Apr-2001

Advanced recycle paraffin isomerisation technology

An examination of how combining chlorinated catalyst, molecular sieves and well designed process configurations can maximise the value of C5/C6 streams

Bruno Domergue, Axens
Russell Matthews, Holborn Europa Raffinerie GmbH

Article Summary

Light naphtha isomerisation can significantly boost octane in light gasoline fractions. It is particularly attractive when employed in conjunction with processes whose objectives are to meet Euro 2005 benzene and sulphur limits in gasoline. Unfortunately, many of these mandated processes have the undesirable side effect of reducing octane. Further complicating the refiner’s octane deficit is uncertainty about future MTBE regulations, highlighting the need for isomerisation processes that produce aromatics and sulphur-free product.

The first half of this article presents isomerisation technologies from conventional once-through and deisohexaniser recycle to advanced schemes involving separation processes. Cost-effective debottlenecking opportunities applying the Ipsorb or Hexorb concepts to an existing isomerisation unit, which can improve product RON by 4 to 6 points, are also discussed.

Following the overview of isomerisation technologies, Holborn Europa Raffinerie (HER), describes an industrial programme that includes isomerisation technology with deisohexaniser recycle to meet Euro 2000 and prepare for anticipated 2005 gasoline specifications. Project considerations that were addressed from the initial concept through start-up and the operating results obtained are covered.

Isomerisation routes Catalyst: the process base
Two types of isomerisation catalyst, zeolite and chlorinated alumina, are generally proposed, both having noteworthy competitive advantages. The choice is between the robustness of the zeolite and the high activity of the chlorinated alumina-based catalyst. The more rugged but less active zeolite catalyst, IP632, can be employed if feed purity prevents use of chlorinated alumina catalyst. Nevertheless, the zeolite operates at higher temperatures and provides a significantly lower octane boost.

Chlorinated alumina’s lower operating temperature results in the highest octane, although its higher sensitivity to feedstock impurities requires strict feed pretreatment to eliminate oxygen-  (including water) containing, nitrogen- and sulphur-containing compounds.

The IS 614A chlorinated alumina catalyst is unique among the chlorinated in that it is regenerable. Should it be poi-soned during a plant upset, a complete shutdown is avoided by using a two-bed reactor system. The contaminated bed is simply taken offline, the catalyst removed, regenerated, reloaded, and put back online. This ensures years of continuous service.

The isomerisation performance for four chlorinated alumina catalyst configurations discussed in this article is summarised in Table 1, based on processing a feed having a C5 to C6 ratio of 40:60. This ratio corresponds to a feed containing benzene precursors that are typically sent to isomerisation units in order to minimise benzene production in catalytic reforming units. Operating costs and production revenue are obtained from typical utilities, catalyst, adsorbent costs and octane-barrel values for the year 2000.

Conventional isomerisation
Once-through isomerisation
For minimum capital investment, a cost effective once-through scheme is available. Going beyond the 83 RON octane limitation (Table 1) the once-through scheme requires recycling the unconverted lower-octane paraffins to the isomerisation reactor. Adding a deisohexaniser downstream from the reaction section is a logical next step towards octane improvement.

Isomerisation with deisohexaniser
In Figure 1 (on previous page), the deisohexaniser separates the higher octane and more volatile C5s and di-methylbutanes (DMBs) as a distillate that is recombined with the bottom fraction to become the final isomerate product.

A side-stream from the bottom half of the column, containing higher concentrations of lower octane methylpentanes (MPs) and unconverted n-hexane, is recycled to the reactor. A recycle flow equivalent to 65 per cent of the fresh feed adds 4.5 octane points to the once-through scheme.

Although adding the deisohexaniser is simple in concept and increases the C6 isomer content, it does not provide for recycling normal pentane (61 RON) which enters the final isomerate product via the deisohexaniser distillate.

Advanced recycle technologies
Molecular sieve separation
For full conversion of all normal paraffins, recycling normal paraffins to extinction is required to convert them entirely to branched isomers. This involves separating and recovering the normal paraffins from their isomers. An efficient separation path uses molecular sieve adsorption technology. Using molecular sieves to separate normal paraffins from their isomers, either in the vapour or liquid phase, is a proven industrial technique and has been applied to isomerisation processes.