Gases, thermal conductivities

The second part of the chapter is devoted to the effect of pressure on heat and mass transfer. After a brief survey on fundamentals the estimation of viscosity, diffusivity in dense gases, thermal conductivity and surface tension is explained. The application of these data to calculate heat transfer in different arrangements and external as well as internal mass transfer coefficients is shown. Problems at the end of the two main parts of this chapter illustrate the numerical application of the formulas and the diagrams. 

Geometry of the body of sulfidic material Particle-size distribution Permeability to water and gases Thermal conductivity and heat capacity... 

In this equation, is the gas thermal conductivity the Hquid density the Hquid heat capacity T, the gas temperature the initial droplet temperature and the droplet boiling point. 

A = Surface area ft based on tube ID C = Gas specific heat. Btu/lb°F d = Tube inner diameter, in. k = Gas thermal conductivity, Btu/ft-h°F L = Tube length, ft N = Total number of tubes in boiler Pr = Gas Prandtl number Q = Duty of the boiler. Btu/h... 

Fig. 10. Sonochemical rates as a function of ambient gas thermal conductivity. [Replotted data from R. O. Prudhomme (95).]...

Some researchers have noted that this approach tends to underestimate the lean phase convection since solid particles dispersed in the up-flowing gas would cause enhancement of the lean phase convective heat transfer coefficient. Lints (1992) suggest that this enhancement can be partially taken into account by increasing the gas thermal conductivity by a factor of 1.1. It should also be noted that in accordance with Eq. (3), the lean phase heat transfer coefficient (h,) should only be applied to that fraction of the wall surface, or fraction of time at a given spot on the wall, which is not submerged in the dense/particle phase. This approach, therefore, requires an additional determination of the parameter fh to be discussed below. 

Gas Thermal conductivity (TC or TCD) Flame ionization (FID) Electron capture (ECD) Mass spectrometry (MS or GC-MS)... 

Gas thermal conductivity sensors are used to detect variations in the composition of mixtures of gases by monitoring changes in the thermal conductivity of the mixture. Such instruments are used (a) as detectors for gas chromatographs (Section... 

A katharometer is employed to determine the concentration of H2 in a H2/CH4 mixture. The proportion of H2 can vary from 0 to 60 mole per cent. The katharometer is constructed as shown in Fig. 6.54 from four identical tungsten hot-wire sensors for which the temperature coefficient of resistance ft, is 0.005 K. The gas mixture is passed over sensors R, and R whilst the reference gas (pure CH4) is passed over sensors R2 and R,. The total current supplied to the bridge is 220 mA and it is known that the resistance at 25°C and surface area of each sensor are 8 Q and 10 mm2 respectively. Assuming the heat transfer coefficient h between gas and sensor filaments to be a function of gas thermal conductivity k only under the conditions existing in the katharometer and that in this case h = k x 10 (h in W/m2K and k in W/mK), draw a graph of the output voltage V0 of the bridge network as a function of mole per cent H2. 

The heat of reaction, H, for carbon oxidation to carbon monoxide is 2340 cal/g. k is the gas thermal conductivity evaluated at the arithmetic mean of particle and gas temperatures e is the particle emissivity and assumed here to be unity and is the Stefan-Boltzmann constant. 

Gases Methods for estimating low-pressure gas thermal conductivities are based on kinetic theory and generally correlate the dimensionless group kM/r C (M = molecular weight, T] = viscosity, C = isochoric heat capacity) known as the Eucken factor. The method of Stiel and Thodos is recommended for pure nonpolar compounds, and the method of Chung is recommended for pure polar compounds. 

Sc is the Schmidt number for mass-transfer properties, Pr is the Prandd number for heat-transfer properties, and Le is the Lewis number K/(Cjpg ), where K is the gas thermal conductivity and is the diffusion coefficient for the vapor through the gas. Experimental and theoretical values of the exponent n range from 0.56 [Bedingfield and Drew, Ind. Eng. Chem, 42 1164 (1950)] to = 0.667 [Chilton and Colburn, Ind. Eng. Chem., 26 1183 (1934)]. A detailed discussion is given by Keey (1992). Values of P for any system can be estimated from the specific heats, diffusion coefficients, and other data given in Sec. 2. See the example below. 

The characteristics of the medium, homogeneous or heterogeneous (e.g. viscosity, vapor pressure, the nature and size of suspended particles, the nature and concentration of any dissolved gas, thermal conductivity etc.). 

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