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12.9 THE EFFECT OF PRESSURE AND TEMPERATURE             271





                         Table 12.1 Amount of Products for Constant Pressure Combustion of
                         Methane in Air Initial Temperature [ 1000 K; Equivalence Ratio [ 1.00
                              Pressure (bar)   No Dissociation  1        10       100

                             Amount of CO 2         1         0.6823    0.7829   0.8665
                              Amount of CO          0         0.3170    0.2170   0.1335
                             Amount of H 2 O        2         1.8654    1.920    1.9558
                                                    0         0.1343    0.0801   0.0444
                              Amount of H 2
                                                    0         0.2258    0.1485   0.0889
                              Amount of O 2
                                                   7.52        7.52     7.52      7.52
                              Amount of N 2
                          Total amount of substance  10.52    10.7448  10.6685   10.6091

               where a 1 is the degree of dissociation of the CO 2 reaction and a 2 is the degree of dissociation of the
               H 2 O reaction. It can be seen that dissociation tends to increase the amount of substance of products,
               and hence the effect of pressure should be to reduce the degree of dissociation. This effect can be seen
               quite clearly in Table 12.1, where the amount of substance of products is compared under four sets of
               conditions: no dissociation, dissociation at p ¼ 1 bar, dissociation at p ¼ 10 bar and dissociation at
               p ¼ 100 bar. The minimum total amount of substance occurs when there is no dissociation, with the
               maximum amount of substance then occurring at the lowest pressure. Figure 12.5 shows how the
               degrees of dissociation for the carbon dioxide and water reactions vary with pressure.


                                 0.35

                                 0.30

                                 0.25
                                Degree of dissociation, α   0.20  Carbon dioxide




                                 0.15

                                 0.10

                                 0.05                     Hydrogen


                                  0.0
                                     1                    10                   100
                                                     Pressure, p / (bar)
               FIGURE 12.5
               Effect of pressure on degree of dissociation.
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