Para-xylene Production via Simulated Moving Bed Adsorption and Dual Isomerization
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Solution Overview
Problem
Current para-xylene production methods face challenges in achieving high purity and yield, particularly in existing units, due to the high cost and complexity of distillation columns for separating isomerization streams, and limitations in co-producing para-xylene with other xylene isomers.
Innovation Solution
The process involves a simulated moving bed adsorption unit producing three effluents: an extract rich in para-xylene, an intermediate raffinate for vapour phase isomerization, and a 2-raffinate for liquid phase isomerization, with additional steps for distillation and crystallization to achieve para-xylene purity of at least 99.7%, while reusing existing equipment and minimizing costs.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a single distillation column is used to separate isomerization streams, then the equipment cost is reduced, but the separation efficiency and purity of para-xylene cannot be sufficiently improved
Solution Approach 1:
The single distillation column is segmented into two separate distillation columns, each performing a specific separation function. The first column separates ethylbenzene from the C8 aromatic stream, while the second column separates para-xylene from meta-xylene and ortho-xylene. This segmentation allows each column to be optimized for its specific separation task, achieving high para-xylene purity without requiring an overly complex single-column design.
Solution Approach 2:
The isomerization unit acts as an intermediary between the adsorption unit and the distillation system. It converts meta-xylene and ortho-xylene from the adsorption raffinate into para-xylene, which then feeds into the distillation system. This intermediary step reduces the burden on the distillation columns by pre-concentrating para-xylene before separation, improving overall separation efficiency.
2Productivity
If existing unit capacity is increased without modifications, then operating cost remains stable, but production capacity cannot be increased
Solution Approach 1:
The adsorption unit is designed with multi-functionality to serve both separation and feed preparation functions. The same adsorption beds that separate para-xylene from the feed also produce a raffinate stream that is fed to the isomerization unit. This multi-functional design allows the existing adsorption equipment to contribute to both the original separation function and the new production capacity expansion through isomerization.
Solution Approach 2:
The isomerization unit performs preliminary action by converting meta-xylene and ortho-xylene into para-xylene before the distillation step. This pre-conversion increases the para-xylene content in the stream entering the distillation system, reducing the separation burden and enabling higher production capacity from the existing distillation equipment.
3Manufacturing precision
If high purity para-xylene is produced through extensive distillation, then product purity is improved, but operating cost and energy consumption increase significantly
Solution Approach 1:
The isomerization unit serves as an intermediary that reduces the energy burden on the distillation system. By converting meta-xylene and ortho-xylene into para-xylene before distillation, it pre-concentrates the desired product, reducing the number of theoretical stages and reboiler duty required in the distillation columns to achieve the same purity level.
Solution Approach 2:
The process changes the compositional parameters of the feed stream to the distillation system through isomerization. By increasing the para-xylene content and reducing meta-xylene and ortho-xylene content in the isomerization effluent compared to the original adsorption raffinate, the distillation system operates under more favorable composition conditions, reducing energy consumption for separation.
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 allows for a 30% increase in para-xylene production capacity at a low cost without significantly increasing operating costs, while maintaining high purity and reducing the need for expensive distillation columns, by effectively utilizing two distinct isomerization steps and crystallization for final purification.
Implementation Method 1
a step for adsorption in a unit operating as a simulated moving bed from which at least three effluents are produced
Implementation Method 2
a step for isomerization of C8 aromatics, in a unit operating in the vapour phase and converting ethylbenzene
Implementation Method 3
a step for isomerization of C8 aromatics in a unit operating in the liquid phase or in the vapour phase, preferably in the liquid phase
Implementation Method 4
crystallization for final purification
Data Source
AI summary
A process for producing para-xylene from a hydrocarbon feed is described in which an adsorption column operating as a simulated moving bed with at least five zones delivers an extract, a 2-raffinate and an intermediate raffinate.The extract is distilled and the distillate is optionally re-crystallized to recover para-xylene with a purity of at least 99.7%. The 2-raffinate is distilled then isomerized in a reactor preferably operating in the liquid phase and at a low temperature. The intermediate raffinate with an enriched ethylbenzene content is distilled then isomerized in the vapour phase.