**6. Evaluation of properties of UCO and biodiesel**

The pH value of UCO varies between 5.13 and 6.61, indicating a weak acid for a biodiesel feedstock. The pH value reported in the literature were not uniform but depended on the type and degree of usage. Food such as sausage triggered higher pH values than fish etc. This is a result of fats from fish being more acidic than those of beef [39]. The value of acidity is reduced in used palm oil after repeated frying due to the effects of thermal degradation and contamination from the food items. UCO which is normally subjected to heating witnesses a reduction in pH as a result of usage. Generally, due to repeated and high cooking temperatures, the acid value of the oil tends to reduce. However, the transformation and the mechanism for the generation of cyclic and noncyclic hydrocarbon in vegetable oil during high-temperature repeated cooking can be difficult to predict as a result of the many reactions that produce many unstable intermediate hydrocarbons. In contrast, the results obtained as reported in the literature show that biodiesel is a feasible feedstock for the production of HDRD which is also known as green diesel. From the previous report research analysis, EN 14214 and other standard test methods were used for the analysis of biodiesel. Density at 15°C and kinematic viscosity at 40°C of the fuel were measured under EN ISO 3675:1998 and EN ISO 3104:1994,with 926 kg/cm3 and 37.3 1mm2 /s values being reported respectively [73]. The acid value and iodine value analysis were

done by titration under EN 14104:2003 and EN 14111:2003, with 0.63 mgKOH/g and 109.98 mgI2/100 g being reported respectively [74]. The flashpoint was measured using the Pensky-Martens method ISO 2719:2002 [75], and the sulfur content was quantified under EN ISO 13032:2012 [76]. The aforementioned properties prove the viability of biodiesel as a potential feedstock for HDRD production. The application of HDRD derived from biodiesel fuel offers high yield, excellent storage stability, high cetane number, and flashpoint.
