Areas, active heat transfer

Syngas (typically a mixture of CO, H, and CO ) reacts over the active catalyst (Cu/Zn/AljOj) dispersed in an inert oil medium. This process offers considerable advantages over the conventional vapor phase synthesis of methanol in the areas of heat transfer, exothermicity, and selectivity toward methanol. However, this process suffers from the drawback that the methanol synthesis reaction is a thermodynamically governed equilibrium reaction. 

These evaporators are necessarily precision machines and therefore are more expensive than other types, particularly so if compared strictly on equivalent heat transfer area. When the performance for a specific evaporation duty is the basis of comparison, the thin-film evaporator is often the more economical choice because the larger heat transfer coefficient and higher driving force mean much less surface is required than for other evaporators (A = Q/U AT). Thin-film evaporator cost per unit area decreases significantly with unit size, and the largest available unit has 430 square feet of active heat transfer surface. 

Feed and removal volumetric flow Concentration of hexamine in feed Molar mass of hexamine Concentration of nitric acid in feed Molar mass of nitric acid Pre-exponential factor Apparent energy of activation Order of the reaction Thermal capacity of hexamine Feed temperature of hexamine Thermal capacity of nitric acid Feed temperature of nitric acid Overall coefficient of heat transfer Area for heat transfer (jacket + coU) Enthalpy of reaction Reaction temperature... 

While the practical reasons referred to above represented the main driving force for the use of supports, this is by no means the whole story. As all the surface is rarely active, the support material can act as a heat sink or source for active sites or zones in exothermic and endothermic reactions, while increasing the total area for heat transfer between solid material and the gas (or liquid) phase. 

Often complex geometries are used for the design of MSR. An example is the channel structure developed by the chemical company LONZA Ltd., Switzerland [16], where mixing elements are integrated in the microchannels (Figure 5.11). In these cases, the estimation of the active heat transfer surface area and the heat transfer coefficients are hardly possible. Therefore, the introduction of an overall volumetric heat transfer coefficient (U ) is useful. 

Vaporizer systems should be designed so that, within their heat-transfer capacity, the rate automatically adjusts to the process demand. The systems discussed here, such as the typical vaporizer in Fig. 9.39, meet this requirement. An increase in demand, for example, causes the line pressure to drop, leading to an increase in flow of liquid chlorine. The level in the vaporizer then goes up, increasing the area of active heat-transfer surface and therefore the rate of vaporization. 

The color development of photochromic compounds can also be utili2ed as a diagnostic tool. The temperature dependence of the fa ding of 6-nitroindolinospiropyran served as the basis for a nondestmctive inspection technique for honeycomb aerospace stmctures (43). One surface of the stmcture to be exarnined was covered with a paint containing the photochromic compound and activated to a violet color with ultraviolet light. The other side of the stmcture was then heated. The transfer of heat through the honeycomb stmcture caused bleaching of the temperature-dependent photochromic compound. Defects in the honeycomb where heat transfer was inhibited could be detected as darker areas. 

Other important aspects include the effect of temperature (via activation energy) and mixing, particularly for multiphase reactions. Both of these can impact on selectivity and thus can be improved in operating continuously using the inherent benefits of heat transfer area and mixing strategies discussed previously. 

Optimized microfabrication and advanced assembly led to the use of thin platelets, in an original version 100 pm thick with a 80 pm micro channel depth, so that very thin walls (20 pm in the case sketched) remain for separating the fluids. Therefore, also the total inner reaction volume with respect to the total construction volume or the active internal surface area is very large. The latter surface amounts to 300 cm (for both the heat transfer and reaction sides) at a cubic volume of 1 cm. Indeed, the micro heat exchangers exhibited high heat transfer coefficients for gas [46] and liquid (Figure 3.10) [47, 48] flows. 

Truly isothermal operation of a tubular reactor may not be feasible in practice because of large enthalpies of reaction or poor heat transfer characteristics. Nor is it always desirable, as, for example, in the case of a reversible exothermic reaction (see Sect. 3.2.4). In an exothermic catalytic reaction, it may be necessary to provide adequate means for heat transfer to prevent the development of local hot-spots on which coking may occur and reduce the catalyst activity. An excessive temperature rise may also cause the catalyst particles to sinter, thereby reducing their surface area and causing an irreversible decrease in catalytic activity. 

Nickel-platinum bimetallic catalysts showed higher activity during ATR than nickel and platinum catalysts blended in the same bed. It was hypothesized that nickel catalyzes SR, whereas platinum catalyzes POX and, when they are added to the same support, the heat transfer between the two sites is enhanced [59, 60]. Advanced explanations were reported by Dias and Assaf [60] in a study on ATR of methane catalyzed by Ni/y-Al203 with the addition of small amounts of Pd, Pt or Ir. An increase in methane conversion was observed, ascribed to the increase in exposed Ni surface area favored by the noble metal under the reaction conditions. 

The effects of non-uniform distribution of the catalytic material within the support in the performance of catalyst pellets started receiving attention in the late 60 s (cf 1-4). These, as well as later studies, both theoretical and experimental, demonstrated that non-uniformly distributed catalysts can offer superior conversion, selectivity, durability, and thermal sensitivity characteristics over those wherein the activity is uniform. Work in this area has been reviewed by Gavriilidis et al. (5). Recently, Wu et al. (6) showed that for any catalyst performance index (i.e. conversion, selectivity or yield) and for the most general case of an arbitrary number of reactions, following arbitrary kinetics, occurring in a non-isothermal pellet, with finite external mass and heat transfer resistances, the optimal catalyst distribution remains a Dirac-delta function. 

All types of catalytic reactors with the catalyst in a fixed bed have some common drawbacks, which are characteristic of stationary beds (Mukhlyonov et al., 1979). First, only comparatively large-grain catalysts, not less that 4 mm in diameter, can be used in a filtering bed, since smaller particles cause increased pressure drop. Second, the area of the inner surface of large particles is utilized poorly and this results in a decrease in the utilization (capacity) of the catalyst. Moreover, the particles of a stationary bed tend to sinter and cake, which results in an increased pressure drop, uneven distribution of the gas, and lower catalyst activity. Finally, porous catalyst pellets exhibit low heat conductivity and as a result the rate of heat transfer from the bed to the heat exchanger surface is very low. Intensive heat removal and a uniform temperature distribution over the cross-section of a stationary bed cannot, therefore, be achieved. The poor conditions of heat transfer within... 

As the temperature difference increases, new active points appear. Finally the number becomes so large that no bare areas can be seen on the metal. This is the condition of maximum heat transfer illustrated in Fig. 5. The temperature difference is the critical AT. 

Based on the computer output a number of curves were produced as shown in Figure 4. To generate the doto necessary to produce the curves it wos assumed thot the overall heat transfer coefficient wos constant for all cases examined and the shell side pressure drop and heat tronsfer coefficient could be maintained by keeping the active tube length constant ond varying the bundle diameter and the number of tubes in order to satisfy the surface area requirements. 

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