HCN emissions in fluid catalytic cracking
Increased attention to refinery emissions of hydrogen cyanide requires detailed understanding of its formation and conversion in the FCC regenerator
XUNHUA MO, BART DE GRAAF and PAUL DIDDAMS
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The fluid catalytic cracking (FCC) unit is a major conversion unit present in many refineries throughout the world. FCC units are highly flexible and able to upgrade feeds comprising many components, ranging from light, sweet hydrotreated vacuum gas oil (VGO) to heavy, sour residues. In addition, FCC feed can include heavy streams from other refinery units, such as coker gas oils, as well as low-value slops of ranging composition. In the vast majority of cases, the FCC feed contains a wide range of contaminants, including metals (nickel, vanadium, copper, iron, calcium and sodium) and heteroatoms (sulphur and nitrogen).
The role of the FCC unit is to convert low-value, high molecular weight (high boiling point) feed to lighter, more valuable products via cleavage of C-C bonds (cracking). Some of the feed (typically 5-6 wt%) is converted to coke as a byproduct of cracking. This is burned in the FCC regenerator, where the heat of combustion is used to provide the energy required to vaporise and crack the feed; the unit is operated in heat balance.
Many of the contaminants in the feed end up in the coke being burned in the regenerator. Metals irreversibly accumulate on the catalyst, where they deactivate/poison the catalyst or cause undesired side reactions such as dehydrogenation and additional coke formation. Their concentration on the catalyst is controlled via catalyst replacement. Some of the feed sulphur- and nitrogen-containing compounds also form coke on the catalyst; these are temporary poisons because they are burned off in the regenerator. Sulphur and nitrogen in coke burns to form a wide range of gaseous sulphur and nitrogen species (SO2, SO3, COS, H2S, N2, NO, N2O, NO2, NH3 and HCN). The exact composition of these gases in the flue gas depends upon the detailed reaction conditions in the regenerator. For example, under full-burn conditions (excess oxygen used in the combustion of coke), the main species are SO2, SO3, N2 and NO, whereas under partial-burn conditions (sub- stoichiometric oxygen levels) much higher levels of the “reduced” S and N species (COS, H2S, NH3 and HCN) may also be present. In partial-burn units, much of the carbon is combusted to CO rather than CO2 in order to decrease the heat of combustion and allow the processing of heavier, higher coke-making feeds. Most partial-burn FCC units have a CO boiler downstream to convert the CO to CO2, to control CO emissions and recover the energy for steam production. Most of the reduced S and N species in the flue gas are converted to more highly oxidised forms in the CO boiler:
COS, H2S → SO2, SO3 and
NH3, HCN → N2, NO, N2O, NO2
so, in both full-and partial-burn operations, the flue gas contaminants are predominantly SOx (SO2 and SO3) and NOx (NO, N2O and NO2). Other species are only present at much lower concentrations.
Increasing environmental awareness has brought in place stringent regulations to limit emissions of S and N gases to the atmosphere. Unit design features, novel catalytic additives, flue gas scrubbers and so on used in modern FCC units have helped to reduce flue gas SOx and NOx emissions. Catalytic SOx control additives have been used to achieve SOx levels in flue gas as low as 5 ppm without the need for capital expenditure. Advances in regenerator design, transition to non-platinum-containing CO promoters and the use of catalytic NOx control additives have all contributed to decreasing NOx emissions.
A lot of attention has been paid to controlling NOx and SOx (and particulate) emissions. Today, FCC unit design and additives have been geared to achieve the lowest levels of both SOx and NOx. Recently, however, hydrogen cyanide (HCN) emissions (present in flue gas at up to about 150 ppm) have come under scrutiny in the US. Suddenly, a “new” species is added to FCC chemistry. In the public domain, the origin of HCN in FCC flue gas was poorly understood. In this article, we explain the origin of HCN, its relationship to NOx, and how modern FCC unit design geared to low NOx emissions can induce a trade-off with HCN emissions. Continuous catalytic additive development for regenerator flue gas control helps to bring flue gas chemistry under control. However, flue gas control is a combination of additive and regenerator design working together.
Origins of HCN
Where does HCN come from? There are many different nitrogen compounds present in FCC feedstocks; these are measured and reported as total nitrogen and basic nitrogen. Typically, about 30-50% of the feed nitrogen is basic. These nitrogen species strongly adsorb on acid sites on the catalyst and are thereby transported with the catalyst into the regenerator, where they are combusted together with the coke. As a rule of thumb, about half of the feed total nitrogen ends up being combusted in the FCC regenerator. Table 1 shows a typical FCC nitrogen balance.
Coke composition varies, depending on feed properties and stripper efficiency. Coke consists of carbon-rich polycyclic aromatic structures containing heteroatoms and contaminant metals as well as unstripped hydrocarbon products (for example, 10-30% of coke may be gasoline, diesel and fuel oil range products that could not be stripped from the pores of the catalyst). Typically, coke has a hydrogen content of 5-8 wt% largely present in the unstripped products.
The levels of sulphur and nitrogen in coke are much higher than in the feed. In the regenerator, carbonaceous coke and unstripped hydrocarbons are combusted to CO2, CO and H2O. Sulphur is oxidised to SO3, SO2, COS and H2S. Nitrogen behaves differently; as oxygen reacts with the coke matrix to form CO2, CO and H2O, much of the nitrogen may initially form HCN.
This is similar to the chemistry observed in coal combustion. HCN is a thermodynamically unstable species under FCC regenerator conditions and, given sufficient time and temperature, HCN will be fully converted and no trace of HCN will be found in the regenerator. Under FCC regenerator conditions, N2 is the most stable nitrogen species, followed by NO, which at thermodynamic equilibrium should reach a stable concentration of about 10 ppm. Evidently, nitrogen chemistry is not at thermodynamic equilibrium, but is under kinetic control.
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