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Chemical Reaction Thermodynamics, Kinetics, and Reactor Analysis
Published in Debabrata Das, Debayan Das, Biochemical Engineering, 2019
Equation 3.19 signifies that the quantity of heat transferred is equal to the enthalpy change of a process for a constant-pressure and mechanically reversible process. It is important to now define the two major forms of reactions that largely depend on the positive or negative value of the heat transferred, Q. A reaction that involves heat liberation from the system is known as exothermic reaction. Heat liberation is associated with the negative value of ΔH, and hence for an exothermic process, one can write Q < 0. Combustion reactions are typically exothermic in nature. On the other hand, if heat is absorbed by a system during a reaction, then such a reaction is called an endothermic process. Clearly, an endothermic process involves a positive value of ΔH and that corresponds to Q > 0. When sodium hydroxide is added to water, the system undergoes an absorption of heat, and hence the nature of reaction is endothermic.
Identify and Assess Process Hazards
Published in James A. Klein, Bruce K. Vaughen, Process Safety, 2017
James A. Klein, Bruce K. Vaughen
The heat (or enthalpy) of reaction (ΔHr) is an important measure for evaluating chemical reactivity hazards. The heat of reaction is the total heat generated (exothermic) or required (endothermic) by the reaction, where the heat of reaction for exothermic reactions is negative. Although endothermic reactions are generally of less concern than exothermic reactions, they must still be evaluated carefully, and in particular, the formation of unstable, toxic, flammable, or gaseous products can be hazardous. The heat of reaction is calculated as: ΔHr=∑(miΔHf,i)products−∑(njΔHf,j)reactants
Thermodynamics of Fuel Cells
Published in Xianguo Li, Principles of Fuel Cells, 2005
where the subscript “P” stands for the product and “R” for the reactant. The lower case q for the heat transfer and h for the absolute enthalpy represent the respective values on a per unit mole (or mass) of one of the reactants, typically fuel for fuel cell reaction or combustion analysis (e.g., see Example 2.3). If enthalpy of reaction, Δhreaction, is negative, heat is released during the reaction, consequently such a reaction is called exothermic; similarly, if the enthalpy of reaction, Δhreaction, is positive, indicating that heat is absorbed by the system in order for the reaction to proceed, such a reaction is often termed endothermic. Therefore, endothermic reaction requires external means to provide the heat needed for the reaction to occur; whereas exothermic reaction can usually proceed by itself, once the reaction is initiated because the heat created during the reaction can usually be transferred to the surrounding medium. If heat generated is not transferred out of the system, then the system temperature will be increased; higher temperature will enhance the rate of reaction, hence even more heat generation, leading to a self-accelerated reaction process.
Kinetic and thermodynamic evaluation of almond shells pyrolytic behavior using Coats–Redfern and pyrolysis product distribution model
Published in Energy Sources, Part A: Recovery, Utilization, and Environmental Effects, 2023
Rumaisa Tariq, Sana Saeed, Muzaffar Riaz, Saad Saeed
The calculation of thermodynamic parameters is based on the kinetic parameters and peak temperature at which maximum degradation occurs, and it can be obtained from DTA data (Naqvi et al. 2019b). Table 5 clearly indicates that the change in enthalpy and change in Gibbs free energy in the first temperature zone is more than that of values in the second temperature zone. In both ranges, the maximum values of ΔH and ΔG were obtained at 20°C/min, the values of both parameters rise with the rise in heating rate from 10°C/min to 20°C/min and again declines as the heating rate further increases in both zones for all reaction mechanisms. In the range, 150–350°C, the maximum values of ΔH and ΔG were 79.11 kJ/mol and 168.19 kJ/mol obtained from the 3-D diffusion-Jander equation model. While in the range 350–550°C, the maximum values of ΔH and ΔG were 77.23 kJ/mol and 111.74 kJ/mol obtained from the 3-D diffusion-Jander equation model and Ginstling-Brounshtein (D4). Almond shell shows a positive value of ΔH in all given reaction models in both ranges. The positive value of enthalpy indicates that the nature of the reaction is endothermic. ∆G is indicative of energy buildup in the system due to reactant consumption (Naqvi et al. 2019b). In both pyrolysis zones, change in entropy is negative for all given reaction mechanism models. The association mechanism might be the cause of negative entropy values, in which degrees of freedom are lost due to the activated complex formation (Naqvi et al. 2018).
COD removal from landfill leachate using a high-performance and low-cost activated carbon synthesized from walnut shell
Published in Chemical Engineering Communications, 2018
A. R. Mahdavi, A. A. Ghoresyhi, A. Rahimpour, H. Younesi, K. Pirzadeh
According to the obtained data, the Gibbs free energy change values were negative and show that adsorption process was spontaneous and feasible. As can be seen from Table 8, there is a direct relation between ΔG0 values and temperature which means that adsorption process is more favorable at higher temperature. The positive sign of enthalpy change indicates that the process is endothermic. In general, the enthalpy value is the key parameter to distinguish whether the adsorption process occurred physically or chemically. If ΔH values lie in the range of 2–20 kJ/mol, the process is controlled by physical adsorption and if these values are in the range of 80–200 kJ/mol, the chemisorption is dominant (Sari et al., 2007). Therefore, from Table 8 it can be deduced that the COD adsorption on ACH4-500 was occurred physically. The positive value of ΔS indicated an increase in randomness and irregularity between COD and ACH4-500 at the interface.
Effect of injection parameters on performance and emission characteristics of a CRDi diesel engine fuelled with acid oil biodiesel–ethanol blended fuels
Published in Biofuels, 2018
S. Rajesh, B. M. Kulkarni, N. R. Banapurmath, S. Kumarappa
As the esterification reaction is endothermic an increase of temperature increases the reaction rates. For the mole ratio selected the reaction was carried out at temperatures of 45, 50, 60 and 65 °C and at atmospheric pressure as shown in Figure 6. Vaporization of methanol from the reaction mixture was rampant at temperatures higher than 650. Hence in the present investigation a maximum permissible temperature of 65 °C at atmospheric pressure is used as at this temperature conversions above 90% are readily obtained within one hour of operation. Vaporization of the methanol–water vapour mixture was also significant at this temperature.