Lecture-III Basics of Pinch Analysis.pdf

Lecture –III
Pinch Analysis and Process Integration
Nigus Gabbiye Habtu (PhD)
2015E.C
Faculty of
Chemical and Food Engineering
Bahir Dar Institute
of Technology

Heat Exchanger Networks
19/12/2022 Lecture on Integrated Process Design by Nigus Gabbiye(PhD) 2
❑ Why Does Pinch Analysis Work?
▄ Approach in Pinch Analysis
▪ Thermodynamic Approach
▪ Energy Targets and Solution/Design
─ The Composite method
─ Grand composite curves
─ The problem table Algorithm
▪ Capital and Total Cost Targets

Pinch Technology and targeting Heat Recovery: the
thermodynamic roots
• Heat Recovery can be used to provide either heating or cooling to
processes to replace hot or cold utilities.
– It is widely applied in industry and has an extensive historical record.
– Systematic methods for performing Heat Recovery have emerged in
the last 40 years inspired by the 1970s oil crises
• Heat Recovery may take various forms:
– transferring heat between process streams,
– generating steam from higher temperature process waste heat,
– preheating a service stream (air for a furnace, as well as air
or feed water for a boiler) by using excess process heat.
• Heat Exchanger Networks (HEN) synthesis – one of the very important
and common tasks of process design – has become the starting point for
the Process Integration revolution in industrial systems design
19/12/2022 Lecture on Integrated process Design by Nigus Gabbiyev(PhD) 3
❑ Why Does Pinch Analysis Work?

thermodynamic roots
▪ A hot process stream can
supply heat to a cold one
when paired: Heat Exchange
matches
Heat Exchanger Matches
➢ Heat Recovery Pinch concept (discovered Linnhoff and Flower, 1978)
was a critical step in the development of HEN synthesis.

thermodynamic roots
• The main idea behind the formulated procedure was to obtain – prior
to the core design steps – guidelines and targets for HEN
performance.
• The hot and cold streams for the process under consideration are
combined to yield
1) a Hot Composite Curve collectively representing the process
• heat sources (the hot streams);
2) a Cold Composite Curve representing in a similar way the
process
• heat sinks (the cold streams)
• This procedure is possible through thermodynamics
principles.
How?

What is pinch analysis?
• The analysis of the heat exchanger network:
– First identifies sources of heat (termed hot streams)
– Second identifies sources of sinks (termed cold streams)
The material
and energy
balance.
▪ Targets can be set for the heat exchanger network to assess the performance of the
complete process design without actually having to carry out the network design.
▪ These targets allow both energy and capital cost for the heat exchanger network
to be assessed.
▪ The targets allow the designer to suggest process changes for the reactor and
separation and recycle systems to improve the targets for energy and capital cost of
the heat exchanger network( Onion Model)

• The second law of thermodynamics implies that heat flows from higher
temperature to lower temperature locations.
– In a heat exchanger the required heat transfer area is proportional to the
temperature difference between the streams.
Figure: Thermodynamic limits on Heat Recovery
• In heat exchanger design, the minimum
allowed temperature difference (ΔTmin) is the
lower bound on any temperature differences
to be encountered in any heat exchanger in
the network
▪ The value of ΔTmin is a design parameter
determined by exploring the trade-offs
between more Heat Recovery and the larger
heat transfer area requirement
1. Heat Recovery = 10 MW, for ΔTmin = 20 °C.
2. Heat Recovery = 11 MW for ΔTmin = 10 °C
More hear is possible to “squeeze out” by lowering the temperature difference

What is pinch analysis? Conceptually!!!!
• When designing heat exchangers and other unit operations, limitations imposed
by the first and second laws of thermodynamics constrain what can be done
with such equipment.
– For example, in a heat exchanger, a close approach between hot and cold
streams requires a large heat transfer area.
Pinch point
➢ Whenever the driving forces for heat or mass exchange are small, the equipment needed
for transfer becomes large and it is said that the design has a pinch.

• Likewise, in a distillation column, as the reflux ratio approaches the
minimum value for a given separation, the number of equilibrium
stages becomes very large.
• .
• The intersection of an operating line and the equilibrium curve
is called a pinch point

▪ The pinch point is the location in the heat exchanger where the temperature
difference between hot and cold fluid is minimum at that location.
The pinch point is important for analyzing heat transfer in thermodynamic
cycles
Definition
➢ When considering systems of many
heat- or mass-exchange devices
(called exchanger networks), there
will exist somewhere in the system a
point where the driving force for
energy or mass exchange is a
minimum. This represents a pinch or
pinch point.

• The general algorithm is presented to give the minimum number of
exchangers requiring the minimum utility requirements for a given
minimum approach temperature.
▪ Pinch analysis is a methodology for minimizing energy consumption of
chemical processes by calculating thermodynamically feasible energy
targets (or minimum energy consumption) and achieving them by
optimizing heat recovery systems, energy supply methods
and process operating condition
➢ Pinch analysis is based on straightforward thermodynamics, and uses it in a
practical way. However, the approach is largely non-mathematical.

▪ The first key concept of pinch analysis is setting energy / temperature
targets. “Targets” for energy reduction have been a key part of energy
monitoring schemes for many years.
▪ Typically, a reduction in plant energy consumption of 10% per year is
demanded.
• The successful design of these networks involves defining where the
pinch exists and using the information at the pinch point to design the
whole network. This design process is designed as pinch technology
• The concepts of pinch technology can be applied to a wide variety of
problems in heat and mass transfer
1. Heat exchanger networks (HENs)
2. mass-exchanger networks (MENs)

• What is the difference between the two reactor systems??
– the way in which the heat exchange takes place
a) Without Heat Integration, b) With Heat Integration
• The savings received over the life of the plant by using heat integration are (–
471,000 + 1,636,000) = $1,165,000!
The heat integration saves money in two ways:
(1) The cooling water utility is reduced and the high-pressure steam is eliminated,
(2) Heat exchanger E-203 is smaller because the duty is reduced, and E-202 is also
smaller due to the fact that hps condenses at 254°C
DME Reactor Feed and Effluent Heat Exchange System

Basic concepts of pinch analysis system
Consider this process(chemical reactor system with heating & cooling
requirements)
Definitions:
▪ The feed, which starts cold and needs to be heated
up, is known as a cold stream.
▪ The hot product which must be cooled down is called
a hot stream.
Mass
flowrate
W(kg/s)
Specific heat
capacity
Cp (kI/kgK)
Hheat
capacity
flowrate
Cp (kW/K)
Initial
Temperature
Ts(oC)
Final/Target
Temperature
Ts(oC)
Heat Load
H(kW)
Cold stream 0.25 4 1.0 20 200 -180
Hot stream 0.4 4.5 1.8 150 50 +180
Stream information
❑ How do you supply those heat loads??
▪ By external heating and cooling! What else???

Can we reduce energy consumption?
▪ Yes; if we can recover some heat from the hot stream and use it to
heat the cold stream in a heat exchanger, we will need less steam
and water to satisfy the remaining duties.
▪ Ideally we can
extract all of
180kW to heat the
cold stream
Cold stream
Heat stream
▪ This is not possible because of temperature limitations.
▪ By the Second Law of Thermodynamics, we can’t use a hot stream at
150°C to heat a cold stream at 200°C!

Basic concepts of Pinch: Data extraction
Data extraction: Heat Recovery problem identification
• For efficient Heat Recovery in industry, the relevant data must be identified
and presented systematically.
– In the field of Heat Integration, this process is referred to as data extraction
• The Heat Recovery problem data are extracted in several steps.
1. Inspect the general process flowsheet, which may contain Heat Recovery
exchangers.
2. Remove the recovery heat exchangers and replace them with equivalent
“virtual” heaters and coolers.
3. Lump all consecutive heaters and coolers.
4. The resulting virtual heaters and coolers represent the net heating and
cooling demands of the flowsheet streams.
5. The heating and cooling demands of the flowsheet streams are then listed
in a tabular format, where each heating demand is referred to as a cold
stream and, conversely, each cooling demand as a hot stream.

Basic concepts of Pinch: Data extraction
Class Activities: Flowsheet with two hot streams and two cold streams
Product 1
40ºC
Coln
40ºC
Off Gas
Reactor 2
ΔH=-30 MW
Reactor 1
Feed 2
80ºC
200ºC
180ºC
230ºC
20ºC
140ºC
250ºC
Product 2
Feed 1
ΔH=27 MW
ΔH=32 MW
ΔH=-31.5 MW
40ºC
o Total hot streams heat duty =
61.5 MW (Surplus)
o Total cold streams heat duty =
59 MW (Deficit)
1. Tabulate the four streams
2. Generate the composite curve
of those data

• Basic concepts of heat exchange: The temperature–enthalpy diagram:
For constant CP
➢ the slope of the line
representing the stream is:

Basic concepts of pinch analysis system:
The temperature–enthalpy diagram:
• The general algorithm is presented to give the minimum number of
exchangers requiring the minimum utility requirements for a given minimum
approach temperature.
1. Choose a minimum approach temperature. This is part of a
parametric optimization
2. Construct a temperature interval diagram.
3. Construct a cascade diagram, and determine the minimum utility
requirements and the pinch temperatures.
4. Calculate the minimum number of heat exchangers above and below
the pinch.
5. Construct the heat-exchanger network

The temperature–enthalpy diagram
➢ The T/H diagram can be used to represent heat exchange:
➢ For feasible heat exchange between the two, the hot stream must at all
points be hotter than the cold stream, vis versal is also true
T/H diagram with ΔTmin 0°C
• How big the heat
exchanger will determine
the overall cost

The temperature–enthalpy diagram:
T/H diagram with ΔTmin 20°C
▪ The cold stream is shifted on the H-axis
relative to the hot stream so that the
minimum temperature difference, ΔTmin is
20°C.
▪ The effect of this shift is to increase the
utility heating and cooling by equal
amounts and reduce the load on the
exchanger by the same amount – here 20
kW – so that 70 kW of external heating
and cooling is required
Two basic facts are emerge.
1. There is a correlation between the value of ΔTmin in the exchanger and the total utility load
on the system. This means that if we choose a value of ΔTmin, we have an energy target
for how much heating and cooling we should be using if we design our heat exchanger
correctly.
2. If the hot utility load is increased by any value α, the cold utility is increased by α as well. As
the stream heat loads are constant, this also means that the heat exchanged falls by α.

The composite Curve(Multiple streams)
▪ To handle multiple streams, we add together the heat loads or heat capacity
flow rates of all streams existing over any given temperature range.
▪ A single composite of all hot streams
▪ A single composite of all cold streams can be produced in the T/H
diagram,

Basic concepts of pinch analysis system: The composite
Curve(Multiple streams)
• a composite segment is formed consisting of:
1. a temperature difference equal to that of the interval
2. a total cooling requirement equal to the sum of the cooling
requirements of all streams within the interval by summing up
the heat capacity flow rates of the streams
Add up
∆H1 = CPA(T1-T2);
∆H2 = (CPA +CPB+ CPC)(T2-T3);
∆H3 = (CPA+CPC)(T3-T4);
∆H4 = (CPA(T4-T5)
▪ The resulting T/H plot is a single
curve representing all the hot
streams, known as the hot
composite curve
➢ A similar procedure gives a cold composite curve of all the cold streams in a problem

• Cold and hot Composite Curves are combined in the same plot in order to
identify the maximum overlap, which represents the maximum amount of heat
that could be recovered. The HCC and CCC for following data
• The overlap between the two Composite Curve s on the Heat
Exchange axis represents the Heat Recovery target – i.e. the
maximum amount of process heat being internally recovered
▪ The targets for external
(utility) heating and
cooling are represented
by the non-overlapping
segments of the Cold
and Hot Composite
Curves.

• Both CCs can be moved horizontally (i.e., along the ΔH axis), but usually the
HCC position is fixed and the CCC is shifted. This is equivalent to varying the
amount of Heat Recovery and (simultaneously) the amount of required utility
heating and cooling. Where the curves overlap, heat can be recovered between
the hot and cold streams. More overlap means more Heat Recovery and smaller
utility requirements, and vice versa.

• The appropriate value for ΔTmin is determined by economic trade-offs.
Increasing ΔTmin results in larger minimum utility demands and increased
energy costs; choosing a higher value reflects the need to reduce heat transfer
area and its corresponding investment cost. Conversely, if ΔTmin is reduced
then utility costs go down but investment costs go up.
➢ Trade-off is illustration for
maximum heat recovery and
capital cost.
▪ How much energy can we extract?
▪ How big should the exchanger be
▪ What will be the temperatures around it?

• Example -2: A typical pair of composite curves for the four-stream
given in Table below.
▪ Overlap between the composite curves represents the maximum amount of heat recovery possible
▪ Overshoot at the bottom represents the minimum amount of external cooling required
▪ Overshoot at the top represents the minimum amount of external heating required

Basic concepts of heat exchange: The composite
➢ There are three possible ways of moving the hot and cold composite
curves closer together by ΔTmin, so that they touch at the pinch.
1. Express all temperatures in terms of hot stream temperatures and
increase all cold stream temperatures by ΔTmin.
2. Express all temperatures in terms of cold stream temperatures and
reduce all hot stream temperatures by ΔTmin.
3. Use the shifted temperatures, which are a mean value; all hot stream
temperatures are reduced by ΔTmin/2 and all cold stream temperatures
are increased by ΔTmin/2.
➢ Approach 3 has been the most commonly adopted, we will follow for
designing of the heat exchanger network

Basic concepts of pinch analysis system: The Problem
Table Algorithm
▪ It is the preferred method, avoiding the need to draw the composite
curves and maneuver the composite cooling curve using, for example,
tracing paper or cut-outs, to give the chosen minimum temperature
difference on the diagram.
▪ The Composite Curves are a useful tool for visualising Heat Recovery
targets. However, they can be time consuming to draw for problems that
involve many process streams. In addition, targeting that relies solely on
such graphical techniques cannot be very precise.
▪ A method of calculating energy targets directly without the necessity of
graphical construction is called the Problem Table Algorithm(Linnhoff
and Flower (1978)
▪ It provides pinch temperatures and the minimum utility requirements;

Basic concepts of pinch analysis system: The Problem
Table Algorithm
• The steps are as follows:
1. Shift the process stream temperatures.
2. Set up temperature intervals.
3. Calculate interval heat balances.
4. Assuming zero hot utility, cascade the balances
as heat flows.
5. Ensure positive heat flows by increasing the hot
utility as needed.

Basic concepts of pinch analysis: The Problem Table
Algorithm
The procedure follows
1. Convert the actual stream temperatures Tact into interval temperatures
Tint by subtracting half the minimum temperature difference(ΔTmin)
from the hot stream temperatures, and by adding half to the cold
stream temperatures:
➢ The use of the interval temperature rather than the actual temperatures allows the minimum
temperature difference to be taken into account. ∆Tmin = 10oC for the problem being
considered;
Example: Table 2: Interval temperatures for ∆Tmin = 10°C

Basic concepts of pinch analysis : The Problem Table
Algorithm
➢ The amount that can be recovered depends on the relative slopes of the
two curves in the temperature interval.
➢ This problem can be overcome if, purely for the purposes of construction, the
hot composite is shifted to be ∆Tmin /2 colder than it is in practice and that the
cold composite is shifted to be ∆ Tmin /2 hotter than it is in practice as shown
in Figure b. The shifted composite curves now touch at the pinch.

Algorithm
2. Rank the interval temperatures in order of magnitude, showing the duplicated
temperatures only once in the order. Temperature intervals are formed by listing all
shifted process stream temperatures in descending order.
Example: Table 3: Interval temperatures for ∆Tmin = 10°C

Algorithm
3. Carry out a heat balance for the streams falling within each temperature
interval: For the nth interval:
Where:-
➢ First, the stream population of the process segments falling within each
temperature interval (the second two columns of Table 3) is identified. The
sums of the segment CPs (heat capacity flow rates) in each interval are
calculated; then that sum is multiplied by the interval temperature difference
(i.e., the difference between the TBs that define each interval). This
calculation is also illustrated in Table 3

Algorithm
4. “Cascade” the heat surplus from one interval to the next down the column
of interval temperatures; see figure below.
• Each box contains the corresponding interval enthalpy
balances.
• The boxes are connected with heat flow arrows in
order of descending temperature.
o The top heat flow represents the total hot utility
provided to the cascade,
o the bottom heat flow represents the total cold utility.
• The hot utility flow is initially assumed to be zero and
this value is combined (summed up) with the enthalpy
balance of the top cascade interval to produce the
value for the next lower cascade heat flow.
• This operation is repeated for the lower temperature
intervals and connecting heat flows until the bottom
heat flow is calculated, resulting in the cascade
shown in Figure

Algorithm
5. Introduce just enough heat to the top of the cascade to
eliminate all the negative values; see Figure 1b.
▪ From the cascading heat flows, the smallest
value is identified; if it is nonnegative then the
heat cascade is thermodynamically feasible. If a
negative value is obtained, then a positive
utility flow of the same absolute value has to be
provided at the topmost heat flow, after which
the cascading described in Step 4 is repeated.
▪ The resulting heat cascade is guaranteed to be
feasible and provides numerical Heat Recovery
targets for the problem. The topmost heat flow
represents the minimum hot utility, the
bottommost heat flow represents the minimum
cold utility, and the TB with zero heat flow
represents the location of the (Heat Recovery)
Pinch. It is often possible to obtain more than
one zero-flow temperature boundary, each
representing a separate Pinch point.
Cold Utility

Algorithm
• Example 2:
Figure: A simple flowsheet with two
hot streams and two cold streams.
Table3: Heat exchange stream data for the flowsheet
Table 4: Shifted temperatures for the data from Table 3
∆Tmin/2 = 5OC

Algorithm
1st interval , 1 stream
2nd interval , 2 streams
3rd interval , 3 streams
4th interval , 4 streams
➢ Rank the interval temperature based
on the shifted temperature!!!!

Algorithm
• Heat balance within each shifted temperature interval.
• Some of the shifted intervals are seen to have a surplus of heat and some have a
deficit

Algorithm
➢ cascade any surplus heat
down the temperature scale
from interval to interval. This is
possible because any excess
heat available from the hot
streams in an interval is hot
enough to supply a deficit in
the cold streams in the next
interval down.
▪ Some of the heat flows in Figure(a) are negative, which is infeasible. Heat cannot be
transferred up the temperature scale.
▪ To make the cascade feasible, sufficient heat must be added from hot utility to make the
heat flows to be at least zero. The smallest amount of heat needed from hot utility is the
largest negative heat flow from Figure (a), that is 7.5 MW.
QHmin = 7.5 MW
QCmin = 10 MW.
• First, assume no heat is supplied to the first
interval from hot utility

Basic concepts of pinch analysis : Grand composite
curve (GCC)
▪ If the composite curves are re-plotted on axes of shifted temperature, we obtain the
shifted composite curves,(see figure). The shifted curves just touch at the pinch
temperature, and show even more clearly than the composite curves that the pinch
divides the process into two.
There is an imbalance which must be supplied by utilities – external heating and cooling.
▪ Above the pinch, ΔQC > ΔQH and the difference must be supplied by hot utility.
▪ Below the pinch ΔQH < ΔQC and the excess heat is removed by cold utility.
▪ Shifted hot and cold composite curves

• The parts with positive slope (i.e., running uphill from left to right) indicate that cold
streams dominate. Similarly, the parts with negative slope indicate excess hot
streams. The shaded areas, which signify opportunities for process-to-process Heat
Recovery, are referred to as Heat Recovery pockets

• Relation between the GCC (left) and the SCC (right)

Basic concepts of pinch analysis: Grand composite
curve (GCC)
Net heat flow(kW)
0 20 40 60 80 100
Shifted
temperature(oC)
0
20
40
60
80
100
120
140
160
180
Heat duty 20 kW
Cooling duty 60 kW
Pinch T(s) = 850C
Grand composite curve(GCC)
Net Heat
flow(kW)
Shifted
Temperature(oC)
0
60 25
75 55
0 85
82.5 140
80 145
20 165
▪ It represents the difference between the heat available from the hot streams and the
heat required by the cold streams, relative to the pinch, at a given shifted
temperature.
.
▪ The Problem Table and its graphical
representation, the GCC, give the same
results (including the pinch location) more
easily.
▪ Energy targeting is a powerful design and
“process integration” aid.

Algorithm
▪ Three golden rules for the designer wishing to produce
a design achieving minimum utility targets
▪ Don’t transfer heat across the pinch.
▪ Don’t use cold utilities above the pinch.
▪ Don’t use hot utilities below the pinch.

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