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Controlling melting time, temperature

Q: Please recommend the best and fastest method of controlling the melting of beef fat in a thermal oil, jacketed vessel. The fat melts at approx 70ºC and must not go over 90ºC. There are two 3-Kw elements. The controller is basically a "on/off" control, turning the elements on when under temperature and turning them off when set point is reached. Would you measure the fat or the oil? If we measure the fat, melting time is drastically reduced, but a huge overshoot is experienced. Oil control is better, but the melting time is increased.

A: This is a classic problem in which an over temperature causes process problems and avoiding the over temperature results in longer cycle times. In my view, the simplest approach to this is to control the oil temperature to a maximum of 90°C and circulate it through the jacket at a very high rate so that the discharge temperature of the oil is still close to 90°C. This may make for a very long melt time but it is the most conservative way of avoiding overshoot. A somewhat more complicated method would be to measure both the oil and the fat and institute a control scheme which reduces the oil temperature to 90°C as soon as the fat temperature exceeds 70°C. Utilizing the existing scheme of controlling the oil temperature can be improved by changing the oil temperature set point upward until the desired cycle time is achieved or the overshoot becomes unacceptable. This approach requires a series of controlled experiments. However, it will define the best operating range within the capabilities of the existing equipment.