Analysis of complex two-phase instabilities in a vertical thermosyphon reboiler operating at sub-atmospheric pressures

Abstract

Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.

One of the characteristics of multiphase flows with which the operation of process units have to contend is that they often manifest instabilities that have no equivalence in single-phase flow (Bouré et al. 1973). These instabilities result often in the occurrence of undesirable large pressure, flow and/or volume fraction oscillations, which, at best, upset the expected behaviour of the multiphase flow system, resulting in a logical decrease in the reliability and life of the components and, at worst, can lead to serious flow stoppage or structural failure. Although two-phase instabilities have been studied extensively, large gaps of unexplained behaviour still remain due to the complex nature of flow, pressure drop and heat transfer mechanisms and the interactions that occur between the three. There is still substantial work ongoing in the field as these instabilities may be detrimental in the smooth operation, control, mechanical integrity and safety of heat exchangers in a multitude of their industrial applications. This paper sheds light on the experimental studies of transient behaviour at reduced pressures conducted on the thermosyphon research facility at the University of Manchester in the Morton Laboratory. These studies were initiated because literature does not contain many references to this mode of operation and existing design techniques do no adequately cover operation in sub-atmospheric pressures. Thus, the studies meant to establish the operating limits of a full scale replica of an industrial sized natural circulation thermosyphon reboiler comprising 50 vertically-mounted 25 mm OD tubes of 3 m length. Water is used as the process fluid and condensing steam is the heating source. A constant fluid level is maintained at the top of the tube-sheet using an overflow line. The behaviour in the flow-induced unstable region, the heat-induced unstable region and the stable region have been investigated. This work attempts to identify the lower and upper thresholds of instability at various reduced process pressures. For the conditions investigated, explicit thresholds are determined for the transitions between the stable region and the two unstable regions. Instability is defined based on the magnitude of oscillations observed in continuously monitored flows around the recirculation loop. The experiments revealed that the region of stable operation is very dependent on process pressure and progressively becomes smaller as the vacuum becomes lower. The use of throttling in the heat-induced unstable region to return to stable operation tends to be over a narrow region, outside of which the sole way to regain stability is to lower the heat load. In the region of flow-induced instability, throttling of the fluid at the inlet is useless and actually makes the situation worse. These instabilities are alleviated by increasing the heat load or flooding the reboiler.