## 2 The Rotary Dryer System
### The Rotary Dryer Process Description  
The proposed controller was developed and applied in an industrial tobacco plant in Zhejiang, China. In this factory, cut tobacco is dried to remove impurities and some free alkali.  
For this study, a cut tobacco dryer system with a production capacity of 4000 kg/h was used. The dryer is a new generation Hauni KLD rotary dryer [24]. The cut tobacco is dried in batches, generally lasting two hours for each batch. After a drying process is completed, the operator carries out the essential maintenance work of the relevant machines involved. After that, the next batch is processed.  
The process of drying tobacco is entirely shown in Figure 1. However, a full description of such a process is beyond the purpose of this paper. To understand how the proposed operating control works, it is necessary to give a succinct description of two machines involved in the whole process: the Sirox conditioner [25] and the rotary drier. It is important to remember that in this plant, they produce two varieties of tobacco that have a different moisture content, respectively, 12.00% and 12.10%. In addition to this, the processed cut tobacco must have a standard deviation of moisture content of less than 0.042 in each batch.  
Figure 1 shows the dryer process under study. In the aforementioned figure, you can see the cut tobacco which is going to be fed into a rotating cylinder that in reality has a length of 9 m and a diameter of 1.9 m. The rotating drum, called the rotary drier in the figure, has a speed of 11 revolutions per minute (RPM). The dryer uses steam as heating energy. Although in the drum both mixed conduction and convection heating operate, conduction is the principal source of heat. The cylinder's external surface is warmed by the steam supply system. The cut tobacco is fed by a vibrating conveyor into the continuously rotating dryer and directly contacts the drum walls. Simultaneously, the hot air and cut tobacco are conveyed in the same direction. The hot air directly contacts the cut tobacco and transfers heat to cut tobacco by convection. The moisture on the surface of the cut tobacco vaporizes and diffuses into the hot air as the cut tobacco absorbs heat from the drum walls and the hot air. The hot air turns into hot and humid air while drives water vapor and enters the dust box from the discharge end of the dryer. When the exhaust gas comes out of the dryer, it must pass through a dust box which collects the entrained products to meet the waste emission regulations. After that, the dried cut tobacco is sent to the winnower for cooling and screening.  
Figure 1: Drying process of cut tobacco.  
Before the tobacco is dried, it passes through a Sirox conditioner. This is used for conditioning and warming the whole batch. Apart from the two machines mentioned above, it is important to bear in mind that at the inlet of dryer, there is an online moisture measurement instrument.  
Normally, the temperature is controlled by the human operator. The mass feed rate, the rotation speed of the drum, and the inclination of the dryer usually follow preset parameters. Moreover, the water increase provoked by SIROX is always the same independently of the type of cut tobacco that they want to produce [26].  
In the dryer, the heat exchange between the steam flow and the cut tobacco varies depending on the opening of the steam valve. The overall temperature of dryer depends on the steam pressure which, in its turn, depends on the opening of the steam valve, to be further examined later. For the system under study, it is empirically assumed that dryer temperature within the range of 130\(\sim\)133 \({}^{\circ}\)C, the speed of the hot air flow is in the range of 0.348\(\sim\)0.351 m/s, and the temperature of the hot air is in the range of 105.00\(\sim\)105.10 \({}^{\circ}\)C. The cut tobacco at the inlet of the dryer has a moisture content between 18% and 19%.