Heat loss assessment

Heat loss assessment is conducted mainly based on temperature and pressure measurement. It is essential that a reference temperature be defined as the first step. The reason for defining a reference temperature in the energy loss audit is that heat above this temperature is considered as recoverable and thus accounted as loss if it is rejected to coolers. The typical reference temperature is 100 °F. The actual reference temperature to be selected for the heat loss calculation depends on the technical feasibility and economic viability for low-temperature heat recovery. 

Heat Loss Assessment for Poor Turndown Operation... 

It is reported that the BBL polymers have a thermal stability as assessed by TGA tests in excess of 600°C whilst they also show negligible heat loss after several hundred hours in air at 370°C. 

Environmental and Dietary Requirements. For reasons of environmental control, ferrets used in safety assessment studies should be housed indoors. It has been suggested that an optimal temperature range for the ferret is 40-65°F (4-18°C), while relative humidity should be maintained in the range of 40 to 65% (Fox, 1988). The ferret does not tolerate heat well due to its lack of well-developed sweat glands the primary method of regulating heat loss appears to be through panting (Moody et al., 1985). 

For a study of methods of assessment of thermal runaway risk from laboratory to industrial scales [2], A more detailed but eminently clear treatment of this and other needful safety considerations on scaling reactions up to production has since been published [3], So slight a scale-up as replacing two charcoal filters by one bigger one may cause a fire because heat loss was reduced [4], A journal largely devoted to scale-up of organic chemical processes has been launched [5]. 

This expression compares the characteristic time of runaway (TMRad) with the characteristic cooling time. Thus, knowing the mass, specific heat capacity, heat transfer coefficient, and heat exchange area allows the assessment. It is worth noting that, since the thermal time constant contains the ratio V/A, heat losses are proportional to the characteristic dimension of the container. 

The major conclusions from the above-described studies are consistent in the asymmetric mode of operation the reaction zones of the exothermic and endothermic reactions inherently repel each other, leading either to an extreme maximum temperature or to poor performance. A noncontinuous heat supply and production during every other semicycle cause obviously strong fluctuations of operation. Moreover, reasonable states of operation are attainable only with an excess of gas during the exothermic semicycle. This contradicts the condition of equal heat capacities for optimal heat recovery (see Section 1.2.1.1). For example, the heat loss in the case displayed in Fig. 1.9 is equal to the heat demand of the endothermic reaction. Different strategies have been assessed with regard to their potential to reduce hotspots during the exothermic semicycle and to improve thermal efficiency. 

If heat losses during compression are only slight then the bulk of the compressed gas will be at the core gas temperature. However, if heat losses during compression are very significant, as in slow compression, then a rather smaller fraction of the compressed charge will be at the core temperature. The extent to which heat losses during compression cause departures from the adiabatic ideal may be assessed from a comparison of (6.16) with the temperature (Tad) which is predicted on an ideal volumetric basis from knowledge of the dimensions of the RCM [50]. That is... 

Instability typically arises from the interaction of two phenomena with different dependences on a reaction parameter In a nonisothermal reaction, the dependence on temperature is exponential for heat generation by the reaction, but linear for heat loss to the cooling coil or environment in a reaction with chain branching, the dependence on radical population is exponential for acceleration by branching, but quadratic for chain termination. A reaction is unstable if acceleration outruns retardation. This can cause an explosion or, in a CSTR, lead to multiple steady states. Feinberg s network theory can help to assess whether an isothermal reaction admits multiple steady states in a CSTR. 

Before discussing the safety assessment of chemical processes under normal as well as under upset conditions in detail, the classical heat explosion theory shall be treated. The first scientists to investigate the so-called runaway of an exothermic chemical reaction were Semenov and Frank-Kamenetzidi [18,19]. They were the pioneers in investigating and describing the self-heating process of reacting systems up to an explosion-like temperature rise in its dependence on different heat loss conditions to the environment. The criteria they derived are still valid today and form the basis of any safety assessment. 

Cena, K., 1974, Radiative heat loss from animals and man, mHeatloss from Animals ondMan Assessment and Control, J. L. Monteith and L. E. Mount, eds., Butterworths, London, pp. 33-58. 

This calculation is approximate because it refers to continuous operation assessed from time averages and does not consider existence of stratification and the consequences of heat loss. A more exact calculation can be made from the discretized model of the storage tank with due regard to heat loss [56,106,107]. 

The minimum temperature at which a runaway reaction will occur is not an absolute value. It is linked to the rate of heat loss from the system and depends markedly on the process conditions and scale of manufacture. Thus, the rate of heat loss due to natural cooling from a 501 reactor is of the order of 0.2 W kg K whereas a typical value for a 20 m- vessel is 0.04-0.08 W kg K . Accurate laboratory assessment of the minimum temperature for onset of runaway reaction requires equipment where the rate of heat loss is the same as it is in the full-scale process. 

---