International Journal of Industrial Chemistry

, Volume 8, Issue 4, pp 373–381 | Cite as

Assessment of the desulfurization of FCC vacuum gasoil and light cycle oil using ionic liquid 1-butyl-3-methylimidazolium octylsulfate

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Abstract

Increasingly stringent fuel sulfur content requirements/legislations have resulted in intensive quest for alternative desulfurization technologies that will ensure the treatment of fuels to acceptable sulfur levels. In this regard, the extraction of sulfur compounds from gasoline and diesel oil by ionic liquids (ILs) may represent a green alternative to common hydrodesulfurization for deep desulfurization (<10 ppm sulfur). This paper investigates the extraction efficiency as well as the regeneration of the IL, 1-butyl-3-methylimidazolium octylsulfate ([BMIM][OcSO4]) for the desulfurization of the fluid catalytic cracking diesel fuels. The IL was found to be effective in the selective removal of sulfur from FCC light cycle oil than from the FCC vacuum gasoil stream, achieving 96 and 22.1% sulfur removals, respectively. These results suggest that fuel sulfur content and stream composition affects the extraction efficiency and effectiveness of IL. In addition to the fundamental experiments with FCC diesel fuels, the regeneration of sulfur loaded ILs was studied and a preliminary strategy for the integration of an IL-extraction process into an existing refinery structure is briefly discussed. Regenerated [BMIM][OcSO4] IL was used for desulfurization of diesel and achieved highest sulfur removal of 95%. The IL was regenerated up to four times without appreciable decrease in efficiency. The results obtained herein show that ILs may be effective in the desulfurization of real diesel oils.

Keywords

1-Butyl-3-methylimidazolium octylsulfate ([BMIM][OcSO4]) Fluid catalytic cracking (FCC) Desulfurization Diesel fuel Regeneration Ionic liquid 

Introduction

Due to the public concern of sulfur compounds on the environment, there is an increase in the number and stringency of legislative actions on sulfur specifications, particularly for transportation fuels [9]. As a result, petroleum refining industries are largely affected since they are perceived as one of the largest air polluters both directly and indirectly. The most recent sulfur specification adopted by many countries is 50 ppm while the Euro 5 specification has adopted a sulfur specification of 10 ppm [20]. In South Africa, the government is planning to reduce the sulfur content from a current value of 50 ppm to 10 ppm by year 2017 [18].

The removal of sulfur compounds from liquid fuels is carried out industrially via catalytic hydrodesulfurization (HDS), which converts sulfur compounds into H2S (and subsequently into elementary sulfur) and hydrocarbons at high temperature (350 °C) and pressure range of 35–270 bars [2]. This extraction takes place in two common extraction pathways. The first pathway is the direct removal of sulfur (hydrogenolysis) which occurs in linear sulfur compounds. The second extraction path is mainly for thiophenic compounds wherein the aromatic ring is hydrogenated first and then sulfur is removed [2].

The commonly used catalysts are the sulfide Co–Mo/Al2O3 or Ni–Mo/Al2O3 catalysts [11]. It has been shown that substituted dibenzothiophene (DBT) species are less reactive because of the presence of the methyl groups that create steric hindrance during the interaction of sulfur with catalysts’ active sites [12]. It has also been shown that the reactivity of the aromatics decreases with the substituent’s addition to the DBT structure [12]. The HDS process faces several challenges i.e., (i) its inability to achieve ultra-deep desulfurization level (<10 ppm) due to the fact that the refractory thiophenic sulfur compounds remain in the treated stream [16], (ii) the catalysts used need to be replaced more frequently due to deactivation and loss of activities [22], iii) the similar boiling point range of diesel and sulfur compounds make the separation process challenging. The boiling point of diesel is in the range of 163–357 °C which is very close to that of the refractory sulfur compounds.

Oxidative desulfurization has also been considered as a promising alternative to deep desulfurization of fuel [21, 26]. This process involves two steps, which are oxidation of sulfur compounds to sulfones or sulfoxides followed by extraction or purification. Usually, it is operated at ambient pressure and low temperatures (0–30 °C) using oxidants and any polar solvent. The widely used oxidants are hydrogen peroxide and peroxy-acids. However, the problem with using these oxidants is that they require to be added in high concentrations which then cause the loss of product quality and safety issues [27].

Recently, extractive desulfurization has also received much attention due to its applicability at low temperatures and pressures. This method implies choosing a solvent in which sulfur compounds are more soluble as compared to the hydrocarbons [1, 5, 19]. A variety of extractants (liquid solvents) have been investigated [12]. The mostly used extractants include acetonitrile, lactones, dimethylformamide (DMF), nitrogen-containing solvents (e.g., amines), and sulfur-containing solvents (e.g., sulfone) [12, 22]. Although, one of the major attraction of this technique is the fact that it can easily be integrated into the conventional refineries since it uses conventional equipment which does not require any special requirements, its biggest challenge remains the choice of an appropriate solvent in which only sulfur compounds are soluble. For instance, several studies conducted using polyethylene glycol showed that although 50–90% of sulfur compounds were removed from light oil [7], there was co-extraction of aromatic molecules resulting in a high loss of diesel products [22]. A suitable solvent for extractive desulfurization should have a high partition coefficient for sulfur components especially aromatic sulfur compounds, negligible cross solubility, high thermal and chemical stability, nontoxicity, environmental compatibility, and low cost for commercial applications [13]. Many organic solvents, such as dimethyl sulfoxide, acetonitrile, 1-methyl-2-pyrrolidinone, dimethylformamide, and polyalkylene glycol have been used as extractants, but none of these solvents conform to all of the above requirements and their performance in removing sulfur from fuels has not been fully satisfactory [15]. As a result, there have been a drive to find more selective solvents able to transform sulfur compounds into more soluble compounds and in this regard, ionic liquids (ILs) have been recognized as promising alternatives to conventional non-desirable organic solvents and have received considerable attention as extractants for desulfurization of liquid fuels [10, 14], or at least as a complementary technology to the HDS process. ILs are environmental-friendly solvents with unique physicochemical properties, such as negligible vapor pressure, high chemical and thermal stabilities, non-flammability, and recyclability. These properties together with high affinity for sulfur-containing compounds, especially aromatic sulfur components, and immiscibility with fuels make ILs desirable extractants for desulfurization of liquid fuels [14]. Moreover, ILs are highly effective for the extraction of some aromatic sulfur components, and can lower the concentrations to desirable low levels (especially thiophenic compounds). Typically, the ionic liquid systems used in the literature consist of halogen containing anions such as [AlCl4]2, [PF6]2, [BF4]2, [CF3SO3]2, or [(CF3SO2)2N]2 which are usually combined with imidazolium or pyridinium cations to form the ionic liquid. The majority of ionic liquids described currently in the literature for catalytic and other solvent applications contain halogen atoms such as Cl or F. However, the presence of halogen atoms may cause-under certain conditions-serious concerns i.e., labile anions [AlCl4]2 and [PF6]2 if water is present. In this case, significant amounts of HCl (or HF) would be liberated under extraction conditions and may require additional efforts and costs to mitigate. Experiments on the selective extraction of sulfur compounds from synthetic diesel oil (mixture of n-dodecane with DBT-derivatives) using chloroaluminate ILs such as [BMIM][AlCl4] showed promising results i.e. a high Nernst partition coefficient (the ratio of the concentration of sulfur in the IL (mg S/kg IL) to the S-concentration in the oil (mg S/kg oil). Nevertheless, the use of chlorometallate ILs is not desired for technical large scale applications due to their very limited hydrolysis stability and in some cases toxicity. Further screening experiments with alkylimidazolium tetrafluoroborate ([BMIM][BF4]) and hexafluorophosphate, ILs ([BMIM][PF6]) were found to also have fairly high partition coefficients for synthetic sulfur compounds, but although ILs of this type are significantly more stable to hydrolysis, their use as a large scale extracting agent is not optimal because of the relatively high price of the starting material. Additionally, the formation of hydrolysis products-especially HF-is observed at elevated temperatures and in the presence of water [25].

To avoid these stability and corrosion problems, the present work focuses on a halogen-free ILs for the extraction of S-compounds: 1-butyl-3-methylimidazolium octylsulfate. This IL is not only halogen-free, but may also be readily accessible from cheap starting materials, which are available on a large scale: sodium octylsulfate is used as a detergent on a larger scale. At first glance, it might look inconsistent to use S-containing ILs for desulfurization, but any leaching of IL into the oil phase is unwanted anyway and is a critical criterion for the selection of a suitable IL. Mutual solubility of ILs and fuel oil is not desired because the solubility of ILs in fuel oil (IL-in-oil solubility) will result into the loss of ILs and the contamination of fuel oil, while the solubility of fuel oil in ILs will require the recovery of dissolved fuel oil and increase separation cost. Therefore, an ideal IL employed in desulfurization should have small or no mutual solubility with fuel oil. The extractive desulfurization with ionic liquids is not only an interesting alternative for deep HDS of diesel oil, but probably even more attractive for FCC diesel fuel as the heavy oil feed of the FCC-unit up to now has not been desulfurized as the expenses needed are too high compared to a “post-desulfurization” of the product streams of the FCC-process. Although, chemically and thermally stable ionic liquids containing an octylsulfate anion and suitable for application in the extraction of sulfur compounds have also been reported on synthetic fuel oils [3], not much research has been devoted to the applicability of the ionic liquids with an octylsulfate anion for the desulfurization of real fuel oils which constitute the essence of this paper which (i) investigates the capacity of the IL, 1-butyl-3-methylimidazolium octylsulfate as a solvent for deep desulfurization of real FCC diesel fuel, (ii) studies the regeneration of the sulfur-loaded IL, and (iii) assesses the efficiency of regenerated IL in the desulfurization of diesel using appropriate solvent(s).

Experimental

Feed materials

  1. (a)
    Real diesel sample: the samples used in this study were the FCC-unit Gasoil feed and the FCC-unit Light cycle oil product. The FCC-unit improves the total refinery Gasoil (diesel) yield to 27.8% weight on crude (woc) and collectively with the Visbreaking unit (VBU) brings the total Gasoil yield to 28.7% woc [17]. This unit converts the VGO into a number of fractions of focus in this study being LCO, which is a high sulfur content fraction containing the unreactive alkylated benzothiophenes, DBT, and alkylated DBT organic sulfur compounds. The properties of the samples used in this study are shown in Table 1.
    Table 1

    Diesel samples properties

    Property

    FCC VGO

    FCC LCO

    Sulfur content (ppm)

    4203

    549.9

    Cetane index

    46.0

    Density at 15 °C (kg/m3)

    820.0

    Polycyclic aromatic hydrocarbons (wt%)

    2.1

    Ignition point (°C)

    >55

    Kinematic viscosity at 40 °C, mm2/s

    3.98

     
  2. (b)

    Ionic liquid: 1-Butyl-3-methylimidazolium octylsulfate (with a purity of ≥95%) was purchased from Sigma-Aldrich. In choosing the appropriate solvents, the main properties considered were good extractive ability for the non-reactive organic S-compounds and insolubility of the solvents in fuel oils and their constituents. The ionic liquid 1-butyl-3-methylimidazolium octylsulfate was proven to be a suitable solvent through screening experiments with model diesel, giving the highest sulfur partition coefficient out of the five studied ILs [3]. The cross solubility of real fuel and BMIMOcSO4 ionic liquid (IL) was also assessed because it may be a key factor in evaluating the extraction efficiency as BMIMOcSO4 solubility in liquid fuel may give rise to extractant loss and liquid fuel contamination i.e., if 1-butyl-3-methylimidazolium octylsulfate ionic liquid in diesel have noticeable solubility, it may contaminate the fuel and further lead to a NOx pollution, as well as increase the cost of the ionic liquid recycling. By analyzing the BMIMOcSO4-saturated diesel sample using HPLC, no IL peak was found. Therefore, it may be concluded that BMIMOcSO4 has negligible solubility in the real diesel. However, the solubility of real diesel in BMIMOcSO4 IL was measured using a gravimetric method by weighing the mass difference of a given amount of ILs and the ILs saturated with diesel and it was found that the Light Cycle Oil (LCO) has a solubility (wt%) of 0.84 in BMIMOcSO4 IL while the Heavy Cycle Oil (LCO) has a solubility of 1.09 (wt%).

     

Experimental methods

The extraction process was carried out in a batch vessel. The batch vessel was placed in a water bath on a heater to control the temperature. For all the extraction experiments, the Heidolph MR 3001 K heater and magnetic stirrer plate with temperature control was used. The batch vessels were maintained at 25 ± 1 °C using heating and cooling loops. The experiment was divided into two parts to investigate the efficiency of ionic liquids in different samples of diesel, one with a higher sulfur content and the other with a lower sulfur content. Furthermore, the reusability as well as the efficiency of regenerated ionic liquids was investigated in addition to the investigation of core parameters affecting the efficiency and effectiveness of ionic liquids in extractive desulfurization namely, number of extraction stages and extraction time. The diesel was fed into a 50 mL round bottom flasks and placed in a temperature controlled water bath. Ionic liquid was added to the flasks in a 1:4 mass ratio. The flask contents were stirred for 30 min to get good contact between the phases. The mixture was then allowed to settle for 30 min to obtain phase splitting and settling, samples of the upper layer, the raffinate were withdrawn and analyzed by ICP-AES. For the experiments investigating the reusability of the ionic liquid, the bottom phase which is the loaded IL phase was fed, together with fresh diesel oil, into a round bottom flask, and allowed to be in contact as explained above. When investigating the optimum number of extraction stages after the first stage, the remaining diesel oil phase was fed to the second stage with fresh ionic liquid. Hexane was used for the re-extraction of sulfur from loaded IL and the efficiency of the regenerated IL was studied, i.e., the extraction ability of regenerated IL was studied and compared with that of fresh IL and reused (without regeneration) IL. After each desulfurization experiment, the solvent-rich phase was kept to recover the ionic liquids. The S-containing ILs were regenerated by dissolution with a suitable solvent and dried off by vacuum evaporation at 100 °C for 10 h for the clear separation of the ionic liquid and solvent in a Vismara V065 vacuum oven connected to a Pall vacuum pump. The purity of the regenerated ILs was determined by 1H and 13C NMR. The extraction performance of the regenerated ILs was investigated and compared with results obtained for fresh ILs. The same procedure described earlier for the desulfurization of diesel was followed and after each experiment the IL was regenerated, this was done up to 4 times, i.e., 4 regeneration cycles.

Chemical analysis

The inductively coupled plasma-atomic spectrometry (ICP-AES) was used for sulfur analysis. Prior to analysis in an ICP-AES, the diesel samples were treated into a solution by microwave digestion:
  1. (a)
    Specified operating parameters:
    • Microwave power level-1600 W, 100%.

    • Ramp time (time take to heat the sample from 20 to 185 °C)-25 min.

    • Pressure-800 psi.

    • Temperature-215 °C.

    • Hold time (time that the sample spend in the microwave under above conditions)-15 min.

     
  2. (b)

    Sample is weighed out into the microwave vessel and then 5 mL HNO3 and 2 mL HCl added into the vessel.

     
  3. (c)

    The filled microwave vessel is then left to stand open for 20 min to predigest and then sealed and heated in the microwave as per above specifications.

     
  4. (d)

    After the digestion, 43 g of deionized water is added to the digested sample to make up 50 mL.

     
  5. (e)

    The digested sample is then analyzed for Sulfur using the ICP-AES.

     
  6. (f)

    The ICP-AES results were corrected for the dilution factor as per digestion procedure.

     

Results and discussion

Diesel fuel desulfurization

The effect of Oil/IL mass ratio

The effect of Oil/IL mass ratio on the extraction efficiency of ILs was also studied. Figures 1 and 2 show the effect of sulfur content as well as Oil to IL ratio on the desulfurization of diesel distillates.
Fig. 1

Multistage extraction of sulfur from VGO, showing initial compositions and compositions after the 1st, 2nd, 3rd, and 4th extraction stages

Fig. 2

Multistage extraction of sulfur from LCO, compositions in initial LCO and after the 1st, 2nd, 3rd, and 4th extraction stages

From the results in Figs. 1 and 2, it can be seen that the extractive desulfurization method is much more efficient in removing sulfur from the less sulfur-containing LCO (549.9 ppm) than VGO which contains 4203 ppm sulfur. This is largely due to the complex chemical composition of the VGO including more (than in LCO) unreactive different types of organic sulfur compounds as shown Table 2. Treating VGO (the higher sulfur-containing sample) to acceptable sulfur levels will require much more extraction stages, high volumes of IL, and more energy which all translates to high processing costs. Figures 1 and 2 also show the effect of diesel oil to IL mass ratio on the extraction efficiency. Generally, a higher mass ratio will result in higher extraction efficiencies. However, from a financial point of view, smaller mass ratios are appropriate as an extractive desulfurization process employing higher mass ratios will require higher volumes of IL and solvents for the regeneration of the IL. The results may also suggest that multistage extraction maybe more sustainable for the reduction of oil sulfur content to allowable levels than increasing the oil/IL mass ratio.
Table 2

Typical sulfur compounds and corresponding refinery streams [23]

Sulfur compounds

Refinery streams

Corresponding fuels

Mercaptanes, RSH, sulfides, R2S, disulfides, RSSR, thiophene (T), and its alkylated derivatives, benzothiophene

SR-naphtha

FCC naphtha Coker naphtha

Gasoline (BP range: 25–225 °C)

Mercaptanes, RSH, benzothiophene (BT), alkylated benzothiophenes

Kerosene

Heavy naphtha, Middle distillate

Jet fuel (BP range: 130–300 °C)

Thiophene, alkylated benzothiophenes, dibenzothiophene (DBT), alkylated dibenzothiophenes

Middle distillate, FCC LCO,

Coker gas oil

Diesel fuel (BP range: 160–380 °C)

Greater than or equal to three-ring polycyclic sulfur compounds, including DBT, benzonaphthothiophene (BNT), thiophene (PT), and their alkylated derivatives and naphthothiophenes (NT)

Heavy gas oils, Vacuum gas oil (VGO), Distillation residues

Fuel oils (non-road fuel and heavy oils)

Effect of time on the desulfurization of diesel using IL

Initially amount of sulfur removed increased with an increase in the time the media (oil and IL) were stirred. From Fig. 3, one can see that 30 min of contact time gave the highest sulfur removal of 87.84%, but the sulfur removal decreased to 44.74% when the oil and IL were stirred for 90 min. Beyond 30 min of stirring, the efficiency of the IL in extracting sulfur from diesel oil decreases. An extraction time of 30 min is enough to achieve equilibrium between the oil and IL and as such was used in performing all other experiments in this study as short extraction time is desired for industrial application for low production costs.
Fig. 3

Extraction of sulfur from light cycle oil (LCO) at different extraction/residence times, (oil to IL ratio = 2:1, initial S content = 549.9 ppm, S content after 30 min extraction time = 46.89 ppm, S content in LCO after 90 min’ extraction time = 243.89)

Efficiency of used/spent IL in extracting sulfur from diesel

There is still a gap of knowledge on the economic, social, and ecological impact of ILs on the environment and human health as research on ILs is mainly directed at their synthesis, measurement of their properties and new application fields while little on their reusability, regeneration and recoverability, toxicity and biodegradability [7] has been given enough attention. However, due to the high ILs costs, it may be essential to assess their reusability and regeneration. In practice, ILs can be reused to a certain extent or until a threshold concentration is reached above which there is noticeable degradation or inefficiency of the ILs as solvents. It was found that [BMIM][OcSO4] can be reused up to three times without significant drop in efficiency in the extraction of sulfur from diesel as shown in Fig. 4. These results are in agreement with the results reported by Dharaskar et al. [4] on the extraction of sulfur from model diesel oil, where it was found that imidazoled ILs can be reused three times without significant decrease in activity. In the case of real diesel oil we found that the ionic liquid was still able to remove sulfur on the 4th reusing cycle; although at a lower efficiency i.e. ~71% sulfur removed compared to 92% removed when treating a fresh real diesel oil sample. The presence of S-compounds in the IL inhibits the extraction of more S-compounds, thus decrease the efficiency and effectiveness of the IL for the desulfurization of diesel fuel.
Fig. 4

Reusing of IL in the extraction of sulfur from LCO, (oil to IL ratio 4:1, Initial S content = 549.9 ppm, 0 on the horizontal axis denotes fresh IL)

Efficiency of extraction of organic S-compounds from loaded ILs

Fernández et al. [7] reported on several regeneration techniques which can be used for the regeneration of ILs in different processes. Of all the processes given by Fernández et al., [7] two regeneration processes have been identified as viable methods due to their efficiency and cost effectiveness (Liu et al. [15], Gabric et al. [8] namely (a) vacuum distillation which is appropriate for the regeneration of ILs, due to their nonvolatility. Gabrić et al. [8] investigated the regeneration of the selected ILs using vacuum distillation and found that for synthetic diesel oil, the regenerated ILs were not totally purified due to the high boiling points of the S-compounds found in diesel oil. Another disadvantage associated with vacuum distillation is the high investment costs and energy costs. (b) Liquid–liquid extraction is considered as the simplest method of removing solutes form ILs where ILs are washed with a solvent then dried off at high vacuum. The key challenge with liquid–liquid extraction is finding a suitable solvent. Previous studies have shown that the purity of the ILs may be retained after regeneration with a promising solvent and drying at high vacuum for 4 h [15]. In this paper, suitable solvents for the re-extraction of sulfur compounds from the IL were studied using thermodynamic criteria. An investigation of the suitability of selected solvents, namely hexane, pentane, and ethyl acetate, for the re-extraction of organic S-compounds from spent/loaded ILs was undertaken. To this end, Aspen Plus was employed to obtain liquid–liquid equilibrium (LLE) data for the following ternary systems, to get an idea of the solubility degree among the components in each system, the distribution ratio (β) and selectivity (S) were calculated:
  • {Hexane + thiophene + [BMIM][OcSO4]}.

  • {Pentane + thiophene + [BMIM][OcSO4]}.

  • {Ethyl acetate + thiophene + [BMIM][OcSO4]}.

Solute distribution ratio (β) and solvent selectivity (S) which were calculated according to the following equations,
$$\beta = \frac{{\left( {x_{1} } \right)^{\text{I}} }}{{\left( {x_{1} } \right)^{\text{II}} }},$$
$$S = \frac{{\left( {x_{1} } \right)^{\text{I}} .\;\left( {x_{2} } \right)^{\text{II}} }}{{\left( {x_{1} } \right)^{\text{II}} .\;\left( {x_{ 2} } \right)^{\text{I}} }},$$
where subscripts, 1-is the solute/S-compound thiophene and 2-is the IL. While superscripts, I-refers to the solvent phase and II- refers to the IL phase, x is the molar composition of component indicated as subscript, in the phase indicated as superscript.

To perform the simulation in Aspen, the universal quasi chemical functional group activity coefficient (UNIFAC) thermodynamic model was chosen.

Hexane was found to be the most thermodynamically suitable solvent exhibiting high β and S values which translate to low solvent requirements and few number of equilibrium stages in the extraction process (Tables 3, 4, 5). From the solute distribution ratios, β values in Table 3 it can be seen that the S-compound thiophene distributes preferentially in hexane than in the IL, more so at low concentration and that the selectivity values are also high. The phase diagram for the system, {Hexane + Thiophene + [BMIM][OcSO4]}, exhibits Type I behavior, indicating that the re-extraction of thiophene from [BMIM][OcSO4] is possible with little cross-contamination (co-extraction of [BMIM][OcSO4] in the solvent) [24]. Ternary system containing pentane shows type II behavior and the thiophene distribution in the pentane system is more in the IL phase at low thiophene concentrations and nearly equal between the IL phase and pentane phase at higher thiophene concentrations [24].
Table 3

LLE Data, solute distribution ratio, and selectivity for the ternary system {Hexane (1) + Thiophene (2) +[BMIM][OcSO4] (3)}

Hexane-rich phase

IL-rich phase

β

S

X1II

X2II

X3II

X1I

X2I

X3I

0.91

0.00

0.09

0.0082

0.0000

0.9918

  

0.77

0.13

0.10

0.0073

0.0007

0.9921

195.36

178.93

0.63

0.26

0.11

0.0060

0.0016

0.9923

161.67

394.49

0.49

0.39

0.12

0.0045

0.0031

0.9925

127.91

705.93

0.34

0.51

0.14

0.0026

0.0055

0.9919

92.57

1379.55

0.20

0.62

0.18

0.0007

0.0118

0.9875

52.69

5092.31

0.07

0.73

0.19

0.0000

0.0563

0.9437

13.04

 
Table 4

LLE data, solute distribution ratio, and selectivity for the ternary system {Pentane (1) + thiophene (2) + [BMIM][OcSO4] (3)}

Pentane-rich phase

IL-rich phase

β

S

X1(II)

X2(II)

X3(II)

X1(I)

X2(I)

X3(I)

1.000

0.000

0.000

0.221

0.000

0.779

  

0.791

0.209

0.000

0.036

0.002

0.961

0.010563

4.369372

0.621

0.379

0.000

0.013

0.041

0.946

0.109074

0.185507

0.473

0.526

0.001

0.008

0.137

0.856

0.259668

0.06498

0.336

0.659

0.005

0.010

0.282

0.708

0.427036

0.070219

0.200

0.782

0.018

0.014

0.466

0.520

0.596016

0.117828

0.042

0.819

0.140

0.031

0.779

0.190

0.952106

0.779908

Table 5

LLE data, solute distribution ratio, and selectivity for the ternary system {ethyl acetate (1) + thiophene (2) + [BMIM][OcSO4] (3)}

Ethyl acetate-rich phase

IL-rich phase

β

S

\({\text{X}}_{ 1}^{\text{II}}\)

\({\text{X}}_{ 2}^{\text{II}}\)

\({\text{X}}_{ 3}^{\text{II}}\)

\({\text{X}}_{ 1}^{\text{II}}\)

\({\text{X}}_{ 2}^{\text{II}}\)

\({\text{X}}_{ 3}^{\text{II}}\)

0.5594

0.0000

0.4406

0.0396

0.0000

0.9604

  

0.5435

0.0002

0.4563

0.0367

0.0000

0.9633

1.68E − 07

1.26E + 07

0.5323

0.0002

0.4675

0.0347

0.0000

0.9653

1.27E − 07

1.63E + 07

0.4987

0.0003

0.5010

0.4947

0.5051

0.0002

1.72E + 03

1.99E − 07

0.4277

0.0004

0.5720

0.3275

0.6723

0.0002

1.87E + 03

1.54E − 07

0.2907

0.0004

0.7089

0.1626

0.8372

0.0002

1.91E + 03

1.28E − 07

0.0000

0.0005

0.9995

0.0000

0.9998

0.0002

1.87E + 03

1.08E − 07

For large scale industrial application, high β values are desired as they translate to low solvent requirements (consumption) for the re-extraction of sulfur compounds from the IL and the higher the selectivity values the fewer the number of equilibrium stages required in the regeneration process [6].

An important parameter to be weary of is the cross-contamination. The results for ethyl acetate indicate that IL will be co-extracted with thiophene. This cross-contamination may give rise to IL loss in the regeneration process. [BMIM][OcSO4], the IL used during this study, was regenerated by means of an extraction process with hexane as a solvent and vacuum for drying. The purity of the IL was analyzed by 1H and 13C NMR spectroscopy and it was verified that its purity was relatively retained. The efficiency in the extraction of sulfur from diesel fuel of the regenerated IL was studied and the results of this are shown in Fig. 5. Similar to reused IL, the efficiency of regenerated IL only starts to drop after the 3rd regeneration cycle (after the IL has been regenerated 3 times).
Fig. 5

Efficiency of regenerated [BMIM][OcSO4] at different oil/IL mass ratio (% sulfur removed after each IL-regeneration cycle)

A Preliminary petroleum refinery configuration incorporating an Ionic Liquid Extraction Unit (ILEU) into an existing refinery scheme is proposed below. In the scheme, the ILEU unit is placed after the conventional HDS unit. During extraction with IL, there is a degree of cross-contamination, therefore it is expected that the HDS will aid in preventing product loss as it will allow for the recycling of diesel from the regeneration unit. In addition, the HDS can be used for the treatment of S-compounds removed in ILEU to form H2S.

In this configuration, gasoil is fed into a multistage ILEU unit and treated with I (an Aspen Plus simulation revealed that 8 stages will be required at a 4:1 diesel to IL mass ratio to treat the gasoil to the allowable sulfur level of 10 ppm). The system can be set-up in a way that the IL is reused 3 times within the column before being routed to the regeneration unit. After regeneration, the IL may be taken back into the ILEU unit. The extracted S-compounds may also be routed to HDS to be converted into H2S. The oil with the S-compounds from IL regeneration can then be processed in other units of the refinery, e.g., in thermal cracking, coking, or can serve as co-feed in partial oxidation of heavy oils or in a power plant. Alternatively, this sulfur-rich oil from IL regeneration can also be recycled and added to the feed of the HDS reactor. By this means, the refractory S-compounds like 4,6-DMDBT are converted in the end to H2S after several cycles (HDS and extraction) and a long total residence time in the HDS, respectively. By this means, the loss of product would be minimized (Fig. 6).
Fig. 6

Preliminary integration of the ionic liquid extraction unit (ILEU) into an existing refinery scheme

Conclusions

In this paper, the extraction efficiency as well as the regeneration of the IL, 1-butyl-3-methylimidazolium octylsulfate ([BMIM][OcSO4]) for the desulfurization of the FCC diesel fuels was studied. It was found that [BMIM][OcSO4] IL is much more efficient in removing sulfur from the less sulfur-containing LCO (549.9 ppm) than VGO which contains 4203 ppm sulfur. This may be due to the complex chemical composition of the VGO including more unreactive different types of organic sulfur compounds and therefore the treatment of VGO to acceptable sulfur levels may require much more extraction stages, high volumes of IL, and more energy which all translates to high processing costs. The optimum extractive equilibrium time of about 30 min in this work is in line with results from others researchers, i.e., it was found that the extraction equilibrium for low viscosity ILs, such as [BMI][N(CN)2] and [EMI][N(CN)2] is 10 min while for high viscosity ILs, such as ([BMIM][BF4], [BPy][BF4],[OPy][NO3], and [BMIM/Cl][FeCl3], [HNMP][HSO4]) it is slightly higher at about 30 min. Sulfur extraction efficiency of different ILs at different IL-oil mass ratios for real fuel oils was also investigated and the results in this work confirm previous results that showed that the desulfurization of real fuel oils is highest at 2:1 and lowest at 1:5 mass ratios for both high and low viscosity ILs. However, the rate of the increase in the S extraction efficiency is not same for different IL-oil mass ratios [28, 29]. The fuels sulfur content and stream composition affects the extraction efficiency and effectiveness of IL, as such it may be advisable to have the desulfurization unit after the FCC rather than before in a refinery network. The results also imply that the reduction of sulfur to considerably negligible amount may be more possible through the increase of the number of extraction stages than increasing the oil/IL mass ratio as for industrial application low IL/oil mass ratios are recommended due to the high IL costs. The reusability of [BMIM][OcSO4] has been also assessed and hexane was used for the regeneration of [BMIM][OcSO4]. [BMIM][OcSO4] IL can be regenerated 4 times before noticeable drop in efficiency. The results show that [BMIM][OcSO4] IL may be used for the extractive desulfurization of refinery streams such as FCC diesel oils especially for deep desulfurization and olefin-rich refinery streams, as the use of ILs does not alter the oil matrix and does not require hydrogen, which could be a bottleneck in some refineries.

Notes

Acknowledgements

The authors acknowledge financial support received from the South African National Research Foundation (NRF).

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Authors and Affiliations

  1. 1.Sustainable Energy and Environment Research Group, School of Chemical EngineeringUniversity of WitwatersrandJohannesburgSouth Africa

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