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 dis-tributed 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 So-viet 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.
Petrochemicals. Worldwide demand for polymers is growing at a significantly higher pace than oil and gas production (FIG. 2) and thus initiating large expan-sions in olefins and aromatics complex-es. Global paraxylene (PX) consump-tion 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, fol-lowed by the Middle East. New aromatic complexes, which include continuous catalyst regeneration (CCR) reforming units, will be required to meet the grow-ing 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, isomeri-zation and alkylation capacity by 2020.
Catalytic reforming of naphtha is central in the production of both high-octane fuel and aromatics to support both rap-idly 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 transform-ing low-octane naphtha from crude oil and hydroprocesstransform-ing 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 need-ed by other refinery units such as hydrotreaters and hydro-crackers. This is accomplished over a heterogeneous catalyst at elevated temperature and preferably low pressure according to Le Chatelier’s principle.
North America
Global demand MMbdoe 2010 88.2
FIG. 1. Worldwide incremental refinery product demand, 2010–2020.
48SEPTEMBER 2012 | HydrocarbonProcessing.com
Refining Developments
Structure. Reforming catalysts are complex composites of a highly active precious metal, platinum (Pt), to efficiently per-form 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-low-octane gasoline, aro-matics and byproduct hydrogen.
Functions. A simplistic representation of the main reactions is shown in FIG. 4 and is linked to the metallic and acid func-tions. 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 paraf-fins 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 oc-cur, 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 con-version, catalyst deactivation by coke deposition, and light-ends handling limitations.
Catalyst performance. The carrier and highly dispersed Pt metal interact in a complex way to accomplish the desired re-forming reactions. Performance of the catalyst is described in terms of activity, selectivity and stability.
Activity is commonly defined in terms of temperature re-quired to achieve a given objective; it is very similar to the def-inition 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 tem-perature. For fixed-bed units, this means longer cycle lengths, and, for CCR units, it means greater operating flexibility with-in unit constrawith-ints.
Selectivity. The selectivity of the catalyst refers to the rela-tive 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 (pres-sure, WHSV, H2/HC). As with most reaction systems, high se-lectivity 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 regenera-tion frequency. Higher catalyst stability can also allow reducing the recycle gas requirement, thus lowering operating costs.
Carrier. The carrier formulation and method of metal im-pregnation have a significant impact on the activity, selectivity and stability of reforming catalysts. But this is only the begin-ning 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 influ-ence, moderate or otherwise change the catalyst activity, se-lectivity 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.
Growth Index
1990 2000 2010 2020 2030
200
FIG. 2. Worldwide growth index in oil, gas and polymer sectors.
Cl M
FIG. 3. Schematic of acid and metal sites on reforming catalyst.
Paraffin and naphthene isomerization Dehydrocyclization
Hydrocracking/dealkylation
Metallic Acid (Cl-Al Carrier)
Hydrogenolysis/ring opening
FIG. 4. Bi-functional reforming catalyst reactions. The desired reactions are labeled in blue, with undesirable side-reactions labeled in red.
Hydrocarbon Processing | SEPTEMBER 201249
Refining Developments
In fixed-bed reformers, promoters have been used for a long time to increase the stability (onstream time) of the cata-lyst by moderating the coke formation rate. Platinum-Rheni-um (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 of-ten added to fine-tune the selectivity and stability of the cata-lyst. 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 bar-rier, as shown in FIG. 5.
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 interac-tions 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 met-als and varying chloride content.
Identification of promising promoter combinations re-quires extensive laboratory work and pilot testing. The ex-act formulation, impregnation method and manufex-acture are highly proprietary. Ultimately, the active site density and loca-tion 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 meet-ing 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.
Uniform distribution of the carrier and metallic components is important to ensure accessibility to these precious constitu-ents and proper function. When the promoter is mainly on the shell of the particle, the metal-to-acid function ratio is not con-stant 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 pro-moter is preferentially on the surface, it is more sensitive to con-tamination, 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 op-erating conditions found in cyclic and CCR units. Earlier work on promoted systems demonstrates that the promoters are ro-bust and do not elute from the catalyst over many regeneration cycles. FIG. 8 demonstrates excellent promoter retention, with-in the analytical accuracy of the test, for various CCR catalysts.
BREAKING THE SELECTIVITY-STABILITY BARRIER
When targeting specific catalyst performance, there are many choices of promoters, method of impregnation and de-sign of the carrier. Two fundamentally different catalyst lines using unique design approaches were recently compared lead-ing to a new family of catalysts that break the selectivity-stabili-ty barrier commonly encountered in catalyst design.
Carrier
FIG. 6. Schematic of complex multi-promoted catalyst system.
Selectivity
FIG. 5. Selectivity – stability trade-off or barrier.
-0.5 -0.4 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5
FIG. 7. CCR catalyst particle composition profile.
50
FIG. 8. Commercial demonstration of promoter retention on CCR reformer catalysts.
50SEPTEMBER 2012 | HydrocarbonProcessing.com
Refining Developments
At the macroscopic level, the two lines of catalyst produced similar results, but at the micro level, one exhibited better car-rier production technique and the other better promoter char-acteristics. 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
selec-tivity 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 con-stant octane in the case of gasoline application. As the test pro-gresses, 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 cata-lyst and the required temperature to maintain the target RON increases (FIG. 9B). Low coke formation and catalyst deactiva-tion 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 comple-mentary 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 sys-tems. 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 stabil-ity does not suffer to attain superior selectivstabil-ity. Significantly, this improvement was obtained while reducing the Pt loading on the catalyst by 20%, thereby offering a substantial cost re-duction for our customers.
When the optimized carrier and promoter system were ap-plied 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%; hy-drogen increased by 0.1 wt% (50 scf/bbl), while the activity and stability are slightly improved.
50 75 100 125 150
FIG. 10. Optimized quad-metallic catalysts comparison: A) reformate yield, B) hydrogen yield, and C) activity/stability.
87
FIG. 9A. Reformate yield vs. promoter system, and 9B. Stability and coke yield vs. promoter system.
Coke 7 wt %
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