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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.
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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
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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
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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
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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?
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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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