A method for recovering 1,3-butadiene from C4 fraction by extractive distillation

Abstract

This invention belongs to the field of chemical separation and purification technology, and discloses a system and method for recovering 1,3-butadiene from C4 fractions through extractive distillation. The invention employs two extraction towers: a first extraction tower and a second extraction tower. The first extraction tower uses NMP as the extractant, while the second extraction tower uses NMP with potassium thiocyanate added as the extractant. The liquid phase collected from the bottom of the first extraction tower enters the second extraction tower, and the key product, butadiene, is obtained at the top of the second extraction tower. The butadiene product purity reaches over 98% (mass fraction). The energy consumption and operating pressure of the entire process are significantly reduced compared to traditional processes, greatly improving economic efficiency and showing promising application prospects.

Description

Technical Field

[0001] This invention belongs to the field of chemical separation and purification technology, specifically relating to a system and method for recovering 1,3-butadiene from C4 fractions through extractive distillation. Background Technology

[0002] Currently, a large amount of C4 components, such as n-butane, butene, 2-butene, butadiene, and alkynes, are present in the byproducts of petroleum steam cracking and some chemical products. These products have high industrial application value. Among them, butadiene, as a basic chemical raw material, has a large global supply and demand market and is particularly important. In the past, due to technological and equipment limitations, most of the byproduct C4 was treated by combustion, resulting in very low recovery and utilization rates of these components. With the advancement and development of petrochemical technology, the technology for recovering and separating C4 components from petroleum cracking steam has become more mature, and large-scale C4 separation processes have been basically formed. Due to the relative volatility of C4 components being close to 1, and the ability of some components to form azeotropes, extractive distillation technology is now widely used as the preferred method for C4 separation. The extractant is one of the important factors affecting the extractive distillation effect. The most commonly used extractants in industry include dimethylformamide (DMF), acetonitrile (ACN), and N-methylpyrrolidone (NMP). Among them, NMP has the highest selectivity for butadiene and is also non-toxic and harmless, making it the most advanced.

[0003] However, there are two main drawbacks to using NMP extraction to distill C4 components: (1) Separating the intermediate component butadiene from multiple C4 components requires the construction of multiple towers. Butadiene is prone to thermal polymerization between multiple towers, which can block the equipment, resulting in a decrease in the yield and purity of butadiene, and also causing a very high heat load. (2) NMP has a very high boiling point, and due to the limitation of the equipment vacuum, it is difficult to meet the process requirements. At the same time, in order to increase the relative volatility between C4 components, an operating pressure of 3-5 MPa is usually required. Industrially, an appropriate amount of water is usually added to NMP to lower the boiling point of the extractant, usually with a water content of 10%. However, adding water extractant also inevitably brings some drawbacks. First, the increased water content can easily affect the purity of the product. The influence of water needs to be considered again in the subsequent separation process. At the same time, the heat capacity of water is very high, which will also lead to an increase in energy consumption. Moreover, adding water extractant still requires a high-pressure environment to increase the relative volatility between components.

[0004] Therefore, it is necessary to develop a method with low heat load, low operating pressure, and high product purity in the process of recovering butadiene from C4 fraction. Summary of the Invention

[0005] In view of the problems and shortcomings of the existing technology, the purpose of this invention is to provide a method for recovering 1,3-butadiene and butane from C4 fraction by extractive distillation.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] The first aspect of the present invention provides a method for recovering 1,3-butadiene from a C4 fraction by extractive distillation, comprising the following steps: feeding the C4 fraction and a first extractant into a first extraction column for extractive distillation, wherein the top stream containing n-butane is separated from the top of the first extraction column, and the side stream containing butadiene is discharged from the side stream of the first extraction column and enters a second extraction column; simultaneously feeding the second extractant into the second extraction column for secondary extractive distillation, wherein the top stream containing butadiene is separated from the top of the second extraction column;

[0008] The first extractant is N-methylpyrrolidone (NMP); the second extractant is salt-containing N-methylpyrrolidone.

[0009] Preferably, the salt in the second extractant can effectively increase the relative volatility between C4 components. However, due to the large variety of salts, it is impossible to experimentally determine the separation effect of each salt. Establishing a thermodynamic model for the salt-containing extractant using simulation methods presents certain difficulties. Conventional electrolyte models cannot adequately describe the thermodynamic state of hydrocarbons in the system, leading to inaccurate calculation results. Furthermore, thermodynamic models based on the group contribution method require a large amount of experimental data to regress the binary interaction parameters between groups, resulting in a significant workload. Therefore, screening for salts that can significantly alter the relative volatility of C4 components is crucial to determining the feasibility of this method.

[0010] The COSMO-RS model is a priori model based on statistical mechanics and quantum chemistry. It only requires structural fragment information for each salt to obtain its σ-profile, enabling the calculation of the activity coefficient of the salt in any solution. Other thermodynamic properties can then be derived through thermodynamic equations, making it very convenient to use. The COSMO-SAC model is an extension of the COSMO-RS model, but the principle remains essentially the same, and it also has the advantage of being open-source. While the COSMO-SAC or COSMO-RS models can easily calculate the activity coefficient of salt-containing NMP systems, their results for other thermodynamic properties are less accurate. The COSMO-UNIFAC model is a combined thermodynamic model that combines the advantages of the group contribution method (UNIFAC model) and COSMO-based models (COSMO-RS or COSMO-SAC models). It can be used to predict the thermodynamic properties of substances such as activity coefficients at infinite dilution, vapor-liquid equilibrium, boiling point, and pressure. This model extends the UNIFAC model by fitting the activity coefficient values ​​at infinite dilution calculated by the COSMO-based model to obtain the missing UNIFAC binary parameters. This model maintains the original UNIFAC model equations and does not change the original model parameters. Research and experiments have shown that the COSMO-UNIFAC model has higher prediction accuracy than the COSMO model. Therefore, the COSMO-RS model can be used to quickly and accurately screen for effective salt types, while COSMO-UNIFAC can be used to calculate the key parameters and thermodynamic properties of salt-containing systems, thus making the extraction and distillation of C4 components with salt feasible.

[0011] Furthermore, the specific method for screening the types of salts in the second extractant is as follows: Structural files of different types of salts are drawn, and the salt structures are optimized at the def-TZVP basis set and BP86 functional level using the TmoleX commercial software embedded in the COSMO therm, resulting in COSMO files containing σ-profile information. These files are then imported into the COSMO therm software, and the infinite dilution activity coefficients of the salts in NMP and the infinite dilution activity coefficients of the C4 components at different concentrations in salt-containing NMP are calculated using the COSMO-RS model. The salts with the best performance are then screened. The screening results show that potassium thiocyanate (KSCN) salt can effectively change the relative volatility between C4 components; therefore, N-methylpyrrolidone copper with potassium thiocyanate salt is used as the second extractant.

[0012] Furthermore, the properties of C4 and KSCN-containing NMP were calculated using the COSMO-UNIFAC thermodynamic model. Specifically, relevant parameters of the KSCN-containing NMP and C4 systems were calculated and regressed. The structure and frequency of KSCN were optimized at the b3ylp / 6-311+g** level using the quantum chemistry software Gaussian 09. Then, the van der Waals volume and van der Waals surface area were calculated using the wavefunction analysis software Multitwfn, thereby calculating the Q of its unifac group. K and R K， Its critical parameter P was calculated using COSMOtherm. C T C V C Furthermore, based on the infinite dilution activity coefficients of different systems, the binary interaction parameters between N-methylpyrrolidone, butane, butene, butadiene and alkynes and the KSCN functional group were calculated.

[0013] Preferably, in the second extractant, the salt is potassium thiocyanate (KSCN); the mass fraction of potassium thiocyanate in the salt-containing N-methylpyrrolidone is 5%-25%, preferably 20%.

[0014] Furthermore, this invention optimizes the commonly used C4 extractive distillation process in industry: the C4 component generated from petroleum vapor cracking first enters the first extraction tower for pre-separation of butane. To maximize butane separation without affecting other components, pure NMP is used as the first extractant. To avoid repeated heating of the butadiene-containing extractant at the bottom of the tower, a portion of the liquid phase is directly collected from the first extraction tower and fed into the second extraction tower, significantly reducing the heat load on the first extraction tower. In the second extraction tower, NMP containing 20% ​​KSCN by mass is used as the second extractant, effectively increasing the relative volatility of butadiene and butene, achieving efficient butadiene separation, and obtaining high-purity butadiene as the key product under normal pressure. In traditional processes, butadiene is collected in the third extraction tower, while the salt-added extractant alters its relative volatility, allowing high-purity butadiene to be obtained in the second extraction tower, avoiding repeated heating and polymerization of butadiene.

[0015] Furthermore, in order to achieve good separation of C4 components, reduce the heat load of the tower, and improve the final yield of butadiene, the following optimizations and specifications were made to a series of operating and design parameters in this process.

[0016] Preferably, the first extraction tower is a packed tower; the theoretical number of plates in the first extraction tower is 100-120, preferably 100; the top pressure of the first extraction tower is 1 bar; and the mass reflux ratio of the first extraction tower is 15-30 (preferably 25-30, more preferably 30).

[0017] Preferably, the C4 fraction is added to the first extraction tower via a feed inlet; the feed inlet is located at the 80th tray from the top of the first extraction tower, the feed temperature is 30-50℃ (preferably 40℃), and the feed flow rate is 2500-4000 kg / h (preferably 3000 kg / h).

[0018] Preferably, the first extractant is added to the first extraction tower through the first extractant inlet; the first extractant inlet is located at the 10th tray from the top of the first extraction tower, the feed temperature is 30-50℃ (preferably 40℃), and the feed flow rate is 6000-8000 kg / h (preferably 7000 kg / h).

[0019] Preferably, the butane-containing feed flows out from the top of the first extraction tower via the butane outlet; the butane outlet is located at the first tray from the top of the first extraction tower, and the outflow rate is 340-350 kg / h (preferably 350 kg / h).

[0020] Preferably, the butadiene-containing side feed flows out from the side feed port of the first extraction tower; the side feed port is located at the 99th tray from the top of the first extraction tower, and the output flow rate is 4000-5000 kg / h (preferably 5000 kg / h).

[0021] More preferably, the bottom of the first extraction tower is also provided with an extractant outlet; the extractant outlet is located at the 100th tray from the top of the tower, and the extraction flow rate is 4500-5000 kg / h (preferably 4500 kg / h).

[0022] Preferably, the second extraction tower is a packed tower; the theoretical number of plates in the second extraction tower is 115-130, preferably 117; the top pressure of the second extraction tower is 1 bar; and the mass reflux ratio of the second extraction tower is 25-30 (preferably 28).

[0023] Preferably, the second extractant is added to the second extraction tower through the second extractant inlet; the second extractant inlet is located at the 10th tray from the top of the second extraction tower, the feed temperature is 35-50℃ (preferably 40℃), the feed pressure is 1 bar, and the feed flow rate is 3000-4000 kg / h (preferably 3000 kg / h).

[0024] Preferably, the butadiene-containing side feed flows into the second extraction tower through the feed inlet; the feed inlet is located at the 105th tray from the top of the second extraction tower, and the feed temperature is 30-40℃ (preferably 40℃).

[0025] Preferably, the butadiene-containing feed flows through the 1,3-butadiene outlet and is separated from the top of the second extraction column; the 1,3-butadiene outlet is located at the first tray from the top of the second extraction column, and the outflow rate is 1000-1200 kg / h (preferably 1200 kg / h).

[0026] Preferably, the bottom of the second extraction column is also provided with a discharge port, through which the bottom liquid containing salts such as NMP, butene and alkynes in the second extraction column flows out for subsequent distillation separation; the discharge port is located at the 119th tray from the top of the second extraction column, and the discharge flow rate is 6500-6800 kg / h (preferably 6800 kg / h).

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] (1) This invention connects two extraction towers in series: a first extraction tower and a second extraction tower. The first extraction tower uses NMP as the extractant, while the second extraction tower uses NMP with potassium thiocyanate as the extractant. Butane is collected from the top of the first extraction tower, and the liquid phase from the bottom enters the second extraction tower. The key product, butadiene, is obtained at the top of the second extraction tower, with a purity exceeding 98% (mass fraction). This invention uses N-methylpyrrolidone with potassium thiocyanate as the extractant and employs an optimized extraction process to achieve low-pressure, low-energy separation of the important raw material butadiene from C4 components with similar boiling points. Simultaneously, it reduces the problems of high equipment heat load, high operating pressure, and low butadiene product purity present in existing C4 extractive distillation processes.

[0029] (2) This invention provides a process for extracting and distilling C4 using N-methylpyrrolidone with added salt. The COSMO-RS model was used to quickly and accurately screen out potassium thiocyanate, a salt that can effectively increase the relative volatility between C4 components. Simulation calculations showed that adding a 20% (w / w) NMP solution of potassium thiocyanate to the second extraction tower significantly increased the relative volatility between the main product, butadiene, and the byproduct, dibutene, thereby significantly reducing energy consumption and operating costs.

[0030] (3) By designing and optimizing the original extraction process, this invention extracts a portion of the liquid phase from the side stream of the bottom tray of the first extraction tower, avoiding the thermal polymerization of butadiene at the bottom of the tower. Simultaneously, it further reduces the overall heat load of the process, preventing the thermal polymerization of butadiene and improving the purity of butadiene. This achieves the goal of separating the key component, butadiene, from the C4 component through atmospheric pressure extractive distillation. Furthermore, this invention also provides detailed design and specifications for the operating conditions, feed conditions, and process flow of the extractive distillation tower.

[0031] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the system for recovering 1,3-butadiene from the C4 fraction by extractive distillation according to the present invention.

[0033] Figure 2 Henry's coefficient curves of butadiene in NMP and NMP containing 20% ​​KSCN (mass fraction) under different temperature conditions of the present invention;

[0034] Figure 3 This is a bar graph showing the Gibbs free energy of butadiene dissolved in NMP and NMP containing 20% ​​KSCN (mass fraction) at infinite dilution under different temperature conditions according to the present invention.

[0035] Wherein, B1 is the first extraction tower, B2 is the second extraction tower, S1 is the NMP feed stream, S2 is the C4 feed stream, S3 is the butane outflow stream, S4 is the butadiene-containing side-stream outflow stream, S5 is the NMP feed stream with KSCN, and S6 is the butadiene outflow stream. Detailed Implementation

[0036] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0037] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0038] Example 1

[0039] This embodiment provides a system for recovering 1,3-butadiene from C4 fractions via extractive distillation, such as... Figure 1 As shown, the system includes a first extraction tower B1 and a second extraction tower B2. The first extraction tower B1 has a first extractant inlet and a butane outlet at its top, and a feed inlet and a side outlet at its bottom. The second extraction tower B2 has a second extractant inlet and a 1,3-butadiene outlet at its top, and a feed inlet and a discharge outlet at its bottom. The side outlet of the first extraction tower B1 is connected to the feed inlet of the second extraction tower B2.

[0040] The first extraction tower B1 is an atmospheric pressure tower; a packed tower; with 100-120 theoretical trays; a top pressure of 1 bar; and a mass reflux ratio of 30. The first extractant inlet is located at the 10th tray from the top of the tower; the butane outlet is located at the 1st tray from the top of the tower; the feed inlet is located at the 80th tray from the top of the tower; the side stream outlet is located at the 99th tray from the top of the tower; and the bottom of the first extraction tower B1 also has an extractant outlet located at the 100th tray from the top of the tower.

[0041] The second extraction column, B2, is an atmospheric pressure column; a packed column; with 115-130 theoretical trays; a top pressure of 1 bar; and a reflux ratio of 28. The second extractant inlet is located at the 10th tray from the top of the column; the 1,3-butadiene outlet is located at the 1st tray from the top of the column; the feed inlet is located at the 105th tray from the top of the column; and the discharge outlet is located at the 119th tray from the top of the column.

[0042] Example 2: Investigating the effect of the total number of theoretical plates in the second extraction column on the purity of butadiene.

[0043] To investigate the effect of the total number of theoretical plates in the second extraction column on the purity of butadiene, the inventors performed the following simulation calculations using ASPEN plus V14 software, based on experimental verification of the accuracy of the thermodynamic model. These simulations correspond to Examples 2-1 to 2-10, with the corresponding theoretical plate numbers of the second extraction column ranging from 110 to 119. The measured butadiene purity results are shown in Table 1.

[0044] Example 2-1

[0045] This embodiment provides a method for recovering 1,3-butadiene from a C4 fraction via extractive distillation, comprising the following steps: A C4 fraction derived from petroleum steam cracking (with mass fractions of n-butane, 2-butene, 1,3-butadiene, and alkynes of 10%, 45%, 44%, and 1%, respectively) is fed into a first extraction tower B1 (feed temperature 40°C, mass flow rate 3000 kg / h) via a feed inlet. Simultaneously, pure NMP (first extractant) is added to the first extraction tower B1 via a first extractant feed inlet (feed temperature 40°C, mass flow rate 7000 kg / h). After feeding, extractive distillation is performed in the first extraction tower B1. After extractive distillation, the butane-containing top feed stream is separated from the butane outlet at the top of the first extraction tower B1 (the n-butane-containing top feed stream has a collection flow rate of 350 kg / h), and the butadiene-containing side feed stream is separated from the first extraction tower B1. After the side stream discharge, the material directly enters the second extraction tower B2 through the inlet (the outflow rate of the butadiene-containing side stream is 5000 kg / h, and the inlet temperature is 40℃). The bottom stream containing NMP extractant is collected from the extractant outlet at the bottom of the first extraction tower B1. At the same time, NMP (second extractant) containing 20% ​​KSCN by mass is added to the second extraction tower B2 through the second extractant inlet for secondary extractive distillation (inlet temperature 40℃, mass flow rate 3000 kg / h). After the secondary extractive distillation, the top stream containing butadiene is separated from the 1,3-butadiene outlet at the top of the second extraction tower B2 (the outflow rate of the butadiene-containing top stream is 1200 kg / h). The bottom liquid containing butene, alkyne, NMP, and KSCN is collected from the outlet at the bottom of the second extraction tower B2 (the components in the bottom liquid can be separated by subsequent distillation).

[0046] Both the first extraction tower B1 and the second extraction tower B2 are packed towers, with a pressure of 1 bar. The first extraction tower B1 has a reflux ratio of 30 and a theoretical number of trays of 100; the second extraction tower B2 has a reflux ratio of 28 and a theoretical number of trays of 110. Further, in the first extraction tower B1, the feed inlet is located at the 80th tray from the top of the tower; the first extractant inlet is located at the 10th tray from the top of the tower; the n-butane outlet is located at the 1st tray from the top of the tower; the side stream outlet is located at the 99th tray from the top of the tower; and the extractant outlet is located at the 100th tray from the top of the tower. In the second extraction tower B2, the feed inlet is located at the 105th tray from the top of the tower; the second extractant inlet is located at the 10th tray from the top of the tower; the 1,3-butadiene outlet is located at the 1st tray from the top of the tower; and the outlet is located at the 119th tray from the top of the tower.

[0047] In Examples 2-2 to 2-10, the theoretical number of plates in the second extraction column was 111-119, and other conditions were the same as in Example 2-1. The specific parameters and butadiene purity data are shown in Table 1 below.

[0048] Table 1. Effect of different theoretical plate numbers in the second extraction column on butadiene purity.

[0049]

[0050] As shown in Table 1 above, the purity of butadiene increases with the increase of the theoretical plate number of the second extraction column. When the theoretical plate number reaches 117, the purity of butadiene reaches 98%, and further increasing the number of plates has limited effect on increasing its purity. Therefore, 117 theoretical plates are preferred for the second extraction column.

[0051] Example 3: Investigating the effect of the top pressure of the second extraction column on the purity of butadiene.

[0052] To investigate the effect of the top pressure of the second extraction column on the purity of butadiene, the inventors conducted the following experiments, namely Examples 3-1 to 3-10, with the corresponding top pressures of the second extraction column being 1-10 bar. The results of the butadiene purity measurements are shown in Table 2.

[0053] Example 3-1

[0054] The method for recovering 1,3-butadiene from C4 fraction by extractive distillation is basically the same as that in Examples 2-8, except that the feed temperature of the C4 fraction is 55°C and the feed temperature of the second extractant is 20°C.

[0055] The top pressure of the second extraction column in Examples 3-2 to 3-10 was 2-10 bar, and other conditions were the same as in Example 3-1. The specific parameters and butadiene purity data are shown in Table 2 below.

[0056] Table 2. Effect of different top pressures of the second extraction column on butadiene purity.

[0057]

[0058] As shown in Table 2 above, the purity of butadiene decreases with the increase of the top pressure of the second extraction column. The purity of butadiene reaches its highest level of 97.777% when the top pressure is 1 bar (atmospheric pressure). Therefore, 1 bar is the preferred top pressure for the second extraction column.

[0059] Comparative Example 1: Investigating the effect of the total number of theoretical plates in the second extraction column on the purity of butadiene when NMP is used as the second extractant.

[0060] To investigate the effect of the total number of theoretical plates in the second extraction column on the purity of butadiene when NMP is used as the second extractant, the inventors conducted the following experiments, namely Comparative Examples 1-1 to 1-10, with corresponding theoretical plate numbers of 110-119 in the second extraction column. The measured butadiene purity results are shown in Table 3.

[0061] Comparative Example 1-1

[0062] The method for recovering 1,3-butadiene from C4 fraction by extractive distillation is basically the same as that in Examples 2-8, except that the second extractant is also pure NMP; and the bottom liquid containing butene, alkyne and NMP is collected from the bottom outlet of the second extraction tower B2.

[0063] The theoretical plate number of the second extraction column in Comparative Examples 1-2 to 1-10 was 111-119, respectively. Other conditions were the same as those in Comparative Example 1-1. The specific parameters and butadiene purity data are shown in Table 3 below.

[0064] Table 3. Effect of different theoretical plate numbers of the second extraction column on butadiene purity when the second extractant is NMP.

[0065]

[0066] As shown in Table 3 above, when the second extractant is NMP, the purity of butadiene increases with the increase of the theoretical plate number of the second extraction column. However, even when the theoretical plate number reaches 119, the highest purity of butadiene is only about 95%, which is far lower than the 98% when the second extractant of this invention is NMP containing KSCN.

[0067] Comparative Example 2: Investigating the effect of the total number of theoretical plates in the second extraction column on the purity of butadiene when both the first and second extractants are NMP + water.

[0068] To investigate the effect of the total number of theoretical plates in the second extraction column on the purity of butadiene when both the first and second extractants are NMP + water, the inventors conducted the following experiments, namely Comparative Examples 2-1 to 2-10, with corresponding theoretical plate numbers of 110-119 in the second extraction column. The measured butadiene purity results are shown in Table 4.

[0069] Comparative Example 2-1

[0070] The method for recovering 1,3-butadiene from C4 fraction by extractive distillation is basically the same as that in Examples 2-8, except that: the first extractant is NMP containing 8% water (mass fraction); the second extractant is NMP containing 8% water (mass fraction); and the bottom liquid containing butene, alkyne and NMP is collected from the bottom outlet of the second extraction tower B2.

[0071] The theoretical plate number of the second extraction column in Comparative Examples 2-2 to 2-10 was 111-119, respectively. Other conditions were the same as in Comparative Example 2-1. The specific parameters and butadiene purity data are shown in Table 4 below.

[0072] Table 4. Effect of different theoretical plate numbers of the second extraction column on butadiene purity when both the first and second extractants are NMP + water.

[0073]

[0074] As shown in Table 4 above, when both the first and second extractants are aqueous NMP, the purity of butadiene increases with the increase of the theoretical plate number of the second extraction column. However, even when the theoretical plate number reaches 119, the highest purity of butadiene only reaches about 93%, which is far lower than the 98% when the second extractant of this invention is KSCN-containing NMP.

[0075] Comparing Comparative Example 1 and Comparative Example 2, it was found that when water was added to the first and second extractants, the extraction efficiency decreased and the purity of butadiene decreased.

[0076] Comparative Example 3: Investigating the effect of the reflux ratio of the second extraction column on the purity of butadiene and the heat load of the reboiler at the bottom of the second extraction column when the first extractant is NMP + water and the second extractant is NMP + KSCN + water.

[0077] To investigate the effects of the total number of theoretical plates in the second extraction column on the butadiene purity and the heat load of the reboiler at the bottom of the second extraction column when the first extractant is NMP + water and the second extractant is NMP + KSCN + water, the inventors conducted the following experiments, namely Comparative Examples 3-1 to 3-11, with corresponding mass reflux ratios of 20-30 for the second extraction column. The measured results of the butadiene purity and the heat load of the reboiler at the bottom of the second extraction column are shown in Table 5.

[0078] Comparative Example 3-1

[0079] The method for recovering 1,3-butadiene from C4 fraction by extractive distillation is basically the same as that in Examples 2-8, except that: the first extractant is NMP containing 8% water (mass fraction); the second extractant is NMP containing 8% water (mass fraction) and 12% KSCN (mass fraction); and the reflux ratio of the second extraction column B2 is 20.

[0080] The mass reflux ratios of the second extraction towers in Comparative Examples 3-2 to 3-11 were 21-30, respectively, and other conditions were the same as those in Comparative Example 3-1. The specific parameters and butadiene purity data are shown in Table 5 below.

[0081] Table 5. Effects of different mass reflux ratios in the second extraction tower on product and equipment when both the first and second extractants contain water.

[0082]

[0083] As shown in Table 5 above, when the first extractant is NMP + water and the second extractant is NMP + KSCN + water, the purity of butadiene and the heat load of the reboiler at the bottom of the column both increase with the increase of the mass reflux ratio of the second extraction column. However, even when the reflux ratio reaches 30, the highest butadiene purity only reaches about 96%, and the reboiler heat load is as high as about 4300 kW. The butadiene purity is far lower than the 98% purity when the second extractant of this invention is NMP containing KSCN.

[0084] Therefore, a comparison between Example 2 and Comparative Examples 1-3 revealed that the extractant has a significant impact on the purity of the key product, butadiene, and the addition of KSCN significantly improves the purity of the butadiene yield. Further analysis of the Henry's law coefficient and Gibbs free energy of butadiene in NMP and KSCN-containing NMP showed that, as... Figure 2 and Figure 3 As shown, the addition of KSCN to NMP promotes the separation between butene and butadiene, thus obtaining butadiene with the highest purity, without significantly increasing the process energy consumption.

[0085] In summary, comparing Example 2 with Comparative Examples 2 or 3 reveals that using NMP containing water or NMP with both water and KSCN as the extractant reduces the extraction efficiency. Furthermore, the presence of water corrodes equipment, reduces product purity, and negatively impacts the process. Therefore, subsequent separation of the product from water must be considered, reducing economic efficiency. In conclusion, the method proposed in this paper, using NMP containing 20% ​​KSCN as the extractant, achieves the best separation effect while avoiding major problems such as butadiene thermal polymerization currently prevalent in the industry, making it the most advanced approach.

[0086] In summary, this invention effectively overcomes the shortcomings of the prior art and has high industrial applicability. The above embodiments are intended to illustrate the substantive content of this invention, but are not intended to limit the scope of protection of this invention. Those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of this invention without departing from the essence and scope of protection of this invention.

Claims

1. A process for recovering 1,3-butadiene from a C4 cut by extractive rectification, characterized in that, The method comprises the following steps: feeding a C4 fraction and a first extractant into a first extractive tower for extractive rectification, discharging a butane-containing top stream from the top of the first extractive tower, and feeding a butadiene-containing side stream into a second extractive tower after being discharged from the side line of the first extractive tower; meanwhile, feeding a second extractant into the second extractive tower for secondary extractive rectification, and discharging a butadiene-containing top stream from the top of the second extractive tower; The first extractant is N-methyl pyrrolidone, and the second extractant is N-methyl pyrrolidone containing potassium thiocyanate; The first extractive tower and the second extractive tower are both packed towers, the theoretical plate number of the first extractive tower is 100-120, and the theoretical plate number of the second extractive tower is 115-130; The C4 fraction is fed into the first extractive tower through a raw material port, the raw material port is located at the 80th theoretical plate from the top of the first extractive tower, the first extractant is fed into the first extractive tower through a first extractant feeding port, the first extractant feeding port is located at the 10th theoretical plate from the top of the first extractive tower, the butane-containing top stream is discharged from the top of the first extractive tower through a butane outlet, and the butane outlet is located at the 1st theoretical plate from the top of the first extractive tower.

2. The method of recovering 1,3-butadiene according to claim 1, characterized in that, The mass fraction of the potassium thiocyanate in the salt-containing N-methyl pyrrolidone is 5%-25%.

3. The method of recovering 1,3-butadiene according to claim 2, characterized in that, The top pressure of the first extractive tower and the top pressure of the second extractive tower are both 1 bar, the mass reflux ratio of the first extractive tower is 15-30, the solvent ratio of the first extractive tower is 2-3, the mass reflux ratio of the second extractive tower is 25-30, and the solvent ratio of the second extractive tower is 0.5-1.

4. The method of recovering 1,3-butadiene according to claim 3, characterized by, The second extractant is fed into the second extractive tower through a second extractant feeding port, the second extractant feeding port is located at the 10th theoretical plate from the top of the second extractive tower, the butadiene-containing side stream is fed into the second extractive tower through a feeding port, the feeding port is located at the 105th theoretical plate from the top of the second extractive tower, and the butadiene-containing top stream is discharged from the top of the second extractive tower through a 1,3-butadiene outlet, and the 1,3-butadiene outlet is located at the 1st theoretical plate from the top to the bottom of the second extractive tower.

5. The method of recovering 1,3-butadiene according to claim 4, characterized in that, The feeding temperature is 30-50℃, and the first extractive tower and the second extractive tower further comprise a condenser and a reboiler; the temperature of the condensing water in the top condenser of the first extractive tower and the top condenser of the second extractive tower is both 10℃, the temperature of the reboiler of the first extractive tower is 80-90℃, and the temperature of the reboiler of the second extractive tower is 15-20℃.

6. The method of recovering 1,3-butadiene according to claim 5, characterized in that, An extractant discharge port is further arranged at the bottom of the first extractive tower to extract N-methyl pyrrolidone in the tower, and the extraction flow rate is 4500-5000 kg / h; the extractant discharge port is located at the 100th theoretical plate from the top.

7. The method of recovering 1,3-butadiene according to claim 6, characterized in that, An outlet is further arranged at the bottom of the second extractive tower, and the outlet is located at the 119th theoretical plate from the top of the second extractive tower.

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