Thermal cracking (2)
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Thermal conversion processes include thermal cracking, visbreaking, coking, and coke calcination. Thermal cracking involves cracking large hydrocarbon molecules into smaller ones at high temperatures. Visbreaking is a mild thermal cracking process used to reduce the viscosity of residues and produce fuel oil, naphtha, and gas oil. Coking involves heating residues to very high temperatures to produce coke and lighter hydrocarbon products.
Thermal cracking (2)
- 1. SECONDARY PROCESSING : Thermal conversion process THERMAL CONVERSION PROCESSES • Thermal Cracking • Visbreaking • Coking • Coke Calcination
- 2. • 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
- 3. 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.
- 4. • 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.
- 5. Gas+ gasoline F Gas oil Cyclone R A Heavy Distillate C + Recycle Long T I Residue O N Furnace A Furnace T O Cracked R Residue Cracked Residue Gas+ Gasoline F R Gas Oil A C T Short I Residue O N A Furnace T O R Cracked Residue Thermal cracking process
- 6. 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 – 500 Furnace 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 Yield, wt. % on feed Long residue Short residue Products (Two furnace operation) (One furnace operation) C1 – C4 4 2 C5 – 1650C 8.5 4 165 – 3500C 23.5 12
- 7. • 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.
- 8. 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 Conventional 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.
- 9. Coil Visbreaker Gas Heater Gasoline Reduced Crude Steam Charge Gas Oil Tar
- 10. Typical operating condition Equipment Operating parameter Furnace Furnace inlet temp. 0C 3400C Furnace outlet temp. 0C 470 – 4900C Furnace inlet pres; kg/cm 2 22 Furnace outlet pres; kg/cm 2 15 Residence time ; minutes 10 Quench temp; 0C 260 Columns Fractionate Stabilizer Top temp. 0C 138 50 Bottom temp 0C 360 160 Pressure kg/cm 2 5.5 8.5 Feed temp. 0C - 100 Flash zone temp. 0C 395 -
- 11. Typical yield pattern Gas plus loss 3 Naphtha 4 Fuel oil (Visbroken 93* residue) *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.
- 12. • 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.
- 13. Gas Soaker Visbreaker Naphtha Steam Steam Gas Oil Soaker Drum Tar Heater Residual Stock
- 14. Typical operating condition Equipment Operating parameter Soaker Drum Pre. 5 – 15 bar (g) Temp. 4400C Vapour Cracking minimum Liquid Cracking Yes Run length 300 days Yield Pattern Product Yield; % of feed Gas 1.7 Naphtha (80-2000C) 3.1 Light gas oil (200-3500C) 13.2 Heavy gas oil (350-5200C) 27.0 Residue (5200C+) 55.0
- 15. 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.
- 16. • 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.
- 17. Decoking Delayed Coking Waterjet Gas Accumulator Naphtha Coke Drum P/A Fractionator Steam Stripper Steam Heater Light gas oil Heavy Steam gas oil Steam Gen. Feed Condensate Drum Coke
- 18. 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.
- 19. • 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.
- 20. 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.
- 21. • Typical operating conditions Furnace outlet temp. 4800 – 5150C Coke Drum Pressure 1.7 Kg/cm2g Fractionator top temp. 1500C Fractionator Bottom temp. 3500C Fractionator Pres. 1.4 – 3.4 Bar Typical yield from delayed coker Feed 0 API 7.4 12.0 12.5 Sulfur wt% 2.76 1.6 0.6 CCR wt % 19.6 9.6 14.8 Yields Gases 13.2 12.0 6.4 Naphtha 11.1 15.7 21.6 Light gas oil 31.4 35.2 46.0 Heavy gas oil - 15.5 - Coke 44.3 21.6 26.0 Sulfur in coke wt% 0.57 1.2 2.2
- 22. • 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.
- 23. • 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 Description Calcination 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.
- 24. To Stock Steam Green Coke Fuel Gas Incinerator Rotary Kiln & Boiler Rotary Cooler Coke Fines Calcined coke Coke calcination process