October 2021

Special Focus: Plant Safety and Environment

Enhance aromatics production with concurrent reduction of environmental footprint—Part 1

Catalytic reforming processes produce olefin contaminants in aromatics streams via paraffin dehydrogenation side reactions.

Catalytic reforming processes produce olefin contaminants in aromatics streams via paraffin dehydrogenation side reactions. Operating catalytic reformers at higher severity results in higher yields of valuable aromatics, but it also results in higher olefins content in the reformate. Olefins must be removed from C6–C7 aromatics streams to meet benzene product specifications, and from C8+ aromatics streams to meet specifications for paraxylene recovery processes.

Historically, olefins have been removed from aromatic streams in treaters using activated clay to promote the acid alkylation of olefins with aromatic molecules, thereby producing heavier compounds further separated by fractionation. While the process is very efficient, it inherently consumes valuable aromatics to remove undesirable olefins. The higher the olefins content, the higher the product loss. Given the current scale of paraxylene facilities, valuable product losses, even in small percentages, can have a massive impact on plant economics.

Selective hydrogenation converts olefins to paraffins and alkylaromatics via a reaction pathway that does not consume aromatics. As a result, aromatics yields are substantially increased, while the use of clay, which may account for up to 80% of the solid waste in a paraxylene complex, is dramatically reduced. This article presents key features and performances for the selective hydrogenation process, and options for its implementation in a plant block flow diagram. It also describes the benefits of selective hydrogenation addition to an existing facility. The benefits of selective hydrogenation integration in a grassroots facility will be addressed in a separate article.

Process background

The recent paraxylene capacity increase in China is adding pressure on existing aromatics complexes to improve overall efficiency to remain competitive. The very large scale of the plants coming onstream1,2 means that even small losses in terms of percentage can have a huge economic impact on these new complexes. Optimized heat integration, reduced utilities consumption, improved selectivities and reduced investments are common themes for all processes at sites currently in operation, as well as for new aromatics facilities.

Since the adsorption process has grown into the industry workhorse for paraxylene separation, the removal of olefins contaminants from C8+ aromatics streams has become a critical step, because even small olefins concentrations can adversely impact the recovery unit’s capacity. Furthermore, product benzene has a very low specification on acid wash color, which is a measure of unsaturated impurities;3 therefore, removal of olefins from C6 aromatics streams is also a necessary step.

Reformed naphtha (or reformate) is the primary feed source for aromatic complexes.4 In the naphtha reforming process, side dehydrogenation reactions of paraffins or paraffinic substituents, as shown in FIG. 1, can take place in addition to the desired naphthene dehydrogenation and paraffin dehydrocyclization reactions. Furthermore, the transition from high-pressure semi-regenerative reforming to low-pressure continuous catalytic reforming has resulted in a substantial increase of unsaturated species in reformate-derived streams.5 The concentration of these contaminants is a function of reformer feed composition, the severity of the reforming operation and the catalyst used in the reforming process, among other elements.

FIG. 1. Examples of dehydrogenation reactions producing olefins in a catalytic reforming unit. 

Loss of aromatic product to remove olefinic contaminants

The bromine index (BI) is an indirect measure of olefins content in an aromatic stream.6 For more than 50 yr, low BI has been achieved on C6–C7 aromatics and on heavy reformate by processing these streams through clay treaters. Two clay treaters per stream, operated in series or in parallel, are normally present. The acid sites of activated clays catalyze the alkylation of olefins with aromatic molecules. The heavy compounds resulting from this alkylation are then fractionated out, and the overall process is very efficient for olefins removal. Furthermore, significant improvements have been accomplished over the years, with increasingly active acidified clays and even zeolite-based catalysts proposed for clay replacement.7

Despite treater technology advances, the fundamental olefins alkylation chemistry remains the same and is illustrated in FIG. 2. Namely, one aromatic molecule is consumed per alkylated olefin molecule. In other words, some of the valuable aromatic product obtained at high cost via catalytic reforming is lost for the purpose of removing olefin contaminants. Moreover, reforming units in aromatic mode (i.e. maximizing aromatics output) usually operate at higher severity, which subsequently yields higher olefins content in the product. As producers increase operating severity to produce more aromatics, they also consume more aromatics to remove olefins.

FIG. 2. Examples of reaction pathways of olefins alkylation with aromatics in clay treaters. 

Two main clay treating processes are present in typical aromatic complexes, one on the benzene-toluene extract stream and one on the heavy reformate stream. BI is usually lower in the benzene-toluene stream because most olefins in C6–C7 aromatics are removed through the extraction process prior to the separation of benzene from toluene. This is especially true when N-formylmorpholine is the solvent used in the extractive distillation process. The examples presented later in this article focus on BI removal from heavy reformate, meaning C8+ aromatic streams.

Selective hydrogenation to avoid aromatic losses

Selective hydrogenation applied to olefins removal from aromatic streams is a simple, low-temperature process operated in an inexpensive vessel using a hydrogen (H2) source—typically reformer H2—and a catalyst. Depending on the unit location in a complex scheme, additional hardware may be required. The reaction pathways involved in selective hydrogenation are illustrated in FIG. 3. Two major benefits can be derived from a molecule management perspective:

FIG. 3. Examples of selective hydrogenation reaction pathways for olefins removal. 
  1. Reaction pathways do not consume reformate aromatics; therefore, valuable product is no longer destroyed in the process of removing olefins.
  2. When the olefin to be removed is an alkenyl aromatic, then hydrogenation produces additional valuable aromatics (e.g., the ethylbenzene in FIG. 3 will be converted to benzene or xylene in a xylene loop). Based on heavy reformate compositions provided by aromatic sites for feasibility studies, alkenyl aromatics can account for more than 95% of the olefins to be removed in a C8+ aromatics stream. This means that the benefit of selective hydrogenation vs. the clay treating process is essentially doubled by alkenyl aromatics conversion to high-value products.

In addition to the aforementioned aromatics production benefits, selective hydrogenation brings numerous advantages:

  1. Solid waste reduction: Clay treaters produce up to 80% of an aromatics complex’s total solid waste. The massive environmental footprint of clay waste is illustrated in FIG. 4, showing solids consumption as a function of time in aromatics facilities. With selective hydrogenation, clay is essentially eliminated, meaning not only a huge reduction in solid waste generation but also significant savings in disposal costs.
    FIG. 4. Cumulated solids consumption in an aromatics complex. 
  2. Operating efficiency: Clay treaters are designed for a target cycle length at the start of aromatics production. With repeated complex debottlenecks via increased feed processing capacity and/or increased reformer severity, it is not unusual for treater cycle length to rapidly decline, forcing operators to unload/reload every 3 mos or even more frequently, with the following consequences:
    a. Clay start-of-cycle unavoidably produces high xylene losses,8 for a period of 1 wk–2 wk, due to high initial clay activity. Xylenes are lost through                transalkylation reactions involving two C8 aromatics that produce a toluene and a C9 aromatic. At sites replacing clay every 3 mos, this means 1      wk–    2 wk every 12 wk at high xylene losses, with a severe economic impact:
        i. In complexes operating a transalkylation process, the impact is essentially energetic (fractionation, recirculation) because toluene and C9                           aromatics will be recombined in the transalkylation unit to produce xylenes.
        ii. In complexes that do not operate a transalkylation process, lost xylenes are irreversibly downgraded since toluene and C9 aromatics are                            exported to the gasoline pool.
    b. Frequent clay change-outs constitute a significant operating (recurrent unloading and reloading events with potential safety concerns) and logistics            (waste handling and disposal) burden, which is eliminated when clay cycles are exponentially extended.
    c. Short clay cycles resulting from complex debottlenecks make it difficult to maintain the low feed BI specification for the adsorption unit. The                      consequence is the potential permanent loss of a fraction of site recovery capacity during high feed BI excursions if clay cannot be replaced fast                enough. With an upstream selective hydrogenation unit, this threat no longer exists.
    d. The transalkylation reactions catalyzed by initial high clay activity, as mentioned above, produce benzene when the C8 aromatic involved in the reaction      is ethylbenzene. Adsorption units have tight benzene specifications, and each clay start-of-cycle constitutes a potential exposure to off-specification            feed for the adsorption unit.
    e. Selective hydrogenation upstream from an extraction process reduces extraction solvent consumption, as paraffins are easier to separate from                  aromatics than olefins.
    f. Selective hydrogenation reduces traffic in the heavy aromatics column bottoms, yielding energy savings.
    g. Pressure drop excursion events can occur in clay treaters and potentially disrupt production when heavy reformate dienes content is high, because            dienes oligomerize over acidic catalysts,9 leading to rapid coke deposition. Selective hydrogenation is very efficient at converting dienes, thereby                eliminating this risk.

Selective hydrogenation process in the aromatics complex

The typical location of the two main clay treating processes in a modern aromatics complex is shown in yellow in FIG. 5. As previously explained, the benzene/toluene extract stream is clay treated to meet benzene product specifications, while the heavy reformate stream is clay treated to achieve separation process specifications on the xylene column overhead stream. In both cases, the heavy compounds resulting from olefins alkylation with aromatics are fractionated out via the toluene column bottoms in the case of the extract stream and via the xylene column bottoms in the case of the heavy reformate stream.

FIG. 5. Options for selective hydrogenation unit location in an aromatics complex. 

Using green dots, FIG. 5 shows potential locations for a selective hydrogenation process in a modern aromatics complex:

  1. Reformer effluent upstream from the reformate splitter.
    In this configuration, the selective hydrogenation process converts olefins for both C6–C7 aromatics and C8+ aromatics streams. This arrangement results in the maximum conservation of valuable aromatic compounds and reduces energy consumption in the extraction unit, as well as in the heavy aromatics column. This arrangement also corresponds to the largest possible feed quantity to be processed, which has implications with respect to the size of the selective hydrogenation unit.
  2. Reformate splitter bottoms. This configuration offers the full benefit of valuable aromatics preservation for the heavy reformate stream, converts alkenyl aromatics to additional valuable aromatics, and significantly reduces the feed quantity, and therefore the size, of the selective hydrogenation unit. As mentioned earlier, the extraction process removes most of the olefinic co-boilers of benzene and toluene; therefore, maintaining low BI on light reformate is usually less challenging.
  3. Reformate splitter overhead. Due to the higher solubility difference in extraction solvent between paraffins and aromatics vs. olefins and aromatics, paraffins separation from aromatics is easier in an extraction unit. Therefore, a selective hydrogenation process on the reformate splitter overhead results in lower energy and solvent consumption in the extraction process. The non-aromatic raffinate disposition scheme10 is also essential. Depending on whether raffinate is fed to a steam cracker or to the refinery gasoline pool, it may or may not be advantageous to hydrogenate olefins prior to extraction.
  4. Benzene/toluene extract.
    In this stream, BI removal is typically less challenging because most olefinic species would have been removed by the extraction process. However, specific site operating conditions or feed constraints may favor the addition of a selective hydrogenation process in this service.

Example 1

The first example considers a 700,000-tpy paraxylene complex producing benzene and exporting toluene to the gasoline pool. In FIG. 6, a commercial case for the potential addition of a selective hydrogenation unit is considered. The heavy reformate stream exhibits a BI of 2,100, which corresponds to approximately 1.05% olefins content.11 Selective hydrogenation brings the following benefits:

FIG. 6. Example 1: Complex producing parayxlene and benzene and exporting toluene to the gasoline pool. 
  • C8 aromatics are no longer consumed to alkylate olefins in the heavy reformate stream. Xylenes are converted to paraxylene instead of being downgraded to motor gasoline. Similarly, ethylbenzene is converted to benzene instead of being downgraded to motor gasoline.
  • Styrene (a fraction of the olefins to be removed) is hydrogenated to ethylbenzene and then converted to benzene instead of being downgraded to motor gasoline.

Economics are a function of the values associated with each stream at the site, the C8+ aromatics stream specific composition and other proprietary information. Considering solely material balance improvement and disregarding all other benefits listed earlier, the complex achieves an approximate credit of $4 MM/yr on higher-value products by implementing a selective hydrogenation process on the heavy reformate stream.

Example 2

The second example considers a 700,000-tpy paraxylene complex producing benzene and maximizing aromatics production with transalkylation of toluene and C9+ aromatics.

In FIG. 7, another commercial case for potential addition of a selective hydrogenation unit is considered. The heavy reformate stream exhibits a BI of 2,600, which corresponds to approximately 1.3% olefins content.11 Selective hydrogenation brings several benefits:

FIG. 7. Example 2: Complex producing parayxlene and benzene and maximizing aromatics production with a transalkylation process. 
  • C8 aromatics are no longer consumed to alkylate olefins in the heavy reformate stream. Xylenes are converted to paraxylene instead of being downgraded to fuel oil. Similarly, ethylbenzene is converted to benzene instead of being downgraded to fuel oil.
  • C9+ aromatics are no longer consumed to alkylate olefins. C9+ aromatics are routed to transalkylation and converted to additional benzene and paraxylene.
  • Styrene (a fraction of the olefins to be removed) is hydrogenated to ethylbenzene and then converted to benzene instead of being downgraded to fuel oil.
  • Alkenyl aromatics are hydrogenated to alkylaromatics and routed to transalkylation for conversion to additional benzene and paraxylene.

Once again, economics are heavily dependent on values associated with each stream at the site, the C8+ aromatics stream specific composition and other proprietary information. Considering only material balance improvement and disregarding additional benefits listed earlier, the complex achieves an approximate credit of $9 MM/yr on higher-value products by implementing a selective hydrogenation process on the heavy reformate stream.

Selective hydrogenation performance: Selectivity is key

High conversion is usually achievable at the appropriate set of conditions with a hydrogenation catalyst. With maximization of aromatics production being the primary incentive for commercial deployment, selectivity is the key parameter for selective hydrogenation to make economic sense.

In streams containing mostly aromatics, even minor hydrogenation side reactions leading to partial loss of aromatics are a huge obstacle. In the two examples listed, where selective hydrogenation respectively yields a credit of $4 MM/yr and $9 MM/yr from a material balance perspective, a few tenths of a percent of non-selective hydrogenation would suffice to erase all credits.

It is essential to hydrogenate olefins selectively—i.e., without loss of aromaticity. Since selectivity is first and foremost a function of the catalyst used in the process, adopting a catalyst with state-of-the-art selectivity is of paramount importance.

Recommendations

This assessment shows that the addition of an inexpensive process with pristine selectivity performance can result in substantial production credits and environmental footprint reduction that will last for the life of an aromatics complex. While no additional capital spending is often the easiest path in an existing facility, making no change is sometimes more costly than investing in incremental process improvement. Disregarding losses that are not monitored can be an expensive proposition in a highly competitive environment.

Other than the potential uses described here, selective hydrogenation has many applications in petrochemical complexes, including potential applications in aromatics facilities. One such application involves olefins removal from the effluent of a toluene methylation unit, which saves the cost of a fractionation step.12

As initially pointed out, product losses of a few percent can have a huge impact on the economics of mega-scale facilities, which is why selective hydrogenation is increasingly integrated into the design of grassroots complexes. The authors aim to address this subject in a separate article. HP

LITERATURE CITED 

  1. “Crude oil to p-xylene,” HengLi Refinery PX Complex, PEP Report 303, IHS Markit, December 2018. 
  2. “Crude oil to p-xylene,” Zhejiang Refinery PX Complex (Phase 1), PEP Report 303A, IHS Markit, October 2019. 
  3. Durham, G. R. and R. S. Hebert, Patent US 5004851, “Process to produce aromatics of low acid-wash color,” 1991. 
  4. Gonçalves da Silva, J. C., “Hybrid separations and adsorption/reaction processes: The case of isomerization/separation of xylenes,” PhD Thesis, University of Porto, Portugal, 2015. 
  5. “Aluminum silicates—Advances in research and application,” Q. A. Acton, Ed., in Ch. 3: “Zeolites,” 2013. 
  6. Lo, F. Y., D. L. Stern, R. J. Cimini and J. L. Propp, Patent US 7214840, “Reduction of the bromine index of linear alkylbenzenes,” 2007. 
  7. Reddy, J. K., S. Lad, K. Mantri, J. Das, G. Raman and R. V. Jasra, “Zeolite-based catalysts for the removal of trace olefins from aromatic stream,” Applied Petrochemical Research, Vol. 10, Iss. 3, 2020. 
  8. Brown, S. H., J. R. Waldecker and K. Lourvanij, Patent US 7815793, “Process for reducing bromine index of hydrocarbon feedstock,” 2010. 
  9.  Alzaid, A. H., “Impact of conjugated olefins on Ni-Mo-S/γ-Al2O3 catalyst deactivation and fouling of naphtha hydrotreaters,” PhD Thesis, The University of British Columbia, Vancouver, Canada, 2016. 
  10. Molinier, M., J. S. Abichandani, J. L. Andrews, T. P. Bender, R. G. Tinger, D. J. Stanley and G. J. Wagner, Patent US 9302953, “Process for the production of xylenes,” 2016. 
  11. Brown, S. H., T. E. Helton and A. P. Wener, Patent US 6781023, “Decreasing Br-reactive contaminants in aromatic streams,” 2004. 
  12. Chen, T. J., J. D. Ou, J. S. Abichandani and G. A. Heeter, Patent US 9416072, “Selective hydrogenation of styrene to ethylbenzene,” 2016.  

The Authors

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