Heat 2bExchanger 2bNetwork 2bDesign 5b1 5d

7/27/2019 Heat 2bExchanger 2bNetwork 2bDesign 5b1 5d

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Heat Exchanger Network Designone aspect of process integration

J. M. Shaw

Instructor CHE 465

I would happily credit the authors who providedthe example but am unable to do so.


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Introduction Process integration provides a discipline which

allows designers to establish

The minimum energy to operate a process

A process design with the lowest energy intensity An optimal investment strategy

Design decisions are made consciouslyandconsistently. Global capital and operating cost

implications and trade-offs become visible. We will focus on the first bullet only and will

introduce the terminology of the discipline as wego along!


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Start with a flow sheet without heat

exchangers installed


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Identify streams that require heating called

cold streams and streams that require cooling

hot streams.

Without heat recovery we require 750 units for heating cold streams

(steam?) and 660 units for cooling hot streams (cooling water?).


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Next compute the minimum energy

required to operate the process

Compute summary heating

and cooling needs for the

process. Do this step by

step to avoid errors!

These are calledcomposite curves

for heating and

cooling requirements.


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What is the theoretical minimum energy

required to operate the process as designed?

Think of your process as asingle giant counter current heat

exchanger with an infinite

surface area!

The minimum approach

temperature of the hot and cold

composite streams is 0 C! The

temperature at which this occurs

is called the pinch 70 C in

this case.

Energy that cannot be supplied

by exchange must be supplied

by utilities: ~ 200 units for

heating and ~ 110 units for

cooling.


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How closely can this minimum energy

requirement be approached in practice?

Our analogy with a heat exchanger still holds and we establish a minimum approach

temperature for the composite streams, in this case 20 C, that arises around the pinch.

300 units for

heating and210 units for

cooling!

Pay twice for

inefficiencies!

Heating and

cooling

requirements

rise together!


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How do we translate these concepts into

practical heat exchanger network designs?

Golden Rules

1. Avoid exchanging heat between streams where one isabove and one is below the pinch.

2. Avoid cooling streams above the pinch using utilities.

3. Avoid heating streams below the pinch using utilities.

Violating the golden rules may be convenient and smaller heat exchangers

will certainly result but because of the excessive entropy generated you

will pay twice for this violation for as long as the plant operates!


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Separate the network design task at the pinch

and treat the two designs separately


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Design task above the pinch.


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Stream Data

Stream 1 is heated from 60 C to 120 C

Stream 2 is cooled from 100 C to 80 C

Stream 4 is heated from 60 C to 80 C

Heat Exchanger Network Design Data


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Option #1: one heater for the ingredients (1) + two heat

exchangers [product (2)(1) and containers (4)(2)]

60

300

40

92 C

70 C


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Option #2: one heat exchanger [ingredients (1)products]

+ two heaters [ingredients (1) and containers (4)]

100

77 C

40

260


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Summary

Follow this up with

a reality check!

Iterate until an acceptable

design is obtained!

Install heat exchangers,

coolers, and heaters onthat none or few of the

constraints are violated.

Heat 2bExchanger 2bNetwork 2bDesign 5b1 5d

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