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Quantitative Risk Assessment For MSP Revamp for Numaligarh Refinery Limited, Assam

The Numaligarh Refinery is located at Morangi, Golaghat district, Assam in India is a refinery owned by Numaligarh Refinery Limited, a joint venture between Bharat Petroleum (61.65%), Oil India (26%) and Govt of Assam (12.35%). The NRL consists of various processing units. The objective of the project was to conduct the QRA study for the revamp in the MSP (Motor Spirit Plant) of the refinery. The MSP Unit of NRL is having designed capacity of 225 TMTPA. The MS block units comprise of the following units for producing MS with required specifications:

  • Naphtha Hydrotreating Unit (NHTU)
  • Catalytic Reforming Unit (CRU)
  • Isomerization Unit (ISOM)

Naphtha Hydrotreating Unit (NHTU): Naphtha from the feed surge drum is pumped to the reaction section. Hydrogen make up is mixed with feed and sent to the reactor. Feed is heated against HDS reactor effluent in the heat exchanger. Complete di-olefins saturation and partial olefin hydrogenation take place in the reactor. At the outlet of reactor, effluent is mixed with hydrogen recycle gas and the mixture is heated in the HDS feed/effluent exchanger. The feed to the HDS reactor is brought upto the reaction temperature by fired heater. The olefin hydrogenation and hydro-desulfurization and denitrification reactions take place in 2nd HDS reactor. From the separator drum, the recycle gas is circulated by recycle compressor. The liquid phase is sent to the downstream stripper column. The overhead product of the stripper column is partly condensed in air condenser before entering reflux drum. The vapour is sent to battery limit as off gas. The liquid distillate is sent as reflux to the top of the column. Stripper bottom product is routed to Naphtha splitter. Naphtha is split into Light Naphtha and Heavy Naphtha in the Naphtha Splitter. Light Naphtha is routed to Naphtha Isomerization Unit and Heavy Naphtha is routed to Reformer. A small part of Hydro treated Naphtha is routed to Naphtha pool.

Catalytic Reforming Unit (CRU): The Reformer feed comes from Naphtha splitter bottom and mixed with the recycle gas. Combined fluids are preheated and injected into the first reactor. Reforming reactions are endothermic and occur successively in other adiabatic reactors. The last reactor effluent is cooled down and the two phases are separated. The vapour phase is separated further into two streams. The first stream (recycle gas) is sent back to reaction section and the second stream is compressed by compressors and mixed with the liquid from separator drum. The mixture feeds the HP Absorber drum after cooling. The H2 gas from Absorber drum is sent to battery limit through Chloride guards. These schemes allow for an optimized LPG/ reformate recovery and produces a high purity H2 rich gas.

The liquid phase of Absorber drum is mixed with the stabilizer overhead vapour and fed into LPG absorber drum to allow C3 and C4 recovery into the vapour distillate. The gas phase is sent under pressure control to fuel gas system. The liquid is withdrawn under level control and feeds the stabilizer after being heated. The overhead product of the stabilizer column is partly condensed before entering reflux drum. The vapor is sent back to LPG absorber drum. The reformate is withdrawn from the stabilizer bottom and after cooling sent to battery limit. This scheme allows for an optimized LPG recovery from economic standpoint. Unstabilized LPG is routed to De-ethanizer. The overhead product of the De-ethanizer column is partly condensed before entering reflux drum. The vapour is sent to Fuel Gas network. The liquid distillate is used as De-ethanizer reflux.

Isomerization Unit: The main objective of the unit is to convert low octane n-paraffins to high octane iso-parafins. Isomerisation occurs in a chloride promoted fixed bed reactor where n-parafins are converted to iso-parafins. The catalyst used is sensitive to incoming contaminants i.e sulfur and water. The desulfarised feed and hydrogen are dried in fixed beds and mixed together. The mixed feed is heated and passes through a hydrogenation reactor to saturate olefins to paraffins and saturate benzene. The hydrogenation effluent is cooled and passes through isomerization reactor. The final effluent is cooled and separated as hydrogen and LPG which typically go to fuel gas, and isomerate product for gasoline blending.

NRL Refinery Assam
NRL Refinery Assam

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