1

Chapter 6Thermal and catalytic conversion

process

Cracking

• Cracking processes break down heavier hydrocarbon molecules (high boiling point oils) into lighter products such as petrol and diesel. These processes include:

1. Thermal cracking(Pyrolysis)2. catalytic cracking3. hydrocracking

Feed Material

RCO from ADU

The general reaction of thermal cracking:CnH2n ---------- Cn/2Hn+2 + Cn/2Hn

Based on the free radical mechanism• Radical production

C5H12 ------ C5H11* + H*O2 ------- O* +O*

• Propagation C5H12 + H* ----- C5H11* +H2

• Termination C5H11* + C3H7* ------ C8H18

H* +H* ------H2

Thermal cracking

Thermal cracking of reduced crude/vacuum residue at 460-520°C and Pressure of 700 kPaMajor products- Gasoline & middle distillates Thermal cracking of Reduced crude oil Cracking is endothermic reaction. It needs heatProduct yields (Vol%) based on T,P, time.Time of cracking ↑ with ↑ API gravity. So heavier fraction easily cracks.

Gas - 1.0Naphtha 42.0 57.5Heating oil 5.0 23.0Residuum 34.0 18.5

Thermal cracking

Thermal cracking

Properties of cracked materials

1. K value decreases

2. Boiling Point, pour point and µ decreases

3. Un-saturation and aromatics increases

4. Octane no increases

5. Oxidation stability decreases.6. Produce large amount of solid,unwanted

coke. Longer period for cleaning.

Thermal cracking

Cracking reaction are generally first order reactions, so rate

constant

is given by

k = 1/t ln a/(a-x) where

a - % of material in feed

x - % of material that disappears in time t.

As per arrhenius eqn rate constant is,

ln k = -E/RT +Constant

Thermal cracking

The heat of decomposition by empirical formulae

Heat of cracking (KJ/gm) = C (Mc-Mp)/McMp

C = - 1150, M-Mol wt of charge and Product

Heat of decomposition = heat of combustion of products-heat of

combustion of feed stocks

In the present days thermal cracking is working in refineries in

the form of vis-breaking and coking

Viscosity-breaking- Cracking to reduce the viscosity

Heavy crudes or residuum's (bottom of the barrel ) are the feed for this operation. Residue from Atmos. / Vac. distillation units can be used

In india capacity touches 6 lakh barrels per day to get middle distillate.

10-15 kg/cm2 pressure at 425-500°C to reduce viscosity/ pour point.

Visbreaking

Coil/Furnace type- high temp. & short residence time cracking

Visbreaking

T : 470-500 CTime: 5-6 minP – 10 Kg/cm2

Soaker type- Low temperature & Long residence time cracking

Visbreaking

Light gases

T : 425-460 CTime: 15-20 minP – 10 Kg/cm2

Visbreaking- Yield pattern (Based on feed stock)

Luisiana Vacuum Residue

Arabian Light Atmos.residue

Feed stockGravity APICarbon residueSulfur wt %

11.910.60.6

16.9

3.0

Product yieldsNaphtha 6.2 7.8

Light gas oil 6.3

Heavy gas oil 70.8

ResiduumGravity APICarbon residueSulfur wt %

88.411.415.00.6

20.91.3

5.0

FEED PRODUCT

Vis-breaking - Comparison

De-coking

Coil type requires frequent de-coking as coke deposit, fouling on tubes/coils. This is quite labor intensive .

Coil type, tubes are de-coked sequentially without the need to shutdown the vis-breaking operation.

In soaker type require far less frequent de-coking but their being taken out of service normally requires a complete halt to the operation.

Fuel Economylower T is used in soaker type, better fuel economy.Soaker type 15% less capital investment and 30% fuel savings.

CATALYTIC CRACKING

While thermal splitting of C-C bond proceeds via radical mechanism, mainly producing linear cracking products, Catalytic cracking proceeds in the presence of acid catalysts via carbocations giving much more branched products with high octane number

1.Houdry fixed bed CC2.Moving bed3.Fluidized bed

Houdry’s Fixed bed CC

Each reactor was equipped with molten salt heat removal system to remove the heat from regeneration step and it was transferred to the reaction step

FCC

Started in the 1930’s finely divided solids flow like liquids.

Refinery process that provides ~50 % of all transportation fuels indirectly.

Provides ~35 % of total gasoline pool directly from FCC produced naphtha.

~80 % of the sulfur in gasoline comes from the FCC naphtha.

FCC

• Major Secondary Refining Process• Conversion of heavy fractions ( VGO -370 C+)

into lighter fuel products (LPG, Gasoline, Diesel)

• Circulating fluid bed reactor system (Reactor –Regenerator configuration)

• Multi component catalyst system

FCC IS THE WORKHORSE FOR REFINERY - MOST PROFITABLE TOO!

Main Reactions in FCC• Cracking of Paraffins,Naphthenes and

side chain of aromatics• Isomerisation of olefins• Dehydrogenation of Naphthenes and

Olefins• Hydrogen Transfer• Cyclization and condensation of olefins• Alkylation and dealkylation

Main Reactions in FCC

FCC

Catalyst in FCC• For cracking of C-C bonds, the acid

catalysts are added.• First Alcl3 is used, but due to corrosion,

and waste treatment was great.• Zeolite-Y was added to the active

alumina catalyst (silica-alumina) structure. (Si/Al =2.5-3)

Catalyst in FCCBroensted Acid centre

In regenerator more than 700 C, the hydrolysis of framework aluminium occurs, causing partial and even total collapse of zeolite structure

Catalyst in FCC

Formation of bronsted acid sites

Dehydroxylated at 750 F to form Lewis acid sites

OR

Partial De-alumination of framework at 800 C with steam

Next slide

Solid Acid Catalyst-Final structure

Partial de-alumination

Catalyst in FCC

In order to increase the thermal and hydrothermal stability , the rare earth salt solution (mixture of Lanthanum, cerium, neodynium and praseodimium chloride) added to replace the sodium ion present in the structure.

CO combustion promotorsTo facilitate the oxidation of CO to CO2 in regenerator pt or palladium based combustion promoters are used.

Catalyst in FCC

SOx reducing additives

To reduce the SOx emission by adding Inorganic oxide (Al2O3 or MgO )

Oxidation of SO2 to SO3 and formation of metal sulphate on the additive.

In the reactor metal sulphate are reduced to H2S which leaves the FCC unit with cracked products.

Composition of FCC catalyst

Stacked FCC Design

CATALYTICHYDRO-CRACKING

Hydro-cracking (HC) Process• The feedstocks are not suitable for catalytic

cracking because of their high metal, sulfur, nitrogen, and asphaltene contents used in HC.

• The process can also use feeds with high aromatic content.

• Products from hydrocracking processes lack olefinic hydrocarbons.

• The product slate ranges from light hydrocarbon gases to gasolines to residues.

• The process could be adapted for maximizing gasoline, jet fuel, or diesel production.

Hydrocracking Hydrocracking is a catalytic hydrogenation process in

which high molecular weight feedstocks are converted and hydrogenated to lower molecular weight products.

The catalyst used in hydrocracking is a bifunctional one. It is composed of a metallic part, which promotes hydrogenation, and an acid part, which promotes cracking.

Hydrogenation removes impurities in the feed such as sulphur, nitrogen and metals.

Cracking will break bonds, and the resulting unsaturated products are consequently hydrogenated into stable compounds.

Hydrocracking Chemistry

1. Alkane hydrocracking

2. Hydrodealkylation

3. Ring opening

4. Hydroisomerization

5. Polynuclear aromatics hydrocracking

-Products from hydrocracking are saturated. i.e. gasolines from hydrocracking units have lower octane ratings. They have a lower aromatic content due to high hydrogenation activity.

- Products from hydrocracking units are suitable for jet fuel use. Hydrocracking also produces light hydrocarbon gases (LPG) suitable as petrochemical feedstocks.

Hydrocracking Process• Mostly single stage, with the possibility of two operation

modes. Once-through and a total conversion of the fractionator bottoms by recyling.

• In once-though operation, low sulfur fuels are produced and the fractionator bottom is not recycled.

• In the total conversion mode the fractionator bottom is recylced to the inlet of the reactor.

• In the two-stage operation, the feed is hydrodesulfurized in the first reactor with partial hydrocracking. Reactor effluent goes to a high-pressure separator to separate the hydrogen-rich gas, which is recycled and mixed with the fresh feed. The liquid portion from the separator is fractionated, and the bottoms of the fractionator are sent to the second stage reactor.

• Hydrocracking reaction conditions vary widely, depending on the feed and the required products. Temperature and pressure range from 400 to 480°C and 35 to 170 atmospheres. Space velocities in the range of 0.5 to 2.0 hr-1 are applied.

Catalytic hydrocracking

Initial investment and operating cost 1.5

times more than FCC

Demerits

52

FURNACE