Annalysis of the Frontrol System application on soy meal plants.
In the soy oil industry, there are basically three stages to produce soy oil and soy meal. After crushing, the material goes to the extraction stage where the solvent, usually hexane, is added to it. The meal separated from oil goes to the desolventizing stage, where the residual solvent is separated and returns to the continuous process. The desolventizer apparatus can have two forms. The simplest one (DT – Desolventizer Tank) does only the extraction of the solvent from the meal. After passing through the desolventizer, the meal goes to a drier.
The other kind of desolventizer can extract the solvent and dry the meal to a moisture level of 13% in only one unity (DTDC - Desolventizing, Toasting, Drying and Cooling). Figure 1 shows an illustration of a DTDC apparatus.
The meal is introduced in the vessel at the top and it goes down with gravity passing through some levels formed by trays. At the bottom, steam is injected, and it passes through the meal. At the top of the vessel, the steam is recovered and goes to a distillation-like process to separate the solvent and other elements from water.
A DT and a DTDC have many important control issues to be taken into consideration. The first one is the level control of the trays along the vessel. If there is a variability in the tray levels, the solvent separation will not perform well. Considering that the upper tray levels influence the levels downward, it is evident the complexity of the strategy of the control.
The second important control issue is the temperature at the top of the tank. Considering that the steam is injected at the bottom and that it passes through the trays, it is also evident the complexity of the control strategy. Firstly, the separation in levels introduce deadtime between the control variable and the top tray. Secondly, the system transfer function is influenced constantly by the level in each tray. It is also influenced by the amount of the meal flow and by the meal inlet temperature. Therefore, this is the case of a multi-variable and nonlinear system.
Frontrol System - Solutions and Results
Frontrol System presents a solution, using a multi-variable fuzzy controller that can both control the levels in each tray and the temperature at the top of the vessel. For the level control, the level in the upper trays and the steam flow are considered as disturbances. On the other hand, for the top temperature control, the levels in all trays are also considered as disturbances. As a result, a huge reduction of variability is observed.
When Frontrol System is not used, the top temperature varies around 68 oC to 73 oC. The operator uses a higher setpoint to avoid losses of solvent in the meal. This is the cause of a high steam use. On the other hand, when Frontrol System is habilitated, since the variability is very small, the operators can use a setpoint of around 70 oC.
Considering that for each Celsius degree, around 500 kg/hour of steam is spent, at least 1500 kg/hour is saved, when Frontrol System is used. The meal quality improvement is also evident, since there is a great reduction of solvent in the meal.