Polyethylene Hexane Purification Reducing Reactor Fouling
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Solution Overview
Problem
The production of polyethylene in slurry processes faces inefficiencies due to high percentages of heavies in recycled hexane, leading to reactor fouling, increased downtime, and elevated costs, as conventional purification processes fail to effectively separate and recycle hexane, resulting in suboptimal heat-mass transfer and process delays.
Innovation Solution
A process and apparatus for manufacturing polyethylene that involves polymerizing ethylene in hexane, separating the polymer, and treating the liquid medium in wax distillation and distillation columns to obtain purified hexane with reduced heavies, followed by adsorption to produce dry hexane, which is recycled, thereby minimizing fouling and optimizing heat-mass transfer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of substance
If conventional purification processes are used to recycle hexane, then hexane can be recovered and reused, but the recycled hexane contains high percentages of heavies that cause reactor fouling and increased downtime
Solution Approach 1:
The purification process is divided into multiple sequential stages: flash distillation column for initial separation, main distillation column for further purification, and adsorption beds for final polishing. Each stage targets specific impurities and operates with distinct parameters, transforming a single inadequate purification step into a multi-stage systematic approach that effectively removes heavies while recovering hexane
Solution Approach 2:
Adsorption beds containing activated alumina or molecular sieves are introduced as intermediary components between the distillation columns and the reactor. These adsorbents act as mediators that selectively capture residual heavies and moisture from the hexane stream, providing an additional purification barrier that prevents fouling without interfering with the main distillation process
2Ease of manufacture
If recycled hexane with high heavies content is used in polymerization, then production cost is reduced through recycling, but heat-mass transfer becomes limiting and plant must operate at lower load
Solution Approach 1:
The multi-stage purification system performs preliminary removal of heavies and moisture from recycled hexane before it enters the polymerization reactor. By pre-cleaning the hexane to specified purity levels (heavies < 0.5%, moisture < 10 ppm), the system ensures that subsequent polymerization operations can proceed at full design capacity without heat-mass transfer limitations, thereby maintaining high productivity while still utilizing recycled feedstock
3Reliability
If frequency of hot boiling reactors is increased to remove fouling, then reactor cleanliness is improved, but downtime increases and process delays occur
Solution Approach 1:
The purification system performs preliminary removal of fouling precursors (heavies and moisture) from the recycled hexane before it contacts the reactor. By preventing fouling accumulation through continuous high-purity hexane supply, the system eliminates or significantly reduces the need for periodic hot boiling cleanup operations, thereby maintaining reactor cleanliness while minimizing interruption to production
Solution Approach 2:
The enhanced purification process skips the need for repeated reactor shutdowns and hot boiling cycles by ensuring fouling-free hexane is supplied continuously. This allows the polymerization process to run uninterrupted at full capacity, rushing through what would otherwise be periodic maintenance intervals and converting them into continuous operation
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
The process significantly reduces the percentage of heavies in recycled hexane, decreasing reactor fouling, downtime, and operational costs, while enabling efficient heat-mass transfer and retrofittable, economic, and user-friendly operation.
Implementation Method 1
The obtained purified liquid medium is then treated in at least one distillation column operating at a temperature in the range of 85° C. to 140° C. and at a pressure in the range of 0.5 kg/cm2 to 1 kg/cm2, to obtain wet hexane comprising hexane and moisture from the top of the at least one distillation column and the heavies from bottom of the at least one distillation column
Implementation Method 2
The moisture from the wet hexane is adsorbed in an adsorption unit to obtain dry hexane
Implementation Method 3
Polymerizing ethylene monomers, and optionally one or more co-monomers, in hexane in the presence of a catalyst to obtain a slurry containing mixture of polymer in a liquid medium The polymerization process is a highly exothermic process
Data Source
AI summary
The present disclosure relates to a process for manufacturing polyethylene. In the process, the purified liquid medium obtained from the wax-distilling process is further fed to a distillation column to obtain wet hexane comprising hexane fraction and moisture from the top of the column and heavies from the bottom of the column. The so obtained wet hexane is then treated in an adsorption unit to obtain dry hexane, which can be back-fed into the polymerization process. The process yields hexane with reduction in the concentration of the heavies, leading to reduction in the fouling of reactors and other equipments, thereby reducing the downtime of the reactors. The present disclosure also discloses an apparatus for manufacturing polyethylene. The apparatus has a simple and economic design and can be retrofitted in the existing plants.