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Technology Profile: Benzene Production from Pyrolysis Gas

By Chemical Engineering |

This column is based on “Benzene Production from Pygas,” a report published by Intratec. It can be found at: www.intratec.us/analysis/benzene-production-cost.

Benzene is the simplest and most important industrial aromatic hydrocarbon. It is a versatile petrochemical building block, and is an intermediate for several industrially important commodity chemicals and polymers, such as ethylbenzene, cumene, cyclohexane and others. Benzene is used in the production of pharmaceuticals, specialty chemicals, plastics, resins, dyes and pesticides.

The process

The following paragraphs describe a process for benzene production from pyrolysis gasoline (pygas; a naphtha-range petroleum product with high aromatics content). Figure 1 presents a simplified flow diagram.

Figure 1. The diagram shows a benzene production process from pyrolysis gasoline

Figure 1. The diagram shows a benzene production process from pyrolysis gasoline

Prefractionator. Pyrolysis gasoline is fed to a prefractionator, in which the C6 fraction is separated from the charge stock and then directed to the hydrogenation treatment section. Two byproduct streams, one composed of C5 and lighter hydrocarbons and the other of C7 and heavier hydrocarbons, are removed from the prefractionator.

Hydrogen treatment. Along with hydrogen, the C6 cut from the prefractionator is fed to a pretreatment reactor, in which di-olefinic material is hydrotreated in the presence of catalysts (cobalt molybdate on alumina). The di-olefins are converted to olefins, which are then saturated. Thiophene and other sulfur compounds in the pygas are converted to H2S.

The reactor effluent is cooled, producing low-pressure steam, so as to condense hydrocarbons with boiling points higher than methane. The stream is fed to a high-pressure separator for the recovery of hydrogen. A gaseous stream rich in hydrogen is routed to the hydrogen purification stage, while a liquid stream is directed to the extractive distillation column.

Hydrogen purification. The gaseous stream from the flash drum is mixed with a reformer-grade hydrogen make-up, and then fed to a multi-stage absorber column for the removal of residual hydrocarbons. The washed hydrogen-rich gas is compressed and recycled. The extract stream, collected from the bottoms of the absorber, is fed to a distillation column, where all the light hydrocarbons absorbed from the hydrogen-rich stream are recovered as the top product. The bottoms product (the regenerated aromatic stream), is recycled to the absorber column.

Extractive distillation. In this step, the liquid C6 hydrocarbons stream from the hydrogenation treatment is fed to the middle section of an extractive distillation column. The solvent for the extraction, N-formylmorpholine (NFM), is supplied to the top of the column.

The top product is raffinate, rich in C6 non-aromatic species. The bottom product from the extractive distillation is fed to the solvent recovery column, for the recovery of benzene. NFM solvent is recovered through the column bottoms and is directed to the extractive distillation column. High purity benzene is obtained from the column overhead.

Production pathways

Until the 1930s, commercial benzene was primarily produced from coal. Since then, as new catalytic processes were developed, petroleum became the main source for this chemical. Figure 2 presents different pathways for benzene production.

 

Figure 2.  Benzene can be produced via several different pathways

Figure 2. Benzene can be produced via several different pathways

Economic performance

The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce benzene was about $1,060 per ton of benzene in the third quarter of 2014. The analysis was based on a plant constructed in the U.S. with capacity to produce 250,000 metric tons per year of benzene.

Edited by Scott Jenkins

Editor’s note: The content for this column is supplied by Intratec Solutions LLC (Houston; www.intratec.us) and edited by Chemical Engineering. The analyses and models presented are prepared on the basis of publicly available and non-confidential information. The content represents the opinions of Intratec only. More information about the methodology for preparing analysis can be found, along with terms of use, at www.intratec.us/che.

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