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CHAPTER 10: ENERGY
INTEGRATION
Chapter 10
EPF 4802
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LEARNING OUTCOME
To understand the basic principles of energy integration
and its application in process design.
To be able to perform pinch analysis
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10.1 INTRODUCTION
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Energy integration
methodology for minimising energy consumption of
chemical / food processes by calculatingthermodynamically feasible energy targets (or minimumenergy consumption) and achieving them by optimising
heat recovery systems, energy supply methods andprocess operating conditions.
a.k.a process integration, heat integration, energyintegration or pinch technology
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10.2 HEAT INTEGRATION
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What is Pinch Analysis?
Pinch Analysis is a method to
Determine utility requirements
Estimate optimal exchanger requirements
Provide an overview of energy flow in the entire process
or across the whole site
Obtain an overall view of the whole steam/power utility
system
All this without designing any heat exchangers.
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Why?
is to achieve financial savings by better process heat
integration (maximizing process-to-process heat recoveryand reducing the external utility loads).
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How?
The process data is represented as a set of energy flows,
or streams, as a function of heat load (kW) againsttemperature (deg C).
These data are combined for all the streams in the plant
to give composite curves, one for all hot streams (releasingheat) and one for all cold streams (requiring heat).
The point of closest approach between the hot and cold
composite curves is the pinch point (or just pinch) with ahot stream pinch temperature and a cold stream pinchtemperature. This is where the design is most constrained.
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Hence, by finding this point and starting the design there,
the energy targets can be achieved using heat exchangersto recover heat between hot and cold streams in twoseparate systems, one for temperatures above pinch
temperatures and one for temperatures below pinchtemperatures.
In practice, during the pinch analysis of an existing design,often cross-pinch exchanges of heat are found between a
hot stream with its temperature above the pinch and acold stream below the pinch. Removal of thoseexchangers by alternative matching makes the process
reach its energy target.
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Pinch technology presents a simple methodology for
systematically analysing chemical processes and thesurrounding utility systems with the help of the First andSecond Laws of Thermodynamics.
The First Law of Thermodynamics provides the energyequation for calculating the enthalpy changes (dH) in thestreams passing through a heat exchanger.
The Second Law determines the direction of heat flow.That is, heat energy may only flow in the direction of hotto cold. This prohibits temperature crossovers of the hotand cold stream profiles through the exchanger unit.
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In a heat exchanger unit neither a hot stream can be
cooled below cold stream supply temperature nor a coldstream can be heated to a temperature more than thesupply temperature of hot stream.
In practice the hot stream can only be cooled to atemperature defined by the temperature approach of theheat exchanger. The temperature approach is theminimum allowable temperature difference (DTmin) inthe stream temperature profiles, for the heat exchangerunit. The temperature level at which DTmin is observedin the process is referred to as "pinch point" or "pinchcondition". The pinch defines the minimum driving forceallowed in the exchanger unit.
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Example of a problem
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How much heating is needed?
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How much cooling is needed?
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Example
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10.4 INTRODUCTION TO HEAT
EXCHANGER NETWORK
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Simple Heat Exchange Network
Look at a simple system:
t1 t2
T1
T2
Hot Stream
Cold Stream
How can we determine the optimal values forHow can we determine the optimal values for
TT22 and tand t22??
t3
T3
Steam
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Simple Heat Exchange Network
T2
T1
T3
Qrec
Duty
Temperature
Tmin
Qhot
Qcold
t2
t1
t3
We can plot temperature vs. duty:We can plot temperature vs. duty:
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Simple Heat Exchange Network
The maximum possible heat recovery is when the two curves pinch
and Tmin = 0
T2
T1
T3
DUTY
T
EMPERATURE
Tmin= 0
Qhot min
Qcold min
t2
t1
t3
Qrec max
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Simple Heat Exchange Network
What happens as Tmin approaches 0? Hot utility (steam) consumption is the lowest.
Cold utility (cooling water) consumption is the lowest.
We still need three heat exchangers
1 process-process exchanger.
1 process-hot utility exchanger.
1 process-cold utility exchanger.
What is Tlm for the process-process exchanger? How big is the process-process heat exchanger?
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Simple Heat Exchange NetworkSimple Heat Exchange Network
We can see that changingWe can see that changing
TTminmin affectsaffects Utility requirements.Utility requirements. Heat exchangerHeat exchanger areasareas
LargeLarge TTminminenergy cost high, overall heat recovery small,energy cost high, overall heat recovery small,
capital cost lesscapital cost less
How can we find an optimumHow can we find an optimum TTminmin?? Design and cost the system for a range ofDesign and cost the system for a range of TTminmin ..
Determine capital costs.Determine capital costs.
Determine operating costs.Determine operating costs.
Combine capital and operating costs to determine an annualizedCombine capital and operating costs to determine an annualized
costcost
Plot annualized cost vs.Plot annualized cost vs. TTminmin
Select the minimumSelect the minimum
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10.4 STEPS IN PINCH ANALYSIS
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7. HEN Design
6. Optimisation of Dtmin
5. Estimation of Heat Exchanger Network
4. Building composite curve
3. Selection of the Initial Dtmin
2. Thermal data extraction
1. Identification of Hot, Cold and utility streams
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Step 1: Identification of the Hot, Cold and
Utility Streams in the Process
Hot Streams are those that must be cooled or are
available to be cooled. e.g. product cooling before storage
Cold Streams are those that must be heated e.g. feedpreheat before a reactor.
Utility Streams are used to heat or cool process streams,when heat exchange between process streams is notpractical or economic. A number of different hot utilities
(steam, hot water, flue gas, etc.) and cold utilities (coolingwater, air, refrigerant, etc.) are used in industry.
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Step 2: Thermal Data Extraction for Process
& Utility Streams
For each hot, cold and utility stream identified, the
following thermal data is extracted from the processmaterial and heat balance flow sheet:
Supply temperature (TS C) : the temperature at which
the stream is available.
Target temperature (TT C) : the temperature the streammust be taken to.
Heat capacity flow rate (CP kW/ C) : the product offlow rate (m) in kg/sec and specific heat (Cp kJ/kg C).CP = m x Cp
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Enthalpy Change (dH) associated with a stream passing
through the exchanger is given by the First Law ofThermodynamics:First Law energy equation: d H = Q WIn a heat exchanger, no mechanical work is beingperformed:W = 0 (zero)The above equation simplifies to: d H = Q, where Qrepresents the heat supply or demand associated with the
stream. It is given by the relationship: Q= CP x (TS - TT).Enthalpy Change, dH = CP x (TS -TT)** Here the specific heat values have been assumed to betemperature independent within the operating range.
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Table 1: Typical Stream Data
Stream
Number
Stream
Name
Supply
Temp
(C)
Target
Temp
(C)
Heat
Cap Flow
(kW/C)
Enth.
Change
(kW)
1 Feed 60 205 20 2900
2 Reac. Out 270 160 18 1980
3 Product 220 70 35 5250
4 Recycle 160 210 50 2500
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Step 3: Selection of the Initial DTmin Value
the temperature of the hot and cold streams at any point
in the exchanger must always have a minimumtemperature difference (DTmin). This DTmin valuerepresents the bottleneck in the heat recovery.
Hot stream Temp. ( TH ) - ( TC ) Cold streamTemp. >= DTmin
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Step 4: Composite Curves
How do we handle multiple streams that have
temperature overlap?
Stream data must be combined in such a way as torepresent the totalenergy sources and totalenergy
demands in each temperature range.
The pinch method creates what is called a compositecurve.
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Temperature - Enthalpy (T - H) plots known as
Composite curves have been used for many years to setenergy targets ahead of design. Composite curves consistof temperature (T) enthalpy (H) profiles of heat
availability in the process (the hot composite curve) andheat demands in the process (the cold composite curve)together in a graphical representation.
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Composite curve
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Composite Curves: Example
Consider a two stage reactor with reheat:
550
510 550
520560
To Next Reactor
Rctr
#1
Rctr
#2
FeedA
B
Streams A and B have overlapping duties between 520 and 550.
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Composite Curves
Range T in T out Streams M*Cp Q
1 510 520 A 1 10
2 520 550 A 1 30
520 550 B 1 30
520 550 A + B 2 60
3 550 560 B 1 10
Multistage reactor exampleMultistage reactor example -- stream datastream data
Plot T vs. Q for each temperature range.
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Composite Curves
500
510520
530
540
550
560
570
0 20 40 60 80 100
Duty (Q)
TEM
PERATURE(T)
Multistage reactor exampleMultistage reactor example -- composite curvecomposite curve
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Composite Curves
500
510520
530
540
550
560
570
0 20 40 60 80 100
Duty (Q)
TEM
PERATURE(T)
There is an easy way to plot the composite curves: just add up the QThere is an easy way to plot the composite curves: just add up the Q
values over each range of Tvalues over each range of T
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Composite Curves for UOP Platforming
Process
0 50 100 150 200 2500
200
400
600
800
1000
Duty (MMBtu/h)
Temperature(F)
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Composite Curves
0 50 100 150 200 2500
200
400
600
800
1000QH
QC
Pinch
We can set targets for hot
and cold utilities using the
composites, while paying
attention to the process
pinch
Duty (MMBtu/h)
Temperature(F)
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Composite Curves
0 50 100 150 200 2500
200
400
600
800
1000QH
QC
Pinch
Since the duty scale is a
difference in enthalpy, we
can slide the composite
curves horizontally,
increasing or decreasing
Tmin
Duty (MMBtu/h)
Temperature(F)
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Composite Curves
0 50 100 150 200 2500
200
400
600
800
1000QH
QC
Pinch
Duty (MMBtu/h)
Temperature(F)
If we decrease Tmin then
our utility targets are
reduced
What is the effect on capital
cost though?
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4. Grand Composite Curve
Tool that is used for setting multiple utility
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Capital Targets
We can then plot area targets against Tmin We can also introduce a correlation for cost vs. area and
hence plot a capital target against Tmin
Hence find optimum Tmin
0
20
40
60
80
100
120
140
0 10 20 30 40 50 60Tmin
Cost(106$
/y)
Utility Costs Annualized Capital Cost Total Cost
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Step 6: Optimization of Tmin To arrive at an optimum DTmin value, the total annual cost
(the sum of total annual energy and capital cost) is plotted atvarying DTmin values
An increase in DTmin values result in higher energy costs andlower capital costs.
A decrease in DTmin values result in lower energy costs andhigher capital costs.
An optimum DTmin exists where the total annual cost ofenergy and capital costs is minimized.
Thus, by systematically varying the temperature approach wecan determine the optimum heat recovery level or theDTminOPTIMUM for the process.
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Optimization of Tmin
What happens asWhat happens as TTminmin is increased?is increased? More heat exchangers are requiredMore heat exchangers are required (extra cost)(extra cost)
Log mean temperature differences arLog mean temperature differences are greatere greater
Each heat exchanger is smallerEach heat exchanger is smaller
The cost for each heat exchanger decreases (cost savings)The cost for each heat exchanger decreases (cost savings)
More utilities are consumedMore utilities are consumed
Cooling water demand increasesCooling water demand increases
Steam demand increasesSteam demand increases
Utility costs increaseUtility costs increase
Note: hot utility increase = cold utility increaseNote: hot utility increase = cold utility increase
How do we decide on the appropriateHow do we decide on the appropriate TTminmin?? Same as the twoSame as the two--stream problemstream problem
Plot Total Annualized Cost vs.Plot Total Annualized Cost vs. TTminmin for the processfor the process
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Optimization of Tmin
TminOPTIMIZATION
0
20
40
60
80
100
120
140
0 20 40 60Tmin
Cost(106$
/y)
Utility Costs Annualized Capital Cost
Total Cost
Tmin opt
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Energy Costs
Energy prices are often assumed to be well knownEnergy prices are often assumed to be well known See Ch6 & lecture on operating costsSee Ch6 & lecture on operating costs
In practice, energy prices are affected by:In practice, energy prices are affected by: Commodity nature of fuelsCommodity nature of fuels
Fuel mixFuel mix
Flaring of waste products (fuel value vs. disposal cost)Flaring of waste products (fuel value vs. disposal cost)
Capital cost implications of fuel substitutionCapital cost implications of fuel substitution
So the actual energy price varies with time and is seldom properlySo the actual energy price varies with time and is seldom properly
capturedcaptured
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Thermodynamic Significance of the Pinch
When the process is pinched it isWhen the process is pinched it is
decomposed into two sub problemsdecomposed into two sub problems
Qhot min
Qcold minQrec max
pinch
Temperature
Duty
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Pinch Decomposition
When the process is pinched it isWhen the process is pinched it is
decomposed into two sub problemsdecomposed into two sub problems
Qhot min
Qcold minQrec max
pinch
Temperature
Duty
Above the pinch we
only put in utility heatand the process acts
as a heat sink
Below the pinch we only reject
heat to cold utility and the
process acts as a heat source
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Pinch Decomposition
What if we put in extra heat above the pinch?What if we put in extra heat above the pinch?
Qhot min
Qcold min
Qrec max
pinch
Temperature
Duty
Qextra
Heat sink is now out
of energy balance
and we have toreject Qextra to a
lower temperature
Qextra
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Pinch Decomposition
What if we put in extra heat above the pinch?What if we put in extra heat above the pinch?
Qhot min
Qcold min
Qrec max
pinch
Temperature
Duty
Qextra
Qextra
Qextra
Now the heat
source is also out of
energy balance and
we have to reject
Qextra to cold utility
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Pinch DecompositionThe overall effect is that both hot and cold utility are increased by the amount of heat
transferred across the pinch = Qextra
Qhot min
Qcold minQrec max
pinch
Temperature
Duty
Qextra
Qextra
Qextra
So a simple rule for achieving energy targets is dont transfer heat across the
pinch!
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Rules
No external heating below the Pinch.
No external cooling above the Pinch.
No heat transfer across the Pinch.
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Constraints
Constructing a composite curve or grand composite
curve is not easy to be done manually.
Graphical constructions are not the most convenientmeans of determining energy needs. A numerical
approach called the "Problem Table Algorithm" (PTA) wasdeveloped by Linnhoff & Flower (1978) as a means ofdetermining the utility needs of a process and thelocation of the process pinch. The PTA lends itself to hand
calculations of the energy targets.
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Problem Table Analysis
Convert Tact into Tint by substracting half the min T
difference from hot stream temperatures, and adding halfto the cold stream T.
Rank the interval temperatures
Obtain the net heat required for each interval
Cascade the heat surplus
Introduce just enough heat to the top of cascade to
eliminate negative values
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Step 7: HEAT EXCHANGER NETWORK
(HEN) DESIGN
Can be represented by grid representation
Hot streams on top, (flow from left to right) cold onbottom, (flow from right to left)
CP are shown at the end of stream
HE are drawn as circles connected by vertical line
HE connect the 2 streams which heat is being exchanged
As the pinch divides the heat exchange system into two
thermally independent regions, HENs for both above andbelow pinch regions are designed separately.
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Having made all the possible matches, the two designs
above and below the pinch are then brought together andusually refined to further minimize the capital cost. Afterthe network has been designed according to the pinch
rules, it can be further subjected to energy optimization.
Optimizing the network involves both topological andparametric changes of the initial design in order tominimize the total cost.
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Data Extraction
We need to extract the data from a flowsheetWe need to extract the data from a flowsheet
to do the pinch analysisto do the pinch analysisExisting
ProcessFlowsheet
New Design
PinchAnalysis
Data
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But there is a catch:But there is a catch:
Existing
ProcessFlowsheet
New Design
PinchAnalysis
Data
Too many constraints
from the existing
design
Data Extraction
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S
Usually the easiest way to start is by readingUsually the easiest way to start is by reading
the heat loads from the flowsheetthe heat loads from the flowsheet
feedfilter
10
H1H2 H3
120
100
25
120 200
15030
15025
70
T
dH
70
2
5
10
150
H1 H2 H3
Data Extraction
How many streams do weHow many streams do we
have?have?
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S
Whats the problem with this approach?Whats the problem with this approach?
feedfilter
10
H1H2 H3
120
100
25
120 200
15030
15025
70
T
dH
70
2
5
10
150
H1 H2 H3
We need to be very careful thatwe do not miss phase changes!
Data Extraction
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When we have a vaporization we need toWhen we have a vaporization we need to
linearize. How do we do this?linearize. How do we do this?
T
dH
70
2
5
10
150
H1 H2 H3
Data Extraction
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When we have a vaporization we need toWhen we have a vaporization we need to
linearize. How do we do this?linearize. How do we do this?
T
dH
70
2
5
10
150
H1 H2 H3
(A) Point to Point
Data Extraction
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When we have a vaporization we need toWhen we have a vaporization we need to
linearize. How do we do this?linearize. How do we do this?
T
dH
70
2
5
10
150
H1 H2 H3
(B) Piecewise best fit
Data Extraction
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When we have a vaporization we need toWhen we have a vaporization we need to
linearize. How do we do this?linearize. How do we do this?
T
dH
70
2
5
10
150
H1 H2 H3
(C) Above the line
Data Extraction
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When we have a vaporization we need toWhen we have a vaporization we need to
linearize. How do we do this?linearize. How do we do this?
T
dH
70
2
5
10
150
H1 H2 H3
(D) Below the line
Data Extraction
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To be conservative we always linearize on the safe side.To be conservative we always linearize on the safe side.
You can always add more detail near to the pinch if you are concernedYou can always add more detail near to the pinch if you are concernedabout the accuracy of the linearizationabout the accuracy of the linearization
T
dH
Data Extraction
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Soft constraintsSoft constraints
100
The constraint is soft and can be changed.Good opportunity for process modification!
& why not?Why?
StorageTank
100
240 110Storage
Tank
110
240
Data Extraction
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Data Extraction Rules
1. Dont incorporate non-essential features of the existing design2. Watch for phase changes
3. Linearize on the safe side
4. Extract data for isothermal mixing
5. Do not extract utilities
6. Adjust soft constraints to improve targets
2012 G.P. Towler / UOP. For educational use in conjunction withTowler & Sinnott Chemical Engineering Design only. Do not copy
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Example
Energy Integration of the Four Streams
Page 124-137
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use in conjunction with
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2007 G.P. Towler / UOP. For educational
use in conjunction with
Streamnumber
Type Cp(kW/C)
Ts (C) Tt (C) Heat load(kW)
1 Hot 3 180 60 360
2 Hot 1 150 30 120
3 Cold 2 20 135 230
4 cold 4.5 80 140 270
Table 3.2: Data for Heat Integration Problem
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2007 G.P. Towler / UOP. For educational
use in conjunction with
Figure 3.21: a) Separate hot streams, b) Composite hotstreams
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2007 G.P. Towler / UOP. For educational
use in conjunction with
Figure 3.22: Hot & cold stream composite curves
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The problem table method- Determine the
pinch temperature
2007 G.P. Towler / UOP. For educational
use in conjunction with
Figure 3.24:Heat cascade
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2007 G.P. Towler / UOP. For educational
use in conjunction with
Figure 3.25: Grid representation
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2007 G.P. Towler / UOP. For educational
use in conjunction with
Figure 3.27: Network design above the pinch
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use in conjunction with
Figure 3.28: Proposed heat-exchanger network for Tmin =10C
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2007 G.P. Towler / UOP. For educational
use in conjunction with
Stream splitting is needed when:
the heat capacities of streams are not possible to make amatch at the pinch without violating the minimumtemperature difference condition. See Example 3.16
There are not enough streams available
The guide rules for devising a network for maximum heatrecovery, refer page 136.
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More examples