Petroleum refining

PETROLEUM REFINING involves the recovery and processing of usable ``Fractions" from crude oil. Crude oil itself is a complex mixture of many different compounds that are predominantly hydrocarbon in nature. To introduce some chemistry (not much I promise you!), these compounds vary from the relatively simple paraffin hydrocarbons (waxes) to the complex cycloaliphatic and aromatic systems. Other organic compounds containing nitrogen, oxygen, and sulphur also occur in petroleum together with smaller amounts of the heavy metals nickel and vanadium that exist predominantly as porphyrins. (This article is partly based on information in the McGraw Hill Encyclopaedia of Science and Technology).

A petroleum refinery is a complex but integrated facility in which the crude oil is ``converted" into a variety of products as shown in the diagram which shows the various processes in a refinery optimised for, or stressing, the recovery of gasoline (petrol). Indian refineries give more importance to the extraction of diesel, which is what most of our crude is converted into. The diesel comes from the middle-distillate, light and heavy gas oil, which is why it is called gas oil in some countries such as Japan.

Indian refineries also `divert' most of the naphtha to produce fertilizers and petrochemicals. Aviation turbine fuel (ATF) is a special type of kerosene and is part of the middle-distillates group that extends from heavy oils to kerosene. Naphtha is also used to produce electricity in some power plants, as are some liquid petroleum products. The heaviest fractions are not shown and are primarily used to produce electrical power and some types of fertilisers. In spite of this multi-stage recovery process some of the heaviest fractions have to be thrown away as sludge in waste ponds.

Production of some fractions (particularly the light and middle ones) can be optimised relative to others, through the `cat cracker' and even more so through the `hydro cracker' shown in the diagram as optimising the production of gasoline. Hydro- crackers have recently been installed in most of our refineries to partly sate our insatiable demand for diesel. Crackers have also proved to be very necessary in producing low sulphur fuels.

The recovery of useable products from petroleum can be conveniently subdivided into four distinct categories: separation, conversion, reorganisation, and finishing. Separation involves the division of crude oil into various streams (or fractions) to isolate the desired products. Conversion is the thermal treatment of petroleum which results in the production of a more saleable product and which usually involves thermal conversion (cracking) of higher molecular weight species to lower molecular weights. Reorganisation procedures involve building the desired product properties by chemical or thermal reaction, including processes such as reforming, isomerisation, and alkylation. Finishing is the ``purification" of the various product streams by a variety of processes (such as hydro treating) to remove deleterious components (such as sulphur) from the streams. It is beyond the scope of this article to give a detailed description of all these stages and I will restrict myself to detailing some of the features of the first two - separation and conversion.

Separation

The first step in a crude oil refining operation is the desalting and dewatering operations. The salt content of the crude that enters the refinery can be as high as 4-5 per cent, and the water content can be high because water is present as an emulsion. The high temperatures of the heater tubes make the introduction of wet crude into them dangerous. In addition, the salt would precipitate onto the tube walls, thereby reducing the efficiency of the heaters. The crude oil is therefore first heated, an emulsion breaker is added, and the resultant mass is settled (or even filtered) to remove the salt and water.

The major separation procedure is distillation, in which the crude is passed through heaters where its temperature is raised to about 340 degree C at which temperature all of the gas, gasoline, jet fuel and light fuel oil fractions are in the vapour (gaseous) phase. The vapour and liquid mixture enters a distillation tower into the bottom of which steam is introduced to make the separation easier. From the top of the tower some gases are evolved and sent to units which process `light ends'. The next higher-boiling fraction is the gasoline followed successively by jet fuel, gas oil, cracking stock and lubricating distillate. Below the feed entrance a non-volatile fraction (the residium) is removed.

Steam distillation is used to increase the amount of distilled products obtainable at a fixed feed temperature. Vacuum distillation is used for additional separation of the crude residue, lube stock, and other fractions to avoid the potentially undesirable thermal decomposition of, for example, lube stock.

Conversion

Conversion processes are those that bring about a change in the number of carbon atoms per molecule and, in some instances, alter the molecular carbon-to-hydrogen ratio. These processes are often referred to as cracking processes and involve the use of high temperatures (over 350 degree C) whereby the higher molecular weight constituents of the crude oil are converted to lower molecular weight products.

Cracking may be achieved either by thermal means (maintaining the heavy fractions at high temperatures) or by catalytic means. In thermal cracking the charge stocks are usually light and heavy gas oils, residual oils, or any of the fractions heavier than gasoline. Catalytic cracking involves the contacting the fraction with a catalyst under lower pressure conditions than in thermal cracking, although the temperatures are almost as high. Catalytic cracking results in much better (in quantity and quality) yields of gasoline and of the middle distillates (diesel and kerosene).

C. Manmohan Reddy

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