What is cracking in the context of secondary refinery processes?

Cracking is a process used in refineries to break down large, complex hydrocarbon molecules into smaller, more valuable ones. This is typically achieved through the application of heat, pressure, and sometimes catalysts. The main goal of cracking is to convert heavy fractions of crude oil into lighter products like gasoline and diesel.

Explain the process of reforming in petroleum refining.

Reforming is a chemical process used in refineries to convert low-octane naphthas into high-octane gasoline components. This is achieved by rearranging or restructuring hydrocarbon molecules, often using a catalyst such as platinum. The process increases the octane number of the fuel, making it more suitable for use in modern engines.

What is hydrotreating and why is it important in refining?

Hydrotreating is a refining process that involves the treatment of petroleum fractions with hydrogen in the presence of a catalyst. The primary purpose is to remove impurities such as sulfur, nitrogen, and metals, which can cause environmental pollution and damage refining equipment. Hydrotreating improves the quality of the final products and ensures compliance with environmental regulations.

Why is catalytic cracking preferred over thermal cracking in modern refineries?

Catalytic cracking is preferred over thermal cracking because it operates at lower temperatures and pressures, making it more energy-efficient. It also produces a higher yield of valuable products like gasoline and olefins, with better control over the product distribution. Additionally, catalytic cracking results in fewer undesirable by-products compared to thermal cracking.

What happens if the catalyst in a reforming process becomes deactivated?

If the catalyst in a reforming process becomes deactivated, the efficiency of the process decreases, leading to lower conversion rates of naphtha to high-octane gasoline. This can result in a drop in the octane number of the output, making it less suitable for use in engines. Regular regeneration or replacement of the catalyst is necessary to maintain optimal performance.

How does hydrotreating affect the sulfur content in diesel fuel?

Hydrotreating significantly reduces the sulfur content in diesel fuel by converting sulfur compounds into hydrogen sulfide gas, which is then removed. This process is crucial for producing ultra-low sulfur diesel (ULSD), which meets stringent environmental regulations and reduces emissions of sulfur oxides when the fuel is burned.

Explain how cracking contributes to the production of olefins.

Breaking a C-C bond in a paraffin gives a smaller paraffin plus an olefin, so all cracking processes make olefins. In a refinery, fluid catalytic cracking is the largest source of propylene and butylenes, recovered from its LPG stream; high-severity FCC variants push propylene yield further. Ethylene, however, comes mainly from steam cracking of ethane or naphtha at about 800-850 °C with very short residence time, which is a petrochemical rather than a refinery process.

Secondary refinery processes: cracking, reforming, hydrotreating

Thermal cracking, FCC, hydrocracking, catalytic reforming and hydrotreating: catalysts, conditions, reactions and hydrogen balances.

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Why it matters

Straight-run distillation gives too much heavy oil, too little high-octane petrol and products with too much sulphur. Secondary processes fix all three: cracking converts heavy fractions to lighter fuels, catalytic reforming raises octane and supplies the refinery's hydrogen, and hydrotreating removes sulphur and nitrogen to meet BS-VI fuel limits (10 ppm S). Refinery margins and emissions compliance depend on these units.

Key ideas

Thermal cracking works by free-radical chain reactions at high temperature, without a catalyst. Products are rich in olefins and light gases; there is little branching.

Visbreaking: mild cracking of vacuum residue (about 450–480 °C, short residence) to reduce viscosity and cut the diluent needed for fuel oil.
Delayed coking: residue is heated to about 490–500 °C in a furnace and allowed to crack in large coke drums over many hours; products are gas, coker naphtha, coker gas oil and petroleum coke. Drums are switched and decoked by hydraulic cutting.

Fluid catalytic cracking (FCC). Feed: vacuum gas oil or treated residue. Catalyst: zeolite Y (usually ultrastable, USY) in a silica–alumina matrix, as a fine powder that flows like a fluid.

Riser reactor: hot regenerated catalyst meets atomised feed; cracking occurs in a few seconds at about 500–540 °C and near 1–3 bar.
Mechanism: carbenium ions on acid sites. This favours branched paraffins, aromatics and olefins in the C3–C4 range, so FCC gasoline has high octane and FCC is the refinery's main source of propylene. Coke (about 4–6 wt % of feed) deposits on the catalyst.
Regenerator: coke is burnt off with air at about 680–750 °C. The combustion heat is carried by the hot catalyst back to the riser and supplies the endothermic cracking and feed vaporisation; the unit runs in heat balance.
Products: dry gas, LPG, gasoline, light cycle oil (LCO) and clarified slurry oil. Catalytic cracking runs at lower pressure than thermal cracking and gives more gasoline of higher octane and less gas.

Hydrocracking combines cracking and hydrogenation over a bifunctional catalyst (acidic zeolite or amorphous silica–alumina for cracking, Ni–Mo or Ni–W sulphide or noble metal for hydrogenation) at about 350–420 °C and 100–200 bar H₂. It gives saturated, sulphur-free middle distillates (diesel, jet fuel) with high yield and little coke.

Catalytic reforming. Feed: hydrotreated heavy naphtha (sulphur and nitrogen must be well below 1 ppm, because they poison platinum). Catalyst: Pt or Pt–Re (or Pt–Sn for continuous regeneration) on chlorided alumina; metal sites dehydrogenate and acid sites isomerise. Conditions: about 480–530 °C; 15–35 bar for semi-regenerative units, about 3.5–5 bar for continuous catalyst regeneration (CCR) units. Reactions:

Dehydrogenation of naphthenes to aromatics (fast, strongly endothermic, produces H₂).
Dehydrocyclisation of paraffins to aromatics (slow, endothermic, produces H₂).
Isomerisation of paraffins (raises octane).
Hydrocracking (unwanted; consumes H₂ and makes gas). Because the main reactions are endothermic, the unit has 3–4 reactors with interheaters. Low pressure favours aromatics and H₂ yield but increases coking; hydrogen is recycled to limit coke. The product, reformate, has RON around 95–102 and is also the main BTX source.

Hydrotreating. Catalyst: Co–Mo or Ni–Mo sulphides on alumina, about 300–400 °C and 30–130 bar H₂ (higher for heavier feeds and deeper desulphurisation). Reactions: hydrodesulphurisation (HDS, S → H₂S), hydrodenitrogenation (HDN, N → NH₃), olefin and aromatic saturation, and metals removal. H₂S is removed by amine scrubbing and converted to sulphur in a Claus unit. Hardest to treat are hindered dibenzothiophenes in diesel.

Fuel quality numbers. Octane number (RON, MON) measures knock resistance of petrol: high for branched paraffins and aromatics, low for normal paraffins. Cetane number measures ignition quality of diesel: high for normal paraffins, low for aromatics. The two are therefore opposite in what they reward.

Formulas

C₁₆H₃₄ → C₈H₁₈ + C₈H₁₆ (cracking: a paraffin gives a smaller paraffin plus an olefin)

C₇H₁₄ (methylcyclohexane) → C₇H₈ (toluene) + 3H₂; C₆H₁₂ (cyclohexane) → C₆H₆ + 3H₂ (reforming)

C₄H₄S (thiophene) + 4H₂ → C₄H₁₀ + H₂S (HDS)

H₂ for HDS (kmol) = (wt fraction S × feed mass / 32.06) × ν

ν: mol H₂ per mol S for the sulphur compound (4 for thiophene; 1 for a thiol R–SH → RH + H₂S).

Normal volume of gas = n × 22.414 m³/kmol (0 °C, 1 atm)

Worked examples

Example 1 (standard): hydrogen for HDS. A gas oil contains 1.2 wt % S, assumed all as thiophene-type rings needing 4 mol H₂ per mol S. Find the H₂ required per tonne for complete desulphurisation (ignore other reactions).

S = 0.012 × 1000 = 12 kg = 12 / 32.06 = 0.3743 kmol.
H₂ = 4 × 0.3743 = 1.497 kmol.
Mass = 1.497 × 2.016 = 3.02 kg.

Answer: about 1.50 kmol, i.e. 3.0 kg of H₂ per tonne (real units use more because of saturation reactions and solution losses).

Example 2 (GATE level): hydrogen from a reformer. A reformer takes 100 t/day of naphtha containing 30 wt % methylcyclohexane (98.19 g/mol) and 20 wt % cyclohexane (84.16 g/mol). Both are 95 % dehydrogenated to aromatics; ignore other reactions. Find the hydrogen produced in t/day and Nm³/day.

MCH reacted = 30 000 × 0.95 / 98.19 = 290.3 kmol/day.
Cyclohexane reacted = 20 000 × 0.95 / 84.16 = 225.8 kmol/day.
H₂ = 3 × (290.3 + 225.8) = 1548 kmol/day.
Mass = 1548 × 2.016 = 3121 kg/day; volume = 1548 × 22.414 = 34 700 Nm³/day.

Answer: about 3.12 t/day, or 3.47 × 10⁴ Nm³/day of H₂.

Common mistakes

Saying FCC is the main source of ethylene; it is a major propylene source, while ethylene comes mainly from steam cracking.
Thinking reforming is exothermic; its main reactions are endothermic, hence interheaters.
Feeding untreated naphtha to a reformer; sulphur poisons platinum.
Confusing octane and cetane trends: aromatics are good for octane and bad for cetane.
Forgetting the regenerator supplies the FCC reactor heat.
Writing impossible formulas (no CₙH₂ₙ₊₄ exists); balance H atoms carefully.

For GATE CH

Expect matching questions (process ↔ catalyst ↔ main product), FCC heat balance and the role of the regenerator, reforming reaction types and the effect of pressure and temperature, octane and cetane trends, and HDS chemistry. Numericals: cracking stoichiometry, hydrogen consumption or production, conversion and yield calculations.

Quick check

What catalyst is used in FCC?
Which reforming reaction is fastest and produces most hydrogen?
What are typical hydrotreating catalysts?
Why does reforming use several reactors with interheaters?

Answers: 1. zeolite Y (USY) in a silica–alumina matrix; 2. dehydrogenation of naphthenes to aromatics; 3. Co–Mo and Ni–Mo sulphides on alumina; 4. the main reactions are endothermic, so the gas cools in each bed and must be reheated.

Interview questions

All Chemical Technology interview questions

Try answering each one aloud before you open it.

1.What is cracking in the context of secondary refinery processes?Concept
Cracking is a process used in refineries to break down large, complex hydrocarbon molecules into smaller, more valuable ones. This is typically achieved through the application of heat, pressure, and sometimes catalysts. The main goal of cracking is to convert heavy fractions of crude oil into lighter products like gasoline and diesel.
2.Explain the process of reforming in petroleum refining.Concept
Reforming is a chemical process used in refineries to convert low-octane naphthas into high-octane gasoline components. This is achieved by rearranging or restructuring hydrocarbon molecules, often using a catalyst such as platinum. The process increases the octane number of the fuel, making it more suitable for use in modern engines.
3.What is hydrotreating and why is it important in refining?Concept
Hydrotreating is a refining process that involves the treatment of petroleum fractions with hydrogen in the presence of a catalyst. The primary purpose is to remove impurities such as sulfur, nitrogen, and metals, which can cause environmental pollution and damage refining equipment. Hydrotreating improves the quality of the final products and ensures compliance with environmental regulations.
4.Why is catalytic cracking preferred over thermal cracking in modern refineries?Application
Catalytic cracking is preferred over thermal cracking because it operates at lower temperatures and pressures, making it more energy-efficient. It also produces a higher yield of valuable products like gasoline and olefins, with better control over the product distribution. Additionally, catalytic cracking results in fewer undesirable by-products compared to thermal cracking.
5.What happens if the catalyst in a reforming process becomes deactivated?Application
If the catalyst in a reforming process becomes deactivated, the efficiency of the process decreases, leading to lower conversion rates of naphtha to high-octane gasoline. This can result in a drop in the octane number of the output, making it less suitable for use in engines. Regular regeneration or replacement of the catalyst is necessary to maintain optimal performance.
6.How does hydrotreating affect the sulfur content in diesel fuel?Application
Hydrotreating significantly reduces the sulfur content in diesel fuel by converting sulfur compounds into hydrogen sulfide gas, which is then removed. This process is crucial for producing ultra-low sulfur diesel (ULSD), which meets stringent environmental regulations and reduces emissions of sulfur oxides when the fuel is burned.
7.Explain how cracking contributes to the production of olefins.Concept
Breaking a C-C bond in a paraffin gives a smaller paraffin plus an olefin, so all cracking processes make olefins. In a refinery, fluid catalytic cracking is the largest source of propylene and butylenes, recovered from its LPG stream; high-severity FCC variants push propylene yield further. Ethylene, however, comes mainly from steam cracking of ethane or naphtha at about 800-850 °C with very short residence time, which is a petrochemical rather than a refinery process.

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