Lecture Lecture 12. 12. Binary Distillation (2)Binary Distillation (2)
[Ch 7][Ch 7]
• Condenser Type
[Ch. 7][Ch. 7]
Condenser Type
• Subcooled Reflux
• Reboiler Type• Reboiler Type
• Condenser and Reboiler Duties
F d P h t• Feed Preheat
• Optimal Reflux Ratio
• Use of Murphree Efficiency
• Multiple Feeds, Side Streams, and Open Steam
Condenser TypeCondenser Type
Total condenser Partial condenser Mixed condenserTotal condenser Partial condenser Mixed condenser
- Partial condenser: liquid reflux and vapor distillate are at equilibrium in the reflux drum In the McCabe-Thiele method, the partial condenser becomes the first equilibrium stagethe partial condenser becomes the first equilibrium stage
- Mixed condenser: can provide both vapor and liquid distillates
SubcooledSubcooled RefluxReflux
• If the condenser outlet pressure is lower th th t t f th lthan the top tray pressure of the column, the reflux is subcooled
• When subcooled reflux enters the top tray
L
When subcooled reflux enters the top tray, its temperature rises and causes vapor entering the tray to condense
• The latent enthalpy of condensation of the vapor provides the sensible enthalpy to heat the subcooled reflux to the bubble
' vapb liPR H RC T
heat the subcooled reflux to the bubble point
subcoolingLPR H RC T
' interanlR R R
C T The McCabe-Thiele
subcooling
internal vap1 LPC TR R
H
The McCabe Thiele construction should be based on the internal reflux ratio
ReboilerReboiler TypeType
Kettle-type reboiler Kettle reboilers are common
Equivalent to oneEquivalent to one equilibrium stage
Vertical thermosyphon-type reboiler
Bottom-tray Thermosyphon reboilers areydowncomer
Thermosyphon reboilers are favored for
(1) Thermally sensitive bottom product compounds
Bottom sump
bottom product compounds
(2) High bottom pressure
(3) Only small T available for heat transfer
(4) Heavy fouling
Condenser and Condenser and ReboilerReboiler Duties (1)Duties (1)
• Energy balance for the entire distillation column
F R D B C lossFh Q Dh Bh Q Q
• Total condenserV
vapCQ V H
P ti l d
( ) ( )1vap vapL D H D R H
• Partial condenser
vapCQ L H L
vapDR H
• Partial reboiler
vapRQ V H vap
BBV H
Condenser and Condenser and ReboilerReboiler Duties (2)Duties (2)
• For bubble-point liquid feed and total condenserF
L V VV
BBV = L+ D = D(R + 1)
Q Q
( )1 vapC
vap
Q D R H
Q BV H
L V
F
FLL
R CQ Q R BQ BV H
• For partially vaporized feed and total condenser F
L V
FVFVVV
( )
vapC
vap vap
Q V H
Q V H V V H
FR C
VQ = Q 1 -D(R + 1)
L V
FL
FLLL
( )p pR FQ V H V V H D(R + 1)
• Saturated steam rate for the reboiler
/ vaps s R sm M Q H
• Cooling water rate for the condenser• Cooling water rate for the condenser
/ ( )2H Ocw C P out inm Q C T T
Feed PreheatFeed Preheat
• (Feed pressure) > (pressure in the column at the feed-tray)
• Deviation of feed temperature from column temperature at the feed location: second-law efficiency
• It is usually best to avoid a subcooled liquid or superheated vapor feed
• The cost of reboiler steam is usually an order of magnitude higher than the cost of cooling water
The feed is preheated and partially vaporized to reduce QR
i i t Q
FV
in comparison to QC
( )1
1F
R CVQ Q
D R
Optimal Reflux RatioOptimal Reflux Ratio
• (minimum reflux) < (reflux in industrial distillation) < (total reflux)
• When reflux ratio is increased from minimum value
Number of plates - Number of plates - Column diameter - Requirement of reboilerRequirement of reboilersteam and condenser cooling water
• The total annual cost is dominated by the steam cost except at the minimum reflux condition
Use of Use of MurphreeMurphree EfficiencyEfficiency
• Concentration changes for a given stage are usually less than predicted by equilibriumpredicted by equilibrium
• Murphree efficiency: (the change in actual composition in the phase) / (the change predicted by equilibrium)
Efficiency based on
vapor
*1
1
n nMV
n n
y yE
y y
Efficiency based on liquid
*1
1
n nML
n n
x xE
x x
vapor phase
liquid phase
yn
Tray n
yn+1
xn-1
xn
yn+1
Multiple Feeds, Side Streams, and Multiple Feeds, Side Streams, and Open Steam (1)Open Steam (1)Open Steam (1)Open Steam (1)
• Two feeds(saturated liquid & saturated vapor)
1V V F 2 2L L F L F
Multiple Feeds, Side Streams, and Multiple Feeds, Side Streams, and Open Steam (2)Open Steam (2)Open Steam (2)Open Steam (2)
• Use of open steam( th th b il )
• One feed, one side stream( d li id) (rather than a reboiler)(saturated liquid)
V V SL L L Open steam can be used if one of the components is water, or if water can form a second liquid phase
[Example] One Feed, One Side [Example] One Feed, One Side Stream (Saturated Liquid)Stream (Saturated Liquid)Stream (Saturated Liquid)Stream (Saturated Liquid)
Operating lines in the rectifying section
1 1n n n x DV y L x Dx
2 2 1 1 DV y L x L x Dx 2 2 1 1s s s s s s DV y L x L x Dx
DL Dy x xV V
s s DL x DxLy xV V
V V V V
Intersection of the operatingthe operating lines
( ) s sL L x L x ( ) s s
x xsL L L
sx x
