Thermosyphon reboilers provide a simple low-cost method of
adding heat to distillation processes. They have no moving parts,
simplifying maintenance and installation. However, proper design
of the process involves careful sizing of equipment to account
for vapor-liquid disengagement and multi-phase fluid flow. Improper
design has led to many unit operation failures. A prime source
of improper design is the lack of detailed information provided
by the intrinsic mass-transfer modelling operations in flowsheet
simulators.
The modelling criteria and flowsheets required to derive sufficient
and correct design information are presented.
The modelling techniques shown are applied to the design of thermosyphon systems for high relative volatility (alpha) distillation systems. Until now, design work in the field has concentrated on the hydrodynamics of low alpha systems. Alpha has a major impact on the simulation and modeling requirements. The high alpha system requires more detailed information for correct design. Additionally, for high alpha systems, the design may be either a minimum-surface-area design or a constant-fluid-flow-driving-force design.
Standard simulation models used in process flowsheet simulators do not generate the required design data and do not allow for the choice between the minimum-surface-area versus constant-fluid-flow-driving-force designs. A modified simulation configuration is shown to handle these.
An example compares the design approaches for a fluid catalytic cracking unit stripper-deethanizer reboiler system. The comparison includes quantification of the impact of using a standard simulation model and the improved technique.
12 pages.
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