Overhead product components

A tower comprised of rectifying (above the feed) and stripping (below the feed) sections is capable of making a more or less sharp separation between two products or pure components of the mixture, that is, between the light and heavy key components. The light key is the most volatile component whose concentration is to be controlled in the bottom product and the heavy key is the least volatile component whose concentration is to be controlled in the overhead product. Components of intermediate volatilities whose distribution between top and bottom products is not critical are called distributed keys. When more than two sharply separated products are needed, say n top and bottom products, the number of columns required will be n — 1. 

A vapor feed is favored when the stream leaves the upstream unit as a vapor or when most of the column feed leaves the tower as overhead product. The use of a vapor feed was a key component in the high efficiency cited previously for the spHtter, where most of the feed goes overhead. Low Column Pressure Drop. The penalty for column pressure drop is an increase in temperature differential ... 

An azeotrope limits the separation that can be obtained between components by simple distillation. For the system described by cui ve B, the maximum overhead-product concentration that could be obtained from a feed with X = 0.25 is the azeotropic composition. Similarly, a feed with X = 0.9 could produce a bottom-product composition no lower than the azeotrope. 

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

If there are components in the feed and bottoms which do not appear in the overhead product, they must gradually be introduced into the calculations. The estimated position above the feed tray to start introducing these components is determined by ... 

Mols of distillate or overhead product, lb mols/hr or batch distillation, mols Mols component, i, in distillate Vaporization efficiency of steam distillation Overall column efficiency Overall tray efficiency Eqg = Murphree point efficiency, fraction Murphree plate/tray efficiency, = E ... 

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... 

Sequencing of columns for separating multicomponent mixtures (a) perform the easiest separation first, that is, the one least demanding of trays and reflux, and leave the most difficult to the last (b) when neither relative volatility nor feed concentration vary widely, remove the components one by one as overhead products (c) when the adjacent ordered components in the feed vary widely in relative volatility, sequence the splits in the order of decreasing volatility (d) when the concentrations in the feed vary widely but the relative volatilities do not, remove the components in the order of decreasing concentration in the feed. 

The correlations given in the previous sections apply to the condensation of a single component such as an essentially pure overhead product from a distillation column. The design of a condenser for a mixture of vapours is a more difficult (ask. 

A distillation column uses a partial condenser as shown in Figure 9.19. Assume that the reflux ratio and the overhead product composition and flowrate and the operating pressure are known and that the behavior of the liquid and vapor phases in the column is ideal (i.e. Raoult s Law holds). How can the flowrate and composition of the vapor feed to the condenser and its liquid products be estimated, given the vapor pressure data for the pure components. Set up the equations that need to be solved. 

Martin and coworkers described an application of optimization to an existing tower separating propane and propylene. The lighter component (propylene) is more valuable than propane. For example, propylene and propane in the overhead product were both valued at 0.20/lb (a small amount of propane was allowable in the overhead), but propane in the bottoms was worth 0.12/lb and propylene 0.09/lb. The overhead stream had to be at least 95 percent propylene. Based on the data in Table E12.4A, we will determine the optimum reflux ratio for this column using derivations provided by McAvoy (personal communication, 1985). He employed correlations for column performance (operating equations) developed by Eduljee (1975). 

Unlike the gasoline Molex process that employs a iso-butane and n-butane desorbent mixture, the MaxEne process employs a heavy desorbent system. A heavy desorbent system means that the bottom product from both the Sorbex extract and raffinate frachonation columns is desorbent while the feed components are recovered as overhead products. In the MaxEne process case, heavy normal paraffin such as n-dodecane is employed as the desorbent though desorbents as light as n-decane and as heavy as n-tetradecane are possible candidates too. 

So far, all our examples have dealt with two-component systems. And many of our towers really just have two components. Also, we have assumed that the reflux rate is large compared to the overhead product rate. And many of our towers do run with a lot of reflux. But we can all... 

Now the overhead product of this tower is a mixture of a hundred different components, ranging from ethane, which has a molecular weight of 30, to decane, which has a molecular weight of 142. Also, while the overhead product rate is 60,000 lb/h, the top reflux rate is only 10,000 lb/h. 

When we raise the top reflux rate to our preflash tower, the tower-top temperature goes down. This is a sign that we are washing out from the upflowing vapors, more of the heavier or higher-molecular-weight, components in the overhead product. Of course, that is why we raised the reflux rate. So the reduction in tower-top temperature is good. 

The distillate product would become increasingly contaminated with heavier components. If this were a refinery crude fractionator, we would say that the endpoint of the naphtha overhead product would increase. 

When neither relative volatility nor concentration in the feed varies widely, remove the components one-by-one as overhead products. 

In a/eotropic distillation, an agent is selected that will form an azeotrope with one of the feed components. In essence, separation is accomplished between this new" azeotrope (as an overhead product) and the other feed component as bottoms product. An agent w ill be selected preferably that will permit easy separation after distillation. 

Heavy materials remaining at the bottom are called the bottoms, or residuum, and include such components as heavy fuel oil (see fuel oil) and asphaltic substances (see asphalt). Those fractions taken in liquid form from any level other than the very top or bottom are called sidestream products a product, such as propane, removed in vapor form from the top of the distillation tower is called overhead product. Distillation may take place in two stages first, die lighter fractions—gases, naphtha, and kerosene-are recovered at essentially atmospheric pressure next, the reamining crude is distilled at reduced pressure in a vacuum tower, causing the heavy lube fractions to distill at much lower temperatures than possible at... 

Entrainment (Fig. 14-33) is liquid transported by the gas to the tray above. As the lower tray liquid is richer with the less-volatile components, entrainment counteracts the mass-transfer process, reducing tray efficiency. At times entrainment may transport nonvolatile impurities upward to contaminate the tower overhead product, or damage rotating machinery located in the path of the overhead gas. 

Component Feed, mol/h Overhead product, mol/h Bottoms product, mol/h... 

Figure 13 shows a McCabe-Thiele diagram, which can be used when the mixture to be distilled consists of only two components or can be represented by two components. Starting at the required overhead product composition x/), an upper-section operating line is drawn hav-... 

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