Unit operations, control distillation column

Different physical modes are sometimes available for the same unit operation. A distillation column can, for example, be modeled on the basis of theoretical stages or using a rate-based model, taking into account the mass transfer on the column internals. A simulation of this kind can be used to extract the data for the design of the process equipment or to optimize the process itself During recent years, dynamic simulation has become more and more important. In this context, dynamic means that the particular input data can be varied with time so that the time-dependent behavior of the plant can be modeled and the efficiency of the process control can be evaluated. 

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. 

The distillation column is probably the most extensively studied unit operation in terms of control. An extensive literature of hundreds of papers and dozens of books has appeared over the last half century discussing methods for controlling distillation columns. 

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

Other control methods. A cychng procedure can be used to set the pattern for column operation. The unit operates at total reflux until equilibrium is established. Distillate is then taken as total draw-... 

In this approach accident cases and design recommendations can be analysed level by level. In the database the knowledge of known processes is divided into categories of process, subprocess, system, subsystem, equipment and detail (Fig. 6). Process is an independent processing unit (e.g. hydrogenation unit). Subprocess is an independent part of a process such as reactor or separation section. System is an independent part of a subprocess such as a distillation column with its all auxiliary systems. Subsystem is a functional part of a system such as a reactor heat recovery system or a column overhead system including their control systems. Equipment is an unit operation or an unit process such as a heat exchanger, a reactor or a distillation column. Detail is an item in a pipe or a piece of equipment (e.g. a tray in a column, a control valve in a pipe). 

In many operations, for instance in a distillation column, it is necessary to understand the fluid dynamics of the unit, as well as the heat and mass transfer relationships. These factors are frequently interdependent in a complex manner, and it is essential to consider the individual contributions of each of the mechanisms. Again, in a chemical reaction the final rate of the process may be governed either by a heat transfer process or by the chemical kinetics, and it is essential to decide which is the controlling factor this problem is discussed in Volume 3, which deals with both chemical and biochemical reactions and their control. 

Optimization implies maximum profit rate. An objective function is selected, and manipulated variables are chosen that will maximize or minimize that function. Unit optimization addresses several columns in series or parallel. It is concerned with the effective allocation of feedstocks and energy among the members of that system. Plantwide optimization involves coordinating the control of distillation units, furnaces, compressors, etc., to maximize profit from the entire operation. All lower-level control functions respond to set points received from higher-level optimizers. 

The dynamic simulation file prepared in Aspen Plus is exported in Aspen Dynamics [10]. We select the flow-driven simulation mode. Aspen Dynamics files have already implemented the basic control loops for levels and pressures. Units with fast dynamics, such as the evaporator or some heat exchangers, may be handled as steady state. The implementation of control loops for the key operational units, chemical reactor and distillation columns, take into account some specific issues from the plantwide perspective, which are developed in detail in Luyben et al. [8]. 

We first review in Part 1 the basics of plantwide control. We illustrate its importance by highlighting the unique characteristics that arise when operating and controlling complex integrated processes. The steps of our design procedure are described. In Part 2, we examine how the control of individual unit operations fits within the context of a plantwide perspective. Reactors, heat exchangers, distillation columns, and other unit operations are discussed. Then, the application of the procedure is illustrated in Part 3 with four industrial process examples the Eastman plantwide control process, the butane isomerization process, the HDA process, and the vinyl acetate monomer process. 

For example, suppose that the distillate flowrate from a distillation column is large compared to the reflux. We normally would use distillate to control level in the reflux drum. But suppose the distillate recycles back to the reactor and so we want to control its flow. What manipulator should we use to control reflux drum level We could potentially use condenser cooling rate or reboiler heat input. Either choice would have implications on the control strategy for the column, which would ripple through the control strategy for the rest of the plant. This would lead to control schemes that would never be considered if one looked only at the unit operations in isolation. 

More work has appeared in the chemical engineering literature on distillation column control than on any other unit operation. Books on this important subject date back to the pioneering work of Rademaker et al. (1975), Shinskev (1977), and Buckley et al. (1985). Some of the more recent developments are discussed in Luyben (1992). The longterm popularity of distillation control is clear evidence that this is a very important and challenging area of process control. Most chemical plants and all petroleum refineries use distillation columns to separate chemical components. Distillation is the undisputed king of the separation processes. 

There are many other aspects of distillation column control in a plantwide context. One of the earliest studies of these issues is the work of Downs (1992). According to Downs, the control strategy for each unit operation must be developed within the framework of the overall component inventory control structure. 7 He presents several... 

The goal of this book is to help chemical engineering students and practicing engineers develop effective control structures for chemical and petroleum plants. Our focus is on the entire plant, not just the individual unit operations. An apparently appropriate control scheme for a single reactor or distillation column may actually lead to an inoperable plant when that reactor or column is connected to other unit operations in a process with recycle streams and energy integration. 

Chemical Co. s methyl acetate reactive distillation process and processes for the synthesis of fuel ethers are classic success stories in reactive distillation. Improvements for the Eastman process are very high five-times lower investment and five-times lower energy use than the traditional process. However, combining reaction and distillation is not always advantageous and in some cases it may not even be feasible. The methyl acetate process based on reactive distillation has fewer vessels, pumps, flanges, valves, piping and instruments. This is an advantage also in terms of safety and maintenance. However, a reactive distillation column itself is more complex (multiple unit operations occur within one vessel) and thus more difficult to control and operate. It is thus not possible to make unique conclusions. 

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). 

Dynamic flowsheeting. The detail of modelling depends on the dynamics of units involved in the plantwide control problem. However, the availability of suitable dynamic models for the wide variety of unit operations Involved in practice is questionable. The simplification of the steady-state Plant Simulation Model to a tractable dynamic model, but still able to represent the relevant dynamics of the actual problem, is a practical alternative. In this case detailed models are necessary for the key units, where impurities are generated and eliminated, as kinetic models for reactors and dynamic models for some distillation columns. For other units, steady-state models are sufficient. 

The temperature of the catalyst is not controlled in the investigated FCC unit yet, so a control loop should be designed to improve the operation of the actual control structure. In the first step, the analysis of degrees of freedom is carried out and a possible manipulated variable is found the coke formation in the reactor can be controll by the feed flow of the bottom product of the main distillation column (BMC) which contains heavy hydrocarbons. (This main distillation column separates the products of the FCC unit.) This flow is free for this control and it is selected as manipulated variable. 

Crude DCE from Rl and R3 are sent to washing/drying operation simulated by the blackbox unit SO. Dissolved gases and very light impurities are removed in the unit SI, and further in the distillation column S4, which is the exit point of the light impurities. We will see that S4 plays a very important role in solving the plantwide control of impurities. 

---