June 2020

Special Focus: Process Optimization

Best practices for aromatics extractive distillation in integrated complexes

Integrated refining/aromatics/olefins complexes are an efficient and effective configuration to maximize crude oil feedstock value and optimize the value of the entire product chain.

Liu, Z., Wang, Y., Zhao, L., Sulzer GTC Technology

Integrated refining/aromatics/olefins complexes are an efficient and effective configuration to maximize crude oil feedstock value and optimize the value of the entire product chain. A single site demonstrates the processing scheme from the raw material to final consumable products, along with the practical perspectives of both reliable operation and sustainable profitability.

Within the last two decades, many existing refineries and petrochemical plants have been transformed to the integrated refining/aromatics/olefins complex through revamps, expansions and additions. In recent years, several grassroots integrated refining/aromatics/olefins mega-projects have been put into commercial operation. Note: Here, the magnitude of “mega” is defined as a crude oil processing capacity for the complex of more than 15 MMtpy (300,000 bpd). At present, more complex projects are under construction.

Integrated facility operations

The purpose of the integrated complex is to maximize the production of aromatics and olefins (and their primary and secondary derivatives chemicals) from crude oil. Depending on market demand for transportation fuels, particularly gasoline, an FCC unit may or may not be included in the complex configuration. Hydrocracking has been the primary conversion route to hydroprocess all distilled cuts from the crude oil, except for straight-run naphtha.

Both straight-run and derived naphtha are sent to the reformer for aromatics production. Naphtha is also sent to the ethylene cracker (as the main cracker feedstock) with other byproduct paraffins-rich streams within the refining section for olefins production. Block flow diagrams of the two integrated complex configurations are shown with FCC (FIG. 1) and without FCC (FIG. 2).

FIG. 1. Process flow for an integrated refining/aromatics/olefins complex with FCC.
FIG. 2. Process flow for an integrated refining/aromatics/olefins complex without FCC.

Naphtha reforming generates aromatics as its main product, while steam cracking of naphtha for ethylene production generates aromatics as a significant byproduct. Both aromatics products have co-boiling, non-aromatics molecules. To obtain the high-purity aromatics molecules (benzene, toluene and xylenes, or “BTX”) for external market sale or for internal downstream processing, the non-aromatics molecules must be separated from the aromatics molecules.

The AED process

The physical separation between aromatics and non-aromatics is achieved by aromatics extractive distillation (AED) technology. The AED unit sits at the junction of the different sections (here referring to three sections—refining, aromatics and olefins) within the integrated complex. The unit receives the feedstock from the refining and olefins sections and supplies product to the downstream derivatives units within the aromatics section.

Extractive distillation (ED) is the method of separating close-boiling components, using an extractive solvent, which alters the volatility between the component molecules in the original mixture. The solvent is a polar component having a higher boiling point than the mixture to be separated. When applied in aromatics/non-aromatics separation, the volatility of the aromatics molecules is depressed relative to the non-aromatic components, such that the non-aromatics can be distilled overhead. The ED configuration for recovering BTX aromatics is shown in FIG. 3.

FIG. 3. Conceptual diagram of technology applicationa for aromatics extractive distillation.

The extractive distillation column (EDC) cleanly separates the aromatics and solvent into the bottoms from the solvent-free non-aromatics in the overhead. The solvent recovery column (SRC) strips the aromatics from the solvent, which is recycled back to the EDC. In this regard, a proprietary processa is offered for AED application. To recover the individual BTX product, the aromatics extract from the processa must be post-fractionated to high-purity BTX products.

Within the integrated complex, the feedstocks to AED from the refining and olefins sections—mainly reformate and hydrotreated pyrolysis gasoline (pygas)—are normally prefractionated to a light cut, heart cut and heavy cut, with the heart cut sent to the AED for aromatics/non-aromatics separation.

Typical paraffins (P), olefins (O), naphthenes (N) and aromatics (PONA) composition (wt%) of the heart cut from reformate is shown in TABLE 1, and hydrotreated pygas is shown in TABLE 2. For the application of AED within an integrated complex, the BTX product specifications usually follow the most stringent ASTM standards; however, sometimes a discrepancy is allowed, depending on the actual downstream processing configuration. TABLES 1 and 2 indicate the most stringent commercial specifications for BTX product.

For a physical separation unit operation, specifying the product purity is not enough. The product recovery must be specified for economic feasibility, as well. TABLE 3 indicates the typical performance parameters of both purity and recovery for BTX product after post-fractionating the processeda aromatics extract.

For an integrated refining/aromatics/olefins complex, maximizing paraxylene (PX) production is an obvious goal. All of the aromatics-rich streams, including reformate and pygas, will be collected together to feed the PX aromatics complex.

FIG. 4 shows a PX aromatics complex configuration. PX is the main product, while benzene is the main byproduct. To maximize PX generation, toluene and C9/C10 aromatics are advantageously used within the complex. Therefore, to optimize the energy consumption in AED and extract post-fractionation, the purity of extracted toluene and xylenes could be relaxed, if the downstream processes within the aromatics complex will not be negatively impacted.

FIG. 4. PX aromatics complex configuration.

Depending on the operating severity of reformer and PONA distribution of the feed to the reformer, some aromatics complexes choose not to send the reformate C8 cut to AED for extraction, since the non-aromatics portion is low enough to meet the feed specification for PX recovery. This is the reason that in some integrated complex projects, reformate C6–C7 cut and pygas C6–C8 cut are combined to send to AED for non-aromatics separation.

In addition to the indigenous benzene from the reformate and pygas, additional benzene will be generated within the aromatics complex. Any surplus benzene will be exported to the open market for sale, since not many integrated complexes will consume all of the in-house generated benzene internally for downstream derivatives. Therefore, benzene product purity requirement is always maintained at 99.9%.

When configuring the location of AED within an integrated complex, depending on the capacity of reformate and pygas, three options exist for consideration in the project definition stage:

  1. Two separate AED units each running independently
  2. One combined AED unit processing both feeds together
  3. Two EDCs (one processing reformate and the other processing pygas) with a combined SRC.

The sketches in FIG. 5 present the concepts of the three options. Each option has pros and cons. The final selection depends on various factors, such as production flexibility, operation management, human resources, ratio between reformate and pygas, plot space limitation, destination of the raffinate, equipment size limitations and more. Each integrated complex project has distinctive features to consider when choosing the configuration. However, when the priority is to maximize the value of each molecule, Option 3 could be an optimized choice.

Fig. 5. Three options for configuring the location of AED within an integrated complex.

The non-aromatics breakdowns in reformate and pygas are different from each other. The reformate raffinate contains about 85% paraffins, which makes it a preferred feedstock for ethylene cracking. Pygas raffinate contains about 60% naphthenes, which makes it a preferred feedstock to reform. Under the current scenario, competition in the global and domestic markets is becoming more intense. Every molecule counts toward the maximization of profitability. Segregating reformate raffinate from pygas raffinate presents an economic way to direct individual molecules to their preferred destination.

As for the CAPEX and OPEX comparison for these three options, data from a case study is included in TABLE 4.


The proprietary BTX technologya is a fully commercialized (FIG. 6) and proven aromatics extractive distillation technology. Compared with other aromatics technologies, it has successfully demonstrated several advantages during normal operation (due to the proprietary solventg and EDC/SRC tower internals):

Fig. 6. BTX unita in an integrated complex in China.
  • Lowest solvent to feed ratio (for the same product purity and recovery)
  • Lowest energy consumption (for the same product purity and recovery)
  • Lowest solvent carryover into the product
  • Lowest solvent makeup in the normal operation
  • Highest product recovery (for the same product purity)
  • Highest product purity (for the same product recovery). HP


        a Sulzer GTC’s GT-BTX process
           b Sulzer GTC’s CrystPX process
           c Sulzer GTC’s GT-STDP process
           d Sulzer GTC’s GT-TransAlk process
           e Sulzer GTC’s GT-IsomPX process (EBI)
            f Sulzer GTC’s GT-IsomPX process (EBD)
            g Sulzer GTC’s TECHTIV solvent

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