Reboiler swell

The reboiler may also experience inverse response, often referred to as reboiler surge or reboiler swell. An increase in heat input may increase the volume of vapor in the reboiler or the pressure drop in the reboiler and its outlet piping. This will temporarily back up liquid into the column bottom, causing liquid level to rise. 

There are diree factors that limit the accuracy of the preceding analysis. The first of these relates to the phenomenon of inverse response discussed in Chapter 13. It is characteristic of valve tray columns and some sieve tray columns operating at low boilup rates. It exercises its most serious effect in those columns where base level is controlled via steam flow. If the level becomes too high, the level controller increases the steam flow. But this causes a momentary increase in base level due to the extra liquid coming down the column (also due to thermosyphon reboiler swell ). Without proper design the level controller can become very confused. This is discussed in detail in Chapter 16. 

If we are not limited in column-base holdup and can design for reasonably well-damped control, then we can treat reboiler dynamics as ne gible. This says that steam flow responds to the flow controller set point immediately, and that boilup follows steam flow without lag. We may Aen prepare the signal flow diagram of Figure 16.7. Note that = steam flow-meter gain = 12/(Wsr)max- This may be partially reduced to the form of Figure 16.8. From this last illustration we can see some of the loop s characteristics as they ate afiected by reboiler swell and inverse response. 

Base level control via steam flow control with inverse response and reboiler swell... 

Change In tube vapor volume with change in heat input. Stepping up heat input generates more vapor and expands the volume of fluids in the tubes. Fluid swell following an abrupt increase in heat input may displace liquid back into the column base. If the liquid to the reboiler comes from a trapout pan, this can flood the bottom tray. This problem is most pronounced at low heat loads (67, 68). 

As a final example, let us look at the control of column-base levd by throttling condensate flow fi om the reboiler. We will ignore possible column inverse response but will take swell into account. Column acoustic impedance, seen fi om the base, will be assumed to be resistive only. The basic, open-loop signal flow diagram for the flooded reboiler is given in Figure 15.13. We need in addition the following relationships ... 

The operators, being unaware of the last three facts, filled the tower almost to the top with naphtha and increased heat flow to the reboiler. The relief valve, set for 50 psig, opened as the liquid in the tower swelled up due to the extra heat. The boiling naphtha partially flashed as it flowed from 50 psig in the tower to zero psig in the blowdown stack. 

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