Cost Minimization And Energy Recovery Using
PINCH TECHNOLOGY
Aakanksha Mahajan
Dr. SS Bhatnagar University Institute of Chemical Engineering
And Technology, Panjab University ,Chandigarh B.E.(Chemical) +
M.B.A(4th year), Roll No. CM9201
AbstractOver the last few years pinch technology has emerged as
somewhat of a maverick development in the process design and energy
conservation scene. Soon after the advent of the technology in the
1970s the process design scene became dominated by the oil crisis.
Energy savings became the primary task of process designers and
thus the primary focus of the technology. Application during this
period typically demonstrated energy cost savings in the range of
20 to 50 percent and payback periods of one year or less. While it
is true that pinch technology is essentially a heat flow based
technique it is also true that it can be used to address a very
diverse range of objectives. This paper gives an outline of the
methodology applied in the technology and its applications in
modern day plants.Keywords
Utility, cold stream, hot stream, DTmin, pinch temperature,
composite curve, grand composite curveIntroduction
Pinch technology is a state of the art technique for design of
energy efficient processing plants. This technique is used to
compute the theoretically minimum utilities consumption for a
process based on the thermal data of process streams i.e. their
temperatures and heat duties in the process. The analysis
establishes the grand composite curve of the process which
represents the net deficit or surplus of heat in the process as a
function of temperature. Pinch point is defined as the temperature
where the net deficit or surplus is zero.
Pinch technology was introduced by B.Linhoff and D.R.Vredeveld
in the year 1978. It is a thermodynamically based method
specifically using the 1st and the 2nd law of thermodynamics. The
1st law helps to calculate the enthalpy of the various streams in
the process and the 2nd law is used to determine the direction of
heat flow.Basic Aims Of The TechnologyThe technology aims to cut
down energy costs by targeting the cost required for utilities in a
plant. A utility is anything that is used to heat or cool a process
stream. Examples include steam, cooling water, brine solution etc.
The technology focuses on two targets:
a) Reduction of amount of utility required
b) Reduction of utility cost by use of intermediate utility.
E.g. Use of cheaper low pressure or medium pressure steam instead
of the costly high pressure steam.
Brief Outline Of The Processa) Identification of hot and cold
streamsThe first step involves the determination of the various hot
and cold streams present in the process. Any stream which has to be
cooled is called a hot stream and any stream which has to be heated
is called a cold stream. The identification of the various streams
involves determining:i. The supply temperature of the streams
ii. The temperature to which the streams have to heated/cooled
i.e. the target temperatureiii. The specific heat capacity of the
streams and the heat capacity flow rateiv. The enthalpy change of
each stream
The following diagram depicts how a typical stream data looks
like:
b) Find the value of DTminDTmin is the temperature approach of a
heat exchanger. It is the minimum temperature difference between
the stream temperature profiles. The value of DTmin has to be
selected very carefully to balance the capital and energy
costs.
A lower value DTmin implies that the temperature difference
between the streams is low and hence a very large area of heat
exchanger is required for the heat exchange which increases the
capital cost. A higher value of DTmin implies that a larger
quantity of utility is required for heating/cooling which increases
the energy cost.
Thus we need to choose an optimum value of DTmin for a
process.
Typical values of DTmin for various industries are:
c) Composite curveA process industry does not contain just one
hot stream or one cold stream. We thus construct a composite curve
which is equivalent to all the hot streams/cold streams in the
process. Thus the next step of the process is to draw the composite
curves for the hot and the cold stream separated by a minimum
temperature difference DTmin. To draw the composite curve we first
draw the temperature enthalpy diagrams for all the hot and cold
streams. The slope of the curves is CP i.e the heat capacity flow
rate of the stream. To draw the hot composite curve we simply add
the slopes of the hot streams i.e. the values of CP for all the
streams. The diagram below depicts how a hot composite curve is
constructed from the various hot streams:
The same process is followed for drawing the cold composite
curve.
The following diagram depicts how a combined composite curve
consisting of both curves the hot composite curve and the cold
composite curve looks like:
The following are the major features of the diagram:i. The
portion of the curves that overlap each other (the highlighted area
in the diagram) depicts the process to process heat exchange i.e.
the heat exchange for which no external utility is required and
process streams can themselves be used to cool and heat each other
thus saving energy.
ii. The off-shoot of the cold composite curve depicts the net
heating duty required i.e. the heating utility requirement.
iii. The off-shoot of the hot composite curve depicts the net
cooling duty required i.e. the cooling utility requirement.
iv. The temperature of minimum separation of the two curves is
DTmin and the point where it occurs is known as pinch point.
Thus a composite curve shows us how energy can be saved by
process to process heat exchange rather than use of external
utility.
d) Shifted composite curve
The next step is to draw the shifted composite curve from the
composite curve. For this we carry out the following changes to the
temperatures of the hot and cold composite curves:
For hot composite curve: -DTmin/2
For cold composite curve: +DTmin/2
Carrying out the above changes and re-plotting the curves gives
us the shifted composite curve.The following diagrams show how a
shifted composite curve is made using a composite curve:
Combined composite curve
Shifted composite curve
e) Grand composite curve
The final step of the process is to construct the grand
composite curve. The curve is constructed by simply subtracting the
enthalpies of the shifted cold and hot composite curves and
plotting the enthalpy differences v/s temperature.
The following is a diagram showing how a grand composite curve
looks like:
The diagram clearly depicts:
i. The heating duty required (20 KW in this case)
ii. The cooling duty required (60 KW in this case)
iii. The pinch temperature (85oC n this case)
This diagram also gives us the definition of the pinch
temperature as the temperature at the point where net heat flow or
surplus/deficit of heat is equal to zero.
Another important thing that can be derived using the grand
composite curve is the use of intermediate utilities.The following
diagram depicts how intermediate utilities can be used to cut down
the cost of utilities used in a process:
Comparing the first two diagrams we see that for the same amount
of heating duty required we can use high pressure steam for a
certain amount of heating duty and very high pressure steam for the
remaining amount of heating duty thus saving energy cost. On
comparing the second and the third diagram we see that the energy
cost has been decreased further by substituting low pressure steam
for a certain amount of duty of high pressure steam.Thus the grand
composite curve enables us to use intermediate utilities and hence
reduce energy cost.
We hence see that the use of the technology has enabled us to
achieve both our aims:
i. Minimizing utility requirement by application of process to
process heat exchange
ii. Minimizing utility cost by use of intermediate utilities by
use of grand composite curve
Basic principles usedThe 3 basic principles involved in the
technology which were used to carry out the above steps are:
No external cooling above the pinch temperature
No external heating below the pinch temperature
No heat transfer between process streams across the pinch
temperature
Violating any of the 3 above principles will lead to higher
energy requirements than the theoretical minimum requirements and
will adversely affect the energy efficiency.Applications
The technology is applicable to any equipment where heat
exchange is taking place. Thus the various applications
(equipment-wise) include: Heat exchangers
Distillation columns
Evaporators
Refrigeration systems
Consequently any process industry using these equipments will
use this technology for cost minimization. Thus the various
applications (industry-wise) include:
Petrochemicals
Petroleum
Bulk chemicals
Pulp and paper
Sugar
Food processing
World and Indian scenario
Various companies around the world have successfully applied
this technology deriving great benefits from it. Some of the
companies are:
Cadbury
British Steel
Shell Oil
Pennzoil
Procter And Gamble
In the Indian sector companies like:
IPCL
Reliance
Indian Oil
Hindustan Petroleum
have successfully used this technology.Indian Pinch
Technologists Dr. Alok Saboo has been the pioneer of pinch
technology in India. He is the director of Midland Plastics Ltd. He
worked with the original team which developed the technology in
U.K. He conducted various pinch studies in leading Indian
companies.
Conclusion
To summarize pinch technology offers a systematic approach for
saving cost and reducing energy consumption. The technology helps
to cut down utility requirement as well as its cost. It thus
provides an alternative to use of various other energy resources
such as heating oil or other fuels which are used as utilities and
are scarce. Although the initial cost for carrying out the analysis
and implementing the changes may be high but the long run benefit
is more than sufficient to cover it.
References
1. Roger Nordman and Thore Berntsson, New Pinch Technology Based
HEN Analysis Methodologies For Cost Effective Retrofitting,
Canadian Journal Of Chemical Engineering, August 2001, Vol 79, page
655-662
2. Linhoff, B and J.R.Flower,Synthesis Of Heat Exchanger
Networks, AIChE Journal Vol 24, No.4, July 1978, page 633-654
3. Linhoff, B and G.T. Polley,General Process Improvements
Through Pinch Technology, Chemical Engineering Progress, June 1988,
page 51-57
4. Tjoe, T.N. and B.Linhoff, Using Pinch Technology For Process
Retrofit, Chemical Engineering, April 1986, page 47-605. Dr. Alok
Saboo and Ms.Mridul Saboo,Optimization Of CHP Systems Using Pinch
Technology
