June 2022

Special Focus: Process Optimization

Xylene-loop scheme for minimized GHG emissions and PX production cost

Molecular management, energy efficiency and carbon dioxide (CO₂) emissions have become major focuses for petrochemical complexes, including aromatics plants.

Molecular management, energy efficiency and carbon dioxide (CO₂) emissions have become major focuses for petrochemical complexes, including aromatics plants. The xylene loop accounts for about two-thirds of the energy consumption and associated emissions in a modern reformate-to-paraxylene (PX) facility. In the xylene loop, xylene vapor-phase isomerization (VPI) units carry out two chemical reactions: the isomerization of para-depleted xylenes to equilibrium xylenes, and the removal of ethylbenzene (EB) by chemical conversion. EB can either be isomerized to xylenes (the EB-reforming VPI process) or dealkylated to benzene (the EB-dealkylation VPI process). With reformate C₈ aromatic cuts containing up to 18% EB, converting EB to benzene or xylene has a significant impact on the total PX vs. benzene production.

With an unprecedented wave of large steam crackers going onstream in China, Korea, the U.S. Gulf Coast and other parts of the world, pygas benzene is expected to be widely available in these regions. The abundance of benzene supply, combined with potential bans on some benzene derivatives (e.g., bisphenol A in Europe), may drive operators of existing and future aromatics complexes to maximize their PX output while minimizing benzene.

In xylene liquid-phase isomerization (LPI), the only reaction taking place is the isomerization of para-depleted xylenes to equilibrium xylenes. Except in specific situations, LPI is normally deployed in conjunction with VPI to manage EB removal from the xylene loop. LPI operates at a significantly lower temperature than VPI and yields hardly any C_7- aromatics and C₉₊ aromatics—therefore, the LPI effluent can be directed to the xylene column without the C_7- aromatics fractionation.

A configuration including LPI with an EB-reforming VPI process in an aromatics complex significantly reduces overall product losses, energy consumption and associated greenhouse gas (GHG) emissions, thereby affording the lowest PX production cost. In an existing facility, the addition of an LPI unit also opens the possibility of revamping an EB-dealkylation process into an EB-reforming process. Case studies showing the synergies between LPI and EB-reforming VPI, including the addition of LPI to an existing facility and grassroots facility combining both technologies, are reported here and discussed.

Xylene loop: Two paths for EB removal

Aromatics producers usually define the xylene loop as the circuit that includes the xylene column, the PX separation process, the xylene isomerization process (the effluent of which returns to the xylene column) and the orthoxylene column, where applicable. In facilities converting reformate to PX, the xylene loop accounts for approximately two-thirds of the capital and operating expenses. Consequently, opportunities to reduce PX production costs and utilities consumption primarily rest with the xylene loop design and operation. The C₈ aromatic cuts processed in PX complexes essentially contain three xylene isomers (orthoxylene, metaxylene and PX) and EB. Ironically, PX—the most desired of the xylene isomers—is also the least abundant at equilibrium. Therefore, PX is continuously separated from C₈ aromatic isomers, and, following this separation step, the para-depleted effluent is re-isomerized to equilibrium xylenes and recirculated in a large loop for further PX separation until all xylene isomers are eventually converted to PX. The fourth C₈ aromatic isomer (EB) cannot be fractionated out, since its boiling point is too close to that of xylene isomers—yet it must be eliminated because it takes up space in the xylene loop and reduces the PX production capacity of any facility.

In VPI units where para-depleted xylenes are re-isomerized to equilibrium xylenes, EB is simultaneously removed via a chemical route. Since two options exist for EB removal, two processes are commercially available:

• An EB-reforming VPI process, where ethylbenzene is converted to xylene
• An EB-dealkylation VPI process, where ethylbenzene is converted to benzene.

While aromatics facilities are usually erected to produce PX, benzene is an important byproduct. Naphtha reformate C₈ cuts may contain up to 18% EB.^1,2 EB content in C₈ aromatic streams vary widely depending on the source, with transalkylate C₈ cuts containing as little as 1% EB,² and pygas C₈ cuts containing more than 50% EB.¹ In a typical modern plant, reformate represents about half of the xylene-loop C₈ aromatic feed, while transalkylate represents the other half, meaning that the feed EB content is approximately 8%–9%. Therefore, the decision to convert EB to xylene or benzene has a significant impact on the overall feed consumption/PX production capacity of the plant.

FIG. 1A shows the reaction pathway for EB conversion to xylene, and FIG. 1B shows the reaction pathway for EB conversion to benzene. The former reaction is an equilibrium reaction under thermodynamic control—hence, conversion per pass is limited. The latter is an irreversible reaction under kinetic control; in this case, conversion per pass can be adjusted with parameters such as temperature, residence time and hydrogen partial pressure.²

State-of-the-art, optimized EB-reforming and EB-dealkylation VPI processes have made substantial progress in minimizing product loss per pass. However, due to the thermodynamically vs. kinetically controlled reaction pathways, the achieved EB conversion per pass is nearly twice as high with EB-dealkylation VPI than with EB-reforming VPI, resulting in higher energy consumption at equivalent EB conversion for the latter process. Conversely, at equivalent PX production, feed consumption will be substantially higher with EB-dealkylation VPI, since EB is converted to benzene instead of xylene. Furthermore, with EB-dealkylation VPI, two out of eight carbons in the EB molecule are converted to light gas, which significantly affects overall plant economics when C₈ aromatic feeds are EB rich.

LPI: Low investment for low product loss and low energy consumption

LPI has been previously discussed in literature.^3,4 This simple and inexpensive process affords lower energy consumption vs. VPI processes—not only because of the absence of feed vaporization and associated hardware, but also because of the lack of side reactions, meaning little to no byproduct generation and a considerably simplified effluent fractionation scheme. Marginal side reactions also mean near undetectable product loss per pass, which is another significant advantage compared to VPI processes. Since the LPI process does not remove EB, it is usually deployed in conjunction with a VPI process to prevent EB accumulation in the xylene loop. The higher the percentage of PX separation raffinate that is processed through LPI vs. VPI, the higher the energy savings and product loss minimization. Conversely, the higher the percentage of PX separation raffinate that is processed through LPI vs. VPI, the lower the overall EB removal. Therefore, the split between LPI-processed and VPI-processed raffinate is the result of a xylene-loop optimization effort.

The two commercial case studies detailed in this article demonstrate that PX production can be maximized via EB-reforming VPI, while both energy consumption and product loss are minimized by the addition of an LPI process. In other words, a scheme comprising EB-reforming VPI and LPI processes effectively leads to the lowest PX production cost and the lowest GHG emissions for a xylene loop.

Case Study 1: LPI process addition to an existing xylene loop operating an EB-reforming VPI process

The flow scheme of this commercial facility is depicted in FIG. 2. The plant has opted for EB-reforming VPI to maximize PX production. A continuous catalytic reforming process and a transalkylation process are feeding two xylene loops operated in parallel and comprising a fractionation section including a xylene column and a xylene splitter, an adsorption unit and an EB-reforming VPI unit. The two xylene splitter bottom streams feed an orthoxylene column. The proposed LPI process addition includes two units (shown in green)—namely, one per each xylene loop.

The added LPI heating duty—and marginally increased loop traffic because of the slightly higher EB content—are more than compensated for by the reduction in:

• VPI traffic and associated duty
• VPI effluent fractionation duty
• The xylene column duty.

Lower traffic in the VPI process also results in significantly lower xylene losses. Overall energy credits and material balance credits are summarized in TABLE 1. In the base case, prior to adding the LPI process, the complex operates with EB-reforming VPI only at design capacity. When LPI is added, the EB-reforming VPI process is set at turndown capacity to maximize the benefits of LPI implementation, which sets the size and feed processing capacity of the LPI unit. At constant PX and orthoxylene production, the xylene-loop feed consumption is cut by ~3%. Another way to substantiate this improvement would be to increase PX production by ~3% at constant feed consumption. Further, and still at constant PX production, electricity consumption decreases by 6% and fuel gas consumption decreases by 2%. The overall PX production cost is reduced by 4% on a $/t basis, and GHG emissions are decreased by 11% on a 1 t CO₂ equivalent/t PX basis. It is worth pointing out that, in an existing facility such as in Case Study 1, the low capital expenditure investment for the LPI addition can be further reduced by revamping onsite equipment, such as converting a clay treater into an LPI reactor or by reusing the previous heat exchanger and/or feed heater, if available. Another important fact is that, at constant PX production, the benefits (such as reduced energy consumption in the reformer and reduced reformate splitter duty) extend well beyond the xylene-loop improvements listed in TABLE 1.

Case Study 2: Process flow scheme integrating an EB-reforming VPI process and LPI process in the design of a grassroots aromatics complex

The flow scheme of this planned facility is depicted in FIG. 3. One of the main objectives for the proposed plant was to minimize benzene production—therefore, EB-reforming VPI was suggested to maximize PX. Reformate and transalkylate feed a single xylene loop comprising a xylene column, an adsorption unit and an EB-reforming VPI unit. The proposed LPI process addition includes a single unit (shown in green) operating in parallel with the EB-reforming VPI unit.

Overall energy credits and material balance credits are summarized in TABLE 2. Energy credits are substantially higher in Case Study 2 (LPI process integration in a grassroots complex) vs. Case Study 1 (LPI process addition to an existing complex). In Case Study 1, the base case heat integration scheme is optimized, while heat integration optimization is limited following the addition of LPI because the constraints associated with existing units cannot be changed.

In Case Study 2, both the base case and the case integrating the LPI process have their equipment size and heat integration scheme optimized—hence, the higher energy credits for the flow scheme including LPI. Feed consumption credits are similar for Case Study 1 and Case Study 2, and they correspond to the expected feed savings associated with the addition of an LPI unit. As pointed out in Case Study 1, feed consumption reduction benefits could also be represented in terms of a production increase at constant feed consumption. However, in an evaluation at constant PX production, feed consumption is cut by 4%, while electricity consumption decreases by 14% and fuel gas consumption decreases by 10%. The overall PX production cost is reduced by 4% on a $/t basis—an outcome like Case Study 1, despite slightly higher feed savings and significantly higher utilities savings. This is because Case Study 2 was realized in a region of the world where feed and energy costs are substantially lower than in the location of Case Study 1. Conversely, GHG emissions were reduced by more than 30% on a 1 t CO₂ equivalent/t PX basis, owing to a very energy-efficient design, along with substantial reductions in both high-pressure and medium-pressure steam consumption, which are not reported in TABLE 2.

Since Case Study 2 was evaluating a grassroots facility, a third case was assessed, combining EB-dealkylation VPI with LPI. As expected, feed consumption at constant PX production increases proportionally to EB content in the fresh C₈ aromatics feed, since EB is converted to benzene instead of to PX in such a scenario. Consequently, of all the possible options, the scheme integrating EB-reforming VPI and LPI processes offers the lowest PX production cost and the lowest GHG emissions/t of PX produced.

Benzene or PX?

Benzene and PX prices have been close in recent years,⁵ to the point where converting EB to benzene rather than to PX constituted no economic hurdle for most aromatics facilities. This situation could change, however, due to a wave of planned steam cracking facilities—a wave unprecedented by both the number and size of these projects, which includes plants in China, Korea and on the U.S. Gulf Coast.^6,7,8

Even if pygas benzene production depends on steam cracker feed (i.e., ethane crackers will produce considerably less pygas than naphtha crackers), benzene is still expected to be widely abundant because of the number and size of naphtha cracking facilities. Furthermore, reformers shifting to aromatics mode because of the decreasing gasoline demand are expected to process more benzene precursors, thus delivering more benzene to the market. While benzene becomes increasingly available, the future of some benzene derivatives is presently being reevaluated. For example, a potential ban on polystyrene is under consideration in multiple locations in the U.S.^9,10 Simultaneously, Europe is assessing more restrictive regulations impacting bisphenol A,¹¹ which may also include future bans. In a world combining increased benzene availability and potentially lower interest in benzene derivatives, converting EB to xylene rather than to benzene—using a combination of EB-reforming VPI and LPI processes—would represent an environmentally friendly and cost-competitive solution for aromatics complexes.

EB-reforming VPI process combined with the LPI process: Additional considerations

The following are additional items for consideration:

• No PX recovery limitation for existing complexes when adding the LPI process: The addition of a technology (such as LPI) that is low cost, and with high energy savings and reduced GHG emissions, is often very attractive to PX producers who are endeavoring to minimize their costs and reduce their environmental footprints. However, there are occasional concerns associated with increased EB traffic in the xylene loop and potentially associated capacity reduction for the PX adsorption process. Experience shows that complexes using the latest molecular sieve technologies have more PX recovery capacity than they can utilize, and that PX capacity limit is a non-issue when adding an LPI unit. The slight increase in EB traffic does not affect the overall PX production capacity, even with the oldest adsorption processes now in operation.
• LPI effluent circulation around the recovery unit and associated additional savings: When an LPI unit is added to an existing xylene loop, part of the LPI effluent can be routed directly to the inlet of the PX separation unit (bypassing the xylene column and generating additional energy credits), so long as feed specifications to the separation process are met.
• EB-dealkylation VPI process retrofit with an EB-reforming VPI process: Since EB-dealkylation VPI processes operate at a substantially higher weight hourly space velocity (WHSV) than EB-reforming VPI processes, it is unusual to retrofit the former process into an EB-reforming VPI unit. While other process parameters (such as temperature and pressure) would generally be compatible, loading an EB-dealkylation VPI reactor with an EB-reforming VPI catalyst would decrease the overall isomerization capacity of the xylene loop under consideration. This is because there would not be enough room in the EB-dealkylation VPI reactor to fit the required EB-reforming VPI catalyst quantity. However, if such a retrofit is carried out in conjunction with the addition of an LPI process, then the xylene loop can be shifted from converting EB to benzene to converting EB to xylene (and, further, to PX) without incurring feed processing capacity loss and with substantial energy credits gain. Similarly, VPI units that were initially designed to operate an EB-reforming process and that were later debottlenecked via conversion to EB dealkylation, can be converted back to EB reforming at no feed processing capacity loss with the addition of an LPI process to the xylene loop.

Xylene loop for the lowest PX production cost and the lowest GHG emissions

Since feedstock price is by far the most important operating cost parameter in aromatics complex economics,¹² EB-reforming VPI—which converts EB to PX—offers significant advantages to sites predominantly targeting PX product. Adding an LPI process further decreases feed consumption by reducing product losses, while substantially reducing energy consumption and associated GHG emissions. For petrochemical facilities maximizing PX output, a xylene loop combining two specific petrochemical processes^a,b is the most efficient and environmentally friendly solution for PX production. HP

NOTES

^a  Axens’ Oparis^® process

^b  ExxonMobil’s Liquid Phase Isomerization process

LITERATURE CITED

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2. Zhu, F., J. A. Johnson, D. W. Ablin and G. A. Ernst, Efficient Petrochemical Processes: Technology, Design and Operation, Wiley, New York City, New York, October 2019.
3. Gonçalves, J. C. and A. E. Rodrigues, “Simulated moving bed reactor for p-xylene production: Dual-bed column,” Chemical Engineering and Processing, June 2016.
4. Mhatre, S., V. Warke, A. Choi, M. Molinier and A. Saple, “A new liquid-phase isomerization process for xylene loop debottleneck and energy savings,” Hydrocarbon Processing, January 2020.
5. Statista, “Price of p-xylene worldwide from 2017–2021,” online: https://www.statista.com/statistics/1259883/price-p-xylene-globally/
6. Argus, “Viewpoint: Asian ethylene sector braces for new supply,” January 5, 2021, online: https://www.argusmedia.com/en/news/2174016-viewpoint-asian-ethylene-sector-braces-for-new-supply
7. Volkova, M., “LG Chem to start up new steam cracker in Yeosu,” MRC, June 7, 2021, online: https://www.mrchub.com/news/388673-lg-chem-to-start-up-new-steam-cracker-in-yeosu
8. Stratas Advisors, “North America: Upcoming cracker projects and capacity outlook,” April 11, 2019, online: https://stratasadvisors.com/Insights/2019/041119-Downstream-North-America-Upcoming-Cracker-Projects-and-Capacity-Outlook
9. OregonCoast.com, “Polystyrene ban,” April 13, 2020, online: https://www.oregoncoast.org/industry/news/polystyrene-ban/
10. Waste360.com, “Washington state bans polystyrene foam, limits single-use plastic at restaurants,” May 18, 2021, online: https://www.waste360.com/plastics/washington-state-bans-polystyrene-foam-limits-single-use-plastic-restaurants
11. Mo, C., “Bisphenol A (BPA) regulations in the European Union: An overview,” August 25, 2020, online: https://www.compliancegate.com/bisphenol-a-regulations-european-union/
12. Omran, H. R., S. M. El-Marsafy, F. H. Ashour and E. F. Abadir, “Economic evaluation of aromatics production, a case study for financial model application in petrochemical projects,” Egyptian Journal of Petroleum, 2017.

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