Heavy key components

If the light and heavy key components form an azeotrope, then something more sophisticated than simple distillation is required. The first option to consider when separating an azeotrope is exploiting change in azeotropic composition with pressure. If the composition of the azeotrope is sensitive to pressure and it is possible to operate the distillation over a range of pressures without any material decomposition occurring, then this property can be used to... 

In normal applications of extractive distillation (i.e., pinched, closeboiling, or azeotropic systems), the relative volatilities between the light and heavy key components will be unity or close to unity. Assuming an ideal vapor phase and subcritical components, the relative volatility between the light and heavy keys of the desired separation can be written as the produc t of the ratios of the pure-component vapor pressures and activity-coefficient ratios whether the solvent is present or not ... 

The value of 0 will lie between the relative volatilities of the light and heavy key components, which must be adjacent. 

LK = subscript for light key Nn, = 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... 

To determine the reasonableness of the top and bottom compositions of a fractionation column, a Hengstebeck plot is fast and easy (Reference 4). First, select a heavy key component and determine the relative volatility (a) of all column components to the heavy key. The a can be otfeed or perhaps more accurately cc = (a,op oCboitom) - Plot In D/B versus In a and the component points should fall close to a straight line. If a fairly straight line does not result, the compositions are suspect. A nomenclature table is provided at the end of this chapter. 

Relative volatility of component i versus the heavy key component... 

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

Select a heavy key component and compute of all components to this key. 

The following is a simplified estimating procedure for recovery in multicompnent distillation. In the working expressions provided below, the parameters b, d, and f rpresent the bottoms, distillate, and feed, respectively. Subscripts i, HK, and LK represent the component i, the heavy-key component, and the light-key component, repsectively. Relative volatility is represented by symbol a. Calculations can be readily set up on an Excel Spreadsheet. 

Yaws [124] et al. provide an estimating technique for recovery of each component in the distillate and bottoms from multicomponent distillation using short-cut equations and involving the specification of the recovery of each component in the distillate, the recovery of the heavy key component in the bottoms, and the relative volatility of the light key component. The results compare very well with plate-to-plate calculations. Figure 8-46, for a wide range of recoveries of 0.05 to 99.93% in the distillate. 

The correlation constants required for Equations 8-127 and 8-128 are obtained by specifying a desired recovery of the light key component LK in the distillate and the recovery of the heavy key component HK in the bottoms. Then the constants are calculated as follows ... 

Assume a multicomponent distillation operation has a feed whose component concentration and component relative volatilities (at the average column conditions) are as shown in Table 8-3. The desired recovery of the light key component O in the distillate is to be 94.84%. The recovery of the heavy key component P in the bottoms is to be 95.39%. 

FsR,k = factor for contribution of sidestream k flow to minimum reflux HK = heavy key component L = liquid flo Tate, mol/h LK = light key component nf = number of feeds ns = number of sidestreams m = number of sidestreams above feed n qp = thermal condition of feed qs = thermal condition of sidestream R = reflux ratio... 

HK = Heavy key component in volatile mixture h = Enthalpy of liquid mixture or pure compound at tray conditions of temperature and pressure, or specified point or condition, Btu/lb mol, or Btu/lb... 

Xjo = Mol fraction light key in overhead expressed as fraction of total keys in overhead xjB = Mol fraction most volatile component in bottoms XhD = Overhead composition of heavy key component, mol fraction... 

Fr, = Intermediate feed, Scheibel-Montross method FL = FH = All mol fractions lighter than light key in feed, Scheibel-Montross method FHK = Heavy key in feed FLK = Light key in feed HK = h = hk = Heavy key component H = Components heavier than heavy key h = Heavy, or heavy or high boiling component in mixture also heavy key component... 

For a multicomponent system, a simple technique to yield conservative results is as follows. Combine a light key component and all lighter components, and a heavy key component and all heavier components into two groups to get XF, XD, XB. For the key groups, use the a of the keys themselves. 

When Component i is the light key component L, and j is the heavy key component H, this becomes1,6-8 ... 

If the assumption is maintained that all of the lighter than hght key components go to the overheads and all of the heavier than heavy key components go to the column bottoms, for cases where the hght and heavy key components are not adjacent in volatility, one more value of 9 is required than there are components between the keys. 

Solution Calculate the liquid composition at the rectifying pinch using Equation 9.57. But first, the root of the second Underwood Equation associated with the heavy key component must be calculated. From Table 9.1, this lies in the range ... 

Assume that components heavier than the heavy key component do not appear in the distillate. The root is determined in Table 9.5. 

For the same feed, operating pressure and relative volatility as Exercise 10, the heavy key component is changed to pentane. Now 95% of the propane is recovered in the overheads and 90% of the pentane in the bottoms. Assuming that all lighter than light key components go to the overheads and all the heavier than heavy key go to the bottoms, estimate the distribution of the butane and the minimum reflux ratio using the Underwood Equations. 

In multicomponent distillation, A and B are the light and heavy key components respectively. In this problem, the only data given for both top and bottom products are for m-and p-xylene and these will be used with the mean relative volatility calculated in the previous problem. Thus ... 

The generation of an initial population of random separation sequences is done first. The sequences describe both in which order the components are separated and which separation method is used. For example the sequence on left in Figure 13 is described by the string 23 12 14 11. The first integer is for the separation method and the second for the heavy key component of the split in the column. The first separation is made by method 2 and the components heavier than no.3 (i.e. 4 and 5) go to bottom. In the next separation method 1 is used and component heavier than 2 (i.e. 3) goes to bottom, etc. 

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