Optimisation of low solids waste treatment processes

Dulux Australia

Mathematicians at the 1997 Mathematics in Industry Study Group workshop have improved a process used to clean paint tanks, allowing Dulux to decrease the processing time and increase solvent recovery.

Paint is a combination of resins (for adhesion), solvents (for application and drying) and pigments (for colour), and it sells mainly on the basis of colour. To ensure each batch of paint is exactly the right colour, the tanks in which paint is manufactured must be scrupulously cleaned between batches. This generates waste containing up to eight different solvents and particles of dried paint and resin.

Dulux treats this waste in a two-stage process to recover and recycle solvents and water. Waste solvent from the cleaning process, known as low solids waste, is treated to recover solvents and water. Sludge left at the end of this first process, together with high solids waste from other parts of the manufacturing process, are steam-treated in what is known as the Dusol process to recover more solvent and water.

The company is beginning to overload its installed capacity to treat the low solids waste. It has responded by using the Dusol vessel to handle batches of the excess. But the Dusol vessel is not designed to cope with low solids waste, and the process is slow. If not handled carefully, the procedure can also generate more waste water than the company can accommodate at its manufacturing plant, or it can result in solids becoming baked onto the wall of the Dusol vessel, requiring expensive maintenance.

Dulux recovers solvents from paint waste in a Dusol vessel. The process can be improved by reducing the pressure in the Dusol vessel; processing time decreases, solvent recovery increases, and the vessel is easier to clean.

Dulux asked the Mathematics in Industry Study Group to investigate whether the Dusol vessel could be operated efficiently to treat low solids waste. Specifically, the group was asked to model the low solids treatment process in the Dusol vessel, determine the best operating conditions and assess if installing a vacuum pump would improve matters. Dulux also wanted to know how it could eliminate baking solids onto vessel walls, and if the process could be run continuously rather than in batches.

The MISG team began by refining a model for a Dusol process in which there is only one solvent; if the solvent with the highest boiling point could be recovered, then all other solvents could be recovered.

A system of equations was produced for mass and energy flows in the system. From this, it was evident that the lower the boiling point of a solvent, the shorter the time needed to process it. But solvent boiling points can be reduced by lowering the pressure.

Then it was realised that a vacuum could be created without having to install a pump. If the condenser were sealed from the atmosphere, the cooling of the hot vapour inside it would be enough to lower the pressure of the system.

The team then went on to look at how the one solvent system behaves under a vacuum in both the Dusol vessel and the attached condenser. What emerged was a series of non-linear differential equations. These equations showed that the evaporation process would be very rapid indeed, and would be controlled by the pressures within the condenser.

The MISG team recommended lowering the pressure in the Dusol vessel to recover more solvents, reduce the processing time and minimise baking of solids. Furthermore, the desired reduction in pressure could be achieved simply by sealing the condenser and separator from the atmosphere.