From this point of view, 20m/m% is the maximum amount of water considered to add into original used oil samples for making emulsions (S1). (It is easier to prepare a specific weight of used oil samples than a specific volume ofused oil. Hence, in this study, oil samples are all prepared by weight.) If samples that are more representative could be acquired in the future (with higher average water content), then more efforts will be made on this subject.
Since the water-in-oil emulsion samples are quite different than any samples ever studied with ultrasonic irradiation (mostly dilute aqueous system, or heavy fossil fuels), there are still some factors that not clearly known which will affect our system. For example, the gas environment or the addition of different chemicals (hydrogen source or radical sources) may have different effects in our system from in other systems. Thus, experiments should be designed in a way to identify the effects made by those factors, positively or negatively. To make water/oil emulsion samples, a domestic blender (Hamilton Beach/ Proctor-Silex, Inc., Model 56200) with 16 speed selection and a LO/HI power range was used. The maximum motor power is 425 Watt. Emulsion samples were prepared with different selections of speed and time. Microscopic examination was used to check the stability of the emulsion. An optimal speed/time combination will be decided for preparation of future samples. For each test run, 75g of raw used oil sample or emulsified sample was transferred into 125-mL reaction bottle and subjected to ultrasonic irradiation.