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Redefining reforming catalyst performance: High selectivity and stability

09.01.2012  |  Le Goff, P.-Y.,  Axens, Ruel-Malmaison, FranceRoss, J.,  Axens North America, Princeton, New JerseyLopez, J. ,  Axens, Salindres, France

Highly developed CCR catalysts are more robust to ensure extended service over 7–9 years.

Keywords: [catalysts] [reforming] [aromatics] [catalytic reforming] [gasoline] [naphtha] [benzene] [paraxylene]

Over the next 10 years, global demand for oil products is forecast to increase at an average rate of 1.2%/yr through 2020. Demand will be just below 100 million barrels per day of oil equivalent (MMbdoe). However, this growth will not be distributed evenly around the world.

Developed markets

In the Organization for Economic Co-operation and Development (OECD) countries, reductions in automobile fuel consumption will decrease oil demand at about 0.5%/yr, thus creating refining overcapacity. The situation is completely different in nations with growing economies where the gross domestic product (GDP) is increasing rapidly and the population aspires to greater mobility. In these (non-OECD) countries, demand for oil products will rise at 2%/yr and will comprise 53% of world demand by 2020.

Developing markets

Concerning gasoline demand over the next 10 years, strong growth is mainly expected in Asia (+2.1 MMbdoe), the Middle East (+0.3 MMbdoe), the Former Soviet Union States (+0.37 MMbdoe) and Latin America (+0.6 MMbdoe), as shown in Fig. 1. In these regions of developing and growing economies, continued strong growth is projected for both gasoline and petrochemical polymers.

 
  Fig. 1. Worldwide incremental refinery product demand, 2010–2020.



Petrochemicals

Worldwide demand for polymers is growing at a significantly higher pace than oil and gas production (Fig. 2) and thus initiating large expansions in olefins and aromatics complexes. Global paraxylene (PX) consumption is forecast to exceed 40 million tons (MMton) by 2015 compared to 32 MMton in 2011. The additional capacity will be located in the Asia-Pacific region, where PX demand is the highest, followed by the Middle East. New aromatic complexes, which include continuous catalyst regeneration (CCR) reforming units, will be required to meet the growing demand in polyester used for bottles and textiles. To meet both aromatics and gasoline demand, capacity additions for light-oil processing are expected in these regions at about 1.5 MMbpd in combined reforming, isomerization and alkylation capacity by 2020.

 
  Fig. 2.  Worldwide growth index in oil, gas
  and polymer sectors.

Catalytic reforming of naphtha is central in the production of both high-octane fuel and aromatics to support both rapidly growing markets. Accordingly, there is a continued strong demand for catalytic reforming units and improved catalysts for new and existing units with a global installed capacity over 13 MMbpd. The present annual worldwide market for reforming catalyst represents several thousand metric tons for fixed bed, cyclic and CCR markets.

CATALYTIC REFORMING FUNDAMENTALS

The role of catalytic reforming is fundamental in transforming low-octane naphtha from crude oil and hydroprocessing units into high-octane transportation fuels and aromatics. The process involves transforming or reforming the paraffinic and naphthenic molecules in the feed into high-octane aromatics and branched components, and coproducing hydrogen needed by other refinery units such as hydrotreaters and hydrocrackers. This is accomplished over a heterogeneous catalyst at elevated temperature and preferably low pressure according to Le Chatelier’s principle.

Structure

Reforming catalysts are complex composites of a highly active precious metal, platinum (Pt), to efficiently perform dehydrogenation and hydrogenation reactions, and an active support or carrier to do complementary reactions. The carrier is a high-purity alumina, with a specific pore structure, designed to have an acid functionality, which can be moderated by controlling the amount of chloride added to the support and/or by the addition of promoters. Together, these “metal” and “acid” components, as shown schematically in Fig. 3, form a dual-function catalytic system capable of transforming low-octane paraffins and naphthenes into high-octane gasoline, aromatics and byproduct hydrogen.

 
  Fig. 3.  Schematic of acid and metal sites
  on reforming catalyst.


Functions

A simplistic representation of the main reactions is shown in Fig. 4 and is linked to the metallic and acid functions. The important dehydrogenation reaction to convert a cyclohexane component into an aromatic is very rapid and easily accomplished by the metal function of the catalyst. For many feeds, in particular hydrocracker and coker derived naphthas, a significant portion of the naphthenic compounds contain cyclopentane elements that require the acid-catalyzed reaction of ring extension or conversion into a cyclohexane-bearing component for subsequent dehydrogenation on the metallic sites. Ring extension and dehydrocyclization of paraffins are all difficult, but they are critical functions that require highly selective catalyst. If the acid and metal functions are not tuned or properly balanced, undesirable side reactions do occur, leading mainly to acid cracking and hydrogenolysis, and, to a lesser extent, dealkylation. In the reforming unit, these side reactions result in the formation of light petroleum gas (LPG), light gas and coke; all contribute to nonselective conversion, catalyst deactivation by coke deposition, and light-ends handling limitations.

 
  Fig. 4.  Bi-functional reforming catalyst
  reactions. The desired reactions are labeled
  in blue, with undesirable side-reactions
  labeled in red.


Catalyst performance

The carrier and highly dispersed Pt metal interact in a complex way to accomplish the desired reforming reactions. Performance of the catalyst is described in terms of activity, selectivity and stability.

Activity is commonly defined in terms of temperature required to achieve a given objective; it is very similar to the definition used to describe hydrotreating catalysts. A more active catalyst is able to achieve the same product yield or severity (gasoline octane or aromatics yield) at a lower reactor temperature. For fixed-bed units, this means longer cycle lengths, and, for CCR units, it means greater operating flexibility within unit constraints.

Selectivity. The selectivity of the catalyst refers to the relative yield of desired product, such as C5+ reformate gasoline or aromatics, compared to another catalyst operating with the same severity target (RONc) under similar process parameters (pressure, WHSV, H2/HC). As with most reaction systems, high selectivity is desired, as long as the performance can be maintained.

Stability is a measure of how long a desired performance can be maintained, and it usually reflects the coking tendency of the catalyst as it affects both activity and selectivity. Higher stability in a fixed-bed catalyst translates into a longer cycle length while meeting process severity targets—i.e., more profitable onstream time. For reforming units equipped with CCR, higher stability means lower coking tendencies and slower regeneration cycles, thereby adding operational flexibility. Such operating flexibility provides opportunities to process more demanding feed, such as higher endpoint feed or increased amounts of coker naphtha, or an increased catalyst life resulting from a reduced regeneration frequency. Higher catalyst stability can also allow reducing the recycle gas requirement, thus lowering operating costs.

Carrier. The carrier formulation and method of metal impregnation have a significant impact on the activity, selectivity and stability of reforming catalysts. But this is only the beginning of catalyst design and production technique.

PROMOTERS AND ENHANCED PERFORMANCE

In addition to the essential alumina carrier and Pt metal, other elements known as promoters are introduced to influence, moderate or otherwise change the catalyst activity, selectivity and stability. When combined effectively, the catalyst system allows the refinery to optimize gasoline yield and cycle-length or regeneration frequency to improve profitability and operability within unit constraints.

In fixed-bed reformers, promoters have been used for a long time to increase the stability (onstream time) of the catalyst by moderating the coke formation rate. Platinum-Rhenium (Pt-Re) catalysts allow for longer cycles or more severe operation at thermodynamically favored lower pressure where the coking tendency is greater. Additional promoters are often added to fine-tune the selectivity and stability of the catalyst. There are trade-offs in performance and response to feed contaminants, such as sulfur, with these promoted catalyst systems. The challenge in catalyst development is to prepare the right catalyst formulation to achieve the best performance with the least degree of compromise. Traditionally, this results in trading selectivity and introduces a selectivity-stability barrier, as shown in Fig. 5. 

 
  Fig. 5.  Selectivity – stability trade-off or barrier.



Metallic and acid functions

The interaction between the metallic and acid functions is complex, and optimizing the relative importance of each function is fundamental to obtain the desired balance of selectivity, activity and stability. With the addition of other promoters, the permutations of interactions increases, and the relative affinity of molecules to either the metal or acid sites can be tuned for the desired effect, as in the case of Pt-Re. Fig. 6 is a catalyst system with multiple metals and varying chloride content.

 
  Fig. 6.  Schematic of complex multi-promoted
  catalyst system.

Identification of promising promoter combinations requires extensive laboratory work and pilot testing. The exact formulation, impregnation method and manufacture are highly proprietary. Ultimately, the active site density and location are critical to achieving both the desired metal and acid functions. Moderating the acid site strength on the carrier is one important way to limit cracking reactions, but this is only possible if uniform deposition of the promoter(s) is achieved. Equally important is the production trials where proprietary techniques are applied to produce commercial product meeting both the target process chemistry and particle mechanical properties. Detailed particle analysis is performed to ensure that the manufacturing method is effective, as shown in Fig. 7.

 
  Fig. 7.  CCR catalyst particle composition profile.

Uniform distribution of the carrier and metallic components is important to ensure accessibility to these precious constituents and proper function. When the promoter is mainly on the shell of the particle, the metal-to-acid function ratio is not constant along the diameter. Thus, hydrocarbons diffusing into the particle encounter a higher acid-to-metal ratio leading to undesired cracking reactions. This reduces the intrinsic catalyst selectivity and increases coke make. Moreover, when the promoter is preferentially on the surface, it is more sensitive to contamination, and its elution increases over time.

When the promoters are properly introduced, they remain effective for the service life of the catalyst, even under harsh operating conditions found in cyclic and CCR units. Earlier work on promoted systems demonstrates that the promoters are robust and do not elute from the catalyst over many regeneration cycles. Fig. 8 demonstrates excellent promoter retention, within the analytical accuracy of the test, for various CCR catalysts.

 
  Fig. 8.  Commercial demonstration of
  promoter retention on CCR reformer catalysts.


BREAKING THE SELECTIVITY-STABILITY BARRIER

When targeting specific catalyst performance, there are many choices of promoters, method of impregnation and design of the carrier. Two fundamentally different catalyst lines using unique design approaches were recently compared leading to a new family of catalysts that break the selectivity-stability barrier commonly encountered in catalyst design.

At the macroscopic level, the two lines of catalyst produced similar results, but at the micro level, one exhibited better carrier production technique and the other better promoter characteristics. There were clearly opportunities to optimize the systems at the micro level to provide better performance. The first products to be explored were the CCR catalysts as used in severe, high-profit-margin aromatics units.

CCR catalyst formulations are built around a platinum-tin (Pt-Sn) base system. This provides significantly greater selectivity over Pt-only catalyst, but requires low pressure for best results and continuous regeneration to overcome the greater coke formation tendency. Additional metals, other than Pt and Sn, can be added as promoters to further optimize the catalyst systems. The importance of promoter selection can be demonstrated in Fig. 9.

Pilot-plant testing results are shown in Fig. 9A of the C5+ reformate yield selectivity over time for four catalyst systems: Pt+Sn (bimetallic), tri-metallic 1, tri-metallic 2 and optimized Quad-metallic. In this batch pilot testing strategy, the unit is operated at a constant RON target to reflect either a constant conversion toward aromatics for aromatics application or a constant octane in the case of gasoline application. As the test progresses, catalyst selectivity is measured by the reformate yield and stability by the rate of reformate-yield decay over time as the fixed batch of catalyst age.

During the test, coke is progressively deposited on the catalyst and the required temperature to maintain the target RON increases (Fig. 9B). Low coke formation and catalyst deactivation is indicated by a slow increase in reactor temperature to maintain the target RON. A small slope of the temperature curve indicates high catalyst stability, while the duration of the C5+ plateau and the slow rate of yield decay is the complementary indicator of the C5+ stability of the catalyst. From a commercial unit perspective, the latter part of the test, where temperature increases sharply to maintain severity, defines the ultimate catalyst stability (cycle life) within unit constraints.

Looking more closely at Fig. 9, the two tri-metallic systems show initial selectivity performance higher than the base Pt-Sn, but the performance falls over time as a result of the lower stability (higher coke yield), shown in Fig. 9B, for these systems. When a fourth metal is properly introduced, the quad metallic or simply Quad system, a superior yield selectivity and equal stability is attained relative to the Pt-Sn system. In this case, the selectivity-stability barrier is broken, and stability does not suffer to attain superior selectivity. Significantly, this improvement was obtained while reducing the Pt loading on the catalyst by 20%, thereby offering a substantial cost reduction for our customers.

 
  Fig. 9A.  Reformate yield vs. promoter system,
  and 9B. Stability and coke yield vs. promoter
  system.

When the optimized carrier and promoter system were applied to the low-density CCR catalyst platform, a new Quad catalyst was developed. Fig. 10 shows the performance of this new system. The reformate yield is increased by 0.8 wt%; hydrogen increased by 0.1 wt% (50 scf/bbl), while the activity and stability are slightly improved.

 
  Fig. 10.  Optimized quad-metallic catalysts
  comparison: A) reformate yield, B) hydrogen
  yield, and C) activity/stability.


ENHANCED PHYSICAL PROPERTIES

CCR moving bed catalysts also require careful attention to the physical properties to ensure mechanical strength and surface-area stability over many regeneration cycles, which is indicative of catalyst life and chlorine retention.

Accelerated aging tests have been performed on the new Quad-metallic catalyst to compare surface area retention to conventional Pt-Sn bimetallic catalyst. Fig. 11 shows the surface area decline over time following such test protocols. A conventional Pt/Sn catalyst reaches its end-of-life surface area (approximately 140 m2/g) relatively quickly, whereas the new Quad catalyst retains a higher surface area in the range of 160 m2/g. Higher surface area is associated with improved regeneration (Pt redispersion) and better chloride (C1) retention. As a consequence, the new Quad catalysts will exhibit longer life, reduced salt deposit in downstream units, and lower chloride content in the hydrogen-rich gas, resulting in longer chloride trap life.

 
  Fig. 11.  Catalyst surface area aging test,
  quad metallic vs. bimetallic Pt-Sn.

Put in quantitative terms, the better surface area retention and intrinsically higher chloride retention resulting from a new quad catalyst with a proprietary promoter system results in 30% lower chloride injection over the life of the catalyst vs. standard Pt/Sn.

The mechanical properties of the catalyst are also important in CCR applications. Unlike fixed-bed catalysts, which are mainly concerned about crush strength to endure the static load forces within a fixed bed, CCR catalysts are spherical and designed to resist the dynamic forces from slow movement in the catalyst beds to pneumatic lifting between reactor and regenerator. These forces lead to particle attrition and fines production. Although the fines or broken pieces of the catalyst are captured within the system, they can lead to fouling of screens and increase pressure drop.

Highly developed CCR catalysts are more robust to ensure extended service over 7–9 years. New formulations are subjected to large-scale circulating test units to accurately represent commercial conditions and the forces leading to attrition. The new carrier and multi-promoted catalyst systems have proven to be as robust as previous-generation catalysts with an excellent track record of low particle attrition. HP

NOTES

As a licensor of catalytic reforming and aromatics chain technologies, and supplier of catalysts for these units, Axens is focused on continuous improvement in both process technology and catalyst development. The recent acquisition of the Criterion catalytic reforming catalyst business in 2011, including production facilities and know-how, provided a unique opportunity to compare, contrast and build upon two different approaches to reforming catalyst development and production. New proprietary formulations and production techniques have emerged from this union resulting in breakthrough catalysts for both fixed-bed and moving-bed CCR units that provide superior selectivity without compromising activity and stability. Re-engineered fixed-bed catalysts are under development and on-track for release in 2013. These new products promise the benefits of higher selectivity and reduced cost through promoter selection and loading optimization.

The authors 

 
  Pierre-Yves Le Goff is Axens’ senior technical manager for reforming catalysts and project leader for catalyst development. Dr. Le Goff started his career as a research engineer at Rhodia, where he specialized in catalyst support design and process development. Dr. Le Goff holds an engineering degree from the École de Chimie de Mulhouse, an MBA from Université de la Sorbonne in Paris, and a PhD from Université de Haute-Alsace. 

 
  Jay Ross is a senior technology and marketing manager for Axens covering the field of transportation fuels including FCC, catalytic reforming, isomerization and biodiesel production. He has over 30 years of experience in the refining and petrochemical industry including process engineering design, R&D, licensing and technical assistance. Mr. Ross is a graduate from Princeton University with a degree in chemical engineering.  

 
  Joseph Lopez is a development and industrialization engineer in Axens’ production plant of adsorbents and catalysts in Salindres, France. He is responsible for the development and scale-up of reforming supports and catalysts. He started his professional career as a research engineer at Rhodia working mainly in the field of heterogeneous and homogeneous catalysis. Dr. Lopez holds an engineering degree from the Ecole Nationale Supérieure de Chimie de Montpellier and a PhD in catalysis from the Université Claude Bernard of Lyon. 




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Labeed Shamil
04.11.2013

Dear,

Can i get good analysis method for determinate Pt in Reforming catalyst by ICP-AES ?

Thank you

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