Compressed supercritical ethylene

Self-Diffusion in Compressed Supercritical Ethylene. The main purpose of our work ( ) was to provide transport data on dense supercritical ethylene and to analyze the data in terms of currently available theories. Ethylene was chosen for the study for a number of reasons. First, it is one of the most widely used solvents in industry due to its easily accessible critical temperature, its relatively low cost and wide availability. Highly accurate compressibility data are available (19-21) in the literature over a wide range of temperature and pressures. These data are necessary for a complete analysis of the transport data. 

Figure 2. Density and temperature dependence of the experimental self-diffusion coefficients of compressed supercritical ethylene.

General overview of several studies of transport and intermolecular interactions in compressed supercritical fluids is presented. The unique aspects of the instrumentation used in these studies are emphasized. First, the results of NMR studies of self-diffusion in supercritical ethylene and toluene are discussed. These experiments used the fixed field gradient NMR spin-echo technique. Second, the novel NMR technique for the determination of solubility of solids in supercritical fluids is described. 

Figure 3. The ratio D/D as a function of packing fraction for supercritical ethylene. A indicates ratios calculated using hard sphere diameters determined from diffusion data. 0 indicates ratios calculated using hard sphere diameters determined from compressibility data. The solid lines are the molecular dynamics results, extrapolated to infinite systems, of Alder, Gass and Wainwright (Ref. 24).

Ethylene is compressed to 2,700 bar and a free-radical initiator, e.g., trace amounts of oxygen or a peroxide, is injected into the feed stream to promote the free-radical polymerization. The polyethylene polymer that is formed remains dissolved in the supercritical ethylene phase at the operating temperature, which ranges from 140 to 250°C. The heat of reaction is removed by through-wall heat transfer when the tubular reactor is used and by regulating the rate of addition of initiator when the autoclave reactor is used. 

Much of the earliest information on ethylene polymerization can be found in the patent literature. In a 1946 patent Krase and Lawrence (1946) describe an SCF reaction process for making ethylene polymers. Ethylene is reacted in the presence of a catalyst at temperatures between 40 C and 400°C and at pressures from 800 to 4,000 bar. The polymer is then recovered using a stepwise reduction in pressure with the objective of reducing compression costs. The authors note in this very early patent that appreciable quantities of the polymer are still solubilized in the supercritical fluid phase at pressures as low as 150 bar. More than likely, the material still soluble at 150 bar consists of... 

The polymerization of vinyl monomers in liquid and supercritical CO2 has been studied extensively. Patents were issued in 1968 to the Sumitomo Chemical Company [81] and in 1970 to Fukui et al. [82] for the preparation of homopolymers of polystyrene, poly(vinyl chloride), poly(acrylonitrile) (PAN), poly-(acrylic acid) (PAA), and poly(vinyl acetate) (PVAc), as well as the random copolymers PS-co-PMMA and PVC-co-PVAc. Additionally, a patent was issued in 1995 to Bayer AG [83] for the preparation of styrene/acrylonitrile copolymers in SCCO2. In 1986, the BASF Corporation was issued a Canadian patent for the precipitation polymerization of 2-hydroxyethylacrylate and various N-vinylcarboxamides in compressed carbon dioxide [84]. In 1988, Terry et al. attempted to homopolymerize ethylene, 1-octene, and 1-decene in SCCO2 for the purpose of increasing the viscosity of CO2 for enhanced oil recovery [85]. These reactions utilized free-radical initiation with benzoyl peroxide and r-butylperoctoate at 71 °C and 100-130 bar for 24-48 h. Although the resulting polymers were not well characterized, they were found to be relatively... 

This brief survey begins in Sec. II with studies of the aggregation behavior of the anionic surfactant AOT (sodium bis-2-ethylhexyI sulfosuccinate) and of nonionic pol-y(ethylene oxide) alkyl ethers in supercritical fluid ethane and compressed liquid propane. One- and two-phase reverse micelle systems are formed in which the volume of the oil component greatly exceeds the volume of water. In Sec. Ill we continue with investigations into three-component systems of AOT, compressed liquid propane, and water. These microemulsion systems are of the classical Winsor type that contain water and oil in relatively equal amounts. We next examine the effect of the alkane carbon number of the oil on surfactant phase behavior in Sec. IV. Unusual reversals of phase behavior occur in alkanes lighter than hexane in both reverse micelle and Winsor systems. Unusual phase behavior, together with pressure-driven phase transitions, can be explained and modeled by a modest extension of existing theories of surfactant phase behavior. Finally, Sec. V describes efforts to create surfactants suitable for use in supercritical CO2, and applications of surfactants in supercritical fluids are covered in Sec. VI. 

I g Catalytic Polymerization of Ol ns in Supercritical Carbon Dioxide Table 8.11 Results of copolymerizations of ethylene and methyl acrylate in compressed carbon dioxide. 

With high pressure extraction, (HPE) [6.82-6.86] highly compressed gases such as carbon dioxide, nitrogen, argon, ethylene, propane, etc., (Table 6-14) are mainly used as solvents in a supercritical state. High-boiling substances are also dissolved ... 

STO Stoychev, L, Peters, F., Kleiner, M., Clere, S., Ganachaud, F., Chirat, M., Foirmel, B., Sadowski, G., and Lacroix-Desmazes, P., Phase behavior of poly(dimethyl siloxane)-poly(ethylene oxide) amphiphilic block and graft copolymers in compressed carbon dioxide, J. Supercrit. Fluids, 62, 211, 2012. 

Refer to Figure 4.1. Fresh ethylene and a chain transfer agent (modifier) are mixed with the low-pressure recycle stream previously compressed in a booster compressor, from almost atmospheric conditions up to 20-50 bar (depending on fresh ethylene conditions). The combined stream is then compressed to an intermediate pressure of 200-300 bar approximately (supercritical conditions) in a multistage primary compressor. Optionally, comonomers such as vinyl acetate, acrylic add, and methyl acrylate can be used for the production of ethylene copolymers, and in this case are typically fed at the discharge of the primary compressor. 

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