How do we treat crude oil, this mixture of molecules ranging from the lightest to the heaviest? We are going to heat it, make it evaporate progressively. For example when we heat the bottom of a saucepan of water, first small bubbles appear before we reach 100°C: this is gas dissolved in the water that is beginning to escape. Then, at 100°C, the water boils vigorously and evaporates completely. On the bottom of the pan, we find whitish residues of salts, which must be heated to a very high temperature if they are to vaporise. For crude oil, the principles are the same.
The crude that arrives in a refinery is going to undergo a series of operations, to finish up with products that we, or industrial users, need in our daily life. There are three main types of operations:
- Separation, to obtain the different types of products from the heaviest to the lightest,
- Transformation (or conversion), to modify the natural proportions of each type of product in order to respond to consumer demand,
- Upgrading, to eliminate undesirable compounds and modify the characteristics of certain products to make them compatible with the norms.
Separation is accomplished by fractional distillation. The crude oil is injected at the base of a 60 m high tower, called a topping unit or an atmospheric distillation column (because the pressure used is close to atmospheric pressure). There, it is heated to 350/400°C. Most of it evaporates and begins to rise inside the column. All that is left at the bottom of the tower are the heavy products, the residues. As the vapour rises, its temperature goes down. The heaviest fractions of these vapours are going to condense into liquids, which can be recovered on trays situated at different levels in the column. And so on, until the top of the column where the temperature is 150°C. There we recover the final vapours that have not been condensed: the petroleum gases. This principle allows recovery of 10 or so different types of products, from bitumens to gas, which are called the petrol cuts or fractions.
Column of an atmospheric distillation plant in the Provence refinery at La Mde (France).
Column of an atmospheric distillation plant in the Provence refinery at La Mède (France).
But the heavy residues of this first distillation have in fact retained a significant portion of average density products. So they are subjected to a second, more vigorous distillation, this time under vacuum. Diesel is recovered at the top of the vacuum distillation column, and fuel oils at the bottom.
Conversion: Market demand is for large quantities of light products, but the separation has given significant proportions of heavy fractions. However, we don’t have to stop there, the heavy molecules can be broken down into small pieces! The major operation to convert heavy products to light products is called catalytic cracking. It is carried out at high temperature: 500°C, in the presence of a catalyser (that is to say a substance which encourages chemical reactions without participating directly in them). This treatment is very drastic: more than ¾ of the heavy fractions are transformed into gas, petrol and diesel.
Catalytic cracking plant with fluidised catalyst in the Donges refinery (France).
Catalytic cracking plant with fluidised catalyst in the Donges refinery (France).
The result is even more efficient if hydrogen is added (hydrogen cracking), or if the procedures of carbon extraction are introduced into the process (deep conversion). In fact, all heavy fractions can be transformed into light ones, but a price has to be paid. Deep conversion operations are large consumers of energy.
Upgrading: the products from the distillation and conversion operations must be stripped of molecules, in particular sulphur, that are corrosive or harmful to the environment,.
The desulphurisation of diesel is undertaken at 370°C, under a pressure of 60 bars and in the presence of hydrogen. The sulphur atoms leave the hydrocarbons and bond with the hydrogen to produce hydrogen sulphide, H 2S. This latter product will then be treated to isolate the sulphur, resulting in those large yellow heaps that one sees in certain refinery yards (today sulphur is stocked in tanks maintained at a temperature sufficient to keep it in a liquid state).
The desulphurisation plant in the Anvers refinery.
The desulphurisation plant in the Anvers refinery.
The European norms concerning sulphur emission into the atmosphere are very severe. They have become even more so since the 1 st of January 2005: the refiners will have several years to bring their plants into conformity, that is to say, to rethink and redesign their desulphurisation units to lower even further the sulphur content of fuels.
Moreover, certain products, such as petrol, cannot be used directly in automobile engines because of their self-igniting properties (when your engine goes “boom”). They have an octane rating that is too low. The octane rating measures the tendency of petrol to avoid self-ignition. The nearer it is to 100, the better it is! The numbers 95 and 98 at the petrol pump represent the octane rating. Modern automobile engines, with a very high compression ratio, need petrol with a high octane rating. The main operation that enables the octane rating to be increased is called catalytic reforming.
Alkylation plant at the La Mde refinery (France).
Alkylation plant at the La Mède refinery (France).
The petrol goes through four successive reactors. 500°C, 10 bars of pressure and platinum as a catalyst. Those are the conditions necessary so that we find a good product at the pump! Other methods also lead to an improvement in the octane rating (alkylation, production of the ethers MTBE (methyl tertiary butyl ether) and ETBE (ethyl tertiary butyl ether), which have excellent anti-self-ignition properties.)
There is another upgrading procedure, applied in this case to butane or propane gas and to kerosene. This is the softening process. The products are washed in soda to rid them of mercaptans, nauseating and corrosive sulphuric products (a mercaptan R-S-H is an alcohol where the sulphur atom has taken the place of the oxygen atom). All these operations must be carried out in conditions of the utmost security.
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