Disturbance dryer

Disturbance noise covariance matrix Q This was set as a diagonal matrix, where q and q22 represent changes in the burner and dryer temperatures as a result of changing heat transfer through the walls of the dryer, due to wind and variations in external temperature. 

Dust explosions usually occur in two stages a primary explosion which disturbs deposited dust followed by the second, severe, explosion of the dust thrown into the atmosphere. Any finely divided combustible solid is a potential explosion hazard. Particular care must be taken in the design of dryers, conveyors, cyclones, and storage hoppers for polymers and other combustible products or intermediates. The extensive literature on the hazard and control of dust explosions should be consulted before designing powder handling systems Field (1982), Cross and Farrer (1982), Barton (2001), and Eckhoff (2003). 

Crystallization from solution is a widely utilized separation and purification technique in chemical industry. It is characterized by the formation of a spectrum of differently sized crystals. This spectrum, called the Crystal Size Distribution or CSD, is highly important for the performance of the crystallizer, the crystal handling equipment like centrifuges and dryers, and the marketability of the produced crystals. However, in many industrial crystallizers, the observed CSD s show large transients due to disturbances or are unstable because of the internal feedback mechanisms of the crystallization process ). The main limitation for effective CSD control was the lack of a good on-line CSD measurement device, but recent developments show that this hurdle is taken (2). 

Fig. 9.16 Typical dryer profile showing disturbance caused by sampling.

Process control systems generally operate with a constant set point. It should be emphasized that exact control is not always necessary and over instrumentation should be avoided. Solar dryers generally have some parts of large capacities that reduce the effects of disturbances. 

The air floater dryers are also similar to those described in the paper-drying section. Hot air is impinged to the web from blow boxes above and below, and the web floats supported by the airflow. The low sheet tension ensnies a greater ability to tolerate disturbances or sheet defects, and the quality of pulp is less affected. Pulp dryers also consist of stacked drying decks, as shown in Figure 39.16. Pulp dryers are larger than paper dryers as pulp is air dried from the press section to the cutter. 

Disturbances or load variables cannot be adjusted by a control system. The most common dryer disturbances are ... 

The relationship between output, manipulated, and load variables constitutes the process control system of the dryer (see Eigure 57.1). In particular, the purpose of a dryer control system is to produce a desired output by changing the manipulated variables so as to compensate for changes in the main load variables (disturbances). Inputs are in the form of commands, which the output is expected to follow, and disturbances, which the automatic control is expected to minimize. 

Inferential control [11,17,18] is an early model-based approach for process control. This control strategy is useful when the main dryer controlled variable (i.e., product moisture content) cannot be measured directly due to some technical difficulties or due to insufficient economic justification for its measurement. Inferential control uses the values of measured outputs (e.g., product or gas temperature and humidity) together with the process model to infer the value of the unmeasured control variable. These estimates are used to adjust the values of the manipulated variables in order to keep the moisture content at the desired levels (Figure 57.5). This control policy can also be used to counteract the disturbances as it is less expensive to infer these disturbances from other available process measurements rather than by measuring them directly. 

Both round jet and slot jet arrays are used. In the early drying period, slots may be preferred to minimize disturbance to the web coating. Jet velocities up to 75 m/s are used. Air-recycling rates up to 92% may be needed for optimal thermal efficiency. To avoid the possibility of condensation on the cold web as it enters the dryer, the web may be preheated using IR lamps. The air jet humidity is typically in the range of 0.016 to 0.25 kg water/kg dry air. 

Direct control and online measurement of the solids moisture content would enable significant improvements in dryer control by providing an immediate measure of the moisture content at the dryer exit and by automatic compensation for factors that disturb the control action. In certain dryers, these are already implemented industrially (e.g., in paper dryers). 

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