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 con-version 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 maximiz-ing catalytic activity within the parameters of the reaction sys-tem. Too much activity will give too low catalyst-to-oil (C/O) ra-tio, and it could lead to catalyst deactivation due to higher Δ coke.
As summarized in TABLE 2, the reactor temperature will usu-ally 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 vari-ables on an FCC operation.4 In this instance, reaction temper-atures 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 gaso-line. Increasing catalyst activity and the C/O ratio (by lowering feed temperature), and reducing the reactor pressure, will in-crease 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 fauja-site zeolite content and type and degree of exchange of the zeo-lite. 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 re-quire 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 mak-ing money for US refiners were the middle distillates, i.e., diesel, kerosine, heating oil and jet fuel. The entire refinery became fo-cused on maximizing middle-distillate products including the
TABLE 3. Major operating variables eff ects on FCCU
Scenarios Base Increase ROT Increase MAT
Case ID 0 1 2 3 4 5 6
ROT, °F 980 990 1,000 1,010 980 980 980
Feed preheat, °F 500 500 500 500 475 450 425
Catalyst MAT 70 70 70 70 71 72 73
C/O 6.32 6.49 6.67 6.83 6.29 6.28 6.27
Δ Coke 0.80 0.79 0.78 0.78 0.83 0.85 0.88
Regenerator temp., °F 1,323 1,329 1,335 1,342 1,334 1,343 1,353
HCO recycle, vol% fresh feed 0 0 0 0 0 0 0
Reactor pressure, psig 28 28 28 28 28 28 28
Yields, wt%
Dry gas 3.45 6.39 3.93 4.16 3.59 3.70 3.84
LPG 17.91 19.39 21.22 22.28 17.98 18.07 18.12
Propylene 5.14 5.75 6.46 6.91 5.17 5.20 5.23
Gasoline 52.45 51.84 50.79 50.42 52.66 52.88 53.06
LCO 14.87 14.08 13.32 12.60 14.50 14.11 13.74
Slurry 5.97 5.97 5.57 5.21 4.90 5.78 5.58
Coke 5.04 5.13 5.23 5.32 5.19 5.36 5.52
Conversion 79.17 80.37 81.49 82.51 79.74 80.32 80.87
Hydrocarbon Processing | JULY 201451
Refinery of the Future
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 gaso-line. A riser would be the preferred reactor design with no back-mixing. Contact time should be short, and the recycle of uncon-verted feed is required since the bottoms yield would be excessive.
Feed injection systems must vaporize the feedstock quick-ly while quenching the hot cataquick-lyst 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 distil-lates that are burned in the regenerator, thus reducing the load-ing to the gas plant. Better strippload-ing provides more operatload-ing flexibility to optimize other operating variables.
Regenerator. Regeneration of the catalyst should be effi-cient. 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 hy-drotreater is being used, then the operator should consider op-erating 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 con-versions, 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 resis-tant 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 pserve the integrity of the regular feed nozzles. Refiners have re-ported 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 ac-tivity.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
TABLE 3. Major operating variables eff ects on FCCU (cont.)
Increase C/O Increase recycle Reduce pressure
7 8 9 10 11 12 13 14 15
980 980 980 980 980 980 980 980 980
450 400 350 500 500 500 500 500 500
70 70 70 70 70 70 70 70 70
6.71 7.11 7.49 6.28 6.26 6.24 6.46 6.61 6.77
0.79 0.79 0.78 0.83 0.86 0.89 0.78 0.76 0.74
1,321 1,319 1,316 1,335 1,346 1,358 1,316 1,308 1,301
0 0 0 5 10 15 0 0 0
28 28 28 28 28 28 26 24 22
3.44 3.39 3.36 3.61 3.73 3.90 3.36 3.27 3.17
18.08 18.23 18.36 17.80 17.71 17.62 18.03 18.14 18.26
5.21 5.26 5.31 5.11 5.08 5.06 5.18 5.22 5.26
52.96 53.42 53.61 52.14 51.86 51.58 52.79 53.12 53.49
14.25 13.67 13.15 15.60 16.31 17.01 14.64 14.41 14.15
5.41 5.66 5.37 5.14 5.34 4.69 4.04 5.85 5.73
5.31 5.59 5.85 5.21 5.38 5.56 5.04 5.02 5.02
80.11 80.98 81.73 79.08 79.01 78.97 79.53 79.88 80.25
52JULY 2014 | HydrocarbonProcessing.com
Refinery of the Future
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.
Matrix activity should be maximized to give higher first-pass cracking, and the catalyst should have mainly interme-diate 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 conver-sions. TABLE 4 summarizes various operating variables to con-trol. Reactor temperatures of 930°F–950°F are used for GO, while resids may require 950°F–960°F to avoid excessive hy-drocarbon 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 im-portant 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. Isobutyl-ene 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. Equilib-rium concentrations of the various C4s are rarely achieved due to the reactivity of the butylenes. The isobutylene equi-librium sets the maximum amount of isobutylene that can be produced in a typical catalytic cracker.8 At equilibrium condi-tions, the percentage of isobutylene in the butenes stream var-ies from 46% at 950°F to 44% at 1,050°F.9 The actual percent-age 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 tem-perature and contact time also is important. Low reactor hy-drocarbon partial pressures are essential for minimizing hydro-gen 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 aro-matic stocks. More paraffinic feeds can operate at higher con-versions and generate more LPG. Typically, LPG has a hydro-gen content well above 15 wt%, so hydrohydro-gen-deficient feeds cannot make as many barrels of isobutylene. The percentage of the olefins may be lower, depending on the operating vari-ables 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
deter-Davison MAT 900°F, 16 WHSV, 3 C/O Midwest refiner feed
API° = 29.4, K = 12.2 Gasoline and distillate
Gasoline
FIG. 1. Effect of zeolite content on LCO yield at constant operating severity.
Zeolite input, wt% (Si, AI basis)
LCO, vol%
10 12 14 16
FIG. 2. Effect of zeolite concentration on LCO yields.
Refinery of the Future
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mine whether C3 or C4 olefins are formed. The main operating variable for producing more isobutylene is reactor tempera-ture. 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 reac-tions; thus, the amount of isobutylene will not be as high as that yielded by increasing reactor temperature. If the conver-sion is taken too high, then the higher reactivity of the isobu-tylene will diminish its yield.
Catalyst properties can have a very large effect on isobutyl-ene yield. The goal is to get conversion, but with minimum hy-drogen 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 desir-able, 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 zeo-lite 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 in-crease the isobutylene concentration since the gasoline that cracks within the medium-pore zeolite produces a concentra-tion 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 re-actor temperatures reach 1,030°F, the base of the riser is near 1,100°F. Since isobutane is a function of hydrogen transfer re-actions, the opposite factors for maximizing isobutylene gen-erally apply.
To minimize thermal cracking, the best conditions are good feed injection system, low slip factor riser, rapid and high sepa-ration of the spent catalyst and reactor products, short dilute phase contact times and/or low temperatures, and a highly ef-fective stripper and regenerator.
Feedstocks higher in hydrogen work best for the same rea-sons 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
TABLE 4. Eff ect of operating variables for increasing LCO yields Adjustment Problem
Catalyst activity Lower Poorer selectivity Carbon on catalyst Increase Poorer selectivity Hydrocarbon partial
pressure
1) Steam rate Increase Minor variable (mixing) 2) Unit pressure Decrease Gas comp. limit Combine feed temp. Increase Regen temp. increase Reactor temperature Lower Octane, olefi ns
Regenerator temp. Increase Metallurgy higher gas make Recycle, HCO Increase Inc. coke capacity Boiling range adj.
1) IBP Lower Poor economics fl ash pt. 10%
2) FBP Increase Color cetane spec.
Feedrate Increase Hydraulic limits
Catalyst to oil Lower Regen temp. increase Contact time
In reactor Lower Bottoms cracking
In dilute phase Lower Bottoms cracking