Alchohol Distillation Principles ...

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AE-117
Purdue University
Cooperative Extension Service
West Lafayette, IN 47907
ALCOHOL DISTILLATION: BASIC PRINCIPLES, EQUIPMENT,
PERFORMANCE RELATIONSHIPS, AND SAFETY
Eric Kvaalen, Doctoral Student in Chemical Engineering
Philip C. Wankat, Professor of Chemical Engineering
Bruce A. McKenzie, Extension Agricultural Engineer,
Purdue University
The purpose of this publication is to help you understand the distillation of ethyl alcohol. It first presents the
basic principles involved in distillation and how the process works. The types of distillation equipment and
systems that might be involved in a small fuel alcohol plant are then discussed, as are the performance and
control criteria needed for a general evaluation of each. The publication concludes with a discussion of safety,
along with some general selection, operation and management criteria useful in evaluating alternatives.
The information presented here hopefully will help you decide if you want to get into alcohol production, and if
so, will help you evaluate the different options that are available to you. We will only cover those distillation
processes and equipment capable of producing alcohol concentrations up to about 95.6 weight percent (wet
basis).
Remember, this publication is
not
a design manual. Rather its goal is to give a general understanding of
distillation processes and the performance of various equipment options in order to aid you in evaluating alcohol
production proposals and give a basis for more detailed self-study. We will not discuss fermentation processes
and equipment, or uses of the finished alcohol concentrate.
ETHYL ALCOHOL--A VIABLE ALTERNATIVE FUEL
The idea of ethyl alcohol as a liquid fuel is not new. It received considerable discussion and publicity in the
1920's and 1930's as a motor fuel. It was used as a fuel in several countries during World War II. Interest
surfaced again in the U.S. in the mid 1970's, with the advent of the oil embargo and cartel and the rapidly
escalating oil prices that resulted.
At the time of these rapid oil price increases, many people, particularly in the farming community, began to look
seriously at ethyl alcohol and gasoline/alcohol blends as alternative fuels. However, by the early 1980's,
increased U.S. oil production plus a significant drop in oil consumption due to high prices brought a
corresponding world oversupply of oil and a marked drop in oil and gasoline prices. As a result, interest in
alcohol fuels diminished sharply. Interestingly, the increased use of unleaded fuels and subsidies for fuels using
10 percent alcohol caused many oil companies to add ethyl alcohol to their gasoline as a non-lead octane
improvement additive. Such fuels are not normally advertised as gasoline/alcohol blends.
If one accepts, however, that the long range price of oil and energy will continue to increase, then ethyl alcohol
as a liquid fuel, especially for internal combustion spark ignition engines, will continue to be a potentially viable
alternative fuel source. The fact that alcohol may be profitably manufactured from a variety of crop and forest
residues, as well as from grains themselves, enhances its appeal to farm producers.
Ethyl Alcohol from "Beer"
Alcohol can be made from a variety of agricultural products by a three basic step sequence:
1. Breaking down the feed-stock (the raw material) chemically by a process which may involve cooking
and adding enzymes.
2. Fermentating, i.e., the action of micro-organisms (usually yeast) to produce a "beer" (The term "beer"
describes the liquid traction of a fermented mixture of water and ground or crushed grain that is usually
no more than 10-12% alcohol, hence the similarity of the process and the final alcohol content to that of
domestic beer.) containing a small percentage of alcohol, along with the remains of the feedstock, the
yeast cells and various other substances dissolved in water.
3. Separating the alcohol from the water and other components in the beer, usually by distillation, to
obtain the alcohol in a pure enough form to be used as fuel.
Fermenting grain (cooking it in water and treating it with enzymes to break down the starch and convert it to
sugars) results in an alcohol concentration of roughly 5-10 percent. The finished concentration or "beer"
depends on the amount of water used, the grain and the quality of the fermentation. This beer is so low in
alcohol content that it is useless as a fuel and must be further concentrated to obtain mixtures that will ignite and
burn. For this reason a distillation column is used to produce a higher alcohol concentration. (Several
publications that discuss fermentation in considerable detail are listed at the end of this publication under
"References.")
DISTILLATION--HOW IT WORKS
First of all, let's look at how distillation works. We are all generally familiar with how distilled water is
produced. The water is heated, and the steam or water vapor conducted away in a tube. If the tube is looped
downward and cooling is applied below the hump, the vapor is condensed and distilled water obtained. This is
"simple" distillation- i.e., removing a volatile substance (water) from non-volatile substances (lime, impurities,
etc.).
"Fractional" distillation is used to separate mixtures of two liquids with different boiling points, such as alcohol
and water. Ethyl alcohol with 4 percent water boils at approximately 173° F, while water boils at 212° F. A
mixture of the two liquids will boil at all temperatures between 173° and 212°, depending on the ratio of alcohol
to water.
Consider a beaker or a glass jug filled partially with a mixture of alcohol and water at some temperature. The
top of the container is closed except for a small hole, to which a balloon is attached to keep air out. Thus, the
vessel is at atmospheric pressure, but the enclosure above the liquid level is essentially undisturbed by air
currents circulating around the jug.
After a period of time, the amount of water vapor and amount of alcohol vapor contained in the gaseous mixture
above the liquid in the container will reach a constant value, depending on the temperature and pressure. The
liquid and vapor mixtures reach an "equilibrium," a condition under which there is no net change in the liquid/
vapor ratio or in the alcohol/water ratio within either the liquid or vapor mixture. However, the ratio of alcohol
to water in the vapor phase is generally greater than the ratio in the liquid phase, because alcohol is usually more
volatile than water (see Figure 1). It is this characteristic of a liquid-versus-vapor state of a substance that
permits us to distill off an increasing concentration of alcohol from the alcohol/water mixture.
By bringing about a controlled series of successive sequences re-evaporation, condensation, re-evaporation and
re-condensation), each re-condensation from the previous vapor state achieves a higher alcohol concentration.
This is because the alcohol in the vapor is at a higher concentration than was the concentration in the liquid
mixture from which it was vaporized.
Figure 1 shows the vapor-versus-liquid composition when the pressure is atmospheric. The dotted line in the
figure represents an equal concentration of alcohol in both the liquid and the vapor state. Note that the alcohol
concentration is consistently higher in the vapor phase than in the liquid phase for most of the range of the
graph. The axes are explained later.
Figure 1. Equilibrium relationship between gaseous and liquid alcohol-water mixtures
(atmospheric pressure).
Azeotropic Mixtures
The previous relationships of alcohol-water mixtures hold true up to alcohol concentrations of about 95.6
percent. At this concentration, the two substances quit boiling separately (i.e., the alcohol in the vapor phase is
no longer more concentrated than in the liquid phase), and fractional distillation no longer works. A mixture of
this composition is called an "azeotropic mixture".
Generally, a third substance must be introduced into the mixture to permit separation by distillation, or some
other separation scheme must be used. The details of separating the azeotrope are discussed briefly later.
Types of Distillation Processes Most Applicable to the Farm
There are two general types of distillation processes that appear applicable to farm-size fuel alcohol production
with present technology. One is the
continuous-feed distillation column system
, in which a beer containing a
constant alcohol content is continuously pumped into a column. The other is a
pot-type distillation system
, in
which a batch of beer, with the heavy solids (spent grain) not removed, is simply boiled in place to vaporize the
alcohol. The alcohol-water vapors are then forced to flow through a distillation column to bring about
concentration.
These two processes are discussed in detail in the following pages. There are other fractional distillation systems
that may or may not use a column as we normally think of such units. They include centrifugal techniques,
mechanical rotating wipers in a tube, etc., and are not discussed here.
CONTINUOUS -FEED DISTILLATION COLUMN PROCESS
A simplified schematic of a continuous distillation column is presented in Figure 2. The column consists of a
long tube, which includes a stripping section (the lower portion) and a rectifying section (the upper portion).
There is a condenser located on the top end of the column and an optional reboiler on the bottom.
Figure 2. A continuous distillation process.
The process involves a controlled flow of liquid beer (preferably preheated and with all solids removed), which
is fed into the top of the stripping portion of the column. The liquid alcohol-water mixture (beer) trickles
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