Energy transfer as work

In Section 7.4a we outline the calculation of the work (or more precisely, the rate of energy transferred as work) required to move fluid through a continuous process system, and in Section 7.4b we review the concepts of intensive and extensive variables introduced in Chapter 6 and introduce the concept of specific properties of a substance. Section 7.4c uses the results of the two preceding sections to derive the energy balance for an open system at steady state. 

If the equation is applied to a closed system, m = 0, and the rate of transfer of energy as shaft work, Ws, must be replaced by the total rate of energy transfer as work, W. 

Different kinds of driving forces tend to bring about different kinds of change. For example, imbalance of meehanical forces such as pressure on a piston tend to cause energy transfer as work temperature differences tend to eause the flow of heat gradients in chemical potential tend to cause substances to be transferred from one phase to another. At equilibrium all sueh forees are in balance. 

Energy Transfer as Work Only For a system that transfers energy only as work (vr ), <7 = 0 therefore, AE = 0 -I- w = w. The possibilities are... 

It is well known from thermodynamic principles that energy transferred as work is more useful than energy transferred as heat. Work can be completely converted to heat, but only a fraction of heat can be converted to work. Furthermore, as the temperature of a system is decreased, heat transferred from the system becomes less useful and less of the heat can be converted to work. A state property that accounts for the differences between heat and work is entropy, S. When heat is transferred into a closed system at temperature T, the entropy of the system increases because entropy transfer accompanies heat transfer. By contrast, work transfer (shaft work) is not accompanied by entropy transfer. When heat is transferred at a rate Q from a surrounding heat reservoir at a constant temperature, Treservoir, into a system, the heat reservoir experiences a decrease in entropy given by... 

From Eq. 3.1.2, the energy transferred as work across the boundary of this system is... 

Cundall has done extensive work on benzene231,237 and acetone243 sensitized isomerizations of the 2-butenes, and in every case reported a photostationary or radiostationary trans/cis ratio of 1.27-1.37. Sato, however, has measured a value of unity for the benzene photosensitized isomerization.510 With higher homologs, from 2-pentene to 2-octene, benzene-sensitized isomerizations yield trans/cis ratios of 1.0,238 while acetone-sensitized isomerization of the 2-pentenes in solution yields a ratio of 1.65.244 At present no explanation is possible for the differences between 2-butene and 2-pentene. Until much more information is gathered relating to rates of triplet energy transfer as functions of olefin structure, sensitizer, and medium, the natural decay ratios of each olefin s common triplet cannot be deduced from photostationary trans/cis ratios. 

Work is a mode of energy transfer which occurs due to the existence of imbalance of forces between the system and the surroundings. When the forces are infinitesimally unbalanced throughout the process in which energy is transferred as work, then the process is said to be reversible. 

Heat in the amount of 5 kJ is added to a system while its internal energy decreases by 10 kJ. How much energy is transferred as work For a process causing the same change of state but for which the work is zero, how much heat is transferred ... 

For an open system at steady state with negligible kinetic and potential energy changes from inlet to outlet and no energy transfer as shaft work, the balance is... 

The number of unknown variables for a single unit is the sum of the unknown component amounts or flow rates for ail inlet and outlet streams, plus all unknown stream temperatures and pressures, plus the rates of energy transfer as heat and work. The equations available to determine these unknowns include material balances for each independent species, an energy balance, phase and chemical equilibrium relations, and additional specified relationships among the process variables. 

There has been considerable interest on the influence of zeolites on several aspects of the photochemical behaviour of ketones. Turro et al. have carried out further work on the influence of zeolites on the photochemical activity of dibenzyl ketones. In this study pentasil zeolites were used. In other work Turro has reported the use of triplet-triplet energy transfer as a probe of surface diffusion rates. The luminescent properties of some aromatic ketones in the presence of the hydrophobic zeolite Silicalite have been examined and a spectral study of trsmsients formed by the irradiation of aryl ketones in zeolites has been reported. ... 

The total change in a system s internal energy is the sum of the energy transferred as heat and/or work ... 

Energy Transfer as Heat Only For a system that does no work but transfers energy only as heat (q), we know that = 0. Therefore, from Equation 6.2, we have A = q + 0 = q. The two possibilities are ... 

Again, it is perhaps better to be able to see the truth of this by simply looking at equation (5.24) rather than by agreeing with the logic of the manipulations. Thus since TAS is the maximum heat that can be transferred in a constant T process energy available as work in a reversible isothermal process w = AU — g) according to our previous discussion, work output is maximized in this case. We note, too, that if m = 0, then the inequality w > AAt requires that... 

Rovibrational Energy Transfer. Little work has been done as yet on the particular influence of combined rotational and vibrational energy transfer. The importance of this effect in the rovibrational relaxation of a diatomic molecule has been clearly shown for Hi, where the bottleneck of the flux is found to be near to the (v = 0, / = 10) -> (v = 1, / = 8) step. Similar importance may be expected in dissociation when the effective dissociation energies vary with angular momentum quantum number J. A factorization of the transition probability into a vibrational and rotational factor together with an exponential model for FG/i) has been employed in ref. 37. Experimental evidence shows that both contributions are in reality coupled. 

As an example of energy transfer by work, consider Figure 9.6(aL where an incompressible liquid at 25 C having a specific volume, V, of 0.001 m /kg is pumped continuously at a rate wof... 

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