More Reboiler Debottlenecking Discussions at the Distillation Experts Conclave (DEC2021)

A large fraction of distillation tower reboilers, possibly most, are heated by condensing vapor. Typical examples are steam reboilers, refrigeration vapor reboilers, and heat-integrated reboilers that condense overhead vapor from same or a different tower. For these reboilers, the control valve can be located either in the steam (or vapor) inlet line or in the condensate outlet line. The two methods are different in principle and operation, have different strengths and weaknesses, and require different measures to achieve trouble-free operation.

The selected method, as well as the features added to overcome its weaknesses, is central for good reboiler operation, performance, and tower stability. The literature has some excellent reviews on this subject, but these reviews have last been updated three decades ago. Recent excellent sources have only partially addressed some of the key considerations. This article aims to fill in the gaps, combining the old knowledge with the recent experiences into a detailed updated guide to the key considerations, features, and practices that can improve reboiler control and operation.

Reboilers using an inlet steam control valve (ISC) arrangement can provide significant economic and control benefits to certain distillation tower installations. By exposing the full surface area of a reboiler to steam for effective heat transfer, the steam pressure can be lowered - which can reduce the fouling that can occur with higher pressure steam and the corrosion that can follow from flooding a tube bundle. Control can improve because steam can adjust to process demand changes much more rapidly than by the alternative of varying the condensate liquid level and exposed area of the tube bundle - such as occurs with an outlet condensate control valve (OCC) arrangement.

Furthermore, unlike OCC, it is generally not possible to lose the condensate seal/reboiler duty when the proper drainage design is implemented. Other potential benefits include mitigation of the stratification that can plague channel head gasketing in horizontal designs and enable the use of “long” low pressure steam to improve the steam balance or reduce generation requirements for medium pressure steam.

However, the reliability of ISC (and OCC) reboilers requires effective condensate drainage from the steam space, and there are multiple instances of reboilers experiencing shock-related issues such as channel head gasket and tube bundle damage or the sewering of condensate. These issues may be more commonly experienced with horizontal type reboilers.

Many of these problems can be avoided/mitigated with a proper condensate drainage design, but it often occurs that the reboiler system experiencing difficulty had improper original design or installation and requires correction for improvement. As an example, it is typical that a level pot/control valve arrangement may be improperly balanced, or a stall condition may not be recognized and mitigated. Additionally, some reboiler operations result in the need to discharge colder, non-flashing condensate into headers containing flash steam-a process that can produce violent water hammer.

It is possible to explore historical negative experiences, analyze why certain problems occur, and provide some specific key design considerations necessary to obtain optimized reboiler performance.