April 2021

Special Focus: Clean Fuels

Meeting the Tier 3 challenge with ultra-clean alkylate

The U.S. Environmental Protection Agency (EPA) introduced Tier 3 gasoline sulfur standards in 2017, requiring all U.S. gasoline producers to adhere to an annual 10-ppm average sulfur limit.

Griffiths, E., KBR; Mukherjee, M., Exelus, Inc.

The U.S. Environmental Protection Agency (EPA) introduced Tier 3 gasoline sulfur standards in 2017, requiring all U.S. gasoline producers to adhere to an annual 10-ppm average sulfur limit. A 3-yr extension was provided for about 30 small refineries, which expired on January 1, 2020. The program includes a 6-yr provision allowing refineries that cannot produce qualifying gasoline to buy credits from other refineries to comply with the Tier 3 requirement. The credits are generated by refineries producing gasoline with an annual sulfur content below 10 ppm. At the end of October 2019, the price of these credits increased by 250%, indicating that demand for the credits were possibly greater than their supply.

Refiners have a limited number of options to reduce sulfur levels in gasoline to meet the new 10-ppm requirement. For Tier 3, removing the remaining, more difficult sulfur molecules may lead to more significant octane loss. Most refiners are meeting the regulations by increasing hydrotreating severity either with pre-treating fluid catalytic cracking (FCC) feed or post-treating FCC naphtha. This option increases the refinery hydrogen consumption and reduces the run length of these hydrotreaters. While post-treating is effective for reducing the sulfur content, it saturates olefins, resulting in octane loss in the FCC naphtha. This can be exacerbated at refineries that consume an increased diet of shale‐derived crudes, which are naturally light and produce low sulfur but also low-octane gasoline. The increasing value of gasoline octane in recent years is illustrated in FIG. 1.

FIG. 1. Gasoline prices and premium-regular retail price differential. Source: U.S. Energy Information Administration (EIA).

Alkylate: The ideal blendstock

Accordingly, alkylate has emerged as a preferred gasoline blending component because it contains no sulfur, no olefins and no benzene, and has a low vapor pressure and a high octane number. U.S. refineries produce 1.3 MMbpd of alkylate, which is produced by reacting isobutane with light olefins, using liquid acids [either hydrofluoric (HF) or sulfuric acid]. The use of these corrosive materials raises maintenance costs. Adding significant costs to the operation are the storage, transport and regeneration of the acid. Solid-acid-catalyzed alkylation eliminates the EHS issues and costs associated with using and regenerating corrosive liquid acids.

A proprietary solid-acid alkylation process

Typical solid-acid catalysts deactivate in minutes. After years of development, a new solid-acid catalyst technologya has reached the point of outperforming these liquid acids. The engineered solid-acid catalyst has been designed at multiple levels to provide more than 24-hr cycle times. It also offers robust resistance to typical poisons (such as mercaptans, diolefins and oxygenates), along with the ability to handle a variety of feedstocks. The proprietary catalyst forms the core of a safe and efficient proprietary solid-acid alkylation processb that generates high-octane alkylate without the hazards and costs associated with liquid acid technology. Additionally, it features a simple fixed-bed reactor design and regeneration using hydrogen.

The integration of catalyst science and reaction engineering allows the proprietary catalyst cycle times that are an order of magnitude longer than most solid-acid catalysts and produces high-octane alkylate from isobutane and light olefins (ethylene, propylene, butylenes and amylenes) from any source. The stable catalyst performance greatly simplifies the overall process design, which reduces the capital cost of the alkylation plant, while lowering energy consumption. Innovations in the proprietary solid-acid catalyst system are shown in FIG. 2.

FIG. 2. Innovations in the proprietary solid-acid catalyst system.

Catalyst performance with various feedstocks

The proprietary solid-acid catalyst has been tested with various feedstocks and produces alkylate with a high octane rating over a wide range of operating temperatures, olefin space velocities and feed compositions. Results from the bench-scale testing are summarized in TABLE 1. The octane values and Reid vapor pressure (RVP) were computed using the gas chromatography product analysis and confirmed by independent engine testing.

In general, the octane values for alkylate produced by the proprietary catalyst tend to be higher than those obtained by either liquid acids or ionic liquids for three main reasons. Those include:

  1. The proprietary catalyst has an inherent functionality that converts n-butenes to a mixture of 1-butene, trans-2-butene and cis-2-butene. As a result, irrespective of the type of normal butene used as feed, the product composition of the alkylate is identical.
  2. The trimethyl pentanes-to-dimethyl hexanes (TMP/DMH) ratio for alkylate produced by the proprietary catalyst is roughly double the value for TMP/DMH ratios produced by liquid acids or ionic liquids. The octane values for TMPs range from 100–109, while those for DMH range from 55–75. High TMP/DMH ratios boost the alkylate octane rating.
  3. The distribution of TMP molecules produced by the proprietary catalyst is different from those produced via other processes. While the dominant TMP produced by most alkylation processes is 2,2,4-trimethyl pentane, which has a RON of 100, the proprietary catalyst tends to favor 2,3,4-trimethyl pentane and 2,3,3-trimethyl pentane, which have RON ratings of 103 and 106, respectively.

A combination of these three effects boosts the octane rating of alkylate produced by the proprietary catalyst vs. other technologies.

Feed contaminants

Poisoning is the strong chemisorption of reactants, products or impurities on acid sites otherwise available for catalysis. Certain feed contaminants act as catalyst poisons and increase the rate of deactivation of the catalyst. Typical feed impurities for alkylation units are shown in TABLE 2.

These feed impurities lead to an increase in liquid acid consumption for sulfuric and HF acid alkylation technologies. In general, the recommended limits for feed contaminants using these liquid acids are less than 10 ppm each for sulfur, dienes and oxygenates. For solid-acid catalysts, the feed contaminant acts as a temporary poison by occupying an acid site, which then becomes unavailable for alkylation.

However, the proprietary catalyst—which is zeolitic in nature—can remove these contaminants without the need for extensive pretreatment. These contaminants—which remain adsorbed on the catalyst surface—are removed during the regular regeneration procedure, allowing the catalyst to recover its full activity. This unique feature has important consequences for Tier 3 regulations. Even though the olefinic feedstocks may contain sulfur or oxygenates, the alkylate produced is essentially sulfur and oxygenate free (FIG. 3).

FIG. 3. The proprietary catalysta can trap most feed contaminants (e.g., mercaptans, dienes and oxygenates), while allowing high-octane alkylate molecules to diffuse out easily. 

Performance with feed contaminants

Equally important is the quality of alkylate produced for feedstocks containing high levels of contaminants. There is no change in the quality of alkylate produced for feedstocks with high levels of contaminants for a mixed C3 and C4 olefin feed (TABLE 3).

Proprietary solid-acid alkylation processb design

A unit process diagram of the proprietary process is shown in FIG. 4. In the reaction zone, isobutane and olefins are reacted to produce alkylate. The reaction takes place over the solid-acid catalyst in fixed-bed reactors. Three fixed-bed reactors with recirculation are used in the reaction section. Two are used for alkylation, while the other is being regenerated in a staggered cycle. The reactors typically contain multiple beds, with olefin feed spargers between each bed. A portion of the olefin feed and the reactor recirculation stream are combined and introduced at the top of the reactor.

FIG. 4. Proprietary solid-acid alkylation process unit diagram.

The alkylation reaction is mildly exothermic. The reaction’s heat is removed by a heat exchanger located in the recirculation loop outside the reactor. As in conventional alkylation units, the reactor effluent is sent to a distillation train consisting of two columns. First, a deisobutanizer is used to recover excess isobutane, which is returned to the reactor. Excess n-butane is removed from the alkylate product in this column, as well. Second, a depropanizer is used to remove light components—mainly propane—from the system. The proprietary process does not require any neutralization or washing equipment to post-treat the alkylate product.

At the end of the 24-hr alkylation cycle, the feeds are switched to the newly regenerated reactor, and the catalyst in the previous (i.e., in-service) reactor is regenerated. A vapor-phase circulating loop containing hydrogen and light hydrocarbons is used to heat the reactor to about 275°C (527°F), at which point catalyst regeneration occurs. Due to the small amount of soft-coke buildup during the reaction cycle, hydrogen consumption is low. After 2 hr at this condition, the loop is used to cool the reactor to the reaction temperature. Fresh isobutane is charged to the reactor, making the reactor ready for the next alkylation cycle. This sequence is controlled by a programmable logic controller.

Techno-economic analysis

Due to lower capital costs and a higher alkylate margin, the proprietary solid-acid alkylation process offers significantly higher return on investment vs. liquid catalyzed alkylation units.

The capital cost savings result from multiple sources. First is the elimination of corrosive acid from the process. Removing the liquid acid eliminates the acid neutralization equipment, product washing vessels and storage tanks for fresh and spent acid. Second is the change in process conditions. The solid-acid catalyst operates optimally at around 50°C (122°F). Sulfuric acid and ionic liquid alkylation units require refrigeration to generate reasonable alkylate octane, operating around 5°C (41°F), which requires expensive compressors and refrigeration loops. Eliminating refrigeration also leads to a considerable reduction in power costs. A summary of the utility requirements is provided in TABLE 4.

Differences in catalyst regeneration procedures also lead to considerable savings, which may be considered either capital or operating costs. The solid-acid catalyst is regenerated in the reactor with hydrogen, generating only a small purge of hydrogen and light hydrocarbons. Sulfuric acid requires a large regeneration plant that is either operated onsite (large capital cost) or by another party (large operating cost).

Liquid-acid alkylation and spent-acid recovery units generally incur high maintenance costs due to the presence of corrosive acid. In numerous refineries, the turnaround and inspection intervals of the alkylation unit are determined by the well-documented challenges associated with acid corrosion. A solid-acid catalyst eliminates these challenges and allows the turnaround interval to be extended to come in line with the upstream FCCU. The maintenance and inspection activities during operation and turnaround periods are also simplified without the presence of liquid acids, thus increasing productivity.

The proprietary solid-acid catalyst outperforms liquid catalyst processes in both alkylate yield and octane, thereby boosting alkylate margins from the available olefin feedstock. Alkylate octane using the proprietary catalyst is typically at least one point higher than any other technology. Alkylate yields are around 5% higher than liquid acid processes.

Performance of a commercial solid-acid alkylation unit

A commercial solid-acid alkylationb unit has been in operation in Shandong Province, China, for more than 2 yr, meeting and exceeding predicted product quality parameters. The unit has demonstrated consistent performance and alkylate quality through more than 300 regeneration cycles per reactor, proving the robustness of the proprietary catalyst. FIG. 5 shows a view of the plant.

FIG. 5. View of a commercial solid-acid alkylationb unit.

Two additional proprietary solid-acid alkylation projects are underway in North America. The first project is related to the revamp of an HF acid alkylation unit, and the second is to revamp a sulfuric acid alkylation unit. HP

NOTES

         a Exelus’ Solid-Acid Catalyst (ExSact) technology
         b KBR’s K-SAATTM technology

LITERATURE CITED

  1. Chung, W., R. Zhang, X. Zhang and D. Song, “Safe and sustainable alkylation: Performance and update on composite ionic liquid alkylation technology,” Hydrocarbon Processing, April 2020.

The Authors

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