CHE 306 StagewiseOperations Fall 2010 -...
Transcript of CHE 306 StagewiseOperations Fall 2010 -...
CHE 306Stagewise Operations
Fall 2010Fall 2010
Staged & Packed Column DesignStaged & Packed Column Design
Instructor: Dr. Housam BinousKFUPM, DhahranKFUPM, Dhahran
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1/ Tray geometry
2/ column efficiency
3/ column diameter
/ d i i4/ downcomer sizing
5/ tray layout5/ tray layout
Student will be able to design column internals for
staged & packed columns
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g p
Sieve trays
Holes punched or drilled in metal plate
Efficiency good at design conditions
Turndown is poor
Not good for fouling applications
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Valve trays
Better turndown properties (more flexible if flow rate varies)
Large holes with fitted vales (= covers that move ↕)
Cage restricts upward movementCage restricts upward movement
Some valves are open while others are closed
Constant efficiency (even if flow varies)
No weeping, expensive, likely to plug
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bblBubble‐cap trays
Riser around each hole, cap with holes over the riserp
Need large tray spacing (18 inches) because of entrainment
Poor turndown characteristics
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Perforated plates
No downcomers
Larger holesLarger holes
Liquid weeps through holes and vapor passesh h f h lthrough center of holes
Not flexible (either works or not at all)
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Single pass
Double‐pass commonIf column diameter is large
Multiple‐pass
If column diameter is largeand liquid flow rates high
Reduce downcomer loading & liquid gradient on the tray
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Oldershaw column used in pilot plants, pipe downcomer
Segmented vertical downcomer: most common, easy to install,allow wide variety of liquid flow rates
Sloped design used when V/L disengagement is difficult
Envelope design for very low liquid flow rates
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Straight horizontal weir (2 to 4 inches height)
cheap, poor turndown properties
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Adjustable weirs: can be source of problems if not
adjusted well such as weeping and dry traysadjusted well such as weeping and dry trays
If needs flexibility in liquid rates: Use notched or picket‐
fence weirs
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Inlets & OutletsInlets should be designed to avoid excessive weeping &Inlets should be designed to avoid excessive weeping &
entrainment when high‐velocity stream is added
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low velocity liquid feedside inlet
feed w/ vaporhigher velocity feed
use baffles
large diameter columnlarge diameter columnrequires a distributor
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Reflux or feed to the top tray:
use fig. 10‐8 E or D if there is vapor in the feed
Intermediate feed:
Use fig. 10‐9 A: for low velocity liquid feeds.
Use fig. 10‐9 B or C with baffles in order to prevent entrainment (when high velocity feeds or feeds with vapor)
Large column diameters: fig. 10‐9 E with distributor.
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Vapor outlet & return
Vapor outlet easy to design, a demister may be used
h f h l id li id iat the top of the column to avoid liquid entrainment
Vapor return at the bottom of the column should
be 12 inches above liquid surge levelbe 12 inches above liquid surge level
G d d i fi 10 10 AGood design: fig. 10‐10 A
Bad design: fig. 10‐10B vapor impinges on liquid
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Side drawoffs
Need to disengage vapor and liquid
Chimney tray is not an equilibrium stage Use downcomer sump and tapout
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Tray efficiency
Overall efficiency
Murphree efficiencyMurphree efficiency
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In reality, we have depletion of liquid phase as it flows across the plate
Thus, we have:
Because one can have sometimes:
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For multicomponent mixtures need to use:
One can find case where:
Murphree and overall efficiency are related as follows:
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Efficiency vs. vapor flow rate
As one lowers vapor flow rate: mass transfer less efficient
High vapor flow rates: causes floodding
L fl t i
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Low vapor flow rates: causes weeping
ency Bubble‐cap trays
y efficie p y
sing tray
Sieve traysIncreas
Valve trays
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: relative volatility of key components
: viscosity at feed composition
in cPoise (1 cP= 10‐3 Pa.s)
Both and are determined at average T and P of the column
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If increses then efficiency decreases
(mass transfer rates are lower)( )
If increases then efficiency decreasesIf increases then efficiency decreases
(mass that must be transferred to obtain equilibrium is larger)
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Column diameter
Ø should be chosen to avoid flooding
Fair’s procedure will be use:Fair s procedure will be use:
1/ determine uflood/ flood
2/ determine uoperating
3/ determine column diameter
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Flooding vapor velocity
: surface tension dynes/cmft/s
: surface tension dynes/cm
C : capacity factorCsb,f: capacity factor
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: density
Capacity factor vs. flow parameter is more accurately
determined using eq. 10‐10 a, b, c, d, e and f given forg q , , , , g
various tray spacing in inches.
f lFor 6‐in tray spacing, for example, one must use eq. 10‐10a:
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: ratio of holes area to active area of tray
Generally,
If =0.08 multiply flood velocity with 0.9ti
If =0.06 multiply flood velocity with 0.8
correctionFactorsif If 0.06 multiply flood velocity with 0.8
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Operational velocity
fraction
generally between 0.65 and 0.9
often one can take 0.75
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Sieve Tray Layout
Holes on sieve plate have pattern
Obj ti fl f & li id th tObjective: ensure even flow of vapor & liquid on the tray
1/8 to 1/2 inch
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shell / inlet downcomer: 2‐3 inches clearance
weir: 3 5 inches clearanceweir: 3‐5 inches clearance
holes 2.5‐5.0 inches apart
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Vapor velocity through holes
chosen to provide flexibility to change feed flow rate
Should be above weeping and below flooding points
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Tray Hydraulics
Tray and downcomer pressure heads caused
by various hydrodynamic effects
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downcomer pressure drop p p
dry‐tray pressure drop
Tray thickness
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Tray thickness
frictional lossfrictional loss
in gallons/minutes
1 inch
g /
Liquid crestover the weir
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Weir correction factor
relative froth densitybecause downcomer is not clear liquidbecause downcomer is not clear liquid
liquid is aerated
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Residence time in downcomers
L+e=total liquid flow rate (including entrainment)L+e=total liquid flow rate (including entrainment)
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Condition for avoiding weeping
Surface tension head: surface tension (dynes/cm)
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Valve tray design
Bolles’s method uses sieve tray design as a basis and makes
some modifications as necessaryy
Eqs. (10‐26) to (10‐30) are unchanged
Eq. (10‐23) is unchangedq ( ) g
Eq. (10‐24) is no longer valid.
Need to computehP,dry differntly
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Different values for all closed or all open valves
valve weight
Correction factor due to turbulence
valve area
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Packed columns internalsceramic/plastic saddle
Pall ringg
structured packing
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