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Oct-2004

FCC additive demonstrations Part 2

An overview of additive technology for reducing SOx and NOx emissions, and a review of CO combustion promoter technology and performance at a refinery

Guido W Aru
Intercat (Johnson Matthey)
Viewed : 3725
Article Summary
SOx reduction additives provide the lowest cost solution to reduce SOx emissions for most refiners.  In addition to their use in complying with EPA Consent Decree requirements, these additives are particularly appropriate in cases where there is an unexpected sulphur increase in the FCC feed due to a FCC feed hydrotreater shutdown or while processing sour crudes. They are also useful for balancing refinery-wide SOx emissions.  The amount of additive used can be adjusted to meet the changing requirements of the refinery. 

SOx reduction additives remove SOx from the regenerator flue gas and release the sulphur as H2S in the FCC reactor (Figure 1). Typical SOx additive levels in the catalyst inventory range from 1–10%. Historical SOx removal rates have typically been in the range of 20–60%, but recent government legislation has meant that more and more refiners are now using SOx additives to economically achieve removal rates of greater than 95%. 

SOx reduction additives are more effective at “sorbing” SO3 than SO2 under regenerator conditions. They do not have a strong affinity towards SO2. Since the sulphur oxide in FCC flue gas normally contains ~90% SO2 and ~10% SO3, there is a requirement to convert the SO2 to SO3. The SOx reduction additive therefore contains two components:
An oxidation catalyst to promote the formation of SO3.  The catalyst used is metal based, and is tailored specifically to favour the following reaction:
    SO2 + 1/2 O2 Æ SO3

A “pick-up agent” to remove the SO3 from the regenerator as a metal sulphate and release it as H2S in the reactor or stripper.  For all currently available additives, this is a magnesium-based material.  The SO3 is chemisorbed onto the additive as magnesium sulphate:
    MgO + SO3 Æ MgSO4

Both components of the additive must work together for maximum SOx removal.

Once the additive has picked-up SO3, it circulates with the catalyst to the reactor.  In the reducing environment of the reactor, hydrogen sulphide is released and the additive reverts to its original state as follows:
   MgSO4 + 4H2  Æ MgO + H2S + 3H20

In addition, the sulphate may be partially reduced to magnesium sulphide, and further reactions take place in the stripper to release H2S:
   MgSO4 + H2 Æ MgS + 4H2O
   MgS + H2O  Æ MgO + H2S

Thus, the sulphur is removed as SO3 from the regenerator and released as H2S in the reactor products. This results in an increase of 5–20 % of total H2S in the off gas, which is then removed in the sulphur recovery unit.

Clearly, FCCU operating conditions will effect SOx emissions and additive performance. The most significant of these variables and their affects are summarised in Table 1.

Feed quality is the most significant factor. The sulphur content of the feed dictates to a large extent the potential SOx emissions. However, the types of sulphur species present are also of primary concern. Typically, 10 wt% of sulphur in feed goes to SOx, but this is very variable with a range from 5 to 30 wt%.

Sulphur associated with paraffins, naphthenes and simple aromatics tends to crack to H2S and other light sulphur compounds (such as mercaptans), and leave the reactor with the products. Sulphur contained in large, highly aromatic feed molecules is laid down as coke and ends up in the regenerator. The subsequent burning of this coke produces SOx.

FCC base catalyst also plays a role in reducing SOx emissions. In a similar way to MgO, the alumina in FCC catalysts can act as a “pick-up” agent for SO3:
   Al2O3 + 3SO3 Æ Al2(SO4)3

Thus, catalysts with high matrix activities, such as bottoms cracking catalysts, tend to reduce SOx emissions.

The presence of CO promoter catalyses the oxidation of SO2 to SO3 and therefore enhances the SOx removal process. Higher concentrations of SO3 are also produced in the presence of excess oxygen, so SOx reduction additives tend to be more effective in full combustion regenerators.

Increasing catalyst circulation rate increases the regeneration rate of the SOx additive and thus its effectiveness. Recall that SOx additives release the absorbed SOx as H2S in the reactor and stripper. SOx absorption rates for these additives in the regenerator are rather fast as compared with typical circulation rates. Therefore, increasing circulation rate will improve the measured performance of an SOx additive, and decreasing circulation rate will reduce the performance.

Higher regenerator temperatures favour SO3 formation but can hinder SO3 absorption. Good air distribution and mixing in the regenerator also enhances SO3 formation as well as absorption.

Large regenerator inventories will reduce the efficiency of an additive as will inefficient strippers. Poor stripper operation can have two negative effects. The first is to increase the amount of sulphur going to the regenerator by leaving sulphur-containing hydrocarbons on the catalyst, and the second is to hinder SOx additive regeneration that occurs in the stripper. Both of these negative effects can significantly affect the meas-ured performance of an SOx additive.
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