Net energy requirement

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. 

An understanding of the theory of electron transfer sensitization is necessary in order to choose sensitizers which can photoreduce the onium salt. For electron transfer to be energetically favorable, the excitation energy of the photosensitizer (E ) must be greater than the net energy required to oxidize the photosensitizer, and reduce the onium salt.( )... 

We have seen that one important reason for the efficiency of chain reaction mechanisms is the manner in which the combination of activation energies for initiation, propagation, and termination steps, equation (1-116), gives a net energy requirement less than that for the initiation step. In this sense, we should discuss the influence of temperature on the behavior of the examples of nearly complex reactions discussed previously. We shall not attempt solution of these systems for set adiabatic conditions now, but shall examine the results for conversion and selectivity in terms of how they are affected by temperature level. 

XfPf is actually the on-site cost of the net energy requirement (NER). NER is part of the gross energy requirement (GER) of the process needed to give the required product less the calorific value (CV) of the product. That is,... 

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. 

Target setting. In this step the desired performance levels are identified ahead of the detailed design for example, a 25% reduction of net energy requirements. 

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. 

Net energy requirement (MJ) Maintenance Production Maintenance (% total requirement)... 

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 ... 

During a thunderstorm, record furnace fuel and steam requirements before, during, and after the storm. Make sure to hold process conditions constant (i.e., furnace outlet and reflux temperatures). For a well-insulated unit, net energy requirements will not increase more than a few percent. 

Lofgreen, G.P. and W.N. Garrett, 1968. Asystem for expressing net energy requirements and feed values for growing and finishing beef eattle. J. Anim. Sei. 27, 793-806. 

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.

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