Based on the principle of orthogonal collocation

this paper demonstrates systematically a new methodology for developing a reduced-ordermodel of multicomponent distillation column. The basic column state variables

such as vapor and liquid flow rates and compositions

are considered to be continuous variables along the column.They can be approximately represented by polynomials

thus the number of equations required to describe the system is drastically reduced. As for the discontinuity introduced by the feeds and sidestreams

it can be partially eliminated by suitable redefining new variables which can be approximated by polynomials. In the reduced-order model given in this paper

the simultaneous effects of mass and heat transfer

tray hydraulics and reboiler dynamics are taken into account. The computation time is significantly reduced as the iterative calculation for vapor-liquid equilibrium is avoided. Simulation studies are carried out to check the accuracy of the model for a deethanizer

which has 70 plates and three feeds. It is shown that the model agrees with the rigorous one fairly well.