CHE 306 StagewiseOperations Fall 2010 - KFUPMfaculty.kfupm.edu.sa/CHE/binoushousam/files/binous... · CHE 306 StagewiseOperations Fall 2010 ... Chimney tray is not an equilibrium

CHE 306Stagewise Operations

Fall 2010Fall 2010

Staged & Packed Column DesignStaged & Packed Column Design

Instructor: Dr. Housam BinousKFUPM, DhahranKFUPM, Dhahran

1Dr. Housam Binous PFUPM CHE306

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


g p

Sieve trays

Holes punched or drilled in metal plate

Efficiency good at design conditions

Turndown is poor

Not good for fouling applications



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



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



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)


Cross‐flow pattern


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


Downcomers & weirs: Control liquid distribution and flow


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


Straight horizontal weir (2 to 4 inches height)

cheap, poor turndown properties


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


Trays, weirs and downcomer need mechanical support


Inlets & OutletsInlets should be designed to avoid excessive weeping &Inlets should be designed to avoid excessive weeping &

entrainment when high‐velocity stream is added


low velocity liquid feedside inlet

feed w/ vaporhigher velocity feed

use baffles

large diameter columnlarge diameter columnrequires a distributor


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.


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


Thermosiphon reboiler with bottom draw‐off


Side drawoffs

Need to disengage vapor and liquid

Chimney tray is not an equilibrium stage Use downcomer sump and tapout


Tray efficiency

Overall efficiency

Murphree efficiencyMurphree efficiency



Generally, we have:

because:


In reality, we have depletion of liquid phase as it flows across the plate

Thus, we have:

Because one can have sometimes:


For multicomponent mixtures need to use:

One can find case where:

Murphree and overall efficiency are related as follows:


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


Low vapor flow rates: causes weeping

ency Bubble‐cap trays

y efficie p y

sing tray

Sieve traysIncreas

Valve trays


O’Connell correlation


: 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


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)


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


Flooding vapor velocity

: surface tension dynes/cmft/s

: surface tension dynes/cm

C : capacity factorCsb,f: capacity factor

: density34Dr. Housam Binous PFUPM CHE306

: density

Capacity factor vs. flow parameter


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:


: 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


Operational velocity

fraction

generally between 0.65 and 0.9

often one can take 0.75


Column diameter


Assume ideal gas law holds


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


shell / inlet downcomer: 2‐3 inches clearance

weir: 3 5 inches clearanceweir: 3‐5 inches clearance

holes 2.5‐5.0 inches apart




Vapor velocity through holes

chosen to provide flexibility to change feed flow rate

Should be above weeping and below flooding points


Tray Hydraulics

Tray and downcomer pressure heads caused

by various hydrodynamic effects


downcomer pressure drop p p

dry‐tray pressure drop

Tray thickness


Tray thickness

frictional lossfrictional loss

in gallons/minutes

1 inch

g /

Liquid crestover the weir


Weir correction factor

Weir correction factor


relative froth densitybecause downcomer is not clear liquidbecause downcomer is not clear liquid

liquid is aerated


Residence time in downcomers

L+e=total liquid flow rate (including entrainment)L+e=total liquid flow rate (including entrainment)


Fractional entrainment


flow parameter

Condition for avoiding weeping

Surface tension head: surface tension (dynes/cm)


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


Dry tray pressure drop

log‐log scale


valve

Aslot per valvegas flow

hole


Different values for all closed or all open valves

valve weight

Correction factor due to turbulence

valve area


Packed columns internalsceramic/plastic saddle

Pall ringg

structured packing


Packed distillation column


HETP: Height Equivalent to a Theoretical Plate


Documents

CHE 306 StagewiseOperations Fall 2010 - KFUPMfaculty.kfupm.edu.sa/CHE/binoushousam/files/binous... · CHE 306 StagewiseOperations Fall 2010 ... Chimney tray is not an equilibrium