(187a) Stable Thermosiphon Reboiler Operation: With or without a Valve in the Inlet Piping?

A reboiler provides the necessary heat duty for the separation in a distillation tower. It boils some of the tower bottom liquid using a heating medium such as steam or hot oil and returns vapor to the tower. The reboiler duty is usually either held constant or manipulated to keep a steady temperature on one of the lower trays. This heat duty sets the vapor load in the tower lower section.

Thermosiphon reboilers are the most widely used reboiler type in distillation systems and are usually considered first. They are relatively compact and economical, require no pumps, and offer relatively high heat transfer rates (small exchanger size) with low residence times in the heated zone.

Design and operation of thermosiphon reboilers were discussed as far back as the 1960’s. A recent review by the FRI Design Practices Committee focused on the reboiler circuits, the integration of the reboiler with its piping and the tower bottom sump. However, to-date, one key consideration received little attention: incorporating a throttling valve in the reboiler inlet line.

In industry it is known that a valve in the inlet allows the operator to throttle the inlet pressure to the thermosiphon reboiler to achieve optimum performance. Yet, around the world there are thermosiphon circuits both with and without a throttling valve at the reboiler inlet. Why is that? What is the need of the valve? How does a flow restriction in the inlet piping affects the performance of the reboiler? And what does this mean for the tower?

The pressure balance in the reboiler circuit and the interaction between fluid flow and heat transfer make designing thermosiphons challenging. The engineer must pay attention to details and understand both static design of the system and the potential dynamic behaviors.

The presentation / paper intend to provide an overview of stable thermosiphon design and operation with or without a valve in the thermosiphon reboiler inlet piping.