Boilup measurement

A2.8.3 Convert the boilup measurement in litres per hour to rates per hour in (mL/h) x cm. Convert also the AP measurements to kPa/m (mm Hg/m) and the holdup measurement to milliliters per theoretical plate. 

In order to overcome the level measurement problem, an isolating chamber arrangement or "bathtub (68) is sometimes used (Fig. 15.96). The chamber overflow weir keeps the bundle submerged, and the column liquid level can be separately controlled. This eirrange-ment may increase column height requirement. If the column level is too close to the isolating chamber, it may overflow, flood the reboiler, back up liquid into the column, and possibly also flood the colunm. When boilup exceeds liquid downflow, or when the isolating chamber... 

When the control temperature and the column control pressure are measured at two different locations, the control temperature may also be affected by changes in the differential pressiure between the two locations. For instance, if column top pressure is controlled, and the control temperature is located near the bottom, a rise in differential pressure will increase the pressure at the temperature control location. The control temperature will rise, the controller will interpret the change as a rise in heavy key concentration, and will coimteract it (e.g., by decreasing boilup). 

When the "free stream in an MB control system (Sec. 16.2) is the boilup rate, it is sometimes manipulated by the differential pressure across a column section or across the entire column (Fig. 19.12a). Column pressure drop is primarily a measure of column vapor load, although it is also influenced by the liquid load. Therefore, controlling differential pressure generally maintains a uniform vapor load in the column. 

Whenever heat input is manipulated through a forward loop, as in Figs. 11.14 and 11.15, dynamic compensation is considerably different. If a step increase in feed rate is converted instantaneously into a proportional increase in boilup, a momentous imbalance in the vapor-liquid distribution is forced up the column, Eventually the flow of reflux ivill increase in order to restore balance, and when equilibrium returns, the material balance will be unaffected. The sudden increase in boilup will, however, carry bottoms product upward, lowering the level in the reboiler, until reflux flow increases commensurately. Consequently, an analyzer or temperature measurement anywhere within the column will indicate a transient overcorrection. 

If composition is measured and is cascade controlled via reflux or boilup, or both, ratio controls should be replaced by impulse feedforward compensation (see Chapter 12) if feed flow turndown is greater than 2 1. 

The column base runs empty and there is just enough Hquid head in the line connecting the column base to the reboiler to overcome the pressure difference between the kettle reboiler and the column base. To protect the tube bundle in the event that boilup temporarily exceeds downflow, a head measurement must be made on the tube bundle chamber. This can be connected to an interlock, or, as shown on Figure 4.7, to suitable overrides. 

Overhead level control may be calculated simply by the method of Chapter 16, Section 3, but base level control by boilup is very difficult. It is normally used only when the average bottom-produa flow is very small. The characteristic time constant th should be at least 15 minutes and other design factors should be as indicated in Chapter 16, Section 7. In most cases base level control by boilup requires a dynamic analysis, and perhaps supplementary plant tests. If steam flow is measured with an orifice, a square root extractor should be used. 

The pressure drop during operation is measured by means of a manometer or pressure transducer connected between the flask and the condenser. A very small flow of nitrogen (8 cmVs) inserted between the manometer and the flask will prevent condensation in the connecting tube. The boilup rate is normally controlled by sensing the pressure drop in the column. 

A 1.6.4 Apply heat to the flask until the test mixture boils, then increase the heat progressively up to the flood point. This will be noted by visible slugs of liquid in the packing or on the plates or by liquid filling the neck of the condenser and a sudden increase in pressure drop. Reduce heat to allow the flooding to subside. Increase the heat gradually to just below the flood point. Record boilup and AP measurements just below the flood point. This is considered the maximum rate. 

A2.7.6 Apply heat to the flask and bring the test mixture to a boil. Adjust the boilup rate to approximately 200 mL/h times the cross-sectional area of the column in cm measured by timing the Ailing of the calibrated bulb. When the desired rate has been established, hold for 30 min noting the pressure drop. 

A2.8.4 Plot all data as ordinates versus boilup rate in Utres per hour and (mL/h) x cm as abscissa. Draw smooth curves through the points for holdup in millilitres and in millilitres per theoretical plate. The boilup versus AP measurements should be compared with those made in the efliciency measurements if available, to ensure that they are in reasonable agreement. 

Figure 12.4 shows a control structure that uses two direct composition measurements. Bottoms product purity (mol% C) is measured and controlled by vapor boilup. The composition of A on tray 5 is measured and controlled by the flowrate of flesh feed Fqa-Throughput is set by flow controlling fresh feed Fob- Reflux-drum level is controlled by manipulating the reflux flowrate, and base level is controlled by manipulating the bottoms flowrate. 

Figure 12.29 shows a control structure that uses two direct composition measurements. The bottoms product purity (mol% C) is measured and controlled by vapor boilup. 

To eliminate steady-state offsets in product compositions, dual composition control is implemented. Figure 12.56 shows that the distillate composition is controlled by changing the reflux ratio, and the bottoms composition is maintained by adjusting vapor boilup. Because the composition analyzer has a slower response, a measurement dead time of 4 min is assumed in the simulation. Relay-feedback tests are performed to find settings for the PI controllers. Table 12.5 shows that because of the dead time in the composition measurement, the reset times for the top and bottoms loops now become 120 and 102 min, compared to 4.3 and 2.9 min in the case of temperature control. 

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