Heat recovery from a spray dryer using a run-around coil system

Abstract

This project at Clayton Aniline Co. (CAC) demonstrates the use of a finned-tube heat exchanger and run-around coil system to recovery heat from a spray-dryer exhaust to pre-heat the inlet air. It was originally envisaged that this would reduce the energy consumption of the dryer by 12%, equivalent to 9.4 therms/h.

Funding was granted by the Energy Efficiency Office of the Department of Energy under the Energy Efficiency Demonstration Scheme for the installation and monitoring of the project. This project is one of a package of demonstrations for heat recovery in spray dryers. A parallel project at ABM Chemicals uses a glass tube heat exchanger to recover energy from the dryer exhaust, and a further project at BIP Chemicals demonstrates the use of a separate heat source to pre-heat the dryer inlet air.

Within the U.K., widespread adoption of heat recovery on spray dryers would lead to energy savings of 2,500 TJ/yr or 100,000 tonnes of coal equivalent (tce)/yr. It is estimated that, within the next 5 yr, around 20% of this will be achieved, resulting in savings of around 20,000 tce/yr, worth £1.6 million yr.

The heat recovery system was installed at CAC in July 1982. Unfortunately, due to the built-up of solids on the fins, the unit had to be removed after only three days operation. The equipment was therefore modified and the changes included:

reducing the number of fins from from 8/in. to 4/in.: this reduction in are meand a 30% loss of potential savings;

improving the exchanger spray wash system by including an additional row of nozzles directed towards the centre of the unit.

The replacement exchanger was installed in July 1984 and, since that time, has operated satisfactoryly. It has not adversely affected the operation of the spray dryer, as the problems of excessive solids deposition and ineffective washing have now been overcome.

Detailed monitoring has not yet been completed, but preliminary information and design calculations estimate that for the modified exchanger, energy savings averaging 6.5 therms/h (191 kW) are probable. These are worth about £17,200 to CAC and hence the payback period on the installation cost of £35,200 is 2 yr.