Antisolvent precipitation solution

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. 

Gas anti-solvent processes (GASR, gas anti-solvent recrystallization GASP, gas antisolvent precipitation SAS, supercritical anti-solvent fractionation PCA, precipitation with a compressed fluid anti-solvent SEDS, solution-enhanced dispersion of solids) differ in the way the contact between solution and anti-solvent is achieved. This may be by spraying the solution in a supercritical gas, spraying the gas into the liquid solution. 

Hutchings and coworkers (78-83) pioneered the use of supercritical antisolvent precipitation to prepare a number of catalyst and support materials including vanadium phosphates. Vanadium phosphate precursor solutions were prepared from VOCI3 and H3PO4 refluxed in isopropanol. In the supercritical antisolvent precipitation method, a solution of the material to be precipitated and supercritical CO2 are pumped through a coaxial nozzle at temperatures and pressures above the critical point of... 

Based on the previous considerations, some authors proposed thermodynamic-based approaches to SAS. De la Fuente Badilla et al attempted to develop a thermodynamic-based criterion for optimum batch antisolvent precipitation (GAS) using a definition of the volume expansion that takes into account the molar volume of the system studied. They analyzed various binary and ternary systems and concluded that the pressure corresponding to a minimum value of the liquid-phase volume expansion coincides with the pressure at which the solute precipitates. In a subsequent work, Shariati and Peters further highlighted the role of SC-CO2 in GAS. It acts as a co-solvent (cosolvency effect) at lower concentrations, whereas at higher concentrations it acts as an antisolvent. 

Gas antisolvent processes can be performed in a semicontinuous mode. In this case the solution and the antisolvent are continuously introduced in the system until the desired amount of the product is formed. The introduction of the solution is then stopped and the DG flux extracts the residual solvent from the system. The system is then depressurized to enable collection of the product. The solution is generally introduced through an atomization nozzle that favors the prompt expansion of the solution and the formation of small particles. Different process configurations have been utilized, i.e., co- and countercurrent introduction of the solution and antisolvent fluxes and various nozzles have been designed. The process is referred to by different acronyms such as ASES (aerosol solvent extraction system), SAS (supercritical antisolvent), SEDS (solution enhanced dispersion by supercritical fluids), PCA (precipitation with a compressed fluid antisolvent), GASR (gas antisolvent recrystallization), GASP (gas antisolvent precipitation). 

Antisolvent Precipitation of Biological Molecules from Organic Solutions... 

An alternative approach to precipitate solutes from aqueous solutions is the use of a water-miscible compressed antisolvent, such as SCF NH3 Tc = 132°C, MPa). However, upon dissolution, NH3 increases the... 

Scrubbing the particle-loaded gas by a liquid solution in which is dissolved a coating agent that will entrap the particles either by co-acervation caused by antisolvent precipitation (37) or by deposition caused by coating agent sursaturation caused by solvent extraction by the fluid (88). 

The optical and scanning electron micrographs presented in this chapter show that the particle size of solid materials, such as polymers, monomers, and intermediate chemicals, can be altered by precipitation from a supercritical fluid solution. The only requirement for carrying out the SCF particle reduction process is that the compound must exhibit some solubility in a supercritical fluid. Because the pressure reduction rates are so rapid during the expansion of the solution, supersaturation ratios can be achieved that are much, much greater than can be achieved by thermal, chemical, or antisolvent precipitation processes. Furthermore, it is conjectured that such rapid nucleation rates can result in the particle formation of some materials with a size distribution or morphology that cannot now be achieved by any other process. 

The line for recycling the SCF is also indicated in Figure 2.3-7. Knowledge of the volumetric expansion of the liquid phase with pressure and of the precipitation pressure at the selected temperature are essential for performing any supercritical antisolvent precipitation experiment. The concentration of HC in the starting organic solution also affects the shape and dimensions of the product obtained. 

Some examples of commercial active component production and production of substances with defined and uniform particle sizes (organic and inorganic materials) realized on pilot plant by using the RESS are given in Table 24.8. Other processes were also tested for synthesis of the particles with uniform size distribution as well as production of particles with specific structure (gas antisolvent recrystallization, GASR precipitation with a compressed antisolvent, PCA solution enhanced dispersion of solids, SEDS particles from gas-saturated solutions, PGSS) as shown in Table 24.9. All these processes are of special interest in pharmaceutical industry and in the production of different polymers. 

In the second method the solution is sprayed through a nozzle into compressed carbon dioxide the process is termed as precipitation with compressed antisolvent (PCA) [33] and liquid or supercritical antisolvents can be employed. In the case of continuous flow of the solution and of the antisolvent the process is termed also as aerosol solvent extraction system (ASES) [34], in the case of countercurrent flow and supercritical antisolvent precipitation (SAS) in the case of co-current flow [35]. 

A key to successful chromatographic analysis lies in proper sample preparation. Ideally, it is preferred to dissolve the sample in a suitable solvent and analyze that solution directly, provided the presence of dissolved polymer does not complicate the chromatographic analysis. These cases are indeed rare. Normally, filtration or precipitation followed by final filtration is desirable to remove interferences in sample components (polymers) and higher-molecular-weight components. This approach works well when the polymers are, first, soluble, and second, can be precipitated with an antisolvent. Less soluble polymers, such as highly crystalline resins, require extraction to remove the components of interest from the resin matrix. Numerous extraction techniques (supercritical fluid extraction, solvent extraction, resin dissolution followed by antisolvent precipitation, etc.) are also available [14]. 

APl/polymer solution Antisolvent Precipitation Filtration Washing Drying Milling... 

ASES aerosol solvent extraction system, GAS gas antisolvent precipitation, PGSS precipitation from gas-saturated solutions, RESS Rapid expansion of supercritical solutions, SAS supercritical... 

Figure 12.8 Process of an antisolvent precipitation of a solution of an organic solute in an organic solvent using near-critical or supercritical carbon dioxide. A solution of...

The variability in an isothermal antisolvent precipitation can best be described in a triangular solubility diagram (Figure 12.15). The tie line for a certain concentration of the active in the organic solvent and the tie line for a certain mixing ratio of the solvent and antisolvent are shown. The intersection of both lines defines the supersaturation at ideal mixing. By varying the solute concentration and the ratio of solvent and antisolvent, different supersaturation can be realized Table 12.1 exemplifies typical values. 

Obrzut, D. L., Bell, P. W., Roberts, C. B., Duke, S. R. [2007]. Effect of process conditions on the spray characteristics of a PLA + methylene chloride solution in the supercritical antisolvent precipitation process,/ Supercrit Fiuids, 42,299-309. 

Microparticles can be produced by a simple technique that consists of spraying a polymer, e.g., PLLA, solution in dichloromethane (or dimethylsulfoxide), through a nozzle into a reactor filled with supercritical carbon dioxide (Reverchon et al, 2000). This process is known as supercritical antisolvent precipitation (SAS). The experimental parameters have a limited influence on the particle size (1-4 /im). A modified version of the process, known as the SAS-EM process, allows nanoparticles of a controlled size (30-50 nm) to be produced (Chattopadhay et al., 2002). In order to restrict the use of an organic solvent. Pack and co-workers fed the SAS reactor with a solution of PLLA prepared by homogeneous ring-opening polymerisation in supercritical HCFC-22 (Pack et al, 2003a). 

Supercritical antisolvent precipitation technique has been used to prepare a new titania catalyst support. The titania precursor was prepared by precipitating TiO(acac)2 from a solution of methanol using supercritical carbon dioxide at 110 bar and 40°C. The titania support was used to prepare a gold catalyst with high activity [44]. 

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). 

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