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Fischer Tropsch

Study of the structure of alkanes is considered as the representative stuy of Organic chemistry and the study of the compounds. Alkanes or saturated hydrocarbons are the simplest and least reactive compounds. They are the sources of wide range of study material with respect to the properties of other organic compounds. In order to study the nature of the element Carbon in its covalent valency state study of variety of alkanes and their properties is a compulsory requirement.

The saturated compounds having carbon and hydrogen are termed as alkanes and any other element or group present in it is called its substitution. The compound is then is known as substituted alkane. Alkanes are least reactive because all the valences of carbon are saturated. They are also called paraffins which means that they show little (para) affinity towards reactions. The addition of one CH2 unit due to the catenation property of carbon gives the gradual change in the physical properties of the homologous series of alkanes.

Alkanes find many uses in everyday life. The safety of not causing any physiological damage to the human life make the alkanes most useful. Vaseline, wax, liquid paraffin, hexane etc find use in the daily life. This property is due to the saturation and less reactivity of alkanes.

Most of the alkanes that are extracted are from petroleum. The extraction procedure and the isolation of different alkanes present in petroleum is a complicated technology like cracking and fractional distillation. Since the amount of petrol extracted from the natural source is not sufficient to satisfy the ever increasing demand of the technologically developing human requirements and luxuries, attempts are being made in various ways either to find an alternative to petrol or to synthesise it from other sources to supplement the demand. In this topic we are going to discuss some of the methods of manufacture of alkanes in general and petrol in particular.

 

Fundamentals of Petroleum Refining

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  1. Petroleum and natural gas are the two main natural sources of Alkanes. Petroleum contains large quantities of volatile and non volatile liquid hydrocarbons along with some solid compounds generally termed as paraffin waxes dissolved in the liquid components.
  2. Alkanes containing upto 40 carbon atoms are present in petroleum. Natural gas contains lower molecular weight ( lower carbon chain) alkanes it contains about 80% methane, 10% ethane and 10% higher alkanes that are gases.
  3. Some nitrogen, hydrogen, and carbon di oxide are also present in natural gas. Ozokerite, the neutral wax is a mixture of higher solid alkanes is found near the oil wells.
  4. An oil well is drilled through the rocky covering acting as protective layer over the petroleum. At first the gases gush out which are called natural gases. This gas that comes of mainly consists of methane and other gaseous homologs.
  5. Along with it comes out some volatile liquid components of petroleum with 3-7 carbon atom chains. This volatile fraction of petroleum which comes out along with the gaseous alkanes is called natural gasoline.
The crude gas obtained from the oil wells is passed through oil which absorbs the natural gasoline fraction. These are called scrubbers and the process is called scrubbing.

Natural gasoline fraction is then recovered from this scrubber oil by the process of distillation. The remaining gases after the scrubbing are natural gas containing mainly methane and ethane in the nearly above mentioned proportions.

Refining of Petrol

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Petroleum refining is done by the process of distillation. Different fractions with different boiling points and boil at definite ranges of temperatures are separated by distillation. The crude petroleum is pumped through heated pipes.

The vaporization takes place here and the vapors formed pass in to a tall fractionating column in which there is gradual decrease in temperature as the vapors go up. Different fractions are withdrawn from the fractionating column at different levels and cooled.
The fractionating column is divided in to compartments. Generally five fractions are collected.
  1. Heavy oil fraction: This fraction is collected from the bottom portion of the distillation column. It is further distilled under reduced pressure which separates the lubricating oils, fuel oil and diesel oil (Low speed diesels). On cooling this fraction wax is obtained which is dissolved in neutral oils to get Vaseline and paraffin wax. This process is called de-waxing.
  2. Gas Oil: High speed diesel is obtained in this fraction. Kerosene or Naphtha: Solvent extraction with liquid sulfur di oxide removes aromatic components leaving these two components.
  3. Gasoline: This fraction is the most important fraction and in demand. Petrol is made from this fraction. It is a complex liquid mixture of hydrocarbons having compounds containing 5 to 10 carbon atom chains. The boiling point range of this fraction gasoline is 40-1800c.
  4. Hydrocarbon Gases: These are collected from the upper portion of the fractionation column. This contains lower boiling members of paraffin and olefin series.

Fischer Tropsch Process

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Fischer-tropsch process involves the following steps.

First step : water gas is produced from coke and steam. Red hot coke and steam are subjected to each other at 1200°c.

C + H2O CO + H2 (water gas)

Second step :
the water gas is mixed with half its volume of H2 and then passed over a mixed catalyst. The catalyst mixture consists of 100 parts of Cobalt, 5 parts of thorium hydroxide, 8 parts of magnesia mixed together in 200 parts of Kieselguhr. Kieselguhr is a diatomaceous earth called diatomite. It is a form of silica composed of the siliceous shells of unicellular aquatic plants. It helps to clarify the product.

The water gas and hydrogen mixture is subjected to the catalyst mixture at 200°C and 5-10 atmospheric pressure. The product is a mixture of hydrocarbons in the petroleum range.

xCO + yH2 mixture of hydrocarbons + H2O

Fischer Tropsch Catalysts

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  • Fischer-tropsch catalysts is a mixed catalyst comprising of metallic and organic material.
  • The catalyst mixture consists of 100 parts of Cobalt, 5 parts of thorium hydroxide, 8 parts of magnesia mixed together in 200 parts of Kieselguhr. Kieselguhr is a diatomaceous earth called diatomite.
  • It is a mixed form of silica composed of the siliceous shells of unicellular aquatic plants. It helps to clarify the product.
  • The water gas and hydrogen mixture is subjected to the catalyst mixture at 200°C and 5-10 atmospheric pressure.
  • The product is a mixture of hydrocarbons in the petroleum range.

Fischer Tropsch Reaction

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The Fischer Tropsch reaction is a three step reaction.

In the first step either methane and oxygen reacts to form carbon monoxide and water, or coal and steam produce carbon monoxide and hydrogen.

2CH4 + 3O2 2CO + 4H2O : C + H2O CO + H2 (water gas)

In the second step Carbon monoxide reacts with hydrogen over catalyst and forms methylene free radical.

CO + H2 HCHO : HCHO HC-OH : HC-OH + H2 CH2 + H2O

In the third step in presence of hydrogen polymerizes to form the desired carbon length hydrocarbon.

(CH2)n + H2 CH3 - (CH2)n – CH3 (where n = 4 – 12)

Fischer Tropsch Mechanism

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This is called controlled polymerization reactions where the production of hydrocarbons with C4-C12 can easily be obtained. Steam passed over carbon produces Carbon monoxide (CO) and hydrogen (H2). Thus on the Catalyst H2, CO, CO2, H2O and O2 are present.

Of these H2 and CO gases are present in saturation. Due to the catalytic interaction carbon monoxide reacts with hydrogen and forms formaldehyde HCHO.

CO + H2 HCHO

HCHO isomerizes in to reactive hydroxy methylene HC-OH. The hydrogen present on the catalyst reacts with this hydroxy methylene and forms Methylene free radical CH2 and H2O.

HC-OH + H2 CH2 + H2O

In presence of hydrogen of the catalyst these •CH2 free radicals combine to form medium range carbon chains. The chain length depends upon the catalyst and its activity.

(•CH2)n + H2 CH3 - (CH2)n – CH3 (where n = 3 – 10 )
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