Oct-2010

Optimising the FCC regenerator for reduced emissions

Increasing scrutiny of FCC stack emissions calls for special attention to be paid to the regenerator’s combustion zone

Ray Fletcher and Martin Evans
Intercat (Johnson Matthey)

Article Summary

Continuous attention is focused upon the optimisation of the FCC conversion section for maximum profitability. However, few process engineers have placed much attention on the regenerator unless the operation is close to emissions constraints or out of compliance. The regenerator combustion zone operates with a highly complex and sometimes competing set of reactions. Distribution of spent catalyst and combustion air within the combustion zone and between cyclones adds an additional level of complexity to the FCC regenerator. The prudent FCC engineer will also ensure that the regenerator has been optimised in order that the FCC unit may operate with as few constraints as possible.

Many techniques have been developed over the years that enable today’s process engineers to troubleshoot and optimise the FCC regenerator combustion zone and related hardware. The following topics will be addressed within this article:
- Regenerator bed level
- Catalyst attrition
- Cyclone integrity
- Analysis and control of afterburning
- Control of SOx emissions
- Analysis and control of NOx emissions.

Bed level
Bubbling bed regenerators are designed with a minimum transport disengaging height (TDH) to ensure the least possible catalyst entrainment in the primary cyclone inlets. Violating minimum TDH constraints results in a large increase in catalyst carryover into the cyclones, leading to overloading and higher losses. It is crucial that the process engineer ensures that the bed level taps are reading accurately whenever catalyst losses exceed baseline levels. When in doubt, these level indication taps should be blown down to ensure accurate measurement. It is recommended that these level taps be continuously purged via a critical-flow restriction orifice, with a fail-safe backup having an exit velocity of 0.9 m/s (3 ft/s).

Some refiners intentionally violate the minimum TDH constraint in order to process additional charge. These refiners will, of course, observe regenerator losses that are greater than optimal. However, there are techniques to minimise loss rates, which include setting a maximum regenerator superficial velocity, and, for those units equipped with CO boilers, employing partial-burn operations.

Catalyst reformulations to a lower apparent bed density (ABD) system cause an increase in regenerator bed level. Most new units are equipped with at least one set of fully submerged taps to enable on-line measurement of bed density. These taps measure the bed density in real-time, which is then cascaded to the level indicator, enabling the most accurate measurement of bed levels. The process engineer monitoring an older unit without such taps needs to estimate the new bed density during change-out and have the bed level calculation updated to ensure the bed level is not actually higher than believed while 
transitioning to a lower density catalyst.

An additional catalyst level not normally considered by those monitoring and troubleshooting the FCC regenerator is the level within the cyclone diplegs.1 Most FCC units operate with negative pressure cyclones, in which the pressure in the cyclone is less than the pressure in the regenerator vessel. The mechanism that enables catalyst in a cyclone operating at a lower pressure to discharge into the higher pressure of the regenerator dense bed is the dipleg catalyst level. The height in the dipleg is a function of both the regenerator bed level plus the differential pressure between the cyclone and the regenerator. The dipleg level increases until these two pressures have equilibrated:

hdl = dPcy + ρbed x hbed
                  ρdl
Where:
hdl = Catalyst height in cyclone dipleg
dPcy = Cyclone pressure differential pressure
ρbed = Regenerator bed density
hbed = Length of dipleg submerged in the regenerator dense bed
ρdl = Cyclone dipleg density

It is recommended that the engineer calculate the dipleg bed levels and plot these levels against catalyst losses. There is typically an inflection point at which catalyst losses increase rapidly above a certain dipleg level. The cyclone dipleg levels may be dropped by reducing the charge rate, increasing the regenerator pressure or 
reducing the combustion air rate. Dropping dipleg levels below this inflection point reduces catalyst losses if the critical dipleg level has been violated.

Catalyst attrition
One of the causes of attrition is through catalyst particles being struck by high-velocity air or steam jets within the regenerator, resulting in particle-to-particle and particle-to-wall collisions. The result of these collisions is fracturing of the catalyst particles into ever smaller particles.

A second mechanism that produces attrition fines is grinding. This is the result of particle-to-
particle abrasion, which results in the breaking off of small surface nodules. Most units have both mechanisms present, with the relative ratio of the two mechanisms changing from unit to unit.