Distillation overhead product

A mixture that contains 46 wt% acetone (CH3COCH3), 27% acetic acid (CH3COOH), and 27% acetic anhydride [(CH3C0)20j is distilled at P = 1 atm. The feed enters the distillation column at T = 348 K at a rate of 15,000 kg/h. The distillate (overhead product) is essentially pure acetone, and the bottoms product contains 1% of the acetone in the feed. 

Be molten product molar flow rate from high-tamperaiure end of a melt crysrallizar D molar flow rate or distillation Overhead product... 

Flow rate of bottoms product from a distillation column Flow rate of distillate (overheads product) in a distillation column Flow rate of feed to a distillation column or flash unit Number of moles of a mixture that are liquid, or the liquid stream in a distillation column or flash unit Panial pressure of species = y,P (kPa)... 

A Hquid-phase isophorone process is depicted ia Figure 4 (83). A mixture of acetone, water, and potassium hydroxide (0.1%) are fed to a pressure column which operates at head conditions of 205°C and 3.5 MPa (- 500 psi). Acetone condensation reactions occur on the upper trays, high boiling products move down the column, and unreacted acetone is distilled overhead ia a water—acetone a2eotrope which is recycled to the column as reflux. In the lower section of the column, water and alkaH promote hydrolysis of reaction by-products to produce both isophorone and recyclable acetone. Acetone conversion is typically ia the range 6—10% and about 70% yield of isophorone is obtained. Condensation—hydrolysis technology (195—198), and other Hquid-phase production processes have been reported (199—205). 

Figure 5 illustrates a typical distillation train in a styrene plant. Benzene and toluene by-products are recovered in the overhead of the benzene—toluene column. The bottoms from the benzene—toluene column are distilled in the ethylbenzene recycle column, where the separation of ethylbenzene and styrene is effected. The ethylbenzene, containing up to 3% styrene, is taken overhead and recycled to the dehydrogenation section. The bottoms, which contain styrene, by-products heavier than styrene, polymers, inhibitor, and up to 1000 ppm ethylbenzene, are pumped to the styrene finishing column. The overhead product from this column is purified styrene. The bottoms are further processed in a residue-finishing system to recover additional styrene from the residue, which consists of heavy by-products, polymers, and inhibitor. The residue is used as fuel. The residue-finishing system can be a flash evaporator or a small distillation column. This distillation sequence is used in the Fina-Badger process and the Dow process. 

Product Recovery. The aHyl chloride product is recovered through the use of several fractional distillation steps. Typically, the reactor effluent is cooled and conducted into an initial fractionator to separate the HCl and propylene from the chloropropenes, dichloropropanes, dichloropropenes, and heavier compounds. The unconverted propylene is recycled after removal of HCl, which can be accompHshed by adsorption in water or fractional distillation (33,37,38) depending on its intended use. The crude aHyl chloride mixture from the initial fractionator is then subjected to a lights and heavies distillation the lighter (than aHyl chloride) compounds such as 2-chloropropene, 1-chloropropene, and 2-chloropropane being the overhead product of the first column. AHyl chloride is then separated in the second purification column as an overhead product. Product purities can exceed 99.0% and commercial-grade aHyl chloride is typicaHy sold in the United States in purities about 99.5%. 

In the use of temperature measurement for control of the separation in a distillation column, repeatability is crucial but accuracy is not. Composition control for the overhead product would be based on a measurement of the temperature on one of the trays in the rectifying section. A target would be provided for this temperature. However, at periodic intervals, a sample of the overhead product is analyzed in the laboratory and the information provided to the process operator. Should this analysis be outside acceptable limits, the operator would adjust the set point for the temperature. This procedure effectively compensates for an inaccurate temperature measurement however, the success of this approach requires good repeatability from the temperature measurement. 

Liquid reaching the bottom of the column is partially vaporized in a heated r eboiler to provide boil-up, which is sent back up the column. The remainder of the bottom liquid is withdrawn as bottoms, or bottom product. Vapor reaching the top of the column is cooled and condensed to liquid in the over head conden.ser. Part of this liquid is returned to the column as r eflux to provide liquid overflow. The remainder of the overhead stream is withdrawn as distillate, or overhead product . In some cases only part of the vapor is condensed so that a vapor distillate can be withdrawn. 

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. 

Entrainment Entrainment in a plate column is that liquid which is carried with the vapor from a plate to the plate above. It is detrimental in that the effective plate efficiency is lowered because hquid from a plate of lower volatility is carried to a plate of higher volatility, thereby diluting distillation or absorption effects. Entrainment is also detrimental when nonvolatile impurities are carried upward to contaminate the overhead product from the column. 

Petroleum products may be treated with various solvents for the removal by selective solubility of undesirable constituents or for the recovery of by-products. The solvent and solute must be separated to yield the desired product and to recover the solvent for reuse. The solvents normally boil at a lower temperature than the products from which they are to be removed and so are generally distilled off as overhead products. The pipe stills used for this service may be single-stage or multi-stage units, depending on the service involved. Some solvents can be removed by the use of steam heated stills. In other cases, the high temperature required necessitates the use of fired heaters and vacuum towers. 

Figure 8-39. Batch distillation with trays constant overhead product.

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 ... 

As in the earlier examinations, the amount of methyl iodide detected in the purged product was still averaged ca. 0.3 wt.%. However, unlike the earher ran with [MePy]I, we took a close look at the effluent from the operation with l,2-dimethyl-5-ethyl-pyridinium iodide ([DMEpy] ). All the product was distilled overhead leaving a residue that upon examination by NMR contained a ca. 3 1 acetate [DMEpyratio. Closer examination by NMR revealed that only about 3% of [DMEpy]l in the overhead distillate had been dealkylated to 2-methyl-5-ethyl pyridium hydroiodide. 

It is desirable that the process variable to be monitored be measured directly often, however, this is impractical and some dependent variable, that is easier to measure, is monitored in its place. For example, in the control of distillation columns the continuous, on-line, analysis of the overhead product is desirable but difficult and expensive to achieve reliably, so temperature is often monitored as an indication of composition. The temperature instrument may form part of a control loop controlling, say, reflux flow with the composition of the overheads checked frequently by sampling and laboratory analysis. 

In a process for the production of acrylic fibres by the emulsion polymerisation of acrylonitrile, the unreacted monomer is recovered from water by distillation. Acrylonitrile forms an azeotrope with water and the overhead product from the column contain around 5 mol per cent water. The overheads are condensed and the recovered acrylonitrile separated from the water in a decanter. The decanter operating temperature will be 20 °C. 

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. 

The bottom product from column (G) passes to the hydroextractive distillation column (H). The water feed rate to column (H) is five times that of the bottom product flow from column (G). It may be assumed that the acetonitrile and other by-products are discharged as bottom product from column (H) and discarded. The overhead product from column (H), consisting of the acrylonitrile water azeotrope, is condensed and passed to a separator. The lower aqueous layer is returned to column (H). 

Example 9.1 A distillation column operating at 14 bar with a saturated liquid feed of 1000 kmoUr1 with composition given in Table 9.1 is to be separated into an overhead product that recovers 99% of the n-butane overhead and 95% of the i-pentane in the bottoms. Relative volatilities are also given in Table 9.1. 

A feed mixture of methanol and water containing a mole fraction of methanol of 0.4 is to be separated by distillation at a pressure of 1 atm. The overhead product should achieve a purity of 95 mole % methanol and the bottoms product a purity of 95 mole% water. Assume the feed to be saturated liquid. Using the x—y diagram constructed in Exercise 1 and the McCabe-Thiele construction ... 

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. 

Bottom Product B with a straight line joining the Distillate D and Entrainer Feed E, as shown in Figure 12.24. Pinch point curves for the middle section can now be constructed by drawing tangents to the residue curves from the difference point (net overhead product). This is shown in Figure 12.25 for the system ethanol-water-ethylene glycol. The area bounded by the pinch point curves defines the middle section operation leaf. 

Figure 14.7 If reflux ratio is fixed in batch distiliation, the overhead product purity decreases. (Reproduced from Smith R and Jobson B, 2000, Distillation in Encyclopedia of Separation Science, by permission of Academic Press Ltd).

Figure 14.9 Reflux ratio can be varied in batch distillation to maintain overhead product purity. (Reproduced from Smith R and...

Then Product 2 is distilled overhead for a period, after which its concentration begins to fall as the concentration of Product 3 increases in the distillate. The distillation is finished by collecting Product 3 overhead. 

Process designers sometimes like to use dephlegmators or partial condensers mounted directly in the top of the distillation column when the overhead product is taken off as a vapor. They arc particularly popular for corrosive, toxic, or hard-to-handle chemicals since they eliminate a. separate condenser shell, a reflux drum, and a reflux pump. Comment on the relative controllability of the two process systems sketched below. 

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