From Industrial Waste Glycerol to Triacetin: Kinetic Modelling, Process Intensification and Scale-up Feasibility

Abstract

Due to the growth of global energy demand and climate change concerns as well as the depletion of fossil-based resources, the search for alternative sources to fossil fuels and petrochemicals has become vital. In this context, biodiesel has emerged as a biofuel, with its market and production continuously growing over the years. It is manufactured through the transesterification of triglycerides obtained from renewable materials also generating crude glycerol (Gly) as by-product. The low cost, high availability and versatility of crude Gly makes it an attractive feedstock for the production of high-value chemicals such as triacetin (TA). TA has gained interest recently due to its growing market and various uses, particularly as a fuel additive. TA is generated from the esterification of Gly with acetic acid (AA), which is a three-step equilibrium reaction that also produces intermediate esters monoacetin (MA) and diacetin (DA) as well as water as a by-product. This thesis investigates the reaction under realistic conditions in which crude Gly (50.9 wt.%) from a 2nd generation biodiesel plant was purified physiochemically and utilised as the feedstock (74.3 wt.%). Experimentally, purified Gly (74.3 wt.%) was first used in esterification reactions for screening and recycling of commercial catalysts. Amberlyst 36 was the best performing attaining and maintaining the highest TA yield at ≈25% over 5 cycles. The reaction was then evaluated at different operating conditions for kinetic modelling on Aspen custom modeller V12.1. Based on model discrimination, Eley Rideal model with a water adsorption term best fits experimental data having a root mean square error and % variation explained of 0.0941 and 98.2%, respectively. The activation energies of the forward reactions range between 38.2 kJ mol-1 and 45.8 kJ mol-1 and the backward reactions between 47 kJ mol-1 and 50.3 kJ mol-1 while the water adsorption term is 28.2 kJ mol-1. As impurities are present, catalytic deactivation models were tested but found to not improve the model fittings. Subsequently, the developed model was exported to Aspen Plus V12.1. A scale-up process was proposed utilising 1,630 kg h-1 of purified Gly (82.0 wt.%) to generate ≈18,000 tonnes of high purity TA (99.6 wt.%). The process consists of four sections Gly pre-treatment, esterification, AA recovery and TA purification. The total investment cost and total annualised cost are £2,515 tonneTA-1 and £1,955 tonneTA-1 leading to a competitive TA minimum selling price of £1,754 tonneTA-1. Furthermore, a life cycle assessment illustrated that the primary energy demand and climate change impact are 370 MJ kgTA-1 and 22.8 kgCO2,eq kgTA-1, respectively. The main hotspots were identified to be MP steam heating demand required by the TA purification section. As the reaction is thermodynamically limited, the integration of a pervaporation membrane reactor (PVMR) was evaluated to enhance TA yields and reduce process costs. Using the commercial membrane Nafion 117, TA yield improved by 41% at lab scale. At larger scale, a PVMR integrated process consisting of a CSTR and subsequent PVMR step increases annual production capacity by 23% up to ≈22,000 tonnes of high-purity TA (99.7 wt.%). The process also required 16% lower total annualised cost of £1634 tonneTA-1 leading the TA minimum selling price to decrease to £1,577 tonne-1. Overall, this work presented realistic feasible processes for the production of TA from waste Gly that are relevant for industrial implementation. The work also provides insights into the utilisation of process intensification to further enhance process efficiency and cost savings.

Date of Award	17 Oct 2025
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Vincenzo Spallina (Main Supervisor) & Jesus Esteban (Co Supervisor)