Thermal Cracking

Is defined as the thermal decomposition, under pressure, of large Hydrocarbon (HC) molecules to form smaller molecules. Lighter, more valuable HC may be obtained from such relatively low-value stocks as heavy gas oils (boiling up to 540 °C (1005 °F)) and residues.

a) Thermal cracking is normally conducted at temperatures varying from 455 to 730 °C.
b) The important reactions that occur are C–C bond scission, dehydrogenation, isomerization, and polymerization.
c) Olefinic hydrocarbons may be formed by the dehydrogenation of paraffins.
d) Coke formation is an additional reaction.
e) Olefins formed undergo cracking and then repolymerize; their hydrogen content continues to decrease.

1) Coking
Coking is a severe cracking operation designed to completely convert residual products such as pitch or tar into gas, naphtha, heating oil, gas oil, and coke.
The gas oil fraction represents the major product obtained and is used as feedstock for catalytic cracking units. The C5–220 °C naphtha may be used as a gasoline blending agent, although its octane number (65–80 RON, unleaded) is lower than desirable. The coke is usually used as fuel.

After something like a ten-year gap, there has been renewed interest in coking. Key reasons are the diminishing relative demand for fuel oils and the increasing focus on reducing atmospheric pollution. For the latter, coking not only concentrates feedstock sulfur in the coke but also yields products that can be readily desulfurized.

The main uses of petroleum coke are as follows:

Fuel
Manufacture of anodes for electrolytic cell reduction of alumina
Direct use as a chemical carbon source for the manufacture of elemental phosphorus, calcium carbide, and silicon carbide
Manufacture of electrodes for electric furnace production of elemental phosphorus, titanium dioxide, calcium carbide, and silicon carbide
Manufacture of graphite

The major coking processes in use today:

1) Delayed Coking:
A semi-continuous process in which the heated charge is transferred to large soaking (or coking) drums that provide the long residence time needed to allow cracking reactions to proceed to completion. The feed to these units is normally atmospheric residue, although cracked tars and heavy catalytic cycle oils may also be used.

Process Description — Delayed Coking
Hot fresh liquid feed is charged to the fractionator two to four trays above the bottom vapor zone. This accomplishes the following:

The hot vapors from the coke drum are quenched by the cooler feed liquid, preventing significant coke formation in the fractionator and simultaneously condensing a portion of the heavy ends for recycling.
Any remaining material lighter than the desired coke drum feed is stripped (vaporized) from the fresh liquid feed.
The fresh feed liquid is further preheated, making the process more energy-efficient.

Vapors from the top of the coke drum return to the base of the fractionator.
These vapors consist of steam and the products of the thermal cracking reaction.

2) Fluid Coking:
A continuous process that uses the fluidized-solids technique to convert residues to more valuable products. The residue is coked by being sprayed into a fluidized bed of hot, fine coke particles. The use of a fluid bed permits the coking reaction to be conducted at higher temperatures and shorter contact times than in delayed coking. Steam is used for fluidizing the bed.

Visbreaking Process

•Visbreaking, a mild form of thermal cracking, significantly lowers the viscosity of heavy crude-oil residue without affecting the boiling point range. Residual from the atmospheric distillation tower is heated (800°-950° F) at atmospheric pressure and mildly cracked in a heater .

•It is then quenched with cool gas oil to control over cracking, and flashed in a distillation tower. Visbreaking is used to reduce the pour point of waxy residues and reduce the viscosity of residues used for blending with lighter fuel oils. Middle distillates may also be produced, depending on product demand. The thermally cracked residue tar, which accumulates in the bottom of the fractionation tower, is vacuum flashed in a stripper and the distillate recycled.

•There are two types of visbreaker operations, coil and furnace cracking and soaker cracking.

•Coil cracking (Figure 1) uses higher furnace outlet temperatures[885–930°F (473–500°C)] and reaction times from one to three minutes,while soaker cracking (Figure 2)uses lower furnace outlet temperatures [800–830°F (427–443°C)] and longer reaction times.

•The product yields and properties are similar, but the soaker operation with its lower furnace outlet temperatures has the advantages of lower energy consumption and longer run times before having to shut down to remove coke from the furnace tubes. Run times of 3–6 months are common for furnace visbreakers and 6–18 months for soaker visbreakers. This apparent advantage for soaker visbreakers is at least partially balanced by the greater difficulty in cleaning the soaking drum.

Catalytic Cracking

Catalyst: Both systems use basically similar catalysts but produced in different forms—beds for moving-bed units and fine powder for fluidized-bed units.
• Acid-treated clays ground to a powder
• Synthetic silica–alumina catalysts of higher activity (amorphous)
• Crystalline synthetic silica–alumina catalysts called zeolites or molecular sieves

The advantages of zeolites over the natural and synthetic amorphous catalysts are:

Higher activity.
Higher gasoline yields at a given conversion.
Production of gasoline containing a larger percentage of paraffinic and aromatic HC.
Lower coke yield.
Increased iso-butane production.
Ability to achieve higher conversion per pass without over-cracking.

It is mainly used for the production of high–octane number hydrocarbons. In catalytic reforming, the change in the boiling point of the feedstock passed through the unit is relatively small, as the HC molecules are not cracked but their structures are rearranged to form higher-octane aromatics. Thus, catalytic reforming primarily increases the octane number of motor gasoline rather than increasing its yield. Typical feedstock is HSR gasoline and naphtha (180 to 375 °F).

In addition to being a gasoline blending stock, reformate is the main source of aromatic bulk chemicals such as benzene, toluene, xylene, and ethylbenzene, which have diverse uses—most importantly as raw materials for conversion into plastics.

The commonly used catalytic reforming catalysts contain noble metals such as platinum and/or rhenium, which are very susceptible to poisoning by sulfur and nitrogen compounds. Therefore, the naphtha feedstock to a catalytic reformer is always pre-processed in a hydrodesulfurization unit, which removes both sulfur and nitrogen compounds. Most catalysts require sulfur and nitrogen content to be lower than 1 ppm.

There are many chemical reactions that occur in the catalytic reforming process
(PONA) {Paraffin, Olefin, Naphthene, Aromatic}:

P → isomerizes to some extent, converted to N, and N is subsequently converted to aromatics.
O → saturated to form P, which then reacts as in (1) {hydrocracking}. The hydrocracking of paraffins is the only reforming reaction that consumes hydrogen.
N → converted to aromatics {dehydrogenation}.
A → unchanged.

Reforming is classified as:

Continuous: Catalyst can be regenerated continuously and maintained at high activity (higher capital cost).
Semi-regeneration: Regeneration is required at intervals of 3 to 24 months depending on operation severity (low capital cost). High H₂ recycle rates and operating pressure are used to minimize coke.
Cyclic: A compromise between the two extremes, having a swing reactor for regeneration.

• Reforming Catalyst: All reforming catalysts contain platinum supported on a silica–alumina base. In most cases, rhenium is combined with platinum to form a more stable catalyst that permits operation at lower pressure.
• Platinum serves as the catalytic site for hydrogenation and dehydrogenation reactions.
• Chlorinated alumina provides an acidic site for isomerization, hydrocracking reactions, and cyclization.
• Space velocity: The volume or weight of gas and/or liquid passing through a given catalyst or reactor space per unit time, divided by the volume (or weight) of catalyst through which the fluid passes. High space velocity corresponds to short reaction time.
• WHSV: Weight hour space velocity = weight of feed per hour per weight of catalyst.
• LHSV: Liquid hour space velocity = volume of feed per hour per volume of catalyst.

Hydrocracking

Hydrocracking is a two-stage process combining catalytic cracking and hydrogenation, wherein heavier feedstocks are cracked in the presence of hydrogen to produce more desirable products. The process employs high pressure, high temperature, a catalyst, and hydrogen. Hydrocracking is used for feedstocks that are difficult to process by either catalytic cracking or reforming, since these feedstocks are usually characterized by a high polycyclic aromatic content and/or high concentrations of the two principal catalyst poisons, sulfur and nitrogen compounds.

• The hydrocracking process largely depends on the nature of the feedstock and the relative rates of the two competing reactions, hydrogenation and cracking. Heavy aromatic feedstock is converted into lighter products under a wide range of very high pressures (1,000–2,000 psi) and fairly high temperatures (750°–1,500° F), in the presence of hydrogen and special catalysts. When the feedstock has a high paraffinic content, the primary function of hydrogen is to prevent the formation of polycyclic aromatic compounds.

• Another important role of hydrogen in the hydrocracking process is to reduce tar formation and prevent buildup of coke on the catalyst. Hydrogenation also serves to convert sulfur and nitrogen compounds present in the feedstock to hydrogen sulfide and ammonia.

• Hydrocracking produces relatively large amounts of isobutane for alkylation feedstock. Hydrocracking also performs isomerization for pour-point control and smoke-point control, both of which are important in high-quality jet fuel.

Thermal Cracking

Delayed Coking Product Yields