Desulfurization of high sulfur petroleum coke by molten caustic leaching

Egyptian Journal of Petroleum 28 (2019) 225–231

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Desulfurization of high sulfur petroleum coke by molten caustic leaching Hadis Askari a, Farhad Khorasheh a,⇑, Saeed Soltanali b,⇑, Shokoufe Tayyebi b a b

Department of Chemical & Petroleum Engineering, Sharif University of Technology, Tehran, Iran Research Institute of Petroleum Industry (RIPI), P.O. Box: 14665-137, Tehran, Iran

a r t i c l e

i n f o

Article history: Received 3 February 2019 Revised 17 March 2019 Accepted 4 April 2019 Available online 11 April 2019 Keywords: Desulfurization Molten caustic leaching Petroleum coke Potassium hydroxide Taguchi L9 design

a b s t r a c t Desulfurization by molten caustic leaching (MCL) at 400–500 °C has been investigated in order to reduce the sulfur content of petroleum coke. Effective parameters on desulfurization of petroleum coke, other than temperature, include alkali to feed (petroleum coke) mass ratio, time and mesh size in the ranges of 0.5–1.5, 1–3 h and 200–600 mm, respectively. In this work, petroleum coke desulfurization conditions using solid KOH have been studied. Maximum petroleum coke desulfurization by MCL method has been obtained by Taguchi L9 design using alkali to feed mass ratio of 1, temperature of 600 °C, time of 2 h and mesh size of 200 mm. The changes in the main groups on the coke surface have been determined using FTIR spectroscopy. In addition, SEM-EDX, TGA and XRD analyses have been used to investigate the changes in coke physical and chemical properties. Ó 2019 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1. Introduction Delayed coking is one of the important processes in upgrading the quality of heavy petroleum cuts [1]. The petroleum coke obtained in this process has different applications such as fuel or anode in the aluminum industry. The sulfur present in petroleum coke causes environmental problems. On the other hand, the coke used in the aluminum industry must contain a certain amount of sulfur. Therefore, petroleum coke desulfurization is of special importance [2–4]. The sulfur present in petroleum coke is dependent on the structure of the coking unit feed. In general, the sulfur is inorganic form and includes sulfur compounds such as sulfates and pyritics as well as elemental sulfur [5,6]. Generally speaking, the sulfur in the petroleum coke may be bound to the aromatic framework lying between the aromatic rings, located on the surface of the aromatic structure or connected to carbon chains. Desulfurization is much harder in the first two cases [7–9]. There are several petroleum coke desulfurization methods. In the solvent extraction method, different solvents are mixed with petroleum coke to find the sulfur selective solvent. Since most sulfur compounds in petroleum coke are aromatics, aromatic solvents Peer review under responsibility of Egyptian Petroleum Research Institute. ⇑ Corresponding authors. E-mail addresses: [email protected] (F. Khorasheh), [email protected] (S. Soltanali).

and solvents with structures similar to the sulfur compounds in coke are more appropriate for this purpose. Researchers have used various chemical solvents such as phenol, ether, hydrochloric acid and ortho-chlorophenol for petroleum coke desulfurization, maximum desulfurization achieved being 25% [10–15]. Another petroleum coke desulfurization method is high temperature calcination, in which the sulfur content of petroleum coke is reduced due to the high temperature of the process. In this method, moisture is completely removed at 100–200 °C [16–18]. The sulfur bands present on the coke surface and the sulfur compounds in the pores are then removed at 500 °C [5, 18 and 19]. The sulfur compounds in C-S2, RSH and COS are also burnt on further temperature increase [18,19]. Hydrogen desulfurization has been widely used in refineries for sulfur removal. However, this method has not yet been applied in petroleum coke desulfurization in industrial scale. The advantage of this method compared with molten caustic leaching is that the coke structure will not appreciably change at higher temperatures. In this method, hydrogen or air is passed through the coke to produce H2S gas. The degree of desulfurization is improved depending on the type of the collision of hydrogen gas or air with coke. The mechanism of the process is as follows [5, 11 and 23]:

ðC  SÞsolid þ H2 $ Csolid þ H2 S

ð1Þ

Desulfurization by molten caustic leaching (MCL) method another method for desulfurization of petroleum coke. Coke mixed with alkali in this method. The solid form or a solution the alkali is mixed with the coke powder and the mixture

https://doi.org/10.1016/j.ejpe.2019.04.001 1110-0621/Ó 2019 Egyptian Petroleum Research Institute. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

is is of is

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ultimately dried. The sample is then placed in an oven at 400– 600 °C in an inert gas atmosphere. Different parameters have been tested in this method to obtain different desulfurization percentages [20,21]. Wang et al. achieved 91% desulfurization at 500 °C during 2 h using NaOH as the alkali [22]. In this research, using Taguchi L9 design, the effective parameters on petroleum coke (green coke) desulfurization process by MCL method have investigated. The parameters studied include alkali to feed (petroleum coke) mass ratio, temperature, time and coke mesh size in the ranges of 0.5–1.5, 1–3 h and 200–600 mm, respectively. Furthermore, FTIR, SEM-EDX, TGA and XRD analyses have been used to investigate the physical and chemical properties of crude coke and coke desulfurized under optimal conditions.

2. Experimental 2.1. Materials Petroleum coke was obtained from the delayed coking pilot of the Research Institute of Petroleum Industry. Potassium hydroxide, hydrochloric acid and sodium hydroxide were supplied by Merck Chemical Company. Deionized water was used throughout the experiments. 2.2. Apparatus and tests procedure Three different sizes of a petroleum coke sample (200, 400 and 600 mm) from the delayed coking pilot of the Research Institute of Petroleum Industry (RIPI) were meshed and dried in an oven for 12 h. The specifications of the crude petroleum coke are given in Table 1. Firstly, 10 g of the crude petroleum coke and a certain amount of the alkali were mixed and fed into a reactor, which was then placed in a furnace. A flow of nitrogen gas was passed through the reactor. The furnace thermal flux was set based on minutes. The process was carried out for 1–3 min when the furnace temperature reached 400–600 °C range. When the reaction was over, the reactor was removed from the furnace and the coke and alkali mixture was washed with distilled water, followed by acid washing. The sample was then dried after washing and filtration. 2.3. Characterization 2.3.1. X-ray diffraction (XRD) X-ray analyses were conducted by XRD to identify the crystal structure using a model D5000 Siemens instrument (scan speed 0.04 s1, range 2h, between 5 and 50°with Cu-ka radiation and 0.154056 wavelengths in 30 kV and 40 mA). The products were characterized by comparison of XRD analysis with reference XRD diagrams.

Table 1 Composition of petroleum coke from pilot of RIPI. Analysis Ash (% wt) volatile (% wt) Moisture(% wt) S (%wt) V (ppm) Ni (ppm) Na (ppm) Fe(ppm) Si (ppm)

2.3.2. Sulfur analysis (FE-SEM) Leco test was performed using a CS 125 model instrument to determine the total sulfur content. 2.3.3. Field Emission Scanning Electron Microscope (FE-SEM) FE-SEM analysis was carried out to determine morphology and particle size using a field emission electronic microscope (FE-SEM), ZEISS, SIGMA Series. 2.3.4. Energy-dispersive X-ray spectroscopy (EDXS) EDXS analysis using a Zeiss instrument was carried out to determine the percentage of the elements in petroleum coke. 2.3.5. Fourier transform infrared spectroscopy (FT-IR) FT-IR spectroscopy experiments were performed by a Bruker, Alpha Series spectrophotometer to analyze sample structures. 2.3.6. Thermal gravimetric analysis (TGA) The thermal behavior of the coke was studied by TGA analysis using a STA 1500-Rheometric Scientific instrument. For this purpose, the sample was heated at the rate of 10 °C/min up to 800 °C in an air atmosphere to determine the weight reduction. 2.4. Taguchi orthogonal array and experimental parameters In the current study, the Design Expert (7.0.0) software was used. The L9 orthogonal array of the Taguchi method was chosen to optimize the desulfurization of petroleum coke by MCL method. The selected factors and their levels are listed in Table 2. Four factors (mass ratio of alkali to coke, time, temperature, and mesh size) with three levels for each factor were investigated. The number of permutations with simple factorial design for the optimization of the assigned three levels of each parameter would be 38. However, the fractional factorial design reduced the number of experiment to 9. 3. Results and discussion The effective parameters on petroleum coke desulfurization process by molten caustic leaching (MCL) method, including alkali to feed (petroleum coke) mass ratio, temperature, time and coke mesh size, have been studied by Taguchi model. The orthogonal array structure, values of effective parameters and percent desulfurization for each test are given in Table 3. Percent desulfurization of crude petroleum coke is calculated using the following formula: Desulfurizationð%Þ ¼

sulfurinraw cokeðwt%Þ  sulfur in treated cokeðwt%Þ sulfur in raw cokeðwt%Þ  100

Table 4 indicates the analysis of variance (ANOVA), which specifies the effect of each parameter. The objective of analysis of variance is the investigation of the parameter, which is the most effective on petroleum coke desulfurization. The results are given in Table 4. As observed, alkali to feed mass ratio is the most effective parameter on coke desulfurization by molten caustic leaching

ASTM method 1.16 17.0 0.3 6.18 700 400 10 1800 1000

ASTM D3174 ASTM D3175 ASTM D3173 ASTM D4292 Wet & AAS Wet & AAS Wet & AAS Wet & AAS Gravimetry

Table 2 Effective parameters and their levels. Parameter

Level 1

Level 2

Level 3

Alkali/coke (g/g) Time (hr) Temperature (°C) Mesh size (lm)

0.5 1 400 200

1 2 500 400

1.5 3 600 600

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H. Askari et al. / Egyptian Journal of Petroleum 28 (2019) 225–231 Table 3 Design of experiments for desulfurization of petroleum coke according to Taguchi method. Sample

Alkali/coke (g/g)

Time (hr)

Temperature (°C)

Mesh size (lm)

sulfur (wt%)

desulfurization (%)

Run-1 Run-2 Run-3 Run-4 Run-5 Run-6 Run-7 Run-8 Run-9

0.5 0.5 0.5 1 1 1 1.5 1.5 1.5

1 2 3 1 2 3 1 2 3

400 500 600 500 600 400 600 400 500

200 400 600 600 200 400 400 600 200

5.37 5.28 4.56 3.45 2.11 3.53 0.97 2.35 3.01

13.10 14.56 26.21 44.17 65.85 42.88 84.30 61.97 51.29

Table 4 ANOVA analysis results for all parameters. Parameter

DOF (f)

Sum of Squares (SS)

Variance (V)

Pure sum (SS’)

P (%)

Alkali/coke Time Temperature Mesh size Total

2 2 2 2 8

3694.91 106.99 896.48 25.80 4724.18

1847.46 53.49 448.24 12.90

3694.91 106.99 896.48 25.80

78.21 2.26 18.97 0.54 100

(MCL) method. The second most effective parameter is temperature while the other two parameters (time and mesh size) are the least effective. 3.1. Effect of parameters on desulfurization 3.1.1. Effect of Alkali/coke ratio Fig. 1(a) shows the effect of alkali to feed mass ratio parameter. As observed, the higher is the alkali to feed mass ratio, the higher is the percent desulfurization. Potassium hydroxide seems to react with the coke to produce potassium sulfide. Thus, the higher is the amount of potassium hydroxide, the greater is the desulfurization. Potassium hydroxide is a strong base and attacks the carbon sulfur bond. The possible reactions are shown below. The major problem of this parameter is the separation of coke from the alkali.

Therefore, the optimal value of this parameter must be obtained to achieve the ideal percent desulfurization.

Cokes  S þ 2KOH ! K2 S þ Cokes  O þ H2 O

ð2Þ

K2 S þ 2H2 O ! 2KOH þ H2 S

ð3Þ

4KOH þ CH2 ! K2 O þ K2 CO3 þ 3H2

ð4Þ

K2 O þ C ! 2K þ CO

ð5Þ

K2 CO3 þ 2C ! 2K þ 3CO

ð6Þ

C þ H2 O ! CO þ H2

ð7Þ

CO þ H2 O ! CO2 þ H2

ð8Þ

Fig. 1. Main effects of (a) mass ratio of alkali/coke, (b) time, (c) temperature, (d) mesh size on the desulfurization of petroleum coke.

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3.1.2. Effect of time The effect of time parameter on petroleum coke desulfurization by MCL method is shown in Fig. 1(b). As shown, no steady trend in desulfurization is observed as the process takes longer. This is due to the interaction of the other parameters. Time has no appreciable contribution in the desulfurization process. In addition, some reverse reactions seem to occur and sulfur returns to the coke structure. 3.1.3. Effect of temperature Desulfurization percent increases as a function of temperature. This effect is shown in Fig. 1(c). However, since increased temperature increases operation costs, the optimal conditions must be determined in order to achieve the desirable percent desulfurization. Considering the results obtained, higher temperature makes KOH more active in the reaction and thus causes increased desulfurization. 3.1.4. Effect of mesh size The effect of mesh size parameter on petroleum coke desulfurization by MCL method is shown in Fig. 1(d). As observed, as the coke mesh size increases, negligible changes are observed in the desulfurization. The diagram shows that mesh size parameter plays a small part in petroleum coke desulfurization. The smaller is the mesh size of the coke, the better the desulfurization must be to increase the desulfurization surface. The desulfurization power seems to be so great that it is independent on the particle mesh size or the difference is not tangible since the mesh sizes are not very different. Bigger difference in mesh sizes would be more observable. DOF for each component expresses the degree of freedom of the parameter and is obtained using Eq. (9). In addition, the variance of each parameter is found by dividing the sum of the squares of each component into its DOF, as shown in Eq. (10). Percent P, which shows the effect of each parameter, is obtained by dividing the sum of squares of each component into the sum of squares [Eq. (11)].

DOF of a factor ¼ number of the level  1

aluminum, making it impure. In addition, the sulfur content has decreased and the specifications have reached those of anode coke. As shown by the data in Table 6, the ash content of petroleum coke has increased, which may be due to the potassium oxide produced during desulfurization. Increasing potassium oxide content of the petroleum coke increases its ash content. Table 5 Optimum conditions for alkali calcination experiment. Parameter

Level

Level description

Alkali/coke Time Temperature Mesh size

3 1 3 2

1.5 1 600 400

Table 6 Specifications of crude petroleum coke, anode coke and desulfurized petroleum coke by MCL method.

Ash (% wt) S (%wt) K (ppm) V (ppm) Ni (ppm)

Crude petroleum coke

coke desulfurized

Anode coke

1.16 6.18 157 700 400

1.696 2.11 350 500 280

0.1–0.3 1.7–3 200 165–460 120–350

ð9Þ

VA ¼

SSA DOF

ð10Þ

PA ¼

SSA  100 SST

ð11Þ

where, SSA is the sum of squares of factor A, SST is the total sum of squares. The higher is the percent p of a parameter, the greater is the significance and effect of that parameter on the results. Therefore, the alkali to feed mass ratio is the most effective parameter in petroleum coke desulfurization, the other effective parameters being temperature, time and mesh size, respectively. R-square for this model, which shows the percent distribution of the predicted values around real values, is 0.9657 for the adsorption capacity. The closer this figure is to one, the greater is the model accuracy. In order to achieve the highest sulfur removal in petroleum coke desulfurization, each of the effective parameters must be optimized. The optimal process conditions are given in Table 5. The best percent desulfurization achieved was almost 85%. The specifications of petroleum coke subjected to desulfurization by molten caustic leaching method are given in Table 6. As observed, during the molten caustic leaching process, nickel and vanadium metal contents have somewhat decreased and the specifications have approached those of anode grade coke. The metals present in petroleum coke cause formation of impurities in

Fig. 2. XRD patterns of (a) raw coke of petroleum and (b) treated coke with MCL method.

Fig. 3. FT-IR spectrum of (a) raw coke and (b) treated coke with MCL method.

H. Askari et al. / Egyptian Journal of Petroleum 28 (2019) 225–231

Degree of crystallinity of coke is determined by XRD analysis. In general, the crystal structure of coke is a criterion for its final application after desulfurization. Due to the application of coke as a starting material for anode, it ought to possess a certain degree of crystallinity. Fig. 2 shows the XRD images of pure and desulfurized coke by molten caustic leaching (MCL) method under optimal conditions. Three peaks in2h, 52, 43.7, 25.42 are observed in the XRD image of crude coke. 0:9k According to Scherrer equation (D ¼ bcosh Þ, the peaks corresponding to smaller particles are wider and have smaller intensities. Therefore, nanoparticle peaks are wider and less intense in comparison with those of other particles. As observed, following

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desulfurization, crystallinity has decreased, which will increase due to calcination at high temperature. The coke crystallinity is expected to increase by its calcination at high temperature. FTIR spectroscopy has been used to determine important groups on the coke surface. Fig. 3 shows FTIR spectra of petroleum coke and desulfurized petroleum coke by molten caustic leaching (MCL) method under optimal conditions. The aromatic CAH is represented by the peak in the 700–900 cm1 range, which has disappeared following desulfurization at 700 °C. The peaks at 1200–1300 cm1 are associated with CAO and CAOAC bonds. The aromatic C@C is shown by the peak at 1600 cm1, which has converted into a larger peak following desulfurization. The peaks

Fig. 4. SEM of (a) crude petroleum coke and (b) coke desulfurized using MCL method.

Fig. 5. SEM-EDX images and EDX patterns of on (a) petroleum coke and (b) treated coke.

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120 100

Weight (%)

80 60 40 20 0 0

100

200

300

400

500

600

700

800

Temperature (oC)

Fig. 6. TGA curves for coke treated by the MCL method.

in the range of 700–900 cm1 are attributed to the vicinal and peripheral hydrocarbons, which are known as very reactive, sulfur containing hydrocarbons. The reduction of the intensity of these peaks in the desulfurized coke indicates desulfurization. In addition, the intense peaks in the range of 1200–1300 and 1600 cm1 show the bonding of oxygen to coke during desulfurization, represented by the following reaction:

Cokes  S þ 2MOH $ M2 S þ Cokes  O þ H2 O

ð12Þ

1

The changes in the peak at 3400 cm , which corresponds to the OAH group, indicate the substitution of OAK band in coke by OAH after washing with water [24–30]. Scanning electron microscopy (SEM) is an important tool to study the surface properties of a petroleum coke particle in nano scale. SEM facilitates the investigation of the structure and morphological details of coke. EDX analysis is used to find sulfur content in the specific areas of the sample. Fig. 4 shows the SEM images of crude petroleum coke and coke desulfurized under optimal conditions at different magnifications. As observed in the Figure, some coke particles have decomposed at high temperature (600 °C). EDX analysis of crude petroleum coke and coke desulfurized under optimal conditions in different areas of the surface has been carried out. Fig. 5(a) and (b) show the SEM and EDX images. EDX helps to find sulfur distribution in different areas of the surface. As observed in the images, sulfur percentages in different areas of crude petroleum coke and coke desulfurized using molten caustic leaching (MCL) under optimal conditions have been determined. Fig. 6 shows the TGA images of crude petroleum coke and coke desulfurized under optimal conditions. The weight of coke powder has not decreased much, indicating that the coke has not been properly demoisturized. Coke obviously absorbs moisture from the air in the molten caustic leaching (MCL) process. However, the lack of weight loss at 100 °C shows that the coke has not been completely dried prior to TGA analysis. Weight reduction gradually occurs by increasing the temperature to 500 °C and reaches 94% of the initial weight. A quick reduction will also happen at higher temperatures due to COx compounds in the petroleum coke, which combine with oxygen and reduce the amount of coke.

to coke mass ratio, temperature, time and mesh size. In addition, Taguchi L9 design has been applied as a statistical tool to study the effects of alkali to coke mass ratio, temperature, time and mesh size parameters in petroleum coke desulfurization by molten caustic leaching method. The effect of alkali to coke mass ratio on coke desulfurization by molten caustic leaching has been investigated using three different values of 0.5, 1 and 1.5. The temperature effect has been investigated using three different values of 400, 500 and 600 °C. The effect of time has been investigated using three different values of 1, 2 and 3 h. The effect of coke mesh size has been investigated using three different values of 200, 400 and 600 mm. According to the results obtained, all the parameters have different degrees of impact on the process. In addition, increasing the temperature and time, as well as alkali to coke mass ratio and reducing the mesh size, increases the percent desulfurization of sulfur. However, since increasing temperature and alkali consumption increase the operation costs, optimal conditions for achieving the desirable percent desulfurization must be determined. According to the ANOVA Table, the degree of the effect of each parameter on petroleum coke desulfurization process is as: Alkali to coke mass ratio > temperature > time > mesh size. Maximum percent desulfurization is obtained using alkali to coke mass ratio, temperature, time and mesh size values of 1.5, 600 °C, 2 h and 200 mm, respectively. The structural differences between crude coke and coke desulfurized under these conditions were studied by comparing the corresponding structures. The FTIR, XRD, SEM, TGA and EDX analytical data of the crude coke and coke desulfurized under optimal conditions were also compared. Major differences in the different groups in the two samples were observed using FTIR analysis. The alkali affects the chemical and physical properties of coke, which causes its increased oxygen and moisture content. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]

4. Conclusions

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Petroleum coke desulfurization has been experimentally studied in this work using molten caustic leaching method. Several experiments have been carried out using different values of alkali

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