1.0 INTRODUCTIONIsomerization technologies can be used across a wide spectrum of refining applications. According to Robinson and Hsu (2006), isomerization occurs has a side-reaction in all conversion processes, but in refining, isomerization process refers specifically to the on-purpose isomerization of n-butane, n-pentane, and n-hexane. There are two distinct processes of isomerization; butane isomerization and pentane/hexane isomerization (C5/C6). Butane isomerization found initial commercial application during the World War II for making high-octane aviation gasoline components and additional feed for alkylation units. Isomerization of C5 and C6 alkanes was commenced towards the end of World War II, to provide additional blending stock for aviation gasoline (Weitkamp, Knozinger and Ertl, 1997). However, there was a decline in the demand of alkylate and this led to majority of the butane isomerization units shut down. In recent years, new isomerization units have been installed due to greater demand for high-octane motor fuel.The n-paraffin components of the lighter gasoline fraction especially butane (C4) to hexane (C6) have poor octane ratings. Isomerization is the conversion of these n-paraffins to their isomers that yield to high gasoline components of high octane rating. Isomerization process is carried out in the presence of a catalyst. This report would focus on C5/C6 isomerization giving a detailed Universal Oil Products (UOP) Penex process description. It would further discuss the important process variables and the impact on the process when these variables are changed. This report will also discuss the technical differences of two C5/C6 isomerization licensors and finally, discuss the important factors considered in selecting a catalyst for the process.

2.0 PENTANE/HEXANE ISOMERIZATION PROCESSIn C5/C6 isomerization process, low octane straight-chained n-pentane is converted into a high octane iso-pentane while hexanes are converted into branched form with double side chains. According to Magee and Dolbear (1998), the C5/C6 refinery feed in isomerization units are more complex than the n-butane feed for isobutene production. This is because the C5/C6 feed contains some benzene and naphthenes (cycloalkane). Conversion is achieved in the presence of a catalyst. This catalyst could be a chlorinated-alumina based catalyst, a zeolite based catalyst or a sulphated metal oxide based catalyst. Isomerization process is reversible and slightly exothermic. There are diverse flow schemes used commercially for isomerization processes. The choice of which to use depends mainly on the feed composition, desired octane number, and the available capital.CH3CH2CH2CH2CH3 CH3CH2CH(CH3)CH3Isopentanen-Pentane

2.1 UOP PENEX PROCESS DESCRIPTIONThe feed to the isomerization unit is light naphtha. It is pre-treated before reaching the isomerization unit through hydrotreating processes to avoid catalyst poisoning. The light naphtha feed and the hydrogen are charged to two different dryer vessels. These vessels are filled with molecular sieves, which remove water to protect the catalyst. The chlorinated-alumina based catalyst is used in this process. The dried feed is then mixed with the hydrogen and passed through a heat exchanger against the reactor effluent. It then enters a charge heater where it is heated to the reactor temperature before entering the reactors. The reactor operates at about 120oC-180oC. The reactor effluent is cooled and sent to the product stabilizer where separation takes place. The stabilizer overhead vapours are caustic scrubbed for removal of HCl formed from organic chloride added to the reactor feed to maintain catalyst activity. After scrubbing, the overhead gas flows to the fuel gas system. The bottom product from the stabilizer is the isomerate and it is sent for gasoline blending.

Figure 1 UOP Penex process flow diagram (Tine, 2004)

2.2 PROCESS VARIABLESTemperatureThe operating temperature is the most effective influence on the equilibrium of the process. Low temperature favours C5/C6 yield. However, the catalyst activity must be high to achieve meaningful conversion levels. Although, even at low temperatures, complete conversions in one pass over catalyst is not possible (Magee and Dolbear 1998). If the temperature is increased undesired hydrocracking reactions would occur.

FeedThis is the most important process variable. If the feed is poisoned it would affect the overall product yield.

PressureThe system pressure considered in conjunction with the hydrogen flow rate to the reactor. Pressure has very little or no effect on C5/C6 isomerization. However, Chlorided-alumina catalyst is more active at higher pressures.

Liquid Hourly Space Velocity (LHSV)LHSV is set during the design phase of any isomerization project and reflects the compromise between residence time and overall catalyst cost. At lower LHSVs, more catalyst is loaded resulting in a longer residence time. As a result lower temperature operation is possible, resulting in higher product.

Hydrogen-to-Hydrocarbon Ratio (H2/HC)Operating at lower hydrogen to feed ratio can increase conversion. However, an increase in hydrogen could prevent coke formation on the catalyst.

3.0 PROCESS LICENSORS COMPARISONThere are various licensors for C5/C6 isomerization process. Table 1 shows some of these licensors.Table 1 C5/C6 isomerization licensors and their processesLICENSORSPROCESSES

UOPPenex, Par-Isom

AxensIpsorb, Hexorb

GTC TechnologyIsomalk 2

Kellogg Brown & Root KBR Isomerization

BPBP Isomerization

UOP Penex uses a high activity chloride-promoted catalyst (Pt/chlorinated Al23). This high-activity catalyst allows low operating temperatures that favour better isomerisation yields. Axens Ipsorb could either use a zeolite based catalyst or the high activity chloride-promoted catalyst depending on the feed. The zeolite catalyst is not very active and therefore needs higher operating temperatures. However, it is resistant to water and sulphur in the feed. The process flow sheets for the two licensors are different. Unlike the UOP process, the Axen Ipsorb process uses a deisopentanizer upstream to separate isopentane distillate from the reaction. This enhances the n-pentane equilibrium conversion while reducing reactor throughput (Domergue and Watripont, 2005). Both processes can use either one or two reactors. The two reactors are in series and are used to achieve high on-stream efficiency. The catalyst in one reactor can be replaced while operation continues in the other.

Figure 2 Ipsorb Process (Dormergue and Matthews 2001)

If the catalyst used in the Ipsorb process is the chlorinated-alumina based catalyst, both processes would operate at almost the same conditions. For instance, the operating reactor temperature for the isomerization reactors for both processes is 120oC-180oC. Hence, a fired heater is not required in the two processes. In both processes, there no recycle gas compressor is needed. This is because only a small amount of hydrogen needs to be present to drive the reaction. The reactor product for the UOP Penex process is sent to a stabilizer for separation. On the other hand, the Axens Iposrb reactor product is sent to a molecular sieve separator.

Table 2 Comparison of UOP Penex and Axens Ipsorb (with a chlorinated-alumina based catalyst) PARAMETERUOP (PENEX)AXENS (IPSORB)

Temperature, oC120-180120-180

Pressure, Mpa3.0-4.02

LHSV, h-11.52

Mole Ratio H2:CH(0.3-0.5):1