September 2018

Process Optimization

Find the best aromatics extraction system for industrial applications

Aromatics are fundamental petrochemical compounds used as raw materials to manufacture hundreds of polymers, solvents and additives; these, in turn, are used to make consumable products that people use every day.

Chou, C., GTC Technology US, LLC

Aromatics are fundamental petrochemical compounds used as raw materials to manufacture hundreds of polymers, solvents and additives; these, in turn, are used to make consumable products that people use every day. As the consumable market grows, higher demand is seen to produce more aromatics, specifically benzene, toluene and xylenes (collectively known as BTX). BTX is the base of all aromatic derivatives, and continues to see increased global demand. As a result, new BTX units or expansions of existing BTX units come online nearly every year.

FIG. 1. BTX production by major sources.
FIG. 1. BTX production by major sources.

Due to the low BTX content in crude oil, BTX is largely created by converting other molecules to BTX molecules. The most common method is by naphtha reforming, which is typically a process unit in refineries, with the purpose of converting non-aromatics from the same carbon numbers to aromatics. Reformate, the product from naphtha reforming, provides roughly one half of the source for BTX. The pyrolysis gasoline (pygas) from naphtha crackers provides roughly one third of the source for BTX. The remainder comes from coke oven light oil (COLO) and other production methods. Fig. 1 shows BTX production by major sources.

When used as petrochemical products, BTX are required by downstream processes to have high purities; otherwise, prices would be significantly lower for gasoline solvent. Despite having BTX as the majority components, significant amounts of non-aromatics are still present in all reformate, pygas and COLO that prevent BTX from reaching the required petrochemical-grade purities by simple distillations. Table 1 shows the typical BTX content in different BTX-rich sources, and the typical BTX purities required as petrochemical products.

As shown in Table 1, with the exception of xylenes from low- to medium-severity reformate, all BTX-rich sources have too many non-aromatics in the same boiling range as BTX, and therefore contaminate the BTX purities.

To obtain high-purity, on-spec BTX from BTX-rich sources, non-aromatics in the BTX boiling range (C6–C8) must be selectively rejected—i.e., the aromatics in the BTX boiling range need to be extracted from all other non-aromatics components. This separation method is called aromatics extraction.

FIG. 2. LLE and ED systems for aromatics extraction.
FIG. 2. LLE and ED systems for aromatics extraction.

Aromatics extraction system types

Two major types of aromatics extraction are being successfully practiced in the industry: liquid-liquid extraction (LLE) and extractive distillation (ED). Both types have long application histories, and have been used for all reformate, pygas and COLO streams. However, due to the nature of different extraction theories, one may be preferred over the other for certain feed sources or certain product requirements. Fig. 2 illustrates the simplified configuration of LLE and ED systems.

The BTX-rich stream is the feed entering the system, where the aromatics and non-aromatics are separated by solvent extraction. The aromatics leave the system as extract product, while non-aromatics leave the system as raffinate product. In both systems, extraction solvent is used for aromatics extraction and is recirculated internally. The solvent containing extracted aromatics is called rich solvent; the recirculated solvent after separation from aromatics is called lean solvent. Only a very small amount of solvent is lost slowly over time (solvent degradation) through the carryover of the raffinate and extract.

Liquid-liquid extraction (LLE) system

LLE, developed in the 1960s, is the oldest type of aromatics extraction system. It consists of four major columns, as shown in Fig. 2: extractor, stripper, water wash column and recovery column. LLE takes place in the extractor. When the extractor is filled with liquid, the solvent dissolves (extracts) the aromatics moving downward, while the undissolved non-aromatics move upward. The liquid moves based on the density difference between the two liquid phases, with the extraction efficiency purely based on the “solubility difference” of aromatics and non-aromatics compounds in the extraction solvent.

FIG. 3. Solubility trend of different compounds in extraction solvent.
FIG. 3. Solubility trend of different compounds in extraction solvent.

Fig. 3 shows the solubility trend of different compounds in the extraction solvent. The aromatics have relatively higher solubility than non-aromatics (paraffins and olefins/naphthenes) in the solvent, which provides the fundamental extraction theory for LLE. However, the hydrocarbons with lower carbon numbers (C5–C6) also have relatively higher solubility than the hydrocarbons with higher carbon numbers (C8–C9), causing significant amounts of light non-aromatics to be dissolved and moved along with solvent to the bottom of the extractor. The extractor bottoms are sent to the stripper, where the light hydrocarbons (including the light non-aromatics) are stripped out and sent back to the extractor to reject additional light non-aromatics. By doing so, the bottom of the stripper can have the minimum amount of non-aromatics to achieve the required aromatics purity. However, a large internal loop develops between the extractor and the stripper, which is responsible for a significant portion of energy consumption in an LLE system.

The rich solvent from the bottom of the stripper is sent to the recovery column, where the aromatics are stripped out as extract product, and the lean solvent is recirculated to the extractor. Due to the high solvent carryover with non-aromatics from the top of the extractor, the water wash column is used to wash and recover the solvent before the raffinate product is collected.

The solvent used in LLE must be stable, nontoxic and simple to separate from aromatics in the recovery column. Additionally, the solvent must provide a decent solubility difference between aromatics and non-aromatics for LLE to work efficiently. Table 2 shows the critical physical properties for different commercially available solvents for LLE. In terms of overall critical solvent properties, Sulfolane and TECHTIV are considered to be the best choices for aromatics extraction solvents for an LLE system.

Extractive distillation (ED) system

ED is a relatively new type of aromatics extraction system, with less equipment and generally higher efficiency compared to LLE. ED was developed in the 1970s, and used over time with different solvents to enhance its performance. The system consists of two major columns, as shown in Fig. 2.

As opposed to solubility difference being the fundamental theory for LLE, the ED separation is based on the “relative volatility” of non-aromatics over aromatics created by a solvent environment. The higher relative volatility allows non-aromatics to be separated from aromatics more easily. The relative volatility can be boosted by the increase of the solvent-to-feed ratio, but with the cost of increased energy consumption and larger equipment. Therefore, the selection of the high-selectivity solvent is critical for an ED system. Table 3 shows the relative volatilities of C7 non-aromatics over C6 aromatics (benzene) under a different solvent environment in ED with a fixed solvent-to-feed ratio.

The solvent environment creates different relative volatilities for different compounds—i.e., it alters the boiling points for different compounds under the influence of the extraction solvent (Fig. 4), which is also what happens in the ED column (EDC). In the solvent environment of an EDC, the non-aromatics, which are relatively lighter (lower boiling point), will be distilled in the overhead of the EDC as the raffinate product. Meanwhile, the aromatics that are relatively heavier (higher boiling point) go to the bottom of the EDC with the solvent. The rich solvent from the EDC bottom is then sent to the solvent recovery column (SRC), where the aromatics are stripped out as extract product, and the lean solvent is recirculated back to the EDC (Fig. 2).

FIG. 4. Boiling point changes of compounds in the ED solvent environment.
FIG. 4. Boiling point changes of compounds in the ED solvent environment.

Fig. 4 also illustrates the impact of using different solvents in an ED system. Since the higher-selectivity solvent provides higher relative volatilities, a boiling point difference can be created between total non-aromatics and total aromatics so that they can be separated in the EDC. On the other hand, if the lower-selectivity solvent that provides lower relative volatilities is used, then the boiling point difference between non-aromatics and aromatics would be smaller (even overlapping, in cases) with the same solvent-to-feed ratio, or it would require a higher solvent-to-feed ratio (unreasonably high, in cases) to have the same boiling point difference.

When the boiling point difference between non-aromatics and aromatics is not enough or overlaps, then the boiling range in the feed needs to be reduced (e.g., BTX extraction reduced to BT extraction, or BT extraction reduced to B extraction); otherwise, the aromatics extract product will be contaminated by the non-aromatics and will fail to meet the required purity. As a result, different solvents in the ED system have different flexibility for the boiling range of the feed and, therefore, have limits on the extracted aromatics product. Table 4 shows the flexibility of the feed boiling range and the aromatics product of each solvent for ED.

LLE system vs. ED system

Looking at capital investment on the basis of the same feed and the same solvent, LLE always has higher investment than ED due to the necessity of more major columns and other equipment. LLE also has higher solvent inventory and requires a larger plot space, which adds to the equipment cost.

Although ED is a better choice in most cases, from an energy consumption point of view, LLE would use less energy consumption than ED in some special cases when the feed has very low aromatics content (< 40%). Normally, the common BTX-rich feed (Table 1) has high aromatics content that favors the ED system; however, in rare cases where the BTX-rich stream is blended with a non-aromatics stream, or taken from straight-run naphtha, such feed may have < 40% aromatics content that favors the LLE system for the purpose of lower energy consumption.

The LLE system has other inherent problems that need to be considered, including a more complicated system (leading to longer startup time), higher corrosion potential, higher risk of no phase separation when aromatics content is too high, and there is lower benzene purity and difficulty in expanding capacity. Table 5 shows a general comparison between the LLE and ED systems.

In comparison, the ED system has clear advantages over the LLE system for aromatics extraction in most aspects. The same trend is also seen in the industry, in that most new aromatics extraction units have chosen ED, and more old LLE units are being revamped to the ED system. Only in rare cases, such as when the feed has < 40% aromatics content or other special exceptions, does a feasibility study favor LLE.

Regardless of which system is chosen, multiple extraction solvents are available on the market for both. It is important that the extraction solvent selection is considered during the aromatics extraction unit design, in accordance with the feed characteristics and product requirements, to ensure optimized design and flexibility for the unit. A change of extraction system (LLE and ED) and/or extraction solvent for the existing aromatics extraction is possible; however, it requires a detailed feasibility study by the process provider that designs the specific solvent system. HP

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