duminică, 7 aprilie 2013


Thermal decomposition of hydrocarbons

Like all organic chemical compounds, hydrocarbons are stable at relatively low temperatures only reinstate. When heated, a longer time is decomposition in [H - (hydro) and carbon (carbide)]. It is known for long that this transformation does not occur directly, but through a series of intermediate transformations that give rise to other hydrocarbons with a relatively higher stability at high temperatures. By this thermal decomposition can be a source of new oil that underlies some important techniques. Although the chemical reactions are the same type usually differ, as the temperature at which the decomposition of hydrocarbons by reactions of cracking or rupture (in6500).Thermal decomposition reactions are never simple, so it does not get a single new hydrocarbon, but forming mixtures more or less complex. Analysis of these mixtures is sometimes grea.De therefore not exactly known thermal decomposition products than simple hydrocarbons. Another complication comes from the fact that new formats can suffer hydrocarbons in turn bring their own transformations giving rise to other hydrocarbons, which are found mixed with primary byproducts of decomposition. Therefore, the research chimicede thermal decomposition reactions have to distinguish between primary byproducts, from initial hydrocarbons and coproducts rezultatidin descompunereaprodusilor mayors.

Thermal decomposition can be studied by a static method, heated to a certain temperature hydrocarbon enclosed in a container and then analyzing the resulting byproducts, either by a dynamic method hydrocarbon passing through a tube heated to deliberate, so that the action of heat to perform in a given time, usually short, which depends on the speed of passage of the tube.Such reaction products are removed from under the influence of heat and the formation of secondary products is prevented or delayed more. Factors influencing thermal decomposition are, first, the temperature and duration of heating. Between them there is a correlation. As the heating time is shorter, the temperature must be higher to achieve the same degree of long descompunere.Timpurile favors secondary reactions. An important factor is the pressure, but less influencing breaking reactions (monomolecular) and especially synthesis (bimolecular).Container or tube surface may also play a role especially iron and nickel accelerates decomposition with charring age and form hydrogen and methane. These metals give rise to heterogeneous surface reactions which play a role catilic. Glass, quartz of chromium metal and chromium steels have high-surface reactions. In pots or tubes made from these materials homogeneous pyrolysis reactions take place.

Methane has a much different attitude to other hydrocarbons, because it has only one carbon atom in molecula.El is distinguished by high stability, which surpasses all hydrocarbons (except acetylene). When not in front catalysts (homogeneous phase), methane is stable up to about 900 degrees (in the presence of catalysts balance somewhere in CH4 = C +2 H2).Alkanes with two or more carbon atoms in the molecule starts to decompose at lower temperatures than methane, namely at lower temperatures so the molecule is larger as 4000-6000.The main primary reactions that occur are: a broken linkC - C, with the formation of an alkane and an alkene of less than plus dehydrogenation. Examples are butane mentioned thermal decomposition at 600 degrees and atmospheric pressure in these reactions are occurring simultaneously:- CH3CH2CH2CH3 ® CH4 + CH2 = CH - CH3- CH3CH2CH2CH3 ® CH3 - CH3 + CH2 = CH2- CH3CH2CH2CH3 ® CH3 CH = CH-CH3 + H2

100 moles of butane which is broken down:- 48 follows the path 1- 36 follows the path 2- 16 follows the path of three.

As seen above reaction is dominated by such reactions come demetanarea.Din large amounts of methane gas found in industrial cracking alkanes hidrocarburilor.La higher breaking bonds C - C is made anywhere of the molecule at random , so we obtain a complex mixture of alkanes and alkenes smaller. Dehydrogenation reaction plays an important role in thermal decomposition only lower alkanes. It is the main reaction in the sealing occurs at a rate of 35-39% for propane, butane 16% toand go falling to higher alkanes.

The isobutane chemical reactions of thermal decomposition are:- (CH3) CH2 - CH3 → (CH3) 2C = CH2 + H2 (64%)- (CH3) 2CH - CH 3 → CH 3 - CH = CH2 + CH4 (36%)

Cicloalcanii break, when thermally decomposed in the same way as alkanes and alkenes give results

The cyclopentane pass in ethylene and propylene:
CH3 - CH = CH2

CH2 = CH2

Thermodynamics cracking reactions
Of all organic reactions, decomposition reactions of hydrocarbons Terem were most carefully studied the thermodynamic point of view which is explained by their great technical importance

The two reactions of decomposition of hydrocarbon dehydrogenation or breakage (cracking) can be represented by the general formula:- CH2 N + 2 → CnH2n + H2 (type .1)- Cm + 2H2 (m + n) + 2 → CmH2m + CnH2n +2 ​​(typ. 2)

Applying these formulas equations of chemical free enthalpy formulation of the elements of alkanes and alkenes is reached by a simple calculation, two free enthalpies variation equations:(At a pressure of 1 atm.)for the dehydrogenation reaction: G0 = 30,200 to 33.8 Tthe cracking reaction: G0 = 18.940 - 33.8 T

As you can see, these reactions are approximately independent of the number of carbon atoms in the molecule. Both reactions were positive free enthalpy positive at low temperatures.From the thermodynamic point of view, cracking is favored over dehydrogenation because for cracking the free enthalpy G0 = 0-2700C and becomes negative above this temperature, while the dehydrogenation G0 = 0-6220.This provision of the theory is tested experimentally for higher alkanes, CAE is virtually cracheaza dehydrogenation, but as seen above, the dehydrogenation of lower alkanes C4 in response to homogeneous uncatalyzed, gas phase plays an important role.

Methane is much more stable than other hydrocarbons provided from thermodynamic considerations, the reaction will be favored by dementare:→ Cn H2n + 2 - n-1 + 1H2 CH4and to a lesser extent, deethanizer. This explains the large amounts of methane gas occurring in thermal cracking. Kinetic Interpretation - Mechanistic methane formation but see further.The mechanism of the reaction of cracking
Cracking reactions in homogeneous gaseous phase, the resulting saturated hydrocarbons by free radicals. A small free radical, eg. CH3 - extract a hydrogen atom in the molecule of higher hydrocarbons.CH3CH2CH2CH2CH2CH2CH2CH2CH2R + CH3CH3CH2CHCH2CH2CH2CH2CH2CH2R + CH4Radical is formed discontinue the B to carbon radical so formed a new alkene and a new radical:→ CH3CH2CH = CH2 + CH2CH2CH2CH2CH2R

In this radical ruptures continue until you reach a methyl or ethyl which initializes a new chain of reactions:CH2CH2CH2CH2CH2R → CH2 = CH2CH2CH2R, etc..

The radical intermediates are produced and the isomerization odd electron in an atom of carbon trecabd radical break now secundar.Noul b, with the formation of propylene:CH2CHCH2CH2CH2R → CH3CH = CH2 + CH2CH2R.On the basis of these reactions can provide quantitative reaction products which arise from an alkane cracking unit. Check experimentation led to results that confirm the exact theory