09_reforming.pdf

Catalytic Reforming Chapter 10

Petroleum Refinery Schematic Gasses Polymerization

Sulfur Plant

Alkyl Feed

Butanes

Fuel Gas LPG

Alkylation Gas Separation & Stabilizer

Polymerization Naphtha

Isomerization

Isomerate Alkylate

Light Naphtha

Aviation Gasoline Automotive Gasoline

Reformate Naphtha Hydrotreating

Heavy Naphtha

Sulfur

LPG

Sat Gas Plant

Gas

Naphtha Reforming

Solvents

Naphtha

Atmospheric Distillation

Crude Oil

Jet Fuels

Kerosene

Desalter

AGO LVGO Vacuum Distillation

Distillate Gas Oil Hydrotreating

Kerosene Hydrocracking

Fluidized Catallytic Cracking

HVGO

Cat Naptha

Solvents Distillate Hydrotreating

Diesel

Cycle Oils

Coker Naphtha

Residual Fuel Oils

SDA Bottoms

Visbreaking

Coker Gas Oil

Vacuum Residuum

Heating Oils

Fuel Oil DAO

Solvent Deasphalting

Treating & Blending

Cat Distillates

Naphtha

Asphalts

Distillates Fuel Oil Bottoms Lube Oil

Lubricant Greases

Solvent Dewaxing

Waxes Waxes Coking

Coke

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Introduction to Naphtha Reforming • Purpose: enhance aromatic content of naphtha »Feed stocks to aromatics complex »Improve the octane rating for gasoline

• Reactions • Dehydrogenation — removal of hydrogen from cycloparaffins (naphthenes) to form aromatics »Aromatics provide octane enhancement of gasoline feed stocks

• Paraffins isomerized to produce branched isoparaffins »High octane »Some isomerized further to cycloparaffins & subsequently to aromatics.

• Hydrogen by-product utilized in hydrotreating

Development of Reforming • In the 1930s gasoline octane improved by use of thermal reforming » Mild thermal cracking of naphtha’s paraffins to olefins

• In late 30s & early 40s, alkylation of olefins developed to improve the octane of aviation gasoline • By the 50s, high performance automotive engines needed high-octane gasoline » Expanded use of alkylation, reforming, & isomerization

• Some large refineries had catalytic reforming capacity based on the cyclic process used in World War II » Benzene for styrene » Toluene for TNT » Aromatics for aviation fuel

• Catalyst regeneration cycle was very short & required elaborate cycle controllers — small refineries could not afford

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Development of Reforming • Platforming commercialized by UOP in 1949 » Platinum catalyst » Could be operated for a year before regeneration was required

• Catalytic reforming became ubiquitous as the smaller refiner quickly adopted it

Overview of Reforming • Produces large amounts of hydrogen that is ultimately used in hydrotreating »Catalytic reforming second only to catalytic cracking in commercial importance to refiners »Almost every refinery in the world has a reformer

• Reformate desirable component for gasoline »High octane number, low vapor pressure, very low sulfur levels, & low olefins concentration »Despite levels of benzene, aromatics, & olefins

• Catalytic reformer contributes the second highest volume to the gasoline pool FCCU

35 vol%

Reformer

30 vol%

Alkylation

20 vol%

Isomeration

15 vol%

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Feeds Stocks & Products • Catalyst is noble metal & very sensitive to sulfur & nitrogen degradation » Feed stocks hydrotreated for sulfur removal » Control of chloride & water also important

• Best applied to naphtha feeds without sulfur » Not catalytic cracker naphtha — Contains olefins & aromatics — typically alkylated to increase octane » Not delayed coking naphtha — Contains high levels of sulfur, olefins, & aromatics

• Not appropriate » Light straight run gasoline — Tend to crack to butanes & lighter » Gas oil streams — Tend to hydrocrack & deposit coke on the reforming catalyst

Feeds Stocks & Products • Developed to improve octane number of naphtha by increasing the aromatic content » Cycloparaffins (naphthenes) dehydrogenated to form aromatics » Paraffins are isomerized to branched isoparaffins —Some of these isoparaffins are reacted to cycloparaffins that are subsequently reacted to aromatics

• Reforming increases the amounts of aromatics in gasoline » Aromatic reformate exhibits a large octane rating spread between research & motor numbers — reduced quality » To improve air quality tolerance for aromatics (primarily benzene) in gasoline drastically reduced

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Feeds Stocks & Products • Severity » Low severity now used for gasoline blend stocks » High severity is used to maximize aromatics when sent to aromatics units

• Reforming still of great importance » Hydrogen produced is the major source of hydrogen utilized in hydrotreating

Reforming Chemistry • Uses a solid catalyst to convert naphthenes to the corresponding aromatics & isomerize paraffinic structures to isomeric forms » Both reactions lead to a marked increase in octane number » Both reactions lead to volume shrinkage

• Correlations permit the use of a PONA analysis of the feed for prediction of yield and quality of the product » Originally feed qualities measured in terms of Watson "K" Factor — a rough indication of amount of paraffins

• Aromatics largely untouched by reactions

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Reforming Chemistry Dehydrogenation

Isomerization

CH3

CH3

+

CH3

3 H2

CH3 CH3 CH2

CH3

Dehydrocyclization CH3 CH3

+

H2

Hydrocracking CH3 CH2

CH3 CH3

+ H2

CH3 + H2

+

Reforming Chemistry • Dehydrogenation of Naphthenes (Cycloparaffins) to Aromatics » Methyl-cyclohexane dehydrogenated to toluene » Reaction significant & relatively easy and rapid

• Isomerization of Naphthenes (Cycloparaffins) » Ethyl-cyclopentane rearranged to methylcyclohexane » Must be done before the methyl-cyclohexane may be dehydrogenated to methyl-benzene (toluene)

• Dehydrocyclization of Paraffins to Naphthenes (Cycloparaffins) » Normal heptane is rearranged ethyl-cyclopentane or methylcyclohexane » Difficult rearrangement but increases the aromatic content and produces excess hydrogen

• Isomerization of Normal Paraffins to Isoparaffins » Normal pentane is rearranged to isopentane » Branched isomers improve octane rating

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Reforming Chemistry • Hydrocracking of Paraffins and Aromatics » Not desired since they produce light gases, consume hydrogen, & lead to coking » Regeneration by burning in an inert atmosphere is necessary to remove the buildup » Some catalysts are more susceptible to coking than other catalysts » The introduction of the bimetal catalyst was a method to decreased coking

The Effects of Process Variables • Primary control for changing conditions or qualities is reactor temperature » Normally about 950°F at reactor inlet » May be raised for declining catalyst activity or to compensate for lower quality feedstock » Higher reactor temperature increases octane rating but reduces yield and run length

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The Effects of Process Variables • Design considerations for improvement in quality will include pressure, recycle ratio, reactor residence time, & catalyst activity » Low reactor pressure increases yield & octane but increases coke make » Increased hydrogen partial pressure due to hydrogen recycle (hydrogen to hydrocarbon ratio) suppresses coke formation, hydrogen yield & octane gain, but promotes hydrocracking » Low space velocity favors aromatics formation but also promotes cracking by operating closer to equilibrium conditions » Higher activity catalysts cost more but increases run lengths and or yields

Conditions Favorable for Reforming Reactions Reaction

Pressure

Temperature

Isomerization of naphthenes

Indeterminate

Indeterminate

Dehydrocyclization of paraffins to naphthenes

Low pressure

High temperature

Dehydrogenation of naphthenes to aromatics

Low pressure

High temperature

Isomerization of normal paraffins to isoparaffins

Slight dependence

Slight dependence

Hydrocracking

High pressure

High temperature

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Hydroforming • Early cyclic process used by several major oil companies to produce toluene for TNT during World War II • Catalytic reforming using molybdenum oxide on alumina catalyst • Rapid coking of the catalyst, requiring a cyclic regeneration of reactors about every four hours » Timing mechanism that used for lawn sprinkler systems was used to switch from reforming to regeneration service » Reactor system included one extra "swing" reactor —Facilitate periodic removal & regeneration of a reactor

Exxon Fluid Bed Process • Desire for a constant activity • Used molybdenum catalyst • Commercial failure » Reforming is a high conversion operation — a back mixed fluid bed does not provide the "push" in either concentration gradients or catalyst activity » Use of platinum catalysts not practical for fluid beds —Catalyst attrition —Loss of fine particles & attendant high operating cost

• Several of these units failed in rather spectacular manner by dispersing themselves over a wide area

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UOP Platforming • A semi-regenerative process — low platinum — only required regeneration once a year • Made naphtha octane improvement accessible to all refiners

UOP Platforming

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UOP Platforming • Naphtha feed desulfurized in a naphtha hydrodesulfurization — heated to 900°F or greater • Typically three reactor beds & furnace preheaters » Dehydrogenation is highly endothermic —Large temperature drop as the reaction proceeds —Multiple reactors with intermediate reheat required » First reactor for naphthene dehydrogenation —Requires less catalyst before reheat is required » Last reactor for isomerization of paraffins —Requires more catalyst —Frequently two reactors in parallel to permit as much as twice the catalyst volume » Typical catalyst distribution: 20%, 30%, 50%

UOP Platforming • Effluent from final reactor is cooled & hydrogen is flashed off » Some hydrogen is cascaded to reformer’s hydrodesulfurization unit » Remainder normally typically used for distillates hydrodesulfurization

• Reformed naphtha is stabilized in a tower » Light ends directed to saturates gas plant

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UOP Platforming • Reformer reactors designed for liquid hourly space velocities of the order of 2.2 » Even though the process is totally vapor phase

• Hydrogen recycle ratio of 6,000 to 10,000 scf/bbl » High hydrogen partial pressure inhibits dehydrogenation of naphthenes but it suppresses coke formation » Trade off is yield vs. run length

• Dehydrogenation reactions make on the order of 1,000 scf/bbl or more H2 » As much as 200 scf/bbl used for desulfurization of feed

UOP Platforming • Catalyst life controlled by coke formation » Expected to be one year » Catalyst life is expressed as volume of feed processed per pound of catalyst » Life of 100 barrels per pound possible —For 100 barrels per pound, 20,000 barrel per day unit will have catalyst charge of 70,000 pounds of catalyst (containing 0.06% platinum) for one year of operation

• Reactor catalyst volume actually determined in terms of space velocity requirements » That catalyst quantity is then converted to life days to estimate run length

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UOP Platforming • Reactor temperature primary control variable » Temperature drop will decrease with time, indicating reduced conversion & decline of catalyst activity » Higher inlet temperature required —Offsets catalyst activity as coking progresses —Decline is gradual & can be projected to end-of-run

• Also necessary to monitor moisture & chloride balances » Precipitous decline in catalyst activity indicates poisoning

• Reactor temperature is a useful indicator of severity » Lower severity means higher yield but lower quality of reformate » Reformate quality usually expressed in terms of RON

UOP Approach to Catalyst Regeneration • Catalyst deactivated because of coke formation & contamination » Coke formation repressed by high hydrogen partial pressure » Normally regenerated once per year, but could be half that depending on severity of the operating conditions » Coke formation can be offset by increasing reaction temperature for a while

• Regeneration complex operation — requires extensive facilities & skilled personnel » Operation performed at infrequent intervals » Easy to ruin load of catalyst costing several millions of dollars

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UOP Approach to Catalyst Regeneration • UOP constructed central catalyst regeneration plant in Shreveport for use by all Platformer process users » Service rendered on a toll basis » UOP field reps worked with refinery personnel —Evaluate performance —Select the time of termination —Assistance with catalyst unloading & recharging

• “Semi-regenerative” process since regeneration was required on a yearly basis

Changes to UOP Process • Higher pressures » Earliest units at 450 psig » Now as low as 100 psig —Low pressures increase rate of coking & requires more frequent regeneration

• Increased rate of coking » In-situ regeneration spread in the 60s » Unit shutdown, regeneration with air burnoff of the coke, followed by hydrogen reduction of the catalyst plus adjustment of the chloride content » Could not totally restore activity — after a few regenerations necessary to deal with catalyst platinum crystal growth that led to permanent decline of activity

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In Situ Regeneration & Competitive Processes • In situ regeneration requires a unit shutdown » "Swing" reactor — extra reactor that can be taken out of service & regenerated » Required couple of days from the reactor going off -line to having a fresh reactor ready to put back in service

• Other refiners developed their own process using more active catalysts & a lower pressure » Required more severe operating conditions & more frequent regeneration » Shorter catalyst life led to development of the cyclic regeneration of individual reactors instead of unit shutdowns for regeneration of all reactors

Catalyst Trends • Platinum concentrations now lower • Chloride still added for isomerization & fragmentation but some catalysts are now on a silica alumina base • Chevron developed a bi-metallic catalyst » Platinum & rhenium » Dual site dehydrogenation

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UOP Continuous Regeneration of Reforming Catalyst

• Responding to search for higher catalyst activity, UOP conceived of a moving bed process

» Continuously regenerating a portion of a moving bed of catalyst to remove coke & sustain activity » Operating pressures lowered to 50 psig

• Three reactors stacked one on top of the other & gravity flow of the catalyst from top to bottom » The catalyst flows down between concentric rings of screen » Reactants pass radially through the catalyst to the inner conduit and then to the next bed » Mode of regeneration is proprietary —Probably employs air or oxygen burning of the coke followed by reduction & acidification

UOP Continuous Regeneration of Reforming Catalyst

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Trends in Catalytic Reforming • Removal of aromatics from gasoline is the major factor » Straight run gasoline & naphtha desulfurized in one hydrotreater unit followed by fractionation at a cut point of about 180°F — Overhead gasoline with benzene & cyclohexane to isomerization unit — benzene hydrogenated to cyclohexane » Fractionator bottoms goes to reforming — This reformer naphtha feed contains no benzene or cyclohexane — Xylenes formed in the reformer from fractionator botttoms, but benzene not produced since cyclohexane removed in overheads » No facility to extract benzene from gasoline » Larger hydrotreater & a splitter are required

• Every refinery must deal with removing benzene from gasoline pool » Unique configurations because of existing units, different crudes, different product markets

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