Mixtures specific heat

Step 5. Solve for mixture specific heat ratio k , using Equation 2.21 9.59... 

Since the mixture specific heat for a perfect gas can be written as a mass-weighted sum of the species specific heats, cp = Ek=l YkCpk,... 

COMMENTS Note that the reference temperature enthalpy cancels out of the sensible enthalpy expression. In some thermodynamic tables, the reference point is chosen differently therefore the absolute enthalpy calculated may be different. However, the enthalpy change between two states will always be the same. Also, there are several ways the air mixture properties could have been calculated. The mixture specific heat for air could have been used and Cp raix (Xi -T ) would have given the molar specific enthalpy change. Also, since tables abound for air, we could have read the enthalpy changes directly from an ideal gas air table. 

The specific heat for a liquid at constant pressure, written as and expressed in kJ/(kg-K), can be calculated for mixtures in two ways ... 

The specific heat of gases at constant pressure is calculated using the principle of corresponding states. The for a mixture in the gaseous state is equal to the sum of the C g of the ideal gas and a pressure correction term ... 

Traditionally, sodium dichromate dihydrate is mixed with 66° Bh (specific gravity = 1.84) sulfuric acid in a heavy-walled cast-iron or steel reactor. The mixture is heated externally, and the reactor is provided with a sweep agitator. Water is driven off and the hydrous bisulfate melts at about 160°C. As the temperature is slowly increased, the molten bisulfate provides an excellent heat-transfer medium for melting the chromic acid at 197°C without appreciable decomposition. As soon as the chromic acid melts, the agitator is stopped and the mixture separates into a heavy layer of molten chromic acid and a light layer of molten bisulfate. The chromic acid is tapped and flaked on water cooled roUs to produce the customary commercial form. The bisulfate contains dissolved CrO and soluble and insoluble chromic sulfates. Environmental considerations dictate purification and return of the bisulfate to the treating operation. 

Cg = humid heat for humidity H in units of kj / (kg-K) and = latent heat of vaporization at / in kj /kg. The slope of the constant-enthalpy adiabatic cooling line is —C j which is the relationship between temperature and humidity of gas passing through a totally adiabatic direct-heat dryer. The humid heat of a gas—vapor mixture per unit weight of dry gas includes the specific heat of the vapor... 

Various types of rapid, adiabatic compressions have caused explosions. With propane at an initial temperature of 25°C, To = 432°K (I59°C) for compression and specific heat ratios of 25 and I.I3, respectively. Assume that now air enters a compressor to bring propane into the flammable range at 5 percent by volume. The mixture then will be mostly air with k = 1.47. The same compression ratio of 25 will elevate the final temperature T2 to 834°K (56I°C), i.e., above the published autoignition temperature of 450°C for propane and perhaps high enough to cause an explosion. 

Physical and Chemical Properties — Physical State at 15 T7 and 1 atm. Liquid Molecular Weight Mixture Boiling Point at I atm. Decomposes Freezing Point 17, -8,265 Critical Temperature Not pertinent Critical Pressure Not pertinent Specific Gravity 1.2 at 20 °C (liquid) Vapor (Gas) Density Not pertinent Ratio cf Specific Heats cf Vapor (Gas) Not pertinent Latent Heal of Vaporization Not pertinent Heat of Combustion -15,700, -8750 -366 Heat of Decomposition -50, -28, -1.2. 

Temperature rc) Humidity kg HjO/kg dry air) Water vapor partial pressure (kPa) Water v K>r partial density (kg/m ) Water vaporization heat M/kg) Mixture enthalpy (kj/kg dry air) Dry air partial density (lKinematic viscosity (I0< mJ/s) Specific heat (kJ/K kg) Heat conductivity (W/m K) Diffusion factor water air (1 O mJ/s) Temperature rc)... 

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