CHAPTER No.9
DESIGN OF DISTILLATION COLUMN
In industry it is common practice to separate a liquid mixture by
distilling the components, which have lower boiling points when they are in
pure condition from those having higher boiling points. This process is
accomplished by partial vaporization and subsequent condensation.
9.1 DISTILLATION:-
"Process in which a liquid or vapor mixture of two or more substances is
separated into its component fractions of desired purity, by the
application and removal of heat".
9.2 TYPES OF DISTILLATION COLUMNS
There are many types of distillation columns, each designed to perform
specific types of separations, and each design differs in terms of
complexity.
Batch columns
Continuous columns
Batch Columns
In batch operation, the feed to the column is introduced batch-wise. That
is, the column is charged with a 'batch' and then the distillation process
is carried out. When the desired task is achieved, a next batch of feed is
introduced.
Continuous Columns
In contrast, continuous columns process a continuous feed stream. No
interruptions occur unless there is a problem with the column or
surrounding process units. They are capable of handling high throughputs
and are the more common of the two types. We shall concentrate only on this
class of columns.
9.3 CHOICE BETWEEN PLATE AND PACKED COLUMN
Vapor liquid mass transfer operation may be carried either in plate or
packed column. These two types of operation are quite different. The
relative merits of plate over packed column are as follows:
1-Plate column are designed to handle wide range of liquid flow rates
without flooding.
2-If a system contains solid contents; it will be handled in plate column,
because solid will accumulate in the voids, coating the packing materials
and making it ineffective.
3-Dispersion difficulties are handled in plate column when flow rate of
liquid are low as compared to gases.
4-For large column heights, weight of the packed column is more than plate
column.
5-If periodic cleaning is required, man holes will be provided for
cleaning. In packed columns packing must be removed before cleaning.
6-For non-foaming systems the plate column is preferred.
7-Design information for plate column is more readily available and more
reliable than that for packed column.
8-Inter stage cooling can be provided to remove heat of reaction or
solution in plate column.
9-When temperature change is involved, packing may be damaged.
Our mixture which is to be processed is "Acrylonitrile, Acetonitrile". I've
selected plate column because:
1-System is non-foaming.
2-Temperature is high i.e. 80C.
CHOCE OF PLATE TYPE
There are three main types, sieve tray, bubble cap, valve tray. I've
selected sieve tray because:
1-They are lighter in weight and less expensive. It is easier and cheaper
to install.
2-Pressure drop is low as compared to bubble cap trays.
3-Peak efficiency is generally high.
4-Maintenance cost is reduced due to the ease of cleaning.
9.4 SELECTION CRITERIA OF TRAYS
Cost
Cost of plate depends upon material of construction used.
For mild steel, the ratio of cost between plates is
Sieve plate : valve plate : bubble-cap plate
3 : 1.5 :
1.0.
Capacity
Sieve tray > valve tray > bubble-cap tray
Operating Range:-
It is the range of liquid and vapor flow rates which
must be above the weeping conditions and below the flooding conditions.
Operating range flexibility comparison is
Bubble cape tray > Valve tray > Sieve tray
For good design, sieve plate gives satisfactory operating range.
Pressure drop
Bubble-cap tray > valve tray > sieve tray
Choice of Tray Type (Sieve Tray)
Sieve plates are lighter in weight and less expensive. It is easier
and cheaper to install.
Pressure drop is low as compared to bubble cap trays.
Maintenance cost is reduced due to ease of cleaning.
If properly designed, sieve tray gives desired separation
9.5 MAIN COMPONENTS OF DISTILLATION COLUMNS
Column internals such as trays/plates and/or packing which are used to
enhance component separations.
A reboiler to provide the necessary vaporization for the distillation
process. The liquid removed from the reboiler is known as the bottoms
product or simply, bottoms.
A condenser to cool and condense the vapor leaving the top of the
column. The condensed liquid that is removed from the system is known
as the distillate or top product.
A reflux drums to hold the condensed vapor from the top of the column
so that liquid (reflux) can be recycled back to the column. The
condensed liquid is stored in a holding vessel known as the reflux
drum. Some of this liquid is recycled back to the top of the column
and this is called the reflux.
A schematic of a typical distillation unit with a single feed and two
product streams is shown above.
9.6 FACTORS AFFECTING DISTILLATION COLUMN OPERATION
Vapor Flow Conditions
Adverse vapor flow conditions can cause:
Foaming
Entrainment
Weeping/dumping
Flooding
Foaming
Foaming refers to the expansion of liquid due to passage of vapor or gas.
Although it provides high interfacial liquid-vapor contact, excessive
foaming often leads to liquid buildup on trays. In some cases, foaming may
be so bad that the foam mixes with liquid on the tray above. Whether
foaming will occur depends primarily on physical properties of the liquid
mixtures, but is sometimes due to tray designs and condition. Whatever the
cause, separation efficiency is always reduced.
Entrainment
Entrainment refers to the liquid carried by vapor up to the tray above and
is again caused by high vapor flow rates. It is detrimental because tray
efficiency is reduced: lower volatile material is carried to a plate
holding liquid of higher volatility. It could also contaminate high purity
distillate. Excessive entrainment can lead to flooding.
Weeping/Dumping
This phenomenon is caused by low vapor flow. The pressure exerted by the
vapor is insufficient to hold up the liquid on the tray. Therefore, liquid
starts to leak through perforations. Excessive weeping will lead to
dumping. That is the liquid on all trays will crash (dump) through to the
base of the column (via a domino effect) and the column will have to be re-
started. Weeping is indicated by a sharp pressure drop in the column and
reduced separation efficiency.
Flooding
Flooding is brought about by excessive vapor flow, causing liquid to be
entrained in the vapor up the column. The increased pressure from excessive
vapor also backs up the liquid in the down comer, causing an increase in
liquid holdup on the plate above. Depending on the degree of flooding, the
maximum capacity of the column may be severely reduced. Flooding is
detected by sharp increases in column differential pressure and significant
decrease in separation efficiency.
Reflux Conditions
Minimum trays are required under total reflux conditions, i.e. there is no
withdrawal of distillate. On the other hand, as reflux is decreased, more
and more trays are required.
Feed Conditions
The state of the feed mixture and feed composition affects the operating
lines and hence the number of stages required for separation. It also
affects the location of feed tray.
State of Trays and Packings
Remember that the actual number of trays required for a particular
separation duty is determined by the efficiency of the plate. Thus, any
factors that cause a decrease in tray efficiency will also change the
performance of the column. Tray efficiencies are affected by fouling, wear
and tear and corrosion, and the rates at which these occur depends on the
properties of the liquids being processed. Thus appropriate materials
should be specified for tray construction.
Column Diameter
Vapor flow velocity is dependent on column diameter. Weeping determines the
minimum vapor flow required while flooding determines the maximum vapor
flow allowed, hence column capacity. Thus, if the column diameter is not
sized properly, the column will not perform well.
9.7 DESIGNING STEPS OF DISTILLATION COLUMN
( Calculation of Minimum Reflux Ratio Rm.
( Calculation of optimum reflux ratio.
( Calculation of theoretical number of stages.
( Calculation of actual number of stages.
( Calculation of diameter of the column.
( Calculation of weeping point.
( Calculation of pressure drop.
( Calculation of thickness of the shell.
Calculation of the height of the column.
Process Condition:-
Process Design:-
Temperature of feed = 80o C
Temperature of top product = 75o C
Temperature of bottom product = 98o C
"COMPONENTS "FEED "DISTILLATE "BOTTOM "
" "XF "XD "XW "
"Acrylonitrile ".93 ".98 ".011 "
"Acetonitrile ".04 ".00004 ".94 "
"Acrolein ".02 ".02 ".005 "
"Water ".002 ".00002 ".05 "
Heaviest Key Component = Water
Light Key Component = Acetonitrile
Relative Volatility
Acrylonitrile = 2.85
Water = 1
Calculation of Minimum Reflux Ratio Rm
Using Underwood equation,
As feed is entering as vapors so, q = 0
By trial, ( = 1.88
Using eq. of min. reflux ratio,
putting all values Rm = 2.5
Actual Reflux Ratio:-
The rule of thumb is:
R = (1.2 ------- 1.5) Rmin
R = 1.3 Rmin
R = 3.25
Calculation of Minimum no. of Plates:-
The minimum no. of stages Nmin is obtained from Fenske relation which is,
Nmin = 6
Theoretical no. of Plates:-
Gilliland related the number of equilibrium stages and the minimum reflux
ratio and the no. of equilibrium stages with a plot that was transformed by
Eduljee into the relation ;
From which the theoretical no. of stages to be,
N= 20.
One plates is removed for reboiler, so N = 20-1 = 19
Location of feed Plate:-
The Kirkbride method is used to determine the ratio of trays above and
below the feed point.
From which,
Number of Plates above the feed tray = ND = 12
Number of Plates below the feed tray = NB = 7
Calculation of actual number of stages:-
Tray Efficiency:
Average temperature of column = 75+98/2 = 87oC
Feed viscosity at average temperature = (L = 0.30 Cp
= 0.6
From equation , overall efficiency of column = 60%
So, No. of actual trays = 19/0.6 = 33
Location of feed point = 7/0.6 = 12
Determination of the Column Diameter:-
"Top Conditions "Bottom Conditions "
"Ln= 1210 kgmol/hr "Lm = 1496 kgmol/hr "
"Vn = 1486 kgmol/hr "Vm = 1486 kgmol/hr "
"Average Mol Wt = 53 kg/kgmol "Average Mol wt = 40 kg/kgmol "
"T = 353 K "T = 390 K "
"Liquid density = dl = 807 kg/m³ "Liquid density = dl = 787 kg/m³ "
"Vapour density = dv = 2.01 kg/m³ "Vapour density dv = 1.71 kg/m³ "
Flow Parameter: (For Stripping Section)
FLV = Liquid Vapor Factor = 0.05
Capacity Parameter:
Assumed tray spacing = 18 in.
From Fig (24) of Appendix-B, sieve tray flooding capacity,
Csb(18) = 0.080 m/Sec
Surface tension of system = ( = 26.3 dynes/Cm
Corrected Csb = Csb(18) = 0.084m/Sec
Now Vnf = = 1.57 m/Sec
Maximum volumetric flow rate of vapour at bottom:
= 8.25m3/sec
Net area required
= 6.9 m2
Total area required AT = An +Ad = 0.9 AT
An = 6.9 m2
Down comer area = Ad = 0.10 AT
AT = D2 = 7.7 m2
D = 3 m
Flow Parameter: (For Rectifying section)
FLV = Liquid Vapor Factor = 0.04
Capacity Parameter:
Assumed tray spacing = 18 in.
From Fig (24) of Appendix-B, sieve tray flooding capacity,
Csb(18) = 0.079 m/Sec
Surface tension of system = ( = 27.3 dynes/Cm
Corrected Csb = Csb(18) = 0.103 m/Sec
Now Unf = = 1.49 m/Sec
Maximum volumetric flow rate of vapour at top:
= 9.88 m3/sec
Net area required
= 8.8 m2
Total area required AT = An +Ad = 0.9 AT
An = 8.8 m2
Down comer area = Ad = 0.10 AT
AT = π D2 /4= 9.6 m2
D = 3.3 m
Now as diameter of Striping and Rectifying section varies les than 10% so
there is no need of varying diameter of column
Tray Selection:-
Number of Tray Passes
For one pass design that the liquid flow rate does not exceed 7 to 13
gpm/in of outlet weir .
At this preliminary point in the calculation assume that the weir length is
0.8 times the tower diameter
So weir length = 0.8(2.504) m = 98 inch
For bottom section
= Ln * Ma/dl * 3600
= 334 gal/min
= 334 gal/min / 98 in
= 3.41 gpm / in
For top section
= Ln * Ma/dl * 3600
= 349 gal/min
= 349 gal/min / 98 in
= 3.56 gpm/in
As both top and bottom liquid flow rates are between 7 - 13 gpm/ in so
We have selected single cross flow sieve tray with segmental down comer.
For this type of tray
Other Tray Specs
Down comer area = Ad = 0.10 AT = 0.77 m2
Weir length = Lw = 0.8 DT = 2.504 m
Selected weir height hw = 1.5 in. =0.0381 m
Hole diameter = 3/16'' = 4.75 mm 0.00475=
Assumed tray spacing = 18 in.
(Tray Hydraulics
Active Area:
Aa = At-2Ad
= 6.16 m2
Hole Area:
12% of Aa
Ah = 0.12 * 6.16
= 0.74m2
"Flooding checking: "
" "
"F = (Vf / Vnf)*100 "
"Vf = qv/An "
"Vf = 1.19 m/sec "
"F = 76 % "
"Tray Pressure Drop "
"Ht =Hd + ( Hw + How ) + Hr "
"Hw = 38 mm "
"(b) Dry Tray Pressure Drop "
"Hd = 51 (Uh/Co) 2 ((V /(L) "
"Hd = Dry tray drop. "
"Uh = Hole velocity = Qv/Ah "
"Uh = = 8.95 m/sec "
"Using Fig. 11.34 of Coulson 6. We find "Co " "
"Co = Orifice Coefficient = 0.83 "
"Hd = 51 "
"= 14.67 mm "
"(c) Weir Crest "
"How = 750 ( Lw/(L* Lw) 2/3 "
"Lw = weir length "
"Lw = .8DT "
"Lw = 2.504 m "
"How = 25 mm "
"(d) Residual Head ( Hr) "
"Hr = = 15.9 mm "
"So, "
"Ht =Hd + ( Hw + How ) + Hr "
"= 14.67 + 38 + 25 + 15.9 "
"= 90 m "
"Total Pressure Drop: "
"Pt = (9.8110e-3) Ht "
"= 9.8110e-390787 "
"= 690 Pa = 0.69kPa "
"Total Pressure drop across all Trays = 0.69 33 "
"= 20kPa "
"Estimation of Weep point: "
" "
" "
"HW =38 mm "
"HOW = 25 mm "
"HW + HOW = 38+25 = 63 mm "
"From graph, "
"K2 = 30.5 "
"= 3.99 m/s "
"Actual Min. Vapour Velocity "
"So there will be no Weeping. "
Calculation of Entrainment
As FLV = 0.05 and F = 76%
From Figure 11.29 of coulson 6, we calculate
( = 0.043
( = Fractional Entrainment factor
Since ( < 0.1, so now process is satisfactory
Total no. of holes
Total no. of holes =
dh = 4.762 mm
ah = (3.14.004762)/ 4 = 1.7710-4 m2
Total no. of holes = 0.7392/1.7710-4 = 4176
Pitch calculation
At lw / Dc = 0.8, graph b/w angle subtended by chord & chord
length:
θc = 108°
Angle subtended at plate edge by unperforated edge strip = 180 –
108= 72°
Length of unperforated edge strips = (D– hw) ( *72 / 180
= 3.9 m
Area of unperforated edge strip =Au = hw*3.9
=
0.15 m2
Area of calming zone = Acz = 2*hw*(lw-2*hw)
= 0.18 m2
Total area available for perforations = Ap = Aa – (Au + Acz)
= 5.83 m2
K = 0.905 for Equilateral Triangular Pitch
So
0.7392/5.83 = 0.905(0.0047/hole pitch)2
Hole Pitch = 0.01256 m
Check
Pitch to Hole Diameter ratio = 0.01256/0.00476
= 2.63
( Correct )it should between 2 and 4
So our selection of equilateral triangular pitch is right.
Downcomer Liquid backup/ liquid height in downcomer:
Let, hap= hw-10
= 38.1 mm = 0.015m
Area under apron = hap*lw
Aap = 0.015*
= 0.037 m2
As Aap is less than Ad = 0.59m2
=10.82 mm
=113.89 mm=0.11 m
Since,
HD < 0.5*(Tray spacing +Weir height)
0.11 < 0.375
So, tray spacing is acceptable.
Residence Time:
= 13 sec
As residence time is greater than 3 sec, so satisfactory.
Height of Distillation Column
No. of plates = 33
Tray spacing = 0.4572 m
Distance between 33 plates = 0.457233 = 13 m
Top clearance = 0.5 m
Bottom clearance = 0.5 m
Tray thickness = 4.75 mm/plate
Total thickness of trays = 0.00475 33 = 0.15675 m
Total height of column = 13 + 0.5 + 0.5 + 0.15675 = 14 m
9.8 NOMENCLATURE OF DISTILLATION COLUMN
Aa = Active area (m2)
Ad = down comer area (m2)
Ah = hole area (m2)
An = Net area (m2)
Ada = area under down comer apron (m2)
C = corrosion allowance (in/year)
Csb(20) = capacity parameter for liquids (( = 20
dynes/cm)
Csb = Capacity parameter of liquid
D = Diameter of tower (m)
Df = flow width normal to liquid flow (m)
Dh = hole diameter (m)
e = total entrainment (Kg/sec)
F = feed (Kgmol/hr)
F* = Design % loading
HF = Enthalpy of feed (J/Kgmol K)
HB(l) = Enthalpy of bottom (J/Kgmol K)
HD(L) = Enthlpy of the distillate (J/Kgmol K)
ha = head loss due to created liquid (cm)
hf = froth height on tray (cm)
had = had loss due to down comer apron (cm)
hdc = height of clear liquid in down comer (cm)
hw = weir height (cm)
how = height of liquid crest over weir (cm)
ho = head loss due to bubble formation (cm)
J = weld efficiency factor
L = liquid flow rate (Kg/hr)
Lw = weir length (m)
Nm = min. no. of plates
(Pdry = dry pressure drop (cm)
(PT = total pressure drop (cm)
QL = Liquid flow rate (Kg/m3)
Qv = Vapour flow rate (Kg/ m3)
Qc = condenser duty (J/ m3)
QR = Rebotr duty (J/hr)
QP = Areatio factor
Rh = Hydraluic radius of aerated mass (m)
Reh = Modified Reynold No.
Rm = Min. reflux ration
R = operational reflux ratio
Tm = thickness of shell (mm)
Uh = vapour velocity through holes (m/sec)
Vm = vapour flow rate at bottom (Kgmol/hr)
WF = mass flow rate of feed (Kgmol/hr)
WB(() = bottoms (Kg/hr)
W D(() = mass flow rate distillate (Kg/hr)
Unf = flooding velocity based upon net area (m/sec)
Vd = down comer liquid velocity (m/sec)
( = relative volatility
(L = density of liquid (Kg/m3)
(v = density of vapor (Kg/m3)
(average = average viscosity of feed (Cp)
( = fractional entrainment factor
( = relative froth density
( = surface tension (J/N2)
( = liquid gradient
9.9 SPECIFICATION SHEET:
"Identification "
"Item "Distillation Column "
"Item # "D-108 "
"Type "Sieve Tray "
"No. of item "3 "
"Function "
"The separation of Acrylonitrile "
"Design data "
"No. of trays "33 "
"Operating Pressure "132 kPa "
"Operating Temperature "98 oC "
"Tray spacing "0.45m "
"Tray thickness "4.75 mm "
"Height "14 m "
"Diameter "3 m "
"Reflux ratio "3.25 "
"Hole size and Arrangement "4.75mm equilateral triangular "
" "pitch "
"Liquid density "787 Kg/m3 "
"Vapor density "1.71 Kg/m3 "
"Material of Construction "Stainless stell 18Cr/8Ni Ti "
" "stabilized "
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