Sox Fcc

NOVEL SOX REMOVAL CATALYSTS FOR THE FCC PROCESS: MANUFACTURE METHOD, CHARACTERIZATION, AND PILOT-SCALE TESTING

Introduction

Sulphur oxides, commonly termed SOx, are a mixture of SO2 and SO3, wherein SO2 is the predominant form found in the lower atmosphere. These noxious gases react with ambient moisture to produce acid rain; they may also adsorb onto fine particulate matter formingsulphate aerosols. The negative effects of SOx are widespread. Short-term human exposure to high concentrations of SOx (above 1000 μg m-3) is associated with reduced lung function and respiratory tract diseases. Prolonged exposure to low concentrations is linked to increased mortality rates.1 Acidification of water bodies due to acid rain disrupts ecosystems, harming both plant and animal life. Acid rainalso damages constructions; it erodes building materials and accelerates metal corrosion. Additionally, the sulphate aerosols produce a haze that reduces visibility.

Environmental awareness has prompted government agencies worldwide to establish strict regulations on SOx emissions. Meanwhile, the scientific community is actively exploring different approaches to minimise SOx emission. Thestrategies are varied, depending on the emission source. SOx is mostly produced by combustion of sulphur-containing fossil fuels. Power stations, oil refineries and other large industrial plants release around 90 wt% of the total SOx emissions. Therefore, various proposals include changing the operating conditions and using fuels with lower Sulphur contents or natural gas. Still, the most popular andinexpensive method for SOx removal is the addition of selective sorbents with the fuel.

The Fluid Catalytic Cracking (FCC) process, in particular, contributes with ~7% of global SOx emissions. In FCC units, SOx emissions can be controlled in-situ by adjusting the composition of the main catalyst, or by incorporating a SOx reducing catalyst. These catalysts decidedly represent a practical,economical and flexible alternative, as they may be added to the FCC unit in relatively low amounts, at any moment, to obtain a quick SOx removal response.

The FCC process is one of the most important refining processes in today’s industry. Valuable products such as gasoline, diesel fuel and light olefins are obtained from the conversion of heavy oil fractions over a Y-zeolite based acidcatalyst. The cracking reactions essentially occur in the riser section of the FCC unit (Scheme 1), where vaporized hydrocarbons crack while flowing upwards together with the hot catalyst coming from the regenerator. The cracked products are roughly separated from the catalyst in the disengage and go on to the main fractionation tower.

The hydrogen to carbon ratio of cracked products is greater thanthat of the feed. Thus, a carbonaceous residue is 75 inevitably formed during catalytic cracking, termed coke. Although coke accretion deactivates the catalyst, it is vital to the FCC process, as its combustion provides the necessary heat for maintaining the auto thermal operation of the unit.

Then, after the cracked products have been separated, the occluded hydrocarbons remaining on catalystparticles are removed in the stripper by steam injection. The coked catalyst flows from the stripper to the regenerator zone, where most of the coke is removed by combustion with air at high temperature. Regenerators of FCC units may be operated in two distinct combustion modes, i. e., partial and full combustion. The former is oxygen deficient, based on the stoichiometric amount required to burnoff all the coke; whereas in the latter, oxygen is in excess.

In addition to hydrocarbons, gas oils cracked in FCC units typically contain a certain amount of hetero-atoms of sulphur, nitrogen and metals. The sulphur content in gas oils usually varies from 0.1 to 4 wt. %. After cracking, sulphur is distributed over the products. Depending on gas oil’s nature, catalyst properties and operating...