Aromatic Production (Part - 1) Chemical Engineering Notes | EduRev

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Chemical Engineering : Aromatic Production (Part - 1) Chemical Engineering Notes | EduRev

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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  

Processes

Description

Coal Carbonisation

(Coke oven plant)

From coke oven plant during carbonisation, light oil is obtained as by product which contains about 2-8 kg, 0.5-2 kg, 0.1-0.5 kg of benzene, toluene and xylene respectively per tonne of coal.

Steam       cracking        of

hydrocarbons

Steam cracking of naphtha and light hydrocarbon like ethane and propane produce liquid product (pyrolysis gasoline) rich in aromatics containing about 65% aromatics about 50% of which is benzene. About 30-35% of benzene produced worldwide is from pyrolysis gasoline.

Catalytic Reforming

Catalytic reforming is a major conversion process, which converts low octane naphtha to high-octane gasoline and produce aromatics rich in BTX. Major reactions involved are dehydrogenation of naphthalenes to aromatics, isomerisation of paraffins and naphthenes, dehydrocyclisation of paraffins to aromatics, and hydrocracking of paraffins.

BP-UOP Cyclar Process

In this process, BTX is produced by dearomatisation of propane and butane. The process consists of reaction system, continuous regeneration of catalyst, and product recovery. Catalyst is a proprietary zeolite incorporated with a non noble metal promoter.

Dearomatisation            of naphtha

Process consists of extraction of aromatics from high aromatic naphtha feed without prior reforming. The process is useful for naphtha having high aromatics.

Hydro dealkylation and disproportionation

Hydrodealkylation: It involves production of benzene by dealkylation of  toluene either by catalytic or thermal process.

Catalytic process: Hydeal, Deltol

Thermal process: HAD (ARCO), THDC Gulf Oil

Disproportionation: It involves conversion of toluene into benzene and xylenes

 

This process consists of conversion of C8 stream into valuable o- and p- xylene having isomerisation and isomer separation stage.

Mitsubishi's Zforming

This process uses metallosilicate zeolite catalyst to promote

Process

dehydrogenation of paraffins followed by oligomerisation and

dehydrocyclisation of paraffins followed by oligomerisation.

KTI Pyroforming

This process uses a shape selective catalyst to convert C2 and C3paraffins to aromatics.

Cheveron'sAromax

It is similar to conventional catalytic reforming processes and L-type zeolite

process

catalyst.

Isomerisation              and

This process consists of conversion of C8 stream into valuable o- and p-

Isomer process

xylene having isomerisation and isomer separation stage.

Mitsubishi's Zforming

Process

This process uses a metallosilicate zeolite catalyst to promote dehydrogenation of paraffins followed by oligomerisation and dehydrocyclisation of paraffins followed by oligomerisation.

KTI Pyroforming

This process uses a shape selective catalyst to convert C2 and C3 paraffins to aromatics.

Cheveron'sAromax

process

It is similar to conventional catalytic reforming processes and L-type zeolite catalyst.

 
Catalytic Reforming 
 
Catalytic reforming is a key conversion process in a petroleum and petrochemical industry.  The catalytic reforming gives flexibility to meet gasoline octave number requirement.  It can also make aromatics of high market value. Catalytic reforming is a refining process that uses selected operating conditions and selected catalysts to convert.  Basic objective of catalytic reforming is
  • To produce high octave blending stock for motor fuel
  • To produce high value aromatic hydro carbon such as BTX

Process Description 

A typical catalytic reforming process includes following three sections:

  • Naphtha Hydrotreating
  • Catalytic Reforming
  • Catalyst Circulation and Regeneration

Basic steps in catalytic reforming involve feed preparation, temperature control, reaction in reformer and product recovery, various types of catalytic reformer are – semi regeneration, nonregeneration cyclic moving bed two types of reformer reactors are in use radial flow and axial flow. Details of this has been covered in Module VI Lecture 6.

Reactions in Catalytic Reforming Process 

Number of reactions takes  place in catalytic reforming. Dehydrogenation is one of the major reactions. these reactions are discussed in detail in module VI lecture 6Some of the major reactions are 

Dehydrogenation 

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 

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