CHAPTER 10. PLATE DISTILLATION COLUMNS
Learning Objectives and Expectations
To be familiar with the main features of plate distillation columns such as
sieve plates, downcomers, weirs, reboiler, condenser/reflux drum, shell and
nozzles.
To be familiar with the factors that affect plate distillation column
performance such as pressure drop per plate, number of stages, gas and
liquid flow rates, reflux ratio.
Understand how to design plate columns to control downcomer back up,
entrainment, coning, flooding, and weeping.
Be able to specify number of stages, column diameter, plate spacing, liquid
flow pattern, downcomer area and plate design (hole area and size).
Calculate the plate pressure drops, backup heights and residence times.
Be familiar with the rules of thumb for distillation column design such as h ,
w
l , l /D , A , d , l , h , t .
w w c h h p ap r
To be able to design plate distillation columns safely and cost effectively.
To be able to produce plate distillation column specification sheets and
equipment drawings.
Chapter 10 Distillation Columns 1
CHAPTER 10. PLATE DISTILLATION COLUMNS
1. NATURE OF OPERATIONS
- Mainly for distillation operations
- To separate one or more species in a mixture
- Relies on the variation in the relative volatility of the species in the
mixture
- Vapour-liquid equilibrium data is essential for design
- Column can be single or multi-stage
- Multi-stage operations are more useful.
2. MAIN FEATURES OF A MULTI-STAGE PLATE COLUMN
- Cylindrical shell - Plates – sieve, bubble cap
- Plate supports - Condenser
- Reboiler - Reflux drum
- Inlet ports - Take off ports
- Recycle ports - Downcomers
- Weirs
Chapter 10 Distillation Columns 2
Chapter 10 Distillation Columns 3
Chapter 10 Distillation Columns 4
(Reflux Drum)
Chapter 10 Distillation Columns 5
Chapter 10 Distillation Columns 6
Liquid
P
Vapour
2
Liquid
P > P > P
0 1 2
P
1
Vapour
Liquid
P
video
0
Vapour https://www.youtube.com/watch?v=mnbQ5dGjmbY
Chapter 10 Distillation Columns 7
Chapter 10 Distillation Columns 8
Sieve
Trays
Perforated
Plates
Chapter 10 Distillation Columns 9
Sieve Trays - Perforated Plates
Chapter 10 Distillation Columns 10
Valve
Trays
Valves open to let
vapor through
video
Chapter 10 Distillation Columns 11
Valve Trays - Valves open to let vapor through
Chapter 10 Distillation Columns 12
Bubble Caps
Let vapor through but
difficult for liquid to go
video through
Chapter 10 Distillation Columns 13
Bubble Caps - Let vapor through but difficult for liquid to go through
Chapter 10 Distillation Columns 14
7. PLATE HYDRAULIC DESIGN
a. Aim
To provide good vapour-liquid contact and achieve a reasonably high plate
efficiency
b. Normal Operations
To achieve a higher efficiency:
- Vapour to travel up the tower through the perforated holes
- Liquid to travel down the tower via the downcomer
- Vapour and liquid travelling via other paths will reduce tray efficiency
- To allow sufficient liquid hold up
- To maintain appropriate liquid and vapour flow rates (hole diameter and
hole number)
- To maintain appropriate level of pressure drop between plates
(downcomer area vs liquid flow rate)
- Otherwise flooding, weeping, coning and excess entrainment can occur
Chapter 10 Distillation Columns 15
c. Weeping
- Vapour flow rate is too low
- i.e. unable to maintain a level of liquid on plate
- Liquid weep through holes
d. Entrainment
- Vapour flow rate is too high
- Liquid drops are carried back to tray above by vapour
e. Coning
- Liquid flow rate is too low
- Vapour jets upward passing through liquid layer quickly without
allowing good vapour/liquid contact
f. Flooding video link
(1’50 – 3’03’) https://www.youtube.com/watch?v=QFGKwD49oNs
- Pressure drop between plates are too high
- Liquid is unable to flow down the tower freely
- Liquid backs up in the downcomer
- Sharp drop in plate efficiency when this occurs
Chapter 10 Distillation Columns 16
Coning
Weeping
Vapour goes up
Liquid goes down
through holes too
through holes
fast
Chapter 10 Distillation Columns 17
Entrainment
Spray goes up
through holes with
vapour
Chapter 10 Distillation Columns 18
Flooding
Normal Pressure in
operation lower plate too
high, pushes
liquid back up
downcomer
Chapter 10 Distillation Columns 19
g. Competing Factors
Tray efficiencies vs operation difficulties
See Fig. 11.26, C&R, Vol. 6, next page
To obtain high tray efficiencies, it is preferable to have:
- Large mass transfer surface (large vapour/liquid interfacial surface area)
- Long vapour/liquid contact time
- This may be provided by deep liquid layer in tray
- High vapour velocity is preferred, this gives small diameter bubbles
(small vapour velocities give large bubbles)
- High vapour velocity also yields high intensity of turbulence
But these operating conditions can cause operational difficulties:
- Entrainment of liquid droplets
- Coning of vapour for very high vapour velocity
- Foaming of liquid, foam fills the tower space – reduces disengagement
volume and tray efficiency in extreme cases
- High pressure drop in the space between trays
- Boiling points may be changed
- High ∆P may be ultimately lead to the flooding of the tower
Chapter 10 Distillation Columns 20
Chapter 10 Distillation Columns 21
Quiz
What are the major likely causes to flooding?
Chapter 10 Distillation Columns 22
3. SPECIFICATIONS OF A DISTILLATION COLUMN
a. Degree of separation e. Details of column internal parts
- Product purity - Liquid flow pattern
- Feed and product flow rates - Weirs and downcomers
- Distributors, number and size
b. Operating conditions - Condenser duty
- Batch or continuous - Reboiler duty
- Pressure and/or temperature
f. Size of column–diameter & height
c. Type of contacting device - Fluid mechanics
- Plate or packed columns - Pressure drops
- Type of plate – sieve or bubble cap
g. Mechanical fittings and support
d. Number of stages - Inlet and outlet ports
- Ideal and real stages - Supports for plate and shell
- Reflux ratio - Downcomer support
Chapter 10 Distillation Columns 23
4. DISTILLATION COLUMN DESIGN – PLATE TYPE – MULTI-STAGE
PROCESS DESIGN
Initial data required:
- Feed composition and flow rate
- Distillate composition and flow rate
- Material balance calculations gives residue composition and flow rate
Apply standard method of data processing:
- Assume a reasonable reflux ratio
- For binary components systems – H&M
- Ponchon and Savarit Method
- McCabe and Thiele Method
For multiple components systems– Advanced H&M
- Lewis Metheson Method
- Thiele-Geddes Method
Calculate and obtain:
- Liquid flow rate for each tray
- Number of ideal trays
- Vapour flow rate for each tray
- Tray efficiency estimation
- Condenser duty
- Number of real trays
- Reboiler duty
- Location of feed tray
Chapter 10 Distillation Columns 24
b. Physical Column Design
- Plate spacing (approximate) – to be revised
- Column diameter (approximate) – to be revised
- Type of plate and downcomer
- Pattern of liquid flow
c. Plate Hydraulic Design
- Column diameter (refined calculations)
- Tray spacing (refined calculations)
- Liquid flow pattern
- Plate area, downcomer area, active area, hole area etc.
- Pressure drop across a plate
At the end of the preliminary design, the following checks are
essential:
- Check weeping rate
- Check downcomer back-up
- Check extent of entrainment
- Check plate layout – hole diameter, pitch, calming area etc.
Chapter 10 Distillation Columns 25
The design process is an interactive one, the values of some
parameters needs to be assumed, and then revised. When all the
criteria are met then, the entire design may be further optimized to
minimize the capital and operating costs.
d. Mechanical Design
- Structural support - external support for stability
- structural support for internal parts
- Downcomer and weirs
- Connection nozzles
Chapter 10 Distillation Columns 26
5. PHYSICAL COLUMN DESIGN
a. Plate Spacing
- To allow sufficient head room for the frothing of the liquid and the
hold up of the liquid in the downcomer area and on trays
- To allow interaction between liquid and vapour phase
- To allow disengagement of liquid drop from vapour
- 0.15 m to 1 m are usually used.
- Depends on tower diameter and flow rates
- Use guidelines given by C&R and Treybal
* C&R, Vol. 6: For column > 1 m, plate spacing = 0.3 – 0.6 m
Use 0.5 m as a first guess
* Treybal, Table 6.1
Column Diameter Plate Spacing
< 1 m 0.5 m
1 – 3 m 0.6 m
3 – 4 m 0.75 m
4 – 8 m 0.9 m
Minimum 0.15 m
Chapter 10 Distillation Columns 27
Chapter 10 Distillation Columns 28
b. Column Diameter
- Usually determined by vapour flow rate.
- The velocity must be below that which would cause excessive
liquid entrainment or pressure-drop
- C&R, vol. 6, Eq. 11.79 and 11.80, see next page
- Use for a first approximation only to calculate maximum
vapour velocity and tower diameter.
- Calculations must be revised later
The maximum allowable vapour velocity is determined as 85% of the
flooding velocity.
The cross-sectional area of the column is determined from the volumetric
flow rate and max allowable vapour velocity A = V / v
The circular cross-section column diameter is then determined from the
area. D =
( 4A/π)1/2
c
Chapter 10 Distillation Columns 29
A = V / v
( ) 1/2
ρ −ρ
( )
(11.79)
uˆ = − 0.171l2 + 0.27l − 0.047 L V
V t t
ρ
V
uˆ
Where = maximum allowable vapour velocity, based on the
V
gross (total) column cross-sectional area, m/s,
l
=plate spacing (tray spacing), m, (range 0.5 – 1.5).
t
The column diameter, D , can then be calculated:
c
ˆ
4V
D = w (11.80)
c
πρ uˆ
V V
Where V ˆ is the maximum vapour rate, kg/s D =
( 4A/π)1/2
c
w
Chapter 10 Distillation Columns 30
c. Type of Plates
- Sieve Plates
- Simplest type and most widely used
- Low cost, satisfactory for most applications
- Vapour flows upwards though perforations of plate
- Liquid may weep through the holes
- 2 mm to ∼ 18 mm holes typically
Chapter 10 Distillation Columns 31
Chapter 10 Distillation Columns 32
Sieve Trays - Perforated Plates
Chapter 10 Distillation Columns 33
Bubble Cap Plates
- Vapour passes through a riser and meets the bubble cap
- Vapour then leaves the bubble cap through the slots in the skirt of
the cap
- Able to handle a wide range of vapour and liquid flow rates
- Much heavier plate
- Higher cost, use this only if very low vapour rate is needed
- Useful to avoid weeping
Chapter 10 Distillation Columns 34
Chapter 10 Distillation Columns 35
Chapter 10 Distillation Columns 36
weeping
coning
Chapter 10 Distillation Columns 37
Bubble Caps - Valves open to let vapor through
Chapter 10 Distillation Columns 38
Valve Plates
- Each hole of plate is covered by a moveable flap
- Flap lifts as vapour flow increases
- Hole diameters usually larger than those used in
sieve plates
- More expensive than sieve plates
- Can handle a wider range of flow rates than sieve
plates (higher turn town ratio)
Chapter 10 Distillation Columns 39
Valve Trays - Valves open to let vapor through
Chapter 10 Distillation Columns 40
Valve Trays - Valves open to let vapor through
Chapter 10 Distillation Columns 41
d. Pattern of Liquid Flow
- Single pass cross flow, simplest, commonly used
- Reverse flow
- Double pass cross flow
- See Fig. 11.17 and 11.21, C&R, Vol. 6
Chapter 10 Distillation Columns 42
6. MECHANICAL DESIGN
a. Shell
- May be a pressure vessel, wall thickness to be specified
- Type of end caps, dished or flat plate
- End caps need to be attached to the shell wall
- Internal parts need to be installed
b. Plates
- Cap design, for bubble caps and valve caps (depends on fluid
mechanics)
- Hole design, for sieve plates, size and number of holes
(depends on vapour flow rate)
- Plate thickness depends on hole diameter
- Mainly stainless steel
Chapter 10 Distillation Columns 43
c. Downcomers
- See Fig. 13.31, Walas, (next page) for cross flow plates
and downcomers
- Downcomer sheet – vertical vs those making an angle
to tower wall
- Downcomers may also be in the form of a circular pipe
- Inlet weirs, outlet weirs
- To allow disengaging of gas from liquid
- To form a seal so that only liquid enters the tray below
- To maintain the required depth of liquid on the tray so
that efficient vapour/liquid contact is allowed
- Weir lengths ≈ 60 – 80 % of column diameter
Chapter 10 Distillation Columns 44
Chapter 10 Distillation Columns 45
d. Other Accessories
- Reflux drum, usually horizontal ones, liquid hold up, 5 min, half full.
- For a 1 m diameter tower, add 1.2 to1.5 m top column for vapour
disengagement.
- Allow 2 m at bottom for liquid reservoir and reboiler vapour return.
- Reboiler, thermal siphon, steam supply piping.
- Condenser, coolant supply
e. Structural Supports
- Structural support for the plate, the hydraulic load, the downcomer
- Must allow access of workers – installation and inspection
(manhole, ladder, etc).
- The weight of the workers must also be supported (may be more
than one)
- External support for tower
Chapter 10 Distillation Columns 46
Chapter 10 Distillation Columns 47
f. Connection Nozzles
- Feed pipe and take off pipes
- Pipes for side streams and reflux streams
- Connections to reboiler and condenser
- Manholes, DN 450, one manhole every 10 trays
- Usually flanged connections designed to take the
required temp. and pressure
Chapter 10 Distillation Columns 48
8. DEFINITION OF PLATE AREAS
Units are in (m2)
A = total column cross-sectional area
c
A = cross-sectional area of downcomer
d
A = net area available for vapour-liquid disengagement
n
= A – A for single pass plate
c d
A = active or bubbling areas
a
= A – 2 A
c d
A = hole area, total area of active holes
h
A = perforated area, hole area plus blanked area
p
A = the clearance area under the downcomer
ap
Chapter 10 Distillation Columns 49
A
A A
n
d a
Liquid
Leaving
Tray Liquid
Entering
Tray
Calming
Zone
A
A + A = A p Edge
n d c
Strip
A = A – Calming Zone – Edge strip
p a
Chapter 10 Distillation Columns 50
9. SOME PRELIMINARY RULES OF THUMB
a. Liquid Flow Pattern
- A function of column diameter and liquid flow rate
- Use fig. 11.28, C&R Vol. 6., next page
- Cross flow is most commonly used
b. Hole Size
- d , hole diameter = usually 2 mm to 10 mm
h
- 5 mm is the preferred diameter
- Holes drilled or punched
- For carbon steel, hole diameter ≈ plate thickness
- For stainless steel, hole diameter ≈ 2 x plate thickness
c. Plate Thickness
- Typically = 5 mm for carbon steel
3 mm for stainless steel
Chapter 10 Distillation Columns 51
. . . . . .
Chapter 10 Distillation Columns 52
d. Hole Pitch, I
p
- Hole pitch = 2.5 to 4.0 hole diameters usually
should be no less than 2 hole diameters
-Square or triangular pitch
- For triangular pitch
2
A d
h = 0.9 h Eq. 11.86, C&R, Vol. 6
A l
p p
This is plotted in Fig. 11.33, C&R, Vol. 6, next page
Thus the maximum hole area available for vapour flow is
d
dictated by the ratio of h
l
p
Where d = hole diameter (mm)
h
l
= pitch diameter (mm)
p
Chapter 10 Distillation Columns 53
Chapter 10 Distillation Columns 54
e. Hole Area, A
h
As a first trial, take A = 0.1A A = A – Calming Zone – Edge Strip
h a p a
A
A A
n
d a
Liquid
Leaving
Tray Liquid
Entering
Tray
Calming
Zone
A
A + A = A p Edge
n d c
Strip
Chapter 10 Distillation Columns 55
f. Weir Dimensions
Weir length – l
w
Usually l ≈ 0.6 – 0.85 of column diameter
w
A first estimate
A
l
Use w = 0 . 7 7 giving d = 0.12
A
D
c
c
i.e. with weir length of 77% of column diameter, the downcomer is ∼
12% total column area
Use Fig. 11.31, C&R, vol. 6, next page for other values of l /D .
w c
Chapter 10 Distillation Columns 56
Chapter 10 Distillation Columns 57
Chapter 10 Distillation Columns 58
Outlet weir height – h
w
- Operating at > 1 atm., h ≈ 40 to 90 mm usually,
w
40-50 mm is recommended for most purposes
- Operating under vacuum, 6-12 mm is recommended
Weir liquid crest
The height of the liquid crest for a segmental downcomer is:
2/3
L
h = 750 w Eq. 11.85, C&R, Vol. 6
ow
ρ l
L w
Where L = liquid mass flow rate, kg/s
w
h = weir crest height, mm liquid
ow
l = weir length, m
w
h should be > 10 mm
ow
Chapter 10 Distillation Columns 59
g. Typical Plate Layout
l
w = 0.77
Calming
D
c
Zone
50 mm
50 mm
A
d = 0.12
A
c
Downcomer area = 12%
Strips for calming zone = 50 mm
l
wide
w D
c
Edge strip for support = 50 mm
wide
A
d A = perforated area
p
A
p
A = downcomer area
Edge Strip 50 mm d
Chapter 10 Distillation Columns 60
10.OTHER CALCULATIONS
a. Flooding Vapour Velocity
Governed mainly by vapour velocity, see fig. 11.27, C&R, Vol. 6, next page
Flooding vapour velocity is:
ρ −ρ
u = K L V Eq. 11.81, C&R, Vol. 6
f 1 ρ
V
( )
K = f F ,l Eq. 11.27, C&R, Vol. 6
Where
1 LV t
L ρ
F = W V Eq. 11.82, C&R, Vol. 6
LV
V ρ
W L
ρ
= density of liquid phase F = Liquid vapour flow factor
L LV
ρ
= density of vapour phase l = Tray spacing (m)
V t
L = liquid mass flow rate (kg/s) K = Constant
W 1
V = vapour mass flow rate (kg/s) u = Flooding vapour velocity, based
W f
on A , net column area
n
For safe design, use u = (0.75 – 0.85) of u where u is velocity based on net
n f, n
area operating velocity. Design for 75-85% flooding at the maximum flow rate
Chapter 10 Distillation Columns 61
Conditions
Chapter 10 Distillation Columns 62
Note:
Fig. 11.27 holds only if:
1. d < 6.5 mm
h
2. h < 15% of tray spacing
w
3. Non foaming liquids
4. A : A > 0.1
h a
Otherwise, some corrections will be needed.
K is applicable only if the surface tension of the liquid is 0.02 N/m,
1
otherwise correction is needed using the following equation
0.2 0.2
K σ K σ
1a = a or 1a = a
K σ K 0.02
1b b 1b
Chapter 10 Distillation Columns 63
b. Weep Point Vapour Velocity
The vapour velocity at the weep point
This is the minimum velocity for stable operations
Weeping will occur if velocity through the hole is below this value
[ ( )]
K − 0.90 25.4 − d
u = 2 h Eq. 11.84, C&R, Vol 6
h ρ1/2
V
u
Where = weep point vapour velocity (m/s)
h
(based on hole area, A )
h
ρ
V = vapour density (kg/m3)
K
= a constant (Fig. 11.30, C&R, Vol. 6, next page)
2
( )
K = f h + h
2 w ow
Check for weeping at 70% flow rate turndown.
Chapter 10 Distillation Columns 64
Chapter 10 Distillation Columns 65
c. Tower Diameter
Say use operating velocity at 80% of flooding velocity (between 75 and
85%)
u = 0.8u
(based on net area)
n f
Then net area required for liquid-vapour disengagement is:
V '
A = w
n
u
n
V '
Where = volumetric vapour flow rate (m3/s)
w
A
= net area (m2)
n
u
= operating vapour velocity (m/s)
n
As a first trial take the downcomer area to be 12 % of total column area.
Chapter 10 Distillation Columns 66
A
If X = d = 0.12 A
A
d A + A n d
d n
A
Then X = n = 0.88
n
A + A
d n
Remember that A = A + A
c d n
A
A = n
total column area
c
X
n
4A
Tower diameter, D = c
c
π
πD2 A + A = A
n d c
or A = c
c
4
Chapter 10 Distillation Columns 67
11. PROVISIONAL PLATE DESIGN
- Column area
- Downcomer area
- Net area (A )
n
- Active area (A )
a
- Hole Area (A ) = 0.1 A as a first approximation
h a
- Weir length (l ) = 0.77 D
w c
- Weir height (h ) ≈ 50 mm
w
- Plate material – stainless steel
- Hole diameter ∼ 3-6 mm
- Plate thickness ∼ 3-5 mm
Chapter 10 Distillation Columns 68
12 PLATE PRESSURE DROP
See Fig. 11.35, C&R, Vol. 6, next page
Total pressure drop between two consecutive plates are:
h = h + h + h + h Eq. 11.90, C&R, Vol. 6
t d w ow r
Where h = head loss due to dry plate (mm)
d
h + h = head loss due to liquid on plate (mm)
w ow
h = residual head (mm)
r
h = total head (mm), pressure drop of total plate
t
Chapter 10 Distillation Columns 69
apron
Chapter 10 Distillation Columns 70
a. Dry Plate Drop
2
v ρ
h = 51 h V Eq. 11.88, C&R, Vol. 6
d
C ρ
o L
Where v = actual vapour velocity thru the holes
h
(based on hole area) m/s
PlateThickness A
C = f , h
o
HoleDiameter A
p
Typically, orifice coefficient C = 0.65 – 0.85
o
Use Fig. 11.34, C&R, Vol. 6. next page
Chapter 10 Distillation Columns 71
Drawing
Chapter 10 Distillation Columns 72
b. Residual Head
A pressure loss due to liquid surface tension, froth density, and froth
height
12.5×103
h =
Eq. 11.89, C&R, Vol. 6
r
ρ
L
For water, h = 12.5 mm
r
c. Liquid Head
Phys.org
Height of liquid on tray = h + h
w ow
h should be > 10 mm (discussed before) , h usually 10 to 20
ow ow
mm
Chapter 10 Distillation Columns 73
13 DOWNCOMER DESIGN
a Downcomer Backup
Height of the liquid in the downcomer is usually termed back-up of liquid.
Back-up of liquid in the downcomer is caused by resistance to flow in the
downcomer and the excess pressure drop across consecutive plates.
Height of (liquid + froth) in the downcomer << l (well below top of weir)
t
h = h + h + h + h Eq. 11.91, C&R, Vol. 6
b ow w t dc
Where h = downcomer back-up (mm), measured from plate
b
surface
h
dc = head loss in the downcomer (mm)
h + h = head loss due to liquid on tray (mm)
o ow
h = head loss of total plate (mm)
t
2
L
h = 166 wd Eq. 11.92, C&R, Vol 6
dc
ρ A
L m
Chapter 10 Distillation Columns 74
2
L
h = 166 wd
dc
ρ A
L m
Where L = liquid mass flow rate in downcomer (kg/s)
wd
A = smaller of A and A (m2)
m d ap
A = downcomer area (m2)
d
A = clearance area under the downcomer (m2)
ap
A = h l
And Eq. 11.92, C&R, Vol 6
ap ap w
Where h = distance between bottom edge of the apron and the
ap
plate surface (m)
h − h − h = h + h
b ow w t dc
( )
h = h − 5 to 10 mm
ap w
Therefore the apron of the downcomer is 5 to 10 mm below the
outlet weir height.
Chapter 10 Distillation Columns 75
P − P = ρgh
1 2 t
h = h + h + h + h
t d w ow r
2
v ρ
h = 51 h V
d
C ρ
o L
h
ow
12.5×103
h =
r
ρ
L
h = h + h + h + h
b ow w t dc
Chapter 10 Distillation Columns 76
b. Height of Froth in Downcomer
Froth produced due to aeration of liquid
Froth density ≈ half that of clear liquid
Safe design requires
1
( )
h < l + h Eq. 11.94, C&R, Vol. 6
b t w
2
So that the top level of froth is unlikely
to reach top of weir.
Chapter 10 Distillation Columns 77
c. Residence Time of Liquid in Downcomer
Sufficient time is needed for the entrained vapour to disengage from the
liquid
To prevent loss in tray efficiency
To prevent gas being carried downwards through the downcomer.
A h' ρ
t = d b L
Eq. 11.95, C&R, Vol. 6
r
L
wd
t = residence time of liquid in Downcomer (s)
r
ρ
= liquid density (kg/m3)
L
L
= Liquid mass flow rate in downcomer (kg/s)
wd
'
h
= downcomer back-up (m)
b
(same as h , but h has the unit of mm while h’ has the unit of meter)
b b b
Typically, t > 3 seconds is recommended.
r
∴ It takes > 3 seconds for liquid /vapour to disengage in downcomer.
Chapter 10 Distillation Columns 78
14 ENTRAINMENT
- Liquid droplets are carried along by gas at high velocity
- See Fig. 11.29, C&R, Vol. 6, next page
- ψ = f(F , % flooding)
LV
Where ψ = fractional entrainment
moles liquid entrained
= ---------------------------------------------
moles of liquid (flow + entrained)
mass liquid entrained
≈ ---------------------------------------------
mass of liquid (flow + entrained)
operating velocity (based on net area)
- % flooding = --------------------------------------------------
flooding velocity (based on net area)
u
n
= Eq. 11.81, C&R, Vol. 6
u
f
- As a rough guide, choose ψ < 0.1
- Ideally ψ→ 0, realistically this does not occur.
Chapter 10 Distillation Columns 79
Chapter 10 Distillation Columns 80
15 PLATE DESIGN PROCEDURE
(1) Established maximum and minimum vapour and liquid flow rates.
(2) Collect system physical properties
(3) Select a trial tray spacing
(4) Estimate tower diameter
- Limiting factor is flooding vapour velocity
- Check entrainment now and reduce % flooding as necessary
(5) Choose the pattern of liquid flow
(6) Make provisional plate design
- Downcomer area, active area, hole area, plate thickness, etc.
(7) Check weeping vapour velocity
Chapter 10 Distillation Columns 81
(8) Check plate pressure drop
(9) Check downcomer back-up
- Tray spacing, residence time.
(10) Check plate layout
- Hole area, perforated area, hole pitch, calming zone, support strips
(11) Check entrainment
(12) Recalculate % flooding based on chosen column diameter
(13) Optimize the design
(14) Finalize the design
Chapter 10 Distillation Columns 82
16 OTHER RULES OF THUMB (WALAS)
1. R = 1.2 to 1.5 x R
operating min
2. For reasonable accessibility, tray spacing ≈ 50 cm – 60 cm
3. Safety factor for number of ideal trays, + ∼ 10%
4. Pressure drop/tray ≈ 7 – 12 cm water head or 0.7-1.2 kPa
5. Tray efficiency ≈ 55 – 90 %
6. Sieve trays ≈ 3 mm to 8 mm hole diameter.
7. Hole area ≈ 10% active area
8. Weir height ≈ 50 mm
Weir length ≈ 75% tray diameter
9. Horizontal reflux drums
Liquid hold-up = 5 minutes @ half full
10. for Tower with ∼ 1m diameter
add 1.2 to 1.5 m at the top – for vapour disengagement
add 2m at bottom for liquid reservoir and reboiler return
11. Limit of tower height
< 53 meter due to wind load and load on foundation
Chap1te2r. 1L0/ DDis DISTILLATION COLUMN WORKED EXAMPLE Methanol is to be recovered from an aqueous waste stream containing 20 mol % methanol using a plate column. The distillate should contain at least 98 mol % methanol and the residue 1 mol % methanol. The feed is a liquid preheated to 81.7oC (saturated liquid) and the reflux is returned at its bubble point. Other data: Flow rate = 8000 kg/hr Turn down ratio @ 80 % of max flow rate DISTILLATION COLUMN DESIGN a. Process design b. Physical column design c. Plate hydraulic design d. Mechanical Design Chapter 10 Distillation Columns 84 (a) Process Design Determine number of ideal stages Feed is a saturated liquid q = 1 q 1 = = ∞ Slope of q line = q −1 0 i.e. q line is a vertical straight line Equilibrium curve and q line intersects at the point of minimum reflux X draw operating line at R D = 0.493 m R +1 m X = 0.98 ⇒ R = 0.988 D m Chapter 10 Distillation Columns 85 Methanol-Water 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Chapter 10 Distillation Columns 86 Mole fraction MeOH in liquid ruopav ni HOeM noitcarf eloM Methanol-Water 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Chapter 10 Distillation Columns 87 Mole fraction MeOH in liquid ruopav ni HOeM noitcarf eloM Methanol-Water 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Chapter 10 Distillation Columns 88 Mole fraction MeOH in liquid ruopav ni HOeM noitcarf eloM X D = 0.493 R + 1 m X = 0.98 D R = 0.988 m Take R = 1.5 R = 1.482 m X D Put this in R +1 X D = 0.395 Y intercept for operating line R +1 Draw operating line (actual), perform McCabe – Thiele stage by stage construction ⇒ 12 ideal stages Chapter 10 Distillation Columns 89 Methanol-Water 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Chapter 10 Distillation Columns 90 Mole fraction MeOH in liquid ruopav ni HOeM noitcarf eloM R = 1.5 R m R = 1.482 X = 0.98 D X D = 0.395 R +1 Methanol-Water 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Chapter 10 Distillation Columns 91 Mole fraction MeOH in liquid ruopav ni HOeM noitcarf eloM Review slides McCabe-Thiele diagram: Operating Lines 1 y i 0.8 R x y = x + D n R +1 n+1 R +1 0.6 slope 0.4 0.2 L Bx y = m x − B m m+1 V V m m 0 x 0 0.2 0.4 0.6 0.8 1 i 92 The q-line with the McCabe-Thiele 1 0 0 1 93 x y q x y = x − F q ( q −1) q ( q −1) x x x B F D Reflux ratio, R Is defined at the top of the column total as the ratio of the liquid rate V condenser n returned to the column to the distillate rate i.e.: L D n Reflux = R L D n 94 Minimum reflux ratio – infinite # of stages 1 0 0 1 x 95 y x D R +1 Decreasing R min q-line x x x B F D What of the following would NOT be typically performed via a distillation process? 1. Separating a crude oil into various fractions 2. Purifying methane from a natural gas stream 3. Concentrating cows milk 4. Recovery of a solvent from an extraction process 96 Rank the relative volatility from highest to lowest for the binary mixtures shown? y i C B A x i 1. ABC 0 0 2. CBA 0 3. BAC 0 4. Cannot not tell from graph alone 97 Which would be considered the less volatile component for this binary mixture? 0 1. Would require a y – x diagram to determine 0 2. Component A 0 3. Component B 0 4. Need more information 98 Methanol-Water 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Chapter 10 Distillation Columns 99 Mole fraction MeOH in liquid ruopav ni HOeM noitcarf eloM Back to normal slides 2 3 4 5 6 7 8 9 10 11 12 Determine no. of plates - Assume column efficiency = 55 % for now, Must check this later. - Assume reboiler to be 1 perfect stage 12 −1 = 20 No. of plates = 0.55 ∴ 20 plates plus 1 reboiler 21 stages all together Chapter 10 Distillation Columns 100 PLATE DESIGN PROCEDURE 1. Establish maximum and minimum vapour and liquid flow rates. 2. Collect system physical properties 3. Select a trial tray spacing 4. Estimate tower diameter Limiting factor is flooding vapour velocity Check entrainment now and reduce % flooding as necessary 5. Choose the pattern of liquid flow 6. Make provisional plate design - Downcomer area, active area, hole area, plate thickness, etc. 7. Check weeping vapour velocity Chapter 10 Distillation Columns 101 (1) Liquid and Vapour Flow Rates Overall Mass Balance Flow rate of feed = 8000 kg/hr MW of CH OH = 32 3 MW of H O = 18 2 Averaged MW of feed = (0.2) (32) + (0.8) (18) = 20.8 kg/kmole 8000kg / hr Molar flow rate of feed = 20.8kg / kmole F = 384.6kmole / hr Chapter 10 Distillation Columns 102 D =? X = 0.98 D D = 75.33 kmole/hr F = 384.6 kmol/hr X = 0.2 F b.p. = 81.7oC F = D + W W = ? 384.6 = D + W overall mass balance X = 0.01 W W = 309.27 kmole/hr 0.2 F = 0.98D + 0.01W component mass balance ⇒D = 75.33 kmole/hr W = 309.27 kmole/hr Molar flow rates (without accent’) Chapter 10 Distillation Columns 103 Convert molar to mass flowrate ( )( )( ) ( )( )( ) D'= 75.33kmole/ hr 0.98 32kg / kmole + 75.33kmole/ hr 0.02 18kg / kmole D'= 2389.59kg / hr ( )( )( ) ( )( )( ) W '= 309.27kmole/ hr 0.99 18kg / kmole + 309.27kmole/ hr 0.01 32kg / kmole W '= 5610.16kg / hr D'+W '= 7999.75kg / hr = 8000kg / hr Mass flow rates (with accent’) Chapter 10 Distillation Columns 104 reflux drum ’ D = 2389 kg/hr F = 8000 kg/hr column How much is the reflux stream? ’ W = 5610 kg/hr reboiler Chapter 10 Distillation Columns 105 Top of tower material balance (reflux drum) L D R = 0 D Mass balance boundary reflux = L ′' L 0 L = 0 G = vapour coming 0 MW up through column L′' D' 0 = R D = MW D' L′' = RD′ 0 ( )( ) L' = 1.482 2389.59kg / hr = 3541.37kg / hr 0 L / D = R ( D and R known) R G = L + D Chapter 10 Distillation Columns 106 Next three slides inserted as review Reflux ratio, R Is defined at the top of the total column as the ratio of the V condenser n liquid rate returned to the column to the distillate rate i.e.: L D n R = L D Reflux n 107 Rectifying section operating line (TOL) as a function of the Reflux Ratio For constant molal overflow: R = L /D. L = L V = V n N n N n V = V = L + D n N n V = L + D n n L L so n = n V L + D n n L L D R n = n = V L D + D D R +1 n n 108 Rectifying section operating line (TOL) as a function of the Reflux Ratio = = + V V L D n N n D D = Also: V L + D n n D D D 1 = = V L D + D D R +1 n n L Dx So: y = n x + D becomes: n n+1 V V n n V’ = L’ / (R / (R +1)) R x y = x + D n R +1 n+1 R +1 109 Back to normal slides (note G is used for vapour flow in this slide, same as V in the review slides) L L′' 0 = 0 Slope of top operating line = G G′ 0.98− 0.2 Slope = = 0.597 0.98− 0.514 ′' L 3541.37kg / hr Slope = 0 = = 0.597 ′ G′kg / hr G 3541.37kg / hr = 5930.96kg / hr = G′ 0.597 Also, G′ = 5930.96kg / hr = L′ + D′ = 3541.37kg / hr + 2389.59kg / hr o Chapter 10 Distillation Columns 110 Methanol-Water 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Chapter 10 Distillation Columns 111 Mole fraction MeOH in liquid ruopav ni HOeM noitcarf eloM 2 3 4 5 6 rise = 0.98 - 0.514 7 = 0.466 8 9 run = 10 slope = 0.98 - 0.2 0.466/0.78 = 0.78 = 0.597 11 R/(R+1) = 1.482 / (1 + 1.482) 12 = 0.597 ’ G = 5931 kg/hr reflux drum ’ D = 2389 kg/hr ’ G = 5931 kg/hr L ’ = ’ o L = 3541 kg/hr o 3541 kg/hr F = 8000 kg/hr column How much vapour returned? ’ W = 5610 kg/hr reboiler Chapter 10 Distillation Columns 112 Bottom of tower material balance (reboiler) V = vapour going Liquid coming m up through column = L Down through m column (CH OH=12+16+4=32) 3 H O=18 2 Mass balance boundary W = residue 0.514 − 0.01 L’ – V’ = W’ (known) Slope = = 2.65 0.2 − 0.01 L’ / V’ = slope (from plot) L' = V ' +W ' Slope(on a mole basis) is defined as L' /V ' m m m m (L ’/MW) / (V ’/MW) = slope = L /V , when MW is the same m m m m Chapter 10 Distillation Columns 113 Review slides McCabe-Thiele diagram: Operating Lines 1 y i 0.8 R x y = x + D n R +1 n+1 R +1 0.6 slope 0.4 0.2 L Bx y = m x − B m m+1 V V m m 0 x 0 0.2 0.4 0.6 0.8 1 i 114 Methanol-Water 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Chapter 10 Distillation Columns 115 Mole fraction MeOH in liquid ruopav ni HOeM noitcarf eloM 2 3 4 5 6 7 8 9 10 slope = rise = 0.504/0.19 0.514- 0.01 = 2.65 11 = 0.504 12 run = 0.2 - 0.01 = 0.19 Convert to molar flow rate L ’ = 2.65 V ’ and L ’ = V ’ + W’ m m m m We had W’ = 5610.16kg/hr ⇒V ’ = 3400.16 kg/hr m L ’ = 9010.21 kg/hr m M.W. (bottom) = (0.99) (18 kg/kmole) + (0.01) (32 kg/kmole) M.W. = 18.14 kg/kmole ⇒ V = 187.44 kmole/hr m L = 497.2 kmole/hr m Chapter 10 Distillation Columns 116 ’ G = 5931 kg/hr reflux drum ’ D = 2389 kg/hr ’ G = 5931 kg/hr L ’ = ’ o L = 3541 kg/hr o 3541 kg/hr F = 8000 kg/hr column ’ V = m 3400 ’ V = 3400 kg/hr ’ kg/hr m L = m 9010 kg/hr ’ W = 5610 kg/hr reboiler ’ L = 9010 kg/hr m Chapter 10 Distillation Columns 117 PLATE DESIGN PROCEDURE 1. Establish maximum and minimum vapour and liquid flow rates. 2. Collect system physical properties 3. Select a trial tray spacing 4. Estimate tower diameter Limiting factor is flooding vapour velocity Check entrainment now and reduce % flooding as necessary 5. Choose the pattern of liquid flow 6. Make provisional plate design - Downcomer area, active area, hole area, plate thickness, etc. 7. Check weeping vapour velocity Chapter 10 Distillation Columns 118 Estimate Operating Conditions & Physical Properties Assume 100 mm water pressure drop / plate Column ∆P = (0.1 m) (1000 kg/m3)(9.81m/s2)(20 plates) = 1.96 x 104 kg/ms2 ∼ 0.2 bar Tower Pressure: Top = 1 bar Bottom = 1.2 bar Surface tension Water ∼ 57 x 10 -3 N/m Methanol ∼ 19 x 10 -3 N/m Chapter 10 Distillation Columns 119 Bottom: 1.2 bar, mainly water, 105 oC (from steam tables) v =11..432785 m3 / kg ⇒ ρ = 0.700kg / m3 g v BOT density calculation v =1.048×10−3 m3 / kg ⇒ ρ = 954kg / m3 L L Top:1.0 bar, mainly MeOH, 82oC ρ = 791kg / m3 L PV = ZnRT n⋅MW P⋅MW TOP density calculation ρ = = v V ZRT assuming Z =1, MW = 32g / mole, T = 355oK ( )( ) 101kPa 32g / mole kg ρ = =1.10 v ( 8.314J / moleK)( 355K) m3 (data from Rogers & Mayhew, pg. 4 & 10) Chapter 10 Distillation Columns 120 DISTILLATION COLUMN DESIGN a. Process design - completed b. Physical column design c. Plate hydraulic design d. Mechanical design Chapter 10 Distillation Columns 121 DISTILLATION COLUMN DESIGN a. Process design b. Physical column design c. Plate hydraulic design d. Mechanical design Chapter 10 Distillation Columns 122 PLATE DESIGN PROCEDURE 1. Establish maximum and minimum vapour and liquid flow rates. 2. Collect system physical properties 3. Select a trial tray spacing 4. Estimate tower diameter Limiting factor is flooding vapour velocity Check entrainment now and reduce % flooding as necessary 5. Choose the pattern of liquid flow 6. Make provisional plate design - Downcomer area, active area, hole area, plate thickness, etc. 7. Check weeping vapour velocity Chapter 10 Distillation Columns 123 (b) Physical Column Design (3) Trial Tray Spacing Select plate column Select cylindrical shell Need to double Select sieve plate Check later if ok Select single pass cross flow Try plate spacing = 0.5m Chapter 10 Distillation Columns 124 PLATE DESIGN PROCEDURE 1. Establish maximum and minimum vapour and liquid flow rates. 2. Collect system physical properties 3. Select a trial tray spacing 4. Estimate tower diameter Limiting factor is flooding vapour velocity Check entrainment now and reduce % flooding as necessary 5. Choose the pattern of liquid flow 6. Make provisional plate design - Downcomer area, active area, hole area, plate thickness, etc. 7. Check weeping vapour velocity Chapter 10 Distillation Columns 125 (4) Estimate Tower Diameter Calculate Flooding Velocity → assume plate spacing = 0.5 m L ρ F = V C & R Eq. 11.82 LV V ρ L 1.10 F = 0.597 = 0.0227 Top LV 791 ⇒ K = 0.090 From Fig. 11.27, next page 1 0.7 Bottom F = 2.65 = 0.0718 LV 954 ⇒ K = 0.083 1 Chapter 10 Distillation Columns 126 Top of column Bottom of column 0.09 0.083 0.0227 0.0718 Chapter 10 Distillation Columns 127 Correction for surface tension for K 0.2 1 K σ b = b Standard surface tension, σ, = 20 x 10-3 N/m K σ a a 0.2 19 ( ) K = 0.09 = 0.089 Top 1 20 0.2 57 ( ) Bottom K = 0.083 = 0.1023 1 20 ρ −ρ Velocity at flooding point = u (vapour) u = K L V f f 1 ρ V 791−1.10 ( ) u = 0.089 = 2.385m / s Top f 1.10 954 − 0.7 ( ) Bottom u = 0.1023 = 3.775m / s f 0.7 If the velocity of vapour in the column exceeds this value, the pressure drop between each plate will become too high and the pressure will push the liquid back up the downcomer. This is known as flooding. Chapter 10 Distillation Columns 128 High vapour velocity Low vapour velocity P P 2 2 P P 1 1 Normal Flooding operation P >> P 1 2 P > P 1 2 Needed to drive vapor at faster rate Chapter 10 Distillation Columns 129 Lecture 24 129 Determine Operating Velocity, u →Design for 85% flooding i.e. Operating Vel. = 0.85 flooding Vel. Top u = (2.385 m/s) (0.85) = 2.03 m/s Bottom u = (3.775 m/s) (0.85) = 3.21 m/s Determine Column Diameter Max. Vapour volumetric flow rate, V ( ) 5930.96kg / hr = = 3 top V ( ) 1.498m / s ( ) 3 3600s / hr 1.1kg / m ( ) 3400.16kg / hr = = 3 Bottom V ( ) 1.349m / s ( ) 3 3600s / hr 0.7kg / m V = A v 130 130 Net area required, A n 3 1.498m / s Top A = = 0.739m2 n 2.03m / s 0.88 0.12 3 1.349m / s A Bottom A = = 0.420m2 n A d n 3.21m / s Liquid Leaving → Assume downcomer is 12% of Tray Liquid total column cross sectional area Entering Tray Total column area, A c 2 0.739m Top A = = 0.840m2 c 0.88 2 0.420m Bottom A = = 0.477m2 c 0.88 A + A = A n d c Chapter 10 Distillation Columns 131 Column diameter, D c Note: πD2 The top of the column (above feed) A = c c is larger in diameter than the 4 bottom of the column (below feed). 0.84×4 Top D = =1.034 In many cases you need to design c π the two different sections (top and 0.477×4 bottom) differently. Bottom D = = 0.779m c π For this example only: Use the same dia. above and below feed, Reduce perforated area for plates below feed Choose the nearest std. Pipe size = DN 1050 with O.D. = 1067 mm Wall thickness: Sch. 40 = 9.5 mm std Sch. 80 = 12.7 mm extra strg Select Sch. 80 to give i.d. = 1042 mm Chapter 10 Distillation Columns 132 Review slide Five Feed Plate Scenarios: L V L V L V n n n n n n F F F A B C L V L V L V m m m m m m L V L V n n n n F F D E L V L V m m m m Chapter 10 Distillation Columns 133 Top of column 0.18 Bottom of column 0.0078 Chapter 10 Distillation Columns 134 0.0718 0.0227 14 ENTRAINMENT Review slide - Liquid droplets are carried along by gas at high velocity - See Fig. 11.29, C&R, Vol. 6, next page - ψ = f(F , % flooding) LV Where ψ = fractional entrainment moles liquid entrained = --------------------------------------------- moles of liquid (flow + entrained) mass liquid entrained ≈ --------------------------------------------- mass of liquid (flow + entrained) operating velocity (based on net area) - % flooding = -------------------------------------------------- flooding velocity (based on net area) u n = Eq. 11.81, C&R, Vol. 6 u f - As a rough guide, choose ψ < 0.1 - Ideally ψ→ 0, realistically this does not occur. Chapter 10 Distillation Columns 135 DISTILLATION COLUMN DESIGN a. Process design b. Physical column design c. Plate hydraulic design d. Mechanical design Chapter 10 Distillation Columns 136 PLATE DESIGN PROCEDURE 1. Establish maximum and minimum vapour and liquid flow rates. 2. Collect system physical properties 3. Select a trial tray spacing 4. Estimate tower diameter Limiting factor is flooding vapour velocity Check entrainment now and reduce % flooding as necessary 5. Choose the pattern of liquid flow 6. Make provisional plate design - Downcomer area, active area, hole area, plate thickness, etc. 7. Check weeping vapour velocity Chapter 10 Distillation Columns 137 (5) Liquid Flow Pattern ' L = m Max. Liquid volumetric flow rate ρ L ( ) 9010.21kg / hr Bottom L = ( ) m ( ) 3 3600s / hr 954kg / m = 2.623×10−3 m3 / s Fig. 11.28, C&R, Vol. 6 next page Use single pass cross flow √ ok Chapter 10 Distillation Columns 138 (5) Liquid Flow Pattern ' L = m Max. Liquid volumetric flow rate ρ L Fig. 11.28, C&R, Vol. 6 next page Use reverse flow √ ok Chapter 10 Distillation Columns 139 0.0026 m3/s bottom 0.0012 m3/s top . . . . . . Chapter 10 Distillation Columns 140 PLATE DESIGN PROCEDURE 1. Establish maximum and minimum vapour and liquid flow rates. 2. Collect system physical properties 3. Select a trial tray spacing 4. Estimate tower diameter Limiting factor is flooding vapour velocity Check entrainment now and reduce % flooding as necessary 5. Choose the pattern of liquid flow 6. Make provisional plate design - Downcomer area, active area, hole area, plate thickness, etc. 7. Check weeping vapour velocity Chapter 10 Distillation Columns 141 (6) Provisional Plate Design (Bottom of the Column) Use a typical plate layout as in notes l A w = 0.76 d = 0.12 D A Calming c c Zone Downcomer area = 12% 50 mm 50 mm Strips for calming zone = 50 mm wide Edge strip for support = 50 mm wide Hole area = 10% of active area l D w c A d A p Edge Strip 50 mm Chapter 10 Distillation Columns 142 Defining some specific areas: D = Col. Dia. = 1.042m c π( 1.042m)2 A = Col. Area = = 0.853m2 c 4 0.12 A = Downcomer area = (0.12) A d c A = 0.102 m2 d A = Net area = A – A = 0.751 m2 n c d A = Active area = A – 2A = 0.648 m2 a c d A = Hole area = (0.1) A = 0.0648 m2 h a A = A – calming zone – edge strip p a 0.76 Chapter 10 Distillation Columns 143 Chapter 10 Distillation Columns 144 Weir & Plate Dimensions Weir length = l = (0.76) D = 0.792 m w c → Assume weir height = h = 50 mm w Stainless steel plate Plate thickness = 3 mm Hole diameter = 5 mm Chapter 10 Distillation Columns 145 PLATE DESIGN PROCEDURE 1. Establish maximum and minimum vapour and liquid flow rates. 2. Collect system physical properties 3. Select a trial tray spacing 4. Estimate tower diameter Limiting factor is flooding vapour velocity Check entrainment now and reduce % flooding as necessary 5. Choose the pattern of liquid flow 6. Make provisional plate design - Downcomer area, active area, hole area, plate thickness, etc. 7. Check weeping vapour velocity Chapter 10 Distillation Columns 146 (7) Check for Weeping ( ) 9010.2kg / h = Max. Liquid rate (bottom) ( ) 3600s / hr L' = 2.503kg / s m Given a max of 80% turn down (flexible operation) L’ (t.d.) = (0.8)(2.503kg/s)=2.002kg/s m Extent of weeping = f (h + h , v ) w ow h Height on the weir and over the weir Vapour velocity through the holes Chapter 10 Distillation Columns 147 2/3 L note : L' = L h = 750 w m w ow ρ l w w 2/3 ( ) 2.503kg / s Max h = 750 =16.64mm ow kg Rate 954 (0.792m) 3 m 2/3 Turned Down ( ) 2.002kg / s h = 750 =14.34mm ow kg ( ) 954 0.792m 3 m h = 50mm w Chapter 10 Distillation Columns 148 Normal Operation h + h = 66.64 mm w ow Turned down operation h + h = 64.34 mm w ow From Fig. 11.30, next page, we get K = 30.4 2 For both (h + h ) w ow K does not vary much for these cases 2 Chapter 10 Distillation Columns 149 30.4 64.3, 66.6 Chapter 10 Distillation Columns 150 K = weeping corrections factor 2 ( ) K − 0.90 25.4 − d u = 2 h h ρ 1/2 V u = vapour velocity at holes with weeping h ( ) 30.4 − 0.90 25.4 −5mm u = =14.39m / s h 1 (0.7kg / m3 ) 2 Turned down operating vapour velocity, v , at holes h ( ) ( ) 3 1.349m / s 0.8 v = =16.6m/ s ( ) h 2 0.065m A h Turned down operating velocity > weeping point velocity Turned down v > u h h 16.6 m/s > 14.39 m/s √ ok. Chapter 10 Distillation Columns 151 Implying: We want v > u h h velocity through holes > weeping velocity d = 5 mm is ok h If weeping occurs, then K & u are too 2 h A = 0.1 A is ok big, h a l = 0.76 D - Reduce d w c h is ok or - Increase l A = 0.12 A w d c or - Reduce h + h w ow h = 50 mm is ok w If weeping occurs, then v is too small, h h ∼ 14-16 mm is ok ow - Reduce A h Chapter 10 Distillation Columns 152 8. Check plate pressure drop 9. Check downcomer back – up - Tray spacing, residence time 10.Check plate layout - Hole area, perforated area, hole pitch, calming zone support strips. 11.Check entrainment 12.Recalculate % flooding based on chosen column diameter 13.Optimize the design 14.Finalize the design Chapter 10 Distillation Columns 153 (8) Pressure Drop Across Plates Total plate pressure drop, h t h = h + h + h + h t w ow d r Max. Vel. of vapour through holes (bottom of the column) ( ) 3 1.349m / s v = = 20.75m / s h ( ) 2 0.065m From, Fig. 11.34, next page Plate Thickness 3mm = = 0.6 Hole dia 5mm A / A ≈ A / A = 0.1 ⇒ C = 0.74 h p h a 0 Chapter 10 Distillation Columns 154 0.74 Chapter 10 Distillation Columns 155 Dry plate head loss, h d 2 v ρ h = 51 h V d C ρ 0 L 2 20.75m/ s 0.7 h = 51 = 29.43mm liquid d 0.74 954 Residue head: 12.5×103 h = r ρ L 12.5×103 h = =13.10mm liquid r 954 Chapter 10 Distillation Columns 156 Total plate pressure drop, h t h = h + h + h + h t w ow d r h = 50 +16.64 + 29.43 +13.10 t = 109.2mm liquid Note that 100 mm was assumed to calculate pressure at bottom of column. √ ok here. Calculations may be revised if h is too big t or too small. But small changes in physical properties have little effect on plate design. Chapter 10 Distillation Columns 157 Note: We assumed 100mm H O head between plates 2 ρ gh = ρ gh liq liq H2O H2O and ρ = 954 kg/m3 at the column bottom liq thus the equivalent water head is (1000) h = (954) 109.2 mm H2O h = 104.2 mm, even closer to 100mm H2O Chapter 10 Distillation Columns 158 Assumptions check: ∆P = 100 mm/plate is ok Estimated temp: Top 82oC Bottom 105oC are ok Estimated physical properties are ok D = 5 mm h Re-confirmed, ok A = 0.1 A h a Chapter 10 Distillation Columns 159 8. Check plate pressure drop 9. Check downcomer back – up - Tray spacing, residence time 10.Check plate layout - Hole area, perforated area, hole pitch, calming zone support strips. 11.Check entrainment 12.Recalculate % flooding based on chosen column diameter 13.Optimize the design 14.Finalize the design Chapter 10 Distillation Columns 160 (9) Downcomer Liquid Back up → Take h = h - 10 ap w h = 50 – 10 = 40 mm ap Area under apron = A =l h ap w ap A =(0.792m) (0.04m) = 0.0317m2 ap A =0.1023m2 (as obtained before) d Use smaller of A & A ap d ∴ Use A = 0.0317 m2 m Chapter 10 Distillation Columns 161 Chapter 10 Distillation Columns 162 Downcomer head loss, h dc 2 L h =166 wd dc ρ A L m ( ) 2 2.503kg / s h =166 =1.137mm liquid ( )( ) dc 954kg /m3 0.0317m2 Back up in the downcomer h = h + h + h + h b w ow t dc h = 50 + 16.64 + 109.2 + 1.14 = 176.98 mm b h ∼ 0.18 m b (available height) = ½ (plate spacing + weir height) = ½ (0.5 + 0.05) = 0.275 m Downcomer Backup < available height 0.18m < 0.275m √ ok So tray spacing is acceptable Chapter 10 Distillation Columns 163 Chapter 10 Distillation Columns 164 Check residence time, t Implying: r ∆P/plate is ok A h'ρ t = d b L r L Tray spacing = 0.5 m is ok wd l & A are ok ( ) ( ) w d ( ) 2 3 0.1023m 0.18m 954kg / m t = r ( ) Apron clearance 2.503kg / s h = 40 mm is ok ap h & h are ok = 7.01s (in downcomer) w ow Vapour flow rate is ok t > 3 sec., ∴ satisfactory r Chapter 10 Distillation Columns 165 DISTILLATION COLUMN DESIGN a. Process design b. Physical column design c. Plate hydraulic design d. Mechanical design Chapter 10 Distillation Columns 166 8. Check plate pressure drop 9. Check downcomer back – up - Tray spacing, residence time 10.Check plate layout - Hole area, perforated area, hole pitch, calming zone support strips. 11.Check entrainment 12.Recalculate % flooding based on chosen column diameter 13.Optimize the design 14.Finalize the design Chapter 10 Distillation Columns 167 (10) Trial layout D = 1.042 m column diameter c Calming l = 0.792 m weir length Zone w 50 mm 50 mm Allow 50 mm unperforated strip around the edge of plate Allow 50 mm wide calming zone l w D c A d A p Edge Strip 50 mm Chapter 10 Distillation Columns 168 Perforated Area l w = 0.76 ⇒ θ = 99o Fig 11.32 c D c Angle subtend by unperforated edge = 180o – 99o = 81o Mean length of unperforated edge strip 2×81 ( )( ) = 1.042 − 0.05m π =1.402m 360 Area of unperforated edge strip =(1.402m) ( 0.05m) = 0.0701m2 Chapter 10 Distillation Columns 169 Chapter 10 Distillation Columns 170 π Circumference = D Chapter 10 Distillation Columns 171 π Length = D (81/360) Chapter 10 Distillation Columns 172 π( Length = D - 50mm) (81/360) Chapter 10 Distillation Columns 173 Straighten out the arc then π( Area = D - 50mm) (81/360) * 50 mm Chapter 10 Distillation Columns 174 Straighten out the arc then π( Area = D - 50mm) (81/360) * 50 mm Chapter 10 Distillation Columns 175 Perforated Area l w = 0.76 ⇒ θ = 990 Fig 11.32 c D c Angle subtend by unperforated edge = 180o – 99o = 81o Mean length of unperforated edge strip 2×81 ( )( ) = 1.042 − 0.05m π =1.402m 360 Area of unperforated edge strip =(1.402m) ( 0.05m) = 0.0701m2 Chapter 10 Distillation Columns 176 ’ Don t worry about minor Error due to Overlapping triangles Chapter 10 Distillation Columns 177 Area of calming zone = (0.792 m ) (0.05m) (2) = 0.0792 m2 Total area for perforation, A p A = A – edge strip – calming zone p a = 0.648 – 0.0701 – 0.0792 m2 = 0.499 m2 Chapter 10 Distillation Columns 178 2 A 0.065m h = = 0.130 2 A 0.499m p l From Fig 11.33 ⇒ p = 2.7 √ ok d h This is satisfactory l p ∼ Between 2.5 to 4.0 is recommended d h l = hole pitch = 13.5 mm for d = 5 mm p h l p > 2.0 is usually satisfactory d h Chapter 10 Distillation Columns 179 0.13 2.7 Chapter 10 Distillation Columns 180 No of holes Area of 1 hole with 5 mm diameter ( )2 π 0.005 Area = =1.964×10−5 m2 4 2 0.065m = No of holes 1.964×10−5 m2 / hole = 3310 holes Chapter 10 Distillation Columns 181 8. Check plate pressure drop 9. Check downcomer back – up - Tray spacing, residence time 10.Check plate layout - Hole area, perforated area, hole pitch, calming zone support strips. 11.Check entrainment 12.Recalculate % flooding based on chosen column diameter 13.Optimize the design 14.Finalize the design Chapter 10 Distillation Columns 182 (11) Check for Entrainment (Bottom of column) calculate % flooding for A n ( ) 3 V 1.349m / s u = m = =1.794m / s ( ) v 2 A 0.7507m n 1.794m / s % flooding = ×100% = 47.52% The % flooding is less than 3.775m / s we specified due to the increased column cross sectional area. operating velocity % flooding = flooding velocity we had F = 0.0718, from Fig 11.29 LV Ψ = 0.0078 acceptable, less than 0.1 Chapter 10 Distillation Columns 183 Review slide Defining some specific areas: D = Col. Dia. = 1.042m c π( 1.042m)2 A = Col. Area = = 0.853m2 c 4 0.12 A = Downcomer area = (0.12) A d c A = 0.102 m2 d A = Net area = A – A = 0.751 m2 n c d A = Active area = A – 2A = 0.648 m2 a c d A = Hole area = (0.1) A = 0.0648 m2 h a A = A – calming zone – edge strip p a 0.76 Chapter 10 Distillation Columns 184 Top of column 0.18 Bottom of column 0.0078 Chapter 10 Distillation Columns 185 Check for Entrainment (Top of Column) 85% flooding (as determined earlier) F = 0.0227 (as determined earlier) LV Ψ= 0.18 not very good because, > than 0.1 Top and bottom of column should really be different designs. We will go back through the whole process later for the top of the column, to specify the top column diameter different from the bottom, and follow through the whole design process for the top section. Chapter 10 Distillation Columns 186 Plate Specification Summary Turn down ratio = 80% max rate Plate thickness = 3 mm Plate diameter = 1.042 m d.c. material = stainless steel Hole size = 5 mm Plate spacing = 0.5m Hole pitch = 13.5 mm ∆ Weir length = 0.792 m mm liquid No of holes = 3310 Pressure drop = 109 Plate mm water Plate material = stainless steel Pressure drop = 104 Plate Chapter 10 Distillation Columns 187 No of plates = 20 + reboiler Man/access hole: One every 10 trays ⇒ 3 holes Residence time = 7 secs Connection nozzles Weir height = 50 mm Reflux drums Liquid Flow = cross flow single pass External supports Apron clearance = 40 mm T & P monitoring devices Chapter 10 Distillation Columns 188 Other Specifications Condenser duty Reboiler duty Pressure vessel for shell? Location of feed inlet - Ideal = 9 0.55 efficiency 9/0.55 = 16th tray Add 1.2 m at top – Vapour Disengagement Add 2 m at bottom – Liquid reservoir Vapour return Chapter 10 Distillation Columns 189 Height = 13.2 m Diameter = 1.042 m 20 plates Feed at 16th plate Flanged Nozzles Equipment Sketch, not drawing Chapter 10 Distillation Columns 190 Chapter 10 Distillation Columns 191 Top of column Revised 0.09 Bottom of column 0.0078 Chapter 10 Distillation Columns 192 0.0026 m3/s bottom 0.0012 m3/s top . . . . . . Chapter 10 Distillation Columns 193 Chapter 10 Distillation Columns 194 16 82 Chapter 10 Distillation Columns 195 Hole area Chapter 10 Distillation Columns 196 Chapter 10 Distillation Columns 197 30.4 65 Chapter 10 Distillation Columns 198 Chapter 10 Distillation Columns 199 Chapter 10 Distillation Columns 200 0.74 Chapter 10 Distillation Columns 201 Apron clearance Small one Chapter 10 Distillation Columns 202 131 backup 9 9 6 Time > 3s Chapter 10 Distillation Columns 203 Chapter 10 Distillation Columns 204 120 0.25 0.88 Chapter 10 Distillation Columns 205 Chapter 10 Distillation Columns 206 Chapter 10 Distillation Columns 207 0.136 2.6 Chapter 10 Distillation Columns 208
