Design of Distillation Columns (1).pdf

Design of Distillation Columns 07/03/2012 18:44
Lectures 1&2
Distillation- fundamental principle
The vapour of a boiling liquid mixture will be richer in the components that have
lower boiling points.
Condensed vapour will contain more volatile components
Distillation is one of the most common processes in industry, & is very energy
intensive (heating and cooling), it can be up to 50% of costs.
Design of distillation columns
Involves determination of number of stages required for a given separation
• Graphically by McCabe- Thiele method (uses operating lines and equilibrium curve)
Involves hydraulic design:
• Selection of tray type or packing materials
• Column sizing (i.e diameter and height of column)
• Determination of pressure drop
• Consideration of flooding, weeping, entrainment and foaming ect. (for plate columns) or HETP,
degree of wetting, flooding etc. (for packed columns)
Two main types of distillation
• Batch- the feed introduced to the column batch wise.
• Continuous- no interruption occurs, ( Binary systems and multicomponent systems)
Column internals: Trays vs. Packings
Trays: stagewise process (used to hold up the liquid to give better separation)
• Type of trays:
• Sieve
• Valve
• Bubble cap
Packings: continuous process (packed columns are used to enhance contact
between vapour & liquid)
• Type of packing:
• Random packings
• Structured packings

Tray Columns
Basic requirements of tray design:
• Intimate mixing between liquid and vapour streams for efficient exchange of
components
• Provide sufficient liquid hold-up for high efficiency mass transfer
• Ability to handle desired rates of vapour and liquid flows without excessive liquid
entrainment or flooding
• Minimal pressure drop on each tray (especially important in vacuum operation)
Typical tray operation
Single pass cross flow liquid arrangement as shown
o Liquid flows across plate and vapour flows up through plate
o Transfer of liquid from plate to plate via downcomers (gravity)
o Outlet weir on edge of plate to retain a level of liquid on plate (i.e liquid
hold-up)
Other types of liquid flow arrangements on trays
o Reverse flow (low liquid rates)
o Double pass (high liquid rates)
o Four pass also possible with bigger trays (for even higher liquid rates)
Different configurations have downcomers fitted at various positions on the tray to
appropriately direct liquid flows
Distillation Trays
Sieve tray: metal, diameter & number of holes are design considerations (cheep and
simple)
Bubble Cap tray: has raised chimneys fitted over each holed, a cap covers the
riser. There is a space between riser and cap to allow the passage of vapour.
The vapour rises through the chimney directed downwards by the cap on
discharging through slots in the cap bubbling through the liquid on the tray.
Valve Tray: perforations are covered by lift able caps, self creating a flow area for
passage of vapour through the liqu\id. The lifting caps direct the vapour to flow
horizontally into the liquid (better mixing)

Recap
Distillation: process in which a liquid or vapour mixture of two or more substances
into its component fractions of desired purity.
• Vertical shell where separation of liquid components is carried out.
• Internal trays of packing; valve trays, bubble caps, side trays.
• Reboiler to provide the necessary tray vaporisation the distillation process.
• Condenser to cool and condense the vapour leaving the top of the column.
• Reflux drum to hold the condensate vapour from the top of the column, so
Which tray type?
Factors to consider:
that liquid (reflux) can be recycled back to the column.
• Cost
• Operation range/ flexibility
• Efficiency
• Pressure drop
Generally sieve plates are satisfactory for many applications (except for very low
vapour flow rates)
Packed columns
Packing characteristics in operation:
• Large surface area for maximum vapour/ liquid contact
• High degree of turbulence to promote rapid, efficient mass transfer
between phases
• Open structure for low resistance to vapour flow, hence low pressure
drops
• Promote uniform liquid distribution on surface
• Promote uniform gas flow across column cross-section
Liquid films flow down over the surface of packing gas flows up through the open
structure of packing close contact between liquid and gas is achieved.
Progressive or continued transfer of more volatile component from liquid to vapour
and less volatile component from vapour to liquid.
Interfacial area, turbulence and free flow area considerations are addressed by
design of packing. Smaller packing: increased capital cost, higher pressure drop,
increased surface area.
Packings- note will need more energy to drive vapour up the column when using packing.
Three types:

• Broken solids; cheapest, hardly used, non-uniformity in size, unreliable
performance, high pressure drop
• Shaped packing/ random packing
• Structured packing
Various random shaped packing including:
• Rasching Rings: simple hollow ring, oldest, cheapest, most widely used,
less effective, not necessarily most economic. Can be made in various
material and ceramic and carbon.
• Lessing Rings: Rasching Rings with partitions across its centre,
increased surface area and strength. Ceramic and metals
• Pall Rings: superior performance, highly effective give better wetting and
distillation. Liquid smaller pressure drop than Rasching under same
conditions, available in metals, ceramics and plastics.
• Berl saddles: less free gas space better aerodynamic shape, ceramic or
plastic.
• (super) Intalox saddles
• Metal Hypac
• Mini Rings
Mini rings and metal saddle design gives high performance and low pressure
drop. Pall rings and interlox saddles most popular of packing, high efficiencies at
low pressure drops.
Selection of random packing
Need to consider:
• Type
• Size
• Material
Choice of size of random packings
Size of packing affects
• Height of column
• Pressure drop
• Cost
Large packings
• Are cheaper (based on unit volume)
• Give lower pressure drop per meter of packing
• Give reduced mass transfer efficiency and therefore result in the need for
taller columns (more separation stages)

• Do not necessarily result in reduced overall cost of column
Typically, packing size ≤ 1/8 of column diameter
Choice of material for random packings
• Ceramics- easily broken not strong
• Plastics- organic solvents, cant be used at high temperatures
• Carbon
Choice based on:
• Nature of fluids
• Operating temperature
• Strength of packing required
Structured Packings
• Wire mesh or perforated metal sheets
• High surface area and high voidage
• Lower HETP (height equivalent of theoretical plate), due to higher
efficiency, and pressure drop than random packings.
• Significantly more expensive than random packings
H:L/N
• L: length of column
• N: number of theoretical plates
Plates or Packing
Choice dependent on the following considerations:
• Column diameter
• Range of operating conditions required
• Liquid flowrates
• Liquid distribution
• Foaming
• Fouling systems
• Corrosive systems
• Heat evolution and sidestream
• Pressure drop
• Liquid hold-up
• Maintenance
• Weight

Lectures 3 & 4
Design of sieve tray columns (sievewise distillation)
Downcomer: liquid falls down through downcomer by gravity from one tray to the
one below it.
Weir: ensure that there is always some liquid hold up
Sieve tray, single pass cross flow liquid arrangement
hdc : height of downcomer
hw : height of weir
how: height of liquid over weir
Ts: Tray spacing
hw + how = static liquid seal of a tray
Design procedure of sieve trays
For specified vapour and liquid flows (and physical properties), need to estimate tray
diameter, spacing and layout which will give acceptable:
• Entrainment
• Approach to flooding
• Weeping
Entrainment
• Pressure drop on each tray
• Height of liquid back-up in downcomer
Entrainment refers to the liquid carried by vapour up to the tray above, it occurs at
high vapour flows and relatively low liquid flows. It is detrimental because: tray
efficiency is reduced (less volatile material is carried to plate holding liquid of
higher volatility), it could contaminate high purity distillate, excessive entrainment
can lead to flooding.
Chocking (blockage) of downcomer can set a limit to the liquid flowrate when the
inflowing gas-liquid dispersion needs more space when large amount of gas is
set free in the downcomer. The separated gas rises counter to the down flowing
gas containing liquid and can induce blocking condition.

Flooding
Brought about by excessive vapour flow
• Liquid entrainment in vapour (entrainment flooding)
• Backing up of liquid in the downcomer (downcomer flooding)
Determines maximum capacity of the column, detected by:
• Sharp increases in column differential pressure
• Significant decrease in separation efficiency
Flooding is brought about by excessive vapour flow causing liquid to be entrained in
the vapour up the column. Depending on the degree of flooding the max capacity
of the column severely reduced.
Entrainment flooding
• Caused by excessive liquid droplets being carried up to tray above
• Occurs at high gas flows and with thin liquid layer on tray (gives rise to tray
spray regime)
• Impact dependent on tray spacing
• Also referred to as bed expansion flooding
Downcomer flooding
• Occurs when height of gas- liquid mixture in downcomer (hdc) exceeds
height of downcomer
Bed expansion flooding sets when the gas-liquid layer on the tray extends to the next
(higher) tray the dispersion height becomes equal to the tray spacing.
Depends on: liquid height on a tray, expansion of liquid.
Equivalent to entrainment flooding, drop entrainment rises rapidly. Downcomer over
flowing- when the height of gas liquid layer in the downcomer exceeds the height of
the downcomer.
Weeping/ Dumping
Caused by low vapour flow:
• vapour is insufficient to hold up the liquid on the tray
o liquid starts to leak through perforations (weeping)
• excessive weeping will lead to dumping
o the liquid on all trays will crash through to the base of the column
(via a domino effect)
o detected by sharp pressure drop and separation efficiency

weep point, the gas flowrate at which the first leakage of liquid starts because the
gas flowing up through perforation is no longer able to counter balance the
hydrostatic head of liquid on a tray.
Pressure drop
Total tray pressure drop, ΔPT, is made up of:
• Dry tray pressure drop, ΔPdry
o Determined by gas flowing through holes in tray
• Pressure drop due to liquid hold up
o Liquid depth on tray offers additional resistance to gas flow
The tray pressure drop is composed of at least two major contributions:
1) a pressure drop by the gas flowing through the perforations
a. Depends on gas flowrate, fractions of free area, pressure drof
coefficients of particular perfations.
b. Depends on relative hole thickness, hole shape nearness of other
holes.
2) a pressure drop caused by the liquid present on the tray. This liquid hold up
effect primarily increases with an increase in outlet weir height, decreases
with an increase in gas flowrate & increase in liquid flowrate.
3) Depends on physical properties of gas/liquid system
hT = hd + h
hd =dry tray pressure drop
h=gas through liquid
Foaming: expansion of liquid due to passages of vapour or gas
Height of liquid back-up in downcomer (hdc)
Caused by:
• Pressure drop over plate
• Resistance to flow in downcomer
Tray spacing, Ts, is a design parameter chosen to prevent liquid in the downcomer
backing up to the tray above. For safe design avoiding downcomer flooding:
hdc ≤ ½ Ts
Most factors that affect column operation is due to vapour flows. Weeping
determines the minimum vapour flow. Flooding determines the maximum
vapour flow.

Tutorial Question
Liquid flowrate, L=0.14 kmol/s
Vapour flowrate, V=0.15 kmol/s
Average molecular mass of liquid vapour =70kg/kmol
Liquid density, ρL = 700kg/m 3
Vapour density, ρV = 2.26kg/m 3
Flooding= 80%
Height of weir hw =50mm
Ts=0.45m
hap=38mm
single pass cross flow from liquid arrangement graph
DT = 2.45m
Ψ< 0.1 è entrainment ok
Weeping use weep point correlation, k2 vs (hw+how) chart
• q=(0.14*70)/700
• =0.014m 3 /s
• Lw=0.77*DT=1.89
how= 750 (q/Lw)^0.67
• how= 750 (0.014/1.89)^0.67
hw+how=50+28=78mm
k2=30.7
Uh min= [k2-0.9(25.4-dh)]/√(ρv)
• Uh min= [30.7-0.9(25.4-4.76)]/√(2.26)
• Uh min= 8.06m/s
Uh actual=QV/Aholes=4.65/0.076AT=4.65/0.076*4.7= 13m/s
Uh actual > Uh min
èno weeping
Pressure drop
ΔΡtotal =ΔΡdry +ha
• ha =ΔΡ due to aerated liquid on tray

ΔΡdry= 50.8 (ρV/ρL)(Uh/Cv0) 2
• Cv0 =discharge coefficient, 0.76
• ΔΡdry= 50.8 (2.26/700)(13/0.76) 2
• =48mm of liquid=330N/m 2
note ΔΡdry is m of liquid to get in N/m 2
ΔΡdry =(mm of liquid) * ρL * g
ha =Qp (hw + how)
=0.6(78)
=46.8m
Qp : depends on how much froth is produced (high flowrate + low Active
area )=low Qp
ΔΡT = 48 + 46.8 =94.8
= 651 N/m 2
axis ion the graph Qp vs Fv=Q/Aa(√ρv)
note : acceptable ΔΡT between 300-1200 N?m 2 per tray for atmospheric
duty.
Height of liquid back up in downcomer
hdc= ΔΡT +(hw +how +Δ+ hda)
Δ: liquid gradient difference in liquid level needed to drive liquid flow across plate. Δ
is negligible for sieve tray, can be significant in vacuum operation.
hda= loss under down comer apron
=165 (q/Ada) 2
Clearance area under downcomer apron
Ada = hap * LW (m 2 ) =0.038 *1.89=0.072m 2

Typically hap=hw-(5 to 10mm)
hda =165 (0.014/0.072) 2
=6.24 mm of liquid
hdc = 94.8 + 50 +28 +0 +6.24 =179mm of liquid
check hdc ≤ ½ Ts
Ts = 0.45m
½Ts = 0.179m
hdc < ½Ts è risk of flooding is low
residence time
tr = (Adc * hdc)/ qL
=0.12*AT*0.179/ 0.014
• =0.12*4.7*0.179/ 0.014=7.25 >3s minimum

07/03/2012 18:44

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07/03/2012 18:44