Thermal cracking is the oldest and, in a way, the simplest cracking process. It basically aims at the reduction of molecular size by application of heat without any additional sophistication such as catalyst or hydrogen. At a temperature level of 450-500 C, the larger hydrocarbon molecules become unstable and tend to break spontaneously into smaller molecules of all possible sizes and types. By varying the time, temperature and pressure under which a particular feedstock remains under cracking conditions, the desired degree of cracking (conversion) can be controlled. Temperature and time (residence time) are important process variables pressure plays a secondary role.
Obviously, the cracking conditions to be applied and the amount and type of cracked products will depend largely on the type of feedstock. In practice, the feedstock for thermal cracking is a mixture of complex heavy hydrocarbon molecules left over from atmospheric and/or vacuum distillation of crude. The nature of these heavy, high molecular weight fractions is extremely complex and much fundamental research has been carried out on their behaviour under thermal cracking conditions.
However, a complete and satisfactory explanation of these reactions that take place cannot be given, except for relatively simple and well-defined types of products. For instance, long chain paraffinic hydrocarbon molecules break down into a number of smaller ones by rupture of a carbon-to-carbon bond (the smaller molecules so formed may break down further). When this occurs, the number of hydrogen atoms present in the parent molecule is insufficient to provide the full complement for each carbon atom, so that olefins or "unsaturated" compounds are formed. The rupturing can take place in many ways, usually a free radical mechanism for the bond rupture is assumed.
However, paraffinic hydrocarbons are usually only a small part of the heavy petroleum residues, the rest being cyclic hydrocarbons, either aromatic or naphthenic in character. In these, the rupture takes place in the paraffinic side-chain and not in the ring. Other side reactions also take place. In particular, the condensation and polymerisation reactions of olefins and of the aromatics are of considerable practical importance, since they can lead to undesirable product properties, such as an increase in the sludge or tar content. Hence, in practice, it is very difficult to assess the crackability of various feedstocks without plant trials. The final products consist of gas, light hydrocarbons in the gasoline and gasoil range and heavier products. By selection of the type of unit, feedstock and operating conditions, the yields and quality of the various products can, within limits be controlled to meet market requirements.
The maximum conversion that can be obtained will be determined by the quality of the bottom product of the thermal cracker, thermally cracked residue. This stream is normally routed to the fuel oil blending pool. When the cracking has taken place at a too high severity, the fuel can become 'unstable' upon blending with diluent streams (see below). Normally, the refinery scheduler will assess what the maximum severity is that the thermal cracking unit can operate on, without impacting on the stability of the refinery fuel blending pool.
When thermal cracking was introduced in the refineries some 80 years ago, its main purpose was the production of gasoline. The units were relatively small (even applying batch processing), were inefficient and had a very high fuel consumption. However, in the twenties and thirties a tremendous increase in thermal cracking capacity took place, largely in the version of the famous DUBBS process, invented by UOP. Nevertheless, thermal cracking lost ground quickly to catalytic cracking (which produces gasoline of higher octane number) for processing heavy distillates with the onset of the latter process during World War II. Since then and up to the present day, thermal cracking has mostly been applied for other purposes : cracking long residue to middle distillates (gasoil), short residue for viscosity reduction (visbreaking), short residue to produce bitumen, wax to olefins for the manufacture of chemicals, naphtha to ethylene gas (also for the manufacturing of chemicals), selected feedstocks to coke for use as fuel or for the manufacture of electrodes.
In modern oil refineries there are three major applications of the thermal cracking process:
- THERMAL GASOIL PRODUCTION
Visbreaking (i.e. viscosity reduction or breaking) is an important application of thermal cracking because it reduces the viscosity of residue substantially, thereby lessening the diluent requirements and the amount of fuel oil produced in a refinery. The feed, after appropriate preheat, is sent to a furnace for heating to the cracking temperature, at about 450-460 degrees C. The cracking takes place to a small extent in the furnace and largely in a soaker (reaction chamber) just downstream of the furnace. At the soaker outlet, the temperature is lower than at the furnace outlet (soaker inlet) because the cracking reactions are endothermic. The products are quenched at the soaker outlet to stop the cracking reaction (to prevent excessive coke formation). After that, the products enter the fractionator at a temperature level of 300- 400 degrees C and from here onward the processing is similar to any normal distillation process. The products are separated into gas, gasoline, kero, gasoil and residue. The residue so obtained has a lower viscosity that the feed (visbreaking), which leads to a lower diluent requirement to make the fuel on specification for viscosity. The up-flow soaker provides for a prolonged residence time and therefore permits a lower cracking temperature than if the soaker was not used. This is advantageous as regards cost in furnace and fuel. Modern soakers are equipped with internals so as to reduce back mixing- effects , thus maximising the viscosity reduction. Since only one cracking stage is involved, this layout is also named one-stage cracking. The cracking temperature applied is about 440-450 degree C at a pressure of 5-10 barg in the soaker. The fractionator can be operated at 2-5 barg, depending on furnace constraints, condenser constraints and fuel cost.
Thermal Gasoil Production
This is a more elaborate and sophisticated application of thermal cracking as compared with visbreaking. Its aim is not only to reduce viscosity of the feedstock but also to produce and recover a maximum amount of gasoil. Altogether, it can mean that the viscosity of residue (excluding gasoil) run down from the unit is higher than that of the feed.
In the typical lay out is the first part of the unit quite similar to a visbreaking unit. The visbroken residue is vacuum-flashed to recover heavy distillates, which are then sent back to a thermal cracking stage, together with heavy distillate recovered from the fractionator, in a second furnace under more severe cracking conditions ( temperature 500 degrees C; pressure 20-25 barg) . More severe conditions are necessary because the feedstock has a smaller molecular size and is therefore more difficult to crack than the larger residue molecules in the first stage. This layout is referred to as tow-stage cracking.
This is an even more severe thermal cracking application than the previous one. The goal is to make a maximum of cracking products - distillates - whereby the heavy residue becomes so impoverished in hydrogen that it forms coke. The term "delayed" is intended to indicate that the coke formation does not take place in the furnace (which would lead to a plant shutdown) but in the large coke drums after the furnace. These drums are filled/emptied batch-wise (once every 24 hours), though all the rest of the plant operates continuously. A plant usually has two coke drums, which have adequate capacity for one day's coke production (500-1500 m2). The process conditions in the coke drum are 450-500 degrees C and 20 - 30 bar. Only one coke drum is on-line; the other is off line, being emptied or standing by. Only the vapour passes from the top of the coke drums to the fractionator, where the products are separated into the desired fractions. The residue remains in the coke drum to crack further until only the coke is left. Often the heaviest part of the fractionator products is recycled to feed.