RadFrac
Process Modeling
using Aspen Plus
Faculty of Chemical and Natural Resources Engineering
Universiti Malaysia Pahang
2008
Objectives
 Enter the minimum input required for the
RadFrac fractionation model
 Implement design specifications
Aspen Workshop 2008
RadFrac Flowsheet Connectivity
Aspen Workshop 2008
Some RadFrac options
 To set up an absorber with no condenser or reboiler, set
condenser and reboiler to none on the RadFrac Setup
Configuration sheet
 Either Vaporization or Murphree efficiency on either a stage or
component basis can be specified on the RadFrac Efficiency
form
 Tray and packed column design and rating is possible
 A second liquid phase may be modeled if the user selects
Vapor-Liquid-Liquid as Valid phase
 Reboiler and condenser heat curves can be generated
Aspen Workshop 2008
RadFrac demonstration
Mole fraction
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RadFrac Specifications
Partial condenser
Kettle reboiler
15 stages
R = 1.5
Distillate to feed ratio = 0.6
Column pressure = 315
psia
 Feed stage = 8
OVHD
C1: 0.26
C2: 0.09
C3: 0.25
nC4: 0.17
nC5: 0.11
nC6: 0.12
FEED
B1
Flow= 1000 lbmol/hr
T= 190 F
BOTM
P= 315 psia
Aspen Workshop 2008
Use the RKS-BM Property
method
RadFrac Configuration Sheet
Specify
 Number of stage
 Condenser and reboiler
configuration
 Two column operating
specifications
 Default: distillate rate and
reflux ratio
 Valid phases
 Convergence
Aspen Workshop 2008
RadFrac Setup Streams Sheet
Specify
 Feed stream location
 Feed stream convention
 Above-stage
 On-stage
 Bottom and overhead
product streams
 Side products
Aspen Workshop 2008
RadFrac Setup Pressure Sheet
Specify one of:
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Top/bottom pressure
Pressure profile
Section pressure drop
Aspen Workshop 2008
Plot Wizard (1)
 Use the plot wizard (on th eplot menu) to quickly
generate plot results of a simulation. You can
use the Plot wizard for displaying results for the
following operations:
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Assay data analysis
Physical property analysis
Data regression analysis
Profiles for all separation models including RadFrac,
MultiFrac, and PetroFrac
 Click the object of interest in the data browser to
generate plots for that particular object
Aspen Workshop 2008
Plot Wizard (2)
 The Plot Wizard guides
you in the basic
operations for generating
a plot
 In Step 2, click the plot
type you wish to
generate, then click
Next> to continue
 Click the Finish button to
generate a plot with
default settings
Aspen Workshop 2008
Plot Wizard Demonstration
Use the Plot Wizard to create plots of temperature, flows, and
compositions throughout the column
Aspen Workshop 2008
Design Specs and Vary
 Design specifications can be specified inside the
RadFrac block using DesignSpecs and Vary forms
 One or more RadFrac inputs can be manipulated to
achieve specifications on one or more RadFrac
performance parameters
 The number of specs should, in general, be equal to the
number of varies
 The DesignSpecs and Varys in a RadFrac are solved in
a “Middle loop”. If you get an error message saying that
the middle loop was not converged, check the
DesignSpecs and Varys you have entered.
Aspen Workshop 2008
Workshop (1)
Methanol Tower
Objective: Set up a methanol tower
B1
DIST
FEED
63.2 wt% water
36.8 wt% methanol
BOTM
Flow= 120000 lb/hr
Pressure= 18 psia
Saturated liquid
Use the NRTL-RK
property method
38 trtays (40 stages)
Feed tray= 23 (stage 24)
Total condenser
Top stage pressure= 16.1 psia
Pressure drop per stage= 0.1
psia
Distillate flow rate= 1245
lbmol/hr
Molar reflux ratio= 1.3
Aspen Workshop 2008
Part A
 Record the column duties:
 Condenser duty:
 Reboiler duty:
 Record composition:
 Mass fraction of methanol in the distillate:
 Mass fraction of water in the bottom:
 Make plots of temperature, flows, and
composition
Aspen Workshop 2008
Part B
 Set up DesignSpecs within the column so
that there is:
 99.95 wt% methanol in the distillate
 99.90 wt% water in the bottom
 Vary the distillate rate (800-1700 lbmol/hr)
and reflux ratio (0.8-2)
 Record the final values for:
 Distillate rate:
 Condenser duty:
Reflux rate:
Reboiler duty:
Aspen Workshop 2008
Part C
 Perform the same calculations after
specifying a 65% Murphree efficiency for
each tray. Assume the condenser and
reboiler have stage efficiencies of 90%.
Determine how these efficiencies affect
the column duties:
 Condenser duty:
Reboiler duty:
Aspen Workshop 2008
Part D
 Perform a tray sizing calculation for the
entire column, given that Bubble Cap trays
are used
 Record the predicted column diameter
Aspen Workshop 2008