Adapting FCCU Operation to Meet Reformulated Gasoline Standards
The clean Air Act of 1990 imposes a huge financial and operational burden on refiners. This burden could be lightened if existing units made more of the components required under the Act's reformulated gasoline standards. Improved catalysis offers one way to do this.
The new law seeks to reduce auto emissions by establishing formulas for gasoline components (benzene, aromatics, and oxygenates) and properties such as Reid vapor pressure. It requires adding some constituents, especially oxygenated compounds, and eliminating others. It also may alter gasoline content indirectly by limiting hydrocarbons, carbon monoxide, and NOX in auto exhausts. The standards for oxygenates and aromatics will affect the operation of fluid catalytic cracking units since nearly half the refinery gasoline pool derives from the FCCU: 38 percent directly from FCC gasoline and 12 percent indirectly from alkylate.
Oxygen content must be boosted to a minimum of 2.7 wt% and 2.0 wt% in carbon monoxide and ozone nonattainment areas, respectively. If all the increase came from the use of methyl tertiary butyl ether (MTBE), this would translate into 15 vol% and 11 vol% MTBE content, respectively, up from less than 1.0 percent at this time. Future MTBE demand would then require an additional 800,000 BPD of isobutylene feedstock above the 100,000 BPD now produced in refinery FCCUs.
Aromatics comprise about 32 percent of the finished unleaded pool and must be reduced to 25 percent or less. This will impact fluid catalytic cracking unit (FCCU) operations because FCC gasoline averages 29 percent aromatics.
Refiners have several options for lowering aromatics in FCC gasoline: process more naphthenic or paraffinic gas oils; lower unit conversion; or undercut full distillation range gasoline. Undercutting is probably the most desirable choice because the FCCU can then be run to maximize desirable light olefins (C3=, C4=, and C5=) while controlling aromatics.
The light olefins can be converted to excellent blending components for reformulated gasoline, e.g., polygasoline, dimate, alkylate, MTBE, tertiary butyl alcohol (TBA), ethyl tertiary butyl ether (ETBE), tertiary amyl methyl ether (TAME), and tertiary amyl ethyl ether (TAEE). Isobutylene and isoamylenes are the olefins in greatest demand. They are made into MTBE and TAME, the preferred oxygenate blending components for reformulated gasoline.
Current Catalytic Options
BASF researchers have found that FCCU light olefin output can be optimized by the proper mix of catalysts, additives and operating conditions.
Yields of C3= may be increased 28 percent by changing from a 4% REO, inactive matrix catalyst to a non rare earth exchanged, ultrastable zeolite catalyst with an active matrix blended with 3% Z-100(BASF's additive containing Mobil's ZSM-5 zeolite). Propylene yield rises further when reactor temperature increases from 960° F to 1030° F (Figure 1).
Propylene is a raw material for three gasoline blending components: dimate, polygasoline and alkylate. Although the first two are high in C6 to C9 olefins, these olefins do not present a problem because they have relatively low vapor pressures and good octanes.
Yields of butylenes, as with propylene, increase as reactor temperature rises, as catalyst hydrogen transfer characteristics decrease, and as the use of Z-100 increases (Figure 2). These changes combined can increase butylene output 30 to 40 percent.
Isobutylene is the most versatile of the butylenes and produces excellent blendstocks for reformulated gasoline when converted to ethers. Its production can be increased by the same methods discussed above to increase the yield of total butylenes. Under these conditions, isobutylene selectivity (iC4=/C4=) increases as well as shown in (Table 1).
Butylenes can be converted to five gasoline blending components: polygasoline via catalytic polymerization; alkylate with the addition of isobutane; MTBE using methanol as a raw material; TBA in the presence of water; and ETBE in reaction with ethanol.
The changes in catalyst types and operating conditions discussed above also increase isoamylenes production. By lowering the rare earth of the zeolite and increasing the matrix activity, the isoamylene selectivity (iC5=/C5=) can be increased from 0.44 to 0.55 (Table 1 ). Isoamylenes can be converted to excellent blendstock ethers, such as TAME and TAEE, that have low RVP and high octane values (more than 103 R+M/2). Alkylate produced from C5=s also possess low RVP (2.0 psi) and good blending octane values (91.3 R+M/2). Aromatics content in any of the above blending components is less than 0.5 vol%.
Current technology thus allows for an increase in light olefins content in the LPG stream and the front end of the gasoline stream. These olefins can then act as feedstocks for oxygenates production.
This discussion placed no mechanical or other limitation on the FCCU, even though most units in the real world operate at some limit. In taking advantage of the catalysts, additives, and operating conditions that maximize light olefins productions, investment will often be required to upgrade the FCCU and associated fractionation and compression sections.
Future Catalytic Options
The work discussed above highlighted yields from current FCC catalyst technologies. A major focus at BASF over the last year has been new catalyst development and design for maximum FCCU iso-olefin yields.
Preliminary results are encouraging. Catalytic chemistry has been identified which can increase isobutylene yield significantly (Figure 3). While the catalysts that provide these yields are experimental, the eventual utilization of these materials in the FCCU has the potential to significantly reduce the capital investment needed to manufacture iso-olefins from normal paraffins.
Major challenges in meeting reformulated gasoline standards include producing increased yields of oxygenates and converting or reducing the formation of heavier aromatics in FCC gasoline.
In the near-term, the yield of light olefins (C3= through C5=s) can be increased moderately by changing to low or zero rare earth USY catalysts with active matrices, using octane additives, and by increasing reactor temperature. These olefins, when further processed by alkylation, etherification and other means, yield excellent blending components for reformulated gasoline.
In the long-term, refineries will probably obtain the required quantity of reformulated gasoline blending components from both new catalysts and capital investment in new units. Given the timetable in the law, these catalysts are targeted to be in refiners' hands in 1992 or 1993. Future issues of the Catalyst Report will look at the development of these catalysts.
By: G. L. Yepsen, Marketing
Manager Cracking Catalyst
A.(Tony) Witoshkin, Director, Cracking Technology
*[Portions of this paper were adapted from: Yepsen, G. and Witoshkin, T., Refiners Have Options to Deal with Reformulated Gasoline, Oil & Gas Journal, 4/8/91]
SCR Catalysts Help Refiners Control NOX Emissions
In addition to meeting the reformulated gasoline and diesel fuel sulfur content standards mandated in the 1990 Clean Air Act (CAA), many U.S. refineries will be challenged to comply with the Title I non-attainment provisions of this legislation. Under Title I, existing and new refinery process units will come under scrutiny for compliance with emission standards related to NOX (oxides of nitrogen), VOCs (Volatile organic compounds) and CO (Carbon monoxide).
The CAA identifies 96 geographic areas as being in non-attainment with Federal standards for ambient ozone concentrations, a primary cause of smog. Petroleum refineries tend to be located in close proximity to many of these ozone non-attainment areas. Because NOX and VOCs react photochemically to produce smog, refinery equipment producing these emissions will require increased controls.
The CAA also identifies 41 areas that are out of compliance with regard to CO levels.
BASF manufactures catalysts that can effectively control NOX , CO, and VOCs to meet federal, state, and local emissions regulations.
The following article discusses the application of selective catalytic reduction (SCR) technology to the control of refinery NOX emissions.
NOX is a mixture of nitrogen oxides, predominantly colorless NO and reddish-brown NO2, which is formed during the high temperature combustion/oxidation in the presence of nitrogen-bearing compounds. NOX is emitted from FCCU's, fuel-fired engines and turbines, and refinery heaters or boilers, as well as other process units.
The process of NOX control via Selective Catalytic Reduction (SCR) involves injecting ammonia (NH3) into the exhaust gas stream, and then passing that mixture over a catalyst to react the NOX and NH3 to form nitrogen and water. Typical systems are designed for 70% to 90% NOX removal, but BASF has also designed and built systems to provide as much as 96% NOX removal.
SCR has proven to be the most effective method for controlling NOX in lean-burn processes (containing excess O2), and consequently is gaining increasing acceptance as Best Available Control Technology (BACT) by federal, local, and state environmental regulators.
The degree of NOX removal depends on several factors, including temperature, the molar ratio of NH3/ NOX in the exhaust, uniformity of NH3-NOX mixing, allowable NH3 slip (i.e., NH3 leakage from the system in the exhaust), oxygen content of the exhaust, size of the catalyst bed, and catalyst activity. This article will focus on the first four of these factors and review how they affect NOX removal performance.
Figure 4 shows that for VNX(TM) vanadia/titania (V/Ti) catalysts, increasing temperature improves NOx conversion efficiency until about 710° F where the undesirable side reaction of NH3 oxidation becomes significant. As temperature is increased further, NOx conversion falls as NH3 is oxidized instead of reacting to destroy NOX. The operating temperature range for VNX(TM) catalysts is typically 525° to 850° F.
BASF ZNX(TM) zeolite SCR catalysts (see Figure 5) are unique in that they do not oxidize ammonia even when operating at temperatures up to 1150°F, so their performance continues to improve with increasing temperature. Extensive aging in both the laboratory and at 6 field aging sites has also shown that the ZNX catalyst formulation is thermally stable for thousands of hours at temperatures in excess of 1100° F.
As a result of its unique high temperaure selectivity and stability, ZNX catalyst is particularly well suited for operation at temperatures well above that suitable for V/TI based catalysts. Currently ZNX catalyst is being used to treat the exhaust of a refinery heater operating at temperatures up to 850° F. and the exhaust from a chemical plant that can reach temperatures as high as 950° F.
NH3/ NOx ratio and mixing are both important factors because the SCR reaction requires that both NOx and NH3 be present at the same place and in the proper ratio for the desired level of NOx control to be achieved. Insufficient NH3 will result in less than the desired degree of NOx removal. Excessive NH3 will result in higher NH3 "slip" escaping in the exhaust, and operating permits typically limit "slip" to 10 ppm. Incomplete mixing results in the worst condition of all; some regions will be starved of NH3 (insufficient NOx removal), and others will have excess NH3 (excessive NH3 slip).
Proper system design, using engineering principals developed and proven over years of experience, is critical to good NH3 distribution and control.
VNX(TM) and ZNX(TM) catalysts are presently operating in a number of refinery applications including cogeneration gas turbines as well as process unit heaters.
An example is shown in Figure 6.
CAA will put additional demands on refinery emission compliance plans. BASF technology for control of NOx is commercially proven and available for refinery applications.
L. W. Morris,
Stationary Source Catalyst
B. K. Speronello,