Ind. Eng. Chem. Res. 2006, 45, 1441-1445
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Adsorption of Ethylmethylamine Vapor by Activated Carbon Filters Yehya El-Sayed,† Teresa J. Bandosz,*,† Hilda Wullens,‡ and Peter Lodewyckx§ Department of Chemistry, The City College of New York and The Graduate School of CUNY, 138th Street and ConVent AVenue, New York, New York 10031, Defence Laboratories Department, NBC Protection, Martelarenstraat, 181, B1800 VilVoorde, Belgium, and Department of Chemistry, Royal Military Academy, Renaissancelaan 30, B1000 Brussels, Belgium
The adsorption of high concentrations of ethylmethylamine (EMA) vapor from a humid air stream by an activated carbon bed has been studied. The adsorption behavior is quite different from the adsorption of trace quantities from an aqueous solution. The introduction of surface groups has no apparent direct effect on EMA adsorption. However, because these surface groups enhance water adsorption from the air stream and because adsorbed water hinders organic vapor adsorption, the modified carbons exhibit a lower apparent capacity and slower adsorption kinetics for EMA than the untreated ones. EMA is adsorbed from the vapor phase by a different mechanism than it is adsorbed from the liquid phase. The effect of humidity was evaluated and shown to influence the breakthrough time. Introduction Adsorption phenomena have long been used to perform different separation and purification processes. Such processes are usually based on the use of a suitable porous solid adsorbent with a high surface area and/or a high micropore volume. The success or failure of an adsorption process depends on the solid performance in both equilibrium and kinetic mechanisms.1 The former controls the adsorptive capacity, whereas the latter is related to finding a way for adsorbate molecules to reach the interior of surface area or micropore volume. Thus, a good solid adsorbent is one that provides a good adsorptive capacity as well as good kinetics. To obtain an adsorbent that satisfies these two requirements, the porous solid must have a reasonably high surface area or micropore volume, and it must also have a reasonably expanded pore network for the transport of molecules to the interior of the material. Among the practical solids used in industries, activated carbon is the most versatile because its surface features satisfy the above-mentioned requirements.1 It is important to mention that the surface of activated carbon is not completely hydrophobic from the chemical point of view. The chemical nature of the activated carbon surface is more complex than its pore network. These adsorbents are made from raw materials that are usually rich in oxygen, and therefore, many oxygen functionalities are found to exist on the surface. Moreover, the presence of oxygen functionalities is always expected as a result of self-oxidation.2 Activated carbons can be further modified with several chemical agents that usually introduce onto the surface a variety of heteroatoms such as oxygen and nitrogen in the form of organic functional groups. The application of activated carbon for odor removal has been reported in the patent literature.3,4 Human sweat is a source for some of many odorous substances. One of these odorous compounds is ethylmethylamine (EMA). In a previous study,5 the influence of both pore size distribution and surface chemistry (amount and type of surface groups) on the adsorption of EMA * To whom correspondence should be addressed. Tel.: (212) 6506017. Fax: (212) 650-6107. E-mail:
[email protected]. Internet: http://www.sci.ccny.cuny.edu/∼tbandosz. † The City College of New York and The Graduate School of CUNY. ‡ NBC Protection. § Royal Military Academy.
from an aqueous solution was investigated. The results showed that surface chemistry controls the adsorption of EMA at both high and low equilibrium concentrations. At high equilibrium concentrations, the amount adsorbed is directly dependent on the total number of acidic and basic groups on the surface, which indicates the importance of hydrogen bonding. At low concentration, the density of the surface acidic groups enhances the adsorption of ethylmethylamine through acid-base interactions and hydrogen bonding. Porosity plays an important but not predominant role in the strength and extent of the adsorption process. When adsorption of EMA from the vapor phase at equilibrium conditions was studied, the mechanism of the process showed that the interactions with the surface acidic groups are energetically more favorable than adsorption in the porous structure. Unfortunately, the kinetics of the process was not evaluated. The objective of this paper is to study the effect of porosity and surface chemistry on the adsorption of high concentrations of EMA from a humid air stream. This research focuses on both adsorption capacity and adsorption kinetics. The analysis is based on the breakthrough capacities obtained under different experimental conditions. To identify the role of surface chemistry and structural features, carbons of various origins were chosen whose porosities and surface groups differed significantly. The carbon surface was further modified by the introduction of either oxygen- or nitrogen-containing groups. To obtain more information on the reaction mechanism, breakthrough tests were carried out. From the adsorbate breakthrough time, the service life of the activated carbon filter can be predicted. A number of semiempirical models for predicting breakthrough times have been proposed in the past. These include the Mecklenburg,6,7 Wheeler-Jonas (WJ),8-10 and Yoon-Nelson11 equations. All of these equations are based on the mass balance between the quantity of vapor entering the carbon bed and the sum of the quantities adsorbed in and penetrating the bed. Among these equations, the Wheeler-Jonas equation9,12,13 has been widely used by several research groups over the past decades to estimate breakthrough times of organic vapors for filter beds filled with granular activated carbon (GAC).
10.1021/ie0509589 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/11/2006
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Ind. Eng. Chem. Res., Vol. 45, No. 4, 2006
Theoretical Section The Wheeler-Jonas equation (eq 1) is a well-known predictive equation for estimating the breakthrough times of physisorbed organic vapors on activated carbon beds.9,12-14 The prediction is based solely on measurable and readily available macroscopic parameters
tb )
(
)
MWe FbWe Cin - Cout ln QCin kvCin Cout
(1)
where tb is the breakthrough time to reach Cout (min), Cin is the vapor inlet concentration in air (g/cm3), Cout is the chosen breakthrough concentration (g/cm3), M is the weight of the carbon bed (g), We is the equilibrium adsorption capacity (g/ gcarbon), Q is the volumetric flow rate (cm3/min), Fb is the bulk density of the carbon bed (gcarbon/cm3), and kv is the overall adsorption rate coefficient (min-1). Two unknown parameters in this equation need to be calculated, namely, We and kv. In the past, several models were derived that allowed their calculation without any prior breakthrough experiments.15-20 We is normally estimated from the Dubinin-Radushkevich equation15
We ) W0dL exp
[
( )]
-BT 2 2 Cs log Cin β2
(2)
where We is the static adsorption capacity (g/gcarbon), W0 is the micropore volume (cm3/g), dL is the liquid density (g/cm3), B is a structural constant of the carbon, β is the affinity coefficient of the organic vapor, T is the adsorption temperature (K), Cs is the saturation vapor concentration (ppm), and Cin is the vapor inlet concentration (ppm). The only unknown parameters in this equation are W0 and B; they can be derived from nitrogen adsorption isotherms at 77 K. Calculation of the overall adsorption rate coefficient kv is more complex. For organic compounds, kv is mainly linked to the effects of surface diffusion. The most recent and complete approach for calculating the value of kv is to use the following semiempirical equation19
kv ) 800β0.33VL0.75dp-1.5
x[ ] We Mw
(3)
where kv is the overall adsorption rate coefficient (min-1), We is the adsorption equilibrium capacity (g/gcarbon), Mw is the molecular weight of the vapor (g/mol), dp is the average diameter of the carbon particle (cm), β is the affinity coefficient of the organic vapor, and VL is the linear velocity through the bed (cm/ s). Experimentally, different breakthrough times can be obtained by varying the weight of the carbon bed (varying only Mw). A plot of breakthrough time versus carbon bed weight should yield a straight line that allows for the calculation of We and kv using the slope and the intercept. Experimental Section Materials. Two activated carbons were chosen for this study.5,21,22 They are BAX (Westvaco-wood origin, chemically activated with phosphoric acid) and BPL (Calgon Carbon, bituminous coal origin). One sample of each carbon was oxidized with 15 N HNO3 for 24 h. Both carbons were impregnated with urea (saturated solution) for 24 h and then heated in nitrogen at 723 K at the rate of 10 K/min for 1 h to
introduce nitrogen groups. A sample of BPL carbon, initially impregnated with urea, was additionally heated at 1223 K. Before further experiments, the initial and modified carbons were washed in a Soxhlet apparatus to remove water-soluble species. The oxidized samples are referred to as BAXO and BPLO, and the urea-modified ones are referred to as BAXN1, BPLN1, and BPLN2, where N1 and N2 stand for heating temperatures of 723 and 1223 K, respectively. Methods. (i) Boehm Titration. The amounts of oxygencontaining surface groups were determined using a simplified version of Boehm titration.23,24 One gram of carbon sample was placed in 50 mL 0.05 N sodium hydroxide or hydrochloric acid. The vials were sealed and shaken for 24 h and filtered; then, 10 mL of each filtrate was pipetted, and the excess of base or acid left in the solution was titrated with HCl or NaOH, depending on the original titrant used. The total amounts of acidic sites were calculated under the assumption that NaOH neutralizes carboxylic, phenolic, and lactonic groups. The number of surface basic sites was calculated from the amount of hydrochloric acid that reacted with the carbon. (ii) Sorption of Nitrogen. Nitrogen isotherms were measured using an ASAP 2010 apparatus (Micromeritics) at 77 K. Before the experiments, the samples were heated at 393 K and then outgassed at this temperature under a vacuum of 10-5 Torr to constant pressure. The isotherms were used to calculate the specific surface area, SDFT; micropore volume smaller than 10 Å, V
