ChE455 Heat Exchanger Networks by 1G6xMwj5

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									Heat Exchanger Networks
 & Utility Minimization
  NMSU Chemical Engineering
         Ch E 455
                       Outline

• Heat Integration
• Design Procedure for MUMNE
  –   Temperature interval diagram
  –   Cascade diagram
  –   Temperature-Enthalpy diagram
  –   Minimum number of exchangers
  –   Design above and below pinch
               Heat Integration

• Heat exchange networks
• It saves money to match streams rather than pay to
  heat one and pay to cool another
• You have already done this on ad hoc basis in design
  projects
              Heat Integration

• There is a rigorous methodology
• We will learn MUMNE (Minimum Utility, Minimum
  Number of Exchangers) method
• Not necessarily (and unlikely to be) economic
  optimum
              Design Procedure

1. Complete energy balance on all streams to
   determine all temperatures, mC p values, and heat
   flows.
2. Choose minimum approach temperature. Typically,
   this is between 5°C and 20°C, but any positive
   number is valid.
3. Complete temperature interval diagram, Each
   stream is drawn and labeled. The heat flow in each
   interval is calculated.
              Design Procedure

4. Complete the cascade diagram. The energy excess
   or deficit is calculated for each interval on the
   temperature interval diagram.
5. Find the minimum hot and cold utility requirements
   and identify the pinch temperature.
6. Complete the composite temperature enthalpy
   diagram. This is a T-Q diagram for the entire
   process.
              Design Procedure

7. Determine the minimum number of heat
   exchangers required above and below the pinch.
8. Design the heat exchanger network.
               Pinch Technology

• Advantages include:
  – simple, does not require elaborate mathematics
  – sets performance targets before actual design (minimum
    required theoretical utility for entire process)
  – analysis provides network design by matching hot and cold
    streams for heat integration
  – graphical representation (composite curve) used to
    increase conceptual understanding of system
  – method table used to predict minimum utility
    requirements
    Application of Pinch Technology

• Design an integrated heat exchanger system with a
  minimum approach temperature of 10°F to
  minimize utility for the following six streams:

              hot streams         cold streams
     str    A       B     C     D       E        F
     m     4000 10000 6000      6000    9000     6000   lb/hr
    Cp     0.65 1.00 0.50       0.70    0.95     0.55   btu/lb°F
    To      410 370       270    260     310      340   °F
     Tf     350 290       250    300     370      390   °F
    mCp    2600 10000 3000      4200    8550     3300   btu/hr°F
                     Example

• The heat flow values of Q (or ΔH) are calculated from
  the energy balance. The sign convention is positive
  for heat available from a stream and negative for
  heat needed by a stream.
• Choose the minimum approach temperature. For
  this problem, it is 10°C.
                      Example

• Draw and label the temperature interval diagram.
  Label the intervals beginning with “A” for the
  highest temperature interval. The heat flow for each
  interval is calculated from
                        Q=   åmC pDT
• where the sum is over all streams existing in that
  interval.
                Temperature Interval Diagram
       Prepare the composite curves,                      A   B   C       F   E   D
       distinguishing interval of temperature
                                                  400°F
       where streams influent/effluent
       temperatures begin/end
             hot streams                  cold streams                1               1
   str      A       B       C         D         E       F
   m            hot streams
           4000 10000 6000            6000      streams
                                           cold 9000 6000 lb/hr 2                     2
                                                 350°F
   Cpstr      A       B
            0.65 1.00 0.50   C          D
                                       0.70      E
                                                0.95     F
                                                        0.55 btu/lb°F
   Tom       4000 370
             410 10000 270  6000        6000 310
                                        260      9000 340 °F
                                                         6000 lb/hr
                                                                                      3
    Cp
   Tf                1.00 0.50
              0.65 290
             350 streams 250             0.70 370
                                        300       0.95 390 °F 3
                                                         0.55 btu/lb°F
    To        cold 370
               410            270         260 8550 3300 btu/hr°F
                                                   310    340 °F                      4
  mCp      2600 10000 3000
           D 350 E290 F               4200       300°F
     Tf                       250         300      370    390 °F
00 mCp     6000cold streams
                    9000 6000
             2600 10000 3000 lb/hr 4200          8550 3300 btu/hr°F4                  5
 C
50            D
           0.70        E
                     0.95      F
                             0.55 btu/lb°F
 000         6000 310  9000 340 °Flb/hr
                               6000                                5
70          260                                  250°F
0.50
50            0.70 370
            300                0.55
                       0.95 390 °Fbtu/lb°F
 270
00         4200         310     340 °F
               260 8550 3300 btu/hr°F
 250           300      370     390 °F
 000         4200      8550 3300 btu/hr°F
           Temperature Interval Diagram
Treating all Hot streams and all Cold              A      B     C           F   E   D

streams together, determine heat flow in   400°F
each interval                                          H1  104,000
                                                                        1
              H  m c p T
                                                                                       1

                                                                        2
  H1  m1c p,1T1
                                          350°F
                                                                                        2


  H1  4,000lb hr                                                    3
                                                                                        3

          0.65 Btu lb  hr  F         300°F
                                                                                        4

          370  410F                                                4               5

  H1  104,000Btu hr                     250°F
                                                                        5
           Temperature Interval Diagram
Treating all Hot streams and all Cold                A     B       C           F     E        D
streams together, determine heat flow in   400°F
each interval                                          H1  104,000
                                                                        1          H1  66,000
              H  m c p T
                                                                                                 1
                                                       H2  252,000                    H2  355,500
                                                                        2                         2
    H2  m1c p,1  m2c p,2 T2
                                           350°F
                                                           H3  600,000


    H2  4,0000.65
                                                                                                  3
                                                                        3                H3  256,500



            10,0001.0 
                                                                               H4  0            4
                                           300°F

                                                         H4  0              H5  168,000

            350  370
                                                                        4                         5
                                                   H5  60,000
                                                                        5
    H2  252,000                         250°F
Composite                      420
                               410
                                          hot
                                          cold
                                                      Define pinch point as temperature approach

                               400                                       Pinch
Curves                         390
                               380
                                                                         Point

                               370
                               360




            temperature (°F)
                               350
                                     only reject
                               340
                                     heat below
                               330    the pinch
                               320      point                                                      Qhot
                               310
                               300
                                                                    Never transfer heat       only add heat
                               290
                                                                    across the pinch point.     above the
                               280       Qcold                                                 pinch point
                               270
                               260
                               250
                                0.E+00       2.E+05        4.E+05      6.E+05      8.E+05     1.E+06      1.E+06
                                                                         H
                  Method Table

•   draw hot/cold temperature scales offset by Tmin
•   plot stream temperatures on appropriate scales
•   determine temperature intervals
•   divide stream’s temperature change into intervals
    based on supply & target temperatures for each
    stream
                             Method Table
int   T(°   mcc -    Hi     cumulative
                                                    A   B   C   F   E   D
       F)    mch                                                               400°F
                                            400°F
                                                                            1
1      10    -2.60    -26.0     -26.0
2      20    +0.70    +14.0     -12.0
                                                                            2
3      10    +9.25    +92.5     +80.5                                       3   pinch
                                                                                point
4      20    -0.75    -15.0     -15.0                                       4    350°F
                                            350°F
5      30    -1.45    -43.5     -58.5
6      10    -10.00   -100.0   -158.5                                       5
7      20    -5.80    -116.0   -274.5
8      20    +4.20    +84.0    -190.5
                                                                            6   300°F
9      20    -2.50    -50.0    -250.5       300°F                           7

heat surplus may be transferred                                             8
to the lower temperature interval
                                                                            9   250°F
                                            250°F
               HOT            Heat Cascade             COLD

Heat Cascade   410
                                 -26000
                                                       400



               400                  -26000             390

                                 14000
               380   Qhot           -12000             370

                     -80500      92500
                                                              pinch
               370                        0            360
                                                              point
                                 -15000
               350                  -15000             340

                                 -43500
               320                  -58500             310

                                -100000
               310                 -158500             300

                                -116000
               290                 -274500             280

                                 84000
               270                 -190500    Qcold    260

                                 -60000       250500
               250                        0            240
       Minimum Energy Network Design
            mcp          A     B    C     D      E     F
(x   10-3   Btu/hr°F)   2.6   10   3.0   4.2   8.55   3.3
                                                             400°C
        400°C




                                                             350°C
        350°C




                                                             300°C
        300°C




                                                             250°C
        250°C
       Minimum Energy Network Design
            mcp          A           B           C         D              E          F
(x   10-3   Btu/hr°F)   2.6         10          3.0       4.2           8.55        3.3
                                                                                               400°C
        400°C

                           402.9°F
                  85.5                                                      85.5

                                              HA,  mc p T
                                                                                              350°C
        350°C
                                          85.5  103  2600T  370
                                                  T  402.9F
                                                                                               300°C
        300°C                                                    HE,  mc p T
                                                                         

                                                                          855010
                                                                          85.5  103 Btu hr
                                                                                               250°C
        250°C
       Minimum Energy Network Design
            mcp           A           B           C          D             E     F
(x   10-3   Btu/hr°F)    2.6         10          3.0        4.2          8.55   3.3
                                                                                          400°C
        400°C

                   18.5                                                            18.5
                            402.9°F



                                                                                          350°C
        350°C
                                           HA,  mc p T
                                                   

                                                  2600410  402.9 
                                                  18.5  103 Btu hr
                                                                                          300°C
        300°C




                                                                                          250°C
        250°C
       Minimum Energy Network Design
            mcp          A           B           C           D            E           F
(x   10-3   Btu/hr°F)   2.6         10          3.0         4.2         8.55         3.3
                                                                                                 400°C
        400°C
                                                                                365.6°F
                                                                                          18.5
                           402.9°F



                                                                                                 350°C
        350°C                                 HF,  mc p T
                                                      

                                          18.5  103  3300T  360
                                                        365.6F
                                                                                                 300°C
        300°C




                                                                                                 250°C
        250°C
       Minimum Energy Network Design
            mcp          A           B          C           D            E           F
(x   10-3   Btu/hr°F)   2.6         10         3.0         4.2         8.55         3.3
                                                                                                 400°C
        400°C
                                                                               366.6°F
                                                                                         QH = 80.5
                           402.9°F



                                                                                                 350°C
                                          HF,  mc p T
                                                  
        350°C
                                                3300390  365.6 
                                                80.5  103 Btu hr
                                                                                                 300°C
        300°C




                                                                                                 250°C
        250°C
       Minimum Energy Network Design
            mcp          A             B              C            D             E            F
(x   10-3   Btu/hr°F)   2.6           10             3.0          4.2          8.55          3.3
                                                                                                          400°C
        400°C
                                                                                        366.6°F
                                                                                                  QH = 80.5
                            402.9°F



                                                                                                          350°C
        350°C
                         52.0               If we do this, stream B will no longer           66.0
                                            have heat available at a sufficiently              14.0
                                            high temperature to supply stream E
                                                                                427.5
                                                                                                          300°C
        300°C

                                      800.0

                                                                   168
                                                                                                          250°C
        250°C
                                                     50.0
       Minimum Energy Network Design
            mcp          A             B              C            D             E             F
(x   10-3   Btu/hr°F)   2.6           10             3.0          4.2          8.55           3.3
                                                                                                           400°C
        400°C
                                                                                         366.6°F
                                                                                                   QH = 80.5
                            402.9°F



                                                                                                           350°C
        350°C
                         52.0               If we do this, stream B will no longer            66.0
                                            have heat available at a sufficiently               14.0
                                            high temperature to supply stream F
                                                                                 427.5
                                                                                                           300°C
        300°C

                                      800.0

                                                                   168
                                                                                                           250°C
        250°C
                                                     50.0
       Minimum Energy Network Design
            mcp          A             B                C      D           E             F
(x   10-3   Btu/hr°F)   2.6           10               3.0    4.2        8.55           3.3
                                                                                                     400°C
        400°C
                                                                                   366.6°F
                                                                                             QH = 80.5
                            402.9°F



                                                                                                     350°C
        350°C
                                  360.3°F
                         52.0                                                           66.0
                                              344.4°F                                     14.0

                                                                          427.5
                                                                                                     300°C
        300°C

                                      800.0
                                                                     Split streams B and F into
                                                               168   two streams to prevent
                                                                     violation of the second law     250°C
        250°C
                                                        50.0
       Minimum Energy Network Design
            mcp          A             B              C      D      E            F
(x   10-3   Btu/hr°F)   2.6           10             3.0    4.2   8.55          3.3
                                                                                             400°C
        400°C
                                                                           366.6°F
                                                                                     QH = 80.5
                            402.9°F



                                                                                             350°C
        350°C
                                  360.3°F
                         52.0                                                   66.0
                                            344.4°F                               14.0

                                                                   427.5
                                                                                             300°C
        300°C                               320.0°F
                         QC = 190.5
                                   116 684

                                        QC = 50.0            168     QCtotal = 240.5         250°C
        250°C
                                                      50.0
Minimum Energy Network Design
        A              B              C   D   E                  F
                                                                     400°C
400°C
                                                       366.6°F
            402.9°F



                                                                     350°C
350°C
                  360.3°F

                            344.4°F




                                                  A – F, E, F        300°C
300°C                       320.0°F               B1 – F, U
                                                  B2 – E, D, U
                                                  C–U
                                                  D – B2
                                                  E – B2, A
                                                  F – B1/A, A, U     250°C
250°C
Minimum Energy Network Design



                              402.9
    410




                                                       370




                                                                                    350
A




                                      370
                 18.5                       85.5                           52.0




                                                                                              340
                               360
    365


                                                                                                    F

                        390




                                                                                                        290
                                                                                    360
                                                                            14




                                                       370
                                                   B




                                                                                   310 320
                        250
           270




    C
                                            360
                                                                           427.5

          A – F, E, F                                                        E
          B1 – F, U




                                                                 260 300
          B2 – E, D, U
          C–U




                                                                                        300
          D – B2                                                           168
          E – B2, A
                                                             D
          F – B1/A, A, U

								
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