Advances in ethylene separation technology

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Advances in ethylene separation technology: Ethylene production serves as an indicator of the overall development level of the petrochemical industry. In 2006, global ethylene production reached 117-Mt, an increase of 4.0% compared to 2005 ； China’s ethylene production reached 9.67 Mt, an increase of 24.3% compared to 2005. China’s ethylene production capacity ranks second in the world, only behind the United States. The petrochemical industry, led by ethylene, has played an important role in the national economy and social development. The ethylene plant is a key facility in the petrochemical industry, and the separation process is the core step in ethylene production. To reduce the energy consumption and equipment costs of separation units, improve operating processes, minimize equipment corrosion, extend operational cycles, and reduce waste generation, various patent holders have developed new ethylene separation unit technologies and separation equipment. This paper provides a review of the recent advances in ethylene separation unit technology and separation equipment. 1 Ethylene separation unit technology 1.1 Catalytic distillation technology Catalytic distillation technology combines catalytic reaction with distillation separation, enabling both reactions and separations to take place simultaneously within a single tower. It features high conversion rate of the target reactant, good reaction selectivity, low energy consumption, long catalyst life, and easy operation. The C3 selective hydrogenation in the ethylene plant of Yanshan Petrochemical Company, a subsidiary of Sinopec Corporation (referred to as Yanshan Petrochemical), utilizes the catalytic distillation technology developed by Lummus Company and CD Tech Company – the CD-Hydro process. This process accomplishes selective catalytic hydrogenation and the distillation separation of light hydrocarbons in one step, within the high-pressure depropanization tower, namely the C3 catalytic distillation column. The CD-Hydro process has the following features: (1) In the depropanization system, selective hydrogenation of acetylene and methacrylenic anhydride (MAPD) is combined with the separation of light hydrocarbons in a single unit. By eliminating the equipment associated with fixed-bed selective hydrogenation units, the CD-Hydro process results in significantly lower investment costs compared to conventional processes ； (2) The constant-pressure boiling system ensures precise control of the temperature in the catalytic distillation zone; lower reaction temperatures and isothermal conditions improve the selectivity of hydrogenation and reduce the formation of green oil ； (3) The flushing effect of liquid-phase reflux **reduces the adsorption of oligomers on the catalyst surface, allowing them to be discharged along with the heavier components from the bottom of the tower, thereby extending the catalyst’s lifespan. However, during operation, as the load on the unit increases, the MAPD content in the feed to the C3 catalytic distillation column rises, causing the MAPD level at the top of the column to exceed the specified limit. This results in an increase in the amount of propylene recovered from the bottom of the column, as well as an increased loss of propylene there. The Beijing Research Institute of Chemical Technology, affiliated with Sinopec Corporation (referred to as the Beijing Research Institute of Chemical Technology), proposed the concept of a modular catalytic distillation unit, which combines a liquid-phase hydrogenation catalyst with mature structured packing to form such a catalytic distillation unit. The newly developed C3 hydrogenation catalytic distillation unit BCD-1 consists of three parts, namely specially designed metal plate corrugated structured packing, a stainless steel mesh cylinder, and spherical catalysts. The Beijing Research Institute of Chemical Technology applied this catalytic distillation technology to the pre-depropanization and pre-hydrogenation process. By installing a catalytic reaction zone in the depropanization tower, it was possible to separate C3- from C4+ in the pyrolysis gas, while simultaneously carrying out selective hydrogenation of the alkynes and dienes present in the C3-. In other words, the traditional separation of C3- from C4+ and the selective hydrogenation of C3- were combined, with both processes taking place simultaneously within one catalytic distillation tower. The advantages of this method are: (1) it eliminates the C3 hydrogenation reactor used in the conventional process of the pre-depropanization step, thereby simplifying the process and reducing costs ； (2) The alkynes and dienes in C2 and C3 can be converted into the corresponding alkenes through catalytic hydrogenation, thereby significantly reducing the load on the gas-phase hydrogenation reactor and minimizing the size of the equipment ； (3) Under the effect of vapor-liquid phase separation in the catalytic distillation column, after selective hydrogenation of alkynes and dienes, they can easily desorb from the active sites, reducing the chances of further hydrogenation and thereby increasing the selectivity of catalytic hydrogenation as well as the yield of olefins ； (4) The polymers formed on the catalyst can quickly leave the catalyst bed and enter the stripping section, thereby extending the catalyst’s service life ； (5) Since the reaction temperature is controlled at the boiling point of the material under the reaction pressure, the heat of reaction can only cause more material to vaporize; thus, the bed temperature does not rise excessively, which helps to increase the yield of olefins ； (6) Butadiene does not enter the reaction zone, ensuring that no butadiene is lost during the hydrogenation process. This method simplifies the process, reduces energy consumption, lowers investment costs, improves the selectivity of the hydrogenation reaction, increases the yield of olefins, and extends the service life of the catalyst. 1.2 Membrane separation technology Membrane separation technology is a method that utilizes the difference in permeability of membrane materials to various components in a mixture in order to separate gas (liquid) phase mixtures. Gas membrane separation technology achieves separation by taking advantage of the different rates at which various components in a mixed gas pass through the membrane under the influence of pressure. Gas membrane separation technology boasts advantages such as high separation efficiency, low energy consumption, and simple operation, offering broad application prospects and strong competitive advantages. Research on the use of membrane separation technology for separating olefins from alkanes began to gain momentum in the 1980s. Membrane separation materials should possess both high permeability to olefins and high selectivity between olefins and alkanes. In recent years, polymer membrane materials used for the separation of olefins and alkanes mainly fall into two categories: one is based on the dissolution-diffusion mechanism, and the other is based on the enhanced transport mechanism. The membrane separation principle based on the dissolution-diffusion mechanism takes advantage of the differences in size and shape between olefin and alkane molecules, utilizing the significant difference in their dissolution-diffusion rates in a polymer medium for separation. The membrane separation principle based on the transport promotion mechanism improves the permeability of olefins through selective and reversible complexation between transition metal carriers in the membrane and olefins, thereby enabling effective separation of olefins from alkanes. Membrane separation technology has been widely applied, but it is still in the research stage for ethylene separation. LeBlanc et al. developed a silver-containing sulfonated polyphenylene oxide membrane, in which Ag+ was incorporated into the polymer matrix via ion exchange. When using this membrane to separate ethylene-ethane mixtures, the selectivity for ethylene can reach 288. Eriksen et al. believe that the most suitable polymer membrane for separating ethylene-ethane mixtures is an ionically cross-linked polymer based on fluorosulfonates, namely the Nation membrane. By converting this polymer membrane into an ion exchange membrane containing Ag+, the selectivity for olefins can be greatly improved due to the coordination between the olefins and Ag+. When used to separate an equal mixture of ethylene and ethane, the purity of the obtained ethylene can exceed 99% ； Eriksen et al. converted the Nation hollow fibers to the Na-type, then transformed them into the Ag-type via ion exchange using a 2 mol aqueous solution of silver nitrate, after which these hollow fibers were installed in a steel permeation device for the separation of ethylene and ethane. The 1000-hour lifetime experiment revealed that the coordination equilibrium constant of ethylene for Ag+ on hollow fibers is roughly equivalent to that of Ag+ in a large amount of aqueous solution. Boom et al. found that when preparing polymer membranes, adding silicoaluminoferric salts, Na-type and Ag-type zeolite powders, etc., to the polymer material can significantly improve the permeability and selectivity of the polymer membrane for olefins. They confirmed this through experiments on the separation of ethylene and ethane, and believed that the reason for the improved selectivity for olefins was that the alkanes had to travel longer paths around the zeolite particles, while the reason for the improved permeability of olefins was the membrane’s better solubility and absorption of olefins. Although membrane separation technology has the advantage of energy savings, its equipment investment is high, and it falls short of the requirements for industrialization; further development and research are still needed. 1.3 Extractive distillation technology: Extractive distillation is a distillation process in which an extractant is added to the liquid feedstock to alter the relative volatility of the various components, thereby enabling their separation. Extractive distillation is commonly used to separate mixtures whose relative volatilities are very close to each other. Its advantage is that the extractant vaporizes continuously during the distillation process, resulting in lower energy consumption ； The difficulty is that it is not easy to choose an appropriate agent. When separating olefins from alkanes using extractive distillation, the extractant and the olefins-alkanes are mixed at the feed inlet and then fed into the distillation column. Alkanes are obtained at the top of the column, while at the bottom a mixture of olefins and the extractant is produced. This mixture is then sent to an extractant recovery column, where olefins are obtained at the top of this column and the extractant is obtained at the bottom; the resulting extractant can be reused. Industrially, the separation of ethylene and ethane is carried out in a distillation column with over 100 trays, operating at a low temperature of -25°C and a high pressure of 2.306 MPa. Due to the drawbacks of low-temperature distillation, such as high compression power consumption, large cooling demand, and a large number of tray levels, researchers have been striving to find an efficient, energy-saving, and cost-effective method for separating olefins from alkanes as a replacement for low-temperature distillation. Yi Bo et al. studied the extractants for separating ethylene from ethane using extraction distillation technology, and proposed a composite solvent of acetonitrile and water as well as a salted system of N,N-dimethylformamide with NaSCN as extractants, which improved the selectivity for ethane and ethylene. 1.4 Adsorption separation technology Adsorption separation technology is a method that relies on the preferential adsorption of certain components in a gas mixture onto solid adsorbents to achieve separation. Depending on the method of adsorbent regeneration, adsorption separation technology is divided into pressure swing adsorption (PSA) and temperature swing adsorption (TSA). In the PSA process, adsorbent regeneration is achieved through pressure changes ； In the TSA process, adsorbent regeneration is achieved through heating. The PSA process has developed rapidly due to its advantages such as wide range of applicable gas sources, high product purity, no environmental pollution, great operational flexibility, high degree of automation, and significant energy savings. The TSA process is used only for purification due to its long cycle time and low gas processing volume. Sometimes the two are also used together. To achieve continuity in the adsorption process, at least two or more adsorbers need to be operated in rotation. In the adsorption process for separating olefins from alkanes, the solid adsorbents commonly used include zeolites, activated carbon, and metal complexes. The patent discloses a method for separating propylene from propane using combined processes such as PSA in the TSA plant and distillation-based PSA; the product can be obtained through vacuum desorption of propylene, and the PSA process employs 4A zeolite as the adsorbent. Later, they made further improvements to this process, using 4A molecular sieves or zeolites and silica gel modified with elements such as K, Ca, Mg, Ag, Cu, etc. (with a exchange capacity of less than 25%) as adsorbents. After distillation and flash evaporation, the pyrolysis gas was separated using a PSA process. The patent discloses another combined process: first, olefins are adsorbed using a PSA process, and then desorbed with a gas desorbent. The desorbent containing olefins is separated from the olefins after distillation; the desorbent is reused while the olefins are recovered. The patent discloses a process for the separation and recovery of ethylene using type A zeolite (with a Na mass fraction of 60%-75% and a K mass fraction of 25%-40%) as an adsorbent, under adsorption pressures of 0.05-5 MPa and adsorption temperatures of 50-200°C. The patent discloses a method for recovering ethylene from the mixture resulting after the oxidation of ethylene using a PSA process; the resulting ethylene is returned to the demethanization tower, which allows the temperature of this tower to be increased (from below -95°C to above -45°C), thereby reducing energy consumption. Kulvaranon et al. used 5A and 13X zeolites as adsorbents to separate propylene from propane via the PSA process. The adsorption cycle was carried out at 25°C and 0.1 MPa, while the desorption temperature was increased rapidly from 50°C to 160°C; propane desorbs preferentially at lower temperatures. For a two-component mixture with 50% mole fractions each, a product with an acrylene mole fraction of 85% can be obtained through an operational cycle that includes medium-temperature desorption. Raymond et al. used adsorption to recover ethylene from low-concentration gases, whose volume composition was 2%-4% ethylene, 5%-15% carbon dioxide, a small amount of oxygen, and the remainder nitrogen. First, 4A zeolite is used as an adsorbent to remove carbon dioxide from the mixture; then, 13X zeolite is used as an adsorbent to selectively adsorb ethylene from the remaining gas mixture. Finally, desorption is carried out to achieve the recovery of ethylene. UOP has developed a process for separating olefins from alkanes—the Olex process. This process uses specialized adsorbents and desorbents, and is combined with UOP’s simulated moving bed adsorption technology. Pilot test results show that this process can achieve an olefin recovery rate of up to 99.7% and olefin products with a purity of up to 99.6%. UOP has proposed using the PSA process to recover ethylene from catalytic cracking off-gases. After entering the C2 fractionation tower, they installed a PSA system. The catalytic cracking off-gases are compressed, dehydrated, and fractionated before entering the C2 fractionation tower; ethylene with a purity of over 95% is obtained at the top of the tower, while ethane containing a small amount of ethylene is obtained at the bottom. This ethane is then sent to the PSA system for ethylene recovery. The recovered ethylene is added back to the catalytic cracking off-gases for further cyclic separation, using zeolite as the adsorbent. 1.5 Absorption separation technology: Absorption separation technology is a process that utilizes the difference in solubility of various components in a gas within a liquid absorbent to separate gas mixtures. It is a gas-liquid mass transfer operation that requires mass transfer between two phases. Unlike distillation, absorption separation technology requires the introduction of a second phase from the outside; separation is achieved through the one-way transfer of the absorbent from the gas phase into the liquid phase, and a purer absorbent component can be obtained only through desorption. Gutierrez developed a process for recovering olefins using bimetallic salts. In this process, copper chloroaluminate is dissolved in toluene to form a liquid absorbent. This absorbent is then sprayed and brought into contact with refinery exhaust gas containing 12% (by volume) ethylene in a complexing unit, causing the ethylene to react with the liquid absorbent and dissolve in it, thereby separating it from the exhaust gas. The method for desorbing the absorbed substance is by reducing pressure and increasing temperature. The ethylene recovery rate is approximately 96%, with a product purity of 99.5%. In the absorption separation process used for separating olefins, the aqueous solution systems employed include both Ag+ and Cu+ systems. AgNO3, AgBF4, AgClO4, AgCF3CO2, etc., can all be used for the separation of olefins. Among them, AgBF4 has a high adsorption capacity, followed by AgClO4 and AgCF3CO2, while AgNO3 has a low adsorption capacity. Douglas et al. developed a process using an aqueous AgNO3 solution as the absorbent, with an operating pressure of 1.65 MPa, an absorption temperature of 30–40°C, and a desorption temperature of 40°C, employing a packed tower. The literature also reports a process using an aqueous AgBF4 solution as the absorbent (operating pressure of 0.2 MPa, adsorption temperature of 30°C, desorption temperature of 80°C) and a process using CuNO3/ethanolamine as the absorbent (operating pressure of 1.2 MPa, adsorption temperature of 30°C, desorption temperature of 50°C); the purity of the ethylene product in these processes is 86.6%, with high yields as well. In the absorption separation process used for separating olefins, the non-aqueous solution system employed is mainly the Cu+ system. Cu+ has a weak complexing effect with organic solvents; therefore, it is more stable in organic solvents than in aqueous solutions and is less prone to disproportionation. Commonly studied chelating agents include CuCF3CO2, CuNO3, Cu2SO4, CuBF4, CuAlCl4, CuAlCH3Cl3, CuAlC2H5Cl3, CuAlCNCl3, and halogenated amines, while aromatic or alkenic solvents are generally used as solvents. The patent discloses a process using CuAlCl4/aromatic solvents as absorbents, namely the ESEP process. When the volume fraction of ethylene in the feedstock is 12%, the ESEP process can achieve an ethylene yield of nearly 96% with a purity of 99.5%. The patent discloses a new process for separating ethylene from ethane and other gases. This process uses a Ni-containing ditthiomophore complex as a binding agent; in the presence of common pollutants, ethylene can selectively bind to the ditthiomophore complex and be recovered reversibly. The binding of ethylene to dithienyl complexes is an equilibrium process that proceeds in the direction of the olefin-metal complex. This process can be used for the separation of C2-6 monoolefins; by reducing the system pressure or increasing the temperature, ethylene can be easily recovered from the complex. This process enables the efficient recovery of ethylene from ethane and other saturated hydrocarbons, without the ligand becoming deactivated. Once this process is put into practical use, it is expected to replace the traditional ethylene-ethane separation methods, which require high investment. 1.6 Combined Process: China University of Petroleum has proposed a combined process that integrates hydration separation, cryogenic separation, and PSA processes for separating the pyrolysis gas from ethylene production units. The main purpose of this combined process is to apply a new type of hydration separation to the separation of pyrolysis gas. First, by controlling the reaction conditions, C2 compounds in the pyrolysis gas are preferentially converted into hydrates and thus separated. Subsequently, the cooling capacity generated by the expansion refrigeration of a mixture of methane and hydrogen is used as the cooling source for the demethanization tower. In combination with the PSA process, this approach enables the separation of hydrogen, methane, and C2 compounds (ethylene and ethane). It eliminates the need for sequential condensation stages and cryogenic tanks found in traditional processes, thereby reducing the cooling load on the demethanization tower and improving economic efficiency. 2 Ethylene separation equipment: Based on improvements to distillation column trays, new types of trays such as the DJ series trays, differential floating valve trays, and series inclined-hole trays have been developed. With the advent of fractional condensers, efficient heat and mass transfer systems such as fractional condensation distillation columns, thermally integrated distillation systems, and partitioned-wall distillation columns were developed. 2.1 DJ series trays: Yao Kejian and others developed a series of high-efficiency DJ series trays featuring high throughput and low pressure drop. The DJ series of trays exhibit excellent hydrodynamic and mass transfer properties, as well as stable operation: (1) They are suitable for operations with high liquid-to-gas ratios and large liquid volumes; since the overflow perimeter is 2-5 times larger than that of conventional trays, the liquid flux of DJ series trays is 30%-60% higher than that of conventional trays ； (2) Stable operation: when the gas-liquid load changes, the liquid level fluctuates little, and so does the pressure drop ； (3) The gas-liquid distribution is uniform, and it possesses a redistribution capability; when high requirements are not placed on the initial distribution, it is possible to modify old towers without altering the original gas-liquid distribution device. Thanks to the 90-degree rotation of the liquid drop pipes between the two plates, the gas-liquid distribution can be quickly made uniform ； (4) The suspended downcomer is not restricted by the liquid level, allowing the bubbling area to increase by 1.5%-20% ； (5) The plate spacing is small, resulting in a high theoretical number of plates per meter of tower height ； (6) The liquid layer is thin, resulting in reduced plate pressure; in towers handling large volumes of liquid, this prevents the formation of an inefficient layer of excessive foam, thereby ensuring efficient mass transfer and saving energy consumption ； (7) As the liquid flows out of the downcomer, it does not need to be deflected, so no slow-flow zone is formed. Coupled with the scouring effect of bubbling on the large-pore sieve plates, it possesses excellent resistance to clogging. The DJ series of trays that have been used in industrial production include models DJ-1, DJ-2, DJ-3, and DJ-5, among others. The DJ-1 tray is a wide-downcomer tray capable of withstanding extremely high liquid loads. The DJ-2 type tray is a tray equipped with a flow guiding device; this device ensures a more uniform distribution of the liquid flow on the tray, making the flow resemble piston flow and reducing splash leakage, thereby improving the efficiency of the tray as well as its operational flexibility. The DJ-3 tray is a composite tray; it is created using an in-situ compounding technique to attach a thin layer of structured packing beneath the DJ-2 tray. In addition to the high-throughput performance characteristic of the DJ series of trays, the DJ-3 tray also offers greater operational flexibility and higher mass transfer efficiency. The DJ-5 type tray uses a rectangular suspended downcomer as the liquid drainage channel, and a new type of fixed valve as the bubbling element. The new type of fixed valve combines the advantages of sieve tray plates and floating valve trays, overcoming the disadvantages of floating valve trays such as high material consumption, high manufacturing costs, easy clogging, as well as the tendency for floating valves to be blown away or stuck, and the issues associated with sieve tray plates including high mist entrainment and severe leakage. The cost of the DJ-5 type tray is close to that of the sieve-tray, and its tray efficiency is comparable to that of ordinary floating valve trays. Lanzhou Petrochemical Company, a subsidiary of China National Petroleum Corporation, carried out an expansion and renovation of the deethanization tower in its ethylene plant, replacing the original Sulzer Vortex trays with DJ-3 type trays. As a result, the ethylene production capacity increased from 160 kt/a to 240 kt/a, with a maximum capacity of 320 kt/a ； DJ-5 type trays were used to replace the floating valve trays in the upper section of the low-pressure depropanization tower, while DJ-2 type trays were used to replace the sieve tray trays in the lower section of the original tower. The modified low-pressure depropanization tower was successfully put into operation in 2003, meeting the desired performance criteria. When Dushanzi Petrochemical Company of China National Petroleum Corporation carried out capacity expansion upgrades on the propylene distillation column and ethylene distillation column in its ethylene plant, it did not modify the column shells but replaced the internal components, using DJ-3 type trays in place of the UOP MD type trays. The facility was officially put into operation in September 2002, and after the renovation, all towers met the design specifications. The separation requirements and production capacity of the ethylene distillation column in the ethylene plant of Fushun Petrochemical Company, a subsidiary of China National Petroleum Corporation (referred to as Fushun Petrochemical Company), have consistently failed to meet the production demands. After replacing the original floating valve trays with DJ-3 type trays, all performance indicators of this column have met or exceeded the design specifications. 2.2 Differential floating valve trays: Differential floating valve trays were developed to address the shortcomings of F1-type floating valve trays. Their features include: (1) The differential floating valve has small valve openings at its top, which allows for full utilization of the mass transfer space above the floating valve, resulting in more uniform gas dispersion and better gas-liquid contact ； (2) Differential floating valves with guiding functions are used locally to eliminate liquid retention on the tray and improve the uniformity of gas-liquid distribution ； (3) The use of a bubbling promoter ensures uniform bubbling across the entire tray, while also promoting a more even gas distribution; this increases the processing capacity of the tray and enhances the mass transfer efficiency ； (4) The downcomer was appropriately improved to increase the area of the bubbling zone ； (5) The valve stem features a new structural design, enabling quick and easy installation of the floating valve; during operation, the floating valve does not rotate easily and will not fall off. The newly built propylene-propane separation tower at the Chemical Plant of Daqing Oilfield, China National Petroleum Corporation, utilizes differential floating valve trays. The original design used F1 floating valve trays, with a tower diameter of 6 m and 205 trays. By using differential floating valve trays, the tower diameter can be reduced to 5.2 meters; only 181 trays are required, and the tower efficiency increases from 75% in the original design to 95%, **resulting in savings in capital investment. The tower was successfully put into operation in October 1999. The propylene-propane separation tower at the Second Oil Plant of Fushun Petrochemical Company was upgraded from 300 kt/a to 450 kt/a. After the upgrade, the height and diameter of the tower remained unchanged; the original tower had 180 layers of F1 floating valve trays, which were replaced by differential floating valve trays after the modification. After the modification of the unit, it was put into operation successfully in June 2000, with smooth operation; the reflux ratio decreased from 18 to 11, **reducing energy consumption. 2.3 Series of inclined-hole trays: An inclined-hole tray is a type of tray in which inclined holes are arranged in an interlaced pattern. The vapor-liquid flow within such a tray is optimized; the gas is ejected horizontally, and the gas emitted from adjacent holes does not collide with each other. This results in a uniform gas distribution, without any overlapping that could accelerate the liquid flow. An appropriate amount of liquid remains on the tray at all times, ensuring sufficient vapor-liquid contact, reduced mist entrainment, a high allowable vapor-liquid load, as well as certain self-cleaning capabilities. It avoids the disadvantages of other types of trays, such as gas being injected vertically upward (screen trays), gas interfering with each other (bubble and floating valve trays), and gas being injected in one direction only (floating jet trays). The characteristics of inclined-hole trays are: (1) high gas-phase load and large production capacity, with the production capacity being 30%-40% higher than that of floating-valve trays ； (2) The tray efficiency is high, generally equal to or slightly higher than that of floating valve trays ； (3) Simple structure, low processing cost ； (4) It has a self-cleaning effect, and the material is not prone to clogging ； (5) The pressure drop is small. The composite inclined-orifice tray was developed on the basis of the inclined-orifice tray. It adds some floating tongue holes to the fixed inclined holes of the inclined-hole tray; these floating tongue holes not only serve to guide the liquid but also increase the operational flexibility of the inclined-hole tray. The new type of multi-overflow composite inclined-orifice tray is a composite tray developed on the basis of inclined-orifice trays, through careful research and analysis of MD sieve trays. It possesses the advantages of the MD sieve tray’s multi-overflow structure while overcoming its disadvantages, and at the same time retains the benefits of inclined-orifice trays. The new type of multi-overflow composite inclined-hole tray uses a downcomer design similar to that of MD trays, but with only one or two downcomers; it has a relatively simple structure, the liquid flows over a longer distance, and thus the efficiency of the tray is high. Furthermore, by using inclined-hole trays instead of sieve-hole trays on the plate surface, the processing capacity of the tower is further improved. Composite inclined-hole tray plates were applied in the 13 distillation columns of Yanshan Petrochemical Company’s 300 kt/a ethylene expansion project. Their production capacity is over 50% higher than that of conventional floating-valve trays, the separation efficiency has been improved, and their cost is only 1/5 of that of similar equipment, resulting in annual economic benefits of 57.5 million yuan. 2.4 Segregative Fractionator The segregative fractionator consists of a plate-fin heat exchanger and a separation tank; heat transfer and mass transfer occur simultaneously in the plate-fin heat exchanger, which is designed with 10–15 theoretical plates. Unlike ordinary plate-fin heat exchangers, the fractional distillation unit features spacious gas-liquid channels, with a gas-liquid separation tank located at its bottom; multiple cold fluid streams pass through the unit to provide cooling capacity. The recovered gas flows from bottom to top within the channel; the higher it goes, the lower its temperature becomes. Part of the gas condenses on the walls of the channel, and the condensate flows downward due to gravity, coming into contact with the gas in the opposite direction. Mass transfer and heat transfer occur between the gas and the liquid film, thereby enabling separation by condensation and fractional distillation. The fractional distillation unit possesses both the heat transfer function of a conventional plate-fin heat exchanger and the separation function of a conventional distillation tower. The technical key to the fractional condenser and fractionator lies in the combination of mass transfer and heat transfer; the condensed liquid forms a downward-flowing film on the fins, where it comes into contact with the upward-moving air flow in order to facilitate mass transfer and heat transfer. Traditional quenching systems only carry out heat exchange, whereas fractional condensers perform both heat transfer and mass transfer, achieving a multi-stage separation effect. In Stone & Websmr’s Advanced Recovery System (ARS), the cryogenic demethanization system is designed with two fractional condensers and fractionators. The top control of the first fractional condenser/fractionator regulates the C3 content; it is known as a thermal fractional condenser/fractionator ； The top of the second fractional condenser controls the ethylene content; it is known as the cold fractional condenser. The cold fractional distiller has a total of 7 streams: one hot stream (the purified pyrolysis gas) and 6 cold streams. The pyrolysis gas entering the bottom separation tank mainly contains hydrogen, C1, and C2; after flashing, the liquid phase enters the demethanization tower, while the gas phase rises through the channels of the plate-fin heat exchanger. It is cooled by the cold stream while also coming into contact with the liquid phase flowing downward for separation, with the ethylene concentration decreasing as one moves toward the top. A portion of the material at the top of the fractional distillation unit is mixed with the vapor from the demethanization tower and sent to the expander to provide cooling for the fractional distillation unit, while the majority of the remaining material goes directly into the hydrogen recovery system. The low amount of ethylene contained in the pyrolysis gas leaving the top of the fractional distiller is ultimately incorporated into the fuel gas. 2.5 Condensative Fractionation Tower The basic structural unit of the condensative fractionation tower (CFT) consists of two sections: the upper section is an improved condensative fractionator, while the lower section is a packed tower; each section is designed with a certain number of theoretical plates. Various types of fractional condensation and distillation columns can also be derived from the basic structural unit. CFT offers better mass and heat transfer performance than fractional condensers and distillers, but it is more difficult to manufacture; however, the equipment size (the plate-fin heat exchanger part) and investment costs are reduced. The full condensation fractional distillation column (CFT-Ⅱ) is a new type of high-energy-efficiency distillation equipment developed based on the technical features of CFT. While continuously enhancing heat and mass transfer in both the distillation section and the stripping section, CFT-Ⅱ enables continuous energy transfer between different energy levels, thereby improving the energy efficiency of distillation equipment, especially that used for low-temperature distillation. CFT-Ⅱ can operate at very low reflux ratios and reboiler loads; to ensure the quality of the product at the tower top as well as the product recovery rate, the reflux heat exchanger and reboiler are typically kept at very low operating loads. CFT-Ⅱ inherits and builds upon CFT’s high energy efficiency, excellent separation performance, and flexibility. 2.6 Heat-Integrated Rectifier System The Heat-Integrated Rectifier System (HRS for short) is an improvement over traditional distillation systems, as well as a significant advancement in split-condenser distillers. HRS thermally integrates conventional plate-fin heat exchangers, separation tanks, and distillation columns; it has no reflux pump and is the core equipment of Stone & Webster’s second-generation ARS technology. Compared to a fractional distiller, its heat transfer efficiency is significantly higher, at about 10 times that of a fractional distiller. When the same separation efficiency is achieved, the equipment size is significantly reduced, and the investment cost is greatly lowered. For a 600 kt/a ethylene plant, the capital investment is reduced by over $6.5 million, and the ethylene loss is decreased from 5% to 0.05%-0.1%. 2.7 Divided Wall Column The Divided Wall Column (DWC for short) is a special type of fully thermally coupled distillation column that offers significant advantages for the distillation of multi-component mixtures. It is currently a focal point of research abroad in terms of equipment integration as well as energy savings and consumption reduction. In the DWC, a vertical partition is installed inside the distillation column, dividing it into an upper section, a lower section, a distillation feed section separated by the partition, and an intermediate product extraction section (as shown in Figure 1 (simplified)). Since the principle and calculation methods of DWC are the same as those of a thermally coupled distillation column, DWC is thermodynamically equivalent to such a column, allowing for a 30% reduction in investment compared to traditional two-column systems (two distillation columns in series). Compared to traditional two-tower systems, DWC reduces energy consumption by about 30%, due to the following reasons: (1) DWC is more thermodynamically efficient. When a three-component mixture of A, B, and C (with A being the light component, C the heavy component, and B the intermediate component) is separated in a two-column system, the concentration of component B in the stripping section of the first column increases as the concentration of component A decreases; it then decreases again as the concentration of component C increases. In other words, the concentration of component B exhibits a peak within the column, resulting in remixing within the column, which is one of the reasons for low separation efficiency. In contrast, when DWC separates a mixture of components A, B, and C, preliminary separation is carried out in the pre-separator, and component B appears simultaneously at both the top and bottom of the tower. The upper part of the pre-separator separates A+B from C, while the lower part separates B+C from A; the upper and lower sections only separate the product components of their respective sections. The same is true for the main tower, effectively preventing remixing in the twin-tower system ； (2) It reduces the mixing effect on the feed plate, minimizing mixing problems caused by differences between the feed concentration and the concentration on the feed plate. Since in the pre-separator, Component B is present both at the top and bottom of the column, the feed composition is similar to the composition on the tray, reducing the mixing effect at the feed tray. In terms of equipment investment, compared to traditional two-column separation systems, DWC requires only one condenser, one reboiler, one column, and a control system; it thus occupies less space. However, a partition is needed, and the column is slightly larger than that of a simple distillation column. As a result, the total equipment investment is reduced by about 30%. 3 Conclusion As separation technologies continue to advance and the demand for energy savings and reduced consumption increases, process technologies that can enhance production while reducing consumption are highly favored. In terms of unit technology, binary/trinary refrigeration technology allows a single refrigeration circuit to meet the cooling requirements at different temperatures and pressures within a pyrolysis unit. The use of this technology in such units helps reduce investment costs and improve reliability. Catalytic distillation technology has the potential for widespread use in ethylene plants, as it integrates catalytic reactions with distillation separation processes, simplifying the process flow and reducing equipment investment. The key to catalytic distillation technology lies in developing efficient catalysts for the reaction process, as well as efficient methods for loading catalysts in the separation process. In terms of separation equipment, the degasification and fractionation tower integrates mass transfer and heat transfer processes for use in demethanization systems, thereby simplifying the process flow. However, it is difficult to manufacture such towers; if new manufacturing techniques can be developed to reduce this difficulty, they hold great potential for use in ethylene separation. Split-wall distillation towers are used for ethylene separation; they not only improve separation efficiency but also reduce the need for equipment, thus saving on investment. However, the challenge in applying split-wall distillation columns lies in the complexity of their control schemes; therefore, it is necessary to enhance research on the dynamic characteristics of such columns in order to determine optimized control strategies. With the application of these advanced unit technologies and separation equipment in ethylene production, it is certain that the production costs of ethylene will be further reduced, the production process will be optimized, and the competitiveness of the products will increase.