Hi 11 Minimum Network Part1

7/28/2019 Hi 11 Minimum Network Part1

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Heat Integration UVa | Synthesis 11. Minimum network 1

HEAT INTEGRATION

Synthesis

11. Minimum network


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Outline

Network design overview

Grid diagram

Pinch topology: feasibility criteria

Pinch topology: stream population

Pinch topology: adequacy of thermal inertia

Sizing heat exchangers

Auxiliary criterionRemanent problem algorithm

Multiple pinches

Threshold problems

Retrofitting


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Network design overview

Two general steps:

First design a minimum energy requirement (MER) network

Pinch design method rules&guides will be followed strictly

Network development (evolution) towards the optimum

No rule to respect, except the best operating, economic network

MER design steps (for each semi-problem):First design the topology at the pinch:

what streams will match among those that start or end at the pinch

minimum temperature difference between matching streams

this is to start designing at the pinch (going from hardest to easiest)

Then solve the rest:

matching streams as wanted (using utility included)

... proceeding from inside outwards (widening driving force)


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Grid diagramSymbolic network representation. Best tool for design understanding

Hot streams run from right to left* in the top (including heating)

Cold streams run from left to right*

in the bottom (including cooling)Symbolic diagram: scale is not take into account

Heat exchangers link together hot and cold streams

[* unlike in the literature, for consistency with the composite curve diagram]



Grid diagram

With design purposes information concerning the pinch and streamdata can be included

This is just one of the possible network configurations ...



Grid diagram

... but basic (problem) data are immutable for a concrete Tmin

:

So the grid diagram is an excellent tool for network design

On this grid network will be woven



Grid diagram

Note the symbolic not to scale nature of the diagram:

At least one stream must start or end at the pinch (to make it)



Pinch topology: feasibility criteria

To solve the matchings at one side of a pinch two design criteria mustbe met:

Feasibility (I): stream populationFeasibility (II): adequacy of thermal inertia

Meeting these two criteria is compulsory to achieve a MER design

Stream population at the pinch:

'The number of streams leaving this side of the pinch must begreater than or equal to the number of streams that reach it'

Adequacy of thermal inertia in pinch matchings:

'The thermal inertia of each stream that leaves this side of the pinchmust be lesser than or equal to that of the stream that matches'

'The slope of each stream that leaves this side of the pinchmust be greater than or equal to that of the stream that matches'

Ensuring that both criteria are met matchings can be freely chosen (onstart-up, operating, safety, proximity... considerations)




Stream population at the pinch:

'The number of streams leaving this side of the pinch must be

greater than or equal to the number of streams that reach it'

We have not enough hotstreams to carry every coldstream to the pinch, and wecan't use heating

We have not enough coldstreams to carry every hotstream to the pinch, and wecan't use cooling




In this cases some streams must be splitted to meet the criterion:

We have now enoughtemperature to carry every coldstream to the pinch

The thermal inertia of thesplitted stream is distributed so

that:

This distribution is our will, butmust meet the 'adequacy of

thermal inertia' criterion

It will be one of the parametersto optimize during the networkdevelopment (evolution) stage

mCP = mCP a mCP b



Adequacy of thermal inertia in pinch matchings:

'The thermal inertia of each stream that leaves this side of the pinchmust be greater than or equal to that of the stream that matches'

'The slope of each stream that leaves this side of the pinchmust be lesser than or equal to that of the stream that matches'


... or the temperature differencedecreases as we move away from

the pinch, becoming less than theminimum

We can match stream 2 with stream5, but not with 1...

...and stream 3 with stream 6, but

not with 4

Looking at the thermal inertia ofstreams arriving and leaving thepinch we can say what are theallowed partners (at the pinch)


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In some cases some streams must be splitted to meet this criterion:

After splitting streams for this reason should be checked that thepopulation criterion (in the pinch) still met

Both hot and cold streamscan be splitted


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Sizing heat exchangers

In dimensioning (sizing) heat exchangers stream eliminationheuristics should be followed:

'For the network to be formed by the minimum number of units eachnew pairing must remove, at least, the requirements of one stream(or part of one stream)'

This is in accordance with Euler's Theorem in Graph theory:

( Nmin= [NA-1] + [NB-1])

In the case of large temperature variations for one or both streamsthis rule may lead to excessively large exchange areas:

Large areas are often divided into several shields to avoidtemperature cross (or cross-over)

One heat exchanger can eliminate the requirements of the twostreams if their duties are equal. The minimum number is then lessthan the expected


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Auxiliary criterion: mCp difference

A very simple example:

Feasible Impossible:

The other match cant' be done

Of course, it is more useful when the number of streams is large


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Remanent Problem Algorithm

You can calculate the impact of a design choice (choosing a match,sizing one duty) without designing by applying recursively the

Problem Table Algorithm and Minimum Exchange Area Estimationprocedure to the problem that remains after that option

Note that, according

to setting the duty ofthe HX, the set ofremaining streamschanges

Hi 11 Minimum Network Part1

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