What is distillation, where is applied, the process description

What is distillation? This is the process of turning the liquid into vapor, which then condenses again into a liquid form. The simplest example is the distillation of water, when the steam from the kettle precipitates in the form of drops on a cold surface.

Application and history

Distillation is used to separate liquids from non-volatile solids, such as distilling alcohol from fermented materials, or to separate two or more liquids with different boiling points, such as the production of gasoline, kerosene and lubricating oils from oil. Other industrial applications include processing of chemical products such as formaldehyde and phenol, desalination of sea water.

The process of distillation was probably used by ancient experimenters. Aristotle (384-322 BC) mentioned that pure water can be obtained by evaporation of the sea. Pliny the Elder (23-79 AD) described a primitive condensation method in which the oil obtained by heating rosin is collected on the wool placed in the upper part of the distillation cube.

Simple distillation

Most distillation methods used in industry and laboratory studies are variations of simple distillation. This basic technology uses a cube or retort in which the liquid is heated, a condenser for vapor cooling, and a container for collecting the distillate. When the mixture of substances is heated, the most volatile of them, or the one whose boiling point is minimal, is distilled first, and then others are distilled or not completely distilled. Such a simple apparatus is perfectly suitable for purifying a liquid containing non-volatile components, and is effective enough to separate substances with different boiling points. For laboratory use, parts of the apparatus are usually made of glass and are connected by plugs, rubber hoses or glass tubes. On an industrial scale, the equipment is made of metal or ceramics.

Fractional distillation

The method, called fractional, or differential, distillation, was developed for refining, because simple distillation for the separation of liquids, the boiling point of which differs little, is ineffective. In this case, the vapor is repeatedly condensed and evaporated in an isolated vertical vessel. A special role here is played by dry-cells, fractional columns and condensers, which allow to return some of the condensate back to the cube. The goal is to achieve close contact between the rising different phases of the mixture so that only the most volatile fractions in the form of vapor reach the receiver, and the rest is returned as liquid to the side of the cube. Purification of volatile components as a result of contact between such counterflows is called rectification, or enrichment.

Multiple distillation

This method is also called multi-stage flash evaporation. This is another type of simple distillation. With its help, for example, water is distilled at large commercial desalination plants. The transformation of the liquid into steam does not require heating. It simply comes from a container with high atmospheric pressure into a container with a lower one. This leads to rapid evaporation, accompanied by the condensation of steam into the liquid.

Vacuum distillation

In one of the versions of the process with reduced pressure, a vacuum pump is used to create a vacuum. This method, called "vacuum distillation," is sometimes used when working with substances that usually boil at high temperatures or decompose on boiling under normal conditions.

Vacuum pumps create a pressure in the column, which is much lower than atmospheric pressure. In addition to them, vacuum regulators are used. Careful monitoring of the parameters is very important, since the efficiency of separation depends on the difference in relative volatility at a given temperature and pressure. Change of this parameter can negatively affect the course of the process.

What is distillation in a vacuum is well known in oil refineries. Conventional distillation methods separate light hydrocarbons and impurities from heavy hydrocarbons. The residual product is subjected to vacuum distillation. This allows high-boiling hydrocarbons, such as oils and waxes, to be separated at low temperatures. The method is also used for the separation of heat sensitive organic chemical compounds and for the recovery of organic solvents.

What is steam distillation?

Steam distillation is an alternative distillation method at temperatures below the normal boiling point. It is used when the distilled substance is not mixed and does not chemically react with water. Examples of such materials are fatty acids and soybean oil. During the distillation, steam is supplied to the liquid, which heats it and causes evaporation.

Distillation in the packed column

Although packed columns are most often used for absorption, they are also used for distilling vapor-liquid mixtures. This design provides a large area of the contact surface, which increases the efficiency of the system. Another name for this design is the rectification column.

The principle of operation is as follows. The raw mix of components with different volatility is fed to the center of the column. The liquid flows down through the nozzle, and the steam moves upward. The mixture in the bottom of the tank gets into the heater and leaves it together with the steam. Gas rushes up through the nozzle, picking up the most volatile components of the liquid, exits the column and enters the condenser. After liquefaction, the product enters a collection of reflux, where it is separated into a distillate and a fraction used for irrigation.

A different concentration leads to the fact that less volatile components pass from the vapor phase to the liquid phase. The nozzle increases the duration and area of contact, which increases separation efficiency. At the outlet, the vapor contains the maximum amount of volatile components, while in the liquid their concentration is minimal.

Nozzles are filled in bulk and in packs. The filler shape can be either random or geometrically structured. It is made of an inert material, such as clay, porcelain, plastic, ceramic, metal or graphite. The filler, as a rule, has dimensions from 3 to 75 mm and has a large surface area in contact with the vapor-liquid mixture. The advantage of filling in the bulk is large flow capacity, resistance to high pressures and low cost.

Metal fillers have high strength and good wettability. Ceramic have an even higher wettability, but they are not so strong. Plastic are strong enough, but poorly wetted at low flow rates. Because ceramic fillers are resistant to corrosion, they are used at elevated temperatures that plastic can not withstand.

Batch nozzles are a structured mesh, the dimensions of which correspond to the diameter of the column. Provide the presence of long channels for fluid and vapor flows. They are more expensive, but they can reduce pressure drops. Batch attachments are preferred at low flow rates and under low pressure conditions. They are usually made of wood, sheet metal or woven mesh.

They are used for the recovery of solvents and in the petrochemical industry.

Distillation in the distillation column

The most widely used columns are plate type. The number of plates depends on the desired purity and complexity of the separation. It affects the height of the distillation column.

The principle of its work is the following. The mixture is fed in the middle of the height of the column. The difference in concentration leads to the fact that less volatile components pass from the stream of vapor into the flow of liquid. The gas leaving the condenser contains the most volatile substances, while the less evaporated gases exit through the heater into the liquid flow.

The geometry of the plates in the column affects the degree and type of contact between the different phase states of the mixture. Structurally, they are made with sieve, valve, cap, lattice, cascade, etc. Sieve trays, in which there are holes for steam, are used to ensure high productivity at low cost. The cheaper valve plates, in which the openings are provided with opening and closing valves, are prone to clogging due to the accumulation of material thereon. Caps are equipped with caps, allowing the vapor to pass through the liquid through the tiny holes. This is the most advanced and expensive technology, effective at low flow rates. The liquid flows from one plate to the other down the drainage vertical pipes.

Belleville columns are often used to restore solvents from process waste. They are also used for the reduction of methanol during the drying operation. As a liquid product comes out water, and the volatile organic waste passes into the vapor phase. That's what distillation is in the distillation column.

Cryogenic distillation

Cryogenic distillation consists in applying general distillation methods to gases cooled to a liquid state. The system operates at temperatures below -150 ° C. For this, heat exchangers and coils are used. The whole structure is called a cryogenic block. Liquefied gases enter the block and are distilled at very low temperatures. Columns of cryogenic distillation can be packed and packed. Batch design is more preferable, since the bulk material is less efficient at low temperatures.

One of the main applications of cryogenic distillation is the separation of air into its constituent gases.

Extractive distillation

Extractive rectification uses additional compounds that act as a solvent to change the relative volatility of one of the components of the mixture. A solvent is added to the extractive column to the substances to be separated. The component of the feed stream to be recovered is connected to the solvent and comes out in the liquid phase. The other component evaporates and goes to the distillate. The second distillation in the other column allows the substance to be separated from the solvent, which then returns to the previous step to repeat the cycle.

Extractive rectification is used to separate compounds with similar boiling points and azeotropic mixtures. Extractive rectification is not as widespread in industry as conventional distillation, due to the complexity of the design. An example is the process for obtaining cellulose. The organic solvent separates the cellulose from the lignin, and the second distillation yields a pure substance.