Method for recycling polyolefin tail gas
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Solution Overview
Problem
Current methods for recovering valuables from vent gas in polyolefin production are inefficient, leading to high energy consumption, low purity of N2 and hydrocarbons, and increased investment and material costs due to suboptimal separation sequences and energy usage.
Innovation Solution
A process involving a compression cooling separation step, heavy hydrocarbon separation, light hydrocarbon separation, N2 purification, and turbo expansion, with a membrane separation procedure, where the steps are arranged in an optimized sequence to reduce energy consumption and improve the purity and recovery of N2 and hydrocarbons, using external cooling with a temperature no lower than ambient.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If compression-condensation method is used for vent gas recovery, then hydrocarbons can be separated, but energy consumption and investment become relatively high when high recovery is required
Solution Approach 1:
The vent gas recovery process is divided into multiple separation steps: compression-condensation separation first removes heavy hydrocarbons, then membrane separation extracts light hydrocarbons, and finally pressure swing adsorption purifies nitrogen. This segmented approach allows each step to target specific components, achieving high overall recovery without requiring excessive energy input at any single stage.
Solution Approach 2:
The process utilizes changes in pressure and temperature parameters across different separation stages. Compression increases pressure to enable condensation of heavy hydrocarbons, while subsequent membrane and adsorption stages operate at different pressure conditions to selectively separate light hydrocarbons and nitrogen. These parameter changes enable efficient separation without maintaining continuously high energy input.
2Productivity
If membrane separation method is used alone, then separation factor and efficiency are higher, but product purity and pressure cannot meet direct reuse requirements
Solution Approach 1:
The process combines three different separation methods: compression-condensation, membrane separation, and pressure swing adsorption. Each method contributes its strength - compression-condensation handles heavy hydrocarbons, membrane separation efficiently extracts light hydrocarbons with high separation factor, and pressure swing adsorption delivers high-purity nitrogen. The merging of these methods achieves both high efficiency and high purity that no single method could provide alone.
3Temperature
If cryogenic technology is used through turbo expansion, then liquefaction is realized, but separation sequence optimization is lacking leading to suboptimal results
Solution Approach 1:
Before applying cryogenic turbo expansion, the process first removes heavy hydrocarbons through compression-condensation and light hydrocarbons through membrane separation. This preliminary action prepares the gas stream by eliminating components that would interfere with or reduce the effectiveness of the subsequent cryogenic separation, thereby optimizing the overall recovery when turbo expansion is applied.
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 compressor energy consumption, enhances the purity and recovery of N2 and hydrocarbons, thereby lowering overall system energy, investment, and material costs, while maintaining effective separation without the need for external cooling below ambient temperatures.
Implementation Method 1
the separation sequence is compression-condensation separation, membrane separation, and pressure swing adsorption separation
Implementation Method 2
the vent gas is generally recovered by a compression-condensation method. Since different components have different boiling points, through reducing a temperature of the vent gas, some gas can be liquefied and separated
Implementation Method 3
the most prominent improvement is the realization of cryogenic technology through turbo expansion or throttle expansion
Data Source
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AI summary
A process for recovering valuables from vent gas in polyolefin production is disclosed. The process includes a compression cooling separation step, a heavy hydrocarbon separation step, a light hydrocarbon separation step, a N2 purification step, and a turbo expansion step in sequence. The N2 purification step comprises a membrane separation procedure. The light hydrocarbon separation step comprises at least one gas-liquid separation procedure. A first gas, which is obtained by the gas-liquid separation procedure and is heated through heat exchange with multiple streams in the light hydrocarbon separation step, enters the heavy hydrocarbon separation step and is further heated; the heated first gas then enters the N2 purification step; a first generated gas, which is obtained by the membrane separation procedure of the N2 purification step, enters the heavy hydrocarbon separation step and the light hydrocarbon separation step in sequence, and is cooled through heat exchange with multiple streams in the heavy hydrocarbon separation step and the light hydrocarbon separation step; and then the cooled first generated gas enters the turbo expansion step. The energy consumption of a compressor can be greatly reduced. An external cooling medium with a temperature lower than an ambient temperature is not needed. The purity and recovery of N2 and hydrocarbons can be improved, which can facilitate reduction of energy consumption of a whole system, an investment, and a material consumption.