1) The document presents the process and mechanical design of a packed bed extractor to extract 96% of benzene from a feed mixture of 78% 1-hexene and 22% benzene using tetra-methylene sulphone as the solvent. 2) Key steps in the process design include determining the equilibrium curve, mass flow rates, minimum solvent rate, dispersed and continuous phase properties, flooding velocity, column diameter, mass transfer coefficient, number of transfer units, and column height. 3) Key steps in the mechanical design include determining the shell and head thickness, stresses from various loads, internal packing support, and distributor specifications. Lessing rings are identified as the optimum packing material.

1. CL-304 (PED-II)
Instructor: Prof. Ashok Kumar Dasmahapatra
Group 8
PROCESS EQUIPMENT DESIGN
TERM PROJECT
Process & Mechanical Design of a Packed Bed Extractor
Group Members:
Namrata Das (130107034)
Nayan Gupta (130107035)
Nilesh Raj (130107036)
Niraj Chetry (130107037)

2.  Problem Statement:
Extraction of Benzene is desired from a mixture of Benzene &
1-Hexene containing 78 mole% 1-Hexene and 22 mole% Benzene.
Flow rate of the feed solution is 6000 kg/hr.
Tetra-methylene Sulphone is to be used as the solvent for 96%
extraction of benzene from the feed mixture.

3.  Ternary equilibrium data:

4.  Description:
Materials:
Carrier : 1-Hexene (A)
Solvent: Tetra-Methylene Sulphone (B)
Solute : Benzene (C)
Continuous Phase Properties:
Density (ρc) = 715.73 kg/m3
Viscosity (µc) = 4.7x10-4 Pa.s
Diffusivity of Benzene (Dc) = 1.62x10-9 m2/s
Dispersed Phase Properties:
Density (ρd) = 1261 kg/m3
Viscosity (µd) = 0.013 Pa.s
Diffusivity of Benzene (Dd) = 4.1x10-9 m2/s
Interfacial tension of dispersed phase (γ) = 0.00728 N/m

5.  Solution Procedure: Process calculations
1. Equilibrium Curve:
o Plot the right-angled ternary plot
2. Raffinate & Extract Phase Mass Flow Rates:
o Use stoichiometric calculations to find these two
3. Minimum Solvent Rate:
o Calculate the minimum solvent rate for extraction by locating Δm on the ternary plot
4. Equilibrium Solute Concentration in Extract:
o Total material balance & solute balance gives => yE
5. Packing Material Specifications:
o Using 3 different types of packing material
- Raschig Rings
- Lessing Rings
- Berl Saddles
(Image Source: Google Image Search)

6. 6. Flooding Velocity:
o The flooding velocity for the dispersed phase & continuous phase is calculated using
the correlation:
(Ref: A.Suryanarayna, page:551, for correlation)
7. Column Diameter:
o Using flooding velocity & mass flow rate, find the column diameter
8. Dispersed phase hold-up (ф):
o Solve the cubic equation - UD + UC (ф/1- ф) = Vo ε ф(1-ф); where, Vo = C[aP ρC / ε3gΔρ]-0.5
to get the values of ф
o Select the root such as ф <0.52
9. Mass Transfer Coefficient (Koca):
o Calculate Schmidt’s No. for both phases and use packing specifications and the above
value of ф along with the avg. coefficient distribution(m) from equilibrium data to
calculate Koca
 Solution Procedure: Process calculations

7. 10. Height of Transfer Units (HtoC):
o Using continuous phase velocity (UC) & mass transfer coefficient (Koca) to get HtoC
11. No. of Transfer Units (NtoC):
o Using yE found from material balance and xF* found from the equilibrium curve, find
NtoC using the following:
NtoC = xRʃxF dx/(x-x*) ~ (xF-xR)/(x-x*)M
where, (x-x*)M = [(xF-xF*)-(xR-xR*)]/[ln(xF-xF*)/(xR-xR*)]
12. Column Height:
o Using the above two values of HtoC & NtoC , we get the column height: H = HtoC * NtoC
13. Comparison of Packing Materials:
o We repeat the steps 1-11 for each of the packing material type and tabulate the results
to compare all 3 types of material choice:
It is evident from the calculated results that “Lessing Rings” would be the optimum
choice as a packing material for the desired extraction
 Solution Procedure: Process calculations

8.  Mechanical Design: Specifications
Diameter of the tower, Di =1m
Height of the tower, H = 2.9m
Working Pressure = 1atm =10.1325 kg/cm2= 0.101325 MPa
Design Pressure, P = 1.13atm = 11.4497 kg/cm2= 0.114497 MPa
Shell Material: Plain Carbon Steel, Grade 2B (IS : 2002-1962)
Permissible Tensile Stress, ft = 950 kg/cm2 ~ 95 MPa
Insulation thickness = 100mm
Density of insulation = 770 kg/m3
Top disengaging space= 1m
Bottom separator space= 1m
Density of material of column = 7700 kg/m3
Wind Pressure = 130 kg/m2 ~ 1.275MPa

9.  Mechanical Design: Calculations
1. Shell Thickness:
o Using the formula: ts = PDi/(2fJ+P) + c , we get the shell thickness
2. Head Design:
o Working Pressure Range: 0.1~1.5 MPa
Choice of Head: Shallow dished & Torispherical
We calculate the thickness of head by: t= PDoC/2fJ
3. Stress calculations:
o Stress in the mechanical design due to various contributors are calculated:
 Axial Stress (compressive): fap= PD/4(ts-c)
 Compressive stress due to weight of shell upto a distance ‘x’ : fds=ρsgx
 Compressive stress due to weight of insulation: fd(ins)= ᴨDinstinsρins / ᴨDmt
 Compressive stress due to weight of liquid and tray: fdl= Wliq/ ᴨDm(ts-c)
 Stress due to weight of attachments: fd(att)= Wa/ᴨDmt
 Total compressive dead weight stress at height ‘x’: fdb= fap+fds+fd(ins)+fdl
 Stress due to wind load at distance ‘x’: fws= 1.4Pwx2/ ᴨDot
 Stress in upwind side: fmax=fws+fap-fds
 Stress in downwind side: fmax=fws+fap+fds
Calculating the failure location ‘x’ verifies the earlier calculated value of “Column Height”

10.  Mechanical Design: Specifications
4. Internal Packing Support:
o For column diameter upto 1.2m, we can use the GIS/EMS Random Packing Support
Grid in such small columns
(Ref: Internals for packed column, SULZER Chemtech)
5. Distributor:
o For low interfacial tension value in LLX, Extraction Distributor VRX can be used.
(Ref: Internals for packed column, SULZER Chemtech)

11.  Results: Design Details
The design specifications based on the optimum choice of packing material are listed below:

12.  Graph:

13.  Bibliography:
 Mass Transfer Operations, 3e, Robert E. Treybal
 Mass Transfer Operations, A. Suryananarayana
 Mass Transfer Operations, B.K.Dutta
 Packed Tower Design & Applications, Ralph F. Strigle
 Perry’s Handbook, 8th Edition, Section-15, Mc Graw Hill Education
 Packed Column Design & Performance, L.Klemas & J.A.Bonilla
 Structured Packings: for Distillation, Absorption & Reactive Distillation,
by SULZER Chemtech Ltd.
 Liquid-Liquid Extraction Technology, by SULZER Chemtech Ltd.
 Design Practice for Packed Liquid-Liquid Extraction Column, by SULZER
Chemtech Ltd.
 Internals for Packed Columns, by SULZER Chemtech Ltd.