Cumulative exergy consumption

Suppose we deal with a process in which iron, Fe, has to be used as a reactant, for example, in a reduction reaction. The standard chemical exergy of Fe is 376.4 kj/mol. If we wish to carry out a thermodynamic or exergy analysis of this process, this value is not appropriate. After all, to put the exergy cost of the product, for which Fe was needed as a reactant, in proper perspective, we need to consider all the exergetic costs incurred in order to produce this product all the way from the original natural resources— iron ore and fossil fuel in this example. The production of iron from, for example, the iron ore hematite and coal has a thermodynamic efficiency of about 30% [1], and therefore it is not 376.4 kj/mol Fe that we need to consider 

The chemical exergy of a molecule in a mixture is smaller than in its pure state, as it will require work to separate the mixture in its pure constituents, the exergy of separation. This exergy will be lost as the exergy of mixing when the pure constituents spontaneously form the mixture. The total exergy of a 

The concept of cumulative chemical exergy consumption is very useful and accounts for the fact that when a compound (e.g., ammonia) is introduced into a process, its chemical exergy has to be corrected for the exergy consumption accumulated since this compound was manufactured from its natural constituents (air and natural gas in the case of ammonia). 

If the thermodynamic efficiency of a process step is calculated, the chemical exergies should be excluded from the calculation if the process step does not include chemical conversions. If it does, it may be appropriate to distinguish between the physical and the chemical efficiency, itphys and T chem, °f the process step. 

Finally, although the exergy concept is not strictly necessary for the calculation of the available work lost in the process, it is an extremely handy tool to calculate losses and efficiencies and for making a quick assessment of process options. Chapter 8 gives some simple illustrations, whereas Part III, Case Studies presents the results of integrated studies in the world of energy and chemical technology. 

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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. 

In Chapter 18 we show that a chain of process steps, starting from natural gas, will result in the production of liquid H2 with an overall thermodynamic efficiency of 57%. Calculate the cumulative exergy consumption of this H2. 

N2 and H2 and the production of H2 from natural gas. Assume physical steps to have efficiencies of 10% and chemical steps of 60%. What is the cumulative exergy consumption of urea Separations take place with the help of compressors with an efficiency of 75%, running on electricity from an advanced natural gas-fired power station with a thermodynamic efficiency of 55%. 

The following derivations are from Szargut (1990). The total consumption of natural resources involved in the production of a product can be expressed by the overall index of cumulative exergy consumption rp... 

Exergy consumption of resources that are renewable should also be considered. The overall cumulative exergy consumption index may help in assessing various energy utilization problems for a specified product, such as the relationships between the amounts of raw materials and the products, or the cost of raw materials and the alternative production technologies available. 

Here, r is the cumulative exergy consumption index for a specified product. For the manufacturing of major products, we have 17 < 1. Sometimes, for a byproduct we may have 17 > 1, if the exergy of the product is greater than the exergy of the substituted product of a certain process. It would not be useful to calculate 77 for certain products, such as cars and airplanes, because their usefulness results mainly from their system features, not from the chemical composition of their components. However, the calculation of r may be beneficial for all kinds of products, because the values of r can be used to compare various design variables and production technologies. 

Cumulative exeigy consumption can be calculated by the balance equations the rkj for the useful products equals the sum of cumulative exergy consumption of all raw materials and semifinished products in the production network. For the link j of the network and for the natural resources k, the balance equations are... 

If r)zm < T)b the value r)u>l results from Eq. (5.18). Some typical values of the cumulative degree of thermodynamic perfection 17 are given in Table 5.1. For small values of r that result from the substitution ratio ziu, the cumulative exergy consumption will be large for the major product j of the specified process technology. 

The difference between cumulative exergy consumption r and exergy consumption of a natural resource represents the cumulative exergy loss (8(f)) involved in all parts of a manufacturing technological network... 

Cumulative exergy consumption The total consumption of exergy of natural resources connected with the fabrication of the considered product and appearing in all the links of the network of production processes, according to J. Szargut. 

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