Mar-2012

FCC catalyst coolers in maximum propylene mode

Catalyst cooling technology for continuous heat removal from the regenerator in maximum propylene operations can avoid damage to FCC catalyst and equipment

Rahul Pillai and Phillip Niccum
KBR

Article Summary

Market demand for propylene has placed a strong emphasis on many FCC units to run in maximum propylene mode. Increasing reactor temperatures in pursuit of higher propylene, without regenerator heat removal, can raise the regenerator temperature to unacceptable levels, resulting in accelerated catalyst deactivation, degraded cracking selectivity and a need for exotic mechanical design to avoid equipment damage.

This article presents FCC modelling that demonstrates the utility of continuous heat removal from the regenerator for maximum propylene operations. Developments in FCC catalyst cooling technology have given refiners a flexible and reliable option to confront the heat balance challenges of maximum propylene FCC operations.

In an unconstrained environment, increasing FCC propylene production can be as easy as increasing the reactor temperature. However, in most cases, increasing the propylene yield is not that easy, as most FCC units are already operating against several physical and economic constraints. More commonly, regenerator coke burning and the vapour recovery unit limit capacity, increasing the reactor temperature without first reducing the FCC feed rate.

In grassroots FCC installations, coke burning and vapour recovery capacity can be built into the unit design, and existing FCC units can be revamped to include the requisite coke burning and vapour recovery capacity for increasing propylene production. However:
• Even with abundant coke burning and vapour recovery unit capacity, a high regenerator temperature can emerge as a major constraint to increasing reactor temperature because of the impact of the higher temperature on the unit heat balance¹
• FCC operators can effect a reduction in equilibrium catalyst activity to offset the increasing regenerator temperature that would naturally come from increasing the reactor temperature, but reducing catalyst activity runs counter to the more basic objective of increasing propylene production.

History of FCC propylene production
The first commercial FCC unit was built by The M W Kellogg Company in Standard Oil of New Jersey’s Baton Rouge, Louisiana, refinery and commissioned in May 1942. Between 1942 and 1944, Kellogg built 22 of 34 FCC units constructed throughout the US, and the FCC process quickly became a major contributor to worldwide propylene and butylene production.

Rare earth-exchanged Y zeolite catalyst was first synthesised by Mobil in 1959. By the late 19⁶0s, over 90% of US FCC units were operating with the Mobil-invented zeolite catalyst. The high activity of the zeolite catalysts, compared to the earlier amorphous catalysts, greatly improved the gasoline yield and reduced coke and dry gas yields from the FCC units, but the catalyst’s high hydrogen transfer characteristic greatly reduced the light olefin yield and gasoline octane.

In the 1970s, after the introduction of zeolite catalyst, FCC unit design and operation evolved to regain some of the lost octane and light olefin yield, primarily with a higher reactor operating temperature and riser cracking.² Increasing reactor temperatures increased the light olefin yield, but this came at the expense of an increased yield of dry gas — a lower-valued FCC product.

During the 1980s, Mobil introduced two new technologies with application to increasing the production of light olefins and octane while limiting incremental dry gas production: Mobil developed the ZSM-5 catalyst additive to crack low-octane (linear) gasoline-boiling-range olefins and paraffins into light olefins, and invented closed cyclones, which minimise product vapour residence time between the riser outlet and the main fractionator.^{3, 4}

In addition to the reduction in dry gas, the closed cyclone riser termination system reduced delta coke, especially on units that previously employed low catalyst separation efficiency riser termination devices. Therefore, the closed cyclone system was especially adept at increasing FCC propylene production because it simultaneously relieved constraints on both vapour recovery unit capacity and regenerator operating temperature.

The Maxofin FCC Process introduced by M W Kellogg and Mobil in 1985 (see Figure 1) is designed to maximise the production of propylene, ethylene and aromatics from traditional FCC feedstocks by combining the effects of FCC 
catalyst, ZSM-5 additive and a high-severity second riser designed to crack surplus naphtha and C4s into incremental light olefins and aromatic naphtha.⁶ Like closed cyclones, the Maxofin FCC Process also provides some relief to the heat balance while operating at high reactor temperatures because of the limited delta coke from recracking the recycled naphtha and C4 feedstocks. The recycled naphtha and C4s essentially act as a regenerator refrigeration system while simultaneously serving to increase propylene production in the high-severity second riser.

KBR dense-phase catalyst cooler
For many decades and until recently, FCC catalyst coolers have been considered only as a means to effectively process high-carbon-
residue FCC feedstocks, where the impact of Conradson carbon residue (CCR) on delta coke is a fundamental driver of the FCC heat balance.

CCR in the feed increases the amount of coke deposited on the catalyst as it passes through the riser. The increased concentration of coke on the catalyst as it passes through the reactor is referred to as delta coke. Without intervention, increasing delta coke leads to a high regenerator temperature, reducing FCC feed conversion due to lowering of the catalyst-to-oil ratio and accelerated catalyst deactivation. To mitigate the impact of increasing feed CCR on regenerator temperature, the heat released during catalyst regeneration must be controlled or heat must be removed from the system. The best option for controlling the heat balance with increasing delta coke is often the use of a catalyst cooler and/or regenerator operation in partial CO combustion mode.