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How to make anything with a catalytic cracker

07.01.2014  |  Dean, C.,  High Olefins FCC Technology Services, LLC, Houston, TexasLetzsch, W. S.,  Technip Stone & Webster Process Technologies, Houston, Texas

Various refined products can be produced via an FCCU only with changes in operating condition, different feedstocks and advanced/specialty catalyst choices.

Keywords: [FCC] [gasoline] [diesel] [catalyst] [isobutylene] [isobutane] [butylenes] [hydrocracking]

The first fluid catalytic cracking (FCC) process was introduced almost 72 years ago; yet, this technology remains a principle conversion process in a modern refinery. Much has changed since the original FCC unit (FCCU) became operational. Properly operated, an FCCU can produce a variety of valuable refined products, including olefins and aromatics in addition to high-quality transportation fuel products. Various refined products can be produced via an FCCU only with changes in operating condition, different feedstocks and advanced/specialty catalyst choices.

Maximizing individual products

Numerous papers have been presented that focus on maximizing the various products produced by an FCCU. These topics usually appear when the economics favor a particular yield. This article can be used as a reference for FCC operators to apply as a guide on what is necessary to maximize (or minimize) the yield of the many diverse FCC products.

To achieve top performance from an FCCU, several factors must be considered. First, the design must be conducive to the making of the particular product. Second, the properties of the feedstock are important since the molecules entering the unit will determine what can potentially be made.

Optimization of the many operating variables is required, along with using proper catalyst. Using the right catalyst in the proper way is the single most important action that a refiner can take to maximize FCC performance.

Gasoline maximization

From the beginning, the FCCU was primarily a gasoline machine. Feedstocks were atmospheric gasoil (AGO) and vacuum gasoil (VGO), and the objective was to produce high yields of gasoline. Originally, the gasoline yield was maximized by recycling both light- and heavy-cycle oils. The coke make was almost twice as high as necessary since the recycle rates could be as much as 100% of the fresh feedrate. Some designs had two risers that were of equal size.

The present-day design parameters for an FCCU to maximize gasoline are:

  1. A straight and vertical reaction system that operates in a gas/solid mode with minimum slip and an optimum residence time.
  2. Feed injectors that vaporize the feed as fast as possible and reduce the incoming regenerated catalyst temperature to the mix temperature quickly.
  3. Rapid separation of the spent catalyst from the reaction products. This quickly terminates the reactions, thus minimizing the dilute-phase residence time and/or the temperature of the product vapors.
  4. Stripping should be as efficient since hydrocarbons carried over to the regenerator are primarily gasoline and diesel. Excessive residence time of the hydrocarbons in the stripper will convert them to light gases, thus reducing gasoline yield.
  5. Regeneration should be essentially complete since the residual carbon left on the catalyst is associated with the large-pore molecular sieves used in the catalyst.

FCC feed

Feedstock has a very marked impact on gasoline yields.1 Since gasoline typically has a hydrogen content of about 13.5 wt%, a feedstock that is higher in hydrogen will produce more gasoline yield. Table 1 shows results from a cracking study done in a circulating pilot plant with a Middle East GO, a severely hydrotreated Louisiana GO, and the atmospheric bottoms from a US shale oil.2 The shale oil data was adjusted to better simulate the expected commercial results.

 


The data show a direct correlation between hydrogen content of the feed and gasoline yield. Even though the tight oil contains some 1,050°F plus material, the gasoline yield exceeds 57 wt% or 67 vol% on fresh feed.

Operating variables must be manipulated to attain high conversion while minimizing coke and light gases make.3 Maximum gasoline yield usually occurs at conversion levels between 80 vol% and 85 vol%. The conversion will be lower when processing aromatic feeds. Highest gasoline yields are achieved by maximizing catalytic activity within the parameters of the reaction system. Too much activity will give too low catalyst-to-oil (C/O) ratio, and it could lead to catalyst deactivation due to higher Δ coke.

As summarized in Table 2, the reactor temperature will usually be between 960°F and 985°F. Lower temperatures adversely affect the stripper operation, and the higher temperatures will overcrack the gasoline produced.

 



Table 3 summarizes the effects of the major operating variables on an FCC operation.4 In this instance, reaction temperatures above 980°F will have lower gasoline yields as did the heavy-cycle oil recycle. Recycle at high conversion is usually used for bottoms cracking rather than for producing more gasoline. Increasing catalyst activity and the C/O ratio (by lowering feed temperature), and reducing the reactor pressure, will increase gasoline yield. While lowering the feed temperature will make more coke, the dry gas yield may be reduced. If heavy feeds are processed, the ability of the feed injection system to vaporize the feed may set a minimum feed temperature.

 



 


Catalyst impact

Catalyst properties must be tuned to the particular FCC operation. Both the feedstock and equipment limitations impact the choice of catalyst and additives. Gasoline and conversion may not be maximized if the unit is operating against multiple constraints, such as the air blower, wet-gas compressor and catalyst circulation.

The main catalyst variables that can be controlled are faujasite zeolite content and type and degree of exchange of the zeolite. Matrix activity, pore structure and total pore volume, and metals passivators are all matrix components, which are varied to optimize the FCCU.

Catalysts containing intermediate pore-size zeolites (ZSM-5) need to be excluded. This is sometimes forgotten when an equilibrium catalyst is used.

The equilibrium unit cell size needs to be optimized as well. For maximum gasoline, values ranging from 24.32 to 24.40 are used. Feeds with few coke precursors would benefit from larger numbers, while heavier or more aromatic feeds normally require lower values. Coke and gas selectivities are usually limited in that case. The starting unit cell size should be as close to the equilibrium value as possible since the fresh catalyst will play a significant role in the overall cracking performance.

Diesel maximization

A few years ago, the only products making money for US refiners were the middle distillates, i.e., diesel, kerosine, heating oil and jet fuel. The entire refinery became focused on maximizing middle-distillate products including the FCCU. As with gasoline maximization, all aspects of the FCCU must be considered.

Diesel is the first product from the cracking reactions, and it reaches a peak before the gasoline yield, as shown in Fig. 1.
When diesel-range material cracks, the primary product is gasoline. A riser would be the preferred reactor design with no back-mixing. Contact time should be short, and the recycle of unconverted feed is required since the bottoms yield would be excessive.

 
  Fig. 1. Effect of zeolite content on lco yield
  at constant operating severity.



Feed injection systems must vaporize the feedstock quickly while quenching the hot catalyst from the regenerator. This minimizes Δ coke and dry gas make. A quick separation of the hydrocarbons and spent catalyst minimizes dry gas make. While bottoms cracking occurs in the dilute phase of the reactor, it is more effective to recycle the unconverted feed.

Efficient stripping minimizes the amount of middle distillates that are burned in the regenerator, thus reducing the loading to the gas plant. Better stripping provides more operating flexibility to optimize other operating variables.

Regenerator. Regeneration of the catalyst should be efficient. However, striving for the lowest carbon on catalyst may not be desirable. The strongest acid sites on the catalyst tend to crack the feed all the way to gasoline. Since the residual carbon is associated with these sites, a carbon level of about 0.1 wt% to 0.2 wt% may be desirable, and it would depend on the catalyst (formulation) used.

Diesel-free feed. The feed to the cracker should not have any diesel present. This material is preferentially cracked to gasoline, and there is a large cetane loss. If a cat-feed hydrotreater is being used, then the operator should consider operating it as a mild hydrocracker. This action is more selective to diesel than an FCCU, and it provides a high-quality product for the diesel pool.

Hydrocracker. If the refinery has a hydrocracker, it should be operated at capacity. This may require additional hydrogen for the plant, but it should be economical. At lower FCC conversions, the light-cycle oil (LCO) is a higher-quality product and requires less-severe post-treating.5

Feedstocks with two and three aromatic rings will make more LCO than paraffins, since the aromatic nuclei are resistant to cracking reactions. Recycle streams and coker GOs are relatively rich in these molecules, and they will produce more middle distillates. These can be processed in the fresh feed riser or in a separate dedicated reactor riser. Coker GOs boiling in the diesel range would go to a middle-distillate hydrotreater rather than the FCCU.

Main fractionators should have a heavy-cycle oil (HCO) draw so that a more coke-selective stream can be sent to the FCC.6 Decant oil should go through dedicated nozzles to preserve the integrity of the regular feed nozzles. Refiners have reported better overall yields when the decant oil is injected well downstream of the fresh feedstock.

Catalysts used to maximize LCO should be very low in activity.7 As shown in Fig. 2, this is true for rare earth Y. Type Y or ultra-stable zeolites have large pores, and they are very effective at cracking diesel-sized molecules. Type Y or ultra-stable zeolites must be limited to about 5%–10% in a catalyst formulation to prevent overcracking of the LCO. Studies also suggest that smaller crystal sizes would help by allowing the LCO produced to diffuse more rapidly out of the sieve.

 
  Fig. 2. Effect of zeolite concentration
  on LCO yields.

Matrix activity should be maximized to give higher first-pass cracking, and the catalyst should have mainly intermediate or low-acid strength sites. Strong-acid sites produce gasoline. An open-pore structure is desired to minimize LCO overcracking. Compositions that include magnesium have been proven to make more diesel due to their unique acid-strength distributions.

Operating variables are manipulated to give low conversions. Table 4 summarizes various operating variables to control. Reactor temperatures of 930°F–950°F are used for GO, while resids may require 950°F–960°F to avoid excessive hydrocarbon carryover from the stripper. Feed preheat may be maximized, and a fired heater would be required for the more crackable feeds.

 



Recycle is essential to improve bottoms cracking with low conversions. Rates of 1%–30% would be required to give LCO/GO ratios of at least 3, and, in many cases, over 5, for these middle-distillate operations.

Iso-C4s

The iso-C4 hydrocarbons are very valuable. Refiners frequently want to maximize or, at least, increase one of them. These molecules are isobutylene and isobutane.3 Both are important feeds to an alkylation unit, and the isobutylene is used to make methyl-tertiary butyl ether (MTBE), the preferred oxygenate used globally except in North America. Isobutylene can also serve as a feed to a catalytic polymerization unit or as a similar process that makes gasoline from light olefins. Isobutane is required for alkylation. Some refiners are short of this material due to a lack of local field butane supplies.

Isobutylene

The C4 hydrocarbons are generally one of the ultimate products from a catalytic cracking unit due to the beta-scission carbenium ion cracking mechanism. Equilibrium concentrations of the various C4s are rarely achieved due to the reactivity of the butylenes. The isobutylene equilibrium sets the maximum amount of isobutylene that can be produced in a typical catalytic cracker.8 At equilibrium conditions, the percentage of isobutylene in the butenes stream varies from 46% at 950°F to 44% at 1,050°F.9 The actual percentage of isobutylene in the butene stream depends on the unit design operating conditions, feedstocks and catalyst. While isobutylene is initially produced, hydrogen transfer reactions can diminish its yield.

Unit design features that help preserve isobutylene are: a short reactor contact time, and rapid separation of the spent catalyst and reactor effluent. Reducing the dilute phase temperature and contact time also is important. Low reactor hydrocarbon partial pressures are essential for minimizing hydrogen transfer, and FCCUs operating above 30 psig may not be able to make the needed isobutylene. Dispersion and/or riser stream will lower the hydrogen transfer reactions.

Like every other product from an FCCU, feedstock plays an important role in isobutylene manufacturing. In general, high K factor feeds will give more isobutylene than more aromatic stocks. More paraffinic feeds can operate at higher conversions and generate more LPG. Typically, LPG has a hydrogen content well above 15 wt%, so hydrogen-deficient feeds cannot make as many barrels of isobutylene. The percentage of the olefins may be lower, depending on the operating variables and conversion levels. Key feedstock parameters would include the amount and configuration of the naphthenes. The length of the side chains on the ring compounds will determine whether C3 or C4 olefins are formed. The main operating variable for producing more isobutylene is reactor temperature. The amount of the C4 iso-olefin increases significantly when the gasoline is over-cracked.

Other variables that increase conversion can also increase isobutylene yield. These include higher catalyst activity and higher C/O ratios. Both can promote hydrogen transfer reactions; thus, the amount of isobutylene will not be as high as that yielded by increasing reactor temperature. If the conversion is taken too high, then the higher reactivity of the isobutylene will diminish its yield.

Catalyst properties can have a very large effect on isobutylene yield. The goal is to get conversion, but with minimum hydrogen transfer. The large-pore zeolite used should be an ultra-stable Y (US-Y) rather than a rare earth Y (ReY). Minimal RE should be used for stabilization. A lower unit cell size is desirable, and it is important that the fresh catalyst also be low in unit cell size. High unit cell sizes mean the acid sites on the zeolite are closer together, and that promotes hydrogen transfer.

An active matrix will provide much needed catalytic activity and has minimal hydrogen transfer activity. Bronsted acid sites would be preferred to Lewis acids since they would promote more skeletal isomerization of the olefins. ZSM-5 tends to increase the isobutylene concentration since the gasoline that cracks within the medium-pore zeolite produces a concentration of isobutylene near the equilibrium value (40% or higher).

Isobutane

Isobutane yield will increase with conversion but can be cracked thermally in the feed injection zone. When reactor temperatures reach 1,030°F, the base of the riser is near 1,100°F. Since isobutane is a function of hydrogen transfer reactions, the opposite factors for maximizing isobutylene generally apply.

To minimize thermal cracking, the best conditions are good feed injection system, low slip factor riser, rapid and high separation of the spent catalyst and reactor products, short dilute phase contact times and/or low temperatures, and a highly effective stripper and regenerator.

Feedstocks higher in hydrogen work best for the same reasons outlined in the previous section (isobutylenes). Tight oils would be expected to produce high quantities of isobutane with the proper unit design and operating conditions.

While high conversions are desired, catalytic conversion is preferred to thermal conversion (reactor temperature). Higher catalyst activity can be achieved by increasing the activity of the catalyst or the C/O ratio by reducing feed temperature. It is normally more economical to first increase the RE content of the catalyst to raise activity, then increase the large-pore zeolite content, and finally raise catalyst additions. A higher UCS is usually desired. Using ZSM-5 should be reduced or eliminated.

Butylenes

Butylenes are the most desirable light olefins in a gasoline-oriented refinery. These often have a value of gasoline or higher since they are the preferred feed to an alkylation unit. Butylenes give the highest octanes and consume less isobutane than propylene. Amylenes are usually only processed to reduce gasoline olefinicity and/or vapor pressure.

All of the caveats that applied to making more isobutylene apply to maximizing butylenes. The unit design and feedstock properties would not be different. Reactor temperature is the key operating variable. Normally, a minimum reactor temperature of 980°F would be used to make light olefins. Many refiners will run over 1,000°F reactor temperature when operating in this mode. Low-hydrogen transfer and controlled activity are desired with an ultra-stable zeolite with a unit cell size of 24.32 angstroms or less. Some units might run as low as 24.27 angstroms, but dry gas or low conversion may become a limitation.

Some catalyst additives are aimed at increasing butylenes vs. propylene. Since the additive suppliers have more experience with their products than a typical refiner, they should be consulted as to a product recommendation and a yield estimate. Increased matrix activity is also desired for maximizing butylenes and modifications to the ZSM-5 carrier may also help. HP

ACKNOWLEDGMENT

This is an updated version of an earlier presentation at the American Fuels and Petrochemical Association Annual Meeting, March 16-18, 2014, at Orlando, Florida.

LITERATURE CITED

1 Bryden, K. J. and E. T. Habib, Jr., “Processing shale oils in FCC: Challenges and opportunities,” Hydrocarbon Processing, pp. 59–64, September 2013.
2 Huovie, C., et. al., “Solutions for FCC Refiners in the Shale Oil Era,” AFPM Annual Meeting, March 2006.
3 “Alternative Routes to High Conversion,” The Catalyst Report, TI-805 Engelhard (BASF).
4 Gim, S., W. Letzsch, H. McQuiston and C. Santner, “FCC Opportunities at Lower Throughputs,” AFPM Annual Meeting, March 2010.
5 Unzelman, G., “Potential Impact of Cracking on Diesel Fuel Quality,” Katalistics’ 4th Annual FCC Symposium, 1983.
6 Niccum, P., “Maximizing diesel production in an FCC-centered refinery—Part 1,” Hydrocarbon Processing, September 2012.
7 Pillai, R. and P. Niccum, “Select new production strategies for FCC light cycle oil,” Hydrocarbon Processing, February 2013.
8 McLean, J. B. , G. S. Koermer and R. J. Madon, “Maximizing catalytic isobutylene selectivity,” paper from Engelhard.
9 McLean, J. B. and A. Witsoshkin, “Iso-olefins for oxygenate production using Isoplus,” NPRA Annual Meeting, March 1993.


The authors
Warren S. Letzsch has 46 years of experience in petroleum refining including petroleum catalysts, refining, and engineering and design. His positions have included R & D, technical service and sales, which led to senior management positions in sales, marketing and technology development and oversight. He was one of the developers of the Technip/Axens R2R process and has authored over 80 technical papers. Mr. Letzsch holds eight patents in the field of fluid catalytic cracking. He was the FCC/DCC Program manager at Stone & Webster for 10 years and is now a senior refining consultant for Technip as well as a private consultant to the refining industry.
Christopher Dean is an independent process engineering consultant with over 37 years in the worldwide refining business with an emphasis on high olefin fluid catalytic cracking (HOFCC) with petrochemical integration. He is the founder and principal consultant for High Olefins FCC Technology Services LLC. His worldwide refining background includes the development and commercialization of the High Severity-FCC Process, the development of several integrated refinery and petrochemical projects, catalyst technical service, process engineering, design and unit operations on a variety of refinery units. He has published or presented over 30 papers and has been issued two patents on FCC gasoline desulfurization and has three other FCC pending process patents. 




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