Temperature Control with Boilup Steam Flow Rate

4 Temperature Control with Boilup (Steam Flow Rate) 

In the boilup scheme, a column temperature controller manipulates a control valve In the steam line to the reboller. The 

The origins of the boilup scheme may go back to the days when the feed flow rate and reflux flow rate to a distillation column were controlled by manual valves and glass rotameters in the control room. A pneumatic temperature controller manipulated an automatic valve in the steam line to the reboiler. This may be difficult to imagine by those who have only experienced modern-day equipment. 

The manipulated boilup scheme is still used in many cases and especially for top-fed stripping columns that have no reflux stream and no rectification section above the feed. A column temperature controller manipulates the steam flow rate to the reboiler. The MRT point for a stripper is often the overhead vapor temperature leaving the column. If the overhead vapor temperature is close to the bottoms temperature, then an overhead vapor flow rate controller can be used to manipulate the steam flow rate. 

The manipulated boilup scheme is quite easy to start up after some liquid is accumulated in the reflux drum. However, this scheme does not shed environmental disturbances very well. When a sudden rain storm hits the distillation tower, the temperature disturbance is shed by the temperature controller increasing the steam flow rate to the reboiler. This can be a problem if the distillation tower needs to be running at its maximum capacity near to flooding. 

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To illustrate the disturbance rejection effect, consider the distillation column reboiler shown in Fig. 8.2a. Suppose the steam supply pressure increases. The pressure drop over the control valve will be larger, so the steam flow rale will increase. With the single-loop temperature controller, no correction will be made until the higher steam flow rate increases the vapor boilup and the higher vapor rate begins to raise the temperature on tray 5. Thus the whole system is disturbed by a supply-steam pressure change. 

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