Page 106 - Applied Process Design For Chemical And Petrochemical Plants Volume II

Distillation 95

hB = total enthalpy of bottoms product, Btu/mol Reflux Ratio: 0.50 (assumed)
or Btu/lb Assumed No. Theoretical Stages: 8 including condenser
hF = total enthalpy of feed, Btu/mol or Btu/lb and reboiler
Summary of input data to computer:
Example 8-30: Tray-By-Tray Multicomponent Mixture
Using Digital Computer
1. Molecular weights
2. Boiling points
This example summarizes a typical short multicompo-
nent distillation using the techniques previously cited (see 3. "K" value equations for each component as a function
Computer Printout). of pressure
The problem was to separate component 4 from com- 4. Equations for calculating enthalpy of liquid of each
ponent 5 while keeping component 5 losses into the over- component as a function of temperature
head at less than 5 weight % of the total overhead or to 5. Equations for calculating enthalpy of vapor of each
recover in the bottoms better than 90% (weight) of the component as a function of temperature
component 5 entering in the feed. 6. Initialvalues for stages to start calculations
The feed composition is: a. linear temperature gradient
b. linear pressure gradient
Component Mols Pounds Boil Point. "F
1 0.623 53.68 155.7 The results of the computer calculation are as summa-
2 7.234 130.36 313.0 rized by copies of the printouts. Note that Stage one is the
3 80.223 7423.03 244.2 product from an overhead condenser and is liquid, as is
4 1.717 127.20 332.6 the bottoms or reboiler outlet product. The results show
5 9.678 1395.28 380.3 that the initial criteria have been met for recovery of com-
6 0.325 85.37 476.6 ponent 5; however, this does not reflect any optimization
100.000 9214.91
of reflux or final number of stages (theoretical trays) that
might be required to accomplish the separation in a final
Enthalpy, Btu/unit flow 2,901.076; lb = 31.48 design.
Feed temperature: 90°F, liquid at stage 5 from top, As an example, if this were the final column selection,
Equimolal overflow not assumed then the column trays = %condenser-reboiler = 6 theoret-
Column Pressure: 0.39 (top) to 0.86 (bottom) psia, dis-
tributed uniformly to each tray (lex6 continued on page 99)

Computer Printout for Multicomponent Distillation
NUMBER OF STAGES = 8 (INCLJDING CONDEBSE3 ABD BEBOILER)
NUMBER OF COHPOBECilS = 6
COMPONENTS BOLECULAB BEIGilT NOBBAL SOILING IOINT, 3ZG. P.
96.175 155.73
ld.023 212.33
92,533 244.20
74.080 332.65
144.170 380.30
162.610 i176. 63
COLUHN PRESSLIE2 = 0.3Y TO 9-86 Fsra
REFLUX RATIO = 0.5330
EI~UILIOLAL OVEBPLOJ NOT ASSUJED
FEED ST2EAilS

3.613 53.66
7.23ir 135. 36
32.223 iu23.23
1.717 !27.L3
9.673 13j5.2d
3.525 $5.37
rn-
l"1AL 1 130.333 Y214.91