Delayed Coking Chapter 5 CHE 735 Navid Omidbakhsh 02

1. Delayed CokingChapter 5CHE 735Navid Omidbakhsh02/11/06

5. Development of Coking Coking evolved from the Dubbs cracker � After World War II railroads shifted from steam todiesel locomotives � Demand for heavy fuel oil sharply declined � Coking increases distillate production & minimizes heavy fuel oil Between 1950 and 1970 coking capacity increase five fold � More than twice the rate of increase in crude distillation capacity

6. Development of Coking

7. Delayed Coking Predominate coking technology is Delayed Coking � Fluid Bed Coking � Flexicoking Delayed Coking technology is relatively inexpensive Considered open art � Companies do license technology emphasizing coke furnaces, special processing modes, & operations

8. Coking Chemistry �Carbon rejection� process � Also removes hydrotreating catalyst poisons May be considered a cycle of cracking & combining � Side chains cracked from thermally stable polynuclear aromatic cores Cleavage of C-C bonds tend to make light hydrocarbons & polynuclear aromatics Liberated side chains can �over crack� to form light gases � Heteroatoms in side chains end up in light products � Polynuclear aromatics combine (condense) to form asphaltenes & ultimately coke Metals & heteroatoms in polynuclear aromatic cores end up in coke

9. Coking Chemistry � High temperatures & low pressures favor cracking � More distillate liquids � Lower yields of coke & hydrocarbon gas High residence time favor the combining reactions Over conversion will reduce distillates & produce coke and hydrocarbon gases

10. Coking Chemistry Cracking reactions � Saturated paraffins crack to form lower molecular weight olefins & paraffins Isomerization is insignificant � Side chains cracked off small ring aromatics (SRAs),cycloparaffins (naphthenes), & polynuclear aromatics (PNAs) Naphthenes may dehydrogenate to aromatics SRAs may condense to larger polynuclear aromatics (PNAs) leave thermally stable PNAs

11. Coking Chemistry Combining reactions. � Low molecular weight olefins form higher molecular weight compounds Two ethylenes to butene � SRAs combine (condensation) to form resins � Resins after cracking off side chains combine their remaining polynuclear aromatics (PNAs) to form asphaltenes � Asphaltenes after cracking off side chains left with large PNAs Partially saturated with short side chains & entangled with heteroatoms The large PNAs precipitate to form crystalline liquids & ultimately solidify to form coke � Metals & sulfur embedded

12. Feed for the Delayed Coker Delayed Coker can process a wide variety of feedstocks � Can have considerable metals (nickel & vanadium), sulfur, resins, & asphaltenes � Most contaminants exit with coke Typical feed is vacuum resid � Atmospheric resid occasionally used Typical feed composition � 6% sulfur � 1,000 ppm(wt) metals � Conradson Carbon Residue (CCR) of 20-30 wt% Feed ultimately depends on type of coke desired

13. Solid Products Coke with large amounts of metals & sulfur may pose a disposal problem � Refiner may have to pay for disposal Product grades � Needle coke � Anode grade � Fuel grade � Shot coke

14. Solid Products Hydroprocessing upstream of delayed coker may be used to make high quality coke Needle coke � FCC cycle oils, gas oils, & resids with low sulfur & aromatics used as feedstocks � Used for electrodes in steel manufacturing Anode grade coke � Resids with small ring aromatics � Used for electrodes in aluminum production

15. Solid Products Fuel grade coke � About 85% carbon & only 4% hydrogen � 11% heteroatoms sulfur, nitrogen, oxygen, vanadium & nickel � Resid high in polynuclear aromatics & sulfur used as feedstock Shot coke � From size of small ball bearings to golf ball � Nearly always try to avoid! Little market Operational problems

18. Light Products Light ends processed in coker gas plant. Liquids � Reduced aromatics & heteroatoms concentrated in coke, but still contain large amounts of sulfur & olefins � Naphtha fraction may be used as catalytic reformer feed after hydroprocessing Only small fraction of gasoline pool � Gas oil fed to catalytic cracker or hydrocracker Hydrocracker preferred � does better job of processing aromatic rings

19. Feedstock Selection Amount of coke formed related to the carbon residue of the feed � Since this parameter correlates well with hydrogen/carbon atomic ratios and indicates coking tendencies, this parameter is especially useful for screening feeds � Three main tests Ramsbottom method (ASTM D-524) Conradson Carbon (ASTM D-189) Microcarbon Residue Test

20. Yields Low yields of liquids relative to hydrocracking � Mass conversion of vacuum resids to liquids about 55% to about 90% for hydrocracking Coke & liquid yields may be estimated by simple equations (misprint pg. 75 � see pg. 89) Coke Yield (wt%) = 1.6 (wt% CCR) Gas (C4-) (wt%) = 7.8 + 0.144 (wt% CCR) Gasoline (wt%) = 11.29 + 0.343 (wt% CCR) Gas Oil (wt%) = 100 - (wt% Coke) - (wt% Gas) - (wt% Gasoline) Gasoline (vol%)=186.5/(131.5+�API) x (wt% Gasoline) Gas Oil (vol%)=155.5/(131.5 +�API )x (wt% Gas Oil)

24. Effect of Operating Variables Pressure and Temperature � Trend to lower pressures Coke drums normally 25 psig but 15 psig common Pressure dictated by the suction pressure of gas compressor removing vapors from fractionator overhead accumulator drum � Lower pressures & higher temperatures increase gas oil yields by favoring cracking reactions Increase the load on the gas compressor Increase probability of shot coke formation, foaming, & coke carryover

25. Effect of Operating Variables Recycle Ratio � Low recycle ratios common � increase gas oil yields & decrease coke yields Contaminants produced with the gas oil, lowering quality Contaminates increase probability of shot coke formation, foaming, & coke carryover

27. Equipment & Operating Modes Typical equipment � Heater (furnace) � Coke drum vessels � Fractionator � Downstream vapor processing vessels Coke drums run in two batch modes � Filling � Decoking Both modes of operation concurrently feed to the fractionator

28. Operating Modes Filling of coke drums � Feed heated in the furnace to cracking temperature � Hot feed routed up from bottom of coke drums � coking reaction occur in drums & solid coke deposited � Gas from top of coke drum fed to fractionator � Continue for full cycle time until coke drum is full Decoking � At end of cycle empty drum switched on-line & filling operations continue � Off-line drum filled with coke undergoes operation Quench step � hot coke quenched with water giving off steam &volatile hydrocarbons Vapors fed to the fractionator Coke drilled out with water drills

29. Operating Modes Fractionator � Vapors compressed & sent to gas plant � Naphtha is condensed from fractionator overhead � Gas oils are two sidestream draws from the fractionator

30. Fresh Feed & Furnace Fresh feed to bottom of fractionator before charging to the furnace Feed heated in furnace � Outlet temperature about 925�F Cracking starts about 800�F Endothermic reactions Superheat allows cracking reactions to continue in coke drums Velocities in the furnace kept high � Prevent coking in the furnace tubes � Delay coking reaction until the coke drums Hence name �Delayed Coking�

31. Fresh Feed & Furnace Steam injected into furnace � Reduce oil partial pressure & increase vaporization � Maintains proper fluid velocities � Provides sufficient wash on tube walls to minimize coking in the tubes Heater design trends due to heavier feeds & longer run lengths � Lower heat fluxes, typically 9,000 Btu/hr-ft2 � More flexible residence times in tubes

32. Coke Drums & Coking Flow direction � Furnace outlet enters from bottom of the coke drum � Vapor flows up the coke drum � Coke solidifies & precipitates in the drum Coking reaction takes about three hours for the coke to fully solidify � Lower molecular weight product exit top of drum as vapors Sent to bottom of fractionator Number of coke drums � Even number of coke drums, typically two or four � One set on-line in a coking step while the other set is being decoked

34. Coke Drums & Coking Increase of throughput capacity � Large coke drums � More coke drums � Good control of the coke fill height Cycle Time � Trend is to decrease cycle time Once typically 24 hour cycle 16 hour cycles now common & preferable 14 hour & 12 hour cycles being designed � Primary purpose for reducing cycle time to increase throughput � Emphasis for decreased cycle time Automated deheading operation Advanced control for heating & cooling

35. Coke Drums & Coking Deheading � Removal of injection equipment & opening of the bottom plate � Automatic deheading may take 15 minutes vs. 45 minutes for manual deheading Impact of heating & cooling cycles � Shorting the cooling, heating, draining, or coke cutting operations may affect fractionator operation & coke size quality � Short cycle times may impose greater heating-cooling cycles on the rum & reduce drum life Thermal stresses of these repeated cycles calls for careful attention to vessel design For example, drum & its supporting skirt are at different differential temperatures during the cycle

36. Coke Drums & Coking Decoking steps � Filled drum removed from service & replaced with empty coke drum � Full drum steamed to remove hydrocarbon vapors � Drum is filled with water until coke bed submerged Controlled cooling rate to minimize thermal effects of temperature swing � Coke cooled to 200�F � Drum drained & deheaded � Coke drilled from the drum � Drum reheaded � Testing & warmed up � Replaces filled drum & switched back into service

38. Coke Drums & Coking Number of Coke Drums � Before 1930, only one coke drum was utilized Generally devoted to producing specialty coke Anodes Completely batch � Currently in sets of 2, 4 or 6 4 drums most common � Number is function of throughput & tradeoff of coke drum diameter & coke cycle time

39. Coke Drums & Coking Size of Coke Drums � Normally 25 feet or more in diameter & three times as high In 1930, typiclly 10 feet in diameter 27 foot drum appeared in 1985, 28 foot in 1995, & 29 foot in 1998 � Diameter set by several factors Ability to cut coke hydraulically Transportation Fabrication capacity Vessel mechanical design

40. Coke Drums & Coking Materials of construction � Low alloy steel clad with stainless Combat hydrogen blistering Sulfide attack Thermal stress Stresses of cutting operation

41. Coke Drums & Coking Coke Drum Fill Height � Filling to maximum height increases capacity of coke drums At end of coking cycle drums are not completely full Volume allowance for foaming � Coke drum instrumentation is used to optimize fill heights

42. Coke Drums & Coking Foaming � Caused by waxy liquid state molecules in top of vessel � Upper portion of coke not hard at end of coking cycle Amorphous mass � large amount volatile material Steam-out step � strips out volatiles & cools agglomerates coke mass into rigid structure � Antifoams commonly used Excessive use increases amount of silicon in product liquids �� degradation of hydroprocessing catalysts Carryover � Foaming � High vapor velocities High capacity boosted by shortened coke cycle times Low operating pressures

43. Coke Product Coke cut out of drum called �green coke� � Varies in size from fines, to particulates, to chunks Shot coke � Hard spheres resembles shot Sizes from small ball bearings to golf balls � High asphaltene content feed requiring very high coking temperatures

44. Automated Drum Switching Provides many benefits � Greater safety � Diminished operational upsets � Reduced hydrocarbons releases to atmosphere � Reduced numbers of personnel required Scheme � Correct set of valves opened & closed automatically as units run through the coke cycle Safety interlocks preventing inappropriate switching

45. Decoking Each coke drum has a drilling rig that raises & lowers a rotating cutting head � Uses high-pressure (4,000 psig) water Steps � Drum cooled & displaced with water to remove volatiles � Pilot hole is drilled through the coke to bottom head � Pilot drill bit replaced with a much larger high-pressure water bit � Cut either from top to bottom or bottom to top � The coke falls from coke drum into a collection system

46. Coke Handling Collection systems � Direct discharge to hopper car Coke drums straddle railroad track Water drains from cars, collected in pit beneath track, & sent to reprocessing Pros Lowest cost option Disadvantages Only used for two drum cokers Logistics of moving cars under the drums Slow cutting rate to avoid damage to cars & give even distribution in cars Personnel required to move cars & distribute load during cutting Fines & dirty water control Dusting controlled by water spray in the cars

47. Coke Handling Collection systems � Pad loading Coke dropped down inclined slide from the coke drum bottom to a pile sitting beside but not under the drum Water drainage from pile on the pad Ports packed with sized coke to retain the fines Coke moved to stockpiling by a front-end loader � Pit & Crane loading Provides for both storage & dewatering of coke Coke loading crane system transfers coke to railway cars Pit minimizes elevation of drums Pit makes dust control easier

48. Coke Handling Collection systems � Slurry loading Most elegant of coke handling options Scheme Coke drops from drum during decoking Passes through crusher Conveyed by a sluiceway to a slurry sump Slurry pump sends coke slurry up into dewatering bins Water from bins drain back to sump Drained coke sent to silos � Must handle slurry water & process discarded water before it goes to refinery water disposal system

49. Coke Handling Other considerations � Method used establishes height coke vessels & drilling derricks above them � Method used determines nature of environmental measures � Coke is handled wet but it dries, leading to dusting & dirty water management � Switching coke drums imposes an incremental cyclic steam & vapor load on the refinery system Steam is consumed in only parts of the decoking cycle Vapor recovery system must handle vapor from flushing a drum before decoking

50. Fractionator Fractionator feeds � Coke drum overhead Early designs Refluxed by cooled fractionator bottom recycle stream Now sent to bottom of fractionator above liquid level � Fresh resid feed Early designs Routed directly to furnace Above hot vapor feed from the coke drum Provided refluxing of hot vapors Now typically sent to bottom of fractionator below the liquid level Strips light ends from the feed Preheats feed Condenses heavy ends from coke drum vapor Suppresses coke formation by cooling and reducing the concentrations of resins in the hot coke drum vapor.

51. Fractionator Flash Zone � Space above the feed to the bottom of the heavy gas oil draw Height of the flash zone about one column diameter(typically 25 feet) � Necessary for liquid disengaging & wash zone equipment � Feeds introduced at liquid pool at bottom of flash zone Quench oil from the gas oil stream injected in the line at the top of the coke drum Suppress coking in line to the fractionator

52. Fractionator Wash Zone � Upper part of the flash zone just below the heavy gas oil draw � Washes vapors coming up the column to remove coke fines, resins & asphaltenes Could cause coking in upper sections of fractionator � Washing is done by a series of spray nozzles across the column area using cooled gas oil Low cost & low pressure drop Current spray designs minimize amount of spray liquid & hence recycle � May use a grid section washed with cooled gas oil Higher pressure drop & prone to clogging � Sieve trays, shed trays, and decks used before grids Often washed with fresh feed slipstream Rarely used Coking, mechanical damage, fluid distribution, high recycle ratios

53. Fractionator Recycle � Ratio of furnace charge (fresh feed plus recycle) to fresh feed Throughput ratio of one indicates no recycle � Used to decrease contaminants (metals, sulfur, Conradson carbon residues) in the heavy gas oil Recycle ultimately cracked & contaminants contained with the coke � Types Internal � condensed liquids to fractionator from coke drum overhead feed External � quench flows to the top of coke drum & gas oil flows to the wash section � Current designs minimize recycle Increased resins & asphaltenes in coke decreases gas oil yields Equipment is larger � Anode & needle grade cokes usually made with higher recycles than fuel grade coke

54. Fractionator Avoiding Coking in Fractionator � Major design consideration � �Black� oil passing down to the surge section of the fractionator is heavy with resins & solublized asphaltenes Produces more coke than gas oil in the coke drum Also likely to coke in bottom of the fractionator � Several methods Surge time limited to several minutes since coking is a function of time Slipstream from the bottoms cooled & recycled back to the surge section Remove some oil from bottom of fractionator & dispose of as No. 6 heavy fuel oil or mix with HGO Historical option Common when the demand for heavy fuel oil was high This reduces the amount of coke made

55. Fractionator Products Gas Product � During coking cycle vapors from the drum go to the fractionator � Fractionator overhead produces liquid naphtha & gas light ends � Light ends compressed and sent to gas plant The gas plant is usually built as integral part of the coker unit It has the same configuration as the saturates gas plant or catalytic light ends gas plant Absorber Deethanizer Sponge oil absorber Fractionator

56. Fractionator Products Naphtha Product � Naphtha fractions high in sulfur & olefins but low in aromatics � If naphtha products processed further in a catalytic reformer they are hydrotreated first Sulfur � hydrogen sulfide Olefins � saturates Aromatics naphthenes

57. Fractionator Products Gas Oil Side Draw � At least one gas oil sidestream but two are common Steam stripped to narrow end point � Part of side draw cooled & pumped back for reflux Cooling also helps to prevent coking Two pump around configurations Cooled liquid is fed back to tray above the draw tray � Ensure sufficient liquid from the draw tray � Mainly for heat transfer Fed to the tray below the draw tray � Fractionation is better & number of trays may be reduced � Amount of oil to be recycled back to coker controlled by varying end point of the lower gas oil draw

58. Coke Products & Calcining Green Coke � coke produced directly by a refinery Sub-classifications � Needle coke Produced from low sulfur & highly aromatic feeds � Sponge coke Largest production from the delayed coker Term comes from its porosity � Shot coke Less desirable coke Shot coke is a spheroid with diameters from 1 to 300 millimeters

59. Coke Products & Calcining Calcining � Green coke heated to finish carbonizing coke & reduce volatile matter to very low levels Heated 1800 � 2400�F Calcining done in rotary kiln or rotary hearth Drained & crushed sponge coke fed to wet end of kiln Opposite end fired by oil or gas & hot gases flow up through the rotating cylinder Coke tumbles down the rotating barrel of the kiln� moisture & evolved gases swept counter currently to wet end of kiln Calcined coke discharged at lower end to a rotary cooler Cooled with water spray followed by air cooling

60. Coke Products & Calcining Calcining does not remove metals About � of green coke is calcined Uncalcined sponge coke has heating values of 14,000 Btu/lb � Primarily used for fuel � Crushed & drained of free water � contains 10% moisture, 10% volatiles, & the rest coke Anode grade coke has low sulfur (< 2%) & low metals concentration to prevent metal contaminant � Needle coke used for anodes in the steel industry � Low sulfur & low metals sponge used for anodes in aluminum industry

61. Fluid Bed Coking & Flexicoking Fluid Coking & Flexicoking are expensive processes that have only a small portion of the coking market Continuous fluidized bed technology � Coke particles used as the continuous particulate phase with a reactor and burner Exxon Research and Engineering licensor of Flexicoking process � Third gasifier vessel converts excess coke to low Btu fuel gas