Heat specific surface area

There are complexities. The wetting of carbon blacks is very dependent on the degree of surface oxidation Healey et al. [19] found that q mm in water varied with the fraction of hydrophilic sites as determined by water adsorption isotherms. In the case of oxides such as Ti02 and Si02, can vary considerably with pretreatment and with the specific surface area [17, 20, 21]. Morimoto and co-workers report a considerable variation in q mm of ZnO with the degree of heat treatment (see Ref. 22). 

Harkins and Jura [21] found that a sample of Ti02 having a thick adsorbed layer of water on it gave a heat of inunersion in water of 0.600 cal/g. Calculate the specific surface area of the Ti02 in square centimeters per gram. 

Make a numerical estimate, with an explanation of the assumptions involved, of the specific surface area that would be found by (a) a rate of dissolving study, (b) Harkins and Jura, who find that at the adsorption of water vapor is 6.5 cm STP/g (and then proceed with a heat of immersion measurement), and (c) a measurement of the permeability to liquid flow through a compacted plug of the powder. 

HEAT TRANSFER SURFACE AREA, m 2 SPECIFIC HEAT OF LIQUID, kJ/kg.K WEIGHT OF BATCH LIQUID, kg. HEATING MEDIUM TEMPERATURE, K INITIAL BATCH TEMPERATURE, K ... 

A = heat transfer surface area c = specific heat of batch liquid C = coolant specific heat M = weight of batch liquid Tj = initial batch temperature Tj = final batch temperature tj = initial coolant temperature U = overall heat transfer coefficient = coolant flowrate 6 = time... 

Specific surface areas of the catalysts used were determined by nitrogen adsorption (77.4 K) employing BET method via Sorptomatic 1900 (Carlo-Erba). X-ray difiraction (XRD) patterns of powdered catalysts were carried out on a Siemens D500 (0 / 20) dififactometer with Cu K monochromatic radiation. For the temperature-programmed desorption (TPD) experiments the catalyst (0.3 g) was pre-treated at diflferent temperatures (100-700 °C) under helium flow (5-20 Nml min ) in a micro-catalytic tubular reactor for 3 hours. The treated sample was exposed to methanol vapor (0.01-0.10 kPa) for 2 hours at 260 °C. The system was cooled at room temperature under helium for 30 minutes and then heated at the rate of 4 °C min . Effluents were continuously analyzed using a quadruple mass spectrometer (type QMG420, Balzers AG). 

Worz et al. give a numerical example to illustrate the much better heat transfer in micro reactors [110-112]. Their treatment referred to the increase in surface area per unit volume, i.e. the specific surface area, which was accompanied by miniaturization. The specific surface area drops by a factor of 30 on changing from a 11 laboratory reactor to a 30 m stirred vessel (Table 1.7). In contrast, this quantity increases by a factor of 3000 if a 30 pm micro channel is used instead. The change in specific surface area is 100 times higher compared with the first example, which refers to a typical change of scale from laboratory to production. 

Micro heat exchangers and also any kind of micro channel devices, heated or cooled externally, offer considerably improved heat transfer owing to their large internal specific surface areas. Hence they offer unique possibilities to steer oxidations to increased selectivity of the partial-oxidation products. 

As indicated above, it has been demonstrated many times that the small reaction volumes in micro reactors and the large specific surface areas created allow one to cope with the release of the large amounts of heat. Knowledge of heat transfer characteristics seems to be a top priority when designing a micro reactor for oxidations. 

Fig. 4.25 represents a steady-state, single-pass, shell-and-tube heat exchanger. For this problem W is the mass flow rate (kg/s), T is the temperature (K), Cp is the specific heat capacity (kJ/m s), A (= 7i D Z) is the heat transfer surface area (m ), and U is the overall heat transfer coefficient (kJ/m s K). Subscripts c and h refer to the cold and hot fluids, respectively. 

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

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