Reboiler oscillation

Reboiler oscillations can often be dampened by increasing flow resistance in the reboiler inlet lines or reducing flow resistance in the reboiler outlet lines. This shifts flow resistance from the outlet to the inlet lines. This is analogous to dampening U-tube oscillations by increasing friction in the bottom of the tube while easing the restriction at the open end of the tube. Design practices for reboiler inlet and outlet lines were discussed in Sec. 15.2. 

In an existing installation, or in a service where oscillations are anticipated, an oscillation problem can be overcome by installing a restriction orifice or a throttling valve (see earlier in this section Fig. 15.4a) in the inlet line to the reboiler (150c, 313,358,360). Figure 15.7 (360) demonstrates the effectiveness of a throttling valve in dampening reboiler oscillations. 

Figure 15.7 Thermosiphon reboiler oscillations and their dampening by throttling reboiler inlet valve ("Experimental Study of a Vertical Thermosiphon Reboiler, K. R. Shellene, C. V. Sternling, DM. Church, and N. H. Snyder, Chemical Engineering Progress Symposium ries, vol. 64, no. 82, p. 102, 1968. Reproduced by permission of the American Institute of Chemical Engineers.)...

Unstable operation often results from control system problems, improper design of the reboiler, or very low flow rates of liquid (leading to pnlsing). Some devices, snch as dnalflow trays (with no downcomers), tend toward oscillating flows. The most common canse of stability is the control system. 

Inlet line This line is usually sized for a cross-sectional area of 25 to 50 percent of the total tube cross-sectional area of the reboiler (124, 253, 358). A relatively high pressure drop in the inlet line is important for preventing excessive circulation, dampening oscillations, and for minimizing preheat zone length. 

Outlet line This line is usually sized so that its cross-sectional area is at least the same as the total cross-sectional area of the reboiler tubes (253, 358, 360). Excessive pressure drop in this line promotes oscillations and elongates the preheat zone. Excessive velocity in this line may also be undesirable at the column inlet (see Sec. 4.1, guideline 5). On the other hand, if the outlet line rises vertically before bending toward the column, velocities should be kept above 15 ft/s (237), or slug flow may result. 

The following discussion generally applies regardless of the variable that regulates boilup and the directness of control. Perhaps the only exception is that the need to avoid sluggish and oscillating boilup manipulation is emphasized when boilup is manipulated to achieve a desired product purity. For simplicity only, most of the examples discussed assume that the heating medium flow rate manipulates the reboiler control valve. 

Sustained oscillation could not be found by means of continuation analysis for the lab-scale methyl formate column. However, oscillations have been observed during experimental investigations by Reder [87] as displayed in Fig. 10.6. Various temperature readings in the reboiler and on different trays are shown in the lower diagram. A step change in heat duty is applied at time 0. Large amplitude temperature oscillations with a period of about 0.3 h can be observed before and after the step change. The oscillations have been reproducible in various experiments. No source of external excitation could be located. An explanation of the phenomenon has not yet been achieved, however. 

Two-phase flows oscillations induced by the flow boiling process are of interest to designers of macroscale thermal systems and equipment, such as thermosyphon reboilers, steam generators, evaporators in the refrigeration industry and chemical... 

These results are perhaps surprising because the side-reboiler for the diabatie design introduces strong cross-coupling effects causing considerable oscillations within the column. However the improved controlled response is due to the removal of the sharp nonlinearity in the temperature profile (Fig 7). Shifts in the internal temperature, used as a measured variable in the control scheme, are less drastic producing a more even elosed loop response. 

Even if is small (for example 2 s ), a large number of trays (for example 40) will result in a considerable effective dead time (80 s.), which, together with the hydraulic dynamics of the reboiler and integration of the bottom level, will result in slow level control (oscillation period larger than 6 times 80 s). 

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