Maximum allowable temperature

There are two general temperature poHcies increasing the temperature over time to compensate for loss of catalyst activity, or operating at the maximum allowable temperature. These temperature approaches tend to maximize destmction, yet may also lead to loss of product selectivity. Selectivity typically decreases with increasing temperature faster deactivation and increased costs for reactor materials, fabrication, and temperature controls. 

Conductor s maximum allowable temperature 6c in -C) for indoors 95 1 Assuming welded... 

Rossiter (1986) demonstrated the procedure for the production process of crystalline common salt from brine. It was found that the optimal median size is determined by the entrainment limit in the crystallizer. The crystallizer had to be operated at maximum allowable temperature and the slurry density measured for quality constraints. It was also suggested that cost discontinuities should be imposed based on temperatures of the available heat sources, possible materials of construction and other intrinsic properties of the system. 

Figure 16-16 shows the performance characteristic of a split-shaft turbine where the only power output limitation is the maximum allowable temperature at the inlet of the turbine section. In actual practice a torque limit, increased exhaust temperature, loss of turbine efficiency, aud/or a lubrication problem on the driven equipment usually preclude operating at very low power turbine speeds. The useful characteristic of the split-shaft engine is its ability to supply a more or less constant horsepower output over a wide range of power turbine speeds. The air compressor essentially sets a power level and the output shaft attains a speed to pnivide the required torque balance. Compressors, pumps, and various mechanical tinvc systems make very good applications for split-shaft designs. 

Parallel reactions, omi = oa2, aai = am = 0, E < E2. The selectivity of the desired product decreases with temperature. However, a low temperature disfavours the reaction rate. A nonuniform temperature-time profile should be applied to maximize the reactor productivity (see Fig. 5.4-70). Initially the temperature should be low to avoid the formation of too much unwanted product. The temperature is gradually raised with time to increase the reaction rate until the maximum allowable temperature is reached. At T u the reaction is completed. 

The maximum allowable temperature is 925 K. See Figure 21.8(b) for a schematic representation of one particular situation between the limits. The slopes of the operating lines AB and CD are equal and constant. The points B, C, and D are not fixed. No reaction occurs during temperature adjustment between stages, or after the second-stage outlet. 

A typical graph of k as a function of temperature is shown in Figure 3.6. The increasing slope shows the importance of determining a maximum allowable temperature in process equipment so that the heat removal capacity is not exceeded. Under adiabatic conditions, the temperature will reach the calculated maximum only if the reactants are depleted. The actual maximum temperature in a system with some heat dissipation will, of course, be somewhat lower than the calculated value. 

In a typical example as shown in Figure 3.6, the rection rate constant increases rapidly with increasing temperature as described in Equation (3-6). It follows, then, that it is necessary to determine the maximum allowable temperature in the system. This is the maximum temperature at which heat... 

One simple case of disordered structure involves many of the long charged side chains exposed to solvent, particularly lysines. For example, 16 of the 19 lysines in myoglobin are listed as uncertain past C8 and 5 of them for all atoms past C/J (Watson, 1969) for ribonuclease S Wyckoff et al. (1970) report 6 of the 10 lysine side chains in zero electron density in trypsin the ends of 9 of the 13 lysines refined to the maximum allowed temperature factor of 40 (R. Stroud and J. Chambers, personal communication) and in rubredoxin refined at 1.2 A resolution the average temperature factor for the last 4 atoms in the side chain is 9.2 for one of the four lysines versus 43.6, 74.4, and 79.3 for the others. Figure 57 shows the refined electron density for the well-ordered lysine and for the best of the disordered ones in ru-... 

Extrusion processes can be rate limited by the maximum allowable temperature of the discharge. For thermally sensitive resins, the extrudate will be required to... 

A single catalytic packed bed reactor is to be designed to treat 100 mol/s of reactant A and produce product R. Feed gas enters at 2.49 MPa and 300 K, the maximum allowable temperature is 900 K unless otherwise noted, the product stream is wanted at 300 K, and the thermodynamics and kinetics of the exothermic reaction are given to us in Fig. 19.11. Prepare a sketch showing the details of the system you plan to use ... 

Scale-up is limited by several factors, such as maximum working velocity in the case of adsorption/ion-exchange upflow fixed beds, and maximum allowable temperature in cases where microorganisms are involved in the process, for example, in the packed bed bioreactors (Michell et al., 1999). 

Characteristics of raw makeup and cooling tower waters, temperatures, maximum allowable temperature, flow rates available, and unit costs. 

The overall heat-transfer coefficient U depends upon the properties of the dry product and the method of heal transfer. The heat-transfer rate A is influenced by the mechanical design of the heating elements and the conditioning of the frozen mass. The temperature gradient AT is limited by the maximum allowable temperatures al the sublimation interface and dry-layer surface. In the constant-rate period, the lirst one-half to two-thirds of the drying cycle, about 8fl + of the water is removed. 

Figure 3.4 Variation of TMRad as a function of temperature. The maximum allowed temperature with respect to thermal stability (TD24) is given at the point where TMRad is equal to 24 hours.

Here Tmax represents a maximum allowed temperature. This can be defined to avoid triggering secondary reactions or to avoid high pressures. In fact, it corresponds to the worst case approach presented in Section 3.4. 

A second-order dimerization reaction is to be performed in a batch reactor. The initial temperature is 50 °C, and the desired reaction temperature 100 °C, whereas the maximum allowed temperature is 120 °C. 

An exothermal reaction is to be performed in the semi-batch mode at 80 °C in a 16 m3 water cooled stainless steel reactor with heat transfer coefficient U = 300 Wm"2 K . The reaction is known to be a bimolecular reaction of second order and follows the scheme A + B —> P. The industrial process intends to initially charge 15 000 kg of A into the reactor, which is heated to 80 °C. Then 3000 kg of B are fed at constant rate during 2 hours. This represents a stoichiometric excess of 10%.The reaction was performed under these conditions in a reaction calorimeter. The maximum heat release rate of 30Wkg 1 was reached after 45 minutes, then the measured power depleted to reach asymptotically zero after 8 hours. The reaction is exothermal with an energy of 250 kj kg-1 of final reaction mass. The specific heat capacity is 1.7kJ kg 1 K 1. After 1.8 hours the conversion is 62% and 65% at end of the feed time. The thermal stability of the final reaction mass imposes a maximum allowed temperature of 125 °C The boiling point of the reaction mass (MTT) is 180 °C, its freezing point is 50 °C. 

Thus, it is possible to define the maximum allowable temperature by considering the acceptable induction time for the thermal explosion. This assessment of the probability of a runaway by using a zero-order reaction model may be too cautious. 

Finally, the application of some of those criteria to the phenol-formaldehyde reaction gives some interesting insights on the thermal behavior of the system and also highlights the operation limits arising from an imposed maximum allowable temperature in the reactor. 

In many practical cases, the conditions for criticality described in the previous sections are only necessary to ensure safe operation. Such conditions do not guarantee, indeed, that the maximum allowable temperature in the reactor, Tma, is not exceeded. For instance, this upper temperature limit can be imposed, in liquid systems, by the bubble point of the reacting solution or by the decomposition temperature of some compounds in it, or, in gaseous systems, by the maximum internal pressure the vessel can comply with. 

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