Apr-2010

Optimisation of product yield and coke formation in a RFCC unit

Simulation studies of a commercial residue fluid catalytic cracking unit indicate the conditions for an optimum balance of product yield with low coke deposition

Sepehr Sadighi, Seyyed Reza Seif Mohaddecy, Omid Ghabouli and Mehdi Rashidzadeh
Research Institute of Petroleum Industry

Article Summary

Residue fluid catalytic cracking (RFCC) is used for upgrading heavy feedstocks to light products such as gasoline. In this study, simulation of a commercial RFCC unit was calibrated against actual plant data, acquired in test runs, and its performance predicted by simulation studies. The effects of key operating variables, such as temperature, input, steam flow rate to the riser, recycle rate of light cycle oil (LCO) on the yield of products and rate of coke deposition on the catalyst, were studied. The simulation results demonstrate that during the study period, and while all other operating parameters are held constant, the rate of injected steam to the riser, and the recycle rate of LCO, can reduce coke formation in the RFCC process.

Fluid catalytic cracking
Fluid catalytic cracking (FCC) is an effective refinery process for the conversion of heavy gas oils to gasoline. It is carried out at high temperature by contacting residue feed with an appropriate catalyst without hydrogen. After separation from the catalyst, the hydrocarbons are separated into desired products, such as LPG, gasoline, distillate and fuel oil. In this process, coke is deposited on the catalyst during the reaction and is burned off in the regenerator. When applied to the upgrading of heavy oil, the catalytic cracking process has been used to maximise the gasoline yield of refineries.

The processing of heavier crude oil sources has resulted in an increase in the boiling point and Conradson carbon residue (CCR) of the feedstock used in FCC plants. This process, using heavy residual oil, is RFCC, which requires different catalysts to those used in the FCC process.¹ RFCC catalysts are designed to be more stable than FCC catalysts because of the higher cracking temperature required for heavier feedstocks. Moreover, they should tolerate high levels of catalyst poisons, compared to FCC catalysts, because of the RFCC feed’s higher content of heavy metals, such as nickel, iron and vanadium.²

To maintain acceptable yields, licensors and catalyst suppliers have modified traditional FCC technology in such key areas as catalyst, feed injection, riser design, separation section and reactor/regenerator design. These improvements have enabled FCC operators to appreciably increase their level of residue processing, so that their conversion rate is 75% of the fresh feedstock.

This new generation of catalysts substantially reduces riser residence time, providing greater selectivity and control over distillate products. Licensors are now looking to shorten this to remove the riser altogether. Efficient and rapid separation of products from the catalyst is essential; otherwise, cracking will continue to lead to unwanted gas and coke production.

Process description
RFCC is an extension of conventional FCC technology, offering better selectivity to produce higher amounts of gasoline and less gas than hydro and thermal processes. In order to control the heat balance and recover part of the heat for steam production, the design of an RFCC unit includes two-stage regeneration, mix temperature control and catalyst cooling.^3,4

The catalyst used for RFCC is an acidic matrix such as crystalline aluminosilicate zeolite (USY or rare earth exchanged HY) in an inorganic matrix, which meets the required physico-chemical properties. The major limitation of the RFCC process is the requirement of high-quality feedstock (high H/C ratio and low metal content) to avoid the deposition of coke, high catalyst consumption and unit operability. Therefore, this process can only treat atmospheric residue, which contains relatively low amounts of metals, sulphur and carbon. However, the availability of such feeds is limited in refineries. Moreover, the crude from which they are derived is relatively high in price.^5-8

As Figure 1 shows, RFCC utilises a riser-reactor, catalyst stripper, first-stage regeneration vessel, second-stage regeneration vessel, catalyst withdrawal well and catalyst transfer lines.

Fresh feed is finely atomised with dispersion steam and injected into the riser through the feed nozzles over a dense catalyst phase. The small droplets of feed make contact with the freshly regenerated catalyst and vapourise immediately. The oil molecules mix intimately with the catalyst particles and crack to form lighter, more valuable products. Steam is injected through nozzles into the mixture of catalyst and vapourised feed. The operation is carried out at a temperature that is consistent with targeted yields.

Products exit the riser-reactor through a high-efficiency, close-coupled, proprietary riser termination device. Spent catalyst is pre-stripped by a high-efficiency packed stripper prior to regeneration. The product vapour is quenched to give the lowest dry gas and maximum gasoline yields attainable. Final recovery of catalyst particles is performed in cyclones before the product vapour is transferred to the fractionation section.
Catalyst regeneration is carried out in two independent stages equipped with proprietary air and catalyst distribution systems, which create fully regenerated catalyst with minimum hydrothermal deactivation.

These benefits are achieved by applying the first-stage regenerator in a partial-burn mode, the second-stage regenerator in full combustion mode, and both regenerators in parallel with respect to air and flue gas flows.

The process is capable of upgrading feeds to about 6 wt% Conradson carbon content without additional catalyst cooling, with less air, lower catalyst deactivation and smaller regenerators than a single-stage regenerator design. Gas oil extraction can be easily retrofitted.

Process simulation
This article reports on the effects of recycling LCO on the yield of products and coke deposition on the catalyst in an Iranian RFCC unit.⁹ The specifications and operating conditions for the riser, as well as a fresh feed analysis, are shown in Tables 1 and 2, respectively.