Yan Jiasong; Yu Shanqing; Long Jun; Gong Jianhong

(Research Institute of Petroleum Processing, SINOPEC, Beijing 100083)

Abstract: Increasing gasoline production in FCC unit can improve the utilization efficiency of petroleum resources and gain economic benefit. This paper discusses the technical principles for increasing FCC gasoline yield from the aspects of feedstock properties, operating conditions, LCO (light cycle oil) recycling, catalyst selection and reactor type, and illustrates the industrial application examples for maximizing gasoline production. The technical measures, such as optimizing the feedstock, properly increasing the catalyst activity and reaction temperature, recycling LCO or hydrotreated LCO, applying high gasoline yield catalyst, and adopting the two-zone riser reactor, are proposed to enhance the gasoline yield.

Key words: fluid catalytic cracking; gasoline; light cycle oil; riser reactor

1 Introduction

Fluid catalytic cracking (FCC), as a main secondary refining process, can convert the crude oil fractions into many commodity fuels and platform chemicals, such as gasoline. Gasoline production from FCC process accounts for over 30% of the total gasoline produced worldwide in the petroleum industry. However, FCC is responsible for about 70% gasoline production in China[1-5]. In the near future, the gasoline demand growth will increase with an average annual rate of 7% for Asia (China) and the Middle East as a result of the automobiles market demand.

Gasoline is the main value-added product of the FCC units, and an increase of gasoline yield can gain more profit. Moreover, boosting gasoline yield in FCC units is an effective way to improve the utilization of petroleum resources[6-7]. Although the FCC technology is quite mature, the research scope is still enormous due to changing FCC feedstock, gradual shifts in market demands, and evolving unit operations. In this paper,the feedstock properties, operating conditions, LCO recycling, catalyst performance, and reactor types are presented and critically analyzed, in order to provide a comprehensive understanding about the current status and some possible technical approaches to increase gasoline production.

2 Properties of FCC Feedstock

The overall performance of FCC process in the production of valuable products is strongly affected by the feedstock properties, which have evolved over a period of commercial application. Conventional FCC feedstocks are mainly vacuum gas oils (VGO), coker gas oils (CGO),and solvent deasphalting oil. With the development of FCC technologies, FCC feedstocks are extended to cover atmospheric residue, hydrogenated heavy oil and even vacuum residue[8].

Density is one of the most basic properties of FCC feedstock. In the similar distillation range, the higher the density is, the less paraffins and the more aromatics are contained in the feed composition. A certain relationship between the yield of FCC gasoline and the composition of different hydrocarbons in feedstock are illustrated[9-10].Paraffins and naphthenes in the feedstock are ideal components for producing FCC gasoline. Aromatics are compounds that have at least one benzene ring. The benzene ring is very stable under the operating conditions of catalytic cracking, while the alkyl side chain on the benzene ring is easy to crack. Monocyclic aromatics are also potential components of gasoline, and can increase the gasoline octane number; polycyclic aromatics are not suitable for the production of FCC gasoline. Therefore,the yield of gasoline mainly depends on the content of saturated hydrocarbons and monocyclic aromatic hydrocarbons in FCC feedstock. Under the same FCC processing conditions, the relationship between the density of feedstock with similar distillation range and the gasoline yield is shown in Figure 1. As the density of the feedstock with similar distillation range increases, the gasoline yield decreases obviously.

Figure 1 Relationship between gasoline yield and feed density

3 Operating Conditions

Due to the limitation of crude oil availability and refinery process scheme, it is difficult for refiners to select FCC feedstock flexibly. Therefore, many refiners focus on optimizing the operating conditions under the given feedstock to maximize the gasoline yield in FCC unit(FCCU). Where is the FCC gasoline from? It is generally known that lower value heavy oil can be converted into higher value transportation fuels, such as gasoline through FCC process. One of the factors affecting gasoline production is the conversion degree of the heavy oil obtained from the process. The relationship between the gasoline yield and the conversion of feedstock is shown in Figure 2. Generally, the yield of gasoline increases rapidly with the increase of conversion when the conversion rate is less than 50%. It is worthy of noting that the increase of gasoline yield slows down and then slightly decreases with the further increase of conversion.When the conversion is 70%—90%, the gasoline yield reaches a maximum value. The conversion rates of different feedstocks may vary considerably when the gasoline yields reach the maximum. As shown in Figure 2,the conversion rate of feed A is higher than that of feed B when the gasoline yield reaches the maximum value. With a further increase of conversion, the amount of conversion of feedstock into gasoline is less than the amount of further conversion of gasoline into other products, which causes a reduction in gasoline production.

Figure 2 Relationship between gasoline yield and conversion of feed

Actually, the conversion in FCCU is generally maintained at about 70% in many refineries, which is usually lower than the achievable maximum gasoline yield, because the yield of low value products such as coke and dry gas will increase more significantly in case of high conversion.Under this condition, most refineries can improve the gasoline yield by increasing the conversion. There are many ways to improve the conversion in FCCU, such as increasing the catalyst activity, increasing the reaction temperature, increasing the catalyst to oil ratio, prolonging the reaction time, reducing the reaction pressure, etc. The most common way to improve the conversion of FCCU is to increase the catalyst activity and/or the reaction temperature.

In order to improve the yield of gasoline, Refinery C improves the catalyst activity by increasing the catalyst consumption to acquire high conversion while keeping other operating conditions constant. The properties of feedstock, catalyst, and product distribution in FCCU at Refinery C are listed in Table 1 and Table 2. Judging from the data listed in Table 1, it can be seen that when the micro activity of the equilibrium catalyst increased by three points, the conversion and the gasoline yield increased by 3.23 percentage points and 2.03 percentage points, respectively, while the LCO yield decreased obviously, and other product yields increased slightly.In order to boost the gasoline production, Refinery D also improves the catalyst activity by increasing the catalyst consumption, and at the same time increases the reaction temperature from 515 °C to 525 °C. The properties of FCC feedstock in Refinery D are listed in Table 1, and the catalyst information and product distribution are listed in Table 3. As shown in Table 3, the increase of equilibrium catalyst activity by 4 points and reaction temperature by 10 °C can lead to the increase of conversion by 4.75 percentage points and gasoline yield by 3.19 percentage points, while the yields of LCO and slurry decrease obviously. The daily statistical data are shown in Figure 3. The statistical data also show that the gasoline yield increases significantly after the increase in catalyst activity and reaction temperature.

Table 1 Properties of feedstock used in Refinery C and Refinery D

Table 2 Performance of FCCU in Refinery C

Table 3 Performance of FCCU in Refinery D

Figure 3 Gasoline yield of FCCU in Refinery D

4 LCO Recycling

LCO containing a certain amount of saturates and monocyclic aromatics is an important byproduct of FCC.When LCO density is less than 0.93 kg/L, more than 50% of components in LCO are saturates and monocyclic aromatics which can be cracked to get more than 30%of gasoline. When the LCO density is over 0.93 kg/L,the content of bicyclic aromatics and tricyclic aromatics,which can hardly be cracked is high. Therefore, LCO with high density and high aromatic content is no longer worthy of direct recycling, and needs to be hydrotreated to reduce its density for recycling.

Hydrotreated LCO (HLCO) with a low content of bicyclic aromatics and tricyclic aromatics, is much more suitable for recycling to gain more gasoline than LCO[11-12]. In order to improve the efficiency of HLCO cracking, the Research Institute of Petroleum Processing(RIPP), the R&D center of SINOPEC, has developed a new processing technology to convert LCO to gasoline with high RON (LTAG)[13-15]. This FCC technology is an integration of the hydrotreating of LCO into HLCO and the FCC processing of HLCO. Di-aromatics that are enriched in LCO are hydrogenated moderately into monoaromatics with a tetralin structure. In the following FCC operation, mono-aromatics can be cracked readily into alkylbenzene, which is a typical gasoline component with a higher RON. Table 4 shows the commercial application results in Refinery E under different processing cases.Base case, Case 1, Case 2, and Case 3 represent the operation without LCO recycling, LCO recycling, HLCO recycling, and HLCO recycling with LTAG technology,respectively. It can be seen from Table 4 that under the same LCO recycling ratio the gasoline yield increases by 4.86 percentage points in Case 1, by 7.31 percentage points in Case 2, and by 8.83 percentage points in Case 3. The results indicate that LTAG technology can significantly boost the gasoline production, and at the same time the octane number of gasoline can increase by 0.6 units. No matter which way is adopted, LCO recycling can improve the gasoline yield apparently.

5 Catalyst Performance

FCC catalysts are highly relevant to the distribution and properties of target products. The excellent physicochemical properties, such as well-defined pores/channels and tailored acidity of FCC catalysts, allow them to function efficiently in FCC process while processing a variety of feedstocks. A number of studies related to FCC catalysts have indicated that the catalyst for increasing gasoline yield should have the following properties[16-18]:1) Optimized pore size distribution resulting in optimum gasoline yield and lowest slurry;

2) High accessibility and stability leading to high bottoms conversion and gasoline yield;

Table 4 Products slates of FCCU with recycled LCO or HLCO in Refinery E

3) Optimum hydrogen transfer activity for achieving maximum gasoline selectivity and octane barrels;

4) Optimum metal contamination tolerance;

5) Appropriate catalyst activity contributing to the cracking reaction, while a too high catalyst activity leading to excessive cracking of gasoline fraction.

RIPP has developed a series of FCC catalysts for refiners targeting an increase to their gasoline production. These catalysts, such as SGC and RCGP, have exhibited excellent performance to serve the needs of FCCU and have been widely used in refineries. The main technologies incorporate the modified silicon-aluminum matrix technology (SAM), the structural optimization of zeolite technology (SOZ), and the metal tolerance technologies. Compared with traditional matrix, the SAM matrix has higher total acidity, especially more weak L acids. Moreover, the proportion of meso-pores and macro-pores is higher, which is beneficial to the pre-cracking of the larger hydrocarbon molecules.The main advantages of the catalyst adopting SAM technology include proper catalytic activity, superior bottoms cracking ability, higher total liquid yield, as well as optimum coke selectivity. The SOZ technology is developed based on the synergistic combination of hydrothermal ultra-stablization technology and rare earth modification. It has more mesoporous structure because of the reduction of non-framework aluminum by cleaning the pores. Compared with conventional USY zeolite, it shows higher crystallinity, larger surface area and pore volume resulting in better stability and activity,which are beneficial to improving the accessibility of zeolite active sites and strengthening cracking activity.Furthermore, these larger pores can significantly improve the diffusion of larger feed molecules into and out of the zeolite crystals of the FCC catalyst, while preventing the valuable cracked products from being potentially converted to undesirable products through thermal cracking, hydrogen transfer or other reactions.So the catalyst adopts SOZ technology can improve the bottoms cracking ability and increase the gasoline yield,while decreasing the coke and dry gas production.

Refinery F has applied the high gasoline yield catalyst SGC-1 to increase gasoline production. The product distribution of SGC-1 catalyst application is listed in Table 5. It can be seen from Table 5 that the mass yield and volume yield of gasoline increased by 4.08 percentage points and 5.50 percentage points, respectively, after the application of the SGC-1 catalyst. The yields of LCO and LPG decreased by 2.76 percentage points and 1.46 percentage points, respectively. The application suggests that the SGC-1 catalyst shows strong ability to boost gasoline production.

Table 5 Products slates of SGC-1 catalyst application in Refinery F

In order to increase the production of gasoline, Refinery G has adopted the high gasoline yield catalyst RCGP-1. The product distribution of catalyst RCGP-1 is listed in Table 6. It can be seen from Table 6 that the mass yield of gasoline increases by 4.01 percentage points after the application of catalyst RCGP-1. The yield of LCO and slurry decreased by 3.41 percentage points and 0.60 percentage points, respectively, while the yields of other products changed little. The results suggest that the RCGP-1 catalyst has excellent gasoline selectivity and good bottom cracking ability.

Table 6 Products slates of RCGP-1 catalyst application in Refinery G

6 Reactor Type

Circulating fluidized beds have been used in FCC processes due to their distinct advantages of uniform temperature distribution, high gas-to-particle mass ratio and heat transfer rates, and flexible operation. However,many disadvantages exist in the oil products produced by fluid catalytic cracking (FCC) riser, such as insufficient cracking of macromolecular feedstock, and overcracking of small molecular products.

According to the reaction mechanism of catalytic cracking, a novel FCC process (Figure 4)[19-21]has been developed by RIPP. The reactor is a combination of a conventional FCC riser (section I) and a dense circulating fluidized bed (section II), acting as two different reaction zones, respectively. The two-zone riser reactor can promote feedstock conversion in section I and inhibit the overcracking reaction in section II. Catalytic cracking process is very complicated,including cracking, hydrogen transfer, isomerization,cyclization, dehydrogenation, alkylation, alkylation transfer, condensation reaction, etc. High temperature is favorable to the cracking reaction, while low temperature is favorable to hydrogen transfer reaction and isomerization reaction. The two-zone riser reactor divides the complex FCC process into two reaction zones. The reaction condition of section I is characteristic of high temperature and short time, which is conducive to cracking reaction. The macromolecules of feedstock are cracked into the molecules mainly composed of gasoline components in section I. The reaction condition of section II features low temperature and long time, which is conducive to the hydrogen transfer and isomerization reaction. Through the hydrogen transfer and isomerization reaction, the quality of gasoline components can be improved[22]. Because of the lower temperature in section II, the overcracking reaction of gasoline components are reduced, which is beneficial to the production of gasoline.

The two-zone riser reactor has been applied in more than 50 FCCUs. The FCCU in Refinery H is taken as an example to identify the commercial application of twozone riser reactor. The product distribution of Refinery H,which has applied the two-zone reactor, is listed in Table 7. It can be seen from Table 7 that the gasoline yield of two-zone riser reactor increases by 5.14 percentage points as compared with conventional riser reactor, and the LCO and slurry yield decrease. The application results indicate that the two-zone riser reactor can boost gasoline production efficiently.

Figure 4 Configuration of the two-zone riser

Table 7 Comparison of normal riser and two-zone riser in Refinery H

7 Conclusions

FCC is a major process for increasing gasoline production in refineries. Technical measures for enhancing FCC gasoline yield have been discussed and proposed as follows: selecting feedstock rich in monocyclic aromatics and saturated hydrocarbons,properly increasing catalyst activity and reaction temperature, recycling LCO, adopting hydrotreated LCO with LTAG technology, selecting catalyst for increasing gasoline yield, and adopting two-zone riser reactor.