(187d) Identifying Turndown Limits for Vertical Tubeside Thermosiphon Reboilers

Operating thermosiphon reboilers at less than design capacity is often necessary, but the lack of knowledge as to how far a unit can be turned down can cause problems. At HTRI, we have recently completed a set of experiments in a thermosiphon flow loop to answer the question of turndown limits. A vertical tubeside thermosiphon reboiler (VTTR) was attached to a separator. Steam supplied to the shellside of the VTTR heated up the tubeside fluid, which came in subcooled and exited two-phase. The density difference between the two-phase fluid in the VTTR tubes and the single-phase fluid inside the column drove the circulation. By controlling the column pressure and systematically lowering the mean temperature difference (MTD) between the steam and the process fluid, the unit was turned down gradually. Below a certain MTD or average heat flux, we observed an end to liquid recirculating back into the column, marking the turndown limit. Based on tests with pure fluids and wide boiling range mixtures, very different turndown mechanisms were observed, indicating different underlying mechanisms responsible for unit shutdown:

• Pure fluid tests indicated two distinct behaviors: tests with a light hydrocarbon (n-pentane) indicated that the unit was operational even with an MTD as low as one Deg-C, the lowest MTD we could attain experimentally. Tests with water and p-xylene indicated that below a minimum MTD or heat flux, the vapor generation rate matched the circulation rate, pointing to a pool boiling type mechanism with all vapor outlet. For pure fluids or fluids with low boiling range (< 5 Deg-C), the only known shutdown criterion is based on the end of onset of nucleate boiling (ONB). Our data clearly indicates that for some pure fluids, the ONB criterion is insufficient.
• Tests with wide boiling range (> 30 Deg-C) mixtures indicated that below a certain minimum heat flux, the circulation abruptly ended. We postulate that just before shutdown, the low heat flux and low circulation rate led to lights boiling off preferentially, resulting in accumulation of the heavy component. This led to a rapid lowering of the effective MTD, causing the unit to shut down.

While this research is not complete, our observations and analysis clearly indicate the need to develop better criteria for turndown limits than those currently available. HTRI is currently working on developing guidelines to help the design, operation and troubleshooting of thermosiphons. Additional experimental work with other fluid mixtures and different turndown schemes (e.g. flooding the VTTR shellside to reduce the effective heat transfer area) are also being considered.