Numerical modelling of heat and mass transfer in a steam-air sterilisation process inside an industrial autoclave

Abstract

Steam sterilisation in autoclaves or retorts, a widely used thermal process in the food processing and pharmaceutical industries, consumes a large amount of energy and is known to cause unwanted peak energy demand when these processes operate in parallel. Traditionally, steam sterilisation is studied through observation so that there is an extensive reliance on expensive experimental measurements. The simultaneous occurrence of multiple heat and mass transfer phenomena within an autoclave renders steam sterilisation a highly complex process to analyse. Fundamentally based mathematical models of thermal processes often focused on a product or on a small-scale process level. The simulation of industrial sized autoclaves, which is not empirically based including an analysis of energy consumption, has yet been established.

A numerical model, which enables solutions to be realised within minutes, is presented in this thesis for evaluating transient heat and mass transfer phenomena within an operating industrial size steam-air sterilisation cycle, by assuming quasi-steady state conditions for all heat and mass transfer rates and known controller settings. The model including the products and controller was validated by comparison with experimental measurements with a 1.9% difference in the total steam consumption and excellent agreement on all the transient performance profiles, in particular the temperature at the centre of the average product.

A major operational factor shown is the presence of air which significantly reduces condensation even with low concentration. The developed model was used to identify that the uninsulated walls were a major source of heat loss, primarily the energy required to heat up the walls. These losses can be reduced by 70% through insulating on the inner side of the walls compared to 30% when insulating the inside walls. It is also shown that the initial temperature of the products significantly affects the total steam consumption. Finally it is shown that when a predetermined cycle is used, there is a limit to the lower allowable upstream pressure, below which the products will not have been properly sterilised.