Antisolvent

Methods for isolation of the product polycarbonate remain trade secrets. Feasible methods for polymer isolation include antisolvent precipitation, removal of solvent in boiling water, spray drying, and melt devolatization using a wiped film evaporator. Regardless of the technique, the polymer must be isolated dry, to avoid hydrolysis, and essentially be devoid of methylene chloride. Most polycarbonate is extmded, at which point stabiUzers and colors may be added, and sold as pellets. 

Supercritical fluids can be used to induce phase separation. Addition of a light SCF to a polymer solvent solution was found to decrease the lower critical solution temperature for phase separation, in some cases by mote than 100°C (1,94). The potential to fractionate polyethylene (95) or accomplish a fractional crystallization (21), both induced by the addition of a supercritical antisolvent, has been proposed. In the latter technique, existence of a pressure eutectic ridge was described, similar to a temperature eutectic trough in a temperature-cooled crystallization. 

Gas AntisolventRecrystallizations. A limitation to the RESS process can be the low solubihty in the supercritical fluid. This is especially evident in polymer—supercritical fluid systems. In a novel process, sometimes termed gas antisolvent (GAS), a compressed fluid such as CO2 can be rapidly added to a solution of a crystalline soHd dissolved in an organic solvent (114). Carbon dioxide and most organic solvents exhibit full miscibility, whereas in this case the soHd solutes had limited solubihty in CO2. Thus, CO2 acts as an antisolvent to precipitate soHd crystals. Using C02 s adjustable solvent strength, the particle size and size distribution of final crystals may be finely controlled. Examples of GAS studies include the formation of monodisperse particles (<1 fiva) of a difficult-to-comminute explosive (114) recrystallization of -carotene and acetaminophen (86) salt nucleation and growth in supercritical water (115) and a study of the molecular thermodynamics of the GAS crystallization process (21). 

D. J. Dixon, "Formation of Polymeric Materials by Precipitation with a Compressed Fluid Antisolvent," Ph.D. Dissertation, University of Texas at Austin, Austin, Tex., 1992. 

At pressures and temperatures above the eritieal point, where liquid and vapour phases beeome indistinguishable, supereritieal fluids (SCFs) exhibit very different properties to those of the liquids or gases at ambient. Partiele formation in SCFs oeeurs as a result of a rapid inerease in supersaturation, either by means of expansion or by antisolvent mixing proeesses. Thus Chang and Randolph (1989) demonstrated that small (<1 pm) uniform partieles of... 

Shekunov, B. Yu., Baldyga, J. and York, P., 2001. Particle formation by mixing with supercritical antisolvent at high Reynolds numbers. Chemical Engineering Science, 56(7), 2421-2433. 

Depolymerization, e.g., polyethylene terephthalate and cellulose hydrolysis Hydrothermal oxidation of organic wastes in water Crystallization, particle formation, and coatings Antisolvent crystallization, rapid expansion from supercritical fluid solution (RESS)... 

Precipitation/Crystallization to Produce Nano- and Microparticles Because fluids such as C02are weak solvents for many solutes, they are often effective antisolvents in fractionation and precipitation. In general, a fluid antisolvent may be a compressed gas, a gas-expanded liquid, or a SCF. Typically a liquid solution is sprayed through a nozzle into CO2 to precipitate a solute. As CO2 mixes with the liquid phase, it... 

Nanoparticles of controllable size can be obtained in the supercritical antisolvent-enhanced mass-transfer (SAS-EM) process, which can... 

FIG. 20 22 Schematic of supercritical antisolvent with enhanced mass-transfer process to produce nanoparticles of controllable size. R, precipitation chamber SCF pump, supply of supercritical COg I, inline filter H, ultrasonic horn P, pump for drug solution G, pressure gauge. 

In an industrial application dissolution/reprecipitation technology is used to separate and recover nylon from carpet waste [636]. Carpets are generally composed of three primary polymer components, namely polypropylene (backing), SBR latex (binding) and nylon (face fibres), and calcium carbonate filler. The process involves selective dissolution of nylon (typically constituting more than 50wt% of carpet polymer mass) with an 88 wt % liquid formic acid solution and recovery of nylon powder with scCC>2 antisolvent precipitation at high pressure. Papaspyrides and Kartalis [637] used dimethylsulfoxide as a solvent for PA6 and formic acid for PA6.6, and methylethylketone as the nonsolvent for both polymers. 

Dixon, J. and Johnston, K. (1991) Molecular thermodynamics of solubilities in gas antisolvent crystallization. AIChE Journal, 37 (10), 1441-1449. 

Reverchon, E. (1999) Supercritical antisolvent precipitation of micro- and nano-particles. Journal of Supercritical Fluids, 15 (1), 1-21. 

Gas analysis, of water, 26 36-40, 41-42 Gas antisolvent (GAS) technique, 24 17 Gas-assisted injection molding (GAIM), 19 552... 

Precipitation inhibitors, dispersants contrasted, 3 686 Precipitation leachate procedure, synthetic, 25 868-869 Precipitation reactions, for niobium and tantalum determination, 27 142-143 Precipitation reagents, protein, 22 133 Precipitation with compressed antisolvent (PCA) process, 24 17, 18 Precipitator dust, in phosphorus manufacture, 19 12 Precipitators, electrostatic, 23 180 Precision agriculture, 23 328 26 269-270 Precision measurement techniques, noble gases in, 27 370 Precision scales, 26 245 Preconcentration, of uranium ores, 25 401 Pre-crosslinked polychloroprene grades, 19 852... 

A number of techniques are based on supercritical fluid technology. Three are of particular pharmaceutical interest, namely the supercritical antisolvent (SAS) system, the rapid expansion of supercritical solution (RESS) method, and the gas antisolvent (GAS) technique [126]. 

If the desired yield target cannot be achieved then an antisolvent system can be selected using the same techniques. The antisolvent should be fully miscible with the primary solvent and have a low solubility for the solute. It should be noted that the addition of an antisolvent to reduce solubility and generate supersaturation may introduce scale up issues, caused by the differences in micro-mixing performance between the laboratory and manufacturing plant. 

Once a set of potential solvents (and antisolvents) have been identified then the solubility behavior should be assessed in the laboratory, confirming the effect of temperature, the isolated solid form and the limits of purification. 

In this example a productivity of 100 kg/m3, yield of >85% and Maximum solids fraction at isolation of 0.3 are specified. The solvents that match these constraints are 1,4-dioxane, Ethanol, Isobutanol, Pentanol, Propanol, Isopropyl alcohol, 1,1,1-Trichloroethane and Water. On the basis of these results the first choice of a cooling crystallization from a single solvent appears feasible and an antisolvent addition should not be required. From this list the alcohols and water are certainly the best choices from a health, safety and environmental... 

This problem encompasses two single compound CAMD problems, namely design of solvent and anti-solvent and then identification of optimal mixture pair and its composition. The single component solvent design problem is the same as in case study 1 (Sub-problems 1, 2 and 3). The 10 molecules that are designed in the first case study are considered here. The single component antisolvent design proceeds as follows... 

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