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.
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