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.