Experimental and theoretical study of VOC removal by adsorption onto activated carbon in a fixed bed column.

In this work, an experimental and theoretical study on the adsorption of vapors of volatile organic compounds (VOCs) in a fixed bed column is presented and developed; the analyzed system is composed by three kinds of VOCs, that is dimethoxy-ethane (DME), isopropyl alcohol (IPA) and methyl-tertbutyl-ether (MTBE) and a commercial activated carbon as adsorbent material. Nowadays VOC are widely used as dissolving and cleaning agents in many industrial processes such as printing, manufacturing of magnetic tapes, electronic chips, pharmaceutical and cosmetic synthesis and are therefore frequently present in the gaseous industrial effluents. Unfortunately VOC are a class of hazardous pollutants, due to their harmful effects of human and animal health and the environments. It is well documented that VOC inhalation and longterm exposure can cause serious health effects; moreover they are included in the “ozone precursor substances”, that means substances which contribute to the formation of ground-level ozone. Therefore, control of VOC emission is mandatory for the industries, according to the environmental regulations issued world-wide. Different techniques are available to destroy VOC by different types of thermal, catalytic or biological oxidation or to remove VOC from gaseous effluents by absorption, adsorption or membrane separation. Among the last type of technologies, adsorption is one of the most extensively used, because of the high selectivity and capacity of the adsorbent material, the ease of operation, the low capital and operating costs, and the compactness of equipment; due to its high separation efficiency even at low concentrations and at low operating costs, adsorption is also employed after one or more previous treatment processes, which operate a more gross separation to reduce pollutant’s concentrations to low levels. Furthermore, adsorption in fixed bed columns is suitable to treat effluents of variable composition and can tackle the problem of accidental increases of pollutant concentration without compromising the effluent quality, but at the expense of a reduction of the column’s useful life. Several adsorbent materials can be used to adsorb VOCs: activated carbon, zeolite, silica gel, MOF, carbon molecular sieve. In particular, activated carbon is a good adsorbent with high adsorption capacities towards polar and non-polar compounds; it can be produced by a great variety of raw materials, such as hazelnut shell, rubber-seed shell, palm kernel shell, coconut shell, rice husk, wood; this large availability of waste material makes the activated carbon relatively cheap and available. However, its removal efficiency can be negatively affected by gas relative humidity, competitive adsorption of different contaminants and slow mass transfer rate. In particular, in the pharmaceutical industry in which the production is predominantly in batch mode, the gaseous streams contain several VOCs and competitive adsorption usually occurs. Furthermore, the gaseous composition can vary periodically in relation to the planning of production. The effluent is treated in a fixed bed column packed with activated carbon before its release into the atmosphere; it is clear that it is difficult to study the column behavior, and hence, to predict its life time in such operating conditions. Indeed, as mentioned before, when a column is fed with a multicomponent stream, the competitive adsorption for the adsorbent sites usually occurs; moreover, because of the discontinuity nature of the feeding stream, the column is subjected to adsorption and desorption phases which take place along the bed until its break occurs. Therefore, the development of a fixed-bed mathematical model able to describe the complex system behavior is crucial if you want to predict the performance of such separation process; to this aim an adequate understanding of both equilibrium and kinetics of adsorption and desorption is essential. In order to obtain this information, the experimental response must be matched with the theoretical response, calculated from the dynamic model of the system. On the other hand, from the perspective of the fixed-bed column design, the model can represent a useful tool to predict the dynamic behavior of the column in a wide range of operating conditions, without recourse to extensive experimentation. Single VOC removal by adsorption process is widely analyzed in the literature, where a variety of VOCs as well as kinds of models to describe their adsorption in several adsorbent materials can be found; far fewer studies focusing on the adsorption of multicomponent mixtures in dynamic conditions are instead available and even fewer studies on the desorption phase have been carried out. In this research work, adsorption and desorption of the three aforementioned organic solvents, widely used in pharmaceutical industries, on a commercial activated carbon are experimentally and theoretically studied in a fixed bed column.