Heat Integration_Setting Energy Targets

7/28/2019 Heat Integration_Setting Energy Targets

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HEAT INTEGRATIONSETTING ENERGY TARGET


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Process Energy Targets

Process energy targets (minimumsteam and cooling waterrequirements) can be obtained from

Composite Curves

Cumulative process heat availability(surplus)

Cumulative process heat requirement(deficit)

Problem Table Algorithm

Process heat surpluses and deficits within

some specified temperature intervals


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Process Energy Targets from

the Composite Curves

Representation of process streams heatcontent on a plot of temperature (T)

versus enthalpy (H)


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Example 1: Two-stream Heat

Recovery Problem

Stream Type Supply

Temp.

(oC)

Target

Temp.

(oC)

H (MW)

1 Cold 40 110 14

2 Hot 160 40 -12


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Two-stream Heat Recovery

Problem


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Streams can be shifted horizontally


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Example 2

A simple flowsheet with two hot streams and two cold streams


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Stream Data For The Flowsheet

Stream Type Supply

Temp. Ts

(oC)

Target

Temp.

TT (oC)

H (kW) Heat Capacity

Flowrate

CP (kW oC-

1)

Reactor 1Feed Cold 20 180 3200 20

Reactor 1

Product

Hot 250 40 -3150 15

Reactor 2Feed Cold 140 230 2700 30

Reactor 2

Product

Hot 200 80 -3000 25


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Effect ofTmin

Tmin = the smallest approach temperature (T)for heat exchange

Bigger Tmin gives:

Less heat rocovery Maximum hot and cold utilities

Maximum energy consumption

Big capital cost

Small Tmin gives: Maximize energy recovery

Minimize hot and cold utilities

Minimize energy consumption


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Summary (i)

Temperature-enthalpy diagrams canbe used to determine heat recoverypotential

Composite curves can be used totarget for many hot streams andmany cold streams

Energy targets set from material andenergy balance and Tmin

Can be varied to different targets


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Working Session 1

a) Set up stream data tableb) Construct composite curvesc) Read energy targets for Tmin = 10C

1

2

4

CP=1.8


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Targeting the Minimum

Energy Requirement using



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What is the problem with the CC??

1) Complicated2) Cant get accurate point for

T pinch, QHmin and Qcmin


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2 Approaches:

Hot and Cold Temperature Intervals

Global (Shifted) Temperature Intervals

General Idea Get net H for each heat/enthalpy interval

Cascade Hnet downwards for cumulative net H

Only cascade for positive heat flow eliminate negativeheat flow

HHOT - HCOLD = (FCpHOT -FCpCOLD) T


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Problem Table Analysis - Example

Stream data, Tmin = 10C

Stream Type Ts (oC) TT (

oC) FCP (kW/K)

C1 Cold 20 180 20

C2 Cold 140 230 30

H1 Hot 250 40 15

H2 Hot 200 80 25


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Hot and Cold Temperature

Intervals Determine the heat surplus (S)/deficit (D) within intervals

COLD HOT FCP,H - FCP,C HNET

(240) 250

230 (240) C2 15 150 (S)

(190) 200 -15 -600 (D)

180 (190) C1 10 100 (S)

140 (150) -10 -400 (D)

(70) 80 20 1400 (S)

(30) 40

H2

-5 -200 (D)

20 (30)

H1

-20 -200 (D)


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Hot and Cold Temperature

Intervals Heat Cascade

0

150

-600

100

-400

1400

-200

-200

150

450

-350

-750

650

-450

250

750

150

-600

100

-400

1400

-200

-200

900

1200

400

0

1400

300

1000

QH,min

QC,min

Pinch

Add the largest (-) heat flow


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Global (Shifted) Temperature

Intervals

Shifting Rule:

Cold Stream +Tmin /2

Hot Stream -Tmin /2


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Shifted temperature intervals


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In each shifted temperature

interval, calculate a simple energybalance form


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Temperature Interval Heat Balance


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Cascade any surplus heat from

high to low temperature


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Heat flows cannot be negative! Add

heat to make them at least 0!


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SUMMARY (ii)

The problem table algorithm:

Step 1: Adjust for Tmin

Step 2: Set up temperature intervals

Step 3: Calculate interval heat balance

Step 4: Cascade for positive heat flows

Then, QHmin, QCmin and pinch location withoutdrawing graphs


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Working Session 2

Use the problem table method to verify your results in working session 1

1

2

4

CP=1.8

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Heat Integration_Setting Energy Targets