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You would expect that replacing an old piece of equipment with a new, seemingly identical one would result in better performance and higher efficiency. But that is not always the case. When it comes to reboiler tubes, old is sometimes better than new.
During an ethylene plant turnaround, a 38-year-old tube bundle within a vertical thermosiphon reboiler was replaced with a new, seemingly identical bundle. After the turnaround, the reboiler was not able to achieve the same performance, and the heat duty loss was substantial.
This article investigates this mystery and serves as a lesson for engineers who are revamping heat-transfer equipment with a like-for-like replacement. It demonstrates a thorough method of troubleshooting a thermosiphon reboiler suffering from poor heat transfer.
Reboiler operation
Circulating vertical thermosiphon reboilers are preferred in the chemical and petrochemical industries (1). They are compact, achieve high heat-transfer rates, have relatively low fouling tendency, require little plot space, and have low capital cost and no pumping cost.
Liquid is supplied to the reboiler from a distillation tower base. Often, a baffle divides the tower base into a constanthead reboiler draw sump and a level-controlled bottom draw sump (Figure 1). The baffle, which may be horizontal (as in Figure 1) or vertical, preferentially diverts the colder liquid from the bottom tray into the reboiler, and supplements it with reboiler return liquid. The liquid overflowing the baffle into the bottoms sump consists mainly of the hotter reboiler return liquid.
Boiling regimes within reboilers
Upon heating liquid inside a reboiler tube, bubbles form by nucleation at the heating surface and are dispersed in the liquid. Nucleate boiling depends on the microscopic properties of the wall surface rather than the macroscopic geometry (2). Vapor bubbles form in the superheated boundary layer within microcavities in the heated surface. The wall super- heat is the temperature difference between the wall and the local liquid saturation temperature. Bubbles grow at the wall and detach. Convective boiling occurs simultaneously with nucleate boiling.
When the wall superheat exceeds a certain value, known as the Leidenfrost point, a stable vapor film forms between the hot tube surface and the bulk liquid and blankets the wall (Figure 2) (3). This is termed film boiling. The vapor film has high thermal resistance,...