Heat balance assessment

Heikkila et al. (1996) have expanded the work of Hurme and Jarvelainen (1995) with environmental and safety aspects (Fig. 11). The alternatives are simulated to determine the material and heat balances and to estimate the physical properties. Then the alternatives are assessed in economic terms for which the internal rate of return is used. The environmental effects are estimated by equivalent amount of pollutant that takes into consideration the harmfullness of the different effluent substances. With environmental risks are also considered aspects of occupational health to choose inherently healthier process. Even though most health related rules are considered later in the work instructions, health effects should also be a part of the decision procedure. The inherent safety is estimated in terms of the inherent safety index as described later. 

In practice, of course, it is rare that the catalytic reactor employed for a particular process operates isothermally. More often than not, heat is generated by exothermic reactions (or absorbed by endothermic reactions) within the reactor. Consequently, it is necessary to consider what effect non-isothermal conditions have on catalytic selectivity. The influence which the simultaneous transfer of heat and mass has on the selectivity of catalytic reactions can be assessed from a mathematical model in which diffusion and chemical reactions of each component within the porous catalyst are represented by differential equations and in which heat released or absorbed by reaction is described by a heat balance equation. The boundary conditions ascribed to the problem depend on whether interparticle heat and mass transfer are considered important. To illustrate how the model is constructed, the case of two concurrent first-order reactions is considered. As pointed out in the last section, if conditions were isothermal, selectivity would not be affected by any change in diffusivity within the catalyst pellet. However, non-isothermal conditions do affect selectivity even when both competing reactions are of the same kinetic order. The conservation equations for each component are described by... 

These points are explained in detail in this chapter. In a first section, the general aspects of reaction engineering for batch reactors are briefly presented. The mass and heat balances are analysed and it is shown that a reliable temperature control is central to the safety of batch reactors. The different strategies of temperature control and their consequences on reactor safety are explained in the following sections. For each strategy, the design criteria and the safety assessment procedure are introduced. The chapter is closed by recommendations for the design of thermally safe batch reactions. 

In this section, different typical heat accumulation situations encountered in the process industry are reviewed and analysed. The next section introduces different types of heat balance used in assessment of heat confinement situations. 

A special section then deals with the use of time-scales for the heat balance, which provides a simple to use assessment technique. The third section is devoted to the heat balance with purely conductive heat removal. The chapter closes on practical aspects for the assessment of industrial heat confinement situations. 

A practical approach of heat balance, often used in assessment of heat accumulation situations, is the time-scale approach. The principle is as in any race the fastest wins the race. For heat production, the time frame is obviously given by the time to maximum rate under adiabatic conditions. Then the removal is also characterized by a time that is dependent of the situation and this is defined in the next sections. If the TMRld is longer than the cooling time, the situation is stable, that is, the heat removal is faster. At the opposite, when the TMRld is shorter than the characteristic cooling time, the heat release rate is stronger than cooling and so runaway results. 

Part I gives a general introduction and presents the theoretical, methodological and experimental aspects of thermal risk assessment. The first chapter gives a general introduction on the risks linked to the industrial practice of chemical reactions. The second chapter reviews the theoretical background required for a fundamental understanding of mnaway reactions and reviews the thermodynamic and kinetic aspects of chemical reactions. An important part of Chapter 2 is dedicated to the heat balance of reactors. In Chapter 3, a systematic evaluation procedure developed for the evaluation of thermal risks is presented. Since such evaluations are based on data, Chapter 4 is devoted to the most common calorimetric methods used in safety laboratories. 

The converged mass and heat balances and the exergy loss profiles produced by the Aspen Plus simulator can help in assessing the thermodynamic performance of distillation columns. The exergy values are estimated from the enthalpy and entropy of the streams generated by the simulator. In the following examples, the assessment studies illustrate the use of exergy in the separation sections of a methanol production plant, a 15-component two-column... 

The total energy efficiency of the plant, which is determined by the total available output energy and total input energy, reaches 72.6%. These heat balance data are available for LCA (life-cycle assessment). 

The most common procedure is pumping a substrate solution of a defined composition (concentration of substrate and other compounds having influence on the enzyme activity) through the microcalorimetric column. The basic information provided by the microcalorimetric measurement is the relation between reaction conditions and the steady-state heat response, ATr, measured as the temperature difference between the column input and output. Figure 2 is an illustration of such measurement. In the next part of this review, the mathematical assessment of the experimental data, based on mass and heat balances, is provided. 

The thermal environment is sometimes very complex. Convection, radiation and conduction are the common means of heat exchange and they vary independently over time and location. The final effects on the surface heat exchange of the human body are important factors for heat balance and for perception of the thermal conditions. Assessment of the thermal environment in a modern office or a car can create difficulties due to the complex interaction of the ventilation system with the situation close to the person and the external, environmental factors (e.g. radiation, air temperature and air movements). Furthermore, measurements in reality, as well as in the laboratory, contain various methodological problems. In this chapter some important aspects of dynamic water vapour and heat transport through fabrics are discussed. 

The tiiermal behaviour of a chemical reactor depends on the thermodynamics of the process, on the reaction rate, vtdiich has already been mentioned above, but also on the mode of exchange with the environment. This is completely described in the overall heat balance of the system. The following sections will present the main balance equations, which are required for the subsequent safety assessment, as well as definitions and interpretation of characteristic numbers used in their presentatioiL... 

Independent of the mode of operation, the most critical point during the course of the process corresponds to the time at which the maximum driving temperature difference between jacket and reaction mixture temperature occurs. Due to this fact the safety assessment focuses its efforts on the most reliable prediction of this point and its stability. In all three cases this critical point of a batch process is mathematically characterized by the condition dT/dt = 0 for the heat balance. 

Content validity is perhaps the simplest but least convincing measure. If each of the items of our measurement device displays the correct content, then validity is established. Theoretically, if we could list all of the possible measures of a phenomenon, content validity would describe how well our measurement device samples these possible measures. In practice it is assessed by having experts in the field judge each item for how well its content represents the phenomenon studied. Thus, the heat balance equation would be judged by most thermal physiologists to have a content that well represents the thermal load on an operator. Not all aspects are as easily validated ... 

Methane is a greenhouse gas which plays a significant role in the heat balance of the atmosphere [1]. It is thus important to assess the rate of removal of methane from the atmosphere, for which the reaction of methane with hydroxyl radicals... 

Accidents happening in polymerization reactors are practically always due to a lack of control of the course of reaction caused by a disturbance of the heat balance, which results in a temperature increase leading to loss of control of the reactor and a runaway reaction. In this section a systematic procedure based on a failure scenario with six key questions, allowing assessment of the criticality of a process, is presented. Since the heat balance is at the center of our concerns in matters of thermal control of reactors, the different terms of the heat balance will be examined. Finally, aspects of the dynamic stability of reactors and of the thermal stability of reaction masses are analyzed. 

The heat balance is important as well for the design of reactors, for their scaleup, for the risk assessment, and especially for the assessment of the reactor stability. 

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