ALL OR PART OF LIQUID OR OIL OR BLACK OIL

OIL AND PETROLIUM

An oil is any substance that is liquid at ambient temperatures and is hydrophobic but soluble in organic solvents. Oils have a high carbon and hydrogen content and are nonpolar substances. The general definition above includes compound classes with otherwise unrelated chemical structures, properties and uses, including vegetable oils, petrochemical oils, and volatile essential oils. All oils can be traced back to organic sources.
In chemistry, a chemical substance is a material with a specific chemical composition.[1]
A common example of a chemical substance is pure water; it has the same properties and the same ratio of hydrogen to oxygen whether it is isolated from a river or made in a laboratory. Some typical chemical substances are diamond, gold, salt (sodium chloride) and sugar (sucrose). Generally, chemical substances exist as a solid, liquid, gas, or plasma and may change between these phases of matter with changes in temperature or pressure. Chemical reactions convert one chemical substance into another.
Forms of energy, such as light and heat, are not considered to be matter, and thus they are not "substances" in this regard.
Definition
Chemical substances (also sometimes referred to as a pure substance) are often defined as "any material with a definite chemical composition" in most introductory general chemistry textbooks.[2] According to this definition a chemical substance can either be a pure chemical element or a pure chemical compound. However, there are exceptions to this definition; a pure substance can also be defined as a form of matter that has both definite composition and distinct properties.[3] The chemical substance index published by CAS also includes several alloys of uncertain composition.[4] Non-stoichiometric compounds are a special case (in inorganic chemistry) that violates the law of constant composition, and for them, it is sometimes difficult to draw the line between a mixture and a compound, as in the case of palladium hydride. Broader definitions of chemicals or chemical substances can be found, for example: "the term 'chemical substance' means any organic or inorganic substance of a particular molecular identity, including - (i) any combination of such substances occurring in whole or in part as a result of a chemical reaction or occurring in nature"[5]
History
The concept of a "chemical substance" became firmly established in the late eighteenth century after work by the chemist Joseph Proust on the composition of some pure chemical compounds such as basic copper carbonate.[6] He deduced that, "All samples of a compound have the same composition; that is, all samples have the same proportions, by mass, of the elements present in the compound." This is now known as the law of constant composition.[7] Later with the advancement of methods for chemical synthesis particularly in the realm of organic chemistry; the discovery of many more chemical elements and new techniques in the realm of analytical chemistry used for isolation and purification of elements and compounds from chemicals that led to the establishment of modern chemistry, the concept was defined as is found in most chemistry textbooks. However, there are some controversies regarding this definition mainly because the large number of chemical substances reported in chemistry literature need to be indexed.
Chemical elements
An element is a chemical substance that is made up of a particular kind of atoms and hence cannot be broken down or transformed by a chemical reaction into a different element, though it can be transmutated into another element through a nuclear reaction. This is so, because all of the atoms in a sample of an element have the same number of protons, though they may be different isotopes, with differing numbers of neutrons.
There are about 120 known elements, about 80 of which are stable - that is, they do not change by radioactive decay into other elements. However, the number of chemical substances that are elements can be more than 120, because some elements can occur as more than a single chemical substance (allotropes). For instance, oxygen exists as both diatomic oxygen (O2) and ozone (O3). The majority of elements are classified as metals. These are elements with a characteristic lustre such as iron, copper, and gold. Metals typically conduct electricity and heat well, and they are malleable and ductile.[8] Around a dozen elements,[9] such as carbon, nitrogen, and oxygen, are classified as non-metals. Non-metals lack the metallic properties described above, they also have a high electronegativity and a tendency to form negative ions. Certain elements such as silicon sometimes resemble metals and sometimes resemble non-metals, and are known as metalloids.
Chemical compounds
A pure chemical compound is a chemical substance that is composed of a particular set of molecules or ions. Two or more elements combined into one substance, through a chemical reaction, form what is called a chemical compound. All compounds are substances, but not all substances are compounds.
A chemical compound can be either atoms bonded together in molecules or crystals in which atoms, molecules or ions form a crystalline lattice. Compounds based primarily on carbon and hydrogen atoms are called organic compounds, and all others are called inorganic compounds. Compounds containing bonds between carbon and a metal are called organometallic compounds.
Compounds in which components share electrons are known as covalent compounds. Compounds consisting of oppositely charged ions are known as ionic compounds, or salts.
In organic chemistry, there can be more than one chemical compound with the same composition and molecular weight. Generally, these are called isomers. Isomers usually have substantially different chemical properties, may be isolated and do not spontaneously convert to each other. A common example is glucose vs. fructose. The former is an aldehyde, the latter is a ketone. Their interconversion requires either enzymatic or acid-base catalysis. However, there are also tautomers, where isomerization occurs spontaneously, such that a pure substance cannot be isolated into its tautomers. A common example is glucose, which has open-chain and ring forms. One cannot manufacture pure open-chain glucose because glucose spontaneously cyclizes to the hemiacetal form.
Substances versus mixtures
All matter consists of various elements and chemical compounds, but these are often intimately mixed together. Mixtures contain more than one chemical substance, and they do not have a fixed composition. In principle, they can be separated into the component substances by purely mechanical processes. Butter, soil and wood are common examples of mixtures.
Grey iron metal and yellow sulfur are both chemical elements, and they can be mixed together in any ratio to form a yellow-grey mixture. No chemical process occurs, and the material can be identified as a mixture by the fact that the sulfur and the iron can be separated by a mechanical process, such as using a magnet to attract the iron away from the sulfur.
In contrast, if iron and sulfur are heated together in a certain ratio (56 grams (1 mol) of iron to 32 grams (1 mol) of sulfur), a chemical reaction takes place and a new substance is formed, the compound iron(II) sulfide, with chemical formula FeS. The resulting compound has all the properties of a chemical substance and is not a mixture. Iron(II) sulfide has its own distinct properties such as melting point and solubility, and the two elements cannot be separated using normal mechanical processes; a magnet will be unable to recover the iron, since there is no metallic iron present in the compound.
Chemicals versus chemical substances
While the term chemical substance is a somewhat technical term used most often by professional chemists, the word chemical[10] is more widely used in the pharmaceutical industry, government and society in general. Thus the word chemical includes a much wider class of substances that includes many mixtures of chemical substances that often find application in many vocations;[11] and is most commonly used only for artificial or processed substances, such as the products of the chemical industry.
Naming and indexing
Every chemical substance has one or more systematic names, usually named according to the IUPAC rules for naming. An alternative system is used by the Chemical Abstracts Service (CAS).
Many compounds are also known by their more common, simpler names, many of which predate the systematic name. For example, the long-known sugar glucose is now systematically named 6-(hydroxymethyl)oxane-2,3,4,5-tetrol. Natural products and pharmaceuticals are also given simpler names, for example the mild pain-killer Naproxen is the more common name for the chemical compound (S)-6-methoxy-α-methyl-2-naphthaleneacetic acid.
Chemists frequently refer to chemical compounds using chemical formulae or molecular structure of the compound. There has been a phenomenal growth in the number of chemical compounds being synthesized (or isolated), and then reported in the scientific literature by professional chemists around the world.[12] An enormous number of chemical compounds are possible through the chemical combination of the known chemical elements. At the last count, about thirty million chemical compounds are known.[13] The names of many of these compounds are often nontrivial and hence not very easy to remember or cite accurately. Also it is difficult to keep the track of them in the literature. Several international organizations like IUPAC and CAS have initiated steps to make such tasks easier. CAS provides the abstracting services of the chemical literature, and provides a numerical identifier, known as CAS registry number to each chemical substance that has been reported in the chemical literature (such as chemistry journals and patents). This information is compiled as a database and is popularly known as the Chemical substances index. Other computer-friendly systems that have been developed for substance information, are: SMILES and the International Chemical Identifier or InChI.

Essential oil
An essential oil is a concentrated, hydrophobic liquid containing volatile aroma compounds from plants. An oil is 'essential' in the sense that it carries a distinctive scent, or essence, of the plant. Essential oils do not, as a group, need to have any specific chemical properties in common, beyond conveying characteristic fragrances. In history, oil has been used by Vikings, Spartans, etc. in war as they believed it made them stronger.[citation needed]
Essential oils are typically extracted by distillation. Other processes include expression, or solvent extraction. They are used in perfumes, cosmetics and bath products, for flavoring food and drink, and for scenting incense and household cleaning products.
Mineral oil
Mineral oils, found in porous rocks underground, originated from organic material, such as dead plankton, accumulated on the seafloor in geologically ancient times. Through various geochemical processes this material was converted to mineral oil, or petroleum, and its components, such as kerosene, paraffin waxes, gasoline, diesel and such. These are classified as mineral oils because they do not have an organic origin on human timescales, and are instead derived from underground geologic locations, ranging from rocks, to underground traps, to sands.
Other oily substances can also be found in the environment; the most well-known of those is asphalt, occurring naturally underground or, where there are leaks, in tar pits.
Petroleum and other mineral oils (specifically labelled as petrochemicals) have become such a crucial resource to human civilization in modern times they are often referred to by the ubiquitous term of "oil" itself.
Organic oils
Organic oils are also produced by plants, animals and other organisms through organic processes, and these oils are remarkable in their diversity. Oil is a somewhat vague term in chemistry; instead, the scientific term for oils, fats, waxes, cholesterol and other oily substances found in living things and their secretions, is lipids.
Lipids, ranging from waxes to steroids, are somewhat hard to characterize, and are united in a group almost solely based on the fact that they all repel, or refuse to dissolve in, water, and are however comfortably miscible in other liquid lipids. They also have a high carbon and hydrogen content, and are considerably lacking in oxygen compared to other organic compounds and minerals.
Synthetic oils
Synthetic oil is a lubricant consisting of chemical compounds which are artificially made (synthesized) from compounds other than crude oil (petroleum). Synthetic oil is used as a substitute for lubricant refined from petroleum, because it generally provides superior mechanical and chemical properties than those found in traditional mineral oils.
Food
Many edible plant and animal oils and fats are used in cooking and food preparation. In particular, many foods are fried in oil much hotter than boiling water. Oils are also used for flavoring and for modifying the texture of some foods e.g. stir fry.
Health advantages are claimed for a number of specific oils such as omega 3 oils, evening primrose oil, olive oil and coconut oil. Trans fats, often produced by hydrogenating vegetable oils, are known to be harmful to health.
Hair
Oil is used on hair to give it a lustrous look. It helps to avoid tangles and roughness to the hair. It also helps the hair to be stabilised and grow faster.[citation needed]
Fuel
Almost all oils burn in aerosol form generating heat, which can be used directly, or converted into other forms of fuels by various means. The oil that is pumped from the ground is then shipped via oil tanker to an oil refinery. There, it is converted from crude oil to diesel fuel (petrodiesel), ethane (and other short-chain alkanes), fuel oils (heaviest of commercial fuels, used in ships/furnaces), gasoline (petrol), jet fuel, kerosene and liquefied petroleum gas.
Electricity generation
Oil and any of its more refined products are often used to create electricity. This is done by means of a steam engine. The steam engine turns the thermal energy into rotary motion, which can then be transformed into electricity, by means of a generator.
Heat transport
Many oils have higher boiling points than water and are electrical insulators, making them useful for liquid cooling systems, especially where electricity is used.
Lubrication
Due to their non-polarity, oils do not easily adhere to other substances. This makes oils useful as lubricants for various engineering purposes. Mineral oils are more suitable than biological oils, which degrade rapidly in most environmental conditions.
Painting
Color pigments can be easily suspended in oil, making it suitable as supporting medium for paints. The slow drying process and miscibility of oil facilitates a realistic style. This method has been used since the 15th century.
Petrochemicals
Main article: Petrochemicals
Crude oil can be processed into petroleum; 'petrochemicals' are chemical products made from raw materials of petroleum or other hydrocarbon origin. They are used in products such as detergents, fertilizers, medicines, paints, plastics, synthetic fibres, and synthetic rubber.
Other uses
Sulfuric acid has been called oil of vitriol in pre-scientific times, due to its viscous consistency. Even in modern times, it is sometimes called vitriolic acid, and caustic personalities are called "vitriolic".[citation needed] Sulfuric acid is not a petrochemical, and in modern parlance, is not an oil.[citation needed]
Religion
Oils have been used throughout history as a fragrant or religious medium. Oil is often seen as a spiritually purifying agent. It is used in religious ceremonies, such as the chrism used in baptism, and has traditionally been used to anoint kings and queens. Oil that is associated with one or more saints is known as "oil of saints" and believed by some to have beneficial properties, as is "oil of martyrs"[1].
Liquid
Liquid is one of the three classical states of matter. Like a gas, a liquid is able to flow and take the shape of a container, but, like a solid, it resists compression. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly constant density. A distinctive property of the liquid state is surface tension, leading to wetting phenomena.
The density of a liquid is usually close to that of a solid, and much higher than in a gas. Therefore, liquid and solid are both termed condensed matter. On the other hand, as liquids and gases share the ability to flow, they are both called fluids.


Surface waves in water
Introduction
Liquid is one of the three primary states of matter, with the others being solid and gas. A liquid is a fluid. Unlike a solid, the molecules in a liquid have a much greater freedom to move. The forces that bind the molecules together in a solid are only temporary in a liquid, allowing a liquid to flow while a solid remains rigid.
A liquid, like a gas, displays the properties of a fluid. A liquid can flow, assume the shape of a container, and, if placed in a sealed container, will distribute applied pressure evenly to every surface in the container. Unlike a gas, a liquid may not always mix readily with another liquid, will not always fill every space in the container, forming its own surface, and will not compress significantly, except under extremely high pressures. These properties make a liquid suitable for applications such as hydraulics.
Liquid particles are bound firmly but not rigidly. They are able to move around one another freely, resulting in a limited degree of particle mobility. As the temperature increases, the increased vibrations of the molecules causes distances between the molecules to increase. When a liquid reaches its boiling point, the cohesive forces that bind the molecules closely together break, and the liquid changes to its gaseous state (unless superheating occurs). If the temperature is decreased, the distances between the molecules become smaller. When the liquid reaches its freezing point the molecules will usually lock into a very specific order, called crystallizing, and the bonds between them become more rigid, changing the liquid into its solid state (unless supercooling occurs].
Types of liquids
Only two elements are liquid at room temperature and pressure: mercury and bromine. Four more elements have melting points slightly above room temperature: francium, caesium, gallium and rubidium.
Pure substances that are liquid under normal conditions include water, ethanol and many other organic solvents. Liquid water is of primordial importance in chemistry and biology; it is believed to be a necessity for the existence of life.
Important everyday liquids include aqueous solutions like household bleach, other solutions (homogeneous mixtures, multiphasic liquids) like mineral oil and gasoline, emulsions like vinaigrette or mayonnaise, suspensions like milk and blood, and colloids like paint.
Liquid crystals, used in LCD displays, cannot be classified within the classical three states of matter; they possess solid-like and liquid-like properties. The same holds for biological membranes.
Properties
Quantities of liquids are commonly measured in units of volume. These include the SI unit cubic metre (m3) and its divisions, in particular the cubic decimetre, more commonly called the litre (1 dm3 = 1 L = 0.001 m3), and the cubic centimetre, also called millilitre (1 cm3 = 1 mL = 0.001 L = 10−6 m3).
The volume of a quantity of liquid is fixed by its temperature and pressure. Unless this volume exactly matches the volume of the container, one or more surfaces are observed.
In a gravitational field, liquids exert pressure on the sides of a container as well as on anything within the liquid itself. This pressure is transmitted in all directions and increases with depth. If a liquid is at rest in a uniform gravitational field, the pressure, p, at any depth, z, is given by

where:
is the density of the liquid (assumed constant)
is the gravitational acceleration.
Note that this formula assumes that the pressure at the free surface is zero, and that surface tension effects may be neglected.
Objects immersed in liquids are subject to the phenomenon of buoyancy. (Buoyancy is also observed in other fluids, but is especially strong in liquids due to their high density.)
Liquids have little compressibility: water, for example, does not change its density appreciably unless subjected to pressures on the order of 100 bars (equivalent to the pressure 1 km below the surface of the ocean). In the study of fluid dynamics, liquids are often treated as incompressible, especially when studying incompressible flow.
The surface of a liquid behaves like an elastic membrane in which surface tension appears, allowing the formation of drops and bubbles. Capillary action, wetting, and ripples are other consequences of surface tension.
Viscosity measures the resistance of a liquid which is being deformed by either shear stress or extensional stress.
Phase transitions


A typical phase diagram. The dotted line gives the anomalous behaviour of water. The green lines show how the freezing point can vary with pressure, and the blue line shows how the boiling point can vary with pressure. The red line shows the boundary where sublimation or deposition can occur.
At a temperature below the boiling point, any matter in liquid form will evaporate until the condensation of gas above reach an equilibrium. At this point the gas will condense at the same rate as the liquid evaporates. Thus, a liquid cannot exist permanently if the evaporated liquid is continually removed. A liquid at its boiling point will evaporate more quickly than the gas can condense at the current pressure. A liquid at or above its boiling point will normally boil, though superheating can prevent this in certain circumstances.
At a temperature below the freezing point, a liquid will tend to crystallize, changing to its solid form. Unlike the transition to gas, there is no equilibrium at this transition under constant pressure, so unless supercooling occurs, the liquid will eventually completely crystallize. Note that this is only true under constant pressure, so e.g. water and ice in a closed, strong container might reach an equilibrium where both phases coexists.
Liquids can display immiscibility. The most familiar mixture of two immiscible liquids in everyday life is the vegetable oil and water in Italian salad dressing. A familiar set of miscible liquids is water and alcohol. Liquid components in a mixture can often be separated from one another via fractional distillation.
Liquids generally expand when heated, and contract when cooled. Water between 0 °C and 4 °C is a notable exception.
Structure
Correlations


Structure of a classical monatomic liquid. Atoms have many nearest neighbors in contact, yet no long-range order is present.
‎In a liquid, atoms do not form a crystalline lattice, nor do they show any other form of long-range order. This is evidenced by the absence of Bragg peaks in X-ray and neutron diffraction. Under normal conditions, the diffraction pattern has circular symmetry, expressing the isotropy of the liquid. In radial direction, the diffraction intensity smoothly oscillates. This is usually described by the static structure factor S(q), with wavenumber q=(4π/λ)sinθ given by the wavelength λ of the probe (photon or neutron) and the Bragg angle θ. The oscillations of S(q) express the near order of the liquid, i.e. the correlations between an atom and a few shells of nearest, second nearest, ... neighbors.
A more intuitive description of these correlations is given by the radial distribution function g(r), which is basically the Fourier transform of S(q). It represents a spatial average of a temporal snapshot of pair correlations in the liquid. g(r) is determined by a relatively simple calculation of the average number of particles found within a given volume of shell located at a distance r from the center. The average density of atoms at a given radial distance from the center is given by the formula:

where n(r) is the mean number of atoms in a shell of width Δr at distance r, and ρ is the mean atom density.[1]
g(r) provides a means of comparison between diffraction experiment and computer simulation. It can also be used in conjunction with the interatomic pair potential function in order to calculate such macroscopic thermodynamic parameters as the internal energy, Gibbs free energy, entropy and enthalpy of the disordered system.


Radial distribution function of the Lennard-Jones model fluid.
A typical plot of g versus r shows a number of important features:
1. At short separations (small r), g(r) = 0. This indicates the effective width of the atoms, which ultimately limits their distance of approach.
2. A number of obvious peaks appear, at increasingly reduced intensities. The peaks indicate that the atoms pack around each other in 'shells' of nearest neighbors. At very long range, g(r) approaches a limiting value of 1 (or unity), which describes the average density at this range.
3. The attenuation of the peaks at increasing radial distances from the center indicates the decreasing degree of order from the center particle. This illustrates vividly the origin of the term "short-range order" in classical liquids and glasses.
Experimental verification of the radial distribution in simple liquids has been obtained by methods relying on the scattering of X-rays, where constructive interference is limited to peaks found within a limited radial distance r. Thus, peaks of decreasing amplitude appear only where the conditions for the constructive interference of X-rays are satisfied. The result is the characteristic periodic arrangement of light and dark bands of local intensity maxima and minima—analogous known to the diffraction pattern of the X-rays reflected from crystalline planes. [2]
Hidden structure
A number of authors have identified a static "hidden structure" and explored the dynamics of structural transitions in liquids. Utilizing molecular dynamics methods, they have separated the study of the liquid state into two parts:
1. Mechanically stable packings of molecules via potential minima;
2. Vibrational motion (generally anharmonic) about those mechanically stable points.
All configurations are "quenched" by a steepest-descent construction into a nearby potential minimum. The systems exhibit a "defect softening" phenomenon, or mean attraction between defects, which influences the spectrum of normal mode vibrational frequencies at the local potential minima for liquids that solidify into body centered cubic crystals. Attempts to reconstitute the equilibrium pair correlations functions by thermally broadening the quenched versions, using Einstein or Debye approximations, were clear failures. Evidently, the true phenomena in such systems entail substantial anharmonicity.[3]
The presence of "hidden structure" in supercooled liquids has been supported by the electron microscopic studies, indicating a well-defined "micellar" structure of glass which is interpreted as being the result of a superlattice of paracrystalline domains. The geometrical disorder of glass is therefore only exhibited at length scales above 10 nanometers (approximately the size of the elementary domain). Various degrees of interdomain ordering can therefore be realized.[4]
Dynamics
Molecular vibrations
Andrade focused his studies on the mechanism of structural transformations (or diffusionless transformations) in liquids. He emphasized that the intermolecular forces in the solid and the liquid state must be quite similar, and cited Lindemann's theory of melting, which has been remarkably successful in yielding accurate values for the atomic vibrational frequencies of the normal modes of vibration of simple solids. Lindemann supposes that melting occurs when the amplitude of the vibrations of the atoms about their equilibrium positions becomes a fixed large fraction of the interatomic separation distance.[5][6]
The essential difference between the liquid and solid state is therefore not the magnitude of the intermolecular force under which the molecule vibrates—but rather the amplitude of the motion. In the liquid state, this is so large that the molecules come into contact quite often. As a result, they are disturbed and the "position of equilibrium", which in a crystalline solid is fixed, is slowly displaced in a liquid. Therefore, a molecule in a liquid can be viewed as vibrating relatively to a slowly displaced equilibrium position. The vibration has the same frequency as (identical) molecules in the solid state.
Frenkel also considered the dynamics of thermal motion of atoms about their static equilibrium positions in the rigid elastic network. The rigidity of crystals is in full agreement with the conception that this 'heat motion' reduces to vibrations of small amplitude about invariable equilibrium positions, while the characteristic fluidity of liquids is due to the fact that the positions of the atoms in a liquid body are not permanent. When the period of atomic or molecular vibration is large compared with the time scale of an applied external force, elastic deformation may occur. If, however, the vibrational period is small compared with the time scale during which the body is acted upon by a force of constant magnitude and direction, it will yield to this force via irreversible plastic deformation. [7]
In the study of the high-frequency dynamics of simple liquids and solids near their melting points, the particular condition of zero vibrational frequency has been referred to as the "thermodynamic limit" (υ → 0). The conclusions of inelastic light scattering studies near the melting point is that there is no discernible difference between the liquid and solid vibrational spectra at sufficiently high frequencies. Thus, on the short time and length scales probed by these experiments, melting causes no discontinuous change in the microscopic dynamics of the substance. The lower the frequency, the larger the discontinuity between liquid and solid behavior—so that in the thermodynamic limit (zero frequency) the transition is first order. [8]
Effects of association
The mechanisms of atomic/molecular diffusion (or particle displacement) in solids are closely related to the mechanisms of viscous flow and solidification in liquid materials. Descriptions of viscosity in terms of molecular "free space" within the liquid[9] were modified as needed in order to account for liquids whose molecules are known to be "associated" in the liquid state at ordinary temperatures. When various molecules combine together to form an associated molecule, they enclose within a semi-rigid system a certain amount of space which before was available as free space for mobile molecules. Thus, increase in viscosity upon cooling due to the tendency of most substances to become associated on cooling.[10]
Similar arguments could be used to describe the effects of pressure on viscosity, where it may be assumed that the viscosity is chiefly a function of the volume for liquids with a finite compressibility. An increasing viscosity with rise of pressure is therefore expected. In addition, if the volume is expanded by heat but reduced again by pressure, the viscosity remains the same.
The local tendency to orientation of molecules in small groups lends the liquid (as referred to previously) a certain degree of association. This association results in a considerable "internal pressure" within a liquid, which is due almost entirely to those molecules which, on account of their temporary low velocities (following the Maxwell distribution) have coalesced with other molecules. The internal pressure between several such molecules might correspond to that between a group of molecules in the solid form.
The viscosity of liquids is characterized by a low activation energy. Thermal fluctuations break joining bonds with higher temperatures resulting in higher concentrations of broken bonds (or configurons). According to Ojovan [11] in the molten state configurons form large clusters which are macroscopically large. Broken bonds facilitate the irreversible plastic deformation (or flow) of the liquid. Also, the viscosity of amorphous solid materials is characterized in the glassy state by a high activation energy. The bond system of an amorphous material changes its Hausdorff dimension from Euclidian 3 below glass transition temperature Tg – where the amorphous material responds mechanically as an elastic solid on most experimental timescales — to fractal 2.55 (± 0.05) above Tg – where the amorphous material exhibits the rheology observed in a classical liquid [12].
Structural relaxation
The mean lifetime of an atom in its equilibrium position has been identified as the relaxation time, as originally described in Maxwell's kinetic theory of gases. In the simplest case of a monatomic liquid, the structural relaxation must reduce to a change of the degree of local order, yielding a more compact arrangement of higher density when the liquid is compressed, or a lower density when expanded. This change in the degree of local order must in general lag with respect to the variation of the volume (or the pressure), since it is connected with a rearrangement and redistribution of mutual orientations. These processes require a certain activation energy, and thus proceeding with a finite velocity. This is the origin of the viscous relaxation due to irreversible plastic deformation in the case of supercooled liquids near the glass transition. [13] [14] [15]
Applications
Liquids have a variety of uses, as lubricants, solvents, and coolants. In hydraulic systems, liquid is used to transmit power.
In tribology, liquids are studied for their properties as lubricants. Lubricants such as oil are chosen for viscosity and flow characteristics that are suitable throughout the operating temperature range of the component. Oils are often used in engines, gear boxes, metalworking, and hydraulic systems for their good lubrication properties.[16]
Many liquids are used as solvents, to dissolve other liquids or solids. Solvents are found in a wide variety of applications, including paints, sealants, and adhesives. Naptha and acetone are used frequently in industry to clean oil, grease, and tar from parts and machinery. Surfactants are commonly found in soaps and detergents. Solvents like alcohol are often used as antimicrobials. They are found in cosmetics, inks, and liquid dye lasers. They are used in the food industry, in processes such as the extraction of vegetable oil.[17]
Liquids tend to have better thermal conductivity than gases, and the ability to flow makes a liquid suitable for removing excess heat from mechanical components. The heat can be removed by channeling the liquid through a heat exchanger, such as a radiator, or the heat can be removed with the liquid during evaporation.[18] Water or glycol coolants are used to keep engines from overheating.[19] The coolants used in nuclear reactors include water or liquid metals, such as sodium or bismuth.[20] Liquid propellant films are used to cool the thrust chambers of rockets.[21] In machining, water and oils are used to remove the excess heat generated, which can quickly ruin both the work piece and the tooling. During perspiration, sweat removes heat from the human body by evaporating. In the heating, ventilation, and air-conditioning industry (HVAC), liquids such as water are used to transfer heat from one area to another.[22]
Liquid is the primary component of hydraulic systems, which take advantage of Pascal's law to provide fluid power. Devices such as pumps and waterwheels have been used to change liquid motion into mechanical work since ancient times. Oils are forced through hydraulic pumps, which transmit this force to hydraulic cylinders. Hydraulics can be found in many applications, such as automotive brakes and transmissions, heavy equipment, and airplane control systems. Various hydraulic presses are used extensively in repair and manufacturing, for lifting, pressing, clamping and forming.[23]
Liquids are sometimes used in measuring devices. A thermometer often uses the thermal expansion of liquids, such as mercury, combined with their ability to flow to indicate temperature. A manometer uses the weight of the liquid to indicate air pressure.[24]

Distillation
Distillation is a method of separating mixtures based on differences in their volatilities in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction.
Commercially, distillation has a number of applications. It is used to separate crude oil into more fractions for specific uses such as transport, power generation and heating. Water is distilled to remove impurities, such as salt from seawater. Air is distilled to separate its components—notably oxygen, nitrogen, and argon—for industrial use. Distillation of fermented solutions has been used since ancient times to produce distilled beverages with a higher alcohol content. The premises where distillation is carried out, especially distillation of alcohol, are known as a distillery.
History


Distillation apparatus of Zosimus, from Marcelin Berthelot, Collection des anciens alchimistes grecs (3 vol., Paris, 1887-1888).
Early types of distillation were known to the Babylonians in Mesopotamia (in what is now Iraq) from at least the 2nd millennium BC.[2] Archaeological excavations in northwest Pakistan have yielded evidence that the distillation of alcohol was known in the Indian subcontinent since 500 BC,[3] but only became common between 150 BC - 350 AD.[3] Primitive tribes of India used a method of distillation for producing Mahuda liquor. This crude and ancient method is not very effective.[4]
Distillation was later known to Hellenistic alchemists from the 1st century AD,[5][6][7] and the later development of large-scale distillation apparatus occurred in response to demands for spirits.[5] According to K. B. Hoffmann the earliest mention of "destillatio per descensum" occurs in the writings of Aetius, a Greek physician from the 5th century.[8] Hypatia of Alexandria is credited with having invented an early distillation apparatus,[9] and the first clear description of early apparatus for distillation is given by Zosimos of Panopolis in the fourth century.[7]
The invention of highly effective "pure distillation" is credited to Arabic and Persian chemists in the Middle East from the 8th century. They produced distillation processes to isolate and purify chemical substances for industrial purposes such as isolating natural esters (perfumes) and producing pure alcohol.[10] The first among them was Jabir ibn Hayyan (Geber), in the 8th century, who is credited with the invention of numerous chemical apparatus and processes that are still in use today. In particular, his alembic was the first still with retorts which could fully purify chemicals, a precursor to the pot still, and its design has served as inspiration for modern micro-scale distillation apparatus such as the Hickman stillhead.[11] The isolation of ethanol (alcohol) as a pure compound through distillation was first achieved by the Arab chemist Al-Kindi (Alkindus).[12] Petroleum was first distilled by the Persian alchemist Muhammad ibn Zakarīya Rāzi (Rhazes) in the 9th century, for producing kerosene,[13] while steam distillation was invented by Avicenna in the early 11th century, for producing essential oils.[14]
As the works of Middle Eastern scribes made their way to India and became a part of Indian alchemy, several texts dedicated to distillation made their way to Indian libraries.[15] Among these was a treatise written by a scholar from Bagdad in 1034 titled Ainu-s-Sana'ah wa' Auna-s-Sana'ah.[15] Scholar Al-Jawbari travelled to India.[16] By the time of the writing of the Ain-e-Akbari, the process of distillation was well known in India.[17]
Distillation was introduced to medieval Europe through Latin translations of Arabic chemical treatises in the 12th century.[18] In 1500, German alchemist Hieronymus Braunschweig published Liber de arte destillandi (The Book of the Art of Distillation)[19] the first book solely dedicated to the subject of distillation, followed in 1512 by a much expanded version. In 1651, John French published The Art of Distillation the first major English compendium of practice, though it has been claimed[20] that much of it derives from Braunschweig's work. This includes diagrams with people in them showing the industrial rather than bench scale of the operation.


Distillation


Old Ukrainian vodka still
As alchemy evolved into the science of chemistry, vessels called retorts became used for distillations. Both alembics and retorts are forms of glassware with long necks pointing to the side at a downward angle which acted as air-cooled condensers to condense the distillate and let it drip downward for collection. Later, copper alembics were invented. Riveted joints were often kept tight by using various mixtures, for instance a dough made of rye flour.[21] These alembics often featured a cooling system around the beak, using cold water for instance, which made the condensation of alcohol more efficient. These were called pot stills. Today, the retorts and pot stills have been largely supplanted by more efficient distillation methods in most industrial processes. However, the pot still is still widely used for the elaboration of some fine alcohols such as cognac, Scotch whisky, tequila and some vodkas. Pot stills made of various materials (wood, clay, stainless steel) are also used by bootleggers in various countries. Small pot stills are also sold for the domestic production[22] of flower water or essential oils.
Early forms of distillation were batch processes using one vaporization and one condensation. Purity was improved by further distillation of the condensate. Greater volumes were processed by simply repeating the distillation. Chemists were reported to carry out as many as 500 to 600 distillations in order to obtain a pure compound[23].
In the early 19th century the basics of modern techniques including pre-heating and reflux were developed, particularly by the French[23], then in 1830 a British Patent was issued to Aeneas Coffey for a whiskey distillation column[24], which worked continuously and may be regarded as the archetype of modern petrochemical units. In 1877, Ernest Solvay was granted a U.S. Patent for a tray column for ammonia distillation[25] and the same and subsequent years saw developments of this theme for oil and spirits.
With the emergence of chemical engineering as a discipline at the end of the 19th century, scientific rather than empirical methods could be applied. The developing petroleum industry in the early 20th century provided the impetus for the development of accurate design methods such as the McCabe-Thiele method and the Fenske equation. The availability of powerful computers has also allowed direct computer simulation of distillation columns.
Applications of distillation
The application of distillation can roughly be divided in four groups: laboratory scale, industrial distillation, distillation of herbs for perfumery and medicinals (herbal distillate), and food processing. The latter two are distinctively different from the former two in that in the processing of beverages, the distillation is not used as a true purification method but more to transfer all volatiles from the source materials to the distillate.
The main difference between laboratory scale distillation and industrial distillation is that laboratory scale distillation is often performed batch-wise, whereas industrial distillation often occurs continuously. In batch distillation, the composition of the source material, the vapors of the distilling compounds and the distillate change during the distillation. In batch distillation, a still is charged (supplied) with a batch of feed mixture, which is then separated into its component fractions which are collected sequentially from most volatile to less volatile, with the bottoms (remaining least or non-volatile fraction) removed at the end. The still can then be recharged and the process repeated.
In continuous distillation, the source materials, vapors, and distillate are kept at a constant composition by carefully replenishing the source material and removing fractions from both vapor and liquid in the system. This results in a better control of the separation process.
Idealized distillation model
The boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the pressure in the liquid, enabling bubbles to form without being crushed. A special case is the normal boiling point, where the vapor pressure of the liquid equals the ambient atmospheric pressure.
It is a common misconception that in a liquid mixture at a given pressure, each component boils at the boiling point corresponding to the given pressure and the vapors of each component will collect separately and purely. This, however, does not occur even in an idealized system. Idealized models of distillation are essentially governed by Raoult's law and Dalton's law, and assume that vapor-liquid equilibria are attained.
Raoult's law assumes that a component contributes to the total vapor pressure of the mixture in proportion to its percentage of the mixture and its vapor pressure when pure, or succinctly: partial pressure equals mole fraction multiplied by vapor pressure when pure. If one component changes another component's vapor pressure, or if the volatility of a component is dependent on its percentage in the mixture, the law will fail.
Dalton's law states that the total vapor pressure is the sum of the vapor pressures of each individual component in the mixture. When a multi-component liquid is heated, the vapor pressure of each component will rise, thus causing the total vapor pressure to rise. When the total vapor pressure reaches the pressure surrounding the liquid, boiling occurs and liquid turns to gas throughout the bulk of the liquid. Note that a mixture with a given composition has one boiling point at a given pressure, when the components are mutually soluble.
An implication of one boiling point is that lighter components never cleanly "boil first". At boiling point, all volatile components boil, but for a component, its percentage in the vapor is the same as its percentage of the total vapor pressure. Lighter components have a higher partial pressure and thus are concentrated in the vapor, but heavier volatile components also have a (smaller) partial pressure and necessarily evaporate also, albeit being less concentrated in the vapor. Indeed, batch distillation and fractionation succeed by varying the composition of the mixture. In batch distillation, the batch evaporates, which changes its composition; in fractionation, liquid higher in the fractionation column contains more lights and boils at lower temperatures.
The idealized model is accurate in the case of chemically similar liquids, such as benzene and toluene. In other cases, severe deviations from Raoult's law and Dalton's law are observed, most famously in the mixture of ethanol and water. These compounds, when heated together, form an azeotrope, which is a composition with a boiling point higher or lower than the boiling point of each separate liquid. Virtually all liquids, when mixed and heated, will display azeotropic behaviour. Although there are computational methods that can be used to estimate the behavior of a mixture of arbitrary components, the only way to obtain accurate vapor-liquid equilibrium data is by measurement.
It is not possible to completely purify a mixture of components by distillation, as this would require each component in the mixture to have a zero partial pressure. If ultra-pure products are the goal, then further chemical separation must be applied. When a binary mixture is evaporated and the other component, e.g. a salt, has zero partial pressure for practical purposes, the process is simpler and is called evaporation in engineering.
Batch distillation


A batch still showing the separation of A and B.
Heating an ideal mixture of two volatile substances A and B (with A having the higher volatility, or lower boiling point) in a batch distillation setup (such as in an apparatus depicted in the opening figure) until the mixture is boiling results in a vapor above the liquid which contains a mixture of A and B. The ratio between A and B in the vapor will be different from the ratio in the liquid: the ratio in the liquid will be determined by how the original mixture was prepared, while the ratio in the vapor will be enriched in the more volatile compound, A (due to Raoult's Law, see above). The vapor goes through the condenser and is removed from the system. This in turn means that the ratio of compounds in the remaining liquid is now different from the initial ratio (i.e. more enriched in B than the starting liquid).
The result is that the ratio in the liquid mixture is changing, becoming richer in component B. This causes the boiling point of the mixture to rise, which in turn results in a rise in the temperature in the vapor, which results in a changing ratio of A : B in the gas phase (as distillation continues, there is an increasing proportion of B in the gas phase). This results in a slowly changing ratio A : B in the distillate.
If the difference in vapor pressure between the two components A and B is large (generally expressed as the difference in boiling points), the mixture in the beginning of the distillation is highly enriched in component A, and when component A has distilled off, the boiling liquid is enriched in component B.
Continuous distillation
Continuous distillation is an ongoing distillation in which a liquid mixture is continuously (without interruption) fed into the process and separated fractions are removed continuously as output streams as time passes during the operation. Continuous distillation produces at least two output fractions, including at least one volatile distillate fraction, which has boiled and been separately captured as a vapor condensed to a liquid. There is always a bottoms (or residue) fraction, which is the least volatile residue that has not been separately captured as a condensed vapor.
Continuous distillation differs from batch distillation in the respect that concentrations should not change over time. Continuous distillation can be run at a steady state for an arbitrary amount of time. Given a feed of in a specified composition, the main variables that affect the purity of products in continuous distillation are the reflux ratio and the number of theoretical equilibrium stages (practically, the number of trays or the height of packing). Reflux is a flow from the condenser back to the column, which generates a recycle that allows a better separation with a given number of trays. Equilibrium stages are ideal steps where compositions achieve vapor-liquid equilibrium, repeating the separation process and allowing better separation given a reflux ratio. A column with a high reflux ratio may have fewer stages, but it refluxes a large amount of liquid, giving a wide column with a large holdup. Conversely, a column with a low reflux ratio must have a large number of stages, thus requiring a taller column.
Continuous distillation requires building and configuring dedicated equipment. The resulting high investment cost restricts its use to the large scale.[clarification needed]
General improvements
Both batch and continuous distillations can be improved by making use of a fractionating column on top of the distillation flask. The column improves separation by providing a larger surface area for the vapor and condensate to come into contact. This helps it remain at equilibrium for as long as possible. The column can even consist of small subsystems ('trays' or 'dishes') which all contain an enriched, boiling liquid mixture, all with their own vapor-liquid equilibrium.
There are differences between laboratory-scale and industrial-scale fractionating columns, but the principles are the same. Examples of laboratory-scale fractionating columns (in increasing efficiency) include:
• Air condenser
• Vigreux column (usually laboratory scale only)
• Packed column (packed with glass beads, metal pieces, or other chemically inert material)
• Spinning band distillation system.
Laboratory scale distillation
Laboratory scale distillations are almost exclusively run as batch distillations. The device used in distillation, sometimes referred to as a still, consists at a minimum of a reboiler or pot in which the source material is heated, a condenser in which the heated vapour is cooled back to the liquid state, and a receiver in which the concentrated or purified liquid, called the distillate, is collected. Several laboratory scale techniques for distillation exist (see also distillation types).
Simple distillation
In simple distillation, all the hot vapors produced are immediately channeled into a condenser that cools and condenses the vapors. Therefore, the distillate will not be pure - its composition will be identical to the composition of the vapors at the given temperature and pressure, and can be computed from Raoult's law.
As a result, simple distillation is usually used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C),[26] or to separate liquids from involatile solids or oils. For these cases, the vapor pressures of the components are usually sufficiently different that Raoult's law may be neglected due to the insignificant contribution of the less volatile component. In this case, the distillate may be sufficiently pure for its intended purpose.
Fractional distillation
For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Therefore, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column. This separation, by successive distillations, is also referred to as rectification [27].
As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component.[28] In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description.
More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.[29]
Steam distillation
Like vacuum distillation, steam distillation is a method for distilling compounds which are heat-sensitive.[30] This process involves using bubbling steam through a heated mixture of the raw material. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure). The vapor mixture is cooled and condensed, usually yielding a layer of oil and a layer of water.
Steam distillation of various aromatic herbs and flowers can result in two products; an essential oil as well as a watery herbal distillate. The essential oils are often used in perfumery and aromatherapy while the watery distillates have many applications in aromatherapy, food processing and skin care.


Dimethyl sulfoxide usually boils at 189 °C. Under a vacuum, it distills off into the receiver at only 70 °C.


Perkin triangle distillation setup
1: Stirrer bar/anti-bumping granules 2: Still pot 3: Fractionating column 4: Thermometer/Boiling point temperature 5: Teflon tap 1 6: Cold finger 7: Cooling water out 8: Cooling water in 9: Teflon tap 2 10: Vacuum/gas inlet 11: Teflon tap 3 12: Still receiver
Vacuum distillation
Main article: Vacuum distillation
Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is referred to as vacuum distillation and it is commonly found in the laboratory in the form of the rotary evaporator.
This technique is also very useful for compounds which boil beyond their decomposition temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure.
Molecular distillation is vacuum distillation below the pressure of 0.01 torr.[31] 0.01 torr is one order of magnitude above high vacuum, where fluids are in the free molecular flow regime, i.e. the mean free path of molecules is comparable to the size of the equipment. The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, rate of evaporation no longer depends on pressure. That is, because the continuum assumptions of fluid dynamics no longer apply, mass transport is governed by molecular dynamics rather than fluid dynamics. Thus, a short path between the hot surface and the cold surface is necessary, typically by suspending a hot plate covered with a film of feed next to a cold plate with a clear line of sight in between. Molecular distillation is used industrially for purification of oils.
Air-sensitive vacuum distillation
Some compounds have high boiling points as well as being air sensitive. A simple vacuum distillation system as exemplified above can be used, whereby the vacuum is replaced with an inert gas after the distillation is complete. However, this is a less satisfactory system if one desires to collect fractions under a reduced pressure. To do this a "pig" adaptor can be added to the end of the condenser, or for better results or for very air sensitive compounds a Perkin triangle apparatus can be used.
The Perkin triangle, has means via a series of glass or Teflon taps to allows fractions to be isolated from the rest of the still, without the main body of the distillation being removed from either the vacuum or heat source, and thus can remain in a state of reflux. To do this, the sample is first isolated from the vacuum by means of the taps, the vacuum over the sample is then replaced with an inert gas (such as nitrogen or argon) and can then be stoppered and removed. A fresh collection vessel can then be added to the system, evacuated and linked back into the distillation system via the taps to collect a second fraction, and so on, until all fractions have been collected.
Short path distillation


Short path vacuum distillation apparatus with vertical condenser (cold finger), to minimize the distillation path; 1: Still pot with stirrer bar/anti-bumping granules 2: Cold finger - bent to direct condensate 3: Cooling water out 4: cooling water in 5: Vacuum/gas inlet 6: Distillate flask/distillate.
Short path distillation is a distillation technique that involves the distillate travelling a short distance, often only a few centimeters, and is normally done at reduced pressure.[32] A classic example would be a distillation involving the distillate travelling from one glass bulb to another, without the need for a condenser separating the two chambers. This technique is often used for compounds which are unstable at high temperatures or to purify small amounts of compound. The advantage is that the heating temperature can be considerably lower (at reduced pressure) than the boiling point of the liquid at standard pressure, and the distillate only has to travel a short distance before condensing. A short path ensures that little compound is lost on the sides of the apparatus. The Kugelrohr is a kind of a short path distillation apparatus which often contain multiple chambers to collect distillate fractions.
Other types
• The process of reactive distillation involves using the reaction vessel as the still. In this process, the product is usually significantly lower-boiling than its reactants. As the product is formed from the reactants, it is vaporized and removed from the reaction mixture. This technique is an example of a continuous vs. a batch process; advantages include less downtime to charge the reaction vessel with starting material, and less workup.
• Pervaporation is a method for the separation of mixtures of liquids by partial vaporization through a non-porous membrane.
• Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture.
• Flash evaporation (or partial evaporation) is the partial vaporization that occurs when a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device. This process is one of the simplest unit operations, being equivalent to a distillation with only one equilibrium stage.
• Codistillation is distillation which is performed on mixtures in which the two compounds are not miscible.
The unit process of evaporation may also be called "distillation":
• In rotary evaporation a vacuum distillation apparatus is used to remove bulk solvents from a sample. Typically the vacuum is generated by a water aspirator or a membrane pump.
• In a kugelrohr a short path distillation apparatus is typically used (generally in combination with a (high) vacuum) to distill high boiling (> 300 °C) compounds. The apparatus consists of an oven in which the compound to be distilled is placed, a receiving portion which is outside of the oven, and a means of rotating the sample. The vacuum is normally generated by using a high vacuum pump.
Other uses:
• Dry distillation or destructive distillation, despite the name, is not truly distillation, but rather a chemical reaction known as pyrolysis in which solid substances are heated in an inert or reducing atmosphere and any volatile fractions, containing high-boiling liquids and products of pyrolysis, are collected. The destructive distillation of wood to give methanol is the root of its common name - wood alcohol.
• Freeze distillation is an analogous method of purification using freezing instead of evaporation. It is not truly distillation, but a recrystallization where the product is the mother liquor, and does not produce products equivalent to distillation. This process is used in the production of ice beer and ice wine to increase ethanol and sugar content, respectively. It is also used to produce applejack. Unlike distillation, freeze distillation concentrates poisonous congeners rather than removing them.
Azeotropic distillation
Interactions between the components of the solution create properties unique to the solution, as most processes entail nonideal mixtures, where Raoult's law does not hold. Such interactions can result in a constant-boiling azeotrope which behaves as if it were a pure compound (i.e., boils at a single temperature instead of a range). At an azeotrope, the solution contains the given component in the same proportion as the vapor, so that evaporation does not change the purity, and distillation does not effect separation. For example, ethyl alcohol and water form an azeotrope of 95.6% at 78.1 °C.
If the azeotrope is not considered sufficiently pure for use, there exist some techniques to break the azeotrope to give a pure distillate. This set of techniques are known as azeotropic distillation. Some techniques achieve this by "jumping" over the azeotropic composition (by adding an additional component to create a new azeotrope, or by varying the pressure). Others work by chemically or physically removing or sequestering the impurity. For example, to purify ethanol beyond 95%, a drying agent or a (desiccant such as potassium carbonate) can be added to convert the soluble water into insoluble water of crystallization. Molecular sieves are often used for this purpose as well.
Immiscible liquids, such as water and toluene, easily form azeotropes. Commonly, these azeotropes are referred to as a low boiling azeotrope because the boiling point of the azeotrope is lower than the boiling point of either pure component. The temperature and composition of the azeotrope is easily predicted from the vapor pressure of the pure components, without use of Raoult's law. The azeotrope is easily broken in a distillation set-up by using a liquid-liquid separator ( a decanter ) to separate the two liquid layers that are condensed overhead. Only one of the two liquid layers is refluxed to the distillation set-up.
High boiling azeotropes, such as a 20 weight percent mixture of hydrochloric acid in water, also exist. As implied by the name, the boiling point of the azeotrope is greater than the boiling point of either pure component.
To break azeotropic distillations and cross distillation boundaries, such as in the DeRosier Problem, it is necessary to increase the composition of the light key in the distillate.
Breaking an azeotrope with unidirectional pressure manipulation
The boiling points of components in an azeotrope overlap to form a band. By exposing an azeotrope to a vacuum or positive pressure, it's possible to bias the boiling point of one component away from the other by exploiting the differing vapour pressure curves of each; the curves may overlap at the azeotropic point, but are unlikely to be remain identical further along the pressure axis either side of the azeotropic point. When the bias is great enough, the two boiling points no longer overlap and so the azeotropic band disappears.
This method can remove the need to add other chemicals to a distillation, but it has two potential drawbacks.
Under negative pressure, power for a vacuum source is needed and the reduced boiling points of the distillates requires that the condenser be run cooler to prevent distillate vapours being lost to the vacuum source. Increased cooling demands will often require additional energy and possibly new equipment or a change of coolant.
Alternatively, if positive pressures are required, standard glassware can not be used, energy must be used for pressurization and there is a higher chance of side reactions occurring in the distillation, such as decomposition, due to the higher temperatures required to effect boiling.
A unidirectional distillation will rely on a pressure change in one direction, either positive or negative.
Pressure-swing distillation
Pressure-swing distillation is essentially the same as the unidirectional distillation used to break azeotropic mixtures, but here both positive and negative pressures may be employed.[clarification needed]
This has an important impact on the selectivity of the distillation and allows a chemist[citation needed] to optimize a process such that fewer extremes of pressure and temperature are required and less energy is consumed. This is particularly important in commercial applications.
Pressure-swing distillation is employed during the industrial purification of ethyl acetate after its catalytic synthesis from ethanol.
Industrial distillation


Typical industrial distillation towers
Main article: Continuous distillation
Large scale industrial distillation applications include both batch and continuous fractional, vacuum, azeotropic, extractive, and steam distillation. The most widely used industrial applications of continuous, steady-state fractional distillation are in petroleum refineries, petrochemical and chemical plants and natural gas processing plants.
Industrial distillation[27][33] is typically performed in large, vertical cylindrical columns known as distillation towers or distillation columns with diameters ranging from about 65 centimeters to 16 meters and heights ranging from about 6 meters to 90 meters or more. When the process feed has a diverse composition, as in distilling crude oil, liquid outlets at intervals up the column allow for the withdrawal of different fractions or products having different boiling points or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column and are often called the bottoms.

Diagram of a typical industrial distillation tower
Large-scale industrial towers use reflux to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower as shown in the schematic diagram of a typical, large-scale industrial distillation tower. Inside the tower, the downflowing reflux liquid provides cooling and condensation of the upflowing vapors thereby increasing the efficacy of the distillation tower. The more reflux that is provided for a given number of theoretical plates, the better the tower's separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux that is provided for a given desired separation, the fewer the number of theoretical plates required.
Such industrial fractionating towers are also used in air separation, producing liquid oxygen, liquid nitrogen, and high purity argon. Distillation of chlorosilanes also enables the production of high-purity silicon for use as a semiconductor.

Section of an industrial distillation tower showing detail of trays with bubble caps
Design and operation of a distillation tower depends on the feed and desired products. Given a simple, binary component feed, analytical methods such as the McCabe-Thiele method[27][34] or the Fenske equation[27] can be used. For a multi-component feed, simulation models are used both for design and operation. Moreover, the efficiencies of the vapor-liquid contact devices (referred to as "plates" or "trays") used in distillation towers are typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a distillation tower needs more trays than the number of theoretical vapor-liquid equilibrium stages.
In industrial uses, sometimes a packing material is used in the column instead of trays, especially when low pressure drops across the column are required, as when operating under vacuum.


Large-scale, industrial vacuum distillation column[35]
This packing material can either be random dumped packing (1-3" wide) 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 is useful to compute a number of "theoretical stages" 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 the 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 will not be correct in the packed bed and the required separation will not be achieved. The packing will appear to not be working properly. The height equivalent of a theoretical plate (HETP) will be greater than expected. The problem is not 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 to it maximum efficiency. Methods of evaluating the effectiveness of a liquid distributor to evenly distribute the liquid entering a packed bed can be found in references.[36][37] Considerable work as been done on this topic by Fractionation Research, Inc. (commonly known as FRI).[38]
Distillation in food processing
Distilled beverages
Carbohydrate-containing plant materials are allowed to ferment, producing a dilute solution of ethanol in the process. Spirits such as whiskey and rum are prepared by distilling these dilute solutions of ethanol. Components other than ethanol, including water, esters, and other alcohols, are collected in the condensate, which account for the flavor of the beverage.