Modelling and optimisation of low solids waste treatment processes

Abstract

Dulux Australia is a subsidiary of the multinational chemical company ICI and is the largest paint manufacturer in Australia. To increase paint production capacity it will be necessary for the company to implement more effective methods of cleaning and waste disposal at the manufacturing site. Paint is a substance composed of resins and pigments which are initially suspended in a mixture of liquid solvents. In this form the paint must be stored in sealed containers to prevent evaporation of volatile solvents. The liquid paint can be applied by brush or spray to a wood or metal surface where it subsequently dries to provide a durable protective coating. The resins are used for surface adhesion, the liquid solvents allow easy application of the paint and are sufficiently volatile to promote quick drying, and the pigments provide colour. Since colour is a particularly important characteristic it is essential that the tanks in which the paint is manufactured are scrupulously cleaned between batches. The cleaning process generates a large amount of liquid waste from which the solvents can be recovered by distillation. Unfortunately the high temperatures normally required to recover the less volatile solvents can result in baking of solid waste onto the walls of the container. In this article we propose a simple model for the distillation process and show that boiling temperatures can be lowered by reducing the pressure in the container. In particular we show that the required pressure reduction can be achieved using a condenser, provided that the heat exchange capacity of the condenser matches the heat exchange capacity of the boiler at the desired boiling temperature. In this way we suggest that baking of solid waste onto the container walls can be avoided. Since both the boiler and the condenser used for this process by Dulux Australia are vacuum rated it may be possible for the company to implement the proposed operation with only minor modifications to the current equipment.