Physical exergy

Physical exergy out of equilibrium with the environment in temperature. 

Table 6.2 lists exergy values for methane. It is clear from this table that methane carries an impressive amount of exergy as chemical exergy. Further, the table shows (1) the influence of increased pressure and temperature on the physical exergy and (2) that this latter contribution of exergy is nearly two orders smaller than the chemical contribution. Chemical exergy is the exclusive subject of Chapter 7. 

Exergy is a convenient concept if one wishes to assign a quantitative quality mark to a stream or a product. This quality mark expresses the maximum available work or potential to perform work because of its possible differences in pressure, temperature, and composition with the prevailing environment. The physical exergy, Exphys, only accounts for the differences in pressure and temperature the standard chemical exergy, Ex/hf.rn, accounts for the difference in composition with the environment at the environment s pressure and temperature. Thus... 

In the last chapter, the concepts of exergy and physical exergy, in particular, were introduced. This chapter deals with three other important concepts, namely, exergy of mixing, chemical exergy, and cumulative exergy consumption, and their numerical evaluation. 

Recall that exergy values reflect the extent to which a compound or mixture is out of equilibrium with our environment. Examples are differences in pressure and temperature with the environment. Differences in temperature lead to heat transfer, while differences in pressure lead to mass flow. Chapter 6 shows that the physical exergy represents the maximum amount of work that can be obtained from a system by converting a system s pressure and temperature to those of our environment. 

This is the definition of the physical exergy of the effluent stream The computation of the terms will yield the physical component of the stream, and the combination with the chemical and mixing components will allow for the computation of the efficiency. The question now remains Why did the computation of the efficiency based on the Carnot factor give the correct number The answer is that since the temperature of the effluent gases is fixed, it mimics an infinite heat reservoir, and therefore n[AH - TqAS] simplifies to nAH[ 1 - T0(AS/AH)] = nAH[ 1 - (T0/T)], since AG = AH - TAS = 0 at equilibrium. 

The losses in the reactor are chemical exergy losses, whereas the cooler losses can be attributed to physical exergy losses. Mixing constitutes physical losses as do the losses in the extruder due to the dissipation of mechanical energy to heat. The losses in the purge vessel (V3) are due to the fact that the gas is incinerated. The sum of all losses equals 4.73MJ/kg PE, or US 0.0465 per kg PE. 

In the absence of nuclear, magnetic, electrical, and sirr-face tension effects, the total exergy of a system Esys can be divided into four components physical exergy Ef, kinetic exergy E , potential exergy E, and chemical exergy E ... 

The subscript sys distinguishes the total exergy and physical exergy of a system from other exergy quantities, including transfers associated with streams of matter. The total specific exergy on a mass basis esys is... 

The physical exergy associated with a thermocfynamic system is given by... 

The rate of physical exergy E associated with a nra-terial stream (subscript ms) is... 

The purpose of a gasifier is to convert the chemical exergy of a solid fuel into the chemical exergy of a gaseous fuel. Due to the chemical reactions involved, the physical exergy of the material streams increase between the inlet and the outlet. 

Physical exergy results from the deviation of temperature and pressure from the environmental values. Chemical exergy results from the deviation of the composition of a component in a system from the composition of the same component in the environment. Chemical exergy of a stream of matter can be defined as the maximum work (useful energy) that can be obtained from it in taking it to chemical equilibrium (of composition) with the environment. In an open system, the specific flow exergy exf is expressed by... 

The physical exergy Eph is equal to the maximum amount of work obtainable when a compound or mixture is brought from its temperature T and pressure P to environmental conditions, characterized by environmental temperature T and pressure Pq. The standard chemical exergy of a pure chemical compound Ech is equal to the maximum amount of work obtainable when a compound is brought from the environmental state, characterized by the environmental temperature To (298.15 K) and environmental pressure Po (1 atm), to the dead state, characterized by the same environmental conditions of temperature and pressure, but also by the concentration of reference substances in a standard environment. 

The physical exergy is defined as the maximum theoretical useful work obtained as a system interacts with an equilibrium state. The physical exergy of a stream is given by [12] ... 

Physical exergy is associated with the temperature and pressure of the reactants and the products in the fuel cell system. The physical exergy is expressed in terms of the differences of enthalpy from those and entropy from those at standard conditions of temperature and pressure of = 298 K and Pq = 1 atm, respectively. The general expression of the physical exergy can be described as ... 

Exergy is composed of two parts, which are physical exergy and chemical exergy, as discussed in Chapter III.5.2.1. Physical equations are calculated by the equation (III.62) for liquids and equation (III.63a) for gases. Chemical exergies are obtained from the table A. 1. 

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