Destructive distillation is the chemical process involving the decomposition of feedstock by heating to a high temperature; the term generally applies to processing of organic material in the absence of air or in the presence of limited amounts of oxygen or other reagents, catalysts, or solvents, such as steam or phenols. It is an application of pyrolysis. The process breaks up or 'cracks' large molecules. Coke, coal gas, gas carbon, coal tar, ammonia liquor, and "coal oil" historically, are examples of commercial products of the destructive distillation of coal.
Destructive distillation of any particular inorganic feedstock produces only a small range of products as a rule, but destructive distillation of organic materials commonly produces very many compounds, often hundreds, though not all chemical products of any particular process are of commercial importance. The molecules distilled off generally are smaller and more volatile than the feedstock molecules, but some reactions polymerise or condense small molecules into larger molecules, including heat-stable tarry substances and chars. Cracking into liquid and volatile compounds, and polymerisation or the formation of chars and solids may all occur in the same process, and any class of the products might be of commercial interest.
Historically the process of destructive distillation and other forms of pyrolysis led to the discovery of many chemical compounds or elucidation of their structures before contemporary organic chemists had developed the processes to synthesise or specifically investigate the parent molecules. It was especially in early days that investigation of the products of destructive distillation, like those of other destructive processes, played parts in enabling chemists to deduce the chemical nature of many natural materials. Well known examples include the deduction of the structures of pyranoses and furanoses.
The process of pyrolysis can be conducted in a distillation apparatus (retort) to form the volatile products for collection. The mass of the product will represent only a part of the mass of the feedstock, because much of the material remains as char and non-volatile tars. This is in contrast to simple combustion, where the fuel is oxidized; typically the combustion products of a hydrocarbon fuel amount to roughly four times the mass of the fuel consumed.
Destructive distillation and related processes are in effect the modern industrial versions of traditional charcoal burning crafts. As such they are of industrial significance in many regions, such as Scandinavia. The modern processes are sophisticated and require careful engineering to produce the most valuable possible products from the available feedstocks.
Destructive distillation is an important chemical process producing thousands of distinct chemical compounds. One tonne (1000kg) of coal produces 700 kg of coke, 100 liters of liquor ammonia, 50 liters of coal tar and 400 m3 of coal gas.
Wood can be used to produce guaiacol, tar, terpenes, turpentine and methanol together with a solid residue of charcoal. Coal is used to produce coal gas and coal tar and coke. The destructive distillation of bituminous coal produces 'coal tar pitch' a volatile organic compound (CTPV) which contains a wide variety of polycyclic aromatic hydrocarbons (PAHs); or polynuclear aromatic hydrocarbons, or (PNAs) including naphthalene and anthracene which sublime readily, which is convenient when it is necessary to refine them and also pyrene. Many PNAs such as benzopyrene are carcinogenic. Isoprene was first isolated by destructive distillation of natural rubber which is the biochemical starting point for the biosynthesis of terpenes and for many industrial syntheses as well.
It also is an increasingly promising method for recycling monomers derived from waste polymers.
- Lunge, George. Coal-tar and ammonia Publisher: Gurney and Jackson,1887. May be downloaded from: http://www.archive.org/details/coaltarandammon00lunggoog
- Speight, James G. The Refinery of the Future. Publisher: William Andrew 2010 ISBN 978-0-8155-2041-2
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- I.L. Finar Organic Chemistry vol 1 ( 4th.ed.) Longmans 1963 plus I.L. Finar Organic Chemistry vol 2 ( 3rd.ed.) Longmans Green & Co. 1964 May be downlaoded from: http://www.archive.org/details/OrganicChemistryVol1 plus http://www.archive.org/details/OrganicChemistryVol2
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