P ROCESSING A water wash has been avoids the need for thermal regeneration found that effectively recov- of the solvent. ers the solvent in the IFPEX-1 offers a number of benefits IFPEXOL process by pulling over glycol technology, including more dissolved methanol into compact design with only one contactor Gas Processing water for subsequent tower (good for offshore locations), no removal. required heating, no foaming in contac- The water-solvent mixture and hydro- tors, reduced corrosion and solvent degra- carbon have a low interfacial tension that dation, and the ability to locate the cold often forms a stable emulsion and is diffi- section at a separate treating location. cult to separate by conventional settling Fig. 1 shows a schematic of IFPEX–1, processes. Effective separation of this emulsion and reuse of the methanol is critical in achieving Liquid-liquid separation technology optimum cost-effectiveness of the process. IFPEXOL is a cryogenic liquids-sepa- including the location for the advanced improves IFPEXOL ration process that uses methanol for coalescer technology. dew point suppression instead of glycol. The inlet natural gas is first split into process economics But the solvent losses resulting from the two streams. One, consisting of 50% of dissolution of methanol in the generat- the total flow, passes through the IFPEX-1 Peter Hampton Petro-Canada Oil & Gas ed hydrocarbon liquids can be very contactor tower against rich methanol, Calgary costly. while the remainder of the gas stream bypasses the tower. Thierry Darde The process The bypass stream and the IFPEX-1 Pall Europe Developed by Institut Francais du contactor outlet stream are recombined, Saint Germain-En-Laye, France Petrole, IFPEXOL is capable of dehydrating makeup methanol is injected, and the gas Robert James (IFPEX-1) and sweetening (IFPEX-2) stream is then fed to a chiller. Pall (Canada) Ltd. sweet and sour natural gas and liquid Condensation of the water phase and Mississauga, Ont. streams.1 2 heavier hydrocarbons occurs during pas- The IFPEX-1 process employs methanol sage through the chiller. Thomas H.Wines Pall Corp. instead of the commonly used triethylene The liquids are separated in the three- East Hills, NY glycol (TEG) to dehydrate the gas.This phase, low-temperature separator. IFXPEX-1 PROCESS* Fig. 1 Make up methanol Cold process Cold dry gas Cold separator Condensed hydrocarbons Methanol-water Inlet raw wet gas Pall Aqua Sep liq.-liq. coalescer Pall prefilter Water injection Water/LPG static mixer LPG product to storage Recovered water Methanol-water *With LPG water wash and liquid/liquid coalescer. Reprinted with revisions, from the April 16, 2001 edition of OIL & GAS JOURNAL Copyright 2001 by PennWell Corporation P ROCESSING Gas flows to sales, while liquid hydro- be successful with conventional separation Stages carbons are further processed to produce devices, including gravity separators or The liquid-liquid coalescing system LPG with a defined specification. mesh pad coalescers. operates in three stages: separation of The heavy methanol, rich aqueous When the drop size created is in the solids, coalescence, and separation of coa- phase (75% MeOH) is recycled to the top micron size range, a stable emulsion lesced drops: of the IFPEX-1 contactor. As the gas rises forms that greatly enhances the extraction • Separation of solids. Solids can in- in the contactor, the methanol is vapor- process by increasing the contacting sur- crease the stability of an emulsion, and re- ized and stripped from the methanol-rich face area, allowing for a smaller overall moving solids can make coalescing easier. aqueous phase so that almost pure water extraction system. Generally, this step can be achieved by is collected from the bottom of the The difficulty then lies in the separa- a separate cartridge filter system or by a IFPEX-1 contactor.The methanol is recov- tion of the stable emulsion, and this can regenerable backwash filter system for ered by condensation in the cold process be accomplished with advanced poly- high levels of solids. In addition, the filtra- section. meric liquid-liquid coalescer technolo- tion stage protects the coalescer and The LPG can be further treated for gy. increases service life. additional methanol recovery with a mix • Coalescence. In the next step in the tank and water-wash stage to recover sol- Advanced technology process, primary coalescence, the pore uble and carryover methanol from the Traditional liquid-liquid coalescers have dimensions begin with a very fine struc- LPG. used glass fiber medium, which works ture and then become more open to allow A high-efficiency polymeric liquid-liq- well for emulsions that have interfacial for void space for the coalescing droplets. uid coalescer system is downstream of the tensions greater than 20 dyne/cm and for In the primary coalescence zone, the water-wash stage to separate water- systems that have neutral water as the dis- inlet droplet dispersion containing fine methanol-LPG emulsions formed in the persed phase. droplets in the size range of 0.2 to 50 µm washing stage. Pall Corp. has developed new coalescer is transformed into a suspension of The expected solubility of methanol media constructed with formulated poly- enlarged droplets in the size range of 500- in the LPG stream is 2,000 ppm under mers and fluoropolymers that are effective 5,000 µm. normal process conditions, and the for emulsions having interfacial tensions The coalescence mechanism involves methanol content can be much higher as low as 0.5 dyne/cm and for harsh adsorption of droplets to the coalescer when free methanol and water are chemical environments.3 4 fibers, followed by translation along the emulsified. Additional water extraction The Pall liquid-liquid coalescer has fibers, and collisions at the junctures can be performed to recover this produced clean petroleum fuels with between fibers.The collisions result in the methanol. sodium levels less than 0.5 mg/l. and droplets merging or coalescing, and the The methanol-recovery process free water concentrations of less than15 viscous drag of the bulk fluid stream then involves the mixing of water with the ppm (vol). causes the enlarged drops to disengage methanol-rich LPG stream to allow for Fig. 2 shows a Pall AquaSep high-effi- from the fibers. sufficient mass transfer or extraction fol- ciency liquid-liquid coalescer in the verti- This process is repeated several times lowed by a final separation stage that cal configuration. through coalescer depth until the large breaks the immiscible mixture into an The system consists of a prefilter sec- coalesced drops exit the coalescer medium. organic and aqueous phase. tion followed by a vertical stacked coa- The necessary condition that droplet- Once the methanol-extract water lescer-separator cartridge stage that is a fiber adsorption occur for coalescing has stream is separated, it is injected into the patented design of Pall. been supported by a number of sources.5 6 IFPEX-1 contactor. Based on solubility The LPG and methanol-water emul- • Separation of coalesced droplets. Once limits of the natural gas feeding the sion first enters at the top of the coa- coalesced, the droplets are now assumed tower, the methanol is absorbed from the lescer housing and flows from the inside to be as large as possible for the given water into the gas while the stripped of the coalescer cartridges radially out- flow conditions.The separation stage is water is collected from the bottom of the ward, causing the enlargement or coa- achieved with hydrophobic separator car- tower. lescing of the inlet dispersion into large tridges that provide an effective barrier to It is possible that stable emulsions can droplets at the outlet of the coalescer car- aqueous coalesced drops but allow the be formed, depending on how the water- tridges. LPG to pass through them. wash stream is contacted with the The aqueous coalesced drops and LPG The methanol-water coalesced drops methanol-rich LPG. then flow axially downward where the are carried with the downward convective Mixing tanks, contactor columns, or flow then goes from the outside of the flow of the LPG and accumulate in a lower static in-line mixing devices can be used separator to the inside. collection sump.The purified LPG passes to increase the surface area of contact The large coalesced drops are repelled through the separators and exits at the between the two phases and enhance by the separators and collected in the bot- bottom of the housing.The methanol- extraction. tom sump. water in the collection sump can be man- Often it is necessary to keep the size of The purified LPG passes through the ually drained periodically or equipped the emulsified drops in the hundreds-of- separators and exits at the bottom of the with an automatic level control and drain micron size range so that separation can housing. system. Emulsion formation, stability cially detrimental to conventional glass Brazeu River gas plant Emulsion formation and stability can fiber coalescers.7 The surfactants can Petro-Canada Oil & Gas operates the be greatly affected by the addition of adsorb at the solid-liquid interface (coa- Brazeau River gas plant in Alberta.The methanol. lescer fibers) or at the liquid/liquid inter- plant processes a water-rich natural gas Methanol acts as a surfactant and face (water/oil). flow of 6,100 kg/hr (70 MMscfd) and a increases the solubility of the water in the When surfactants concentrate on the water-rich LPG flow of 38.5 kg/hr. LPG, thereby bridging the gap between coalescer fibers, this is known as “disarm- The IFPEX-1 process was selected so the two immiscible fluids (LPG and water) ing,” and the coalescer fibers are shielded that the existing plant infrastructure could and consequently strongly reduces the from the passing aqueous droplets.This be utilized, even though the capacity of interfacial tension.