Basic control design scheme

The process flow diagram, or PP D. is a pictorial description of the process. It gives the basic processing scheme, the basic control concept, and the process information from which equipment can be specified and designed. It pror ides the basis for the der elopment of the P l diagram and also serves as a guide for the plant operator. The process flow diagram usiuilly includes... 

The basic control scheme used for the perfect control analysis, a single inline feedback loop, gives 10 times the allowable concentration variation at the exit of the second tank for the predicted worst case. Including feedforward reagent addition would be insufficient by itself to give the tenfold improvement required, due to a 20% error in the estimated load (inferred from pH), so an additional in-line ratio feedback controller was added between the two tanks. As an additional actuator was therefore available at no extra cost, lead-lag feedforward from the load error at the first controller to the second controller actuator was added. To reduce feedforward dynamic mismatch, the lag was set to approximately the residence time of the first tank. The lead constant was added to the design parameters. 

There are several issues which need to be addressed when designing the basic control for the column. The first is one of pairing. We will see that on our simple column that there are five PVs that we must control - pressme, reflux drum level, column base level, distillate composition and bottoms composition. We normally have available five MVs - distillate flow, bottoms flow, reflux flow, reboiler duty and condenser duty. We need therefore to decide which MV is going to be used to control which PV. Theoretically there are 5 , or 120, possible combinations. While many of these are nonsensical, a large number of feasible schemes are possible. 

In the next sections we will not focus on this issue, but rather focus on the processes. The sequence of the subjects might suggest a design procedure, from which one might deviate. It is recommended, however, to use the hst of subjects as a checkhst , after a basic control scheme has been developed. 

Table 33.1. Checkhst, i.e. basic control scheme design procedure.

In this chapter, we have tried to indicate a systematic procedure for the design of the basic control scheme. In addition, some attention was paid to protection and optimal control. This did not lead to a cookbook recipe, which produces ready to install control schemes. On the contrary, the final selection of the process control system is still a matter of different aspects, affected by local circumstances. In addition, close cooperation between people from different disciplines is necessary. It is probably a good idea to highlight a few of these circumstances, although this hst will not be complete. 

In the previous chapter the procedure for the design of control schemes was discussed. The procedure was illustrated on a reactor with recycle. The selection of appropriate combinations of controlled and manipulated variables was relatively simple, since the interactions were limited. In this chapter the procedure will be applied to a distillation column. This is a unit operation with many interactions between the corrections that are made. Using a basic knowledge of the process dynamics, a basic control scheme is designed. Subsequently, two control schemes will be compared a basic control scheme based on material balance control and a control scheme based on "energy balance control. The distillation column can also be used to demonstrate the optimization of the control scheme. The principle is that the control scheme should be designed in such a way, that an objective function can be maximized. 

Steady-state simulation and design methods for separation processes, with emphasis on distillation, have been presented in detail in many references, a few of which are listed in the references for this chapter. This chapter will present a discussion of the basic control schemes for distillation columns. Let us start by stating the obvious tbe amount of literature on separation processes, particularly distillation, is colossal. Particularly readable books and references are those by Buckley and co-workers [1,2], King [3], Tyreus [4], Seborg et al. [5], Shinskey [6], Smith and Corripio [7], Svrcek and Morris [8], and Wilson and Svrcek [9]. 

In the above schemes the two quantities (/ and / ,) are not separated. Initially it w as not easy to separate them and the whole phasor ( was varied to achieve a speed variation. Yet close speed control was possible but the motor s basic parameters were essential to achieve more accurate speed control. Since it may not be practical to obtain all the parameters of each motor promptly, the drive software is designed so that the name plate particulars ofa motor (V, N, and kW) alone arc enough to determine the machine s required parameters through the motor s... 

The detailed design and specification of the automatic control schemes for a large project is usually done by specialists. The basic theory underlying the design and specification of automatic control systems is covered in several texts Coughanowr (1991), Shinskey (1984) (1996) and Perry et al. (1997). The books by Murrill (1988) and Shinskey (1996) cover many of the more practical aspects of process control system design, and are recommended. 

The development of stable and practical reactors and effective control systems for the three types of classical reactors are covered. Notice that reactors are included, not just control schemes. Underlying the material and approaches in this book is my basic philosophy (theology) that the design of the process and the process equipment has a much greater effect on the successful control of a reactor than do the controllers that are hung on the process or the algorithms that are used in these controllers. This does not imply that the use of models is unimportant in reactor control, since in a number of important cases they are essential for achieving the desired product properties. 

While the control structure proposed in Fig. 5.16 satisfies the basic demands of heat management, it is not a scheme we would recommend building. First, we have reservations about the design of the recycle column reboiler and its hot bypass. As shown in Fig. 5.16, the hot gas temperature is over 4253C. When we add the fact that the operating pressure is close to 500 psia and that the stream contains mostly methane and hydrogen, we have to wonder how to design the reboiler and its bypass valve so they will operate safely and reliably. 

There are different approaches to implementing the feedback concentration control for the direct design. Various schemes to implement the concentration control for direct design are described in the literature for cooling and antisolvent crystallizations. " The basic steps are as follows (i) the solution concentration is estimated from IR absorbances and temperature or solvent-antisolvent ratio using the calibration model that relates IR spectra to concentration and (ii) the temperature or antisolvent flow rate setpoint is calculated from the concentration, solubility curve, and the user-specified supersaturation setpoint. 

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