Flash degassing

In the case of flash degassing, the polymer solution is first heated under pressure to above the boiling point of the volatile components and decompressed directly into the ZSK. The polymer and solvent (monomer) spontaneously separate from each other inside the ZSK and the majority of the volatile components are released via the back venting system. Depending on the pressure and the temperature, up to 90% of the solvent can be removed in this way. Efficiency depends on the temperature of the polymer solution at the feed intake, the pressure drop in the back vent, and the material properties of the feeding system. The back vent is located upstream from the polymer or polymer solution feeding port (see Fig. 10.2). In this case, there is no melt in the screw channel so that the entire screw cross-section is available for the removal of gas or vapors. 

Figure 10.2 Process and screw design in the flash degassing area...

The effectiveness of flash degassing and the temperature evolution along a ZSK 58 McPlus was determined experimentally as shown in Fig. 10.3. Degassing via the back vent continues until the solvent reaches the boiling point. The temperature increase of the polymer melt is relatively small due to the high content of solvent and therefore low viscosity in the vacuum zones 1 and 2 in addition, the polymer is constantly cooled by the enthalpy of vaporization of the solvent. 

Small concentrations of volatile components in a liquid mixture may accumulate in the vapor space of a container over time and appreciably reduce the flash point relative to the reported closed-cup value. This may be the result of degassing, chemical reaction or other mechanism. An example is bitumen [162]. Similarly, if a tank truck is not cleaned between deliveries of gasoline and a high flash point liquid such as kerosene or diesel oil, the mixture might generate a flammable atmosphere both in the tmck tank and the receiving tank. Contamination at the thousand ppm level may create hazards (5-1.4.3 and 5-2.5.4). Solids containing upward of about 0.2 wt% flammable solvent need to be evaluated for flammable vapor formation in containers (6-1.3.2). 

A solution of phenol (188 mg, 2 mmol) and benzonitrile (2.06 g, 20 mmol) in McCN (20 mL) was degassed by bubbling nitrogen through it and irradiated with a 16-W low-pressure mercury arc lamp (Applied Photophysics Ltd, APQ40) for 24h. The crude product was separated by flash chromatography (EtOAc/ petroleum ether 1 5) to give yellow crystals yield 79 mg (20%) mp 53-55 C. 

A solution of 3.7I g (0.013 mol) of tributyltin hydride in 10 mL benzene is added dropwise to a solution of 3.09 g (0.011 mol) of S-[(S)-4-(benzyloxy)-3-pentenyl] 5-methyl carbonodithioate in 25 mL of degassed, anhyd benzene under an atmosphere of argon, followed by 5 mg of AIBN. The mixture is heated under reflux for 2.5 h and then is concentrated under reduced pressure. Flash chromatography of the residue using ht,0/petroIeum ether 1 50 containing 1 % triethylamine as eluant gives a colorless oil yield 3.87 g (78%) [a] - 26 (e = 1. CHCl3). 

In the slurry process, propylene monomer is dissolved in a hydrocarbon diluent in which the polymerization process occurs. The polymerization products are either soluble (the highly atactic components) or insoluble. Both the insoluble and soluble components are collected and form separate product streams. The insoluble species form a slurry in the solvent, from which they are removed by centrifugation. The soluble, atactic component is removed with the solvent as another product stream. To separate the atactic polymer from the solvent, the solution is heated allowing the solvent to flash off, leaving the atactic polymer behind. Any un reacted monomer is degassed from the solution and recycled to the start of the polymerization process. 

Because excited triplet states decay more slowly than excited singlet states, it is much easier to determine the excited triplet-state lifetime 3t than H. Phosphorescence emission from a degassed sample at low temperature (77K) lasts for longer than 1ms and may even be several seconds. The molecules in the sample are irradiated with a short ( 1 ps) flash and the decay of the phosphorescence signal is monitored using an oscilloscope. Any accompanying fluorescence signal will decay too rapidly to be observed. The excited triplet-state lifetime is obtained as the time taken for the emission intensity to fall to 1/e of its initial value. 

Electron transfer [Eq. (1)] would occur at a rate near the diffusion limit if it were exothermic. However, a close estimate of the energetics including solvation effects has not been made yet. Recent support of the intermediacy of a charge transfer complex such as [Ph—NOf, CP] comes from the observation of a transient (Amax f 440 nm, t =2.7 0.5 ms) upon flashing (80 J, 40 ps pulse) a degassed solution (50% 2-propanol in water, 4 X 10 4 M in nitrobenzene, 6 moles 1 HCl) 15). The absorption spectrum of the transient is in satisfactory agreement with that of Ph—NO2H, which in turn arises from rapid protonation of Ph—NOf under the reaction conditions ... 

Direct flash excitation (>400 nm) or the triplet-acetone-sensitization of nitrosamide X in degassed water or benzene solutions gives the amidyl radical transient exhibiting lax 335-350 nm. This transient is not observed with undegassed solution of X, indicating that oxygen has intercepted the precursor of the amidyl radical at least, with the diffusion controlled rate... 

Isopiperitenone (0.5 g, 3.3 mmol), l,2-bis(trimeLhylsiloxy)cyclobutene (3.83 g, 16.7 mmol) and degassed pentane (50 111L) were placed in an Ace photochemical reactor and irradiated at rt with a 250-W Hg immersion lamp. Progress of the reaction was monitored by GC. When the peak corresponding to isopiperitenone had disappeared (3.5 h), the solvent was removed under reduced pressure and the residue distilled, bp 120 C/O.l Torr. The distillate was subjected to flash chromatography (petroleum ether hp 30-60 C/5% EtOAc), giving the desired product yield 0.31 g (24%). 

Evidence supporting this mechanism is presented for the case of acrylamide polymerization sensitized by riboflavin, but not for the case of fluorescein and its halogenated derivatives. Irradiation with a millisecond flash in the presence of air leads to polymer formation after an induction period of one hour. In contrast, when the irradiation is carried out with degassed solutions, polymerization starts only after the sample is exposed to atmospheric oxygen. 

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