vapor partial pressure profile has two 2149



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Table 5. Kinetic and Heat Parameters Employed in the Breakthrough Experiment Simulations parameter

value

2 D/r μ μ (m /s)

0.9

Ea (kJ/mol) Cps (J/kg 3 K)

16.0 880

Cpw (J/kg 3 K)

500

Fw (kg/m )

8238

Fp (kg/m3)

1060

3

parameter

value

U296 (W/m2 3 K)

15

U301 (W/m2 3 K) U321 (W/m2 3 K)

15 25

U323 (W/m2 3 K)

25

hw (W/m2 3 K)

60

U347 (W/m2 3 K)

30

different plateaus. The first portion of the curve corresponds to unfavorable adsorption equilibrium, where the concentration wave travels rapidly because of a small value of the derivative of the isotherm: According to eq 5, the velocity of the concentration wave is inversely proportional to the derivative of the isotherm. The second portion of the curve reflects the favorable shape of the adsorption equilibrium at higher partial pressure of water vapor with the consequent lower velocity of the concentration wave. The two zones of the isotherm can also be observed in the temperature profiles within the column. It can be noted that, at the thermocouple near the exit (0.073 m from the feed inlet), the temperature reflects the adsorption from the initial unfavorable portion of the isotherm (smaller T peak), followed by a second (and higher) temperature increase resulting from the adsorption of a higher amount of water vapor. It should also be mentioned that, for all of the breakthrough curves measured in this work, it was observed that the concentration wave traveled more slowly than the temperature wave. It was previously reported that this happens when the following relation is ~ pΔqi)/(CpsΔCi) > 1.0 satisfied:2 (C In Figure 5, we report a breakthrough curve measured at a higher water vapor partial pressure in the presence of unfavorable and favorable portions of the isotherm. In Figure 6, the results of run 6 are shown. In this case, the feed partial pressure (0.011 bar) is before the inflection point (and thus only within the unfavorable part of the isotherm). It can be observed that, in this case, the shape of the breakthrough curve in run 6 has only one plateau. Because the experiment was within the unfavorable portion of the isotherm, it was very difficult to achieve adsorption equilibrium, and the experiment took a long time to be completed. Also, only one temperature peak was observed, with a much smaller intensity because of the lower loading at this partial pressure. Regeneration by External Heating (TSA Mode). Several desorption experiments were performed using an external heating device to increase the temperature of the column. Table 6 lists the experimental conditions of all of the runs performed. The performance factors, productivities, and energy consumptions are also reported in Table 6. Productivity was defined as R theat=elect productivity ¼

0

Rt FH2 O jz ¼ 0 dt þ 0cooling FH2 O jz ¼ 0 dt tcycle wads ð10Þ

where FH2O represents the molar flow rate of water vapor exiting the column, tcycle is the time duration of one cycle, and wads is the adsorbent weight within the bed.

Figure 6. (a) Breakthrough curve for an inlet partial pressure of 0.011 bar at 296 K (O, experimental data; ;, simulation). (b) Temperature history of the CMS 3K bed at (O) 1.5 and (b) 7.3 cm. Lines represent simulation data.

Energy consumption per mole of water desorbed is defined by energy consumption ¼ R theat=elect 0

VItheat=elect Rt FH2 O jz ¼ 0 dt þ 0cooling FH2 O jz ¼ 0 dt

ð11Þ

In the initial experiment (run 1), the system was only heated without other thermal insulation. However, it was observed that much energy was lost to the surroundings, and for this reason, in the other experiments, the column was thermally insulated from its surroundings. Figures 7-9 show the experimental data together with simulated results obtained for runs 1, 2, and 5, respectively. The mathematical model could predict the behavior of the system with good accuracy. The length-to-diameter ratio of this column is quite small, which is why axial dispersion effects are quite important. In fact, it can be observed that temperature increase and decrease in both positions within the column are nearly the same. The thermal Peclet number of this system was 9.5-10.5 in all simulations. The horizontal dashed lines in the pressure outlet history represent the saturation pressure at the 2150



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Table 6. External Heating Experimental Conditions and Performance Parametersa run 1

b

2

c

3c

4c

5c

no. of cycles

1

1

1

1

3

PH2O (bar)

0.023

0.024

0.024

0.021

0.026

tfeed (s) theating (s)

2000 4200

2000 4200

2000 3100

2000 1000

2000 1000

tcooling (s)

10468

35370

19403

19120

9000

Qfeed (L/min)

1.55

1.58

1.54

1.56

1.57

Qheating/cooling (L/min)

0.46

0.46

0.45

0.46

0.47

Pfeed (bar)

1.01

1.00

1.05

1.00

1.01

Pheating (bar)

1.08

1.08

1.09

1.07

1.09

Pcooling (bar)

1.08

1.08

1.09

1.08

1.08

T (K) power (W)

297 73.7

298 79.2

296 74.1

297 74.7

296 75.2

productivity (mol/h 3 kg) energy consumption (kJ/mol)

0.17

0.085

0.086

0.17

0.22

6914

7324

5391

1664

1398

a

For all experiments, the feed step was performed in co-currently, whereas the heating and cooling steps were performed in countercurrent mode. b Noninsulated run. c Insulated run.

experimental operating temperature. It should be noted that it was possible to measure the partial pressures above the condensation pressure at the feed stream because the tubes where the sensor was placed were heated to higher temperatures. Comparing the results from Figures 7 and 8, it can be observed that, in the case of the thermally insulated column, for the same heating time, the cooling step was much longer. The longer cooling step reduced the productivity factor (0.085 mol/h 3 kg) when compared with shorter cooling steps as in the case of run 1 (0.17 mol/h 3 kg). Run 2 also shows that a higher maximum temperature was reached (for the same heating time) because the electric power employed was slightly higher and, especially, because of the insulation material placed around the column. However, when the column was insulated, a higher fraction of the energy consumed was used to effectively heat the adsorbent. Figure 9 shows the experimental and simulated results of the last experimental run performed (run 5), where three cycles were performed continuously. It can be observed that there was a slight increase of the feed relative humidity in the successive cycles. This is due to a slight increase (1 K) in the ambient temperature of the laboratory due to the thermal cycles. The solid lines in Figure 9 correspond to the simulations without any fitting parameter. It can be observed that the predictions obtained with the mathematical model (solid lines in Figure 9) are in very good agreement with the results obtained in the initial cycles of the experiment. It can be observed that the time for cooling the cycle was not long enough, and for this reason, more cycles were necessary to achieve cyclic steady state (CSS). Run 5 present a high productivity (0.22 mol/h 3 kg) (because of the shorter cooling step employed). The amounts of energy employed in the desorption of water in runs 4 and 5 were the same. However, because a high partial pressure of water was employed in run 5, more moles of water could be recovered, thereby decreasing the net consumption of energy per mole of water. The analysis of the adsorbed and temperature internal profiles, obtained by simulation, shows that CSS was reached after four cycles under the conditions considered for run 5.

Figure 7. TSA cycle (run 1/noninsulated) experimental data (points) and simulation (lines). (a) Pressure history at the lower column end during one cycle. (b) Temperature history of the CMS 3K bed at (O) 1.5 and (b) 7.3 cm.

The results obtained in the fitting of the experimental data allowed for the validation of the mathematical model employed. Further simulation studies were performed to study the possibility of improving the TSA performance. Simulations performed for the same conditions as the ones reported for run 2, modifying only the heating step time to 2500 s, were performed. The temperature reached inside the column with this shorter heating step was the same as was obtained experimentally in run 1 and provided a basis for comparison between the column behavior with and without thermal insulation. The results obtained from the simulation showed that, with the shorter heating step time, the energy consumption decreased to 3772 kJ/mol, compared with the 6914 kJ/mol obtained experimentally in run 1. The productivity obtained from the simulation was 0.14 mol/h 3 kg. This value is still lower than that obtained experimentally for run 1, because of the long cooling time needed to cool the thermally insulated column. Electrothermal Desorption (ESA Mode). TSA processes are, in general, characterized by having long cycles and, consequently, low productivity values. The heat transfer from the energy source (hot gas or an external heating source) to the adsorbent is normally a time-consuming task, as is the cooling step needed to return the bed to the cold temperature for a new cycle. 2151



Figure 8. TSA cycle (run 2/insulated) experimental data (points) and simulation (lines): F, feed; H, heating; C, cooling. (a) Pressure history at the lower column end during one cycle. (b) Temperature history of the CMS 3K bed at (O) 1.5 and (b) 7.3 cm.

Electric swing adsorption (ESA) is a separation process that can significantly reduce the heating step and thus improve the unit productivity. Other potential applications of ESA technology are when hot gas is not available. Also, ESA can be more energetically efficient because heat is applied directly to the adsorbent. In this work, ESA technology was tested in the presence of water vapor. Water vapor from deionized water should be a nonconductive gas; however, it was necessary to demonstrate that, upon application of an electric current, the water adsorbed in the carbon molecular sieve will not remove some functional groups from the surface and produce a massive short circuit in the equipment. This safety issue would automatically remove the possibility of application of ESA to many streams saturated of water, such as CO2 capture from flue gases9,10,41 or biogas.42 As mentioned in the Experimental Section, the column employed for ESA experiments was electrically insulated from the rest of the unit. The electrothermal desorption was performed after a feed step of 2000 s with a partial pressure of water vapor of 0.027 bar (conditions similar to those of run 5 of the TSA experiments). The operating conditions used for the ESA experiments are detailed in Table 2. Feed was performed in

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Figure 9. TSA (run 5/insulated) experimental data (points) and simulation (lines): F, feed; H, heating; C, cooling. (a) Pressure history at the lower column end during three cycles. (b) Temperature history of the CMS 3K bed at (O) 1.5 and (b) 7.3 cm.

cocurrent mode, and electrification and cooling were performed in countercurrent mode (Figure 2b). The experimental and simulated results are shown in Figure 10. The electrification was performed by applying a constant voltage of 14 V dc to the CMS 3K bed. The temperature at the lower thermocouple (0.10 m) reached a value of 334 K, whereas at 0.21 m, the temperature reached 370 K. This large difference in the temperatures obtained at the different thermocouples is due to a variable electrical resistance inside the column. In previous work carried out with an activated carbon honeycomb monolith, it was observed that the resistance of the material decreased as its temperature increased.9 However, variations of electrical resistance with the pressure applied over packed columns were reported previously.43 A linear dependence of the electric resistance on the bed length was considered to correctly describe the temperature profiles inside the column. Figure 10a presents large differences between the experimental results and the predictions of the mathematical model. This difference is due to two different experimental factors. The first one is related to the variations of electrical resistance observed when the column was heated. It can be observed that, in the electrification step, electrical resistance increased with temperature, which was not expected (Figure 11). This increase is related 2152



Figure 10. ESA cycle experimental data (points) and simulation (lines). (a) Pressure history at the lower column end during one cycle. (b) Temperature history of the ESA bed at (O) 10 and (b) 21 cm.

Figure 11. Electric resistance history of the whole bed during the ESA cycle.

to a larger thermal expansion of the column and column jacket, which reduced the compression of the pellets, thereby increasing

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the resistance. When the column cooled, the materials contracted again, and the electrical resistance returned to its original value. This result was also obtained by heating the column only in a stream of helium, so it is not related to the water adsorbed in the CMS. A more complicated system for electrodes should be developed to solve this problem and increase the efficiency of the process. The second source of error is related to the condensation of water inside the column. In the TSA experiments, all of the system was heated (including the sensor), and no condensation was possible. In the ESA experiments, only the adsorbent was heated directly. For this reason, water could condense in the lower portion of the column, that is, before reaching the water sensor. Because the experimental ESA system was not perfectly adiabatic, when the hot gases heated the stainless steel jacket and Teflon column, water evaporation took place at completely different rates. Solving this problem might be more difficult in the future because the water sensor should be placed immediately close to the adsorbent and with an independent heating source with low power consumption to avoid interference with the temperature of the feed stream. However, it is very important to mention that the ESA technique could be used for water adsorption/desorption without any kind of safety problems, which allows for the use of ESA for water-containing streams. However, it should be mentioned that, to improve the unit productivity of ESA processes, water condensation must be avoided. Avoiding water condensation will allow a significant decrease in the time needed for the adsorbent regeneration, and acceptable productivity values should be obtained. The ESA experimental results were deeply affected by the experimental problems inside the designed ESA column. Because of the experimental problems reported above, the separation performance was poor. The productivity obtained in the process was only 0.12 mol/h 3 kg (for a recovery of 80%, instead of the 100% considered in the TSA mode experiments). Despite all of the problems verified in the experiments performed, the energy consumption was evaluated at 687.9 kJ/mol, which is lower than the values obtained for the external heating experiments (see Table 6). The experimental results demonstrated the deep impact of the experimental problems obtained. Problems such as water condensation inside the ESA column and the electric resistance variation due to materials expansion with heating must be solved so that the process can be feasible. The problem of the lack of homogeneity in the temperature distribution inside the adsorption bed can be solved if a contant electric resistance inside the column is achieved. A constant value of the electrical resistance within the adsorption bed can be obtained by employing honeycomb monoliths.37 A simulation was performed to evaluate the possibility of employing ESA to remove water from a gaseous stream in the case of having constant electric resistance throughout the column. A resistance of 5 Ω (similar to the value at the start of the experimental run) was considered for simulation. The conditions employed in the simulation performed are the same as those employed in the experimental ESA run, shown in Table 2. The simulation results show that, when a constant value is employed for the electric resistance within the column, the temperature is almost homogeneous. When higher temperature can be achieved in the whole column length, the water desorbs much faster from the entire column, achieving a higher local concentration, as can be seen in Figure 12a. Because more water is desorbed more rapidly, the velocity of regeneration is 2153



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the overall bed capacity, and also the cooling steps will take longer times.

’ CONCLUSIONS Adsorption equilibrium of water vapor on CMS 3K (Takeda, Tokyo, Japan) was evaluated at temperatures between 296 and 347 K. The isotherms obtained shown type V behavior, being well-described by the virial model. The adsorption kinetics of water vapor in CMS 3K was also studied through the analysis of fixed-bed behavior, on the basis of several breakthrough experiments at different temperatures (296-347 K). The fixed-bed mathematical model employed was able to fit the data well. TSA and ESA processes were tested by employing a simple three-step cycle: feeding, heating/electrification, and cooling. TSA simulations showed that CSS was reached after only four cycles of the process. The ESA experimental results show that improved electrode systems should be used when extrudates or pellets are employed to avoid an undesired increase of the electric resistance with temperature. These results also show that this technique can be employed in separations where the feed stream is saturated with water with lower energy requirements than a TSA process. In large units, massive water condensation inside the ESA column should be prevented to avoid contact of water with the lower electrode.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ351 22 508 1618. Fax: þ351 22 508 1674. E-mail: [email protected].

Figure 12. Simulation of an ESA cycle: (a) pressure history at the lower column end during one cycle and (b) temperature history of the ESA bed at (O) 10 and (b) 21 cm.

improved, taking less that 6000 s compared to 12000 s (Figure 10) when the resistance changes with temperature. The simulation performed allowed a productivity (0.17 mol/h 3 kg) to be obtained that was similar to the values obtained for the TSA experiments. The value was still conditioned by the need for a long cooling time. The interesting result in this simulation is the fact that the ESA process can, in fact, allow for a significant decrease in the energy consumption of the separation. The energy consumption obtained was 405.5 kJ/mol, which is less than onehalf of the best result obtained experimentally in the TSA mode experiments. Based on the energy consumption value obtained with this simple cycle simulation, we expect that, with the heterogeneity electric resistance problem solved, the ESA process can be a feasible alternative to the TSA process. Further studies are needed before scaling up the process. The scaleup of the process must take into account that continuous feed streams should be processed, and an appropriate scheduling for a predefined number of columns must be employed in the adsorbent regeneration. Also, the possibility of employing continuous energy consumption should be explored. Another scaling-up issue is the fact that larger columns behave adiabatically.44 The effect of adiabatic beds in overall cyclic performance might be significant: the heat of adsorption in the feed step might significantly reduce

’ ACKNOWLEDGMENT The authors would like to thank financial support from the Portuguese Foundation for Science and Technology (FCT) through project PTDC/EQU-EQU/65541/2006. Rui Ribeiro also acknowledges financial support from FCT (SFRH/BD/ 43540/2008). ’ NOMENCLATURE A = virial coefficient (kg/mol) a = area to volume ratio (m-1) B = virial coefficient (kg/mol) Bii = Biot number of component i ci = average concentration in the macropores of component i (mol/m3) C = concentration (mol/m3) Ci = concentration of component i in the gas phase (mol/m3) ~ p = molar constant-pressure specific heat of the gas mixture C (J/mol 3 K) ~ Cps = constant-pressure specific heat of the adsorbent (J/kg 3 K) ~ Cpw = specific heat of the column wall (J/kg 3 K) 3 C ~t = total gas concentration (mol/m ) Cv = molar constant-volume specific heat of the gas mixture (J/mol 3 K) ~ Cvi = molar constant-volume specific heat of component i (J/mol 3 K) ~ Cv,ads,i = molar constant-volume specific heat of component i adsorbed (J/mol 3 K) dp = extrudate diameter (m) 2154


Industrial & Engineering Chemistry Research Dax = axial dispersion coefficient (m2/s) Dk,i = Knudsen diffusivity of component i (cm2/s) Dm,i = molecular diffusivity of component i (m2/s) Dp,i = pore diffusivity of component i (m2/s) Dw = internal diameter of the column (m) Dμ,i = micropore diffusivity of component i (m2/s) = limiting diffusivity at infinite temperature of compoDμ,i nent i (m2/s) e = wall thickness (m) Ea,i = activation energy of micropore diffusion for component i (kJ/mol) FH2O = molar flow rate of water exiting the column (mol/s) hf = film heat-transfer coefficient between the gas and the solid phase (W/m2 3 K) hw = film heat-transfer coefficient between the gas phase and the column wall (W/m2 3 K) I = electric current intensity (A) kfi = film mass-transfer coefficient (m/s) KH = adsorption equilibrium constant (Henry constant) of component i (kPa-1) Kp,i = LDF constant for mass transfer in the macropores for component i (m/s) Kμ,i = LDF constant for mass transfer in the micropores for component i (m/s) K¥ = adsorption equilibrium constant of component i in the limit T f ¥ (kPa-1) Lc = column length (m) P = total pressure (kPa-1) qi = adsorbed-phase concentration of component i (mol/kg) qi* = adsorbed-phase concentration in the equilibrium state of component i (mol/kg) æ = extrudate-average adsorbed-phase concentration Æq hi (mol/kg) Q = flow rate, SLPM (L/min) rc = radius of the crystal (m) Rc = radius of the column (m) Re = Reynolds number Rg = universal gas constant (J/mol 3 K) Rp = radius of the extrudate (m) Sc = Schimdt number t = time (s) T = temperature (K) Tg = temperature of the gas phase (K) Ts = temperature of the solid phase (K) Tw = wall temperature (K) T¥ = external temperature (K) U = global external heat-transfer coefficient (W/m2 3 K) uc = velocity of propagation of the concentration front (m/s) ui = interstitial gas velocity (m/s) V = electric potential (V) wads = weight of adsorbent within the bed (kg) yi = mole fraction of component i z = axial distance along the column (m) Greek Letters

rw = ratio of the internal surface area to the volume of the column wall (m-1) Rwl = ratio of the logarithmic mean surface area of the column shell to the volume of the column wall (m-1) (-ΔHi) = isosteric heat of adsorption of component i (kJ/mol) εc = porosity of the column εp = porosity of the extrudate

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λ = axial heat dispersion (W/m2 3 K) Fb = gas density in the bulk (kg/m3) Fc = column density (kg/m3) Fg = gas density (kg/m3) Fp = extrudate density (kg/m3) Fw = column wall density (kg/m3) θ = coefficient of effective energy employed for adsorbent heating

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