reactive. To maintain low pressure in the equilibrium system and at the same time a safe pressure in the cracking vessel, steam is used. Since steam is not part of the actual equilibrium, it has no effect on the behaviour of the equilibrium system. To all intents and purposes, the steam is acting as an inert gas and while it contributes to the total pressure in the cracking vessel, it does not have any impact on the position of equilibrium.
………………………
………..ethylene glycol …………………………………………….. ethene
……….. CH2OHCH2OH …………………………………………….. CH2=CH2
The term petrochemical is also often applied to substances such as ammonia, NH3, because the hydrogen used in the reaction N2(g)+3H2(g)⇌2NH3(g), is derived from natural gas. Amonia reaction is a reversible reaction …
What are Reversible Reactions?
A reversible reaction is one that can occur in both directions. They are given the symbol… ⇌
For example Ammonia is made by a reversible reaction. This is summarized by the equation below…
Nitrogen+Hydrogen⇌Amonia
N2(g)+3H2(g)⇌2NH3(g)
As you can see from the equation above – ammonia is made by reacting Nitrogen and Hydrogen together. In terms of chemistry it takes 1 molecule of Nitrogen and 3 molecules of hydrogen to make 2 molecules of Ammonia.
Reversible reactions occur in both directions, that is Nitrogen and Hydrogen make ammonia and Ammonia breaks down to form Nitrogen and Hydrogen. Each reversible reaction has an equilibrium – that is where an equal amount of reactants and products are being made so it would appear that there is no net reaction (no overall reaction).
Controlling a reversible reaction
We can control a reversible reaction in two main ways…
Nitrogen+Hydrogen⇌Amonia
N2(g)+3H2(g)⇌2NH3(g)
..↖..↗..........↖
4 Particles…………………..2 Particles
Less pressure ⟷ More pressure
1. Pressure – by increasing the pressure we force the particles closer together so the reaction will occur in the direction of least particles – in this case increasing the pressure makes more ammonia.
Nitrogen+Hydrogen⇌Amonia
N2(g)+3H2(g)⇌2NH3(g)
Endothermic…………………. Exothermic
More heat ⟷ Less heat
2. Temperature – If we increase the temperature the reaction will favour the endothermic reaction (the one that takes in heat). In this case the reverse reaction (making nitrogen and hydrogen) is endothermic and would increase.
So therefore, to increase the yield of ammonia we can increase the pressure and decrease the temperature.
Petroleum is a naturally occurring material containing over 200 useful compounds, mainly saturated hydrocarbons. It may contain solids and gases as well as liquids.
Natural gas is a mixture of low molecular mass hydrocarbons – 85% methane, CH4, and 10% ethane CH3CH3.
Crude oil, the name by which unrefined petroleum is usually known, is a vital commodity and dominates world trade. However, un refined crude oil has few direct applications. It contains straight and branched chain saturated hydrocarbons, cycloalkanes, aromatic compounds and generally up to five per cent sulfur.
Fractional distillation of crude oil
Crude oil is separated into fractions of similar molecular mass (and boiling point) in a fractionating tower, represented in the diagram below.
Fractional distillation involves heating crude oil to temperatures in excess of 4000C, a temperature high enough to vaporise most of the hydrocarbons present in the mixture. The hot vapours rise up the fractionating column. The temperature gradually falls as they rise up the column and different hydrocarbons will liquefy as the temperature drops below their boiling temperatures.
Bubble caps in the column allow vapour to continue to rise up the column through condensed liquid which collects on trays.
Depending on the temperature, certain hydrocarbons in the vapour stream liquefy on each tray. The mixture of hydrocarbons that collects on a particular tray is called a fraction. Each fraction contains a set of hydrocarbons with a narrow range of boiling temperatures.
The higher boiling temperature fractions are collected near the bottom of the fractionating column. The lower boiling temperature fractions rise to the top of the fractionating column.
Unfortunately, the quantities in which the hydrocarbon fractions are obtained via fractional distillation do not match marketplace demand.
To produce greater quantities of common fuels (which – generally contain the smaller hydrocarbon molecules) – from a particular quantity of crude oil, fractions containing large hydrocarbon molecules are converted to smaller molecules via the process of cracking. This involves heating, in the presence of a catalyst, in the absence of air, and converts high molecular mass hydrocarbons to low molecular mass hydrocarbons.
Cracking is used to increase the yield of petrol from crude oil and to produce unsaturated hydrocarbons, such as ethene and propene, which are extensively utilised in the petrochemical industry. As stated earlier, chemicals produced from crude oil, and via the conversion of components of crude oil (petroleum), are known as petrochemicals. The production of petrochemicals is a vital part of the modern chemical industry.
Common petrochemicals include plastics, medicines, fertilisers and insecticides. Polyethylene wrap used for food, non-stick materials such as Teflon, fibres such as nylon and polyester, and simple medicines such as aspirin are all linked to the petrochemical industry.
Three main uses of crude oil
The fractions obtained from crude oil have three main uses, namely:
- as fuels; for example, petrol
- for their physical properties; for example, oils as lubricants
- as a source of petrochemicals
