FCC-Catalyst Coking - American Chemical Society

Article pubs.acs.org/IECR

FCC-Catalyst Coking: Sources and Estimation of Their Contribution during Coker Gas Oil Cracking Process Gang Wang,†,* Ze-kun Li,† Yin-Dong Liu,‡ Jin-sen Gao,†,* Chun-ming Xu,† Xing-ying Lan,† Guo-qing Ning,† and Yong-mei Liang† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, 102249, China CNPC Research Institute of Petrochemical Technology, Beijing, 100195, China

‡

ABSTRACT: The coking phenomenon of coker gas oil (CGO) feedstock during fluid catalytic cracking (FCC) using a commercial equilibrium catalyst was investigated. Different types of coke formed via coker gas oil (CGO) catalytic cracking were also analyzed. The coke formed was composed of adsorption coke (Cad), dehydrogenation condensation coke (Cdh), and hydrogen transfer coke (Cht). Cad, derived from nitrogen compounds, adsorbed on the acid sites of the catalyst and accounted for 37 wt % of the total coking content under conventional reaction conditions. Cdh was formed by the dehydrogenation condensation of polycyclic aromatic hydrocarbons and accounted for about 43 wt % of the total coking content. The coking content of Cht was greatly determined by the degree of the secondary reaction. Coke selectivity can be decreased and Cht yield can be controlled by simultaneously increasing the reaction temperature and shortening the reaction time.

1. INTRODUCTION Fluid catalytic cracking is an important process for converting high-boiling-point petroleum fractions into gasoline and other low-boiling-point fractions. The utilization of coker gas oil (CGO) as one of the main feedstock materials for fluid catalytic cracking (FCC) has become an outstanding problem. CGO use has affected the efficient use of heavy oil with the rapid expansion of the production capacity of a coking process.1 For several decades, CGO has been supposed as difficult to process because of its hydrogen deficiency as well as high aromatic and nitrogen contents, especially basic nitrogen contents.2−4 These characteristics have resulted in lower conversion, inferior product distribution, and a limited blending ratio of CGO in the CGO−FCC process. To enhance the blending ratio and avoid the unpleasant effects of CGO catalytic cracking, extra feedstock treatments or alternative processes have been recently proposed. One process involves pretreating CGO before being used as FCC feedstock. Such pretreatment processes include hydro-treatments,5 uses of solid adsorbents,6 extraction using an immiscible solvent,7,8 and neutralization by acid additives.9 The other method involves specialized processes such as denitrified catalytic cracking,10 twostage riser FCC,11 and partitioning conversion FCC.12 In these extra treatments or alternative processes optimal options include removing unfriendly components in the first step, or processing the CGO in a different reaction zone. However, remarkable achievements have not been reached. Therefore, additional insight on the CGO catalytic cracking process is required to further improve catalytic processes and selectivity. Catalytic cracking is a parallel-series reaction. A portion of the feedstock is converted into coke and gas as final products. The coke temporarily deactivates the active sites of the catalyst by poisoning, pore blockage, or both.13,14 Jiménez-Garcι ́a et al.15,16 proposed a theoretical approach to estimate the kinetic frequency factors with improved accuracy from lab-data of the catalytic cracking process. They think the pore blockage might disable a number of active sites more than site coverage. © 2012 American Chemical Society

The catalytic cracking processes of vacuum gas oil (VGO) and vacuum residues (VR) have different composition and deactivation mechanisms. Compared with raw feedstocks such as VGO and VR, the composition of CGO is very different. CGO is characterized by high contents of both nitrogen and basic nitrogen compounds.17,18 Therefore, further investigation of the catalytic cracking chemistry of CGO is necessary. Nitrogen compounds in CGO and their effects on acid sites have been systematically investigated by some researchers.4,19,20 The presence of basic nitrogen molecules in CGO is responsible for the temporary deactivation of FCC catalysts due to preferential adsorption onto acid sites. This deactivation results in the decreased density of acid sites and the retardation of the CGO−FCC performance. However, the simple acid−base theory can only partly explain the results of catalytic cracking process for CGO due to its complex composition. The coking path during a catalytic cracking process should fully explain the reaction mechanism, which will be helpful for controlling CGO coking process. The work presented here investigated the catalytic cracking of CGO and analyzed the coking behavior of each component. The purpose is to reveal the restrictive factors and influencing mechanisms of the process. VGO and CGO were used as feedstocks to analyze the constraints in CGO catalytic cracking. The coked catalysts obtained from the VGO and CGO catalytic cracking reactions were characterized by multiple techniques. Additionally, to obtain deep understanding of coking in CGO catalytic cracking, the coke type and its formation mechanism were analyzed.

2. EXPERIMENTAL SECTION 2.1. Feedstocks and Catalyst. Dagang CGO was chosen as the experimental feedstock, and Daqing VGO was used as Received: Revised: Accepted: Published: 2247

June 8, 2011 December 13, 2011 January 5, 2012 January 5, 2012 dx.doi.org/10.1021/ie2012328 | Ind. Eng.Chem. Res. 2012, 51, 2247−2256

Industrial & Engineering Chemistry Research

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in a gas buret by water displacement. And the catalysts were unloaded from the reactor and characterized. The gas products were analyzed by an Agilent 6890 gas chromatograph. The collected liquid products were weighed and then analyzed by simulated distillation using another Agilent 6890 gas chromatograph according to the ASTM-2887-D method. The purpose was to determine the yields of gasoline (IBP to 205 °C), light cycle oil (LCO; 206 to 350 °C), and heavy oil (above 350 °C). Coke content over catalysts was measured by a HV-4B microcomputer digital display analyzer. 2.3. Physicochemical Characterization of the Catalyst Samples. The spent catalysts were characterized by various physicochemical techniques. The Fourier transform infrared absorption (FT-IR) spectra of chemisorbed pyridine (Py-IR) were conducted using a FT-IR spectrometer (BIO-RAD, FTS3000) equipped with an in situ cell containing CaF2 windows. Brönsted and Lewis acid sites were distinguished by the bands of chemisorbed pyridinium ion at 1540 cm−1 and coordinative bonded pyridine at 1450 cm−1, respectively. The band at 1490 cm−1 was associated with pyridine adsorbed on both Brönsted and Lewis acid sites. The Brunauer−Emmett− Teller surface areas and pore volume measurements of the catalysts were conducted on a volumetric adsorption apparatus (ASAP 2020 M, Micromeritics Co., Atlanta, GA) at −196 °C using N2 as adsorbate. The samples were evacuated at 350 °C for 5 h under a vacuum of 1.33 MPa prior to analysis, and the pore volume and pore-size distributions of the catalysts were derived from the adsorption branches of the isotherms using the Barrett−Joyner−Halenda method. Scanning electron microscopy (SEM) images were observed and energy disperse spectroscopy (EDS) results were obtained on a Cambridge S-360 electron microscope.

the contrast feed. Their properties are listed in Table 1 and Table 2. CGO was characterized by high contents of nitrogen Table 1. Properties of Feeds feeds

density, kg·m−3

carbon residue, wt %

H/C ratio

S, wt %

N, wt %

basic nitrogen, wt %

CGO VGO

910 801

0.123 0.050

1.62 1.89

0.78 0.46

0.54 0.02

0.16 0.02

Table 2. Group Composition of Feeds group composition, wt % heavy medium light aromatic aromatic aromatic saturated feeds hydrocarbon hydrocarbons hydrocarbons hydrocarbons resin asphaltene CGO VGO

60.84 83.00

6.87 7.50

13.49 3.50

10.11 4.50

8.35 1.30

0.34 0.00

(especially basic nitrogen), heavy aromatics, and residues compared with VGO. A commercial equilibrium FCC catalyst (Cat E) was employed. Its details of properties are listed in Table 3. Table 3. Properties of Catalyst heavy metal, μg·g−1

microactivity

surface area, m2·g−1

66

102

micropore volume, cm2·g−1

packing density, g·mL−1

Ni

0.031 0.90 12806 Particle Size Distribution, v%

V

Na

492

3600

0−20 μm

20−40 μm

40−60 μm

60−80 μm

80−105 μm

>105 μm

0

13

30

25

18

14

3. RESULTS AND DISCUSSION 3.1. Analysis of the Constraints in CGO Catalytic Cracking. Table 4 shows the catalytic cracking performance of VGO and CGO on CatE under the same reaction conditions. The conversion of CGO is 20.39 wt % lower than that of VGO. The coke selectivity (coke yield/conversion) of CGO is 2.4 wt % higher than that of VGO. Their coke and dry gas yields show a little difference, which indicates that the coking tendency of CGO is much more serious. From the comparison of VGO and CGO with respect to properties and catalytic cracking performance that have been described above, a further study on the catalytic cracking of CGO is necessary for understanding its catalytic cracking constraints. Normally, the depth of feedstock-conversion during FCC reactions is restricted internally by its own cracking nature, and externally by the changes in the catalytic activity. Therefore, experiments were implemented by gradually increasing the catalyst-to-oil mass ratio (CTO) to create more catalyst-active sites for investigating the cracking potential of CGO. The results are shown in Figure 3. The conversion of CGO increases by 14 wt %, when CTO increases from 4.5 to 8. Given adequate catalytic active sites, the conversion of CGO reaches higher than 85 wt %, and can be further promoted if other impetuses are supplied. This finding shows that the conversion potential of CGO is high. To understand the reason for the low conversion of CGO in FCC reactions further, VGO with few nitrogen, heavy aromatics, and residues was chosen as the contrast feed, to evaluate the reaction performance of the coked catalysts Catvc and Catcc obtained from VGO and CGO catalytic cracking, respectively. The results shown in Table 5 indicate the large

During the tests, all the mass balances were between 97 and 100 wt % of the feed. In the present study, conversion was defined as the sum of dry gas, liquid petroleum gas (LPG), gasoline, diesel, and coke. The total liquid products were referred to as the sum of liquid petroleum gas, gasoline, and diesel yields. The selectivity was defined as the percentage of yield to conversion. 2.2. Experimental Apparatuses and Product Analyses. The researches were mainly performed in a fixed fluidized bed reactor and a pilot test riser FCC unit. The coked catalysts were prepared in the pilot riser FCC unit shown in Figure 1. The FCC unit comprised a feed injection system, a reaction-regeneration system, a product recovery and measurement system, a control system, as well as other auxiliary equipments. The equipment adopted a level of parallel flow design, continuous reaction−regeneration operations, and electric heating devices. Small spent plug, flue gas control, and cracking gas control valves were installed to accommodate the reaction temperature, catalyst-to-oil ratio, and residence time. Comparison tests between the CGO and VGO coked catalysts were carried out in a fixed fluidized bed reactor. The schematic of this apparatus is presented in Figure 2. The details about the system can be found elsewhere.21 As a whole, it is a batch system operated in a fluidized mode. The fully regenerated catalyst was used at the start of each run. Feed was introduced into the reactor by an oil pump. During the reaction, the liquid products were collected in the glass receivers located at the exit of the reactor, and a cold bath was kept for the liquid products. The gaseous products were collected 2248

dx.doi.org/10.1021/ie2012328 | Ind. Eng.Chem. Res. 2012, 51, 2247−2256


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Figure 1. Pilot test riser FCC unit.

Figure 3. Effect of the catalyst-to-oil mass ratio on the reaction (fixed fluidized-bed reactor, reaction temperature, 510 °C; WHTV, 15 h−1).

Figure 2. Schematic diagram of the fixed fluidized-bed reactor: (1) constant-temperature box; (2) steam furnace; (3) feedstock; (4) electronic balance; (5) oil pump; (6) water tank; (7) water pump; (8) preheater; (9) reactor furnace; (10) thermocouple; (11) reactor; (12) inlet and outlet of thecatalyst; (13) filter; (14) condenser; (15) collecting bottle for liquid products; (16) gas-collect tank; (17) beaker; (18) gas sample bag.

activities of Catcc significantly decrease, whereas the conversion and liquid yield of VGO decrease by 13.34 and 13.81 wt %, respectively. Therefore, the coking paths of CGO and VGO during FCC are different, and resulted in huge differences in their catalyst-active sites and cracking performances.

difference in the reaction performances between the two catalysts at similar coking rates. Compared with Catvc, the

Table 4. Product Distribution of CGO-FCC and VGO-FCC (Reaction Temperature, 510 °C; CTO, 6; WHTV, 15 h−1) product distribution, wt % feed

dry gas

LPG

gasoline

diesel

heavy oil

coke

conversion, wt %

selectivity of coke, %

VGO CGO

1.86 1.91

18.32 11.56

49.92 27.61

18.21 26.37

5.83 26.22

5.86 6.33

94.17 73.78

6.22 8.58

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Table 5. Reaction Performance Comparison of Coked Catalysts for VGO (Reaction Temperature, 510 °C; CTO, 6, WHTV, 15 h−1) product distribution, wt % catalyst

dry gas

LPG

gasoline

diesel

heavy oil

coke

conversion, wt %

liquid yield, wt %

Catvc Catcc

1.25 0.69

18.32 10.08

40.63 32.86

19.85 22.05

16.61 29.95

3.34 4.37

83.39 70.05

78.80 64.99

Coke formation during CGO catalytic cracking decreases the catalyst activity, which is one of the key factors in achieving a low feedstock CGO conversion. 3.2. Characterization of the Coked Catalysts. 3.2.1. Brö nsted and Lewis Acidity of Catalysts. Brönsted (B) and Lewis (L) acid sites are active sites on FCC catalysts. Corma22 and Chen23 studied the effects of the two acid sites on catalysts during VGO catalytic cracking reactions. The results show that alkane molecules need the proton offered by L-acid sites at the initial cracking reaction. The initial cracking reaction rate accelerated as L-acid sites increase. As unsaturated components in the products increase, B-acid site gradually takes on an important role in succeeding reactions. The concentration of the catalyst-active sites decreases due to coke deposition during FCC reactions. To reveal the effects of coke from different feedstock compositions on acid sites, FT-IR spectra of chemisorbed pyridine (Py-IR) were used to distinguish Band L-acid sites on the Catvc and Catcc. The results are listed in Figure 4. The changing contents of acid sites according to the peaks changes in FT-IR spectra are presented in Table 6. Figure 4 displays the Py-IR spectra of CatE, Catvc, and Catcc. In the region 1400−1600 cm−1, three peaks due to C−C stretching vibrations of pyridine are observed. The peak at about 1450 cm−1 is assigned to pyridine adsorbed on L-acid sites, the peak at about 1540 cm−1 is characteristic of pyridine adsorbed on B-acid sites, while the peak at 1490 cm−1 appears commonly for pyridine adsorbed on both B- and L-acid sites. From the Py-IR spectra, we can see that the number of acid sites on Catvc is much larger than that on Catcc. Table 6 shows that coke from different sources retards the acid sites at different degrees. The total acid sites contents of Catvc and Catcc decrease by 36% and 88%, respectively, compared with CatE. The loss of L- or B-acid sites are decreased by 30%−40% after VGO coking (Catvc) and by more than 75% after CGO coking (Catcc); moreover the decrease of peak amplitude for L-acid sites (Figure 4) is higher than that for B-acid sites. These results show that the present process tends to form coke selectively on L-acid sites. Basic nitrogen compounds are condensed aromatics with three or more rings.17−20 They can easily provide electron pairing to form strong chemical adsorptions. Therefore, the basic nitrogen compounds in feedstocks are the main chemical species for the decrease of the L-acid sites content on the Catcc surface. Previous studies of the coking phenomenon from VGO and VR fractions indicated that the coke from VGO uniformly deposited on the catalyst channel and covered parts of acid sites.24 Gum and asphaltene were not able to go straight into the catalyst channel. Instead, they were slightly deposited on acid sites, leading to a higher activity of the VR coked catalyst than that of VGO. Moreover, the effect on the catalyst activity of CGO coking during the reactions obviously exceeded that of VGO. Therefore, the process must have its own characteristics, which may explain the fact that the blending ratio of CGO is much lower than that of VR.

Figure 4. Py-IR spectra of CatE, Catvc, and Catcc: (a) at 200 °C; (b) at 350 °C.

Table 6. Changes in Acid Sites Content of Coked and Equilibrium Catalysts (Reaction Temperature, 510 °C; CTO, 6; WHTV, 15 h−1)

CatE Catvc

Catcc

weak acid

strong acid

catalyst

L

B

L

B

total acid sites content

initial acid sites content, au residual acid sites content, au loss ratea,% residual acid sites content, au loss ratea,%

3.6

2.0

4.7

1.8

12.1

2.5

1.2

2.7

1.3

7.7

31 0.2

40 0.5

43 0.4

28 0.3

36 1.4

94

75

91

83

88

Loss rate = (initial acid sites content − residual acid sites content)/ initial acid sites content.

a

3.2.2. Surface Area and Pore Volume. There are two coking paths leading to catalyst deactivation. One is the coverage of acid sites, and the other is the blockage of pores. The strong acid sites of a catalyst are supplied by a molecular sieve, whereas parts of the weak acid sites are supplied by the matrix. The formation of adsorption coke covered some acid sites, but the relation of the decreased Catcc acid sites content with the block2250



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age of pores needs further discussion. Zeolite, which has abundant porous structures, greatly increased the specific surface areas and catalytic efficiencies of catalysts. The static cryogenic adsorption volumetric method was used to characterize the pore distributions and surface areas of the catalysts pre- and postcoking to determine the coking effects. The results are shown in Figure 5, Figure 6, and Table 7.

Table 7. Changes in the Specific Surface Areas and Pore Volumes of Coked and Equilibrium Catalysts (Reaction Temperature, 510 °C; CTO, 6; WHTV, 15 h−1) Catvc items SBETa, m2·g−1 Smicra, m2·g−1 Sextea, m2·g−1 Vmicrb, mL·g−1 Vpb, mL·g−1

CatE

residual value

Catcc

loss rate, %

residual value

loss rate,c%

95.2

83.2

12.6

76.6

19.5

54.1

48.5

10.4

40.5

25.1

41.5

41.1

1.0

34.2

17.6

0.0282

0.0254

9.9

0.0201

28.7

0.143

0.139

2.8

0.117

18.2

BET method. bt-Plot method. cLoss rate = (initial value − residual value)/initial value.

a

pore and macropore increase with decreased micropore, especially the mesopore. The micropores of the FCC catalyst are all on the molecular sieve, and the mesopores are all on the matrix. Changes in the specific surface areas and pore volumes between the coked and not-coked catalysts show that the catalyst capacity of the molecular sieve is 50 wt % stronger than that of the matrix.26 The catalytic activity of the molecular sieve is also decided by the micropores. Obviously, feedstock molecules diffuse into the channel of the molecular sieve in VGO cracking reactions, and their activities were fully utilized. Compared to Catvc, the change in the pore distribution of Catcc is smaller. This result implies that the coking reaction of CGO is a multivariate process and not solely concentrated on the micropore of the molecular sieve. The coking distribution needs to be analyzed further. Table 7 shows that the cokes produced by the two feedstocks all lead to decreasing specific surface areas and pore volumes, especially for Catcc. Compared with CatE, one obvious characteristic of Cat cc is the loss of micropore aggravates. The external specific surface area decreases by 17.6% at the same time. The external specific surface area of Catvc only decreases by 1% because the surface property of Catvc is mainly caused by the micropore. The coking sites can also be concluded as different. When the data of pore distribution are combined, it is concluded that coking during VGO catalytic cracking takes place in the micropore, and a little amount is deposited outside the channel. On the

Figure 5. N2 adsorption−desorption isotherms for catalysts (The coked catalysts were obtained under the reaction conditions of reaction temperature, 510 °C; CTO, 6; WHTV, 15 h−1).

Figure 5 shows the N2 adsorption−desorption isotherms of CatE, Catvc, and Catcc. From Figure 5, one can find that the isotherms of all the three samples exhibited IV type isotherm curve. The adsorption isotherms gradually rise below P/P0 = 0.8, and ascend sharply beyond P/P0 = 0.8, but do not reach stable status even when P/P0 = 1.0, which shows that nitrogen vapor confined in the pore of sample particles condenses at a pressure lower than its saturation pressure, which is called “capillary condensation”. In the case of Catvc, a long and narrow hysteresis loop was observed in the isotherms, extending even to the low pressure area, which suggests that the character of micropore molecular sieves was reserved after being reacted with VGO,25 while for the Catcc, the “capilillary condensation” phenomenon was not observed in the low pressure area, indicating that the coke had covered the micropores. What’s more, the hysteresis loop broadened for the Catcc, suggesting that the coke jammed the pore mouth, and therefore, the diffusion resistance within pores increased. Figure 6 shows that the pore distributions for Catvc and Catcc changed in different degrees compared with CatE. The meso-

Figure 6. Pore size distributions in the coked and equilibrium catalysts (The coked catalysts were obtained under the reaction conditions of reaction temperature, 510 °C; CTO, 6; WHTV, 15 h−1). 2251



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Figure 7. SEM images of the coked and equilibrium catalysts (coked catalysts were obtained under the reaction conditions of reaction temperature, 510 °C; CTO, 6; WHTV, 15 h−1).

3.3. Characteristic Analyses of Coking in CGO− FCC. 3.3.1. Determination of the Reaction Type for Coking Reactions. Previous studies have revealed the formation of different coke species and distributions on the catalyst surface after CGO catalytic cracking compared with that of conventional FCC feedstock. Their effects on the catalyst activity were much more serious such that re-evaluating the coking reaction had become necessary. Researchers have deeply investigated the coke precursors and the kinetics of coke formation during VGO and VR catalytic cracking reactions. The cokes produced by VGO were stably formed in the secondary reaction during catalytic cracking. During VR catalytic cracking, the severity of coking from the initial reaction step decreased from external to internal surface area of the catalyst, and the opposite took place during the secondary reactions. Therefore, coke yields under different conversions were obtained by a series of experiments. For each reaction product, the yield was plotted against the conversion. The plots were enveloped by a single curve, called the optimum performance envelop (OPE), which corresponded to the selectivity curve for that product.27,28 The OPE is used to determine whether the observed reaction products are primary or secondary, and to calculate the initial selectivity by determining the slopes of the curves at zero conversions. As shown in Figure 8, a tangent was drawn through the origin of the OPE curve to characterize the initial selectivity of coke to estimate the reaction type of coking. Figure 8 shows that the initial selectivity of coke

other hand, for CGO, coke must also have been deposited outside the channel. At similar coking rates, the coking content in the micropore of Catcc is obviously lower, but the loss rate of the micropore volume is higher by 14.7% than that of Cat vc. This result shows that the pore blockage in Catcc is more serious. According to the results of distillate oil, all the coke is concentrated in the catalyst micropore when coke formation is low. As coke content accumulates (1−3 wt %), parts of the coke are disposed in large channels. With further increased coke content, the pore entrance is blocked, and the coke is disposed on the external surface area. Consequently, the catalyst is deactivated. The additional coke on the matrix surface has a little effect on the catalyst activity before pore entrance blockage. The coke formed during CGO catalytic cracking process accumulates on the external surface even under a lower coke rate. Consequently, there is a serious loss in the activity sites. The coking material possibly does not go deep into the catalyst channel, but preferentially deposits on the particles outside the surface. The catalyst capability of the external surface is limited, resulting in the blockage of the catalyst channel. The morphologies of the coked catalysts need to be observed to verify this hypothesis. High-resolution scanning electron microscopy (SEM) is an efficient method to realize this goal. 3.2.3. SEM Analysis. To clarify changes in the catalysts after CGO−FCC performance, SEM was used to characterize their surface morphologies of catalysts before and after reaction with VGO (Catvc) and CGO (Catcc). Figure 7 shows the SEM results of the coked catalyst surface and the severity of its coverage. Compared with CatE, the image of Catvc can be clearly seen, whereas the catalyst morphology of Catcc cannot, since its surface is surrounded by batt-like coke. On the basis of the foregoing results, coke can be inferred as disposed on the catalyst surface during CGO catalytic cracking reactions. This phenomenon leads to the formation of a strong “shielding effect” by the packed batt-like materials leading to pore mouth blockage. Consequently, most pore channels of the catalyst particles are blocked, and the reaction molecules cannot freely go in and out. Hence, further reactions are constrained, even when the active sites within pores are still active. These findings show that coke disposed on the particle surface is another important cause of catalyst deactivation during CGO catalyst cracking reactions. To clarify changes in the catalysts after CGO−FCC performance, SEM was used to characterize the surface morphologies of the catalysts before and after reaction with VGO (Catvc) and CGO (Catcc).

Figure 8. OPE curve of the coke yield in CGO−FCC (the fixed fluidized-bed reactor).

is above zero and that the OPE curve is above the tangent. The coke yield and initial selectivity tangent deviate a little when the conversion is low (below 50 wt %). Moreover, the coke content 2252



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ascends when the conversion reaches a certain value. This indicates that coke is formed even in the beginning of the reaction, and considered to be produced by the primary reaction, which is defined as primary coking reaction. As the conversion increases (above 50 wt %), the coke yield quickly increases due to the increasing contribution of the secondary reaction to coking, and we define this process as secondary coking reaction. Coke formed in the primary reaction is mainly caused by the coking potential precursor (components which tend to form coke in the feedstock during catalytic cracking reactions, such as condensed aromatics, basic or nonbasic nitrogen compounds) in the feedstock, and its limit yield (at the turning point of the curve) can be regarded as the measure of the potential feedstock coking content. In the present study, the number is 4.2 wt % for the feedstocks. 3.3.2. Analysis of the Coking Mechanism for CGO Catalytic Cracking. Normally, there is a connection between the coking mechanism and feedstock composition. The characteristic coking in CGO−FCC and the feedstock composition properties are inseparable. As previously mentioned, CGO was characterized by high aromatic and nitrogen contents (Table 1 and Table 2). Therefore, we sought to determine the changes in the coking mechanism during CGO catalytic cracking. The competitive adsorption effects of these two nonideal components during CGO catalytic cracking have already been previously discussed.29 When CTO is low, the acid sites do not balance the adsorbate species within the feedstock. Therefore, components with strong absorption abilities are preferentially adsorbed, and thus hinder normal reactions for hydrocarbon molecules with strong reaction abilities. The catalyst pore shows a step distribution. Large molecules are not able to enter the microspores owing to diffusing limits. They have to be cracked into smaller molecules on the surface of the matrix or in a larger channel. As a result, the initial cracking performance of every component is very important. A previous study showed that basic nitrogen compounds in oils are in the forms of pyridine, quinoline, benzopyridine, and dibenzopyridine.18 Nonbasic nitrogen compounds are types of pyrrole, benzopyrrole (indole), and dibenzopyrrole (carbazole).30 The adsorption abilities on the acid sites of these nitrogen compounds are far stronger than those of hydrocarbons, but their reaction abilities are much weaker. As a product of deep thermal cracking, CGO contains nitrogen compounds with even higher condensation degrees. Consequently, these compounds in CGO are preferentially adsorbed on the catalyst surface, and coke is formed. We refer to these cokes as the adsorption coke (Cad). The distribution of nitrogen in the products of CGO catalytic cracking had been reported in literature.31 Nitrogen content in coke ascends with the descending of CTO, implying that due to competitive adsorption, the adsorption of nitrogen compounds becomes dominant, thus leading to the decrease of active acid sites supplied by catalysts. A shorter catalyst−oil contact time corresponds to lower nitrogen contents detected in heavy oil. Therefore, the conversion of nitrogen compounds is higher than that of other components when the catalyst−oil contact time is short. Table 8 shows the nitrogen distribution in the products of CGO catalytic cracking under different temperatures. More than 60 wt % nitrogen from the feedstock is transformed into coke, less than 8 wt % into diesel, and less than 1.4 wt % into gasoline. These results sufficiently proved that most nitrogen compounds preferentially formed coke on

Table 8. Nitrogen Distribution (wt %) in the Products during Catalytic Cracking Reaction (CTO, 6; WHTV, 15 h−1) reaction temperature, °C fractionated products

470

490

510

530

550

gas gasoline diesel heavy oil coked catalyst total

0.04 1.09 7.43 25.99 65.45 100.00

1.82 1.27 7.77 24.51 64.64 100.00

4.35 1.37 6.24 23.96 64.09 100.00

5.94 1.31 5.71 23.76 63.27 100.00

9.34 1.36 5.53 23.51 60.27 100.00

the catalyst surface rather than being cracked into small molecules. Cokes on the catalyst surface in the primary reaction formed by the dehydrogenation condensation of polycyclic aromatic hydrocarbon cannot be ignored. Polycyclic aromatic hydrocarbons occupy a major portion of CGO. Their molecule sizes are larger and their adsorption abilities are stronger. They are adsorbed on catalyst surfaces and larger pore channels, enabling them to undergo easily dehydrogenation condensation reactions that lead to the accumulation of coke. We defined this kind of coke as dehydrogenation condensation coke (Cdh). The coking reactions of polycyclic aromatic hydrocarbons cannot be directly measured by present characteristic methods. However, useful information can be obtained from the composition of dry gas. Hydrogen is mainly produced by thermal cracking and dehydrogenation condensation during the catalyst cracking reactions. H2, C1, and C2 are supposed to be produced randomly by thermal cracking. Therefore, the proportion of a dehydrogenation reaction can be measured by the proportion of the volume fraction of H2, with the total volume fractions of C1 and C2 in the cracking gas. This relationship can be defined as the dehydrogenation index DHI:

DHI = ¢H2 /[¢C1 +

∑ ¢C2]

DHI is used to measure the proportion of a dehydrogenation reaction. A higher DHI corresponds to a larger proportion of the dehydrogenation condensation reaction. The changes in DHI under different conversions during CGO catalyst cracking are presented in Figure 9. With increased conversion, DHI initially increases, and then decreases before gently stabilizing. This trend shows that nitrogen adsorption has the advantage early in the reaction. The role of polycyclic aromatic hydrocarbons in the coking reaction gradually becomes predominant as the reaction proceeds. The reaction proportions of the other active components quickly increase as the reaction progresses. Ultimately, the characteristics of the primary reaction become no longer evident. When the “shielding effect”of coke formed by the primary reaction on catalyst particles is avoided, olefin and naphthene in feedstocks, which have strong FCC reaction abilities, are able to increase contact with the acid sites. As a consequence, they are able to enter the catalyst pore and complete the reaction. In this case, the proportions of secondary reactions increase, especially the hydrogen transfer reaction. Therefore, C ht formed by hydrogen transfer reaction increases with increased conversion. This indicates that coke in secondary reactions is only formed after the elimination of the effect of the initial reaction on the catalyst activity. Therefore, it is not 2253



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where W(Ccat) is the Ccat yield equal to the sum of the yields of Cdh and Cht. Rc is the catalyst-to-oil weight ratio, A is a coefficient corresponding to the feedstock and catalyst and operation conditions, and tc is the catalyst-to-oil contact time (in seconds). Therefore, coke yield in CGO−FCC can be represented by

W (Ct ) = R cAtc n + W (Cad )

(3)

where A and n are the constants for the given feedstocks and catalysts at a fixed reaction temperature. To determine the latent contents of “adsorption” coke formed during CGO− FCC, an experiment was implemented under a fixed time. The important relationship between the coke yields and Rc for CGO catalytic cracking is shown in Figure 10. The data show Figure 9. Changes in the dehydrogenation index in reaction processes (the fixed fluidized-bed reactor).

the main constraint of the FCC reaction. Overcoming the “shielding effect” of coke formed by the initial reaction on catalyst particles is significant for increasing CGO conversion. 3.3.3. Classification for Coke in CGO−FCC. Cimbalorn32 and Habib33 classified FCC coke into four categories. One is “catalytic” coke (Ccat), which is associated with acid-catalyzed cracking reactions. This coke is dependent on the feedstock and catalyst, but more importantly, the optimal amount laid down is dependent on residence time. Two is “additive” coke (Cadd), which depends on feedstock composition, that is, the Conradson carbon percentage of feedstock, the asphaltene content, and multiring high-molecular-weight aromatic molecules. Three is “contaminant” coke, which is derived from the catalytic action of heavy metal poisons. Four is “cat-to-oil” coke (Cst), which in essence measures unstripped but potentially strippable hydrocarbons. These do not belong to coke material but to residual feedstock material due to inefficient stripping of the products. Moreover, some types of coke material are formed from not-vaporized feedstock ingredients when heavy oil is catalytically cracked. In the present research, coked catalysts are obtained from initial reactions on commercial and equilibrium FCC catalysts. When the temperature is high enough to ensure completely vaporized feedstocks, sufficient stripping is managed after the reaction. Therefore, “contaminant,” “cat-to-oil,” and unvaporized cokes can be ignored. “Additive” coke is not the right term for CGO catalytic cracking because of its low percentage of Conradson carbon and thermal condensation probability. Therefore, cokes on Catcc surfaces are composed of “adsorption” (Cad), “condensation” (Cdh), and “hydrogen transfer” (Cht) cokes. The yields can be presented as

W (Ct ) = W (Cad ) + W (Cdh ) + W (Cht )

Figure 10. Effect of the catalyst-to-oil mass ratios on the coke yield (the fixed fluidized-bed reactor).

that coke yield, expressed in percentage feedstock, is directly proportional to Rc when Rc is low. Coke yield rapidly increases with increased Rc due to the aggravating interactions of the secondary products. By prolonging the line along the decreasing direction, the point of the intersection of Rc at zero and y-axis can be obtained. This is the theoretical coke yield for reaction molecules adsorbed on acidic sites before catalytic cracking reactions occur. Therefore, the Cad yield in our research can be confirmed as 1.9 wt % of feedstocks, as shown in Figure 10. This is about 37 wt % of the coke yield under ordinary FCC conditions (catalyst-to-oil ratio = 6). The limit coke yield of the initial reaction [W(Cdh + Cad)] is the sum of Cad and Cdh, which can be regarded as the coke content before the start of the secondary coking formation reaction. Therefore, the latent coke content formed by the dehydrogenation condensation components in feedstocks can be calculated according to the sum of Cad and Cdh, which detract the adsorption coke previously gained. The yield of (Cdh + Cad) can be determined as 4.2 wt % from Figure 8, and the yield of Cdh can be computed as 2.3 wt %, which occupies 45 wt % of the total coking content. Cdh is the main source of coking in CGO-FCC under ordinary conditions (catalyst-to-oil ratio = 6). Figure 8 shows that secondary reactions have obvious effects on coke yield. The proportion of Cht in coke rapidly increased with increased secondary reactions. To increase the conversion of feedstock and avoid the effect on the heat balance of FCC plants due to excess of coke formation, Cht content needs to be reasonably controlled. Cracking hydrocarbons is a strong decalescence reaction with a faster rate. A hydrogen transfer reaction is an exothermic reaction with a slower reaction rate. Therefore, coke selectivity can be decreased by properly enhancing the reaction temperature and

(1)

where W(Ct) in eq 1 is the total coke yield. W(Cad), W(Cdh), and W(Cht) are the coke yields for the corresponding classifications. The coke yield is relevant to the type of catalyst, the feedstock, and the operating conditions during catalytic cracking reaction. Voorhies34 found an intrinsic uniformity in the way that carbon deposited on a catalyst increased with time, as well as in the way that the content of coke deposition on a catalyst increased with prolonged reaction time. Consequently, the classical “coke clock”, or Voorhies equation, was used. The catalytic coke yield can be calculated by

W (Ccat ) = R cAtc n

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(4) Wang, G.; Liu, Y. D.; Wang, X. Q.; Xu, C. M.; Gao, J. S. Studies on the catalytic cracking performance of coker gas oil. Energy Fuels 2009, 23, 1942−1949. (5) Yong, D. Research of hydrotreated CGO as FCC feedstock. Liaon. Chem. Ind. 2004, 33, 287−290. (6) Luan, X. L.; Li, C. Y.; Chen, W. Y.; Wang, H. Y. Study on removal of basic nitrogen in coker gatch oil using adsorption method. J. Petrochem. Univ. 1999, 12, 15−18. (7) Wang, G.; Huang, H.; Xun, C. M.; Gao, J. S. Study on integrated process of catalytic cracking and solvent extraction for Liaohe lowquality coker gas oil. Pet. Ref. Eng. 2009, 39, 7−10. (8) Qi, J.; Yan, Y. Z.; Fei, W. F.; Su, Y. X.; Dai, Y. Y. Solvent extraction of nitrogen compounds from catalytically cracked diesel oil by metal ion complexation. Fuel 1998, 77, 255−258. (9) Chen, J.; Zhang, A. M.; Li, D. S.; Liu, J. Removing basic nitrogen compounds from coker gas oil by complexation reaction. Pet. Process Petrochem. 2008, 39, 51−55. (10) Zhang, R. C.; Shi, W. Y. Denitrifled catalytic cracking (DNCC) technology for coker gatch processing. Pet. Process Petrochem 1998, 29, 22−27. (11) Yuan, Q. M.; Wang, Y. L.; Li, C. Y.; Yang, C. H.; Shan, H. H. Study on conversion of coker gas oil by two-stage riser fluid catalytic cracking. J. China Univ. Petro. (Ed. Nat. Sci.) 2007, 31, 122−126. (12) Gao, J. S.; Xun, C. M.; Lan, X. Y.; Meng, X. H.; Cao, B. An equipment and method for CGO processing. China Patent: CN 200510114533.3, 2005. (13) Cerqueira, H. S.; Ayrault, P.; Datka, J.; Magnoux, P. m-Xylene transformation over a USHY zeolite at 523 and 723 K: Influence of coke deposits on activity, acidity, and porosity. J. Catal. 2000, 196, 149−157. (14) Roncolatto, R. E.; Cardoso, M. J. B.; Cerqueira, H. S.; Lam, Y. L.; Schmal, M. XPS study of spent FCC catalyst regenerated under different conditions. Ind. Eng. Chem. Res. 2007, 46, 1148−1152. (15) Jiménez-Garcι ́a, G.; Quimtana-Solόrzano, R.; Maya-Yescas, R. Improving accuracy in the estimation of kinetic frequency factors from laboratory data to model industrial catalytic cracking risers. Ind. Eng. Chem. Res. 2011, 50, 2736−2745. (16) Jiménez-Garcι ́a, G.; Quimtana-Solόrzano, R.; Aguilar-Lόpez, R.; Maya-Yescas, R. Modelling catalyst deactivation by external coke deposition during fluid catalytic cracking. Int. J. Chem. Reactor Eng. 2010, 8, No. Article 2 (http://www.bepress.com/ijcre/vol8/S2). (17) Li, Z. K.; Wang, G.; Liu, Y. D.; Shi, Q.; Gao, J. S. Structure analysis of key components in coker gas oil and their effects on catalytic cracking reaction. Acta Pet. Sin. (Pet. Process. Sect.) 2010, 26, 691−699. (18) Li, Z. K.; Wang, G..; Shi, Q.; Xu, C. M.; Gao, J. S. Retardation effect of basic nitrogen compounds on hydrocarbons catalytic cracking in coker gas oil and their structural identification. Ind. Eng. Chem. Res. 2011, 50, 4123−4132. (19) Shi, Q.; Xu, C. M.; Zhao, S. Q.; Chung, K. H.; Zhang, Y. H.; Gao, W. Characterization of basic nitrogen species in coker gas oils by positive-ion electrospray ionization fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2010, 24, 563−569. (20) Zhu, X.; Shi, Q.; Zhang, Y.; Pan, N.; Xu, C.; Chung, K. H.; Zhao, S. Characterization of nitrogen compounds in coker heavy gas oil and its subfractions by liquid chromatographic separation followed by fourier transform ion cyclotron resonance mass spectrometry. Energy Fuels 2011, 25, 281−287. (21) Meng, X. H.; Xun, C. M.; Gao, J. S. Studies on the reactions of Daqing atmospheric residue catalytic pyrolysis. Chem. React. Eng. Technol. 2003, 19, 358−365. (22) Wojciechowski, B. W.; Corma, A. Catalytic Cracking: Catalysts, Chemistry, and Kinetics. In Chemical Industries; Marcel Dekker: New York, 1986. (23) Chen, N. Y.; Weisz, P. B. Molecular engineering of shape selective catalysts. Chem. Eng. Prog. Symp. Ser. 1967, 63, 86−93. (24) Li, B. H.; Wang, G. S.; Yang, G. H. Deposition and distribution of coke on residue cracking catalyst. J. Pet. Univ. (Sci. Technol.) 1989, 13, 65−72.

shortening the reaction time. Consequently, the main reactions are strengthened, whereas hydrogen transfer reactions are restrained.

4. CONCLUSIONS Compared to that of VGO, the conversion of CGO catalytic cracking is lower and CGO more easily forms coke. Excessive coke formation leads to a remarkably decreased catalyst activity, which is the main cause for the low conversion. Coking during CGO catalytic cracking leads to a severe loss of catalyst acid sites. Coking formed by basic nitrogen compounds adsorbed on L acid sites is an important factor in decreasing catalyst activity. Coke is usually deposited on the catalyst micropores during VGO catalytic cracking. On the other hand, during CGO catalytic cracking, coke is congregated on the catalyst surface and forms a batt-like appearance. This phenomenon leads to the “shielding effect” on the catalyst particles and results in a greatly decrease utilization efficiency of the catalyst. Coke formed during CGO−FCC process includes adsorption coke (Cad), catalyst condensation coke (Cdh), and hydrogen transfer coke (Cht). Cad and Cdh are formed by feedstocks in the initial reaction step, whereas Cht is formed in the secondary reaction. Cad is formed by basic nitrogen compounds in feedstocks adsorbed on L acid sites, which is one of the main factors for the greatly decreased catalyst activity. This formation accounts for 37 wt % of the total coking content under conventional reaction conditions. Cdh is formed on the catalyst surface by the potential precursor of coke in feedstocks, which is a main source of coke formed under ordinary conditions in a CGO−FCC reaction. This formation accounts for about 43 wt % of the total coking content. Cad and Cdh are deposited on the catalyst surface and formed batt-like coke. They induce a “shielding effect” on the catalyst particle, which is a fundamental cause for FCC catalyst deactivation. Catalyst deactivation by coking on the catalyst surface can be weakened, and feedstock conversion can be improved via two methods. One is by increasing the catalyst-to-oil ratio and the L acid concentration on the catalyst surface. The other is by decreasing the congregation of coke on the catalyst surface. Coke selectivity can be decreased and Cht yield can be controlled by simultaneously enhancing the reaction temperature and shortening the reaction time.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 8610-8973-3993. Fax: 8610-6972-4721. E-mail: jsgao@ cup.edu.cn. Tel.: 8610-8973-3085. Fax: 8610-6972-4721. E-mail: [email protected].

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ACKNOWLEDGMENTS The authors acknowledge the financial support provided by the National Natural Science Foundation for Young Scholars of China (20906103) and the National Natural Science Foundation of China (21176252).

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REFERENCES

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