1. Process Modeling using Aspen Plus
RadFrac
Process Modeling
using Aspen Plus
Faculty of Chemical and Natural Resources Engineering
Universiti Malaysia Pahang
2008

2. Objectives
Enter the minimum input required for the RadFrac fractionation model
Implement design specifications
Aspen Workshop 2008

3. RadFrac Flowsheet Connectivity
Aspen Workshop 2008

4. 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

5. RadFrac demonstration
RadFrac Specifications
Partial condenser
Kettle reboiler
15 stages
R = 1.5
Distillate to feed ratio = 0.6
Column pressure = 315 psia
Feed stage = 8
Mole fraction
C1:
C2:
C3:
nC4: 0.17
nC5: 0.11
nC6: 0.12
Flow= 1000 lbmol/hr
T= 190 F
P= 315 psia
Use the RKS-BM Property method
Aspen Workshop 2008

6. RadFrac Configuration Sheet
Specify
Number of stage
Condenser and reboiler configuration
Two column operating specifications
Default: distillate rate and reflux ratio
Valid phases
Convergence
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7. RadFrac Setup Streams Sheet
Specify
Feed stream location
Feed stream convention
Above-stage
On-stage
Bottom and overhead product streams
Side products
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8. RadFrac Setup Pressure Sheet
Specify one of:
Top/bottom pressure
Pressure profile
Section pressure drop
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9. 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:
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
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10. 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
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11. Plot Wizard Demonstration
Use the Plot Wizard to create plots of temperature, flows, and compositions throughout the column
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12. 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

13. Workshop (1)
Methanol Tower

14. Objective: Set up a methanol tower
63.2 wt% water
36.8 wt% methanol
Flow= lb/hr
Pressure= 18 psia
Saturated liquid
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
Use the NRTL-RK property method
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15. Part A Record the column duties: Record composition:
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

16. 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 ( lbmol/hr) and reflux ratio (0.8-2)
Record the final values for:
Distillate rate: Reflux rate:
Condenser duty: Reboiler duty:
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17. 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:
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18. 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