ADSORPTION PROCESS FOR VOC (VOLATILE ORGANIC COMPOUNDS)

REMOVAL

CONTENTS

Adsorption Volatile Organic Compounds (VOCs) Source & Impact of VOCs VOC Removal techniques Adsorption Process VOCs Adsorption Fixed Bed Adsorption Theory Important Parameter That effect the performance of Adsorption Carbon Based Adsorption:

BAC(Bead Activated Carbon for Gas treatment system) Continuous Adsorption / Desorption System Indoor VOC air cleaning with activated carbon fiber (ACF) filters Adsorption Isotherms of Benzene and Toluene.

ADSORPTION:

• Definition• Adsorption is a process that occurs when a gas or liquid solute accumulates on

the surface of a solid or, more rarely, a liquid (adsorbent), forming a molecular or atomic film (adsorbate).

• Word "adsorption" is also used to describe the transition of a fluid when it transforms into an adsorbed phase.

Adsorption – a phenomenon general which involves the preferential partitioning of substances from the gaseous or liquid phase onto the surface of a solid substrate

Adsorbent

Gas phase Adsorbate

Adsorbed phase

Physical adsorption is caused mainly by van der Waals forces and electrostatic forces between adsorbate molecules and the atoms which compose the adsorbent surface.

Thus adsorbents are characterized first by surface properties such as surface area, structure (roughness) and polarity.

ADSORPTION

Volatile organic compounds are organic chemicals that have a high vapour pressure and easily form vapours at normal temperature and pressure. More precisely, if an organic compound has a vapour pressure greater than 0.1 mm Hg at 20 °C, it is considered volatile. Hundreds of VOCs can be found in the air and have been documented from a variety of sources.

VOLATILE ORGANIC COMPOUNDS

SOURCES & IMPACT OF VOLATILE ORGANIC COMPOUNDS

VOC REMOVAL TECHNIQUES

ADSORPTION PROCESS

Huang et al. (2005) defines adsorption as the separation of a substance from its gas phase and

accumulation of the substance in its adsorbed phase on a solid surface.

Adsorption also involves the competition for binding sites on the surface of the adsorbent by the

adsorbing molecules.

Researchers have reported that although this competition also exists between water and pollutant

molecules on desiccants, their capacities for pollutant or moisture removal remain relatively

unaffected (Lee et al., 2000).

A particular adsorption pattern, called the adsorption isotherm defines the relationship between

the gas phase and the adsorbed phase concentration at a constant temperature (Das et al., 2004;

Huang et al., 2005).

Most isotherms take the form of a variant of a sigmoid shape.

VOCs ADSORPTIONAdsorption of VOCs at low coverage all models tend to transform into a linear model and this is called Henry’s Law (Ruthven, 1984). The constant of proportionality (K) is called the Henry constant. The expression for the Henry constant is given as:

q = Kcwhere K can be due to concentration or due to pressure. C = Concentration of adsoprbed phase.

In fixed bed adsorption the exiting fluid may be absolutely free of adsorbate until the bed approaches saturation.

Generally plug flow systems are characterised by a stoichiometric equilibrium front, the “shock wave”, which is a sharp concentration front moving through the bed as shown in Fig. 2.

Upstream of the front the adsorbent is saturated with adsorbate and the concentration of solute in the fluidis that of the feed, co.

FIXED BED ADSORPTION THEORY

Modelling of isothermal, single transition systems

Starting from an initially sorbate free column or bed (Fig. 2) to either a step change in sorbate concentration at bed/column inlet or to the injection of small pulse at bed/column inlet (step and pulse input, respectively), mass balance and kinetic equations for an element of the adsorption column are derived. For an axially dispersed plug flow the dynamic behaviour at very low concentrations is given by the partial differential equation:

The first term accounts for axial dispersion and the DL component accounts for Eddy diffusion. The next twoterms permit an axial variation in fluid velocity and the fourth term is the volume-average adsorbate loading perunit mass (Ruthven, 1984), The linear driving force model for mass transfer is given by an expression in Eq. (3),

where q* is the adsorbate loading in equilibrium with the solute concentration, c, in the bulk fluid. Additionally, c’ is the concentration in equilibrium with the average loading q-, and k is the overall mass-transfer coefficient, which accounts for both external and internal transport resistances.

The constant K derives from the equilibrium adsorption isotherm, as Henry’s law in this case, q = Kc. (Das et al., 2004; Ruthven, 1984; Adsorption notes; Crittenden and John Thomas, 1998)The fluid mass balance:

Rate Equation:

where θ = c/co, and ψ = q-/q and the dimensionless time and bed length parameters are

Modelling desorption

For desorption or regeneration of the adsorbent Eq. (11) applies.

Adsorption capacity and linear adsorption isothermsThe adsorption capacity is the amount of the molecule adsorbed (the adsorbate) per unit mass of the adsorbent at a given gas-phase concentration under equilibrium conditions. It corresponds to one point on the adsorption isotherm.Dubinin postulated that the amount of vapour adsorbed (W) by an activated carbon source, at a relative pressure (P/Ps), is a function of the thermodynamic potential (A), with A expressed as:

Several tests on organic compounds such as benzene led to the classical expression of Dubinin and Radushkevich (the D–R equation) shown below:

Benzene is used as a reference compound for carbonaceous materials and its β is assigned the value 1. In orderto simulate the adsorption of VOCs on adsorbents, the equilibrium constant, K, was obtained by using the D–Requation. The D-R equation was also used to determine the amount of benzene adsorbed on the silica gel. The saturation pressures of benzene at different temperatures were approximated by using the Claussius–Clapeyron equation (Ruthven, 1984). The Claussius–Clapeyron estimation was applied under the assumptions that: 1. the saturated vapour is an ideal gas. 2. the molar volume of the saturated vapour is much greater than the molar volume of the saturated liquid. 3.the heat of vapourisation is constant over the temperature range of interest.

Adsorption capacity and linear adsorption isotherms

IMPORTANT PARAMETERS THAT EFFECT THEPERFORMANCE OF ADSORPTION PROCESSES

• Initial gas phase concentration• Initial adsorbed phase concentration• Bed temperature (K)• Increasing/decreasing bed length• Changing interstitial velocity• Particle diameter• Desorption and regeneration

Fig: Effect of initial gas phase and adsorbed phase concentrations on breakthrough of benzene over 4 X 6 silica gel particles

Effect of bed temperature on the adsorption characteristics of benzene on silica gel.

Effect of bed length on breakthrough of benzene over silica gel particles.

Effect of particle radius on the overall effective mass-transfer resistance (k)

•High resistance against abrasion, less dust generation•Small spherical grains which enable high-speed adsorption•Excellent flowability with high sphericity suitable for fluidized bed system•Low ash content•Almost no deterioration over repeated reactivations

BAC(Bead Activated Carbon for Gas treatment system)

Bead Activated Carbon Conventional CoconutActivated Carbon

CARBON BASED ADSORPTION

CONTINUOUS ADSORPTION / DESORPTION SYSTEM

1.Adsorption Section (Fluidized bed)• Form fluidized bed of 15~20mm thickness• Adsorb VOC, Solvent and Odor from effluent

gas• Clean treated gas is released to the

atmosphere

2.Desorption / Condensation Section• BAC used for adsorption is transferred to the

that exchanger and heated under counterflow contact with the desorption gas which flows upward

• Solvent vapor is transferred to the condenser and liquefied for recovery

3.Transfer Section with Carrier Gas• Desorbed BAC is transferred to the top of

the adsorption for the next adsorption process

VOC Treatment:Flow Scheme

Fig. The apparatus holding the ACF media was removed, rotated, and reinstalled during regeneration. The direction of air flow through the ACF during regeneration was opposite the direction of air flow during periods of VOC adsorption

Indoor VOC air cleaning with activated carbon fiber (ACF) filters:

M.A. Sidheswaran et al. / Building and Environment xxx (2011) 1-11

Adsorption Isotherms of Benzene and Toluene on Corn Grain-Based Carbon

Monolith at (303.15, 313.15, and 323.15) KThe isotherm data were correlated with the Toth isotherm model which is widely known and highly applicable for correlation of adsorption measurements involving heterogeneous adsorbents. The Toth isotherm is a simple and highly accurate one at both low and high pressures

where P is the equilibrium pressure; N is the moles adsorbed and m, b, and t are the model parametersThe total micropore volume of the carbon monolith was also evaluated using the Dubinin-Astakhov

(DA) equation which is based on potential theory. This isotherm equation is reasonably good in explaining adsorption equilibria of organic vapors onto microporous activated carbon.

Figure: Adsorption isotherms of benzene (●, 303.15 K; ■, 313.15 K; ►,323.15 K; -, Toth isotherm) (a) and toluene (O, 303.15 K; □, 313.15 K; ∆, 323.15 K; -, Toth isotherm) (b) on corn grain-based carbon monolith

Adsorption Isotherms of Benzene and Toluene on Corn Grain-Based Carbon

Monolith at (303.15, 313.15, and 323.15) K

References:

1. Faisal I. Khan *, Aloke Kr. Ghoshal, Removal of Volatile Organic Compounds from polluted air, Journal of Loss Prevention in the Process Industries 13 (2000) 527–545.

2. Meera A. Sidheswaran , Hugo Destaillats ,*, Douglas P. Sullivan , Sebastian Cohn , William J. FiskEnergy efficient indoor VOC air cleaning with activated carbon fiber (ACF) filters, Building and Environment xxx (2011) 1-11.

3. Matiwaza Ncube , Yuehong Su, The removal of volatile organic compounds from supply air using a desiccant column – A theoretical study, International Journal of Sustainable Built Environment (2012) 1, 259–268.

4. Debasish Das, Vivekanand Gaur, Nishith Verma , Removal of volatile organic compound by activated carbon fiber, Carbon 42 (2004) 2949–2962.

5. C. J.Nwali, Volatile Organic Compounds Removal by Adsorption on Activated Carbon Filters, International Journal of Advanced Research in Chemical Science (IJARCS) Volume 1, Issue 3, May 2014, PP 38-43.

6. Kwang-Hyun Park, M.S. Balathanigaimani , Wang-Geun Shim, Jae-Wook Lee , Hee Moon, Adsorption characteristics of phenol on novel corn grain-based activated carbons, Microporous and Mesoporous Materials 127 (2010) 1–8.

7. M. S. Balathanigaimani, Wang-Geun Shim, Min-Joo Lee, Jae-Wook Lee, and Hee Moon,Adsorption Isotherms of Benzene and Toluene on Corn Grain-Based CarbonMonolith at (303.15, 313.15, and 323.15) K, J. Chem. Eng. Data 2008, 53, 732–736.

8. Debasish Das, Vivekanand Gaur, Nishith Verma ,Removal of volatile organic compound by activated carbon fiber, Carbon 42 (2004) 2949–2962