Continuous distillation

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Continuous distillation is one form of distillation. Briefly, distillation is a liquid-liquid separation process, and is one of the unit operations of chemical engineering. Continuous distillation is used widely in the chemical process industries which is mainly where large quantities of liquids have to be distilled.<ref name=Kister>Kister, Henry Z. (1992). Distillation Design, 1st Edition, McGraw-Hill. ISBN 0-07-034909-6.</ref><ref name=Perry>Perry, Robert H. and Green, Don W. (1984). Perry's Chemical Engineers' Handbook, 6th Edition, McGraw-Hill. ISBN 0-07-049479-7.</ref> Such industries are the petroleum processing, petrochemical production, coal tar processing, brewing, liquified air separation, and hydrocarbon solvents production and similar industries but it finds its widest application in petroleum refineries. In such refineries, the crude oil feedstock is a very complex multicomponent mixture that must be separated and yields of pure chemical compounds are not expected, only groups of compounds within a relatively small range of boiling points, also called fractions and that is the origin of the name fractional distillation or fractionation.

[edit] Main Equipment

Although small size units, mostly made of glass, are used in laboratories, industrial units are large, vertical steel cylinders (see Image 1) known as "distillation towers" or "distillation columns" with diameters ranging from about 65 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters or more. The tower is normally provided inside with horizontal plates or trays as shown in Image 4, each of which has one or more weirs to retain some of the liquid on the tray, with overflows to the tray below. Each tray also has a number of very short pipes and on top of each pipe is a little cap, called a "bubble cap", often with a serrated edge. Each tray is designed so that the liquid level ensures that liquid flowing downward through the tray meets the rising vapours and contacting takes place.

Depending on their purpose, distillation columns may have liquid outlets at intervals up the length of the column as shown in Image 3.

[edit] Principle

The principle of distillation comes from the fact that when a liquid mixture is heated to its boiling point, the composition of the vapour above the liquid will be different from that of the liquid. If this vapour is then separated and condensed into a liquid, we find that it has become richer in the lower boiling component of the original mixture. If we do the same again with the just condensed liquid, further enrichment occurs. If we carried out this operation an infinite number of times, theoretically we would finish off with a pure liquid, and we could do the same with the liquid phase, and finish off with another pure component.

This is exactly what happens in a continuous distillation column. A mixture is heated up in either one or more heat exchangers or a fuel-fired furnace or both, and routed into the distillation column. If the feed is from a source at a pressure higher than the distillation column pressure, it is simply pressured into the column. Otherwise, the feed will be pumped or compressed into the column. The feed may be a superheated vapor, a saturated vapor, a partially vaporized liquid-vapor mixture, a saturated liquid (i.e., liquid at its boiling point at the column's pressure) or a sub-cooled liquid. If the feed is a liquid at a much higher pressure than the column pressure and flows through a pressure let-down valve just ahead of the column, it will immediately expand and undergo a partial flash vaporization resulting in a liquid-vapor mixture as it enters the distillation column.

On entering the column, the liquid portion of the feed starts flowing down but part of it, richer in lower boiling component(s), vaporises and rises. However, as it rises, it cools and while part of it continues up as vapour, some of it (enriched in the less volatile component of that tray) begins to descend again.

The liquid downflow meets the vapours rising and contacting takes place, whereby the combined mixture on each tray separates with each vapour phase richer in more volatile, each liquid phase richer in less volatile component. As the liquid goes down, the vapour continues rising until, because there is both a pressure and a temperature gradient in the column (lower pressure and temperature on top, higher at the bottom) the more volatile (so called lighter) component reaches the top, while the less volatile (so called heavier) component descends to the bottom.

To compensate for heat loss and to improve bottom product purity, heat is most often added to the bottom of the column by a reboiler, and the purity of the top product can be improved by recycling some of the externally condensed top product liquid as reflux. Of course, if no product were withdrawn, the system would blow up, so the combined product withdrawal rate has to equal the feed rate. During steady operation after startup, and with no external disturbance, the system is said to be in equilibrium. This means that: the feed rate, composition and temperature are constant, the withdrawal rate of top and bottom products is constant, and the temperature and pressure on each tray in the column is constant.

The attraction of continuous distillation, apart from the minimum amount of (easily instrumentable) surveillance, is that if the feed rate and feed composition are kept constant, product rate and quality are also constant.

Image 2 depicts a continuous fractional distillation tower for separating a feed stream into two fractions, an overhead distillate product and a bottoms product. The overhead stream may be cooled and condensed using a water-cooled or air-cooled condenser. The bottoms reboiler may be a steam-heated or hot oil-heated heat exchanger, or even a gas or oil-fired furnace.

In a distillation tower (such as in Images 2 and 3), separation of the components in the feed mixture is achieved by having a series of coexisting vapor and liquid equilibrium stages, each of which is at a different temperature and pressure. The stage at the tower bottom has the highest pressure and temperature. Progessing upwards in the tower, the pressure and temperature decreases for each succeeding stage. The vapor-liquid equilibrium for each feed component in the tower reacts in its unique way to the different pressure and temperature conditions at each of the stages. That means that each component establishes a different concentration in the vapor and liquid phases at each of the stages, and this results in the separation of the components.

[edit] Design and theory

Design and operation of a distillation column depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method <ref name=Perry/><ref name=Beychok>Beychok, Milton (May 1951). "Algebraic Solution of McCabe-Thiele Diagram". Chemical Engineering Progress.</ref><ref name=SeaderHenley>Seader, J. D., and Henley, Ernest J.. Separation Process Principles. New York: Wiley. ISBN 0-471-58626-9.</ref> or the Fenske equation<ref name=Perry/> can be used to assist in the design. For a multi-component feed, computerized simulation models are used both for design and subsequently in operation of the column as well.

[edit] Reflux

The separation efficiency of a distillation column can be enhanced by using more reflux or by using more vapor-liquid contacting devices such as plates or trays, or packing.

Large-scale industrial fractionation towers use reflux to achieve more complete separation of products.<ref name=Kister/><ref name=Perry/> Reflux refers to the portion of the condensed overhead liquid product from a distillation tower that is returned to the upper part of the tower as shown in images 3 and 4. Inside the tower, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficacy of the distillation tower. The more reflux that is provided, the better is the tower's separation of the lower boiling from the higher boiling components of the feed.

[edit] Plates or trays

Distillation towers use various vapor and liquid contacting methods to provide the required number of equilibrium stages. Such devices are commonly known as "trays" or "plates" and some example trays are depicted in image 4. An even more detailed, expanded image of two trays can be seen in the Theoretical plate article.

If each physical tray or plate were 100% efficient, than the number of physical trays needed for a given separation would equal the the number of equilibrium stages or theoretical plates. However, that is very seldom the case. Hence, a distillation column needs more plates than the required number of theoretical vapor-liquid equilibrium stages.

[edit] Packing

Another way of improving the separation in a distillation column is to use a packing material instead of trays. These offer the advantage of a low pressure drop across the column, beneficial when operating under vacuum. This packing material can either be random dumped packing such as Raschig rings or structured sheet metal. If a distillation tower uses packing instead of trays, the number of necessary theoretical equilibrium stages is first determined and then the packing height equivalent to a theoretical equilibrium stage (known as the HETP) is also determined. The total packing height required is the number theoretical stages multiplied by the HETP.

Liquids tend to wet the surface of the packing and the vapors pass across this wetted surface, where mass transfer takes place. Unlike conventional tray distillation in which every tray represents a separate point of vapor-liquid equilibrium, the vapor-liquid equilibrium curve in a packed column is continuous. However, when modeling packed columns it is useful to compute a number of theoretical plates to denote the separation efficiency of the packed column with respect to more traditional trays. Differently shaped packings have different surface areas and void space between packings. Both of these factors affect packing performance.

[edit] Overhead system arrangements

Images 2 and 3 assume an overhead stream that is totally condensed into a liquid product using water or air-cooling. However, in many cases, the tower overhead is not easily condensed totally and the reflux drum must include a vent gas outlet stream. In yet other cases, the overhead stream may also contain water vapor because either the feed stream contains some water or some steam is injected into the distillation tower (which is the case in the crude oil distillation towers in oil refineries). In those cases, if the distillate product is insoluble in water, the reflux drum may contain a condensed liquid distillate phase, a condensed water phase and a non-condensible gas phase, which makes it necessary that the reflux drum also have a water outlet stream.

[edit] Examples

[edit] Continuous distillation of crude oil

Petroleum crude oils contain hundreds or more different hydrocarbon compounds: paraffins, naphthenes and aromatics as well as organic sulfur compounds, organic nitrogen compounds and some oxygen containing hydrocarbons such as phenols. Although crude oils generally do not contain olefins, they are formed in many of the processes used in a petroleum refinery.<ref> Gary, J.H. and Handwerk, G.E. (1984). Petroleum Refining Technology and Economics, 2nd Edition, Marcel Dekker, Inc.. ISBN 0-8247-7150-8.</ref>

The crude oil fractionator does not produce products having a single boiling point, rather, it produces fractions having boiling ranges. For example, the crude oil fractionator produces an overhead fraction called "naphtha" which will become a gasoline component after it is further processed through a catalytic hydrodesulfurizer to remove sulfur and a catalytic reformer to reform its straight-chain hydrocarbon molecules into more complex molecules with a higher octane rating value.

The naphtha "cut", as that fraction is called, has very many different hydrocarbon compounds. Therefore it has an "initial" boiling point of about 35°C and a "final" boiling point of about 200°C ... that is what is meant by the "boiling range" of each "cut" produced in the fractionating columns. At some distance below the overhead, the next "cut" is withdrawn from the side of the column and it is usually the jet fuel cut also known as a kerosene cut. It also contains very many different hydrocarbons and the boiling range of that cut is from an initial boiling point of about 150°C to a final boiling point of about 270°C. The next cut further down the tower is the diesel oil cut with a boiling range from about 180°F to about 315°C. Note the overlap of boiling range between any cut and the next cut because the distillation separations are not perfectly sharp.

Then there are the heavy fuel oil cuts further down the column with even more different hydrocarbons and a wider boiling range. Finally there is the bottoms product with a great many more hydrocarbons and a very wide boiling range. All these cuts are processed further in subsequent refinng processes.

[edit] Naphtha debutanizer

The naphtha cut from the crude oil distillation may be further distilled in a "naphtha debutanizer" as may also be done with the naphthas produced in many of the other process units in a refinery.

All of these naphthas need to have their butane content content reduced to avoid the gasoline made from the naphthas having too high a vapor pressure (which causes "vapor lock" in automotive engines). The name debutanizer sounds as if the debutanizers simply separate butane from naphtha, but they are designed to remove any normal butane, isobutane, normal and isobutylenes, propane, propylene, ethane, ethylene, methane and even some hydrogen from the various raw naphthas.

It should be noted that naphtha debutanizer are also sometimes referred to as naphtha "stabilizers".

[edit] See also

• Azeotropic distillation
• Extractive distillation
• Fractional distillation
• Fractionating column
• Steam distillation
• Vacuum distillation

[edit] External links

• Distillation Theory by Ivar J. Halvorsen and Sigurd Skogestad, Norwegian University of Science and Technology, Norway
• Distillation, An Introduction by Ming Tham, Newcastle University, UK
• Distillation by the Distillation Group, USA

[edit] References

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