CN113293024A

CN113293024A - Method for preparing biodiesel by three-stage selective hydrodeoxygenation

Info

Publication number: CN113293024A
Application number: CN202010337008.2A
Authority: CN
Inventors: 刘洪�; 崔华礼
Original assignee: Jiangxi Zunchuang Energy Co ltd
Current assignee: Jiangxi Zunchuang Energy Co ltd
Priority date: 2020-04-26
Filing date: 2020-04-26
Publication date: 2021-08-24
Anticipated expiration: 2040-04-26
Also published as: CN113293024B

Abstract

The invention belongs to the technical field of preparation of biodiesel and discloses a method for preparing biodiesel by three-stage selective hydrodeoxygenation, which comprises the following steps: mixing the pretreated raw material waste oil with hydrogen, exchanging heat, and adding the mixture into a first fixed bed reactor at the temperature of 200-280 ℃ for hydrogenation saturation; then, adding the mixture into a second fixed bed reactor, and firstly carrying out hydrodemetallization on a first bed layer at the temperature of 310-360 ℃; then carrying out hydrodeoxygenation on the second bed layer at the temperature of 330-380 ℃; and finally, adding the mixture into a third fixed bed reactor at the temperature of 345-380 ℃ for hydrodesulfurization, and performing post-treatment to obtain the biodiesel. The invention divides the hydrogenation process into three sections to be carried out, and increases the selectivity of hydrogenation saturation, hydrogenation deoxidation and hydrogenation refining by grading the catalyst and controlling the reaction temperature of each section so as to improve the quality of the biodiesel, effectively avoid the inactivation of the catalyst and avoid the problem of bed layer scaling.

Description

Method for preparing biodiesel by three-stage selective hydrodeoxygenation

Technical Field

The invention belongs to the technical field of preparation of biodiesel, and particularly relates to a method for preparing biodiesel by three-stage selective hydrodeoxygenation.

Background

At present, the biodiesel generally adopts esterification or ester exchange technology, the product is mainly fatty acid methyl ester, the quality is poor, the heat value is low, the acid value is high, the mixing proportion is small, the production cost is high, the biodiesel cannot be independently used, the mixing proportion is too small, and the biodiesel is difficult to accept in the market.

Although the quality of the product is improved greatly by the fixed bed cracking deoxidation process technology, the fatty acid methyl ester is eliminated. However, since the reaction and regeneration are switched, the heat cannot be comprehensively utilized, and the energy consumption and production cost of the device are high. The product structure (more low-quality gas byproducts and heavy byproducts) is unreasonable, the product quality is poor (the product has high aromatic hydrocarbon and olefin content, high product density and low cetane number), and the processing flow has to be prolonged for improving the product quality.

The technology for producing the second-generation biodiesel by catalytic hydrodeoxygenation is rapidly developed in recent years, and the product quality is greatly improved. However, the waste animal and plant oleic acid has extremely high value and the contents of organic calcium, organic chloride and iron ions are extremely high, so that the catalyst of the reactor is inactivated, a bed layer is scaled, equipment is corroded, and the device is in a state of starting and stopping. In addition, the prior biodiesel production device basically adopts an oil refinery diesel hydrofining device from a catalyst to a reactor to the process flow, and a plurality of technical problems are not solved.

Disclosure of Invention

In view of the above, the invention provides a method for preparing biodiesel by three-stage selective hydrodeoxygenation, aiming at the problems of easy catalyst deactivation, easy bed layer scaling and poor product quality in the production process of the existing biodiesel preparation method.

The invention is realized by the following method:

a method for preparing biodiesel by three-stage selective hydrodeoxygenation comprises the following steps:

s1, mixing the pretreated raw material waste oil and fat with hydrogen, carrying out heat exchange, adding the mixture into a first fixed bed reactor at the temperature of 200-280 ℃, and carrying out hydrogenation saturation reaction under the action of a catalyst to obtain hydrogenation saturated generated oil;

s2, adding the hydrogenation saturation produced oil into a second fixed bed reactor, and carrying out hydrogenation demetalization reaction on the hydrogenation saturation produced oil on a first bed layer at the temperature of 310-360 ℃ under the action of the catalyst and the protective agent; carrying out hydrodeoxygenation reaction on the second bed layer at the temperature of 330-380 ℃ under the action of the catalyst to obtain hydrodeoxygenated product oil;

s3, adding the hydrodeoxygenation produced oil into a third fixed bed reactor at the temperature of 345-380 ℃, and carrying out hydrodesulfurization reaction under the action of the catalyst to obtain hydrofined produced oil;

and S4, carrying out heat exchange and cooling on the hydrofined oil, and then carrying out gas-liquid separation, stripping and fractionation to obtain the biodiesel.

Preferably, in the S1, the reaction pressure of the first fixed bed reactor is 5.5 to 6.5MPa, the reaction volume space velocity is 0.8 to 1.1, and the hydrogen-oil ratio is (820 to 870): 1.

preferably, in the S2, the reaction volume space velocity of the second fixed bed reactor is 0.3 to 0.7, and the hydrogen-oil ratio is (780 to 820): 1, the temperature of the second bed layer is 330-360 ℃.

Preferably, in the step S3, the reaction temperature of the third fixed bed reactor is 345-360 ℃, the reaction volume space velocity is 0.3-0.7, and the hydrogen-oil ratio is (780-820): 1.

preferably, the reaction temperature of the S1, the S2, and the S3 is controlled by injecting cold hydrogen.

Preferably, adding (1.2-1.8) times of hydrogenated cycle oil into the pretreated raw material waste oil and fat to control the reaction temperature of the S1, the S2 and the S3; the hydrogenated circulating oil is the hydrofined oil of the S3, and the liquid is separated out after the oil enters a thermal high-pressure separator after heat exchange and cooling.

Preferably, the catalyst is a catalyst with an active component of one of molybdenum or tungsten and an auxiliary agent of nickel.

Preferably, the pore diameter of the protective agent is more than 50nm, and the void ratio is more than 70%.

Preferably, the pretreatment process of the raw material waste oil comprises the following steps: carrying out coarse filtration and cleaning on waste oil; then carrying out electrochemical refining desalination, settling to discharge sewage, backwashing and filtering to obtain the pretreated raw material waste oil.

Preferably, the S4 is specifically:

carrying out heat exchange and cooling on the hydrorefining generated oil, and then carrying out hot high-pressure gas-liquid separation and cold high-pressure gas-liquid separation to obtain hydrogen, hydrogenated circulating oil and hydrodeoxygenation mixed oil; mixing the hydrogenated circulating oil and the pretreated raw material waste oil and fat, and then feeding the mixture into a reaction system; and carrying out steam stripping and fractionation on the hydrodeoxygenation mixed oil to obtain the biodiesel.

Compared with the prior art, the invention adopting the scheme has the beneficial effects that:

the invention divides the hydrogenation process into three sections to be carried out, the reaction efficiency of hydrogenation saturation, hydrogenation deoxidation and hydrogenation refining is increased mainly by controlling the reaction temperature of each section, and coking, scaling and nickel carbonyl reaction are effectively inhibited, thus effectively avoiding catalyst deactivation and scaling of a bed layer on the premise of improving the quality of the biodiesel.

Particularly, through hydrogenation saturation, the condensation coking on the surface of the hydrodeoxygenation catalyst is reduced; through hydrodemetallization, the surface scaling of the hydrodeoxygenation catalyst and the pressure drop of a reaction bed layer are relieved; through hydrodeoxygenation, more than 99% of oxygen in the waste animal and vegetable oil is removed, and pure hydrocarbon biodiesel without lipid is produced. The pure hydrocarbon biodiesel produced by the method is superior to the national six-diesel standard in the main indexes of cetane number, aromatic hydrocarbon content, density, sulfur content and the like. The product yield of the invention reaches more than 85 percent.

Drawings

FIG. 1 is a process flow diagram of a method for preparing biodiesel by three-stage selective hydrodeoxygenation provided by the invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The embodiment provides a method for preparing biodiesel by three-stage selective hydrodeoxygenation, which comprises the following steps:

s2, adding the hydrogenation saturation produced oil into a second fixed bed reactor, and carrying out hydrogenation demetalization reaction on the hydrogenation saturation produced oil on a first bed layer at the temperature of 310-360 ℃ under the action of the catalyst and the protective agent; carrying out hydrodeoxygenation reaction on the second bed layer at the temperature of 330-380 ℃ under the action of a catalyst to obtain hydrodeoxygenated product oil;

Since there are inevitable side reactions during the hydrodeoxygenation reaction of biodiesel, such as the reaction of monoolefine and diolefin in the waste animal and vegetable oil with hydrogen for hydrogenation saturation, which contributes to the improvement of the quality of biodiesel, it is necessary to ensure the smooth proceeding of the reaction;

for example, metallic nickel in the catalyst reacts with carbon monoxide to generate carbonyl nickel to deactivate the catalyst, iron ions, calcium ions, magnesium ions and the like in the raw materials are hydrogenated and reduced into metallic simple substances to be adsorbed on the surface of the catalyst, so that the pressure drop of a bed layer of the reactor is increased, even the bed layer is blocked, and obviously, the reaction needs to be inhibited;

in addition, for example, in the hydrodeoxygenation process, side reactions such as decarboxylation and decarbonylation may occur; decarboxylation and decarbonylation can reduce the number of carbon atoms of a carbon chain, so that the yield of the biodiesel is reduced, and the reaction also needs to be inhibited;

in order to improve the quality of a target product, namely, biodiesel, the hydrogenation process is divided into three sections, the selectivity of hydrogenation saturation, hydrogenation deoxidation and hydrogenation refining is increased mainly by grading of the catalyst and controlling the reaction temperature of each section, and the reaction of carbon monoxide and active components of the catalyst is effectively inhibited, so that the catalyst can be effectively prevented from being deactivated on the premise of improving the quality of the biodiesel, and the problem of bed layer scaling is avoided.

In the embodiment, in a first fixed bed reactor at a temperature of 200-280 ℃, a hydrogenation saturation reaction is carried out under the action of a catalyst, the main purpose is to hydrogenate and saturate double bonds in diolefin and monoolefin into single bonds, and detection shows that the diene acid saturation rate can reach 100% and the monoolefin acid saturation rate can reach 95%.

The control range of the reaction temperature in the section is critical, the reaction temperature is too high, namely higher than 280 ℃, double bond condensation coking is easy to generate, and the generated coke covers the surface of the catalyst, so that the activity of the catalyst is reduced; and the reaction temperature is too low, for example, below 180 ℃, the active component of the catalyst, such as metallic nickel, is liable to react with CO in the circulating hydrogen (i.e., cold hydrogen) to form nickel carbonyl, resulting in loss of metallic nickel as the active component of the catalyst.

Because the metal ions such as iron ions, calcium ions and the like in the waste animal and vegetable oil are high in content, the hydrodeoxygenation catalyst is easy to scale and cover the active center of the catalyst, and the pressure drop of the reactor is increased. The metal ions such as iron ions, calcium ions and the like are easily removed under the hydrogen condition, and the simple substance metal reduced by hydrogenation is easily adsorbed on the surface of the catalyst to form a hard shell which covers the active center of the catalyst to block the gap of the catalyst, so that the catalyst is inactivated, the pressure drop of the bed layer of the reactor is increased, and even the bed layer is blocked.

In the implementation, a special hydrodemetallization reaction bed layer is arranged in the second fixed bed reactor, namely, hydrodemetallization is carried out on the first bed layer at the temperature of 310-360 ℃ under the combined action of the catalyst and the protective agent, wherein the protective agent can enable elemental metal generated by hydrogenation reduction to be adsorbed on the surface of the protective agent, and the problems of catalyst inactivation and bed pressure drop increase cannot be caused in a certain start-up period due to large aperture and porosity of the protective agent, so that the purposes of protecting the activity of the hydrodemetallization catalyst and bed pressure drop are achieved; and the detection shows that the demetallization rate of the hydrodemetallization reaction reaches 95 percent. And then, carrying out hydrodeoxygenation reaction on the second bed layer at the temperature of 330-380 ℃ under the action of a catalyst, and detecting until the hydrodeoxygenation deoxidation rate reaches 98% and the olefin hydrogenation saturation rate reaches 100% in the step.

And then carrying out main reaction hydrodesulfurization, denitrification, degumming and the like in a third fixed bed reactor at the temperature of 345-380 ℃ under the action of a catalyst.

And in the actual production process, hydrodeoxygenation, hydrodedecarboxylation and hydrodecarbonylation are parallel reactions. The hydrodeoxygenation is a reaction which needs to be carried out smoothly, the carbon number of a carbon chain of a product after the hydrodeoxygenation is not reduced, and the product yield is high. The reaction of hydrogenation decarboxylation and hydrogenation decarbonylation can produce the breakage of alpha carbon bond, the carbon chain of the product can be reduced by one carbon atom, and the total yield can be reduced; in addition, hydrogenation decarboxylation and decarbonylation generate CO and CO₂Wherein CO is a poison of the hydrogenation catalyst, and can form nickel carbonyl with active components of the catalyst, such as metallic nickel, at the reaction temperature of below 180 ℃, so that the loss of the active components is caused, and the activity and the service life of the catalyst are reduced.

Therefore, the whole reaction process of the embodiment is carried out in the environment of lower than 380 ℃, so that the hydrogenation decarboxylation and decarbonylation can be effectively inhibited, and the quality of the biodiesel is further improved.

Further, in S1, the reaction pressure of the first fixed bed reactor is 5.5-6.5 MPa, the space velocity of the reaction volume is 0.8-1.1, and the hydrogen-oil ratio is (820-870): 1. preferably, the reaction pressure of the first fixed bed reactor is 5.9MPa, the space velocity of the reaction volume is 1, and the hydrogen-oil ratio is 850: 1.

wherein because the reaction temperature of S1 is lower, and the combination of 850: the high hydrogen-oil ratio of 1 can reduce double bond condensation coking as much as possible on the premise of ensuring that double bonds in diolefin and monoolefin are hydrogenated and saturated to form single bonds, and avoids the deactivation of the active center of the catalyst due to the adsorption of coke.

Further, in S2, the reaction volume space velocity of the second fixed bed reactor is 0.3-0.7, and the hydrogen-oil ratio is (780-820): 1, the temperature of the second bed layer is 330-360 ℃. Preferably, the space velocity of the reaction volume of the second fixed bed reactor is 0.5, and the hydrogen-oil ratio is 800: 1, the second bed temperature is 350 ℃.

Further, in S3, the reaction temperature of the third fixed bed reactor is 345-360 ℃, the reaction volume space velocity is 0.3-0.7, and the hydrogen-oil ratio is (780-820): 1. preferably, the reaction temperature of the third fixed bed reactor is 350 ℃, the space velocity of the reaction volume is 0.5, and the hydrogen-oil ratio is 800: 1

Since hydrodeoxygenation is an exothermic reaction and hydrodedecarboxylation and decarbonylation are endothermic reactions, if the reaction temperature is reduced, the reaction temperature can be reduced to a certain extent to inhibit the hydrodedecarboxylation and decarbonylation reactions, so that the reaction temperatures in S2 and S3 are reduced; in addition, in order to ensure smooth progress of hydrodeoxygenation, the reaction temperature is not required to be excessively lowered, so that the hydrogen-oil ratio is increased in combination with better inhibition of decarboxylation and decarbonylation reactions, and the low-temperature high-hydrogen-oil ratio is favorable for the progress of hydrodeoxygenation.

Further, the reaction temperature of S1, S2, and S3 was controlled by injecting cold hydrogen.

The cold hydrogen is used to control the reaction temperature and the temperature rise of each bed layer by entering the catalyst bed layers, so that the catalyst load of each bed layer is similar, and the efficiency of the catalyst is generated to the maximum extent.

Further, adding (1.2-1.8) times of hydrogenated cycle oil into the pretreated raw material waste oil to control the reaction temperature of S1, S2 and S3; the hydrorefining product oil of which the hydrocycle oil is S3 enters a thermal high-pressure separator to be separated out liquid after heat exchange and cooling.

Preferably, 1.5 times of hydrogenated cycle oil is added into the pretreated raw material waste oil and fat.

Furthermore, the catalyst is a catalyst with an active component of one of molybdenum or tungsten and an auxiliary agent of nickel.

Furthermore, the aperture of the protective agent is larger than 50nm, and the void ratio is larger than 70%. Because the protective agent with large aperture and large porosity is adopted, the elemental metal generated by hydrogenation reduction can be adsorbed on the surface of the protective agent, and the problems of catalyst inactivation and bed pressure drop increase can not be caused in a certain startup period due to the large aperture and large porosity of the protective agent.

The protective agent generally consists of an inert substance, a catalyst with a trace or small amount of hydrogenation activity. Such as alumina with a minor amount or amount of hydrogenation active catalyst.

Further, as shown in fig. 1, the pretreatment process of the raw material waste oil is as follows: carrying out coarse filtration and cleaning on waste oil; then carrying out electrochemical refining desalination, settling to discharge sewage, backwashing and filtering to obtain the pretreated raw material waste oil.

Further, S4 specifically includes:

carrying out heat exchange and cooling on the third hydrorefined oil, and then carrying out hot high-pressure gas-liquid separation and cold high-pressure gas-liquid separation to obtain hydrogen, hydrogenated cycle oil and hydrodeoxygenation mixed oil; after being purified, the hydrogen enters a circulating system for circulation; mixing the hydrogenated circulating oil and the pretreated raw material waste oil and fat, and then feeding the mixture into a reaction system; and carrying out steam stripping and fractionation on the hydrodeoxygenation mixed oil to obtain the biodiesel.

Preferably, the hydrorefining generated oil is subjected to heat exchange in heat exchange heat recovery equipment, and after being cooled in a cooler, gas-liquid separation is performed in a hot high-pressure gas-liquid separator in a hydrogenation device and a cold high-pressure separator in the hydrogenation device to obtain hydrogen, hydrogenation cycle oil and hydrodeoxygenation mixed oil;

after being purified, the hydrogen enters a circulating system for circulation; mixing the hydrogenated circulating oil and the pretreated raw material waste oil and fat, and then entering a reaction system again; and the hydrodeoxygenation mixed oil enters a stripping tower for dehydration, and then enters a fractionating tower for fractionation to finally obtain the required biodiesel.

Example 1

s1, performing coarse filtration and cleaning on the raw material waste oil; then carrying out electrochemical refining desalination, carrying out sedimentation in a sedimentation tank to discharge sewage, backwashing and filtering to obtain pretreated raw material waste oil;

mixing the pretreated raw material waste oil and fat with hydrogen, carrying out heat exchange in heat exchange heat recovery equipment, adding the mixture into a first fixed bed reactor with the temperature of 200 ℃, the pressure of 5.5MPa, the reaction volume airspeed of 0.8 and the hydrogen-oil ratio of 820:1, and carrying out hydrogenation saturation reaction under the action of a catalyst to obtain hydrogenation saturated generated oil;

during this process, the reaction temperature was controlled by feeding cold hydrogen (recycle hydrogen) to the first fixed bed reactor;

s2, adding the hydrogenated saturated oil into the reaction mixture, wherein the space velocity of the volume of the reaction mixture is 0.3, and the hydrogen-oil ratio is 780: 1, in a second fixed bed reactor, carrying out hydrogenation demetalization reaction on the hydrogenation saturated generated oil in a first bed layer at the temperature of 360 ℃ under the action of a catalyst and a protective agent; then carrying out hydrodeoxygenation reaction on the second bed layer at the temperature of 380 ℃ under the action of a catalyst to obtain hydrodeoxygenated product oil;

during this process, the reaction temperature was controlled by feeding cold hydrogen into the second fixed bed reactor;

wherein the aperture of the protective agent is more than 50nm, and the void ratio is more than 70%;

s3, adding the hydrodeoxygenation product oil into the reaction kettle at the temperature of 380 ℃, wherein the space velocity of the reaction volume is 0.3, and the hydrogen-oil ratio is 780: 1 in a third fixed bed reactor, and carrying out hydrodesulfurization impurity removal reaction under the action of a catalyst to obtain hydrofined product oil;

during this process, the reaction temperature was controlled by feeding cold hydrogen to the third fixed bed reactor;

s4, carrying out heat exchange on the hydrorefining generated oil in heat exchange equipment, cooling in a cooler, and then carrying out gas-liquid separation in a hot high-pressure gas-liquid separator in a hydrogenation device and a cold high-pressure separator in the hydrogenation device to obtain hydrogen, hydrogenation cycle oil and hydrodeoxygenation mixed oil;

after being purified, the hydrogen enters a circulating system for circulation; mixing the hydrogenated circulating oil and the pretreated raw material waste oil and fat, and then entering a reaction system again; and the hydrodeoxygenation mixed oil enters a stripping tower for stripping, and then enters a fractionating tower for fractionating to finally obtain the required biodiesel.

Through detection, the hydrogenation saturation rate of the method in the embodiment reaches 100%, the hydrodemetallization rate reaches 96%, and the hydrodeoxygenation rate reaches 98%; the prepared biodiesel is superior to the national diesel standard in cetane number, aromatic hydrocarbon content, density and sulfur content, and the yield of the product reaches 89%.

Example 2

mixing the pretreated raw material waste oil and fat with hydrogen, carrying out heat exchange in a heat exchange heat recovery device, adding the mixture into a first fixed bed reactor with the temperature of 240 ℃, the pressure of 5.9MPa, the reaction volume airspeed of 1 and the hydrogen-oil ratio of 850:1, and carrying out hydrodeoxygenation saturation reaction under the action of a catalyst to obtain hydrogenated saturated generated oil;

s2, adding the hydrogenated saturated oil into the reactor at a reaction volume space velocity of 0.5 and a hydrogen-oil ratio of 800: 1, in a second fixed bed reactor, carrying out hydrogenation demetalization reaction on the hydrogenation saturated generated oil in a first bed layer at the temperature of 335 ℃ under the action of a catalyst and a protective agent; then carrying out hydrodeoxygenation reaction on the second bed layer at the temperature of 360 ℃ under the action of a catalyst to obtain hydrodeoxygenated product oil;

s3, adding the hydrodeoxygenation product oil into the reactor at the temperature of 360 ℃, wherein the space velocity of the reaction volume is 0.5, and the hydrogen-oil ratio is 800: 1 in a third fixed bed reactor, and carrying out hydrodesulfurization impurity removal reaction under the action of a catalyst to obtain hydrofined product oil;

after being purified, the hydrogen enters a circulating system for circulation; mixing the hydrogenated circulating oil and the pretreated waste oil and fat, and then entering a reaction system again; and the hydrodeoxygenation mixed ring oil enters a stripping tower for stripping, and then enters a fractionating tower for fractionating to finally obtain the required biodiesel.

Through detection, the hydrogenation saturation rate of the method in the embodiment reaches 100%, the hydrodemetallization rate reaches 99.5%, and the hydrodeoxygenation rate reaches 99%; the prepared biodiesel is superior to the national diesel standard in cetane number, aromatic hydrocarbon content, density and sulfur content, and the yield of the product reaches 93 percent.

Example 3

mixing the pretreated raw material waste oil and fat with hydrogen, carrying out heat exchange in a heat exchange heat recovery device, adding the mixture into a first fixed bed reactor with the temperature of 280 ℃, the pressure of 6.5MPa, the reaction volume space velocity of 1.1 and the hydrogen-oil ratio of 870:1, and carrying out hydrodeoxygenation saturation reaction under the action of a catalyst to obtain hydrogenation saturated generated oil;

s2, adding the hydrogenated saturated oil into the reactor at a reaction volume space velocity of 0.7 and a hydrogen-oil ratio of 820:1, in a second fixed bed reactor, carrying out hydrogenation demetalization reaction on the hydrogenation saturated generated oil in a first bed layer at the temperature of 310 ℃ under the action of a catalyst and a protective agent; then carrying out hydrodeoxygenation reaction on the second bed layer at the temperature of 330 ℃ under the action of a catalyst to obtain hydrodeoxygenated product oil;

s3, adding the hydrodeoxygenation product oil into the reactor at 345 ℃, wherein the reaction volume space velocity is 0.7, and the hydrogen-oil ratio is 820:1 in a third fixed bed reactor, and carrying out hydrodesulfurization reaction under the action of a catalyst to obtain hydrofined ring oil;

s4, carrying out heat exchange on the hydrorefining generated oil in heat exchange equipment, cooling the hydrorefining generated oil in a cooler, and then carrying out gas-liquid separation in a hot high-pressure gas-liquid separator of a hydrogenation device and a cold high-pressure separator of the hydrogenation device to obtain hydrogen, hydrogenation cycle oil and hydrodeoxygenation mixed oil;

after being purified, the hydrogen enters a circulating system for circulation; mixing the hydrogenated circulating oil and the pretreated waste oil and fat, and then entering a reaction system again; and the hydrodeoxygenation mixed oil enters a stripping tower for stripping, and then enters a fractionating tower for fractionating to finally obtain the required biodiesel.

Through detection, the hydrogenation saturation rate of the method in the embodiment reaches 100%, the hydrodemetallization rate reaches 99%, and the hydrodeoxygenation rate reaches 98%; the prepared biodiesel is superior to the national diesel standard in cetane number, aromatic hydrocarbon content, density and sulfur content, and the yield of the product reaches 91%.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are included in the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.

Claims

1. A three-stage selective hydrodeoxygenation biodiesel preparation method is characterized by comprising the following steps:

2. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to claim 1, wherein in the S1, the reaction pressure of the first fixed bed reactor is 5.5-6.5 MPa, the space velocity of the reaction volume is 0.8-1.1, and the hydrogen-oil ratio is (820-870): 1.

3. the method for preparing biodiesel by three-stage selective hydrodeoxygenation according to claim 1, wherein in S2, the reaction volume space velocity of the second fixed bed reactor is 0.3-0.7, and the hydrogen-oil ratio is (780-820): 1, the temperature of the second bed layer is 330-360 ℃.

4. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to claim 1, wherein in S3, the reaction temperature of the third fixed bed reactor is 345-360 ℃, the reaction volume space velocity is 0.3-0.7, and the hydrogen-oil ratio is (780-820): 1.

5. the three-stage selective hydrodeoxygenation biodiesel production process of any of claims 1-4, wherein the reaction temperature of said S1, said S2 and said S3 is controlled by injecting cold hydrogen.

6. The three-stage selective hydrodeoxygenation biodiesel preparation method according to any one of claims 1 to 4, wherein the reaction temperature of the S1, the S2 and the S3 is controlled by adding (1.2 to 1.8 times) of hydrogenated cycle oil to the pretreated raw material waste oil; the hydrogenated circulating oil is the hydrofined oil of the S3, and the liquid is separated out after the oil enters a thermal high-pressure separator after heat exchange and cooling.

7. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to any one of claims 1 to 4, wherein the catalyst is a catalyst with an active component of one of molybdenum or tungsten and an auxiliary agent of nickel.

8. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to any one of claims 1 to 4, wherein the pore size of the protective agent is greater than 50nm, and the porosity is greater than 70%.

9. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to any one of claims 1 to 4, wherein the pretreatment process of the raw waste oil is as follows: carrying out coarse filtration and cleaning on waste oil; then carrying out electrochemical refining desalination, settling to discharge sewage, backwashing and filtering to obtain the pretreated raw material waste oil.

10. The three-stage selective hydrodeoxygenation biodiesel production method according to any one of claims 1 to 4, wherein S4 is in particular: