Propylene recovery

Fuel. Propylene has a net heating value of 45.8 MJ/kg (19,700 Btu/lb) and is often contained in refinery fuel-gas streams. However, propylene is diverted from streams for refinery fuel use in large quantities only when economics for other uses are unfavorable, or equipment for propylene recovery does not exist or is limited in capacity. Propylene is also contained in Hquid petroleum gas (LPG), but is limited to a maximum concentration of 5 vol % in certain grades (83) (see Liquefied PETROLEUM gas). 

In the depropanizer tower the propane and lighter gases are taken overhead to become feed to the ethylene and propylene recovery facilities. Separation is accomplished at a relatively low overhead temperature of -25°F to minimize reboiler fouling by olefin polymerization. 

Figure 8-4. A flow diagram for the hydration of propylene to isopropanol (1) propylene recovery column, (2) reactor, (3) residual gas separation column, (4) aqueous - isopropanol azeotropic distillation column, (5) drying column, (6) isopropyl ether separator, (7) isopropyl ether extraction.

The mixture leaving the bottom of the reactor consists of unreacted propylene and water, IPA, and diisopropyl ether (DIPE). In subsequent steps it is cooled, depressurized, and waterwashed. The unreacted propylene and by-product DIPE are flashed off and separated in a propylene recovery column. The unreacted propylene is compressed and recycled. 

IPA in concentrations of 91% or 99% is recovered in the same manner described in the indirect hydration route. Approximately 5% DIPE forms as a by-product in this process and comes out the bottom of the propylene recovery column. 

Steam cracking of ethane is the most widely used process for making ethylene. U.S. 6,578,378 (to Technip-Coflexip) gives a typical ethane cracker product composition and describes an improved separation process for ethylene recovery. U.S. 5,990,370 (to BP) gives yields for ethane, propane, and mixtures. U.S. 5,271,827 (to Stone Webster) gives details of furnace design and yields for a naphtha feed. Several other separation schemes for ethylene and propylene recovery are described in the literature. Estimate the cost of production for a new steam cracking facility that produces 1 million metric tons per year of ethylene and 600,000 metric tons per year of propylene. What feedstock would you recommend ... 

At present there is no question that the propylene shortage is a reality, and there is need to assess the potential as well as the actual sources of propylene. In this respect, the price that propylene should seek will provide a frame of reference. Consider that the gasoline value of propylene equals approximately 2.8 cents/lb (12.5 cents/gal -s- 4.35 lb/gal) and that propylene recovery costs should not exceed 0.2 cents/lb. The total—3.0 cents/lb—is a break-even area for a refiner. He will be sorely tempted to release propylene above this price and satisfy gasoline commitments by other means. The exact price, of course, will vary for each refinery and will be a function not only of its raw materials vs. sales picture but also of its accounting techniques. The 3.0 cents/lb figure could vary by as much as 0.5 cents/lb, and will depend on surplus of refining capacity (pool octane), isobutane, refining processes in place, etc. 

Description The ROG unit is broken down into sections including feed contaminant removal, ethylene recovery and propylene recovery. Feed contaminants including acid gases, oxygen, NO, arsine, mercury, ammonia, nitrites, COS, acetylenes and water must be removed. It is critical that the designer of the unit be experienced with feedstock pretreatment since many of the trace components in the offgas can have an impact on the ultimate product purity, catalyst performance and operational safety. 

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...

Figure 14. Simulated propylene recovery vs propylene mole fraction on free inert gas base for the ISA and the VSA without inert gas.

Alkaline effluents coming from polymerization and propylene recovery pH 11 to 12,... 

Operational changes or equipment modifications that result in enhanced propylene recovery in an FCCU absorber-stripper will inevitably increase hydrogen sulfide recovery. Moreover, the percent recovery of hydrogen sulfide from sour fuel gas may be an order of magnitude greater than the increased percent recovery of propylene from fuel gas. 

Overhead from the depropanizer is fed to the 03 splitter, after hydrogenation to convert the highly unsaturated methyl acetylene and propa-diene to propylene. The primary function of this tower is production of polymer grade or, in some cases, chemical grade propylene. A secondary function is propylene recovery because the bottoms from this column either is recycled to an ethane/propane cracking furnace or is used as propane fuel. 

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. 

Kim et al. (2009) fabricated membrane for propylene recovery from off-gas by incorporating silica nanoparticles of size 12-400 nm in a PDMS layer coated on a polysulfone (PSf) support. Hydrophilic fumed silica (Aerosil 200, particle size 12 nm) or silica nanoparticles prepared by a sol-gel method (particle size 300-A00 mn) were incorporated in the PDMS layer to make asymmetric MMMs. To prevent particle agglomeration, silane coupling was attempted using three types of silane agents 3-mercaptopropyltrimethoxysilane, 3-aminopropyl-trimethoxylsilane, and 3-methacryloxypropyl-trimethoxylsilane. Propylene/nitrogen mixture (volume ratio 15/85) was cooled to about 0°C before entering the membrane cell as feed to make propylene more condensable and a vacuum was applied on the permeate side. 

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