Gas separation by membranes has acquired increasing importance in the petrochemical industry and is now a relatively well-established unit operation, especially in the production of polymers. The process of polymer degassing is necessary to suit polymer for extrusion and pelletizing, increasing safety, environmental, and product quality aspects. Nitrogen is generally used for this purpose, resulting in a vent gas primarily composed of monomers and nitrogen.
In early polyolefin plants, these streams were often used as boiler fuel. However, considering the current tight monomers market, particularly in propylene, and the benefits of membrane-based recovery processes, major polyolefin producers around the world already employ them in new state-of-the-art plants. In order to enhance the competitiveness of older plants, the use of a recovery solution is becoming mandatory.
In this report, the recovery of propylene and nitrogen from a polypropylene degassing vent stream is reviewed and compared with its use as boiler fuel. Included in the analysis is an overview of the technology and economics of a process similar to MTR VaporSep®. Both the capital investment and the operating costs are presented for a propylene recovery unit (PRU) operating in the US Gulf Coast, Germany, and Brazil.
The economic analysis presented in this report is based upon a recovery unit installed in a 450 kta polypropylene plant. The estimated CAPEX for such unit is USD 5.8 million on the US Gulf Coast, the lowest figure among the regions under analysis. Due to propylene scarcity, which raised its market value, and the low fuel prices due to natural gas growing offerings, installation of a membrane unit for this type of recovery is advantageous. This fact is proven by the calculated internal rate of return of more than 40% per year in the region.
Furthermore, local propylene and fuel market dynamics are the key drivers in evaluating a membrane separation unit’s attractiveness. If fuel can be obtained from inexpensive sources and propylene is priced high, the unit’s installation could be considered. This improvement is already part of state-of-the-art polypropylene technologies such as INEOS InnoveneTM and Lummus Novolen®, and is a remarkable solution for this type of recovery.
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