epf 4802 chap 10 heat integ

8/12/2019 Epf 4802 Chap 10 Heat Integ

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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.

2012 G.P. Towler / UOP. For educational use in conjunction withTowler & Sinnott Chemical Engineering Design only. Do not copy


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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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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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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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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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T

dH

70

2

5

10

150

H1 H2 H3

(A) Point to Point

Data Extraction



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T

dH

70

2

5

10

150

H1 H2 H3

(B) Piecewise best fit

Data Extraction



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T

dH

70

2

5

10

150

H1 H2 H3

(C) Above the line

Data Extraction



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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



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Example

Energy Integration of the Four Streams

Page 124-137

2007 G.P. Towler / UOP. For educational

use in conjunction with


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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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Figure 3.21: a) Separate hot streams, b) Composite hotstreams


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Figure 3.22: Hot & cold stream composite curves


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The problem table method- Determine the

pinch temperature



Figure 3.24:Heat cascade


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Figure 3.25: Grid representation


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Figure 3.27: Network design above the pinch


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Figure 3.28: Proposed heat-exchanger network for Tmin =10C


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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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