Distillation control fundamental

We cannot hope to cover the vast subject of distillation fundamentals and control in a single chapter in this book. Our objective here is to review some of the basic principles about distillation and then to summarize the essentials of distillation column control, particularly as it relates to the plantwide control problem. Many more details are available in the books cited above. In the first section we review some of the important fundamentals about distillation. Sections 6.3 through 6.8 discuss distillation column control mostly from the perspective of an isolated column or column system, This treatment presents what may appear to be a laundry list of control structures for different types of columns. Although we feel this presentation is valuable, particularly for the young inexperienced student or engineer, it is also vital to retain a broader plantwide perspective. Section 6.9 addresses some of these plantwide distillation control issues. In the next section we review the process fundamentals of distillation as they relate to process operation and control. 

In this chapter we have presented some fundamental concepts of distillation control. Distillation columns are without question the most widely used unit operation for separation in the chemical industry. Most final products are produced from one end or the other of a distillation column, so tight control of product quality requires an effective control system for the column. However, the column is usually an integral part of an entire plant, so its control scheme must also be consistent with the plantwide control structure. 

BramhUla, A. (2014) Distillation Control and Optimization Operation Fundamentals Through Software Control, McGraw-Hill Professional, New York, US. 

One of the fundamental concepts in distillation control is to prevent rapid changes in pressure. If pressure increases too quickly, vapor rates through the trays decrease, which... 

Having discussed some general points about sampled-data control techniques and algorithms, we now look specifically at how sampled-data control is applied to a distillation column. Fundamentally we are interested in achieving good control in the face of set-point changes and load disturbances—traditionally called servo and regulator control. 

Distillation is one of the most important unit operations in chemical engineering. It forms the basis of many processes and is an essential part of many others. It presents a more difficult control problem then with many other unit operations, as at least five variables need to be controlled simultaneously and there are at least five variables available for manipulation. Thus, a distillation column provides an example of a multiple-input-multiple-output control problem. It is critical that variable pairing is done appropriately between controlled and manipulated variables. The overall control problem can usually be reduced to a 2 x 2 conposition control problem since the inventory and pressure loops frequently do not interact with the composition loops. This workshop will highlight some fundamental mles of distillation control and show how a basic distillation control scheme can be selected. 

As might be expected, the vapour phase may offer the controlling resistance to mass transfer in high pressure distillations. Values for tray efficiencies at elevated pressure are scarce [23, 24]. The prediction of tray efficiency may be approached in several ways. One way is to utilize field performance data taken for the same system in very similar equipment. Unfortunately such data are seldom available. When they are available, and can be judged as accurate and representative, they should be used as a basis for efficiency specification [25], Another way is to utilize laboratory-or pilot-plant efficiency data. For example a small laboratory-Oldershaw tray-column can be used with the same system. Of course, the results must be corrected for vapour-and liquid mixing effects to obtain overall tray efficiencies for large-scale design [26], Another approach is the use of empirical or fundamental mass-transfer models [27-30],... 

In this chapter it was shown that equilibrium theory, which was first developed for nonreactive chromatographic and nonreactive distillation processes is readily extended to many integrated reaction separation processes with fast reversible reactions. The theory provides an easy understanding of the dynamics of these processes and therefore has many useful applications in process control. Further, the theory nicely predicts inherent limitations of integrated reaction separation processes, which may not be obvious from first glance. It emphasizes fundamental features, which are common to many of these processes. 

Constantinides Applied Numerical Methods with Personal Computers Coughanowr and Koppel Process Systems Analysis and Control Douglas Conceptual Design of Chemical Processes Edgar and Himmelblau Optimization of Chemical Processes Gates, Kalzer, and Sdniib Chemistry of Catalytic Processes Holland Fundamentals of Multicomponent Distillation... 

The Hull site is a top-tier COMAH establishment and the individual plant safety reports were used as a base to identify the major hazard scenarios. While the Hull plants produce different final products, the key operational stages are the same, i.e., feed system, reactor section, initial separation and recycling, and final distillation train. The Risk Control Systems (RCS) were therefore fundamentally the same but each plant was reviewed in isolation. A total of eight different RCS were considered as part of the review and the process described in HSG 254 was used for each one. The Workbook produced during the review for one of the plants as part the process is attached in Appendix 1. ... 

It is assumed that the reader is familiar with the fundamentals of process control of the key unit operations, as reactors, distillation columns, etc. A concise description of essential topics in Process Control can be found in the recent book of Luyben Luyben (1998). We also strongly recommend the recent monograph of Luyben and Tyreus on plantwide control (1999). Basic issues in dynamics and process control can be found in the classical textbooks of Stephanopoulos (1984), Luyben (1990), Marlin (1995) and Ray Ogunnaike (1998). 

The fundamental scheme of arrangement as applied in the first edition has been retained. Section 5.1.3 has been extended to cover pilot plant distillation. Section 4.2 now deals with fluid and interface dynamics. Chapter 8 could be drastically shortened as there are a variety of components of distillation apparatus and the pertaining measuring and control devices commercially available. The nomograms, which were presented separately, have been inserted in the text. The references for the various chapters have been rearranged and important new items added to them. A great number of review articles serve to provide comprehensive lists of references for a longer period. 

Distillation fundamentals do not change, nor does the importance of distillation in our energy-intensive society. What does change is the range of applications and methods of analysis that provide more insight and offer improvements in steady-state design and dynamic control. In the seven years since the first edition was published, a number of new concepts and applications have been developed and published in the literature. 

An enormous amount of work has gone into the study of contacting devices, and the literature on their performance characteristics is extensive. Most of the work dealing with laiger equipment has been based on plant observations, but a significant portion of it has been based on controlled experiments in commercial-scale equipment by Fractionation Research, Inc. (FRI). The methods for analysis and design of distillation columns, present in this section, are based on a combination of fundamental research papers, results released by FRI, and many repotted plant tests. 

This book focuses on the fundamentals of process control of distillation columns. It covers the following topics ... 

---