Aromatic Production (Part - 2) Chemical Engineering Notes | EduRev

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Effect of Feed Quality on Aromatic Yield

  • Naphthenes dehydrogenate very fast and give rise to aromatics. Therefore, N + 2A is taken as index of reforming. Higher the N + 2A, better is quality to produce high aromatics. 

                   N = Naphthenes %                 A = Aromatics % 

  • Lighter fraction have a poor naphthene and aromatic content are, therefore, poor feed for reforming. Low IBP feed results in lower aromatics and H2 yield
  • Heavy fractions have high naphthene and aromatic hydrocarbon content. Therefore, good reforming feed but tendency of coke formation is high General recommended feed ranges for production 

              (1) Benzene   = 60-90oC Fraction  

              (2) To luene   = 90-110oC Fraction 

              (3) Xy lenes   = 110-140oC Fraction 

              (4) Octane blending stock  = 90-140oC Fraction 

Aromatic Production (Part - 2) Chemical Engineering Notes | EduRev

Figure M-VII 7.1: Aromatics Complex 

P-Xylene 

The p-xylene plant consists of five units are:

Pretreatment Unit: This unit is used for reducing sulphur content to 5 ppm (max.) by dehydro - desulphurisation which takes place at 330–370 oC and 24 kg/cm2 pressure in presence of cobalt molybdenum catalyst.

Reformer Unit: To get maximum amount of C8 aromatics by reforming process (Process similar to described earlier).

Fractionation Unit: For separation of o-, m-, and p-xylenes from combined C8 reformate and isomerisate from isomerisation unit (after clay treatment).

Parex Unit: This unit is for the separation of p-xylene by selective adsorption using molecular sieve followed by desorption. Other method for separation of p-xylene is by crystallisation process.

Isomerisation: Isomerisation of C8 stream from Parex unit rich in m- and o-xylene and ethyl benzene to p-xylene, which is sent to fractionation unit for separation of high component. The bottom of the column is recycled for further recovery of xylenes.

Aromatic Conversion Processes 

Because of higher demand of benzene and p-xylene in comparison to toluene and m-xylene various processes are commercially available for conversion of toluene and m-xylene to more value added products like benzene and p-xylene. processes are also avialble for conversion of paraffins into aromatics. Some of the major processes are: 

Aromatic conversion Process

Process details

Isomeration

Isomerizatin of meta xylenes to para and ortho

xylenes

Transalkylation  and Disproportion

Transalkylation and Disproportionation of C7 and C9

Toluene Disproportionation

Toluene disproportionation to xylenes and benzene

 

Selective                           Toluene

Disproportionation

Selective conversion of toluene to para xylene by

disproportionation

Xyelene Isomerizations

Maximization of p-xylene, ethyl benzene (EB)

conversion and EB dealkylation process

Aromataziation

Conversion of light hydrocarbons to benzene,

toulenes and xylenes

Paraffin Aromatizatio processes: 

Process

Licensor

Cycler

UOP-BP

Aroforming

IFP-Sheddon Technology

Management

M-2 Forming

Mobil

Z-Forming

Research Association of

 Cyclar Process 

Cyclar process inexpensive and plentiful LPG requires minimal feed pretreatment and product purification requirements and simplicity in operation. Reaction involved in the cycler process is shown in Figure M-VII 7.2. Process flow diagram of UOP-BP cycler process for LPG aromatisation is shown is Figure M-VII 7.3.

Feed: Propane, Butane, Pentanes or mixture

Liquid Product: Largely BTX essentially free from C6-C9 paraffinic & naphthalenes Preparation of Benzene Toluene and xylene charges very little with the composition of feed. Aromatic yield:

 63.6% of Feed  for Propane 

67.5% of Feed  for Butane 

→ Very high H2 yield of 5.5 – 6% for feed

→H2 purity of about 95%.

Feed                          Intermediate                                     Products

Propane

Butanes 

Pentanes 

Aromatic Production (Part - 2) Chemical Engineering Notes | EduRev

Aromatic Production (Part - 2) Chemical Engineering Notes | EduRev

Figure M-VII 7.2: Reactions involved in Cyclar Process 

Aromatic Production (Part - 2) Chemical Engineering Notes | EduRev

Figure M-VII 7.3: UOP-BP Cyclar Process for LPG Aromatisation 

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