Adsorption of butyl acetate in air over silver-loaded Y and ZSM-5 zeolites: Experimental and modelling studies

Abstract

Adsorption behaviours of butyl acetate in air have been studied over silver-loaded Y (Si/Al = 40) and ZSM-5 (Si/Al = 140) zeolites. The silver metal was loaded into the zeolites by ion exchange (IE) and impregnation (IM) methods. The adsorption study was mainly conducted at a gas hourly space velocity (GHSV) of 13,000 h⁻¹ with the organic concentration of 1000 ppm while the desorption step was carried out at a GHSV of 5000 h⁻¹. The impregnated silver-loaded adsorbents showed lower uptake capacity and shorter breakthrough time by about 10 min, attributed to changes in the pore characteristics and available surface for adsorption. Silver exchanged Y (AgY(IE)) with lower hydrophobicity showed higher uptake capacity of up to 35%, longer adsorbent service time and easier desorption compared to AgZSM-5(IE). The presence of water vapour in the feed suppressed the butyl acetate adsorption of AgY(IE) by 42% due to the competitive adsorption of water on the surface and the effect was more pronounced at lower GHSV. Conversely, the adsorption capacity of AgZSM-5(IE) was minimally affected, attributed to the higher hydrophobicity of the material. A mathematical model is proposed to simulate the adsorption behaviour of butyl acetate over AgY(IE) and AgZSM-5(IE). The model parameters were successfully evaluated and used to accurately predict the breakthrough curves under various process conditions with root square mean errors of between 0.05 and 0.07.

Introduction

Volatile organic chemicals (VOCs) are emitted as gases from certain solids or liquids which contain organic compounds. Paints, varnishes, and wax all contain organic solvents, as do many cleaning, disinfecting, cosmetic and degreasing products. All of these products can release organic compounds while using them, and, to some degree, when they are stored. When these organic compounds released to atmosphere, they become a key contributor of smog formation. Smog is hazardous because it decreases visibility.

Adsorbent-based processes for the separation of multi-component gaseous mixtures are becoming increasingly popular. The new generation of synthetic and more selective adsorbents developed in recent years has enabled the adsorption-based technology to compete successfully with the traditional gas-separation techniques, such as cryogenic distillation [1]. Adsorption technology nowadays is being implemented successfully for its commercial applications in the removal of VOCs. For example, SORBATHENE unit technology, based on the pressure-swing adsorption principle, developed and patented by Dow Chemical Company in 1987, has been installed as an economical alternative for the recovery of VOCs [2].

The adsorbent acts as a separation medium for the process. The primary requirements of an absorbent are the selectivity, in which it determines the preferential adsorption of one or more component based upon equilibrium and/or kinetic mechanisms. Besides, a good adsorbent gave the maximum possible loading of VOC on the adsorbent and it must be chemical and physical stable under various operating conditions. Activated carbons are generally used in many adsorption processes due to their higher adsorption capacity and lower price. However, their regeneration is very difficult because of their thermal and chemical instabilities that may cause significant safety problems [3]. As an alternative to activated carbon, high silica zeolites have several advantages. It is reported that at relatively high humidity, carbon takes up appreciable quantities of moisture thereby limiting their effectiveness for VOC uptake [4]. A part of that, zeolites are inorganic and hence, they can be regenerated in air, subject to flammability considerations. The use of hydrophobic zeolites is attracting more and more attention due to their resistance to high thermal operation and their high adsorption affinity for VOC in humid conditions [4].

The common operating experimental variables in the adsorption studies are the gas flow rate, VOC concentration and the type of VOC. Additional variables are the amount of adsorbent and the temperature of the adsorbent bed. The presence of moisture in the gaseous stream is likely to affect the adsorption process. For adsorption systems that are well designed and operated, continuous VOC removal efficiencies of greater than 95% are achievable for a variety of solvents [5]. It has been observed that at higher gas flow rate, longer time is needed to reach the steady-state concentration under identical operating conditions. This indicates the overall drop in the adsorption rate at higher flow rate [6], [7]. A more detailed analysis has revealed that the heat of adsorption varies strongly with VOC concentration range [8]. However, vent streams with high VOC concentration can be diluted with air or inert gases. Second, the type of model VOC chosen with very high molecular weight compounds (MW ≥ 130) and are characterized by low volatility (boiling point >204 °C) are strongly and readily adsorbed on carbon, making it difficult to be desorbed during regeneration. Conversely, low molecular weight compounds (MW < 45) do not adsorb readily on carbon. Tao et al. [9] proposed hydrophobic zeolite as a promising adsorbent, which is superior to activated carbon due to their resistance to humidity and their non-flammability, the use of hydrophobic molecular sieves such as high silica zeolites are gaining importance for the adsorption of VOC.

Gas stream relative humidity above 50% can affect working adsorption capacity at VOC concentrations below 1000 ppm [10]. Chou and Chiou [11] reported that the capacity of a specified adsorbent to adsorb a particular VOC depends mainly on vapour pressure of the VOC, moisture and temperature of the VOC-laden gas stream. Biron and Evans [12] studied the effect of water vapour on the organic mixture adsorption of cyclohexane, acetonitrile, 2-butanone, iso-propanol, n-heptane, 1-hexanol, diacetone and methylphosphonate by activated carbon. It was found that the adsorption capacities for the organic mixture decreased with increasing relative humidity. Hydrophobic adsorbent has been intensively investigated for humid VOC adsorption as they can eliminate problems associated with suppressing VOC uptake in the presence of high humidity. Zeolites have some advantages because they are hydrophobic and this property changes with an increase in their Si/Al ratio [4].

Combined adsorption–catalytic combustion is an innovative technology for VOC removal in which, the metal loaded adsorbent can also act as good oxidation catalyst for decomposition of VOC during the desorption process at high temperature [13], [14]. The adsorption process of the organic vapour onto the adsorbent will be carried out until saturation and subsequently, the desorption stream at high organic concentration will be effectively oxidized by the same material at high temperature. The presence of suitable active metal in the adsorbent is critical during the desorption process to ensure the success of the combustion of the organic molecules. In the present study, silver-loaded zeolite was selected as the dual function adsorbent–catalyst as the metal is reported to be very effective active component for many VOC combustion catalysts [3], [15], [16]. The high activity of this metal in the oxidation process is associated with its ability to present at multiple oxidation states (+1 to +3) under the reaction conditions and the easy dissociation of oxygen molecules on surface of this metal [16]. Ag loading at low concentration into zeolites is not expected to significantly influence the adsorption behaviour of organic as the adsorption of organic molecules onto metal sites is not usually favoured as compared to that on high surface area zeolites. However, the presence of Ag in those zeolites is crucial in the catalytic combustion stage of the combined adsorption–catalytic combustion process as this metal can act as a good catalyst for the oxidation process.

The objective of the present research is to obtain the breakthrough curves of butyl acetate as a model VOC in a fixed bed of silver-loaded zeolite adsorbent as the first process of the combined VOC adsorption–catalytic decomposition. A suitable adsorption model to evaluate process parameters from the experimental breakthrough data is also proposed. The adsorption process variables are VOC concentration, gas hourly space velocity (GHSV) and the presence of water vapour in the feed. The simulated breakthrough curves are compared with the experimental one. However, the catalytic combustion during the desorption step is beyond the scope of the present study.