Temperature profile selectivity considerations

Selection of Optimum Reactor Temperature Profiles. . . Thermodynamic and Selectivity Considerations 375... 

SELECTION OF OPTIMUM REACTOR TEMPERATURE PROFILES. . . THERMODYNAMIC AND SELECTIVITY CONSIDERATIONS... 

Fluidized and transport bed processes have also been developed for better management of the heat released. The former prevents the occurrence of hot spots in the catalyst bed through a more uniform temperature profile. The concentration of the -butane can also be higher, even within the explosion limits, thanks to the barrier to flame propagation constituted by the fluidized bed of particles. Selectivity is not however dissimilar to that in fixed bed operation due to considerable back-mixing of the products and longer residence times. 

The yields of the main products vary, as indicated in Table 2. The relative amounts depend on the purity and particle size of the silicon and the copper catalyst, the amount and nature of the additives, the temperature profile and flow velocity in the reactor, and the history of the particular contact mass. A major consideration is to maximize the selectivity for the desired Me2 SiCl2 over MeSiCb, which is often obtained in more than useful amounts. Early in the reaction, selectivity for Me2SiCl2 is high as the silicon becomes largely consumed, this selectivity decreases. The by-products MesSiCl and MeSiHCb are both usefiil in further processing. 

Periodic flow reversal inducing forced unsteady-state conditions [339]. The flow to the reactor is continuously reversed before the steady state is attained. A dual hot-spot temperature profile, characterized by a considerably lower temperature than in the single hot spot that would develop in the traditional flow configuration, forms in exothermic oxidation reactions. An increase in selectivity and better reactor control (lower risk of runaway) is possible over fixed-bed reactor operations, but compared... 

In the injection molding process, setting the temperature involves optimization of the temperature profile of the plasticating unit (extruder barrel), temperatures of the mnners and gates, (aU these determine the molten polymer temperature) as well as the mold temperature. The temperature setpoints depend on the material type (viscosity profile, thermal and shear stability, thermal properties) as well as machine or process considerations (machine capacity to shot size ratio, screw design, mold and part design, cycle time, etc.). Temperatures of the two basic units, the injection system and the mold, should be discussed separately since their selection stems from very different considerations. 

It has been experimentally shown that the pyrolysis selectivity of two coils of different geometry, tube diameters and temperature profiles can be identical provided their average residence times and hydrocarbon partial pressures fall on the same selectivity line. The same conclusions have also been drawn from kinetic considerations. This approach has been used to extend the pyrolysis selectivity lines into the millisecond region. 

The combustion of natural gas with air is accompanied by formation of nitrogen oxides. This becomes significant above 1400-1500 C, which is easily surpassed in normal diffusion flames. The problem is solved by use of low NOx burners and further reduction can be obtained by cleaning the flue gas using selective catalytic reduction (SCR) with ammonia [510]. Low NOx burners have longer flames in order to limit the maximiun temperatures, but this has a significant impact on the heat flux and temperature profiles in the reformer, since it tends to make the flux profiles wider. The burner type must be taken into consideration in the design. 

Rigorous modelling must take the selected geometry into consideration and this usually requires CFD. It must, in addition to heat transfer across the wall and film mass and heat transfer between the gases and catalysts, also include axial heat conduction in the metals due to steep temperature profiles. In addition transient behaviour and interaction between the steep temperature profiles must be understood for a proper design, especially when a reasonable catalyst deactivation is... 

The selection of a coating system can be complicated by a number of considerations. The temperature and time of cure has to be considered versus the heat stability of the substrate plastic or paper. The degree of dimensional change and/or moisture loss of the substrate during cure at a particular time and temperature profile is a factor, also. Another consideration is the tendency of the uncured coating to penetrate the substrate before and during cure, which is also related to the attainability of a pinhole-free coating for efficient release. 

These three criteria—molar mass interval, eluent, and working temperature—are fixed by the group of samples to be analyzed and considerably restrict the number of suitable columns. The selection has to be done from current lists of the manufucturers. It is useless to collect these data here, as such tables would be antiquated before this book is printed. This chapter deals with the quality of the selected columns. At this stage, columns of the same application profile are compared. The most important properties are (1) the number of... 

Selection of the ligand to be used in construction of an aflfinity column requires careful consideration. Possible candidates for this include substrate analogues, effectors, enzyme cofactors, and under certain circumstances the enzyme substrate. When a substrate is used, conditions must be arranged such that the enzyme does not function catalytically. This may be accomplished by omission of required metal ions, a change in pH if the Km and Kcat pH profiles are different or low temperatures. For those enzymes which catalyze biomolecular reactions, one substrate may be easily used if (1) the other needed substrate of the reaction is... 

The optimisation of the extraction time was also based on similar theory to the effect of increasing extraction temperature. Similar results were found when samples were extracted for different periods of time, from 20 to 50 min (Fig. 3). The effect of extraction time on the chromatographic profile of the sulfur compounds was much less than the effect of temperature. Prolonging the extraction time to 35 min gradually increased the peak areas of most compounds. Increasing the extraction time from 30 to 35 min did not increase the peak areas of the heavy sulfur compounds very much, but led to a decline in peak areas for some of the lighter compounds such as D6-DMS, CS2, and also DMDS and DEDS. Therefore, an extraction time of 30 min was selected, with a practical consideration being the desire to keep the extraction time reasonably short. 

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