Acknowledgments

am grateful to the following individuals who played key roles in this ook's completion: Warren Letzsch of Stone & Webster Engineer- g Corporation; Terry Reid of Akzo Nobel Chemicals, Inc.; Herb elidetzki of KBC Advanced Technologies, Inc.; and Jack Olesen of race/Davison provided valuable input. My colleagues at RMS ngineering, especially Shari Gauldin, Larry Gammon, and Walt Broad ent the "extra mile" to ensure the book's accuracy and usefulness.

Preface to the

Second Edition

The first edition of this book was published nearly five years ago. he book was well received and the positive reviews were overhelming. My main objective of writing this second edition is to rovide a practical "transfer of experience" to the readers of the nowledge that I have gained in more than 20 years of dealing with arious aspects of the cat cracking process.

This second edition fulfills my goal of discussing issues related to e FCC process and provides practical and proven recommendations improve the performance and reliability of the FCCU operations. he new chapter (Chapter 9) offers several "no-to-low" cost modificaons that, once implemented., will allow debottlenecking and optimizaon of the cat cracker.

I am proud of this second edition. For one, I received input/feedback om our valued clients, industry "FCC gurus," as well as my colleagues RMS Engineering, Inc. Each chapter was reviewed carefully for ccuracy and completeness. In several areas, I have provided additional iscussions to cover different FCCU configurations and finally, both e metric and English units have been used to make it easier for aders who use the metric system.

Unfortunately, the future of developing new technologies for petroum refining in general, and cat cracking in particular, is not promis- g. The large, multinational oil companies have just about abandoned eir refining R&D programs. The refining industry is shrinking pidly. There is no "farm system" to replace the current crop of chnology experts. In cat cracking, we begin to see convergence and milarity in the number of offered technologies. Even the FCC atalyst suppliers and technology licensers have been relatively quiet

developing "breakthrough" technologies since the introduction of

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CHAPTER

Process Description

Fluid catalytic cracking (FCC) continues to play a key role in an tegrated refinery as the primary conversion process. For many finers, the cat cracker is the key to profitability in that the successful peration of the unit determines whether or not the refiner can remain mpetitive in today's market.

Approximately 350 cat crackers are operating worldwide, with a tal processing capacity of over 12.7 million barrels per day [1]. Most the existing FCC units have been designed or modified by six major chnology licensers:

1.ABB Lummus Global

2.Exxon Research and Engineering (ER&E)

3.Kellogg Brown & Root—KBR (formerly The M.W. Kellogg Company)

4.Shell Oil Company

5.Stone & Webster Engineering Corporation (SWEC)/IFP

6.UOP (Universal Oil Products)

Figures 1-1 through 1-3 contain sketches of typical unit configuraons offered by some licensers. Although the mechanical configuration individual FCC units may differ, their common objective is to grade low-value feedstock to more valuable products. Worldwide, out 45% of all gasoline comes from FCC and ancillary units, such

the alkylation unit.

Since the start-up of the first commercial FCC unit in 1942, many provements have been made. These improvements have enhanced e unit's mechanical reliability and its ability to crack heavier, lowerlue feedstocks. The FCC has a remarkable history of adapting to ntinual changes in market demands. Table 1-1 shows major developents in the history of the process.

The FCC unit uses a microspheroidal catalyst, which behaves like liquid when properly aerated by gas. The main purpose of the unit

Process Description

Vent to Main Column

cSi—^

Slurry

I

Feed Preheater

Figure 1-5. Typical feed preheat system.

ncreasing preheat temperature allows increased throughput. The effects f feed preheat are discussed in Chapter 6.

ISER—REACTOR—STRIPPER

The reactor-regenerator is the heart of the FCC process. In a modern at cracker, virtually all the reactions occur in 1.5 to 3.0 seconds efore the catalyst and the products are separated in the reactor.

From the preheater, the feed enters the riser near the base where it ontacts the regenerated catalyst (see Figure 1-6).The ratio of catalyst- o-oil is normally in the range of 4:1 to 9:1 by weight. The heat bsorbed by the catalyst in the regenerator provides the energy to heat he feed to its desired reactor temperature. The heat of the reaction ccurring in the riser is endothermic (i.e., it requires energy input). The irculating catalyst provides this energy. The typical regenerated catalyst mperature ranges between 1,250°F to 1,350°F (677°C to 732°C).

Pivot

Cyclone Dipleg

Restraint

PLAN

Pivot

Cyclone Dipleg*

Restraint

ELEVATION

Figure 1-8. Typical trickle valve (courtesy of Emtrol Corporation),

tripping Section

As the spent catalyst falls into the stripper, hydrocarbons are adsorbed n the catalyst surface, hydrocarbon vapors fill the catalyst pores, and e vapors entrained with the catalyst also fall into the stripper. tripping steam, at a rate of 2 to 5 Ibs per 1,000 lbs (2 kg to 5 kg r 1,000 kg,) is primarily used to remove the entrained hydrocarbons tween catalyst particles. Stripping steam does not address hydrorbon desorption and hydrocarbons filling the catalyst pores. Hower, reactions continue to occur in the stripper. These reactions are

Fluid Catalytic Cracking Handbook

Products

Reactor

Regen

Catalyst

Standpipe

LiftAir

ir Blower

Figure 1-10. A typical Model II using lift air to transfer spent catalyst.

Figure 1-13. A typical FCC main fractionator circuit.

The heaviest bottoms product from the main column is commonly alled slurry or decant oil. (In this book, these terms are used interhangeably.) The decant oil is often used as a "cutter stock" with acuum bottoms to make No. 6 fuel oil. High-quality decant oil (low lfur, low metals, low ash) can be used for carbon black feedstocks. Early FCC units had soft catalyst and inefficient cyclones with ubstantial carryover of catalyst to the main column where it was bsorbed in the bottoms. Those FCC units controlled catalyst losses o ways. First, they used high recycle rates to return slurry to the actor. Second, the slurry product was routed through slurry settlers.

The hydrocarbon vapors flow to the wet gas compressor. This gas ream contains not only ethane and lighter gases, but about 95% of e C3 and C4 and about 10% of the naphtha. The phrase "wet gas" fers to condensable components of the gas stream.

The hydrocarbon liquid is split. Some is pumped back to the main olumn as reflux and some is pumped forward to the gas plant, ondensed water is also split. Some is pumped back as wash to the verhead condensers and some is pumped away to treating. Some ight be used as wash to the wet gas compressor discharge coolers,

AS PLANT

The FCC gas plant (Figure 1-14) separates the unstabilizedgasoline d light gases into the following:

•Fuel gas

•C3 and C4 compounds

•Gasoline

C3's and C4's include propane, propylene, normal butane, isobutane, d butylene. Propylene and butylene are used to make ethers and kylate, which are blended to produce high-octane gasoline. Most gas ants also include treating facilities to remove sulfur from these products. The gas plant starts at the wet gas compressor. A two-stage centrifugal mpressor is typically employed. This type of compressor generally corporates an electric motor or a multistage turbine that is driven pically by high-pressure steam. The steam is exhausted to a surface ndenser operating under vacuum. It should be noted that there are CC units in which single-stage wet gas compressors are used.

In a two-stage system, the vapors from the compressor's first stage scharge are partially condensed and flashed in an interstage drum. he liquid hydrocarbon is pumped forward to the gas plant, either to e high pressure separator (HPS) or directly to the stripper.

The vapor from the interstage drum flows to the second-stage mpressor. The second-stage compressor discharges through a cooler the high pressure separator. Gases and light streams from other finery units are often included for recovery of LPG. Recycle streams om the stripper and the primary absorber also go to the high pressure parator. Wash water is injected to dilute contaminants, such as

The overhead product is totally liquefied in the overhead condensers. portion of the overhead liquid is pumped and returned to the tower s reflux. The remainder is sent to a treating unit to remove H2S and ther sulfur compounds. The mixed C3's and C4's stream can then be d to an ether or an alkylation unit. It can be fed to a depropanizer wer where the C3's are separated from C4's. The C3's are processed or petrochemical feedstock and the C4's are alkylated.

The debutanized gasoline is cooled, first by supplying heat to the tripper reboiler or preheating the debutanizer feed. This is followed y a set of air or water coolers. A portion of the debutanizer bottoms pumped back to the presaturator or to the primary absorber as lean il. The balance is treated for sulfur and blended into the refinery asoline pool.

asoline Splitter

A number of refiners split the debutanized gasoline into "light" and heavy" gasoline. This optimizes the refinery gasoline pool when lending is constrained by sulfur and aromatics. In a few gasoline plitters, a third "heart cut" is withdrawn. This intermediate cut is low octane and it is processed in another unit for further upgrading.

Water Wash System

The cat cracker feedstock contains low concentrations of organic ulfur and nitrogen compounds. Cracking of organic nitrogen comounds liberates hydrogen cyanide (HCN), ammonia (NH3), and other itrogen compounds. Cracking of organic sulfur compounds produces ydrogen sulfide (H2S) and other sulfur compounds.

A wet environment exists in the FCC gas plant. Water comes from e condensation of process steam in the main fractionator overhead ondensers. In the presence of H2S, NH3, and HCN, this environment conducive to corrosion attacks. The corrosion attack can be any or ll of the following types [2]:

* General corrosion from ammonium bisulfide * Hydrogen blistering and/or embrittlement

* Pitting corrosion under fouling deposits

Ammonium bisulfide is produced by the reaction of ammonia and ydrogen sulfide [2]:

1.Catalyst Addition Rate

A higher catalyst addition rate dilutes the concentration of metals and allows less time for the vanadium to get fully oxidized,

lkaline Earth Metals

Alkaline earth metals in general, and sodium in particular, are etrimental to the FCC catalyst. Sodium permanently deactivates the atalyst by neutralizing its acid sites. In the regenerator it causes the eolite to collapse, particularly in the presence of vanadium. Sodium omes from two prime sources:

•Sodium in the fresh catalyst

«Sodium in the feed

Fresh catalyst contains sodium as part of the manufacturing process. Chapter 3 discusses the drawbacks of sodium that are inherent in the resh catalyst.

Sodium in the feed is called added sodium. For all practical purposes, he adverse effects of sodium are the same regardless of its origin.

Sodium usually appears in the form of sodium chloride. Chlorides end to reactivate aged metals on the catalyst and allows them to cause more damage.

usually water soluble and are removed from raw crude in the desalter. However, some of these salts can be carried over with desalted crude.

•Processing of the refinery "slop." A number of refiners process the refinery slop in their desalter. This can adversely affect the

tep 2: 10%-90% slope

Mope = T(90%) - T(10%) = 1080 - 652

Slope = 5.3°F/percent off

tep 3: From Appendix 2, corrections to VABP are approximately -34°F for MeABP and -10°F for CABP.

Therefore:

MeABP = 851 - 34 = 817°F = 436°C

0.913

MeABP = 816°F (435°C)

n the absence of full distillation data, the K factor can be estimated sing the 50% point in place of MeABP.

In summary, the K factor can provide information about the aromaticity r paraffinicity of the feed. However, within the narrow range (K = 1.5-12.0), it cannot differentiate between ratio of paraffins, naphthenes, nd aromatics. To determine these ratios, other correlations, such as OTAL or n-d-M, should be employed.

he refractive index at this temperature. To use the n-d-M method, efractive index at 20°C needs to be estimated using published corelations. For this reason the n-d-M method is usually employed in onjunction with other correlations such as TOTAL. Example 2-3 can e used to illustrate the use of the n-d-M correlations.

Example 2-3

Using the feed property data in Example 2-1, determine MW, CA, CN nd Cp using the n-d-M method.

tep 1: Molecular weight determination by ASTM method.

1.Obtain viscosity at 100°F (37.8°C)

a. Plot viscosities at 130°F (54.4°C) and 210°F (98.89°C).

b.Extrapolate to 100°F, VIS = 279 SSU.

2.Convert viscosities from SUS to centistoke (csT):

a.From Appendix 6, viscosity @ 100°F = 60.0 cSt.

b.Viscosity @ 210°F = 7.37 cSt.

3.Obtain molecular weight:

a.From Appendix 5, H = 372 and MW = 430.

tep 2: Calculate refractive index @ 20°C from the TOTAL correlation.

RI(20) = 1 + 0.8447 x (SO)1'2056 x (VABP(deg C) + 273.16r°'0557

x (MW)-0'0044

RI(20) = 1 + 0.8447 x (0.913)!'2056 x 728.2^'0557 x 446^0044

RI,20) = 1.5046

tep 3: Calculate n-d-M Factors.

V = 2.51 x (RI(20) - 1.4750) - (D20 - 0.8510) = 0.0271

V = 2.51x (1.5046 - 1.4750) - (0.90 - 0.8510) = +0.0271

w = (D20 - 0.8510) - 1.11 (RI(20J - 1.4750) = +0.0226

w - (0.90 - 0.8510) - 1.11 x (1.5046 - 1.4750)= +0.0226

RI(2m = RI(t) + 6.25 x (t - 20) x 10"4

t = temp, °C

Example 2-5

With the refractive index @ 78°C = 1.4810, determine the refracive index @ 20°C.

RI(2()) = 1.4810 + 6.25 x (67 - 20) x 10"4

RI(20) = 1.5104

Note that the calculated RI(20) closely matches that using the TOTAL orrelation.)

Pretreatment of FCC feedstock through hydroprocessing has a umber of benefits including:

•Hydrodesulfurization (HDS)

•Hydrodenitrogenation (HDN)

•Hydrodemetallization (HDM)

•Aromatic Reduction

•Conradson Carbon Removal

Desulfurization of FCC feedstocks reduces the sulfur content of FCC