The fluid catalytic cracking (FCC) process can produce a wide range of products. FCC technology was introduced almost 72 years ago to facilitate the production of high-octane fuels, and many units are still operated for that purpose. However, the FCC unit (FCCU) can also be used to produce petrochemicals. On- going changes in ethane cracking operations do not produce suf- ficient propylene to meet growing demand. Petrochemical yields from the FCCU is an area of increasing interest as more compa- nies try to integrate refining and petrochemical operations.1
PROPYLENE MAXIMIZATION
In the last 10 years, the FCCU has typically been designed to produce large amounts of propylene. This has been true for cat- alytic crackers running both conventional and hydrotreated gas- oils (GOs) and atmospheric resids. Several factors are contrib- uting to this trend. Steam crackers are getting larger, and more are operating on ethane rather than naphtha. Ethane produces very little propylene, and other sources must be found to meet the required propylene demand. To make the situation even more acute is that propylene demands are once again expected to outpace ethylene demand. FCCUs are also getting larger. While the average FCCU processes about 40 Mbpd, new units typically range from 50 Mbpd–120 Mbpd. These units are large enough to support world-scale polypropylene (PP) facilities.
To produce maximum levels of propylene, higher unit con- versions are required. The increase in propylene yield comes primarily at the expense of overcracking the C6–C10 olefins in
the gasoline boiling range. These higher conversions are ob- tained by operating in more severe cracking conditions, i.e., higher reactor temperatures, increased catalyst circulation rates for higher catalyst/oil (c/o) ratios, and/or higher catalyst activ- ity. All of the commercial processes that maximize propylene use a pentasil (medium-sized pore) zeolite to overcrack the gasoline. Without exception, feeds that are higher in hydrogen content produce more propylene.
FCCU designs. Unit designs for producing propylene enable increased severity in the reaction zone. Variations in the design parameters include:
• Increasing cracking residence times by riser modifications or the addition of bed cracking
• Using a downflow reaction scheme
• Using advanced feed injectors with high levels of steam injection for feed atomization and optimal hydrocarbon partial pressure in the reaction system
• Applying reactor-termination technology that reduces excessive dry gas and Δ coke
• Using higher c/o ratios due to the endothermic heat of cracking and operating at elevated reactor temperatures • Recycling cracked naphtha
• Modifying the regenerator design to allow for the addition of extraneous fuel to maintain regeneration kinetics • Using modified and unique downstream product
recovery sections
• Adding product treating sections for producing a chemical- or polymer-grade product for petrochemical purposes
• Using reactor designs that are compatible with the required temperatures for maximum propylene.
Dual risers. There are options with dual riser designs. One configuration has two parallel reactor risers terminating into a common reactor-disengaging vessel, where the riser product ef- fluents are combined and are recovered in a single fractionation and gas-plant recovery section. A second option is to have two reactors (riser or down flow) with separate termination vessels. The reaction products are segregated to produce fuel- and poly- mer-grade products. This design option allows for different op- erating modes and feedstocks to produce distillates or gasoline in one riser along with propylene in the second reactor. With the two reaction zones, these units can achieve propylene yields at the 12 wt% level.2, 3
Fractionator concerns. The main fractionator and gas con- centration plants have different concerns. Due to the high con- versions and better quality feedstock, the bottoms yields are minimized. This requires a careful review of the main column bottoms circuit and heat integration in the gas concentration unit.4 Additionally, a propane/propylene splitter may be includ-
ed in the gas concentration to produce chemical- or polymer- grade propylene. If this is the case, additional processing units are included for treating propylene for contaminant removal.
Performance. TABLE 1 shows the gasoline and light-olefin
yields for a conventional gasoline FCCU vs. a high-olefin FCCU (HOFCCU) for propylene.5 One drawback to producing maxi-
mum propylene is that it comes at the expense of gasoline yields and gasoline composition. While higher-severity operations can easily double or triple propylene yields, gasoline make will be reduced by 25%–50%. The gasoline composition is 2 to 3
80NOVEMBER 2014 | HydrocarbonProcessing.com
Petrochemicals
times higher in total aromatics.6 Further breakouts of propylene
for the current operating modes are shown in TABLE 3.
Gasoline production. Conventional FCCU units were de-
signed to meet gasoline demand by cracking heavy GOs (HGOs) or resids that generally produce propylene yields from 3 wt%–5 wt% in a maximum gasoline mode. With the addition of a ZSM- 5 additive, the propylene is increased about 3 wt% on average.
The high-severity FCCU (HSFCCU) mode utilizes more severely hydrotreated feedstocks or GOs from highly paraffin- ic crude oils to produce 12 wt% propylene yield. The catalysts and more severe operating conditions are similar to those in the traditional operation. However, these HSFCCUs are lim- ited in processing flexibility to shift from propylene to fuels. Due to the recovery sections, these units are also limited in feedstock flexibility.
High-olefin operation. The HOFCCUs were developed
to produce propylene yields from 15 wt% to 20+ wt% and will yield high levels of other light olefins. The HOFCC gaso- line is highly aromatic, and it is preferentially a petrochemical
feedstock. However, it can be used in unique gasoline-blending pools. For example, if a refinery has isobutane available, then the HOFCCU can produce enough mixed butylenes for an alkyla- tion process. In this case, the HOFCC gasoline may be blended with alkylate to meet fuel specifications. TABLE 2 summarizes
the directional changes in the operating variables to raise pro- pylene production, and the concerns regarding unit operation.
Operating at elevated reactor temperatures is a key to pro- ducing higher propylene and other olefin yields from maximum gasoline operations. Gasoline modes have reactor temperatures ranging from 920°F to 1,000°F, while HSFCCUs require riser temperatures above 1,020°F and a cold-wall riser reactor design.7
Higher cat/oil ratios are needed from heat balance consid- erations and to help achieve the required high conversions. Higher reactor temperatures require increased catalyst circu- lation rates, as does the higher endothermic heat of cracking common to propylene processes.7 Catalyst circulation is a
dependent variable; however, it is set by the heat load and Δ coke. This can limit the quality of the feed for units designed for c/o ratios above 12. If the feed quality is very high, a fired heater may be desirable.
Hydrocarbon partial pressure should be minimized for pro- ducing propylene. This is achieved from lowering reactor pres- sure and/or by increasing steam usage. A riser steam usage of 10 wt% on fresh feed is not uncommon, and it can be as high as 30 wt%. Main fractionators need to be packed to achieve the lowest
TABLE 3. Propylene yields for FCC designs and ZSM-5
Conventional FCCU HOFCCU
FCC FCC + ZSM-5 HSFCC HSFCC + ZSM-5
C3= yield 3 wt%–5 wt% 6 wt%–8 wt% 10 wt%–13 wt% 15%–20% + wt%
TABLE 1. Typical product yields for conventional gasoline vs. HOFCC comparison
Typical product ranges Gasoline FCC HOFCC
Wt% on fresh feed Dry gas 1.5–3 3–12 Ethylene 0.5–1.5 2–7 Total LPG 16–22 32–44 Propylene 4–7 12–22 Butylenes 4–8 8–14 Gasoline 47–53 30–40
TABLE 2. Eff ects of variables on propylene yield
Adjustment Concerns
Reactor temperature Increase Metallurgy
Cat/oil ratio Increase Cat circulation, slide valve Δ Ps Residence time, space velocity Increase Δ coke
Preheat Increase Furnace limit, regenerator temperature control
Hydrocarbon partial pressure
Unit pressure Decrease Higher gas make, larger compressor product recovery section
Steam rate Increase Increase sour water recovery Recycle
Recycle cracked-light naphtha Increase Increase in product recovery
Recycle heavy oil Increase May back out feed, not enough to meet heat requirements
Catalyst
Catalyst activity (circulating) Increase High cat additions, less metal tolerant, high hydrogen transfer High Z/M ratio Increase High H2 transfer
High catalyst ZSM-5 additives Increase Lower cracking catalyst activity
Unit cell size Decrease Low cat activity
Feedstock
Higher quality More hydrogen Low Δ coke