Everything about Continuous Distillation totally explained
Continuous distillation, a form of
distillation, is an ongoing separation in which a mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams as time passes during the operation. A distillation is the
separation or partial separation of a
liquid feed mixture into components or fractions by selective
boiling (or
evaporation) and
condensation. A distillation produces at least two output fractions. These fractions include at least one
volatile distillate fraction, which has boiled and been separately captured as a vapor condensed to a liquid, and practically always a bottoms (or residuum) fraction, which is the least volatile residue that hasn't been separately captured as a condensed vapor.
An alternative to continuous distillation is
batch distillation, where the mixture is added to the unit at the start of the distillation, distillate fractions are taken out sequentially in time (one after another) during the distillation, and the remaining bottoms fraction is removed at the end. Because each of the distillate fractions are taken out at different times, only one distillate exit point (location) is needed for a batch distillation and the distillate can just be switched to a different receiver, a fraction-collecting container. Batch distillation is often used when smaller quantities are distilled. In a continuous distillation, each of the fraction streams is taken simultaneously throughout operation; therefore, a separate exit point is needed for each fraction. In practice when there are multiple distillate fractions, each of the distillate exit points are located at different heights on a
fractionating column. The bottoms fraction can be taken from the bottom of the distillation column or unit, but is often taken from a
reboiler connected to the bottom of the column.
Each fraction may contain one or more components (types of
chemical compounds). When distilling
crude oil or a similar feedstock, each fraction contains many components of similar volatility and other properties. Although it's possible to run a small-scale or laboratory continuous distillation, most often continuous distillation is used in a large-scale industrial process.
Industrial application
Distillation is one of the
unit operations of
chemical engineering. Continuous distillation is used widely in the chemical process industries where large quantities of liquids have to be distilled. Such industries are the
natural gas processing,
petrochemical production,
coal tar processing,
brewing,
liquified air separation,
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, which are called
fractions. These
fractions are the origin of the term
fractional distillation or
fractionation. It is often not worthwhile separating the components in these fractions any further based on product requirements and economics.
Industrial distillation is typically performed in large, vertical cylindrical columns (as shown in images 1 and 2) known as "distillation towers" or "distillation columns" with diameters ranging from about 65 centimeters to 11 meters and heights ranging from about 6 meters to 60 meters or more.
Principle
The principle for continuous distillation is the same as for normal distillation: when a liquid mixture is heated so that it boils, the composition of the vapor above the liquid differs from the liquid composition. If this vapor is then separated and
condensed into a liquid, it becomes richer in the lower boiling component(s) of the original mixture.
This is what happens in a continuous distillation column. A mixture is heated up, and routed into the distillation column. On entering the column, the feed starts flowing down but part of it, the component(s) with lower boiling point(s), vaporizes and rises. However, as it rises, it cools and while part of it continues up as vapor, some of it (enriched in the less
volatile component) begins to descend again.
Image 3 depicts a simple continuous fractional distillation tower for separating a feed stream into two fractions, an overhead distillate product and a bottoms product. The "lightest" products (those with the lowest boiling point or highest volatility) exit from the top of the columns and the "heaviest" products (the bottoms, those with the highest boiling point) exit from the bottom of the column. 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 continuous distillation, the system is kept in a
steady state or approximate steady state. Steady state means that quantities related to the process don't change as time passes during operation. Such constant quantities include feed input rate, output stream rates, heating and cooling rates,
reflux ratio, and
temperatures, pressures, and compositions at every point (location). Unless the process is disturbed due to changes in feed, heating, ambient temperature, or condensing, steady state is normally maintained. This is also the main attraction of continuous distillation, apart from the minimum amount of (easily instrumentable) surveillance; if the feed rate and feed composition are kept constant, product rate and
quality are also constant. Even when a variation in conditions occurs, modern
process control methods are commonly able to gradually return the continuous process to another steady state again.
Since a continuous distillation unit is fed constantly with a feed mixture and not filled all at once like a batch distillation, a continuous distillation unit doesn't need a sizable distillation pot, vessel, or reservoir for a batch fill. Instead, the mixture can be fed directly into the column, where the actual separation occurs. The height of the feed point along the column can vary on the situation and is designed so as to provide optimal results. See
McCabe-Thiele method.
A continuous distillation is often a
fractional distillation and can be a
vacuum distillation or a
steam distillation.
Design and operation
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 or the
Fenske equation Each of these plates or trays 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. Some example trays are depicted in image 5. A more detailed, expanded image of two trays can be seen in the
theoretical plate article. The reboiler often acts as an additional equilibrium stage.
If each physical tray or plate were 100% efficient, than the number of physical trays needed for a given separation would equal the number of equilibrium stages or theoretical plates. However, that's very seldom the case. Hence, a distillation column needs more plates than the required number of theoretical vapor-liquid equilibrium stages.
Fractionation Research, Inc. (commonly known as FRI) has performed research on all types of trays measuring their capacity, pressure drop and efficiency in hydrocarbon systems from full vacuum to 500 psia.
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 lower pressure drop across the column (when compared to
plates or trays), beneficial when operating under vacuum. 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
height equivalent to a theoretical plate (HETP), is also determined. The total packing height required is the number theoretical stages multiplied by the HETP.
This packing material can either be random dumped packing such as
Raschig rings or
structured sheet metal. 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's 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.
Another factor in addition to the packing shape and surface area that affects the performance of random or structured packing is liquid and vapor distribution entering the packed bed. The number of
theoretical stages required to make a given separation is calculated using a specific vapor to liquid ratio. If the liquid and vapor are not evenly distributed across the superficial tower area as it enters the packed bed, the liquid to vapor ratio won't be correct in the packed bed and the required separation won't be achieved. The packing will appear to not be working properly. The
height equivalent to a theoretical plate (HETP) will be greater than expected. The problem isn't the packing itself but the mal-distribution of the fluids entering the packed bed. Liquid mal-distribution is more frequently the problem than vapor. The design of the liquid distributors used to introduce the feed and reflux to a packed bed is critical to making the packing perform at maximum efficiency. Methods of evaluating the effectiveness of a liquid distributor can be found in references.. Considerable work as been done on this topic by Fractionation Research, Inc.
Overhead system arrangements
Images 4 and 5 assume an overhead stream that's totally condensed into a
liquid product using water or air-cooling. However, in many cases, the tower overhead isn't 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.
Examples
Continuous distillation of crude oil
Petroleum crude oils contain hundreds or more different
hydrocarbon compounds:
paraffins,
naphthenes and
aromatics as well as, and some
oxygen containing hydrocarbons such as
phenols. Although crude oils generally don't contain
olefins, they're formed in many of the processes used in a petroleum refinery.
The crude oil fractionator doesn't 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's further processed through a catalytic
hydrodesulfurizer to remove
sulfur and a
catalytic reformer to
reform its 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's 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's 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°C 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.
After these come the heavy fuel oil cuts and finally the bottoms product, with very wide boiling ranges. All these cuts are processed further in subsequent refining processes.
Further Information
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