What is distillation? This is the process of converting a liquid into steam, which then condenses again into liquid form. The simplest example is water distillation, when steam from a kettle is deposited in the form of drops on a cold surface.
Application and History
Distillation is used to separate liquids from non-volatile solids, as in the distillation of alcoholic beverages from fermented materials, or to separate two or more liquids with different boiling points, as in the production of gasoline, kerosene and lubricating oils from petroleum. Other industrial applications include the processing of chemicals such as formaldehyde and phenol, desalination of sea water.
The distillation process was probably used by ancient experimenters. Aristotle (384-322 BC) mentioned that clean 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 wool placed in the upper part of the distillation cube.
Simple distillation
Most distillation methods used in industry and laboratory research are variations of simple distillation. This basic technology uses a cube or retort in which the liquid is heated, a condenser to cool the steam, and a distillate collection tank. When a mixture of substances is heated, the most volatile of them is distilled in the first place or the one at which the boiling point is minimal, and then others are distilled, or not at all. Such a simple apparatus is perfect for cleaning liquids 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 connected with corks, rubber hoses or glass tubes. On an industrial scale, equipment is made of metal or ceramic.
Fractional distillation
The method, called fractional, or differential, distillation, was developed for oil refining because simply distilling to separate liquids whose boiling points differ little is inefficient. In this case, the vapor condenses many times and evaporates in an isolated vertical container. A special role here is played by dry steam vessels, fractional columns and condensers, which allow returning 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 in the form of liquid towards the cube. The purification of volatile components as a result of contact between such countercurrents 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, distillation of water is carried out at large commercial desalination plants. The conversion of liquid to steam does not require heating. It simply falls from a tank with high atmospheric pressure into a tank with a lower pressure. This leads to rapid evaporation, accompanied by condensation of the vapor in the liquid.
Vacuum distillation
In one type of reduced pressure process, 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 when boiled under normal conditions.
Vacuum pumps create pressure in the column, which is much lower than atmospheric pressure. In addition to them, vacuum regulators are used. Careful control of the parameters is very important, since the separation efficiency depends on the difference in relative volatility at a given temperature and pressure. Changing this parameter can negatively affect the process.
What is vacuum distillation 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 the separation of high boiling hydrocarbons, such as oils and waxes, at low temperatures. The method is also used in 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 does not mix 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.
Packed column distillation
Although packed columns are most often used for absorption, they are also used for the distillation of vapor-liquid mixtures. This design provides a large contact surface area, which increases the efficiency of the system. Another name for this design is a distillation column.
The principle of operation is as follows. A raw mix of components with different volatility is fed to the center of the column. The liquid flows down through the nozzle, and the vapor moves up. The mixture at the bottom of the tank enters the heater and exits with the steam. Gas rushes up through the nozzle, picking up the most volatile components of the liquid, leaves the column and enters the condenser. After liquefaction, the product enters the phlegmy collector, where it is separated into distillate and the fraction used for irrigation.
Different concentrations lead to the fact that less volatile components pass from the vapor phase to the liquid. The nozzle increases the duration and contact area, which increases the separation efficiency. At the outlet, the vapor contains the maximum amount of volatile components, while their concentration in the liquid is minimal.
Nozzles are filled in bulk and in bags. The shape of the filler can be either random or geometrically structured. It is made from an inert material such as clay, porcelain, plastic, ceramic, metal or graphite. The filler, as a rule, has sizes from 3 to 75 mm and has a large surface area in contact with the vapor-liquid mixture. The advantage of filling in bulk is its high throughput, resistance to high pressures and low cost.
Metal fillers have high strength and good wettability. Ceramic has an even higher wettability, but they are not so durable. Plastic are strong enough, but poorly wetted at low flow rates. Since ceramic fillers are corrosion resistant, they are used at elevated temperatures, which the plastic does not withstand.
Batch nozzles are a structured grid, the dimensions of which correspond to the diameter of the column. Provide long channels for fluid and vapor flows. They are more expensive, but can reduce pressure drops. Batch nozzles are preferred at low flow rates and under low pressure conditions. They are usually made of wood, sheet metal or woven mesh.
Used for solvent recovery and in the petrochemical industry.
Distillation column distillation
The most common columns are plate-type. The number of plates depends on the desired purity and complexity of separation. It affects how high the distillation column will be.
The principle of her work is as follows. The mixture is fed in the middle of the column height. The difference in concentration leads to the fact that less volatile components pass from the vapor stream to the liquid stream. The gas leaving the condenser contains the most volatile substances, while the less vaporized ones exit through the heater into the liquid stream.
The geometry of the plates in the column affects the degree and type of contact between different phase states of the mixture. Structurally, they are made of mesh, valve, cap, grate, cascade, etc. The mesh plates, in which there are holes for steam, are used to provide high performance at low cost. Cheaper valve plates, in which the openings are equipped with opening and closing valves, are prone to clogging due to accumulation of material on them. The cap is equipped with caps that allow the steam to pass through the liquid through tiny holes. This is the most advanced and expensive technology, effective at low flow rates. Liquid flows from one plate to another down the drain vertical pipes.
Dish columns are often used to recover solvents from process waste. They are also used to restore methanol during the drying operation. Water comes out as a liquid product, and volatile organic waste goes into the vapor phase. This is what distillation column is.
Cryogenic distillation
Cryogenic distillation involves the application of 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 unit and are distilled at very low temperatures. Cryogenic distillation columns may be packed and batch. Batch design is preferable because bulk material is less effective at low temperatures.
One of the main applications of cryogenic distillation is the separation of air into its constituent gases.
Extractive distillation
Extractive distillation uses additional compounds that act as a solvent to alter the relative volatility of one of the components of the mixture. Solvent is added to the extractive column to the substances to be separated. The component of the feed stream to be recovered is combined with the solvent and exits in the liquid phase. Another component evaporates and enters the distillate. The second distillation in another column allows you to separate the substance from the solvent, which then returns to the previous stage to repeat the cycle.
Extractive distillation is used to separate compounds with close boiling points and azeotropic mixtures. Extractive distillation is not as widespread in industry as conventional distillation, due to the complexity of the design. An example is the process for producing cellulose. An organic solvent separates cellulose from lignin, and a second distillation produces a pure substance.