March 2022

Special Focus: Petrochemical Technology

Enhance aromatics production with concurrent reduction of environmental footprint—Part 2

Part 1 of this article (October 2021) discussed how catalytic reforming processes produce olefin contaminants in aromatics streams via paraffin dehydrogenation side reactions.

Part 1 of this article (October 2021) discussed how 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.

That article presented key features and performances for the olefins selective hydrogenation process, and options for its implementation in an aromatics plant block flow diagram. It also described the benefits of selective hydrogenation addition to an existing facility. The benefits of selective hydrogenation integration in a grassroots facility will be addressed here.

Olefins selective hydrogenation integration in the aromatics block of a crude-to-chemicals complex

The most significant benefits achieved by adding an olefins selective hydrogenation process to an existing aromatics complex have been recently described1 and can be summarized as:

  • End the downgrade of valuable aromatic molecules to fuel oil in the process of removing olefins
  • Yield additional valuable aromatic molecules by hydrogenation of alkenyl aromatics
  • Lower the environmental footprint of aromatic plants by considerably reducing solid waste production.

Furthermore, process selectivity—that is, olefins hydrogenation without loss of aromatic rings—is key to additional aromatics production.

Material balance improvements are incremental; they can be very substantial for smaller existing sites, but they become of immense proportion when olefins selective hydrogenation is part of the aromatics section design in a new crude-to-chemicals (CTC) complex, due to the large production capacity of these facilities. Similarly, the environmental benefits are massive, as the generation of millions of kilograms (kg) of clay waste can be avoided every year.

The olefins selective hydrogenation unit can be integrated upstream from the reformate stabilization column for maximum advantages. In this configuration, the full reformate stream is processed, yielding the following benefits:

  • The reduced olefins content in the feed to the extraction unit results in lower solvent and energy consumption for the extraction process
  • Aromatics preservation is maximized on both the C6/C7 aromatics stream and C8+ aromatics stream
  • Aromatics net gain is maximized on the C8+ aromatics stream
  • Diolefins (which may shorten clay cycles2) are efficiently removed on both the C6/C7 aromatics stream and C8+ aromatics stream.

This arrangement takes advantage of synergies such as desirable feed temperature and the use of the reformate stabilization column for removal of unreacted hydrogen and light ends, without any need for a flash drum or other stabilization means. Therefore, the low-cost, once-through process requires minimum equipment, namely a vessel and a heat exchanger. The impact on the stabilization column design is negligible because olefins to be hydrogenated represent a small fraction of the stream to be processed and the technology operates at low excess hydrogen. The source of make-up hydrogen is usually the catalytic reforming unit. The configuration is depicted in FIG. 1.

FIG. 1. Olefins selective hydrogenation unit inserted between the CCR and the reformate stabilization column.

Heavy reformate stream: Incremental aromatic production doubled

Using selective hydrogenation rather than clay treating for olefins removal [or bromine index (BI) removal] from aromatic streams prevents the loss of valuable aromatics in the benzene/toluene and heavy reformate streams. However, as previously reported1, the olefins to be removed in a heavy reformate stream are predominantly alkenyl aromatics. Plant sample analyses suggest that the prevalent molecules are methyl styrene, dimethyl styrene and styrene, accounting for most of the heavy reformate olefins content. FIG. 2 illustrates how these molecules, when hydrogenated via selective hydrogenation rather than being converted to heavy aromatics by alkylation with aromatics of equivalent molecular weight in clay treaters, contribute to the overall complex aromatics production. Namely:

FIG. 2. Reaction pathways for the production of additional valuable aromatics produced by olefins selective hydrogenation.
  • Methyl styrene is converted to methyl ethylbenzene via selective hydrogenation. Then, in the transalkylation process, methyl ethylbenzene is dealkylated to produce a toluene molecule. Finally, still in the transalkylation process, toluene is transmethylated either with another toluene to yield a benzene and a xylene molecule, or with a trimethylbenzene to yield two xylene molecules (FIG. 2A).
  • Dimethyl styrene is converted to dimethyl ethylbenzene via selective hydrogenation. Then, in the transalkylation process, dimethyl ethylbenzene is dealkylated to produce a xylene molecule (FIG. 2B).
  • Styrene is converted to ethylbenzene via selective hydrogenation. If the complex operates an ethylbenzene-dealkylation isomerization, ethylbenzene is converted to benzene, while if the complex operates an ethylbenzene-reforming isomerization, ethylbenzene is converted to xylene (FIG. 2C).

Consequently, with olefins selective hydrogenation on the heavy reformate stream, not only is aromatics consumption for olefins removal avoided, but olefin-substituted aromatics are converted to additional valuable aromatics.

Process conditions, catalyst, selectivity and impact on C6/C7 and C8+ aromatic streams

Since typical olefins content in reformate aromatic streams is in the order of 1%–2% by weight3, the hydrogenation process is only useful if the olefins can be hydrogenated selectively, i.e., without simultaneous hydrogenation of aromatic compounds.3,4 In the literature, palladium catalysts have been reported to be more selective than nickel catalysts.4,5 In pyrolysis gasoline hydrogenation, palladium catalysts are usually preferred for the selective hydrogenation of styrene and diolefins, while nickel catalysts are preferred when some amount of aromatic saturation is targeted.5 Further, conditions such as mild temperature and low excess hydrogen favor olefins hydrogenation while minimizing aromatic saturation, and also result in slower coke deposition, allowing longer cycles and softer coke (lower C/H ratio) formation that renders catalyst regeneration easier.

Diolefins selective hydrogenation, as well as the selective hydrogenation of the olefinic function of olefin-substituted aromatics, are achievable at milder conditions than the selective hydrogenation of aliphatic olefins, because the former molecules adsorb more strongly on catalysts’ surface than the latter.5,6 An olefins selective hydrogenation process positioned upstream from the reformate splitter must operate at mild conditions to avoid undesirable aromaticity loss. This drives BI removal efficiency differences on the C6/C7 and C8+ aromatics streams, but these differences work to the advantage of the user due to the aromatics complex configuration:

  • On the heavy reformate stream (C8+ aromatics), where BI mostly results from the presence of alkenyl aromatics, the BI removal achieved by selective hydrogenation processing the full reformate stream is very high. Moreover, this stream is diluted downstream of the clay treater by the isomerate recycle C8 aromatics stream containing very low BI, and—where a transalkylation unit is part of the process flow scheme—diluted by the transalkylate C8+ aromatics stream, which also contains very low BI. Finally, any heavy olefinic species is removed from the xylene loop via the xylene splitter bottoms. All these conditions ensure that the BI feed specification to the paraxylene recovery unit is achieved for extremely long periods. Hence, the clay treater becomes a guard bed present for infrequent upsets or unusual feed contaminants; under normal conditions, the clay treater is expected to operate for many years without any need for clay change-out.
  • On the light reformate stream, BI mostly results from the presence of aliphatic aromatics. BI removal achieved by selective hydrogenation processing the full reformate stream is substantially lower than on heavy reformate. However, light reformate is then processed in the extraction unit where additional BI is removed. The combined BI reduction by selective hydrogenation and aromatics extraction process yields a benzene/toluene extract stream with extremely low BI. Here as well, the clay treater becomes a guard bed essentially present for infrequent upsets or unusual feed contaminants, and under normal conditions, the clay treater is expected to operate for many years without any need for clay change-out. In some plant designs, the clay treater has even been eliminated.

Commercial example: Selective olefins hydrogenation process in a CTC complex.

FIG. 3 depicts the aromatics block of a major CTC complex that has been producing paraxylene for 3 yr. Multiple continuous catalytic reformers produce aromatic-rich effluents, and each reformate stream is processed through an olefins selective hydrogenation unit prior to separation of light and heavy fractions in the reformate splitter. On the light reformate side, a BI reduction of > 60% is achieved. This stream is then fed to an extraction unit, where additional olefins are removed as raffinate. The raffinate from an aromatics extraction unit is a paraffinic cut that has low octane value, making it unattractive for disposition in the Mogas pool.

FIG. 3. Schematic representation of the aromatics block of a commercial CTC complex.

Where a steam cracker is in operation, this raffinate provides much higher value as steam cracker feed, and the upstream olefins removal by selective hydrogenation makes it an even more suitable steam cracker feed. On the heavy reformate side, a BI reduction of > 90% is achieved. The heavy reformate stream is then combined with low-BI transalkylate and feeds multiple xylene loops for paraxylene recovery.

In typical clay treater operation on both C6/C7 and C8+ aromatic streams, temperature is usually adjusted to maintain a BI removal (olefins conversion by alkylation) that meets the complex requirements for acid wash number (benzene/toluene extract) and paraxylene recovery unit specifications (heavy reformate). When the temperature reaches the maximum achievable by the clay treater, then spent clay is removed and replaced by fresh clay. While every complex is different, typical clay treater cycles can range from a few weeks up to 12 mos in aromatic plants.8,9 However, in this commercial CTC facility in service for 3 yr, clay has never been replaced in either C6/C7 or C8+ aromatics service and the temperature has never been increased to compensate for loss of activity, meaning that both treaters are fed with streams containing extremely low BI levels. The clay in these treaters is expected to remain active for a long time.

FIG. 4 shows the stable operation as a function of time onstream of one of the selective olefins hydrogenation units in use at this facility. As can be seen, the site has decided to target 65%–70% olefins conversion to achieve (1) extremely low BI on the C6–C7 stream following additional olefins removal in the extraction unit, (2) extremely low BI on the C8 aromatics stream via selective hydrogenation conversion combined with dilution effects explained above, (3) undetectable aromatic ring loss through the selective hydrogenation process, and (4) catalyst cycle exceeding complex turnaround requirements (≥ 5 yr). The catalyst can then be regenerated with very high activity recovery and reused for subsequent cycles.

FIG. 4. BI removal by a commercial selective olefins hydrogenation unit on a full reformate stream in the aromatics block of a CTC complex.

Assessing small incremental improvements can be challenging in a pilot plant and even more in a commercial environment. Data are usually scattered with a significant error bar, because small variations on large gas chromatography peaks are difficult to quantify10 and also because such data are subject to flowmeter calibration variations. However, an attempt to measure valuable aromatic molecules gain in the plant through a selective olefins hydrogenation unit, as the result of the conversion of molecules such as styrene, methyl styrene and dimethyl styrene to alkyl substituted aromatics (as discussed earlier), is shown in FIG. 5. Time onstream monitoring suggests close to constant and consistent additional valuable aromatics generated via olefins selective hydrogenation throughout the year of data displayed in FIG. 5.

FIG. 5. Valuable aromatics gain by alkenyl aromatics hydrogenation through a selective olefins hydrogenation unit processing a full reformate stream.

Selective olefins hydrogenation benefits in a CTC complex: Selectivity is key

The integration of a selective hydrogenation process on the full reformate of the aromatics block in a CTC complex offers massive benefits. For a 4-MMt paraxylene plant, additional production at constant feed rate exceeds 110,000 tpy when only taking into consideration the heavy reformate stream. This does not include the incremental benzene/toluene production on the light reformate stream, nor the incremental production obtained by avoiding frequent clay change-outs and associated start of cycle periods at high xylene losses.

For the commercial operation described in this article, it is estimated that 3 yr of operation without any clay change-out have already spared the purchase of about 7 MMkg of clay, as well as associated solid waste generation and disposal costs. This represents a massive environmental footprint reduction for this large aromatics facility.

Additional benefits include reduced traffic in the heavy aromatics column and associated energy consumption, as well as reduced solvent, energy consumption and benzene losses in the aromatics extraction process. Selective olefins hydrogenation is a must-have process for plants of CTC scale, and indeed all new CTC facilities coming onstream or under construction include a selective olefins hydrogenation process in their aromatics block flow diagram. Yet the selection of catalytic technology and process operating conditions remains crucial: it is advisable to operate at slightly lower conversion where the catalyst nears 100% selectivity to avoid any loss of aromaticity, which could be fatal in streams with inherently high aromatics content. For the commercial operation described in this article, economic calculations have shown that 0.2% aromatics loss through hydrogenation would reduce by 60% the credits associated with the selective olefins hydrogenation process, while 0.5% aromatics loss would erase these credits almost entirely.

Takeaway

A selective olefins hydrogenation process with the appropriate selectivity has become an indispensable technology in the block flow diagram of very large aromatics complexes. Given the extent of aromatics losses and environmental footprint associated with clay treaters, selective olefins hydrogenation should also be considered by producers operating smaller aromatics facilities. HP

 

LITERATURE CITED

  1. Claire, F., A. Cotte and M. Molinier, “Enhance aromatics production with concurrent reduction of environmental footprint—Part 1,” Hydrocarbon Processing, October 2021
  2. Maisel, D. S., B. I. Smith and H. W. Scheeline, “Treatment of aromatic and unsaturated distillates,” Patent US2778863, 1957.
  3. Jani, P. J., S. M. Banerjee and D. W. Ablin, “Methods and apparatuses for selective hydrogenation of olefins,” Patent US9738572, 2017.
  4. Frey, S. J. and R. E. Marinangeli, “Process for the selective hydrogenation of olefins,” Patent US6977317, 2005.
  5. Ali, J., “The hydrogenation of pyrolysis gasoline (pygas) over nickel and palladium catalysts,” PhD thesis, University of Glasgow, 2012.
  6. Justino, G. T., C. S. A. Vale, M. A. P. da Silva and A. R. Secchi, “Modeling styrene hydrogenation kinetics using palladium catalysts,” Brazilian Journal of Chemical Engineering, Vol. 33, No. 03, 2016.
  7. Vaidyanathan, S., J. Zhou and S. Kapur, “Improve integration opportunities for aromatics units—Part 1,” Hydrocarbon Processing, November 2012.
  8. Brown, S. H., J. R. Waldecker and K. Lourvanij, “Process for reducing bromine index of hydrocarbon feedstock,” Patent US7815793, 2010.
  9. Düker, A., “Optimization of integrated aromatic complexes,” Digital Refining, March 2012.
  10. Barwick, V. J., “Sources of uncertainty in gas chromatography and high-performance liquid chromatography,” Journal of Chromatography, 1999.

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