The Volatile Organic Compounds (VOCs) are very pollutants and present in the gas released by the food, aerospace and electronics industries. They can be hydrophilic or hydrophobic. They have a direct impact on the environment but also on human health. They are sometimes mutagenic, teratogenic, carcinogenic, and are often smelly. Therefore, regulatory constraints on VOC emissions are becoming more stringent in the world, so manufacturers have to find methods of treatment inexpensive and effective for all families of VOCs.

The aim of our study was therefore to develop at the laboratory scale a new absorption pilot valuable for the treatment of hydrophilic and hydrophobic VOCs, simple, adapted for study various parameters and that can be modeled to further industrial scale-up.

In this context, the proposed process in this study for treatment of the gaseous VOCs is an innovative method by spraying emulsion oil in water. The objective by using emulsion oil in water is to enable the capture of both hydrophobic and hydrophilic compounds respectively from oil and water and to promote the transport of the organic solvent in the drops of water by reducing the viscosity of the spray liquid. At the same time, this type of spraying process is also very convenient for the treatment of dust present in the flue gas.

Material and methods:

As organic solvent, the use of vegetable oils has been proposed to reduce the impact on the environment. Vegetable oils have shown affinity for the capture of VOCs such as toluene (R.TATIN 2008).

The pilot laboratory used is a transparent PMMA plastic column of 0.2 m in diameter and 0.7 m in height (Figure 1).

  Pump through a nozzle placed on top of the column sprayed emulsion prepared in advance in a tray for 2 hours. An air flow polluted with a controlled amount of toluene is introduced at counter-current at the bottom to simulate pollution. The liquid emulsion and polluted air are flowing in the column in an open circuit. Thanks to a PID system (RAE3000), Input and Output Toluene concentrations in the column are measured to track versus time the evolution of the absorption efficiency of VOCs by the emulsion. The oil viscosity, surface tension and density were also measured. The Henry's constants of the toluene with oils under test were determined thanks to an equilibrium method. Using a high speed camera NIKON, droplet size distributions and their velocities are measured during the experiments. After a rest, separation of the emulsion is obtained and the oil can be regenerated by thermal effect.

Results:

The results obtained with these devices show that vegetable oils with a high rate of Triacilglycerol monooleate have good absorption capacity of toluene. These vegetable oils have a large molecular weight which gives them some stability as the solvent.

From our laboratory pilot, VOCs absorption efficiencies reached with different types of vegetable oils-water emulsion were measured. For our column size and conditions of gas and liquid flow the absorption efficiencies obtained with the best oils are close to 50%. Characterization of potential oil absorbent was performed for each oil thanks to an equilibrium method; constants sharing m obtained (m = Henry / pressure) for selected oils are close to 0.05. The sizes of emulsion drop are near to 0.5 mm (Figure 2).

  A mass transfer model issued from double film theory was developed to predict the global mass transfer coefficient KG° for a vegetable oil. Variations of absorption efficiency can be well predicted by the model when gas or liquid flows are modified.

  The dynamic regenerations of oils were conducted in a balloon. Following these studies, some oils were considered as "good solvents" especially with their potential absorbent (denoted by the Henry constant) and also for their ability to be regenerate thermally. Their regenerative capacity is a major issue because it limits the viability of the process and the possible reuse of VOCs. Finally, a successful attempt for regenerating a sufficient volume of oil was produced and then used as a solvent in our pilot to validate the viability of large scale regeneration. 90% of its potential absorption has been recovered.

Romuald. TATIN, PhD : « Absorption Physique de Composés Organiques Volatils par Pulvérisation d’Emulsion d’Huile dans l’Eau », LISBP , INSA Toulouse, 3 October 2008.