Challenge of Polynuclear Aromatics in Hydrocracking Units

Polynuclear aromatics (PNAs), also known as polycyclic aromatic hydrocarbons (PAHs) or polyaromatic hydrocarbons (PAH), are a group of organic compounds that consist of fused aromatic rings. They are formed by the fusion of two or more benzene rings or other aromatic ring structures. Polycyclic Aromatic Hydrocarbons (PAH) can vary in size and complexity, ranging from simple molecules with a few rings to large and complex structures with numerous rings.

Heavy Polynuclear Aromatics (HPNA) are aromatic molecules consisting of 7+ fused aromatics rings and are formed by the condensation of lower aromatics and dehydrogenation of polynaphthenes. The HPNA in the industry are named “Red Death” because of their famous red colour and deleterious impacts on the unit operation. HPNA molecules can lead to increased coking on catalysts or cause equipment fouling in Hydrocracking units.

Once heavy polynuclear aromatics (HPNA) compounds are produced it is almost impossible to convert them into lighter ones. In once-through single-stage Hydrocracking processes, HPNA do not accumulate and therefore do not cause serious operational problems. In a recycle operation with two-stage hydrocracking units, the buildup of HPNA may cause severe operational problems, and “HPNA management is required to achieve ~ 100 % conversion.

Important examples of Polynuclear Aromatics include naphthalene (C10H8), anthracene (C14H10), chrysene (C18H12), pyrene (C16H10), perylene (C20H12), coronene (C24H12), and ovalene (C32H14). They are condensed hydrocarbons containing several fused aromatic rings.

Properties of Polynuclear Aromatics

Some of the major properties of Polynuclear Aromatics are as follows;

• PNA are more hydrogen deficient than benzene; primarily are composed of Hydrogen and Carbon only.
• These compounds are similar to asphaltenes, as they consist of multiple condensed ring aromatics. However, their molecular weight tends to be lower than naphthenes.
• As the side chains or Alkyl groups of aromatics are shortened or disappear, the solubility of HPNA decreases.
• PNA are preferentially adsorbed on the catalyst because they are more polar than most other feed components. The HPNA deposited on the catalyst are coke precursors and accelerate catalyst deactivation.
• HPNA are high-boiling compounds and thus concentrate in the fractionator bottom stream, which in recycle hydrocracking is routed back to the reactor.
• HPNA are difficult to crack and their concentration increases as new feed is added. As the buildup of HPNA surpasses their solubility at temperatures used for the condensation and separation of the reactor products, some HPNA precipitate and foul the reactor product condenser.
• These materials have very low solubility in both aromatic and aliphatic hydrocarbons.
• The presence of HPNA material in the recycle liquid causes its appearance to change. As the amount of HPNA material increases, the recycle liquid will begin to appear somewhat reddish. With increasing HPNA concentrations, a reddish-brown color is seen.
• Because of its relatively low solubility, HPNA compounds were precipitate at a temperature corresponding to values typically seen in the reactor effluent air cooler.

Factors Affecting HPNA Formation

The formation of HPNA in hydrocracking units depends on several factors. Most of the time HPNA formation depends on the concentrations of precursors in the feed and operating conditions.

• Lower pressure units and high conversion operations are unfavorable conditions, which increase HPNA formation.
•  Light Polynuclear aromatics (PNAs), with two to six rings, are found in straight-run VGO. Larger Heavy Polynuclear Aromatics (HPNAs) with more than six rings are found in heavy cracked stocks such as heavy coker gas oil (HCGO) or heavy FCC cycle oil (HCO). HCGO contains high concentrations of aromatics, polyaromatics, and other unsaturated compounds, as well as high micro carbon residue (MCR) or Conradson Carbon Residue (CCR).
• The formation of HPNA depends on feedstock composition and boiling range. An increase in feedstock endpoint results in an increase in HPNA formation because heavier feedstocks contain more HPNA precursors.
• The higher reactor temperature required for higher conversions or catalyst deactivation favours an increase in HPNA because it enhances dehydrogenation and condensation. The temperature effect on the HPNA concentration becomes more significant when conversion approaches 100%.
• If the two-stage Hydrocracking units are designed for high conversion, unconverted oil is recycled from the fractionation section back to the reactor section for further conversion. As conversion increases, there comes a point where the generation of HPNA material in the reactor becomes greater than its removal via the recycle unconverted oil stream. The concentration of HPNA material will build up in the recycle liquid and its presence can be seen as a characteristic reddish-orange colour of the recycle liquid.
• Under similar operating conditions, amorphous catalysts form fewer HPNA than zeolite-based catalysts. This may be due to the higher cracking rate of bulky hydrocarbon molecules over amorphous solid acids than over zeolite catalysts.

Formation of Heavy Polynuclear Aromatics  

The HPNA are formed in the hydrocracking unit by (a) condensation of two or more smaller Polynuclear Aromatics (PNA) present in the hydrocracker feed; (b) dehydrogenation of larger hydrogenated polycyclic compounds; or (c) cyclization of side chains on preexisting PNA, followed by dehydrogenation. An increase in reactor temperature increases dehydrogenation.

Effects of Polynuclear Aromatics on Hydrocracking Unit

If HPNA are not managed well they can cause problems in the following areas;

1. Coke Formation on Hydrocracking Catalyst

The coking of the catalyst begins with the adsorption of high-molecular-weight hydrocarbons and a low hydrogen/carbon ratio containing hydrocarbon rings. The HPNA material present in the recycle liquid will eventually end up on the catalyst as a coke, with the result that the end of run temperature is reached prematurely.

2. Blocking of downstream equipment

The other problem is that since HPNA material has low solubility in reactor effluent material, it will precipitate when a characteristic temperature and concentration are reached. If the unit is configured with a single high-pressure separator, the entire effluent material will be cooled to the separator temperature and will foul the air-cooled reactor effluent cooler. The result is a gradual but steady fouling of the exchanger surface with precipitated HPNA material. An indication of exchanger fouling is an increase in the outlet temperature of the effluent air cooler.

Managing Polynuclear Aromatics  (HPNA) in Hydrocracking Units

Successful hydrocracking unit operation requires effective HPNA management. Techniques are commercially proven to effectively control HPNA achieving a very high ∼100% conversion. Compared to the early days when only controlling feed endpoint or purging fractionator bottoms were the only solutions. The new techniques while requiring extra capital investment represent a more favourable solution and pay back quickly.

There are various options available for managing PNA;

• To control HPNA, a small slipstream (5~10%) can be removed from the Fractionator bottom to purge HPNA out of the unit to prevent HPNA accumulation in the reactor. Purging a small stream of unconverted oil reduces overall conversion and negatively impacts operating economics, but this is an economic trade-off against downtime and lost opportunities.
• Moderate Operation of the Hydrocracker units to achieve the desired conversion is recommended to avoid HPNA formation. But high conversions can be achieved when the unit is operated at end-of-run conditions.
• Utilizing a high-activity Zeolites-based catalyst which would require lesser temperatures for high conversions as compared to Amorphous-based catalysts which require high temperatures.
• Installation of a Hot Separator at the reactor effluent will remove the HPNA before going to Air Cooled Exchanger before High-Pressure Separator. HPNA is bypassed and the fin fan exchanger is avoided from fouling. Although a hot separator design resolved the issue of premature shutdown due to fouling of equipment, the issue of catalyst deactivation due to the deposition of HPNA material in the recycled material on the catalyst as coke remained unsolved.
• HPNA Removal By Adsorption: HPNA can also be removed physically from the recycle oil by applying absorbents (e.g., on activated carbon). This technique has been commercialized and allows the unit to operate at 100% conversions processing a heavy feedstock without any unconverted oil purge thus providing longer run lengths and higher conversion. But the two main disadvantages of this method are carbon handling and disposal because of its huge use of carbon and high change over frequency.

• HPNA Removal by Fractionation/Stripping: Due to the disadvantages of the Carbon process alternative fractionation technique was adopted by which the HPNA material in the recycle liquid is concentrated and rejected as a small purge stream amounting to as little as 0.5 vol% of the feed. A slipstream of recycle material is routed to a small HPNA stripping zone and the lighter hydrocarbons are stripped and returned to the product fractionator and the concentrated HPNA-rich bottoms are purged from the unit.

Commercial Solutions

1. Haldor Topsoe HPNA Trim Process 

TOPSOE™ has developed HPNA Trim™ (patent pending) to cost-effectively manage HPNAs in full conversion hydrocrackers. This uniquely simple process takes advantage of the heavy HPNA compounds’ extremely high boiling points.

2. UOP’s HPNA RM® and SSF Techniques

UOP offers two major techniques HPNA RM (Absorbent Based) and Split-Shell Fractionator (SSF) in which a portion of unconverted oil is reprocessed in the SSF concentrating HPNA to the small 0.5 volume% purge out of unit.

 Top References

1. https://www.digitalrefining.com/article/1002803/enhancing-flexibility-in-two-stage-hydrocrackers
2. https://uop.honeywell.com/en/news-events/2020/12/tupras-uses-honeywell-technology 
3. Handbook of Petroleum Processing edited by Steven A. Treese, Peter R. Pujadó And David S. J. Jones
4. Hydroprocessing for Clean Energy by Frank (XIN X.) ZHU, Richard Hoehn, Vasanr Thakkar, Edwin Yuh
5. Hydrocracking Science and Technology By Scherzer, Julius.; Gruia, Adrian

6. Enhancing flexibility in two-stage hydrocrackers, www.dgitalrefining.com

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