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Study on shale oil diesel refining.

Introduction

Large quantities of diesel fuel are imported every year to fill the vacancy in China. Therefore, much work has been done on the research of diesel, such as FCC diesel, the production of diesel from waste plastics and tar, even the bio-diesel, etc. [1-6]. These methods have not been widely used in industry. Oil shale resources in China are estimated to be 400 billion tonnes, equivalent to 16 billion tonnes of shale oil [7]. China is one of several countries in the world which own shale oil industry. The annual shale oil production in Fushun reached 100,000 tonnes and the scale will be further enlarged. Diesel fraction of shale oil contains too much unsaturated hydrocarbons as well as oxygen-, nitrogen-, and sulfur-containing compounds. Its gum content is high, sediments may form and stability is low. Therefore, the diesel must be refined to obtain a stable and qualified diesel product.

Up to now, there are two methods to refine diesel--hydrofining and non-hydrofining. The quality of oil refined through hydrofining is high. This method cannot be used in a small refinery because of high investments and prime costs. The operation of a non-hydrofining process is simple and investmens low. The research of non-hydrofining has been focused on FCC diesel [8-9]. Little work has been performed on the non-hydrofining of diesel from shale oil. This paper presents the results of extractive refining of shale oil diesel fraction using a multicomponent solvent.

Experimental

Sample

Fushun shale oil fraction 180-360[degrees]C was used in this study. The properties of the diesel fraction listed in Table 1 are compared with those of diesel No. 0. One can see that the content of carbon residue and gum in the diesel fraction of Fushun shale oil does not meet the standard. Especially the content of gum is 12 times more than that of the standard.

Method

Diesel fraction of Fushun shale oil was continuously extracted three times by using a solvent, containing 7-10% additive, at the ratio of solvent to oil of 0.1 to 0.6, at temperature 10-30[degrees]C. Diesel fraction was treated with a multicomponent solvent in a separating funnel. Two layers were formed, the upper layer was the oil. After separating, the diesel yield was calculated and the gum content determined. The extracted diesel was treated with aqueous alkaline solution, the effect of alkali on stability of the treated diesel fraction was investigated.

Results and Discussion

The Choice of Solvent

Oxygen-, nitrogen- and sulfur-containing compounds are all polar compounds. According to the principle that similar compounds are soluble in each other, solvent A was chosen for extraction. It was found that the effect of solvent A was not satisfactory because the removal of gum was only 63% even at the ratio of 1.2 (of solvent to oil). The content of gum was still higher than established by specification. Consequently, it was necessary to add an additive in order to lower the content of gum. The effect of additive concentration on gum removal is seen in Fig. 1 (the ratio of solvent to oil 0.5).

As shown in Fig. 1, the content of gum decreases with an increase in additive concentration while there was only little effect on oil recovery. This can be explained by the fact that the additive can remove gum but it is also efficiently selective. That is to say that the additive reacts with unstable components of diesel. The decrease in gum content was not remarkable when additive concentration exceeded 10%. So, the additive concentration was chosen between 7% and 10%.

[FIGURE 1 OMITTED]

The Influence of the Solvent to Oil Ratio

Tables 2 and 3 show the results of extraction using the solvent containing 9% additive at different ratios of solvent to oil.

The results in Table 2 and Table 3 show that the rate of gum removal increased. The color of refined diesel became lighter with increasing ratio of solvent to oil. The more times extracted, the higher the extraction efficiency at the same ratio of solvent to oil. There was a little effect on the rate of gum removal when the ratio of solvent to oil exceeded 0.6. Therefore, the suitable ratio of solvent to oil was found to be 0.5-0.6.

The Effect of Alkali Concentration

The stability of diesel fraction was greatly improved through treating with a multicomponent solvent, but the content of gum remained above the specification limit. The effects of alkali on diesel stability are shown in Table 4.

Emulsification can be observed during shale oil refining with aqueous alkali. The lower the alkali concentration and the larger the ratio of alkali to oil, the more intensive the emulsification. The full effect cannot be obtained even using electrical demulsification. The concentration of alkali should be increased and the dosage of the alkali reduced to liquidate the unstable composition testified in Table 4. For example, when the alkali concentration is 25% and the ratio of alkali to oil 1 : 150, the gum content of diesel is 65 mg/100 mL which is within the limitation, 70 mg/100 mL. The yield of refined oil is as high as 98.86%.

The Optimal Temperature and Shaking Time

The effect of temperature on oil recovery and gum removal is shown in Table 5. The additive concentration is 9%.

[FIGURE 2 OMITTED]

It was observed that the recovery decreased gradually with an increase in temperature. This can be explained by higher solubility of oil in the solvent at higher temperatures. A part of diesel was removed with the solvent. On the contrary, higher temperature is unfavorable for removing gum. However, lower temperature is not advisable as the fluidity of diesel is low and viscosity high, both disadvantageous to operation. So, the optimal temperature is 10-30[degrees]C.

The transfer of oil and solvent is very fast so the balance can be reached in a short time (Fig. 2). 5-10 minutes are enough to shake the mixture.

The Recovery of Solvent

The boiling point of solvent A is low, so the mixture can be distilled at atmospheric pressure. The recovery of solvent is above 97%. In addition, the extraction effect of recovered solvent, which is reused and added to fresh solvent and additive, is investigated. The result is illustrated in Fig. 3.

It can be seen from Fig. 3 that there is no notable change in the gum content and the recovery of refined oil with increasing proportion of recovered solvent, and the gum content meets the requirements of the national diesel specification. Therefore, it is practical to add fresh solvent and additive to the recovered solvent when refining the diesel.

[FIGURE 3 OMITTED]

The residual liquid after solvent recovering is extremely viscous at the room temperature (20[degrees]C) and can be burned easily. It can be added to the heavier fraction of shale oil used as fuel oil.

The Elemental Composition of the Residue and Refined Oil

The stability of refined oil is improved greatly. To get the knowledge of which compounds are removed during solvent refining, the ultimate analysis is prerequisite. Table 6 presents the elemental composition of several samples.

It can be seen from Table 6 that nitrogen content of the diesel fraction is reduced by 62.2% in the refined diesel compared with the diesel fraction. At the same time, nitrogen content of the residue is 3 times higher than that of the diesel fraction. The removal of oxygen-containing compounds is efficient using solvent refining. Oxygen content decreases from 3.53 to 0.45%, and the removal rate is as high as 87.3%. About 58.6% of sulfur can be removed. Sulfur content can meet the national specification ([less than or equal to] 0.5%). Therefore, nitrogen-, oxygen- and sulfur-containing compounds can be removed by the method of solvent refining, and thus the stability of diesel is improved.

Conclusions

The multicomponent solvent is of efficient selectivity but also of good ability to remove gum. The optimum operating conditions are: temperature 10-30[degrees]C, additive concentration 7-10%, the total ratio of solvent to oil 0.5-0.6, and three extraction stages. 62.2% of nitrogen, 87.3% of oxygen and 58.6% of sulfur present in the diesel fraction can be removed. The stability of diesel can be improved greatly.

A small quantity of heteroatomic compounds in the diesel after refining with multicomponent solvent can be removed by alkali washing. All the items can meet the requirements of the specification.

A lot of advantages have been shown such as cheapness, abundance, low toxicity and easy recyclability of the solvent. The additive is cheap and abundant, and its dosage is small.

Acknowledgements

The present study was financially supported by Beijing Municipal Education Committee (Project No. XK 114140479).

Presented by Qian Jialin Received November 9, 2004

REFERENCES

[1.] Xing Ji, Xiaolin Xi, Linhe Kong et al. Prospect and technology progress of biodiesel industry // Engineering Science. 2002. Vol. 4, No. 9. P. 86-93 [in Chinese].

[2.] Yiping Wang, Yi Zhai, Jinli Zhang et al. Research progress of biodiesel preparation // Chemical Industry and Engineering Progress. 2003. Vol. 22, No. 1. P. 8-12 [in Chinese].

[3.] Yongkui Fang, Aner Qiu, Xuegang Gong. The study on using waste and spent plastics to produce gasoline and diesel // Oil Refining and Chemical Engineering. 2002. Vol. 13, No. 4. P. 19-21 [in Chinese].

[4.] Ning Luo, Zhizhong Liu, Zhaode Mu. Pyrolysis and fueled technology of waste plastics // Enviromental Science in Chongqing. 1996. Vol. 18, No. 3. P. 41-44 [in Chinese].

[5.] Jin'an Zhao, Cunrui Guo, Zhizhong Wang. Oil-making process through coprocessing of low-temperature tar and waste PE // Coal Chemical Industry. 1998. No. 1. P. 42-44 [in Chinese].

[6.] Ziqiang Tang, Jin'an Zhao, Zhizhong Wang. Study of coprocessing of low temperature coal tar and waste plastics // Journal of Fuel Chemistry and Technology. 1999. Vol. 27, No. 5. P. 403-407 [in Chinese].

[7.] Xianglin Hou. Shale Oil Industry in China.--Beijing: The Hydrocarbon Processing Press, 1984.

[8.] Fengtao Zhan, Zhifeng Lv, Lin Li et al. Review of studies on diesel fuels refining by using non-hydrogenation techniques // Journal of the University of Petroleum, Beijing. 2000. Vol. 24, No. 3. P. 116-120 [in Chinese].

[9.] Ruijun Fan, Fandi Zeng, Ping Mei et al. Development of FCC light diesel oil non-hydrogenative refining technology in China // Oil Refining and Chemical Engineering. 1999. Vol. 13, No. 4. P. 40-42 [in Chinese].

YAOLING CHI * (a), SHUYUAN LI (a), XIAOBIN LI (b)

(a) State Key Lab of Heavy Oil Processing, University of Petroleum Beijing 102249

(b) Institute of Process Engineering, Chinese Academy of Sciences Beijing 100080

* E-mail: cylxju@163.com, lishuyuan@hotmail.com, xbli@home.ipe.ac.cn
Table 1. Properties of Diesel Fraction
from Fushun Shale Oil

Items Specification Diesel

Sulfur, % <1.0 0.87
Water, % Trace Trace
Ash, % <0.02 0.013
Cetane number >45 51
Distillation 50% <300 [degrees]C 298
Distillation 90% <355 [degrees]C 335
Distillation 95% <365 [degrees]C 358
Water-soluble acids and alkalis no no
Mechanical impurities, % no no
Solidification point, [degrees]C <0 -2
Carbon residue, % <0.4 2.12
Gum, mg 100 m[L.sup.-1] <70 804
Flash point, [degrees]C >65 81

Table 2. Effect of Solvent to Oil Ratio on Gum Removal
at Single-Stage Extraction

Item Solvent/oil ratio

 0.1 0.2 0.3

Recovery, % 91.6 86.1 83.2
Gum, mg x 100 m[L.sup.-1] 612 497 376
Gum removal, % 23.89 38.18 53.23

Item Solvent/oil ratio

 0.4 0.5 0.6

Recovery, % 81 79.5 79
Gum, mg x 100 m[L.sup.-1] 297 205 192
Gum removal, % 63.06 74.5 76.12

Table 3. Effect of Solvent to Oil Ratio on Gum Removal
at Multi-Stage Extraction

Item Solvent/oil, total process

 0.3 0.4

 Solvent/oil, each stage

 0.2 0.1 0.2 0.2

Recovery, %, each stage 86.1 92.5 86.1 90.9
Recovery, %, total 78.5 78.3
Gum, mg x 100 m[L.sup.-1] 341.1 259.3
Gum removal, % 57.59 67.8

Item Solvent/oil, total process

 0.5

 Solvent/oil, each stage

 0.2 0.2 0.1

Recovery, %, each stage 86.1 90.9 98.8
Recovery, %, total 77.4
Gum, mg x 100 m[L.sup.-1] 182.8
Gum removal, % 77.26

Item Solvent/oil, total process

 0.6

 Solvent/oil, each stage

 0.2 0.2 0.2

Recovery, %, each stage 86.1 90.9 98.6
Recovery, %, total 77.2
Gum, mg x 100 m[L.sup.-1] 146.4
Gum removal, % 81.8

Table 4. Effect of Alkali on Diesel Stability

Alkali Alkali/oil ratio
concen-
tration 1 : 5-1 : 50 1 : 75 1 : 100 1 : 150 1 : 200

 Gum, mg x 100m[L.sup.-1]

5% Areas with Decreasing emulsification gradually
10% serious
15% emulsif- 88 110 132
20% ication 73 86 89
25% 47 65 72
30% 35 52 66

Table 5. Effect of Temperature on Oil Recovery
and Gum Removal

 T, [degrees]C

 15 25 32 40 48

 Recovery, %

Solvent/oil ratio:
0.2 88.3 87.5 86.6 86.1 85.3
0.2 93.6 90.5 91.4 90.9 89.8
0.1 98.9 98.9 98.7 98.8 97.1
Total recovery, % 81.7 78.3 78.1 77.4 74.4
Gum, mg x 100 [mL.sup.-1] 35 46 68 77 96

Table 6. Elemental Composition of Several Samples

Samples C, % H, % N, %

Shale oil 84.58 11.50 1.27
Diesel fraction 82.59 12.03 0.98
Refined diesel 85.60 12.49 0.37
Residue 83.25 9.88 2.56

Samples O, % S, % H/C

Shale oil 2.49 0.583 1.63
Diesel fraction 3.53 0.87 1.75
Refined diesel 0.45 0.36 1.75
Residue 4.50 0.98 1.42
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Author:Chi, Yaoling; Li, Shuyuan; Li, Xiaobin
Publication:Oil Shale
Date:Sep 1, 2005
Words:2320
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