Design of thermosyphon reboiler

Thermosiphon reboilers in chemical industry has wide scope for usage. The PDF will provide introduction, types and design methodology that.

1. 1














DESIGN
OF
THERMOSIPHON
REBOILER
IN
EXCEL

- Harshad Vaghela

2. 2

ABSTRACT

A reboiler is an exchanger which provides heating requirement of distillation
column. It is very important equipment in industries. Since the pump is required to transport
liquid from the bottom of column to reboiler, it will add an additional capital cost, operating
cost, maintenance cost of pump. The use of thermosiphon would able to eliminate these
costs. The thermosiphon reboiler drives the liquid to the reboiler by hydrostatic head
available from the liquid at bottom of the column. The thermosiphon reboiler can be
oriented horizontally or vertically. It is very important to understand the design of each type
of reboiler. The design of each type of reboiler involves the heat transfer fundamentals.
Microsoft Excel provides a very helpful platform for designing an equipment. Excel is user
friendly and transparent design tool.

3. 3

Table of Contents

CHAPTER – 1 INTRODUCTION .................................................................................. 7
1.1 Outline of report ....................................................................................................... 7
CHAPTER - 2 REBOILER............................................................................................. 8
2.1 Types of Reboiler ...................................................................................................... 8
2.1.1 Forced circulation reboiler ............................................................................... 8
2.1.2 Natural circulation reboiler .............................................................................. 9
2.2 Reboiler arrangement ............................................................................................ 10
CHAPTER – 3 FEATURES OF MICROSOFT EXCEL ............................................ 11
CHAPTER - 4 THERMOSYPHON REBOILER ....................................................... 13
4.1 Types of Thermosyphon reboiler .......................................................................... 13
4.1.1 Horizontal thermosyphon reboiler ................................................................. 13
4.1.2 Vertical Thermosyphon Reboiler ................................................................... 14
CHAPTER - 5 DESIGN PROCEDURE FOR HORIZONTAL THERMOSYPHON
………………… REBOILER .......................................................................................... 16
CHAPTER – 6 DESIGN PROCEDURE FOR VERTICAL THERMOSYPHON
………………… REBOILER .......................................................................................... 20
CHAPTER - 7 DESIGN PROBLEM OF HORIZONTAL THERMOSIPHON
……….…..…… REBOILER…………...……………………………………………….24
CHAPTER – 8 DESIGN PROBLEM OF VERTICAL THERMOSIPHON
………………… REBOILER .......................................................................................... 26

4. 4

LIST OF FIGURES

Figure no. Title Page no.
2.1 Forced circulation reboiler 11
2.2 Natural circulation reboiler 11
2.3 Once-through circulation reboiler 12
2.4 Recirculating reboiler 12
3.1 Solver dialogue box 14
4.1 Horizontal thermosiphon reboiler 16
4.2 Vertical thermosiphon reboiler 17
5.1
Correlation of Natural circulation boiling and
sensible heating coefficient
21

5. 5

NOMENCLATURE
Symbol Title
First
used on
page no.
µ Viscosity 19
µw Viscosity at wall temperature 19
A Area for Heat transfer 19
a1 Flow area of one tube 25
AF Flow area 25
B Baffle spacing 26
BWG Birmingham wire gauge 19
CL Tube count constant 26
CP Heat capacity 18
CTP Tube layout constant 26
DE Equivalent diameter 21
di Internal diameter of tube 19
do Outside diameter of tube 21
Ds Diameter of shell 25
f friction factor 21
FT Correction factor for LMTD 19
G Mass flux 19
g Gravitational accelaration 20
hi Heat transfer coefficient at tube side 19
HL1 Enthalpy of liquid at its inlet temperature 18
HLs Enthalpy of liquid at its saturation temperature 18
ho Heat transfer coefficient at shell side 20
hs Heat transfer coefficient for sensible heat transfer 20
Hv Enthalpy of vapor 18
hv Heat transfer coefficient for vaporization 20
k Thermal conductivity 18
LMTD Log-mean temperature difference 18
LT Length of tube 19
mH Flowrate of hot stream 18
min Input flowrate to reboiler 18
mv Vapor production rate 18
n Number of passes 18
NT Number of tube 20
PLag Static pressure at reboiler leg 22

6. 6

PR Pitch ratio 25
PT Pitch 21
Q Total heat transfer 18
qs Sensible heat transfer 18
qv Heat transfer for vaporization 18
R Temperature efficiency 19
Rd Fouling factor 20
Re Reynolds number 18
S Temperature effectiveness 19
t1 Cold stream outlet temperature 18
T1 Hot stream inlet temperature 18
t2 Cold stream inlet temperature 18
T2 Hot stream outlet temperature 18
tmean Cold stream mean temperature 20
Tmean Hot stream mean temperature 20
tw Wall temperature 20
Uc Overall heat transfer coefficient (clean) 20
UD Overall heat transfer coefficient (design) 19
v Velocity of stream 25
Vin Specific volume at reboiler inlet 22
VL Specific volume of liquid 28
VO Specific volume at reboiler outlet 22
Vv Specific volume of vapor 28
W Mass flowrate 20
ΔPs Pressure drop at shell side 21
ΔPt Pressure drop at tube side 21
Δt Temperature difference between wall and fluid 20
ρavg Average density in reboiler 21
ρL Density of liquid 28
ρo Average density in reboiler outlet 28
ρv Density of vapor 28

7. 7

CHAPTER - 1
INTRODUCTION


Reboiler is a heat exchanger used to supply heat requirement of distillation column.
Since the distillation is very important operation in petroleum, petrochemical and chemical
industry, proper reboiler operation is also very important. Design aspects of reboiler
depends on which type of reboiler is used. Reboiler is classified in two categories, Forced
and Natural circulation reboiler. Forced circulation reboilers are suitable for vaporization
of fluid with high viscosity and fouling charecteristics. Natural circulation reboilers are
used in petrochemical, petroleum and chemical industries.
The design aspects of each reboiler involves heat transfer operation fundamentals.
Since the design is iterative, it requires the platform for calculation. Microsoft Excel
provides very helpful platform for this and is used to create the code for designing of
thermosiphon reboiler. This report focusses on the design of thermosiphon reboiler which
is natural circulation reboiler.
The horizontal thermosiphon reboiler is designed for the vaporisation of naphtha
and vertical thermosiphon reboiler is designed for vaporisation of n-butane and methanol-
water systems. The results are well matching with the same given in reference book.

1.1 Outline of report
Chapter 1 of report consists of introduction of report. Chapter 2 consists of the
reboiler and its types and arrangement in details. This chapter also deal with recirculation
in reboiler. Chapter 3 includes the various features of Microsoft Excel and applications.
Chapter 4 include the mechanism of thermosiphon reboiler, types of thermosiphon reboiler,
details, advantages and disadvantages of each type. Chapter 5 and 6 deals with the step by
step design procedure and design equations respectively. Chapter 7 and 8 includes the
design problems for horizontal and vertical thermosiphon reboiler respectively.

8. 8

CHAPTER - 2
REBOILER


A reboiler or vaporiser fulfil the heat requirement of distilling system. Usually
reboilers are shell and tube heat exchangers. A liquid from bottom of column is fed to the
reboiler where it is converted to vapor by means of service stream (hot fluid such as steam,
hot oil, etc.). A liquid from column is vaporised again and again therefor it is termed as
reboiler.
With the help of reboiler, 100% vaporisation can be done (Kern, 1997). But residue
accumulates and to remove that blowdown must be provided. It is not preferable to lose the
valuable feed so blowdown provision is not recommended. If there is any dirt present in
feed that will accumulate in equipment and cause fouling rapidly. This is also undesirable.
If the reboiler is overdesigned, extra surface is provided for vaporisation, then vapor will
be superheated which will require an additional surface for de-superheating. This will
reduce the performance of distillation column. Due to these reasons, less than 100 %
vaporisation is preferred. Usually 80% of feed vaporisation is favourable. In reboiler, the
term Recirculation Ratio is specified, which is defined as the ratio of the flowrate of liquid
at outlet of reboiler to the flowrate of vapor at outlet of reboiler.
���??????������??????�� ���??????�=
�������� �� �??????��??????� �� ����??????��� ������
�������� �� ����� �� ����??????��� ������


2.1 Types of Reboiler
The reboilers are mainly classified into two categories based on circulation method
used.
(A) Forced circulation reboiler (Pump through circulation reboiler)
(B) Natural circulation reboiler
2.1.1 Forced circulation reboiler
When the liquid feed is fed with the help of pump or a gravity flow from the column,
these reboilers are termed as forced circulation reboiler (figure 2.1). The use of pump allows

9. 9

us to control the rate of feeding to reboiler. To achieve high het transfer coefficient and
high recirculation, the use of pump is preferable. The high velocity of feed permits the
operation without excess fouling for longer period. Though this will increase the cost of
pumping power but the maintenance cost of reboiler is decreased. the use of pump is
preferred when installation is small and the liquid is highly viscous and so pressure drop is
high through piping and reboiler.

2.1.2 Natural circulation reboiler
When the liquid is driven is by hydrostatic head available from column, the
circulation is Natural. It is shown in figure 2.2. For this, the column needs to be elevated.
Vapor + Liquid
Vapor
Liquid
Reboiler
Liquid
level
Pump
Bottom
Produc
t
Figure-2.1 Forced circulation reboiler
Vapor + liquid
Hydrostatic
head available
to drive the
liquid.
Liquid vaporises,
and density of
mixture gets
lower down.
Vapor
Liquid
Bottom
produc
t
Figure- 2.2 Natural circulation reboiler

10. 10

Recirculation is driven by the density difference between the inlet and outlet of reboiler.
The use of this reboiler saves a cost of pump, power but requires more head from column.
The recirculation rate cannot be controlled easily in this case.

2.2 Reboiler arrangement
When reboilers are used, the space below the is used for disengagement of vapor
and liquid. The unvaporised liquid can be recirculated through the reboiler. The circulation
of liquid can be achieved in two ways (Kern,1997).
As shown in figure 2.3, the liquid from downcomer of bottom plate is fed to the
reboiler and liquid from outlet of reboiler is withdrawn as product after disengagement.
This arrangement is Once-through circulation reboiler. This provides poor control of
circulation rate.
-

In figure 2.4, the liquid from bottom plate and liquid from reboiler outlet after
disengagement is mixed and then again fed to the reboiler and part of bottom liquid is
withdrawn as a product. This is known as recirculating reboiler. This type has less chances
of fouling and better control of circulation rate.

Vapor +
Liquid
Vapor
Liquid
Vapor +
Liquid
Liquid
Figure-2.3 Once-through
circulation reboiler
Figure-2.4 Recrculating
reboiler

11. 11

CHAPTER - 3
FEATURES OF MICROSOFT EXCEL



Microsoft Excel is part of the Microsoft Office suite. The whole suite also includes
the Word, PowerPoint, OneNote, Outlook, Publisher and Access. Excel is the spreadsheet
application that provides a platform to easily perform calculations of data by using
functions and formulas. A spreadsheet is a collection of text and numbers laid out in a
rectangular grid.
Spreadsheets are often used in business for budgeting, management, and decision
making. Excel is a program used to create electronic versions of spreadsheets. MS Excel
has numerous tools and features that helps to work easily and to save time also. Microsoft
Excel provides an interface to organize nearly any type of information. The data can be
managed in any layout and structure in Excel. Excel allows you to work with text, numbers,
and date information in a relatively open way.
Excel allows to do mathematical calculations and to store important data in the form
of charts or spreadsheets. It is also used for billing, data management, analysis, finance,
business tasks, complex calculations, etc.
Microsoft Excel can store larger amounts of data which can be analysed to discover
trends. With the help of graphs and charts, data can be summarized and stored in an
organized way. Excel allows to perform a what-if analysis in which one or more values in
a spreadsheet can be changed and changes on the calculated values can be assessed.
Excel is also very helpful in engineering also. With the use of it, complex
engineering challenges can be solved. It provides useful tools used daily in engineering.
Unit conversion, plotting charts, fitting an equation from plots and data, creation of user
defined functions, and many can easily be done with the use of Excel.
It also helps in auditing the spreadsheet. The various tools are available which helps
to fix the problem. It can also show the step by step calculation of input formulas. It is also
possible to prevent errors because of improper inputs. Data validation can help for this. The

12. 12

input in cell can be limited for user. It is very easy to update data, charts and even
calculations if any changes in input occur. Excel provides a large number of built-in
functions that can be used to perform specific calculations. These functions are grouped as
logical, financial, statistical, mathematical, trigonometric, lookup functions.
Solver and goal-seek are very useful features of Excel which are used for what-if
analysis. When the calculated results are set to the desired goal value, the input value for
this can be calculated using these features. Goal-seek helps only when there is one input
variable. When there is more than one input variable then solver can be used.

Solver (figure 2.1) can be used for finding out an optimal (maximum or minimum)
value in one cell which is called as objective cell subject to constraints of other formula
cells. Solver works with group of cells, called variable cells. It adjusts the values of variable
cell to satisfy the limits of constraint and shows the result for the objective cell.

Figure-3.1 Solver dialogue box

13. 13

CHAPTER - 4
THERMOSYPHON REBOILER


Thermosyphon reboiler are natural circulation reboiler. The liquid from column
flows under column pressure and hydrostatic head available in bottom. The liquid enters to
reboiler and where it is vaporised. The two-phase mixture from outlet of reboiler enter to
just below the bottom tray into vapor space above bottom liquid. The vapor flow upward
through the bottom tray. The density difference between the inlet liquid and outlet two-
phase mixture drives the circulation in thermosyphon reboiler (Ludwig, 2001).

4.1 Types of Thermosyphon reboiler
According to orientation, the thermosyphon reboiler has been divided in two categories:
1.Horizontal thermosyphon reboiler
2.Vertical thermosyphon reboiler

4.1.1 Horizontal thermosyphon reboiler
For horizontal thermosyphon reboiler, the process fluid is always on shell side, with
heating medium on tube side. For these units, the TEMA type G, H, and X are common in
use. Figure 3.1 shows the horizontal unit located at bottom of column. The horizontal units
have several advantages. It provides high surface area. It is capable of moderately high heat
transfer rates. It provides better flexibility to control high liquid flowrate. The requirement
of hydrostatic head of bottom liquid is comparatively less hence, the less boiling point
elevation is there. This will lead to high heat transfer rate. Tube bundle can be removed
easily for maintenance purpose as it is oriented horizontally. The horizontal type reboilers
are mostly used in refineries (Ludwig,2001).
It has several disadvantages also. The space and extra piping required is more. The
recirculation provided is usually less. The residence time in reboiler is more therefore with

14. 14

fouling or corrosive fluid, it may create problem. There are chances of thermal degradation
of reboiler if process fluid is heat sensitive.
*
4.1.2 Vertical Thermosyphon Reboiler
For this type of reboiler, the process fluid is on tube side, with the heating fluid is
on shell side. The TEMA type E shell are mostly used. The requirement of space and extra
piping are comparatively less as it can be set closer to column. The recirculation provided
is usually more than 4:1 hence the residence time in reboiler is less therefore it can better
handle the fouling or corrosive fluid. This reboilers are well suitable for heat sensitive
material. The vertical thermosyphon reboilers are mostly used in petrochemical and
chemical industries. The vertical unit is shown in figure 4.2.
The requirement of hydrostatic head is more compared to horizontal unit. This will
lead to higher boiling point elevation. The vertical unit determines the height of bottom
plate in column above grade. The removal of tube bundle is much difficult. The
maintenance and cleaning can be awkward.

Vapor + Liquid
Vapor
Liquid
Bottom
product
Liquid
Figure-4.1 Horizontal thermosiphon reboiler

15. 15


Vapor + Liquid
Liquid
Vapor
Bottom
product
Figure-4.2 Vertical thermosiphon reboiler

16. 16

CHAPTER - 5
DESIGN PROCEDURE FOR HORIZONTAL
THERMOSYPHON REBOILER

The first step to design horizontal thermosiphon reboiler is the calculation of heat
duty requirement. This follows the calculation of LMTD, selection of suitable tube
dimensions, calculation of number of tubes and number of passes required, heat transfer
area and overall heat transfer coefficient. This follows the estimation of properties at film
temperature. The design proceeds with the calculation of tube side heat transfer coefficient
and shell side boiling coefficient. From this, overall heat transfer coefficient is computed.
And the pressure drop at tube and shell side is calculated. The detailed procedure and
equations for designing the horizontal thermosiphon reboiler as suggested by Kern (1997)
is given below

1. Evaluate the heat load (Q) for the unit.
For sensible heat transfer,
�
�=�
??????�
�∆� …(5.1)
For vaporisation,
�
�=�
�� …(5.2)
Total heat load
�=�
�+�
� …(5.3)
If enthalpies are known instead of properties then
�
�=�
??????(�
??????,�−�
??????,1) …(5.4)
and �
�=�
�(�
??????−�
??????,�) …(5.5)

2. Determine the LMTD and correction factor FT.

����=
(�
1−�
1)−(�
2−�
2)
��(
�
1−�
1
�
2−�
2
)

…(5.6)

17. 17


�
??????=
√�
2
+1∙��(
1−�
1−��
)
(�−1)��
2−�(�+1−√�
2
+1)
2−�(�+1+√�
2
+1)

…(5.7)
Where,
�=
�
2−�
1
�
2−�
1
…(5.8)

�=
�
2−�
1
�
1−�
1

…(5.9)
The equation is derived for 1,2 exchanger but it will result only 2% error while calculating
FT for 1,8 exchanger (Kern, 1997).
3. From maximum allowable heat flux, calculate the area required.
The maximum allowable (critical) heat flux for natural circulation is 37,800 W/m
2
and for
forced circulation is 63,100 W/m
2
(Kern,1997).

4.Select suitable dimensions (OD, ID, BWG) of tube and by assuming the length, calculate
the number of tube required.
To decide number of passes required, calculate velocity of service stream form volumetric
flowrate and flow area. If the velocity is not enough then increase number of passes.

Calculate the heat transfer area (A) provided and from that calculate overall heat transfer
coefficient (UD).

5.Estimate the properties (such as density, viscosity, heat capacity, thermal conductivity,
latent heat) of process and service stream (Green and Perry, 2008).

6. Determine the tube-side film coefficient for convection or condensation as required.
If tube side fluid undergoes no phase change,
For Re < 2100,

ℎ
??????&#3627408465;
??????
&#3627408472;
=1.86[(
&#3627408465;
??????&#3627408442;
&#3627409159;
)(
&#3627408438;
&#3627408451;&#3627409159;
&#3627408472;
)]
1
3
⁄
(
&#3627409159;
&#3627409159;
&#3627408484;
)
0.14
…(5.10)
For Re > 4000,

ℎ
??????&#3627408465;
??????
&#3627408472;
=0.027(
&#3627408465;
??????&#3627408442;
&#3627409159;
)
0.8
(
&#3627408438;
&#3627408451;&#3627409159;
&#3627408472;
)
1
3
⁄
(
&#3627409159;
&#3627409159;
&#3627408484;
)
0.14

…(5.11)

18. 18

If tube side fluid undergoes phase change,
ℎ
??????=1.51(
4&#3627408442;
&#3627409159;
&#3627408467;
)
−
1
3
⁄
(
&#3627409159;
&#3627408467;
2
&#3627408472;
&#3627408467;
3
&#3627409164;
&#3627408467;
2
&#3627408468;
)
−
1
3
⁄
…(5.12)
where

&#3627408442;=
&#3627408458;
0.5&#3627408447;&#3627408449;
??????

…(5.13)

7. Determine the shell-side coefficient.
a. Evaluate tube wall temperature.
&#3627408481;
&#3627408484;=&#3627408481;
&#3627408474;&#3627408466;??????&#3627408475;+
ℎ
??????&#3627408476;
ℎ
??????&#3627408476;+ℎ
&#3627408476;
(&#3627408455;
&#3627408474;&#3627408466;??????&#3627408475;−&#3627408481;
&#3627408474;&#3627408466;??????&#3627408475;) …(5.14)
and ∆&#3627408481;=&#3627408481;
&#3627408484;−&#3627408481;
&#3627408474;&#3627408466;??????&#3627408475; …(5.15)
b. Evaluate boiling coefficient from figure 5.1 or from equations.
When there is a boiling range, the overall heat transfer coefficient can be weighed average
of sensible and latent heat transfer.

The equation for individual heat transfer coefficients are as follows,
ℎ
&#3627408480;=17.10(∆&#3627408481;)
0.308
…(5.16)
ℎ
&#3627408483;=2.547(∆&#3627408481;)
2
+5.321(∆&#3627408481;)−7.393 …(5.17)

The limit for organics and water is given as 1700 and 5700 W/m
2
K respectively by Kern
(1997).
From these coefficients (hs and hv), the weighted average coefficient can be found out.

ℎ
&#3627408476;=
&#3627408452;
&#3627408478;
&#3627408480;
ℎ
&#3627408480;
+
&#3627408478;
&#3627408483;
ℎ
&#3627408483;

…(5.18)

8. Calculate the required area, based on the film coefficient of steps 6 and 7.
&#3627408456;
&#3627408464;=
ℎ
??????&#3627408476;ℎ
&#3627408476;
ℎ
??????&#3627408476;+ℎ
&#3627408476;
…(5.19)
9. Calculate fouling factor.
&#3627408453;
&#3627408465;=
1
&#3627408456;
&#3627408439;
−
1
&#3627408456;
&#3627408464;
…(5.20)
8. The clean overall heat transfer coefficient (UC) should be greater than the design overall
heat transfer coefficient (UD) and should provide reasonable fouling factor

19. 19

9. Determine the tube side pressure drop.
∆&#3627408451;
&#3627408481;=
2&#3627408467;&#3627408447;
??????&#3627408442;
2
&#3627408475;
&#3627408465;
??????&#3627409164;
…(5.21)

10. Determine the shell-side pressure drop.
∆&#3627408451;
&#3627408480;=
2&#3627408467;&#3627408447;
??????&#3627408442;
2
&#3627408439;
&#3627408440;&#3627409164;
??????&#3627408483;&#3627408468;
…(5.22)
The equivalent diameter can be found out from following equation.
For square pitch,
&#3627408439;
&#3627408440;=
4×(&#3627408451;
??????
2
−&#3627409163;&#3627408465;
&#3627408450;
2
/4)
&#3627409163;&#3627408465;
&#3627408476;
…(5.23)
For triangular pitch,
&#3627408439;
&#3627408440;=
4×(0.86 &#3627408451;
??????
2
−&#3627409163;&#3627408465;
&#3627408450;
2
/4)
&#3627409163;&#3627408465;
&#3627408476;
…(5.24)
Figure-5.1 Correlation of Natural circulation boiling and
sensible heating coefficient (inside and outside tubes)

20. 20

CHAPTER - 6
DESIGN PROCEDURE FOR VERTICAL
THERMOSYPHON REBOILER


The design procedure of vertical thermosiphon reboiler follows the calculation of
heat duty, LMTD, selection of suitable dimension, calculation of number of tubes, heat
transfer area and overall heat transfer coefficient. By assuming the proper recirculation
ratio, static pressure at leg and frictional pressure loss at tube side is calculated. The
combined pressure drop is compared with driving force. The heat transfer coefficient at
shell side and tube side is calculated. From this, overall coefficient is calculated. Finally,
the pressure drop at shell side is calculated. The detailed procedure and design equations
for designing the vertical thermosiphon reboiler as suggested by Kern (1997) is given
below.

1. Evaluate the heat load (Q) for the unit using equation (5.1) to (5.5).

2. Determine the LMTD and correction factor FT using equation (5.6) and (5.7).
Usually, vertical units are 1,1 exchanger therefor FT = 1.

3.Estimate the properties (such as density, viscosity, heat capacity, thermal conductivity,
latent heat) of process and service stream (Perry,2008).

4.From maximum allowable heat flux, calculate the area required.

5.Select suitable dimensions (OD, ID, BWG) of tube and by assuming the length, calculate
the number of tube required. Calculate the heat transfer area provided and from that
calculate overall heat transfer coefficient (UD).

6.Select suitable recirculation ratio (4:1). Calculate static pressure at lag (Plag).
&#3627408451;
&#3627408473;??????&#3627408468;=
&#3627408447;∙&#3627408468;
&#3627408457;
&#3627408476;−&#3627408457;
??????
&#3627408473;&#3627408475;(
&#3627408457;
&#3627408450;
&#3627408457;
??????
) …(6.1)

21. 21

7.Calculate the frictional pressure drop in tube side (ΔPt).
∆&#3627408451;
&#3627408481;=
2∙&#3627408467;∙&#3627408447;∙&#3627408442;
2
&#3627408465;
??????∙&#3627409164;
??????&#3627408483;&#3627408468;
…(6.2)

8.Calculate the driving force which can be provided and compare it with the total pressure
drop (ΔPt + Plag).
If total pressure drop is more than the driving force, actual the recirculation ratio will be
less than 4:1 hence, frictional resistance has to be reduced by choosing the shorter tubes.
If the driving force exceeds the total resistances then assumed value of recirculation ratio
is assured.

9.Calculate heat transfer coefficient at shell side.
If sensible heat transfer is only involved, equation (5.10) and (5.11) can be used. Where di
is replaced by DE. Equivalent diameter (DE) is found out from equation (5.23) and (5.24).

If condensing stream is used, Nusselt equation can be used.

ℎ
&#3627408476;=0.943(
&#3627408472;
&#3627408467;
3
&#3627409164;
&#3627408467;
2
&#3627409158;&#3627408468;
&#3627409159;
&#3627408467;&#3627408447;∆&#3627408481;
&#3627408467;
)
1
4
⁄

…(6.3)

10.Calculate heat transfer coefficient at tube side using either seider-tate or dittus-boelter
equation shown below.
For Re < 2100,

ℎ
??????&#3627408465;
&#3627408472;
=1.86[(
&#3627408465;
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&#3627409159;
)(
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)]
1
3
⁄
(
&#3627409159;
&#3627409159;
&#3627408484;
)
0.14
…(6.4)
For Re > 4000,

ℎ
??????&#3627408465;
&#3627408472;
=0.027(
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)
0.8
(
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)
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3
⁄
(
&#3627409159;
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)
0.14

…(6.5)

11.Calculate overall heat transfer coefficient (Uc) and fouling factor (Rd) using equation
(5.19) and (5.20).

12.Calculate pressure drop at shell side using equation (5.22).

22. 22

CHAPTER – 7
DESIGN PROBLEM OF HORIZONTAL THERMOSIPHON
REBOILER


17500 kg/hr of Naphtha is fed to Horizontal thermosiphon reboiler and produce
13200 kg/hr of vapor in temperature range of 430 K to 442 K. The heat is supplied by hot
oil with a temperature range of 547 K to 478 K. Use 14 BWG, 1” OD tube.

23. 23

24. 24

25. 25

A horizontal thermosiphon reboiler is designed to vaporize the 11500 kg/hr of CCl4. The
remperature of vaporisation is 382 K. Isothermal vaporization can be assumed. Steam at
0.38 MPa is available for heating.

26. 26

27. 27

28. 28

CHAPTER - 8
DESIGN PROBLEM OF VERTICAL THE RMOSIPHON
REBOILER


The vertical thermosiphon reboiler is designed to provide 18500 kg/hr of Vapor of almost
pure n-Butane which is boiling isothermally at 382 K. Heat is supplied by steam at 0.86
MPa. Use 16 BWG, 0.75” OD tube and recirculation ratio of 4:1 is employed.

29. 29

30. 30

31. 31

RESULT


Vertical Thermosiphon Reboiler
Particulars
Cold fluid Hot fluid
n-Butane Steam
Mass flow (kg/hr) 18500 2196.527
Heat transfer coefficient (W/m
2

K)
1700 8500
Pressure drop (kPa) 4.256 0.234

Heat transferd (W) 1246948.115
Area (m
2
) 33.271
LMTD corrected (K) 64.474
Fouling factor (m
2
K/W) 1.014E-03
Tube OD (mm) , BWG 19,16
Length (m) 3.657
No. of tubes 152
No. of Passes 1
Pitch Triangular
Pitch Ratio 1.25
Shell diameter (m) 0.35

32. 32




















Horizontal Thermosiphon Reboiler
Particulars
Cold fluid Hot fluid
Naphtha Hot Oil
Mass flow (kg/hr) 13200 23628.8877
Heat transfer coefficient (W/m
2
K) 1177.07102 2228.50139
Pressure drop (kPa) 24.131953 3.441

Heat transferd (W) 1232495.779
Area (m
2
) 32.55816318
LMTD corrected (K) 72.81931641
Fouling factor (m
2
K/W) 0.000483206
Tube OD (mm) , BWG 25.4, 14
Length (m) 3.657421518
No. of tubes 112
No. of Passes 8
Pitch Square
Pitch Ratio 1.25
Shell diameter (m) 0.5
Particulars
Cold fluid Hot fluid
CCl4 Steam
Mass flow (kg/hr) 11500 890.116
Heat transfer coefficient
(W/m
2
K)
1702.5 8500
Pressure drop (kPa) 299.754 45.922

Heat transferd (W) 531086.611
Area (m
2
) 14.029
LMTD corrected (K) 33
Fouling factor (m
2
K/W) 0.000142
Tube OD (mm) , BWG 19, 16
Length (m) 3.657
No. of tubes 68
No. of Passes 1
Pitch Triangular
Pitch Ratio 1.25
Shell diameter (m) 0.35

33. 33

CONCLUSION


The thermosyphon reboiler is designed with the use of Microsoft EXCEL & its
Solver feature. The design of vertical thermosiphon reboiler is designed for the n-Butane.
As a result, the area for heat transfer required is 33.271 m
2
. The horizontal thermosyphon
reboiler is designed for the naphtha and carbon tetrachloride. As a result, the area for heat
transfer is 32.658 m
2
and 14.03 m
2
.
The Microsoft Excel provides very transparent platform for calculation. As one can
observe the changes in all steps involved in calculations with the change in one or more
variables. With the use of Excel and its features, iterative calculations can be done easily.

34. 34

REFERENCES


1. Kern D Q. Process Heat Transfer. McGraw-Hill. New Delhi. 1997; 453-491
2. Ludwig E E. Applied Process Design for Chemical and Petrochemical Plants-Vol
3. GPP. New Delhi. 2001; 160-184
3. Green D W, Perry R H. Perry’s Chemical Engineers’ Handbook-Ed 8. McGraw-
Hill. New York. 2008; 2.96-2.450
4. Couter G, Marquis A. Mastering Microsoft Excel. Willey. New York. 2006