P-xylene Crystallization Separation via Simulated Moving Bed
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Solution Overview
Problem
The separation of p-xylene from a mixture of isomers with similar boiling and melting points is challenging, requiring complex processes like multi-stage cryogenic crystallization or simulated moving bed molecular sieve adsorption, and achieving industrially acceptable yields is difficult when p-xylene concentration in the feedstock is low.
Innovation Solution
A process involving feeding a mixed xylenes stream with greater than or equal to 60% p-xylene to a crystallization unit for cooling crystallization, followed by filtration and purification using a simulated moving bed or multiple moving beds, to obtain high-purity p-xylene crystals and mother liquor, optimizing conditions such as temperature and bed layer configurations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If multi-stage cryogenic crystallization separation is used to separate p-xylene from isomers with similar boiling points, then separation capability is improved, but device complexity and process difficulty increase significantly
Solution Approach 1:
The patent changes the separation parameter from boiling point (distillation) to melting point (crystallization). By cooling the mixed C8 aromatic hydrocarbons to −50°C to −20°C, p-xylene crystallizes while other isomers remain liquid, achieving separation based on melting point differences rather than boiling point differences.
Solution Approach 2:
The patent utilizes the phase transition of p-xylene from liquid to solid through cooling crystallization. The mixed C8 aromatic hydrocarbons are cooled to induce p-xylene crystallization, and the resulting slurry is then heated to melt the crystals and separate the liquid p-xylene from other components.
2Productivity
If simulated moving bed molecular sieve adsorption separation is used to separate p-xylene, then separation efficiency is improved, but device complexity and operational difficulty increase
Solution Approach 1:
The patent replaces the complex adsorption separation system with a simpler thermal-based crystallization system. Instead of using molecular sieves and simulated moving bed mechanisms, the invention uses temperature control to induce crystallization, followed by simple filtration and phase separation.
3Manufacturing precision
If rectification process is used to separate o-xylene with the highest boiling point, then separation is achieved, but the number of theoretical plates required exceeds one hundred and reflux ratio becomes relatively high
Solution Approach 1:
The patent changes the separation approach from boiling point-based distillation to melting point-based crystallization. This parameter change allows separation of o-xylene through its higher melting point relative to other isomers, avoiding the need for excessive theoretical plates and high reflux ratios required by distillation.
4Manufacturing precision
If rectification process is used to separate ethylbenzene with the lowest boiling point, then separation is achieved, but the difficulty is much higher compared to o-xylene separation
Solution Approach 1:
The patent changes the separation parameter from boiling point to melting point. Ethylbenzene separation becomes easier because it has a significantly lower melting point (−94.95°C) compared to p-xylene (13.3°C), allowing clear separation through crystallization without the operational difficulties encountered in distillation.
5Productivity
If two-stage crystallization process is used when p-xylene concentration in feedstock is low, then industrially acceptable yield is achieved, but process complexity increases
Solution Approach 1:
The patent performs preliminary concentration of p-xylene through selective crystallization at controlled temperatures. By cooling to specific temperature ranges and controlling crystallization conditions, the process enriches p-xylene in the crystal phase, allowing high yield in a single stage rather than requiring two-stage processes.
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 process achieves high chemical purity and yield of p-xylene, with recovery yields ranging from 35% to over 87%, and reduces the complexity of separation by leveraging the differences in selectivity and melting points of the isomers.
Implementation Method 1
cooling crystallization
Implementation Method 2
obtain a slurry comprising p-xylene crystals
Implementation Method 3
filtration and purification unit
Data Source
AI summary
Provided is a process for crystallization separating p-xylene, comprising: a) feeding a mixed xylenes stream comprising greater than or equal to 60% by weight, of p-xylene, such as 60% to 98% by weight of p-xylene, to a crystallization unit to perform cooling crystallization, to obtain a slurry comprising p-xylene crystals; and b) feeding the slurry to a filtration and purification unit, to obtain a mother liquor, washings, and p-xylene, wherein the filtration and purification unit uses a simulated moving bed or a combination of multiple moving beds.