Increase Gasoline Octane and
Light Olefin Yields with ZSM-5
Volume 5 Issue 5
Mobil's patented ZSM-5 has been used successfully in over 50 Fluid Catalytic Cracking (FCC) units worldwide. It provides refiners with a rapid method to improve FCC gasoline octane numbers and light olefin yields without affecting dry gas, coke, or unit conversion. This catalyst report presents an overview of the role of ZSM-5 and application strategies to maximize FCC profits.
How ZSM-5 Works
ZSM-5 is a 10-membered sieve containing channel openings of 5.1 to 5.6 Angstroms. These openings are smaller than the 7-8 Angstrom cage openings for a comrnercial fluid catalytic cracking Y zeolite. Hence, access of molecules larger than monomethyl aliphatics into ZSM-5 is restricted, and reaction of molecules with critical diameters greater than 6 Angstroms is severely diffusion limited. Besides controlling accessibility of hydrocarbons, ZSM-5 contains strong Bronsted acid sites necessary for cracking hydrocarbons, and does not catalyze coke formation as readily as a Y-zeolite(Ref. 1).
The properties of ZSM-5 stated above allow this material to be used as an FCC additive for boosting gasoline octane number(Ref.2). ZSM-5 also increases C3 and C4 LPG selectivity with a concurrent decrease in gasoline selectivity. It should be noted that the increased LPG fraction consists predominantly of olefins. There is essentially no change in dry gas (C2-) or coke selectivity. There is, however, a significant change in gasoline composition. The effect of ZSM-5 on gasoline structure(Ref. 3) may be summarized as follows:
Since the rate of olefin cracking is more than two orders of magnitude greater than paraffin cracking,(Ref. 4) the decrease of C7+ olefins is due to their being cracked to give mainly C3-C5 olefins. In the presence of such reactive olefins, paraffin cracking is substantially reduced. There are several pathways for gasoline paraffin formation during catalytic cracking. One secondary pathway involves hydrogenation of primary olefins. By cracking these primary olefins ZSM-5 reduces the reactant olefins needed to produce paraffins(Ref. 3). Thus, low octane paraffins in gasoline are decreased as a direct result of olefin cracking.
The decrease in gasoline molecular weight and increase in C5 hydrocarbons are important reasons for the increase in gasoline octane number. There is essentially no incremental formation of aromatics and naphthenes due to the presence of ZSM-5. However, since the total volume of gasoline is reduced due to the decrease in C7+ aliphatics, the concentration of aromatics and naphthenes in the gasoline is increased. This again helps in enhancing gasoline octane numbers.
The above summary describes generally how ZSM-5 works. However, the balance between the increase in octane number, C3 and C4 olefin yields, and the decrease in gasoline yield will depend on factors such as feed characteristics, operating conditions, and the type of Y zeolite catalyst used.
ZSM-5 Performance Indicators
FCC feed quality has a significant influence on FCC gasoline properties. Directional indicators of ZSM-5's potential on a given feed include the following feedstock characteristics: aniline point, refractive index, gravity, and nitrogen content. These analyses describe a feed's relative aromaticity or paraffinicity.
FCC Feed Aniline Point
Aniline point indicates feedstock aromaticity. Aniline point increases as the aromatic content of the feed decreases. Highly paraffinic feeds will yield large amounts of olefins in the gasoline and show a greater yield shift when ZSM-5 is added. Table 1 compares feedstock and yield shifts for two commercial operations using two different mid-continent feeds. This comparison shows that aniline points of 180°F/82°C and 200°F/93°C, respectively, significantly affect yield and octane shifts for similar base octane at the same ZSM-5 addition rate. The 20°F/11°C higher aniline point for refinery B indicates higher feed paraffinicity resulting in higher gasoline aliphatic content, and, therefore, a higher content of intermediate olefins that can be readily cracked by ZSM-5. This results in higher incremental LPG production and gasoline yield loss.
FCC Feed Nitrogen Content
ZSM-5, like traditional Y zeolite, may be poisoned by high levels of nitrogen. Small molecules like NH3 and pyridine can easily enter ZSM-5 and poison strong acid sites, whereas polycyclic nitrogen compounds are unable to enter the sieve and affect activity. Since nitrogen in FCC feed is mostly associated with large bulky hydrocarbons, ZSM-5 is not appreciably affected by feed nitrogen contamination. Commercial and laboratory data show ZSM-5 to be effective even with feeds containing 1,000 wppm basic and 3,000 wppm total nitrogen.
ZSM-5 Usage Strategies
ZSM-5's shape selectivity provides certain benefits to the refiner. Unlike operation with full octane catalysts, addition of ZSM-5 increases gasoline octane number without changes in conversion, or increases in coke and dry gas yields. Furthermore, ZSM-5 affords the refiner flexibility. Trim addition of ZSM-5 can allow the refiner to easily adjust FCC gasoline octane number and LPG olefin yield. Such flexibility can quickly increase profits. This strategy has been used by some refiners to reduce octane variability with changing feed quality.
The use of ZSM-5 may be influenced by operating constraints. Some constraints and applications involving ZSM-5 are shown in the following examples. The cases were computer simulated based on laboratory and commercial data.
Case 1 Limitations: Reactor
Temperature Feed Preheat
Table 2 shows the effect of ZSM-5 at constant reactor temperature and feed preheat. RON and MON are higher by 1.0 and 0.4 numbers respectively, with LPG yield up by 1.4 vol% and gasoline yield reduced by 1 vol%. Alkylate gasoline yield plus octane values are also given, assuming all C3 and C4 olefins are alkylated. In this example, total gasoline yield is actually increased 1.1 vol% with a 0.7 and 0.4 gain in RON and MON respectively.
Case 2 Limitation: Wet Gas
Table 3, shows the effect of reducing reactor temperature by 10°F/5°C to reduce dry gas (ethane and lighter products), while catalyst addition is increased to maintain conversion. The reduction in dry gas yield eases the compressor limit while the lower reactor temperature reduces gasoline octane number. ZSM-5 increases gasoline octane numbers until the increased LPG yield meets the wet gas compression limit. Total FCC and alkylate octane-barrels are also increased for this operation.
Case 3 Limitation: Gasoline
Table 4 shows the effect of ZSM-5 on yields, gasoline octane number and bromine number (BN). Note that gasoline volume is reduced, with the largest losses being from the paraffins and olefins. Aromatic and naphthene concentrations increase. RON increases by 0.6 numbers, while BN decreases by 1.0 number. The use of ZSM-5 without an increase in the BN of gasoline is particularly important in BN sensitive areas such as California and Japan.
Refiners have found ZSM-5 to be a very flexible catalyst additive for increasing gasoline octane numbers and light olefins. The latter may be upgraded into chemicals or high octane blending components such as methyl tertiary butyl ether, polygasoline, dimate, and alkylate.
1. Chen. N.Y., Garwood, W.E., and Dwyer, F.G., "Shape Selective Catalysis in Industrial Applications" Marcell Dekker, Inc., 1989.
2. Schipper, P.H., Dwyer, F.G., Sparrell, P.T., Mizrahi, S., and Herbst, J.A., "Zeolite ZSM-5 in Fluid Catalytic Cracking: Performance, Benefits, and Applications", ACS Symposium Series 375, Ed., M.L. Occelli, page 64, 1988.
3. Madon, R.J., "Increasing Gasoline Octane Number During Fluid Catalytic Cracking - Role of ZSM-5 and USY", Advances in Catalytic Chemistry IV, Snowbird, Utah, 1989.
4. Haag, W.O., Lago, R., and Weisz, P.B., Disc. Faraday Soc. 72, 317 (1981).