Temperature increase requirement

Industrially, catalyst activity maintenance is often screened via "temperature increase requirement" (TIR) experiments. In these experiments, constant conversion is established and the rate of temperature increase required to do so is used as a measure of the resistance of the catalyst to deactivation. However, this type of operation may mask the effect of particle size, temperature, temperature profile, and heat of reaction on poison coverage, poison profile, and the main reaction rate. This masking may be particularly important in complicated reactions and reactor systems where the TIR experiment may produce positive feedback. 

Figure 2.2 Temperature increase required to double the reaction rate constant (A 7 ) for different values of the activation energy (a) E, = 50 kJ mol1 (b) E. = 150 kJ mol1 (c) Ea = 300 kJ mol1.

Depending on tlie time. scale of deactivation, the catalytic activity can be restored in different ways. For example, in fluid catalytic cracking, where the deactivation is very fast, a recirculating leacTor is used for continuous catalyst regeneration. However, if the deactivation is slow and constant conversion is desired 10 meet certain environmental regulations as in VOCoxidation, the temperature level can be used to compensate fur the loss of intrinsic catalytic activity. Under such additions, the deactivation effects are measured by the temperature increase required to maintain constant conversion. 

The second separation scheme involves an increase in temperature under iso-baric conditions. This can be achieved in the retrograde region, and again an order of magnitude reduction in solubility can occur with a modest increase in temperature. This separation method is probably more favorable than the first method in terms of energy consumption because the first method involves a significant recompression step after separation. However, a separation based on a temperature increase requires much more specific information on the solubility behavior of the solute. In contrast, a separation based on a pressure reduction mainly involves consideration of the critical pressure of the solvent. A near 100% separation can be achieved once the pressure is reduced to below the critical pressure. 

In previous sections we have dealt with nonisothermal effects arising from the thermochemistry of the reactions involved. There is another type of thermal effect that appears in the operation of large-scale reactors such as those used in hydrotreating. These reactors are normally subject to slow catalyst deactivation, and constant conversion operation is required in order not to upset subsequent processing units. Here the reactor temperature is used to cope with the loss of intrinsic catalyst activity and the thermal parameters of the main and deactivation reactions, particularly the activation energies, have a great influence on the operation. Further, it has been common practice in many industrial laboratories for many years to evaluate catalyst activity and activity maintenance for such processes in laboratory experiments which are also conducted under constant-conversion conditions. In this procedure, catalyst deactivation effects are manifested in the rate of temperature increase needed to maintain constant conversion, that is, a temperature increase required (TIR). 

Micro-organisms are killed faster at high temperature than at low temperature. In order to quantify the change in resistance of a micro-organism in response to a change in temperature, the Z-value has been introduced. The Z-value is the number of degrees of temperature increase required to decrease the D-value by a factor 10. 

Akin to the wet granulation process, the dried material is miUed to control the particle size for downstream processing such as flow, uniformity, and compaction (Jones 2011). Air jet or impact mills such as hammer or conical mills can be used for this purpose. For pharmaceutical operations, hammer milling is sufficient to reduce the particle size, however due to the risk of heat generation, for some sensitive material air j et milling may be necessary. As alluded earlier, any process involving temperature increase requires careful assessment with respect to the quality attributes. 

A parameter closely related to the energy of activation is the Z value, the temperature dependence of the decimal reduction time (D). The Z value is the temperature increase required for a one-logjo reduction (90% decrease) in the D value. The Z value can be determined from a plot of logjo D versus temperature (Fig. 12.5). The temperature dependence of the decimal reduction time can be expressed in linear and nonlinear forms ... 

The equilibrium constant for this reaction decreases with increase in temperature but the higher temperature is required to achieve a reasonable rate of conversion. Hydrogen chloride is now being produced in increasing quantities as a by-product in organic chlorination reactions and it is economic to re-convert this to chlorine. 

Secondary and pnmary alcohols do not react with HCl at rates fast enough to make the preparation of the conespondmg alkyl chlorides a method of practical value There fore the more reactive hydrogen halide HBr is used even then elevated temperatures are required to increase the rate of reaction... 

Initiators. The degree of polymerization is controlled by the addition rate of initiator(s). Initiators (qv) are chosen primarily on the basis of half-life, the time required for one-half of the initiator to decay at a specified temperature. In general, initiators of longer half-Hves are chosen as the desired reaction temperature increases they must be well dispersed in the reactor prior to the time any substantial reaction takes place. When choosing an initiator, several factors must be considered. For the autoclave reactor, these factors include the time permitted for completion of reaction in each zone, how well the reactor is stirred, the desired reaction temperature, initiator solubiUty in the carrier, and the cost of initiator in terms of active oxygen content. For the tubular reactors, an additional factor to take into account is the position of the peak temperature along the length of the tube (9). 

Corrosivity. Corrosivity is an important factor in the economics of distillation. Corrosion rates increase rapidly with temperature, and in distillation the separation is made at boiling temperatures. The boiling temperatures may require distillation equipment of expensive materials of constmction however, some of these corrosion-resistant materials are difficult to fabricate. For some materials, eg, ceramics (qv), random packings may be specified, and this has been a classical appHcation of packings for highly corrosive services. On the other hand, the extensive surface areas of metal packings may make these more susceptible to corrosion than plates. Again, cost may be the final arbiter (see Corrosion and corrosion control). 

Power dissipation can lead to temperature increases of up to 40°C in the mass. Note that evaporation of liquid as a result of this increase needs to be accounted for in determining liquid requirements for granulation. Liquid should be added through an atomizing nozzle to aid uniform hquid distribution in many cases. In addition, power intensity (kW/kg) has been used with some success to judge granulation end point and for scale-up, primarily due to its relationship to granule deformation [Holm loc. cit.]. Swept volume ratio is a preliminary estimate of expected power intensity. 

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