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Petroleum Refining

Study of Organic chemistry and the study of the compounds starts with the study of alkanes. These are the most simple and less reactive compounds which give a wide range of ideas about the properties of other organic compounds. In order to study the nature of the element Carbon in its covalent valency state, one has to study the variety of Alkanes.

Alkanes are the saturated compounds of carbon and hydrogen. They are least reactive because all the valences of carbon are saturated and they are also called as paraffin because of this non reactive nature. The catenation property of carbon in an order of increasing by one CH2 unit gives the gradual change in the physical properties.
Petroleum is one of the chief sources of alkanes. The extraction procedure and the isolation of different alkanes present in petroleum is a complicated technology.

 

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.
  5. This gas that comes of mainly consists of methane and other gaseous homo logs. Along with it comes out some volatile liquid components of petroleum with 3-7 carbon atom chains.
  6. 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.

Petroleum Refining Process

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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.

Preparation of Alkanes

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There are many methods for the preparation of alkanes. Some important ones are

1. Sabetier and Senderen’s method


It is by a reduction catalyst Nickel on unsaturated hydrocarbons By this method alkane with same number of carbon atoms can be obtained.

-HC=CH- + H2 $\xrightarrow[3000C]{Ni}$ -CH2-CH2- and

-C≡C- + 2H2 $\xrightarrow[3000C]{Ni}$ -CH2-CH2-

Heating an alcohol with excess of concentrated HI in presence of red Phosphorus at 150°C alkanes with same number of carbon atoms can be prepared. The purpose of red P is to regenerate HI from Iodine that is formed. This reaction can also be performed by using R-COOH instead of R-OH but in a sealed tube at 200°C or RCO-R(ketone) under pressure at 150°C.

Reduction of alkyl iodides with nascent hydrogen produced in situ by Cu, Zn couple in alcohol, or by Pd catalyst in presence of hydrogen or by decomposition of Grignard reagent in water.

2. By Wurtz reaction

  1. Alkyl halides when treated with metallic sodium in ether medium produces alkane with double the number of carbon atoms that are there in alkyl halide.
  2. When two alkyl halides of different alkyl groups are subjected to the Wurtz reaction, a mixture of compounds with the alkyl group combinations is formed.
  3. Alkyl halides heated in an inert solvent with powdered zinc also gives alkanes and this reaction is called Frankland’s method.

3. Kolbe’s Electrolytic method


In this method sodium or potassium salt of a carboxylic acid is subjected to electrolysis. This produces an alkane with one carbon atom less than the carboxylic acid taken. One carbon atom escapes as carbon di oxide.

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