Net energy requirements for

Considerable amounts of energy are required to run most chemical processes. Engineers use energy balances to account for the energy that flows into or out of each unit of a process, to determine the net energy requirement for the process, and to design ways to reduce the energy requirement so as to improve process profitability. 

The chemical value assigned to ethylene is based on the net energy requirement for ethylene production by steam cracking. 

We have just seen that the net energy requirements for acetylene are higher than for ethylene, and the difference is reflected in the energy requirements for all downstream products. To give an indication of how energy balances can continue to be applied the following table has been compiled from fairly modern sources. Energy inputs of 2 0 and 3 0 MJ/mol have been used for ethylene and acetylene respectively. 

We noted earlier that the activation energy of a chemical reaction plays a critical role in its kinetics. Catalysts increase the rate of reaction by providing a new reaction pathway that lowers the activation energy. Figure 11.15 illustrates this concept for the conversion of two ozone molecules to three oxygen molecules. Despite the fact that the addition of chlorine atoms requires an extra step relative to direct collisions of two ozone molecules, the net energy required for the reaction is lower. 

In the US system, NE values for lactation (NEj) are calculated fromTDN, DE or NE by an equation similar to that used in the Dutch system. For example, in the US system, foods containing 10 or 12 MJ ME/kg DM would be calculated to provide 6.0 and 7.1 MJ NEi/kg DM, respectively, whereas in the Dutch system, they would be calculated to provide 5.8 and 7.2 MJ NEi/kg DM, respectively. Net energy requirements for body maintenance and milk synthesis are expressed as NE, as they are in the Dutch and related European systems. 

Energy requirements for lactation involve an estimate of the gross energy value (EVi) of the milk, which may be used along with the yield to estimate the net energy requirement for milk production. 

The ME required for weight gain (Mg ) is calculated from the net energy requirement for gain (EVg) by an equation similar to that of ARC (1980) as follows ... 

Figure 2. The cost of growing a steer from 300 to 500 kg when increasing feed intakes relative to maintenance are maintained. The requirement for net energy for the gain (G) of the 200 kg remains constant, but the net energy required for ntairitenance is reduced as the number of days required for the gain is reduced.

This reaction has a positive free energy of 422.2 kj (100.9 kcal) at 25°C and hence energy has to be suppHed in the form of d-c electricity to drive the reaction in a net forward direction. The amount of electrical energy required for the reaction depends on electrolytic cell parameters such as current density, voltage, anode and cathode material, and the cell design. 

The theoretical energy requirement for the burning of Portiand cement clinker can be calculated from the heat requirements and energy recovery from the various stages of the process. Knowledge of the specific heats of the various phases, and the heats of decomposition, transformation, and reaction then permits calculation of the net theoretical energy requirement of 1760 kj (420 kcal) for 1 kg of clinker from 1.55 kg of dry CaCO and kaolin (see Clays) (8). 

Chemical reactions involve rearrangements of atoms Some chemical bonds break, and others form. Bond breakage always requires an input of energy, and bond formation always results in a release of energy. The balance between these opposing trends determines the net energy change for the reaction. 

The net heating energy required for a deodorization system can be calculated as ... 

Using ammonia as the starting material for nitric oxide preparation bypasses the enormous energy requirement for elemental nitrogen bond dissociation, and in so doing achieves the product of interest via a net exothermic (thermodynamically favorable) process. 

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