CN102559393B - Method for preparing biodiesel with low acid value - Google Patents

Abstract

A method for preparing biodiesel with low acid value, comprising: providing oil and monohydric alcohol to a first-stage reactor for reaction, the temperature in the reactor is 100-330°C, and the pressure is >3-12MPa, starting from the first-stage The material containing at least fatty acid ester from the reactor enters the second-stage reactor, the temperature of the second-stage reactor is 100-330° C., and the pressure is 0.1-3 MPa, and biodiesel with reduced acid value is collected. By adopting the method of the invention, not only the acid value of biodiesel can be effectively reduced, but also the reaction can be carried out continuously, and the acid value of the oil which is difficult to reduce the acid can be reduced to below 3mgKOH/g.

Description

A kind of preparation method of low acid value biodiesel

technical field

The invention relates to a method for preparing biodiesel by reacting oil and monoalcohol.

Background technique

Biodiesel can be obtained through the transesterification reaction of fats, triglycerides and low-carbon alcohols. The reaction products include fatty acid esters, monoglycerides, diglycerides, glycerol, alcohols and unreacted triglycerides, which can be obtained by separation Biodiesel, which is mainly fatty acid monoalkyl esters, also contains small amounts of monoglycerides and diglycerides.

The preparation methods of biodiesel can be divided into acid catalysis, base catalysis, enzyme catalysis and supercritical method.

CN1412278A discloses an acid catalysis method, a method for preparing biodiesel from waste animal and vegetable oil with high acid value, which uses sulfuric acid as a catalyst, and proceeds according to the procedures of dehydration, esterification, phase separation and decolorization of raw materials, and the raw materials are first heated at 60-100 °C , dehydration under vacuum, and then, at 40-85°C, add 2-6% sulfuric acid, react for 6 hours, separate fatty acid methyl ester phase, decolorize with activated clay at 90-125°C to obtain biodiesel.

The problem with acid catalysis is that the reaction speed is slow, and a large amount of waste acid and waste water will be produced, causing serious pollution.

The reaction speed of the alkali-catalyzed method is faster than that of the acid-catalyzed method, but the raw material requirements are strict. Generally, the raw material is low acid value and refined oil. If there is a lot of free fatty acid in the oil, the direct use of alkali catalyst will produce fatty acid soap. In this case The base catalyst must be excessive, which will cause the separation of the fatty acid ester layer and the glycerin layer to become difficult. In order to avoid these problems, DE3444893 discloses a method, first, carry out pre-esterification treatment to oil, use acid catalyst, normal pressure, 50～120 ℃, carry out esterification with free fatty acid and alcohol, then add excess alkali metal catalyst Neutralize the acid catalyst remaining in the pre-esterification, and then carry out the transesterification reaction.

CN1031070C uses concentrated sulfuric acid, phosphoric acid or p-toluenesulfonic acid, toluenesulfonic acid and naphthalenesulfonic acid as catalysts to carry out pre-esterification reaction of oil and alcohol in a stirred tank reactor at 80-160°C. After the reaction is completed, add alcohol and excess alkali to neutralize the acidic catalyst and residual free fatty acid first, and the remaining alkali is used as a catalyst for transesterification to carry out transesterification

Using the above-mentioned pre-esterification process, the remaining acid catalyst will be neutralized by alkali, the demand for alkali metal catalyst will increase significantly, the processing flow will be long, there will be many steps, equipment investment, and energy consumption will increase significantly. The removal of active catalysts from the reaction process and products requires a large amount of washing water and a large amount of waste water.

US5713965A discloses a method, in the presence of lipase, using hexane as a solvent to prepare fatty acid methyl esters through the reaction of oil and alcohol, although the reaction conditions are mild using an enzyme catalyst, the reaction time is long and the efficiency is low, and a solvent is required , and the cost of the enzyme is high and easily inactivated.

PCT/JP99/0543 discloses a method for preparing fatty acid ester by reacting oil and monohydric alcohol. At 270-280° C. and 11-12 MPa, methanol is reacted with oil, and the yield of fatty acid methyl ester is 55-60%. According to the data disclosed in this patent, the reaction conditions are harsh, the equipment requirements are high, the fixed investment is large, and the yield of fatty acid methyl ester is not ideal.

Contents of the invention

The invention provides a method for preparing biodiesel with low acid value, which can effectively reduce the acid value of the biodiesel and significantly reduce the energy consumption of the device.

The preparation method of the low-acid-value biodiesel provided by the present invention comprises: providing grease and C ₁ -C ₆ monohydric alcohols to a first-stage reactor for reaction, wherein the temperature in the reactor is 100-330° C., and the pressure is > 3~12MPa, the material containing at least fatty acid ester from the first stage reactor enters the second stage reactor, wherein, the temperature of the second stage reactor is 100~330℃, the pressure is 0.1~3MPa, and the collected acid value is reduced of biodiesel.

Said reactor is a fixed bed reactor, preferably a tubular reactor. The reactor may contain packing. Grease and monohydric alcohol can be supplied to the first reactor separately, or they can be supplied to the first reactor after they are pre-mixed. Before being supplied to the reactor, the materials can be preheated by a preheater, or can be directly fed into the reactor. Grease and monohydric alcohol can enter from the bottom of the first-stage reactor and flow upward; they can also enter from the top of the first-stage reactor and flow downward, preferably enter from the bottom of the first-stage reactor and flow upward.

The material from the first-stage reactor can directly enter the second-stage reactor, or can be flashed under reduced pressure to evaporate part or all of the monohydric alcohol and water, so that the remaining liquid phase material containing fatty acid ester can enter the second-stage reaction It can also be decompressed and flashed to evaporate part or all of the monohydric alcohol and water, and the remaining liquid phase materials are phase-separated, so that the ester phase enters the second-stage reactor. That is to say, the material that enters the second-stage reactor at least comprising fatty acid ester can be: (1) the material that comes out directly from the first-stage reactor; (2) the material of (1) is flashed under reduced pressure, The remaining liquid phase material after distilling off part or all of the monohydric alcohol and water; (3) the ester phase material obtained by phase-separating the material in (2).

The pressure achieved by decompression flash evaporation can be the same as or different from the pressure of the second-stage reactor, and the position where the material containing fatty acid ester enters the second-stage reactor can enter from the lower part of the second-stage reactor and flow upward; It can also enter from the upper part of the second-stage reactor and flow downward, preferably enter from the lower part of the second-stage reactor and flow upward. The monohydric alcohol supplemented to the second-stage reactor enters the second-stage reactor from the lower part of the second-stage reactor and flows upward.

In the first stage reaction, the temperature in the reactor is 100-330°C, preferably 150-300°C, more preferably 180-260°C. The pressure is >3-12MPa, preferably 4-10MPa, more preferably 5-8MPa. The hourly space velocity of the oil and fat liquid can vary within a wide range, generally 0.1-6h ^-1 , preferably 0.2-4h ^-1 , more preferably 0.5-2h ^-1 . The mass ratio of monohydric alcohol to oil is 0.05-2:1, preferably 0.1-1:1.

In the second stage reaction, the temperature in the reactor is 100-330°C, preferably 150-300°C, more preferably 180-260°C, and the pressure is 0.1-3MPa, preferably 0.5-3MPa, more preferably 0.7-3MPa. The liquid hourly space velocity of the oil can be changed in a wide range, and the liquid hourly space velocity is 0.1~6h ^-1 , preferably 0.2~4h ^-1 , more preferably 0.5~2h ^-1 ; Monohydric alcohol needs to be supplemented so that the mass ratio of monohydric alcohol to oil (according to the amount added in the first stage) is 0.05-2:1, preferably 0.1-1:1. The acid value of the material entering the second stage should be ≤20mgKOH/g, preferably ≤15mgKOH/g, most preferably ≤10mgKOH/g.

Said oil is fatty acid triglyceride, mainly derived from animal oil or vegetable oil, and also includes oil plants from microorganisms, algae and other substances. 20-160 mgKOH/g, more preferably 30-130 mgKOH/g. The higher the acid value of the raw material oil, the more difficult it is to reduce the acid value to below 3mgKOH/g. The oil with high acid value within the above acid value range can be crude oil, waste oil or deteriorated oil. Among them, crude oil refers to unrefined or unrefined oil that has not reached the product standard. The refining process includes but not limited to degumming, alkali refining, dephosphorization, decolorization, deodorization and other processes. High acid value oils within the above acid value range can also be obtained by additionally adding free fatty acids to the oil.

The monohydric alcohol refers to C ₁ -C ₆ monohydric aliphatic alcohol, which may be saturated alcohol or unsaturated alcohol. Such as methanol, ethanol, propanol, isopropanol, allyl alcohol, n-butanol and its isomers, pentanol and its isomers, etc. Alcohols alone or their mixtures can be used, preferably methanol and/or ethanol .

According to the invention, the reaction can be carried out with or without the presence of a basic metal compound. Preference is given to adding the basic metal compound in the second stage of the reaction. The basic metal compound includes but is not limited to one or more of the hydroxides, alcoholates, oxides, carbonates, bicarbonates and fatty acid salts of IA and IIA elements in the periodic table, preferably periodic One or more of C ₁₂ -C ₂₄ fatty acid salts of IA and IIA elements in the table, more preferably one or more of sodium and potassium C ₁₂ -C ₂₄ fatty acid salts. The amount of the basic metal compound is 0.001-0.07 wt%, preferably 0.003-0.05 wt%, based on the weight of the raw oil.

According to the method provided by the present invention, in order to obtain biodiesel with an acid value meeting the requirements, the above process can be carried out once or several times.

By adopting the method of the invention, not only the acid value of the biodiesel can be effectively reduced, but also the reaction can be carried out continuously, the operation steps are simplified, no waste acid is produced, and there is basically no waste water, which is beneficial to environmental protection and subsequent processing. Since the reaction pressure of the first stage is higher than that of the second stage, there is no need for a heat-resistant high-pressure pump between the two stages, which can significantly reduce investment and energy consumption. For oils that are difficult to reduce acidity, the acid value of the oil can be reduced to below 3mgKOH/g, and biodiesel can be obtained with high yield.

Detailed ways

The present invention is further illustrated by examples below, but the present invention is not limited thereto.

In the embodiment, "yield" is defined as follows:

Yield=(product weight/feed oil weight)×100%

Example 1

Oil and methanol with an acid value of 125mgKOH/g are used as raw materials, the liquid hourly space velocity is 1h ^-1 , and the mass ratio of alcohol to oil is 0.25:1, and they are put into the first-stage tubular reactor. The reactor temperature is 270°C and the pressure is 5.5Mpa. After a little reaction, methanol is removed from the material, and the acid value of the thick ester is 6mgKOH/g. The ester phase and methyl alcohol collected after the first-stage reaction enter the second-stage reactor from the bottom, and the KOH (calculated in potassium) of 0.027w% of the oil weight is added simultaneously. ), the temperature of the second-stage reactor is 260°C, the pressure is 2.5MPa, the liquid hourly space velocity is 0.95h ^-1 , the mass ratio of alcohol to oil is 0.16:1 (the oil is based on the amount added in the first stage), and the reaction crude product is obtained by removing methanol and separating After the glycerol, the obtained crude ester was distilled under reduced pressure to distill high-purity fatty acid methyl ester with a yield of 92.3w% and an acid value of 0.4mgKOH/g.

Example 2

Oil and methanol with an acid value of 130mgKOH/g are used as raw materials, the liquid hourly space velocity is 1h ^-1 , and the mass ratio of alcohol to oil is 0.2:1, and it enters the first-stage tubular reactor. The temperature of the first-stage reactor is 260°C and the pressure is 6.5 Mpa, get a little reacted material to remove methyl alcohol, measure thick ester acid value 7mgKOH/g, the ester phase that collects after the first stage reaction and methyl alcohol enter the second stage reactor from the bottom, add the NaOH of grease weight 0.006w% simultaneously ( Calculated as sodium), the temperature of the second-stage reactor is 250°C, the pressure is 1.5MPa, the liquid hourly space velocity is 0.95h ^-1 , and the mass ratio of alcohol to oil is 0.14:1. The obtained crude esters were distilled under reduced pressure to distill high-purity fatty acid methyl esters with a yield of 90.2w% and an acid value of 0.6 mgKOH/g.

Example 3

Oil and methanol with an acid value of 146mgKOH/g are used as raw materials, the liquid hourly space velocity is 1.2h ^-1 , and the mass ratio of alcohol to oil is 0.17:1, which is supplied to the first-stage tubular reactor. The reactor temperature is 252°C and the pressure is 8Mpa. Take a little After the reaction, the methanol was removed from the material, and the acid value of the crude ester was measured to be 11.2mgKOH/g. The material coming out of the first-stage reactor and the methanol added in the second-stage enter the second-stage tubular reactor from the lower part, and the second-stage reactor The temperature is 274°C, the pressure is 0.7MPa, the liquid hourly space velocity is 1.2h ^-1 , and the mass ratio of methanol to oil in the second stage is 0.13:1. After removing methanol and separating glycerol, the obtained crude ester is subjected to vacuum distillation. , high-purity fatty acid methyl ester was obtained at the top of the tower, the yield was 89w%, and the acid value was 1.5mgKOH/g.

Example 4

Oil and methanol with an acid value of 122mgKOH/g are used as raw materials, the liquid hourly space velocity is 0.9h ^-1 , and the mass ratio of alcohol to oil is 0.3:1, and they are put into the first-stage tubular reactor. The reactor temperature is 210°C and the pressure is 4Mpa. After a little reaction, the methanol was removed from the material, and the acid value of the crude ester was measured to be 15 mgKOH/g. The ester phase collected after the first stage reaction entered the second stage reactor from the upper part, and the methanol entered the second stage reactor from the lower part. The second stage reactor The temperature is 220°C, the pressure is 0.2MPa, the liquid hourly space velocity is 1h ^-1 , and the mass ratio of alcohol to oil is 0.4:1 (the oil is based on the amount added in the first stage). After removing methanol and separating glycerin, the crude ester obtained is Distill under reduced pressure to distill out high-purity fatty acid methyl esters with an acid value of 2.2 mgKOH/g.

Comparative example 1

The same acidified oil and methanol as in Example 3 are used as raw materials, the reaction pressure is 8Mpa, the liquid hourly space velocity is 1.2h ^-1 , and the mass ratio of alcohol to oil is 0.17:1, and the reaction temperature is 252°C to obtain After methanol was removed from the crude reaction product and glycerin was separated, the obtained crude ester was distilled under reduced pressure to distill high-purity fatty acid methyl ester with a yield of 89w% and an acid value of 10mgKOH/g.

Comparative example 2

The same acidified oil and methanol as in Example 3 were used as raw materials, the reaction pressure was 0.7MPa, the liquid hourly space velocity was 1.2h ^-1 , and the mass ratio of alcohol to oil was 0.13:1, which was supplied to the first-stage tubular reactor, and the reactor temperature was 274 ℃, to obtain the reaction crude product, remove methanol and separate glycerin, then carry out vacuum distillation on the obtained crude ester to distill high-purity fatty acid methyl ester, the yield is 79w%, and the acid value is 11mgKOH/g.

Claims (4)

1. a preparation method for low acid number biofuel, comprising: by grease and C ₁~ C ₆unitary fatty alcohol is provided in first step reactor and reacts, and wherein, grease and unitary fatty alcohol enter from the bottom of first step reactor, upwards flow, and in reactor, temperature is 180 ~ 260 DEG C, and pressure is 5 ~ 8MPa, liquid hourly space velocity 0.5 ~ 2h ^-1, alcohol oil quality is than 0.1 ~ 1: 1, enter from the first step reactor material at least comprising fatty acid ester out from the bottom of second stage reactor, upwards flow, control the acid number≤20mgKOH/g entering the material of second stage reactor, in the reactor of the second stage, add alkaline metal cpds, collect the biofuel that acid number reduces, second stage temperature of reactor is 180 ~ 260 DEG C, pressure is 0.7 ~ 3MPa, liquid hourly space velocity 0.5 ~ 2h ^-1, alcohol oil quality is than 0.1 ~ 1: 1, and said reactor is tubular reactor, and said acid value of oil and fat is 1 ~ 200mgKOH/g.

2. in accordance with the method for claim 1, wherein, be selected from from the first step reactor material at least comprising fatty acid ester out: (1) is directly from first step reactor material out; (2) by the material of (1) through vacuum flashing, steam remaining liquid phase material after part or all of unitary alcohol and water; Or the material of (2) is carried out the ester phase materials that phase-splitting obtains by (3).

3. in accordance with the method for claim 1, wherein, described monohydroxy-alcohol is methyl alcohol and/or ethanol.

4. in accordance with the method for claim 1, wherein, described alkaline metal cpds is selected from the C of sodium and potassium ₁₂~ C ₂₄one or more in soap, the amount of alkaline metal cpds counts 0.001 ~ 0.07wt% of raw oil material weight with metal.