Paraffins isomerisation options
Isomerisation technologies are available, from once-through and deisohexaniser recycle processes to recycle schemes involving molecular separation processes
Bruno Domergue and Laurent Watripont, Axens
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Light naphtha isomerisation is a process that significantly boosts octane in light gasoline fractions. It is a particularly important process step when employed in conjunction with processes whose objectives are to meet the most stringent gasoline specifications, such as those for reducing benzene and sulphur content in the gasoline pool. Unfortunately, many of these mandates have the undesirable side effect of reducing octane in the gasoline pool too. Furthermore, uncertainty about future MTBE regulations underlines the need for isomerisation processes that produce aromatics and sulphur-free product. The isomerisation route offers a solution to current and future gasoline pool quality and production challenges.
Catalyst: the process base
The Axens isomerisation process generally employs a high-activity chlorinated alumina isomerisation catalyst like IS-614A, developed by IFP in the early 1990s. This high-activity catalyst allows low operating temperatures that favour better isomerisation yields and high space velocities with negligible hydrocracking of the C5–C6 molecules. Cracking is to be avoided, as it produces low-value light ends as well as heavy compounds that foul the catalyst and shorten the cycle.
However, chlorinated alumina catalyst does require strict feed pretreatment to eliminate oxygen (including water), nitrogen and sulphur-containing compounds. The more rugged but less active zeolite catalyst, IP 632, can be employed if feed contamination is a problem. Nevertheless, this catalyst operates at a higher temperature and results in a significantly lower octane boost.
The refining industry is under constant pressure to reduce operating expenses, among which are catalyst costs. The challenge is to bring an isomerisation catalyst to market offering high activity at a lower cost with a reduced platinum requirement. In response, Axens and Albemarle Catalysts have jointly developed a new chlorinated alumina catalyst, ATIS-2L, which is now available commercially.
In-house pilot plant experiments were conducted under constant-catalyst-volume conditions to compare the performance of ATIS-2L with Axens’ IS-614A catalyst, as well as with another catalyst available on the market. The following results were obtained from testing a very heavy reactor feed of C5–C6s containing 25–30% of higher-boiling naphthenes, methylcyclopentane and cyclohexane, and for which the C5:C6 ratio was about 0.25. The presence of naphthenes in this feedstock is representative of the current trend to remove benzene precursors from reformer feeds in order to minimise benzene production in the reformer.
Compared to the previous-generation IS-614A catalyst, the new ATIS-2L catalyst exhibits higher isomerisation activity at constant reactor volume, making reduced operating temperatures possible, which result in even more favourable isomer distributions. This is illustrated in Figure 1, where a 10°C reduction in operating temperature is shown to yield a one-point octane increase.
Moreover, since ATIS-2L has a lower bulk density than IS-614A, even while achieving better equilibrium
, the initial catalyst load is reduced by 22% and the platinum inventory falls by 10% at constant reactor volume. Combined, the two effects lead to a 16% reduction in catalyst investment.
Pilot tests were also conducted for a lighter feed (0.29 C5:C6 feed ratio with 15% naphthenes), where ATIS-2L was compared with a conventional commercial catalyst used for isomerisation. Based on equal reactor volumes and despite its lower bulk density, ATIS-2L demonstrated an octane benefit of almost one point, again due to higher volumetric activity. When the improved RON and lower loading density features are taken into consideration, ATIS-2L offers the most attractive economical solution.
Conventional isomerisation technologies
The isomerisation data reported hereafter are based on chlorinated alumina catalysts operating with fresh feeds, for which the concentration of C5s is 65% of that for C6s. When capital investment must be minimised, Axens proposes once-through schemes without recycle. The reaction system consists of two liquid-phase reactors in series, with special valving arrangements allowing each reactor to be operated in the lead or tail position. Hydrogen utilisation is fully achieved in this once-through scheme, requiring neither recycle compressor nor separator drum.
With the chlorinated alumina catalyst, a very high equilibrium conversion of normal molecules to higher branched isomers is attained. In order to remove potential catalyst contaminants, the feed and make-up gas undergo pretreatment steps such as adequate hydrotreating and molecular sieve dryers.
Even the most active isomerisation catalyst can only produce a limited octane gain in a simple once-through isomerisation scheme. Isomerate RONs of 83–84 can be obtained from a feed having a C5:C6 ratio of 0.65. For a somewhat higher RON product, a deisopentaniser can be placed upstream of the isomerisation section. The high RON isopentane distillate is removed from the reaction, thus enhancing normal pentane equilibrium conversion while reducing reactor throughput. To go beyond the once-through limitations requires recycling the unreacted lower-octane paraffin components to the isomerisation reactor. This may be achieved with a deisohexaniser.
For still higher RON isomerate, a deisohexaniser can be added downstream from the reaction section. In the scheme shown in Figure 2, the higher octane and more volatile isohexanes (dimethylbutanes) are removed by distillation together with the C5s. The distillate is combined with the deisohexaniser bottom to become the final isomerate product.
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