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TUTORIAL: LIGHT NAPHTHA ISOMERIZATION(1)
TUTORIAL: LIGHT NAPHTHA ISOMERIZATION
Veronica G. Deak, Richard R. Rosin, and Dana K. Sullivan UOP LLC Des Plaines, Illinois
The objectives of light naphtha isomerization have traditionally been to increase the octane of light naphtha streams and to reduce benzene content. These objectives are still relevant today as refiners search for new sources of octane to offset octane losses from MTBE phase-out or gasoline desulfurization and as refiners look for economical ways to comply with the MSAT II limits on gasoline benzene content. Light naphtha isomerization processes have been an effective solution for these objectives for more than forty years. New catalysts and new process technologies are making light naphtha isomerization technology more economical and more effective for meeting the challenges of today's refinery.
This tutorial will review the fundamental thermodynamics and process concepts for light naphthaisomerization. It will then review and compare the common commercial catalysts and processconfigurations. The tutorial will focus on the importance of light naphtha isomerization as part of a refiner's MSAT II compliance strategy and on the comparative economics of various processes for both new units and unit revamps.
For decades light naphtha isomerization has played an important role in refineries in increasing gasolineoctane and managing benzene content. With the current limits on gasoline olefin content, benzeneconcentration, and endpoint, the paraffinic composition of isomerization product makes it a very usefulgasoline blending component. Isomerization catalysts and processes continue to evolve to meet the newdemands posed by MTBE phase-out, gasoline desulfurization, compliance with MSAT II regulations, theincrease of ethanol in the gasoline pool, and the rapidly increasing cost of platinum.
This paper reviews chemistry, thermodynamics, and processing schemes. It also includes acomparison of the three major types of commercial catalysts, their process configurations, and economics for both new units and conversion of existing equipment. The use of light naphtha isomerization in meeting MSAT II gasoline benzene limits is also discussed.
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Thermodynamics and Reaction Network
The main objective of light naphtha isomerization processes is to increase the octane value of light naphtha by isomerizing the C5/C6 paraffins. The isomerization of normal pentane to iso-pentane results in a 31 RON increase and the isomerization of normal hexane to methyl-pentanes or dimethyl-butanes results in an average increase of 44 RON or 70 RON, respectively. The octane values of light naphtha paraffins are shown in the table below.
Table1: Light Naphtha Blending Octane Values
2,2 di-methyl butane
2,3 di-methyl butane
Unlike many reactions in refinery processes, paraffin isomerization reactions are equilibrium limited under most commercial process conditions. A common measure of how close a product or feed stream is to the equilibrium composition is the iso-ratio of a particular paraffin. The iso-ratio is the weight percentage of the particular paraffin divided by the total weight percentage of all isomers of that paraffin.
The iso-ratio for i-C5 is simply:
i-C5 iso-ratio = wt% i-C5 / ( total wt% C5 paraffins ) = wt% i-C5 / (wt% n-C5 + wt% i-C5 )
For 2,2 di-methylbutane (22DMB) the iso-ratio is:22DMB iso-ratio = wt% 22DMB / ( total wt% C6 paraffins )= wt% 22DMB / (wt% n-C6 + wt% 2 MP + wt% 3 MP + wt% 23DMB + wt% 22DMB)
Iso-ratios are stream properties similar to octane, they do not imply a reaction rate or a conversion. Sinceparaffin isomerization reactions are mildly exothermic, equilibrium conversion to the higher-RON, branched-paraffins is favored by lower reaction temperatures. The figure below shows the iso-ratios for i-C5 and 22 DMB for an equilibrium mixture and effect of temperature on the iso-ratios.
The exact composition of light naphtha can vary widely depending on a refiner’s crude slate, processing strategy, and the design and operation of the naphtha splitter. Typically, light naphtha contains more than just C5 and C6 paraffins, it will contain C5 and C6 naphthenes, benzene, and C7 paraffins. The reactions that take place in light naphtha isomerization units fall into three broad categories: paraffin isomerization, cyclic reactions, and cracking. The catalyst formulation of commercial light naphtha isomerization catalysts contains a metal function and a strong acid function. These are the catalytic functions that influence the reactions.