Process control cascade

Cascade control strategies are among the most popular and usehil process control strategies. Modem control systems have made thek implementation and operation both easier from the standpoint of operations personnel, and cost effective as they are implemented in software rather than hardwiring the connections. 

While the single-loop PID controller is satisfactoiy in many process apphcations, it does not perform well for processes with slow dynamics, time delays, frequent disturbances, or multivariable interactions. We discuss several advanced control methods hereafter that can be implemented via computer control, namely feedforward control, cascade control, time-delay compensation, selective and override control, adaptive control, fuzzy logic control, and statistical process control. 

The resulting hierarchical control structure is represented schematically in Figure 5.2. Note that, while controller design proceeds in a bottom-up manner, starting from the fastest time scale, during the operation of the process there will exist a tight top-down interconnection via control cascades between the supervisory and regulatory layers. 

The carbolithiation of unactivated alkenes has also proven very successful for the synthesis of complex polycyclic systems. This has typically been achieved by reaction sequences utilizing an intramolecular carbolithiation process to generate a variety of carbocycles185 and heterocycles186. To achieve the intermolecular carbolithiation reaction required to initiate a controlled cascade reaction sequence for the generation of indole ring scaffold, Kessler and coworkers44 have expanded the synthetic utility of the styrene... 

Greg Shinskey (1988), over the course of a long and productive career at Foxboro, has proposed a number of advanced control" structures that permit improvements in dynamic performance. These schemes are not only effective, but they are simple to implement in basic control instrumentation. Liberal use should be made of ratio control, cascade control, override control, and valve-position (optimizing) control. These strategies are covered in most basic process control textbooks. 

As demonstrated elsewhere [34], it is possible to utilize such a different thermal dependence of the enzymes involved in the cascade system for process control. The appropriate choice of operational conditions allowed the enzyme activities to be controlled and directed the selectively of the process to the first reaction product, the amide or to the second reaction product, the acid. A higher process temperature and residence time (20 °C and r = lOh) favor production of the acid (amidase activity prevails), while a lower temperature and residence time (5 °C and t = 5 h) allow the production of the intermediate amide (amidase activity negligible) [34]. 

A pilot-scale distillation column located at the University of Sydney, Australia is used as the case study [60]. The 12-tray distillation column separates a 36% mixture of ethanol and water. The following process variables are monitored temperatures at trays 12, 10, 8, 6, 4, and the reflux stream, bottom and top levels (condenser), and the flow rates of bottoms, feed, steam, distillate and reflux streams. The column is operated at atmospheric pressure using feedback control. Three variables are controlled during the operation top product temperature, condenser level, and bottom level. Temperature at tray 8 is considered as the inferential variable for top product composition. To maintain a desired product composition, PI controllers cascaded on flow were used to manipulate the reflux, top product and bottom product streams. 

The three major types of process heater cascade control systems are shown in Figures 10-2, 10-3, and 10-4. Each system has a primary and a secondary loop. The secondeiry loop detects and acts on as many upsets as possible so that the primary loop has the least disturbances. 

The design procedure for series cascades of units was proposed three decades ago (P. S. Buckley, Techniques of Process Control, 1964, Wiley, New York) and has been widely used in industry for many years. The first step is to lay out a logieal and consistent material balance control structure that handles the inventory eontrols (liquid levels and gas pressures). The hydraulie strueture provides gradual, smooth flow rate changes from unit to unit. This filters flow rate disturbances so that they are attenuated and not amplified as they work their way down through the cascade of units. Slow-aeting, proportional level eontrollers provide the most simple and the most effective way to aehieve this Bow smoothing. 

Automatic feedback control is the continuous or repetitive modification of some operating parameters based on measurement data. Due to the dynamic nature of the process, the control algorithm is usually also dynamic and has to be designed carefully to avoid instability. We have to distinguish between two types of process control standard and advanced one. Standard controllers have a static algorithm like PID-controller type or cascade mode for the temperature control. Advanced control schemes use a more sophisticated algorithm that could be based on shortcut models or dynamic process models. 

Feedforward Control Cascade Control Set points are given by the sensed conditions upstream of the process. Chemical process control... 

Bubble logic that implements universal Boolean logic in physical fluid dynamics has been demonstrated in [4]. A bubble traveling in a microchannel can represent a bit of information as well as carry a chemical payload. Therefore, it is possible to integrate chemistry with computation for process control. Since bubble logic preserves the information representation from input to output, devices can be cascaded to implement combinational and sequential Boolean logic. 

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