January 2019

Special Focus: Process Optimization

Octane upgrading technology to boost value of light paraffinic feeds

In November 2017, US crude oil production eclipsed the 10-MMbpd mark, setting the highest production record in history.

Aggus, B., Massa, M., INVISTA Performance Technologies

In November 2017, US crude oil production eclipsed the 10-MMbpd mark, setting the highest production record in history. The 2017 growth in production was primarily driven by the Permian basin, situated in West Texas and Southeast New Mexico. Permian producers added 460,000 bpd between 2016 and 2017. Year-over-year US onshore production grew 1.4 MMbpd from December 2016 to December 2017, and as a result of the increase in production from these onshore tight oil basins, the US crude slate is becoming increasingly lighter.1 This trend is expected to continue, creating an excess of light paraffinic naphtha destined for export as cracker feed, since the gasoline pool is unable to absorb these low-octane, high-Reid-vapor-pressure (RVP) materials.

Octane upgrading vs. isomerization

Targeting this market need, a commercially proven octane upgrading technologya isomerizes and aromatizes C5 through C7+ paraffinic streams, such as straight-run naphtha, topped oil, raffinate oil, Fischer-Tropsch naphtha and/or natural gasoline. This innovative technology simultaneously raises the octane, lowers the RVP and reduces sulfur content by approximately 90%. It utilizes standard refining equipment and unit operations, such as fixed-bed reactors, absorption and fractionating columns, shell-and-tube exchangers, etc. The robust catalyst system is tolerant to impurities that can be catastrophic to alternate technologies that process these feeds (Fig. 1).

FIG. 1. An operating plant in 2013.
FIG. 1. An operating plant in 2013.

Isomerization, which requires complex feed preparation and treatment, is the primary alternative for these types of feeds. The isomerization process isomerizes a C5/C6 feed stream, boosting octane by converting the n-paraffins to iso-paraffins. The process cannot tolerate sulfur in the feed and, therefore, requires a hydrotreater with makeup hydrogen and associated compression. The feed must be tightly fractionated to minimize C7 materials in the feed, which will coke the catalyst. In addition, the feed must be dry to prevent catalyst deactivation due to the presence of water. An isomerization process with recycle has advantaged yields at the price of significantly higher CAPEX and OPEX.

In contrast, octane upgrading technology requires no pretreatment to remove sulfur, provided the sulfur in the feed is less than 200 ppmwt. This eliminates the CAPEX and OPEX associated with naphtha hydrotreating. In addition, hydrogen is not required in the process, so sourcing and handling of hydrogen are eliminated. The catalyst is water tolerant, eliminating the potential for catalyst damage due to inadvertent introduction of wet feed. The key to this technology is the proprietary catalyst and the associated chemistry.

Catalyst and chemistry

The catalyst is a complex-metal-modified, nano-size ZSM-5 type. It provides an immediate octane boost by isomerizing alkanes, which subsequently can also crack, oligomerize and aromatize to form additional high-octane, value-added compounds. While zeolites are commonly used as catalysts and adsorbents in refining, the marriage of dispersed metals, tailored acidity and customized carrier structure makes this catalyst unique. The catalyst structure is orchestrated to deliver the optimum crystal size, cell size, carrier porosity, pore distribution and metal crystallite distribution to facilitate access to the active sites (Fig. 2).

FIG. 2. The catalyst structure is orchestrated to deliver the optimum specifications to facilitate access to the active sites.
FIG. 2. The catalyst structure is orchestrated to deliver the optimum specifications to facilitate access to the active sites.

The catalyst metals play a critical role in the alkane activation, isomerization and dehydroaromatization chemistry. The catalytic process is initiated with the isomerization and subsequent cracking of alkanes, which is facilitated by the uniquely tailored acid functionality of the zeolite. This cracking process creates branched carbocations that can either recombine to form higher-molecular-weight branched species of higher octane value, or undergo β-scission to form olefin components. The olefinic compounds predominantly undergo dehydroaromatization, forming three moles of hydrogen per mole of aromatic ring. The catalyst favors formation of toluene, xylenes and C9 aromatics with low benzene formation.

The unique metal combination and deposition provides catalyst stability, with long run lengths of up to 60 d between catalyst regeneration cycle times in commercial operation. Coking is the primary deactivation mechanism of the catalyst, requiring periodic oxidative regeneration to restore activity. The typical catalyst regeneration cycle is 7 d–10 d, and the overall catalyst life is longer than 2 yr.

Process description

Two or more reactors are operated in parallel, with the feed equally distributed among the reactors during normal operation. When one reactor is being regenerated, 100% of the feed is processed through the remaining reactor(s) (Fig. 3).

FIG. 3. Process flow diagram showing two or more reactors operated in parallel, with the feed equally distributed among the reactors during normal operation.
FIG. 3. Process flow diagram showing two or more reactors operated in parallel, with the feed equally distributed among the reactors during normal operation.

The feed to the reactor is first processed through a series of heat recovery exchangers, followed by a fired heater to achieve the desired inlet temperature, typically 570°F–790°F (300°C–420°C). The overall reaction is endothermic. However, olefin-containing liquefied petroleum gas (LPG) can be co-fed to the reactor in proportion to the naphtha feed to minimize heat input. Oligomerization of the olefins in the LPG feed is a highly exothermic reaction. This co-fed LPG increases yields, reduces energy consumption and improves the octane of the final product by three to five points.

The reactor is a fixed-bed reactor with typically three to five beds per reactor, depending on the throughput. The catalyst gradually cokes during operation, and the feed temperature is correspondingly increased to maintain the desired yield and octane. As the temperature reaches the 790°F–840°F (420°C–450°C) range, the rate of coking increases and the reactor is taken offline for regeneration.

Product leaving the reactor is passed through feed/product heat exchangers, where it is partially condensed. This stream is passed through the stabilizer feed drum, in which the liquid and gas products are separated, with both streams being fed forward to the stabilizer column. The stabilizer reboiler duty is adjusted to optimize the RVP of the gasoline blendstock.

If LPG is co-fed to the process, then a second column can be added to facilitate better separation of the offgas (C1, C2 and H2), LPG and gasoline products.

Recommendation

If a refinery is getting the economic short end of long naphtha, the octane upgrading technology described in this article can generate higher value from light paraffinic streams destined for export.

This low-CAPEX, operator-friendly process converts low-value paraffinic streams into high-value gasoline blendstock. Actual yields and octane enhancement depend on the feed composition and the final product octane target. The six units in operation and three units under construction are designed for a variety of feeds. Typical yield is approximately 70 wt% C5–85 wt% C5. The octane enhancement increases as the feed octane decreases.

With a primarily C6/C7 feed, the octane enhancement can exceed 20 points. The degree of RVP improvement is dependent on the feed composition. Benzene generation is less than 0.7 wt%, and product sulfur content is less than 10% of the feed content. The catalyst is tolerant of sulfur and water. The process utilizes standard refining equipment and has a wide operating window, which is an ideal combination for capturing return on investment. HP

Note

a IsoA™ Technology is offered by Koch Industries affiliates INVISTA Performance Technologies and Koch-Glitsch.

Literature cited

1 US Energy Information Administration, “This week in petroleum,” March 14, 2018.

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