SOX Emission Reduction
The environmental impact of sulfur oxide (SOX = SO2 + SO3) emissions has gained much attention over the past ten years. While presently there are no federally mandated standards, the United States Environmental Protection Agency recently has proposed a limit on SOx emissions from new, modified, and reconstructed fluid catalytic cracking unit (FCCU) regenerators equal to 9.8 kg SOx/ 1000 kg of coke burn-off (about 300 vppm). Several local and state agencies have already set limits on SOx emissions from FCCU regenerators. For example, the Southern California Air Quality Management District (SCAQMD) Board, whose jurisdiction covers the refinery rich Los Angeles Basin, has a current limit on existing FCC units of 60 kg SOx /1000 barrels of feed (about 300 vppm). In addition, the current limit for new FCC units is even more stringent than the 1997 limit for existing units. Clearly, the subject of SOx emissions is an important one for refiners today.
This issue of the Catalyst Report discusses the FCCU sulfur balance, the mechanism of SOx emission reduction, FCCU variables affecting SOx emission reduction, and the inherent ability of BASF's Fluid Cracking Catalysts to reduce SOx emissions.
FCCU Sulfur Balance
The amount of SOx emitted from an FCC unit regenerator is a function of the quantity of sulfur in the feed, coke yield and conversion. As illustrated in Figure 1, 45% to 55% of feed sulfur is converted to H2S in the FCC reactor, 35% to 45% remains in the liquid products, and about 5% is deposited on the catalyst in the coke. It is this sulfur in the coke which is oxidized to SOx in the FCCU regenerator (generally in a mixture of about 90% SO2 and 10% SO3).
Mechanism of SOX Reduction
The discussion in this section will refer to Figure 2 entitled "Mechanism of Catalytic SOx Emissions Reduction in FCC Units". in the figure, MxO refers to any metal oxide such as Al2O3, which is an integral part of BASF's catalyst matrix or other types of metal oxides that react with SOx.
In the regenerator, sulfur bearing coke is burned to SO2, CO, CO2 and water. A portion of the SO2, in the presence of excess oxygen will combust further to SO3. The SO3 can now either exist in the flue gas or react with catalyst or an additive metal oxide to form metal sulfate. The reaction to metal sulfate can be considered the "capture reaction".
Moving to the reactor, the metal sulfate will react with hydrogen to form either metal sulfide and water or metal oxide. H2S and water. In the stripper, metal sulfide reacts with steam to form metal oxide, and H2S. Finally, the sulfur leaves the system as H2S in the product stream, rather than as SOx in the flue gas.
Variables Affecting SOX Reduction
Figure 3, illustrates the key variables, which impact SOx reduction. These variables and their effects on SOx emissions are discussed below.
Both the absolute level of feed sulfur and the types of sulfur compounds in the feed will affect SOx emissions. Refering to the FCCU sulfur balance section of this report, as the weight percent of feed sulfur increases, so will the potential SOx emissions.
Another significant feed stock parameter affecting SOx capture is the type of sulfur present in the feed. Sulfur associated with paraffins will tend to crack to H2S and thus leave the system with the reactor products. However, sulfur associated with aromatic rings will tend to lay down as Coke. Burning sulfur bearing coke in the regenerator will tend to increase emissions. Therefore, to determine the effect of increased feed sulfur, the types of sulfur compounds present need to be known.
Consider this reaction which occurs in the regenerator:
SO2 + 1/2 O2 <---> SO3
Any oxygen present above the stoichiometric amount needed to form SO3 will drive the above reaction to the right thus forming SO3 which can be captured by the alumina matrix or other metal oxide sites. In this way, increasing excess oxygen will reduce SOx emissions.
FCC Catalyst Properties
The second reaction occurring in the regenerator is as follows:
Al2O3 + 3SO3 -> Al2 (SO4)3
This reaction is the so called "capture reaction". The higher the number of alumina or other metal oxide sites available for SOx capture, the greater the sulfur emission reduction.
Catalyst CO Promoter
CO oxidation promoters also promote the oxidation of SO2 to SO3. Increasing CO promoter content will decrease SOx emissions.
The regenerator temperature will affect both reactions occurring in the regenerator section. It will act on the kinetics of the first reaction in which SO2 reacts with O2 to form SO3 according to Le Chatelier's principle. Any increase in temperature in a gas system in equilibrium will tend to drive the equilibrium to increase total moles, i.e. the reaction is driven to the left. SO2 and oxygen are formed at the expense of SO3. Because of this, SO3 is not available to combine with the metal oxide sites.
An increase in temperature also impacts the formation of metal sulfate. Higher temperatures favor the degradation of metal sulfate, or the reverse of the "capture reaction". Thus, increasing regenerator temperature will increase SOx emissions.
Catalysts Circulation Rate
Catalyst circulation rate will have two effects. An increase in catalyst circulation rate will increase the number of metal oxide sites available for SOx capture. This effect will tend to reduce emissions. However, increased catalyst circulation will increase conversion thus increasing the amount of sulfur-bearing coke entering the regenerator. This may increase sulfur emissions if sufficient metal oxide sites are not concurrently available for SOx capture. Normally however, increasing catalyst circulation will have the effect of reducing SOx emissions.
BASF's in-situ FCC Catalysts Reduce SOX Emissions
The key FCC catalyst properties necessary to reduce SOx emissions are high Al2O3 content coupled with high matrix surface area to allow access to Al2O3 sites, and the presence of an oxidation promoter.
BASF's patented in-situ manufacturing process results in FCC catalysts which have higher Al2O3, surface area than competitive FCC products.
An Al2O3 rich, high surface area matrix, enhances the SOx capture reaction. SO3 captured in the regenerator is released in the reactor as H2S. The net result of these in-situ catalyst features is a level of sulfur emission reductions which cannot be achieved by competitive catalysts without the use of additives.
The results of three commercial FCC trials, highlighting the SOx reduction of each trial are illustrated in figures 4, 5 and 6. As can be seen, substantial SOx emission reductions were seen with the use of BASF FCC catalysts. For reference, the fresh catalyst properties are included in each figure.
The Catalyst Report is a publication of the Petroleum Catalysts Group of BASF Corporation. Its purpose is to provide the refining industry with technical information related to cracking/hydroprocessing catalysts or processes. For more information on any subject in The Catalyst Report, contact your BASF Petroleum Catalyst Group Representative at the office nearest you:
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