External reboiler in distillation column simulation
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In distillation column simulations, sometimes, in order to reflect actual operating conditions, it is necessary to use an external reboiler and adopt a column model without a reboiler. As shown in the figure below: During the aforementioned simulation, one additional iteration was added (convergence is already difficult in tower simulations that involve loops in the first place). I would like to ask everyone what issues need to be taken into consideration in such simulations with external reboilers in order to achieve better convergence. Thank you. . .
Generally, no external consideration process is required, unless a reboiler needs to be designed separately, in which case a detailed heat exchanger design model is needed.
Because the stream that is heated in a recirculation loop within the reboiler (green line) has a complex composition, in actual processes the fluid entering the reboiler is first pressurized by a pump before being forced to circulate; the outlet of the reboiler ensures a fully liquid state, and after pressure reduction via a control valve, a two-phase mixture of gas and liquid is formed, which then enters the bottom of the tower. Can the above process be clearly described by using the reboiler built into the tower? 1. Determine the flow rate of the heating fluid in the reboiler and its temperature after heating ； 2. The heating fluid undergoes one stage of pressure reduction ； 3. The fluid at the reboiler inlet is pressurized to prevent the formation of a gas phase at the reboiler outlet
Compared to an external reboiler, it causes greater disruption to the composition at the bottom of the tower; forced circulation is used instead of natural settling, along with reheating