Dry distillation
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Dry distillation is the
If there are no chemical changes, just phase changes, it resembles classical distillation, although it will generally need higher temperatures. Dry distillation in which chemical changes occur is a type of destructive distillation or cracking.
Uses
The method has been used to obtain liquid fuels from coal and wood. It can also be used to break down mineral salts such as sulfates (SO2−
4) through thermolysis, in this case producing sulfur dioxide (SO2) or sulfur trioxide (SO3) gas which can be dissolved in water to obtain sulfuric acid. By this method sulfuric acid was first identified and artificially produced. When substances of vegetable origin, e.g. coal, oil shale, peat or wood, are heated in the absence of air (dry distillation), they decompose into gas, liquid products and coke/charcoal. The yield and chemical nature of the decomposition products depend on the nature of the raw material and the conditions under which the dry distillation is done. Decomposition within a temperature range of 450 °C to about 600 °C is called carbonization or low-temperature degassing. At temperatures above 900 °C, the process is called coking or high-temperature degassing.[2] If coal is gasified to make coal gas or carbonized to make coke then coal tar is among the by-products.
Wood
When wood is heated above 270 °C it begins to carbonize. If air is absent, the final product (since there is no oxygen present to react with the wood) is charcoal. If air (which contains oxygen) is present, the wood will catch fire and burn when it reaches a temperature of about 400–500 °C and the fuel product is wood ash. If wood is heated away from air, first the moisture is driven off. Until this is complete, the wood temperature remains at about 100–110 °C. When the wood is dry its temperature rises, and at about 270 °C, it begins to spontaneously decompose. This is the well known exothermic reaction which takes place in
In the common practice of charcoal burning using internal heating of the charged wood by burning a part of it, all the by-product vapors and gases escape into the atmosphere as smoke. The by-products can be recovered by passing the off-gases through a series of water to yield so-called wood vinegar (
See also
References
- ^ Price, Overton W.; Kellogg, R.S.; Cox, W.T. (1909). Forests of the United States: Their Use. Government printing office.
- ISBN 978-3-11-011451-5.
- ISBN 9251013284. Retrieved 15 February 2015.