Distillation control device

Distillation control schemes may be analyzed either on a steady-stale (sensitivity analysis) or on a dynamic basis. The latter requires a dynamic model that takes into account the dynamic response of the column and the control loope. An example of a dynamic model is described by McCune and Gailier,6 but il should be apparent from the material presented enrlier that the holdup characteristics of distillation columa devices can vary widely, and snch variation should be accommodated by the model. The development of the naw high-efficiency packings has caused a new look at the system dynamics when the liquid holdup in the column is quite low, and thus the existing models for trays may not be adjustable to application to packings. The use of a tray-type dynamic model is described in the article by Gailier and McCune 7 so work to date has been reported for packed column dynamic models. 

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. 

There is also a tendency to manufacture standardized distillation apparatus, beginning with circulation stills, including also combinations of non-glass flasks and glass columns, and ending up with automatic continuous pilot plants with electro-magneticaUy operated reflux heads, level control and flow control devices (Fig. 142). The maintenance-free operation of such plants requires numerous glass valves. 

With the aid of the columns and other apparatus described it is possible to separate liquid mixtures continuously at normal and reduced pressures as well as at small overpressures (chap. 5.4.5) as sharply a.s in batch distillation. This is clear from the distillation curves of the products obtained in the continuous distillation of a mixture of crude fatty acids in the C4 to Ci range (Fig. 168). A further example shows that by the use of control devices it was possible to separate a mixture of phenols into its main components with great constancy. Fig. 169 illustrates the results of the first separation in this sequence, which was made between the ortho- and Twcfa/pom-cresol fractions. 

The extent to which measuring and control devices are employed in batch and continuous distillation depends on the nature of the work and, to a considerable degree, on financial considerations [12]. In practice a distinction can be made between semi- (or partly) automatic and fuUy automatic apparatus. In the latter all operations except starting up are performed automatically in partly automatic apparatus only some of the functions are governed by regulating devices. 

The flow sheet of a process for the production of absolute alcohol by pressure swing distillation is shown in Fig. 11.0-1. The process consists, in essence, of two distillation columns, several heat exchangers, several vessels, and a complex network of pipelines (see Sect. 11.3.2). Also shown are measuring and control devices. According to international standards (e.g., DIN 19227, ISO 3511) these devices are illustrated by thin circles with a letter code indicating the function of the device, for instance, first letters F = flow, L = liquid level, P = pressure, T = temperature subsequent letters A = alarm, C = control, F = fraction, etc. 

A variety of other configurations and modifications of the basic design shown in Figures 3-6 and 3zZ are possible. Valve trays (see Chapter lOi are popular. Downcomers can be chords of a circle as shown or circular pipes. Both partial and total condensers and a variety of reboilers are used. The column may have multiple feeds, sidestream withdrawals, intermediate reboilers or condensers, and so forth. The column also usually has a host of temperature, pressure, flow rate, and level measurement and control devices. Despite this variety, the operating principles are the same as for the sinple distillation column shown in Figure 3-6. 

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. 

Thirty years ago these computed variables were calculated using pneumatic devices. Today they are much more easily done in the digital control computer. Much more complex types of computed variables can now be calculated. Several variables of a process can be measured and all the other variables can be calculated from a rigorous model of the process. For example, the nearness to flooding in distillation columns can be calculated from heat input, feed flow rate, and... 

Figure 11.5a shows a typical implementation of feedforward controller. A distillation column provides the specific example. Steam flow to the reboiler is ratioed to the feed flow rate. The feedforward controller gain is set in the ratio device. The dynamic elements of the feedforward controller are provided by the lead-lag unit. 

A device described by Sawyer and Dixon [13] was used for the determination of alcohol and acid in beer and stout. Attempts to improve the reliability of this method and to improve the signaTto-noise characteristics of the measurements prompted a critical design described by Lidzey et al. [14]. This unit overcomes many of the fluctuations in results observed with use of the first unit in this a number of possible sources of surging were indicated and these were not controlled owing to the varying conditions in the coil. In addition, the separation of the waste involatile material from the volatile phase took place outside the heated flask distillation unit. Air bubbles present in the segmented stream were also responsible for considerable surging. 

The glass floats, which control the liquid levels in the distillation chambers, leaked and sank and thus caused problems with liquid level control in the distillation chambers. New floats were made, but minor leakage persisted. The float has since been eliminated, and a more sensitive level detecting device is now used. 

Entrainment generally limits the capacity of distillation trays and is commonly a concern in vaporizers and evaporators. Fortunately, it is readily controllable by simple inertial entrainment capture devices such as wire mesh pads in gravity separators. 

A typical control scheme for a distillation column is shown in Fig. 19. Flow controllers (FCs) regulate the flow rates of the feed and overhead products. Each flow rate is measured by a device such as an orifice plate placed upstream... 

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