Process Control Scheme

The process control scheme proposed for the column is based upon similar suggested layouts used for the control of distillation columns (Ref. A8). Modifications were made in accordance with the individual operating characteristics and requirements of this system. Final verification of the proposed scheme is achieved by consulting Refs. A7 and A9. 

---

The analyzer is installed as part of a larger process improvement effort, such as installation of a distributed control system (DCS) and is part of the process control scheme. 

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. 

The process control scheme for the absorption column is presented in Figure 9.6. It was designed from the recommendations presented in the HAZOP analysis, the results of which are reported in Section 9.8. It features ratio control on the make-up water stream. The signals from flow transmitters on this line and on the gas input line are fed to the ratio controller, whereby the make-up water stream is adjusted. 

The process control scheme suggested for the steam superheater is shown in Figure 10.2. This flowscheme features a control valve on the saturated-steam inlet line. A temperature controller taking its signal from the steam outlet line ensures the flow is regulated to maintain the superheater temperature of 380°C. Steam pressure is... 

FIGURE 10.2 Steam superheater process-control scheme. 

The process-control scheme contains loops for maintaining concentration of the absorbent (by manipulating the fresh FeEDTA2- feed), the pH and the Redox potential in the bioreactor (by addition of acid/base and ethanol, respectively). Other control loops maintain the liquid level in the sump of the absorber, and the level in the bioreactor. 

If the test indicates that control performance may be adequate for several different processes/control schemes, which of these should be pursued in detail first ... 

Process monitoring capabilities and ease of integration into process control schemes... 

Martin, E.N. (1970). The design and development of process control schemes using analogue and scale modelling techniques. Chemical Age of India, 21, 184-194. 

Advanced control systems are becoming a necessity because of the ever increasing pressures toward product quality and reliability. All material processing operations can benefit from improved process control. However, increased control on the more complex systems, such as composites, compounds and blends, and on the more critical applications will yield the highest benefits. In addition, process control schemes based on process variables alone are not sufficient anymore... 

Advanced process control schemes can be either smart combination of simple controllers, or nonlinear controllers using short-cut or even dynamic models. In both cases the design of such feedback loops is nontrivial when several actuated... 

Figure 4.19 Advanced inline dilution process control scheme.

The inputs or incoming signals to a process control scheme can be from a variety of measurement sources. In general these can either be measurements of the process conditions or measurements of the process materials involved in the operation. Temperature, pressure and flow are the most common process conditions used. As process control becomes more demanding then chemical and physical measurements of the materials being processed or produced can be used. These include for example colour, density and chemical composition. This is the important link to chemical measurements and the spectroscopic applications, which are the subject of this section of this handbook. 

Process analysis is the general term apphed to analytical techniques used to provide information for controlling processes. Such methods can provide a wide range of information relating to the physical and chemical properties of the material being examined and also the properties and characteristics of the products that will result as a consequence of the process conditions being used. This information, when used effectively, can provide a significant input into process control schemes which impact upon the quahty consistency and usefulness of the product as well as the efficiency of the operation. 

In general, the processes controlled in chemical industry are not hazardous but still dangerous. Hence, the monitoring and control of chemical production plants is an important topic in theory and practice. A process control scheme for chemical processes can be... 

Inlet air pressure (and implicitly discharge air flow rate) to the air knife, however, can be adjusted on-line. That adjustment can be part of a process control scheme because it provides a much more substantial gain. 

Significant inherent variability in process outputs Requiring controller design techniques specifically for dealing with the stochastic nature of the measurements such as statistical process control schemes. Such designs may place limits on achievable classical closed-loop dynamic performance. 

Although many process variables are easily measured, lack of on-line sensors for key polymer properties renders quality control of polymer plants difficult. Process control schemes based on process variables p, T, flow-rate and feedstock compositions) alone are no longer sufficient, because these cannot reveal all material property variations. Significant efforts are being spent on improvements to process control systems, as exemplified by the numerous attempts to monitor polymer properties during processing, such as composition, density, viscosity and dispersion of a minor phase, etc., all of which are somehow difficult to measure. The development of an on-line inferential system for polymer property is a very active research area of polymerisation reactor control [1]. A schematic of inferential systems is illustrated in Fig. 7.1. For highest quality... 

The environmental model is the product of a problem analysis. It forms the basis for the development of behavioral models of the process. This holds for physical models, in which the internal dynamic mechanism is described by pltysical laws, as well as empirical (black box) models, in which only overall dynamic relationships between process inputs and outputs are formulated. The environmental model can also be used for the development of a process control scheme. By evaluating the static (power of control) and dynamic relationships (speed of control) between process inputs and outputs, the process control scheme can be selected. In general no dynamic model of the process is required yet at this stage. This is the starting point that is used in the chapters on process control. 

The chosen system boundaries have to agree with the boundaries as they are shown on the process flowsheet. Investigate whether the given process boundaries are also the right ones for the process control scheme. Pay special attention to the sources of energy and material, which belong specifically to the process and which can be manipulated. They can lead to the selection of a larger area for the process boundaries. Specifically the separation between what can be influenced and cannot be influenced (external influences) determines the system boundaries. 

In many cases feed flow and composition distuibances are present and setpoint changes are normally not required. Process control schemes are designed to rednce the effect of disturbances on the operation of the process. In the case of a distillation colnmn, in which the qnality of both products is controlled, a material balance control scheme and an energy balance control scheme will bring the process back to a steady situation after a disturbance has taken place. However, different control schemes will show different dynamic responses to disturbances. In addition, there are control schemes that exhibit better self-regulation of the product quahty to distuibances. In that case there will be a better response of the quality towards disturbances. 

---