By – Mohamed Shakur
While demand for gasoline may have peaked in North America and Europe, other parts of the world are experiencing rapid growth in demand for gasoline as well as a move toward clean fuels specification. Many refiners have utilized oxygenates, alkylate and isomerate to offset the loss of volume and octane that results from the reduction in sulfur, olefins and aromatics in the gasoline pool. Isomerate, a gasoline blending component from light paraffin isomerization, is an ideal choice because it is paraffinic with no benzene, aromatics or sulfur. With UOP’s light naphtha isomerization processes, a stream with an octane number of 60 to 70 containing C5 and C6 components can be economically upgraded to isomerate with an octane number as high as 93. UOP offers a full portfolio of catalysts and processes as well as unique revamp options that will allow any refiner to upgrade their light naphtha stream to a high-value isomerate product.
A full portfolio of catalysts
Isomerization of C5/C6 streams is an equilibrium limited reaction for which branched paraffin isomers are generally favored by low temperatures (see Figure 1). UOP’s catalyst portfolio consists of the most active catalysts capable of operating at the lowest temperatures and producing the highest octane products. There are three main catalyst types currently used in the production of isomerate: chloride alumina, sulfated zirconia and zeolitic.
The main choice of customers today is either chlorided alumina or sulfated zirconia catalysts.
Chlorided alumina catalysts offer the highest activity and yield. However, chlorided alumina catalysts are water sensitive, non-regenerable and require an organic chloride promoter. Additionally, feed dryers and caustic scrubbing are required which increase capital and operational expenses. Currently, UOP offers the chlorided alumina I-82, I-84 and I-122 catalysts, which are replacing the previous generations of lower-performance I-4, I-8, I-80 and I-8 Plus catalysts. The fill cost of the reactors can be reduced significantly by combining I-84 and I-122 catalysts, which have a lower quantity of platinum than I-82. In a properly operated UOP PenexTM process unit, catalyst life of over eight years has been achieved, especially when the UOP Drier Regeneration Control System (DRCS) has been used to automate and optimize the operation of the feed dryers.
Sulfated zirconia catalysts have activities significantly higher than the older, zeolitic catalysts, but are still a little less active than chlorided alumina catalysts. However, sulfated zirconia catalysts are contaminant tolerant and are regenerable. UOP currently offers the PI-242 and PI-244 catalysts whose performance approaches chlorided alumina catalysts. PI-242 and PI-244 catalysts have been chosen for new unit designs where there is a concern about feed type and contaminant levels, the use of chloride or caustic or where the catalyst has been shown to be economically competitive to chlorided alumina catalysts. The platinum quantity for chlorided alumina and sulfated zirconia catalysts available from UOP ranges from 0.12 Wt% to 0.33 Wt%. Each catalyst system has a unique set of operating parameters that affect both unit costs and product octane as well as potential revamps of existing units. Sulfated zirconia catalysts require higher hydrogen to hydrocarbon ratio and so a recycle compressor and separator are currently required.
A full portfolio of processes
UOP pioneered the development of naphtha isomerization in the 1950s and since that time, has steadily improved both the process and catalyst. The refiner’s blending needs and economics dictate the selection of the most suitable isomerization technology for a location. UOP offers two primary types of flowschemes used for the isomerization of light hydrocarbons; hydrocarbon-once-through and hydrocarbon-recycle. Hydrocarbon-once-through flowschemes increase the octane of a naphtha stream by passing the material through the reactor one time and are capable of producing products with research octane numbers ranging from 78 to 86 depending on feed composition and catalyst. UOP’s Penex process (see Figure 1), which utilizes chlorided alumina catalyst, is the most popular hydrocarbononce- through process because of its high product octanes and yields. The UOP Par-Isom process which utilizes a sulfated zirconia catalyst has become quite competitive to the Penex process. The Par-Isom process has been selected by refiners with one or all of the following objectives:
• A process with higher tolerance to feed contaminants
• Avoid the use of chloride
• Avoid the disposal of spent caustic
• Revamp an idle semi-regen unit for isomerate production at relatively low cost
Hydrocarbon-recycle flowschemes make use of either molecular sieves or fractionation columns to separate normal paraffins and lower octane isomers out of the product and/or feed. The lower octane isomers are then recycled back to the reaction section for enhanced overall conversion to higher octane isomers. Depending on the recycle scheme employed, the isomerate from hydrocarbon-recycle units can have product research octane numbers of 87 to 93 (almost 5 to 9 numbers higher than once-through units). The most common of the hydrocarbon-recycle designs incorporates a Penex process unit with a deisohexanizer (DIH) column (commonly referred to as a Penex/DIH unit) but a variety of other configurations are available. Of the more than 220 UOP isomerization units that have been commissioned, approximately 60% are hydrocarbononce- through units. The remaining 40% utilize either molecular sieve separation or fractionation in a hydrocarbon-recycle flowscheme. Most new units utilize fractionation because molecular sieve schemes are require higher capital costs. The selection of a separation flowscheme is dependent upon feed composition, availability of utilities, and required product octane.
Maximize asset utilization – Revamp with the Par-Isom process
UOP’s Par-Isom process, using PI-242 catalyst, has been used for unit conversions or where project drivers emphasize tolerance to feed contaminants or simplicity of operation more than the need for the highest product octane and yield. As a drop-in replacement, PI-242 and PI-244 catalysts are ideal for the conversion of existing reforming units or for increasing the capacity or performance of isomerization units currently using a zeolitic catalyst. One example of a low-cost revamp that has been done utilizing PI-242 catalyst is a Par-Isom side-car unit, in which an isomerization reactor is added to an existing semi-regenerative reforming unit. The two units share a common recycle compressor and may share product separation and stabilization facilities depending on existing equipment characteristics and product requirements. UOP has also developed several lower cost revamps by converting existing, idle reforming units to a Par-Isom process units using PI-242 catalyst. Very minor changes were required to convert a reforming unit for use as a Par-Isom process unit. The cost for the conversion to a Par-Isom unit is very low since the recycle compressor, reactors, reactor section heat exchangers, separator, and stabilizer can be reused.
Lowest Capital and Operating Costs
The high activity of the UOP catalysts ensures that the reactors are operated at the lowest possible temperatures which results in the lowest utilities. Both Penex and Par-Isom units are fully heat integrated to recover the maximum heat. Product yields are very high since cracking is minimized. Catalyst deactivation in the Penex unit is by contaminant ingress, particularly oxygenates, into the unit. UOP’s broad portfolio of chlorided alumina type isomerization catalysts with platinum content varying from 0.12 Wt% to 0.24-Wt%, allows for the matching the platinum content to the application and ultimately lowering the total cost of filling the reactors.
A hydrocarbon-once-through Par-Isom unit and a Par-Isom unit with a DIH column (commonly referred to as a Par-Isom/DIH unit) have EECs that are roughly 75% of the comparable hydrocarbon-once-through Penex units and Penex/DIH units. Par-Isom units tend to have utilities that are 10 to 15-% higher than the corresponding Penex unit.
Consider your options by leveraging UOP’s expertise
UOP’s highly experienced specialists and service personnel can quickly identify the best options for any refiner regardless of the feed type, source, composition or the processing objectives. UOP can also determine the optimum product octane number and the best processing solution for a particular refiner’s gasoline pool while considering the capital constraints. Contact your local Sales Representative or visit uop.com for more information.