Chemical heat balance

Typically, the biggest lost that occurs in chemical processes is in the combustion step (6). One-third of the work potential of natural gas is lost when it is burned with unpreheated air. Eigure 3 shows a conventional and a second law heat balance. The conventional analysis only points to recovery of heat from the stack as an energy improvement. Second law analysis shows that other losses are much greater. 

Figure 1-36. Process engineering costs (1975), based on process engineering charged at 14 per manhour. Chemical plant engineering operations, includes flowsheet development and drafting, material and heat balances, equipment designs, ratings, checking, and bid reviews and selection of equipment. By permission, E. E. Ludwig [7].

Steady state models of the automobile catalytic converter have been reported in the literature 138), but only a dynamic model can do justice to the demands of an urban car. The central importance of the transient thermal behavior of the reactor was pointed out by Vardi and Biller, who made a model of the pellet bed without chemical reactions as a onedimensional continuum 139). The gas and the solid are assumed to have different temperatures, with heat transfer between the phases. The equations of heat balance are ... 

In chemical processing the most fundamental constraint is that of the thermodynamics of the system. This constraint defines both the heat balance of the process and whether or not the processes in the reactor will be equilibrium limited. These constraints will limit the range of chemical engineering solutions to the problems of designing an economically viable process that can be found. 

Just as in the case for the hydrosphere, the atmosphere participates in all of the major biogeochemical cycles (except for phosphorus). In turn, the chemical composition of the atmosphere dictates its physical and optical properties, the latter being of great importance for the heat balance of Earth and its climate. Both major constituents (O2, H2O) and minor ones (CO2, sulfur, nitrogen, and other carbon compounds) are involved in mediating the amounts and characteristics of both incoming solar and outgoing infrared radiation. 

Hub, L. and T. Kupr, "Heat Balance Calorimetry and Automation of the Testing Procedure," in Proceedings of the Runaway Chemical Reaction Hazard Symposium, IBC, London, England (1987). 

Two limiting modes of operating chemical reactors employ (a) a vessel so well stirred that the composition and temperature are the same throughout (b) a vessel typified by a tube without mixing in which all molecules have the same residence time, and in which gradients of composition and temperature exist. Material and heat balances on these devices utilize the conservation law,... 

Part 1 Control of Chemical Processes. Some common problems in chemical processes are presented and either classical solutions or physical interpretation of controllers are discussed. Thus, the first chapter includes modeling and local control whereas the second chapter is focussed on nonlinear control design from heat balance on chemical reactors. The three first chapters deal with regulation problems while the last one is devoted to a tracking one. 

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... 

Emission Inventory scaling, proposed by (24), uses the relative emission rates of two source types subject to approximately the same dispersion factor (e.g., residential heating by woodstoves and natural gas) to approximate the source contribution from the source type not included in the chemical mass balance (e.g., natural gas combustion). The ratio of the emission rates is multiplied by the contribution of the source type which was included in the balance. 

An implicit relationship for the burning rate at low pressure may now be found by substituting Lnf ch from Equation 8 for Ln in the heat balance (Equation 3a). Inherent in this step is the assumption that the pyrolyzed fuel and oxidizer gases start to react immediately on leaving the propellant surface. When = Ts (e.g., where the first gaseous reaction stage is very fast) the burning rate at low pressure where it is chemical-reaction rate controlled can be written as ... 

A classic chemical engineering problem of the form under consideration here is that of a non-isothermal reaction occurring in a catalytic particle or packed bed into which a single gaseous participant diffuses from a surrounding reservoir (Hatfield and Aris 1969 Luss and Lee 1970 Aris 1975 Burnell et al. 1983). This scenario is also appropriate to the technologically important problem of spontaneous combustion of stockpiled, often cellulosic, material in air (Bowes 1984). If we represent the concentration of the gaseous species as c, the mass- and heat-balance equations for reaction in an infinite slab are... 

Heat release or consumption by surface reactions contributes to the energy balance at a gas-surface interface. Diffusive and convective fluxes in the gas phase are balanced by thermal radiative and chemical heat release at the surface. This balance is stated as... 

Action Preparation of individual mass and energy balances for each dent of chemical engineering equipment (or heat balances over several items of equipment). 

The introduction of this knowledge and a presentation of these methods are the objective of this book. In the present chapter, the essential theoretical aspects of thermal process safety are reviewed. Often-used fundamental concepts of thermodynamics are presented in the first section with a strong focus on process safety. In the second section, important aspects of chemical kinetics are briefly reviewed. The third section is devoted to the heat balance, which also governs chemical... 

In chemical thermodynamics, the convention is that exothermal effects are negative and endothermal positive. Here, since we consider the heat balance for practi-... 

If the deviation was an uncontrolled temperature increase, the temperature increase will continue and accelerate the reaction until the accumulated reactant has been converted. Therefore, it is important to know quantitatively the degree of reactant accumulation during the reaction course, as it predicts the degree of conversion, which may occur after interruption of the feed. This can be done by chemical analysis or by using a heat balance, for example from an experiment in a reaction calorimeter [4]. Since the accumulation is the result of a balance between the amount of reactant B introduced by the feed and the amount converted by the reaction, a simple difference between these two terms calculates the accumulation [5, 6]. 

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