Light olefins, particularly ethylene
and propylene, are the most crucial building blocks for the petrochemical industry.1 The mounting demands for the
production of light olefins are much higher than for other chemicals, yet the
current worldwide supply of these important raw materials is inadequate to satisfy
demand. Moreover, the formidable challenges—such as excess manufacturing
capacity of the oil refining industry, the volatility of plant utilization rates,
and the forecasted plateau of refined oil consumption—compel oil-refining
companies to embark on a structural adjustment and transformation to their
operations.
Over the past few decades, the steam-cracking
process has been the dominant route to produce ethylene and propylene. However,
high energy consumption and low olefin selectivity do not align with increasing
environmental regulations and the current demand for light olefins.2
Consequently, the catalytic cracking process to produce light olefins was
adopted in the refining industry due to its low energy consumption, low carbon
dioxide (CO2) emissions and high selectivity of light olefins.3,4
Another factor that influences the
production of light olefins is raw materials; however, the proportion of heavy
feedstocks in raw materials has continuously increased in recent years. Therefore,
it is imperative to develop new units and corresponding catalysts to address these
challenges. This article will detail the design of a new catalyst to increase
the production of ethylene and propylene, along with an industrial application
of this new catalyst.
Catalytic pyrolysis process. Over the past several
decades, many companies have developed new technologies based on the modified
fluid catalytic cracking (FCC) process, which converts heavy feedstocks to
light olefins with high selectivity. One of these processes was invented in the
1980s by the authors’ company, which used a deep catalytic cracking (DCC) schemea.5
Although this process scheme is like
the traditional FCC process, some modifications have been made in the reactor
section of the DCC scheme, including the addition of a riser and a fluidized
dense-bed reactor. The addition of the fluidized dense-bed reactor is to
maximize the propylene yield by prolonging the contact time between the
catalysts and the gasoline products generated in the riser reactor. Further,
the authors’ company developed a proprietary catalytic pyrolysis processb
(CPP) by changing the operating parameters, catalyst formulation and unit
configuration of the original DCC scheme to maximize both ethylene and
propylene production from heavy crude oil.
Using the operating condition of the traditional
FCC process as the benchmark, TABLE 1 reveals the difference in the operating conditions
among FCC, DCC and the advanced CPP technologies. The
operating conditions of the CPP are much more severe than the DCC process,
which requires the corresponding catalyst to have excellent hydrothermal
stability while simultaneously maintaining good reactivity.
The first-generation catalystc
for the proprietary CPP technology was developed by the authors’ company and
first used in the Daqing Refining and Chemical Co.6,7 The phosphorus
and alkaline earth metal modified ZSM-5 was the main active component in the first-generation
catalystc.
When using vacuum gasoil (VGO) as the
raw material, the first-generation catalyst showed good stability
and selectivity in ethylene and propylene production. In addition, with the
modification of operating conditions, the P/E ratio could be further adjusted.
VGO is quite suitable for producing ethylene and propylene with high
selectivity since its major component is paraffin hydrocarbons, which are apt
to be transformed into light olefins. However, globally, feedstocks have become
heavier, which has resulted in decreased activity of the conventional first-generation
catalystc.
As shown in TABLE 2, the properties of the raw
materials used by Shaanxi Yanchang Coal Yulin Energy and Chemical Co. were more
challenging than the ones used in Daqing Refining and Chemical Co.’s facility.
The content of paraffin hydrocarbons decreased by 13.9%, while the content of
aromatics increased by 13.9%. Therefore, based on the property of the raw
materials, the authors’ company further designed and developed a new catalyst for
these more challenging feedstocks being processed by Shaanxi Yanchang Coal
Yulin Energy and Chemical Co. This was done by promoting the ring-opening
reaction of naphthenes and naphthenic aromatics. Meanwhile, the hydrothermal
stability of the new catalyst was further improved by modification.
Developing the new CPP catalyst. The primary component
of the CPP catalyst is ZSM-5 zeolite. Due to its strong acidity, good
hydrothermal stability, low hydrogen transfer activity and special pore
structure, ZSM-5 zeolite displays an excellent selectivity in CPP technology. However,
the pore size of ZSM-5 zeolite (0.54 nm × 0.56 nm) is smaller than the
molecular size of naphthene and naphthenic aromatics, which limits the
diffusion and the accessibility to the active sites of these molecules.8,9
Therefore, the authors’ company developed a new hierarchical ZSM-5 zeolite with
abundant mesopores, which increased the diffusion coefficient of the large
molecules and their accessibility to the acidic sites in the ZSM-5 channels.
This novel hierarchical ZSM-5 zeolite
has a crystal size of approximately 1.4 μm, and the size of the mesopores is approximately
10 nm (FIG. 1).
A nitrogen adsorption experiment was conducted to measure the surface area of the
new ZSM-5 zeolite. The results of the Brunauer-Emmett-Teller (BET) surface area
analysis are shown in TABLE
3. Compared to the conventional ZSM-5 zeolite, the new hierarchical
ZSM-5 zeolite provided higher SBET (392 m2·g-1)
and Vpore (0.255 mL·g-1). Additionally, the
surface area and the pore volume of the mesopores in the new ZSM-5 were
remarkably higher vs. conventional ZSM-5 zeolites. These results indicated that
the new hierarchical ZSM-5 preserved many more mesopores than conventional
ZSM-5 zeolites, which provided more easily accessed active sites on the
catalysts.
Besides the structured design of the ZSM-5
zeolite, a proper metal modification is generally considered as another way to
improve reaction conversion and selectivity. Metal salt was chosen to modify
the hierarchical ZSM-5, and this modified ZSM-5 was marked as M/ZSM-5. It was
found that the metal modification could significantly enhance the total acid
amount and the strong acid amount (TABLE 4), which could help promote the
reaction depth, thus converting more hydrocarbons into gas-phase products.10
FIG. 2 shows that, when using M/ZSM-5 as a catalyst,
both the yield and selectivity of light olefins are significantly improved vs.
the unmodified ZSM-5. A density functional theory (DFT) calculation was
conducted to investigate the mechanism of how the metal improved selectivity.
It was found that the metal ions accumulated at the corner of the zigzag
channel, causing a reduction in the volume of the corner. Due to this reduction, the energy barrier of the cracking
reaction, cyclization reaction and polymerization increased to different
extents—among which the energy barrier change of the cracking reaction was the
smallest, which, in turn, improved the selectivity of the catalytic cracking
reaction.
It was also found that the
modification of the metal enhanced the hydrothermal stability of the new ZSM-5
zeolite.11 Compared to the unmodified hierarchical ZSM-5, the metal
modification enhanced the retention of the total acid amount (TABLE 5). Further
study on the mechanism of this improvement of hydrothermal stability revealed
the unique structure of the M/ZSM-5, including the connection between metal ions
and framework oxygens, which impeded attacks on the aluminum framework from
water molecules (FIG. 3).
In addition to the development of new
zeolite material, it was also indispensable to focus on a new matrix. One of
the notable issues was that, due to the low efficiency of the cyclone used in the
regenerator, there was a significant loss of the fresh catalyst (20 μm–40 μm). Therefore, the
authors’ company considered reducing the amount of fresh catalysts to 0 μm–40 μm, which required a much
more stable catalyst.
Generally, it is contradictory to
increase the stability of catalyst and also maintain the accessibility of catalyst
simultaneously.12 The authors’ company managed to use hyperviscous
alumina sol as the binder in preparation of the new catalyst. As shown in TABLE 6, compared
with the previous catalyst that used regular alumina sol as a binder, the new
catalyst showed the same pore volume and a lower attrition index, indicating
better mechanical stability and an unchanged accessibility. The amount of Al13
aggregation in hyperviscous alumina sol is much higher vs. regular alumina sol,
which enhances the adhesive property of the binder. It is well known that a
catalyst’s matrix not only serves as the support for zeolite, but also as a Lewis
acid site to improve the conversion of raw materials. To adjust the amount and
strength of Lewis acid sites in the matrix, the authors’ company introduced a different
metal oxide into the conventional matrix. FIG. 4 shows the relationship between the
conversion of heavy oil and the coke yield. The modification of the matrix by
M1 and M3 can simultaneously improve the conversion and significantly reduce
the coke yield.
The industrial application of a new catalyst. Based on the experimental
results, a new catalystd was successfully developed. After a series
of laboratory experiments, the new proprietary catalystd was further
applied in an industrial CPP unit on November 12, 2021, at Shaanxi Yanchang
Coal Yulin Energy and Chemical Co.’s refinery. The devised processing
capability of the CPP unit was approximately 1.5 MMtpy. The authors’ company
designed the reaction-regenerator system. Considering the standard conditions
of changing catalysts and the common reasons for catalyst loss, the storage of
the novel proprietary catalystd in the unit reached 100% by the end
of November 2021. The catalyst changeout was performed without any aberrant
loss of catalyst.
The following are two
different case studies that reflect the potential benefits of using the novel
proprietary catalystd. The program guaranteed test run (PGTR) was
conducted November 12–15, 2021, using this novel proprietary catalystd.
The base case (BC) data was from 2020, when a different catalyst was used in
the same unit. It should be noted that, except for the increase of the inlet
rate, the differences between the basic operating conditions during the BC and the
PGTR were negligible (TABLE
7).
Properties of raw oil. It is necessary to analyze
the properties of raw materials, as they can exert a notable influence on the
catalytic performance. The properties of raw materials during the BC and the PGTR
are shown in TABLE 8.
Basically, the properties from both cases are quite similar, yet some
disparities remain. The carbon residue in the PGTR was about 5.5%, which is
0.81% higher than the carbon residue in the BC. The higher amount of carbon
residue would cause this increase in coke formation; therefore, compared to the
raw materials used in the BC, those used in the PGTR had an adverse effect on
the catalytic activity of the new proprietary catalystd.
Properties of equilibrium catalyst (Ecat). The properties of the
Ecat are shown in TABLE
9. The specific surface area, the microporous specific surface area
and the specific surface area of micropores of the Ecat in the PGTR were
discernibly higher than those used in the BC. The larger specific surface area is
conducive to the interaction between the organic molecules and acid centers on
the catalyst, and to the diffusion of big molecules in catalyst channels, thus improving
the conversion of heavy oil. These results indicated that the novel proprietary
catalystd achieved the designed goal with a greater capacity for the
conversion of heavy oil. In addition, the content of poisonous metal in the
novel proprietary catalystd is much higher than the catalyst used in
the BC, which indicated that the new catalyst has a better metal-tolerance
ability.
The distribution of CPP products and their properties. To show the potential
of the novel proprietary catalystd, the yields of the primary
products between the BC and the PGTR are provided here, as well. As shown in TABLE 10, with similar
operating conditions, the ethylene yield increased from 12.97% to 14.05%, and the
yield of propylene increased by 1.49%, which increased the total yield of light
olefins by approximately 2.6%. Moreover, coke yield decreased from 10.92% to
9.58%, which resulted in a decrease in CO2 emissions of
approximately 21,000 tpy. According to the processing capability and the
average prices of raw materials and light olefins, the increment of the profit
from the light olefins portion could reach $80 MM/yr.
Takeaways. Due to the trend of raw materials becoming
heavier, the requirements of new, efficient and more stabilized CPP catalysts
for the catalytic cracking of heavy feedstocks under harsher conditions are growing
rapidly. With the proper design of the zeolite material, the authors’ company produced
a novel proprietary catalystd. The industrial application of this
novel proprietary catalystd is promising. The yield of light olefins
could reach 36.5%, which is higher than the previous catalyst, while the yield
of coke is around 9.58% under operating conditions that are harsher than the
traditional DCC process. The authors believe that this specific catalyst shows
great potential and competence for the future when heavy oil will be the dominant
feedstock in processing operations. HP
NOTES
a Sinopec’s -Deep
-Catalytic Cracking technology
b Sinopec’s Catalytic
Pyrolysis Process
c Sinopec’s CEP-1
catalyst
d Sinopec’s Epylene
catalyst
LITERATURE CITED