Adsorption Dynamics of Organic Compounds and Water Vapor in

Ind. Eng. Chem. Res. 2006, 45, 2303-2314

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Adsorption Dynamics of Organic Compounds and Water Vapor in Activated Carbon Beds Nan Qi, W. Scot Appel, and M. Douglas LeVan* Department of Chemical Engineering, Vanderbilt UniVersity, NashVille, Tennessee 37235

John E. Finn Astrobiology Technology Branch, NASA Ames Research Center, Moffett Field, California 94035

Breakthrough behavior is investigated for an activated carbon bed fed with constant concentrations of organic vapors and a stepped concentration of water vapor. The organics are present as single components and binary mixtures, with both hydrophobic and hydrophilic organics considered. Experiments are performed using a fully automated fixed-bed apparatus, which enables breakthrough curve studies for gas mixtures over a wide range of concentrations, from much less than 1 ppm up to saturation. The adsorbed-phase mass transfer coefficient of water is found to be a strong function of water loading. The system response is complex, with behaviors for hydrophilic and hydrophobic compounds being quite different. A model incorporating an accurate description of the highly nonideal adsorption equilibria is developed to simulate all of the fixed-bed experiments and gives good agreement in time, trend, extent, and shape of the breakthrough curves. Introduction Fixed-bed adsorption using activated carbon is applied widely to remove volatile organic compounds from air. Water can be present in ambient air and gas streams at a high concentration and can fluctuate with time. This is known to greatly affect the adsorption capacity of a system for organic compounds. However, with few exceptions, designs do not account in any rigorous way for the fluctuating relative humidity (RH) that is common in the feed streams to these adsorption beds. This paper examines the role of humidity variations on adsorption dynamics for activated carbon beds. We consider the adsorption of pure water vapor first to establish a foundation and then study the impact of humidity variations on the adsorption of trace organic vapors as pure compounds and binary mixtures. There are a few published studies on the adsorption dynamics of pure water vapor in activated carbon particles, and these have appeared mainly in the past decade. Lin and Nazaroff1 found that there is a strong relationship between water vapor adsorption-desorption kinetics and equilibrium on activated carbon. They found that pore diffusion is dominant below 60% RH and surface diffusion is dominant above 60% RH. They proposed an analytical intragranular transport model accounting for both pore and surface diffusion to interpret rate data. Kinetics of water vapor adsorption on activated carbon has also been studied by Foley et al.,2 Harding et al.,3 and Cossarutto et al.4 Similar results were obtained in their experimental studies, including that water adsorption kinetics follows Glueckauf’s linear driving force (LDF) mass transfer model and that the rate coefficient kn varies markedly with the shape of the isotherm and adsorption mechanism. The largest kn values were observed for adsorption on primary sites at low pressures. kn was found to decrease as the partial pressure of water increases, until it reaches a minimum where the growth of water clusters on primary sites induces bulk adsorption in micropores and a sharp rise in loading occurs in the isotherm. As pressure further increases, kn starts to increase again, which is believed to be caused by the growth of clusters filling mesopores. * To whom correspondence should be addressed. Tel.: (615) 3222441. Fax: (615) 343-7951. E-mail: [email protected].

Removal of single volatile organic compounds from a dry feed by fixed-bed adsorption has been studied in some depth. This includes the removal of organics such as trichloroethylene,5 acetone, 1,2-dichloroethane, ethyl acetate, ethyl alcohol, methyl ethyl ketone, and toluene6 from air. Gas streams for purification normally contain more than one adsorbable component. Multicomponent fixed-bed experiments have been conducted to examine the effects of adsorbate-adsorbate interactions and adsorption kinetics on breakthrough curves. Systems reported in the literature include mixtures of methane-ethane-propane,7,8 n-pentane-isopentane,9 2-chloropropane-chlorobenzene,10 carbon tetrachloride-chlorobenzene,10 acetone-toluene,11 and methyl ethyl ketone-toluene.11 Experiments for investigating water vapor and organic coadsorption dynamics in fixed beds are usually performed in three ways:12 (1) organics are passed into a bed preloaded with water, (2) mixtures of organics and water vapor are passed into a dry bed, and (3) mixtures of organics and water vapor are passed into a bed preloaded with water. Fixed-bed experiments for a single organic compound with water vapor have been performed by a few research groups. Cho13 observed that preadsorbed water can greatly decrease the adsorption capacity of activated carbon for CFC-113. Jedrzejak et al.14 found that breakthrough curves for toluene-water gas mixtures fed to a dry activated carbon bed showed a mutual exclusion effect for both components. Experiments with humidified vinyl chloride vapor fed to a dry bed and dry vinyl chloride vapor fed to a humidified bed were reported by Scamehorn.15 Takeuchi and Furuya11 studied single organic adsorption in a wet fixed bed for methanol, MEK, and toluene to examine the effects of moisture content on organic breakthrough behavior. They found that preloaded water under 40% RH increased the bed capacity for methanol. However, preloaded water under 40% RH did not affect breakthrough curves of MEK and toluene. The water loading at 40% RH was not large, and there was still some pore volume left for adsorption of the organic, e.g., MEK or toluene. Very few fixed-bed adsorption experiments have been performed for gas mixtures of multiple organics with water. Toluene-dichloromethane mixtures at 15, 65, and 90% RH were

10.1021/ie050758x CCC: $33.50 © 2006 American Chemical Society Published on Web 02/23/2006

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studied by Gong and Keener.12 They observed that, with increasing humidity, the breadth of the breakthrough curve increased for toluene and decreased for dichloromethane. Adsorption capacities of both components decreased because of the humidity. The effect was quite significant for toluene at low concentrations and for dichloromethane at high concentrations. Biron and Evans16 studied fixed-bed adsorption of three gas mixtures, 2-butanone-acetonitrile-cyclohexane, 2-propanol-n-heptane, and 1-hexanol-diacetone alcohol-dimethylmethylphosphonate, on prewetted activated carbon at 73% RH and 89% RH. Each mixture consisted of water-soluble and water-insoluble components. The authors reported that water solubility was not the main influence on organic loadings in the prewetted bed. They believed that, if preloaded water did not saturate the bed, organics could be adsorbed on the dry area of the adsorbent surface and displace the preloaded water. High humidity, e.g., 89% RH, dramatically decreased the bed capacities for organic adsorbates, and the adsorbent surface was no longer available for organics, even for the water-soluble compounds. Organic diffusion through condensed water in the pores was not believed to occur by the authors in this situation. In this work, first, pure water vapor adsorption equilibrium and fixed-bed dynamics with humidity subjected to step changes are investigated on BPL activated carbon. Water adsorption kinetics is examined, and an equation is proposed to describe the water mass transfer coefficient as a function of water loading. Then, we investigate a series of interesting and complex breakthrough behaviors of trace organics as single components and binary mixtures with humidity steps. Cooperative and competitive coadsorption of organics with water vapor are observed. Effects of adsorbate chemical properties (e.g., hydrophilicity and volatility) and molecular interactions on breakthrough curves are studied. The possibility of regenerating an activated carbon bed using a humidity step is considered also. A mathematical model is developed to simulate the experimental fixed-bed humidity-step dynamics. Mathematical Model The mathematical model consists of material balances, rate equations, and a description of highly nonideal multicomponent adsorption equilibria using the virial mixing coefficient (VMC) model.17 Conservation Equations. Material balances were written for each adsorbable component i, organics and water vapor, as

∂nji ∂ci ∂(Vci) Fb + + )0 ∂t ∂t ∂z

(1)

where nji is the average loading of the adsorbate within the adsorbent particle, ci is the fluid-phase concentration,  is the void fraction of packing (extraparticle volume fraction), Fb is the bulk density of the adsorbent, and V is the interstitial fluid velocity. Since the fixed bed was operated at isothermal conditions (25 °C) in all experiments of this work, heat effects could be neglected and an energy balance was not needed. This assumption is discussed in detail below with the evaluation of actual temperature variations. Adsorption Equilibrium. Pure water vapor adsorption equilibrium was described using the adsorption isotherm model of Qi and LeVan18 given by

p)

n ξ0 + ξ1n + ξ2n2 + ξ3n3 + ξ4n4

(2)

where p is the equilibrium pressure, n is the equilibrium loading, and the ξ’s are model parameters. For pure organic compound adsorption equilibria, we used the Toth isotherm written in the form

n)

ap (1 + bpt)1/t

(3)

where a, b, and t are constants. Multicomponent adsorption in this study involves highly nonideal systems such as binary mixtures of water and a hydrophobic organic compound and ternary mixtures of water and both hydrophobic and hydrophilic organic compounds. The adsorption equilibria of these systems are treated using the VMC model,17 which we proposed for nonideal systems. The VMC model was developed from an equation of state and is thermodynamically consistent. For a binary system, partial pressures are given by

2 3 3 ln p1 ) ln ppure1 + B12n2 + 2C112n1n2 + 2C122n22 + ... A A 2A (4) 2 3 3 ln p2 ) ln ppure2 + B12n1 + 2C122n1n2 + 2C112n12 + ... A A 2A (5) For a ternary system, the partial pressure of component 1 is

2 2 3 ln p1 ) ln ppure1 + B12n2 + B13n3 + 2C112n1n2 + A A A 3 3 3 3 C n n + 2C122n22 + 2C123n2n3 + 2C133n32 + ... 2 113 1 3 A 2A A 2A (6) Bij and Cijk are virial mixture coefficients with i, j, and k being component indices, and ppurei is the pressure of pure component i at loading ni. ppurei and ni are related by pure-component adsorption equilibrium. Rate Equations. The rate equations based on the tworesistance model are given for each adsorbable component i as

∂nji ) kni(n/i - nji) ) kci(ci - c/i ) ∂t

(7)

The superscript * indicates values at the fluid-phase/stationaryphase interface, where adsorption equilibrium is assumed to be achieved instantaneously. n/i and c/i are related by adsorption equilibrium as described in eq 2 for pure water vapor, in eq 3 for pure organics, and in eqs 4-6 for mixtures. kci is the fluidphase film mass transfer coefficient and can be calculated from the Sherwood number using19

3(1 - )kfi Fbrp

(8)

kfidp Re 0.5 0.33 ) 1.15 Sc Di 

(9)

kci ) Shi ≡

( )

where kfi is the external mass-transfer coefficient, Di is the fluidphase diffusion coefficient, Sc is the Schmidt number, Re is the Reynolds number based on particle diameter, and rp and dp are the particle radius and diameter, respectively. kni is the LDF coefficient for intraparticle mass transfer. kni for water describes complex behavior considered in depth later. kni for organic

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compounds was determined from the experiments and simulation as described later as well. Solution Method. The pressure drop through the bed in our experiments was small (∼2.5% of the absolute pressure), but it was significant enough for us to include it in the mathematical model. This was accomplished as follows. If a packed bed is fed with inert gas or the bed is in equilibrium with the feed, the relationship between volumetric flowrate and absolute pressure is

PfeedFfeed ) PF ) PoutFout

(10)

where P and F are local values within the bed. In our experiments, Pout ) 1 atm. ∆P, the total pressure drop across the packed fixed bed of length L, was determined using Ergun’s equation.20 The Reynolds number based on particle diameter was 1.4. Assuming that the fluid phase behaves as an ideal gas and that adsorption occurs isothermally, the material balance, eq 1, can be written

PoutFout ∂yi FbRgT ∂nji ∂yi )∂t PA ∂z P ∂t

(11)

where yi is the fluid-phase mole fraction of adsorbable component i, A is the cross-sectional area of the bed, Rg is the gas constant, and T is the system temperature (298.15 K). Thus, the full model consists of material balances, an equation for pressure drop, rate equations, and adsorption equilibrium relations. In the numerical solution of the equation set, the axial derivative appearing in eq 11 was written using backward differences. The material balance for bed element j is given by the first-order ordinary differential equation

PoutFout 1 yi,j - yi,j-1 FbRgT 1 dnji,j dyi,j )dt A Pj ∆z  Pj dt (j ) 1, 2, ..., N) (12) with ∆z ) L/N, where N is the total number of elements. To prevent the bed discretization from affecting the results, a fine mesh of elements (N ) 200) was used. A further increase in N did not give a discernible change in the simulation results. For a uniformly packed bed with a small pressure drop, a linear decrease in pressure can be assumed of the form

Pj ) Pout +

∆P (N - j) N

(13)

The rate equation given by eq 7 was written for each element. The LDF coefficient for water in bed element j will be a function of the adsorbed-phase water concentration in that element, i.e., kni,j ) kni(nji,j). The breakthrough curve concentration is given by

ci(t) )

Pout y RgT i,N

(14)

The set of coupled equations was integrated using a Gear’s method solver. Details are given by Qi.21 Experiments Systems. Pure water vapor and two systems of organic compounds were studied on a fixed bed with humidity steps.

Table 1. Properties of Organic Compounds compound

boiling point (K)

solubility in water (wt % @ 20 °C)

ethanol DCM MEK toluene

351.55 312.90 352.75 383.75

infinity 2.00 26.8 0.074

The two organic systems are (1) ethanol and dichloromethane (DCM) and (2) methyl ethyl ketone (MEK) and toluene, with experiments conducted for the organics as pure components and binary mixtures. Some properties of these chemicals are given in Table 1. From normal boiling points, ethanol and MEK have medium volatilities, DCM is relatively light, and toluene is relatively heavy. As for solubilities in water at 20 °C, MEK is hydrophilic, ethanol is very hydrophilic, DCM is hydrophobic, and toluene is very hydrophobic. For comparison purposes, the two organics in a gas mixture were studied at the same feed concentrations. Ethanol and dichloromethane were investigated at feed concentrations of 3 ppm. MEK and toluene were examined at 100 ppm because these two compounds have higher adsorbed-phase loadings, and consequently, propagation times are longer. Apparatus. The fixed-bed apparatus, shown in Figure 1, was developed, fully automated, and used in the humidity-step experiments. It enabled breakthrough curve studies for gas mixtures over a wide range of concentration, from ,1 ppm up to saturation. Some adsorption equilibrium data for both singlecomponent and multicomponent systems were measured with this apparatus also. A two-stage water sparger, two secondary adsorption beds (Beds 2 and 3), and the test bed (Bed 1) were placed in an environmental chamber (Thermotron SE-300L) for temperature control. Two sets of two-stage glass sparging vessels for different organic liquids were placed inside a chilling bath (PolyScience model 1196) capable of maintaining a temperature as low as -40 °C. These sparging vessels contained organic liquids and could be used to preload Beds 2 and 3. These beds were loaded with pure organic compounds at high temperature, cooled to the test temperature, and used to produce stable trace concentrations of the organics over a long period. Three streams of pure nitrogen gas at flow rates F1, F2, and F3 were fed to the system to generate saturated vapors of two organics and water. They were mixed with a pure nitrogen gas stream with flow rate F4 to create the feed stream for Bed 1. To avoid any possible condensation of water vapor, the tube from the outlet of the water sparger to its connection to the main gas stream was heated to a controlled temperature. Desired concentrations of water and organic vapors in the mixture could be precisely achieved over a wide range using mass flow controllers to set F1-F4. At the outlet of Bed 1, the breakthrough concentrations were monitored with time. A gas chromatograph (GC, HewlettPackard 6890) with a flame ionization detector (FID) was used to measure the concentrations of the organic compounds. A slight vacuum was pulled from the bed outlet stream through 1/16 in. o.d. stainless steel tubing with a flow restriction. A very small flow rate of gas (∼2 cm3/min) was continuously withdrawn from the bed outlet stream and passed through a gas sampling valve for GC analysis at desired time intervals. An RH sensor (Vaisala HMP238 with sintered cap removed to increase the speed of response) was used to detect water vapor effluent concentrations. To have a higher accuracy in measuring the pure-water adsorption isotherm, a thermal conductivity detector (TCD) in the GC was used to analyze the water vapor breakthrough concentrations.

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Figure 1. Apparatus for adsorption dynamics of organic compounds and water vapor in activated carbon beds.

Several experimental difficulties were overcome to create the trace concentration feeds and measure the trace breakthroughs. These include removing all lubricants from valves to prevent absorption of organics, heating sample transfer lines, using a heated gas sampling valve enclosure, and using stainless steel tubing treated for surface passivation throughout the apparatus. The whole fixed-bed apparatus then offered excellent sensitivity, reproducibility, and precision for gas-mixture generation and gas-sample analysis, including at extremely low concentrations (,1 ppm). Details are provided by Qi.21 A data acquisition and process control system was developed using National Instruments LabView software to set and read the flow rates of all mass flow controllers for F1-F4, send gas samples to the GC, start the GC analysis procedures, and read the RH sensor via a RS 232C serial bus. All system parameters for the humidity-step experiments are given in Table 2. Materials. BPL activated carbon in 40 × 50 mesh granular form was used as the adsorbent. It was prepared from BPL activated carbon in 6 × 16 mesh form (Calgon Carbon Corp. Lot No.4814-J) by grinding and sieving. The adsorbates were DCM (PRA grade, 99.9+% pure), ethanol (anhydrous, 99.5+% pure), MEK (high-performance liquid chromatography (HPLC) grade, 99.8+% pure), toluene (anhydrous, 99.8% pure), and water (deionized). Zero-grade nitrogen gas was used as the carrier gas. Operating Procedure. About 0.85 g of activated carbon contained in Bed 1 (0.65 cm i.d.) was regenerated at 150 °C for 12 h with helium flowing through at a rate of 300 cm3/min. Bed 1 was then cooled to room temperature and connected inside the environmental chamber. The temperature of the environmental chamber was always set at 25 °C so that water spargers, Beds 1-3, and all other parts inside the chamber were at the same temperature.

Table 2. System Parameters for Humidity-Step Experiments L (m) r (m) R (m) T (K) F (m3/s)

Bed Properties 0.05 Fb (kg/m3) 3.25 × 10-3  4.80 × 10-3 dp(m) 298.15 wc(kg)

517 0.41 2.20 × 10-4 8.50 × 10-4

Feed Properties 3.33 × 10-6 Vs (m/s)

0.10

Rate Properties compound

kc (m3/(kg s))

kn (s-1)

ethanol DCM

3.68 3.49

MEK

3.10

toluene water

2.90 5.32

1.0 × 10-3 6.0 × 10-4 (DCM as pure compound) 1.5 × 10-3 (DCM in any mixture) 9.0 × 10-4 (MEK without water vapor) 1.0 × 10-1 (MEK with water vapor) 1.3 × 10-3 in eq 19 β0 ) -5.44 β1 ) -7.40 × 10-1 β2 ) 6.02 × 10-2 β3 ) -1.15 × 10-3

Before any new experiment started with a clean dry bed or before any humidity step was made in the feed stream, the gas flow was bypassed around Bed 1. Flow rates F1-F4 were adjusted and set to create a feed stream at the desired adsorbate concentrations (water and organics) and total flow rate. Then, valves around Bed 1 were switched to let this feed stream pass through the bed. The adsorbate concentrations of the bed effluent were recorded as a function of time. After the breakthrough concentrations of all components reached their steady values, which corresponded to the feed stream adjusted for pressure drop, Bed 1 was bypassed and a new feed stream was created by adjusting flow rates F1-F4. Humidity-step experiments for pure water vapor were started with a clean, dry bed. One experiment had four steps (0% f

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20% f 40% f 60% f 80% RH). A second experiment was conducted with a single, large humidity step (0 f 85% RH). For organics with humidity steps, a dry feed stream containing single or binary organic adsorbates was created and passed through the bed. After the adsorption equilibrium was approached, the humidity was stepped from 0 to 20% RH in the feed stream while keeping the same organic concentration(s). The same procedure was followed for humidity steps to 40%, 60%, and 80% RH. The organic concentrations in the feed stream were kept constant throughout the four RH steps. The total flow rate of the feed stream was always keep constant at 200 cm3/min at 25 °C with the superficial velocity through the bed being 10 cm/s. Results and Discussion Adsorption Equilibria. Water vapor adsorption equilibria on 40 × 50 mesh BPL activated carbon were measured with the fixed-bed apparatus using the breakthrough method. A feed stream with a preset water vapor concentration was passed into a clean bed at a flow rate of 60-160 cm3/min at 1 atm. After water vapor had fully broken through the bed, an equilibrium point was calculated from the amount of water that accumulated in the bed. Then, a feed stream with a new water vapor concentration was fed to the bed until breakthrough was again complete and a new equilibrium point could be determined. Allowing for pressure drop, we calculated equilibrium points using

∆n )

Fout [c φ(t - t1) wc feed 2

∫t t c(t) dt] 2

(15)

1

which gives the amount of adsorbate accumulating in the bed between times t1 and t2, with the pressure drop correction factor given by

φ)

Pout Pout + ∆P

(16)

where cfeed is the feed stream concentration in the bypass mode (P ) 1 atm), c(t) is the breakthrough concentration, and wc is the mass of carbon in the bed. The effective equilibrium vaporphase concentration reached in the bed is given by

c ) cout

φ+1 2φ

(17)

where cout is the bed outlet concentration measured at the equilibrium state with no further adsorption taking place. The pure water vapor adsorption isotherm at 25 °C is shown in Figure 2. The curve in the figure is a plot of eq 2, which describes the data accurately. The corresponding isotherm parameters are given in the caption to Figure 2. A similar breakthrough method was used to measure some adsorption equilibrium data for pure organics, organic-water mixtures, and organic-organic mixtures. Integrations over breakthrough curves were used to determine equilibrium loadings. The adsorption equilibrium data for pure ethanol and DCM were measured experimentally in this work, and data for pure MEK and toluene obtained by Qi and LeVan22 were utilized. The Toth isotherm parameters were regressed using the objective function

e)

exp 2 (ln pcal ∑ m - ln pm ) m

(18)

Figure 2. Isotherm for water vapor adsorbed on 40 × 50 mesh BPL activated carbon at 25 °C measured using the breakthrough method. Solid curve is a plot of the model, eq 2, with the parameters ξ0 ) 0.402, ξ1 ) 0.410, ξ2 ) 0.0205, ξ3 ) -1.95 × 10-3, and ξ4 ) 4.00 × 10-5. Table 3. Toth Isotherm Parameters for Pure Organic Compounds on BPL Activated Carbon at 25 °C compound

a (mol/(kg kPa))

b ((kPa)-t)

t

ethanol DCM MEK toluene

3.22 × 3.41 × 103 2.22 × 106 7.59 × 108

0.687 2.67 5.96 12.1

0.0264 0.187 0.148 0.136

109

where pexp m is the experimentally measured pressure with data index m, and pcal m is the corresponding model calculated pressure from the loading measured experimentally. Parameters for the Toth isotherm, eq 3, for the pure organic compounds are listed in Table 3; these pertain to adsorption equilibrium over the range of interest in our experiments. Isotherms for the four organic compounds are shown in Figure 3. MEK and toluene are adsorbed much more strongly than DCM and ethanol. The Toth equation describes the data very well in all cases. The VMC model through the B-term (using 1 parameter, i.e., B12) or through the C-terms (using 3 parameters, i.e., B12, C112, and C122) was used to describe all multicomponent adsorption equilibrium data. When similar fitting results were obtained, truncation after the B-term was used in order to reduce the number of system parameters. The VMC model parameters are listed in Table 4. VMC model parameters given by Qi and LeVan22 for the two binary systems MEK-water and toluenewater were used here. The original equilibrium data for these mixtures were measured on 6 × 16 mesh BPL activated carbon at 25 °C with a volumetric apparatus.22 All other VMC parameters were determined from mixture adsorption equilibrium data measured in this work on 40 × 50 mesh BPL activated carbon using the breakthrough method at 25 °C. Pure Humidity Steps. Humidity-step experiments were conducted at 25 °C, with relative humidity increased in four steps, 0% f 20% f 40% f 60% f 80%. The water vapor breakthrough curves are shown in Figure 4. It is apparent that the water breakthrough curves are sharp in response to the humidity steps for 0% f 20% RH and 20% f 40% RH, indicating that the amounts of water adsorbed in these steps is low. Water breakthrough curves become significantly delayed in response to the humidity steps for 40% f 60% RH and 60%

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Figure 3. Adsorption equilibrium data and Toth isotherm descriptions (solid curves) for dichloromethane, ethanol, MEK, and toluene on BPL activated carbon: (a) all four organics over a full range on semilogarithmic axes and (b) expanded view for dichloromethane and ethanol on rectangular axes.

f 80% RH, meaning that the amounts of water adsorbed at these higher water concentrations increase considerably. This observation is consistent with the water vapor adsorption isotherm shown in Figure 2 and with what has been reported about water adsorption in activated carbons in the literature.1-4 To describe the experimental results shown in Figure 4, the LDF coefficient kn must depend on the adsorbed-phase concentration of water. Otherwise, the set of breakthrough curves cannot be described well. We found kn to be a strong function of the water average loading nj. We modeled this relationship using the equation

kn ) exp(β0 + β1nj + β2nj2 + β3nj3)

(19)

where the values of the parameters β are constants given in Table 2. These parameters were obtained by numerical regres-

sion21 using the breakthrough curves of Figure 4. Several equations in different mathematical forms describing a variety of function shapes were constructed and tested; the form of eq 19 is the best one we found to describe our experimental results. Figure 5 shows the variation of kn as a function of the adsorbed-phase water concentration. The value of kn drops quickly at low water loading and reaches its lowest values between 5 and 12 mol/kg, where the growth of clusters of water molecules on primary sites occurs and the corresponding adsorption isotherm shows a sharp rise. Water clusters block the access to pores at these loadings and reduce adsorbed-phase mass transfer rates. As water loading further increases, kn increases dramatically for increased surface diffusion. This result is consistent with the literature.1-4,23 With kn described by eq 19, the breakthrough behavior for the four humidity steps was simulated, and the result is shown in Figure 4 as the solid curve. Simulation results are in excellent agreement with the experiments. As a check, a one-step humidity-step experiment (0% f 85% RH) was conducted giving the water breakthrough curve shown in Figure 6. Water vapor began to break through the bed very quickly at first. After the bed outlet concentration rose above 50% RH, the water breakthrough slowed, and it took a long time for the bed to reach equilibrium with the feed stream. This observation is consistent with the water vapor adsorption equilibrium, which shows low loadings at low water pressures and much higher loadings at high water pressures. It is also consistent with the water adsorption rates on activated carbon, with kn decreasing first, passing through the minimum, and increasing again along with water loading, as shown in Figure 5. Numerical simulation was performed to predict the breakthrough behavior of this one humidity-step experiment with the water adsorbed-phase mass transfer coefficient described by eq 19. The predicted breakthrough behavior is shown in Figure 6 as the solid curve. The prediction depicts the experimental breakthrough time and shape very well. Organic Compounds with Humidity Steps. The experiment for ethanol at 3 ppm with four humidity steps (0% f 20% f 40% f 60% f 80% RH) gave the breakthrough curves for ethanol and water shown in Figure 7. Note that, in this figure as well as in remaining figures, to more clearly show the effluent concentration waves, the time axis is not shown back to zero time, when the dry feed of organic(s) was begun. The arrows indicate the time when the new feed stream at a different RH level was fed to the bed. After dry pure organic broke through the bed and the bed was in equilibrium with the feed stream, the organic loading was calculated. At the end of each humidity step, the percentage of organic left in the adsorbed phase was evaluated relative to the amount of the organic that is adsorbed from a dry pure feed at the same concentration, corresponding to a value of 100%. These percentages of relative organic loadings are positioned on the figures between two consecutive time arrows for each corresponding humidity step. It can be seen that low RH (e40%) did not affect ethanol (3 ppm feed) adsorption very much. At 60% RH, a small extra amount of ethanol (5%) was adsorbed from the gas phase onto the activated carbon bed; thus, ethanol and water show slight cooperative coadsorption with adsorbed water promoting the adsorption of ethanol. However, at 80% RH, competitive adsorption is apparent, with a large amount of ethanol having been driven off of the adsorbent, leaving its loading at 63% of the loading for a dry feed of 3 ppm. The water breakthrough

Ind. Eng. Chem. Res., Vol. 45, No. 7, 2006 2309 Table 4. VMC Model Parameters for Mixtures on BPL Activated Carbon at 25 °Ca mixture

B12 -1.77 × 8.28 × 10-2 1.77

ethanol(1)-water(2) DCM(1)-water(2) ethanol(1)-DCM(2) ethanol(1)-DCM(2)-water(3) MEK(1)-water(2) toluene(1)-water(2) MEK(1)-toluene(2) MEK(1)-toluene(2)-water(3) a

Bij and Cijk have units of

m2/mol

C112 10-2

4.55 × 10-2 9.95 × 10-2 1.87

-1.12 ×

C122 10-1

3.05 ×

C123

10-3 0.000

-2.99 × 10-1

-3.32 × 10-1 0.000

and

m4/mol2,

respectively.

Figure 4. Breakthrough curves for pure water vapor: 0 f 20 f 40 f 60 f 80% RH.

Figure 6. Breakthrough curve for pure water vapor: 0% f 85% RH.

Figure 7. Breakthrough curves for ethanol (3 ppm) with humidity steps. Solid curves are model predictions. Figure 5. Mass transfer coefficient kn for water vapor adsorption on BPL activated carbon of 40 × 50 mesh at 25 °C.

behavior is almost identical to the pure-water behavior under the same humidity steps shown in Figure 4. This is because ethanol at a low concentration of 3 ppm has only a small influence on water adsorption. Figure 8 shows the breakthrough behavior of DCM at 3 ppm under humidity steps. DCM was continuously removed from the adsorbent, showing competitive coadsorption with water at

all RH steps. Competition for adsorbent surface increased as RH increased. There was only 12% of the initial loading of DCM left at 80% RH. Water breakthrough curves were not affected much by DCM at 3 ppm. Humidity-step experiments for a binary organic mixture of ethanol and DCM, both at 3 ppm, were performed with the results shown in Figure 9. When the bed was initially fed with a dry mixture of ethanol and DCM, ethanol broke through the bed earlier, indicating a lower loading than for DCM. The percentages shown in Figure 9 represent the loadings of the

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Figure 8. Breakthrough curves for DCM (3 ppm) with humidity steps. Solid curves are model predictions.

Figure 9. Breakthrough curves for binary mixture of ethanol (3 ppm) and DCM (3 ppm) with humidity steps. Solid curves are model predictions.

organic compounds relative to the corresponding dry, purecomponent loadings at the same feed concentration; the numbers over the bars are for the hydrophilic compound, here ethanol, and the numbers under the bars are for the hydrophobic compound, here DCM. It can be seen that the loadings for ethanol and DCM from the initial dry feed stream were somewhat lower than corresponding single-component loadings because of the slight interaction of the two adsorbates on the carbon surface. The small roll up of ethanol at its initial breakthrough with water-free feed is also a sign of the interaction of two adsorbates. It indicates that ethanol broke through the bed before DCM, and as the DCM concentration wave followed through the bed, a small amount of ethanol was displaced from the adsorbent to the gas phase by the competition for surface area. For the four humidity steps, the breakthrough curves of two organics at 3 ppm showed similar behavior to their single-component response at the same organic concentrations (3 ppm) and at the same humidity steps. In other words, for the humidity steps, the breakthrough curves of ethanol in Figure 9 are similar to the ones in Figure 7, and the breakthrough curves of DCM in Figure 9 are similar to the ones in Figure 8. This

Figure 10. Breakthrough curves for MEK (100 ppm) with humidity steps. Solid curves are model predictions.

similarity is the result of the organics being at low levels (3 ppm for both) and the adsorbate interactions on the surface being weak because of fairly low loadings. So, each compound in the binary mixture of organics responds to RH steps almost as if it were a single component. Water did not affect ethanol adsorption very much at low RH (e40% RH), slightly increased the ethanol loading at 60% RH, and desorbed ethanol considerably at 80% RH. There was 71% of the ethanol (referred to the pure-component loading) left in the adsorption bed at 80% RH. Water and DCM showed competitive coadsorption in all RH steps, and essentially all of the DCM was driven out of the bed at 80% RH. The apparent reversal in selectivity caused by water in Figure 9 is particularly interesting. For the original dry feeds, DCM was the preferred adsorbate as evidenced by its rolling up the ethanol breakthrough concentration. But at 80% RH, ethanol is clearly the preferred component, as very little DCM is left in the bed. The humidity steps are very efficient for removing DCM and inefficient for removing ethanol. Thus, fixed-bed regeneration based on increases in humidity could be used effectively for DCM. It is also apparent by comparing Figures 7, 8, and 9 that the water breakthrough curves were not changed appreciably by the presence of the two organics at 3 ppm. Figure 10 shows the results for MEK at 100 ppm with RH steps. Water did not influence MEK adsorption appreciably below 40% RH, but it decreased the MEK loading and showed competitive coadsorption with MEK when the RH was 60% and above. There was 63% of the MEK left on the carbon at 80% RH. Water broke through the bed more quickly, and the breakthrough curves are sharper than for the pure-water RH steps shown in Figure 4. This observation is more obvious for the RH steps from 40% to 60% and from 60 to 80%. Competition for the limited surface area by MEK and water decreased the bed capacities for both components. Figure 11 shows the results of toluene at 100 ppm with RH steps. Although toluene and DCM are both hydrophobic chemicals, toluene is much less volatile and less water-soluble than DCM. Compared to the experimental results for DCM, toluene at 100 ppm showed both similar and different behaviors under the same RH steps. The similar phenomenon is that water and toluene showed competitive coadsorption at all RH steps, and the competition for surface area by the two components

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Figure 11. Breakthrough curves for toluene (100 ppm) with humidity steps. Solid curves are model predictions.

Figure 12. Breakthrough curves for binary mixture of MEK (100 ppm) and toluene (100 ppm) with humidity steps. Solid curves are model predictions.

increased as RH increased. The different point is that water did not displace toluene very much. There was still 96% of the toluene left in the adsorption bed at 80% RH. As a highly hydrophobic, heavy, nonpolar compound, toluene has higher affinity for the nonpolar activated carbon surface than for water. Adsorption-bed regeneration by humidity increases to remove toluene will be ineffective. On the other hand, because of the high affinity of toluene for the carbon surface, water adsorption from the mixture with toluene was greatly reduced in comparison with the pure water. This can be seen from the very sharp breakthrough curves for water in all RH steps in Figure 11. Figure 12 shows experimental results for a binary organic mixture of MEK and toluene, both at 100 ppm, with RH steps. After the bed was initially fed with a dry mixture of MEK and toluene, hydrophilic MEK broke through the bed earlier (because of a lower loading) than hydrophobic toluene. As the toluene concentration wave gradually passed through the bed, a large amount of MEK was displaced from the activated carbon bed because of the intensive competition for surface area by toluene. The equilibrium loading of toluene was almost the same as that

of pure toluene at 100 ppm, 94%. However, the MEK loading for the mixture feed was much lower than the loading of pure MEK at 100 ppm, 12%. The bed was similar to a bed saturated by dry, pure toluene feed at 100 ppm. The breakthrough curves of toluene under humidity steps in Figure 12 are very similar to those of pure toluene at 100 ppm in Figure 11. Toluene and water showed competitive coadsorption in all RH steps, and the competition for surface area became more intense as RH increased. Water did not remove much toluene from the adsorbed phase, even at 80% RH. However, the breakthrough curves for MEK in Figure 12 are totally different from those for pure MEK at 100 ppm in Figure 10. Water did not have much effect on MEK adsorption, and the MEK loading at 80% RH was almost unchanged after RH steps. Humidity-step regeneration will not effectively remove MEK and toluene. Compared to the pure-water breakthrough curve, the sharp water breakthrough curves in Figure 12 indicate that water adsorption was greatly reduced for the mixture feed of MEK and toluene with both at 100 ppm. Simulation results for organics with humidity steps have been shown as solid curves in Figures 7-12. Although these experiments are difficult to model because of complex adsorption equilibria and kinetics, our simulation predictions are in good agreement with the experiments in the time, trend, extent, and shape of the breakthrough curves. All adsorption equilibria used in the mathematical model were measured and modeled independently. In this work, adsorption equilibrium was found to have a much stronger effect than the mass transfer properties of the adsorbates on breakthrough behavior and simulation. In the simulation of the binary organic mixture of MEK (100 ppm)toluene (100 ppm) with four RH steps on 40 × 50 mesh BPL activated carbon at 25 °C, the VMC parameters regressed from binary adsorption equilibrium data for MEK-water and toluenewater on 6 × 16 mesh BPL activated carbon at 25 °C22 were used with good results. This is because the VMC parameters represent molecular interactions between different adsorbates, not the adsorbate-adsorbent interactions.17 For the two BPL activated carbons, which differ only in mesh size, the VMC parameters are independent of the adsorbent particle size. Also, for each of the two binary organic mixtures with humidity steps, the interactions of any two of the three components were described first using the binary VMC parameters regressed from corresponding adsorption equilibrium data. Then, the ternary cross term was sought. We found that C123 could be set equal to zero with good fixed-bed simulation results. This suggests that the ternary adsorbate interactions for both mixture systems of two organics with water have only small effects on mixture adsorption equilibria in our experiments. Nonisothermal Effects. Nonisothermal effects on the fixedbed humidity-step experiments were carefully considered. The major components of the experimental apparatus (e.g., the water vapor generation system, the fixed bed, and the breakthrough concentration sampling site) were all thermostated in the environmental chamber at 25 °C. However, the heat of adsorption can be considerable and may lead to a significantly nonisothermal fixed-bed adsorption process. If a large amount of adsorbate is adsorbed on the carbon surface in a short time and heat transfer is not efficient from carbon particles to the fluid phase, from the fluid phase to the bed wall, and from the wall to the environment, the nonisothermal effects can be large. In the two sets of pure water vapor humidity-step experiments conducted in this work, the largest amount and fastest rate of water vapor adsorption occurred in the one-step humidity-step

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experiment (0% f 85% RH), where the highest bed temperature rise should be observed. The corresponding bed temperature rise was calculated to be 42 °C using the adiabatic adsorption calculation method of LeVan et al.19 To investigate the real nonisothermal effects, the temperature rise of the fixed bed was measured experimentally. A separate one-step fixed-bed humidity-step experiment (0% f 85% RH) was conducted with two thermocouples placed along the center of the bed cross section, with one in the middle of the packed bed and the other at the bed outlet. The highest temperature rise found was 1.2 °C, which was observed in the experiment at an early time in the adsorption process and in the middle part of the bed. This is small and can be neglected. Since the organics were present at trace concentrations in this study, their heat of adsorption can be ignored. For the four-step humidity step experiment, the highest temperature rise will be