9/27/99 Simple Distillation Design 1
CONCEPTUAL DESIGN OF
SIMPLE DISTILLATION COLUMNS
Goals:
u Determine feasibility of proposed separation
u Estimate tray and reflux requirements
u Estimate column diameter and height
9/27/99 Simple Distillation Design 2
DEFINITION OF SIMPLE DISTILLATION
u A distillation column with a single feed and no side
streams
u Vapor-liquid equilibrium
e Ideal (Raoult’s Law)
e Nearly ideal (modified Raoult’s Law)
e No azeotropes
u Modest heat effects - no substantial variations in vapor
and liquid traffic above and below the feed
9/27/99 Simple Distillation Design 3
REQUIREMENTS FOR A FEASIBLE DESIGN
u Achieve product specifications
e Purity
e Fractional recovery
u Reboil and condense using availability utilities
u Column diameter and height should be within
fabricatable limits
e Diameter < 7 m
e Height < 35 m
9/27/99 Simple Distillation Design 4
Determine
Column Specs
and Constraints
Set Column
Operating
Pressure
Calculate
Mininum Tray
Requirement
(Fenske)
OK?
Modify
No
Nmin< 30?
Yes
No
Calculate
Minimum Reflux
Requirement
(Underwood)
Choose Reflux Rate
and Estimate
Theoretical Tray
Requirement
(Gilliland)
Calculate Vapor
and Liquid
Traffic
VB,VT,LB,LT
Calculate Column
Diameter
Above & Below
Feed
(Fair)
Dmax < 7 m?
Yes
YesNo
Estimate Overall
Tray Efficiency
(O'Connell,
Murphree)
Calculate Actual
Trays Required
and Column
Height (HCOL)
HCOL < 30 m?
Simulate and
Size Auxiliary
Equipment
Yes
No
Distillation Column Design Procedure
9/27/99 Simple Distillation Design 5
PRELIMINARY MATERIAL BALANCE
u Calculate distribution of key components based on
separation specifications
u Assume that all components lighter than the light key
distribute entirely to the distillate
u Assume that all components heavier than the heavy key
distribute entirely to the bottoms
u Calculate the distribution of components with volatilities
between the keys by assuming infinite reflux and using
the Fenske equation
9/27/99 Simple Distillation Design 6
DETERMINE THE OPERATING PRESSURE
u Determine the bubble point temperature of the distillate
e Tdbub = Tcw + D Tmin
u Calculate the bubble point pressure
e Pdbub = Psat,i(Tdbub) xdi
u Do same for the bottoms- determine Tbbub and Pbbub
u In order to be able to condense the overhead vapor we
must have Pcol > Pdbub
u And to reboil the bottoms,we need Pbbub < Pcol
u So,choose Pcol such that Pdbub < Pcol < Pbbub
u If not,either choose utilities with a wider temperature
range or relax the separation specs
9/27/99 Simple Distillation Design 7
DETERMINE THE MINIMUM TRAY
REQUIREMENT Nmin
u Calculate Nmin using the Fenske Equation
u As a rule of thumb,the column height should not exceed
30 m,Normal tray spacing is 2 ft or 0.6 m,Thus the
maximum number of trays we can have is about 50 trays
u As another rule of thumb,the number theoretical trays
should be about 1.5 Nmin to 2.0 Nmin,So,if Nmin > 30,
the column height guideline will probably be exceeded
u If so,either
eadjust the separation specs (if possible)
eaccept the additional height requirement,or
ego to a different separation,e.g,extractive distillation
9/27/99 Simple Distillation Design 8
ESTIMATE THEORETICAL TRAY
REQUIREMENT
u Calculate Rmin (Underwood Equation)
u Choose an operating reflux ratio R
e Rule of thumb,1.05 Rmin < R < 1.1 Rmin
e Estimate theoretical tray requirement (Gilliland
correlation)
u Or choose the number of theoretical trays Nt
e Rule of thumb,1.5 Nmin < Nt < 2.0 Nmin
e Estimate the reflux ratio (Gilliland correlation)