Wiped Film Evaporator - Technical Information
Wiped Film Evaporator - Technical Information
The wiped film evaporator unit operation provides the capability to separate solvents and/or monomers from a polymer melt. A wiped film evaporator should be used when the removal of volatiles from a viscous polymer melt is diffusion limited. The blades inside the wiped film evaporator continually mix and spread a thin film of the melt on the wall of the evaporator. As the melt moves down the evaporator, the volatiles diffuse out of it and into the vapor space of the evaporator. The volatiles are pulled out of the evaporator under vacuum.
A wiped film evaporator (WFE) has wiper blades attached to a central shaft. The shaft rotates with a fixed angular velocity causing the blades to pass over the polymer melt. As the wiper blades pass over the melt, excess polymer melt accumulates along the front of the blade. This is called the bow wave. In the model it is assumed that the polymer melt on the wall is stationary and all of the movement of the polymer melt is in the bow wave. The downward movement of the polymer melt in the bow wave is called the axial velocity.
Calculation Method
The wiped film evaporator (devolatizer) can model several heating conditions: isothermal (using the combined feed temperature or a user-specified temperature), adiabatic (allowing heat input only through friction of the wiper blades), and at steady state (allowing heat input from a heating/cooling jacket). Alternatively, you may specify a temperature profile.
You must select Rating or Design calculations. In rating mode, you must specify evaporator height and you may specify blade configuration, number of blades, etc. Then the WFE model will calculate the amount of volatiles removed from the feed polymer melt. In design mode, you must specify the starting component and ending component numbers for the design specification. That specification is the target weight fraction of those components in the outlet polymer melt.
Ratio of Bow Wave to Film Area
This is the ratio of the bow wave cross-sectional area to the film cross-sectional area.
Shear Rate at Blade Tip
The default is calculated by assuming a straight line profile. The unit of measure is 1/sec.
Power Parameter
This is the fraction of the calculated power required to push the bow wave. The value you specify does not affect the power required to overcome the drag force of the polymer on the blade.
Viscosity Parameter
The viscosity parameter is omega, where effective viscosity = omega * viscosity under shear + (1-omega) * viscosity at zero shear. The default is 1.
Polymer Components
Mass diffusivity data for polymers must be entered for the component data.
Feed and Product Streams
The WFE unit can have up to 10 feed streams. The wiped film evaporator models a rate limited process. All feed streams should be liquid. If vapor is present in the feed stream, it will be assumed that it was the product of an equilibrium flash. The calculations will attempt to condense the vapor and set the feed to the non-equilibrium state specified by the temperature and pressure. Then the wiped film evaporator model will calculate the rate limited devolatization of the polymer melt as it tries to reach thermophysical equilibrium. The inlet pressure is taken to be the lowest pressure of all the feed streams, or you may specify the inlet pressure, or you may specify the pressure drop between the feed stream and the inlet.
Both an overhead and bottoms product stream must be specified. The bottoms contains the outlet polymer melt with less volatiles than the feed. The overhead is a vapor stream containing the volatiles that were removed.
Additional information
For additional information regarding the use of PRO/II's Wiped Film Evaporator, please see the PRO/II User's Guide.
Nguyen, T.K., et. al., Development of a Wiped Film Evaporator Model for Computer Simulation - Final Report, Engineering Interdisciplinary Clinic, California State Polytechnic University, Pomona, CA, June 1996.
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