Residue Upgrading in Slurry Phase over Ultra-fine NiMo/γ-Al2O3Catalyst

(SINOPEC Research Institute of Petroleum Processing, Beijing 10083)

Residue Upgrading in Slurry Phase over Ultra-fine NiMo/γ-Al2O3Catalyst

Tong Fengya; Yang Qinghe; Li Dadong; Dai Lishun; Deng Zhonghuo

(SINOPEC Research Institute of Petroleum Processing, Beijing 10083)

In this article, residual oil hydroconversion was studied in slurry phase in the presence of fine solid NiMo/γ-Al2O3catalyst and the effects of operating conditions were carefully studied. The results showed that residue conversion was only affected by the reaction temperature and reaction time. The coke yield increased with a higher reaction temperature, a bigger catalyst particle size, a longer reaction time, a lower initial hydrogen pressure and a lower catalyst concentration. Heteroatoms removal rate increased with a higher reaction temperature, a longer reaction time, a higher initial hydrogen pressure, a higher catalyst concentration, and a smaller catalyst particle size. The role of catalyst in the slurry bed technology was discussed and its function could be stated as follows: the metal was applied to activate the hydrogen atoms for removing heteroatoms and saturating aromatics, while the support of the catalyst was used to prevent the mesophase coalescence for reducing coke formation.

slurry phase; residual oil; NiMo/γ- Al2O3; upgrading; mechanism

1 Introduction

The aim of residue oil upgrading is removing the heteroatom compounds such as S, N, nickel, vanadium and carbon residue to a certain degree to offer high quality feed for residue fluid catalytic cracking (RFCC), and finally producing lower boiling point distillates with higher commercial value than the residuum feedstock[1]. Two technologies can be used to upgrade residual oil, which are the carbon rejection technique and the hydrogen addition technique. Compared to the carbon rejection method, the hydrogen addition method can obtain higher liquid yields and better quality products to become a popular technology in today’s oil industry.

The hydrogen addition technology can be divided into four kinds, viz.: the fixed-bed, the moving-bed, the ebullated-bed and the slurry-bed. The fixed-bed technology is, in principle, simpler and can result in a stable and reliable performance. However, it is strongly limited by feedstock characteristics, because the fixed bed catalyst is quite sensitive to poisons contained in the feed and to coke formation. Inefficiency due to fast deactivation of the catalyst and an uneven temperature profile in the catalyst bed has prevented the extensive use of this technology for the hydroconversion of heavy feeds[2]. Compared to the fixed bed process, the ebullated bed technology is more flexible to the feedstock slate, because it can process more inferior feeds containing high amount of metals and coke, but its limitation on the overall conversion rate that usually reaches less than 80% may cause concern.

The slurry-bed technology is reliable for processing the inferior feedstock to achieve a high conversion, and will become a potential technology in the future. However, several challenges exist in the slurry phase hydroconversion. The cost of catalyst is an important one. Till now, the catalyst of slurry bed hydrocraking can be divided into two kinds, among which the first one can be named the additive, which has no hydrogen addition ability, and the second one can be named the catalyst, which has a high catalytic activity[3].The effect of the additive has a significant effect on the residual oil hydroprocessing only at a high residue conversion, and can be concluded as the physical adsorption of the mesophase, which could form coke. The function of the catalyst is offering active hydrogen atoms to prevent the aggregation of macro free radicals and finally reduce the coke formation. Nowadays,the catalysts used in slurry phase hydroconversion by the researchers[4-6]and by the companies[7-10]are almost dispersed MoS2, which are produced through a costly precursors in situ. These catalysts have a good function for reducing the coke formation and removing the heteroatoms, and the heteroatoms have nowhere to deposit because MoS2has a smaller surface area. For this purpose, some solid additives need to be added. The supported NiMo/ γ-Al2O3catalysts with a comparatively low price and large surface area, which are mostly used in fixed-bed residue hydrotreating, show a high activity in residue processing, and may be a good choice to solve the catalyst cost challenge and other problem related with the dispersed catalyst. Researcher[11]once used spent HDS catalyst for residual oil hydrocracking in a pilot plant, and found out that it had a good activity and was more economical than the dispersed MoS2catalyst, but this statement still lacked a deepened study.

In this paper, the fine supported NiMo/γ-Al2O3catalyst was used for processing residual oil and the effects of operating conditions were studied.

2 Experimental

2.1 Feedstock and catalyst properties

The feedstock used in the experiments was a commercial mixture of VR and HCO, with its typical properties shown in Table 1. The supported NiMo/γ-Al2O3catalyst was sized to certain mesh, and its properties covered: a NiO content of 2.5%, a MoO3content of 15.4%, a BET surface area of 105.6 m2/g, and a total pore volume of 0.565 cm3/g.

2.2 Experimental procedure

The experiments were carried out in a stirred batch autoclave (Parr 4757). A 0.5-L stainless autoclave was filled with 200 g of residue (heated to about 100 ℃) and a certain amount of catalyst. A theoretical CS2dosage was sprinkled to the hot residue and immediately stirred. Then the autoclave was flushed with hydrogen to reach the reaction pressure, and after this procedure the operation was started. When the temperature reached 350 ℃, the catalyst was presulfided for two hours. After the reaction was completed, the gas was vented. The liquid and catalyst mixture were dissolved in toluene and then filtered. Toluene was then removed from the filtrate by a rotary evaporator to collect the liquid product. The total liquid product was weighed carefully. The liquid product was analyzed to determine the S, MCR and vanadium content.

2.3 Measurement

The removal rates in this paper are defined by the general equation as shown below:

where A is one of MCR, S content, and vanadium (V) content; AFis the mass fraction of A in feedstock, %; WFis the mass of A in the feedstock, g; APis the mass fraction of A in liquid product, %; and WPis the mass of A in the liquid product, g.

Residue conversion = (the mass of >500 ℃ fraction in feedstock - the mass of >500 ℃ fraction in liquid product)/the mass of >500 ℃ fraction in feedstock×100%.

Coke yield = (the mass of the feedstock- the mass of gas– the mass of liquid)/the mass of feedstock × 100%.

Table 1 Properties of the feed

3 Results and Discussion

3.1 Effect of temperature on residue hydroconversion

The effect of temperature on the hydroconversion of residue was studied at an initial hydrogen pressure of 10 MPa, a reaction time of 4 h and a catalyst dosage of 3% (based on the catalyst support). The catalyst was ground to a size smaller than 0.106 mm. The results are shown in Figure 1. Figure 1 clearly shows that with an increasing tem-perature the residue conversion, the sulfur removal rate and the coke yield all increased. When the temperature was increased from 360 ℃ to 400 ℃, the MCR conversion increased, but when the temperature was higher than 400 ℃, the MCR conversion decreased. During the experiment, at a higher temperature the sulfur, vanadium, nitrogen and nickel removal rate, as well as the residue conversion were all higher. The residue conversion was mainly realized through thermal reaction, so the temperature had a great effect on the conversion rate[12]. The researcher[12], upon studying the Athabasca bitumen residuum upgrading under nitrogen, hydrogen, or hydrogen gas in the presence of a hydrotreating catalyst at different residue conversion rates, found out that little difference was identified among the relative rates of conversion under the same operating condition. It was concluded that at the first 30%—40% of residuum conversion, the phenomenon in which the labile carbon-to-carbon bonds were broken up to produce distillate that constituted the main reaction, while the additional conversion led to the formation of an aromatic-carbon and aliphaticcarbon bi-radical intermediate, which was formed from hydroaromatic structures. The activation energy of the hydrodesulfurization and hydro-MCR was 37.4kJ/mol and 109.4 kJ/mol, respectively, which could illustrate that the MCR conversion was higher than sulfur conversion at a varying temperature[13].

Figure 1 Heteroatoms removal rate and coke yield as a function of temperature

The MCR represents the coke formation tendency of the oil and it is mainly composed of polycyclic aromatics[14]. The removal of MCR means hydrogen is added to the aromatics and the number of rings in polycyclic aromatics is reduced. Figure 1 shows that the MCR conversion at first increased and then decreased with an increasing temperature. This occurred because at higher temperature hydrogen could readily produce free radicals to hydrogenate more aromatic compounds. But at 400 ℃, the MCR decreased, because at higher temperature the polycyclic aromatics could coalesce to form molecules with higher carbon residue.

The effect of temperature indicated that neither lower temperature nor higher temperature was beneficial to residue conversion.

3.2 Effect of reaction time on hydroconversion of residue

The experiments of heteroatoms removal rate and coke yield as a function of reaction time were conducted at an initial hydrogen pressure of 10 MPa, a reaction temperature of 400 ℃, and a catalyst dosage of 3% (based on the catalyst support) with a particle size of less than 0.106 mm, with the test results presented in Figure 2. Figure 2 clearly shows that as the reaction time increased, the heteroatoms removal rate and coke yield all increased. Upon comparing Figure 1 with Figure 2, it can be seen that to achieve a residue conversion of 80%, the coke yield was approximately 3% through changing the reaction time, while the coke yield was 4% through changing the reaction temperature. This means that an appropriate reaction time or reaction temperature could all lead to an increase in residue conversion, but the conversion was more sensitive to temperature which would also lead to higher coke yield. To solve this problem, ENI’s commercial plant operates at a lower temperature coupled with a longer residence time.

Figure 2 Heteroatoms removal rate and coke yield as a function of reaction time

3.3 Effect of pressure on hydroconversion of residue

The experiments for investigating the heteroatoms removal rate and coke yield as a function of reaction pressure were conducted at a reaction temperature of 400 ℃, a reaction time of 4 h and a catalyst dosage of 3% (based on the catalyst support) with a catalyst particle size of less than 0.106 mm, with the results shown in Figure 3. It can be seen from Figure 3 that the reaction pressure obviously did not affect the heteroatoms removal rate, but could significantly affect the coke yield. When the pressure increased from 6 MPa to 12 MPa, the coke yield decreased from about 2.0% to 0.9%. Coke was produced via the coalescence of large condensed nuclei of free radicals of aromatic compounds, and any material that could prevent its coalescence would reduce the coke formation. Higher pressure could promote the dissolving of hydrogen in the liquid oil. The dissolved hydrogen could provide a hydrogen free radical to the aromatic free radical, and finally inhibit the coalescence of aromatic free radicals.

It can be seen from Figure 3 that residue conversion was not affected by hydrogen pressure. Beaton[15]found out that residue conversion was primarily a thermal reaction and involved a free-radical mechanism to convert residue molecules to hydrocarbon gases, naphtha, distillates, and gas oils. He also found out that hydrogen must be present to prevent the retrograde polymerization reactions in the process, but the level of conversion was independent of hydrogen partial pressure. The results of this paper were the same as those referred to in the literature.

Figure 3 Heteroatoms removal rate and coke yield as a function of pressure

3.4 Effect of catalyst concentration on hydroconversion of residue

The experiments for investigating the heteroatoms removal rate and coke yield as a function of the concentration of catalyst with a particle size of less than 0.106 mm were conducted at a reaction temperature 400 ℃, a reaction time of 4 h, and a pressure of 10 MPa, with the results shown in Figure 4. The residue conversion was not affected by the catalyst concentration, because it was a thermal reaction process. At a higher catalyst concentration, the coke yield was lower, because the role of catalyst was to supply hydrogen to the heavy oil fraction and prevent the coke formation[12]. The catalyst also promoted the heteroatoms removal and aromatics saturation in the light oil fractions, leading to an increase in the heteroatoms removal rate.

Figure 4 Heteroatoms removal rate and coke yield as a function of catalyst concentration

Hydrotreating catalysts contain expensive metals such as cobalt, nickel, and molybdenum. A higher catalyst concentration means a higher cost. From the economics view point, the catalyst concentration should be at a proper range. For the slurry residue hydroconversion process, the proper catalyst concentration should be less than 5 wt% (based on the feedstock).

3.5 Effect of catalyst size on hydroconversion of residue

The experiments for investigating the heteroatoms removal rate and coke yield as a function of catalyst particle size were conducted at a reaction temperature of 400 ℃, a reaction time of 4 h, an initial hydrogen pressure of 10 MPa and a catalyst dosage of 3.0% (based on the catalyst sup-port), with the results shown in Figure 5. In Figure 5, the number “1, 2, and 3” represents a catalyst with a particle size of <20 mesh, 20—150 mesh, and >150 mesh, respectively. Figure 5 shows that the heteroatoms removal rate increased with a decreasing catalyst particle size. The residue was composed of a group of macromolecules which concentrated most of the heteroatoms. During the process for removing the heteroatoms, the macromolecules at first entered the catalyst pores and then were adsorbed on the active sites, which actually controlled the reaction rate. When the catalyst was small in size, its active sites were easily exposed which would be more available for the macromolecules, and at last could achieve a higher heteroatom removal rate. For this reason, the slurry bed could use a lower catalyst concentration to obtain a higher heteroatom removal rate than the fixed bed. However, the small catalyst might bring about some problems associated with separation of the catalyst from the liquid products.

Figure 5 Heteroatoms removal rate and coke yield as a function of catalyst size

3.6 The role of fine NiMo/γ-Al2O3in the residue hydroconversion

No matter what kind of the four technologies of residue hydroconversion was adopted, coke formation should be prevented. So the catalyst added to the hydroconversion process was not only aimed at reducing the heteroatoms content but also at reducing the coke formation.

Sanford[12]has proposed a mechanism of residue hydrocraking. He assumes that the main role of catalyst is to activate the hydrogen atoms that can enter into reaction with the aromatic carbon radicals to form the cyclohexadienyl type intermediate. Once the intermediate is formed, the molecule would decompose through a series of reactions to form distillate and gases. This is the process that can reduce coke formation otherwise the intermediate would very fast turn into coke (also named as solids). The researchers[16]in UOP have assumed that during the residue hydroconversion, the role of catalyst can be divided into two parts. The first part is that the catalyst can activate the hydrogen to produce hydrogen atoms, and the hydrogen atoms then take part in the reaction to remove the heteroatoms and saturate the aromatic hydrocarbons. The second part is that the catalyst in the oil phase can prevent the mesophase coalescence which is the main way leading to coke formation. To some extent, this suggestion seems more consummate.

4 Conclusions

(1) The residue conversion was only affected by reaction temperature and reaction time, while other operating conditions had little effect on it.

(2) The coke yield increased with an increasing reaction temperature and catalyst particle size, while it decreased with an increasing reaction time, initial hydrogen pressure, and catalyst dosage.

(3) The heteroatoms removal rate increased with an increasing reaction temperature, reaction time, initial hydrogen pressure, and catalyst concentration, while it decreased with an increasing catalyst particle size.

(4) To gain the same residue conversion, the increase of reaction temperature led to more coke yield than that achieved by the increase of the reaction time.

(5) The role of catalyst in the slurry bed technology was also discussed, and its function could be assumed as follows. The metal could activate the hydrogen atoms for removing the heteroatoms and saturating the aromatics, and the support of the catalyst was dispersed in the oil phase to prevent the mesophase coalescence to reduce the coke formation.

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Catalyst for Manufacture of 1,4-Butanediol (BDO) Developed by FRIPP Passed Technical Review and Appraisal

The project for improving the catalyst technology for manufacture of 1,4-butanediol (BDO) undertaken by the Fushun Research Institute of Petroleum and Petrochemicals (FRIPP) has passed the review and appraisal tests organized by the Sinopec’s Science and Technology Division. This technology includes the development of the improved DZH type catalyst for esterification of maleic anhydride and the development of the improved FHE type catalyst for hydrotreating of dimethyl maleate. Hence the research staffs by means of enhancing the catalyst exchange capacity, the catalytic reaction activity and its mass transfer efficiency, coupled with changes in the solvent for stabilizing the active radicals, have significantly improved the catalytic activity, thermal stability and service life of the improved DZH catalyst for esterification of maleic anhydride. The commercial application tests of this catalyst in the 10 kt/a unit have revealed that the acid number of the reaction product was decreased to 0.29 mg KOH/g, while the service life of the first batch of catalyst was increased to 4568 hours capable of reducing the operating cost, with its catalyst performance being on a par with similar overseas catalysts.

The research staffs, by means of adjusting the formulation of the FHE type catalyst for hydrotreating of dimethyl maleate, optimizing the pore structure and reducing the total acidity of this catalyst, have prepared the improved FHE-1/2 type catalyst with higher activity and better stability. The commercial application tests of this catalyst in the 10 kt/a unit have revealed that the conversion of dimethyl maleate was equal to 99.6%, and the total BDO, tetrahydrofuran and γ-butyrolactone selectivity reached 99%, with the overall performance of this catalyst being on a par with similar overseas catalysts.

date: 2015-03-13; Accepted date: 2015-05-24.

Yang Qinghe, Telephone: +86-10-82368123; E-mail: yangqh.ripp@sinopec.com.