Hexane as a byproduct of Isomerization Unit using a Top Dividing Wall Column

Kondapalli Shreya Chaudhary
Chemical Engineer
Petroleum Corporation Limited
Mumbai Refinery, India

Prateek A. Shanware
Chemical Engineer
Petroleum Corporation Limited
Mumbai Refinery, India

Isomerization process is gaining importance in the present refining context due to limitations on gasoline pool benzene, aromatics and olefin content. The isomerization process saturates benzene and upgrades the octane of light naphtha fractions. Isomerate product contains very low sulfur and benzene, making it an ideal blending component for the refinery gasoline pool. This process complements the catalytic reforming process, in upgrading the octane of refinery naphtha streams.

Post commissioning of a continuous catalytic regeneration reformer at Bharat Petroleum Corporation Limited (BPCL) Mumbai Refinery, the existing semi regeneration catalytic reformer unit would become redundant. In view of this anticipated redundancy, a process study had been carried out along with the setting up of the continuous catalytic regeneration reformer. Conversion of the catalytic reformer unit to an isomerization unit would not only enable the refinery to increase its Bharat Stage (BS) IV motor spirit production from 40 to 100 percent of the pool, it would also build the capability of producing some quantity of more stringent quality, BS VI motor spirit.

Considering this, a global notice inviting tender for the conversion of catalytic reformer unit to isomerization unit was floated. The scope for this revamp was prepared in-house without any Project Management Consultant (PMC). Hexane of world class specification was included, as a byproduct from isomerization unit, as a part of this tender. The possibility of production of hexane from an Isomerization unit was explained to the interested bidders as hexane is traditionally produced by solvent extraction method. Economic evaluation, by comparing the net present value, of the proposals received, was also carried out in-house. In March 2017, Isomerization unit, licensed by M/s GTC Technology, LLC was commissioned at Mumbai Refinery.

The challenge of installing a divided wall column in the isomerization unit was taken up, although it had never been used to produce stringent quality hexane ever before, as BPCL is a trend setter in the Industry. We also recognized the phenomenal outcome of our disruptive thinking to co-produce hexane from an isomerization unit, which was being installed to take India on the road of total BS IV motor spirit sufficiency.

Isomerization Complex

The isomerization unit has a capacity of 0.62 million metric tonnes per annum. The salient feature of the unit design is co-production of petrochemical feedstock like Special Boiling Point (SBP) product and Hexane along with motor spirit blend component, isomerate.

The revamped unit comprises of a naphtha hydrodesulphurization unit to remove impurities from wide cut (IBP-140oC) naphtha and naphtha splitter 1 top cut (IBP - 60oC) to meet the stringent quality of motor spirit blend component, isomerate and specialty products, SBP and Hexane.

The desulphurized naphtha is split in naphtha splitter 2 into three cuts. Top cut is routed to the isomerization unit along with other light naphtha cuts from refinery units like hydrocracker and reformer. Middle cut from the naphtha splitter can augment motor spirit production/reformer feed while the bottom cut can be routed to motor spirit pool/high speed diesel pool/ reformer.

Isomerization Process

The isomerization process improves the research and motor octane number of light naphtha, predominantly C5/C6, so that it can be blended into motor spirit pool. The light naphtha fraction of the refinery is typically high in normal paraffin content resulting in low octane number, around 68. Isomerization process converts an equilibrium proportion of these low octane normal paraffins into their higher octane branched isomer.

The isomerization reaction is carried over a fixed bed non-chlorinated mixed -metal oxide catalyst in a hydrogen environment. The unit operates at moderate operating conditions (130 - 180oC / 27 kg/cm2g), low hydrogen partial pressure and high space velocity. These operating conditions promote the isomerization reaction and minimize hydrocracking.


Figure 1: Isomerization of Paraffins



Figure 2: Typical Isomerization Process Scheme

Figure 2 above, shows the schematic of a typical Isomerization unit with a De Iso Hexanizer to split the stable isomerate. In our case, the De Iso Hexanizer is used to co-produce hexane along with motor spirit blend component, isomerate.
Hexane, so produced, is treated in a polishing unit.

Divided Wall Column

The De Iso Hexanizer column installed in the isomerization unit, at Mumbai Refinery, is a divided wall column.

Although the concept of a divided wall column has been around for decades and has been applied to a wide variety of processes but it is the first time that a divided wall column has been installed in an isomerization unit and that also for co-production of very stringent quality hexane along with isomerate.

BPCL chose to implement a divided wall column, for this service, in line with its aptitude of 'the first mover status in the Industry'. The high energy saving potential as compared to a multi column system gave the final push in this decision. This energy saving potential is achieved as the column design avoids the back mixing of a concentrated lighter stream into the heavier bottom stream, after separation.

In addition, a divided wall column reduces plot area requirement - four column in series vs single column installation.

The divided wall column is categorized into three types based on the location of the wall inside the column: the top, the middle and the bottom . Out of around 150 divided wall columns being operated world vide, the divided wall column commissioned at Bharat Petroleum Corporation Limited Mumbai Refinery in 2017 is the first grass root divided wall column in India and the world's first top Divided Wall Column.

Energy reduction due to use of a divided wall column in the isomerization unit for co-production of hexane and isomerate is as tabulated:

This 27% decrease in reboiler duty due to use of divided wall column along with change from medium pressure steam reboiler to fired heater reboiler resulted in a saving of ` 63.1 crores per annum translating into 9.5 cents per barrel of crude processed in the refinery.

Hexane production

Mumbai Refinery used to produce hexane from its solvent extraction unit, which is the traditional process for producing hexane. The hexane so produced had a benzene content of 130–240 ppm wt.

Since March 2017, hexane of world class specification, is being co -produced from the isomerization unit commissioned at Mumbai Refinery:

Incorporation of high purity hexane production into the design of the isomerization unit, paved the way for dismantling of a 1989 installed, solvent extraction unit thereby making plot area available for upcoming projects in the refinery. It has also reduced the variable operating cost for production of hexane (6700 per metric tonne of hexane produced from the solvent extraction unit). Variable operating cost of hexane polishing unit, in the isomerization unit, is around-204 per metric tonne of hexane produced.

By implementation of this project, BPCL Mumbai Refinery has successfully changed their product slate of producing 'food grade' hexane to 'pharmaceutical grade' hexane. This has helped BPCL in increasing its market share.

Benefits

Installation of a divided wall column and coproduction of superior quality hexane from the isomerization unit is accruing savings of 95.7 crores per annum i.e. 14.4 cents per barrel of crude processed in the refinery.

Epilogue

This success story, which began with BPCL Mumbai Refinery's foresight to pursue the path not trodden resulted in making India the first in the history of refining with the production of very stringent quality hexane as a byproduct along with the main product, motor spirit blend component (isomerate) from an isomerization unit. This isomerization unit also has the world's first top Divided Wall Column.

Acknowledgements

We would like to thank P. Padmanabhan, C. J. Iyer, P. V. Ravitej and A. C. Prabhune of BPCL for their unwavering support & guidance and GTC Technology for their understanding of our process requirements and unit design to meet the same