Extractive Distillation Column Overhead System for Aromatics Recovery
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Solution Overview
Problem
Existing extractive distillation processes face challenges in achieving high purity of aromatic hydrocarbons, particularly for full-range feedstocks, due to high levels of C8 naphthenic impurities, which affect subsequent xylene isomerization and purification units and result in significant benzene loss to the overhead raffinate stream.
Innovation Solution
A novel configuration for the extractive distillation column (EDC) involves recycling a solvent-rich stream to the lower portion to extract benzene from the overhead raffinate stream, reducing heavy non-aromatics in the EDC bottom and enhancing benzene recovery, while introducing a solvent-rich stream to the overhead system to separate benzene and other aromatics, thereby reducing C8 naphthenic content in the aromatic product.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a conventional extractive distillation column is used to recover aromatic hydrocarbons from full-range feedstock, then the process is simpler to implement compared to liquid-liquid extraction, but the aromatic product contains high levels of C8 naphthenic impurities
Solution Approach 1:
The extractive distillation column is divided into two functional sections: an upper extraction section where solvent contacts feed to separate aromatics from non-aromatics, and a lower enrichment section that enhances separation of C8 naphthenes from aromatic products. This segmentation allows each section to optimize its function, achieving high purity while maintaining process simplicity.
Solution Approach 2:
A solvent-rich stream is introduced as an intermediary medium in the lower enrichment section. This solvent-rich stream acts as a mediator to selectively interact with C8 naphthenes, enhancing their separation from aromatic products without requiring complex additional equipment or processes.
2Manufacturing precision
If the solvent is configured to keep benzene at the EDC bottom to achieve high aromatic purity, then aromatic recovery is improved, but heavy non-aromatics are driven to the overhead causing significant benzene loss
Solution Approach 1:
Different quality requirements are applied to different sections of the column. The upper section maintains conditions favorable for keeping benzene at the bottom (high aromatic purity), while the lower section creates local conditions that prevent benzene loss by enhancing the separation of heavy non-aromatics through the solvent-rich stream.
Solution Approach 2:
The solvent-rich stream is introduced in advance in the lower enrichment section to pre-separate heavy non-aromatics before they can contaminate the aromatic product. This preliminary action prevents the need for aggressive solvent configurations that would cause benzene loss.
3Productivity
If the EDC is operated to maximize aromatic recovery, then productivity is improved, but the level of naphthenic impurities in the C8 aromatic product increases
Solution Approach 1:
The column is segmented into extraction and enrichment sections, allowing the extraction section to maximize aromatic recovery while the enrichment section simultaneously removes C8 naphthenic impurities. Both functions operate in parallel, achieving high productivity and high purity together.
Solution Approach 2:
The solvent-rich stream serves as an intermediary that enables simultaneous achievement of high aromatic recovery and high C8 aromatic purity by selectively interacting with and removing naphthenic impurities in the lower enrichment section.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach significantly improves the purity of C8 aromatic hydrocarbons, reducing non-aromatic contaminants and maintaining benzene quality as a suitable gasoline blend stock, achieving aromatic recovery comparable to liquid-liquid extraction processes.
Implementation Method 1
mixing a third solvent-rich stream having the same composition as said first solvent-rich stream, with said non-aromatic HCs-rich stream in step (b) to generate a solvent phase and a raffinate phase
Implementation Method 2
In ED, a nonvolatile polar solvent is added to an extractive distillation column (EDC) to increase the relative volatility between the more-polar and less-polar components that have close-boiling points
Implementation Method 3
The overhead vapor is condensed and a portion of the condensate is recycled to the top of the EDC as reflux
Data Source
AI summary
Extractive distillation for recovering aromatic hydrocarbons employs an extractive distillation column with a novel overhead system including a partial condenser. The process enables (i) efficient removal of heavy non-aromatics, particularly C8 naphthenic compounds, from the EDC bottom stream to increase the purity of aromatic products, especially of the mixed xylenes and (ii) reduction (or better control) of benzene loss to the raffinate product stream to maintain its quality as a gasoline blend stock and, as a result, enhance benzene recovery in the aromatic products. Feedstock includes a full-range feedstock, such as pyrolysis gasoline, or a narrow-range feedstock, such as reformate.