An intensified approach for transesterification of biodiesel from Annona squamosa seed oil using ultrasound-assisted homogeneous catalysis reaction and its process optimization

Context & Rationale

The global transition towards renewable and sustainable energy has intensified interest in second-generation biodiesel derived from non-edible, non-food-competing feedstocks. Annona squamosa L. (custard apple), a tropical fruit tree cultivated extensively across Asia, Africa, and the Americas, produces seeds that are toxic, inedible, and therefore unsuitable for food or pharmaceutical applications ??? yet represent a significant and largely underutilised source of lipid-rich biomass. The conversion of such agricultural waste streams into viable biodiesel feedstocks offers both environmental and economic advantages; however, the efficiency of both oil extraction and transesterification processes from this source had not previously been subjected to systematic, multi-variable optimisation.

Scope of the Study

This study presents a comprehensive, integrated investigation into the ultrasound-assisted extraction (UAE) of oil from Annona squamosa seeds and its subsequent conversion to fatty acid methyl esters (FAME) via ultrasound-assisted transesterification (UAT) using KOH as catalyst. Both processes were independently optimised using Response Surface Methodology (RSM) with face-centred central composite design, encompassing four key process variables each: liquid-to-solid ratio, extraction temperature, sonication time, and sonication amplitude for oil extraction; and oil-to-methanol ratio, catalyst concentration, reaction temperature, and sonication time for transesterification. Quadratic polynomial models were developed and validated through ANOVA at the 95% confidence level.

Optimisation & Process Performance

The RSM-guided numerical optimisation identified process conditions under which both oil yield and FAME conversion attained values that compare favourably ??? in both magnitude and resource efficiency ??? with those previously reported for this feedstock using conventional transesterification approaches. The specific optimised conditions and their corresponding model-predicted and experimentally validated yields are presented and discussed in full within the manuscript, and represent a notable advancement in the intensification of biodiesel production from custard apple seed oil.

Characterisation of the Biodiesel Product

The physicochemical properties of both the crude seed oil and the resultant FAME were characterised against ASTM D6751 standards. Spectroscopic and thermal characterisation of the biodiesel ??? encompassing FT-IR, GC???MS, thermogravimetric analysis, ¹H NMR, and fluorescence spectroscopy ??? collectively confirm the identity and quality of the product. The thermal decomposition profile of the ASSO-derived FAME, in particular, warrants attention as a point of comparison with commercially established biodiesel sources, and is detailed in the full publication.

Keywords

Annona squamosa  ??  Biodiesel  ??  Ultrasound-assisted extraction  ??  Transesterification  ??  Response surface methodology  ??  FAME  ??  KOH catalyst  ??  Process optimisation  ??  Non-edible feedstock  ??  Fluorescence spectroscopy