Current status and progress of p-xylene production technology

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l. Selective Toluene Disproportionation Process: In the mid-to-late 1980s, Mobil Corporation (now ExxonMobil) developed a selective toluene disproportionation process (MSTDP) that uses a chiral catalyst to produce xylene products rich in p-xylene. ExxonMobil has sold patent licenses for this technology to some production facilities around the world (such as Coke and Trust Company). Recently, it has stopped providing licenses for the MSTDP process, but continues to offer licenses for its conventional toluene disproportionation process. ExxonMobil has developed an improved toluene disproportionation process called PxMax, and has recently licensed the patents for this technology to South Korea’s LG-Caltex. Since 1997, UOP has been offering patent licenses for its selective toluene disproportionation technology, known as PXPlus. Later, GTC Company (a subsidiary of Foster Wheeler) was granted the exclusive rights to sell Indopetrol’s selective toluene disproportionation process, GT-STDP. The xylene-rich product obtained in the selective toluene disproportionation (STDP) process can be directly fed into single-stage crystallization or a small Parex unit to recover high-purity p-xylene product. However, this setup also produces unwanted mixed xylene, as well as large amounts of benzene; the mass ratio of benzene to xylene is close to 1.0. Each process has its own advantages. The STDP process can produce p-xylene in high concentration (greater than 80%) from toluene as the raw material, along with large amounts of benzene as a by-product ； In conventional toluene disproportionation technology, C9 aromatics can be processed together with toluene to yield an equilibrium mixture of xylene (with p-xylene content of about 20%–25%), but with fewer benzene by-products. The conventional toluene disproportionation technique utilizes both the toluene disproportionation reaction and the transalkylation reaction. The choice of process depends on the specific needs of the user. (1) ExxonMobil’s PxMax process. The PxMax process using MTPX catalyst was first put into industrial use in 1996 at a refinery in Louisiana, United States, with another unit coming online at ExxonMobil’s chemical plants in Baytown and Beaumont, Texas. The process flow is similar to MSTDP, except for the catalyst. ExxonMobil has filed many patents related to its HZSM-5 catalyst. The most promising molecular sieve catalysts seem to require activation with deposited silica and treatment under conversion conditions using a silica-containing selective reagent for p-xylene. The silica-modified HZSM-5 catalyst (containing 5%-10% SiO2/HZSM-5) exhibits a selectivity of approximately 98% for xylene at a toluene conversion of 20%–25%. The silicate coating deposited on the surface of the zeolite reduces surface activity while increasing selectivity. It is generally believed that the advantage of MTPX is that the reactants cannot approach the acidic centers on the outer surface. The acidic sites on the outer surface of the catalyst can re-isomerize p-xylene in the catalyst pores into an equilibrium mixture with the other two isomers, thereby reducing the content of p-xylene in xylene to 24%. A relatively high content of p-xylene can be obtained by reducing the proximity of p-xylene to these acidic sites within the catalyst pores. The MTPX catalyst prevents p-xylene from coming into contact with these external acidic sites by chemically modifying the surface acidic centers using a highly selective reagent for p-xylene. ExxonMobil’s patent data shows that as the temperature increases, the selectivity for p-xylene decreases, while the conversion rate of toluene increases ； As the weight-based space-time velocity (WHSV) increases, the toluene conversion rate decreases while the selectivity for p-xylene increases ； As the hydrogen/hydrocarbon ratio increases, the toluene conversion rate decreases, while the xylene selectivity increases. Further improved MTPX catalysts can reduce unwanted by-products, primarily by lowering the formation of ethylbenzene. This is achieved by enhancing the catalytic hydrogenation or dehydrogenation capabilities, for example by adding metal compounds such as platinum (0.01%-2%). Patents show that when 0.25% platinum is added for every 10% of SiO2/HZSM-5, the production of ethylbenzene can be reduced by 3–4 times, while the selectivity for xylene remains above 98%. Furthermore, the production of C9 aromatics can also be reduced by 3 times. This PxMax process enables efficient conversion, reduces the formation of ortho and meta isomers, and facilitates the production of more p-xylene product. Most of the examples in the patents show that the reactor temperature in the PxMax process is slightly higher than that in the MSTDP process (440–443°C), while the WHSV and hydrogen/hydrocarbon ratios are very similar. The conversion rate of toluene is significantly lower than that in the MSTDP process, but the selectivity for xylene is higher. It is expected that the process for PxMax will be similar to that of the MSTDP process, and existing MSTDP plants can be modified to use MTPX catalysts. (2) UOP’s PXPlus process. UOP’s PXPlus process was put into industrial use at the end of 1998. This process is very similar to Mobil’s MSTDP, both in terms of operation and workflow. This PX process is also used in situations where a large amount of benzene and p-xylene are both required. Unlike UOP’s Tatory process, the PX Plus and MSTDP processes do not support the transalkylation reaction between toluene and C9 aromatics, which reduces benzene yield. When used in combination with the Raytheon/Niro crystallization technology, this technique is known as the PXPlusXP process. UOP claims that this process can produce mixed xylene with a p-xylene content of over 80%, and even as high as 90%, whereas the equilibrium value of p-xylene in conventional toluene disproportionation is only 25%. At a toluene conversion of 30%, the yield of light components per pass in this process is less than 2%. A standalone PXPlus unit includes benzene and toluene columns along with a single-stage crystallization recovery unit. Compared to UOP’s Tatoray process, the PX Plus process has a relatively simpler flow diagram. Fresh toluene is mixed with the recycled toluene from the toluene column and the recycled hydrogen-rich stream; the feed is preheated by reactor effluent and then passed through a fixed-bed heater to reach the desired reaction temperature. The hot feed enters a fixed-bed reactor, which can be of either downflow or plug-flow design. The reaction products from the feed/product heat exchanger are cooled and condensed, and then sent to a gas-liquid separator. The gas coming from the separator contains recycled hydrogen; it is necessary to discharge a portion of this gas in order to prevent the accumulation of inert substances, and to add fresh hydrogen to maintain a high purity level of hydrogen. The liquid from the separator is sent to the stripping tower, where light components are produced as a by-product to stabilize the product. The stabilized bottom product is sent to the benzene and toluene distillation column. High-purity benzene is recovered from the top of the benzene tower. The overhead product from the second column contains toluene, which is recycled to the front end of the plant; the bottom product of the toluene column contains xylene (with up to 90% p-xylene), and it is sent to the xylene reprocessing column. The top product of this tower enters a single-stage crystallizer, where the p-xylene product is recovered in a separate unit. If PX Plus is part of a large aromatic complex, the concentrated p-xylene can be sent to the Parex adsorption separation unit from the xylene reprocessing tower, along with fresh mixed xylene and recycled isomers. (3) ExxonMobil’s MSTDP process. ExxonMobil’s first-generation toluene disproportionation process is Mobil’s Selective Toluene Disproportionation (MSTDP) process, which produces xylene in which p-xylene accounts for about 90%. The key to high selectivity is a ZSM-5 catalyst subjected to coking pretreatment. Molecular sieves are a type of selective catalyst that controls chemical reactions based on the size of the surface pores and the volume of the internal chambers where the reactions take place. An important feature of the crystal structure of these catalysts is that it provides selective and controlled entrances and exits, offering free space within the crystal by defining pore volume and pore windows. Compared to the meta and ortho isomers, which have larger spatial volumes, p-xylene is more likely to escape from the pores of the pre-treated catalyst, while the other two isomers re-equilibrate within the catalyst pores, resulting in more p-xylene being produced. This selective toluene disproportionation process has been in industrial operation since 1988 at the Eni Chemicals plant located in Gela, Italy. Other MSTDP units were built by Exxon (now ExxonMobil) and Coke Company. When the selective toluene disproportionation process is used, the toluene conversion rate is only 30%, which increases the material handling volume in the BTX plant; however, due to the high content of xylene in the xylene mixture, the separation workload in the adsorption or crystallization units can be significantly reduced. Furthermore, from an economic perspective, there is no need to recycle the small amount of other xylene isomers back to the isomerization unit. The process flow is similar to that of selective and non-selective toluene disproportionation processes. The dry toluene feed is preheated by the reactor effluent together with the recycle gas through indirect heat exchange, then heated by a flame heater before entering the fixed-bed reactor. The reactor product is cooled and then passed through a phase separator. Most of the hydrogen-rich gas is recycled, with a small portion being released to maintain an appropriate hydrogen partial pressure. The liquid in the separator is stabilized, a small amount of light components are removed, and a small amount of olefins is removed using clay treatment. Reactor conditions vary depending on the specific process. The pressure in a conventional toluene disproportionation process is generally 4–4.5 MPa, with a temperature of 320–500°C. The operating pressure for MSTDP technology is generally 2.2–3.5 MPa, with a temperature of 400–470°C. The initial pretreatment is carried out at higher temperatures and lower pressures. (4) Recovery of p-xylene from the products of the selective toluene disproportionation process. Several industrial crystallization techniques can be used to recover p-xylene from the products of selective toluene disproportionation processes. As mentioned above, these products have a high xylene content; feedstocks with a xylene content of over 70% are attractive for many separation technologies that still present challenges. ①The melt static crystallization process for BEFSPROKEM. A division of John Brown Company, BEFSPROKEM, developed a batch process for melt static crystallization (MSC) in a single step. An important MSC device is a crystallizer designed specifically for the recovery of p-xylene. The crystallizer includes heat transfer surfaces for heating and cooling, as well as specialized internal components that facilitate better separation between the crystalline solid phase and the liquid phase. The temperature must be reduced below the existing control levels in order to form large crystals, which ultimately result in a crystal network or crystal bed. Depending on the design of the crystallizer and the static operating conditions, the liquid phase does not have the opportunity to develop, resulting in p-xylene with a purity of nearly 100% that is suitable for crystallization. The mother liquor containing impurities is discharged by gravity. This mother liquor can be processed in existing adsorption or crystallization units, or sold directly as mixed xylene. After the discharge is complete, the crystal network inside the crystallizer acts like the packing in a mass transfer tower. Other parts of the process include removing impurities adhering to the crystals. The precipitated crystals are washed with molten pure product, diluting the impurities in the liquid film surrounding the crystals. The purity of this crystalline cake can be increased to the specified value, reaching over 99.9%. The final step in the process is to melt the crystals and discharge pure p-xylene into the product tank ； ②The heat pump crystallization system from Sulzer Chemtech. The heat-pump crystallization system is a newly developed technology for producing pure p-xylene from a mixture of xylene isomers. Sulzer claims that the purity of p-xylene can reach 99.95%, and the equipment requires low investment, as well as low energy consumption and maintenance costs. The key project designed by Sulzer is the heat-pump crystallizer. This type of crystallizer can switch between two operating modes: cooling with a liquid refrigerant and heating to evaporate the refrigerant. Two crystallizers are the minimum requirement; if the scale of the plant is larger, more crystallizers can be used. When one crystallizer operates in crystallization mode as an evaporator, the other operates as a condenser for surface condensation or in melting mode. The equipment is essentially composed of a standpipe system that provides heat transfer surfaces. The xylene mixture enters from the top of the tube. The liquid is distributed on the outer tube surface as a downward-flowing film. The refrigerant used for cooling is distributed at the top of the tube through the inner tube, wetting the interior of the crystallization tube ； ③Badge/Niro crystallization process. Badge/Niro states that their technology also offers the advantages of low investment and low utility consumption. This process also uses a simple crystallizer design (a vertical scraper-type crystallizer), but it features the use of Niro’s screw-type washing tower (as opposed to centrifugal operation). It is claimed that this process can produce p-xylene with a purity of 99.93% (by mass), and the recovery rate can reach 95% when the purity of the feed material is 90%. The slurry from the crystallizer enters the bottom of the washing tower, where a screw device inside the tower pushes the slurry upward. As the mother liquor is washed away by counter-current p-xylene, the crystals are compacted. The crystals are scraped off the top of the bed and fluidized at the top of the circulating pure xylene. The resulting slurry is heated to melt the crystals. The stream flowing out of the melter splits into two streams: one is the pure product, and the other flows back to the washing tower. To obtain a high-purity product, both the BEFS and Sulzer processes require at least two crystallizations, unless feedstock with a high p-xylene content is used. Both techniques lose additional efficiency due to separation and crystallization from the mother liquor (either by washing with p-xylene product or through surface condensation of the crystals). The Badge/Niro process appears to be the most energy-efficient, as the product crystallizes only once, and no p-xylene is recycled due to the separation of crystal from liquid. Furthermore, among the three technologies, only the Badge/Niro technology can be effectively used to modify existing crystallization units. 2. Toluene methylation process: Toluene methylation, which involves the alkylation of toluene with methanol to produce xylene, has been a topic that many companies have devoted significant effort to researching; these companies include Amoco (now BP), DuPont, Union Carbon (now Dow), ExxonMobil, Union Oil, and UOP. Recently, GTC Technologies has begun selling the toluene methylation process (GT-TolAkl) developed by Indian Petrochemicals Limited (1PCL). GTC states that with a proprietary high-silica molecular sieve catalyst, the selectivity for xylene can exceed 85%. The reaction is carried out in a fixed-bed reactor in the presence of hydrogen and water. The recovery of p-xylene is generally carried out in a crystallization system. The operating conditions of the GT-TolAkl system are as follows: temperature of 400–450°C, pressure of 100–500 kPa, weight space velocity of 1–2 h^-1, and p-xylene selectivity of 80%–90% (by mass). Compared with the STDP process, the advantages of the toluene methylation route are: ① the amount of toluene required per ton of p-xylene can be reduced from about 2.8 tons to 1.0 ton ； ②Methanol is readily available and relatively inexpensive (for example, in the first quarter of 2001 it was 79 cents per gallon, the highest price in 5 years) ； ③The production of benzene is negligible (0.006 gallons of benzene per pound of xylene). According to the conceptual design of the toluene methylation process, the additional toluene and methanol are vaporized and combined with recycled toluene and hydrogen, preheated by the reactor effluent, and further heated to 400°C in a heating furnace. This feed is fed into a methylation reactor to produce xylene and various by-products (such as benzene, ethylbenzene, carbon monoxide, carbon dioxide, and hydrogen). Due to the exothermic reaction, the temperature rose to 450°C. The reactor effluent is cooled by heat exchange with the reactor feed, and then passes through a partial condenser, where some organic products such as benzene, ethylbenzene, toluene, and xylene are condensed. The remaining gaseous products (carbon monoxide, carbon dioxide, and hydrogen) are separated from the organic liquid in a separation tank; part of the gas is recycled to provide the hydrogen required for the reaction, while the rest is released as a fuel by-product. The liquid product is sent to the benzene tower, where benzene is recovered at the top of the tower as a by-product. The benzene tower bottom product is sent back to the toluene recovery tower. Due to the low one-pass conversion of toluene in the reactor, the liquid stream exiting the reactor contains a large amount of toluene; therefore, a larger toluene recovery tower and higher steam consumption are necessary. In the two-stage toluene distillation mode, the operating pressure in the first stage is higher than that in the second stage; compared with the single-stage mode, the steam consumption in the two-stage mode can be significantly reduced. The steam consumption can be further reduced by customizing the number of distillation stages. The overhead product from the high-pressure toluene distillation column can be used as energy for the reboiler of the low-pressure column. Through condensation in the low-pressure column reboiler, toluene combines with the overhead product of the low-pressure column and is recycled back to the methylation reactor. The bottom product of the low-pressure column, which contains a mixture of xylene and ethylbenzene, is sent to the crystallization unit. In the stream containing mixed xylene, 80%-90% is p-xylene, with small amounts of ethylbenzene also present. In the crystallization process, mixed xylene is cooled and then fed into the first crystallization stage, which consists of one or two series-connected crystallizers, depending mainly on the composition of the feed. A stream of crystallized slurry flows into the continuous centrifugation section, where 80%-90% of the p-xylene crystals are separated from the filtrate. After being exchanged with fresh feedstock, the first stage filtrate leaves this unit and is used as feedstock for the xylene isomerization unit. The first stage of crystallization is melted, and the resulting material is sent to a two-stage crystallizer. The crystal slurry resulting from this crystallization process enters a second stage of continuous centrifugation, where the crystals are separated from the liquid. The liquid filtrate contains a high concentration of p-xylene and is recycled back to the first stage. The crystal was washed with toluene during centrifugation, removed from the centrifuge, and melted. The p-xylene material then enters the final crystallization stage to produce high-purity p-xylene. 3. Separation process for p-xylene (1) UOP’s Parex process. The development of molecular sieve adsorbents with strong affinity for p-xylene and weak adsorption for other C8 aromatic isomers has made it possible to develop an adsorption process for recovering p-xylene from C8 aromatics. The Parex process was developed by UOP in the 1960s, and it enables the continuous adsorption of p-xylene from liquid-phase mixed C8 fractions. The company has sold technical licenses for multiple Parex units, and 58 such units are currently in operation around the world. This process is usually combined with an isomerization process to produce p-xylene in high yield. The raw material is C8 aromatics with a balanced composition. The C8 aromatics and mixed xylene stream from the bottom of the deheptanization section column are fed into the xylene separation column; xylene and lighter components are obtained from the top of the column, while C9+ aromatics are obtained from the bottom of the heavy fraction column and used as a raw material for gasoline. The material at the top of the tower is sent to the Parex unit. This device is a fixed-bed using molecular sieves. The separation of xylene is achieved through the selective adsorption of xylene by molecular sieves. It is a process similar to liquid chromatography. To recover p-xylene from molecular sieves, a liquid is required that has a stronger affinity for the molecular sieves than p-xylene does, in order to desorb p-xylene. Separation is carried out at 120–170°C under moderate pressure. The boiling point difference between the desorbent and p-xylene is large enough to allow them to be separated by distillation. The one-way recovery rate of p-xylene is 90%-97% (compared to only 60%-70% for the crystallization method). The adsorbent is usually ADS-27, a zeolite for the exchange of barium and potassium ions, which allows the main raw material components to enter its pore structure. The adsorption chamber of the Parex process uses a continuous fixed-bed adsorption technology with a simulated moving bed. This is achieved by moving the feed and desorbent inlet as well as the product outlet of the adsorption bed. Multiple feed lines are connected to a unique, patented distributor located within a distribution valve and an adsorption bed. 4 additional pipelines are connected to the valve, delivering 4 types of process fluids (namely mixed xylene feed, desorbent, raffinate, and extract) to the adsorber column and the distillation column (raffinate and extract). All 4 types of fluid flow are properly controlled to maintain a constant flow rate. All 4 types of fluid streams pass through a rotary valve, which directs the fluid streams at predetermined times to another pipeline inlet or outlet connected to the lower part of the bed. The switching between these 4 types of fluids takes place continuously in the same direction; at regular time intervals, the flow is transferred from one set of pipelines to another adjacent set, with the switching speed being adjusted to match the flow rate of those fluids. The entry and exit points move from one position to an adjacent one at synchronized intervals, as if the molecular sieve could slowly and continuously move through the adsorption bed via fixed entry and exit points, while taking in or releasing liquid. The liquid circulates from the bottom to the top of the adsorption tower through a pipeline independent of the rotary valve. The movement of the adsorption bed is a physical simulation achieved by rotating the rotating components of the distributor. The extract enters a distillation column to recover p-xylene, while the desorbent is discharged from the bottom of the column. p-Xylene from the extraction tower is purified in the refining tower by washing with recycled toluene. P-xylene products can be obtained from this tower. The raffinate is sent to a raffinate distillation tower, where ethylbenzene, m-xylene, and o-xylene are recovered from the top of the tower, while the desorbent is obtained from the bottom. Although the overhead product from the raffinate tower can be used as a raw material for blended gasoline, it is more commonly used as feed for the isomerization reactor in an integrated adsorption/isomerization unit. For most adsorption and extraction operations, it is necessary to use a reprocessing tower to maintain the quality of the desorbing agent. In this process, the desorbing agent (usually diethylbenzene) is sent to the reprocessing tower, where a portion of the heavier impurities is separated off in order to prevent their accumulation. Similar to IFP’s EluxyI process, UOP also offers some combined designs. The Hysorb XP process is used to purify mixed xylene, producing concentrated xylene material that serves as a raw material for crystallization units. The purity of xylene produced by UOP units is generally 99.9% (by mass). All new Parex units designed after 1987 are capable of producing p-xylene with a purity of 99.9%. A total of 73 patent licenses for Parex units were sold starting in 1971, of which 23 were sold after 1994. (2) IFP’s Eluxyl adsorption process. IFP has developed an adsorption process technology for the separation and purification of p-xylene using Eluxyl, and provides patent licenses for it. Eluxyl shares a similar concept to UOP’s Parex technology, but the equipment design is different. IFP has its own high-performance adsorbent (SPX 3000), which was used in the first industrial plant (S-Oil in South Korea) to produce products with a purity of up to 99.9%. This technology uses nearly 120 individual on-off valves, rather than a proprietary large rotary valve with multiple inlets and outlets as used by UOP. IFP states that the cost of a large number of small valves is lower than that of UOP’s single rotary valve, and these valves can be repaired during maintenance. Online maintenance was also successfully tested on the first set of industrialized units. IFP uses Raman spectroscopy to measure the concentration profile inside the tower. This innovative analysis method utilizes optical fibers to transmit spectra, enabling real-time and accurate reflection of the concentration profiles within the tower; combined with computer-based control of valve sequencing, it serves to optimize and regulate operations. IFP has also optimized the design of its internal components, reduced dead volume, and improved efficiency. Apart from the differences in valves, the IFPEluxyl process is estimated to be similar to UOP’s Parex process. A process that combines these two approaches – adsorption properties and crystallization technology – can be used to modify existing crystallization units. The Eluxyl unit is located upstream of the crystallization unit and produces p-xylene with 95% purity; this stream enters the single-stage crystallizer. This combined unit uses toluene as a solvent, has a smaller number of stages, and less amount of adsorbent; it employs two distillation columns (namely a stripping column and a redistillation column) instead of four columns. The reduced investment in combined units is mainly due to the lower purity of the p-xylene stream in the Eluxyl unit (i.e., 95% rather than 99%), as well as the less stringent specifications regarding the content of C9 aromatics in the feed. The material at the bottom of the extraction tower enters the crystallizer, while the filtrate from the crystallizer is recycled back to the adsorption tower. The raffinate is sent to the isomerization unit. Operation has improved significantly due to the high p-xylene content in the feed to the crystallization unit. The purity of p-xylene obtained from the crystallization unit can reach over 99.9%. From January 1995 to May 1996, a set of Eluxyl demonstration units was operated at Chevron’s Pascoagula refinery, producing 8,000–10,000 tons per year of xylene. The first set of industrial facilities was put into operation in December 1997 at the S-Oil refinery in Ulsan, South Korea, with a capacity of 500,000 tons per year. The second set of industrial units was put into operation in May 1998 at Chevron’s Pascaugaula refinery, with a capacity of 450,000 tons per year. Later on, IFP sold the Eluxyl technology license to Sinopec’s Zhenhai Refinery, with a capacity of 4,527 tons per year. IFP’s first and world’s largest unit is said to now have a capacity that exceeds its rated design capacity. Currently, the maximum design capacity per unit for EluxyI is 750,000 tons per year. Eight patent licenses for EluxyI units have been sold, and several of these units are already in operation. (3) Crystallization separation technology. The crystallization method for separating p-xylene is an older process that is still in use today; it relies on a combination of crystallization and centrifugation to separate p-xylene from its isomers. Until the development of the molecular sieve adsorption method, the crystallization method was the only way to produce p-xylene; combining stepwise crystallization with isomerization could significantly increase the yield of p-xylene compared to using a single crystallization unit. The yield of p-xylene using a combination of crystallization/isomerization can reach 80% of the starting material, whereas the yield of p-xylene using only crystallization is only 12% of that from reformation. Some processes, such as GTC’s GT-CrystPX, have seen a significant improvement in competitiveness due to the enhanced reliability of the equipment and the ability to increase its scale (which reduces the number of series and rotating components). A mixture of xylene and the recycled product from the isomerization section enters the heavy fraction column. If o-xylene needs to be recovered, m-xylene and p-xylene are taken out from the top of the tower, while the product at the bottom of the tower will contain o-xylene and C9+ aromatics. The product at the bottom of the tower enters the o-xylene reprocessing tower; the recovered o-xylene can be used as a product or fed back into the isomerization reactor. The bottom product of the o-xylene reprocessing tower, C9+ aromatics, is typically used as a blending component in high-octane gasoline. If o-xylene does not need to be recovered, o-xylene and the light components can be taken out from the top of the tower, while the bottom product containing C9+ aromatics is sent outside the boundary area as a by-product. In both cases, the top product from the recombination column is sent to the step crystallization section. The operating temperature for the first stage of crystallization is -62 to -67°C. The first-stage crystallizer is usually a tubular heat exchanger with scrapers on its surface, or a batch-type crystallizer. Inside the crystallizer, the spirally arranged blades scrape the p-xylene crystals off the walls. The crystals formed in the first stage are relatively small, and it is necessary to strictly control their particle size to ensure recovery in the centrifugation and filtration stages. The growth of crystals requires careful control of the relationship between time and temperature for specific raw materials. The first paragraph states that increasing the residence time at a relatively low quenching rate can promote crystal growth. Significant progress has been made in the development of efficient solid-liquid separation facilities. Most modern installations use continuous solid bowl centrifuges in the first stage. The two bowls rotate horizontally at different speeds, resulting in a spiral motion on the outer surface of the inner bowl. This spiral motion causes the solid to be removed from the sedimenting slurry tank through a discharge section, and what is discharged is an almost dry filter cake. Centrifuges can be equipped with backwashing, but whether it is beneficial for the separation of xylene remains to be studied. Other configurations either use heat exchangers with scrapers on their surface, or are equipped with scrapers on a central rod; the central rod provides mixing and helps maintain an optimal heat exchange surface. The residence time is about 3 hours, with cooling provided by ethylene. In the first stage of crystallization, it tends to grow into long and thin monoclinic needle-like crystals, which are difficult to remove. A considerable portion of the mother liquor remains trapped at the interfaces between the p-xylene crystals. By adjusting the centrifugation speed, the differences in the bowls, and the depth of the slurry tank, the purity of p-xylene obtained in the first stage can reach 85%. The filtrate from the first stage, from which p-xylene has been separated, is sent to the isomerization reactor to produce more p-xylene-rich feedstock for use as input to the crystallization unit. The crystallized product from the first stage is melted or partially remelted and recrystallized in a second crystallizer, at an operating temperature of 0°C, with propane being used for cooling. The crystals produced in the second stage are cylindrical in shape, with dimensions of 200X 360 μm. Additionally, since the temperature in the second stage is relatively high, the viscosity of the mother liquor is low (1 cP in the second stage compared to 5 cP in the first stage), which results in far fewer problems when discharging the crystals. The discharge section of the second crystallizer utilizes a propeller plate mechanism, which facilitates the discharge of material. Since the filtrate from the second stage is still rich in p-xylene, it is recycled as feed for the crystallizer in the first stage. The crystals from the second crystallizer are generally washed with a circulating toluene stream to melt them, and then separated from toluene in a toluene/p-xylene separation column. Toluene is recovered at the top of the separation tower and recycled to the washing process located in the second crystallizer. p-Xylenes are obtained from the bottom of the separation tower. Operating in this manner yields p-xylene with a purity of 99.5% or higher. The improved crystallization setup replaces the second centrifugation stage with a Niro scrubber, enabling the product purity to reach 99.9%. Chevron Phillips, BP (formerly Amoco and Arco), Krupp Wood, and Japan’s Maruzen have all developed crystallization processes and brought them into industrial use. Chevron technology is the most widely used technology in the world, but until the early 1990s and the wave of MSTDP construction during that decade, more than half of the p-xylene production capacity in the United States utilized Amoco’s technology. GTC Technology also provides licenses for the crystallization technology (GT-CrystPX) developed by Lyondell Chemical. (4) Isomerization process. The main industrial isomerization technologies include those of ExxonMobil, UOP, IFP, GTC/IPCL, and Engelhard. The processes of these competing technologies are very similar. In UOP technology, the liquid feed consists of C8 aromatics that reduce the contents of para- and ortho-xylene isomers in other processes. This feed is mixed with make-up and recycled hydrogen; after being heated through indirect heat exchange and a heating furnace, it passes through a fixed-bed catalytic reactor. The reactor effluent is cooled by heat exchange before entering the phase separator. The hydrogen-rich gas phase is recycled, with a portion of the gas being discharged to maintain the required minimum hydrogen concentration. The liquid from the separator is sent to the deheptane tower to separate the light hydrocarbons in the isomers, after which the isomers proceed to the zeolite tower. In the clay tower, trace amounts of dienes are polymerized to protect the p-xylene separation adsorbent (if used) and to bring o-xylene into compliance with the acid-washing color specifications. The Baitu Tower uses a switching system, with the offline unit being regenerated using steam. The isomer treated with white clay is circulated within the unit to form part of the mixed xylene feed for the xylene separation tower. The isomerization process uses a bifunctional catalyst (i.e., acid function and metal function), with an operating pressure of 2.2 MPa. Process logistics connected to **balance. Using UOP’s I-9 catalyst, ethylbenzene can be converted into xylene. UOP’s I-210 catalyst can reduce ring loss and has been in industrial use since April 1998. Some catalyst systems using IFP (such as Opairs) can also convert ethylbenzene into xylene. Aromatic production plants that attempt to maximize the production of p-xylene and o-xylene from a fixed amount of raw materials usually choose this catalyst system. ExxonMobil offers several types of isomerization processes. Shell’s high-active isomerization (MHA process) uses a shape-selective zeolite catalyst, ZSM-5, with typical reaction conditions of 420–425°C and 1.6 MPa. EM-4500 is ExxonMobil’s newly developed xylene isomerization catalyst, with the p-xylene content in the isomerized mixture being 102%–104% of the equilibrium level. Its advantages include a long operating time between catalyst regenerations, with examples showing over 4 years. Performance remains stable throughout the entire operation. The latest generation of heterogenization technology, XyMax, uses the EM-4500 catalyst. XyMax features the use of a proprietary highly selective catalyst, which enables higher yields, better operational flexibility, and greater potential for overcoming bottlenecks. Its ethylbenzene conversion rate can be increased to over 80%, with losses reduced by more than 50%. Other processes that can convert ethylbenzene dealkylation into benzene include the BPAMSAC process, GTC’s GT-IsomPx process, and UOP’s Isomar process using I-100 or I-300 catalysts. The advanced MHAI (AMHAI) process utilizes a unique dual-catalyst bed system, which enables the optimization of the ethylbenzene conversion, non-aromatic hydrocarbon cracking, and isomerization processes. This catalyst system is often used when benzene is needed. Another process operates under low pressure and in a hydrogen-free cycle; it is known as Mobil’s Low-Pressure Xylene Isomerization (MLPl) process, and the catalyst used in this process is also based on ZSM-5 zeolite. ()