Stripping Column Design Steps

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					CHAPTER 6                                             DESIGN OF EQUIPMENTS

       Before going in details of stripping column design first we see what is
stripping and what its industrial uses are.

       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.


       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.

       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.

       Stripping is mainly conducted in trayed towers (plate columns) and packed
columns, and less often in spray towers, bubble columns and centrifugal contactors.

       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 the
liquid 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 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.

   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 be
       installed on the plates.

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   6. It is easier to make provision for the withdrawal of side-streams from plate
   7. If the liquid causes fouling, or contains solids, it is easier to make provision for
       cleaning 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 the
       equivalent plate column.
   9. The liquid hold-up is appreciably lower in a packed column than a plate
       column. 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 than
       0.6 m, where plates would be difficult to install, and expensive.
Packed column is selected for our operation.

       The principal requirements of a packing are that it should:

              Provide a large surface area: a high interfacial area between the gas and
              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 proprietary
       structured packings.
   2. Random packings: rings, saddles and proprietary shapes, which are dumped
       into 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.

       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

       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.

       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 the
larger 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

             <0.3 m                          <25 mm

             0.3 to 0.9 m                    25 to 38 mm

             >0.9 m                          50 to 75 mm

       The term structured packing refers to packing elements made up from wire
mesh or perforated metal sheets. The material is folded and arranged with a regular
geometry, to give a high surface area with a high void fraction. A typical example is
shown below.

                                  Structured Packing

       Structured packings are produced by a number of manufacturers. The basic
construction and performance of the various proprietary types available are similar.
The advantage of structured packings over random packing is their low HETP
(typically less than 0.5 m) and low pressure drop (around 100 Pa/m). They are being
increasingly used in the following applications:

   1. For difficult separations, requiring many stages: such as the separation of
   2. High vacuum distillation.
   3. For column revamps: to increase capacity and reduce reflux ratio requirements.
       The applications have mainly been in distillation, but structured packings can
also be used in absorption; in applications where high efficiency and low pressure
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CHAPTER 6                                           DESIGN OF EQUIPMENTS

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.

   Factors to be considered

   1. Void fraction

   2. Effective surface

   3. Packing size

   4. Maximum operating temperature

   5. Mechanical strength

   6. Material selection

   Packing used here is 0.038m ceramic intalox saddle because

   1. One of the most efficient packings

   2. Little tendency to nest and block areas of bed

   3. Gives a fairly uniform bed

   4. Higher flooding point

   5. Lower pressure drop


 Nominal size                  0.038mm

 Packing factor F              170             Specific gravity (g/cm3)       2.3

 Package density (kg/m3)       580             Water absorption (%)           <0.3

 Free volume (%)               80              Acid resistance (%)            >99.6

 Surface area (m2/m3)          180             Max operating temp.            1100℃

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  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)                                                   STRIPPED GAS (11)

Mass flow rate= 14339.44kg/hr                                        Mass flow rate=9201.94kg/hr

Molar flow rate= 203.53kgmol/hr                                      Molar flow rate= 574.1kgmol/hr

Mole Fraction:                                                       Mole Fraction:

Propylene: 0.023                                                     Propylene: 0.0083

n-Butanal: 0.936                                                     Hydrogen: 0.476

iso-Butanal: 0.021                                                   CO:          0.506

Propane:    0.0049                                                   N-butanal: 0.0064
                                                                       Product (19)
STRIPPING GAS (10)                                                   Iso-butanal:.0.0021
Hydrogen: 0.0046                                                        Mass flow rate= 13839.5 kg/hr
Mass flow rate= 8712kg/hr                                            Propane: 0.0017
CO:         0.0085                                                     Molar flowrate= 195.57kgmol/hr
Molar flowrate= 566.04kgmol/hr
                                                                       Mole Fraction:
Mole Fraction:
                                                                       N-Butanal: 0.956
Hydrogen: 0.484
                                                                       Iso-Butanal: 0.0218
CO:        0.516
                                                                       Propylene: 0.000097

                                                                       Propane:       0.000073

                                                                       Hydrogen:      0.0093
                                                                                                  Page 74
                                                                       CO:            0.12
CHAPTER 6                                         DESIGN OF EQUIPMENTS


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      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

   1. Determining the diameter of column.

   2. Determining the HETP of packing

   3. 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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CHAPTER 6                                              DESIGN OF EQUIPMENTS

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

       ρg = Density of gas

       ρl = 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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CHAPTER 6                                                DESIGN OF EQUIPMENTS

HETP is calculated as

                    HETP =


        A= Size of packing                           = 38mm

        σ= Surface tension of liquid                 = 29.2 mN/m

        µ= Overall viscosity of feed stream = 0.000414 Pa s

HETP = 0.0357m

Number of transfer units is calculated as followed.

                                      ����                ���� ���� − ��������
                  ������������������������ =           �������� ���� − ����              + ����
                                   ���� − ����              ���� ���� − ��������

        β=L/HG                             = 0.0045

        L=Molar liquid flow rate           = 203 kmol/hr

        G=Molar gas flow rate              = 566 kmol/hr

        H=Henry’s 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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CHAPTER 6                                                        DESIGN OF EQUIPMENTS

        Height of overall gas transfer unit is calculated as followed.

                                                            ���� − ����
                                    ������������   = ����������������
                                                        �������� ���� ����

                                              Hog = 1.45m

        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 requires the calculation of the superficial velocity that is
given as

                       Vog      =     G/Aρg

                       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

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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CHAPTER 6                                                   DESIGN OF EQUIPMENTS

Gas mass velocity is calculated with following formula.


        mv = Mass flow rate of gas stream

        A = Area of column
                                 G v
                                 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.

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

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CHAPTER 6                                                   DESIGN OF EQUIPMENTS

Shell weight is calculated as

Shell Weight = Volume of shell × Density of shell material

Shell weight = 12670 kg

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

                                    ���� ���� =
                                              ������������ + ����. ��������

        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

Weight of elliptical head is calculated as

                                       �������� �������� − �������� �������� − �������� ����
                           ���� = ��������
                                              W = 58kg

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        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

                                                �������� ����
                                         �������� =

        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

                                ������������ =
                                           ���� �������� + ���� ���� �������� ���� ����


               Ms = Wind loading

               Ds = Inside diameter of shell

               ts = Thickness of support

               σbs= 0.81 N/mm2

Test compressive strength of support

                              ������������ (����������������) =
                                                    ���� �������� + ���� ���� ���� ����

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CHAPTER 6                                                          DESIGN OF EQUIPMENTS


        W= Dead weight of column when full of water

        σws (test) = 0.044 N/mm2

Operational compressive strength of support

                              ������������ (������������������������������������) =
                                                              ���� �������� + ���� ���� ���� ����


        W= Total weight of column

        σws (operational) = 0.359 N/mm2

Maximum tensile strength of support

                    ������������ �������� ���������������������������� = ������������ − ������������ (��������������������������������������������)

                                  Max σs (Tensile) = 770 kPa

Maximum compressive strength of support

                     ������������ �������� �������������������������������������������� = ������������ − ������������ (����������������)

                             Max σs (Compressive) = 455 kPa

Check for taken thickness of support

Following two conditions must be satisfied.


                                    �������� (����������������������������) < �������� ������������������������


        fs= Design stress          = 240N/mm2

        J= Joint efficiency        = 85%

        θs=Base angle (normally taken as 90°)

0.0226 < 0.770

Condition 1 is satisfied.

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       E= Young Modulus of elasticity = 11.35 N/mm2

                                         0.455 < 0.518

Condition 2 is satisfied.

So thickness of support = 220mm

       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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CHAPTER 6                                           DESIGN OF EQUIPMENTS

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


  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

                                                                               Page 84

Description: 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 trayed column. Stripping is mainly conducted in trayed towers (plate columns) and packed columns, and less often in spray towers, bubble columns, and centrifugal contactors