Influence de l'énergie de surface d'un tambour deshuileur sur son efficacité de relevage

Influence of the Surface Energy of a Deoiling Drum on Its Gathering Efficiency

¹ Institut National des Sciences Appliquées
² Ministry of Industry

Résumé

Au cours de cette étude est analysée l'influence de l'énergie de surface d'un tambour déshuileur sur son efficacité et sa sélectivité de relevage. Quatre tambours en acier inoxydable, en polychlorure de vinyle, en polypropylène et en dérivé fluorocarboné de tensions critiques de mouillage différentes sont testés comparativement en fonction de l'histoire de leur expositionvis-à-vis de phases hydrocarbure (kérosène) et eau. Les essais réalisés montrent que seuls les tambours en dérivé fluorocarboné de très faible tension critique de mouillage s'avèrent intéressants pour des utilisations industrielles. En effet, ils présentent un comportement indépendant de l'histoire de leur exposition ce qui les rend très faciles à utiliser, sans précautions particulières, du fait de leur très grande sélectivité vis-à-vis des hydrocarbures. Par contre, les autres tambours, qui présentent des contributions polaires non négligeables à leur énergie de surface, ont des comportements vis-à-vis des hydrocarbures fonction de l'histoire de leur exposition et ne s'avèrent pas sélectifs vis-à-vis des hydrocarbures à récupérer.

Abstract

The aim of this article is to analyze the influence of the surface energy of a deoiling drum on its gathering efficiency and selectivity. The project consists in testing drums having the same size but made of materials having different surface energies so as to evaluate the influence of this key parameter that had previously been poorly analyzed. Four materials were chosen : stainless steel, polyvinyl chloride, polypropylene, and a fluorocarbon derivative. These materials are representative, a priori, of the behavior of most materials because of the position of their critical wetting tension (gamma c) compared to the superficial tension values of water (72 dyn/cm) and the hydrocarbon phases to be gathered (25 to 35 dyn/cm) (Fig. 1). It should be noted that the fluorocarbon derivative drum tested was patented by our laboratory and is currently marketed by Société Elf. To simulate industrial conditions of the use of deoiling drums, we chose a tangential rotation speed of 0. 24 m/s, appreciably corresponding to the rotation speed of Elf drums. Likewise, the pollution sequences performed in the laboratory were examined to simulate the different working conditions of an industrial drum. Figures 5, 6, 7 and 8 show the volumes of kerosine and water gathered for each of the four drums as a function of time, together with the cumulative volumes of kerosine added to the test tank (Fig. 2) to simulate sequential pollution. The experimental results obtained reveal the different types of behavior by deoiling drums according to the nature of the material making them up :1) If all the drums tested were first immersed in and imbibed by the kerosine phase before being immersed in water, they were able to recover the polluting kerosine phase under excellent conditions. No appreciable difference in behavior was found among the different materials (Fig. 17). 2) At the end of the recovery of the initial pollution, with the drums rotating in water, three different types of behavior could be seen as a function of time. The steel drum immediately forms a very large film of water over its entire surface area (Fig. 5). The PVC and polypropylene drums are not immediately covered by water after the disappearance of the kerosine film. However, after several hours, a slight rise in the water film can be seen in preferential areas of the drums. This dynamic phenomenon increased in time, and after 24 hours of operating the entire surface area of the drums was covered by a homogeneous water film (Figs. 6c and 7c). Only the fluorocarbon-derivative drum, after having entirely recovered the hydrocarbon phase, was not covered by water after 24 hours. It thus proves to be very advantageous from the industrial standpoint since it is highly selective with regard to hydrocarbons. 3) After this conditioning phase in water during 24 hours, recovery of the second pollution phase led to two very different behaviors between the steel drum and the three plastic drums. The stainless-steel drum continued, despite the presence of a layer of 3 cm of kerosine on the surface, to bring up the same amount of water. On the contrary, the three other drums immediately raised a very large film of kerosine. To conclude, these tests reveal three types of extreme behaviors that are representative, a priori, of the materials as a whole. Materials with high surface energy of the metal type cannot raise a hydrocarbon phase under good conditions unless they are previously wetted hence conditioned by this hydrocarbon phase. Such materials are thus of little interest for the continuous specific recovery of hydrocarbon phases. Materials with low surface energy such as plastics can be classified in two groups :- Standard plastics with a critical wetting tension of greater than 30 dyn/cm, and which often have appreciable polar contributions to the surface energy to be recovered without prior conditioning of the drum by a hydrocarbon phase. However, after prolonged contact with water, such drums take up large amount of water. In effect, they are suitable only for sequential operating under the control of an operator. - On the other hand, drums made of fluorocarbon derivatives with very low critical wetting tension proved to be highly selective with regard to hydrocarbons, and above all they do not take up water. Such drums marketed on a large scale by Société Elf thus lend themselves to continuous autonomous operations.