Heat transfer control

In the case of thermodynamics, the designer can investigate the nature of the reaction heat and whether the reaction is reversible. If these exothermic reactions are irreversible, attention may be focused on the influence of reactor design on conversion and with heat transfer control. An objective of reactor design is to determine the size and type of reactor and mode of operation for the required job. The choice... 

Equation (l) shows the rate of polymerization is controlled by the radical concentration and as described by Equation (2) the rate of generation of free radicals is controlled by the initiation rate. In addition. Equation (3) shows this rate of generation is controlled by the initiator and initiator concentration. Further, the rate of initiation controls the rate of propagation which controls the rate of generation of heat. This combined with the heat transfer controls the reaction temperature and the value of the various reaction rate constants of the kinetic mechanism. Through these events it becomes obvious that the initiator is a prime control variable in the tubular polymerization reaction system. 

Equations 1 and 2 in the forms stated are attributable to Probert 98), and derivations for the case of heat transfer controlling are given by Kumm 70), Marshall and Seltzer 87), Godsave 37-39), and Goldsmith and Penner 42). The case of mass transfer controlling is derived by Fuchs 31) and compared by Gilbert, Howard, and Hicks 36) to isothermal evaporation into a maintained vacuum. 

Gohrbandt s data for camphor spheres (40, 97) afford comparison of rates with diffusion controlling and with heat transfer controlling. Extrapolation to low temperatures of the heat transfer portion indicates sufficient heat transfer but inadequate diffusion. Similarly, extrapolation to high temperatures of the diffusion portion indicates sufficient diffusional driving force but inadequate heat transfer to maintain the surface temperature. 

From the extensive experimental and model development work performed at CSM (during a period of over 15 years), it has been demonstrated that a heat transfer controlled model is able to most accurately predict dissociation times (comparing to laboratory experiments) without any adjustable parameters. The current model (CSMPlug see Appendix B for details and examples) is based on Fourier s Law of heat transfer in cylindrical coordinates for the water, ice, and hydrate layers, and is able to predict data for single- and two-sided depressurization, as well as for thermal stimulation using electrical heating (Davies et al 2006). A heat transfer limited process is controlled by the rate of heat supplied to the system. Therefore, a measurable intermediate (cf. activated state) is not expected for heat transfer controlled dissociation (Gupta et al., 2006). 

Proximity to the phase boundary, and the need to supply energy to dissociate the hydrates, will control the rate of dissociation and thus the economics. Because conductive heat transfer controls hydrate dissociation, hydrates closer (in temperature and pressure) to the phase boundary will be most economical to dissociate. Heat transfer limitations indicate that high surface areas (thin layers) are most economical. 

Equation (17) represents the general case of uptake under complete heat transfer control and equation (18) gives the corresponding adsorbent temperature profile. Furthermore, if kg is large compared to h, (s << 1), equation (10) can be approximated as p 3 s and equations (17) and (18) reduce to n... 

The equilibrium data [28] was used to evaluate p for the system. A heat of adsorption of 14.9 kcal/mole was estimated for the system. Curves b and c, respectively, show the corresponding uptakes for external film heat transfer control and isothermal sorption using the same k as in curve a. 

Figure 5.5 Heat transfer control by variable heat transfer area in flooded condenser.

Figure 5.6 Utility exchanger without heat transfer control.

TABLE 14.2 Evaporative Drying of Spherical Green Body, by Heat Transfer Controlling Steps... 

Control of the temperature throughout the reforming catalyst bed can be established by use of a monolithic catalyst. The heat transfer control can be accomplished by combining three effects that monolithic catalyst beds can impact significantly (1) direct, uniform contact of the catalyst bed with the reactor wall will enhance conductive heat transfer (2) uniformity of catalyst availability to the reactants over the length of the flow will provide continuity of reaction and (3) coordination of void-to-catalyst ratio with respect to the rate of reaction will moderate gas-phase cracking relative to catalytically enhanced hydrocarbon-steam reactions. This combination provides conditions for a more uniform reaction over the catalyst bed length. 

A wood cylinder is vertically positioned in the uniformly heated zone of the reactor, through a suspension system, which is connected to a precision balance. The sample is exposed to the same radiative heat flux along the lateral surface. For each chosen radiation intensity, steady temperatures of the radiant heater are achieved within a couple of minutes (maximum heating rates of about 750K/s) but, given the thick sample, pyrolysis takes place under heat transfer control. 

For all the wood varieties, the temperature field shows an outer and an inner region, along the cylinder radius, determined by the simultaneous and the sequential degradation of the components, respectively. Though the details of process dynamics depend on the external heat flux and the wood category, the differences tend to disappear for heat fluxes above 40kwW, when a shift from chemical reaction to internal heat transfer control occurs. 

Di Biasi C. (1996) Kinetic and heat transfer control in the slow and flash pyrolysis of solids. Ind. Eng. Chem.Res., 35, 27-46. 

In attempting to model the heat transfer-controlled behavior, we were surprised to find, in the literature, a significant variability in the key thermal properties that such a model required. Quite different predictions could result from using different assumptions regarding the thermal properties. To address this obviously significant impediment to reliable model development, an experimental program focusing on thermal property measurements was initiated. The results are summarized below. 

Assume a planet in which the upper mantle has about 0.2% water, and the composition is broadly peridotitic, with heat production as on Earth. The geotherm depends on the vertical distribution of the heat production in the interior and on the surface temperature, and on the heat transfer controls. The surface temperature is maintained by the radiative balance of the atmosphere and any greenhouse increment, and is only indirectly dependent on interior processes. If the state of the atmosphere means that the surface is cold, say — 100°C to —200°C, then the lithosphere will be thick. The top of the mantle adiabat will be forced to cool. The deeper mantle will thus store heat until melting extracts it as... 

The method gives no information about sohds residence time or dryer length. A minimum drying time can be calculated by evaluating the maximum (unhindered) drying rate assuming gas-phase heat-transfer control and estimating a gas-to-solids heat-transfer coefficient. The simple equation (12-60) then apphes ... 

The second important aspect of circnlation is the heat balance between absorption in the endothermic reaction and release in the exothermic zone. The circulation system design depends on whether the overall scheme is deactivation controlled or heat transfer controlled. The final eqnations, with a brief reference to the principles, are provided for the two extreme cases. 

FIGURE 11.39 Fluidized-bed circulation system for heat transfer control. 

In chemical process industries, stirred tank reactors are frequently used. For hydrogenation/oxi-dation applications, the heats of reaction are large, and the overall operation may be heat transfer controlled. In reactions where selectivity is of importance (for example, oxidation, hydrogenation,... 

In the case of catalytic packed-bed reactors, provision of extra volume should be considered to accommodate additional catalyst in case of selectivity/activity loss over time. In some heat transfer-controlled reactions, it may be possible to partly evaporate the solvent itself, condense it in a reflux condenser, and send the solvent back to the reactor. The thermal sensitivities of all the species in the reactor must be known. Some compounds may decompose dangerously at certain conditions, and suitably lower temperatures must be chosen for operation. 

Prefer monolithic confignration for intensification with surface area/volume usually 1.5 to 4 times greater than traditional pellets. Excellent for mass transfer controlled reactions. For gas reacting with solid nsnally heat transfer controlled, because these are highly exothermic or endothermic reactions. Particle size and size distribution are critical. These reactions may follow different patterns ... 

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