Research on New Silica Sol Matrix Used in Fluid Catalytic Cracking Reaction

Zhang Zhongdong; Liu Zhaoyong; Yan Zifeng; Wang Yi;Zhang Haitao; Wang Zhifeng

(1. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580; 2. Lanzhou Petrochemical Research Center, Petrochemical Research Institute, PetroChina, Lanzhou 730060)

Research on New Silica Sol Matrix Used in Fluid Catalytic Cracking Reaction

Zhang Zhongdong1,2; Liu Zhaoyong1,2; Yan Zifeng1; Wang Yi1;Zhang Haitao1; Wang Zhifeng2

(1. State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Qingdao 266580; 2. Lanzhou Petrochemical Research Center, Petrochemical Research Institute, PetroChina, Lanzhou 730060)

A new silica sol binder was obtained by mixing the acid-modified aluminium sulfate and water glass. The effect of SiO2concentration in sodium silicate, pH value and polymerization was investigated. The new silica sol binder, which possessed abundant pore volume and suitable acid amount, was an ideal component for preparing cracking catalyst. As a result, the corresponding catalyst comprising the new binder showed excellent performance. Compared with the reference sample, the liquefied petroleum gas (LPG) and propylene yield obtained over this catalyst increased by 3.49 and 1.20 percentage points, respectively. The perfect pore structure and suitable Lewis acid amount of new silica sol were the possible reason leading to its outstanding performance.

silica sol; binder; pH value; LPG; propylene; catalyst

1 Introduction

As we all know, fluid catalytic cracking (FCC) unit converts low value heavy hydrocarbons into a series of more valuable products such as gasoline, diesel and light olefins[1–3]. Higher conversion of heavy feedstock to these products is more desirable for FCC unit. In China, fluid catalytic cracking reaction unit at petroleum refineries is a very important source of gasoline, diesel and LPG nowadays. Although FCC units have been commercially deployed for over 60 years, the technology is still in need of further development to meet new challenges[4–6]. Modern FCC units can process a wide variety of feedstocks and operator could adjust operating conditions to maximize the production of gasoline, middle distillate (LCO) or light olefins to meet different market demands.

To take full advantage of marketing opportunities, some refiners are blending residual oil with gas oil as the feed to FCC units[7–9]. In fact, the majority of FCC units have been designed to process residual oil. In comparison with FCC catalysts used for cracking of vacuum gas oils, the FCC catalysts for residual oil cracking should have much more macropores to increase the mass transfer of large residue molecules inside the pores of catalysts.

Heavy metals such as vanadium and nickel existing in petroleum feedstock have the most significant impact on the FCC catalyst’s performance. During the cracking process, these metals are deposited on the catalyst and affect both catalytic activity and selectivity. The major effect of nickel is to produce additional hydrogen and coke, while vanadium, in addition to increasing gas and coke production, can cause partial destruction of the zeolite in the presence of steam. In the case of nickel, the lower activity of catalyst is primarily ascribed to coke deposition and the catalyst can therefore be regenerated. In the case of vanadium, the loss in activity is primarily due to zeolite destruction which is irreversible.

Commercial FCC catalyst often contains Y zeolite, ZSM-5, kaolin, binder, chemical elements, etc.[10-11]The Y zeolite provides most of the cracking activity of the FCC catalyst, and the matrix fulfills both physical and auxiliary catalytic functions[5]. However, the price of Y zeolite is costly. In order to decrease the cost, people cannot help reducing the dosage of Y zeolite, because reduction of Y zeolite content will influence the conversion of heavyoil. It seems that it is difficult to take into account the relationship between the Y zeolite content and the conversion of heavy oil. People think the only purpose of using binders in fluid catalytic cracking (FCC) operation is the need for increased strength of catalyst in the course of reaction. However, with a better understanding of the FCC process, scientists believe that the role and type of binder usually vary a lot. Up to date, AlPO4, silica sol, Al sol and pseudo-boehmite are often used as binder according to the literature information[12–15]. However, these binders have many defects. For instance, the pore volume of Al sol is low, and the strength of pseudo-boehmite is weak. In this paper, a new porous binder material, which has the advantage of both suitable meso-macro pores, proper bond strength and heavy metal resistance, is prepared and applied in fluid catalytic cracking reaction.

Silica sol, which is also called silicate sol, is a colloidal solution of multi-molecular polymer silicate formation. Its molecular formula is SiO2·xH2O. The average particle size of silica sol is 7—20 nm. Silicate polymer particles can be dispersed in water and organic solvent, which is a chemical binder. A variety of methods can be used for preparation of silica sol, which can be divided into two categories. One of the methods is polymerization by which silica sol is prepared via polymerization of silicic acid and integration of large particles, and the other is depolymerization. These two categories can be subdivided into the electrolysis-electrodialysis method[16], the ion exchange method[17]and the acidification method[18].

The processing of two former methods is not only complex, but also entails high production cost. However, the last one is a simple and low cost method. The defect is that the silica content in silica sol is low, and is readily transformed into gel. In this paper, a special “one-step”method is introduced. The silica content is high, and the colloid is stable. Furthermore, the catalyst containing this silica sol shows a good product distribution.

Silica sol according to this principle was prepared just as follows[19]:

2 Experimental

2.1 Preparation of new silica sol and catalyst

Acid-modified aluminium sulfate (AMAS) was prepared from aluminium sulfate solution and 1 mol/L H2SO4solution, and then mixed with the soluble water glass solution under normal pressure and temperature. After suitable treatment, the remaining liquid was used as the silica sol matrix. The industrial catalyst was prepared as follows. During the catalyst preparation process, the ZSM-5 zeolite (manufactured by the Catalyst Plant of Nankai University), the kaolin matrix (manufactured by the Suzhou Kaolin Company), and the silica sol matrix were mixed together and shaped through spray-drying to obtain a micro-spheroidal catalyst. This catalyst was named as the ADD-new. The old catalyst was prepared through adding the Al sol (manufactured by the Catalyst Plant of Lanzhou Petrochemical Company, PetroChina) to replace the silica sol. The old catalyst was marked as the ADD-old.

2.2 Catalytic cracking tests

The catalyst, named as the CAT-new, was prepared by mixing the ADD-new with the commercial catalyst LBO-16[20](manufactured by the Catalyst Plant of Lanzhou Petrochemical Company, PetroChina), and the catalyst CAT-old was prepared by mixing the ADD-old with LBO-16 catalyst. The catalytic cracking performance of the catalysts was evaluated in a fixed-fluidized bed unit (FFB).

The mass ratio of binders to catalyst was 6:94. The tests were carried out under the typical conditions for FCC units: a cracking temperature of 500 ℃, a catalyst to oil mass ratio of 4.0, and a weight hourly space velocity of 15 h-1. Prior to the FFB test, the CAT-old sample and CAT-new sample were steam-deactivated at 800 ℃ for 10 h in a fluidized bed in the presence of 100% steam. The chemical composition of the product FCC gasoline was determined by an online GC-MS. The feedstock (as shown in Table 1) was a mixture of 70% Xinjiang vacuum gas oil (VGO) and 30% Xinjiang vacuum tower bottom (VTB).

Table 1 Main properties of the FCC feedstock

2.3 Physicochemical characterization

The elemental content was analyzed by the X-ray fluorescence spectrometric (XFS) method on a Rigaku ZSX primus instrument operated at 50 kV and 40 mA. The textural properties were determined by N2adsorption at 77 K on an ASAP-2010 instrument (Micromeritics, USA). Prior to measurement, the sample was outgassed at 573 K for 12 h. The micropore volume and external surface area were calculated from the t-plot method.

3 Results and Discussion

3.1 SiO2concentration of sodium silicate

Literature information and research[21]show that, silica sol is a dispersion system, which contains a lot of SiO2particles. Silica sol is a thermodynamic instable system, because its surface has a lot of free energy, and is prone to coalescence into large particles to produce silica gel. Therefore, the stability time of silica sol has a close relationship with the initial concentration of SiO2. In order to investigate the effect of initial concentration of raw water glass for preparation of silica sol, different concentration of raw water glass was mixed with AMAS, with the results shown in Table 2. As shown in Table 2, the less the concentration of water glass, the longer the stabilizing time of silica sol would be. Therefore, we should select a small concentration of water glass.

Table 2 Effect of initial concentration of raw water glass

3.2 pH value

The effect of pH value in the range of 1—5 on the stabilizing time of silica sol was investigated, with the results shown in Figure 1. It can be seen from Figure 1 that a low pH value can prolong the stabilizing time. The reason can be ascribed to different states of aggregation in different range of pH values[22].

Figure 1 Relation between pH value and stabilizing time of colloids

In acidic solution, the polymerization reactions are mainly carried out through the cations of neutral molecules or are related with the hydroxyl bond effect. Therefore, at a higher acidity, the reaction can hinder or slow the reaction rate of polymerization, and then increase the stabilizing time of colloids. Therefore, the pH value should be controlled within a certain range of acidity in the course of preparation of silica sol, and a pH value of between 2.5 and 3.5 was suitable.

3.3 Polymerization

In order to improve the stability of silica sol, aluminum sulfate was added to prolong the stabilizing time of silica sol, with the results shown in Table 3. It can be seen from Table 3 that during the polymerization process of silica sol, aluminum ions can slow down the polymerization rate of silica sol and generate smaller particles. In addition, the aluminum ions could change the surface energy of silica sol.

Table 3 The effects of concentration of aluminum sulfate

3.4 Physicochemical properties of new silica sol

The prepared silica sol material existed in the form of slurry, and no precipitation was observed after settling for 48 h in the laboratory. The corresponding surface area and pore volume were characterized, with the results presented in Table 4. It can be seen from Table 4 that the silica sol had considerable surface area and large pore volume. Among them, the bulk density of newly prepared silica sol was lower than that of old binder. The surface area and pore volume of old binder was obviously smaller than those of new silica sol. Compared with the pure Al sol, this new porous binder, which had obvious advantages to function as cracking catalyst filler material, was an ideal cracking catalyst component.

Table 4 Textural properties of the silica sol

3.5 Catalytic cracking tests

The product distribution and the property of the resulting gasoline fraction are shown in Table 5. The data show that, compared with the CAT-old sample, the LPG and propylene yield obtained during catalytic reaction over the CAT-new sample increased by 3.49 and 1.20 percentage points, respectively. The overall reaction performance of CAT-new sample was obviously superior to the CAT-old sample. We all know if the ZSM-5 zeolite content of catalyst decreases, the ability to achieve maximum LPG yield will reduce and the product distribution will deteriorate. The activity of catalyst was improved despite the decreased active components in the catalyst, which could evidence the advantages of the silica sol binder.

Table 5 Product distribution of the prepared FCC catalysts

Compared with the CAT-old sample, MON and RON of gasoline fraction obtained during catalytic cracking over the CAT-new sample could increase by 0.2 and 1.0 unit, respectively. The result was very ideal although the molecular sieve content reduced, which showed that the synergism effect of both ZSM-5 zeolite and new silica sol binder had fully achieved the desired catalytic properties. The high LPG yield and octane number of gasoline obtained during catalytic reaction over the new catalyst showed that the new silica sol binder was an effective component for preparing this FCC catalyst. The reason can be attributed to two aspects. On one hand, the silica sol binder had a perfect pore structure. The use of new material provided a favorable space for adsorption and diffusion of hydrocarbons of gasoline fraction and other molecules, which at the same time was helpful to LPG yield and product distribution improvements. More accessible space also facilitated the cracking of interim products. On the other hand, the acid amount of new silica sol binder was excellent. The suitable acid amount was conducive to providing a high dispersion of active components, and could bring into full play the excellent catalytic properties of zeolites.

4 Conclusions

A new silica sol binder was obtained by mixing the acidmodified aluminium sulfate and the water glass, and applied to prepare catalytic cracking catalyst. The effect of SiO2concentration of sodium silicate, pH value and polymerization process were investigated in this paper. The new silica sol binder possessed abundant pore volume and suitable acid amount and was an ideal component for preparing cracking catalyst. The new prepared catalystfeatured high Lewis acid amount and large pore volume. As a result, the corresponding catalyst showed excellent performance. Compared with the reference catalyst, the LPG and propylene yield of catalytic reaction over the CAT-new sample increased by 3.49 and 1.20 percentage points, respectively. MON and RON of gasoline obtained from catalytic reaction over the CAT-new sample could increase by 0.2 and 1.0 unit, respectively. The possible reason of obtaining prominent performance was ascribed to the perfect pore structure and suitable Lewis acid amount of the new silica sol.

Acknowledgements:The authors thank the Department of Science and Technology Management of PetroChina for providing financial support.

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Recieved date: 2013-10-18; Accepted date: 2014-04-22.

Dr. Liu Zhaoyong, Telephone: +86-931-7981914; E-mail: lzy0539@126.com.