When steam-heated reboilers are used in distillation columns and the heating medium is steam, a flow control loop for the steam will typically be incorporated. The control valve can then be placed either in the steam or condensate line. Steam pressure compensation is always recommended in these cases, and the controls are illustrated in the figure below:
Note that there are two possible locations for the control valve: either in the steam supply line or in the steam condensate line. When the control valve is placed in the steam supply line, it is imperative that a steam trap is used to drain the condensate away but retain the steam. However, when the control valve is located in the condensate line, no steam trap is needed, but the valve should be fitted with a positioner because the loop behaves more like a level (because of its response to a control signal) than a flow control loop. A positioner assists in stabilizing it, but when the control valve is placed in the steam supply line, a positioner is not required. A positioner is not fitted to the valve located in the steam line because it would destabilize the control loop, making the steam flow respond immediately to the controlled output and precede the heat transfer. On the other hand, changes in the condensate have no direct effect on the steam flow but only on the heat transfer, which has a much slower response. The other benefit is the fact the valve located in the condensate line can be considerably smaller in size – approximately one-third smaller because of the square law relationship between the valve Cv and pipe diameter and additionally, this valve is of a lower temperature grade and hence of lower cost.
Reboiler Control System
Consider the figure below shows an illustration of a kettle reboiler, which is normally connected to the bottom of a column from where it receives the liquid bottoms product.
Under normal circumstances, only a small amount of liquid is held at the bottom of a column and re-boiling whatever amount remains allows the lighter components to be extracted. The unit shown above is mounted externally to the column; however, the heating element can sometimes be fitted directly into the base of the column where it accomplishes the same function of vaporizing the liquid. The design of the equipment ensures that no liquid is discharged along with the vapor. The figure also shows that the liquid collects in the section formed by the weir, from where it is pumped away as the final bottoms product. In the event of a failure of the level controller, the weir is intended to prevent the heating tubes from being uncovered and therefore damaged. However when a loss of column pressure occurs, rapid changes in boil-up result in the liquid vaporizing faster than it is returned, thus allowing the tubes to become uncovered. Suitable precautions should therefore be taken to minimize these occurrences. This type of reboiler is called an immersed type as long as liquid covers the tubes
Related: Furnace Control System for a Rapidly Changing Demand
