Work potential

Exeigy, E, is the potential to do work. It is also sometimes called availabiUty or work potential. Thermodynamically, this is the maximum work a stream can deflver by coming into equiUbrium with its surroundings ... 

Compaiison of actual use of work potential against the rninirnurn allows calculation of an efficiency relative to the best possible separation ... 

The two effects almost cancel one another to yield an approximation for the minimum work potential used in a distillation (3,4). 

The high T] values above conflict with the common behef that distillation is always inherendy inefficient. This behef arises mainly because past distillation practices utilized such high driving forces for pressure drop, tedux ratio, and temperature differentials in teboilets and condensers. A teal example utilizing an ethane—ethylene sphtter follows, in which the relative number for the theoretical work of separation is 1.0, and that for the net work potential used before considering driving forces is 1.4. 

Unlike the conservation guaranteed by the first law, the second law states that every operation involves some loss of work potential, or exergy. The second law is a very powerful tool for process analysis, because this law tells what is theoretically possible, and pinpoints the quantitative loss in work potential at different points in a process. 

Typically, the biggest lost that occurs in chemical processes is in the combustion step (6). One-third of the work potential of natural gas is lost when it is burned with unpreheated air. Eigure 3 shows a conventional and a second law heat balance. The conventional analysis only points to recovery of heat from the stack as an energy improvement. Second law analysis shows that other losses are much greater. 

Converting Heat to Work. There has been a historic bias in the chemical industry to think of energy use in terms of fuel and steam (qv) systems. A more fundamental approach is to minimise the input of work potential embedded in the fuel and feedstock, as well as work purchased direcdy as electricity. Steam is really just a medium of exchange, like money in an economy. 

Cog enera.tion in a. Steam System. The value of energy in a process stream can always be estimated from the theoretical work potential, ie, the deterrnination of how much power can be obtained by miming an ideal cycle between the actual temperature and the rejection temperature. However, in a steam system a more tangible approach is possible, because steam at high pressure can be let down through a turbine for power. The shaft work developed by the turbine is sometimes referred to as by-product power, and the process is referred to as cogeneration. 

Depolarization times greater than 48 hours should be allowed when applying the 100-mV criterion. Comparison of measured potentials is only possible where conditions of temperature and moisture are similar. Figure 19-8 shows the dependence on season and temperature which can probably be ascribed to different aeration. In repair work, potential probes should be provided at different places on the object to reduce IR errors in off potential measurements arising from equalizing currents. 

Eq. (2.22) may be interpreted in terms of exergy flows, work output and work potential (Fig. 2.5). The equation may be rewritten as... 

Energy is the capacity to do work. Potential energy is the energy possessed by an object as a result of its position. Heat, another form of energy, ran be thought of in molecular terms as frictional losses of the uncoordinated motion of molecules. 

As can be seen in Fig. 50, the average working potential is about 3.6 V and re-chargeability is reasonably good. The capacity of LiCo02 was 150 mAhg-1. 

The concentration of the remaining oxidation centered on the relaxed film at any oxidation time is defined by the difference between the density of charge stored in the point at which the film attains an oxidation steady state at the working potential and large polarization times and the charge density stored after a given polarization time [< j(0]-So the diffusion flow of ions is given by... 

Depending on current density, the working potential of steady-state methanol oxidation varies within the range 0.35 to 0.65 V (RHE). Therefore, the working voltage of a methanol-oxygen fuel cell will have values between 0.4 and 0.7 V. 

In many cases the concentration of a substance can be determined by measuring its steady-state limiting diffusion current. This method can be used when the concentration of the substance being examined is not very low, and other substances able to react in the working potential range are not present in the solution. 

The polarographic method can be used to analyze a large group of solutes qualitatively and quantitatively (even when they are present simultaneously) that can be reduced within the working potential range of the DME. It is an advantage of the method that solutions with low concentrations of the test substances can be analyzed, approximately down to (1 to 5) X lO M. The volume of the solution sample needed for analysis can be as small as 1 mL or less. Hence, one can detect less than 0.01 mg of the substance being examined. The error limits of analysis are 2% when appropriate conditions are maintained. 

Energy—Capacity for doing work. "Potential energy" is the energy inherent in a mass because of its spatial relation to other masses. "Kinetic energy" is the energy possessed by a mass because of its motion (SI unit joules) ... 

Accessible work potential is called the exergy that is the maximum amount of work that may be performed theoretically by bringing a resource into equilibrium with its surrounding through a reversible process. Exergy analysis is essentially a TA, and utilizes the combined laws of thermodynamics to account the loss of available energy. Exergy is always destroyed by irreversibilities in a system, and expressed by... 

Fig. 5.35 SNIFTIRS spectrum from a polished Pt electrode in 0.5 m LiC104 in propylene carbonate. Reference potential 2.00 V versus Li/Li+ electrode working potential 3.20 V versus Li/Li+ electrode. According to P. Novak et al.

Few (4 to 8) [26] spectrometer scans are taken, averaged and stored while the electrode potential is held at a reference potential and then at a working potential. In this way only a short time is passed between the transition from the reference to the working condition. The sequence is repeated many times in order to improve the signal-to-noise ratio. 

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