SECONDARY PROCESSINGSECONDARY PROCESSING : :Thermal conversion processThermal conversion process

THERMAL CONVERSION THERMAL CONVERSION PROCESSESPROCESSES

• Thermal CrackingThermal Cracking• VisbreakingVisbreaking• CokingCoking• Coke CalcinationCoke Calcination

• THERMAL CRACKING The processes is which hydrocarbons are decomposed at elevated temperatures to from material of lower Mol. wt. are called thermal conversion process. Any fraction of the crude from Naphtha to Vac. Residue – can be processed thermally. The most important types of thermal conversion process are thermal cracking, visbreaking and coking. Thermal cracking is used for conversion of residues and higher Mol. wt. hydrocarbons into more useful products by cracking the large hydrocarbon molecules into smaller ones at a temp. level of 4500 – 5000C. Cracking activity varies with the type of hydrocarbons and decrease in the following order:n–paraffin > Isoparaffins > cycloparaffins > aromatics > aromatics / naphthenics > polynuclear aromatics. Olefins crack to smaller olefins and di-olefins

Important variables in thermal cracking are Temp.; pressure and residence time. Cracking reactions begin to occur at Temp. of 315 – 3700C. Pressure determines the phase in which the cracking reactions take place. Thermal cracking conversion increases with temp. and residence time. Under very severe thermal cracking conditions, there is tendency for coke formation. Cracking also generates double bonded hydrocarbons (olefins). Side reactions like condensation and polymerization reactions also occur leading to gum formation and tar-like polymerization products. (To avoid this, gasoline or diesel blend produced from thermal cracking process are hydrotreated to make them stable usable product).Since products of thermal cracking have very poor stability and require further treatment ; Fluid catalytic cracking FCC finds more favors with refiners.

• Simplified Thermal Cracking Process :

Simple Thermal cracking process produces gas, naphtha, middle distillates and thermal tar from almost all variety of charge stocks from distillates to the heaviest crude and residual oils.

The feed is heated to cracking temp. 4500 – 5000C and the cracked products containing gas and Full Boiling Range distillates enters the fractionators after passing through an intermediates separator vessel.

LongResidue

ShortResidue

Furnace

Furnace

Furnace

CrackedResidue

Cyclone

Gas+ gasoline

Gas oil

Heavy Distillate + Recycle

Cracked ResidueGas+ Gasoline

Gas Oil

Cracked Residue

FRACTIONATOR

FRACTIONATOR

Thermal cracking process

TYPICAL operating conditions and The Yield Patterns in The SHELL process for Long Residue (Atmospheric Distillation column bottoms) and Short Residue (Vacuum Distillation column bottoms) are : Cracking temperature, 0C 450 – 500Furnace outlet pressure, kgf/cm2

(a) For residue 4(b) For heavy residue 20

Typical Yield Pattern The yield of fractions in the Shell process for long residue and short residue

ProductsProducts

Yield, wt. % on feedYield, wt. % on feed

Long residueLong residue Short residueShort residue

(Two furnace operation)(Two furnace operation) (One furnace operation)(One furnace operation)

CC11 – C – C44 44 22

CC55 – 165 – 16500CC 8.58.5 44

165 – 350165 – 35000CC 23.523.5 1212

Separator vessel bottoms (residual oil)Separator vessel bottoms (residual oil) 4545 --

Fractionator bottoms (residual oil)Fractionator bottoms (residual oil) 1919 8282

• VISBREAKING

Visbreaking – an abbreviation for viscosity breaking or viscosity lowering – is a liquid phase thermal conversion process to reduce the viscosity of Atmospheric (long reduce) and Vacuum (short residue) to produce specification fuel oil. Small quantities of LPG and a fair amount of naphtha are also produced.

Visbreaking is a mild thermal cracking process and help in reducing the viscosities and pour point of long and short residues. Refinery production of heavy oils can be reduced by 30% using visbreaking. Visbreaking also produces gas, gas oil stock and gasoline which go for further processing.

The principal reaction which occur during The Visbreaking are : Cracking of the side chains attached to cycloparaffins and aromatic rings. Cracking of resins to light hydrocarbons (primarily olefins) Some cracking of naphthene rings under higher temp.

of operations (5000C) 2. types of Visbreaking operations :

Conventional Visbreaking (Furnace or coil cracking) Soaker Visbreaking

Also known as coil cracking, the process uses Furnace outlet temp. of 475-5000C and reaction time from one to three minutes. This process produces minimum of Naphtha and a maximum of fuel oil from long and short residues and other heavier feed stocks.

Gas, Naphtha and light gas oils are recovered from the top section of the fractionators. Gas and Naphtha can be further processed in a gas concentration unit for the recovery of LPG.

Conventional Visbreaking :

Coil Visbreaker

ReducedCrude Charge

Tar

Gas Oil

Gasoline

Steam

Gas

Heater

Typical operating condition

EquipmentEquipment Operating parameterOperating parameter

FurnaceFurnaceFurnace inlet temp. Furnace inlet temp. 00CC 34034000CC

Furnace outlet temp. Furnace outlet temp. 00CC 470 – 490470 – 49000CC

Furnace inlet pres; kg/cm Furnace inlet pres; kg/cm 22 2222

Furnace outlet pres; kg/cm Furnace outlet pres; kg/cm 22 1515

Residence time ; minutesResidence time ; minutes 1010

Quench temp; Quench temp; 00CC 260260Columns

FractionateFractionate StabilizerStabilizer

Top temp. Top temp. 00CC 138138 5050

Bottom temp Bottom temp 00CC 360360 160160

Pressure kg/cm Pressure kg/cm 22 5.55.5 8.58.5

Feed temp. Feed temp. 00CC -- 100100

Flash zone temp. Flash zone temp. 00CC 395395 --

Typical yield pattern

Gas plus lossGas plus loss 33

NaphthaNaphtha 44

Fuel oil (Visbroken Fuel oil (Visbroken residue)residue)

9393**

*If diesel production is to be maximized, a second cracking furnace is added to the stream and the heavy fraction boiling between 350 – 5000C, obtained by distilling the visboken residue under vacuum is recycled to second furnace for further cracking. Run lengths of 3 – 6 minutes are common for coil visbreakers.

• Soaker Visbreaking :

In this process a soaker drum is added between the furnace and the fractionator. This drum provide large residence time for the feedstock. The cracking reactions take place in soaking drum. Since higher residence time allows improved conversion at lower temps. ; Soaker Visbreaking Technology is more energy efficient and provides higher run lengths as coking reactions in the furnace coil are significantly reduced. Run lengths to 6-18 months for Soaker Visbreaker. Other advantages of Soaker Visbreaking are : Lower capital cost (10/15% lower) Smaller furnace ; less waste Heat Recovery equipment Less pressure drop through furnace Lower fuel consumption (15% less fuel : 0.2% on feed) Better and more selective yields.

Soaker Visbreaker

Tar

Gas Oil

Naphtha

Steam

Gas

ResidualStock

Heater

Steam

SoakerDrum

Typical operating condition EquipmentEquipment Operating parameterOperating parameter

Soaker DrumSoaker Drum

Pre.Pre. 5 – 15 bar (g)5 – 15 bar (g)

Temp.Temp. 44044000CC

Vapour CrackingVapour Cracking minimumminimum

Liquid CrackingLiquid Cracking YesYes

Run lengthRun length 300 days 300 days

Yield PatternYield Pattern

ProductProduct Yield; % of feedYield; % of feed

GasGas 1.71.7

Naphtha (80-200Naphtha (80-20000C)C) 3.13.1

Light gas oil (200-350Light gas oil (200-35000C)C) 13.213.2

Heavy gas oil (350-520Heavy gas oil (350-52000C)C) 27.027.0

Residue (520Residue (52000C+)C+) 55.055.0

Visbreaking is an effective and cost – effective way to produce more valuable products from heavy residues. Earlier, it used to reduce the viscosity and/ or pour point of a fuel oil but now it is employed to obtain Cat. Cracker feed and to reduce fuel oil production.

• COKING Coking is the most widely practiced means of reducing the C – H ratio of residual oils, Of the 2 main process – delayed coking and fluid coking – more than 90% capacity is in delayed coking units. Delayed coking is a thermal cracking process in which a hydrocarbon feedstock, mainly residue is converted to lighter and more valuable products and coke. Main advantage of the process is that it can take residual stocks from a wide variety of process unit in a Refinery Coking Furnace and the coke drums are the key elements in the process. Cracking is initiated in the furnace tubes where short residence time is allowed. Coking of the feed material is delayed until it reaches large coking drums with larger retention time; downstream of the coking heater. Three types of coke structures can be produced shot, sponge or needle coke.

Accumulator

Fractionator

Naphtha

Gas

P/A

Steam Stripper

Steam

Light gas oil

SteamHeavy gas oil

Feed

Steam Gen.

CondensateDrum

Coke

Heater

DecokingWaterjet

Delayed Coking

Coke Drum

Process Description

• Delayed coking is a semi continuous process in which the heated charge is transferred to large coking drums which allow the long residence time needed to allow the cracking reactions to proceed to completion feed to these units is normally heavy atmospheric residues, although heavy catalytic cycle oils and cracked tars may also be used.

• Feedstock gets pre heated by exchange of heat from outgoing products and is partially vaporized in a specially designed coking furnace. Mild cracking takes place in the furnace where thermal cracking temps; of 4850 to 5050C are reached.

• From the furnace, the liquid-vapor mixture goes to the coking drum (operating in batch-1 coking, the other decoking). The vapors under-go cracking as they pass through the coke drum.

•The cracked products go to fractionate where cracked gas, Naphtha, Kerosene and gas oils are separated. The petroleum coke is formed in the drum due to high residence time of cracking in the drum.

• The feed stream is regularly switched between the operating drum and drum under decoking. Decoking is done using high pressure water jets. This generally fallows a 12-16 hr. cycle.

decoking operation

Following procedure is used to remove the coke :

(i) The coke deposit is cooled with water.(ii) One of the heads of the coking drums is removed to permit the drilling of a hole through centre of the deposit.(iii) A hydraulic cutting device, which uses multiple high pressure water jets, is inserted into the hole and the wet coke is removed from the drum.(iv) After the removal of coke from the coke drum, it is flushed with water and is readied for reuse.

• Typical operating conditionsFurnace outlet temp.Furnace outlet temp. 48048000 – 515 – 51500CC

Coke Drum PressureCoke Drum Pressure 1.7 Kg/cm1.7 Kg/cm22gg

Fractionator top temp.Fractionator top temp. 15015000CC

Fractionator Bottom temp.Fractionator Bottom temp. 35035000CC

Fractionator Pres.Fractionator Pres. 1.4 – 3.4 Bar1.4 – 3.4 Bar

Typical yield from delayed cokerTypical yield from delayed coker

FeedFeed00 API API 7.47.4 12.012.0 12.512.5

Sulfur wt%Sulfur wt% 2.762.76 1.61.6 0.60.6

CCR wt %CCR wt % 19.619.6 9.69.6 14.814.8

YieldsYields

GasesGases 13.213.2 12.012.0 6.46.4

NaphthaNaphtha 11.111.1 15.715.7 21.621.6

Light gas oilLight gas oil 31.431.4 35.235.2 46.046.0

Heavy gas oilHeavy gas oil -- 15.515.5 --

CokeCoke 44.344.3 21.621.6 26.026.0

Sulfur in coke wt%Sulfur in coke wt% 0.570.57 1.21.2 2.22.2

• Fluid Coking

Fluid coking is a continuous process that uses the fluidized solids technique to convert residue including vacuum pitches to more valuable products. Fluid coking uses 2 vessels – a Reactor and a Burner. Both the reactor vessel and the burner vessel contain fluidized beds with coke particles circulating between the two vessel by fluidized solids technique. Coke particles are circulated to transfer heat to the reactor. The residuum is cooled by distributing it as a thin film of liquid on the outside of the hot-coke particles. The vapor products pass through cyclones that remove most of the entrained coke.

• COKE CALCINATION PROCESS Petroleum coke is produced as delayed sponge coke, delayed needle coke, fluid coke. Calcination of raw petroleum coke (green coke) is needed to transform it into useable material. Calcined coke is mostly used by the Aluminum Industry in the manufacture of anodes for Alumina Reduction, Calcined needle coke is used to manufacture. Graphite products and prebaked graphite electrodes for use in electro – metallurgical furnaces.

• Process DescriptionCalcination of green coke is essentially a high temp. treatment involving drying, devolatilization and dehydrogenation by which the C/H ratio of the feed is increased from about 20 to 1000. It may be carried out in a rotary kiln.

To StockSteam

Green Coke

Fuel Gas

Rotary Cooler

Calcined coke

Rotary Kiln

Coke Fines

Incinerator&

Boiler

Coke calcination process