Control systems distillation columns

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. 

EXAMPLE 4.2. Composition control in distillation columns is Ifequently done by controlling a temperature somewhere in the column The location of the best temperature control tray is a popular subject in the. process control literature. The ideal location for controlling distillate composition xo with reflux flow by using a tray temperature would be at the top of the column for a binary system (see Fig. 4.9a). This is desirable dynamically because it keeps the measurement lags as small as possible. It is also desirable from... 

T. L. Tolliver, Control of distillation columns via distributed control systems, in Practical Distillation Control, van Nostrand-Reinhold, 1992, p. 351. 

The basic components of a plate distillation column include a feed line, feed tray, rectifying or enriching section, stripping section, downcomer, reflux line, energy-balance system, overhead cooling system, condenser, preheater, reboiler, accumulator, feed tank, product tanks, bottom line, top line, side stream, and an advanced instrument control system. Plate columns hold trays that may be bubble-cap, valve, or sieve. Figure 6-19 shows the basic components of a plate distillation column. 

Ultimately, the importance of process control is seen through increased overall process efficiency, allowing the plant engineer to get the most from the process design. This is especially true of distillation control. Most distillation columns are inherently flexible, and a wide range of product yields and compositions can be obtained at varying levels of energy input. A key requirement of any control system is that it relates directly to the process objectives. A control system that does not meet the process objectives or produces results that conflict with the process objectives does not add value to the process. 

Distillation columns are controlled by hand or automatically. The parameters that must be controlled are (/) the overall mass balance, (2) the overall enthalpy balance, and (J) the column operating pressure. Modem control systems are designed to control both the static and dynamic column and system variables. For an in-depth discussion, see References 101—104. 

The reaction takes place at low temperature (40-60 °C), without any solvent, in two (or more, up to four) well-mixed reactors in series. The pressure is sufficient to maintain the reactants in the liquid phase (no gas phase). Mixing and heat removal are ensured by an external circulation loop. The two components of the catalytic system are injected separately into this reaction loop with precise flow control. The residence time could be between 5 and 10 hours. At the output of the reaction section, the effluent containing the catalyst is chemically neutralized and the catalyst residue is separated from the products by aqueous washing. The catalyst components are not recycled. Unconverted olefin and inert hydrocarbons are separated from the octenes by distillation columns. The catalytic system is sensitive to impurities that can coordinate strongly to the nickel metal center or can react with the alkylaluminium derivative (polyunsaturated hydrocarbons and polar compounds such as water). 

Devise a control system for the distillation column described in Chapter 11, Example 11.2. The flow to the column comes from a storage tank. The product, acetone, is sent to storage and the waste to an effluent pond. It is essential that the specifications on product and waste quality are met. 

While we laud the virtue of dynamic modeling, we will not duphcate the introduction of basic conservation equations. It is important to recognize that all of the processes that we want to control, e.g. bioieactor, distillation column, flow rate in a pipe, a drag delivery system, etc., are what we have learned in other engineering classes. The so-called model equations are conservation equations in heat, mass, and momentum. We need force balance in mechanical devices, and in electrical engineering, we consider circuits analysis. The difference between what we now use in control and what we are more accustomed to is that control problems are transient in nature. Accordingly, we include the time derivative (also called accumulation) term in our balance (model) equations. 

Distillation columns are expensive items in any plant, and are tricky to control. They should initially be built large enough to accommodate a proposed expansion. The reboilers, condensers, and pumps, however, do not need to be designed to handle any more than the initial throughput. Figure 5-1 shows how the auxiliary system may be expanded by placing similar equipment in parallel when the plant capacity is increased. 

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). 

Several case studies will be discussed in this chapter to show the use of and possible implementation methods for the ideas discussed in the previous chapters in a practical environment. The examples of application consist of two industrial cases for which real plant data were available, and an on-line application for the monitoring and control of a distillation column through a distributed control system. 

The third case study consists of a well-instrumented experimental distillation column that has been interfaced to an industrial distributed control system. In this... 

The experimental facility is a pilot-scale distillation column connected to an industrial ABB MOD 300 distributed control system, which in turn is connected to a VAX cluster. The control system consists of a turbo node (configuration, history, console) remote I/O, and an Ethernet gateway, which allows communication with the VAX-station cluster through the network. This connection allows time-consuming and complex calculations to be performed in the VAX environment. Figure 10 shows the complete setup. 

Finally, a well-instrumented experimental distillation column that has been interfaced to an industrial distributed control system was used to show the implementation of the techniques described in previous chapters in an actual on-line framework, using industrial hardware. In this case, the usefulness of data reconciliation, prior to process modeling and optimization, was clearly demonstrated. 

Buckley, P. S., Luyben, W. L. and Shunta, J. P. Design of Distillation Column Control Systems (Edward Arnold, New York, 1985). 

Example 1.3. Our third example illustrates a typical control scheme for an entire simple chemical plant. Figure 1.5 gives a simple schematic sketch of the process configuration and its control system. Two liquid feeds are pumped into a reactor in which they react to form products. The reaction is exothermic, and therefore heat must be removed from the reactor. This is accomplished by adding cooling water to a jacket surrounding the reactor. Reactor elHuent is pumped through a preheater into a distillation column that splits it into two product streams. 

Process designers sometimes like to use dephlegmators or partial condensers mounted directly in the top of the distillation column when the overhead product is taken off as a vapor. They arc particularly popular for corrosive, toxic, or hard-to-handle chemicals since they eliminate a. separate condenser shell, a reflux drum, and a reflux pump. Comment on the relative controllability of the two process systems sketched below. 

Design liquid level control systems for the base of a distillation column and for the vaporizer shown bdow. Steam flow to the vaporizer is held constant and cannot be used to control level. Liquid feed to the vaporizer can come from the column and/or from the surge tank. Liquid from the column can go to the vaporizer and/or to the surge tank. 

To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

The classic example of an interacting system is a distillation column in which two compositions or two temperatures are controlled. As shown in Fig. 8.9h, the upper temperature sets reflux and the lower temperature sets heat input. Interaction occurs because both manipulated variables affect both controlled variables. 

To illustrate the concept, consider a single distillation column with distillate and bottoms products. To produce these products while using the minimum amount of energy, the compositions of both products should be controlled at their specifications. Figure 8.13u shows a dual composition control system. The disadvantages of this structure arc (1) two composition analyzers are required, (2) the instrumentation is more complex, and (3) there may be dynamic interaction problems since the two loops are interacting. This system may be difficult to design and to tune. 

Figure 11.3d shows a process where the manipulated variable affects the two controlled variables and in parallel. An important example is in distilla tion column control where reflux flow aSecte both distillate composition and a tray temperature. The process has a parallel structure and this leads to a parallel cascade control system. 

At Merck KGaA in Darmstadt (Germany), in-line UV spectroscopy has been used for monitoring a distillation setup for solvent purification." A transmission probe was implemented at the top of the column. Solarization-resistant UV fibers guided the light to a diode array spectrometer. From the spectra, a quality parameter was extracted and fed to the process control system (PCS). As an example, the quality parameter would exhibit a transient behavior upon column startup. Below a defined threshold value of the parameter, the PCS would assume sufficient product quality and switch the exit stream from waste to product collection. The operation of the spectrometer does not take place in a direct manner, but rather via the PCS. 

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