Destructive distillation is achemical process in whichdecomposition ofunprocessed material is achieved by heating it to a high temperature; the term generally applies to processing oforganic material in the absence of air or in the presence of limited amounts of oxygen or otherreagents,catalysts, orsolvents, such assteam orphenols. It is an application ofpyrolysis. The process breaks up or "cracks" large molecules.Coke,coal gas,gaseous carbon,coal tar,ammonia liquor, andcoal oil are examples of commercial products historically produced by the destructive distillation ofcoal.
Destructive distillation of any particular inorganic feedstock produces only a small range of products as a rule, but destructive distillation of many organic materials commonly produces very many compounds, often hundreds, although not all products of any particular process are of commercial importance. The distillate are generally lower molecular weight. Some fractions howeverpolymerise orcondense small molecules into larger molecules, including heat-stabletarry substances andchars. Cracking feedstocks into liquid and volatile compounds, and polymerising, or the forming of chars and solids, may both occur in the same process, and any class of the products might be of commercial interest.
Currently the major industrial application of destructive distillation is tocoal.[1][2]
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 the 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.[3] Well known examples include the deduction of the structures ofpyranoses andfuranoses.[4]
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In his encyclopedic workNatural History (Latin:Naturalis Historia) the Roman naturalist and authorPliny the Elder (23/24 –79 CE) describes how, in the destructive distillation of pine wood, two liquid fractions are produced: a lighter (aromatic oils) and a heavier (pitch). The lighter fraction is released in the form of gases, which are condensed and collected.[5]
In Europe, tar is extracted from the torch-tree [Pinus mugo] by the agency of fire; it is employed for coating ships and for many other useful purposes. The wood of the tree is chopped into small billets, and then put into a furnace ... The first steam that exudes flows in the form of [a liquid] into a reservoir made for its reception: in Syria this substance is known as “cedrium” [cedar oil]; and it possesses such remarkable strength, that in Egypt the bodies of the dead, after being steeped in it, arepreserved from all corruption. The liquid that follows is of a thicker consistency, and constitutes pitch.
The process ofpyrolysis 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, ash, and non-volatile tars. In contrast,combustion consumes most of the organic matter, and the net weight of the products amount to roughly the same mass as the fuel and oxidant consumed.
Destructive distillation and related processes are in effect the modern industrial descendants of traditionalcharcoal 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.[6][7]
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