CE 408 UniSim Lecture #2: Distillation

Chemical Engineering Plant Design

Lecture 05 Distillation in UniSim

Instructor: David Courtemanche

CE 408

CE 408 UniSim Lecture #2: Distillation

• Content: shortcut and rigorous distillation columns

• Topics:• Performance versus design models

• Short Cut Distillation

• Distillation Column

• Custom Column

• Multicomponent Distillation

• A word about partial condensers

Performance versus Design Models

• Any distillation process always has 4 degrees of freedom (things that you can specify; once they are chosen, the distillation is completely specified/determined)

Design Model / Calculations

• Boundary value design method – what we did almost all of the time in CE 407

• Choose xD, xB, R = L/D and optimal feed location• Could locate feed at another location

• Graphical calculation of N, number of stages• N is most likely not an integer

Performance Model / Calculation

• This is a prediction of what will emerge from a completely specified column. It is what is usually done by a process simulator.

• Examples:• Choose R, D/F, Nr , Ns

• Choose R, ത𝑉/B, Nr , Ns

• Guess xD , calculate xB , construct McCabe-Thiele diagram; step from top to bottom of column; check whether last step ends at xB , iterate until it does

Comments

• Both design and performance calculations involve either graphical or algebraic/computational tray-to-tray calculations

• Performance model involves an outer loop for adjusting distillate composition until a certain set of equations is satisfied

• Design and optimization involves a further outer loop whereby parameters (specifications) of a performance model are adjusted until an optimum design is reached

Use reflux ratio = 1.48

Hand calculations give solution as:35 stages required, feed enters at stage 33 from the top

Short Cut Distillation

Short Cut Distillation does design calculation approximately; can be very wrong – use as a rough estimate only!