(96a) Analysis of Energy Consumption in Industrial Distillation Columns

Distillation is the commonly applied separation process in the chemical process industry as heat is used as the non-contaminating separating agent.  However, the process is also inefficient and hence leads to considerable energy consumption.  Methods have been proposed in literature for quantifying the energy inefficiencies for ideal columns.  Suitable recommendations based on Column Grand Composite Method (CGCC) analysis such as reduced reflux ratio, feed preheating and use of side condensers/reboilers have been suggested to improve the efficiency after assuming the vapor and liquid exiting the stages to be in equilibrium.  However, industrial columns are rarely ideal and hence the conclusions based on ideal column assumption may not work well.  To overcome this limitation in the analysis, either the rigorous rate based approach or the simpler efficiency approach may be adopted. 

In the present work, we have carried out the energy utilization analysis for ideal distillation columns handling non-azeotropic and azeotropic systems.  Considerable differences were noticed between the CGCC curves of ideal and non-ideal columns. The assumption of leaving liquid and vapor streams  in a distillation column are in equilibrium with each other cannot be incorporated in case of real industrial columns. CGCC curves will be dissimilar for ideal and non ideal columns. Based on this we have shown that the recommendations for improved energy utilization from ideal columns do not work well for actual columns. Column modifications recommended by thermodynamic analysis uses the available CGCC curves and hence the differences will lead to poor retrofits when accounting for a real industrial columns.

The rate based model calculations are difficult and require many parameters such as multi component transport coefficients, tray dimensions and holdup etc.  Hence, the relatively simpler efficiency approach was adopted and these were estimated independent of the rigorous rate based method.  The methodology based on Murphree efficiencies was developed to carry out the CGCC calculations. The recommendations from the simplified model based on these efficiencies were in close agreement with those made from the rate based model. 

Incorporating this methodology in actual practice will considerably reduce the energy consumption from popular separation processes.