CN113293024B - Method for preparing biodiesel by three-stage selective hydrodeoxygenation - Google Patents

Abstract

The invention belongs to the technical field of biodiesel preparation, and discloses a method for preparing biodiesel by three-stage selective hydrodeoxygenation, which comprises the following steps: mixing the pretreated raw material waste oil and fat with hydrogen, exchanging heat, and adding the mixture into a first fixed bed reactor with the temperature of 200-280 ℃ to be saturated by hydrogen; then, adding the catalyst into a second fixed bed reactor, and carrying out hydrodemetallization on a first bed layer with the temperature of 310-360 ℃; then hydrodeoxygenation is carried out on a second bed layer with the temperature of 330-380 ℃; finally, adding the mixture into a third fixed bed reactor with the temperature of 345-380 ℃ for hydrodesulfurization, and performing post-treatment to obtain the biodiesel. The hydrogenation process is divided into three sections, and the selectivity of hydrogenation saturation, hydrodeoxygenation and hydrofining is increased by the catalyst grading and the reaction temperature control of each section, so that the quality of biodiesel is improved, the catalyst is effectively prevented from being deactivated, and the problem of bed scaling is avoided.

Description

Method for preparing biodiesel by three-stage selective hydrodeoxygenation

Technical Field

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

Background

At present, biodiesel is generally prepared by adopting an esterification or transesterification technology, and the product is mainly fatty acid methyl ester, which has poor quality, low heat value, high acid value, small mixing and mixing proportion, high production cost and the like, can not be independently used, and is difficult to accept in the market.

The fixed bed cracking deoxidation technology has improved product quality and no fatty acid methyl ester. However, as the reaction and regeneration are switched operation, the heat cannot be comprehensively utilized, and the energy consumption and the 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 (high aromatic hydrocarbon and olefin content, high product density and low cetane number) and the processing flow has to be prolonged in order to improve the product quality.

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

Disclosure of Invention

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

The invention is realized by the following steps:

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

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

s2, adding the hydrogenation saturated generated oil into a second fixed bed reactor, and carrying out hydrogenation demetallization reaction on the hydrogenation saturated generated oil under the action of the catalyst and the protective agent in a first bed layer with the temperature of 310-360 ℃; then, carrying out hydrodeoxygenation reaction under the action of the catalyst in a second bed layer with the temperature of 330-380 ℃ to obtain hydrodeoxygenation generated oil;

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

s4, carrying out heat exchange and cooling on the hydrofined oil, and then carrying out gas-liquid separation, steam stripping and fractionation to obtain the biodiesel.

Preferably, in the step 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 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, wherein 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 space velocity of the reaction volume is 0.3-0.7, and the hydrogen-oil ratio is (780-820): 1.

preferably, the reaction temperatures of the S1, the S2 and the S3 are controlled by injecting cold hydrogen.

Preferably, the reaction temperature of the S1, the S2 and the S3 is controlled by adding (1.2 to 1.8) times of hydrogenation circulating oil to the pretreated raw material waste oil; the hydrogenation circulating oil is liquid separated from the hydrofining generated oil of S3 by heat exchange and cooling and then entering a hot high-pressure separator.

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

Preferably, the pore size of the protective agent is greater than 50nm and the void fraction is greater than 70%.

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

Preferably, the S4 specifically is:

carrying out heat exchange and cooling on the hydrofined oil, and then carrying out hot high-pressure gas-liquid separation and cold high-pressure gas-liquid separation to obtain hydrogen, hydrogenation circulating oil and hydrodeoxygenation mixed oil; the hydrogenated circulating oil and the pretreated raw material waste oil are mixed and reentered into a reaction system; the hydrodeoxygenation mixed oil is subjected to steam stripping and fractionation to obtain biodiesel.

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

the hydrogenation process is divided into three sections, the reaction efficiency of hydrogenation saturation, hydrodeoxygenation and hydrofining is increased mainly by controlling the reaction temperature of each section, and coking, scaling and nickel carbonyl reaction are effectively inhibited, so that the catalyst deactivation can be effectively avoided and the problem of bed scaling is avoided on the premise of improving the quality of biodiesel.

Specifically, through hydrogenation saturation, condensation coking on the surface of the hydrodeoxygenation catalyst is reduced; through hydrodemetallization, the surface scaling of hydrodeoxygenation catalyst and the pressure drop of reaction bed are relieved; through hydrodeoxygenation, more than 99% of oxygen in the waste animal and vegetable oil is removed, and the pure hydrocarbon biodiesel without lipid is produced. The pure hydrocarbon biodiesel produced by the invention is superior to the national standard of six diesel 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

The present invention will be described in further detail with reference to specific embodiments in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.

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

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

s2, adding the hydrogenation saturated generated oil into a second fixed bed reactor, and carrying out hydrogenation demetallization reaction on the hydrogenation saturated generated oil under the action of the catalyst and the protective agent in a first bed layer with the temperature of 310-360 ℃; then, hydrodeoxygenation reaction is carried out on the second bed layer with the temperature of 330-380 ℃ under the action of a catalyst, so as to obtain hydrodeoxygenation generated oil;

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

s4, carrying out heat exchange and cooling on the hydrofined oil, and then carrying out gas-liquid separation, steam stripping and fractionation to obtain the biodiesel.

Because side reactions, such as hydrogenation saturation of mono-olefins and di-olefins in waste animal and vegetable oils and fats with hydrogen, inevitably exist in the hydrodeoxygenation reaction process of biodiesel, the reaction helps to improve the quality of biodiesel, so that the smooth progress of the reaction needs to be ensured;

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

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

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

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

The control range of the reaction temperature in the section is critical, the reaction temperature is too high, namely, the temperature is 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; while the reaction temperature is too low, e.g., below 180 ℃, the active component of the catalyst, e.g., metallic nickel, is susceptible to react with CO in the recycle hydrogen (i.e., cold hydrogen) to form nickel carbonyl, resulting in loss of the active component metallic nickel of the catalyst.

Because the content of metal ions such as iron ions, calcium ions and the like in the waste animal and vegetable oil is high, 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 and calcium ions are easy to remove under the hydrogen condition, and the simple substance metal reduced by hydrogenation is easy to adsorb on the surface of the catalyst to form a crust, cover the active center of the catalyst, block the gaps of the catalyst, deactivate the catalyst, increase the pressure drop of the reactor bed, and even block the bed.

In the implementation, a special hydrodemetallization reaction bed layer is arranged in the second fixed bed reactor, namely, hydrodemetallization is carried out under the combined action of a catalyst and a protective agent at the temperature of 310-360 ℃, wherein the protective agent can enable simple substance metal generated by hydrodeoxygenation to be adsorbed on the surface of the protective agent, and the problems of catalyst deactivation and bed pressure drop rise are not caused in a certain starting period due to the large aperture and void ratio of the protective agent, so that the purposes of protecting hydrodeoxygenation catalyst activity and bed pressure drop are achieved; and the detection shows that the demetallization rate of the hydrodemetallization reaction reaches 95 percent. Then, hydrodeoxygenation reaction is carried out under the action of a catalyst in a second bed layer with the temperature of 330-380 ℃, and the hydrodeoxygenation rate reaches 98% and the saturation rate of olefin hydrogenation saturation reaction reaches 100% after detection until the hydrodeoxygenation reaction reaches the step.

Then the main reactions of hydrodesulfurization, denitrification, degelation and the like are carried out in a third fixed bed reactor with the temperature of 345-380 ℃ under the action of a catalyst.

Also because hydrodeoxygenation and hydrodecarboxylation, hydrodedecarbonylation are parallel reactions in the actual production process. Hydrodeoxygenation is a reaction which needs to be carried out smoothly, and the product carbon after hydrodeoxygenationThe carbon number of the chain is not reduced, and the product yield is higher. The reaction of hydrodecarboxylation and hydrodecarbonylation can generate the fracture of alpha carbon bonds, the carbon chain of the product can be reduced by one carbon atom, and the total yield can be reduced; in addition, hydrodecarboxylation and decarbonylation, CO and CO are produced ₂ Wherein CO is a poison to hydrogenation catalysts, and at reaction temperatures below 180 ℃ forms nickel carbonyls with the active components of the catalyst, such as metallic nickel, causing loss of the active components, reducing catalyst activity and lifetime.

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

Further, in S1, the reaction pressure of the first fixed bed reactor is 5.5-6.5 MPa, the reaction volume space velocity 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 reaction volume space velocity is 1, and the hydrogen-oil ratio is 850:1.

wherein because the reaction temperature of S1 is low, and in combination 850: the high hydrogen oil ratio of 1 can reduce double bond condensation coking as much as possible on the premise of ensuring double bond hydrogenation saturation in diolefin and monoolefin to form single bond, and avoid the deactivation of the active center of the catalyst caused by 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 second fixed bed reactor has a reaction volume space velocity of 0.5 and a hydrogen to oil ratio of 800:1, the second bed temperature was 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 reaction volume space velocity is 0.5, and the hydrogen-oil ratio is 800:1

Since hydrodeoxygenation is an exothermic reaction, hydrodecarboxylation and decarbonylation are endothermic reactions, if the reaction temperature is reduced, the hydrodecarboxylation and decarbonylation reactions are inhibited to a certain extent, so that the reaction temperature in S2 and S3 is reduced; in addition, in order to ensure smooth hydrodeoxygenation, the reaction temperature is not excessively reduced, so that the decarboxylation and decarbonylation reaction are well inhibited, and the hydrogenation oil ratio is increased, so that the hydrodeoxygenation reaction is facilitated due to the low-temperature high-hydrogenation oil ratio.

Further, the reaction temperatures of S1, S2 and S3 are 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 layer, so that the catalyst load of each bed layer is similar to the maximum efficiency of the catalyst.

Further, the reaction temperature of S1, S2 and S3 is controlled by adding (1.2-1.8) times of hydrogenation circulating oil to the pretreated raw material waste oil; the hydrorefining generated oil of the hydrogenation circulating oil S3 is subjected to heat exchange and cooling and then enters a hot high-pressure separator to be separated out of liquid.

Preferably, by adding 1.5 times of hydrogenated cycle oil to the pretreated raw waste oil.

Further, the catalyst is one of molybdenum or tungsten as an active component, and nickel as an auxiliary agent.

Further, the pore diameter of the protective agent is more than 50nm, and the void ratio is more than 70%. Because the protective agent with large aperture and large void ratio is adopted, simple substance metal generated by hydrogenation reduction can be adsorbed on the surface of the protective agent, and the problems of catalyst deactivation and bed pressure drop rise can not be caused in a certain start-up period because the aperture of the protective agent is large and the void ratio is large.

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

Further, as shown in fig. 1, the pretreatment process of the raw material waste oil comprises the following steps: coarse filtering and cleaning the waste oil; and then carrying out electrochemical refining desalination, settling to discharge sewage, back flushing and filtering to obtain pretreated raw material waste oil.

Further, S4 is specifically:

carrying out heat exchange and cooling on the third hydrofined oil, and then carrying out hot high-pressure gas-liquid separation and cold high-pressure gas-liquid separation to obtain hydrogen, hydrogenation circulating oil and hydrodeoxygenation mixed oil; after being purified, the hydrogen enters a circulation system for circulation; the hydrogenated circulating oil and the pretreated raw material waste oil are mixed and reentered into a reaction system; the hydrodeoxygenation mixed oil is subjected to steam stripping and fractionation to obtain biodiesel.

Preferably, the hydrofining generated oil is subjected to heat exchange in heat exchange heat recovery equipment, cooled in a cooler, and then subjected to 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 circulating oil and hydrodeoxygenation mixed oil;

after being purified, the hydrogen enters a circulation system for circulation; the hydrogenation circulating oil and the pretreated raw material waste oil are mixed and reentered into a reaction system; the hydrodeoxygenation mixed oil enters a stripping tower for dehydration, and then enters a fractionating tower for fractionation, so that the required biodiesel is finally obtained.

Example 1

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

s1, coarse filtering and cleaning raw material waste oil; then carrying out electrochemical refining desalination, settling in a settling tank to discharge sewage, back flushing and filtering to obtain pretreated raw material waste oil;

mixing pretreated raw material waste oil and hydrogen, exchanging heat 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 space velocity 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;

in this process, the reaction temperature is controlled by passing cold hydrogen (recycle hydrogen) into the first fixed bed reactor;

s2, adding the hydrogenated saturated generated oil into the reaction volume with the space velocity of 0.3, wherein the hydrogen-oil ratio is 780:1, the hydrogenation saturation generated oil is subjected to hydrodemetallization reaction under the action of a catalyst and a protective agent in a first bed layer with the temperature of 360 ℃; then, hydrodeoxygenation reaction is carried out on the second bed layer with the temperature of 380 ℃ under the action of a catalyst, so as to obtain hydrodeoxygenation generated oil;

in the process, the reaction temperature is controlled by introducing 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 hydrodeoxygenation generated oil into the reaction volume space velocity with the temperature of 380 ℃ and the hydrogen-oil ratio of 780:1, and carrying out hydrodesulfurization and impurity removal reaction under the action of a catalyst to obtain hydrofined oil;

in the process, the reaction temperature is controlled by introducing cold hydrogen into the third fixed bed reactor;

s4, carrying out heat exchange on the hydrofined 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 circulating oil and hydrodeoxygenation mixed oil;

after being purified, the hydrogen enters a circulation system for circulation; the hydrogenation circulating oil and the pretreated raw material waste oil are mixed and reentered into a reaction system; the hydrodeoxygenation mixed oil enters a stripping tower for stripping, and then enters a fractionating tower for fractionation, so that the required biodiesel is finally obtained.

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

Example 2

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

s1, coarse filtering and cleaning raw material waste oil; then carrying out electrochemical refining desalination, settling in a settling tank to discharge sewage, back flushing and filtering to obtain pretreated raw material waste oil;

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

in this process, the reaction temperature is controlled by passing cold hydrogen (recycle hydrogen) into the first fixed bed reactor;

s2, adding the hydrogenated saturated generated oil into the reaction volume with the space velocity of 0.5, wherein the hydrogen-oil ratio is 800:1, the hydrogenation saturation generated oil is subjected to hydrodemetallization reaction under the action of a catalyst and a protective agent in a first bed layer with the temperature of 335 ℃; then, hydrodeoxygenation reaction is carried out on the second bed layer with the temperature of 360 ℃ under the action of a catalyst, so as to obtain hydrodeoxygenation generated oil;

in the process, the reaction temperature is controlled by introducing 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 hydrodeoxygenation generated oil into the reaction volume space velocity with the temperature of 360 ℃ and the hydrogen-oil ratio of 800:1, and carrying out hydrodesulfurization and impurity removal reaction under the action of a catalyst to obtain hydrofined oil;

in the process, the reaction temperature is controlled by introducing cold hydrogen into the third fixed bed reactor;

s4, carrying out heat exchange on the hydrofined 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 circulating oil and hydrodeoxygenation mixed oil;

after being purified, the hydrogen enters a circulation system for circulation; mixing the hydrogenated circulating oil with the pretreated waste oil and fat, and re-entering a reaction system; the hydrodeoxygenation mixed ring oil enters a stripping tower for stripping, and then enters a fractionating tower for fractionation, so that the required biodiesel is finally obtained.

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

Example 3

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

s1, coarse filtering and cleaning raw material waste oil; then carrying out electrochemical refining desalination, settling in a settling tank to discharge sewage, back flushing and filtering to obtain pretreated raw material waste oil;

mixing the pretreated raw material waste oil and hydrogen, exchanging heat in heat exchange heat recovery equipment, 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 hydrogenated saturated product oil;

in this process, the reaction temperature is controlled by passing cold hydrogen (recycle hydrogen) into the first fixed bed reactor;

s2, adding the hydrogenated saturated generated oil into the reaction volume with the space velocity of 0.7, wherein the hydrogen-oil ratio is 820:1, the hydrogenation saturation generated oil is subjected to hydrodemetallization reaction under the action of a catalyst and a protective agent in a first bed layer with the temperature of 310 ℃; then, hydrodeoxygenation reaction is carried out on the second bed layer with the temperature of 330 ℃ under the action of a catalyst, so as to obtain hydrodeoxygenation generated oil;

in the process, the reaction temperature is controlled by introducing 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 hydrodeoxygenation generated oil into the reaction volume space velocity with the temperature of 345 ℃ and the hydrogen-oil ratio of 820:1, and carrying out hydrodesulfurization reaction under the action of a catalyst to obtain hydrofined ring oil;

in the process, the reaction temperature is controlled by introducing cold hydrogen into the third fixed bed reactor;

s4, carrying out heat exchange on the hydrofined oil in heat exchange equipment, cooling 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 circulating oil and hydrodeoxygenation mixed oil;

after being purified, the hydrogen enters a circulation system for circulation; mixing the hydrogenated circulating oil with the pretreated waste oil and fat, and re-entering a reaction system; the hydrodeoxygenation mixed oil enters a stripping tower for stripping, and then enters a fractionating tower for fractionation, so that the required biodiesel is finally obtained.

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

The present invention is not limited to the above-mentioned embodiments, and any changes or substitutions that can be easily understood by those skilled in the art within the technical scope of the present invention are intended to be included in the scope of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope of the claims.

Claims (7)

1. The three-stage selective hydrodeoxygenation method for preparing biodiesel is characterized by comprising the following steps of:

s1, mixing pretreated raw material waste oil and hydrogen, performing heat exchange, and then adding the mixture into a first fixed bed reactor with the temperature of 200-280 ℃ to perform hydrogenation saturation reaction under the action of a catalyst to obtain hydrogenation saturated generated oil; the reaction pressure of the first fixed bed reactor is 5.5-6.5 MPa, the reaction volume space velocity is 0.8-1.1, and the hydrogen-oil ratio is (820-870): 1, a step of;

s2, adding the hydrogenation saturated generated oil into a second fixed bed reactor, and carrying out hydrogenation demetallization reaction on the hydrogenation saturated generated oil under the action of the catalyst and the protective agent in a first bed layer with the temperature of 310-360 ℃; then, carrying out hydrodeoxygenation reaction under the action of the catalyst in a second bed layer with the temperature of 330-380 ℃ to obtain hydrodeoxygenation generated oil; 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, a step of;

s3, adding the hydrodeoxygenation generated oil into a third fixed bed reactor with the temperature of 345-380 ℃, and carrying out hydrodesulphurization reaction under the action of the catalyst to obtain hydrofined generated oil; the space velocity of the reaction volume is 0.3-0.7, and the hydrogen-oil ratio is (780-820): 1, a step of;

s4, carrying out heat exchange and cooling on the hydrofined oil, and then carrying out gas-liquid separation, steam stripping and fractionation to obtain the biodiesel.

2. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to claim 1, wherein the reaction temperatures of the S1, the S2 and the S3 are controlled by injecting cold hydrogen.

3. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to claim 1, characterized in that the reaction temperature of the S1, the S2 and the S3 is controlled by adding (1.2 to 1.8) times of hydrogenated circulating oil to the pretreated raw material waste oil; the hydrogenation circulating oil is liquid separated from the hydrofining generated oil of S3 by heat exchange and cooling and then entering a hot high-pressure separator.

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

5. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to claim 1, wherein the pore size of the protective agent is more than 50nm and the void fraction is more than 70%.

6. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to claim 1, wherein the pretreatment process of the raw waste oil is as follows: coarse filtering and cleaning the waste oil; and then carrying out electrochemical refining desalination, settling to discharge sewage, back flushing and filtering to obtain pretreated raw material waste oil.

7. The method for preparing biodiesel by three-stage selective hydrodeoxygenation according to claim 1, wherein the step S4 is specifically:

carrying out heat exchange and cooling on the hydrofined oil, and then carrying out hot high-pressure gas-liquid separation and cold high-pressure gas-liquid separation to obtain hydrogen, hydrogenation circulating oil and hydrodeoxygenation mixed oil; the hydrogenated circulating oil and the pretreated raw material waste oil are mixed and reentered into a reaction system; the hydrodeoxygenation mixed oil is subjected to steam stripping and fractionation to obtain biodiesel.

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