Petroleum Refining Industry
The U.S. petroleum processing industry refines 97 percent of the nation's transportation fuels and produces raw materials for other industries, such as the chemical industry. The petroleum refining industry has annual shipments worth $136 billion and employs 77,000 workers. This industry is extremely energy intensive, with an annual energy consumption of 6 quadrillion Btus, and produces 180 million tons of waste per year (OIT, 1997c).
The petroleum refining industry uses the same traditional chemical engineering separation technologies as the chemical industry, including distillation, crystallization, adsorption, membrane processes, absorption and stripping, and extraction. Like the chemical industry, the petroleum refining industry would benefit from separation technologies with improved energy efficiency, raw materials efficiency, and cost effectiveness.
The petroleum refining industry would benefit from improved technologies for basic organic/organic separations, such as the separation of benzene from gasoline and the separation of aromatics from jet and diesel fuels. New membrane technologies are being developed for organic/organic separations, especially to address the problem of membrane swelling. Copolymers with hard segments for stability and
soft segments for selectivity could overcome this limitation. Recently, a new pervaporation technology based on a polyimide/polyester copolymer membrane system was developed for the separation of heavy catalytically cracked naphtha into an aromatics-rich permeate for gasoline blending and an aromatics-lean retentate for distillate (jet and diesel fuel) blending (Ho et al., 1996). Current pervaporation membrane systems will probably be most effective in hybrid pervaporation/distillation processes where pervaporation performs a first, crude, low-energy, low-cost separation, leaving the polishing operation for distillation. Higher selectivity/flux membranes and modules that can withstand aggressive organic mixtures at relatively high temperature pervaporation conditions (about 120°C to 250°C) would be more cost-effective and have more applications. Controlling membrane swelling is critical to improving selectivity. If high selectivity could be achieved with pervaporation, it could replace distillation in many separation processes.
Currently, the separation of olefins (ethylene, propylene) from paraffins (ethane, propane) on a commercial scale is accomplished almost exclusively via cryogenic distillation. These two separation processes are very energy intensive and, in 1988, accounted for 6.3 percent (about 0.15 quadrillion Btus) of the total distillation energy used by the chemical and petrochemical industries (Humphrey et al., 1991). A less energy-intensive method for separating olefins from paraffins would therefore be extremely beneficial.
The separation of olefins from paraffins could be accomplished with less energy if selective facilitated transport membranes were used. Facilitated transport membranes with a high olefin/paraffin selectivity of about 200 include cross-linked polyvinylalcohol-containing AgNO3 membranes (Ho and Dalrymple, 1994) and Ag+-exchanged perfluorosulfonic acid membranes (Koval et al., 1992; Davis et al., 1993). Both have the potential to save large mounts of energy from hydrocarbon feeds that do not contain poisonous sulfur compounds. Silver and cuprous complexing agents could also be incorporated into inorganic membranes in ways similar to those described for adsorbents. Olefin-complexing adsorbents, including an Ag+-exchanged polymer resin and a near-monolayer of CuCl dispersed on γ-Al2O3, might also be used to separate olefins from paraffins (Yang et al., 1997). For feeds containing sulfur compounds, a new membrane with a sulfur-resistant complexing agent and high selectivity will have to be developed.
Recently, polyimide membranes showing moderate selectivities of about 20 have been reported (Tanaka et al., 1996). A complete olefin/paraffin separation with membrane processes alone would require membranes with considerably higher selectivities, however, perhaps by an order of magnitude. A propylene/
propane separation of 100 using a carbon molecular sieve membrane has also been cited (Suda and Haraya, 1997), indicating that these membranes have potential for olefin/paraffin separations.
The conversion of methane to higher-value products is important for the petroleum refining industry and is essential for the development of remote natural gas reserves. Higher-value products that can be produced from methane include hydrocarbon liquids (methanol), synthetic lubricants and fuels, and olefins (ethylene).
Liquid hydrocarbons can be prepared from the synthesis gas generated from the partial oxidation of methane. Ethylene can be synthesized from the oxidative coupling of methane (Wang and Lin, 1995; Ramachandra et al., 1996; Zeng and Lin, 1997). All of these reactions require high-purity oxygen, however. Therefore, a source of inexpensive, high-purity oxygen would be an enabling separation technology. The alternative, using air, eventually requires a separation (e.g., of nitrogen from the synthesis gas product), which is as difficult as producing high-purity oxygen in the first place.
Inexpensive oxygen for use in the conversion of methane to higher-value products could be produced from air by the use of dense perovskite-type oxide membranes. High-purity oxygen could also be produced via a polymer-inorganic (zeolite) hybrid membrane, a zeolite molecular sieve membrane, or a carbon molecular sieve membrane.
Improved isomer separations could significantly reduce the energy consumption required for the manufacture of certain chemical products. One method uses molecular sieve membranes, including zeolite, carbon molecular sieve materials, and the (currently classified) metal and metal oxide membranes being developed at Oak Ridge National Laboratory. The isomer separations that could be achieved by this method include the separation of p-xylene from other xylenes and the separation of linear olefins (e.g., 1-butene) and paraffins (e.g., n-octane and n-dodecane) from their corresponding branched olefins (e.g., isobutylene) and paraffins (e.g., 2,2,4-trimethylpentane [iso-octane] and 2,3-dimethyldecane). Molecular sieve membranes could also be used in the membrane reactor configuration for the manufacture of p-xylene and linear olefins and paraffins if their corresponding isomerization reactions were enhanced. In this case, a chemical synthesis would be performed in conjunction with a closely coupled, but separate, membrane separation device. For example, para-xylene could be selectively produced by an equilibrium redistribution of mixed isomeric xylenes coupled with a selective transport of the product through a membrane (Scouten, 1997).
Separation of Asphaltenes from Petroleum
A lower energy method for separating asphaltenes from petroleum liquid would be useful to the industry and would significantly decrease the energy required for deasphalting with solvents (e.g., propane). The separation of asphaltenes from petroleum liquid could be achieved via high-temperature ultrafiltration. The removal of waxes from petroleum liquid could be accomplished either through ultrafiltration or microfiltration. Microfiltration via a membrane with 300 Å pores could be used to catch catalyst particles in a residual hydrotreater blowdown at about 450°C. In this microfiltration process, colloidal asphaltenses and some metals might also be captured and recycled to the hydrotreater with the catalyst particles. These applications will require the development of thinner nanostructured inorganic membranes (about 2,000, Å or less) to increase flux/productivity and to reduce module size. The industry would also benefit from lower cost fabrication of high-temperature membranes and modules, as well as fouling-resistant membranes and/or effective fouling control techniques, such as compositional doping, hydrodynamics, shear rate, and pressure (Ho and Ying, 1997).
Dehydrogenation of Ethylbenzene and Paraffins
Currently, the dehydrogenation of ethylbenzene to produce styrene and the dehydrogenation of paraffins to produce olefins are highly energy-intensive processes. Less energy-intensive separation methods would be useful for the petroleum refining industry.
Membrane reactors have the potential to significantly reduce the energy required for the dehydrogenation of ethylbenzenes and paraffins. High-temperature H2-selective membranes could be used as membrane reactors, including palladiumbased, zeolite, ceramic, silica, glass, and carbon molecular sieve membranes. These membranes could be prepared using several techniques: synthesis of thin, nanostructured, dense palladium-based films (Bryden and Ying, 1996; Mardilovich et al., 1996); molecular template-directed synthesis of nanostructured materials; novel procedures to plug large pores or reduce pore size in ceramic and other membranes; and the sol-gel technique using uniform nanosized sol particles (Chu and Anderson, 1996).
Direct Conversion of Hydrogen Sulfide to Hydrogen and Sulfur
Hydrogen sulfide (H2S) is produced along with methane from many natural gas fields, as well as from the hydrodesulfurization of crude oils containing sulfur compounds (e.g., thiophene, benzothiophene, and dibenzothiophene). Currently, the typical technology is to partially oxidize H2S and convert it to elemental sulfur
(S) and water in the Claus process, which recovers the elemental sulfur but loses the valuable hydrogen (H2). It would be advantageous to recover both the H2 and S through the direct conversion of H2S to these two products because the demand for H2 is increasing, and crude oils are getting more sour with higher sulfur content. A method for separating H2S into H2 and S with some input of energy would be of great benefit to the petroleum refining industry.
The recovery of both H2 and S through the direct conversion of H2S to these two products could be accomplished using a high-temperature, H2-selective membrane. A high-temperature (about 800°C) membrane reactor containing a molybdenum sulfide catalyst and a H2-selective membrane could effect the catalytic decomposition of H2S to H2 and S (Ma et al., 1994).
Hydrogen Gas Separation
The petroleum industry has a growing need for the separation of hydrogen from other gases. Examples include H2 recovery from refinery purge gases, such as hydrodesulfurization purge streams or H2/CH4 separation; H2 recovery from ammonia purge gases or H2/N2 separation; synthesis gas H2/CO ratio adjustment or H2/CO separation; and the recovery of H2 from gas containing H2 and H2S in coal gasification.
New membrane materials are promising technologies for hydrogen separation, including polyimides, polyaramides, and polypyrroldones. Thinner membranes (< 500 Å), which will decrease costs and increase flux/productivity, could reduce overall costs and expand markets. One potential application is the purification of a reformate gas to produce high-purity H2, which would require a high H2/CO2 selectivity membrane. If CO2 is the minor component in H2, a membrane with a high CO2/H2 selectivity that would allow the retention of H2 at feed pressure would be desirable. The recovery of H2 from gas containing H2 and H2S in coal gasification will require a high-temperature H2-selective membrane. This membrane would have the same requirements, e.g., high-temperature stability and high H2/H2S selectivity, as the membrane for the direct conversion of H2S to H2 and S described above.
Removal of Acid Gases
The petroleum refining industry needs better separation methods for the removal of acid gases from natural gases, such as the removal of CO2 and H2S from natural gas containing between 15 and 50 percent acid gases and 50 percent or more CO2. Potential applications include the recovery of CO2 from large-scale enhanced oil recovery projects.
The efficient separation of H2S from petroleum refinery and coal gasification process streams represents a continuing challenge. Amine-based acid gas scrubbers
for removing H2S from petroleum hydrodesulfurization streams are currently used. As noted above, it would be desirable to recover the hydrogen value from the extracted H2S. In coal gasification, where the removal of H2S and COS in a continuous process directly from the hot product gases is still a major unresolved technical challenge, considerable economic benefits could be achieved.
For the removal of acid gases from other gases, thinner membranes (< 500 Å) and new membrane materials with high selectivity for CO2 and H2S separation from CH4 would reduce costs by increasing productivity and reducing methane loss to the permeate. Potential applications for natural gas containing 50 percent CO2 (about 400 psi) or more will require a new membrane material with low plasticization by CO2. This could be achieved via new polymer-inorganic hybrid materials, in which the inorganic would control plasticization and swelling of the highselectivity polymer, or via copolymers. A CO2/CH4 selectivity of 80–100, or higher, would be desirable.
Removal of Volatile Organic Compounds from Gases
One important separation need in the petroleum refining industry is the removal of VOCs from various gases. Examples include the removal of gasoline vapor from storage-tank vents in distribution stations (Ohlrogge et al., 1995); the removal of chlorinated and chlorofluorinated hydrocarbons from air; and the removal of solvents from air streams originating from coating and finishing processes and other operations involving the vaporization of solvents. More efficient and cost-effective methods for the removal of VOCs from these gas streams would be extremely beneficial to the industry. Gas separation membranes with higher selectivity could reduce costs and would have a wide variety of applications.
Removal of Organics from Wastewater
The removal of organic compounds from process wastewater represents another challenge for the petroleum refining industry. Examples include the removal of hydrophobic solvents, such as 1, 1, 2-trichloroethylene, chloroform, and ethylene dichloride; and the removal of hydrophilic solvents, such as methanol, ethanol, isopropanol, acetic acid, formic acid, and acetone. The industry needs better and more cost-effective separation technologies to recover solvents and to remove organics from waste streams that might otherwise be released to municipal wastewater systems.
Pervaporation is the most promising membrane process for the removal and recovery of organics from wastewater. Baker and Wijmans (1992) have commercialized this technology for the removal of organic solvents, such as acetone, from wastewater. Currently, silicone robber type membranes are sufficiently permselective
to remove hydrophobic solvents, but they cannot remove hydrophilic solvents sufficiently, partly because of excessive membrane swelling. Thus, the industry needs new membranes that are selective for hydrophilic solvents.
Removal of Particulates
The petroleum refining industry needs better methods for removing particulates from gas streams, such as flue gases from traditional combustion units. In addition, the industry needs methods of removing submicron-sized particulates from coal liquefaction liquids.
High-temperature microfiltration is a promising approach for the removal of submicron particulates from coal liquefaction liquids (Baker et al., 1990), as well as from heavy crudes, shale oils, and tar sands. In addition, this separation technology could also be used for the treatment of used lubricating oil, particularly automotive crankcase oil, in which most of the contaminants are particulates. The membrane requirements for this application include high temperature stability, thinness, low-cost modules, and fouling resistance.
The removal of particulates from gas streams, such as flue gases from traditional combustion units, could be achieved via microfiltration membranes (e.g., modules composed of cordierite honeycomb monoliths coated with an inorganic microfiltration membrane, such as α-Al2O3). Removal efficiencies of greater than 99.9 percent can be achieved with this method (Bishop et al., 1994). However, the thickness (< 2,000 Å) of the nanostructured microfiltration membrane will have to be decreased to increase flux/productivity, and the costs associated with the fabrication of the membrane module will have to be reduced. Another emerging application is the microfiltration of particulates from hot gases generated from advanced coal conversion technologies (e.g., the pressurized fluidized bed combustor). Although alumina-mullite has sufficient stability for this application, nanostructured microfiltration membranes with uniform pore size would be extremely useful.
Like the chemical industry, the sophisticated separation technologies that the pertoleum refining industry uses could be improved to increase energy efficiency, raw materials efficiency, and cost effectiveness. Specific areas for improvement include the following:
- technologies for basic organic/organic separation processes, including the separation of benzene from gasoline and aromatics from jet and diesel fuels
- methods for separating olefins from paraffins
- methods of producing lower cost oxygen for methane conversion to synthesis gas
- processes for isomer separations, including p-xylene from the other xylenes and linear olefins and paraffins from their corresponding branched olefins and paraffins
- methods for separating asphaltenes from petroleum liquid
- methods for the dehydrogenation of ethylbenzene to produce styrene and the dehydrogenation of paraffins to produce olefins
- methods for recovering H2 and S from H2S with some input of energy
- methods for separating hydrogen from other gases, such as refinery purge gases, ammonia purge gases, and synthesis gas; methods for producing high-purity hydrogen
- methods for removing acid gases from natural gas
- methods for removing VOCs from air and various industrial process gases
- methods for removing organic compounds from wastewater and contaminated groundwater
- methods for removing particulates from gas streams and for removing submicron-sized particulates from coal liquefaction liquids