The enhancement on the waste management of spent hydrotreating catalysts for residue oil by a hydrothermal–hydrocyclone process
Graphical abstract
Introduction
The impact of catalysis and catalysts is substantial [1]. Today over 90% of all industrial processes are produced with the aid of catalysts including the refining of fossil fuel, the manufacture of chemicals, the production of synthetic materials and the treatment of the environmental pollutants. The consumption of the industrial catalysts was up to 800,000 ton/year in 2012, which brought the sales exceeding $16.3 billion [2]. Among them, the refining catalyst accounts for about 24% of the world market and the catalysts for hydrotreating (HDT) and fluid catalytic cracking (FCC) contribute the largest share of refining catalysts [3]. The deactivation of the catalyst is inevitable and the spent catalysts generally discard as the solid wastes. It is estimated that the total amount of spent HDT catalyst generated worldwide is 150,000–170,000 ton/year [4]. Potentially harmful constituents over a spent catalyst may be classified into two groups, (i) those that are present in the fresh catalyst, and (ii) those that are deposited during use [5]. Thus, from the aspect of environmental, health and safety, there is a demanding concern of the spent catalyst due to the catalyst itself or the deposited hazardous volatile or persistent organics [6].
Nowadays, the demand of heavy fuel oil as marine engineering fuel has decreased significantly, whereas, the need for light and clean fuel has moved toward the opposite direction. It has been reported that NiMo/SiO2-Al2O3 mixed oxide supported catalysts are suitable for such a purpose, because of their acidic and textural properties [7]. However, these catalysts would deactivate during time on stream mainly due to coke formation and poisoning [8]. About 20–30% of the initial surface area and pore volume was lost due to the coke deposition. Meanwhile, with the times on stream increases, more metal-sulfide compounds would deposit in the pores of the catalysts [9]. The deposition of coke, metal and oil would make the spent catalysts heavier than the fresh catalysts, which generates more industrial waste. For instance, the feedstock of the residue oil in 2015 will increase 6 times of that in 2010 [10], [11]. More commercial units devoted to heavy oil hydroprocessing with moving beds, such as H-Oil and LC-Fining, which could compensate for catalyst deactivation through adding catalysts at the at the top of the reactor and withdrawing at the bottom [12]. The consumption of moving bed is larger than fixed bed. It is obvious that the quantity of the spent catalysts, especially that of HDT catalyst of residue oil, will greatly increase simultaneously.
The non-regenerable catalysts were either sent to metal reclaimers or disposed of in commercial landfills [13]. The de-oiling and the following mechanical (hydraulic) separation are essential operations for the management of spent HDT catalysts. The weight reduction of nearly 20% will be achieved and the toxic substances (such as PAHs, PCBs and PCPs) with high flexibility will be controlled [14]. Various techniques were proposed for the removal of oil and toxic organic compounds attaching on the surface of porous solid materials. These techniques were widely applied not only in the treatment of spent catalysts, but also in the management of other solid wastes, namely oily sludge, contaminated soil. Thermal treatment is one of the most popular techniques [15], [16]. However, the overheating of the spent catalysts will lead to the sintering of the metal and take adverse effects on the reclamation of the metal. Specifically, the treatment temperature for the residue-oil contaminated system should be over 400 °C. The application of organic solvent extraction will facilitate the release of oil from the spent catalysts. But the recycle of the solvent is a necessary operation [17]. The hydrothermal method is a more versatile technique on the removal efficiency, the instrumental investment and the operational time. The addition of other reagents, such as NaOH and organic surfactants [18], [19], will accelerate the release of oil and the application of ultrasonic or sparging air can also enhance the process significantly [20], [21].
In the present research, the hydrothermal desorption was carried out in a novel two-step procedure, namely hydrothermal–hydroclone process. The spent catalysts were continuously treated by a stirring hydrothermal tank and a subsequent hydrocyclone. To the best of our knowledge, it is the first time a hydrocyclone was used for the management of spent catalysts. The artificial catalysts and the actual spent catalysts discarded from a pilot plant of residue-oil hydroprocessing in FRIPP (SINOPEC, China) were both applied as the model porous materials. Generally, hydrocyclones belong to a class of fluid–solid classifying devices that separate dispersed material from a fluid stream [22]. Compared with the traditional separation apparatus, such as filters and centrifuges, hydrocyclone is a powerful separation equipment with numerous advantages, such as large processing capacity, low cost, easy to scale up, and high stability of operation [23]. Additionally, it was concluded that the application of the hydrocyclone can greatly enhance the release of contaminated hydrocarbon from the contaminated catalysts due to the forced self-rotation of the catalyst particles within the hydrocyclone. The application of hydrocyclone and the design process can also be used for the treatment of other solid wastes, including contaminated soil, oily sludge and so forth.
Section snippets
Materials
The commercial wash-oil from JLPEC (SINOPEC, China), 50 tabulated bed residue hydrocracking process (STRONG, Sinopec Technology of Residue Oil New Generation) was used for the simulation of the oil-containing catalysts. The physical chemical properties of the wash-oil were shown in Table 1. Two kinds of catalysts obtained from the pilot plant (4 L/day) in FRIPP (SINOPEC, China) were also applied in this study (see Table 2). FES-2 is the fresh catalyst used for the residue-oil hydrotreating and
Characterization of the spent catalysts
The fabrication of the porous spent catalyst was studied by N2 adsorption-desorption method at 77 K. The isothermal curve was shown in Fig. 1a. According to the catalog of IUPAC, the catalyst showed a type IV N2 isotherm, which was characteristic of meso- microporous materials. On the basis of the adsorption branch, the specific surface area of the sample was calculated to be 231.6 m2 g−1 by the BET model. The distribution of the pore size and the pore volume can be found in Fig. 1b. The pore size
Conclusions
The hydrothermal–hydrocyclone process is an efficient treatment route for the management of the oily spent catalyst. Within an experimental system with a semi-pilot scale (feedstock 802 L/h), 93.5% of the contaminated oil was removed from the surface of the spent catalysts. The oil-containing washing water was cycling within the unit for 30–40 times till the oil content reached as high as 0.5 wt%. In following, the oil was removed from water by an oil-water separator, then gathered with other
Acknowledgment
The authors are grateful to the sponsorship from the National Key Basic Research Program of China (2014CB748500), the National Science Foundation for Distinguished Young Scholars of China (51125032), the National Science Foundation (51308215, 51401295), the Fundamental Research Funds for the Central Universities (222201313001, 22220141416001, 22220141414011), International Cooperation Project of Shanghai Ministry of Science and Technology (14230710700) and the Chenguang Project of Shanghai
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