Thermal Cracking - Part 2

Applications

    Thermal cracking is currently used to "upgrade" very heavy fractions or to produce light fractions or distillates, burner fuel and/or petroleum coke. Two extremes of the thermal cracking in terms of product range are represented by the high-temperature process called "steam cracking" or pyrolysis (ca. 750 °C to 900 °C or more) which produces valuable ethylene and other feedstocks for the petrochemical industry, and the milder-temperature delayed coking (ca. 500 °C) which can produce, under the right conditions, valuable needle coke, a highly crystalline petroleum coke used in the production of electrodes for the steel and aluminium industries.

In modern oil refineries there are three major applications of the thermal cracking process:

VISBREAKING

THERMAL GASOIL PRODUCTION

COKING

** Visbreaking **

  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.

** Delayed Coking **

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.

Product Quality

Thermally cracked products - distillates - are not suitable for commercial use as produced in other units; they require further refinement or treatment in order to improve their quality, particularly sulfur and olefins content. Formerly, wet treating processes, for example treatment with caustic or an other extraction medium, were applied to remove or "sweeten" the smelly sulfur products, but nowadays the catalytic hydrotreating is employed almost without exception, both for gasoline and for gas oil range products. Of course, the gases too have to be desulphurised before being used as fuel gas within the refinery.

The residual products from thermal cracking are normally not treated any further, except for coke, which may be calcined if the specifications require it to be treated. The cracked residue is normally disposed of as refinery or commercial fuel. Here a very important aspect of the process is the stability of the cracked residues or of the final fuels after blending with suitable diluents. Residue contains asphaltenes,which are colloidally dispersed uniformly in the oil in a natural way. In the cracking process, the character of the asphaltenes as well as of the oil changes, and if the cracking is too severe the natural balance of the colloidal system can be affected to the extent that part of the asphaltenes precipitates in the equipment or in the storage tanks, forming sludge. If the sludge formation is excessive, i.e. above a certain specified limit, the product (fuel) is considered to be unstable.

Plant Operations/Decoking

A practical aspect of operation of thermal cracking units is that, in spite of good design and operating practice, furnaces, and sometimes also other equipment, gradually coke up, so that the unit has to be shut down and decoked. Furnaces can be decoked by " turbining" (using special rotary tools to remove coke from inside furnace pipes) or by steam-air decoking process. In the latter case, the coke is burnt off in a carefully controlled decoking process in which air and steam are passed through the tubes at elevated temperatures. Air serves to burn coke, where as the steam serves to keep the burning temperatures low so that they do not exceed the maximum tolerable temperatures.

More recently, a new decoking method using studded 'pigs' propelled with water, is getting more popular. The plastic pigs have a size slightly smaller than the tube inside diameter and are equipped with metal studs. When the pigs are pumped through the furnace pipes, they move around in a rotating fashion, thus scraping the cokes from the inside of the furnace tubes.

Other coked equipment is usually cleaned by hydrojetting techniques. Owing to these unavoidable stops for decoking, the on-stream time i.e. on stream days per annum, for thermal cracking units is slightly shorter than for most other oil processes.

The main  Products  of thermal cracking

Off-Gases: These light non-condensable gases are typically all consumed in the fired heater.

Naphtha: This light hydrocarbon liquid has a calorific value and all or part of it can be consumed for process heat generation if the fired heater is properly designed to accommodate a liquid fuel. The naphtha does not have a very high octane value. Chlorides in the feed tend to find themselves in the naphtha.

Gasoil: This is the main product of the thermal cracking plant and yields of upwards of 80 per cent by weight of dried feed are achievable. The gasoil will have a sulphur content that is dependent on the sulphur level in the used oil feed and the stabilization method used. The [ROBYS.sub.TM] system reduces sulphur in the gasoil whereas the other methods do little if any sulphur reduction.

Gasoil has a value that is high relative to heavy distillate fuel oils or residual replacement fuels. Gasoil is cleaner burning and the markets for gasoil are very large. From an economic viewpoint, this favours the used oil processing plant since there is not a small group of buyers that would influence or dictate selling price. Also, gasoil is marketable anywhere in the world. Thus the technology has potential application world wide. This could be a significant benefit to the world environment in aiding to solve the used oil problem.

Heavy Residual Fuel: The bottoms from the cracking section have a high calorific value and are usable as a residual replacement fuel. This is dependent on local legislation. 


Read also Thermal Cracking - Part 1