Light key components

Ideally, the K value for the light key component in the phase separation should be greater than 10, and at the same time, the K value for the heavy key should be less than 0.1. Having such circumstances leads to a good separation in a single stage. However, use of phase separators might still be effective in the flowsheet if the K values for the key components are not so extreme. Under such circumstances a more crude separation must be accepted. 

Phase separation in this way is most effective if the light key component is significantly above its critical temperature. If a component is above its critical temperature, it does not truly condense. Some, however, dissolves in the liquid phase. This means that it is bound to have an extremely high K value. 

Assuming a sharp separation with only the light key and lighter-than-light key components in the overheads and only the heavy key... 

Minimum total reflux (lbs or mols/hr) corresponding to given total feed will be greater than if only the actual total mols of heavy and light key components were present. Reflux need will be less than if the actual total mols of feed were present, but composed only of light and heavy keys. The more closely non-keyed components are clustered to volatilities of the keys, the nearer are reflux needs to that calculated for the binary and total feed volume. 

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... 

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... 

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. 

Diffusivity of the liquid light key component is calculated by the dilute solution equation of Wilke-Chang [243]. 

Bto = mols total batch charge to still V = total mols per hour vapor overhead XiD = mol fraction light key component in overhead product... 

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%. 

Determine approximate pinch zone liquid composition for light key component... 

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... 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

XjD = Mol fraction light key component in overhead product or, any light component (Colburn) x b = Mol fraction light key component in keys in original charge... 

LK = Light key component L = Liquid, Scheibel-Montross method only or components lighter than light key M = min = Minimum... 

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. 

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 ... 

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 the fractionation of multicomponent mixtures, the essential requirement is often the separation of two components. Such components are called the key components and by concentrating attention on these it is possible to simplify the handling of complex mixtures. If a four-component mixture A-B-C-D, in which A is the most volatile and D the least volatile, is to be separated as shown in Table 11.3, then B is the lightest component appearing in the bottoms and is termed the light key component. C is the... 

The first objective of a shortcut method is selection of two key components and then setting all conditions around these components. Call the light key component LK and the heavy key HK. The fractionation objective is to separate these two key components per a given specification. The distribution (distillate product analysis and bottoms product analysis) of a multicomponent system is also accomplished in this proposed shortcut fractionator program, Hdist. 

Enter an alpha value if you have chosen F or T for the method. Enter a K value for a light key component if you chose A. Input the factor alpha or K. Alpha is defined as simply the light key K divided by the heavy key K component. The K factor is simply the particular component s vapor phase mole fraction divided by its liquid mole fraction. The alpha value is therefore a ratio of the chosen two key components. These key components should be those that readily point to how well the fractionator is doing its job of separation. For example, for a depropanizer tower, choose propane as the light key component and butane as the heavy key, since you wish to separate the propane from the butane to make a propane product specification. For a multicomponent system, you may try several components to determine a controlling alpha and/or to factor an average tray efficiency. 

This factor should be inputted if you have chosen the two-film method. Enter the value of the chosen light key component liquid phase mole fraction. This value should be obtained from an equilibrium curve, a two-component binary system, or a tray-to-tray computer program printout. The program uses this factor to calculate the slope of the equilibrium curve. Be sure to select the liquid light key mole fraction at the proper curve point or tray condition that corresponds to the K value used. 

Step 1. The first step in the procedure is to determine the key component equilibrium curve slope M. This is for one component only. You must choose a light key and a heavy key component for this tray efficiency calculation. Select components that are keys in the fractionator split. A single K = Y/X equilibrium value is to be applied to absorbers and strippers as this M value. The change of Y per the change of X is sought out for the light key component. 

X1 = light key component mol fraction in liquid phase Yi = M slope ratio-derived Y axis vapor phase, first factor Y2 = M slope ratio-derived Y axis vapor phase, second factor... 

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