Distillation Terminology

To provide a better understanding of the distillation process, the following briefly explains the terminology most often encountered.

Solvent Recovery

The term solvent recovery often has been a somewhat vague label applied to the many and very different ways in which solvents can be reclaimed by industry.

One approach when an impure solvent contains both soluble and insoluble particles is to evaporate the solvent from the solids. This requires the use of a small forced-circulation-type evaporator that combines a heat exchanger, external separator, and vacuum system with a special orifice that causes back pressure in the exchanger and arrests vaporization until the liquid flashes into the separator. Although this will recover a solvent, it will not separate solvents if two or more are present.

A further technique is available to handle an airstream that carries solvents. By chilling the air by means of vent condensers or refrigeration equipment, the solvents can be removed from the condenser. Solvents also can be recovered by using extraction, adsorption, absorption, and distillation methods.

Solvent Extraction

Essentially a liquid/liquid process where one liquid is used to extract another from a secondary stream, solvent extraction generally is performed in a column somewhat similar to a normal distillation column. The primary difference is that the process involves two liquids instead of liquid and vapor.

During the process, the lighter (ie, less dense) liquid is charged to the base of the column and rises through packing or trays while the more dense liquid descends. Mass transfer occurs, and a component is extracted from one stream and passed to the other. Liquid/liquid extraction sometimes is used when the breaking of an azeotrope is difficult or impossible by distillation techniques.

Carbon Adsorption

The carbon adsorption technique is used primarily to recover solvents from dilute air or gas streams. In principle, a solvent-ladened airstream is passed over activated carbon and the solvent is adsorbed into the carbon bed. When the bed becomes saturated, steam is used to desorb the solvent and carry it to a condenser. In such cases as toluene, for example, recovery of the solvent can be achieved simply by decanting the water/solvent two-phase mixture that forms in the condensate. Carbon adsorption beds normally are used in pairs so that the airflow can be diverted to the secondary bed when required.

On occasion, the condensate is in the form of a moderately dilute miscible mixture. The solvent then has to be recovered by distillation. This would apply especially to ethyl alcohol, acetone-type solvents.


When carbon adsorption cannot be used because certain solvents either poison the activated carbon bed or create so much heat that the bed can ignite, absorption is an alternative technique. Solvent is recovered by pumping the solvent-ladened airstream through a column counter-currently to a water stream that absorbs the solvent. The air from the top of the column essentially is solvent-free whereas the dilute water/solvent stream discharged from the column bottom usually is concentrated in a distillation column. Absorption also can be applied in cases where an oil rather than water is used to absorb certain organic solvents from an airstream.


During distillation, some components form an azeotrope at a certain stage of the fractionation and require a third component to break the azeotrope and achieve a higher percentage of concentration. In the case of ethyl alcohol and water, for example, a boiling mixture containing less than 96% by weight ethyl alcohol produces a vapor richer in alcohol than in water and is readily distilled. At the 96% by weight point, however, the ethyl alcohol composition in the vapor remains constant, ie, the same composition as the boiling liquid. This is known as the azeotrope composition. Further concentration requires use of a process known as azeotropic distillation. Other common fluid mixtures that form azeotropes are formic acid/water, isopropyl alcohol/ water, and isobutanol/water.

Azeotropic Distillation

In a typical azeotropic distillation procedure, a third component such as benzene, isopropyl ether, or cyclohexane is added to an azeotropic mixture such as ethyl alcohol/water to form a ternary azeotrope. Because the ternary azeotrope is richer in water than the binary ethyl alcohol/water azeotrope, water is carried over the top of the column. The azeotrope, when condensed, forms two phases. The organic phase is refluxed to the column whereas the aqueous phase is discharged to a third column for recovery of the entraining agent.

Certain azeotropes such as the re-butanol/water mixture can be separated in a two-column system without the use of a third component. When condensed and decanted, this type of azeotrope forms two phases. The organic phase is fed back to the primary column, and the butanol is recovered from the bottom of the still. The aqueous phase, meanwhile, is charged to the second column, with the water being taken from the column bottom. The vapor from top of both columns is condensed, and the condensate is run to a common decanter (Fig. 1).

Extractive Distillation

This technique is somewhat similar to azeotropic distillation in that it is designed to perform the same type of task. In azeotropic distillation, the azeotrope is broken by carrying over a ternary azeotrope at the top of the column. In extractive distillation, a very high boiling compound is added and the solvent is removed at the base of the column.


In distillation terminology, stripping refers to the recovery of a volatile component from a less volatile substance. Again, referring to the ethyl alcohol/water system, stripping is done in the first column below the feed point where the alcohol enters at about 10% by weight and the resulting liquid from the column base contains less than 0.02% alcohol by weight. This is known as the stripping section of the column. This technique does not increase the con-

Butanol column

Steam i

Condenser I ?

Cooling water



Organic phase ro o

Aqueous phase


Aqueous column


Figure 1. System for recovering butanol from butanol/water mixture.

Reboiler J__A A A Butanol product

Preheater Water crpM^

Butanol/water feed centration of the more volatile component but rather, decreases its concentration in the less volatile component.

A stripping column also can be used when a liquid such as water contaminated by toluene cannot be discharged to a sewer. For this pure stripping duty, the toluene is removed within the column while vapor from the top is decanted for residual toluene recovery and refluxing of the aqueous phase.


For rectification or concentration of the more volatile component, a top section of column above the feed point is required. By means of a series of trays and reflux back to the top of the column, a solvent such as ethyl alcohol can be concentrated to more than 95% by weight.

Batch Distillation

When particularly complex or small operations require recovery of the more volatile component, there are batch-distillation systems of various capacities. Essentially a rectification-type process, batch distillation involves pumping a batch of liquid feed into a tank where boiling occurs. Vapor rising through a column above the tank combines with reflux coming down the column to effect concentration. This approach is not very effective for purifying the less-volatile component.

For many applications, batch distillation requires considerable operator intervention or, alternatively, a significant amount of control instrumentation. Although it is more energy intensive than a continuous system, steam costs generally are less significant on a small operation. Furthermore, it is highly flexible and a single batch column can be used to recover many different solvents.

Continuous Distillation

The most common form of distillation used by the chemical, petroleum, and petrochemical industries is the continuous-mode system. In continuous distillation, feed constantly is charged to the column at a point between the top and bottom trays. The section above the feed point rectifies the more volatile component while the column section below the feed point strips out the more volatile component from the less volatile liquid. In order to separate N components with continuous distillation, a minimum of N — 1 distillation columns is required.


The turndown ratio of a column is an indication of the operating flexi bility. If a column, for example, has a turndown ratio of 3, it means that the column can be operated efficiently at 33% of the design maximum throughput.


The following briefly defines the many components required for a distillation system and the many variations in components that are available to meet different process conditions.

Column Shells

A distillation column shell can be designed for use as a free-standing module or for installation within a supporting steel structure. Generally speaking, unless a column is of very small diameter, a self-supporting column is more economical. This holds true even under extreme seismic 3 conditions.

There are distillation columns built of carbon steel, 304 stainless steel, 316 stainless steel, Monel, titanium, and Incoloy 825. Usually, it is more economical to fabricate columns in a single piece without shell flanges. This technique not only simplifies installation but also eliminates danger of leakage during operation. Columns more than 80 feet long have been shipped by road without transit problems.

Although columns of more than 3-ft diameter normally have been transported without trays to prevent dislodge-ment and possible damage, recent and more economical techniques have been devised for factory installation of trays with the tray manways omitted. After the column has been erected, manways are added and, at the same time, the fitter inspects each tray.

With packed columns of 20-in. diameter or less that use high-efficiency metal mesh packing, the packing can be installed before shipment. Job-site packing, however, is the norm for larger columns. This prevents packing from bedding down during transit and leaving voids that would reduce operating efficiency. Random packing always is installed after delivery except for those rare occasions when a column can be shipped in a vertical position. Access platforms and interconnecting ladders designed to Occupational Safety and Health Administration (OSHA) standards also are supplied for on-site attachment to free-standing columns.

Installation usually is simple because columns are fitted with lifting lugs and, at the fabrication stage, a template is drilled to match support holes in the column base ring. With these exact template dimensions, supporting bolts can be preset for quick and accurate coupling as the column is lowered into place.

Column Internals

During recent years, the development of sophisticated computer programs and new materials has led to many innovations in the design of trays and packings for more efficient operation of distillation columns. In designing systems for chemical, petroleum, and petrochemical use, full advantage can be taken of available internals to assure optimum distillation performance.

Tray Devices

Although there are perhaps five basic distillation trays suitable for industrial use, there are many design variations of differing degrees of importance and a confusing array of trade names applied to their products by tray manufacturers. The most modern and commonly used devices are sieve, dual-flow, valve, bubble-cap, and baffle trays—each with its advantages and preferred usage. Of these, the sieve- and valve-type trays currently are most often specified.

For a better understanding of tray design, Figure 2 defines and locates typical tray components. The material of construction usually is 14 gauge, with modern trays adopting the integral truss design, which simplifies fabrication. A typical truss tray is shown in Figure 3. For columns less than 3 ft in diameter, it is not possible to assemble the truss trays in the column. Trays therefore must be pre-assembled on rods into a cartridge section for loading into the column. Figure 4 shows this arrangement in scale-model size.

The hydraulic design of a tray is a most important factor. The upper operating limit generally is governed by the flood point, although, in some cases, entrainment also can restrict performance. By forcing some liquid to flow back up the column, entrainment reduces concentration gradients and lowers efficiency. A column also can flood by down-comer backup when tray design provides insufficient down-comer area or when the pressure drop across the tray is high. When the down-comer is unable to handle all the liquid involved, the trays start to fill and pressures increase. This also can occur when a highly foaming liquid is involved. Flooding associated with high tray pressure drops and small tray spacing takes place when the required liquid seal is higher than the tray spacing. Down-

Flat seal

Downcomer area, top

Downcomer area, bottom

Outlet weir

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