Selectivity temperature gradient effect

It is obvious that nonisothermal conditions induced by microwave heating lead to very different results from those obtained under conventional heating conditions. In summary microwave effects like superheating, selective heating and hot spots, can all be characterized by temperature gradients ranging from macroscopic to molecular scale dimensions. 

Furfural exhibits high selectivity at elevated temperatures (175° to 250° F.) this characteristic results in the benefits of reduced viscosity and high temperature gradient and permits operation on waxy stocks. At ambient temperatures, kerosenes and gas oils may be extracted. Water in furfural has a bad effect on its extraction efficiency therefore, solvent recovery systems must include dehydration facilities. From its chemical composition aldehyde furfural appears to be unstable. However, when handled in accordance with standard operating procedures, decomposition and polymerization are negligible. 

Lateral transport of heat and reactants across the diameter of the reactor This feature is important when the reaction has a high heat effect or when insufficient mixing of the reactants would have a strong adverse effect on the performance. Examples are selective oxidation processes (highly exothermic), where lateral or radial temperature gradients would decrease the selectivity of the conversion. Another example is the selective catalytic reduction process of nitric oxide with ammonia, where mixing of ammonia with the flue gas is often a point of great concern. Because the void space in a BSR is continuous... 

Experimental systems used for electrochemical measurements should be selected to take maximum advantage of well-imderstood phenomena such as mass transfer so as to focus attention on the less-understood phenomena such as electrode kinetics. For example, the study of electrochemical reactions in stagnant environments should be avoided because concentration and temperature gradients give rise to natural convection, which has an effect on mass transfer that is difficult to characterize. It is better to engage in such experimental investigations in systems for which mass transfer is well defined. To simplify interpretation of the impedance data, the electrode should be uniformly accessible to mass transfer. 

After the preliminary selection of 300 series stainless steel, these alloys were subjected to further tests to study the effect of stress in the presence of chloride, oxygen, and water vapor, the effect of sensitization, and the rate at which the alloying constituents are leached and transported under the influence of a temperature gradient. A molten salt loop has been in operation for nearly a year as a part of these tests. The... 

Isothermal temperature control in the fluid-bed reactor was easily maintained under all process conditions investigated. The temperature gradients in the catalyst bed did not exceed 5 C even at mean temperature gradients of 200 to 300 C between the catalyst bed and the heat transfer medium. The plant accumulated 17 months-on-strearn of MTG/MTO operation, including 5 months at MTO conditions. MTO operation started with sensitivity studies to determine the effects of temperature and pressure on selectivity. Deactivation periods to vary catalyst activity and to Drovide a comDarison with the 4 B/D Dilot Diant were Derformed... 

Thus, to calculate Vj one has to select an appropriate expression for the energy density gradient and then integrate Eq 4.8 within the limit of composition in both phases, ( ) and ( )g. Huggins-Flory, and Cahn-Hilliard theories were used with good success to predict the temperature gradient, but poor as far as the effects of molecular weight were concerned. 

In conventional experiments, the gas and catalyst are maintained at the same temperature. In microwave experiments the power is deposited within the catalyst, which is cooled by the gas flow and thermal conduction to the surroundings. If the catalyst bed is not thick, the gas is always at a lower temperature than the solid catalyst. The increased loss factor of the catalyst favors the formation of CH3 radicals because they are produced at active O2 sites and these specific sites are preferentially excited by the microwave fleld. Hence, the observed enhancement of C2 selectivity is, in fact, a thermal phenomenon in that it can be completely explained by temperature gradients within catalyst particles, i.e. by locally excited catalytic sites. This observed effect can be explained by assuming that under the action of micro-wave irradiation the temperature of the reaction sites is higher than the mean temperature of the catalyst bed (Section 13.3.3). 

Microscopic hot spots, for example temperature gradients between the metal particles and the support, which cannot be detected and measured because they are close to micro scale, i.e. have molecular dimensions, are dose to selective heating of active sites. Unfortunately, microwave radiation effects at the molecular level are not well understood. 

---