Internal Heat and Mass Transfer

All reactor-cells used had a volume of 30 ml and have been shown to be well mixed over the range of flowrates employed in the present study (22). Both external and internal mass and heat transfer limitations have been shown to be negligible (12,22). Reactants were certified standards of ethylene diluted in N2 and Matheson zero grade air. They could be further diluted in N2. Reactants and products were analyzed by one line Gas Chromatography. The carbon dioxide concentration in the product stream was also continuously monitored using a non-dispersive IR CO2 Analyzer (Beckman 864). 

These phenomena are called external not only because they take place outside the catalyst par ticle, but also because they ar e examined independent of the chemical reaction, in contrast to the internal mass and heat transfer phenomena. 

At the beginning of this chapter it was pointed out that both external and internal resistances are frequently, but not always, important, and general criteria were discussed. Quantitative methods of evaluating Q — and 7] — 7 were developed in Secs. 10-3 and 10-4. Quantitative criteria for the significance of internal mass- and heat-transfer resistances were given... 

In most investigations concerning the reactor modelling, simple pseudohomogeneous (t = 1) reactor models were used. The effect of external and internal mass and heat transfer resistances on the effectiveness factors using realistic complex reaction network has not been widely investigated. The simple linear kinetics proposed by... 

The first step in heterogeneous catalytic processes is the transfer of the reactant from the bulk phase to the external surface of the catalyst pellet. If a nonporous catalyst is used, only external mass and heat transfer can influence the effective rate of reaction. The same situation will occur for very fast reactions, where the reactants are completely exhausted at the external catalyst surface. As no internal mass and heat transfer resistances are considered, the overall catalyst effectiveness factor corresponds to the external effectiveness factor,... 

Negligible pressure drop no internal mass and heat transfer limitations because of the small particle sizes that can be employed... 

In the previous chapters tve discussed the influence of internal mass and heat transfer by neglecting external transport phenomena. Hence, we assumed that concentrations and temperature at the outer surface of the catalyst particle and the bulk of the fluid are the same. But this assumption is not justified under certain conditions and concentration and temperature profiles inside and outside the porous catalyst must be considered. 

In comparing external versus internal mass and heat transfer, Yang (1987) concludes that the major resistance is within the pores for mass transfer and in the gas film for heat transfer. However, the heat transfer rate is not normally used in absorber designs the gas is simply assumed to be at the same temperature as the particles with which it is in contact. 

The essential feature of a Jluidized-bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat transfer rates and good mixing. Heat transfer coefficients in the order of 200 W/m-°C between jackets and internal coils are typically obtained. The solids may be a catalyst, a reactant (in some fluidized combustion processes), or an inert powder added to promote heat transfer. 

Most of the actual reactions involve a three-phase process gas, liquid, and solid catalysts are present. Internal and external mass transfer limitations in porous catalyst layers play a central role in three-phase processes. The governing phenomena are well known since the days of Thiele [43] and Frank-Kamenetskii [44], but transport phenomena coupled to chemical reactions are not frequently used for complex organic systems, but simple - often too simple - tests based on the use of first-order Thiele modulus and Biot number are used. Instead, complete numerical simulations are preferable to reveal the role of mass and heat transfer at the phase boundaries and inside the porous catalyst particles. 

Details on microfabrication and on the internals in the stacked plates have not been substantially disclosed so far. Accordingly, no information on the mechanisms of mass and heat transfer was reported. In one version, geometrically focused multilamination is used for mixing liquid streams [55]. 

Similarly, when internal gradients correspond to differences in concentration or temperature between the external surface of the catalyst particle and its centre, the rate in the particle is substantially different from that which would prevail if the concentration or temperature were the same throughout the particle. The catalytic reaction is then said to be influenced by internal mass or heat transfer, and, when this influence is the dominant one, the rate corresponds to a regime of internal mass or heat transfer. 

Since industrial catalysts are employed as pellets, the mass- and heat-transfer effects can play an important role. The internal diffusion is often the critical step controlling the overall process rate. The use of an efficient catalyst is the decisive element in designing a competitive process. 

The intcrfacial mass- and heat-transfer resistance between any two adjacent regions is negligible compared to the corresponding internal mass- or heat-transfer resistances in each region. 

The bottom example shows the opposite situation internal rate control. In the case of heating from the top, internal control refers to a slow rate of mass transfer from the bulk of the material to the surface of the material. Diffusion, convection, and capillary action (in the case of porous media) are possible mechanisms for mass transfer of moisture to the surface of the slab. In the internal rate control situation, moisture is removed from the surface by the air faster than moisture is transported to the surface. This regime is caused by relatively thick layers or high values of the mass- and heat-transfer coefficients in the air. Internal rate control leads to the observation of a falling-rate period drying curve. 

Essentially all of the surface, of porous catalyst pellets is internal (see page 295). Reaction and mass and heat transfer occur simultaneously at any position within the pellet. The resulting intrapellet concentration and temperature gradients cause the rate to vary with position. At steady state the average rate for a whole pellet will be equal to the global rate at the location of the pellet in the reactor. The concentration and temperature of the bulk fluid at this location rhay not be equal to those properties at the outer surface of the pellet. The effect of such external resistances can be accounted for by the procedures outlined in Chap. 10. The objective in the present chapter is to account for internal resistances, that is, to evaluate average rates in terms of the temperature and concentration at the outer surface. Because reaction and transport occur simultaneously, differential... 

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