Benzene Recovery via Liquid-Liquid Extraction in Extractive Distillation
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Solution Overview
Problem
Current benzene recovery processes in extractive distillation face inefficiencies due to MCH contamination in the raffinate stream, leading to benzene loss and increased energy consumption, as well as low column efficiency caused by low non-aromatic content in the feedstock.
Innovation Solution
The process involves modifying conventional extractive distillation columns by introducing a lean solvent stream recycled from the bottom of a solvent recovery column into the upper portion of the EDC, using a liquid-liquid extraction column to separate benzene from MCH, and optionally adding a stripping agent to prevent channeling, thereby reducing benzene loss and MCH contamination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the EDC reboiler duty is increased to prevent MCH from descending into the column, then MCH contamination in benzene product is reduced, but benzene loss to overhead raffinate stream increases and energy consumption increases
Solution Approach 1:
A liquid-liquid extraction column is introduced as an intermediary unit between the EDC overhead and the final raffinate product. This extraction column uses a selective solvent to preferentially extract benzene from the overhead stream, allowing MCH to remain in the raffinate while recovering benzene in the extract phase. This mediator system enables MCH separation without requiring excessive reboiler duty in the EDC.
Solution Approach 2:
The invention changes the separation parameters by operating the extraction column at conditions optimized for benzene-selective solvent interaction, rather than relying solely on temperature-based distillation in the EDC. By adjusting solvent flow rate, solvent composition, and extraction stage conditions, the system achieves selective benzene recovery without increasing EDC energy consumption.
2Manufacturing precision
If the EDC reboiler duty is increased to prevent MCH from descending into the column, then MCH contamination in benzene product is reduced, but energy consumption in the EDC increases
Solution Approach 1:
The extraction column serves as a mediator that handles MCH removal functionality, allowing the EDC to operate at lower, more energy-efficient conditions. The extraction process uses mass transfer rather than thermal energy to achieve separation, decoupling the MCH removal function from high-energy reboiling.
Solution Approach 2:
The invention replaces the thermal-mechanical separation mechanism (distillation based on volatility differences) with a chemical-mass transfer mechanism (liquid-liquid extraction based on solvent selectivity). This substitution allows separation at lower temperatures and reduces reliance on high reboiler duty.
3Productivity
If a top distillation column with reduced diameter is used, then column efficiency is improved, but high external reflux is required to prevent channeling
Solution Approach 1:
The extraction column acts as an intermediary that handles the bulk of the separation load, allowing the overhead distillation column to be smaller in diameter without requiring excessive reflux. The extraction process compensates for the reduced column capacity, maintaining overall efficiency without channeling problems.
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 minimizes benzene loss to the raffinate product, reduces MCH contamination in the benzene product, increases benzene recovery, and decreases energy consumption, while extending HDS catalyst life by allowing more flexible operation of the hydrodesulfurization unit.
Implementation Method 1
using a liquid-liquid extraction column to separate benzene from MCH
Implementation Method 2
extractive distillation (ED) with a non-aqueous selective solvent to produce a high purity benzene product
Implementation Method 3
introducing a lean solvent stream recycled from the bottom of a solvent recovery column
Data Source
AI summary
Recovering high purity benzene from hydrocarbon feedstock containing aromatics and non-aromatics is implemented by simple and low-cost modifications to conventional extractive distillation columns (EDCs). Methyl cyclohexane (MCH) that is generated through non-selective hydrogenation of toluene in hydrodesulfurization (HDS) units is a major contaminant in benzene production. To meet MCH specifications, often times the extractive distillation (ED) process for recovering purified benzene is operated with excessive benzene loss to the overhead raffinate stream, producing a lower quality non-aromatic product. Novel techniques (1) remove operational constrictions of the HDS unit on MCH production, thus lengthening the catalyst life and (2) allow the EDC to drive essentially any amount of MCH away from the bottom benzene product without concerns with benzene loss to the overhead raffinate stream and (3) recover benzene from the overhead raffinate stream to upgrade the quality of non-aromatic product and increase the benzene product recovery.