Part 1: Maximizing Catalytic Isobutylene Selectivity
By J.B. McLean, G.S. Koermer, R.J. Madon
As refineries enter the reformulated gasoline era, fluid catalytic cracking units (FCCUs) will be used as major suppliers of light olefin feedstocks for methyl tertiary butyl ether (MTBE) and tertiary amyl methyl ether (TAME). Operators will need new catalysts to maximize isobutylene and isoamylene production. BASF has developed a new family of catalysts, called IsoPlus(TM) , that are the first commercially available catalysts that maximize isobutylene and isoamylene yields for reformulated gasoline production. IsoPlus Catalysts increase isobutylene yields 50 to 200 percent compared with currently available FCCU catalysts. Compared with operational options such as raising riser temperature and use of ZSM-5 additives, IsoPlus catalyst offer selectivity advantages.
These catalysts may require some plant changes, e.g., expansion of the FCCU gas plant, if they are to fulfill their potential. Even, so, they appear to offer an economical isobutylene route compared with butane isomerization and dehydrogenation.
This article is the first of a two part "Catalyst Report', discussing the fundamental catalytic and thermodynamic principles effecting isobutylene production. IsoPlus MAT and circulating pilot plant yields are also presented. The succeeding article (Part II) will discuss IsoPlus yields as they impact on overall refinery gasoline production.
The Clean Air Act Amendments of 1990 requires major changes in gasoline, especially at it concerns oxygenates (Ref. 1). Refiners must meet the November 1992 deadline for oxygenated gasoline in carbon monoxide nonattainment areas and the 1995 deadline for reformulated gasoline in ozone nonattainment areas. MTBE (methyl tertiary butyl ether), the most commonly available oxygenate, is produced by reacting isobutylene and methanol. The FCCU is the primary source of isobutylene for many refiners. An alternative oxygenate, TAME (tertiary amyl ether) is produced from 2 specific isomers in the amylenes or C5 olefin fraction 2-methyl-2-butene and 2-methyl-1 butene. In most refineries this fraction currently goes directly to the gasoline pool as part of the cat gasoline.
Current refinery production of isobutylene and isoamylenes cannot meet the entire projected demand for oxygenates. Several processes and catalytic options exist for increasing light olefin yields from the FCCU (Ref. 2), including the use of low rare earth USY catalysts, higher reactor temperatures and use of ZSM-5 containing additives. Although, each of these is effective in increasing isobutylene yieid, each of these changes has an upper limit on isobutylene yield.
Catalyst Effects on Isobutylene Yields
BASF has assessed a broad range of commercial FCCU catalysts for isobutylene yields using microactivity testing (MAT). These catalysts included high rare earth catalysts for maximum gasoline yield, moderate rare earth catalysts for maximum octane barrels, and low rare earth/USY and zero rare earth USY catalysts for maximum octane.
All catalysts tested has isobutylene yields that held steady or increased slightly as conversion increased (Figure 1), whereas isobutane increased dramatically with conversion (Figure 2).
Isobutylene yields differed by about a factor of two (Table 1 ) among the catalyst tested. Highly rare earth exchanged catalysts gave the least isobutylene and very low rare earth content USY catalysts gave the most. These results imply that zeolite unit cell size (UCS) has an inverse relationship to isobutylene production and therefore lowering unit cell size increases isobutylene production. Since lower rare earth zeolites will equilibrate at a lower UCS, the lower rare earth catalyst show higher isobutylene selectivity. This effect is not unexpected since lower UCS is known to decrease hydrogen transfer reactions and produce more olefins (Ref. 3).
Other than catalyst type, refiners are potentially limited in isobutylene production by composition at thermodynamic equilibrium. The composition at equilibrium is expressed by the isobutylene concentration in the total butenes. This value is approximately 0.45 for reactor temperatures in the 950°F to 1000°F range typical of current FCC unit operation, and decreases as reactor temperature rises (Figure 3). Most laboratory and commercial operating data show 0.2 to 0.35 for this ratio of isobutylene to total butenes, illustrating substantial room for improvement of isobutylene yield. Both catalyst and operating conditions can potentially impact this ratio.
Understanding the reaction network among the C4's and the mechanism for production of isobutylene is key to developing a catalyst to produce more iso-olefin up to this thermodynamic limit. This reaction network is pictured in Figure 4. This reaction network is consistent with the well-known fact that linear butenes do not directly isomerize to isobutylene(Ref. 4). Instead the reaction path proceeds by carbenium ion surface intermediates which are at thermodynamic equilibrium as implied by the data in Table 2. These data show the ratio of total iso-C4's to total C4's to be approximately 0.6 Note also that decreasing the hydride transfer reaction such as that illustrated in Figure 5 helps to preserve the isobutylene as well as total butenes.
Given these preceding comments, there are two basic ways to increase isobutylene yield:
BASF's IsoPlus Catalyst uses both of these principles to provide an overall increase in isobutylene production.
Having established that reduction of hydride transfer is important to the production of isobutylene, it also has other benefits.
1. Fewer naphthenes are converted to aromatics, a clear benefit to the refiner interested in lowering gasoline pool aromatics content.
2. Carbenium ion catalytic cracking is less inhibited allowing production of more LPG (at the expense of gasoline).
3. Gasoline olefinicity is increased leading to increased yields of isoamylenes for TAME manufacture.
4. Gasoline octane is increased.
The IsoPlus series of catalysts is available in either the 1000 series or 2000 series, depending on the level of isobutylene production required. The IsoPlus 1000 series raises isobutylene yields 20 to 50 percent compared to current USY catalysts, while fitting into today's unit operating constraints. This catalyst has immediate application for the refiner having spare MTBE production capacity.
The IsoPlus 2000 series raises isobutylene yield by 100% or more and has greater potential application in preparation for meeting the oxygenate requirements for gasoline in 1995.
Figure 6 shows cracking results for an isoparaffin model reaction used to investigate hydride ion transfer reactions. At 65 percent conversion, IsoPlus 1000 catalysts have significantly reduced hydride ion transfer activity, and therefore increased selectivity to isobutylene. Similarly, IsoPlus 2000 catalyst had significantly greater isobutylene yield at a MAT conversion of 65 percent compared with a severely steamed Y zeolite. (Figure 7).
Table 3 illustrates laboratory data at 70% conversion for a number of catalysts that are used commercially. The yields on these various catalysts will be used to provide commercial projections in Part II of this series. The RE-USY base case is typical of what many refiners use today. Although ZSM-5 additive increases isobutylene yield, IsoPlus catalysts are considerably more selective. The IsoPlus 1000 series shows incremental changes in other selectivities, such as gasoline and coke, while IsoPlus 2000 catalysts show more substantial yield shifts.
These catalysts were also run in BASF's circulating pilot unit to verify the MAT results yield increases of isobutylene. Directionally the results show the same trend though reduced in magnitude, as illustrated in Figure 8. This difference is primarily due to the higher yield in the pilot unit for the base case, as well as differences in reaction temperature, hydride transfer and time averaging of MAT yields. Figure 9 shows that the pilot unit confirmed the effect noted in the MAT that the IsoPlus catalysts gave improved incremental selectivity to isobutylene compared to ZSM-5 addition. Gasoline products from the pilot unit runs were analyzed by BASF's PIONA system (Ref. 5) to compare the impacts of these catalysts on isoamylene yields and gasoline aromatics. Table 4 shows that isoamylene yields were increased in a similar fashion to isobutylene, while gasoline aromatics were reduced relative to the base catalyst.
The information developed from MAT and circulating pilot plant experiments, shows that increase in isobutylene yield of up to 100% can be achieved with BASF's IsoPlus series catalyst. The next catalyst report will examine the implications of these yields for the overall refinery gasoline production.
1. "U.S. Refiners Scramble to Meet Reformulated Gasoline Mandate", Oil & Gas Journal, Jan. 27, 1992.
2. "Refiners Have Options to Deal with Reformulated Gasoline", G.L. Yepsen and A. Witoshkin, Oil & Gas Journal, April 8, 1991.
3. L. Pine, P. Maher and A. Wachter, J. Catal., 85, 466, 1984.
4. G. Olah, G. Prakash, R. Williams, L. Field and K. Wade, Hypercarbon Chemistry, J. Wiley and Sons, New York, 1987.
5. "The Impact of Microactivity Test Conditions on Product Yields and Properties", M.J. Margolis and J.B. McLean, AlChE National Meeting, November 1991.