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TUTORIAL: LIGHT NAPHTHA ISOMERIZATION(2)
Paraffin isomerization requires both a metal function and acid function. The exact mechanisms are still debated but relative importance of the catalytic functions on reactions is well known. The reactions for C5and C6 paraffin isomerization are shown below.
C5 and C6 paraffin isomerization requires both metal and acid catalytic functions. C5 isomerization can take place in the presence of relatively weak metal function. C6 paraffin isomerization requires strong metal function to isomerize n-hexane to the high octane di-methyl butanes. Isomerization of n-hexane to methyl pentanes does not require as strong of a metal function as isomerization to di-methyl butane.
Thermodynamically, hydrogen does not take part in paraffin isomerization reactions. However, kinetically, some partial pressure of hydrogen is required for all commercial catalysts. The exact mechanism is not clear but the rate determining step for paraffin isomerization over most commercial catalysts appears to be desorption of iso-paraffin carbo-cation. Hydrogen may have a role in the desorption of carbo-cations or in the reduction of coke-like deposits on the catalyst.
Increasingly, refiners are looking to light naphtha isomerization units to manage benzene in the naphtha stream. Light naphtha isomerization catalysts are effective benzene saturation catalysts because of the metal function required for paraffin isomerization. Benzene can significantly effect the performance of a light naphtha isomerization unit thermodynamically because of the very high heat of reaction and
kinetically because the basic nature of benzene can interfere with the acid sites of the catalyst. Cyclohexane formed from benzene saturation is readily isomerized to methyl-cyclopentane in the presence of acid function. In the presence of a strong metal function cyclohexane can ring open to nhexane. In the presence of a weak metal function or low hydrogen partial pressure, cyclohexane and paraffins can alkylate to heavy naphthenes.
C6 naphthenes do have high octanes, but naphthenes will attenuate the catalyst’s ability to isomerize paraffins. For this reason, naphthene ring opening is a desired catalyst feature and heavy naphthene formation is undesirable.
Heavy paraffins, C7’s, will appear in light naphtha to varying degrees depending on the naphtha splitter efficiency. C7paraffins should ideally be in the heavy naphtha and reformed into toluene and hydrogen. Some portion of C7 paraffins in the light naphtha will crack. The presence of C7 paraffins in light naphtha does not significantly influence the maximum isomerate octane, rather the C7 paraffin cracking will influence the isomerate yield. Any light naphtha isomerization catalyst with sufficient acid strength to isomerize C5 and C6 paraffins will tend to acid crack C7 paraffins to iso-butane and propane.
Typically light naphtha operation at maximum octane and maximum yield results in half of the C7 paraffins cracked to light ends. Operations can be tuned to lower severity resulting in higher C7+ yields but lower octanes will result.
Light naphtha isomerization units catalyze many reactions in addition to light paraffin isomerization reactions. It is the content of compounds other than C5/C6 paraffins in light naphtha that will affect the performance of the light naphtha isomerization unit. It is common to express the severity of a light naphtha feed in terms of the concentration of these compounds. A common measure is the X-factor of the feed.
X-Factor = wt% Feed Benzene + wt% Feed C6 Naphthenes + wt% Feed C7+ compounds Light Naphtha Isomerization Catalysts
There are three different commercial catalyst types for light naphtha isomerization: zeolitic catalyst, amorphous chlorided alumina catalyst, and sulfated zirconia catalyst. These catalyst types differ by the chemistry of the acid function. UOP is the only vendor who develops and manufactures all three types of catalysts to address the widest range of processing needs. Paraffin isomerization reactions require a combination of Bronsted and Lewis acid sites which promote varying levels of protonation activity
needed for isomerization reactions to proceed. Light naphtha isomerization units carry out many different reactions, many of which require a metal function, as described above. All of the commercial isomerization catalysts include platinum in the formulation to create metal function. Platinum is preferred for its sulfur tolerance. With platinum, sulfur is not a permanent poison since it can be stripped.
As discussed above, isomerization of paraffins in C5/C6 streams is an equilibrium limited reaction where branched paraffin isomers are generally favored by low temperatures. The most active catalysts, capable of operating at the lowest temperatures, will produce the highest octane products. Only chlorided alumina type catalysts have enough acid activity for a commercially viable C4 isomerization process.
To assist in aromatics saturation, there are a variety of elements that can be used. UOP offers platinum based benzene saturation catalysts not only because of the superior sulfur resistance of platinum but also because platinum based benzene saturation catalysts can operate at significantly higher temperatures than, for example, nickel catalysts. The use of platinum results in a more robust benzene saturation process with higher yields and smaller recycle streams.
Zeolitic catalysts have the lowest activity of the isomerization catalysts, so must operate at the highest temperatures. High temperature operation results in lower product octane due to less favorable equilibrium for branched paraffins. In addition to operating at high temperatures, zeolitic catalysts are the least selective isomerization catalysts, giving lower product yields. UOP HS-10™ catalyst is a platinum impregnated zeolite. The main benefit of zeolitic catalysts is that they are not permanently deactivated by
water or other oxygenates and are fully regenerable. Long catalysts lives have been commercially realized for this type of catalyst, typically exceeding 10 to 15 years. Feed hydrotreating is not an absolute requirement, but it is recommended for optimum catalyst performance. Zeolitic catalysts do not require a halide promoter, so there is no chloride injection or caustic scrubber.
Chlorided Alumina Catalysts
Amorphous chlorided alumina catalysts, such as UOP’s I-82™ catalyst, I-84 catalyst, and I-122 catalyst are the most active light naphtha isomerization catalysts currently available. Their high activity means that they operate at the lowest isomerization temperatures and thus achieve the highest octane products and highest yields. However, these catalysts are permanently deactivated by oxygenate compounds, like water and CO, so feed and makeup gas driers are needed. Well maintained driers can facilitate a long catalyst life, in excess of 10 years. Process units designed for chlorided alumina catalysts also require continuous chloride injection to maintain high activity. A caustic scrubber is needed to neutralizeresulting HCl in the off-gas. These catalysts are not regenerable.
Platinum helps to saturate aromatics and ring-open naphthenes, so high platinum containing catalysts are needed only for feeds that have more aromatics and naphthenes, higher X-factor feeds, or for protection from sulfur upsets. In 2006, UOP introduced I-84 and I-122 catalysts as lower platinum alternatives for low severity or low X-factor isomerization feeds for light naphtha isomerization.
Chlorided alumina catalysts are irreversibly deactivated when acid sites are converted by oxygenates. Catalysts with the highest acid site densities will have the longest catalyst life. UOP favors chlorided alumina catalysts with higher support densities to achieve higher acid site density for highest initial activity and longer life.