Methane heat capacity

Figure A2.5.29. Peak positions of the liquid-vapour heat capacity as a fiinction of methane coverages on graphite. These points trace out the liquid-vapour coexistence curve. The frill curve is drawn for p = 0.127. Reproduced from [31] Kim H K and Chan M H W Phys. Rev. Lett. 53 171 (1984) figure 2. Copyright (1984) by the American Physical Society.

Explain why the heat capacities of methane and ethane differ from the values expected for an ideal monatomic gas and from each other. The values are 35.309 J-K " mol 1 for CH4 and 52.63 J-K -mol 1 for C2He. 

Table 22.1), which has been modified by appropriate substitutions to yield the desired molecule. Thus, aliphatic hydrocarbons can be built up from methane by repeated substitutions of methyl groups for hydrogen atoms. Other compounds are formed by substitution of functional groups for CHn groups. The heat capacity constants are those for a cubic polynomial in the temperature, which are similar to those discussed in Chapter 4. 

Handa, Y.P. (1986a). Compositions, enthalpies of dissociation, and heat capacities in the range 85 to 270 K for clathrate hydrates of methane, ethane, and propane, and enthalpy of dissociation of isobutane hydrate, as determined by a heat-flow calorimeter. J. Chem. Thermodynamics, 18 (10), 915-921. 

At high pressure methane becomes thermodynamically stable and does not enter into endothermic reactions which are accompanied by an increase in the number of molecules. Under these conditions the flegmatizing action of excess methane proves weak. Evidently, the higher heat capacity of methane is compensated for by the increase in the chemical reaction rate for increased methane concentration. 

Rueff, R.M., The Heat Capacity and Heat of Dissociation of Methane Hydrates A New Approach, Disssertation, Colorado School of Mines, Golden, CO (1985). 

In the CSM laboratory, Rueff et al. (1988) used a Perkin-Elmer differential scanning calorimeter (DSC-2), with sample containers modified for high pressure, to obtain methane hydrate heat capacity (245-259 K) and heat of dissociation (285 K), which were accurate to within 20%. Rueff (1985) was able to analyze his data to account for the portion of the sample that was ice, in an extension of work done earlier (Rueff and Sloan, 1985) to measure the thermal properties of hydrates in sediments. At Rice University, Lievois (1987) developed a twin-cell heat flux calorimeter and made AH measurements at 278.15 and 283.15 K to within 2.6%. More recently, at CSM a method was developed using the Setaram high pressure (heat-flux) micro-DSC VII (Gupta, 2007) to determine the heat capacity and heats of dissociation of methane hydrate at 277-283 K and at pressures of 5-20 MPa to within 2%. See Section 6.3.2 for gas hydrate heat capacity and heats of dissociation data. Figure 6.6 shows a schematic of the heat flux DSC system. In heat flux DSC, the heat flow necessary to achieve a zero temperature difference between the reference and sample cells is measured through the thermocouples linked to each of the cells. For more details on the principles of calorimetry the reader is referred to Hohne et al. (2003) and Brown (1998). 

For AH R, the mean heat capacities for air and methane are required. The value for air is given above. For methane the temperature change is very small use the value given in Table C.l for 298 K 4.217 R. 

For the example of methane steam reforming, Eq. (8) yields an acceleration factor of 4. Accordingly, the axial displacement of the reaction zone is a multiple of the axial displacement of thermal fronts. The difference of the axial displacement between the reaction front and the thermal front determines the axial profile of heat demand during the subsequent exothermic semicycle. Efficient heat recovery requires equal heat capacities of the process streams during both semicycles. The initial state can be restored by discrete heat sources distributed at equal distances along the catalytic part of the reactor. Each point source initiates a thermal wave that covers the distance to the next heating point (Fig. 1.13, right). This concept features... 

The enthalpy change for this reaction at 25°C is AH = —802.2 kJ. Hence, when 1 mol of methane is converted to 1 mol of carbon dioxide gas and 2 mol of water vapor at 25°C and 1 atm, 802.2 kJ of energy is released as heat. If none of this heat escapes to the surroundings (hence the term adiabatic), all of it goes into heating the carbon dioxide and water vapor. To calculate the final temperature, the adiabatic flame temperature, one needs to know how the heat capacity of the material being heated, carbon dioxide and water vapor in this case, varies with temperature. 

Example 4.1 The constants in Table 4.1 require use of Kelvin temperatures in Eq. (4.4). Equations of the same form may also be developed for use with temperatures in °C, (R), and (°F), but the constants are different. The molar heat capacity of methane in the ideal-gas state is given in Table 4.1 as... 

Example 3.2 Total entropy change in a polytropic compressing of methane We compress methane from an initial state at 100 kPa, 300 K, and 20 m3 to 250 kPa and 400 K. The compression process is polytropic (PVa = constant). The average heat capacity of methane is Cpw = 40.57 J/(mol K). Estimate the supplied work and the total entropy change if the surroundings are at 290 K. 

Assume that the methane is an ideal gas. The heat capacity is constant. 

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