+
Improved tower models
SCDS
Tower
+
Quick cost
estimates
Tower cost, installed = $40,000
+
+
Convert reboiler and condenser
duties to costs
Short-cut distillation
in black
R
1
2
4
5
15
Tower #1 design in red
kJ/kg-K
kJ/kg
4.187
2270
latent heat
Cp, water of steam
condenser duty,,
reboiler
kJ/h duty,CW,
kJ/hkg/h
Cw, m 3 /h
CW cost, $/h steam, kg/h steam, lb/h utilities, $/yr
-2.43E+06
2.46E+06
29018
29
$0.58
1.08E+03
$15.71
$130,352
-3.34E+06
3.96E+06
39885
40
$0.80
1.74E+03
$25.30
$208,743
-6.54E+06
7.16E+06
78099
78
$1.56
3.15E+03
$45.74
$378,381
-8.06E+06
8.68E+06
96250
96
$1.93
3.82E+03
$55.44
$458,960
-2.15E+07
2.21E+07
256747
257
$5.13
9.74E+03
$141.17 $1,170,419
Suggested recycle range
The tower is inexpensive relative to the operating costs.
+
Two choices for
detailed column
design
+
SCDS:
non-ideal K-value chemical systems in distillation, absorption,
strippers,… 2 or 3 phase systems
+
TOWR
Note: one of the least likely control methods in the field is to
use mole fraction. Have you ever used a mole fraction
meter?
Rather, a number of variables that are easy to measure and
control are the actual choices. These can be set in ChemCad,
linking the solution to the control methodology. We will not
address this feature in this course.
+
It is easy to build the
TOWR model on the
same pdf as the shortcut column. This eases
comparisons of the
results. The two sheets
are separable later.
+
TOWR #1
Set-up for the MeOH/2-PrOH
problem
Select TOWR #1 from the ChemCad 5
list
+
Problem design
basis table
Set column bottoms to 20% MeOH
10 actual stages @ 48% efficiency
gives 5 theoretical stages
VLE from NRTL
F = 50:50 molar mixture of MeOH, 2PrOH
R = {5, 7, 10, 12, 15}
Reboiler power = {0.5 kW to 1.75 kW}
Feed tray @ 8 or 5; 30 mL/min feed
+
Format – engineering units
+
Thermophysical properties
+
Stream 1 - feed
+
TOWR#1 - General
+
TOWR#1 – specifications
long list of possible specifications
+
TOWR#1 - convergence
+
TOWR#1 – cost estimate
Please
choose
sieve tray
instead
+
Report/Stream compositions/all
streams
+
Plot/Unit ops/Column profile
+
Column profile
Feed on
stage 6
+
Size/Distillation/Trays
+
Sieve tray
+
Tray 2
+
Tray 6
+
Tray 9
+
Design starts
Sensitivity analysis
- feed tray location – let’s find an
optimum location based on reboiler
and condenser duties…

… it is time to exercise the little gray
cells…

Sensitivity analysis

Evaluate feed location from trays 2 to
9

Record reboiler duty, condenser duty,
and reflux ratio

Run TOWR#1 at feed stage = 2 prior
to running the sensitivity analysis
(gives a better chance for a clean
run)
+
Feed @ tray 2 –
note the discontinuity in temperature profile
+
Run/sensitivity study/feed loc/
adjusting
+
recording
+
Run/sensitivity/Plot results