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7/27/2019 Stripping Coloumn
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CHAPTER 6 DESIGN OF EQUIPMENTS
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DESIGN OF STRIPPING COLUMN
Before going in details of stripping column design first we see what is
stripping and what its industrial uses are.
STRIPPING
Unit operation where one or more components of a liquid stream are removed
by being placed in contact with a gas stream that is insoluble in the liquid stream.
OR
Stripping is a physical separation process where one or more components are
removed from a liquid stream by a vapor stream. In industrial applications the liquid
and vapor streams can have co-current or countercurrent flows. Stripping is usually
carried out in either a packed or tray column.
THEORY
Stripping works on the basis of mass transfer. The idea is to make the
conditions favorable for the more volatile component in the liquid phase to transfer to
the vapor phase. This involves a gas-liquid interface that the more volatile component
must cross.
EQUIPMENT USED FOR STRIPPING
Stripping is mainly conducted in trayed towers (plate columns) and packed
columns, and less often in spray towers, bubble columns and centrifugal contactors.
PLATE COLUMN
Packed columns consist of a vertical column with liquid flowing in from the
top and flowing out the bottom. The vapor phase enters from the bottom of the column
and exits out of the top. Inside of the column are trays or plates. These trays force theliquid to flow back and forth horizontally while forcing the vapor bubbles up through
holes in the trays. The purpose of these trays is to increase the amount of contact area
between the liquid and vapor phases.
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PACKED COLUMN
Packed columns are similar to plate columns in that the liquid and vapor flows
enter and exit in the same manner. The difference is that in packed towers there are no
trays. Instead, packing is used to increase the contact area between the liquid and
vapor phases. There are many different types of packing used and each one its
advantages and disadvantages. The gas liquid contact in a packed bed column is
continuous, not stage-wise, as in a plate column. The liquid flows down the column
over the packing surface and the gas or vapor, counter-currently, up the column. In
some gas-absorption columns co-current flow is used. The performance of a packed
column is very much dependent on the maintenance of good liquid and gas
distribution throughout the packed bed, and this is an important consideration in
packed-column design.
CHOICE OF PLATE OR PACKED COLUMN
The choice between a plate and packed column for a particular application can
only be made with complete assurance by costing each design. However, this will not
always be worthwhile or necessary, and the choice can usually be made on the basis of
experience by considering main advantages and disadvantages of each type; which are
listed below:
1. Plate columns can be designed to handle a wider range of liquid and gas flow-rates than packed columns.
2. Packed columns are not suitable for very low liquid rates.3. The efficiency of a plate can be predicted with more certainty than the
equivalent term for packing (HETP or HTU).
4. Plate columns can be designed with more assurance than packed columns.There is always some doubt that good liquid distribution can be maintained
throughout a packed column under all operating conditions, particularly in
large columns.
5. It is easier to make provision for cooling in a plate column; coils can beinstalled on the plates.
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6. It is easier to make provision for the withdrawal of side-streams from platecolumns.
7. If the liquid causes fouling, or contains solids, it is easier to make provision forcleaning in a plate column; manways can be installed on the plates. With small
diameter columns it may be cheaper to use packing and replace the packing
when it becomes fouled.
8. For corrosive liquids a packed column will usually be cheaper than theequivalent plate column.
9. The liquid hold-up is appreciably lower in a packed column than a platecolumn. This can be important when the inventory of toxic or flammable
liquids needs t be kept as small as possible for safety reasons.
10.Packed columns are more suitable for handling foaming systems.11.The pressure drop per equilibrium stage (HETP) can be lower for packing than
plates; and packing should be considered for vacuum columns.
12.Packing should always be considered for small diameter columns, say less than0.6 m, where plates would be difficult to install, and expensive.
Packed column is selected for our operation.
TYPES OF PACKING
The principal requirements of a packing are that it should:
Provide a large surface area: a high interfacial area between the gas andliquid.
Have an open structure: low resistance to gas flow. Promote uniform liquid distribution on the packing surface. Promote uniform vapor gas flow across the column cross-section.
Many diverse types and shapes of packing have been developed to satisfy these
requirements. They can be divided into two broad classes:
1. Packings with a regular geometry: such as stacked rings, grids and proprietarystructured packings.
2. Random packings: rings, saddles and proprietary shapes, which are dumpedinto the column and take up a random arrangement.
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Grids have an open structure and are used for high gas rates, where low pressure
drop is essential; for example, in cooling towers. Random packings and structured
packing elements are more commonly used in the process industries.
RANDOM PACKING
The principal types of random packings are shown
Rasching Rings Pall Rings
Berl Saddles Intalox Saddles
Super Intalox Saddles Metal Hypac
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Raschig rings are one of the oldest specially manufactured types of random
packing, and are still in general use. Pall rings are essentially Raschig rings in which
openings have been made by folding strips of the surface into the ring. This increases
the free area and improves the liquid distribution characteristics. Berl saddles were
developed to give improved liquid distribution compared to Raschig rings. Intalox
saddles can be considered to be an improved type of Berl saddle; their shape makes
them easier to manufacture than Berl saddles. The Hypac and Super Intalox packings
shown in can be considered improved types of Pall ring and Intalox saddle
respectively.
Ring and saddle packings are available in a variety of materials: ceramics,
metals, plastics and carbon. Metal and plastics (polypropylene) rings are more
efficient than ceramic rings, as it is possible to make the walls thinner.
Raschig rings are cheaper per unit volume than Pall rings or saddles but are
less efficient, and the total cost of the column will usually be higher if Raschig rings
are specified. For new columns, the choice will normally be between Pall rings and
Berl or Intalox saddles.
The choice of material will depend on the nature of the fluids and the operating
temperature. Ceramic packing will be the first choice for corrosive liquids; but
ceramics are unsuitable for use with strong alkalies. Plastic packings are attacked by
some organic solvents, and can only be used up to moderate temperatures. So are
unsuitable for distillation columns. Where the column operation is likely to be
unstable, metal rings should be used, as ceramic packing is easily broken.
PACKING SIZE
In general, the largest size of packing that is suitable for the size of column
should be used, up to 50 mm. Small sizes are appreciably more expensive than thelarger sizes. Above 50 mm the lower cost per cubic meter does not normally
compensate for the lower mass transfer efficiency. Use of too large a size in a small
column can cause poor liquid distribution.
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CHAPTER 6 DESIGN OF EQUIPMENTS
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Recommended size ranges are:
Column diameter Use packing size
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drop are needed. The cost of structured packings per cubic meter will be significantly
higher than that of random packings, but this is offset by their higher efficiency.
Selected packing is random because its cheaper and there are no difficult or
vacuum separation requirements.
CHOICE OF RANDOM PACKING
Factors to be considered
1. Void fraction2. Effective surface3. Packing size4. Maximum operating temperature5. Mechanical strength6. Material selectionPacking used here is 0.038m ceramic intalox saddle because
1. One of the most efficient packings2. Little tendency to nest and block areas of bed3. Gives a fairly uniform bed4. Higher flooding point5. Lower pressure drop
PACKING PROPERTIES
Nominal size
1.5"
0.038mm
Packing factor F 170 Specific gravity (g/cm3) 2.3
Package density (kg/m3) 580 Water absorption (%) 99.6
Surface area (m2/m3) 180 Max operating temp. 1100
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MATERIAL BALANCE
Component 10 17 11 19
Propylene 202 201.23 0.80
Hydrogen 549 1.88 547.27 3.64
n-Butanal 13726.88 266.37 13460.5
Iso-Butanal 315.94 8.83 307.11
CO 8163 48.64 8141.61 69.98
propane 44.40 43.76 0.633
Total 8712.00 14339.44 9201.94 13839.50
Material In = Material Out
Stream 10 + Stream 17 = Stream 11 + Stream 19
Total = 23041.44 kg/hr = Total = 23041.44 kg/hr
STRIPPER FEED (17)
Mass flow rate= 14339.44kg/hr
Molar flow rate= 203.53kgmol/hr
Mole Fraction:
Propylene: 0.023
n-Butanal: 0.936
iso-Butanal: 0.021
STRIPPING GAS (10)
Mass flow rate= 8712kg/hr
Molar flowrate= 566.04kgmol/hr
Mole Fraction:
Hydrogen: 0.484
CO: 0.516
STRIPPED GAS (11)
Mass flow rate=9201.94kg/hr
Molar flow rate= 574.1kgmol/hr
Mole Fraction:
Propylene: 0.0083
Hydrogen: 0.476
CO: 0.506
Product (19)
Mass flow rate= 13839.5 kg/hr
Molar flowrate= 195.57kgmol/hr
Mole Fraction:
N-Butanal: 0.956
Iso-Butanal: 0.0218
Propylene: 0.000097
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PROCESS CONDITIONS
Stream Temperature (K) Mass Flowrate (kg/hr)
Liquid Inlet 313 14339.79
Liquid Outlet 388 13842
Gas Inlet 483 8712
Gas Outlet 317 9209
Components\Mole
fraction
10 17 11 19
Propylene 0.0236 0.00834 0.00009
Hydrogen 0.4849 0.00463 0.4767 0.00932
n-Butanal 0.9366 0.00644 0.9559
Iso-Butanal 0.02155 0.00021 0.02180
CO 0.5150 0.00853 0.50655 0.01278
Propane 0.00495 0.00173 0.00007
DESIGN APPROACH
1. Determining the diameter of column.2. Determining the HETP of packing3. Determining Number of transfer units for the required separation.4. Determining the height of overall transfer units.5. Determining the total height of column.6. Determining the flooding velocity.7. Verifying the pressure drop across the column.8. Mechanical Design
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DIAMETER OF COLUMN
The column diameter is calculated by following formula
= .
..
G= Mass flowrate of gas
G= Mass flux of gas
To find G first find the flow parameter X as followed
L= Mass flow rate of liquid stream
g = Density of gas
l = Density of liquid
x = 0.236
Pressure drop range for strippers and absorbers is 147Pa to 490Pa.
Pressure drop of 294 Pa/m of a packed bed is selected.
Value of gas mass flux G from figure 12 Chapter 1 Rule of thumbs for chemical
engineers 3ed.
G=0.7 kg/m2 s Diameter of packed column is 0.603m.
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HEIGHT EQUIVALENT OF THEORETICAL PLATE (HETP)
HETP is calculated as
HETP =
Where
A= Size of packing = 38mm
= Surface tension of liquid = 29.2 mN/m
= Overall viscosity of feed stream = 0.000414 Pa s
HETP = 0.0357m
NUMBER OF TRANSFER UNITS (NTU)
Number of transfer units is calculated as followed.
= +
Where
=L/HG = 0.0045
L=Molar liquid flow rate = 203 kmol/hr
G=Molar gas flow rate = 566 kmol/hr
H=Henrys Law Constant = 79.52 Pa/mol fraction
x2=Solute contents in liquid inlet stream mol fraction = 0.0083
x1=Solute contents in liquid exit stream mol fraction = 0.00009
y1=Solute contents in gas at bottom mol fraction = 0
Ntotal= 4.5 ~ 5
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HEIGHT OF OVERALL GAS TRANSFER UNIT (HOG)
Height of overall gas transfer unit is calculated as followed.
=
Hog = 1.45m
COLUMN HEIGHT
Packing height is calculated as followed
Htotal = Hog x Ntotal
Htotal = 7.28m
Giving 0.457m allowance for disengagement of vapors at top and at bottom
for liquid. Htotal = 8.194 m
FLOODING VELOCITY
Flooding velocity requires the calculation of the superficial velocity that is
given as
Vog = G/Ag
Vog = 5.88m/s
As general rule superficial velocity is 40% to 60% of the flooding velocity.
Taking superficial velocity as 60% of the flooding velocity, then the flooding velocity
is given as
VF = 9.8m/s
CHECK FOR PRESSURE DROP
For pressure drop calculation we required flow factor and gas mass velocity.
Flow factor X is calculated as
X = 2.66
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Gas mass velocity is calculated with following formula.
Where
mv = Mass flow rate of gas stream
A = Area of column
G = 0.703 kg/m2
s
Now the Y ordinate of figure 12 Chapter 1 Rule of thumbs for chemical engineers 3ed
is calculated by the given formula.
=
.
Y = 0.723
Value of pressure drop for this value of Y is 294Pa/m of packing height.
MECHANICAL DESIGN
THICKNESS OF SHELL
Material selection: Stainless Steel 304
Shell thickness is calculated as given below
ts =Thickness of shell
p=Design pressure = O.P. 1.1 = 55.265 N/mm2
D=Inside diameter = 0.602 m
f=Design stress = 145 N/mm2
J=Joint efficiency = 85%
c= Corrosion allowance = 2mm
ts = 82mm
A
vm
G
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SHELL WEIGHT
Shell weight is calculated as
Shell Weight = Volume of shell Density of shell material
Shell weight = 12670 kg
HEAD SELECTION AND THICKNESS
2:1 Elliptical head has been selected because it is used for high pressure requirements
and its manufacturing is easy as compared to other types. Material of construction is
low alloy steel.
Thickness of elliptical head is calculated with following formula
= + .Where
th =Thickness of head
p =Design pressure = O.P. 1.1 = 55.25N/mm2
Cs=Stress concentration factor = 1.77
Rc=Crown Radius = 0.602m
F =Design stress = 240N/mm2
J =Joint efficiency = 85%
C = Corrosion allowance = 2
th = 83 mm
HEAD WEIGHT
Weight of elliptical head is calculated as
= W = 58kg
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SUPPORT DESIGN
Type of support selected is skirt type support for vertical vessels. Material of
construction is construction stainless steel SS-301.
First we find maximum dead weight of vessel when full of water.
Max. Dead weight = 25.5 kN
Weight of column = 202 kN
Weight of Packing = 2.364 kN
Wind Loading
=
Where
w= Dynamic wind pressure = 2790N/m2
x= Length of column = 9.11m
Ms = 69813 N
Take test thickness of support say 220mm.
Tensile strength of support
= + Where
Ms = Wind loading
Ds = Inside diameter of shell
ts = Thickness of support
bs= 0.81 N/mm2
Test compressive strength of support
() = +
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Where
W= Dead weight of column when full of water
ws (test) = 0.044 N/mm2
Operational compressive strength of support
() = + Where
W= Total weight of column
ws (operational) = 0.359 N/mm2
Maximum tensile strength of support
= ()Max s (Tensile) = 770 kPa
Maximum compressive strength of support
= ()Max s (Compressive) = 455 kPa
Check for taken thickness of support
Following two conditions must be satisfied.
1.
() < Where
fs= Design stress = 240N/mm2
J= Joint efficiency = 85%
s=Base angle (normally taken as 90)
0.0226 < 0.770
Condition 1 is satisfied.
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2.
E= Young Modulus of elasticity = 11.35 N/mm2
0.455 < 0.518
Condition 2 is satisfied.
So thickness of support = 220mm
PACKING SUPPORT
The best design of packing support is one in which gas inlets are provided
above the level where the liquid flows from the bed; such as the gas-injection type.
These designs have a low pressure drop and no tendency to flooding. They are
available in a wide range of sizes and materials: metals, ceramics and plastics.
Gas-injection type packing support
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LIQUID DISTRIBUTER
The pan-type construction provides liquid level balance. Vapor passage is provided by
circular gas risers as well as around the periphery of the pan.
Pan-type distributer with bottom holes
SPECIFICATION SHEET
Name of equipment Stripper
Type Packed column
No. of equipment 1
Type of packing 0.038m ceramic Intalox saddles
Material of construction Low alloy steel 950X
Diameter of column 0.602m
Area of column 1.138m2
NTU 5
Hog 1.45m
Height of column 9.11m
Weight of shell 12671kg
Pressure drop 294Pa/m of packing