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HEN Design

# HEN Design - DESIGN OF MAXIMUM ENERGY RECOVERY NETWORKS MER...

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DESIGN OF MAXIMUM ENERGY RECOVERY NETWORKS

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MER NETWORKS Networks featuring minimum utility usage are called MAXIMUM ENERGY RECOVERY (MER) Networks.
DIVISION AT THE PINCH RECALL THAT No heat is transferred through the pinch. This makes the region above the pinch a HEAT SINK region and the region below the pinch a HEAT SOURCE region.

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Pinch Minimum heating utility Minimum cooling utility Heat Source Heat is released to cooling utility Heat Sink Heat is obtained from the heating utility 7.5 12.0 14.0 0.0 -6.0 1.0 -4.0 4.0 3.0 9.0 -2.0 -2.0 14.0 1.5 10.0 T H
CONCLUSION One can analyze the two systems separately, that is, Heat exchangers will not contain heat transfer across the pinch.

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PINCH MATCHES Consider two streams above the pinch Q/FCp H > Q/FCp C T p - T min T C,out Q T H,in T p T C,out = T p - T min + Q/FCp C T H,in = T p +Q/FCp H But T H,in > T C,out + T min. Thus replacing one obtains FCp H < FCp C Golden rule for pinch matches above the pinch. T min Violation when FCp H > FCp C T min
EXERCISE A similar rule is obtained below the pinch. Derive it and illustrate it.

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ANSWER T H,out T p - T min T C,in Q T p T C,in = T p - T min - Q/FCp C T H,out = T p -Q/FCp H But T H,out > T C,in + T min. Thus replacing one obtains FCp C < FCp H Golden rule for pinch matches below the pinch. T min T min Violation when FCp C > FCp H
CONCLUSION Since matches at the pinch need to satisfy these rules, one should start locating these matches first. Thus, our first design rule: START BY MAKING PINCH MATCHES

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QUESTION Once a match has been selected how much heat should be exchanged?
ANSWER As much as possible! This means that one of the streams has its duty satisfied!! THIS IS CALLLED THE TICK-OFF RULE

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HANDS ON EXERCISE Stream Type Supply T Target T H F*Cp ( o C) ( o C) (MW) (MW o C -1 ) Reactor 1 feed Cold 20 180 32.0 0.2 Reactor 1 product Hot 250 40 -31.5 0.15 Reactor 2 feed Cold 140 230 27.0 0.3 Reactor 1 product Hot 200 80 -30.0 0.25 T=140 0 C T=20 0 C H=27 MW REACTOR 2 H=-30 MW H=32 MW REACTOR 1 H=-31.5 MW
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