Aromatic Production (Part - 1) | Chemical Technology - Chemical Engineering PDF Download

Aromatic Production

Aromatic hydrocarbons especially benzene, toluene, xylene, ethyl benzene are major feedstock for a large number of intermediates which are used in the production of synthetic fibers, resins, synthetic rubber, explosives, pesticides, detergent, dyes, intermediates, etc. Styrene, linear alkyl benzene and cumene are the major consumer of benzene.

 Benzene also finds application in the manufacture of a large number of aromatic intermediates and pesticides. As per CMAI, demand for benzene is forecast to grow at an average annual rate of 2.8% per year through 2020 resulting in nearly 57 million tonnes of demand by 2020. Originally, the aromatics were produced from coal tar distillation, which is the by-product of destructive distillation (carbonisation).

 Major application of toluene is as solvent. Other uses are in the manufacture of benzoic acid, chloro derivatives, nitro toluenes, toluene sulphonic acid, toluene sulphonamide, benzaldehyde, etc Xylenes are another important aromatics. Various sources of aromatics is mention in Table M-VII 7.1.

Amongst the xylenes, about 80% of the production is of p-xylene. Finds application in the manufacture of terephthalic acid/DMT. o-Xylene used in the manufacture of phthalic anhydride and m-xyleneIsohthalic acid. Typical yield of benzene, toluene, xylene in kg per tonne of coal carbonised is about 2.8, 0.5-2, and 0.1-0.5 kg.

Table M-VII 7.1: Various Sources of Aromatics  

Methyl Cyclohexane     →       Toluene  +  H₂

MCP     →       Benzene  +  H₂   

Isomerisation

n-Hexane      →        Neohexane 

Dehydrocyclisation of paraffins, i-paraffins to aromatics

n-heptance      →     Toluene  +  H₂ 

Hydrocracking 

Btx from Petroleum 

Major Units of Aromatic Complex

• Heavy Naphtha Pretreatment Unit
• Catalytic reforming (Platformer Unit,CCR Unit Continuous Catalyst Regenerator)
• Recovery Plus
• PSA (Pressure Swing Adsorption)
• BTX separation
• Xylene Fractionation Unit for separation of o-xylene from m- and p- xylene
• p-xylene  and m-xylene separation by crystalisation, adsorption

Process steps in aromatic production: Figure M-VII 7.1 gives the details description of aromatics complex. The various steps involved in aromatic production are given below:

• First step in making BTX is to distill off a suitable fraction rich in natphthenes which serves as precursors for aromatics
• Catalytic reforming or a team cracking to produce an aromatic gasoline. Detail of the catalytic reforming is given in Lecture 6 , Module VI Lecture 6 ,
• Preliminary treatment of this cut: fractionation and /or selective hydrogenations essentially pyrolysis gasoline
• Solvent extraction to eliminate non-aromatic from aromatics
• Distillation to produce pure benzene and toluene and in cased reformates used alone or blended art a pyrolysis gasoline, the following additional treatment
• Distillation aromatic C₈ to yield by super fractionation ethyl benzene and O-xylene, after passage through a separation column in a light cut and a heavy cut (splitter)
• Production of p-xylene at low temperature with a mother liquor by product rich in mxylene
• Isomerisation / delakylation /disporoportionation of m-xylene to p-xylene

Separation of Aromatics: As non aromatics and some of the aromatics have close boiling points, various methods used for  their separation are

• Liquid – Liquid Extraction (DEG, TEG, Tetra methylene Sath NMP-EG, Monoethyle methyl forma midamorphine, DMF)
• Distillation,Eextractive or Azeotropic distillation.
• Adsorption
• Crystallisation

Process Variables:

Various process variables in the catalytic reforming for the production of aromatics are

• Feed quality and N+2A
• Temperatures:
• Space velocity
• Hydrocarbon hydrogen ration
• presence of impurities

Details of these parameters are discussed in Module VI  Lecture 6