CN100460373C - Method for extracting and rectifying separation of benzene - Google Patents

Abstract

A method for extracting and rectifying the separation of benzene, comprising introducing a hydrocarbon mixture containing olefins and benzene into the middle of an extractive distillation tower, contacting with an extraction solvent introduced from the upper part of the extractive distillation tower to carry out extractive rectification, and extracting and rectifying the tower The benzene-containing extraction solvent discharged from the bottom enters the solvent recovery tower to separate benzene and extraction solvent, and the overhead stream is discharged from the system after being cooled; the benzene fraction discharged from the top of the solvent recovery tower is condensed and then passed into the clay tower for refining, and then sent to the benzene flash Steam tank flash evaporation, a part of the extraction solvent discharged from the bottom of the solvent recovery tower enters the benzene flash tank reboiler as a heat source for the reboiler, and then mixes with another part of the extraction solvent and returns to the extractive distillation tower for recycling. The stream discharged from the top of the steamer is condensed and used as benzene product, and the components at the bottom of the tank are discharged from the system. The method is suitable for separating benzene from hydrocarbon raw materials containing a small amount of olefins, can increase the stability of the clay tower operation, and basically does not increase the energy consumption of the process.

Description

Method for extracting and rectifying separation of benzene

technical field

The invention relates to a method for extracting and rectifying the separation of benzene, specifically, a method for extracting and rectifying the separation of benzene from hydrocarbon mixtures containing olefins.

Background technique

There are two main methods for separating benzene from reformed oil and pyrolysis hydrogenated gasoline. One is liquid-liquid extraction, and the other is extractive distillation. The liquid-liquid extraction method currently uses water-containing sulfolane as the extraction solvent. Its disadvantage is that it is not suitable for raw materials with high aromatic content, and the benzene separated by extraction will affect its freezing point due to water content. More benzene towers, water is separated at the top of the benzene tower, pure benzene is obtained from the side line, and a small amount of heavy impurities are discharged from the bottom of the tower. The process of extracting and separating benzene is relatively complicated, and the equipment and operating costs are relatively high. Especially in the field of catalytic reforming, due to the low content of benzene in the extraction raw material, the use of aqueous sulfolane solvent is not suitable because its dissolving power is too small. Compared with the liquid-liquid extraction method, the extractive distillation method has the advantages of simple process and operation, low equipment investment, and low energy consumption. Therefore, it is more advantageous to use extractive distillation to separate pure benzene when the content of aromatics in the raw material is high.

Most of the existing extraction and rectification separation methods for benzene are extractive distillation-solvent recovery-product refining process. The benzene fraction obtained from the top of the solvent recovery tower is directly used as benzene product after removing trace olefins with clay. The selected extraction solvent Mainly N-methylpyrrolidone (NMP), N-formylmorpholine (NFM) or sulfolane. The problem with this method is that when dealing with raw materials with high olefin content, such as continuous reforming to produce oil, the quality of the obtained benzene product is unstable. The olefins contained in the extractive distillation feedstock are more difficult to separate from aromatics than alkanes, especially cycloolefins, diolefins and triolefins, such as cyclopentadiene, cyclohexadiene, etc. The boiling point of these olefins is close to that of benzene, and the structure is similar to that of benzene. Approximately, benzene products tend to enter in the process of extractive distillation, which affects the pickling color number of benzene products, so frequent replacement of clay is required. The national quality standard for high-grade pure benzene stipulates that the pickling color number of benzene is not greater than 1, but the trace amount of olefins in pure benzene will cause the pickling color number to be greater than 1. When the crude benzene containing a small amount of olefins passes through the clay, the small molecular olefins are polymerized into macromolecular heavy olefins under the action of the acidic center of the clay. This heavy olefin may be composed of two olefin molecules or three olefin molecules or even multiple molecules. Their components are numerous and the content of each component is very small, so that the clay has a certain adsorption capacity at the initial stage of use. When the above heavy olefins When olefins are adsorbed on the surface of clay, the adsorption saturation will be reached soon, and the heavy olefins in crude benzene will flow out together with benzene. Although the content is very small, it is enough to increase the acid washing color number of benzene products.

At present, the ethylene cracking unit has a pyrolysis gasoline hydrogenation unit because of the high content of olefins and impurities in the pyrolysis gasoline. needs, and only a small amount of olefins are produced in the reforming process, and the raw material is often only hydrogenated before reforming, so the reformed gasoline contains a small amount of olefins, while the reformed gasoline of the continuous reforming unit has relatively higher olefins content, especially It is the C ₅ -C ₇ cycloalkenes contained therein that have a greater impact on the quality of benzene products.

CN1163879A proposes a method for producing pure aromatics from reformed gasoline. First, the reformed gasoline is selectively hydrogenated, mainly olefins, diolefins and triolefins in the raw material are selectively hydrogenated, and then the selective hydrogenated Extractive distillation and/or liquid-liquid extraction of raw materials to separate aromatics and non-aromatics. Although this method can effectively remove olefins in the raw material, it is also possible to hydrogenate the benzene in the raw material to cyclohexane, thereby reducing the yield of benzene. In addition, adding hydrogenation equipment to the extractive distillation system requires a large investment, high device operating costs, and the need to consume hydrogen resources, so the practical application of this method is limited.

CN1393507A proposes a method for extracting, distilling and separating aromatics and a composite solvent used. The composite solvent includes a main solvent and a co-solvent. When the composite solvent is used to extract and distill benzene from pyrolysis hydrogenated gasoline, it can significantly ease the operating conditions of solvent recovery, improve the recovery rate of aromatics, and obtain high-quality pure benzene. However, when the raw material is continuous reforming to produce oil, the benzene product The pickling color number is unqualified. Even if it is refined by clay, when the air velocity of the clay tower feed is 0.5 hours ^-1 , the device runs for about two weeks, and the pickling color number of the benzene product will be greater than 3 and unqualified, and the clay needs to be replaced , bringing operational fluctuations and increasing production costs.

Contents of the invention

The object of the present invention is to provide a method for extracting and rectifying the separation of benzene from hydrocarbon raw materials with relatively high olefin content. The benzene product obtained by the method is stable in properties and basically does not increase the energy consumption of the extracting and rectifying process.

The method for extracting and rectifying separation of benzene provided by the invention comprises the following steps:

(1) Introduce the hydrocarbon mixture containing olefins and benzene into the middle of the extractive distillation tower, contact with the extraction solvent introduced from the upper part of the extractive distillation tower to carry out extractive distillation, and the benzene-containing solvent discharged from the bottom of the extractive distillation tower enters the solvent The recovery tower separates benzene and extraction solvent, and the overhead stream is discharged from the system after being cooled;

(2) Pass the benzene fraction discharged from the top of the solvent recovery tower into the clay tower for refining after condensation, and then send it to the benzene flash tank for flash evaporation, and a part of the extraction solvent discharged from the bottom of the solvent recovery tower enters the benzene flash tank reboiler as After the heat source of the reboiler, it is mixed with another part of the extraction solvent and then returned to the extractive distillation tower for recycling.

(3) The stream discharged from the top of the benzene flash tank is condensed as a benzene product, and the components at the bottom of the tank are discharged from the system.

In order to improve the stability of benzene product quality and ensure that the benzene product has a qualified pickling color number, the present invention refines the benzene fraction obtained by extraction and rectification with white clay, and then performs flash distillation, so that the heavy olefins contained in the benzene fraction Retained at the bottom of the flash tank, pure benzene is discharged from the top of the flash tank, and the heat source used for heating the flash tank is provided by the extraction solvent recovered from the solvent recovery tower, that is, the heat in the system is used to flash the benzene fraction. The whole process can The consumption has not increased, but the benzene flash tank has been increased. Using the method for extracting and rectifying to separate benzene can treat extracting and rectifying raw materials with high olefin content, and obtain benzene products with stable quality.

Description of drawings

Figure 1 is a schematic flow chart of the present invention for extracting and rectifying the separation of benzene.

Detailed ways

In the method of the invention, a benzene flash tank is added in the extraction and rectification system to separate heavy olefins from benzene in the benzene fraction refined by a clay tower, thereby obtaining a benzene product with stable properties. In the method, the waste heat recovered from the solvent in the extractive distillation system is used to provide heat for the reboiler of the benzene flash tank, so increasing the benzene flash operation basically does not increase the energy consumption of the extractive distillation system.

In order to make the benzene flash tank more effectively separate the olefins in the benzene fraction, its pressure is preferably controlled at 0.05-0.15MPa, the tank top temperature is preferably 75-95°C, the tank bottom temperature is 80-120°C, and the reflux ratio is 0-0.5 . The theoretical plate number of the benzene flash tank is 0-5, more preferably 2-5.

The method of the present invention utilizes the solvent heat recovered by the solvent recovery tower to provide the heat required for the evaporation of the benzene fraction in the benzene flash tank, that is, utilizes the heat generated by the temperature difference between the recovered solvent and the solvent entering the extractive distillation tower to make the benzene flash tank The benzene fraction evaporates, and the benzene is distilled from the top of the tank, while the heavy olefin components remain at the bottom of the benzene flash tank and are discharged periodically. The mass ratio of the extraction solvent entering the benzene flash tank reboiler to the benzene flash tank is 5-15:1, preferably 9-12:1. The extraction solvent entering the reboiler of the benzene flash tank accounts for 25-100% of the total mass of the extraction solvent, preferably 50-75%.

In the inventive method, the number of theoretical plates of the extractive distillation tower is preferably 35～65, and the extraction solvent enters the tower temperature preferably 80～130 DEG C, and the temperature at the bottom of the tower is 140～180 DEG C, and the tower top pressure is 0.15～0.35MPa, and the solvent ratio (solvent The mass ratio to the extraction and rectification raw material) is 3.0-6.0, and the reflux ratio (the mass ratio of the reflux solvent to the overhead distillate material) is 0.2-1.0. The material discharged from the top of the extractive distillation operation is non-aromatic hydrocarbons, and the non-aromatic hydrocarbons are discharged from the system after condensation, and the bottom effluent of the extractive distillation column is an extraction solvent containing benzene.

The benzene-containing extraction solvent is separated in the solvent recovery tower, the benzene fraction is discharged from the top of the tower, and the extraction solvent is discharged from the bottom of the tower. One part enters the reboiler at the bottom of the benzene flash tank to provide a heat source for it, and the other part enters the benzene The solvent in the flash tank reboiler is mixed and then recycled back to the extractive distillation column for reuse. The solvent recovery tower is operated under reduced pressure. The suitable control pressure at the top of the tower is 0.035-0.05MPa, the temperature at the top of the tower is 50-70°C, the temperature at the bottom of the tower is 160-185°C, and the reflux ratio is 0.4-1.0. The number of plates is preferably 8-28.

In the method, the clay tower is used to refine the benzene fraction containing a small amount of olefins obtained from the top of the solvent recovery tower to absorb the olefins therein. The adsorbed benzene fraction is flashed to remove the heavy olefins, thus increasing the service life of the clay. The clay tower is operated under pressure, the suitable pressure is 0.5-1.8MPa, the temperature is 120-180°C, and the mass space velocity of the benzene fraction is preferably 0.05-1.0h- ¹ .

The hydrocarbon mixture described in the method of the present invention is mainly catalytically reformed gasoline or pyrolysis hydrogenated gasoline, which mainly contains benzene, _C5 - _C7 alkanes and a small amount of olefins and cycloalkanes. The olefin content in the hydrocarbon mixture is 1.0-8.0% by mass, and the benzene content is 15-90% by mass.

The extraction solvent is selected from sulfones, glycols or morpholine compounds containing 7 to 8 carbon atoms, preferably sulfolane, tetraethylene glycol, pentaethylene glycol, N-formyl morpholine or mixtures thereof, more Sulfolane is preferred.

The present invention will be described in detail below with reference to FIG. 1 . In Fig. 1, the hydrocarbon mixture containing benzene and a small amount of olefins enters the heat exchanger 101 from the pipeline 1, and enters the extractive distillation column 103 from the middle through the pipeline 2 after exchanging heat with the extraction solvent. The extraction solvent enters the extractive distillation column 103 from the upper part through the pipeline 18 . After extractive distillation, the non-aromatic components are discharged from the top of the extractive distillation tower, and enter the reflux tank 104 through the pipeline 3, part of which flows back into the extractive distillation tower, and the other part is discharged from the system through the pipeline 4. The extraction solvent containing benzene at the bottom of the extractive distillation tower is discharged from the bottom of the tower, enters the middle part of the solvent recovery tower 105 through the pipeline 5, and is distilled under reduced pressure. After being heated by the heater 107, it enters the clay tower 108 through the pipeline 8, and the benzene fraction enters the benzene flash tank 109 through the pipeline 9 after being refined in the clay tower. system. The extraction solvent obtained at the bottom of the solvent recovery tower is divided into two streams through the pipeline 12, and one stream is controlled by the regulating valve 113 through the pipeline 13 and enters the reboiler 110 of the benzene flash tank 109 as a heat source for heating the benzene fraction, and then passes through the pipeline 15 It is mixed with another extraction solvent separated from the pipeline 14 and then enters the pipeline 16, then enters the heat exchanger 101 to exchange heat with the hydrocarbon mixture, enters the solvent water cooler 102 through the pipeline 17 for temperature adjustment, and then enters the extraction essence through the pipeline 18. The upper part of the distillation tower is recycled. The liquid level controller 112 of the benzene flash tank 109 controls the opening of the regulating valve 113 according to the liquid level in the tank, thereby controlling the amount of solvent passing through the reboiler of the benzene flash tank, that is, controlling the heating amount. In actual operation, the solvent regeneration part is also included, which is not marked in Fig. 1 .

The present invention will be further described in detail below by examples, but the present invention is not limited thereto.

Example 1

According to the flow chart in Figure 1, the _C6 fraction of reformed gasoline is used as the raw material, and pure benzene is separated by extraction and rectification, and sulfolane is used as the extraction solvent. The composition and flow rate of raw materials in the extractive distillation process are shown in Table 1, the main operating conditions are shown in Table 2, the circulation flow rate and heat exchange temperature of the solvent are shown in Table 3, and the separation results of extractive distillation are shown in Table 4. The pickling color number of the product is shown in Table 5, and the pickling color number is determined by the GB/T2012 method.

comparative example

Separation of aromatics by the method of Example 1, the difference is that no benzene flash tank is established in the extractive distillation system, the extraction solvent obtained from the solvent recovery tower directly exchanges heat with the extractive distillation column feed, the operating conditions are shown in Table 2, the solvent content The circulation flow rate and heat exchange temperature are shown in Table 3, the separation results of extraction and rectification are shown in Table 4, and the pickling color number of the benzene product refined with white clay is shown in Table 5.

It can be seen from Table 3 that in the comparative example, in order to adjust the temperature at which the recovered solvent enters the extractive distillation tower, the hot solvent in line 17 at 135° C. was cooled to 105° C. with a water cooler, during which a huge energy loss was incurred. However, in Example 1 of the present invention, after the benzene flash operation is added, the waste heat recovered from the solvent adjustment is fully utilized, and it is used for the heat supply of the benzene flash operation, so that the temperature difference between the front and rear of the solvent water cooler is only 12°C. The energy consumption of the whole process in Table 4 shows that the increased benzene flashing operation of the method of the present invention basically does not increase the total energy consumption of the device operation.

The data in Table 5 shows that the pickling color number of benzene obtained by the process of the present invention is qualified, and it is still relatively stable after a long period of operation, thereby increasing the service life of the clay and reducing the cost increase and operational fluctuations caused by frequent switching of the clay. However, in the comparative example without benzene flashing operation, the pickling color number of benzene obtained after the extraction and rectification device operated for a short time was unqualified, and the white clay needed to be replaced.

Example 2~3

According to the flow chart of Fig. 1, the benzene cut that the solvent recovery tower top obtains is taken as raw material, and the influence of the number of theoretical plates on the top of the flash tank on the extraction and separation results is investigated. The composition of the benzene cut is shown in Table 6, the composition of the clay tower and the flash tank The operating conditions are shown in Table 7, and the separation results are shown in Table 8.

From the data in Table 8, it can be seen that when the number of theoretical plates on the top of the flash tank is 0 or 5, the benzene product with qualified pickling color number can be obtained, but the yield of benzene is low when the number of theoretical plates is 0.

Table 1

Table 2

table 3

Table 4

Total heat consumption refers to the sum of extractive distillation and benzene refining heat.

table 5

Table 6

Table 7

Table 8

Claims (8)

1. the method for an extracting rectifying and separating benzene comprises the steps:

(1) will contain the middle part that the hydrocarbon mixture of alkene and benzene is introduced extractive distillation column, contact with the extraction solvent of introducing from extractive distillation column top and to carry out extracting rectifying, the extraction solvent that contains benzene of discharging at the bottom of the extracting rectifying Tata enters solvent recovery tower Separation of Benzene and extraction solvent, and overhead stream is the discharge system after cooling off;

(2) feeding clay tower after the benzene cut condensation that the solvent recuperation column overhead is discharged makes with extra care, and then send into the flash distillation of benzene flash tank, after the part of the extraction solvent of discharging at the bottom of the solvent recovery tower enters the thermal source of benzene flash tank reboiler as reboiler, again with return extractive distillation column after another part extraction solvent mixes and recycle

(3) after the logistics condensation that the benzene flash drum overhead is discharged as benzaldehyde product, a jar end component is discharged system.

2. in accordance with the method for claim 1, it is characterized in that described benzene flash tank pressure is 0.05～0.15MPa, reflux ratio is 0～0.5, and the tank deck temperature is 75～95 ℃, and a jar end temperature is 80～120 ℃.

3. in accordance with the method for claim 2, the theoretical plate number that it is characterized in that the benzene flash tank is 0～5.

4. in accordance with the method for claim 1, it is characterized in that entering the extraction solvent of benzene flash tank reboiler and the charging mass ratio of benzene flash tank is 5～15:1.

5. in accordance with the method for claim 1, it is characterized in that the extraction solvent that enters benzene flash tank reboiler accounts for 25～75% of extraction solvent total mass.

6. in accordance with the method for claim 1, the service temperature that it is characterized in that described clay tower is that 120～180 ℃, pressure are that the mass space velocity that 0.5～1.8MPa, benzene cut pass through is 0.05～1.0 hour ^-1

7. in accordance with the method for claim 1, it is characterized in that olefin(e) centent is 1.0～8.0 quality % in the described hydrocarbon mixture, benzene content is 15～90 quality %.

8. in accordance with the method for claim 1, it is characterized in that described extraction solvent is selected from tetramethylene sulfone, Tetraglycol 99, five glycol, N-formyl morpholine or their mixture.