No abstract

Exxon Mobil Corporation and W.R. Grace have developed an enhanced process for recovering solvents used in the refining of lubricants. The membrane‐based process provides higher yields of lube oil, while simultaneously consuming less energy, decreasing cooling water use, and potentially reducing the level of volatile organic emissions from the dewaxing operation. The yield increase equates to a process savings, for a typical commercial plant, of about 2 million barrels of crude oil per year. By recovering and recycling the dewaxing solvent while bypassing the energy intensive parts of the plant, a single facility could reduce fuel oil consumption by 36,000 barrels/year, and greenhouse gas emissions by about 20,000 tons/year for each plant. The same plant would reduce cooling water use by nearly 4 million gallons/day, and, the release of dewaxing solvents, which are volatile organic materials, could be decreased by 50–200 tons/year per plant depending on the mechanical condition of its equipment. This technology was first implemented commercially at Exxon Mobil's Beaumont, Texas, Refinery. It can be easily retrofitted into existing plants or incorporated into new plant designs, and is currently available for license as the trademarked process MAX‐DEWAX®.

Illinois coal was ground and wet-sieved to prepare three powder stocks whose particle-size distributions were characterized. Three suspending fluids were used (glycerin, bromonaphthalene, Aroclor), with viscosities ~/$ that differed by a factor of 100 and with very different chemistries, but whose densities matched that of the coal. Suspensions were prepared under vacuum, with coal volume fractions ¢ that ranged up to 0.46. Viscosities were measured in a cone-and-plate over a shear rate (9) range 10-3-102s -1. Reduced viscosity ~/r=r//r/$ is correlated in the high-shear limit (qw) with rk/OM~o, where CMoo is the maximum packing fraction for the high-shear microstructure, to reveal the roles of size distribution and suspending fluid character. A new model that invokes the stress-dependence of Cg is found to correlate ~/r well under non-Newtonian conditions with simultaneous prediction of yield stress at sufficiently high q~; a critical result is that stress and not governs the microstructure and rheology. Numerous experimental anomalies provide insight into suspension behavior.

In petrochemical or refinery operations, feed streams containing aromatics such as benzene, toluene, or xylenes can be selectively enriched or depleted in aromatics content by hyperfiltration. The development program for STARMEM included production of asymmetric Lenzing P84 polyimide membranes with various permeate fluxes and aromatics selectivities. Both 5-cm-diameter and commercial-scale 20-cm-diameter spiral-wound elements were designed and built containing these membranes. Tests of these modules at 55 bar (800 psi) and 50°C for up to 2000 h with a toluene recycle stream from a refinery showed stable operation in hot hydrocarbons. The high efficiencies of these modules can allow for high stage cuts (50% and greater) from aromatic process streams. By combining existing refinery operations with this membrane process, favorable cost savings through better purity, recovery, or throughput can be expected. With these membranes acting as organic solvent nanofiltration devices, the permeation properties can be interpreted through a solutiondiffusion equation for molecular transport. The different levels of nonaromatics rejection for the various membranes suggest that different distributions of nanopore dimensions are set during initial membrane formation.

Permselective separation of aromatic from non-aromatic hydrocarbons in feed streams can be accomplished by using membrane systems in hyperfiltration mode. In petrochemical or refinery operations, feed streams containing aromatics such as benzene, toluene, or xylenes can be selectively enriched or depleted in aromatics content. The development program for the STARMEM™ series of membranes led to a commercial process for their large-scale manufacture. A range of membranes in terms of flux and selectivity has been made, and both 2-inch and commercial scale 8-inch diameter spiral wound elements manufactured from these membranes. Tests at 800 psi (55 bar) and 50 °C with a toluene recycle stream have shown stable operation of these modules in hot hydrocarbons. The high efficiencies of these modules allow for high stage cuts (50% and greater) from process streams of thousands of barrels per day. By combining existing refinery operations such as distillation with these new membrane units, cost savings through better purity, recovery, or through-put can be achieved. With these membranes acting as tight hyperfiltration devices, the permeation properties can be modeled through a solution-diffusion equation for molecular transport. Since these membranes also act as solvent resistant nanofiltration devices, unusual benefits can be found while practicing aromatics enrichment of refinery streams, including fractionation of different molecular weight species.