Estimate relative volatility quickly

A plant engineer often needs an estimate of the relative volatility of two components. Here is a simple expression that relates the average relative volatility to the normal boiling points and the latent heats of vaporization of the two components, in the temperature range of their boiling points  

L2 = latent heat of vaporization for Component 2 at Th2y kcal/kmole. 

If a compound s latent heat is not known, it can be estimated from the normal boiling points and molecular weight, M, using the following equations  

The average relative volatility of benzene and toluene can be determined using the following data T/,/, = 353.3 K, T i = 383.8 K, Li, = 7,352 kcal/kmole, and L( = 7,930 kcal/kmole (where the subscripts 6 and t denote benzene and toluene, respectively). Substituting these values into Eq. (4) above, we find that  

This compares with a value of 2.421 for a determined using vapor-pressure/temperature charts. 

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For quick estimates, a relative volatility can be estimated as follows ... 

A quick estimate of the overall column efficiency can be obtained from the correlation given by O Connell (1946), which is shown in Figure 11.13. The overall column efficiency is correlated with the product of the relative volatility of the light key component (relative to the heavy key) and the molar average viscosity of the feed, estimated at the average column temperature. The correlation was based mainly on data obtained with hydrocarbon systems, but includes some values for chlorinated solvents and water-alcohol mixtures. It has been found to give reliable estimates of the overall column efficiency for hydrocarbon systems and can be used to make an approximate estimate of the efficiency for other systems. The method takes no account of the plate design parameters and includes only two physical property variables. 

The Fenske Method gives a quick estimate for the minimum theoretical stages at total reflux. N 1 Xhk)d( hk/Xlk)b] " ln(aLKHK),vG Nomenclature LK = subscript for light key Nm = minimum theoretical stages at total reflux Xhk = mol fraction of heavy key component Xlk = mol fraction of the light key component otLK/HK = relative volatility of component vs the heavy key component... 

Thus, in order to gain a quick assessment of which design is superior, we can use the TT approach outlined in this section, which can be simply done with the DODS package. The DODS package estimates the relative volatilities of benzene and toluene with respect top-xylene as 5.28 and 2.24, respectively. We shall first consido the side-stripper unit in our evaluation the sidestream therefore cannot contain any p-xylene as this is the bottoms product. The sidestream composition is therefore X5=[0.03, 0, 0.97]. 

Smith-Brinkley shortcut method A quick procedure used to estimate the components in a multicomponent mixture leaving the top and bottom of a disfillation column operating with continuous feed. The procedure is applicable to any stage-wise separafion process. For a distillation column with a single feed and a total condenser, the fractional recovery of any component in the bottom product is calculated from details that include the reflux ratio, internal flows of liquid and vapour above and below the feed point (i.e., the rectifying and stripping sections), and the relative volatilities of the components. In the calculation, the reboiler counts as stage one. 

We see that the individual As can take on values greater or less than one, but never negative values. As x, 1.0, Kt —> 1.0 for each species. The relative volatility is always positive and greater than 1 if there is no azeotrope present (Section 8.4.4). For quick estimates of the difficulty of a separation by distillation, the relative volatility a is the chemical engineer s favorite. If a is greater than 1.5 to 2 over the whole range of composition values, then distillation will almost always be the cheapest separation method. If a is less than 1.1, then we seriously consider other separation... 

Volatile components of a deposit will he lost relatively quickly and others may be degraded or absorbed by the surface to which they have been applied and disappear or become ineffective after a short interval. Stable compounds with a long residual life may be evaluated over a limited period using a set regime which can produce data allowing estimation of the length of effective control. 

There are several nonspecific methods available that can determine the total amount of solvent(s) in a sample. Loss on drying (LOD) determines the amount of volatile components that are released from a sample under specific temperature and/or vacuum conditions. Thermal gravimetric analysis (TGA) measures the loss of volatile components from a sample over a temperature gradient. The advantage of these methods is that they give an estimate of the volatile component content of a sample relatively quickly. The disadvantages of these methods are that they do not speciate and cannot account for volatile components that are trapped in the lattice structure of the compound. By accepting the limitations of these methods, a total solvent amount can be... 

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