Catalytic Reforming (Part - 1) | Chemical Technology - Chemical Engineering PDF Download

• Feed preparation: Naphtha Hydrotreatment
• Preheating: Temperature Control,
• Catalytic Reforming and Catalyst Circulation and Regeneration in case of continuous reforming process  
• Product separation: Removal of gases and Reformate by fractional Distillation
• Separation of aromatics in case of Aromatic production

Naphtha Hydro Treatment 

Naphtha hydrotreatment is important steps in the catalytic reforming process for removal of the various catalyst poisons. It eliminates the impurities such as sulfur, nitrogen, halogens, oxygen, water, olefins, di olefins, arsenic and other metals presents in the naphtha feed stock to have longer life catalyst. Figure M-VI 6.1 illustrate hydrotreatment of naphtha.

• Sulphur: Mercaptans, disulphide, thiophenes and poison the platinum catalyst. The sulphur content may be 500 ppm.
• Maximum allowable sulphur content 0.5 ppm or less and water content <4 ppm.  
• Fixed bed reactor containing a nickel molybdenum where both hydro de sulphurisation reactions and hydro de nitrification reactions take place.  
• The catalyst is continuously regenerated. Liquid product from the reactor is then stripped to remove water and light hydrocarbons.  

Various sections in the naphtha hydro treatment unit are:

Charge Heater: Preheating reactor feedstock to reaction temperature of 340 ^oC. Charge heater has four passes four gas burners. Heater tubes are made up of SS-321  

Reaction Section: The reactor consists of two catalyst beds.

Stripping Section: Stripping section uses air for stripping the light ends mainly hydrogen sulfide from reactor product, stripper temperature 14 kg/ cm² and temp. 172 ⁰C

 Stripper Reboiler: Stripper reboiler supply heat required for striper  

Operating Variables Naphtha Hydrotreatmernt

• Reactor temperature
• Space velocity
• Hydrogen partial pressure
• H₂/HC ratio, feed quality
• Stripper bottom temperature 

Figure M-VI 6.1: Hydrotreatment of Naphtha

Classification of Processes 

• Semi Regenerative catalytic reforming
• Cyclic catalytic reforming
• Continuous catalytic reforming(CCR) 

Various Types of Catalytic Reformers

Semi-Regenerative Fixed Bed reactors: In this type of reformers the catalyst generally 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 offline 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.

 Semi- regenerative Catalytic Reforming Process

A semi-regenerative process uses low platinum and regeneration is required only once a year. The process consists of typically three reactor beds & furnace preheaters. The dehydrogenation is highly endothermic and large temperature drop as the reaction proceeds. Multiple reactors with intermediate reheat is required. Dehydrogenation of naphthene takes place in first reactor and requires less catalyst. Preheat of feed is required. Last reactor for isomerization of paraffins. Typical catalyst distribution in three reactors are 20, 30 and 50percent. Figure M-VI 6.2 shows typical catalytic reforming process.

Catalyst Regeneration 

Performance of the catalyst decreases with respect to time due to deactivation. Reasons for deactivation  

• Coke formation
• Contamination on active sites
• Agglomeration
• Catalyst poisoning Activity could be restored if deactivation occurred because of coke formation or temporary poisons.

Objective of Regeneration

• Surface area should be high
• Metal Pt should be highly dispersed
• Acidity must be at a proper level
• Regeneration changes by the severity of the operating conditions
• Coke formation can be offset for a time by increasing reaction temperatures. 

Figure M-VI 6.2: Catalytic Reforming Process

 Reactions in Catalytic Reforming 

Following are the most prevalent main reactions in catalytic reforming

Desirable

• Dehydrogenation of naphthenes to aromatics
• Isomerisation of paraffins and naphthenes
• Dehydrocyclisation of paraffins to aromatics

Non-Desirable 

• Hydrocracking of paraffins to lower molecular weight compounds

Dehydrogenation & Dehydrocyclization: Highly endothermic, cause decrease in temperatures, highest reaction rates, aromatics formed have high B.P so end point of gasoline rises Dehydrogenation reactions are very fast, about one order of magnitude faster than the other reactions. The reaction is promoted by the metallic function of catalyst  

Methyl cyclohexane → Toluene + H₂

MCP→Benzene + H₂

Dehydrocyclisation: It involves a dehydrogenation with a release of one hydrogen mole followed by a molecular rearrangement to form a naphthene and the subsequent dehydrogenation of the naphthene.

 i-paraffins to aromaticsof paraffins  

n-heptane à toluene + H₂

Favourable Conditions: High temperature, Low pressure, Low space velocity, Low H₂/HC ratio 

Isomerisation: 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  

n-Hexane → Neo hexane

Naphthenes dehydro-Isomerisation: A ring re-arrangement reaction, Formed alkyl-cyclohexane dehydrogenate to aromatics." Octane increase is significant, Reaction is slightly exothermic 

Coking: Coking is very complex group of chemical reactions. Linked to heavy unsaturated products such as poly-nuclear aromatics. Traces of heavy olefines and di-olefines promote coking. High feed FBP favors coking. Poor feed distribution in the reactor promotes coking favored by high temperature

Hydrocracking: Exothermic reactions, slow reactions, consume hydrogen, produce light gases, Lead to coking, Causes are high paraffin conc feed

Favourable conditions: High temperature, High pressure