Process control, automatic temperature

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

Specialized equipment for industrial measurements and automatic control have been developed (18) (see Process control). In general, the pH of an industrial process need not be controlled with great accuracy. Consequendy, frequent standardization of the cell assembly may be uimecessary. On the other hand, the ambient conditions, eg, temperature and humidity, under which the industrial control measurements are made, may be such that the pH meter must be much more robust than those intended for laboratory use. To avoid costiy downtime for repairs, pH instmments may be constmcted of modular units, permitting rapid removal and replacement of a defective subssembly. 

Shutters mounted above the cooling sections serve to protect them from overhead wind, snow, ice, and hail. In addition they are also used to regulate, either manually or automatically, the flow of air across the finned tubes and thus control the process fluid outlet temperature. 

Control of the process. Prevention of hazardous deviations in process variables (pressure, temperature, flow), by provision of automatic control systems, interlocks, alarms, trips together with good operating practices and management. 

The process is divided into five steps (see Fig. 9) Ecovio is first processed by the extruder to form a melt stream with constant melt temperature and output rate (Fig. 9, 1). In the forming section, the melt streams of several extruders can be merged into one using an adapter feed block or a multilayer die. These devices control the flow of each stream to obtain an even layer distribution (Fig. 9, 2) [30]. Then, the melt stream is transformed into a flat film in the film die. The thickness distribution is controlled automatically by means of expansion bolts. 

Total unit heat dnty will typically be in the range of 500-1000 BTU per pound of feed to the unit. This set of process heat requirements establishes the amount of heat that must be supplied by combustion of coke. Because of the process control schemes that are normally employed in FCCUs, the unit operation will automatically adjust itself so that the energy produced via coke combustion equals the heat requirements of the process. If the balance is shifted by changes to the feed quality or operating conditions, shifts in catalyst circulation rate and regenerator temperature will occur until a new equilibrinm set of conditions is established. 

Microelectronics and computer programming have allowed the printing processes to be analysed, and sensed remotely so that many of the variables in the printing are monitored, adjusted, and controlled automatically from a console. Such fundamentals as flow of ink from the duct, colour strength, ink viscosity, temperature, and humidity all are monitored and checked against pre-determined limits, so that they can be adjusted to maintain consistency of print quality. 

Both modes usually are conducted with constant vaporization rate at an optimum value for the particular type of column construction. Figure 13.9 represents these modes on McCabe-Thiele diagrams. Small scale distillations often are controlled manually, but an automatic control scheme is shown in Figure 13.9(c). Constant overhead composition can be assured by control of temperature or directly of composition at the top of the column. Constant reflux is assured by flow control on that stream. Sometimes there is an advantage in operating at several different reflux rates at different times during the process, particularly with multicomponent mixtures as on Figure 13.10. 

The Leeds Northrup Co. has developed a two-couple continuous curve recorder for automatically heating furnaces to definite temperatures. As applied in carburizing and tempering of steel one thermocouple is placed in the furnace chamber and the other in contact with the steel piece. The controller automatically maintains the furnace for instance at 1,000°C. until the steel has reached 900°C. and then it drops the furnace to 900 C. Such an arrangement for control can be applied to a great many processes. 

A control system or scheme is characterized by an output variable (e.g., temperature, pressure, liquid level, etc.) that is automatically controlled through the manipulation of inputs (input variables). Suppressing the influence of external disturbances on a process is the most common objective of a controller in a chemical plant. Such disturbances, which denote the effect that the external world has on a process, are usually out of reach of the human operator. Consequently, a control mechanism must be introduced that will make the proper changes on the process to cancel the negative impact that such disturbances may have on the desired operation of the process. Control engineers usually refer to the combination of a sensing element and a control device with a set point as a control loop. ... 

Traditional schedules result in a drying rate that decreases with time, only partly countered by increases in the dry-bulb temperature and the wet-bulb depression as the schedule proceeds. Modem automatic process control means that the kiln schedule can be adjusted continuously so ensuring a more constant rate of heat transfer and evaporation. Also this avoids any shock that an abrapt change in the schedule imposes on the timber. 

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