Distillation columns exercises

A direct sequence of two distillation columns produces three products A, B and C. The feed condition and operating pressures are to be chosen to maximize heat recovery opportunities. To simplify the calculations, assume that condenser duties do not change when changing from saturated liquid to saturated vapor feed. This will not be true in practice, but simplifies the exercise. Assume also that the reboiler duty for saturated liquid feed is the sum of the reboiler duty for saturated vapor feed plus the heat duty to vaporize the feed. Data for the two columns are given in Tables 21.7 and 21.8. 

An alternative control strategy fixes the reactor-inlet toluene flow rate [16]. Fresh toluene is fed into the condenser drum of the last distillation column, on level control. Production-rate changes can be achieved by changing the setpoint of the toluene reactor-inlet flow, or the setpoint of the reactor-inlet temperature controller. When this control structure is used, the whole range of conversion becomes stable. Drawing of this control structure is left as an exercise to the reader. 

There are many excellent texts that discuss the design of distillation columns using equilibrium stage calculations. Some of them were cited in Chapters 12-14. These texts provide a wealth of examples that could be used as the basis for a design using the nonequilibrium model described in Chapter 14. We adapt one such example below (Exercise 14.1) in order to indicate how this might be done. 

Care must be exercised not to specify more control objectives than the available number of degrees of freedom. In such a case the system becomes overspecified and it is impossible to design a control system that satisfies all the desired control objectives. Thus it is impossible to design a control system for the ideal binary distillation column that can satisfy the following six operational (control) objectives ... 

A simulation exercise can show the distillation column has more effect on the response than the reactor, because of higher order dynamics. Modifications on reactor, as increasing the kinetics and simultaneously reducing the volume, will improve only... 

This example follows the simulation of the complete toluene hydrodealkylation process at the end of Section 4.3 and is presented without a solution because it is the basis for Exercise 5.4. To recycle the biphenyl to extinction, the flowsheet in Figure 4.20 is modified to eliminate the last distillation column, and unreacted toluene from the second column is recycled with biphenyl. This is accomplished by the reversible reaction... 

Shown below is a portion of a process that produces styrene from benzene and ethylene (see Exercise 2.12). Benzene is recycled to an alkylation reactor and the by-product ethylbenzene is recycled to a dehydrogenation reactor. Stream 2 is 28.0% of stream 1. Also, 97.0% of the ethylbenzene in stream 3 leaves distillation column 2 via stream 4. Calculate the flow rate and composition of stream 5. 

Design a distillation column to separate a mixture of ethanol and water into a water-rich stream (98 mol% water) and an ethanol-rich stream (80 mol% ethanol). The feed to the distillation colunm is a liquid-vapor mixture equilibrated such that Methanol = 0.443 and A ethanol = 0.10. Use the ethanol-water vapor-liquid diagram from Exercise 4.24 to determine... 

Use the experimental data given in Exercise 4.20 to specify the operating conditions in the distillation column below. The column has nine trays. Determine operating conditions that will maximize L/V in the lower trays (stripping section) and minimize L/V in the upper trays (rectifying section). Also, specify the temperature of stream 1 so the vapor-liquid composition of the input matches that of the feed tray. 

Design a process that starts with 20 mol% acetonitrile and yields two streams with compositions >98 mol% acetonitrile and <2 mol% acetonitrile. Indicate the compositions of all streams in your process and the number of stages in the distillation column(s). You may wish to use the liquid-vapor equilibrium data for acetonitrile/water mixtures at 1.0 atm (Exercise 4.21) as well as the data at 0.2 atm (shown below). 

The book is divided into five parts. Part I provides an introduction to process control and an in-depth discussion of process modeling. Control system design and analysis increasingly rely on the availabihty of a process model. Consequently, the third edition includes additional material on process modeling based on first principles, such as conservation equations and thermodynamics. Exercises have been added to several chapters based on MATLAB simulations of two physical models, a distillation column and a furnace. These simulations are based on the book. Process Control Modules, by Frank Doyle, Ed Gatzke, and Bob Parker. Both the book and the MATLAB simulations are available on the book Web site (www.wiley.com/college/seborg). National Instruments has provided multimedia modules for a number of examples in the book based on their Lab VIEW software. 

Note In the following exercises, a Simulink model is used to approximate the reactor and distillation column units discussed in this chapter. Information is given in Appendix LI. 

Prelab Exercise Study the glassware diagrams and be prepared to identify the fractionating column, Claisen distilling hjead, ordinary distilling head, vacuum adapter, simple bent adapter, calcium chloride tube, Hirsch funnel, and Buchner funnel. 

Again the number of external streams which may be specified independently is equal to the number of columns plus the number of sidestreams withdrawn (in addition to the distillate and bottoms). Thus, for the system shown in Fig. 3-14, only eight of the twelve external streams may be specified independently. Furthermore, care must be exercised in the selection of the streams to be specified in order to obtain an independent set. For example, an examination of this system shows that all of the W s may be specified independently, and for this set of specifications, the normalized g functions become... 

Purpose. This exercise explores the classic reactions of carboxylic acids (RCO2H) with alcohols (R OH), in the presence of acid catalyst, to yield esters (RCO2R ) plus water (H2O, a small stable molecule).The physical properties of these esterification products are examined and the techniques of distillation and column chromatography are applied to the purification of these materials. 

The feed to the high-pressure column (5 atm) is air thus, the composition is Xp 1=0.79 and we assume that the fraction of liquid is 0.5. The distillate is mainly nitrogen. Let s assume Xp, = 0.99 since it is fed to the top of the low-pressure column. The residue is assumed to have a composition in nitrogen of x j = 0.63. This residue, assumed as liquid. Op = 1, in Figure 4.15, is fed to the medium part of the low-pressure column (1 atm). The distillate, mainly nitrogen, is assumed to have a composition of Xp, 2 = 0.996 in nitrogen, while the residue of the low-pressure column has a composition of x, 2 = 0.008. The problem is to compute the number of trays needed for such a separation. As an exercise, the reader can evaluate the effect of different compositions. 

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