Wednesday, October 13, 2021

SS3 CHEMISTRY LESSON NOTE: PETROLEUM REFINING


 

1.0     Introduction

Fractional distillation is the process by which oil refineries separate crude oil into different, more useful hydrocarbon products based on their relative molecular weights in a distillation tower. This is the first step in the processing of crude oil, and it is considered to be the main separation process as it performs the initial rough separation of the different fuels. The different components that are separated out during this process are known as fractions. Fractions that are separated out include gasoline, diesel, kerosene, and bitumen. Fractional distillation allows a lot of useful products to be made from crude oil, with many environmental consequences for the use of those useful products

1.1     Process

The process of fractional distillation is fairly simple, but is powerful in the way that it separates all the different, complex components of crude oil. First, the crude oil is heated to vapourize it and is fed into the bottom of a distillation tower. The resulting vapour then rises through the vertical column. As the gases rise through the tower, the temperature decreases. As the temperature decreases, certain hydrocarbons begin to condense and run off at different levels. Each fraction that condenses off at a certain level contains hydrocarbon molecules with a similar number of carbon atoms. These boiling point 'cuts' allow several hydrocarbons to be separated out in a single process. It is this cooling with the height of the tower that allows for the separation.

After this rough refinement, individual fuels may undergo more refinement to remove any contaminants or undesirable substances, or to improve the quality of the fuel through cracking.

1.1.1  Fractions of petroleum

There are several ways of classifying the useful fractions that are distilled from crude oil. One general way is by dividing into three categories: light, middle, and heavy fractions. Heavier components condense at higher temperatures and are removed at the bottom of the column. The lighter fractions are able to rise higher in the column before they are cooled to their condensing temperature, allowing them to be removed at slightly higher levels. In addition to this, the fractions have the following properties:

  • Light distillate is one of the more important fractions, and its products have boiling points around 70-200°C. Useful hydrocarbons in this range include gasoline, naphta (a chemical feedstock), kerosene, jet fuel, and paraffin. These products are highly volatile, have small molecules, have low boiling points, flow easily, and ignite easily.
  • Medium distillate are products that have boiling points of 200-350°C. Products in this range include diesel fuel and gas oil - used in the manufacturing of town gas and for commercial heating.
  • Heavy distillates are the products with the lowest volatility and have boiling points above 350°C. These fractions can be solid or semi-solid and may need to be heated in order to flow. Fuel oil is produced in this fraction. These products have large molecules, a low volatility, flow poorly, and do not ignite easily.


However, there are two major components that are not accounted for in these three categories. At the very top of the tower are the gases that are too volatile to condense,such as propane and butane. At the bottom are the "residuals" that contain heavy tars too dense to rise up the tower, including bitumen and other waxes. To further distill these they undergo steam or vacuum distillation as they are very useful.

1.2     Locations of petroleum refinery in Nigeria

The downstream industry in Nigeria is well established. NNPC has four refineries, two in Port Harcourt (PHRC), and one each in Kaduna (KRPC) and Warri (WRPC). The refineries have a combined installed capacity of 445,000 bpd. A comprehensive network of pipelines and depots strategically located throughout Nigeria links these refineries. 

Cracking of hydrocarbons

The process of breaking higher hydrocarbons with high boiling points into a variety of lower hydrocarbons that are more volatile (low boiling), is called cracking (or pyrolysis). For example, a higher hydrocarbon C10H22 splits according to the reaction.

The process of cracking, increases the relative amounts of the lower hydrocarbons. During cracking, carbon-carbon bonds get broken in a random manner, leading to various kinds of products being formed.

Types of cracking

Thermal cracking

Breaking down large molecules by heating at high temperature and pressure is termed as thermal cracking. Thermal cracking is further classified into the following classes.

  • Liquid phase thermal cracking process: The higher boiling fractions e.g., fuel oil, lubricating oil are converted into low boiling fractions by heating the liquids at a temperature of 750 K, under a pressure of about 10 atmosphere.
  • Vapor phase thermal cracking process: Low boiling fraction e.g., kerosene is cracked in the vapor phase at a temperature of about 875 K and under a pressure of 3 atmosphere.

Catalytic cracking

Higher hydrocarbons can also be cracked at lower temperature (600 - 650 K) and lower pressure (2 atm) in the presence of a suitable catalyst. Catalytic cracking produces gasoline of higher octane number and therefore this method is used for obtaining better quality gasoline. A typical catalyst used for this purpose is a mixture of silica (SiO2), 4 parts; alumina (Al2O3), 1 part, and manganese-dioxide (MnO2), 1 part.

Steam cracking

Here, higher hydrocarbons are mixed with steam in their vapor phase and heated for a short duration to about 900°C, and cooled rapidly. This process is suitable for obtaining lower unsaturated hydrocarbons.

1.3     Reforming

Reforming, in chemistry, processing technique by which the molecular structure of a hydrocarbon is rearranged to alter its properties. The process is frequently applied to low-quality gasoline stocks to improve their combustion characteristics. Thermal reforming alters the properties of low-grade naphthas by converting the molecules into those of higher octane number by exposing the materials to high temperatures and pressures. Catalytic reforming uses a catalyst, usually platinum, to produce a similar result. Mixed with hydrogen, naphtha is heated and passed over pellets of catalyst in a series of reactors, under high pressure, producing high-octane gasoline.

Reforming or aromatisation involves the conversion of open chain (aliphatic) hydrocarbons and/or cycloalkanes in the presence of a catalyst, into aromatic hydrocarbons (arenes) containing the same number of carbon atoms. Aromatisation involves reactions of the type, dehydrogenation, cyclisation, and isomerisation.

In reforming (or aromatisation), cyclic and acyclic alkanes containing six to eight carbon atoms are heated at about 670 K in the presence of palladium, platinum or nickel as catalyst. Platinum seems to be the best catalyst and so the process is sometimes called platforming.

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