pinch analysis tool

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Pinch Analysis & Process Integration

May 2010

Using the SEAC Pinch Analysis Tool


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Background

Based upon the laws of thermodynamics and proven methodology

Highlights the scope for reduced utility energy demands and as a consequence CO2

emissions i.e. Shows what a process is capable of achieving Simple and intuitive user interface no in-depth knowledge of engineering or process

thermodynamics required.

Process Integration allows a step-change in energy consumption to be made over and abovetraditional techniques.

Using this methodology total energy savings of between 5 and 30% have been identified.

The SEAC Pinch analysis tool has been developed tounderstand and improve the thermal energy demands ofexisting and new processes.


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Typical Improvement Process

5.0

4.0

3.0

2.0

1.0

0.0

6.0

Last Process Existing Process New Design

Energy ConsumptionConsistent units

New Designby traditional

methods

Modified flow sheet based onsystematic techniques for

thermal integration

Minimum

Successive plants


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Process Synthesis

Site Heat & Power Systems

Heat Exchanger Network

Separation

Reaction

Chemical Synthesis

Process Development

Heat Recovery

Utility Heating/Cooling, pumps & compressors


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Process Synthesis

The core of the process is the reaction / mixing / formulation step

Product composition and feed requirements dictate the

separation tasks (including recycles)

Then and only then can the design of various heating and

cooling duties for the streams be defined.

The design of the reactor / mixer is dictated by yield and conversionconsiderations.

The separation stage allow recycle of un-reacted feed etc.

The operating conditions of these units are taken as being accepted.

The design problem then becomes one of getting the optimumperformance out of the system of heat exchangers, heaters andcoolers.

Typically, this is an area where little thought or effort has taken place


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Limitations

Application to existing processes can limit the recovery potential dueto other limiting factors such as cost, layout, fluid rheology etc.

Tighter designs which reduce the temperature driving force use lessutilities and the overall system heat load decreases.

However, with less driving force the area required to meet the duty

increases Higher capital costs (,$.) But, because of the reduced heat load the provision of utility costs

decreases.

This is an important trade off


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Pinch Analysis Tool

The Pinch Analysis Tool aids the identification of the heat recoverypotential and hence energy savings of both new and existingprocesses.

It does NOT specify how this is done but highlights the scope forsavings or opportunities

Heat Recovery = 450 kW


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Pinch Terminology

Any flow which requires to be heated or cooled but does not changein composition is defined as a Stream.

Streams which are being heated are known as COLD streams

Conversely, streams which are being cooled are known as HOTstreams.

Reaction processes are NOT streams as generally there is a changein composition.

Similarly, make ups which are not heated or cooled are also not

defined as streams.


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Adding Data - Input

Each Stream in the process under investigation (up to max. 50

streams) is added via the Input worksheet.

Using available information add the temperature and flow information(various units sets available to match the data source)

Heat Capacity Flow Rate is the most commonly used method inprocess integration to define heat flows (kW/K)


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Terminology

CP = Heat Capacity Flowrate (kW/K) = mass flow W (kg/s) x Specific

Heat Cp (kJ/kgK)

Supply Temperature (ST)= Temperature the stream is being heatedor cooled from. (C)

Target Temperature (TT= Temperature the stream is being heated orcooled to. (C)

Based upon the information provided the tool calculates thefollowing:

Stream Heat Load (kW)

Stream type (Hot or Cold)

Shifted Supply & Target Temperatures*

* Further details of shifted temperatures are given further into this presentation


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The tool contains two methods for calculating the Pinch:

Composite Curves

Problem Table Analysis

Pinch Analysis Tool

Both methods should deliver the same answer and so in effect are a cross

check that the analysis is correct


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For feasible heat exchange between streams, the HOTstream must at all points be hotter than the COLD stream.

The overlap between the composite curves represents themaximum amount of heat recovery possible within theprocess under investigation.

The overshoot at the bottom of the HOT compositerepresents the minimum amount of external cooling

required.

The overshoot at the top of the COLD compositerepresents the minimum amount of external heatingrequired.

DTmin is the controlling design parameter. This is the

approach temperature. As previously discussed, closeapproach temperatures reduce the driving force and wouldtherefore increase the area required to accomplish heattransfer.

In general, DTmin occurs at only one point of closestapproach This is the PINCH.

Pinch Analysis Tool Composite Curves


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Composite Curves Temperature Intervals

To handle multiple streams, the heat loads or heat capacity flow rates of all streams

existing over any given temperature range are added together.

Thus a single composite of all HOT streams and a single composite of all COLDstreams can be produced.

In figure (a) three HOT streams are plotted separately, with their supply and targettemperatures defining a series of interval temperatures T1 T5

Between T1 T2 only stream B exists so the heat available is given by CPB(T1T2)

Between T2 T3 all three streams exist and so the heat available in this interval is

given by (CPA + CPB + CPC)(T2T3)


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Pinch Analysis Tool Composite Curves


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Using the same Four streams that were used to develop the Composite curves

these will now be analysed by the Problem Table Analysis method.

Again, the information required can be input based upon the informationavailable (in this case Heat Capacity Flowrate is used), Supply Temp (TS C) &Target Temp (TT C)

The tool calculates the Shifted temperatures.


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Shifted Temperatures

In the construction of the composite curves it was described how the enthalpy

balance intervals were set up based on stream supply and target temperatures.

The same can be done for hot and cold streams together, to allow for maximumpossible heat exchange within each temperature interval.

To ensure this occurs it is required that HOT and COLD streams are at leastDTmin apart.

This is done by using Shifted temperatures, which are set at DTmin (5C inthis example) below HOT stream temperatures and DTmin above COLDstream temperatures.


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In this diagram the four streams are represented on a vertical temperature scale

with interval boundaries superimposed:

In interval 2, between shifted temperatures 145C and 140C, streams 2 & 4 (HOT

streams) run from 150C to 145C, and stream 3 (the COLD stream) from 135C to140C.

Setting up the intervals in this way ensures guarantees that full heat interchangewithin any interval is possible.

Each interval will either have a net surplus or net deficit of heat as dictated by

enthalpy balance. NEVER BOTH!


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Knowing the stream population as shown in the previous slide, enthalpy balances

can easily be calculated for each interval by:

DHi = (Si Si+1) (SCPHSCPC)i

The tool calculates this enthalpy balance and indicates whether within eachinterval is in heat surplus or heat deficit.

It would therefore be possible to produce a feasible network design based on theassumption that all surplus intervals rejected heat to cold utility (cooling water)

and all deficit intervals took heat from hot utility (steam)

This would not be sensible because it would involve accepting and rejecting heatat inappropriate temperatures.


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Any interval (Si) in the table displayed in the previous slide is hot enough to supply

any duty in interval Si +1

Using intervals 1 and 2 as an example, instead of sending the 60kW of surplusheat from interval 1 to cold utility it can be sent down to interval 2.

It is therefore possible to set up a heat cascade. The tool develops the cascades.


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Infeasible & Feasible Heat Cascades

Assuming that no heat from hot utility is

supplied to the hottest interval (1) the surplusfrom interval 1 (60kW) can be cascaded intointerval 2.

There it joins the 2.5kW surplus from interval 2(making 62.5kW) to be cascaded into interval

3. Interval 3 has a 82.5kW deficit which leaves a

deficit of 20kW after accepting the heat frominterval 2.

Clearly, passing on a deficit or negative flow of

20kW between intervals 3 & 4 isthermodynamically infeasible. (Heat cannot bepassed from a cold stream to a hot stream).

This is an Infeasible cascade.


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To make the cascade feasible 20kW of

heat must be added from hot utility andcascaded right through the system asshown in the diagram.

By enthalpy balance this means that allflows are increased by 20kW

The net result of this example is that theminimum utilities requirements havebeen predicted (i.e. 20kW Hot and 60kWcold)

The position of the PINCH has also

been located.

This is at the interval boundary with ashifted temperature of 85C (i.e. Hotstreams at 90C and cold streams at80C) where the heat flow is zero.

Infeasible & Feasible Heat Cascades


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Comparison of results

The answers developed by both methods are the same:


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Summary

Both methods which are included in the tool calculate the heat

recovery potential and the resulting minimum heating and coolingrequirements of the process being studied.

The tool does NOT tell you how the heat recovery should be donei.e. Heat Exchanger Network design only the scope for improvement

The tool can accommodate up to 50 streams.

The example used in this presentation is the example used in thetool. A simplified version is given in the worksheet examples.

The tool can be used to assess New or existing designs.

Provides SEAC with a unique methodology (no other groups looking

at Pinch Analysis) to assist categories to meet the Compasschallenge.

The pilot work conducted with Aspentech indicated savings ofbetween 5 & 30% of total site energy can be achieved by applyingthis methodology.

pinch analysis tool

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