1.0 INTRODUCTION Isomerization 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 onpurpose 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 highoctane 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, Kno¨zinger 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 PROCESS In 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 n-Pentane
CH3CH2CH(CH3)CH3 Isopentane
2.1 UOP PENEX PROCESS DESCRIPTION The 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 chlorinatedalumina 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.
off-gas to scrubber
Reflux drum hydrogen
drier
stabilizer
drier
reactors
chloride injection
feed isomerate
Figure 1 UOP Penex process flow diagram (Tine, 2004)
2.2 PROCESS VARIABLES Temperature The 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.
Feed This is the most important process variable. If the feed is poisoned it would affect the overall product yield.
Pressure The 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 COMPARISON There are various licensors for C5/C6 isomerization process. Table 1 shows some of these licensors. Table 1 C5/C6 isomerization licensors and their processes
LICENSORS
PROCESSES
UOP
Penex, Par-Isom
Axens
Ipsorb, Hexorb
GTC Technology
Isomalk 2
Kellogg Brown & Root
KBR Isomerization
BP
BP Isomerization
UOP Penex uses a high activity chloride-promoted catalyst (Pt/chlorinated Al2О3). 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 chloridepromoted 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)
PARAMETER
UOP (PENEX)
AXENS (IPSORB)
Temperature, C
120-180
120-180
Pressure, Mpa
3.0-4.0
2
1.5
2
(0.3-0.5):1
<1
o
LHSV, h
-1
Mole Ratio H2:CH Compressor Chlorine compound injection and caustic soda wash Feed drying Fire-heater (Yasakova and Sitdikova 2010)
Absent Is necessary Is necessary Absent
The major difference between the Axens Ipsorb and Hexorb process is that the Ipsorb uses a deisopetanizer upstream while Hexorb uses a deisohexaniser downstream. UOP processes differ with the catalyst used for the process.
4.0 ISOMERIZATION CATALYST SELECTION CRITERIA Selecting the right catalyst is very significant for a process. Catalyst selection has a major impact on the plant efficiency, the plant rate and, the achievement of desired turnaround schedule. The following factors can be used to select the right catalyst for an isomerization process;
Catalyst Vendor: The information on factors such as the feed, its temperature and pressure should be given to a minimum of two catalyst suppliers. The suppliers should recommend the catalyst to operate in this envelope because these factors may vary. It is important to contact more than one catalyst supplier.
Cost: The cost of the isomerization catalyst is an important selection criteria. The cost of the catalyst for different suppliers should be compared. The catalyst could be hired or paid for in instalments. The catalyst supplier should be able to give a guarantee for the catalyst life.
Reliability: It is significant to know the catalyst is reliable. The catalyst supplier should be able to provide references of clients who have used the catalyst for a number of years with good results.
Catalyst Size: The optimum size of the catalyst should be considered. Catalyst particle size influences the rate of isomerization. The smaller the particles, the larger the surface area and thus, the activity per unit mass of the catalyst. However, if the catalyst is too small it could cause pressure drop in the vessel. To avoid this pressure drop, a porous catalyst should be used as the feed would pass through the pores.
Catalyst Recycle/Reprocess: It is important to know if the catalyst can be used after it has been deactivated. Catalysts become deactivated with time on-stream; some isomerization catalysts can be regenerated but others cannot. Even the regenerable catalysts gradually lose their initial properties and would have to be replaced. The spent catalyst then has to find utilization instead of causing environmental problems.
Performance: Long term stability of the catalyst is significant. However, in most cases, catalysts would be poisoned after some. In addition to stability, activity and selectivity are also important performance factors. The catalyst must be selective to avoid unwanted side reactions. Activity of the catalyst is very important for isomerization process due to the low temperature operating condition.
Supply/Delivery time/Availability: The supply time, delivery time and availability of the catalyst has to be known. It is significant to know if the vendor can supply the catalyst in short notice. Also, the catalyst should be delivered in such a way that loading it into the vessel would not be difficult.
Catalyst Strength: The catalyst has to be strong to avoid breakage when loading into the vessel. If the catalyst is not strong enough it would break in the vessel and produce dust which would plug up vessel and other equipment downstream. Furthermore, the strength of the catalyst should not decrease within a short period of time.
Catalyst Shape: The shape of the catalyst also affects the surface area. A spherical catalyst is the wrong shape to use in an isomerization process because it would hold the maximum volume per surface area. However, it is the easiest shape for the catalyst vendor to make and it is cheaper than other shapes.
Catalyst Longevity: The catalyst should be able to last for a long time without completely losing its activity or stability.
Poison Resistance: The catalyst should be able to withstand poisons without significant change in performance.
Catalyst Handling: The risks involved in storing the catalyst should be understood before purchase.
5.0 CONCLUSION Light naphtha isomerization has become increasingly significant in helping refiners improve octane number and meet the production demands for reformatted gasoline. UOP is the leader in licensing of isomerization technology; more than 220 units in the world are operating by its technology currently (Yasakova and Sitdikova, 2010).
Process flow schemes for the isomerization process differ with respect to the catalyst used. For example, when a zeolite based catalyst is used feed drying is not required as the catalyst is resistant to water. However, different equipment may be used by the process licensors to make their processes more efficient.
Low temperature favours C5/C6 yield. Hence, it is important to use a high activity catalyst. Selecting the best catalyst for the process is significant. The catalyst must have high stability, selectivity and activity.
