The theories, methods and processes of treating Marpol oil waste were reviewed. Experiments were carried out to select suitable demulsifier, to examine the efficiency and optimise the conditions for treating Marpol oil waste generated in Hong Kong. Methods to improve the operational effectiveness when oil-water separation was incorporated in recovery system were discussed and presented.
Shipping operation contributes the most quantity to the Marpol oil waste in Hong Kong. How to treat and dispose of them raised much concerns in recent years. The first step of such waste treatment is the removal of the large amount of water typically 2.4% to 50.8%. This thesis first deals with this process, so that water separation from Marpol oil waste.
The oil waste is characterised as a stable water-in-oil emulsion with its stability depending on the water content. The temperature, demulsifier and water content of the oil are the major influencing factors on the rate of separation. Cationic surfactants were found to be completely ineffective as demulsifiers, which may act as emulsifiers. Anionic and nonionic surfactants, which were effective demulsifiers, exhibited good oil/water separation. The efficiency of the nonionic demulsifiers increases with the dosage up to a certain extent after which the efficiency falls. At higher concentrations, nonionic and anionic blended with nonionic surfactants would also act as emulsifiers. However, anionic demulsifiers do not show any decrease in efficiency at higher doses.
Gravitational settlement and centrifuging at elevated temperature does not show significant effects on water separation, and therefore, the usage of anionic demusifier was required. Increasing the operating temperature speeded up the separation process, but could not exceed 90oC due to loss of low boiling point fraction of oil. Convection currents caused by the heat may also disrupt the separation during the settling period.
In the settling system of plant trial, the water removal efficiency of the sample at 85 oC was faster than the one at 50°C. In centrifuge system of plant trial, increasing the rotating speed of the centrifuge could improve the oil/water removal efficiency. Decrease of water content in emulsions and subsequent reduction in viscosity, improve pumpability, reduce transfer and discharge time, and can reduce Marpol oil waste handling, and disposal costs. Recommended treatment system for Marpol oil waste is proposed as a combination of operations of settling and centrifugation, and can be summarized as follows: (1) the untreated waste oil flows through strainer, after which, it is heated and demulsified with an anionic surfactant; (2) it is then placed in a settling tank for removal of bottom water layer; (3) the recovered products in the settling tank are decanted, and the residual ray layer transferred to a buffer storage tank awaiting processing in the next stage; (4) the rag layer is then heated again and sent to a centrifuge to recover remaining oil from the ray layer.