METHOD FOR PRETREATING WASTE FAT, OIL AND GREASE AND CO-PRODUCING FIRST-GENERATION BIODIESEL

Abstract

Provided is a method for pretreating waste fat, oil and grease (FOG) and co-producing a first-generation biodiesel, including: S1, feeding waste FOG, a liquid acid catalyst, and methanol into a pre-esterification reactor, and conducting pre-esterification to obtain a pre-esterification mixed liquid; S2, removing waste residues from the pre-esterification mixed liquid through a filter to obtain a filtrate, and separating the filtrate by a liquid-liquid separator to obtain an organic phase and an aqueous phase; S3, introducing the organic phase into a methanol recovery tower I and conducting separation to obtain a pre-esterification product and crude methanol; introducing the aqueous phase into a methanol recovery tower II and conducting another separation to obtain a liquid acid catalyst and crude methanol; S4, separating the pre-esterification product through a biodiesel refining tower to obtain a first-generation biodiesel product, and a pretreated waste FOG at a tower bottom.

Claims

1. A method for pretreating waste fat, oil and grease (FOG) and co-producing a first-generation biodiesel, comprising: S1, feeding waste FOG, a liquid acid catalyst, and methanol at a certain proportion into a pre-esterification reactor, and conducting pre-esterification reaction to obtain a pre-esterification mixed liquid; S2, removing waste residues from the pre-esterification mixed liquid after the pre-esterification reaction through a filter to obtain a filtrate, and separating the filtrate by a liquid-liquid separator to obtain an organic phase and an aqueous phase; S3, introducing the organic phase after the pre-esterification reaction into a methanol recovery tower I and conducting separation to obtain a pre-esterification product and crude methanol; introducing the aqueous phase into a methanol recovery tower II and conducting another separation to obtain a liquid acid catalyst and crude methanol; and purifying the crude methanol through a methanol refining tower; and S4, separating the pre-esterification product through a biodiesel refining tower to obtain a first-generation biodiesel product, and a pretreated waste FOG at a tower bottom.

2. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein the waste FOG in step S1 comprises one selected from the group consisting of acidified oil, hogwash oil, and swill-cooked dirty oil or a mixture thereof.

3. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein in step S1, the liquid acid catalyst is a mixture of an inorganic acid and an acidic ionic liquid; the inorganic acid comprises one or more selected from the group consisting of concentrated sulfuric acid, phosphoric acid, and hydrochloric acid; the acidic ionic liquid comprises one or more selected from the group consisting of an imidazolium hydrogensulfate ionic liquid, a N-alkylammonium hydrogensulfate ionic liquid, a pyridinium hydrogensulfate ionic liquid and a pyrrole hydrogensulfate ionic liquid; and a molar ratio of the inorganic acid to the acidic ionic liquid is in a range of (0.1-0.5):1.

4. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein in step S1, a molar ratio of the waste FOG to the methanol is in a range of (1-10):1, and a molar ratio of the liquid acid catalyst to the waste FOG is in a range of (0.01-0.1):1.

5. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein in step S1, the pre-esterification reaction is conducted at a temperature of 60 C. to 120 C. and a pressure of 0.1 MPa to 1 MPa for 1 h to 10 h.

6. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein in step S2, an operating temperature of the filter is in a range of 50 C. to 80 C.

7. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein in step S2, the liquid-liquid separator is a chromatography device with an operating temperature of 50 C. to 80 C. and an operating time of 0.5 h to 2.0 h.

8. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein in step S3, both the methanol recovery tower I and the methanol recovery tower II have an operating temperature of 60 C. to 100 C., and an operating pressure of 0.1 MPa to 1.0 MPa.

9. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein the methanol refining tower in step S3 is a normal-pressure fractionation tower with an operating temperature of 67 C. to 75 C. and an operating pressure of 0.1 MPa to 0.2 MP at a tower top.

10. The method for pretreating the waste FOG and co-producing the first-generation biodiesel according to claim 1, wherein in step S4, the biodiesel refining tower is a reduced-pressure fractionation tower with an operating temperature of 100 C. to 250 C. and an operating pressure of 0.5 kPa to 2.0 kPa at a tower top.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to illustrate the specific embodiments of the present disclosure more clearly, the drawing needed to be used in the specific embodiments will be briefly introduced below. Obviously, the drawing in the following description is some embodiments of the present disclosure, and other drawings could be obtained by those skilled in the art without creative effort according to the drawing.

[0026] FIGURE is a flow chart showing a method for pretreating waste FOG and co-producing a first-generation biodiesel according to an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0027] The technical solution of the present disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described examples are some, not all, of the embodiments in the present disclosure. Based on the examples of the present disclosure, all other embodiments obtained by those skilled in the art without creative effort shall fall within the scope of the present disclosure.

Example 1

[0028] As shown in FIGURE, a method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0029] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 10:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of 98% concentrated sulfuric acid to 1-butyl-3-methylimidazolium hydrogensulfate acidic ionic liquid was 0.1) was fed therein at a temperature of 70 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 10% of that of the waste FOG. After that, a reaction was performed for 2 h to obtain a pre-esterification mixed liquid. [0030] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was set to 60 C.) to remove waste residues, and a resulting filtrate was transferred to a liquid-liquid separator (i.e., a chromatography device) and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 50 C., and the separation was conducted for 1 h. [0031] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 80 C. and 100 C. respectively, and the operating pressures of both were 0.2 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 75 C. and an operating pressure of 0.2 MPa. [0032] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 250 C. and the operating pressure was 1.0 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 98%.

Example 2

[0033] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0034] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 10:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of concentrated sulfuric acid to 1-butyl-3-methylimidazolium hydrogen sulfate acidic ionic liquid was 0.1) was fed therein at a temperature of 60 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 10% of that of the waste FOG. After that, a reaction was performed for 1 h to obtain a pre-esterification mixed liquid. [0035] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was set to 50 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 50 C., and the separation was conducted for 1 h. [0036] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 80 C. and 100 C. respectively, and the operating pressures of both were 0.2 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 75 C. and an operating pressure of 0.1 MPa. [0037] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 250 C., and the operating pressure was 1.0 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 93%.

Example 3

[0038] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0039] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 8:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of concentrated sulfuric acid to 1-butyl-3-methylimidazolium hydrogensulfate acidic ionic liquid was 0.1) was fed therein at a temperature of 70 C. under mechanical stirring (at a speed of to 400 rpm), where the composite acid catalyst was added in a molar amount of 10% of that of the waste FOG. After that, a reaction was performed for 3 h to obtain a pre-esterification mixed liquid. [0040] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was set to 60 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 60 C., and the separation was conducted for 1 h. [0041] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 80 C. and 100 C. respectively, and the operating pressures of both were 0.2 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 75 C. and an operating pressure of 0.2 MPa. [0042] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 250 C., and the operating pressure was 1.0 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 96%.

Example 4

[0043] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0044] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 5:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of concentrated sulfuric acid to N-methylpyridinium hydrogensulfate acidic ionic liquid was 0.2) was fed therein at a temperature of 60 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 6% of that of the waste FOG. After that, a reaction was performed for 5 h to obtain a pre-esterification mixed liquid. [0045] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was 65 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 60 C., and the separation was conducted for 2 h. [0046] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 100 C. and 60 C. respectively, and the operating pressures of both were 0.1 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 67 C. and an operating pressure of 0.1 MPa. [0047] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 200 C., and the operating pressure was 0.5 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 92%.

Example 5

[0048] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0049] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 5:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of concentrated sulfuric acid to N-ethyl-N-methylpyrrole hydrogensulfate acidic ionic liquid was 0.3) was fed therein at a temperature of 100 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 5% of that of the waste FOG. After that, a reaction was performed for 2 h to obtain a pre-esterification mixed liquid. [0050] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was set to 65 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 80 C., and the separation was conducted for 2 h. [0051] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 80 C. and 100 C. respectively, and the operating pressures of both were 0.1 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 75 C. and an operating pressure of 0.1 MPa. [0052] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 200 C., and the operating pressure was 1.0 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 95%.

Example 6

[0053] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows. [0054] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 2:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of concentrated phosphoric acid to methyltributylammonium hydrogensulfate acidic ionic liquid was 0.1) was fed therein at a temperature of 120 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 5% of that of the waste FOG. After that, a reaction was performed for 7 h to obtain a pre-esterification mixed liquid. [0055] S2. The pre-esterification mixed liquid after the reaction was filtered by using a filter (the temperature of the filter was set to 80 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 60 C., and the separation was conducted for 0.5 h. [0056] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 60 C. and 80 C. respectively, and the operating pressures of both were 0.1 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 70 C. and an operating pressure of 0.1 MPa. [0057] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 150 C., and the operating pressure was 2.0 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 91%.

Example 7

[0058] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0059] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 1:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of concentrated sulfuric acid to 1-butyl-3-methylimidazolium hydrogensulfate acidic ionic liquid was 0.5) was fed therein at a temperature of 70 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 5% of that of the waste FOG. After that, a reaction was performed for 5 h to obtain a pre-esterification mixed liquid. [0060] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was set to 60 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 60 C., and the separation was conducted for 1.5 h. [0061] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II. Respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 80 C. and 100 C. respectively, and the operating pressures of both were 0.5 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 75 C. and an operating pressure of 0.1 MPa. [0062] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 250 C., and the operating pressure was 1.0 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 98%.

Example 8

[0063] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0064] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 4:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of concentrated sulfuric acid to 1-butyl-3-methylimidazolium hydrogensulfate acidic ionic liquid was 0.1) was fed therein at a temperature of 80 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 1% of that of the waste FOG. After that, a reaction was performed for 1 h to obtain a pre-esterification mixed liquid. [0065] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was set to 65 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 65 C., and the separation was conducted for 2 h. [0066] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 70 C. and 70 C. respectively, and the operating pressures of both were 0.2 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 67 C. and an operating pressure of 0.2 MPa. [0067] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 100 C., and the operating pressure was 2.0 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 90%.

Example 9

[0068] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0069] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 4:1 were fed into a pre-esterification; and a composite acid catalyst (in which a ratio of concentrated sulfuric acid to 1-butyl-3-methylimidazolium hydrogensulfate acidic ionic liquid was 0.1) was fed therein at a temperature of 110 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 2% of that of the waste FOG. After that, a reaction was performed for 3 h to obtain a pre-esterification mixed liquid. [0070] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was set to 65 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 60 C., and the separation was conducted for 1 h. [0071] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 80 C. and 100 C. respectively, and the operating pressures of both were 1 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 75 C. and an operating pressure of 0.2 MPa. [0072] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 200 C., and the operating pressure was 1.5 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 90%.

Example 10

[0073] A method for pretreating waste FOG and co-producing a first-generation biodiesel was conducted as follows: [0074] S1. Waste FOG and methanol at a molar ratio of alcohol to oil of 2:1 were fed into a pre-esterification reactor; and a composite acid catalyst (in which a ratio of concentrated sulfuric acid to 1-butyl-3-methylimidazolium hydrogensulfate acidic ionic liquid was 0.1) was fed therein at a temperature of 70 C. under mechanical stirring (at a speed of 400 rpm), where the composite acid catalyst was added in a molar amount of 1% of that of the waste FOG. After that, a reaction was performed for 10 h to obtain a pre-esterification mixed liquid. [0075] S2. The pre-esterification mixed liquid after the reaction was filtered through a filter (the temperature of the filter was set to 80 C.) to remove waste residues, and a resulting filtrate was transferred to a chromatography device and conducted separation therein to obtain an organic phase and an aqueous phase, where the temperature of the chromatography device was set to 60 C., and the separation was conducted for 2 h. [0076] S3. The organic phase and the aqueous phase after the pre-esterification were introduced into a methanol recovery tower I and a methanol recovery tower II, respectively, and then conducted separation, so as to obtain a pre-esterification product, crude methanol and a liquid acid catalyst, and the liquid acid catalyst was recovered. The temperatures of the methanol recovery tower I and the methanol recovery tower II were set to 100 C. and 100 C. respectively, and the operating pressures of both were 0.2 MPa. The crude methanol was further purified through a methanol refining tower with an operating temperature of 70 C. and an operating pressure of 0.2 MPa. [0077] S4. The pre-esterification product was transferred to a biodiesel refining tower and conducted fractionation therein to obtain a first-generation biodiesel product and pretreated raw oil (i.e., pretreated waste FOG), where the operating temperature at the top of the tower was 250 C., and the operating pressure was 0.5 kPa. The purity of the first-generation biodiesel and the acid value of the pretreated raw oil were measured by GC-MS and KOH titration, and the pre-esterification efficiency reaches 92%.

[0078] Obviously, the above-described embodiments are only examples for clear illustration and are not intended to limit the embodiments. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. An exhaustive list of all embodiments is neither necessary nor possible. The obvious changes or modifications derived therefrom are still within the scope of the present disclosure.