Chemistry and technology of petroleum By Dr. Dang Saebea

Catalytic Reforming andIsomerization

IntroductionCatalytic reforming of heavy naphtha and isomerization of light naphtha constitute a very important source of products having high octane numbers which are key components in the production of gasoline.

Catalytic ReformingCatalytic reforming is the process of transforming hydrocarbons with low octane numbers to aromatics and iso-paraffins which have high octane numbers. It is a highly endothermic process requiring large amounts of energy.

Role of Reformer in the Refinery and Feed PreparationThe catalytic reformer is one of the major units for gasoline production in refineries. It can produce 37 wt% of the total gasoline pool. Other units - fluid catalytic cracker (FCC) - alkylation unit - isomerization unit

Octane NumberAn octane number is a measure of the knocking tendency of gasoline fuels in spark ignition engines.The octane number of a fuel is determined by measuring its knocking value compared to the knocking of a mixture of n-heptane and isooctane (2,2,4- trimethyl pentane).Pure n-heptane zero octanePure isooctane100 octaneExample : an 80 vol% isooctane mixture has an octane number of 80.

Main feed stockThe straight run naphtha from the crude distillation unit is hydrotreated to remove sulphur, nitrogen and oxygen which can all deactivate the reforming catalyst.

The hydrotreated naphtha (HTN) is fractionated into light naphtha (LN) - light naphtha is mainly C5C6 hydrocarbons - heavy naphtha (HN) is mainly C7C10 hydrocarbons.

Environmental regulations limit on the benzene content in gasoline. If benzene is present in the final gasoline, it produces carcinogenic material on combustion.Elimination of benzene forming hydrocarbons, such as, hexane will prevent the formation of benzene, and this can be achieved by increasing the initial point of heavy naphtha. These light paraffinic hydrocarbons can be used in an isomerization unit to produce high octane number isomers.

Main feed stock

Product from catalytic reforming

FEEDPRODUCTParaffins30-7030-50Olefins0-20-2Naphthenes20-600-3Aromatics7-2045-60

The role of the heavy naphtha (HN) reformer in the refineryHydrogen, produced in the reformer can be recycled to the naphtha hydrotreater, and the rest is sent to other units demanding hydrogen.

REACTIONS4 major reactions are categorized as

Dehydrogenation of naphthenes to aromaticsDehydocyclization of paraffins to aromatics Isomerization

HydrocrackingUndesirableDesirable

Dehydrogenation & DehydrocyclizationHighly endothermicCause decrease in temperaturesHighest reaction ratesAromatics formed have high B.P so end point of gasoline rises

Favourable conditionsHigh temperatureLow pressureLow space velocityLow H2/HC ratioDehydrogenationDehydrocyclization

IsomerizationBranched isomers increase octane ratingSmall heat effectFairly rapid reactions

Favourable conditionsHigh temperatureLow pressureLow space velocityH2/HC ratio no significant effect

HydrocrackingExothermic reactionsSlow reactionsConsume hydrogenProduce light gasesLead to cokingCauses are high paraffin concentration feed

Favourable conditionsHigh temperatureHigh pressureLow space velocity

Coke DepositionCoke can also deposit during hydrocracking resulting in the deactivation of the catalyst. Coke formation is favoured at low partial pressures of hydrogen.Hydrocracking is controlled by operating the reaction at low pressure between 525 atm, not too low for coke deposition and not too high in order to avoid cracking and loss of reformate yield.

Thermodynamics of Reforming ReactionsThe dehydrogenation reactions are the main source of reformate product and are considered to be the most important reactions in reforming. These are highly endothermic reactions and require a great amount of heat to keep the reaction going. The dehydrogenation reactions are reversible and equilibrium is established based on temperature and pressure. It is usually important to calculate the equilibrium conversion for each reaction.

ExampleThe Gibbs free energy of the following reaction at 500 C and 20 atm is calculated to be 20.570 kcal/mol.

Calculate the reaction equilibrium conversion and barrels of benzene formed per one barrel of cyclohexane. The hydrogen feed rate to the reactor is 10,000 SCF/bbl of cyclohexane. Cyclohexane density of 0. 779 g/cm3, 1mol H2 = 379 SCF

Reaction Kinetics and CatalystsThe catalyst used for reforming is a bifunctional catalyst composed of platinum metal on chlorinated alumina. Platinum the centre for the dehydrogenation reaction

Iridium (Ir) is added to boost activity, Rhenium (Re) is added to operate at lower pressures and Tin (Sn) is added to improve yield at low pressures. Reaction Kinetics and Catalysts The use of Pt/Re is now most common in semi-regenerative (SR) processes with Pt/Sn is used in moving bed reactors.

Reaction Kinetics and Catalysts Impurities that might cause deactivation or poisoning of the catalyst include: coke, sulphur, nitrogen, metals and water.

The reformer should be operated at high temperature and low pressure to minimize coke deposition.

Process description of catalytic reforming processThe old technologies are fixed bed configuration.Moving bed technology has also recently been introduced Semi-regenerative Fixed Bed Process Continuous Regenerative (moving bed) Process

Semi-regenerative Fixed Bed ProcessThreereactorsfixed bed of catalystSuch a unit is referred to as asemi-regenerative catalytic reformer(SRR).All of the catalyst is regeneratedin situduring routine catalyst regeneration shutdowns (6 to 24 months) by burning off the carbon formed on the catalyst surface

Semi-regenerative Fixed Bed Processfirst reactorReactions such as dehydrogenation of paraffins and naphthenes which are very rapid and highly endothermic

Semi-regenerative Fixed Bed Processsecond reactorReactions that are considered rapid, such as paraffin isomerization and naphthens dehydroisomerization, give moderate temperature decline

Semi-regenerative Fixed Bed ProcessThird reactorslow reactions such as dehydrocyclization and hydrocracking give low temperature decline.

Semi-regenerative Fixed Bed ProcessThe temperature and concentration profile in each reactor

At the top of the stabilizer residual hydrogen and C1 to C4 are withdrawn as condenser products, which are then sent to gas processing,Part of the liquid product (C3 and C4) is returned from the reflux drum back to the stabilizer.

Semi-regenerative Fixed Bed ProcessRecycling some of the hydrogen produced. Some light hydrocarbons (C1C4) are separated from the reformate in the stabilizer.The main product of the column is stabilized reformate, which is sent to the gasoline blending plant.

Semi-regenerative Fixed Bed ProcessA slight modification to the semi-regenerative process is to add an extrareactor to avoid shutting down the whole unit during regeneration. Three reactors can be running while the forth is being regenerated. This modified process is called the cyclic fixed bed process

Continuous Regenerative (moving bed) Process

In this process, three or four reactors are installed one on the top of the other.

Continuous Regenerative (moving bed) Process

The effluent from each reactor is sent to a common furnace for heating.

Continuous Regenerative (moving bed) Process

The catalyst moves downwards by gravity from the first reactor (R1) to the forth reactor (R4). The catalyst is sent to the regenerator to burn off the coke and then sent back to the first reactor R1. The final product from R4 is sent to the stabilizer and gas recovery section.

Typical operating conditions of three reforming processes

PROCESS VARIABLES Chosen to meet refiners yield, activity and stability need

Primary control of changing conditions or qualities in reactor. High temp increase octane rating. High temp reduce catalyst stability but may be increased for declining catalyst activity.

Pressure effects the reformer yield & catalyst stability. Low pressure increases yield & octaneCatalyst typeTemperaturePressure

PROCESS VARIABLESLow space velocity favors aromatic formation but also promote cracking.Higher space velocity allows less reaction time.

Moles of recycle hydrogen / mole of naphtha chargeIncrease H2 partial pressure or increasing the ratio suppresses coke formation but promotes hydrocracking.

Space velocityH2 / HC ratio

Reformer correlationsRONF= research octane number of feed; RONR=research octane number of reformate; C+5 vol% =volume percent of reformate yield; SCFB H2=standard cubic foot of H2 produced/barrel of feed;K =characterization factor (TB)1/3/SG; TB= absolute mid-boiling of feed, (R); SG = specific gravity of feed; N = napthenes vol % and A aromatics vol %

Example 100 m3/h of heavy naphtha (HN) with specific gravity of 0.778 has the following composition: A =11.5 vol%, N = 21.7 vol% and P= 66.8 vol% is to be reformed to naphtha reformate of RON =94. Calculate the yields of each product for that reformer.

SolutionThe material balance for the reformer is presented in the following table:

Isomerization of Light NaphthaIsomerization is the process in which light straight chain paraffins of low RON (C6, C5 and C4) are transformed with proper catalyst into branched chains with the same carbon number and high octane numbers.

Light naphtha from the hydrotreated naphtha (HTN) C5=80 C is used as a feed to the isomerization unit.

Isomerization ReactionsIsomerization is a reversible and slightly exothermic reaction:

The conversion to iso-paraffin is not complete since the reaction is equilibrium conversion limited. It does not depend on pressure, but it can be increased by lowering the temperature. However operating at low temperatures will decrease the reaction rate. For this reason a very active catalyst must be used.

Isomerization CatalystsTwo types of isomerization catalysts The standard Pt/chlorinated alumina with high chlorine content

The Pt/zeolite catalyst

Standard Isomerization CatalystThis bi-functional nature catalyst consists of highly chlorinated alumina responsible for the acidic function of the catalyst.

Platinum is deposited (0.30.5 wt%) on the alumina matrix.

Platinum in the presence of hydrogen will prevent coke deposition, thus ensuring high catalyst activity.

The reaction is performed at low temperature at about 130 C to improve the equilibrium yield.

Standard Isomerization CatalystThe standard isomerization catalyst is sensitive to impurities such as water and sulphur traces which will poison the catalyst and lower its activity. For this reason, the feed must be hydrotreated before isomerization.

The zeolite catalyst, which is resistant to impurities, was developed.

Zeolite CatalystZeolites are used to give an acidic function to the catalyst.

Metallic particles of platinum are impregnated on the surface of zeolites and act as hydrogen transfer centres.

The zeolite catalyst can resist impurities and does not require feed pretreatment, but it does have lower activity and thus the reaction must be performed at a higher temperature of 250 C (482 F).

A comparison of the operating conditions for the alumina and zeolite processes

Isomerization YieldsThe reformate yield from light naphtha isomerization is usually very high (>97 wt%). Typical yields are given in Table

ExampleLight naphtha with a specific gravity of 0.724 is used as a feed to the isomerization unit at a rate of 100 m3/h. Find the product composition.

SolutionIsomerization yields

The End

*****The main feed comes from hydrotreated heavy naphtha, and some feed comes from hydrotreated coker naphtha.)

The straight run naphtha from the crude distillation unit is hydrotreated to remove sulphur, nitrogen and oxygen which can all deactivate the reforming catalyst.

The hydrotreated naphtha (HTN) is fractionated into light naphtha (LN), which is mainly C5C6 (that have carbon atom about 5-6 atom), and heavy naphtha (HN) which is mainly C7C10 hydrocarbons.

*********************************Standard Isomerization Catalyst The standard isomerization catalyst is sensitive to impurities such as water and sulphur traces which will poison the catalyst and lower its activity. For this reason, the feed must be hydrotreated before isomerization.Furthermore, carbon tetrachloride must be injected into the feed to activate the catalyst. The pressure of the hydrogen in the reactor will resultin the elution of chlorine from the catalyst as hydrogen chloride. For all these reasons, the zeolite catalyst, which is resistant to impurities, was developed.*Zeolite CatalystZeolites are crystallized silico-aluminates that are used to give an acidic function to the catalyst. Metallic particles of platinum are impregnated onthe surface of zeolites and act as hydrogen transfer centres. The zeolite catalyst can resist impurities and does not require feed pretreatment, but it does have lower activity and thus the reaction must be performed at a higher temperature of 250 C (482 F).

A comparison of the operating conditions for the alumina and zeolite processes is shown in Table 5.6.****