Purification method of waste edible oil

Abstract

The invention discloses a method for purifying waste edible oil, which comprises the following steps of: extracting water-soluble impurities in the edible oil by using water to prevent the impurities from influencing the adsorption effect of the subsequent steps, and then adsorbing and removing free fatty acids and polar substances by using magnesium silicate salt with high specific surface area in a circulating filtration manner to improve the color and luster of the edible oil. Finally, residual water and small molecular impurities are removed in a vacuum distillation mode, the acid value, the polar substance content, the water content, the color and luster and the like of the purified waste edible oil are greatly improved, and the waste edible oil can be reused.

Description

Technical Field

[0001] The invention relates to the technical field of waste cooking oil treatment, and in particular to a method for purifying waste cooking oil. Background Art

[0002] It is of great significance to develop domestic waste cooking oil purification technology so that waste cooking oil can be reused or used as a raw material for the preparation of biodiesel.

[0003] Prolonged use at high temperatures will cause a large amount of free fatty acids (FFA) and oxidized fatty acids (OFA), as well as polar compounds such as carbonyl, carboxyl, ketone, and aldehyde groups, to accumulate in the cooking oil, turning it into high-acid, high-polarity waste cooking oil. The increase in FFA will reduce the smoke point of the oil, and OFA and its conversion byproducts will cause the oil and fried foods to have odors. In addition, foods containing carbohydrates and proteins will undergo browning reactions when exposed to hot oil, causing the color of the waste cooking oil to darken. Since the service life of cooking oil is short and frequent replacement will incur huge costs, the industry has been looking for an effective way to extend its service life.

[0004] It is generally believed that when the residual moisture in cooking oil exceeds 0.2%, the acid value exceeds 3.0mgKOH / g, the polar matter (TMP) exceeds 27%, and the color becomes dark, the cooking oil is not suitable for use. The residual moisture of waste cooking oil is usually 1-10%, the acid value is usually more than 5mgKOH / g, and the polar matter content is usually more than 35%.

[0005] At present, the main method used in the purification technology of edible oil is adsorption. For example, patent US4681768 mentions that the use of magnesium silicate to adsorb and treat waste edible oil can effectively reduce the impurity content so that it can be used again. However, the purification effect of this single method is not ideal, because the components of waste edible oil are extremely complex, and some water-soluble impurities can easily destroy the structure of the adsorbent and block the pores, resulting in a decrease in the adsorption effect. Patent US5597600 proposes the use of magnesium silicate and alkaline substances in combination to treat waste edible oil. Patents CN104508099A and US9295810 propose the use of magnesium silicate and organic acids in combination to treat waste edible oil. These technologies are also based on a single adsorption method, and the introduction of alkaline substances or organic acids will induce hydrolysis of edible oil and bring new impurities, resulting in a weakening of the purification effect. Failure to treat these new impurities may cause food safety problems. Summary of the invention

[0006] The purpose of the present invention is to provide a method for purifying waste cooking oil. The method provided by the present invention utilizes a combination of extraction, adsorption and distillation to treat the waste cooking oil. The three methods complement each other and exert a synergistic effect, thereby greatly reducing the acid value, polar substance content and water content of the waste cooking oil, improving the color of the waste cooking oil, and enabling it to be reused.

[0007] The technical solution of the present invention is: a method for purifying waste cooking oil, comprising the following steps: Step 1: extracting and removing water-soluble impurities in waste cooking oil using water; Step 2: Under an inert atmosphere, the waste cooking oil from which water-soluble impurities have been removed is circulated and filtered using magnesium silicate to remove free fatty acids and polar substances therein until the acid value no longer decreases; Step 3: The filtered waste cooking oil is subjected to vacuum distillation to remove residual water and small molecule impurities to obtain purified cooking oil.

[0008] In step 1, the mass ratio of waste cooking oil to water is 1:1 to 1:5. Water is a safe and non-toxic extractant and is immiscible with waste cooking oil. It can extract water-soluble impurities in waste cooking oil to prevent impurities from affecting the adsorption effect of subsequent steps. If the amount of water is too little, the extraction effect is poor, and if the amount of water is too much, the treatment efficiency is low.

[0009] The operating temperature range of the circulating filtration is 100-150°C, and the operating atmosphere is an inert gas. At room temperature, the viscosity of edible oil is relatively large and its fluidity is poor, which is not conducive to the operation of circulating filtration. Increasing the temperature will reduce the viscosity of the edible oil and enhance the adsorption effect of the adsorbent, but too high a temperature will cause the decomposition of the edible oil. The operating atmosphere is an inert gas, which provides an oxygen-free environment to prevent the edible oil from oxidative decomposition. The time of circulating filtration depends on the specific filtering equipment. When the acid value of the waste edible oil no longer decreases with the filtration time, it means that the adsorption saturation effect has been reached and the circulating filtration can be stopped.

[0010] The magnesium silicate adsorbent is magnesium silicate or magnesium aluminum silicate. The inventors have found that these two adsorbents can effectively adsorb various acidic impurities in waste cooking oil and significantly reduce the acid value of waste cooking oil. The particle size and usage of the adsorbent can be adjusted according to the specific type, processing volume and initial acid value of the waste cooking oil. The present invention does not limit this. However, the specific surface area of ​​the adsorbent has a relatively large impact on the adsorption capacity. Therefore, the present invention limits the specific surface area of ​​the magnesium silicate adsorbent to 500~800m 2 / g, too small a specific surface area will lead to poor purification effect of waste cooking oil, and a specific surface area of ​​more than 800m 2 / g of magnesium silicate or magnesium aluminum silicate are rare and not easy to obtain.

[0011] The pressure range of the vacuum distillation is 1000~3000Pa. The inventors found that in order to ensure that residual water and small molecular impurities are removed from the waste cooking oil as quickly as possible, the operating pressure at room temperature needs to be set to be less than or equal to 3000Pa. Common vacuum equipment can reach and maintain a vacuum capacity of 1000Pa, but if a vacuum below 1000Pa is to be maintained stably for a long time, the requirements for the vacuum equipment are high, which will increase the investment cost of the equipment.

[0012] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention uses water as an extractant to remove water-soluble impurities from waste cooking oil. Water extraction of waste cooking oil can remove some water-soluble impurities that cannot be removed by adsorption, reduce the acid value and polar substance content to a certain extent, improve the color of the oil, and avoid the toxic effect of impurities on the adsorbent, preventing impurities from affecting the adsorption effect of subsequent steps.

[0013] (2) The present invention adopts a circulating filtration operation under an inert atmosphere, so that the waste cooking oil is continuously in contact with the adsorbent, and impurities are continuously adsorbed and removed. The inert atmosphere prevents the oxidative decomposition of the waste cooking oil, and the continuous contact and circulating operation can ensure that the adsorbent can fully exert its effectiveness, thereby minimizing the acid value and polar substance content and improving the color of the oil.

[0014] (3) The present invention adopts a specific surface area of ​​500~800m 2 / g of magnesium silicate or magnesium aluminum silicate, the higher specific surface area ensures the adsorption effect of the adsorbent.

[0015] (4) The present invention uses vacuum distillation to remove residual water and small molecular impurities. The vacuum distillation method can remove residual water and small molecular impurities that cannot be treated by adsorption, and at the same time, the decomposition of waste cooking oil can be prevented under vacuum.

[0016] (5) The present invention uses a combination of extraction, adsorption and distillation to treat waste cooking oil. The three combined treatment methods play a synergistic role and can maximize the removal effect of various impurities, so that the indicators of the purified waste cooking oil reach the level of standard cooking oil. DETAILED DESCRIPTION

[0017] In the description of the present invention, it should be noted that the waste cooking oil used in the examples is a mixture of various types of waste cooking oils, including rapeseed oil, soybean oil, peanut oil, etc. If no specific conditions are specified in the examples, the conventional conditions or the conditions recommended by the manufacturer are followed. If the manufacturer of the reagents or instruments used is not specified, they are all conventional products that can be purchased commercially, and the inert gas used is nitrogen, which is easy to obtain and relatively cheap.

[0018] The technical solutions provided by the present invention will be described clearly and completely below in conjunction with the specific embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention. Example 1

[0019] Take 1000g of waste cooking oil and 1000g of water in a mass ratio of 1:1, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 2000Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S1. Example 2

[0020] Take 1000g of waste cooking oil and 2500g of water in a mass ratio of 1:2.5, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium aluminum silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 3000Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S2. Example 3

[0021] Take 1000g of waste cooking oil and 5000g of water in a mass ratio of 1:5, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium aluminum silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 1000Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S3. Example 4

[0022] Take 1000g of waste cooking oil and 2000g of water in a mass ratio of 1:2, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 1500Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S4. Example 5

[0023] Take 1000g of waste cooking oil and 3000g of water in a mass ratio of 1:3, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium aluminum silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 2500Pa until there is no distillate, and finally a purified edible oil is obtained, which is recorded as S5. Example 6

[0024] Take 1000g of waste cooking oil and 4000g of water in a mass ratio of 1:4, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 3500Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S6. Comparative Example 1

[0025] 1000 g of waste cooking oil and 1000 g of water were mixed in a mixer-settler at a mass ratio of 1:1 and extracted. The separated waste cooking oil was recorded as S7. Comparative Example 2

[0026] Take 1000g of waste cooking oil and 1000g of water in a mass ratio of 1:1, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered waste cooking oil is recorded as S8. Comparative Example 3

[0027] Take 1000g of waste cooking oil and 500g of water, with a mass ratio of 1:0.5, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 2000Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S9. Comparative Example 4

[0028] Take 1000g of waste cooking oil and 1000g of water in a mass ratio of 1:1, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter without inert gas nitrogen protection. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 2000Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S10. Comparative Example 5

[0029] Take 1000g of waste cooking oil and 1000g of water in a mass ratio of 1:1, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 2000Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S11. Comparative Example 6

[0030] Take 1000g of waste cooking oil and 1000g of water in a mass ratio of 1:1, and mix and extract them in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 2000Pa until there is no distillate, and finally the purified edible oil is obtained, which is recorded as S12. Comparative Example 7

[0031] Take 1000g of waste cooking oil and 1000g of water in a mass ratio of 1:1, and carry out mixed extraction in a mixer-clarifier. The separated waste cooking oil enters a circulation filter and is protected by inert gas nitrogen. 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 5000Pa until there is no distillate, and finally a purified edible oil is obtained, which is recorded as S13. Comparative Example 8

[0032] Take 1000g of waste cooking oil, put it into the circulation filter, pass inert gas nitrogen protection, and heat it at 125℃ with a specific surface area of ​​650m 2 / g of magnesium silicate is continuously contacted until the acid value no longer decreases, and the filtered oil is distilled at 2000Pa until there is no distillate, and finally a purified edible oil is obtained, which is recorded as S14. Test Example 1

[0033] The acid value of edible oil is expressed as the number of milligrams of potassium hydroxide required to neutralize free fatty acids in each gram of oil. The acid values ​​of waste edible oil, samples S1 to S14 in the examples and comparative examples at room temperature were determined by acid-base titration. Test Example 2

[0034] The total polar matter content (TPM) is an indicator for measuring the content of polar matter in edible oil. The TMP values ​​of waste edible oil, samples S1 to S14 in the embodiments and comparative examples at room temperature were determined by rapid column chromatography. Test Example 3

[0035] The transmittance haze meter was used to fix the wavelength of the light source at 610 nm to measure the transmittance of the waste cooking oil, samples S1 to S14 in the embodiments and comparative examples. The higher the transmittance, the better the color improvement effect. Test Example 4

[0036] The moisture content in edible oil will affect the use of edible oil. The moisture content of waste edible oil, examples and comparative examples S1 to S14 was measured using a Karl Fischer moisture titrator.

[0037]

[0038] As can be seen from the table, after the waste cooking oil is treated by the purification method of the present invention, the acid value, TMP and water content are significantly reduced, reaching the standards used in the industry. The light transmittance is significantly improved, indicating that the color is significantly improved.

[0039] Comparative sample S7 was only subjected to the first step of water extraction treatment, which removed water-soluble impurities, slightly reduced the acid value and polar component content, and slightly improved the color. There was no subsequent adsorption and distillation treatment, so its impurity removal effect was very poor. The first step of mixing and separation treatment with water increased the water content in the waste cooking oil, and there was no subsequent water removal treatment, so the water content was relatively high.

[0040] Comparative sample S8 underwent water extraction and adsorption treatment, but without the third step of distillation. Most of the free fatty acids and polar impurities were removed. However, due to the lack of final distillation, some small molecules remained in the edible oil, resulting in high acid value and TMP value. Also, due to the lack of distillation to remove water, the water content in the waste edible oil was relatively high.

[0041] Compared with sample S9, the first step uses less water, and the other treatment processes are the same as sample S1. The small amount of water used will result in a slightly poorer water extraction effect in the first step, resulting in a slight weakening of the adsorption effect of the adsorbent, making the acid value and TMP value slightly higher than sample S1, and the color improvement effect is poor.

[0042] In contrast, sample S10 was not protected by inert gas nitrogen during the circulation filtration, and other treatments were the same as sample S1. Failure to pass inert gas nitrogen protection would cause a certain degree of oxidation and decomposition of the edible oil during the circulation filtration, and the impurities formed could not be adsorbed and processed in time, and eventually remained in the edible oil, resulting in an increase in acid value and TMP value, and poor color improvement effect.

[0043] Compared with sample S11, the temperature of the circulation filtration is 200°C, and the other treatments are the same as sample S1. When the temperature is too high during the circulation filtration, the edible oil will decompose to a certain extent at high temperature, and the impurities formed cannot be adsorbed and processed in time, and eventually remain in the edible oil, resulting in an increase in the acid value and TMP value, and a poor color improvement effect.

[0044] Compared with sample S12, the specific surface area of ​​the adsorbent used is 350m 2 / g, and other treatments were the same as sample S1. The low specific surface area of ​​the adsorbent will lead to a significant weakening of the adsorption effect, incomplete removal of impurities, an increase in acid value and TMP value, and a poor color improvement effect.

[0045] Comparative sample S13, the vacuum distillation pressure is 5000Pa, and other treatments are the same as sample S1. The vacuum pressure is not low enough, resulting in poor vacuum distillation effect, incomplete removal of small molecules and water, resulting in high acid value and TMP value, especially higher water content.

[0046] Comparative sample S14 does not undergo the first step of water extraction, and the other treatments are the same as sample S1. The water-soluble impurities in the waste cooking oil affect the adsorption effect, resulting in incomplete impurity removal, high acid value and TMP value, and poor color improvement effect.

[0047] In summary, the method provided by the present invention can effectively reduce the acid value, polar substance content and water content of waste cooking oil, and achieve excellent purification effect by combining the three means of extraction, adsorption and distillation to treat the waste cooking oil.

[0048] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for purifying waste cooking oil, characterized in that: The steps include: Step 1: extracting and removing water-soluble impurities in waste cooking oil using water; Step 2: Under an inert atmosphere, the waste cooking oil from which water-soluble impurities have been removed is circulated and filtered using magnesium silicate to remove free fatty acids and polar substances therein until the acid value no longer decreases; Step 3: The filtered waste cooking oil is subjected to vacuum distillation to remove residual water and small molecule impurities to obtain purified cooking oil.

2. The method for purifying waste cooking oil according to claim 1, characterized in that: The mass ratio of the waste cooking oil and water used in step 1 is 1:1 to 1:

5.

3. The method for purifying waste cooking oil according to claim 1, characterized in that: The operating temperature range of the circulating filtration is 100-150°C.

4. The method for purifying waste cooking oil according to claim 1, characterized in that: The magnesium silicate adsorbent is magnesium silicate or magnesium aluminum silicate; the specific surface area of ​​the magnesium silicate adsorbent is in the range of 500-800m 2 / g.

5. The method for purifying waste cooking oil according to claim 1, characterized in that: The absolute pressure range of the vacuum distillation is 1000~3000Pa.

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