Propylene Recovery Unit - Full Scheme

PP Splitter: play a key role in Petrochemical sector because the main goal is to obtain from hydrocarbon stream chemical grade propylene 95% vol. or polymer grade 99.5% vol.

1. In the PP Splitter the propylene is separated from the propane and because of the close boiling points of propylene & propane, a tower with large number of trays - typically 200 or more, working at high reflux to feed ratio, greater than 7.0 is required to achieve the desired propylene purity for polymer grade specification and to achieve an economical propylene recovery. In order to make the PP Splitter attractive from economical point of view, heat pumped is use to reduce operating pressure and utilties. Also high capacity trays are available from different vendors and usually a tighter approach temperatures is used for reboiler to reduce heat pump utilities. Propylene as liquid product, is taken from the overhead, while the propane is the bottom of the PP Splitter and sent to storage.

2. Design Tips:
- Binary interaction parameters are mandatory for this application
- Reboiler minimum approach temperature to 7-8°C – do an optimization analysis to determine the minimum approach versus reboiler surface
- Check with Client the propylene specification to see what is maximum amount of C2- accepted – this will determine the presence or not of the de-C2 tower
- Also the dimensions constrains shall be verified with Client – will affect the design 1 (one) or 2 (two) columns
- COS as contaminat could not be removed by fractionation due to very close boiling point in respect to propylene (-50C / -48C)
- High capacity trays are an option, but check with vendors for references, ask about FF and operating range
- The reboiler surface could be reduce by using special tubes which increase heat transfer coeficient
- If heat pump is used ask for turndown to understand the limits of system (remember that the inlet of compressor is coming from PP Splitter top).

3. Important aspect: if you want to pass from polymer grade to chemical grade in operation/design check the overhead stream - propylene, for methyl acetylene & propadiene (usualy the methyl acethylene / propadiene leave the PP Splitter with bottom product - propane, but when you decrease the propylene purity the mentioned coumpound concentrates in overhead)!

4. Action to remove methyl acetylene & propadiene from system: keep the unit as stable as possible to maximize accumulation of methyl acetylene & propadiene between feed tray and tower bottom (methyl acetylene & propadiene have a volatility little higher than propane and slightly lower than propylene) - when the level of buil-up shall be sufficient the methyl acetylene & propadiene will start to exit with propane (bottom splitter stream).

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