Deisohexanizer and Membrane Separation for Normal Hexane Recovery
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Solution Overview
Problem
Current processes for isomerization of paraffins to enhance octane rating in gasoline face challenges with high reflux ratios and heat duty in deisohexanizers, and membrane separation systems are not economically competitive due to large membrane surface areas required for effective separation of linear and branched paraffins.
Innovation Solution
The process involves operating a deisohexanizer to separate and recycle normal hexane from the bottoms stream using a membrane separation system, reducing heat duty and membrane surface area requirements, while maintaining high octane ratings by recycling a permeate fraction rich in normal hexane back to the isomerization reactor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a deisohexanizer is used to separate normal hexane from isomerate, then the octane rating of the isomerate product is improved, but the heat duty and operating costs increase substantially
Solution Approach 1:
The patent changes the separation parameter from thermal (distillation) to membrane-based separation. By using a deisohexanizer operating at reduced reflux ratios combined with membrane separation technology, the process achieves the required normal hexane removal while dramatically reducing the heat duty from 15+ kcal/L to significantly lower levels, thus resolving the contradiction between product quality and energy consumption
2Manufacturing precision
If membrane separation is used to separate normal paraffins from branched paraffins, then the separation efficiency is improved, but the membrane surface area required becomes excessively large
Solution Approach 1:
The patent segments the separation process into two stages: first, a deisohexanizer performs bulk separation of normal hexane at reduced reflux ratios, and second, a membrane separation unit performs final purification. This segmentation allows each unit to operate at optimal conditions, achieving high separation efficiency without requiring excessively large membrane surface areas
3Manufacturing precision
If high reflux ratios are used in the deisohexanizer to achieve complete separation, then the purity of the isomerate product is improved, but the operating costs and energy consumption increase
Solution Approach 1:
The patent applies partial action by using reduced reflux ratios in the deisohexanizer rather than maximum reflux, achieving sufficient separation for blending purposes. The remaining separation requirements are met by the membrane unit, which operates more efficiently. This partial approach to thermal separation combined with membrane separation reduces operating costs while maintaining adequate product purity
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 reduces operating costs by at least 10% and achieves a research octane rating of at least 88.5 in the deisohexanizer overhead, with a significant portion of normal hexane recovered from the bottoms stream, thus enhancing the efficiency and cost-effectiveness of the isomerization process.
Implementation Method 1
a significant portion of the normal hexane contained in the feed to the deisohexanizer is removed with the bottoms stream and is recovered from the bottoms stream through a membrane separation
Data Source
AI summary
In an isomerization process where the isomerization effluent (114) is fractionated in a deisohexanizer (116) to provide a lower boiling fraction (122) containing dimethylbutanes, a higher boiling fraction containing methylpentanes and a bottoms stream (120) containing normal hexane and heavies, the bottoms stream is contacted with a selective permeation membrane (124) to provide a normal hexane-containing permeate (128) suitable for recycle to the isomerization. The energy consumption of the deisohexanizer (116) can be reduced without adversely affecting the octane rating of the overhead (122).