.ThermalSeparation.FilmModel.BaseClasses.EquilibriumEffectiveDiff

Information

There is the possiblity to assume thermodynamic equilibrium on each stage. This class sets the parameter EQ = true. If so, there is no resistance for mass transfer and the compositions in the bulk phase and at the phase boundary are the same. However for numerical reasons nevertheless a molar flow rate N_dot of each component is calculated using the equation

N_dot = A k · (x_star-x) for each component i.

The mass transfer coefficient k shall be chosen high enough to ensure that x_star → x for every component. However if K is too high the simulation becomes very slow or - even worse - the nonlinear solver fails to solve the problem. Also the optimal value for K may change during the simulation. Therefore only a start value for K is given by the user and then K is adapted using a PI controller. Up to now there is only one PI controller for the vapour phase and one for the liquid phase. The PI controller aims to get the maximum difference of the (x_star-x) of all substances on the first stage to zero.

Murhpree tray efficiency

This model also includes the Murphree tray efficiency. The Murphree efficiency ηMurphree is only used for tray columns (for packed columns the efficiency is set to one) and used together with an equilibrium model (parameter EQ=true): in this case it is said that the thermodynamic equilibrium is attained on every plate (plate column). However, if this is not the case but detailed information for a mass transfer model is missing, than the deviation from equilibrium can be expressed using the Murphree efficiency. The Murphree efficiency is defined as the ratio of the real concentration change on the tray to the maximal concentration change (which would be if equilibrium was obtained). The tray efficiency takes a concentration gradient in the liquid on the tray into account: The composition at the tray outlet is different than the composition on the tray (which is not modelled here: in the model for one stage the composition in the stage equals the composition at the stage outlet). Therefore it is possible for the tray efficiency to become greater than 1 (see for example Perry: Perry's chemical engineers' handbook, 8th ed.). If no equilibrium model is used, ηMurphree is set to one. In a binary mixture ηMurphree is equal for both components; in a multicomponent mixture the efficiency is different for every component and can vary between -inf and inf.

Contents

Name	Description
ThermoEquilibrium
StateSelection