The growing demand for propylene has intensified interest in maximizing propylene from refinery fluid catalytic cracking units (FCCUs). Refiners in Asia and the Middle East are setting the pace, with numerous new FCC units planned or already on- stream to take advantage of this opportunity. Each of these new units includes the most modern technology to achieve record high propylene and conversion.
On one hand, new records are set in the heaviness of feeds posing challenges to maintain activity; conversely, some FC- CUs are specifically designed for low sulfur (hydrotreated) gasoil (GO) feeds that impose a real challenge to meet the heat balance requirement of the FCCU. Achieving record high pro- pylene yield and conversion from a wide range of feed quality offers considerable challenges to the catalyst design, as well. High accessibility (diffusion), advanced zeolite and low rare earth (RE) technologies are critical components of modern catalyst designs required to meet the desired objectives. Also, the feed composition impact and process variables on the yields and heat balance are significant and thus requires a good under- standing of the chemistry to design the right FCC catalyst for individual FCCUs.
Naphtha cracking played a dominant role in manufacturing propylene for a long time, but the primary product is ethylene. The growth rate of propylene demand is outpacing that of eth- ylene. More recently, the FCC process is often a better solution for refiners to invest in. The FCC process has become a major refinery process to generate propylene. With many new units an- nounced and several units already in the process of engineering and construction, the importance of the FCC process to meet the growing propylene demand will increase. These new FCC units often operate at higher severity and up to the edge of what is possible today. The highest reactor temperatures and very high levels of shape selective ZSM-5 additives are almost never suffi- cient to meet new propylene targets, which seemingly hover at a plateau due to over-cracking, activity dilution and the absence of key catalyst design features. Many refiners are facing this plateau and have difficulty lifting the yield of propylene. A good under- standing of the cracking mechanisms involved is required, as is a focus on the quintessence of FCC catalyst design to maximize propylene yield and the mechanisms of propylene generation.
Understanding the mechanisms. In the refining industry,
several cracking processes are applied to cleave carbon-carbon bonds: hydrocracking, thermal cracking and, most importantly,
catalytic cracking. Hydrocracking will not be discussed further as it is of nearly no importance for the production of propylene. Cracking is either an acid catalyzed or a thermal, not a catalytic process. Though FCC is predominantly a catalytic process, some thermal reactions also take place.
Thermal cracking proceeds through a free radical process
and involves three steps: initiation, propagation and termina- tion. At high temperatures, radicals are formed in the initiation step. These radicals react further whereby ethylene is formed and new radicals, leading predominantly to ethylene, followed by methane and propane.
Catalytic cracking, however, makes use of acid sites in the
FCC catalyst and involves carbocations intermediates. More- over, in FCC, a higher degree of branching is found, giving evi- dence to isomerization reactions. FIG. 1 compares the products
of thermal and catalytic cracking of n-hexadecane at 500 °C.1 In
thermal cracking very little branched products are found. Looking back in FCC history, some remarkable changes occurred when going from amorphous cracking catalysts to zeolite-based cracking. This step change in the FCC catalyst technology led to a drastic increase in gasoline yield. Those are all improvements that were highly desired from the FCC pro- cess. But at the same time, the gasoline research octane number (RON) dropped dramatically, which was associated with the lower olefin content of the gasoline.
These yield and quality shifts were attributed to hydrogen transfer (HT) reactions, by which initially formed olefins are converted to more stable paraffins. And while gasoline olefins
0 20 40 60 80 100 120 140 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13 C14 C-number
Moles product/100 moles cracked
Thermal cracking quartz chips Catalytic cracking amorphous SiO2/Al2O3
FIG. 1. Comparison of products observed in thermal and catalytic cracking of n-hexadecane at 500°C.
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were reduced, light olefins also decreased with the change to zeolite-based cracking (TABLE 1).
HT is a bimolecular reaction, whereby hydrogen is, for in- stance, transferred from naphthenes to olefins, producing par- affins and aromatics. Brønsted acid sites of the Y-zeolite play a crucial role in these reactions. Catalyst manufacturers have several tools to optimize the characteristics and functionality of the Y-zeolite, such as unit cell size (UCS), silica-to-alumina ratio (SAR) and content of RE and sodium.
For the generation of propylene, other shape selective zeolites exist and are commonly applied. For example, the invention of ZSM-5 created another step change in FCC catalyst technology. Since then, other proprietary advanced zeolite technologies have been developed for maximum propylene applications.12 Howev-
er, because it is well-documented public knowledge that ZSM-5 additives are effective to create light olefins, including propylene, the fundamental chemistry discussed here will use ZSM-5 to help illustrate the mechanisms of propylene production.
Zeolite, such as ZSM-5 with a pore entrance smaller than of Y-zeolite, drives different catalytic reactions. The smaller pores restrict the access of branched and cyclic hydrocarbons into its interior and allow straight-chain and mono-methyl paraffins and olefins to enter, hereby generating predominantly propyl- ene and also butylenes and ethylene. The reactants are particu- larly in the C6 and C7 range, but other gasoline molecules with
a carbon atom number between 5 and 10 can also be converted by ZSM-5 to some extent. FIG. 2 shows clear evidence of the in-
creased production of C3, C2 and C4, and the strong reduction
in C6 and C7 components.
FCC gasoline can be typically classified into five types of hy- drocarbons: paraffins (P), isoparaffins (iP), olefins (O), naph- thenes (N) and aromatics (A). Taking a closer look at the dif- ferent types shows that isoparaffins and olefins with six or seven carbon atoms are mostly cracked by ZSM-5 (FIG. 3). When an
olefin is cracked by ZSM-5, two smaller olefins are produced. The generation of C6 and C7 olefins in the primary cracking steps
of FCC is therefore a valuable mechanism that needs managing.
The numbers on the bars in FIG. 3 show a reduction in the
specific hydrocarbon type. The decrease in light isoparaffins is the largest, but this is speculated to be due to reduced forma- tion of light isoparaffins in the presence of ZSM-5 and other reactions, including cracking to light olefins. The light olefins, however, contribute the most to propylene make, as olefins are more reactive and they crack to form two smaller olefin mol- ecules. In general, gasoline-range olefins are the primary reac- tants for propylene.
The molecules that can be readily cracked by ZSM-5 are in the gasoline boiling range and are often referred to as light-ole- fin precursors. Though isoparaffins are good light-olefin pre- cursors, the small pores of ZSM-5 restrict access to only those paraffins with a methyl branch. Longer branches or multiple branches are consequently undesirable. As Y-zeolite catalyses isomerization reactions, these reactions should be carefully controlled. The right-hand end of the x-axis in FIG. 3 shows
benzene and methylbenzene, which are two of the few gasoline components to increase slightly with the use of ZSM-5.
FCC manufacturers have a variety of tools available for de- signing the optimal catalyst for maximum propylene applica- tions, as these different catalyst components affect cracking, HT and isomerization.
Y-zeolite is the component most talked about. Its acidity is high and contributes to a high level of cracking. Though Y-zeo- lites can be varied in terms of UCS, RE content and SAR, their HT activity is relatively high, and they also possess some isom- erization activity. Lower RE content is thus strongly preferred for lowest HT.
ZSM-5 zeolite is also very high in acidity and thus has high cracking activity, though the cracking rate is highly determined by the size and shape of the reactants. Its high SAR is also re- sponsible for isomerization. Conversely, ZSM-5 has oligomer- ization activity, whereby propylene can be consumed.
Finally, matrices play another crucial role in FCC catalysis. Matrices are predominantly known as the components respon- sible for pre-cracking large molecules before they enter the zeo- lites. A wide range of matrices that vary in functionality, such as bottoms conversion capability and metal capturing power, can be applied. With their differences in functionality, the acidity is different and can be rated as low to moderate compared to Y- zeolite. Their cracking activity varies from low to high, while the HT is typically lower than that of Y-zeolite, as is the isomeriza- tion rate. FIG. 4 compares two different matrices, with different
0 2 4 6 8 10 12 14 16 18 C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C-number Yield, wt% FCC catalyst FCC catalyst + ZSM-5
FIG. 2. The effect of ZSM-5 on products distributed by carbon number.
-2.8 -2.1 -2.0 -1.7 -1.5 0.3 0.5 0 1 2 3 4 5
iP6 O6 iP5 iP7 O7 A7 A6 C-numbers and component type
Yield, wt% on feed
FCC catalyst FCC catalyst + ZSM-5
FIG. 3. Gasoline components that are most and least affected by ZSM-5.
TABLE 1. The change from amorphous cracking catalyst to zeolite-based cracking leads to lower olefi ns
Conversion Gasoline, vol% Propylene, vol%
Silica-alumina gel 75.5 47.5 8.5
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HT power. The one lowest in HT (as shown on the second Y- axis) leads to the highest C3=.
Next to HT, several other secondary reactions take place, such as cyclization and aromatization reactions. Moreover, propylene molecules can undergo oligomerization reactions, which means it is possible that propylene can be consumed and its yield decreases.2 Longer residence times are detrimental for
maximum propylene yields.
In summary, several factors must be controlled for maxi- mum propylene yield:
1. Maximize generation of gasoline precursors, which are predominantly C6 and C7 olefins and iso-paraffins
2. Minimize HT, which consumes valuable olefins
3. Control isomerization reactions to form methyl branches 4. Minimize other unwanted secondary reactions, such as cyclization, aromatization and oligomerization.
Optimizing unit conditions. The FCC process is very ver- satile and has the flexibility to be operated in different modes which fit specific yield slates and meet market demands. To achieve maximum propylene, the FCC unit is typically oper- ated in a high-severity mode, whereby a high conversion is achieved through high reactor temperature and high cat-to-oil ratios. These parameters can be optimized by the operators with relative ease. Other conditions that positively influence the pro- pylene yield are low hydrocarbon partial pressure and/or high steam partial pressure, use of naphtha recycle and the choice of a hydrogen-rich feed. The latter options, however, are not always easy to control. While process conditions can elevate the propyl- ene yield by a few percent, the choice of the catalyst or catalyst system can have a much larger impact on propylene, on the order of 6 wt%–8 wt% additional propylene on feed.
A high-severity operation is most favorable for propylene production. When unit limitations permit, the highest reactor temperature is commonly applied. Increasing reactor tempera- ture, however, also enhances thermal cracking reactions, with consequently more dry-gas production. A ratio that is commonly looked at in FCC is C4/(C3 + C4). A ratio of 0.68 denotes a high
degree of catalytic cracking, whereas values near 0.50 indicate a high contribution of thermal cracking.3 (These values are appli-
cable to FCC catalysts not designed for maximum propylene.) The effect of temperature on additional propylene make is dependent on the starting condition. In maximum propylene applications, reactor temperatures are already quite high and the amount of ZSM-5 is substantial. Under those conditions, the effect of an increased reactor temperature of 10°C modestly enhances propylene yield by about 0.8 wt%–0.9 wt%, as shown in TABLE 3.
It is generally accepted that cracking processes have higher effective activation energy than bimolecular HT reactions, so raising the reactor temperature increases the tendency for gaso- line olefins to crack to light olefins rather than to undergo HT.4
The impact of temperature on propylene make is evident in a thermal cracking process. Also, high severity FCC-type cata- lytic processes operate under much higher temperatures than FCC in order to maximize propylene.5, 6, 7
The effect of feedstock is important. Though a refiner may not have a wide choice of options, any considerations are valu- able. It is generally accepted that cleaner feeds with higher hy-
drogen content are easier to crack and increase propylene yield through higher conversion. Unwanted are feeds that are high in aromatics, since these components cannot be converted to gasoline precursors and thus not lead to propylene. A poten- tially attractive option to consider, though, is the use of recycle streams of specific components which are valuable as reactants for propylene production, such as light naphtha recycle or the addition of products of C4/C5 oligomerization. These can ei-
ther be added in the main riser or, even better, in a second riser when present.
An interesting case for demonstration purposes was pre- sented in 2007, in which Fisher Tropsch (FT) wax was cracked in a pilot plant.9 Blends in different ratios of equilibrium cata-
lyst and ZSM-5 additive olefins and octane maximization are applied. High conversion rates are obtained due to the high crackability of FT wax. While in common FCC operation the FCC catalyst contributes to the highest conversion. With the use of pure ZSM-5 additive, conversion levels above 90 wt% are achieved with a propylene yield of 18 wt%, as shown in TABLE 5.
Catalyst design considerations. With the understanding
of the chemistry and influence of operating conditions, what catalyst design should be applied for optimum propylene? In FCC, there is no generic design that fits all, since all FCC units
9 11 13 15 17 40 50 60 70 80 90 Conversion, wt% C3 = yield, wt% 0.00 0.10 0.20 0.30 0.40 HTI C3=: Matrix 2 C3=: Matrix 1 HTI: Matrix 2 HTI: Matrix 1
FIG. 4. Comparing the effect of different matrices on HT and propylene yield.
TABLE 2. Comparing FCC key components
Acidity Cracking HT Isomerization
ZSM5-Zeolite Very high High* – High
Y - Zeolite High High High Moderate
Matrices Low-
moderate
Low-High Low to moderate
Low
*Size and shape of molecules determines cracking rates
TABLE 3. The eff ect of reactor temperature on propylene yield
Unit 1 Unit 2
Reactor temp., °C 522 543 519 530
C3=, wt% Base +1.6 Base +1.0
C4/(C3+C4), - 0.58 0.56 0.59 0.57
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and operations are unique. Important is a good understanding of the specific unit, its constraints and the bigger picture of the type of operation, for instance, with respect to cleanliness or contamination of feed.
ZSM-5 additives are a very effective solution for increasing propylene yield in the FCC unit. This approach gives the refiner a great deal of flexibility, as additive usage can be adjusted ac- cording to changes in propylene demand and to optimize opera- tion within unit constraints such as wet-gas compressor loading. As explained before, ZSM-5 generates propylene by se- lectively cracking olefins in the gasoline boiling range. As the amount of ZSM-5 additive in the catalyst inventory increases, the incremental yield of propylene produced per percent of additive decreases. Propylene yield reaches a plateau once the ZSM-5 crystal concentration in the catalyst inventory reaches around 10%. The diminishing effectiveness of ZSM-5 at higher concentrations occurs largely because olefins in the gasoline be- come depleted.
It is the base FCC catalyst design and technology, not ZSM- 5 concentration, that define the maximum propylene yield that can be achieved in the FCC unit. FIG. 5 shows the amount of
propylene produced by a variety of ZSM-5 additives when used with four different host FCC catalysts. The yield response of
the ZSM-5 changes according to the properties of the host cata- lyst, and at high concentrations of ZSM-5, the propylene yield reaches a plateau level that is determined solely by technology inherent in the base catalyst.
The pore size of zeolite is too small for access by feed mol- ecules, and so FCC catalysts contain some alumina matrix components to pre-crack the feed into smaller components. The product of this primary cracking by the matrix is very ole- finic, and generates the gasoline olefin precursors that are then cracked into light olefins.
We can estimate the relative activity of FCC catalyst for gen- erating secondary reactions, using the hydrogen transfer index (HTI) for catalysts tested under constant conditions with the same feed. The HTI can be defined in several ways. One of them is the ratio of isobutane over total C4s.
Catalysts with a lower HTI generate fewer secondary reac- tions, and more gasoline olefins are preserved for cracking into light olefins. The four base catalysts tested in FIG. 5 demon-
strate the effect of HTI on propylene generation, with propyl- ene yield increasing as the HTI of the base catalyst is reduced.
Further testing under constant conditions demonstrates that suppressing HT is the key to maximizing propylene. In FIG. 6,
multiple catalysts were mixed with varying amounts of ZSM-5 additive. The relative propylene shows a strong correlation with the HTI of the base catalyst.
As suggested by the correlation obtained in FIG. 6, suppress-
ing HT reactions is the key to maximizing propylene yield, by maximizing the availability of olefin precursors for cracking.
There are several ways that catalyst properties can be modi- fied to suppress HT reactions. It is acid sites density and the close proximity of these acid sites in the zeolite that promote HT. One approach is to reduce the number of these acid sites by lowering the zeolite UCS. A lower zeolite UCS is typically achieved by reducing the RE content. A consequence of reduc- ing catalyst zeolite content or UCS is a reduction in catalyst ac- tivity for conventional catalyst technologies.
Low RE technology is critical to enable both maximum pro- pylene yield and overall performance at a lower UCS, or equiva- lently lower RE.10 Specialized low RE systems deliver minimum
HT without the conventional loss in activity and therefore prof- itability11 Key features for success include a high zeolite SAR for
maximum stability; advanced zeolite technology for propylene; strong, selective matrices for cracking with minimal HT; and a TABLE 4. Example showing eff ect of hydrocarbon partial
pressure on propylene yield
RxT, °C 553 551
RxP, barg 2.1 2.8
Steam, mol% on feed 71% 60%
C3=, wt% on feed 11.6 9.9
TABLE 5. Cracking FT-wax in FCC pilot plant leads to high propylene yields
Feed properties Operations
Type FT-Wax Unit Pilot plant
API, ° 43.1 RxT, °C 538
SG 0.81 CTO 4
S, ppmw 2.5 Catalyst ZSM-5 additive olefi ns and octane maximization
Sim Dist, 5%, °C 368 Conv, wt% 92
Sim Dist, 95%, °C 474 C3=, wt% 18 0 1 2 3 4 5 6 7 8 9 0 2 4 6 8 10 12 14 16 18 20 ZSM-5 crystal, wt% ⌬ propylene yield, wt%
Decreasing base catalyst HTI
FIG. 5.⌬ propylene yield vs. ZSM-5 crystal.
Δ C3 = log (1 + %ZSM-5) R2 = 0.95 1.0 1.5 2.0 2.5 3.0 3.5 0.26 0.28 0.30 0.32 0.34 0.36 0.38 0.40 Base catalyst HTI at 72 wt% conversion
SCT-RT test data, resid
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catalyst architecture with unsurpassed high mass transfer or dif- fusion character, tantamount to the highest catalyst accessibility. Another method of reducing HT is to produce catalyst par-