February 2021

Process Optimization

Maximize margins in a light naphtha isomerization unit by producing additional C₆ products

The increasing demand for more efficient and low-emissions fuels due to stringent environmental policies and growing environmental awareness is leading refiners to find options to enhance the research octane number (RON) of the final product.

The increasing demand for more efficient and low-emissions fuels due to stringent environmental policies and growing environmental awareness is leading refiners to find options to enhance the research octane number (RON) of the final product. This has made the isomerization process an important and intrinsic part of almost all refineries.

The isomerization process is not only capable of upgrading the octane number of naphtha fractions—particularly C₅ and C₆—but also simultaneously reducing the benzene content of naphtha by saturation of the benzene fraction. This efficiently converts low-grade, straight-run naphtha to more marketable and valued product due to the improved RON. Isomerization is preferred not only because it is simple and cost effective for octane enhancement as compared to other octane-improving processes, but also because it produces isomerate product with very low sulfur and benzene, making it an ideal blending component in the refinery gasoline pool.

Structurally, the straight-chain paraffins get converted into their branched-chain isomers, which improves the RON of the isomerate and has increased the popularity of the process. Technology companies offer various options of isomerization processes, whether once-through or recycle. The benefits of improved RON of the final product paired with the CAPEX and OPEX involved in revamps govern refiners’ decisions for their isomerization units.

A few examples of this transformation of a compound into any of its isomeric forms—which have the same chemical composition but different structures and physical and chemical properties—through the process of isomerization are depicted in FIG. 1.

As the isomerization process is mildly exothermic, low temperatures favor the reaction in a forward direction; therefore, highly active catalyst are employed. To avoid the formation of olefins in a low-temperature process, the feed to the isomerization plant is premixed with hydrogen (H₂). Processes are capable of upgrading low-octane C₅/C₆ streams to products with octane ranging from 80 RON–93 RON. It has been observed that once-through processes can produce product from 80 RON–84 RON, while recycle processes with deisopentanizer and deisohexanizer columns can give products with an RON as high as 93.

Process flow scheme in an isomerization unit with recycle

Besides the isomerization reactor, a typical isomerization unit consists of three columns. The deisopentanizer (DIP) column stands at the front end of the unit (before the reactor), while the stabilizer and the deisohexanizer (DIH) columns are at the back end of the unit (i.e., after the reactor). The product stream from the isomerization unit is stabilized and then sent to the DIH column. The column splits the isomerate stream into three streams (i.e., the light isomerate, DIH recycle and heavy isomerate). The light isomerate and the heavy isomerate streams are combined and sent to the battery limits for storage. The DIH recycle stream is sent back to the isomerization reactor. The block flow diagram of a typical isomerization unit is shown in FIG. 2, as are the compositions of the light isomerate and the heavy isomerate stream.

The light isomerate is primarily composed of four components: isopentane, N-pentane, 2-methyl butane (2MB) and 3-methyl butane (3MB). The focus on improving the RON of this stream has made technology companies invest time and money for further improvements in processes and catalysts. The recycle stream that is sent back to the reactor is mainly a mixture of 2-methyl pentanes (2MP), 3-methyl pentanes (3MP) and N-hexane. The lower the quantities of 2MP and 3MP in the recycle stream, there is an increased tendency of the equilibrium to shift in a direction to lower values of 2MB and 3MB in the product stream. These are critical components that contribute to the RON of the total isomerate. Therefore, the recycle stream should have requisite amount of 2MP and 3MP to obtain desired product specifications. However, this eventually leads to a buildup of the third component of the recycle stream (N-hexane) in the recycle loop, affecting the throughput of the isomerization reactor and creating a bottleneck when it comes to capacity augmentation.

This potential bottleneck led to the novel idea of drawing N-hexane out of the recycle loop and using it to make a few value-added products. N-hexane drawn from the loop can be converted to marketable products, such as food-grade/polymer-grade/pharma-grade hexanes and isohexanes. Not only do these marketable products bring in additional revenue to the facility, but as the recycle to the isomerization reactor decreases, more feed can be pushed through the reactor. This is an added benefit for facilities, particularly where the isomerization unit is bottlenecked.

FIG. 3 further describes the compositions of feed and product streams of the DIH column.

ALTERNATE C₆ PRODUCTS FROM THE ISOM UNIT

The recycle stream in an isomerization unit can be used to produce a variety of C₆ products that are widely used in the industry and are produced through alternate processing routes:

• Food-grade hexane (FGH): Food-grade hexane is a colorless solvent primarily used in the extraction of edible oils. This calls for very high purity levels of hexane, followed by safe and careful storage. It also finds usage in the preparation of rubber adhesives, can sealing compounds, etc.
• Polymer-grade hexane (PGH): PGH is a fast evaporating hydrocarbon solvent that consists essentially of hexane isomers. A concentration of approximately 40% makes n-hexane the major component in this mixture. PGH is used as a polymerization medium and in the manufacture of catalysts.
• Isohexane: This compound, which can also be drawn from the recycle stream, is a solvent used in industrial, professional and consumer applications, such as a manufacturing process solvent, metal working and coatings. It is not sold directly to the public for general consumer uses; however, this product may be an ingredient in consumer and commercial product applications, such as cleaning agents and coatings.
• Special boiling point (SBP) spirit: Depending on market demand, SBP spirit 55/115 can also be produced from the recycle stream. It is used in the rubber industry, particularly during the process of vulcanization in tire manufacturing or in preparation of certain rubber mixes, cements and adhesives. It is also used as a thinner for varnish, paint and printing inks formulation where quick drying is required, and as diluent for lacquer, enamels and high-grade leather drops.

TABLE 1 provides the typical specifications of the products discussed here.

Dividing wall columns (DWCs)

DWCs have gained popularity both in grassroots and revamps in the petrochemical industry. The technology works on improving conventional distillation columns, which are the most energy intensive areas in the refining and chemical industries. Facilities are reaping maximum benefits from this technology, and many refineries are undertaking revamps to harness the benefits of DWCs in areas that otherwise are bottlenecked.

Revamps of conventional columns to DWCs can provide the following benefits:

• Ideal alternative for revamp of side-cut columns when high purity is required from the three product streams
• Lower footprint as equipment count is reduced by half
• CAPEX and OPEX can be reduced by approximately 20%–50%.

Structurally, the exterior of a DWC looks like a conventional distillation column, but inside a defining wall in the column separates the tower into two sections, creating different fractionation zones. The zone in the column where the feed is introduced works to effectively separate the heaviest and the lightest key. Because this wall removes the intrinsic mixing that takes place in the conventional column by creating different separation zones, these columns are thermodynamically more efficient compared to their counterparts—therefore providing benefits in terms of operating cost.

DWCs in a DIH column for producing C₆ cut

As demand for C₆ products surged, refiners foresaw an additional source of revenue and hurried to generate C₆ product.

The usual way of obtaining FGH from the recycle stream is by installing two new columns post the DIH column. FIG. 4 shows the typical configuration of producing FGH by the DIH route. In this sequence, two new columns are installed downstream of the isomerization recycle stream to produce FGH.

An attractive alternative to this sequence would be to revamp the existing DIH column using DWC technology to produce four cuts.

For isomerization facilities, the revamp of a DIH column to DWC can have significant benefits. The revamped DIH column produces light and heavy isomerate as top and bottom products, along with FGH and the recycle stream as the other two cuts. FIG. 5 shows how a middle DWC handles the overlap of the heavy isomerate and the recycle stream, reducing the number of stages required for the desired specifications compared to the conventional column—the spare stages are available in obtaining the fourth cut.

This option of getting four cuts from the DIH column is not only attractive in terms of lower CAPEX and OPEX, but is flexible as the column is capable of operating in two modes: the FGH mode, in which the column will produce a fourth cut of FGH; and the DIH mode, in which the column operates in conventional mode with recycle to the isomerization unit without FGH production. To produce the fourth cut, the alignment of the wall inside the column is customized to meet the desired product specifications, quantities and also target to minimize the heat loads. The advantages of this process are sufficient to prompt facilities to utilize it for beneficial production of FGH.

FIG. 6 shows the process flow and the components of the four cuts of the DIH column post revamp. Benefits include:

• Energy consumption is 30% less than the conventional column sequence.
• With the drawing of FGH (i.e., n-hexane from the recycle stream as the fourth cut), the recycle rate to the isomerization reactor is reduced. This helps in pushing more feed through the reactor and is helpful in capacity augmentation.

C₆ production from other routes compared to DWC in isomerization unit

C₆ products are commonly produced via extraction by dearomatization of light naphtha fraction post hydrotreating. In this process, sulfolane is used as a solvent to remove aromatics from the C₆ cut. Although this technology is widely used, it has shortcomings when compared with the production of FGH post the isomerization unit because:

• The latter does not require the installation of an aromatics removal unit (ARU) to get dearomatized naphtha.
• When FGH is produced from an isomerization unit stream (e.g., from the DIH column), no additional column is required, while the former requires columns in series.
• Not only does the revamp to DWC produce FGH from the DIH column bring additional revenue, it also debottlenecks the isomerization unit in terms of capacity by at least 15%–25%.
• There is an increase in total octane barrel by the route of FGH production through isomerization.

Case study

The case study presented here is based on an operator in Asia. The refinery has a light naphtha isomerization unit processing 39 t/hr of fresh feed with an isomerate product RON of 88.

Cost basis analysis and typical payback for the unit are summarized in TABLE 3, providing the investments vs. the total revenue post revamping a DIH column into a DWC for the facility. Net profitability is calculated based on the margin difference between new FGH and isomerization products. The project payback is detailed in TABLE 4.

The following conclusions can be drawn from the case study:

• Revenue increased by producing high-value hexane products
• The RON of isomerate products remains unchanged
• The isomerization unit can process an additional 15%–25% of fresh light naphtha feed without any modifications to the reactor section
• Typical project payback is 5 mos to < 1 yr.

Takeaway

Refineries are taking up the production of food-grade/pharma-grade hexane through the use of DWC technology by revamping the DIH column to a four-cut column. This is a promising venture, requiring some modification in the existing DIH column in terms of the installation of a dividing wall and change limited to a few trays. The revamp can be done easily in 20 d, which are typically available during the annual shutdown in the facility. With rising demand for C₆ products, this is an attractive venture. HP

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