Thermosiphon Reboiler Modeling
Background
Distillation column reboilers are typically one of the following types,
- kettle,
- fired,
- forced-circulation, or,
- thermosiphon.
In the operation of thermosiphon reboilers, flow through the reboiler is achieved naturally via the density difference between,
- the higher-density single-phase (liquid) feed from the column bottoms, and,
- the lower-density two-phase (vapor-liquid) reboiler outlet.
In the case of a horizontal thermosiphon, the liquid from the column passes in crossflow over the tube bundle with the heating fluid passing through the tubes. The resulting vapor-liquid mixture then flows back to the column. We’ll consider modeling a horizontal thermosiphon reboiler but the general principles are the same for vertical reboilers. A flow diagram with a typical set of temperature, pressure, and flow measurements for a thermosiphon reboiler is shown in Figure 1.
Figure 1: Horizontal Thermosiphon Reboiler
Process Modeling
The circulation rate is rarely measured for thermosiphon reboilers but specifying a constant flowrate (e.g. the design circulation rate) isn’t appropriate because the circulation rate varies with reboiler heat duty in the operation of the column. Therefore, in most cases, it’s preferable to specify a constant liquid fraction (\Phi_L) for the reboiler outlet stream.
When the return temperature T_{Co} is measured it may be possible to calculate the vapor fraction \Phi_L but the success of this approach depends on the sensitivity of the return temperature to the liquid fraction. Figure 2 illustrates the sensitivity of equilibrium temperature to liquid fraction for narrow- and wide-boiling hydrocarbon mixtures at 103 and 115 KPa-a pressure. As can be seen, the wide-boiling mixture has a well-defined liquid fraction at 110 C that’s much less sensitive to pressure changes compared to the narrow-boiling mixture.
Figure 2: Wide- and Narrow-Boiling Mixtures
Recommendation
When the bottoms stream is narrow-boiling (i.e. relatively pure) the outlet liquid fraction should be fixed at a nominal value. When the bottoms stream is wide-boiling (i.e. relatively impure) calculation of the outlet liquid fraction should be investigated.