Selective synthesis of alpha monoglycerides by a clean method: Techno-economic and environmental assessment

Abstract

This work proposes an alternative green and selective biocatalytic route for Glycerin Monostearate (α-monostearin) production. The conventional method of production uses an elevated temperature. Apart from the high energy consumption, such high temperatures darken the final product's color, lead to random reactions, and produce high orders of diglycerides and triglycerides instead of monoglycerides. The proposed production process was performed by esterifying stearic acid with glycerin in an organic medium using Candida antarctica lipase (Novozym 435) at a mild temperature. The reaction conditions were optimized using the response surface methodology (RSM): optimum conditions were a temperature of 60 °C, glycerin to stearic acid molar ratio of 8:1, and Novozym 435 amount of 6% w/w. The solvent addition remarkably improved the α-monostearin yield to nearly 80% without the need for the energy-intensive distillation step. The conventional autocatalytic esterification (AUT) process was also performed to investigate the comparative monoglyceride yield, and it was found to be 22.5%. Proton nuclear magnetic resonance and gas-chromatography confirmed that α-monostearin could be produced with the highest purity using the proposed enzymatic method (ENZ). Economic and environmental analyses were also conducted for the proposed ENZ process, and the results were compared with those of the AUT process. The total capital investment of α-monostearin production, considering a projected capacity of 4950 t year−1 and 11% interest for the proposed ENZ process, was favorably 2.5 times lower than that of the AUT process, suggesting a promising investment opportunity. However, the total production costs showed unfavorable negative net present value (NPV) and return on investment (ROI) for the ENZ process and favorable positive NPV and ROI for the AUT process, indicating that the proposed venture is not profitable for α-monostearin production. However, the process can be profitable at improved operational stability of Novozym 435 up to 1 kg per 3-ton product. The carbon footprint was calculated on the basis of the given capacity and conditions of 50 and 656 t CO2 eq./year for the ENZ and AUT processes, respectively. The synthesis of α-monostearin using the proposed route can be considered a building block toward a cleaner large-scale production of α-monoglycerides.