Control Tray Composition

As stated in the design procedure, the composition of reactant A on the first tray in the reactive zone (tray 6 in the base case) is controlled by changing fresh feed Fqa- The composition on this tray is 30 mol% A in the base case. In this section we explore how changing the specified value of a affects the design. 

At the other end of the range where the values of are 30 mol%, vapor boilup again 

These results indicate that the design is fairly insensitive to the value of the A composition controlled on the internal tray. 

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Figure 5.15 Effect of control tray composition on temperature profile.

Example The location of the best temperature-control tray in a distillation column is a popular subject in the process-control literature. Ideally, the best location for controlling distillate composition xa with reflux flow by using a tray temperature would be at the top of the column for a binary system. See Fig. 8.9o. This is desirable dynamically because it keeps the measurement lags as small as possible. It is also desirable from a steadystate standpoint because it keeps the distillate composition constant at steadystate in a constant pressure, binary system. Holding a temperature on a tray farther down in the column does not guarantee that x will be constant, particularly when feed composition changes occur. 

If bottoms composition is to be controlled by vapor boilup, the control tray should be located as dose to the base of the column as possible in a binary system. In multicomponent systems with heavy components in the feed which have their highest concentration in the base of the column, the optimum control tray moves up in the column. 

A distillalion column is used to separate two close-boiling components that have a relative volatility close to one. The reflux ratio is quite high (IS) and many trays are required (150). To control the compositions of both products the flow rates of the product streams (distillate D and bottoms B) an manipulated. Gas chromatographs are used to measure the product compositions. Base level is controlled by steam flow rate to the icboiler and reflux drum level is controlled by reflux flow rate. 

When the temperature on a possible control tray is insensitive to the composition, which is particularly the case when high purity overhead is being made,... 

Since we have two control degrees of freedom, our objectives in distillation are to control the amount of LK impurity in the bottoms product ( b.lk) and the amount of HK impurity in the distillate ( 5>Hk) Controlling these compositions directly requires that we have composition analyzers to measure them. Instead of doing this, it is often possible to achieve fairly good product quality control by controlling the temperature on some tray in the column and keeping one manipulated variable constant. Quite often the best variable to fix is the reflux flowrate, but other possibilities include holding heat input or reflux ratio constant. 

But why is this a good way to select the best control tray It works because we want to find a tray where temperature is significantly affected by the compositions of the LK and HK components. Since temperature is also affected by other variables (pressure and other components), it is important that the effects of these variables are small compared with the effects of the compositions of the key components. 

Figure. 14 Dynamic response to HHK feed composition disturbance, (a) With control tray 6 (i) with control tray 14.

Remark.- Energy balance and overall mass balance are neglected. The only dynamic element is the tray composition. Consequently, this model might be useful for some dynamic studies, when the composition dynamics is relevant, but the process itself is rather slow. This model is not suitable for control, where an accurate description of the initial response is required. 

However, sometimes selection of the appropriate controlled variable is not so easy. For example, in a distillation column it is frequently difficult and expensive to measure product compositions directly with sensors such as gas chromatographs. Instead, temperatures on various trays are controlled. The selection of the best control tray to use requires a considerable amount of knowledge about the column, its operation, and its performance. Varying amounts of non-key components in the feed can significantly affect the best choice of control trays. 

Most column control schemes (see Sec. 16.5) use the composition (or temperature) controller to manipulate either the reflux or reboil, directly or indirectly. The stream which is not controlled is commonly "free, i.e., on flow control. This "free stream is usually manipulated during flood testing, while the stream on temperature control will be automatically adjusted to maintain product composition. For instance, if reflux rate is on temperature control and reboil rate is on flow control, flood testing is performed by raising the reboil rate. This warms up the control tray and increases condensation. The temperature controller will call for more reflux, and the column will reach new stable conditions with both reboil and reflux increased. 

Another djmamic consideration is associated with the column feed. Glenerally, most disturbances enter the column at the feed. If the temperature controller is far from the feed, the disturbance may propagate a long way before the corrective action is taken. On the other hand, if the control tray is located near the feed, it will tend to take excessive corrective action and may be imstable. This consideration is most important when feed composition changes are frequent, and is detailed elsewhere (400). 

Figure 18.3 shows examples of applying this procedure to benzene-toluene columns with different feed points and different feed compositions. Accordingly, trays 7,10, and 5 or 10 are the best control trays in Fig. 18.3a, b, and c, respectively. Figure 18.4, based on the column in Fig. 18.3a, shows how a variation in control tray temperature affects product composition with a correctly located and an incorrectly located control tray. When the temperature variation is caused by a change of pressure or in the concentration of a nonkey component, it will produce a steady-state offset in product composition. A disturbance in the material or energy balance will cause a similar temperature variation until corrected by the control action in this case, the offset will only be temporary. Figure 18.4 shows that the offset in either case is minimized when the control tray is selected in accordance with Tolliver and McCune s procedure (403). A dynamic analysis by these authors (403) indicated that the control tray thus selected tends to have the fastest, most linear dynamics. 

Figure 18.4 Effect of variation in control tray temperature on product composition with a correctly and an incorrectly located control tray. Same column as in Fig. 18.3a. (a) Overhead composition (6) bottom composition. (Reprinted by permission. Copyright Instrument Society of America 1980. From T. L. Tolliver and L. C. McCune, In Tech- ptember 1980.)...

Figure 18.5a illustrates an important consideration for sharp splits. A top section control tray (e.g., tray 15) will adequately control both top and bottom product compositions for plots 2 to 6. For plots 1 and 8 to 10, this control tray will retain the top product on-spec, but will permit wide variations of bottom product composition. Conversely, a control tray located in the bottom section (e.g., tray 30) will give good control of the bottom composition at all plots. It will also give good control... 

A differential temperature control (e.g., Fig. 18.8o) is in essence a pressure-compensated temperature control. The control tray temperature is measured in the usual manner. A second temperature is measured at a point where temperature is relatively insensitive to composition, such as near the bottom or the top of the column. The second temperature is subtracted from the first, giving a differential temperature measurement. This differential temperatiire is used for control. Since the second temperature varies little with composition, the differential temperature will reflect the composition variations measured by the first temperature. When column pressiu-e changes, both temperatures change equally, but the temperature difference remains constant. 

A good knowledge of the column temperature profile, coupled with a good choice of control trays, can sometimes make the differential temperature control technique work even in the presence of nonkeys and when neither product is pure. Vermilion (411) made this system work in another deisobutanizer, the feed to which contained a substantial fraction of both light and heavy nonkeys. The bottom sections contained 40 trays Vermilion used tray 14 [from the bottom this is similar to Webber (418)] as the first temperature control location, and tray 34 as the second. In that case, composition and temperature variations near the feed were relatively small (411). Figure 18.8c demonstrates the importance of correctly choosing the differential temperature measurement location. 

The control tray is best positioned close enough to the top to assure good correlation with the product composition, but at the seune time far enough from the top to suit the analyzer sensitivity or selectivity requirements. About 5 to 15 trays from the top is typical in high-purity separations. The top tray should be avoided. Poor mixing on... 

Minimum Product Variability Criterion Choose the Tray that Produces the Smallest Changes in Product Purities When it is Held Constant in the Face of Feed Composition Disturbances. Several candidate tray locations are selected. The temperature on one specific tray is fixed, and a second control degree of freedom is fixed such as reflux ratio or reflux flow rate. Then the feed composition is changed over the expected range of values, and the resulting product compositions are calculated. The procedure is repeated for several control tray locations. The tray is selected that produces the smallest changes in product purities when it is held constant in the face of feed composition disturbances. 

Minimum Product Variabiiity Critarion. Figure 6.12 shows how product impurities change for two different temperature control trays as feed composition changes. In the first case, the temperature of Stage 38 is fixed at 344.28 K. In the second case, the temperature of Stage 41 is fixed at 349.71 K. 

The results shown in Figure 7.44 show fairly large transient disturbances in both temperature and compositions for feed flow rate disturbances. The change in feed flow rate enters the column and must impact the control tray temperature before any corrective action in reboiler heat input occurs. 

Tray temperature control is used in most distillation columns to infer product composition, but changes in pressure on the control tray can adversely affect the estimation of composition. Pressure is typically controlled in the condenser, not on the control tray, so changes in vapor flow rates will change tray pressure due to changes in tray pressure drops. Pressure-compensated temperature control was proposed over four decades ago to solve this problem. Measurements of both temperature and pressure on the control tray are used to estimate composition. The method has been qualitatively described in many practical distillation control books, but the author is not aware of any quantitative evaluation of its effectiveness that has appeared in the open literature. 

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