SHORT-CUT EVALUATION OF DISTILLATION SEQUENCES

For screening sequences of distillation columns, heuristic rules may b e used, but the many contradictions and limitations of heuristics are well known. Vapour load is often used as a cost indicator for separati on sequences. However, capital costs, which vapour load does not incor porate, also make a significant and variable contribution to the total annual costs of distillation processes.A ''boiling capacity variable' ' has been developed, which is calculated from the number of stages an d internal flow rates in a column. It measures the total internal vapo rization rate in a process. That is, for a distillation column, the ov erall boiling capacity is the sum over all stages (including the reboi ler and the condenser) of the molar vapour flow rate. The short-cut me thods of Fenske, Underwood, Gilliland and others exist for calculating the reflux ratio and number of stages of distillation columns. The ov erall capacity variable can therefore be calculated using these short- cut methods. The optimal separation sequence for an ideal five-compone nt mixture is investigated in this work. Distillation sequences are ev aluated using both rigorous simulation with detailed cost analysis and the capacity variable and short-cut methods. The short-cut results ar e found to agree with the rigorous results, but have the benefits of r equiring fewer detailed assumptions and being less computationally dem anding.