SOx Emission Reduction

SO_X Emission Reduction

Background

The environmental impact of sulfur oxide (SO_X = 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 SO_x emission reduction, FCCU variables affecting SOx emission reduction, and the inherent ability of Engelhard'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 SO_X Reduction

The discussion in this section will refer to Figure 2 entitled "Mechanism of Catalytic SOxEmissions Reduction in FCC Units". in the figure, M_xO refers to any metal oxide such as Al₂O₃, which is an integral part of Engelhard'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 SO_X Reduction

Figure 3, illustrates the key variables, which impact SO_x reduction. These variables and their effects on SOx emissions are discussed below.

Feedstock Effects
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.

Excess Oxygen
Consider this reaction which occurs in the regenerator:

SO2 + 1/2 O2 <---> SO₃

_{Any oxygen present above the
stoichiometric amount needed to form SO}_₃_{will drive the above reaction to the right thus forming SO}_₃_{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}
_{Catalyst Al}_₂_O_₃_{The second reaction occurring in the regenerator is as
follows:}

_Al_₂_O_₃_{+ 3SO}_₃_{-> Al}_₂_(SO_₄₎_₃

_{This reaction is the so
called "capture reaction". The higher the number of
alumina or other metal oxide sites available for SO}_{_x}_{capture, the greater the sulfur emission reduction.}

_{Catalyst CO Promoter
Concentration

CO oxidation promoters also promote the oxidation of SO}_₂_{to SO}_₃_{.
Increasing CO promoter content will decrease SO}_{_x}_emissions.

_{Regenerator Dense
Temperature}
_{The regenerator temperature will
affect both reactions occurring in the regenerator section. It
will act on the kinetics of the first reaction in which SO}_₂_{reacts with O}_₂_{to form SO}_₃_{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. SO}_₂_{and oxygen are formed at the expense of SO}_₃_{.
Because of this, SO}_₃_{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 SO}_{_x}_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 SO}_{_x}_{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 SO}_{_x}_{capture. Normally however, increasing catalyst circulation will
have the effect of reducing SO}_{_x}_emissions.

_{Engelhard's in-situ FCC
Catalysts Reduce SO}_{_X}_Emissions

_{The key FCC catalyst
properties necessary to reduce SO}_{_x}_{emissions are high Al}_₂_O_₃_{content coupled with high matrix surface area to allow access to
Al}_₂_O_₃_{sites, and the presence of an oxidation promoter.}

_{Engelhard's patented in-situ
manufacturing process results in FCC catalysts which have higher
Al}_₂_O_₃_{,
surface area than competitive FCC products.}

_{An Al}_₂_O_₃_{rich, high surface area matrix, enhances the SO}_{_x}_{capture reaction. SO}_₃_{captured
in the regenerator is released in the reactor as H}_₂_{S.
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 SO}_{_x}_{reduction of each trial are illustrated in figures 4, 5 and 6. As
can be seen, substantial SO}_{_x}_{emission reductions were seen with the use of Engelhard FCC
catalysts. For reference, the fresh catalyst properties are
included in each figure.}

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