October 2018

Catalyst

More residue processing in FCCUs

Resid units have more than 2 wt% Conradson carbon residue (Concarbon) in feed and more than 5,000 parts per million (ppm) of nickel (Ni) and vanadium (V) on equilibrium catalysts (Ecat).

Genç, M., Karani, U., Er, I., TÜPRAŞ; Kandaz, S., Albemarle Corp.

Resid units have more than 2 wt% Conradson carbon residue (Concarbon) in feed and more than 5,000 parts per million (ppm) of nickel (Ni) and vanadium (V) on equilibrium catalysts (Ecat). Processing residue influences catalyst, additive selection, unit equipment and operation. It is essential to select higher-activity catalysts with metal traps to handle residue, while keeping catalyst properties the same.

In 2010, China limited rare earth oxide (REO) exports, which led to an increase in global prices. This action led to a significant decrease in REO content in fluid catalytic cracking (FCC) catalysts around the world. FCC operators that produced more propylene preferred to use low-rare-earth and high-matrix catalysts to avoid boosting hydrogen (H2) transfer reactions and saturating the olefins. Although the average REO in FCC catalysts decreased, higher total surface areas (TSAs) compensated for the overall catalyst activity, since activity is related to both REO and TSA.

FIG. 1. Ecat V content.
FIG. 1. Ecat V content.

The units can be limited from either coke or H2, while processing residues. Coke must be burned off to restore the catalyst activity. Main air blower or wet gas compressor capacities may be exceeded with the increases in H2 and delta coke. Upper boundaries for the temperature in the regenerator and stripping efficiencies are other bottlenecks, since they are strongly interrelated with the coke remaining on the spent catalyst. If the hardware in the regenerator or regenerator design cannot cope with the temperature increase or afterburn, this will also be a limiting factor.

Catalyst that can be used for the feed containing residue must have a coke-selective matrix and an optimal zeolite-to-matrix ratio to obtain selective cracking. Catalysts need to be tailored according to the requirements of the refineries (e.g., maximizing conversion, and increasing metals tolerance and distillate or propylene yield). Increased Ni content can be mitigated by the addition of antimony (Sb) to the catalyst inventory. When Ni on the Ecat exceeds 1,000 wppm, Sb can be injected into the feed—the H2/methane (CH4) ratio is continously monitored and kept below 0.8. Sb reacts with Ni in the FCC feed and forms the Sb oxide NiSb2O4. An Ecat Sb:Ni ratio of 0.2–0.4 is generally used. With this mechanism, the dehydrogenation activity of Ni is highly reduced. As a result, H2 yield and delta coke are decreased, resulting in lower wet gas volume, lower regenerator temperature and better yields. Sb can also inhibit the performance of carbon monoxide (CO) promoters. Therefore, it is crucial to balance the Sb amount required for Ni tolerance and afterburn that can be increased in the regenerator.

FIG. 2. Ecat Ni content.
FIG. 2. Ecat Ni content.

V is a mobile metal, in contrast to Ni, and can deactivate the catalyst severely. V results in the formation of vanadic acids that can remove sodium (Na) from the zeolite framework and be converted to sodium hydroxide (NaOH) via hydrolysis reactions. Hydroxyl groups in NaOH can cause zeolite destruction. Therefore, it is better to keep Na on fresh catalyst at low levels so as not to enhance V poisoning. V traps can either be incorporated into the catalyst matrix or added separately. Traps must be effective in immobilizing V and hindering it from reaching the acid sites.

Results and discussion.

Tupras’ Izmir refinery’s FCCU was designed in 1972 for a feedrate of 2,225 m3/d (14,000 bpd) of heavy vacuum gasoil (HVGO) and lube oil byproducts. The unit is a side-by-side configuration with a bubbling bed regenerator operating in partial burn, a riser disengaging system with rough-cut cyclones, and feed injection at the bottom of the riser.

The Izmir refinery’s FCCU can process lube oil extracts, HVGO with APIs between 20 and 22, sulfur up to 2.5 wt%, nitrogen between 1,000 ppmw and 1,600 ppmw, and total metals around 2 wppm. The unit started using proprietary catalysts in 2012.a Ecat Ni and V can be approximately 1,000 wppm and 5,000 wppm, respectively, while 70 wt% micro-activity test (MAT) can still be preserved in the unit. Ecat V and Ni can be as high as in a resid FCCU, although no residue is used in the Izmir refinery’s FCCU. One of the HVGO streams processed in this unit can be very heavy, with a Concarbon of 2.5 wt%–3.5 wt%, Ni of 1 ppmw and V of 3 ppmw–4 ppmw.

FIG. 3. Ecat fluid simulation test (FST) activity.
FIG. 3. Ecat fluid simulation test (FST) activity.

Ecat properties obtained during the trial are shown in FIGS. 1–3. Although feed Concarbon is between 0.6 wt% and 0.85 wt%, heavy tails present in a feed with a 3.5 wt% Concarbon level can build up on Ecat, resulting in an increase in delta coke. When an additive with high cerium oxide (CeO2) content or a catalyst with a high rare-earth (La2O3) amount is used in partial-burn units, this promotes the formation of delta coke. For units that have a high amount of V in the feed, V traps can be incorporated, blended into the catalyst or used separately. Therefore, it is important to control the rare-earth level in the inventory so as not to increase the formation of coke. It is also possible that V traps can become sulfonated in the regenerator by attracting sulfur oxides because of their basic structure.

Basic nitrogen in the feed can neutralize the active acid sites of the catalyst, but it is a temporary poison. A basic nitrogen amount is assumed to be one third of the total nitrogen content. The riser outlet temperature and Ecat activity must be increased to compensate for the increase in feed nitrogen. Nitrogen and sulfur also influence flue gas emissions. Approximately 40% of nitrogen ends up on coke and 25% of it is converted to nitric oxide (NOx), while 3%–5% of feed sulfur remaining on coke leaves as sulfur oxide (SOx). As the feeds get heavier, sulfur and nitrogen remaining in the products increase, along with SOx and NOx. Consequently, catalyst consumption, operating costs of treating units and additive consumption to decrease flue gas emissions increase with the increase in sulfur and nitrogen.

Processing residue also directly affects the coke remaining on catalysts. Types of coke include catalytic coke, additive coke, contaminant coke and cat-to-oil coke. Additive coke is related to the feed properties of basic nitrogen and Concarbon, while contaminant coke is affected by dehydrogenation metals, such as Ni, copper (Cu), V and iron (Fe).

Catalytic coke is affected by the cracking activity of the catalyst, while stripper efficiency is associated with cat-to-oil coke. Nearly 60% of the coke present on Ecat results from catalytic coke, 25% from stripper coke, 10% from contaminant coke and 5% from the feed coke. Therefore, it is important to keep Ecat activity at a certain level and to refrain from sudden loadings of fresh catalyst that may simultaneously increase catalyst activity and coke formation on Ecat. The goal is to keep Ecat activity at the same level during the trial, since this may also affect the conversion. Periods for which feed properties, amount and operating conditions (such as riser outlet temperature, pressure and feed inlet temperature) are similar should be compared for a fair evaluation. Catalyst concentration in the inventory is preferred to be above 70 wt% during the trials. During the period when a proprietary catalyst was used,b a substantial increase in the gasoline yield (2.3 wt%) was seen, while the conversion increase was 4.2 wt%. The octane barrel increase was 1,245 (TABLE 1). This aided the blending of heavier streams to the feed blend and converting more bottom-of-the-barrel to valuable products in one of the cracker units.

Takeaway.

Refiners must take advantage of processing heavier opportunity crudes, while meeting the challenges in processing different feed types. Additional hydrotreating capacities, new catalyst formulations and new investments are inevitable for residue processing. The FCCU is the main cracking unit that can handle more residue in refineries. Therefore, selecting the optimum zeolite-to-matrix ratio, coke-selective matrix, low fresh-catalyst Na content and metal traps are important for determining yield requirements and decreasing delta coke production.

The other option is to conduct revamps in the reactor-regenerator internals or downstream equipment in response to the expected increases in coke and H2 yields. Refineries that can process more residue with gasoil will be able to supersede other producers by increasing their overall profits. HP

NOTES

  a Refers to Albemarle catalysts

  b Refers to Albemarle’s GO-ULTRA catalysts

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