Propane Purging in Polypropylene Manufacturing via Membrane Separation
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Solution Overview
Problem
In polypropylene manufacturing processes, the buildup of propane in the reactor loop reduces catalyst activity and productivity, as existing methods are inadequate for efficiently purging propane while minimizing propylene loss and recycling high-purity propylene.
Innovation Solution
A process involving polymerization, separation, and membrane separation, where the effluent from the polymerization zone is sent to a splitter to produce a propylene-rich overhead and a propane-rich bottom stream, with the bottom stream being washed and sent to a membrane separation zone to enhance propylene content and reduce propane content, allowing for recycling of the permeate back to the polymerization zone and purging of the propane-rich stream.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If conventional venting is used to control propane buildup, then propane content is maintained at steady-state, but propylene is lost and recycling efficiency is reduced
Solution Approach 1:
The vent stream is segmented into different components through membrane separation, allowing selective removal of propane while retaining propylene for recycling. The membrane divides the mixed stream into permeate (propylene-rich) and retentate (propane-rich) streams.
Solution Approach 2:
A membrane separation unit acts as an intermediary between the reactor loop and the purge system. This intermediary selectively transports propane molecules through the membrane while blocking propylene, enabling propane removal without propylene loss.
2Loss of substance
If membrane separation is used to separate propylene from propane in vent stream, then propylene recovery is achieved, but the membrane separation zone requires large size and high operating temperature
Solution Approach 1:
Before the membrane separation zone, a flash separation step is performed to remove heavy components and concentrate the propylene-propane mixture. This preliminary action reduces the load on the membrane system, allowing smaller membrane units to achieve the same separation effectiveness.
Solution Approach 2:
The process operates at lower temperatures compared to conventional membrane separation by adjusting the pressure and composition parameters. The flash separation step creates favorable conditions for membrane operation at reduced temperature, decreasing energy requirements and equipment complexity.
3Reliability
If propane is continuously purged to maintain steady-state, then catalyst activity is maintained, but overall reactor productivity is reduced
Solution Approach 1:
The system implements feedback control by monitoring propane levels in the reactor loop and adjusting the purge rate accordingly. The membrane separation unit provides continuous selective removal, maintaining propane content within optimal ranges without requiring excessive purging that would reduce productivity.
Solution Approach 2:
The membrane separation operates continuously to remove propane as it accumulates in the reactor loop, rather than periodic purging. This continuous action maintains catalyst activity consistently while minimizing disruption to reactor productivity through selective and efficient propane removal.
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 effectively reduces propane levels in the reactor loop, maintains high propylene recycling, and reduces the size and operating temperature requirements of the membrane separation zone, thereby enhancing catalyst activity and reactor productivity.
Implementation Method 1
sending the washed bottom stream to a membrane separation zone thereby producing a permeate having an enhanced propylene content and a reduced propane content and a purge having an enhanced propane content and a reduced propylene content
Data Source
Figure 1
AI summary
The present invention relates to the purging of propane in a polypropylene manufacturing process comprising, a) polymerizing propylene in a polymerization zone, b) withdrawing from said polymerization zone an effluent comprising polypropylene, propylene, propane and optionally a solvent, c) sending said effluent to a separation zone thereby producing polypropylene, optionally the solvent and a stream comprising propylene and propane, d) optionally recycling the solvent to the polymerization zone, e) sending said propylene and propane stream to a splitter thereby producing an overhead consisting essentially of propylene recycled to the polymerization zone and a rich propane bottom stream comprising a minor part of propylene, f) sending the bottom stream to a membrane separation zone thereby producing a permeate having an enhanced propylene content and a reduced propane content and a purge having an enhanced propane content and a reduced propylene content, g) recycling the permeate to the polymerization zone. In an embodiment the bottom stream produced in the splitter of stepe) is washed in a scrubber to remove contaminants and the overhead of said scrubber is sent to the membrane separation zone. The present invention is also the debottlenecking of a polypropylene manufacturing facility comprising, a polymerization zone, a separation zone to recover the polypropylene, optionally the solvent and a stream comprising propylene and propane and a splitter to separate propane and propylene, wherein, a membrane separation zone is provided, said membrane separation zone is fed with the splitter bottom thereby producing a purge and a permeate recycled to the polymerization zone. In an embodiment a scrubber is inserted between the splitter bottoms and the membrane separation zone stream to wash the splitter bottom stream to remove contaminants and the overhead of said scrubber is sent to the membrane separation zone.