Propylene recovery system

Figure 14.12 Photograph of a membrane propylene recovery system installed at a polypropylene plant. This unit recovers approximately 450 kg/h of hydrocarbons. Image courtesy of www.mtrlnc.com, Copyright 2010 mtrinc...

It is prudent at this stage to briefly consider the problems that can be experienced in either refrigeration or heat recovery systems when water treatment is required to prevent freezing. The antifreeze treatment of pure water may be achieved by various means, typical ones being various brines, ethylene glycol, and propylene glycol. 

Stone Webster Eng., Corp. Ethylene/propylene Ethane, propane, butane naphtha Uses Ultra-Selective Conversion, USC, for furnace and Advanced Recovery System, ARS 120 2001... 

In this type of process, polymerization takes place in liquid propylene without the use of an inert diluent. This is a significant simplification over the traditional diluent slurry process, as propylene can be separated from the polymer by flashing, and there is no need for the extensive diluent recovery system. 

Because of the very high percentage of propylene recovery, plus high product purity specified for polymer grade, the reflux ratio required for a C-3 splitter usually is greater than 10 to 1. This system also is complicated by a shift in relative volatility of propylene to propane with the liquid-phase composition. The relative volatility drops from 1.14 in the propane-rich bottom section of the column to only 1.08 in the propylene-rich top section. Revamping such a column with an IMTP system provides up to a 20% increase in the number of theoretical stages available (as compared to the trayed tower) without reduction in column capacity. 

Gas expansion (heat recovery) and external cascade using ethylene and propylene systems supply refrigeration. The main features of Tech-nip s patented technology are ... 

The cracked gases are cooled and fractionated to remove fuel oil and water (2-5) then compressed (6), processed for acid-gas removal (8) and dried (9). The C3 and lighter material is separated as an overhead product in the depropanizer (10) and acetylene is hydrogenated in the acetylene converter (11). The acetylene converter effluent is processed in the demethanizer system (12-14) to separate the fuel gas and hydrogen products. The demethanizer bottoms is sent to the deethanizer (15) from which the overhead flows to the C2-splitter (16), which produces the polymer-grade ethylene product and the ethane stream, which is recycled to the furnaces as a feedstock. The deethanizer bottoms flows to the C3-splitter (18) where the polymer-grade propylene is recovered as the overhead product. The C3-splitter bottoms product, propane, is typically recycled to the furnaces as a feedstock. The depropanizer bottoms product, C4S and heavier, flow to the debutanizer (19) for recovery of the mixed-C4 product and aromatic-rich pyrolysis gasoline. 

The methyl-acetylene and propadiene in the C3 cut are hydrogenated to propylene in a liquid-phase reactor. Polymer-grade propylene is separated from propane in a C3- splitter. The residual propane is either recycled for further cracking, or exported. C4s and hght gasoline are separated in a debutanizer. Gas expansion (heat recovery) and external cascade using ethylene and propylene systems supply refrigeration. 

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