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Utilization of exhausted dust from FCC flue gas as material for FCC catalyst preparation

Abstract

In this paper, new matrix and corresponding fluidized catalytic cracking (FCC) catalysts were prepared by reutilizing the exhausted dust which was derived from fluidized catalytic cracking flue gas. The results showed that the new matrix has wider mesopore size distribution than pseudo-boehmite binder. With the increase of the additional amount of this new matrix, FCC catalyst possessed increased micro-activity test conversion of vacuum gas oil (VGO) from 68 % to 71 %. The catalytic cracking performance on a fixed fluidized bed unit showed that compared with CAT-1, the conversion of VGO on CAT-5 increased by 1.19 %, heavy oil yield and coke yield decreased by 2.16 and 1.65 %, respectively. Besides, the total liquid yield and light oil yield increased by 2.27 and 2.26 %, respectively. The reaction performance of CAT-3 and CAT-5 is obviously superior to CAT-1. The appropriate pore structures and crystallized Y zeolite component on this new matrix improve the cracking ability of FCC catalysts.

Introduction

Air-particle pollution in China, which is caused by increasing fossil fuel consumption and industrial particulate emissions, gives rise to the deterioration of air quality and severe haze weather. Haze weather can cause serious harm to the respiratory system and cardiovascular system of humans and other creatures. The fluid catalytic cracking (FCC) unit is an important industrial particulate source [1, 2].

FCC process is one of the most important technologies to convert heavy oil into liquified petroleum gas (LPG), gasoline, and diesel. FCC catalyst with particle sizes in the range of 80–180 μm cycles between FCC riser and regenerator for hydrocarbons cracking and reactivation [3]. However, due to the collision, hydrothermal collapse, and mechanical friction in recycle, spent FCC catalyst becomes dust and escapes with the FCC flue gas when its particulate size is less than 30 μm (more than 50 % is <10 μm), leading to higher concentration of atmospheric particulate matter pollution. It is necessary to control the emission of FCC flue gas dust.

Element analysis shows that the spent catalyst dust is composed of alumina and silica as well as trace elements such as Ni, V, and Fe that comes from FCC feedstock. However, the traditional landfill of spent catalyst causes resource waste and soil pollution. In recent years, reports show that more spent catalyst has been reutilized to produce cement, flocculant, and wax hydrofining catalyst [4, 5]. Alumina and silica of flue gas dust originate from the FCC fresh catalyst. So, it is reasonable to reutilize the flue gas dust to prepare FCC catalyst referring to their compositional similarity [6].

This study aims to explore an environmental friendly and effective way to recycle the solid particle in FCC flue gas. Flue gas dust was first recrystallized to form a Y zeolite-containing new matrix. This new matrix was further used to prepare FCC catalyst by partial substitution of pseudo-boehmite binder. Finally, the catalytic cracking properties of the as-prepared FCC catalyst were tested in the conversion of vacuum gas oil (VGO).

Experimental section

Raw materials

Industrial pure raw materials, including sodium silicate (Na2SiO3·9H2O), sodium hydroxide (NaOH), alumina sol, and flue gas dust, were provided by the refinery of Zhongyuan Oilfield Company, SINOPEC. Rare earth Y zeolite (REY) was provided by Qingdao Huicheng Petrochemical Technology Co. Ltd. Kaolin clay was purchased from China Kaolin Company. Pseudo-boehmite and VGO was purchased from Shandong Aluminum Company and Luqing Petrochemical Company, respectively.

Synthesis of new matrix and FCC catalyst

FCC catalyst was synthesized following steps shown in Scheme 1.

Scheme 1
scheme1

The diagram of new matrix and FCC catalyst with flue gas dust

The guiding agent for NaY synthesis was prepared by mixing sodium silicate, deionized water and sodium aluminate and then standing at 26 °C for 17 h.

The flue gas dust was retrieved from a commercial FCC unit. The dust was treated by acid and calcined at 700 °C for 1 h before use. A suspension of sodium silicate, sodium hydroxide and deionized water were stirred at 60 °C for 10 min. Then the treated dust was added into the suspension as aluminum source and stirred at 60 °C for 40 min. The Na2O/Al2O3 mass ratio was changed from 3.2 to 9.0, respectively. Thereafter, the guiding agent was added into the above suspension and this suspension was continuously stirred and hydrothermally treated at 92 °C for 26 h to form the matrix precursor. New matrix containing zeolite was finally obtained after filtration, washing, and modification by RE2O3. And the chemical composition of the original dust and the new matrix is listed in Table 1.

Table 1 The chemical composition of the flue gas dust and new matrix

The above zeolite-containing new matrix was adopted to partially replace pseudo-boehmite which plays the role of the binder in conventional FCC catalyst. Before use, the new matrix was lapped to be smaller than 5 μm in particle size. New matrix, pseudo-boehmite, alumina sol, kaolin clay, REY, and water were mixed to be a suspension and stirred at room temperature for 24 h. Finally, the new catalysts were prepared by the treatment of the mixture in a sequence of spray drying, washing, drying, and heating at 420 °C for 1.5 h. The catalysts were marked as CAT-1, CAT-2, CAT-3, CAT-4, and CAT-5, respectively, depending on the solid mass ratio of the new matrix (varying from 0, 2, 4, 6, 8 %).

Characterization

XRD patterns were recorded on a Bruker Axs diffractometer (Germany) using Cu-Kα radiation operating at 40 kV and 40 mA, scanning from 5° to 75° and at a speed of 0.01°/s. N2 isothermal sorption measurements at −196 °C were carried out on QUADRASORB SI analyzer. The samples were outgassed at 300 °C for 4 h with a vacuum pressure less than 10−2 torr prior to N2 sorption measurement. Specific surface area was calculated by the Brunauer–Emmett–Teller (BET) method using experimental points in the relative pressure range of 0.05–0.25 [7]. The pore volume and pore size distribution were derived from the desorption branch of sorption curves, using the Barrett–Joyner–Halenda (BJH) method.

Catalytic evaluation

Catalytic activity tests of catalysts were performed on an automated bench-scale micro-activity test (MAT) unit. 5 g of the prepared catalyst (treated at 800 °C for 17 h with 100 % of water) was filled in a fixed-bed microreactor. The reactions occurred at 460 °C for 70 s in a tubular stainless steel reactor with 1.56 g feed. Vacuum gas oil (VGO) as heavy hydrocarbon feedstock.VGO was purchased from Qing Dao An Bang Refinery Co. Ltd. (Table 2 as provided by vendor). The resulting cracking products were collected and analyzed by a SHIMADZUN2000 gas Chromatograph (GC) analyzer equipped with a flame ionization detector (FID).

Table 2 Properties of feedstock

The effects of the addition of zeolite-containing matrix on catalytic performances of new FCC catalysts were further tested on a fluid fixed bed (FFB) unit. The catalysts were first hydrothermally treated at 800 °C in a 100 % water steam for 17 h before testing. The resulting liquid products and gases were analyzed by Agilent GC analyzer. The amount of coke deposited on the spent catalyst was measured by analyzing the quantities of CO2 and CO after combustion.

Results and discussion

The crystalline phases of the new matrix prepared with varying Na2O/Al2O3 ratios were measured by XRD analysis shown in Fig. 1. All samples possess the characteristic peaks of Y-type zeolite, compared to NaY zeolite. However, there is another weak peak at 2θ degree of 12.5° for all samples except the one with Na2O/Al2O3 of 6.0. This peak is ascribed to the reflection of P-type zeolite at the similar 2θ position. P zeolite has thermodynamic steady state but it is ineffective in hydrocarbon cracking [3]. Therefore, the new matrix prepared at Na2O/Al2O3 ratio of 6.0 was adopted to be further characterized and used.

Fig. 1
figure1

XRD patterns of standard NaY and matrix (filled diamond) P zeolite

To clearly compare the change of crystalline phases, the XRD patterns of flue gas dust and new matrix prepared at Na2O/Al2O3 ratio of 6.0 were shown in Fig. 2. Flue gas dust possesses both alumina phase and Y zeolite phase, originating from the debris of wearing FCC catalyst in FCC system. In contrast, the new matrix has increased peak intensity of Y zeolite and undetected peaks of alumina phase. It indicates that the new matrix contains high content of Y zeolite by recrystallization in the presence of silica source, alumina source, and flue gas dust.

Fig. 2
figure2

XRD patterns of flue gas dust and new matrix prepared at Na2O/Al2O3 ratio of 6.0

The results in Fig. 3 show that the new matrix possesses bi-model mesopores compared with the raw dust. Considering the crystalline phase in Fig. 2, the amorphous phase must be large in proportion to Y zeolite on the new matrix since Y zeolite has sole microporous structure. It coincides with the increases of total surface areas and micropore surface areas of new matrix compared with dust (Table 3).

Fig. 3
figure3

Pore size distributions of flue gas dust and new matrix prepared at Na2O/Al2O3 ratio of 6.0

Table 3 Textural properties of FCC flue gas dust and as-prepared new matrix

The physical properties of FCC catalysts are shown in Table 4. The results show that with increasing the addition of new matrix, the attrition index of FCC catalysts increased obviously while the surface areas and pore volumes slightly increase. It is attributed to the lower cohesiveness of the new matrix compared with pseudo-boehmite binder. The attrition index of FCC catalyst is a key parameter and it represents the mechanical strength of catalyst. To some extent, the lower attrition index means longer lifetime of catalyst. The as-prepared FCC catalysts can meet the requirements of industrial application since their attrition index is less than the industrial limit value of 2.5. MAT conversion of VGO increases with increasing addition of new matrix in FCC catalysts (Table 4). It can be ascribed to the increase of the mesopore sizes of this new matrix and  the Y zeolite content in the  new  matrix (Figs. 24a)

Table 4 Physical properties of FCC catalysts and their MAT conversion for VGO
Fig. 4
figure4

Pore size distributions of a binder and new matrix, and b Cat-1 and Cat-4, respectively

The pore structure of catalyst is important for reactions. For FCC catalysts, three types of pores are involved according to the classification criterion by IUPAC. Micropores with pore diameter less than 2 nm, which are provided by the active component such as Y zeolite, act as the main repository for hydrocarbon cracking. However, heavy oil molecules are too large to enter into the micropores for their diameters are in the range of 3–10 nm. It turns out that the diffusion of molecules in porous channels is not limited when the optimized pore sizes are 2–6 times bigger than the diameter of molecules. Therefore, the macropores (>50 nm) inside new matrix contribute more effectively to the pre-cracking of heavy oil macromolecules compared to pseudo-boehmite binder (Fig. 4b). In contrast, the mesopores (2–50 nm), which are composed of secondary pores of zeolites and amorphous matrix, can be used for the cracking cycloparaffins and side-chain aromatics into light cycle oil (LCO). Besides, the acidity on mesopores should be mild and its numbers are usually limited.

SEM images (Fig. 5) show that all FCC catalysts have spherical particles. The addition of new matrix has no effect on the particle morphology.

Fig. 5
figure5

SEM images of as-prepared FCC catalysts

CAT-1, CAT-3, and CAT-5 were tested on the FFB equipment to check the catalytic cracking peroperties of FCC catalysts with new matrix, and the results are shown in Table 5. It is obvious that FCC catalysts with new matrix have higher VGO conversions than conventional CAT-1 catalyst. Compared with CAT-1, the VGO conversion of CAT-5 increases by 1.19 % while the coke yield decreases by 1.65 %. Besides, the diesel yield increases by 1.40 % while heavy oil yield decreases by 2.59 %. As a result, the total liquid yield and light oil yield increased by 2.27 and 2.26 %, respectively. The overall reaction performances of CAT-3 and CAT-5 are obviously superior to CAT-1. The higher cracking ability of heavy oil is highly related with the properties of the new matrix.

Table 5 Product distribution on different catalyts

Conclusions

An environmental friendly method was introduced to utilize the FCC flue gas dust in this study. A new matrix was first prepared with exhausted FCC flue gas dust. Then FCC catalysts were prepared by partial substitution of pseudo-boehmite binder by this new matrix. Compared with conventional catalyst, the FCC catalysts with new matrix have better heavy oil conversion and coke selectivity, and increased light oil yield and total liquid yield. Therefore, the utilization of flue gas dust in this new strategy has a good application prospect and remarkable economic benefits.

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Acknowledgments

This work was financially supported by the Refinery of Zhongyuan Oilfield Company, SINOPEC and Qingdao Huicheng Petrochemical Technology Co., Ltd (Project No. 14-ZC0607-001).

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Correspondence to Zejun Zuo.

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Zuo, Z., Lv, L., Liu, C. et al. Utilization of exhausted dust from FCC flue gas as material for FCC catalyst preparation. Appl Petrochem Res 5, 215–220 (2015). https://doi.org/10.1007/s13203-015-0126-x

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Keywords

  • Matrix
  • Flue gas dust
  • FCC catalyst
  • Fixed fluidized bed