Heavy nonkey

A widening of the temperature differences across columns, since light nonkey components cause a decrease in condenser temperature and heavy nonkey components cause an increase in the reboiler temperature. 

Key components are the two components in the feed mixture whose separation is specified. The more volatile of these components is the light key. and the less volatile is the heavy key. In Example 2.4, propane is the light key and n-butane the heavy key. Other components are termed turnkeys. The nonkeys which are more volatile than the keys are termed light nonkeys (methane and ethane in Example 2.4), and those less volatile are heavy nonkeys (pentane and hexane in Example 2.4). 

The key components appear to a significant extent in both overhead and bottom products. Light nonkeys end up almost exclusively in the overhead product, and heavy nonkeys end up almost exclusively in the bottom product,... 

There is a significant concentration of all components at the feed stage. The heavy nonkeys, C6 and Ce, dia out rapidly in both the liquid and vapor above the fsed because of their low volatilities. Pentane persists longer than hexane because it is more volatile. A similar behavior le exhihited by the light nonkeys (methane and ethane) balow the feed. 

The heavy nonkeys, C6 and Ce, have relatively constant mole fractions in the liquid and vapor below the feed until about three or four stages above the bottom. These heavy non keys are merely trans-... 

For the rectifying section, it is assumed that light nonkeys are at their limiting compositions and that heavy nonkeys are absent, For the stripping section, it is assumed that heavy nonkeys are at their limiting composition and that light nonkeys are absent. Equations (2,46) to (2,49) can therefore be written for the zone of constant nonkey mole fraction in the rectifying section as follows ... 

Calculate the limiting flow rate of the light nonkeys in the rectifying section and the limiting flow rate of the heavy nonkeys in the stripping section. These limiting flow rates for each nonkey can he calculated from Eqs. (2.50) and (2,51) as follows ... 

Warning Minimum reflux ratios calculated by this method are somewhat lower than true values. This is because the method neglects the presence of light nonkeys just below the feed and heavy nonkeys just above it (see Sec. 2.3.6). 

This behavior results from the presance of light nonkeys below the feed and heavy nonkeys above it. The pinch zones in multicomponent... 

Light nonkeys raise the optimum key ratio at the feed stage, while heavy nonkeys lower it. Rules 1 and 2 therefore become less reliable when there are a lot more light than heavy nonkeys or vice versa, or when the amount of nonkeys exceeds the amount of keys,... 

The first rule of thumb tends to work better when there are more light than heavy nonkeys, while the second tends to work better when there are more heavy than light nonkeys,... 

Stupin and Lockhart (27) also noted that as reflux is lowered from total to minimum, the separation of nonkeys first worsens (curve 2, Fig, 2,21), then improves (curve 3, Fig. 2,21), The intermediate keys follow the converse pattern, At a reflux ratio of about 1,2 to 1,5 times the minimum, component distribution resembles that of the total reflux component distribution. Detailed discussion is elsewhere (7,27). Figure 2,16 demonstrates that light nonkey6 are fractionated out in the stripping section and heavy nonkeys in the rectifying section. The d/b plot depicts this behavior (Sec. 2,4,2). 

Find the key components If any components have similar volatilities to one of the keys, and end up in the same product, lump them with the keys. Convert all mole fractions to the equivalent binary [Eqs, (2.46) and (2.47)], An alternative, simpler procedure is to lump all light keys and light nonkeys into a single light pseudocomponent, and all heavy keys and heavy nonkeys into a single heavy pseudocomponent, This procedure (used in Fig, 2.22) is preferred by the author and others (28), Whichever method is preferred, it must be consistently applied. 

The above applies to both binary and multicomponent distillation. In multicomponent distillation, once the above are specified, other components will distribute according to the equilibrium relationship. Frequently, a product spec sets the maximum concentration of impurities that can be tolerated in the product. Product specs are less than" specifications. The one impurity which is dependent on the column separation and is most difficult to achieve sets the composition specification in the column. This is illustrated in Table 3.1 for a propylene-propane separation (Ca splitter). Since the light nonkeys (hydrogen, methane, ethylene, ethane, and oxygen) end up in the distillate, their concentration in the distillate is independent of the column. Of the others, the most difficult purity to achieve sets the composition specification, Similarly, the heavy nonkeys (MAPD, C4 and... 

The Colburn method (39) This method calculates the minimum reflux ratio of the key components as if they formed a binary system, then corrects this value for light and heavy nonkeys. The Colburn method assumes constant molar overflow and constant relative volatility in each zone of constant composition in the column. This method is more elaborate, but has been recommended (28) as probably the most accurate shortcut method for minimum reflux. 

A component is said to be distributed (or distributing) at minimum reflux if it appears both in the distillate and the bottoms at minimum reflux. Usually, nonkeys are nondistributed (or nondistributing), that is, at minimum reflux the heavy nonkeys are totally contained in the bottoms and the light nonkeys in the distillate. A nonkey component may be distributed if... 

Since the light nonkeys appear to all go to the distillate, and the heavy nonkeys all to the bottom, only the keys needs to be considered. In addition... 

Multicomponent efficiency profiles. Figure 7.11 (198,199) shows typical variations of Murphree plate efficiency for a ternary system made up of a light key (LK) component, a heavy key (HK) component, and a heavy nonkey (HNK) component (benzene, toluene, and m-xylene, respectively). On the basis of experimental data, point efficiency was assumed the same for all components throughout the column. 

The Underwood equation requires a trial-and-error solution and a subsequent material balance to estimate the minimum reflux ratio. First, the unknown, q>, is determined by trial and error, such that both sides of the following equation are equal. The unknown value of q> should lie between the relative volatilities of the light and heavy key components. The key components are those that have their fractional recoveries specified. The most volatile component of the keys is the light key and the least volatile is the heavy key. All other components are referred to as nonkey components. If a nonkey component is lighter than the light key component, it is a light nonkey if it is heavier than the heavy key component it is a heavy nonkey component. 

The -hexane is the light key (LK), the n-heptane is the heavy key (HK), and the -octane is a heavy nonkey (HNK), which goes almost entirely to the bottoms. The product compositions are found by mass balance assuming no -octane and 0.99 mole fraction n-hexane in the distillate. Basing the calculations on a feed rate of 100 mol/h,... 

DISTRIBUTED AND UNDISTRIBUTED COMPONENTS. A distributed component is found in both the distillate and bottoms products, whereas an undistributed component is found in only one product. The light key and heavy key are always distributed, as are any components having volatilities between those two keys. Components more volatile than the light key are almost completely recovered in the distillate, and those less volatile than the heavy key are found almost completely in the bottoms. Whether such components are called distributed or undistributed depends on the interpretation of the definition. For a real column with a finite number of plates, all components are theoretically present in both products, though perhaps some are at concentrations below the detectable limit. If the mole fraction of a heavy nonkey component in the distillate is 10 or less, the component may be considered undistributed from a practical standpoint. However, in order to start a plate-by-plate calculation to get the number of plates for the column, this small but finite value needs to be estimated. 

For the case of minimum reflux, the distinction between distributed and undistributed components is clearer, since heavy nonkey components are generally absent from the distillate, and light nonkey components are not present in the bottoms. The concentrations of these species can go to zero because of an infinite number of plates in the column and conditions that lead to a progressive reduction in concentration for each plate beyond the feed plate. 

Between the lower and upper invariant zones, the mole fraction of both keys in the vapor phase decreases, and the ratio of light key to heavy key decreases. This region of the column serves to remove the light nonkey components from the liquid flowing down and the heavy nonkey component from the material that will flow up and form the distillate. The small amount of reverse fractionation shown for the key components is an interesting phenomenon that is often found in real columns operating at close to the minimum reflux ratio. 

In multicomponent distillation neither the distillate nor the bottoms composition is completely specified because there are not enough degrees of freedom to allow complete specification. This inability to completely specify the distillate and bottoms compositions has major effects on the calculation procedure. The components that do have their distillate and bottoms fractional recoveries specified (such as component 1 in the distillate and component 2 in the bottoms in Table 6.3) are called key components. The most volatile of the keys is called the light key (LK), and the least volatile, the heavy key (HK). The other components are nonkeys (NK). If a nonkey is more volatile than the light key, it is a light nonkey (LNK) if it is less volatile than the heavy key, it is a heavy nonkey (HNK). 

Fortunately, in many cases it is easy to make an excellent first guess. If a sharp separation of the keys is required, then almost all of the heavy nonkeys will appear only in the bottoms, and almost all of the light nonkeys will appear only in the distillate. If there are only light nonkeys or only heavy nonkeys, then an accurate first guess of compositions can be made. 

This appears to be a straightforward application of overall material balances, except that there are two variables too many. Thus, we will have to assume the recoveries or concentrations of two of the components. The normal boiling point of propane is 231.1 K, that of n-butane is 272.7 K, 309.2 for n-pentane, and 341.9 for n-hexane. Thus, the order of volatilities is propane > n-butane > n-pentane > n-hexane. This makes propane the light key, n-butane the heavy key, and n-pentane and n-hexane the heavy nonkeys (HNKs). Since the recoveries of the keys are quite high, it is reasonable to assume that all of the HNKs appear only in the bottoms. Based on this assumption, the overall material balances yield D = 907.9 kmol/h, W = 1092.1 kmol/h. The estimated compositions are as follows. 

If there are nondistributing heavy nonkeys present, a pinch point of constant composition will occur at minimum reflux in the rectifying section above where the... 

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