CATALYTIC REFORMING

CATALYTIC REFORMINGIntroductionWith the fast growing demand in aromatics and demand of high - octane numbers, catalytic reforming one of the most important unit processes in the petroleum and petrochemical industry.The reforming process is a catalytic process which converts low octane naphtha into higher octane reformate products for gasoline blending and aromatic rich reformate for aromatic production.Naphtha feeds to catalytic reforming include heavy straight run naphtha.It transforms low octane naphtha into high-octane motor gasoline blending stock and aromatics rich in benzene, toluene, and xylene with hydrogen and liquefied petroleum gas as a byproduct.Various Types of Catalytic ReformersSemi-Regenerative Fixed Bed reactors: In this type of reformers the catalyst has a life of one or more years between regeneration. The time between two regeneration is called a cycle. The catalyst retains its usefulness over multiple regeneration.Cyclic Fixed Bed Reformers: Cyclic reformers run under more severe operating conditions for improved octane number and yields. Individual reactors are taken by a special valving and manifold system and regenerated while the other reformer unit continues to operate.Continuous Reformers: In these reformers the catalyst is in moving bed and regenerated frequently. This allows operation at much lower pressure with a resulting higher product octane, C5+, and hydrogen yield. These types of reformers are radial flow and are either separated as in regenerative unit or stacked one above the other.Reactions in catalytic reformingThe major reactions in catalytic reforming are:Dehydrogenation of naphthenes to aromaticsIsomerization of paraffins and naphthenesDehydrocyclization of paraffins to aromaticsHydrocarcking of paraffins to lower molecular weight of naphthenes.

Some of the other reactions are:DemethylationHydrodealkylation Catalyst cokingDehydrogenation :Highly endothermic, and it causes a decrease in temperatures as the reaction progresses.Highest reaction rates, aromatics formed so end point of gasoline rises.Favorable Conditions: High temperature, Low pressure, Low space velocity, Low H2/HC ratio increases the yield.Cyclohexane Benzene + H2Methyl cyclohexane Toluene + H2

ISOMERIZATION:Two types of reactions are involved:Isomerization of normal paraffins to Iso-paraffins AND Iso-paraffins to aromaticsIsomerization of cyclopentanes to cyclohexanes plus subsequent conversion into benzene.High temperature, low space velocity, and low pressure increase Isomerization yield.Reaction is exothermic

Methyl cyclopentanes Cyclohexane Benzene + H2Dehydrocyclization:The most critical reactions in reforming are much slower and are favored by high temperature and low pressure.A minimum six carbon chain needed to cyclize to form aromatics.The conversion of paraffins to naphthenes increases with an increase in paraffin carbon number.This reaction is promoted by catalytic functions.Favorable Conditions: High temperature, Low pressure, Low space velocity, Low H2/HC ratio increases the yield

Hydrocracking:Hydrocracking is the breaking of C-C bond in the forming process giving to irreversible formation of lower molecular weight paraffins with less valuable products.Conversion of normal, iso, and cyclo paraffins to lower molecular weight paraffins increases with an increase in temperature and pressure and the conversion increases as space velocity decreases.Cracking of cyclo paraffins also increases with temperature and residence time in the reactors.These are endothermic.C7H16 + H2 C3H8 + C4 H10 n-heptane n-propane n-butaneC10H22 + H2 C6H14 + C4H10 n-decane n-hexane n-butaneC10H22 + H2 C7H16 + C3H8 n-decane n-heptane n-propane

Hydrodealkylation:It is favored at high temperature, low pressure and low space velocity.The reactions are endothermic.Toluene + H2 Benzene + CH4

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.

Coke formation:

It is a natural byproduct and results from intensive cracking of hydrocarbons and is favored by high temperature and low pressure and this affects the activity of the catalyst.

The activity of the catalyst and its selectivity for the desired product decreases over a period of time due to formation of coke on the surface of catalyst.

The rate of coke deposition is dependent on operating conditions, feed composition and nature of the catalyst.Favorable Conditions for Different Reforming ReactionsReactionPressureTemperatureDehydrogenation ofnaphthenes to aromaticsLow pressureHigh temperatureIsomerization of naphthenesIndeterminateIndeterminateDehydrocylization ofparaffins to aromaticsLow pressureHigh temperatureHydrocrackingHigh pressureHigh temperatureThe first catalyst used in catalytic reforming was molybdenum oxide on alumina to catalyze hydrogenation and dehydrogenation reactions to produce high octane gasoline.Reforming catalyst activity is a function of surface area, pore volume and active platinum and chlorine content.Reforming catalyst contain platinum with or without promoting metals dispersed uniformly on high purity chlorinated alumina support containing chlorine, rhenium, iridium, tin or germanium.Some of the advantages of the bimetallic catalysts are: Enhanced resistance to coking, Lower pressure and low H/HC operation resulting high octane number High aromatics and high yield of desired product Better yield stability Lower temperature requirement Better tolerance to high temperature High ultimate life

Reforming catalystReforming ProcessesThe various reforming processes can be characterized in four major categories depending upon the frequency of catalyst regeneration.Semi Regenerative (SR);Continuous Catalyst Regenerative (CCR)Cyclic and Hybrid ;

Catalytic reforming SR, CCR, Cyclic and HybridMagnaforming SR and Semi cyclicPlatforming SR, CCRPower forming- Cyclic and SRSR is the most commonly used technology from moderate to serve type of operations and uses series of fixed bed reactors.CCR uses moved bed technology and operates at much lower pressure compared to SR.The Cyclic process is a compromise between the SR & CCR and is characterized by an additional swing reactor, which is brought on stream when any one of the reactors is to be regenerated.Hybrid process is a combination of SR and CCR or Cyclic type.

operating conditions of three reforming processes

The catalytic reforming process involves treatment of naphtha feedstock for removal of impurities such as sulphur, nitrogen, and metals which poison the reformer catalyst.The treated naphtha mixed with hydrogen is preheated and charged to series of four reactors with interposed re-heaters.Operating conditions: temperature- 450-5250C; Pressure 7-30 bar; H/HC 5-8;As the reaction in reforming endothermic, the product from each reactor is reheated In the inter stage re-heaters to maintain temperature range of 475-5250C.The reactors are may be radial or axial flow type.It should provide good flow distribution and lower pressure dropThe reactor effluent from the last reactor at a temperature of 5000C goes to product recovery section which separation of hydrogen and light hydrocarbons takes place in the flash separator from where the hydrogen rich gases are recycled.The liquid stream from separator goes to the fractionator from where the bottom reformate to sent to storage tank for further processing.Catalyst activity reduced due to formation of coke, olefins, diolefins and sulphur, nitrogen water and halogens.

Process variables in Catalytic ReformingFollowing variables affect the reformate yield and quality of the product in the reforming process:Reaction temperature, Space velocity, Reaction pressure, H2/HC ratio, Feedstock Characteristics, Catalyst type and Catalyst poisons.Aromatic yield can be increased by raising the temperature, lowering the pressure and lowering the H2/HC ratio.

Feed Quality:The feedstock properties greatly affect the reformate yield and aromatic content of reformate.Naphthenes dehydrogenate very fast and thus high naphthenic feed gives the highest reformate yield whereas feedstock having paraffins require more severe conditions and give lower reformate yields.N + 2A are commonly used to judge the reformer yield.With higher N + 2A , aromatic yield will be high.Characterization factor = 12.6 (N+2A)/100Light fractions have poor naphthene and aromatic content and poor feed for reforming. It results lower aromatic and H2 yield.Heavy fractions which contain higher naphthene and aromatic content are preferred feedstock for reforming.

Temperature:It is most important operating parameter in the reforming process and a simple change in the reactor inlet temperature can affect the octane number and product quality.Normally reforming reactor inlet temperature is maintained at 470-5500C. An increase in temperature will result in increase in octane number, decrease in the yield C5+ fraction, decrease in H2 purity, increase in coke deposit.Space velocity:The space velocity in the reforming process is linked with the contact time in the reformer.Lower space velocity the severity will be higher keeping the all other conditions same.Lower space velocity results in increase in octane number, decrease in the product yield, decrease in the H2 purity, and increase in the coke deposit.Higher naphthenic feedstock requires high space velocity whereas more paraffinic stocks requires lower space velocities.Reforming space velocity range is 1.0-3.0 hr-1

REACTION PRESSUREReforming reaction pressure ranges (5 35 kg/sq. cm.). Decreasing pressure increases dehydrogenation of naphthenes and dehydrocyclization of paraffins which favors an increase in production of aromatics and hydrogen (increase catalyst coking and shorter cycle life). Higher pressure causes higher rates of hydrocracking reducing reformate yield but decreases coking of catalyst resulting in longer cycle life.

Hydrogen: Hydrocarbon RatioHydrogen: Hydrocarbon Ratio = Moles of H2 in Re cycle Gas/ Moles of Hydrocarbons.Higher the H2/HC ratio, higher the cyclic length. Two main reasons for reducing H2:HC ratio :Reduction in energy costs for compressing and circulating H2.Favors naphthene dehydrogenations and dehydrocyclisation reactionLowering of H2/HC Ratio, From 8 to 4 carbon increase in 1.75 times and from 4 to 2 carbon increase 3.6 timesCatalyst Poisoning: Temporary Poisons: Temporary poisons are those impurities which can be removed during various pretreatment process like sulphur, nitrogen,Permanent Poisons: Permanent Poisons are those impurities present in the feed which is irreversible damage to the catalystSource and maximum level of catalyst poisons are given in Table.