My apologies of the rather long interlude from my last posting. I am not getting any younger and some health issues have had to take precedence. The good news is that I now appear to have reached a stable plateau for the time being (until next time).

In this briefing I will go into detail on Fluid Catalytic cracking which is one of 3 main technologies for upgrading heavy distillates.

As usual I have attached the full presentation to the post- see bottom.

Exxon developed the first FCC (model 1) at Baton Rouge LA in 1942.  Since then FCC technology has evolved via many iterations. Primarily a process for gasoline production, it extended the amount of gasoline that could produced from a barrel of oil. Notwithstanding the FCC gasoline has very desirable octane values which reduced the overall demand for octane boosting additives.

FCC’s also produce significant amounts of olefinic C3 (propene) and C4 (butenes) olefines which are used in alkylation units and for the production of propylene which can be used at petrochemical feedstocks. Alkylation and ether (MTBE/ETBE) units are the main outlet for the butenes both of which are widely used gasoline ( ethers not in the US). Increasingly  FCC’S are swinging their production towards a mixture of fuels and petrochemicals, with newer units being able to product up to 35% wt C3’s and C4’s.

The main feedstock for FCC units is vacuum gas oil (VGO) which has a boiling range of about 375-500 deg C and is recovered in a vacuum distillation unit processing atmospheric residue. Vacuum residue comes in many forms and FCC units prefer waxy crude low in asphaltenes and refractory carbon.  See slide below

Suitable crudes types are typically low sulphur paraffinic type crude oils ,such as Brent, Bonny Light, Azeri Light, Hibernia, and other light/ medium crudes. One of the key requirements in fuels production is maintaining a high hydrogen to carbon ratio in the produced fuels. This is done by taking carbon out or adding hydrogen in. The FCC is a carbon out process. The FCC process uses a catalyst that is built around zeolites. Zeolites is defined as a crystalline aluminosilicate material characterized by a three-dimensional framework structure that contains uniform sub-nanometer- or nanometer-scale pores. These pores function as molecular sieves, selectively allowing smaller molecules to pass while excluding larger ones. The FCC catalyst resembles a find sand in appearance and circulates like a fluid mixed with the hydrocarbon feedstock and steam.

The mechanism of the FCC is simple on paper, but fiendish in its operation. consists of 3 main parts. The riser reactor, the regenerator and the fractionator.

Fresh feed is introduced into the Riser along with steam and catalyst. The catalyst to oil ratio is critical and is typically around 5:1. At the top of the reactor cyclones separate the catalyst from the hydrocarbon vapours which pass to the fractionator to be separated. In addition to the products indicated there is off gas ( hydrogen sulphide, ammonia, methane, ethane and ethylene) which was typically used as fuel gas. Coke is deposited on the catalyst. The recovered catalyst from the reactor flows by gravity into the regenerator where air is introduced to combust the coke on the catalyst. Higher coke loadings requires more burning and higher temperatures result. The hot regenerated catalyst provides the process heat for the cracking reactions and regenerator temperature is controlled by varying the air flow to achieve full or partial combustion. Often partial combustion is preferred using oxygen enrichment of the combustion air which result in a low BTU fuel gas which can be used for other process heating such as a CO boiler. The severity of the cracking process is controlled by the riser temperature and the catalyst to oil ratio. The higher severity of cracking leads to more catalyst coke which can require cooling. This is more likely with residue cracking, which is becoming more popular but typically requires upgrading. Crude oil composition is especially important for FCC units.

FCC feedstocks are mainly vacuum gas oil, hydro-treated vacuum gas oil and in some cases hydro-treated atmospheric residue. The critical parameters are the nickel and vanadium content both of which are catalyst poisons and nickel promotes hydrogen production, Conradson carbon and C7 insolubles which promote heavy catalyst coking, and sulphur. The lower the sulphur content the better the finished products.

The product slate from the FCC is strongly biased to gasoline production. The main products is gasoline, olefinic LPG, light cycle oil (gas oil), and slurry oil which contains catalyst fines. The LC and slurry oil contain significant sulphur concentrations. The FCC gasoline is the main source of sulphur in the gasoline pool and requires treating to meet the 10 ppm sulphur limit in ULS gasoline. The LPG stream is of particular importance and propylene is frequently recovered for petchem use. The C4 olefines are typically used for ether and alkylate production which are high value octane components. Ether are banned in the US but are widely used in other locations. 

In integrated refinery petrochemical plants the FCC is being increasingly used for petchem feedstocks, and some FCC's can produce very high concentration of C3 and C4 olefines. The are a number of ways that this can be done, either by increasing severity, recycle cracking of the gasoline stream, or a dedicated second riser for cracking of the FCC gasoline. Yields are in %wt of feed

FCC gasoline is generally the mainstay of the gasoline pool, making up something like 35-50% of the gasoline pool by volume. As can be seen the RON and MON of FCC gasoline is quite high and does not require significant octane blending. The other mainstay components tend to be reformate and alkylate, along with minor products like light naphtha and n-butane when specifications permit. Ethanol is a high octane ( RON 132 & MON 106) blending component, but not without several drawbacks. The old days of low octane gasoline and lead alkyls is long gone. FCC olefines are good and bad. The good is that olefines are high octane, the bad is that olefines are bad for smog formation. For this reason, gasoline has olefine limits which limit just how much FCC gasoline can be used in the fuel pool. Gas

oline also has limits on aromatics.

LCO is a poor-quality diesel. It can be used as heating oil, blended in small quantities into diesel or upgraded by hydro-processing. Sometimes it is recycled back to the FCC but is a prolific coke producer.  The high aromatic content it rather limits where is can be used. It has a very low cetane number and the aromatic content promote smoke in diesel engines.

FCC units operate and relatively low pressure usually < 2 bar  and moderate temperatures. The 4 examples below are 3 types of FCC and a steam cracker that cracks naphtha for petrochemical production.  The resident time in the reactor is all examples low < 2 seconds and the steam cracker < 0.5 seconds. Cracking reactions tend to be fast and the cracked products need to be quenched very quickly to avoid side reactions. DCC 1and DCC2 are two Chinese FCC designs but have never been licensed in the West, and have been optimised for petrochemical production. Similar and better design are in use around the world. As can be seen the amount of catalyst to oil ratio requires a large amount of catalyst in circulation. Catalyst consumption can be high due to losses in the separator and the need to add fresh feed to maintain activity.

There will be a part 2 to this discussion, hopefully in the near future. The ppt attached has been sanitized and some of the colours are a little gawdy to say the least. Note units tend to be wt%, deg C and bar.

If you have any questions put them in the comments and I will do my best.