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Distillation

Introduction

Distillation separates chemicals by the difference in how easily they vaporize. The two major types of classical distillation include continuous distillation and batch distillation.Continuous distillation, as the name says, continuously takes a feed and separates it into two or more products. Batch distillation takes on lot (or batch) at a time of feed and splits it into products by selectively removing the more volatile fractions over time.

Other ways to categorize distillation are by the equipment type (trays, packing), process configuration (distillation, absorption, stripping, azeotropic, extractive, complex), or process type (refining, petrochemical, chemical, gas treating).

In all cases, what must be kept in mind is that distillation involves both equipment and theory. Sound analysis with basic principles underlies any successful distillation process. However, putting basics into practice requires real equipment. Process design tells us what equipment needs to accomplish to meet our plant goals. Equipment limits set what a specific unit can achieve. Putting successful distillation units in place requires combining both the theoretical knowledge of the distillation fundamentals along with equipment understanding. The Distillation Group puts both of these areas together to work in your process. We approach troubleshooting, equipment design, process analysis, and revamps by combining knowledge of fundamentals and of how equipment really works. This gives reliable results and effective (and profitable) plant operation.

The following information gives a short background to the rest of the distillation information in this site.

Historical Background

Distillation has been around for a long time. Earliest references are to Maria the Jewess who invented many types of stills and reflux condensers.Common Middle Ages and Renaissance uses of distillation included the manufacture of brandy and other spirits from wine. Another early use was the manufacture of perfumes and essences. Other early users of distillation include the Alchemists. Of course, the history of distillation does not end there. Today we use it for more than just spirits.

Many industries use distillation for critical separations in making useful products. These industries include petroleum refining, beverages, chemical processing, petrochemicals, and natural gas processing.

The beverage industy is the one of the oldest users of distillation. Distillation of ethanol for both consumption and other uses was one of the first major industries ever developed. Ethanol has often been considered as a fuel. At times, this has even been done. F. B. Wright published a major work on production and distillation of fuel ethanol in 1906. A copy of the second edition (1907) from the DGI collection of historical material can be viewed or downloaded. This is a large document (293 pages, 2.43M). A smaller version using low resolution graphics is also available (1.47M).

Natural gas processing started using distillation in the early 1900's. An interesting historical document, Condensation of Gasoline from Natural Gas, documents some early steps in this industry.

Recent developments energy shortages have re-focused attention on major industrial energy users. Distillation is a major energy consumer. During the energy 'crisis' of the 1970s much effort was put into making distillation more efficient. A good example of this work is summarized in the Distillation Operations Manual from the Texas Industrial Commission.

Distillation Today

Distillation Categories

Distillation services can be sorted out into many different categories. Here are some basic definitions:

System composition

System refers to the chemical components present in the mixture being distilled. The two main groups are binary distillation and multicomponent distillation.

Binary distillation is a separation of only two chemicals. A good example is separating ethyl alcohol (ethanol) form water. Most of the basic distillation teaching and a lot of theoretical work starts with looking at binary distillation; it's a lot simpler.

Multicomponent distillation is the separation of a mixture of chemicals. A good example is petroleum refining. Crude oil is a very complex mixture of hydrocarbons with literally thousands of different molecules. Nearly all commercial distillation is multicomponent distillation. The theory and practice of multicomponent distillation can be very complex.

Processing Mode

Processing mode refers to the way in which feed and product are introduced and withdrawn from the process. Distillation occurs in two modes, continuous distillation and batch distillation.

Continuous distillation is feed is sent to the still all the time and product is drawn out at the same time. The idea in continuous distillation is that the amount going into the still and the amount leaving the still should always equal each other at any given point in time.

Batch distillation is when the amount going into the still and the amount going out of the still is not supposed to be the same all the time. The easiest example to use is like old fashioned spirit making. The distiller fills a container at the start, then heats it, as time goes by the vapors are condensed to make the alcoholic drink. When the proper quantity of overhead (drink) is made, the distiller stops the still and empties it out ready for a new batch. This is only a simple case, in industrial usage what goes on gets very complex.

Both continuous and batch distillation are very important to industry. Continuous distillation is most often used with big volume products like jet fuel, benzene, plastic monomers. Batch distillation is most often used with smaller volume products and in plants that make lots of different things and use the same still for many products (in different batches).

Processing sequence

Fractionation systems have different objectives. The major processing objectives set the system type and the equipment configuration needed. The common objectives include removing a light component from a heavy product, removing a heavy component from a light product, making two products, or making more than two products. We will call these major categories are called stripping, rectification, fractionation, and complex fractionation.

This terminology may be a little confusing because we also use the terms stripping and fractionation when we discuss heat flow options through the unit. This confusion results from historical use of the terms and you just need to keep the context in mind when reading or discussing the material. With a little practice you will find that the reason for using the same terms is that many of the systems called stripping or fractionation systems have the same characteristics regardless of using a processing sequence or heat flow analysis of the unit.

Stripping systems remove light material from a heavy product.

Rectification systems remove heavy material from a light product.

Fractionation systems remove a light material from a heavy product and a heavy material from a light product at the same time.

Complex fractionation makes multiple products from either a single tower or a complex of towers combined with recycle streams between them. A good example of a multiple product tower is a refinery crude distillation tower making rough cuts of naphtha (gasoline), kerosene (jet fuel), and diesel from the same tower. A good example of a complex tower with internal recycle streams is a Petlyck (baffle) tower making three on-specifications products from the same tower.

System type

The behavior of the chemicals in the system also determines the system configuration for the objectives. The three major problems that limit distillation processes are close-boilers, distributed keys, and azeotropes. Other problems that may require using special system configurations include heat sensitive materials.

Close boiler systems include chemicals that boil at temperatures very close to each other. So many stages of distillation or so much reflux may be required that the chemicals cannot be separated economically. A good example is separation of nitro-chloro-benzenes. Up to 600 theoretical separation stages with high reflux may be required to separate different isomers.

Distributed keys are systems where some chemicals that we do not want in either the heavy or the light product boil at a temperature between the heavy and the light product.

Azeotropic systems are those where the vapor and the liquid reach the same composition at some point in the distillation. No further separation can occur. Ethanol-water is a perfect example. Once ethanol composition reaches 95% (at atmospheric pressure), no further ethanol purification is possible.

Close boilers and distributed keys are economic problems. The compounds can be separated, but it costs a lot. Azeotropic systems are fundamental thermodynamic problems. At the distillation conditions, the products can only be distilled to a certain point, no further.

Different ways to get around these problems include using other techniques (membranes, crystallization, adsorption, adduction, extraction, precipitation), using complex distillation configurations, changing system conditions, or adding extra chemicals to the process. Adding extra chemicals includes azeotropic distillation, extractive distillation, or salt distillation.

System type

Azeotropic and extractive distillation use the addition of a mass separating agent (MSA) to modify the thermodynamic behavior of the system. Many different azeotropic and extractice distillation configurations are in use.

Azeotropic distillation uses a MSA that forms a minimum boiling azeotrope with some of the feed components is used. The azeotrope is taken overhead and the MSA rich phase decanted and returned to the column as reflux.

Extractive distillation uses a MSA that increases the volatility difference between the compounds to be separated. A good example is sulfolane to increase the relative volatility difference between similar molecular weight aromatic and paraffinic hydrocarbons. The sulfolane unit combines liquid-liquid extraction, extractive distillation, and solvent stripping in one process.

Salt distillation adds a salt to the system to modify the thermodynamic behavior of the system. The salt is normally added to the liquid supply of a batch distillation system.

All of these types of systems are normally considered complex systems. Other equipment is needed to separate and reuse the added MSA. Very complex configurations can result. Good understanding of system thermodynamics is required to predict behavior.

Heat flow

Energy transfer is required to make separations work. Heat flow refers to the arrangement of the distillation column to its heat source and heat sink. The major categories are fractionation (distillation), absorption, stripping, and contacting.

This terminology may be a little confusing because we also use the terms stripping and fractionation when we discuss processing sequence options in distillation. This confusion results from historical use of the terms and you just need to keep the context in mind when reading or discussing the material. With a little practice you will find that the reason for using the same terms is that many of the systems called stripping or fractionation systems have the same characteristics regardless of using a processing sequence or heat flow analysis of the unit.

Fractionation refers to units that have both a reboiler and a condenser. Something is attached to the bottom of the tower to put heat into the tower and something attached to the top of the tower to take heat out of the tower.This is what is normally called distillation

Absorption is a unit that has no method at the top of the tower to take heat out. An external stream is supplied from outside the system to absorb material from the vapor.

Stripping is a unit that has no method at the bottom of the tower to put heat in. An external stream is supplied from outside the system to strip material from the liquid.

Contacting is a unit that has neither a method at the top of the tower to remove heat nor a method at the bottom of the tower to put heat in. Two streams run countercurrent to each other. Both streams are generated outside the mass-transfer system.

What can make things unclear is that these terms have both other meanings and can be used imprecisely. Also, towers can have intermediate heat input and heat removal equipment in the middle. This confuses the picture. But we will use the strict definitions above. An absorber is a tower without a condenser. A stripper is a tower without a reboiler. A contactor has neither and a fractionator has both.

Reaction

Reactive distillation uses a reaction in the distillation equipment to help the separation. The reaction may or may not use a catalyst. DMT manufacture uses reactive distillation without a catalyst. One process to make methy-tert-butyl-ether uses a catalyst inside the distillation tower. The reaction changes the composition, allowing the distillation to work better.

Equipment Type

Distillation equipment includes two major categories, trays and packing.

Trays force a rising vapor to bubble through a pool of descending liquid.

Packing creates a surface for liquid to spread on. The thin liquid film has a high surface area for mass-transfer between the liquid and vapor.

Photograph Gallery

Distillation units come in many different configurations. The configuration depends upon the service, capital versus operating cost optimization, and technology available when the unit is constructed. To see a selection of different units visit our photo gallery.

References

Condensation of Gasoline from Natural Gas

Single file version in HTML

Distillation and De-naturing

Single file version with low resolutions graphics in Adobe PDF.

Single file version with high resolution graphics in Adobe PDF.

Distillation Operations Manual

Multiple file version (for slow connections) in HTML.

Single file version (for fast connections) in HTML.

Single file version in PDF for viewing or download.

 

This page updated 11 January 2002.
© 2001 The Distillation Group, Inc. All rights reserved.