Stripping Coloumn

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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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

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Stripping Coloumn