Conversion factors heat transfer coefficients

Polymerization processes are characterized by extremes. Industrial products are mixtures vwth molecular weights of 10 to 10. In a particular polymerization of styrene the viscosity increased by a factor of 10 as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks vwth high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h ft °Fj). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow for 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature vwthin limits. Initiators of the chain reactions have concentration of 10 g mol/L so they are highly sensitive to poisons and impurities. 

An engineer who is working on the heat transfer analysis of a house in English units needs Ihe convection heat transfer coefficient on the outer surface of the house. But the only value he can find from his handbooks is 14 W/m - °C, which is in SI units. The engineer does not have a direct conversion factor between the two unit systems for the convection heat transfer coefficient. Using the conversion factors between W and Btu/h, m and ft, and C and "F, express the given convection heat transfer coefficient in Btu/h - °F. 

The objective of sca)ing-up a vacuum pyrolysis reactor is to achieve the desired capacity and conversion of feedstock by defermining the dimension of the reactor. The feedstock conversion in a vacuum pyrolysis reactor mainly depends on three factors the heat transfer coefficient from the reactor to the feedstock the residence time of the feedstock in the reactor and the kinetics of the feedstock pyrolysis reactions. The heat transfer coefficient and the residence time determine the quantity of energy transferred and thus the temperature distribution throughout the feedstock in the reactor. The tert erature distribution and the kinetics determine the final conversion achieved at the reactor outlet. 

After the input has been read and sorted, heat transfer coefficients and other thermodunamic data are calculated at the beginning of each catalyst zone. Temperature and conversion profile in the catalyst bed is then calculated by an axial integration. The mathematical model used in the integrations is described in. This model allows in principle the determination of diffusion restrictions and calculation of effectiveness factors for each reaction in cases where several reactions take place simultaneously. In such cases the concept of effectiveness factor may become rather dubious as shown below for the methanol synthesis, and this may be reflected in difficulties in the calculations. 

As a rule of thumb, axial dispersion of heat and mass (factors 2 and 3) only influence the reactor behavior for strong variations in temperature and concentration over a length of a few particles. Thus, axial dispersion is negligible if the bed depth exceeds about ten particle diameters. Such a situation is unlikely to be encountered in industrial fixed bed reactors and mostly also in laboratory-scale systems. Radial mass transport effects (factor 1) are also usually negligible as the reactor behavior is rather insensitive to the value of the radial dispersion coefficient. Conversely, radial heat transport (factor 4) is really important for wall-cooled or heated reactors, as such reactors are sensitive to the radial heat transfer parameters. 

Whenever the kinetics of a chemical transformation can be represented by a single reaction, it is sufficient to consider the conversion of just a single reactant. The concentration change of the remaining reactants and products is then related to the conversion of the selected key species by stoichiometry, and the rates of production or consumption of the various species differ only by their stoichiometric coefficients. In this special case, the combined influence of heat and mass transfer on the effective reaction rate can be reduced to a single number, termed the catalyst efficiency or effectiveness factor rj. From the pioneering work of Thiele [98] on this subject, the expressions pore-efficiency concept and Thiele concept have been coined. 

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