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

Thermal Cracking - Part 2

7:09:00 AM Cracking

Thermal Cracking - Part 1

History

The thermal cracking method (also known as "Shukhov cracking process") was invented by Russian engineer Vladimir Shukhov and patented in 1891 in the Russian empire. This process was modified by the American engineer William Merriam Burton.

William Merriam Burton developed one of the earliest thermal cracking processes in 1912 which operated at 700 - 750 °F (370 - 400 °C) and an absolute pressure of 90 psi (620 kPa) and was known as the Burton process. Shortly thereafter, in 1921, C.P. Dubbs, an employee of the Universal Oil Products Company, developed a somewhat more advanced thermal cracking process which operated at 750°F -860 °F (400°C - 460 °C) and was known as the Dubbs process The Dubbs process was used extensively by many refineries until the early 1940s when catalytic cracking came into use.

What is thermal cracking?

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.

In the thermal cracking process, the compound to be cracked is subjected to high heat and pressure. Sometimes a catalyst is added to control the chemical reactions which occur during the process, with the goal of promoting the development of specific molecules. Fractions such as gasoline which have a low boiling point will be released first. As the thermal cracking proceeds, fractions of various molecular weights can be extracted and processed further for additional uses, or packaged for transport and sale.

This process creates free radicals at the sites where molecular bonds are broken, which can be harnessed in chemical reactions such as polymerization to create new chemical compounds. A wide variety of compounds are extracted or derived through thermal cracking processes, making this process a valuable part of petroleum refining. The process can be supervised by petroleum engineers or chemists who are familiar with the needs of the market, the product being worked with, and the cracking process.

Sometimes known as pyrolysis because it involves the controlled decomposition of a chemical compound under heat and pressure, thermal cracking is designed to create more useful fractions. The cracking can be adjusted to meet needs like a rise in demand for a particular product, or a shortage of a product caused by changes in refinery capacity. Thermal cracking of petroleum is also often discussed in chemistry classes while introducing students to basic chemical concepts which come up in the refinery industry.

Also we can said The term “thermal cracking” is also used in reference to concrete, asphalt, and similar materials. In this case, low temperatures make the material prone to cracking. Things like asphalt are elastic

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 behavior 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.

Any given molecule in the feedstock has a certain probability of being cracked at any given point on the length of the molecule. Therefore, the cracking process applied to a given feedstock will yield a range of hydrocarbons of differing boiling points. The discipline of process engineering focuses on creating process conditions that will favour the formation of more of the desired molecules and less of the undesirable ones. A certain distribution of cracked products with differing boiling points will emerge from any thermal cracking process. This is referred to as a yield pattern. This is influenced by several factors.

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.

If the [C.sub.30] molecule in the example is not given sufficient time in the thermal cracking section to crack, it will boil over and end up in the gasoil product. This is undesirable since the gasoil will be too heavy as a result. The process conditions that can be varied to accomplish the goal are temperature, pressure, and residence time in the thermal cracker. Pressure raises the boiling point of the hydrocarbon giving it the needed time to have the probability of cracking. Temperature will, to some extent, determine where along the length of the molecule the cracking will occur. Higher temperatures tend to crack pieces off the end of the molecule and the result is lighter cracked products such as gases and naphtha.

 Related to thermal cracking

 Stability :

When a paraffin is cracked, it cracks to smaller paraffins and olefins. Olefins are unstable and reactive. Reactive compounds bond with each other and the result is colour change as well as gum and tar formation in the gasoil. Part of any used oil thermal cracking plant is a stabilization system.

Flash Point :

An important parameter that defines the safety of handling of a particular hydrocarbon is flash point. The flash point of a hydrocarbon is the temperature at which the vapours emitting from a heated sample of product will combust or "flash" when exposed to a source of ignition. In Europe, a flash point above 55[degrees]C is considered a combustible product and relatively safe to handle. A flash point below this value is a flammable liquid. Automotive gasoline is an example of a flammable liquid.

A Commercial Used Oil Thermal Cracker:

A new category of used oil processing has emerged in the past few years. Thermal cracking used oil to gasoil is a process that was previously little known to the used oil processing industry. In a thermal cracking process, large hydrocarbon molecules are broken or cracked into smaller ones by the application of heat under the right process conditions. In this fashion, larger molecules of more viscous and less valuable hydrocarbons are converted into less viscous and more valuable liquid fuels.

Coarse Screening:

The used oil is first passed through mesh screening and as much as possible of the coarse abrasive solids are removed.

Dehydration (Flash Evaporator):

In the next stage of the process, used oil is subjected to sufficient heat to drive off any water and light ends present. These light ends could be gasoline or solvents. Collectively they are referred to as naphtha.

The used oil is preheated by cross exchanging it with hot residual product from the cracking section. In this way, heat balance optimum is achieved. Used oil contains emulsified water at concentrations of anywhere from 6% to as high as 15% or more. Plants can be designed for higher water content feeds if desired. It is simply a matter of proper heat exchange medium being designed to a higher degree of dehydration. The waste water from this stage of the process is separated from the naphtha. The water contains impurities and must be further treated in the plant operator's water treatment facility.

An off-gas consisting of any light non-condensable hydrocarbons such as hydrogen, methane, ethane, propane etc. is routed to plant fuel from this section.


Keep reading :Thermal Cracking - Part 2

Thermal Cracking - Part 1

5:37:00 PM Cracking