Plant-wide control of coupled distillation columns with partial condensers

Abstract

Conventional distillation control processes use vapor distillate flowrate to control column pressure and condenser heat removal to control the reflux drum level. These intuitive control systems work well for isolated columns or columns with total condensers. However, these controls are not effective when columns with partial condensers occur in series. The pressure and reflux drum level interact in such systems in ways that defeat conventional control systems, rendering them unable to maintain product purities in the presence of large feed flowrate and composition disturbances. This investigation documents a plant-wide control structure that can address this issue by controlling pressure through reflux heat removal rate and reflux drum level by reflux flow rate. This control system demonstrates its capability to handle large disturbances in throughput and feed composition through a series of Aspen simulations. This alternative system is no more complicated than the conventional system and should work on distillation columns of nearly all designs, not just the coupled partial condenser designs for which it is essential.Common natural gas processing provides a specific example of this alternative control system. Natural gas commonly includes high concentrations of CO2 that must be removed prior to pipeline or LNG distribution. The existence of a minimum-boiling temperature azeotrope between ethane, virtually always present in natural gas, and carbon dioxide complicates the separation of CO2 from the hydrocarbons. This separation commonly employs extractive distillation with high-molecular-weight hydrocarbons. Our recent paper Ebrahimzadeh et al. (2016) discusses in detail the steady-state economic design of a new extractive distillation strategy for the CO2–ethane azeotrope separation with three columns. This strategy shows a 5% reduction in capital and 15% reduction in operating costs when compared to optimized versions of the conventional process. The new strategy also produces CO2 in a liquid rather than a vapor phase, which simplifies transport, storage, and handling. Two columns of the proposed design use partial condensers and are the focus of this investigation.