The urea extraction crystallization is a process for separating

linear paraffins In mathematics, the term ''linear'' is used in two distinct senses for two different properties: * linearity of a '' function'' (or '' mapping''); * linearity of a ''polynomial''. An example of a linear function is the function defined by f(x)= ...

(n-paraffins, n-alkanes) from

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

mixtures through the formation of

urea Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula . This amide has two Amine, amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest am ...

-n-paraffin-

clathrates A clathrate is a chemical substance consisting of a lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin (), meaning 'with bars, latticed'. Most clathrate compounds are polymeric and completely envelop the ...

. The process is primarily used to lower the

pour point The pour point of a liquid is the temperature below which the liquid loses its flow characteristics. It is defined as the minimum temperature in which the oil has the ability to pour down from a beaker. In crude oil a high pour point is generally ...

of petroleum products, by-products of the process are n-paraffins in high purity. The method may also applied for the separation of

fatty acids In chemistry, in particular in biochemistry, a fatty acid is a carboxylic acid with an aliphatic chain, which is either saturated or unsaturated. Most naturally occurring fatty acids have an unbranched chain of an even number of carbon atoms, ...

and

fatty alcohols Fatty alcohols (or long-chain alcohols) are usually high-molecular mass, straight-chain primary alcohols, but can also range from as few as 4–6 carbon atoms to as many as 22–26, derived from natural fats and oils. The precise chain length vari ...

. In addition to urea also

thiourea Thiourea () is an organosulfur compound with the formula and the structure . It is structurally similar to urea (), with the oxygen atom replaced by sulfur atom (as implied by the '' thio-'' prefix). The properties of urea and thiourea differ s ...

is used in the process.

History

In 1939 German chemist Friedrich Bergen was trying different extractants to separate serum proteins from

milk Milk is a white liquid food produced by the mammary glands of lactating mammals. It is the primary source of nutrition for young mammals (including breastfeeding, breastfed human infants) before they are able to digestion, digest solid food. ...

at low temperature. When he tried urea, he noticed that something weird was going on with milk lipids. A treatment with

octanol Octanols are alcohols with the formula C8H17OH. A simple and important member is 1-octanol, with an unbranched chain of carbons. Other commercially important octanols are 2-octanol and 2-ethylhexanol. Some octanols occur naturally in the form o ...

serendipitously Serendipity is an unplanned fortunate discovery. The term was coined by Horace Walpole in 1754. The concept is often associated with scientific and technological breakthroughs, where accidental discoveries led to new insights or inventions. Man ...

revealed that it combines with urea in large crystals. Bergen investigated different lipids, alkanes and alcohols and found out that at least six carbon atoms are required, and that branched hydrocarbons don't participate in the phenomenon. Not being an expert in hydrocarbons and urea, he cooperated with from

BASF BASF SE (), an initialism of its original name , is a European Multinational corporation, multinational company and the List of largest chemical producers, largest chemical producer in the world. Its headquarters are located in Ludwigshafen, Ge ...

IG Farben I. G. Farbenindustrie AG, commonly known as IG Farben, was a German Chemical industry, chemical and Pharmaceutical industry, pharmaceutical conglomerate (company), conglomerate. It was formed on December 2, 1925 from a merger of six chemical co ...

and then with Wilhelm Schlenk, filing for patents with the latter in 1940, which were awarded in 1953. They didn't publish their findings until 1949 because German authorities classified the discovery during the

World War II World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

, but the patent applications were confiscated by Allies' Technical Oil Mission after the war so Sonneborn was able to put a pilot oil dewaxing plant in

Petrolia, Pennsylvania Petrolia is a borough in Butler County, Pennsylvania, United States. The population was 212 at the 2010 census. Geography Petrolia is located in eastern Butler County at (41.017964, −79.718204), in the valley of the South Branch of Bear Creek ...

into operation already in 1950.

DEA AG DEA Deutsche Erdoel AG was an international oil and gas company headquartered in Hamburg, Germany. It was a subsidiary of L1 Energy. In 2018, DEA owned stakes in oil and gas licenses in various countries and operated natural gas underground storag ...

followed the suit in 1954 and

Standard Oil Standard Oil Company was a Trust (business), corporate trust in the petroleum industry that existed from 1882 to 1911. The origins of the trust lay in the operations of the Standard Oil of Ohio, Standard Oil Company (Ohio), which had been founde ...

in 1956, and worldwide research in the topic took off in the 1950s.

Raw materials

In addition to the n-alkanes are also unbranched fatty acids with more than four carbon atoms, their esters and unbranched fatty alcohols can migrate into the channels of the crystallized urea and form a

clathrate A clathrate is a chemical substance consisting of a lattice (group), lattice that traps or contains molecules. The word ''clathrate'' is derived from the Latin language, Latin (), meaning 'with bars, Crystal structure, latticed'. Most clathrate ...

. A deviation from the linear molecular geometry, for example, by C=C-double bonds in the molecule, leads to a less stable inclusion compound. Thus stearic acid (C18: 0) forms more stable

urea adducts Urea can crystallise with other compounds. These can be called urea adducts or if a solvent is involved, a urea solvate, and the process is called urea extraction crystallization The urea extraction crystallization is a process for separating line ...

compared to oleic acid (C18: 1 cis -9) or linoleic acid (C18: 2 cis -9, cis -12). A branching in the fatty acid molecule or an autoxidation result in a large deviation from the straight-chain molecular structure, so that these compounds do not form urea adducts. This is used as part of the fatty acid analysis and for the separation or enrichment of specific fatty acids.Hans-Dieter Belitz, Walter Grosch: ''Lehrbuch der Lebensmittelchemie'', Springer, 1992, , p. 151-155

Process

For the separation of n-paraffins from other hydrocarbon compounds, urea is added with an approximately 20-fold molar excess. The urea crystallizes in a

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is d ...

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

with about 5.5 to 5.8 Å wide channels. In these channels the n-paraffins are included. If the concentration of n-paraffins in the mixture is too high, the mixture is diluted with a solvent. In general, the reaction proceeds according to the scheme: :urea + adduct component -> urea adduct The equilibrium of the reaction is dependent on the concentrations of the reactants, the solvent and the temperature.

Friedrich Asinger Friedrich Asinger (26 June 1907 in Freiland/Niederdonau (Austria); – 7 March 1999 in Aachen) was an Austrian chemist and professor for Technical Chemistry. He is well known for his development of a multi-component reaction, the Asinger reacti ...

: ''Chemie und Technologie der Paraffinkohlenwasserstoffe''. Akademie Verlag, 1956, p. 53–59. The necessary quantity of urea for the formation of inclusion compounds varies from about 1 to 0.8 mole of urea per methyl- and methylene group in a carbon chain. The urea is added as a

supersaturated In physical chemistry, supersaturation occurs with a solution when the concentration of a solute exceeds the concentration specified by the value of solubility at equilibrium. Most commonly the term is applied to a solution of a solid in a ...

aqueous solution to compensate for losses due to adduct formation during the process. In order to avoid a too high concentrations of adducts in the dewaxed oil a

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

such as

methyl isobutyl ketone Methyl isobutyl ketone (MIBK, 4-methylpentan-2-one) is an organic compound with the structural formula, condensed chemical formula (CH3)2CHCH2C(O)CH3. This ketone is a colourless liquid that is used as a solvent for gums, resins, paints, varnishes ...

methylene chloride Dichloromethane (DCM, methylene chloride, or methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odor is widely used as a solvent. Although it is not miscible with ...

is used for dilution. The ratio of oil to water phase is about 1 to 0.5. The mixing of the oil and water phases occurs at slightly elevated temperatures of about 35 °C. In the course of the reaction the mixture is cooled to room temperature. Lower temperatures are advantageous for the formation of inclusion complexes. The urea-paraffin-adduct can be filtered off and thereby separated from the iso-paraffins and other non-paraffinic components. By washing with a solvent a solid adduct residue is obtained. The washing of the clathrates with hot water at about 75 °C breaks up the clathrates and releases the paraffins. The obtained n-paraffins have a purity of about 99%. Losses of urea are small, the hot urea solution can be returned directly back into the process.

Literature

* Kenneth D. M. Harris: ''Fundamental and Applied Aspects of Urea and Thiourea Inclusion Compounds.'' In: ''Supramolecular Chemistry.'' 19, 2007, p. 47-72, .

References

{{reflist Chemical processes