Heat Transport Phenomena

Consequently, for a given design, the material properties are expected to widely contribute to the heat transport phenomena, with SiC appearing to be the most attractive option under this criterion. 

Catalysts are generally developed for a particular process, i.e. for a certain reaction in a certain reactor under certain conditions. Mass and heat transport phenomena put their... 

First of all, the physical structure of the packed bed in the conversion system is defined. The fuel bed structure can be divided into three phases, namely the interstitial gas phase, the intraparticle solid phase, and the intraparticle gas phase. By means of this terminology it is easier to address certain mass and heat transport phenomena taking place on macro and micro scale inside the packed bed during the thermochemical conversion, see Figure 8. 

Compared to the ring, the use of several, smaller chanels changes the trade-off between mechanical strength of the pellet and mass- and heat-transport phenomena. 

Yuan, J., Rokni, M. and Sunden, B. (2003) Three-dimensional computational analysis of gas and heat transport phenomena in ducts relevant for anode-supported solid oxide fuel cells, International Journal of Heat and Mass Transfer 46, 809-821. 

In this sense rate procurement is one of the core businesses of catalysis. It should, however, be carried out in the correct way. Rate measurements can be disguised by slow mass or heat transport phenomena inside and outside the catalyst particle or by the reactor configuration, through which not intrinsic rates are determined, thus rendering the data and efforts useless [1]. 

As was mentioned in Section I, heat transport phenomena of longitudinal flow through rod bundles (i.e., transport from the rods to the fluid v.v.) have been studied quite extensively in the past and are still receiving constant attention, due to the importance of compact heat exchangers and nuclear reactors. This has led to many theoretical and empirical relations to predict Nusselt numbers as a function of the relative pitch and (for nonlaminar flow) the Reynolds number a relatively recent review of those relations that pertain to smooth rod bundles is by Rehme [6]. 

The results confirm that TGA experiments are not significantly affected by heat transport phenomena if low initial sample masses as well as the described TGA configuration and experimental procedures are applied. The temperature gradients inside the samples are sufficiently small to allow the fitting of formal kinetic models to the experimental mass loss curves assuming a homogeneous sample temperature. Cellulose samples with initial sample masses of around 5 mg and higher can only be submitted to kinetic analyses under consideration of the enthalpy balance. 

The technical aspects outlined above illustrate that for a process optimization all process parameters must be considered, since they strongly affect each other. It should also be pointed out that although vapor permeation and pervaporahon are membrane separation process, their selectivity and effectiveness may be significantly determined by mass and heat transport phenomena occurring beyond the membrane surfaces. 

When testing catalytic properties, it is of utmost importance that other phenomena than those occurring at the catalyst s active sites do not become a limiting factor. Only then the observations can be directly related to the catalyst properties. Two types of other phenomena are likely to affect the observations, that is, the mass and heat transport phenomena at the catalyst pellet scale and the reactor flow pattern nonidealities at the reactor scale. 

This chapter deals with the microkinetics of gas-solid catalytic reaction systems. An applied approach is adopted in the discussion, which starts with the formulation of intrinsic rate equations that account for chemical processes of adsorption and surfece reaction on solid catalysts and then proceeds with the construction of global rate expressions that include the individual and simultaneous effects of physical external and internal mass and heat transport phenomena occurring at the particle scale. 

The heat transport phenomena conduction, radiation, convection, and heat transfer are of central importance in all calorimeters. On the one hand, the occurrence of a temperature difference causes a heat flow and thus creates the possibility of heat losses toward the surroundings (heat leaks) - namely, heat flows not detected by the measuring sensor and therefore not measured by the calorimeter. On the other hand, no heat exchange can take place in the absence of a temperature difference. The experimenters find themselves in a dilemma to be measured, heat must be made to flow, but every heat exchange is associated with temperature differences that create errors in measurement (e.g., heat leaks). 

Another important implication arising from the discussion of heat transport phenomena will be discussed in detail in Chapter 6 but should be mentioned here. Indeed, the measured signal recorded by the calorimeter does not reflect exactly the time course of the underlying process inside the sample. Because the measuring probe that detects the heat flow rate is necessarily located at some distance from the sample, the heat exchange between the sample and the probe necessary for the measurement requires a temperature difference, and this requires some time. Thus, both the temperature change and the time course of processes in the sample are, in effect, not exactly reproduced by the calorimeter but are distorted the measured signal is smeared. ... 

To sum up, to yield repeatable results and to be capable of calibration, a calorimeter must be constructed in such a way that the entire heat exchange of the sample with the surroundings takes place in a defined manner through the measuring system only. In any uncertainty analysis (see Section 6.5), heat transport phenomena and their consequences must be taken into account. 

We first consider the consequences of heat transport phenomena, which take place inside the measuring systems or substances to be studied with the calorimeter. Afterward we shall discuss additional effects associated with the specific type of calorimeter used. This chapter also provides an outline of methods for the elimination of heat conduction effects by mathematical means and ends with the presentation of some special methods of evaluation, including the assessment of the measurement uncertainty. 

The concept of criteria for exclusion of interparticle mass and heat transfer effects is the following. Since during a reaction non-zero gradients of concentration and/or of temperature always exist in the fixed bed reactor (albeit sometimes they are very small), a somewhat arbitrary assumption has to be made about the maximum deviation up to which the reaction can be considered not to be influenced by axial and radial mass and heat transport phenomena. The maximum deviation commonly used is 5%, for example, of the reaction rate compared to the zero-gradient rate or of the reactor length compared to the length of an ideal PFR. 

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