Component efficiency

Types of columns and packings. A slow distillation rate is necessary to ensure that equilibrium conditions operate and also that the vapour does not become superheated so that the temperature rises above the boiling point. Efficiency is improved if the column is heat insulated (either by vacuum jacketing or by lagging) and, if necessary, heated to Just below the boiling point of the most volatile component. Efficiency of separation also improves with increase in the heat of vaporisation of the liquids concerned (because fractionation depends on heat equilibration at multiple liquid-gas boundaries). Water and alcohols are more easily purified by distillation for this reason. 

The injection of coolant air in the turbine rotor or stator causes a slight decrease in turbine efficiency however, the higher turbine inlet temperature usually makes up for the loss of the turbine component efficiency, giving an overall increase in cycle efficiency. Tests by NASA on three different types of cooled stator blades were conducted on a specially built 30-inch turbine cold-air test facility. The outer shell profile of all three blade types was the same, as seen in Figure 9-24. 

We describe a parametric point calculation of the efficiency of a simple CCGT plant, firstly with no feed heating. It is supposed that the main parameters of the gas turbine upper plant (pressure ratio, maximum temperature, and component efficiencies) have been specified and its performance (t)o)h determined (Fig. 7.3 shows the T,s diagram for the two plants and the various state points). 

There are two major factors that impact the COP temperature difference and system component efficiency. A heat pump requires energy to move heat from a lower temperature to a higher one. As the difference in the two temperatures increases, more energy is required. The COP of a heat pump is high-... 

The order in which the components are separated will determine the capital and operating costs. Where there are several components the number of possible sequences can be very large for example, with five components the number is 14, whereas with ten components it is near 5000. When designing systems that require the separation of several components, efficient procedures are needed to determine the optimum sequence of separation see Doherty and Malone (2001), Smith (1995) and Kumar (1982). 

Uses of Oldershaw columns to less conventional systems and applications were described by Fair, Reeves, and Seibert [Topical Conference on Distillation, AIChE Spring Meeting, New Orleans, p. 27 (March 10-14, 2002)]. The applications described include scale-up in the absence of good VLE, steam stripping efficiencies, individual component efficiencies in multicomponent distillation, determining component behavior in azeotropic separation, and foam testing. 

For effective liquid chromatographic separations, a column must have the capacity to retain samples and the ability to separate sample components, efficiently. The capacity factor, k R, of a column is a direct measure of the strength of the interaction of the sample with the packing material and is defined by the expression... 

The figure shows results for both HTE and low-temperature electrolysis (LTE). In addition, an efficiency curve for the SI thermochemical process is shown (Brown, 2003). The results presented in Figure 3 indicate that, even when detailed process models are considered, with realistic component efficiencies, heat exchanger performance, and operating conditions, overall hydrogen production efficiencies in excess of 50% can be achieved for HTE with reactor outlet temperatures above 850°C. For reactor outlet temperatures in the range of 600-800°C, the supercritical C02/recompression power... 

Component efficiencies. In binary mixtures, the efficiencies of each of the two components are identical. In multicomponent separations, component efficiencies are all different because... 

Few commercial simulations are geared to handle rigorous multi-component efficiency computations. 

Individual component efficiency method. Another simple procedure, it was recommended lor the AIChE tray efficiency correlation (125). It yields individual component efficiency but takes no account of diffu-sional interaction. It proceeds as follows (12) ... 

Diffuslonal interaction methods. These calculate component efficiencies, but account for diffusional interactions. The calculation procedure is based on the Maxwell-Stefan diffusion equations, as developed by Krishna et al. (200,201). The equations are complex and are presented in the original reference. Lockett (12) has an excellent summary. For a ternary system, the steps below are followed (12) ... 

For multicomponent systems (i.e., those with more than two components) there are c - 1 independent component efficiencies, and there are sound theoretical reasons as well as experimental evidence for not assuming the individual component efficiencies to be alike indeed, they may take values between plus and minus infinity. Component efficiencies are more likely to differ for strongly nonideal mixtures. While models exist for estimating efficiencies in multicomponent systems [see chapter 13 in Taylor and Krishna, (op. cit.) for a review of the literature], they are not widely used and nave not (yet) been included in any of the more widely used commercial simulation programs. 

The fact that component efficiencies in multicomponent systems are unbounded means that the arithmetic average of the component Murphree efficiencies is useless as a measure of the performance of a multicomponent distillation process. Taylor, Baur, and Krishna [AIChE J., 50, 3134 (2004)] proposed the following efficiency for multicomponent systems ... 

Individual component efficiencies can vary as much as they do in this example only when the diffusion coefficients of the three binary pairs that exist in this system differ significantly For ideal or nearly ideal systems, all models lead to essentially the same results. This example demonstrates the importance of mass-transfer models for nonideal systems, especially when trace components are a concern. For further discussion of this example, see Doherty and Malone (op. cit.) and Baur et al. [AIChE J. 51,854 (2005)]. It is worth noting that there exists extensive experimental evidence for mass-transfer effects for this system, and it is known that nonequilibrium models accurately describe the behavior of this system, whereas equilibrium models (and equal-efficiency models) sometime... 

Investigating metabolism or stability of prostanoids, radiolabeled precursors or analytes are often used. The tritiated or -labeled compounds can easily be detected without any further derivatization using online or off-line liquid scintillation, which is not impaired by any interferences derived from matrix components. Efficient but less sensitive PG analysis is possible by UV detection (190-210 nm) of underivatized substances demanding the remove of interfering contaminants or simple sample matrices like buffers or some cell supernatants. 

The Murphree point efficiencies conventionally defined by Eq. 13.1.3 may be expressed in terms of the elements of the matrices [g]. For a ternary mixture there are two independent compositions and two independent efficiencies. Using Eq. 12.2.3 we can show that the component efficiencies and <91/2 given by... 

Only when [g] is a diagonal matrix with all elements on the main diagonal equal to one another (i.e., [g] reduces to the form g[/]) will the three component efficiencies EQy2 ov2> have the same value. This will be the case in mixtures made up of components of a similar nature (e.g., close boiling hydrocarbons or mixtures of isomers). For mixtures made up of chemically dissimilar species, that is, mixtures with large differences between the binary pair diffusivities, we must except to have significant nondiagonal... 

It is worth pointing out that the composition of the vapor above the froth can be calculated without computing the component efficiencies. As noted later, the use of component efficiencies may well be the cause of convergence difficulties that have been encountered when using these multicomponent tray efficiency models in column simulations. ... 

---