LDPE Reactor Shutdown via Lock-out Valves and Nitrogen Purging
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Solution Overview
Problem
In high-pressure low-density polyethylene (LDPE) production, the shutdown of sub-systems is lengthy due to exposure to oxygen, leading to decomposition, increased temperatures and pressures, loss of ethylene and co-monomer, and VOC emissions, necessitating costly purging with nitrogen before restart.
Innovation Solution
Implementing a process that isolates sub-systems by closing lock-out valves and introducing nitrogen purge gas at controlled pressures to depressurize and purge the reactor, reducing the need for extensive nitrogen purging and minimizing oxygen exposure, allowing for concurrent maintenance and cleaning of other sub-systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of repair
If all sub-systems are shut down together during maintenance, then the targeted sub-system can be serviced, but the shutdown time increases and other sub-systems remain idle
Solution Approach 1:
The system is divided into multiple independently controllable sub-systems (reactor, compressors, separators, cooling recycle system). Lock-out valves create isolated segments that can be shut down individually for maintenance while other segments continue operating. This segmentation allows parallel maintenance activities and reduces overall shutdown time.
Solution Approach 2:
Before shutting down a sub-system, lock-out valves are closed to isolate it from the rest of the system. This preliminary isolation action enables the sub-system to be depressurized and maintained independently, while other sub-systems remain pressurized and operational, preparing the system for selective maintenance without full shutdown.
2Ease of repair
If sub-systems are exposed to atmosphere during shutdown, then they can be accessed for maintenance, but oxygen causes decomposition of LDPE product and increases safety risks
Solution Approach 1:
Nitrogen gas is used to purge and maintain an inert atmosphere within sub-systems during shutdown and maintenance. The nitrogen displaces oxygen, preventing decomposition of LDPE product and eliminating fire hazards. This allows maintenance personnel to safely access sub-systems without exposure to reactive atmospheres.
Solution Approach 2:
Nitrogen gas acts as an intermediary substance between the atmospheric environment and the sub-system interior during maintenance. It provides a protective barrier that prevents direct contact between oxygen and the LDPE product, enabling safe maintenance operations while maintaining system integrity.
3Object-affected harmful factors
If repeated nitrogen purging is performed at near-atmospheric pressure to remove oxygen, then oxygen is eliminated, but ethylene and co-monomer are lost to atmosphere and purging time increases
Solution Approach 1:
Oxygen is selectively extracted and removed from sub-systems using nitrogen purging, while valuable ethylene and co-monomer gases are retained within the system through proper valve control and pressure management. The purging process is optimized to remove only the harmful oxygen component.
Solution Approach 2:
The purging process utilizes pressure differential changes to control gas flow. By maintaining positive pressure within sub-systems during purging and controlling valve sequences, the system allows oxygen to be displaced while preventing loss of valuable hydrocarbon gases to the atmosphere.
4Object-affected harmful factors
If decomposition of LDPE product occurs due to oxygen exposure, then temperatures and pressures increase, but this creates safety hazards and requires longer shutdowns
Solution Approach 1:
Lock-out valves are closed and nitrogen purging is performed before any maintenance activities begin, preemptively preventing oxygen from contacting the LDPE product. This preliminary protective action eliminates the conditions that would lead to decomposition, temperature rise, and potential safety incidents during the maintenance period.
Solution Approach 2:
An inert nitrogen atmosphere is established in advance within sub-systems before maintenance operations. This cushioning protective environment prevents oxidative decomposition of LDPE, stabilizing temperatures and pressures, and providing a safety buffer throughout the maintenance process.
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 significantly reduces shutdown time, minimizes ethylene and co-monomer loss, and decreases VOC emissions by maintaining reactant gases within sub-systems during shutdown, enabling more efficient purging and reducing the number of nitrogen purging cycles.
Implementation Method 1
introducing purge gas comprising N2 into the reactor component through a purge gas inlet at a pressure greater than about 0.5 MPag and less than about 5.0 MPag
Data Source
AI summary
Processes for reducing shutdown time of a sub-system/ reactor component in an LDPE process. The process includes closing one or more pairs of upstream lock-out valves, each pair of upstream lock-out valves being located in an inlet stream upstream of the reactor component and configured to cease fluid flow into the reactor component through said inlet stream when said pair of upstream lock-out valves is closed; closing one or more pairs of downstream lock-out valves, each pair of downstream lock-out valves being located in an outlet stream downstream of the reactor component and configured to cease fluid flow out of the reactor component through said outlet stream when said pair of downstream lock-out valves is closed; depressurizing the reactor component; introducing purge gas comprising N2 into the reactor component at and withdrawing the purge gas from the reactor component.