Method for preparing low-sulfur biodiesel

Abstract

The present invention relates to a method for preparing low-sulfur biodiesel. The method includes the following steps: subjecting raw crude oil to heat exchange, preheating, glycerol esterification reaction, flashing, cooling, transesterification, dealcoholization, sedimentation, and methyl ester rectification to obtain low-sulfur biodiesel up to the standard, where in the heat exchange process, the raw crude oil exchanges heat with flashed oil to recycle some heat, and glycerol and fatty acids in a distilled product subjected to flashing are reused for the glycerol esterification reaction; the dealcoholization process removes methanol in a system after the transesterification, and refined methanol recycled by methanol rectification is reused for the transesterification reaction; crude glycerol generated in the sedimentation process does not need to be treated and is directly used for glycerol rectification to obtain refined glycerol, and the refined glycerol is reused for the glycerol esterification reaction.

Claims

1. A method for preparing low-sulfur biodiesel, comprising the following steps: subjecting raw crude oil to heat exchange, preheating, glycerol esterification reaction, flashing, cooling, transesterification, dealcoholization, sedimentation, methyl ester rectification to obtain low-sulfur biodiesel up to the standard, wherein in the heat exchange process, the raw crude oil exchanges heat with flashed oil to recycle some heat, and glycerol and fatty acids in a distilled product subjected to flashing are reused for the glycerol esterification reaction; the dealcoholization process removes methanol in a system after the transesterification, and refined methanol recycled by methanol rectification is reused for the transesterification reaction; crude glycerol generated in the sedimentation process is subjected to glycerol rectification to obtain refined glycerol, and the refined glycerol is reused for the glycerol esterification reaction; wherein in the glycerol esterification reaction, the reaction conditions are normal pressure, a molar ratio 0.5:1-2:1 of glycerol to fatty acid, 200 to 240° C. and nitrogen purging, and the reaction time is 2-4 hours; and wherein in the flashing process the oil is flashed while the oil is hot after the glycerol esterification reaction, and the flashing pressure is 100-1000 Pa.

2. The method for preparing low-sulfur biodiesel according to claim 1, wherein the raw crude oil subjected to heat exchange is preheated to 200-240° C. in the preheating step.

3. The method for preparing low-sulfur biodiesel according to claim 1, wherein in the cooling process, the oil having exchanged heat with the raw crude oil is cooled to 50-70° C. by water.

4. The method for preparing low-sulfur biodiesel according to claim 1, wherein the amount of methanol added in the transesterification is 20-40% the weight of the oil, the amount of KOH added is 0.5-2% the weight of the oil, and the reaction is carried out at 50-70° C. for 1-2 hours.

5. The method for preparing low-sulfur biodiesel according to claim 1, wherein in the dealcoholization process, methanol and a trace amount of water in the system after the transesterification are removed by a multi-layer evaporation device.

6. The method for preparing low-sulfur biodiesel according to claim 1, wherein in the sedimentation step, the oil stands and sediments at 40-60° C. for 1-3 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic flow chart of an embodiment of the present invention.

(2) Description of reference numerals of the accompanying drawing: 1. heat exchange, 2. preheating, 3. glycerol esterification reaction, 4. flashing, 5. cooling, 6. transesterification, 7. dealcoholization, 8. methanol rectification, 9. sedimentation, 10. glycerol rectification, 11. methyl ester rectification.

DETAILED DESCRIPTION

(3) The present invention will be described in detail below with reference to the accompanying drawings and embodiments of the description, but the protection scope of the present invention is not limited thereto.

(4) A crude biodiesel raw material used in Example 1 is waste oil (gutter oil), and the fatty acid methyl ester content distribution thereof is shown in Table 1:

(5) TABLE-US-00001 TABLE 1 Table of content of each fatty acid methyl ester in biodiesel prepared from gutter oil Ingredient C.sub.14:0 C.sub.16:0 C.sub.18:0 C.sub.18:1 C.sub.18:2 C.sub.18:3 Mass fraction 0.764 18.897 2.237 35.998 36.923 3.869

(6) As shown in FIG. 1, the raw crude oil is subjected to heat exchange 1 with oil subjected to flashing 4, and undergoes preheating 2 to 240° C., glycerol is added for glycerol esterification 3 (fresh glycerol is added during the first production and reused glycerol is added later), the temperature of the glycerol esterification 3 is controlled to 240° C., the reaction time is 3 h, and then flashing 4 (a distilled product subjected to flashing 4 can be reused for next glycerol esterification), the top temperature of the flashing 4 is 191° C. and operational absolute pressure is 500 Pa, the oil after the flashing 4 and the raw crude oil are subjected to heat exchange to recycle heat and then are subjected to cooling 5 to 60° C., methanol accounting for 40% the weight of the oil and KOH accounting for 1% the weight of the oil are added for transesterification 6, the temperature of the transesterification 6 is 60° C., and the reaction time is 1 h; after transesterification is completed, dealcoholization 7 (methanol is subjected to methanol rectification 8 and then reused for transesterification) is performed, and then sedimentation 9 is carried out to obtain crude glycerol and crude methyl ester, glycerol undergoes rectification 10 to obtain refined glycerol which can be repeatedly used for glycerol esterification 3, and methyl ester rectification 11 is performed to obtain low-sulfur biodiesel up to the standard, which conforms to the S10 standard in GBT 20828-2015; and a tower kettle obtains heavy components of the bio-asphalt.

(7) Waste oil is used to prepare the biodiesel through the process. The results of mass and sulfur content of each material are shown in Table 2:

(8) TABLE-US-00002 TABLE 2 Table of results of mass and sulfur contents of various materials in the process of preparation of low-sulfur biodiesel Sulfur Feed/output Material type content/ppm mass/g Gutter oil raw material 239.1 1000 Glycerol esterification residue 163.7 1115.2 Oil generated by glycerol esterification 852.2 4.2 Water generated by glycerol esterification 146.1 64.2 Distilled product subjected to flashing 41.2 91.6 Flashing residue 146.3 1020 Distilled methyl ester 8.2 806.8 Heavy component of bio-asphalt 316 102.6

(9) The raw material of crude glycerol used in Example 2 is crude glycerol obtained by dealcoholization and sedimentation after transesterification in the process of preparing biodiesel by glycerol esterification. 60.1 g of crude glycerol is taken and directly rectified at an absolute pressure of 500 Pa and a top gas phase temperature of 161° C. without treatment. The raw material composition and rectification data is shown in Table 3. It can be seen from Table 3 that the crude glycerol obtained by the process can be directly distilled without the conventional acidification and soap removal process, and the glycerol recovery rate reaches 97.4%, which solves the problem of acid-containing wastewater generated by acidification of the glycerol recovery process.

(10) TABLE-US-00003 TABLE 3 raw material composition and rectification data Mass Name Mass/g Component content/% Crude glycerol 60.1 KOH 3.8 raw material Glycerol 60 Others (colloid and 36.2 polyglycerol) Refined glycerol 35.2 Glycerol 99.8 Rectification residue 21.2 Glycerol 1.5 Glycerol rectification yield 97.4%

(11) The above embodiments are only used to explain the present invention, but do not limit the protection scope of the present invention. Besides the above-mentioned embodiments, the technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope of the present invention.