Pinch analysis technique to optimize heat exchanger

Heat Exchanger Network Design using
Pinch Analysis

Submitted to the Training In-charge, ONGC Hazira, in partial fulfilment of Summer Technical
Training, 2014.
K Vivek Varkey
IIT Hyderabad

 Acknowledgements:
We would like to thank our mentor for providing motivation and the required data in full
detail, due to which the process was highly hassle-free. Also we would like to convey
gratitude to the ONGC Hazira administration for arranging the training and allowing us to
pursue this project.

 Introduction
Any process is an energy sink, demanding high amount of heat addition or removal, that ask for individual high
costs, especially in a large process plant. A process may employ numerous heat exchangers to bring the process
stream at required temperatures at the various stages. Operation of these all heat exchangers with their individual
cooling utilities and heating utilities demand a high operating cost on a daily basis.
It is hence beneficial to strategize the heat exchanger network layout so as to couple the hot and cold process
streams, so that minimum heat duty is to be supplied or removed using the external utilities.
However design of an optimal heat exchanger network (HEN) so as to employ minimum operating costs keeping in
mind the installation costs are also not out of proportion, is a complicated method. Hence several algorithms and
famous approaches exist.
This project employs the PINCH ANALYSIS of the energy flow to determine the optimal HEN structure for the CFU in
the ONGC Hazira plant. The report does not deal with the derivation of the pinch technique, as several can be found
in literature.
The data are taken from the design report existing in the plant. The particular case deals with an inlet temperature
of 33oC and inlet pressure of 93Kg/cm2.
The heat duties are calculated in an ideal scenario, from the material balances, temperatures and enthalpy data
taken from the web.
We deal with the 5 heat exchangers namely, E701, E702, E703, E705 and E706.
E704 is not dealt with for reasons of unavailability of data.

S
T
R
I
P
P
E
R
L
P
G
Reflux
Drum
Preheater
Off Gas
Cooler
LPG Condenser
NGL
Cooler
Reboiler
Feed
Off Gas to
GSU
LPG to
CW
Naptha to
storage
 Process Flow Diagram
Vapour
Condensate
Feed from Slug Catcher at 33oC and 93Kg/cm2
PFD shown only for elements associated with this project.
1
5
7
3
4
2
8
6

 Material Balance
Component 1 2 3 (liquid) 3(vapor) 4(liquid) 4(vapor) 5 6 7 8
N2 0.02 0.01 0 0.01 0 0 0 0 0 0.02
H2S 1.36 0.13 0.75 0.48 0 0 0 0 1.23 1.36
CO2 40.46 7.15 12.11 21.2 0 0 0 0 33.31 40.46
C1 266.56 72.14 40.82 153.6 0 0 0 0 194.42 266.56
C2 68.51 7.5 34.02 26.99 0 0 0 0 61.01 68.51
C3 90.32 4.76 68.18 17.38 12.69 45.56 58.25 0 27.31 32.07
iC4 28.99 0.9 25.22 2.87 7.18 16.42 23.6 0 4.49 5.39
nC4 45.74 1.13 41.21 3.4 13.32 25.95 39.16 0.11 5.34 6.47
iC5 19.49 0.27 18.55 0.67 8.22 9.93 0.11 18.04 1.07 1.34
nC5 22.4 0.26 21.54 0.6 10.14 11.04 0.01 21.17 0.96 1.22
C6 32.03 0.18 31.54 0.31 19.39 11.95 0 31.34 0.51 0.69
C7 38.59 0.11 38.35 0.13 28.22 10.03 0 38.25 0.23 0.34
C8 47.64 0.07 47.51 0.06 39.45 8.02 0 47.47 0.1 0.17
C9 25.29 0.02 25.26 0.01 22.58 2.67 0 25.25 0.02 0.04
C10 15.5 0.01 15.49 0 14.5 0.99 0 15.49 0 0.01
C11 17.73 0 17.73 0 17.02 0.71 0 17.73 0 0
C12+ 7.96 0 7.96 0 7.77 0.19 0 7.96 0 0
All flows are in Kmol/hr.

 Cp values for the required components

The values of the components in the CFU unit is
obtained. Following which the respective mole
fraction in each stream is also obtained. These two
data are used simultaneously and the weighted mean
is to obtained the overall specific heat value.

 PROCEDURE
The method used is called Temperature Interval Method of Pinch Analysis
1. The Cp value obtained is multiplied by the flow rate to obtain the heat capacity flow
rate (C).
2. Now we create the following table for the further calculation.
The approach temperature is taken to be 10 degree Celsius. This is subtracted from
both the inlet and outlet temperatures of the hot stream for the purpose of
calculation, so as to obtain the pinch where the net heat exchange is supposed to
be 0. this is put in is Tout* and Tin* in the tables.

E-702 (Stipper Reboiler) C2
Assumptions:
• Stripper reboiler operates at constant 156oC, and main heat consumption is to cause phase change.
• Stripper column bottoms is saturated liquid and the outlet composition is same as the feed to the LPG column.
• Heat duty is calculated by determining the amount of latent heat required.
T = 156oc
M = 344 Kmol/hr
Q = - 959.03 KW
E-703 (LPG Condenser) H1
Assumptions:
• Constant operation temperature of 43oC
• Feed is saturated vapour from the LPG column top and outlest stream is saturated liquid.
• Heat duty is calculated from the latent heat calculations.
T = 43oC
M = 121.13 Kmol/hr
Q = + 627.55 KW

E-705 (NGL Cooler) H2
Assumptions:
• The inlet and outlet temperatures are respectively 175oC and 45oC.
• The composition is known from the mass balance.
• No phase change.
Tf = 45oC
Ti = 175oC
Cp = .244 KJ/mol K
M = 222.81 Kmol/hr
M*Cp = 15.10 KW/K
Q = + 1963.0 KW

E-706 (Off Gas Cooler) H3
Assumptions:
• Inlet and outlet temperatures are respectively 125oC and 40oC.
• No phase change.
• Composition is given in the material balance.
Tf = 40oC
Ti = 125oC
Cp = 0.044 KJ/mol K
M = 424.65 Kmol/hr
M*Cp = 5.20 KW/K
Q = + 442 KW
Hence summing all the enthalpy requirements of the streams we get,
C1+C2+H1+H2+H3 = + 1801.52 KW
Hence the Q min, req is + 1801.52 KW that must the removed somewhere in the process using cooling
utilities

 PROCEDURE
3. Now we draw a diagram where we label the adjusted temperatures in order from
coldest to hottest and we draw arrows to designate streams overlapping these
temperature intervals.

 PROCEDURE
4. Now for each interval in the previously drawn figure we find the enthalpy of each
intervals. This is obtained by adding the heat capacity flow rates ( for hot streams it
is considered positive and negative for the cold stream ). This is then multiplied by
the temperature interval for which we are calculating and the values are written
right next to the respective intervals in the figure labelled under Q.
5. These values are then added cumulatively from the top to the bottom ( higher
temperature to colder temperature ). The obtained data is then written in a new
column Qres.
6. The least value in the Qres column is called the
pinch value and the temperature of that interval is
called the pinch temperature.
7. The pinch value is then added from the beginning
of the values in Qres. It is observed that it becomes
0 at the pinch point.
8. The pinch value is the hot utility added and the
final value obtained ( in this case 2005.55 ) becomes
the cold utility required.

Hence we determine our pinch temperature as 115oC for the Cold streams and 125oC for
the Hot streams.
Also MER (minimum energy requirement) targets:
• Qhot utility = -204.43 KW (to be added by steam)
• Qcold utility = 2005.55 KW (to be removed by cooling water)
As per the method the hot utility can only be used above pinch, and cold utility can only
be used below pinch.
Before we start coupling a hot stream with a cold stream we should keep one thing in
mind. Let the specific heat flow rate of the hot stream be Ch and that of the cold stream
be Cc . If we are trying to couple in the hot side of the pinch it has to be made sure that
Cc > Ch of the respective streams. Similarly when we are trying to couple in the cold side of
pinch it should be taken care that Ch > Cc . Otherwise the stream coupling will become
infeasible.
In the next page we draw a pinch decomposition of the streams and determine an optimal
heat exchanger network.

PROOF
Let us assume a counter current pair of hot and cold stream.
Thi and Tho are respectively the inlet and outlet temperature of the hot stream.
Tci and Tco are respectively the inlet and outlet temperature of the cold stream.
ΔT1 and ΔT2 are respectively the difference between hot inlet and cold inlet and hot inlet
and cold outlet.
Q is the energy exchanged
Ch Cc are respectively the specific heat flow rates for the hot and cold streams.

PROOF
Q = Ch * [ Thi - Tho ]
Q = Cc * [ Tco - Tci ]
After rearranging we get,
Thi - Tho = Q/Ch
Tco - Tci = Q/Cc
now we subtract the equations to get
ΔT2 - ΔT1 = Q*[Cc - Ch ]/Cc Ch

PROOF
HOT SIDE OF THE PINCH
ΔT1 = ΔTMIN
Hence,
ΔT2 = ΔTmin + Q*[Cc - Ch ]/Cc Ch
now, ΔT2 has to be greater than ΔTmin
hence,
Cc > Ch
COLD SIDE OF THE PINCH
ΔT2 = ΔT min
Hence,
ΔT1 = ΔTmin - Q*[Cc - Ch ]/Cc Ch
now ΔT1 has to be greater than ΔTmin
hence,
Ch > Ch

In the next page we draw a pinch decomposition
of the streams and determine an optimal heat
exchanger network.

H1
H2
H3
C1
C2
175 125
156 156
125 45
125
43
43 33
40
43
P
I
N
C
H
H=204.03
H=755
H=272
H= 627.55
H=936
H=442
Heating Utility
Heat Exchanger
Cooling Utility
Heat loads of Exchangers mentioned are in
KW.
Temperature is in Celsius
Above pinch Below pinch

The above the diagram can be explained by taking an example from the actual streams.
So in the hot side of the pinch we see a hot stream E-705 going from 175˚C to 125 ˚C with
a heat capacity flow rate of 15.1 KW/K. this would give out a heat of
15.1*[175-125] KW = 755 KW
Now, when we couple this hot stream with a cold stream , E-702, this 755 KW of energy
can be utilised by the cold stream which actually requires energy of 959.03 KW. For the
excess 204.03 KW, that is required for the cold stream, we provide it with a hot utility of
the same amount.
Similarly we obtain the other hot and cold utilities required.

 Results and Discussions:
The exchanger network drawn allows for the MER targets . The streams are coupled and
utilities are distributed keeping in mind the temperature interval of 10, and the
subsequent corollary that heating utilities can only be used above pinch and cooling
utilities can only be used below pinch.
The main purpose of this design is to allow for energy recycle, a term synonymous with
the modern era and the global challenges we face. Though in this process it is not
significant, some processes can self sustain themselves, that is we can achieve all
temperature targets using very little external utilities. This leads to economic benefits and
environmental benefits. The temperature interval method and pinch analysis as shown
here can be utilised for all processes for energy recycle. The CFU here is only one example.
The project has several assumptions of ideality and some figures are rounded off to allow
for smooth calculations. Hence for application of the process, this is a rough schematic,
more accurate measurements, considerations and calculations are required. Also
installation costs and restrictions must be considered when determining heat exchanger
area.
As per the calculation shown,
• 204.03 kW hot utilities
• 2005.55 kW cold utilities
is required.

 Results and Discussions:
Current energy requirement
Heating utilities
QE-701 + QE-702
= 27.2*[33-43] + [-959.03]
= 1231.03 kW
Cooling utilities
QE-7.03 + QE-705 + QE-706
= 627.55 + 15.1*[175-45] + 5.2*[125-40]
= 3032.55 kW
Therefore the energy that can be saved in terms of percentage :-
• 83.4% for heating utilities
• 33.8% for cooling utilities

 Comments:
Upon supervision by the mentor the following drawbacks were observed :-
1. The NGL cooler is not always in functioning. It is used generally only during abnormal
functioning of C-702
2. In this project it was taken into assumption that everything in reboiler vaporised at
100%. However, this is not the case.
3. E-704 was not used in the calculation.

 References:
www.engineeringtoolbox.xom
www.wikipedia.com
www.cheresources.com
Process and Product Design (Seeder)
Nptel open courseware

Pinch analysis technique to optimize heat exchanger

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Pinch analysis technique to optimize heat exchanger