Heat Integration and Heat Exchanger Network Design

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HEAT INTEGRATION
Department of Chemical Engineering University of
Engineering And Technology, Lahore
HEAT INTEGRATION
HEAT EXCHANGER NETWORK DESIGN
A.N.Tabish 2009-MS-Chem-25
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INTRODUCTION
HEAT INTEGRATION
A cold process stream can be heated by using
steam or any hot utility, available at a
temperature higher than the target temperature of
process stream and a hot process stream can be
cold by using cooling water or any cold utility,
available at a temperature lower than the target
temperature of process stream. Situation refers
the maximum use of utility that is merely an
operating cost, and maximum annualized cost that
demands a larger rate of return to make the
process profitable. A cold process stream can be
used to lower the temperature of hot process
stream, which heats up the cold stream as well.
Hence minimizing the use of utility and
ultimately operating cost and total annualized
cost.
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Heat Integration, enables the maximum heat
exchange between process streams using Pinch
Technology, that revealed various methods to
maximize process-to-process heat exchange and
minimized the use of utilities through an
integrated network of heat exchangers. Applicatio
n of Pinch analysis minimizes the energy
consumption of chemical processes by calculating
thermodynamically feasible energy targets and
achieving them by optimizing heat recovery
systems, energy supply methods and process
operating conditions. It is also known as process
integration, heat integration, energy integration
or pinch technology.
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Improvement in the energy consumption obtained
after successive designs of given product.
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PROFITABILITY OPTIMIZATION
Design for Minimum Capital Cost
Design for Minimum Energy Cost
Optimum Design for Minimum Energy Cost
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Simple process flow sheet
Heat integrated flow sheet
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PINCH TECHNOLOGY

1. Data Extraction To formulate the process flow
   sheet and perform heat and mass balance.
2. Minimum Approach Temperature To select the
   minimum temperature difference that can be
   allowed across any heat exchanger in the network.
3. Composite Curves To draw the enthalpy vs.
   temperature graph for cold streams as well as for
   hot streams
4. Minimum utility targets To calculate the minimum
   heating and cooling requirements that must be
   supplied by utility system.

1. Heat Exchanger Network (HEN) Design To design a
   network of heat exchangers to exchange the energy
   between process and utility streams.
2. Network Relaxation and Optimization To modify
   the network to eliminate the small exchangers
   which are not cost effective.
3. Process Change To alter the process conditions
   of unit operations and other streams to maximize
   heat integration.

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DATA EXTRACTION

1. Process Flowsheet
2. Operating Conditions for process streams unit
   operations
3. Heat and mass balance
4. Hot and cold stream allocation

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MINIMUM APPROACH TEMPERATURE (?Tmin)
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Area and cost variation with ?TLM
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COMPOSITE CURVES
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The hot streams plotted separately
The composite hot stream
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The cold streams plotted separately
The composite cold stream
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MINIMUM UTILITY TARGET

• For an efficient network synthesis it is
  required to evaluate MAXIMUM ENERGY RECOVERY
  (MER) so that MINIMUM UTILITY TARGET can be
  selected and network may be designed so as to
  satisfy the energy requirements for each process
  stream that is the determination of minimum hot
  and cold utility requirements in the process.
• Three methods are widely used for the estimation
  of minimum utility requirement.
• Graphical method
• Algebraic method
• Computer based methods

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MINIMUM UTILITY TARGET (Graphical Method)
Production of Maleic Anhydride from Benzene
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HEAT EXCHANGE STREAM DATA
Stream Supply Temperature (oC) Target Temperature (oC) Enthalpy Change rate (kJ/sec)
Streams to be heated Streams to be heated Streams to be heated Streams to be heated
2 30 110 399
4 117 380 5270
17 34 95 310
28 105 161 253
Streams to be cooled Streams to be cooled Streams to be cooled Streams to be cooled
8 400 76.3 -7170
11 76.3 60 -450
Salt 360 340 -7667
32 136 30 -154
36 202 30 -558
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INITIAL TEMPERATURE INTERVAL TABLE
Stream Source Temperature (oC) Target Temperature (oC) Enthalpy Change rate (kJ/sec) Initial Enthalpy Selection Initial Enthalpy Selection
Streams to be Heated (for enthalpy baseline value of 11000 kJ/sec) Streams to be Heated (for enthalpy baseline value of 11000 kJ/sec) Streams to be Heated (for enthalpy baseline value of 11000 kJ/sec) Streams to be Heated (for enthalpy baseline value of 11000 kJ/sec) Enthalpy Temp
2 30 34 5 11000 30
2 17 34 95 397 11005 34
2 95 105 310 11402 95
2 28 105 110 35 11712 105
28 110 117 50 11747 110
5 28 117 161 1025 11797 117
5 161 380 4410 12822 161
Total 6232 Total 6232 17232 380
Streams to be cooled (for enthalpy baseline value of 17000 kJ/sec) Streams to be cooled (for enthalpy baseline value of 17000 kJ/sec) Streams to be cooled (for enthalpy baseline value of 17000 kJ/sec) Streams to be cooled (for enthalpy baseline value of 17000 kJ/sec)
8 400 360 -894 17000 400
8 salt 360 340 -8118 16106 360
8 340 202 -3020 7988 340
8 36 202 136 -1790 4968 202
8, 36 32 136 76 -1645 3178 136
11, 36 32 76 60 -480 1533 76
36 32 60 30 -53 1053 60
Total -16000 Total -16000 1000 30
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Initial composite diagram with ?Tmin 28 oC
Hot stream pinch Temperature 358 oC Cold
stream pinch Temperature 330 oC. So, ?Tmin at
pinch point is 28 oC. While allowable ?Tmin for
chemical processes is 3 - 10 oC.
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Revision of temperature interval table
Slop of the curve at pinch point 0.0475
Intercept of the curve - 440 For y mx
c Modified enthalpy change rate at 358 oC is
16800 kJ/sec. This value of 16800 must be
increased to 17000 to make the temperature
approach at the pinch point to 10oC. Therefore
200 kJ/hr must be added to every enthalpy change
rate value associated with the streams to be
heated. Hence, Difference of baseline enthalpy
change rates 200 kJ/sec So, Revised baseline
enthalpy change rate for streams to be cooled
17200 kJ/sec.
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Revised temperature interval table
Stream Source Temperature (oC) Target Temperature (oC) Enthalpy Change rate (kJ/sec) Initial Enthalpy Selection Initial Enthalpy Selection
Streams to be Heated Streams to be Heated Streams to be Heated Streams to be Heated Enthalpy Temp
2 30 34 5 11000 30
2 17 34 95 397 11005 34
2 95 105 310 11402 95
2 28 105 110 35 11712 105
28 110 117 50 11747 110
5 28 117 161 1025 11797 117
5 161 380 4410 12822 161
Total 6232 Total 6232 17232 380
Streams to be cooled Streams to be cooled Streams to be cooled Streams to be cooled
8 400 360 -894 17200 400
8 salt 360 340 -8118 16306 360
8 340 202 -3020 8188 340
8 36 202 136 -1790 5168 202
8, 36 32 136 76 -1645 3378 136
11, 36 32 76 60 -480 1733 76
36 32 60 30 -53 1253 60
Total -16000 Total -16000 1200 30
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Revised composite diagram with ?Tmin 10 oC
Revised composite diagram with ?Tmin 10 oC
Minimum heating utility required 17232 17200
32 kJ/sec Minimum cooling utility required
11000 1200 9800 kJ/sec
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MINIMUM UTILITY TARGET (Algebraic Method)
Algebraic method is also termed as Problem Table
Method. Although composite curves can be used to
set energy targets, yet they are inconvenient
because they are based on a graphical
construction. The problem table is the name given
by Linnhoff and Flower to a numerical method for
determining the pinch temperatures and the
minimum utility requirements Linnhoff and Flower
(1978) also termed as Temperature Interval
Method. Once understood, it is the preferred
method, avoiding the need to draw the composite
curves.
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Hot stream Temperature
Cold stream Temperature
Shifted temperatures for data
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Shifted temperatures for data
The temperature interval heat balance
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HEAT EXCHANGER NETWORK DESIGN
HEAT EXCHANGER NETWORK DESIGN
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PROBLEM STATEMENT
Given a number NH of process hot streams (to be
cooled) and a number NC of process cold streams
(to be heated), it is desired to synthesize a
cost-effective network of heat exchangers that
can transfer heat from the hot streams to the
cold streams. Given also are the heat capacity
of each process hot streams, FCP,u its supply
(inlet) temperature, Ts,u and its target
(outlet) temperature, Tt,u, where u 1, 2 ,.,
NH. Given also are the heat capacity of each
process cold streams, FCP,v its supply (inlet)
temperature, Ts,v and its target (outlet)
temperature, Tt,v, where v 1, 2 ,., Nc.
Available for service are NHU heating utilities
and NCU cooling utilities whose supply and target
temperatures (but not flowrates) are known.
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CAPITAL AND ENERGY COSTS
Heat exchanger network that would seem
appropriate to most when energy is cheap and
capital expensive.
Heat exchanger network that would seem
appropriate to most when energy is expensive and
capital cheap.
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CAPITAL AND ENERGY COSTS Contd..
Capital Cost f(Thermodynamic effects)
f(Driving forces, Heat loads) Evidently, as we
go to tighter designs (i.e. to reduce driving
forces) we need less utility and the overall heat
load decreases. Capital cost then increases with
reduced driving forces (we all know that) but
decreases with reduced heat load (we rarely
consider this point).
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CAPITAL AND ENERGY COSTS Contd..
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HEAT EXCHANGER NETWORK DESIGN
The process streams are drawn as horizontal
lines, with the stream numbers shown in square
boxes. The Hot streams are drawn at the top of
the grid, and flow from left to right. The cold
streams are drawn at the bottom, and flow from
right to left. The stream heat capacities CP are
shown in a column at the end of the stream lines.
Heat exchangers are drawn as two circles
connected by a vertical line. The circles connect
the two streams between which heat is being
exchanged that is, the streams that would flow
through the actual exchanger. Heater and coolers
are drawn as a single circle, connected to the
appropriate utility.
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THE PINCH DESIGN METHOD

• If the energy target set by the composite curves
  is to be achieved, the
• design must not transfer heat across the pinch
  by
• Process-to-process heat transfer
• Inappropriate use of utilities

QCmin
QHmin
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THE PINCH DESIGN METHOD Contd..
Stream matching is started at the pinch point to
avoid the violation of the assumption of ?Tmin.
If such matches are not made, the result will be
either use of temperature differences smaller
than ?Tmin or excessive use of utilities
resulting from heat transfer across the pinch.
If the design is started away from the pinch at
the hot end or cold end of the problem, then
initial matches are likely to need follow-up
matches that violate the pinch or the ?Tmin
criterion as the pinch is approached.
If the design is started at the pinch, then
initial decisions are made in the most
constrained part of the problem. This is much
less likely to lead to difficulties later.
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
At the pinch, the match starts with a
temperature difference equal to ?Tmin. The
relative slopes of the temperatureenthalpy
profiles of the two streams mean that the
temperature differences become smaller moving
away from the pinch, which is infeasible. On the
other hand, in second Figure match involving the
same hot stream but with a cold stream that has a
larger CP. The relative slopes of the
temperatureenthalpy profiles now cause the
temperature differences to become larger moving
away from the pinch, which is feasible. Thus,
starting with ?Tmin at the pinch, for temperature
differences to increase moving away from the
pinch.
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
If a cold stream is matched with a hot stream
with smaller CP, (i.e. a steeper slope), then the
temperature differences become smaller (which is
infeasible). If the same cold stream is matched
with a hot stream with a larger CP (i.e. a less
steep slope), then temperature differences become
larger, which is feasible. Thus, starting with
?Tmin at the pinch, for temperature differences
to increase moving away from the pinch.
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
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THE PINCH DESIGN METHOD Contd..
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HEAT EXCHANGER NETWORK DESIGN