multicomponent distillation concepts
TRANSCRIPT
MULTICOMPONENT DISTILLATION
SINGLE STAGE FLASH
• USES TWO PHASE EQUILIBRIUM– FLASH– BUBBLE POINT– DEW POINT
• CAN BE TWO
LIQUID PHASES
• USES RELATIVE VOLATILITY http://www.hills2.u-net.com/chemical/distil/fig17.gif
j
ijiiii K
KxKy ,
METHODS TO OBTAIN Ki
• EMPIRICAL DATA– SET UP EXPERIMENTS TO MEASURE
y = y(x,P,T)– FIT TO STANDARD CORRELATIONS – DATA CAN BE BASED ON BINARY SETS -
SEE PERRY'S SECTION 13 FOR BINARY DATA
– GENERAL CORRELATIONS FOR SPECIFIC TYPES OF MIXTURES - UNIQUAC
http://www.intelligen.com/images/screenshots/BinaryChartDlg_v8.jpg
METHODS TO OBTAIN Ki
• DEPRIESTER CHARTS FOR HYDROCARBON SYSTEMS
• LOW TEMPERATURE DATA
• PERRY’S FIGURE 13-14, P. 13-17
METHODS TO OBTAIN Ki
• DEPRIESTER CHARTS FOR HYDROCARBON SYSTEMS
• HIGH TEMPERATURE DATA
• PERRY’S FIGURE 13-14, P. 13-18
METHODS TO OBTAIN Ki
• AIR SEPARATION PLANT DATA
http://www.cryogenic-consulting.com/argon.pdf#search=%22AIR%20SEPARATION%20EQUILIBRIA%22
http://www.ecs.umass.edu/che/henson_group/research/air/air_separation_plant.png
FLASH CALCULATIONS
• BUBBLE POINT PRESSURE (DIRECT)– T AND xi ARE GIVEN, FIND yi AND P
• BUBBLE POINT TEMPERATURE (T & E)– P AND xi ARE GIVEN, FIND yi AND T
yP x
Piisat
i 1
i
iii SYS
iiii xK
P
Pxy ).(
*57111
i
iii SYS
iiii xK
P
Pxy ).(
*57111
FLASH CALCULATIONS
• DEW POINT PRESSURE (DIRECT)– T AND yi ARE GIVEN, FIND PSYS AND xi
• DEW POINT TEMPERATURE ( T & E)– P AND yi ARE GIVEN, FIND T AND xi
i i
i
i ii
SYSii K
y
P
Pyx ).(
*77111
i i
i
i ii
SYSii K
y
P
Pyx ).(
*77111
FLASH CALCULATIONS• PARTIAL FLASH ∑zi = 1
– P, T AND zi ARE GIVEN, FIND xi & yi
– FLASH LINES ARE DEVELOPED FOR EACH COMPONENT WITH A COMMON f
– SOLVE BY T&E FOR VALUES OF V IN f = V/F
i i
ii
i REFiREF
iF
KV
KzOR
Kf
x0
11
11
11 )(
)(
)( ,
).(, 107111
iREFiREFiF
ii xKf
xx
f
fy
MULTICOMPONENT FRACTIONATION
• GENERAL DESIGN CONSIDERATIONS– SPECIFY PRESSURE– DETERMINE NUMBER OF
EQUILIBRIUM STAGES– ADJUST NUMBER OF
STAGES FOR STAGE EFFICIENCY
– DESIGN TRAYS– DESIGN COLUMN
http://www.eia.doe.gov/kids/energyfacts/sources/non-renewable/images/FCCDistCol.jpg
SHORTCUT METHOD - PSEUDOBINARY SEPARATION
• ASSUMES TWO PRIMARY (KEY) COMPONENTS CAN BE USED TO MODEL THE PROCESS
• NON-KEYS ARE DISTRIBUTED BASED ON αi,KEY
• KEY COMPONENTS– DISTRIBUTED TO DISTILLATE & BOTTOMS– MAJOR COMPONENTS IN FEED– NON-KEY COMPONENTS– LIGHTER THAN LIGHT ARE ASSUMED TO
GO TO DISTILLATE– HEAVIER THAN HEAVY ARE ASSUMED TO
GO TO BOTTOMS– INTERMEDIATE ARE ASSUMED TO
DISTRIBUTE TO DISTILLATE AND BOTTOMS
http://image.absoluteastronomy.com/images/encyclopediaimages/d/di/distillation_column.png
COMPONENT DISTRIBUTIONS
• EXAMPLE FROM AIR SEPARATION
• KEYS– O2– N2
• NON-KEY– Ar
http://www.cryogenic-consulting.com/argon.pdf#search=%22AIR%20SEPARATION%20EQUILIBRIA%22
SHORTCUT METHOD• DISTRIBUTE KEYS BASED ON TARGET
PRODUCT COMPOSITION
• COMPLETE DISTRIBUTION TO YIELD FEED AND PRODUCT MATRICES– xFi, xDi, xBi
• CALCULATE NMIN WITH FENSKE EQUATION USING KEY VALUES
).()ln(
ln
,
12711
avgHKLK
BLKDHK
BHKDLK
MIN
BDxxBDxx
N
SHORTCUT METHOD
• AVERAGE RELATIVE VOLATILITY, αLK,HK
– FEED VALUE
– TOP/BOTTOM AVERAGE
– GEOMETRIC AVERAGES– OVERALL– TOP/BOTTOM
FHKLKavg ,
2BD HKLKHKLK
avg,,
BD
BDF
HKLKHKLKavg
HKLKHKLKHKLKavg
,,
,,,
3
SHORTCUT METHOD
• CHECK OF NON-KEY DISTRIBUTION– USE FENSKE EQUATION RESOLVED FOR
NON-KEY RELATIVE TO KEY:
).(, 14711
Bx
Dx
Bx
Dx
HKB
HKDHKi
iB
iD
SHORTCUT METHOD
• MINIMUM REFLUX RATIO– OPERATING LINES FOR EACH
COMPONENT AT TOTAL REFLUX– RECTIFICATION SECTION
– STRIPPING SECTION
i i
Di
i
Dii KVL
x
V
D
KVL
x
V
Dy
101
1.
i i
Bi
i
Bii KVL
x
V
B
KVL
x
V
By
101
1.
SHORTCUT METHOD• UNDERWOOD EQUATIONS COMPLETE
BALANCE OVER THE COLUMN
• NUMBER OF θ VALUES ARE (1 + NUMBER OF COMPONENTS BETWEEN KEYS)
• VALUE OF θ IS BETWEEN VALUES OF αij
i i
DiidMIN
i i
Fii
xR
xq
1
SHORTCUT METHOD• USE GILLILAND OR ERBAR-MADDOX
CORRELATIONS TO DETERMINE N FOR A SPECIFIC Rd
1
1
2.11711
4.541exp1
1 5.0
R
RR
N
NN
MIN
MIN
Molokanov, International Chemical Engineering, 12(2), 209, 1972
McCabe, Smith and Harriontt, Unit Operations of Chemical Engineering, Ed. 4, p, 578, 1985.
SHORTCUT METHOD
• LOCATION OF FEED TRAY
• IS CRITICAL TO COLUMN EFFICIENCY– LOSS OF SEPARATION – INCOMPLETE SEPARATION
• BASIS FOR ESTIMATE
).(log.log,
,
,
, 217112060
2
DHK
BLK
FLK
FHK
STRIP
RECT
x
x
D
B
x
x
N
N