cbe 417 “unit operations” -...
TRANSCRIPT
CBE 417“Unit Operations”
Lecture: 8
20 Sep 2010
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Overview• Flash Unit OperationFlash Unit Operation• Staged systems• McCabe-Thiele
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Cascade Flash Summary:• Additional “flash” stages improve purity but recovery is poorAdditional flash stages improve purity, but recovery is poor• Recycle of intermediate streams allows better recovery while
preserving good purity• Intermediate stages operated adiabatically – minimizing the
need for intermediate HX equipment, pumps, or valvesH t id d i b tt t id “b il ”• Heat provided in bottom stage provides vapor “boilup”
• Heat removed from top stage provides liquid “reflux”• This allows for a cascade separation to be done in one piece ofThis allows for a cascade separation to be done in one piece of
equipment – called a distillation column
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Add 3rd stage w/recycle &Middle stage adiabatic & dd e s age ad aba c &Higher “reflux” & With stage below and recycle:
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Top of “Column”
Rectifying (enriching) ti f di till tisection of distillation
column
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Equilibrium “Stage”Liquid and vapor leaving a stageLiquid and vapor leaving a stage
(tray) are assumed to be in equilibrium
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Bottom of “Column”Stripping section of distillation column
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Distillation Column
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Distillation ColumnColumn
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Distillation ColumnP Column
Design (binary)•Specify Z X X (more volatile)
XD Za, XD, XB (more volatile) Reflux Ratio (Lo/D) Optimum feed Stage location P column (condenser) F (feed flowrate) F (feed flowrate) Feed condition
•Find N (number of stages) N (feed stage)
Za
Nfeed (feed stage) D, and B (flowrates) Heat duties Diameter, height
Simulation (binary)•Use existing column Simulate to see performance Any needed modifications?
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XB
Any needed modifications?
Distillation ColumnP
QC
ColumnOverall Column Balances (SS)
XD hD Material Balance (MB):
BDF BxDyFz aaa
Za hF Energy Balance (EB):
BhDhQQFh BhDhQQFh BDRCF
• heat added is (+)• heat removed (-)heat removed ( )• adiabatic (well insulated)
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XB hBQR
McCabe-Thiele Graphical Method (binary)
Used to simplify analysis of binary distillation (ease of understanding)Used o s p y a a ys s o b a y d s a o (ease o u de s a d g)Assumptions:• Pure components a, b have equal latent heats of vaporization / mole ( ) and they
stay constant.i
y• are much larger than
• Sensible heat changes• Heats of mixing
• Column is adiabatic (well insulated)
i
• Column is adiabatic (well – insulated)• Constant pressure (P) throughout the column (i.e. no P in the column)
Called Constant Molal Overflow (CMO)• Assumes for every 1 mole of light material vaporized that 1 mole of heavy material• Assumes for every 1 mole of light material vaporized that 1 mole of heavy material
condenses from the vapor phase• Net result:
• Total molar flowrates (i.e. L and V) remain constant within that column section (rectifying or stripping, or other)
• Do not need a stage by stage energy balance
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McCabe-Thiele is done with MB and Thermodynamic information.
MB on Rectifying SectionAt Steady State (SS) for light material y ( ) g(LK) MB: Moles In = Moles Out
DxLxVy DNNNN 11
DN
NN
NN x
VDx
VLy
111
For CMO
VVLL NN 1;LL
RRatioReflux
General Operating
LineDNN x
VDx
VLy 1
DL
DLR o
LineVV
1
RR
VL
111 R
xxR
Ry DNN
1311
RV
D 11 RR
Rectifying SectionGeneral Operating Line: Rectifying Section
xR D General Operating Line: Rectifying Section
111 R
xxR
Ry DNN
?:: 1 NDN ythensoxxLet
Equilibrium 0.8
0.9
1
(xD, y1)yN
xN-1
Stage
0.5
0.6
0.7
Ya
Y=X i i i i
xN yN+1
0.2
0.3
0.4
1RxD
Equilibrium Line:• Relates composition of liquid leaving
stage N (i.e. xN) to the composition of vapor leaving stage N (yN)
Operating Line:• Relates composition of liquid leaving
stage N (i.e. xN) to the composition of i N ( )
0
0.1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Xa
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vapor entering stage N (yN+1)Xa
Tie Together Equilibrium & Operating Lines
0.9
1
D
y1 V1
L(x1, y1)
0.6
0.7
0.8
Ya
(xD, y1) D
y1xDxD
LO
(x2, y3)(x1, y2)(x3, y3)
(x2, y2)
1
0.3
0.4
0.5 Y=X
1RxD
x1 y2
2
0
0.1
0.2 y3
x2
3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1Xa
x3 y4
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MB on Stripping SectionAt SS and CMO still assumed, so: constantVL &
BL,
Bmm BxyVxL 1
constantVL &
Operating Line Stripping Section
Bmm x
VBx
VLy 1
0.8
0.9
1
0.5
0.6
0.7
Ya
Y=X
VVRatioBoilup B
0.2
0.3
0.4
(xB, yB)
(xN, yN)
(x y )
(xN-1, yN)
BVVB
0
0.1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
X
(xB)
(xN, yB)
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Xa
Feed StageL V
D di F d “ di i ” illF
FV
Depending on Feed “condition” will get changes to vapor and liquid flowrates…ZF
L V
FL
L V
Suppose:
Define q = Moles of liquid flow in Strippingsection that result from one mole of feed.
Suppose:• q = 1
• q = 0F
LLq
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• q = 0
Questions?
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