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2/16/2015 NPTEL PHASE II :Petroleum Refinery Engineering

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Cracking Feedstocks and reactors,Effect of processvariables

REFINING[1,2,12]

Process to produce useful fuels from crude oil.Separation, Conversion, Treatment and Blending.

Primary processes fractionationSecondary Processesconversion processes

improve the product quality.upgrade heavier ends to lighter products.Predominantly catalytic processes.

Fig:4.8 A typical sketch of refinery operations integrated withits profit margin, product pattern shift, with deteriorating crude

quality and stringent emission control parameters.

Crack carboncarbon bonds in gas oils.Fine catalyst in fluidized bed reactor allows for immediateregeneration.Lowers average molecular weight and produces high yields offuel products.Produces olefins.

Attractive feed characteristics.Small concentrations of contaminants Poison the catalyst.Small concentrations of heavy aromatics Crack and depositcoke on catalyst.



Products may be further processed.Further hydrocracked.Alkylated to improve gasoline antiknock.Early Fixed and Moving Bed Catalytic Cracking.

Cyclic fixed bed catalytic cracking commercialized in late 1930s.Houdry Process Corporation formed in 1930.First Houdry catalyst cracker started up at Sun Oil’sPaulsboro, New Jersey, refinery in June 1936.Three fixed bed reactors and processed 2,000 barrels/day12,000 barrels/day commercial unit went on stream at Sun’sMarcus Hook Refinery in 1937.Other adoptees: Gulf, Sinclair, Standard Oil of Ohio, and TheTexas Company.

Sun & Houdry Process Corporation started development on amoving bed process in 1936.

Pilot Thermofor catalytic cracker was started in 1941.First commercial 20,000barrel/day unit commissioned atMagnolia’s Beaumont Refinery in 1943.

Catalytic cracking[1,2,12]

The process flows of both types of processes are similar. The hotoil feed is contacted with the catalyst in either the feed riser lineor the reactor.As the cracking reaction progresses, the catalyst is progressivelydeactivated by the formation of coke on the surface of thecatalyst.The catalyst and hydrocarbon vapors are separated mechanically,and oil remaining on the catalyst is removed by steam strippingbefore the catalyst enters the regenerator.The oil vapors are taken overhead to a fractionation tower forseparation into streams having the desired boiling ranges.The spent catalyst flows into the regenerator and is reactivated byburning off the coke deposits with air. Regenerator temperaturesare carefully controlled to prevent catalyst deactivation byoverheating and to provide the desired amount of carbon burnoff.This is done by controlling the air flow to give a desired CO2/COratio in the exit flue gases or the desired temperature in theregenerator.The flue gas and catalyst are separated by cyclone separators andelectrostatic precipitators.The catalyst in some units is steamstripped as it leaves theregenerator to remove adsorbed oxygen before the catalyst iscontacted with the oil feed.



FLUIDIZEDBED CATALYTIC CRACKING[1,2,12] The FCC process employs a catalyst in the form of very fine particles[average particle size about 70 micrometers (microns)] which behaveas a fluid when aerated with a vapor.The fluidized catalyst is circulatedcontinuously between the reaction zone and the regeneration zone andacts as a vehicle to transfer heat from the regenerator to the oil feedand reactor.

Two basic types of FCC units in use today are:sidebyside type,where the reactor and regenerator are separatevessels adjacent to each other,and the Orthoflow, or stacked type,where the reactor is mounted on top of the regenerator.

One of the most important process differences in FCC units relates tothe location and control of the cracking reaction. Until about 1965,most units were designed with a discrete densephase fluidizedcatalyst in the reactor vessel.The units were operated so most of thecracking occurred in the reactor bed The extent of cracking was controlled by varying reactor bed depth(time) and temperature. Although it was recognized that crackingoccurred in the riser feeding the reactor because the catalyst activityand temperature were at their highest there, no significant attemptwas made to regulate the reaction by controlling riser conditions.Afterthe more reactive zeolite catalysts were adopted by refineries, theamount of cracking occurring in the riser (or transfer line) increased tolevels requiring operational changes in existing units. The fresh feed and recycle streams are preheated by heat exchangersor a furnace and enter the unit at the base of the feed riser where theyare mixed with the hot regenerated catalyst.The heat from the catalystvaporizes the feed and brings it up to the desired reactiontemperature. The mixture of catalyst and hydrocarbon vapor travels upthe riser into the reactors. The cracking reactions start when the feedcontacts the hot catalyst in the riser and continues until the oil vaporsare separated from the catalyst in the reactor. The hydrocarbon vaporsare sent to the synthetic crude fractionator for separation into liquidand gaseous products. The catalyst leaving the reactor is called spent catalyst and containshydrocarbons adsorbed on its internal and external surfaces as well asthe coke deposited by the cracking. Some of the adsorbedhydrocarbons are removed by steam stripping before the catalystenters the regenerator. In the regenerator, coke is burned from thecatalyst with air.



The regenerator temperature and coke burnoff are controlled byvarying the air flow rate. The heat of combustion raises the catalysttemperature to 1150 to 1550oF (620oC–845oC) and most of this heat istransferred by the catalyst to the oil feed in the feed riser. Theregenerated catalyst contains from 0.01 to 0.4 wt% residual cokedepending upon the type of combustion (burning to CO or CO2) in theregenerator. The regenerator can be designed and operated to burn the coke on thecatalyst to either a mixture of carbon monoxide and carbon dioxide orcompletely to carbon dioxide.Older units were designed to burn tocarbon monoxide to minimize blower capital and operating costsbecause only about half as much air had to be compressed to burn tocarbon monoxide rather than to carbon dioxide.Newer units aredesigned and operated to burn the coke to carbon dioxide in theregenerator because they can burn to a much lower residual carbonlevel on the regenerated catalyst. This gives a more reactive andselective catalyst in the riser and a better product distribution resultsat the same equilibrium catalyst activity and conversion level. For units burning to carbon monoxide, the flue gas leaving theregenerator contains a large quantity of carbon monoxide which isburned to carbon dioxide in a CO furnace (waste heat boiler) to recoverthe available fuel energy.The hot gases can be used to generate steamor to power expansion turbines to compress the regeneration air and togenerate electric power.



Fig:4.9 Older fluid catalyticcracking (FCC) unit

configurations[1,2]



Fig:4.10 UOP reduced crude cracking(RCC)[1,2]

NEW DESIGNS FOR FLUIDIZEDBED CATALYTIC CRACKINGUNITS[1,2,12] Zeolite catalysts have a much higher cracking activity than amorphouscatalysts, and shorter reaction times are required to preventovercracking of gasoline to gas and coke. This has resulted in unitswhich have a catalystoil separator in place of the fluidizedbed reactorto achieve maximum gasoline yields at a given conversion level. Manyof the newer units are designed to incorporate up to 25% reducedcrude in the FCC unit feed. Schematic drawings of Exxon Research andEngineering’s FLEXI CRACKING IIIR design. These units incorporate a high heighttodiameter ratio lowerregenerator section for more efficient singlestage regeneration and anoffset sidebyside or stacked vessel design. UOPdesigned units utilizehigh velocity, low inventory regenerators, and Kelloggdesigned unitsuse plug valves to minimize erosion by catalyst containing streams . All



units are now designed for both complete combustion to CO2 and COcombustion control. The large differential value between residual fuel and other catalyticcracking feed stocks has caused refiners to blend atmospheric andvacuum tower bottoms into the FCC feed. Residual feed stocks haveorders of magnitude higher metals contents (especially nickel andvanadium) and greater coke forming potential(Ramsbottom andConradson carbon values) than distillate feeds. These contaminantsreduce catalyst activity, promote coke and hydrogen formation, anddecrease gasoline yield. It has been shown that catalyst activity lossdue to metals is caused primarily by vanadium deposition, andincreased coke and hydrogen formation is due to nickel deposited onthe catalyst. The high coke laydown creates problems because of the increase incoke burning requirement with the resulting increased air or oxygendemand, higher regenerator temperatures, and greater heat removal.For feeds containing up to 15 ppm Ni V, passivation of nickel catalystactivity can be achieved by the addition of organic antimonycompounds to the feed with reductions in coke and hydrogen yields upto 15% and 45%, respectively. For feeds containing 15 ppm metalsand having Conradson carbon contents above 3%, hydroprocessing ofthe FCC feed may be required to remove metals, reduce the carbonforming potential, and to increase the yield of gasoline and middledistillates by increasing the hydrogen content of the feed. Feeds for Catalytic Cracking[1,2,12]

Aromatic rings typically condense to cokeNo hydrogen added to reduce coke formation.Amount of coke formed correlates to carbon residue of feednormally 37 wt% CCR.

Catalysts sensitive to heteroatom poisoningSulfur and metals (nickel, vanadium, and iron).Feeds normally hydrotreated.

Atmospheric and vacuum gas oils are primary feedsCould be routed to the hydrocracker for diesel production Notas expensive a process as hydrocracking.Dictated by capacities of gasoline/diesel economics.



Fig:4.11 FCC Products[1,2]



Fig:4.12 FCC unit with coke productions during cracking

reactions[1,2]


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Fig:4.13 FCC regeneration unit[1,2]

PRODUCT YIELDS

Produces high yields of liquids small amounts of gas coke Massliquid yields are usually 90%93%; liquid volume yields that areoften more than 100%(Rule of thumb) Remaining mass yield splitbetween gas and coke.The yield pattern is determined by complex interaction of feedcharacteristics and reactor conditions that determine severity ofoperation.Conversion defined as relative to what remains in the originalfeedstock boiling range of Conversion =100%(Gas Oil Yield).

Catalytic Cracking Catalysis & Chemistry[1,2,819]

Zeolite catalysts



High activity.Good fluidization properties.High gasoline and low coke yields.

Acid site catalyzed cracking and hydrogen transfer via carboniummechanism.

Basic reaction —carboncarbon scission of paraffins,cycloparaffins aromatics to form olefins lower molecularweight paraffins.Olefins exhibit carboncarbon scission and isomerization withalkyl paraffins to form branched paraffins.Cycloparaffins will dehydrogenate (condense) to formaromatics.Small amount of aromatics and olefins will condense toultimately form coke.

Research continues by catalyst suppliers and licensorsRecognition that both crackability of feed and severity ofoperations are factors.Theoretical basis for cracking reactions lead to more precisecatalyst formulation.Catalyst can be tailored to maximize gasoline yield orincrease olefin production.

CATALYTIC CRACKING[1,2,12]



Fig:4.14 Catalytic Cracking[1,2]



Fig:4.15 Exxon FLEXCRACKING IIIR FCC unit(courtesy of Exxon

Research and Engineering)

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variables Cracking Feedstocks and reactors,Effect …//nptel.ac.in/courses/103102022/thermal%20and%20catalytic%20cracking/Cracking%20feedstocks%20and%20reactors%20effect%20of%20process%20v…