How Do You Get Petrol From Thick Black Oil? Break the Molecules Up

Distilling crude oil separates it into fractions by boiling point, which gives gases, petrol, kerosene, diesel and a large quantity of heavy residue, and the proportions are determined by the crude rather than by what anybody wants. Demand does not match that distribution at all, since transport fuels are wanted in far greater quantity than heavy residue is. Cracking resolves the mismatch by breaking the long molecules of the heavy fractions into shorter ones that fall into the ranges that are wanted, which roughly doubles the yield of usable fuel from a given quantity of crude. Refineries are therefore built around cracking units rather than around distillation alone.

The modern arrangement is continuous and circulates its catalyst:

• •Heavy oil is sprayed onto extremely hot powdered catalyst
• •The mixture flows up a reactor as a fluidised suspension, reacting in seconds
• •Products are separated and sent onwards for further processing
• •Carbon deposits on the catalyst and deactivates it within seconds
• •The catalyst flows into a regenerator where that carbon is burned off
• •Burning it supplies the heat the reaction needs, and the catalyst returns

Breaking long hydrocarbon molecules can be done with heat alone, which was the earlier method and which requires very high temperatures and produces a poor mixture of products. A catalyst lowers the temperature required and steers the reaction towards the branched and aromatic molecules that make good petrol rather than towards straight chains and coke. Modern catalysts are built around zeolites, which are aluminosilicate minerals with a regular framework of pores of a precise size, so molecules enter the pores and react at sites inside them, and the pore size determines which molecules can enter and which products can leave. That shape selectivity is why the catalyst controls the product distribution rather than merely speeding the reaction.

Breaking large molecules is one of several conversion processes and they work in different directions. Reforming rearranges straight-chain molecules into ring and branched ones, which raises the octane rating of petrol and produces hydrogen as a by-product that the rest of the refinery consumes. Alkylation does the opposite of cracking, joining small molecules into larger ones in the petrol range. Hydrotreating adds hydrogen under pressure to remove sulphur and nitrogen, which is required to meet fuel regulations and which produces the sulphur that is sold as a commodity. Coking takes the heaviest residue and drives it to the extreme, producing light products and solid carbon. A modern refinery runs all of these and adjusts the balance between them to match demand.

The development was driven by fuel demand and by war. Thermal cracking was introduced from 1913 and increased petrol yields substantially. Eugene Houdry developed a catalytic process in the 1930s using a fixed bed of catalyst that had to be taken offline for regeneration, which worked and was cumbersome. The fluidised design, in which powdered catalyst circulates continuously between reactor and regenerator, was developed by a consortium during the Second World War and came into operation in 1942, supplying high-quality aviation fuel in quantities that are generally credited with a meaningful effect on the air war. The design has been refined continuously since and remains the dominant arrangement worldwide.

Distillation yields whatever the crude contains, which is far more heavy residue than the market wants, so breaking long molecules into shorter ones roughly doubles the usable fuel. Powdered catalyst circulates continuously between a reactor where it works for seconds and a regenerator where burning off carbon deposits supplies the heat. Zeolite pores of a precise size steer the products, and the fluidised design entered service in 1942.