Results of Frontrol System application on a soy meal plant.
Soymeal is an important byproduct obtained from the soy oil production. It is mostly used as animal food for cattle and pets. After the extraction of the oil, the residual solvent must be extracted from soymeal and its moisture and protein must be controlled. In some industrial plants, the solvent extraction, the moisture, and the protein control are performed in the desolventizer. In some others, there is a rotary dryer (Fig. 1), specially designed for controlling the soymeal moisture.
In this kind of dryer, the soymeal is introduced continually. Inside the dryer, there is a heat exchanger receiving steam in such a way that if the steam flow is increased, the soymeal moisture decreases. A subproduct from the process of the oil refinery, usually known as gum, can also be introduced if the moisture and protein of the meal are kept within the desired margins. The common disturbances also present in the system are the moisture and protein of the meal and its flow. Another difficulty in the control process is the high time constant of the dynamic system which is usually around five minutes. That means that a change in the incoming characteristics would be reflected at the output, which would be around 20 minutes later. The dynamic system is also time dependent, since as time goes by, the meal sticks to the dryer and to the heat exchanger modifying its thermal parameters. If the dryer is shut down and cleaned, its' parameters go back again. All those characteristics make the process control hard to be adjusted if a simple PID (proportional, integral, and derivative) controller is used. Then, a high variability can be observed in most of these kinds of dryers. Considering that the moisture should be around 13%, and although it fluctuates constantly, the operators establish a low setpoint to guarantee a product with moisture below the acceptable value. This procedure significantly increases the steam usage.
Considering the explanation presented above, Frontrol System uses a fuzzy controller for this purpose, taking into consideration all the disturbances in its rule base. Usually, an infrared sensor for moisture and protein is installed at the output flow, and if possible, another one at the input flow. As a result, a significant reduction of variability of more than 25% can be obtained in standard deviation of the control error. A reduction of more the 10 kg/h per ton of soy is observed in many plants where Frontrol System is installed.