Demystifying thermosiphon operation

Siddharth Talapatra, Group Lead, Research

Engineers focused on the thermal and hydraulic aspects of heat exchangers sometimes forget that exchangers do not operate as standalone units. A thermosiphon reboiler is one application where understanding the flow loop around the heat exchanger is vital.

Essential to refineries and chemical processing plants, a thermosiphon reboiler operates by natural circulation—there is no external pumping source. Figure 1 shows a vertical tubeside thermosiphon reboiler (VTTR). In this sketch, a column of liquid drives the flow through the inlet piping, the reboiler, and the outlet piping back into the column based on the density difference between the single-phase fluid in the column and the two-phase fluid in the reboiler and outlet piping. Strongly coupled thermal-hydraulic performance makes operation and prediction of such units difficult. Poorly performing thermosiphon reboilers remain a challenging problem for the process industry, costing millions of dollars in lost production.

Figure 1. Schematic of a VTTR, with column level control and liquid draw off

Different configurations of thermosiphon reboilers are possible. Our recent research efforts have focused on VTTRs. There are three main operational concerns with VTTR that are not encountered with other exchanger operations:

Startup: Because boiling the fluid in the reboiler is needed to start the circulation, and effective boiling is only possible once circulation is established, we are left with a chicken-and-egg problem. From our operational experience of running a VTTR at HTRI, the following are some guidelines to help establish circulation.

  • Slowly heat up the unit to avoid thermal shocks. A high mean temperature difference (MTD) does not help speed up startup when circulation has not been established.
  • After some limited pool boiling has started, pull a vacuum on the overhead condenser vent (if possible). This may help kick-start the unit.
  • Starting with a high liquid column height makes it easier to establish circulation.

Instabilities: There are several instability mechanisms attributed to VTTRs. Our research has indicated that Ledinegg instability (or flow incursion) is unlikely in a VTTR. Bouré instability (also called density wave oscillations) is the most common mechanism responsible for oscillatory behavior. Installing a valve in the inlet piping can help resolve this issue at the operational stage. Predictive methods are far from accurate, and HTRI is actively working on improving them.

Turndown limits: HTRI is collecting data to understand how far a VTTR can be turned down before liquid recirculation stops. Preliminary analysis indicates that all available criteria to predict the onset of shutdown are inadequate. Several interesting trends have been observed.

  • For some pure fluids, maintaining a sufficient MTD for the onset of nucleate boiling prevents unit shutdown.
  • For other pure fluids, normal operation requires a minimum heat flux, below which the unit transitions to a pool-boiling mode.
  • For a wide boiling range (40 – 55 °C) binary mixture, rapid shutdown may occur below a threshold heat flux, which is a strong function of the column height to tube length ratio and a weak function of the two-phase density ratio.

HTRI is planning to publish multiple reports on VTTR operation in FY 2019. We hope our improved guidelines and methods will help demystify the challenging task of designing, operating, and troubleshooting thermosiphon reboilers.