STIFFNESS REDUCTION OF GAS SYSTEM MODELS

Gas systems like heat exchangers, adsorption towers, membranes and rea ctors are often modeled as sequences of mixing tanks. This approach re sults in stiff models, and the stiffness increases with a greater numb er of mixing tanks. An alternative modeling approach is described in t he present paper. The pressures of all mixing tanks is assumed proport ional. This assumption imposes constraints on the holdups and makes th e resulting model semi-explicit index two. The index is reduced from t wo to one by an index reduction algorithm, i.e. new equations which ex press the constraints are developed by manipulations and nonlinear tra nsformations of the equations set. The index-reduced model is non-stif f and semi-explicit index one, and it gives practically the same simul atioopn results as the original model. The index-reduced model is thus well suited for simulation by explicit integration algorithms and com mercially-available process simulators.