Question

Exchanger
1. Problem Statement & Scope of Work
Ethylbenzene is manufactured by alkylation of benzene with ethylene. The reaction
products are separated in a series of distillation columns. Ethylbenzene is obtained as
the distillate from the last column (called the ethylbenzene column). In a 70 ton per
day ethylbenzene plant, the final product leaves the overhead condenser at 1350C.
This is required to be cooled to 400C before pumping it to the storage tank. Cooling
water is available at 300C for cooling. Please provide a proposal for the design of a
multi-pass counter-current Shell & Tube Heat Exchanger (Figure-1) that fulfils the
specifications in Section 3 (Process Data) and includes the following items:
Item 1: Do energy balance and find heat duty (Q) and water flow rate
Item2: First start trial with 1-1 pass counter current exchanger and do LMTD
calculation and find out area, assuming overall heat transfer coefficient
based on outside area, Udo
Item 3: Select tube BWG (i.d, o.d and length) from available market and number of
tubes required and linear velocity of water (a velocity above 1 m/s should be
maintained).Do selection of tube side and shell side fluid with proper
justification
Item 4: Start second trial with suitable tentative selection of tube passes (say 1-4
pass) and repeat calculation and find out tube specifications and number of
tubes (You may use Tube-sheet layout and tube count of a shell and tube
heat exchanger Table which are available in standard data book)
Item-5: Select 25% cut segmental baffles with 0.15 m (6 inch) baffle spacing and
shell specification
Item 6: Estimate the tube-side and shell-side heat transfer coefficient (hi) using any
suitable correlation. Number of correlations are available such as Dittus-
Boelter equation, Colburn jH factor etc. At this stage there are number of
decision options on baffle spacing, number of tube passes etc are available to
get reasonably high local heat transfer coefficients.
Item 7: Calculate the ‘clean’ Overall coefficient, U on the outside tube area basis.
and repeat the above calculation to get area required, tube specifications,
number of required tubes. Neglect dirt factor calculation for U.
Item-8: No need to do pressure-drop calculation exercise but you may discuss this
variable and its role on design calculation.
N.B. You may be referred for various data from standard data book or from
Chemical Eng handbook- by Perry if required
2
go through S&T H.E. design calculation from Process Heat Transfer book by D.
Q. Kern. My expectations are your approaches and method of calculations
2. Project Report
This is an individual self-study mini-project on Shell & Tube Heat Exchanger
equipment design. Individual student is to require a report writing covering all
sections of design items. The report should also include an executive summary, along
with brief conclusions and recommendations. Your approaches and method of
calculations are also important and carry significant weightage
3. Process Data
The allowable pressure drop is 0.15 kg/cm2 on both tube and shell sides.
Ethylbenzene at the mean liquid temperature (87.50C):
Density: 840 kg/m3
Specific heat: 2.093 kJ/kg.K
Viscosity: 0.33 cp =3.3 x 10-4 kg/m-s
Thermal conductivity: 0.1156 W/m K = 0.0994 kcal/h m K
Water at 350C (mean temperature)
Density: 993 kg/m3
Viscosity: 8 x 10-4 kg/m s
Specific heat: 4.175 kJ/kg K = 1.0 kcal/kg K
Thermal conductivity: 0.623 W/m.K = 0.536 kacl/h m2K
.

Answer 1