Oct-2012

RFCC units set new standard for propylene production

RFCC units are being commissioned and operated to convert hydrotreated and straight-run resid to high levels of propylene

PATRICK WALKER and RAYMOND PETERMAN
UOP LLC, a Honeywell Company

Article Summary

Worldwide demand for propylene continues to rise at a rapid pace. Consequently, propylene continues to command a substantial price premium over most other refined products. Refiners have long exploited this price structure by designing and operating their conventional fluid catalytic cracking (FCC) units to produce higher levels of propylene. In recent years, refiners have extended this practice to converting residual feedstocks to propylene. This article explores the technical challenges and opportunities presented when targeting high propylene production from a residual fluid catalytic cracking (RFCC) unit. The discussion will focus on the operation of propylene-targeted grassroots RFCC units designed by UOP and commissioned in the past few years. The discussion will also include a summary of advances in RFCC operations and reliability.
 
Unique features of resid FCC operations
Resid FCC operations differ from conventional VGO FCC operations in several important ways: feed characteristics, operating conditions, catalyst and equipment design.

RFCC feed characteristics
Resid is the heaviest fraction in the barrel of crude oil boiling at the highest temperatures. Relative to VGO, resid contains larger quantities of polycyclic aromatics and smaller quantities of paraffins. Relative to VGO, resid has a lower API gravity and lower hydrogen content. Conradson carbon and heterocyclics of sulphur, nitrogen and oxygen, as well as organometallics of nickel and vanadium tend to concentrate in the heaviest fraction. See Figure 1 for a description of these phenomena.¹

Aromatic rings cannot be opened in an FCC unit due to the low hydrogen partial pressure. Consequently, the potential conversion of resid feedstocks to smaller, more valuable molecules is limited. Often, about 75% of the Conradson carbon in the feedstock will wind up as additional coke,² which is subsequently burned in the regenerator, releasing large quantities of heat. 

RFCC operating conditions
When processing heavily contaminated feedstock in an RFCC unit, contaminant coke will deposit on the catalyst as it passes through the reactor. Contaminant coke is unique to RFCC unit operations and is formed in two ways: Conradson carbon in the feedstock will preferentially wind up as coke; metals in the feedstock, which accumulate on the catalyst particles, catalyse the formation of coke. Collectively, these two types of contaminant coke contribute very little to conversion. Additional coke (catalytic coke) must be produced in sufficient quantity to ensure an acceptable level of conversion. A third type of coke (catalyst circulation coke) can also be carried into the regenerator due to imperfect stripping.³ Catalyst circulation coke is not tightly adhered to the catalyst particle but rather constitutes heavy hydrocarbon remaining in the emulsion phase.⁴ Thus, efficient stripping is critical for resid operations. The total coke yield (lb coke/lb feed) is controlled by the operating conditions of the plant. For a clean feedstock, reasonable conversion might be achieved with a coke yield of around 5 wt% on feed. For a resid feedstock, the coke yield might be greater than 10 wt% on feed in order to reject the contaminant coke while maintaining catalytic coke at a quantity sufficient for reasonable conversion.  

The weight fraction of coke (of all types combined) that deposits on the catalyst during a single pass through the reactor is known as Δ coke (lb coke/lb catalyst). Δ coke is a function of the feed properties, catalyst properties, operating conditions and reactor technology. For heavily contaminated feedstock, delta coke can be greater than 1.0 wt%.

The regenerator temperature is a strong function of Δ coke. At constant operating conditions, higher Δ coke resulting from elevated contaminants in the feedstock and higher metals on the catalyst leads to a higher regenerator temperature. Operations at higher regenerator temperatures can reduce the useful working life of the regenerator internal equipment while increasing operating costs due to accelerated catalyst deactivation.

Therefore, when presented with heavily contaminated feedstock, operating conditions must be adjusted to cool down the regenerator. One way to do this is to operate the regenerator in partial combustion, with a flue gas CO2/CO mole ratio of about 2 to 6. This is effective because burning coke partially to CO releases about one-third as much heat as burning coke all the way to CO2. However, this technique alone is insufficient for heavily contaminated feedstock and would likely result in a severe operating cost penalty due to high catalyst consumption rates. For heavily contaminated feedstock, direct removal of heat from the catalyst in the regenerator via a catalyst cooler is required in addition to operating in partial combustion.

Decreasing the CO2/CO ratio or increasing the catalyst cooler duty have a similar impact on the operation of the unit. Both measures result in a reduction in the regenerator temperature, increasing the catalyst-to-oil ratio, and thereby increasing the conversion of feed to gasoline and lighter products. The coke is ultimately burned in the regenerator and recovered as superheated high-pressure steam via the catalyst cooler and the heat recovery section downstream of the CO incinerator. This way, incremental contaminant coke is rejected from the feedstock and burned to produce high-quality steam suitable for driving large turbines such as the main air blower or the wet gas compressor.

RFCC catalyst
Cracking catalyst for resid feedstock should have several features beyond those characteristics of cracking catalyst for VGO feedstock. More specifically, the catalyst structure must be designed to accommodate the large polycyclic aromatic molecules associated with resid feedstocks. This is usually accomplished by the incorporation of matrix components that contain a significant pore volume. The catalyst also needs a low Δ coke character to counter the high Δ coke tendencies of the contaminated feedstock. Finally, the catalyst needs to be tolerant of the metals in the feed, notably nickel and vanadium.