Chemistry and technology of petroleum By Dr. Dang Saebea

Catalytic Reforming and Isomerization

Introduction Catalytic 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 Reforming Catalytic 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 Preparation The 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 Number Pure n-heptane zero octane Pure isooctane 100 octane An 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 octane Pure isooctane 100 octane Example : an 80 vol% isooctane mixture has an octane number of 80.

Main feed stock 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) - light naphtha is mainly C5–C6 hydrocarbons - heavy naphtha (HN) is mainly C7–C10 hydrocarbons.

Main feed stock 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.

Product from catalytic reforming FEED PRODUCT Paraffins 30-70 30-50 Olefins 0-2 Naphthenes 20-60 0-3 Aromatics 7-20 45-60

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

REACTIONS 4 major reactions are categorized as Dehydrogenation of naphthenes to aromatics Dehydocyclization of paraffins to aromatics Isomerization Hydrocracking Desirable Undesirable

Dehydrogenation & Dehydrocyclization Highly endothermic Cause decrease in temperatures Highest reaction rates Aromatics formed have high B.P so end point of gasoline rises Favourable conditions High temperature Low pressure Low space velocity Low H2/HC ratio Dehydrogenation Dehydrocyclization

Isomerization Branched isomers increase octane rating Small heat effect Fairly rapid reactions Favourable conditions High temperature Low pressure Low space velocity H2/HC ratio no significant effect

Hydrocracking Exothermic reactions Slow reactions Consume hydrogen Produce light gases Lead to coking Causes are high paraffin concentration feed Favourable conditions High temperature High pressure Low space velocity +

Coke Deposition Coke 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 5–25 atm, not too low for coke deposition and not too high in order to avoid cracking and loss of reformate yield.

Thermodynamics of Reforming Reactions The 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.

Example The 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 Catalysts The catalyst used for reforming is a bifunctional catalyst composed of platinum metal on chlorinated alumina. the centre for the dehydrogenation reaction Platinum an acidic site to promote structure changes - cyclization of paraffins - isomerization of the naphthenes. chlorinated alumina

Reaction Kinetics and Catalysts 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. 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 process Semi-regenerative Fixed Bed Process Continuous Regenerative (moving bed) Process The old technologies are fixed bed configuration. Moving bed technology has also recently been introduced

Semi-regenerative Fixed Bed Process Three reactors fixed bed of catalyst All of the catalyst is regenerated in situ during routine catalyst regeneration shutdowns (6 to 24 months) by burning off the carbon formed on the catalyst surface Such a unit is referred to as a semi-regenerative catalytic reformer (SRR).

Semi-regenerative Fixed Bed Process Reactions such as dehydrogenation of paraffins and naphthenes which are very rapid and highly endothermic first reactor

Semi-regenerative Fixed Bed Process Reactions that are considered rapid, such as paraffin isomerization and naphthens dehydroisomerization, give moderate temperature decline second reactor

Semi-regenerative Fixed Bed Process slow reactions such as dehydrocyclization and hydrocracking give low temperature decline. Third reactor

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

Semi-regenerative Fixed Bed Process Recycling some of the hydrogen produced. 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. Some light hydrocarbons (C1–C4) 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 Process A 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 Catalyst type Temperature Pressure 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 & octane Temperature Pressure

PROCESS VARIABLES Space velocity H2 / HC ratio Low space velocity favors aromatic formation but also promote cracking. Higher space velocity allows less reaction time. Moles of recycle hydrogen / mole of naphtha charge Increase H2 partial pressure or increasing the ratio suppresses coke formation but promotes hydrocracking. Space velocity H2 / HC ratio

Reformer correlations RONF= 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 ผ 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.

Solution The material balance for the reformer is presented in the following table:

Isomerization of Light Naphtha Isomerization 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 Reactions Isomerization 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 Catalysts Two types of isomerization catalysts The standard Pt/chlorinated alumina with high chlorine content The Pt/zeolite catalyst

Standard Isomerization Catalyst This bi-functional nature catalyst consists of highly chlorinated alumina responsible for the acidic function of the catalyst. Platinum is deposited (0.3–0.5 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 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. The zeolite catalyst, which is resistant to impurities, was developed.

Zeolite Catalyst Zeolites 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 Yields The reformate yield from light naphtha isomerization is usually very high (>97 wt%). Typical yields are given in Table

Example Light 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.

Solution Isomerization yields

The End