Vacuum flash cooling

Figure 8.44. Forced circulation a) evaporating crystallizer, b) vacuum flash cooling) crystallizer...

Before equation 8.23 can be applied to vacuum (flash cooling) crystallization, the quantity V must be estimated ... 

Vacuum flash processes, which operate under the atmospheric boiling point of the solution, include the Uhde—LG. Farbenindustrie process and the closely related Kestner process (22). In these, ammonia, nitric acid, and recirculated ammonium nitrate solution are fed into the neutralizer. Hot solution overflows to an intermediate tank and then to a flash evaporator kept at 18—20 kPa (0.18—0.2 atm) absolute pressure. Partial evaporation of water at this point cools and concentrates the solution, part of which is routed to evaporation. The rest is circulated to the neutralizer. 

Cooling Section For the cooling section, a quench cooler with adequate heat removal capacity is effective. Another technique is to inject the hot medium through an expansion valve into a vacuum chamber, which is known as flash cooling. Both of these take a very short time therefore, the sterilization during the cooling period can be assumed to be negligible. 

The substrate (1 -5 mmol) was dissolved in CHjClj (5-10mL), and the solution was cooled to - 40 C and added to a solution of MeOF in MeCN (70 mL) at the same temperature. The mixture was warmed to rt, and after 40 min it was poured into HjO. extracted with CH2CI2, washed with H2O until neutral, and dried (MgSO ). The pure products were isolated by vacuum flash column chromatography (Merck silica gel 60H, increasing concentrations of EtOAc/petroleum ether). 

Purification comprises a preliminary removal of excess aniline by vacuum flash distillation at about 170 C, under 8 to 9 kPa absolute. The water and amine recovered at the top are condensed and settled The liquid fraction is again flashed at about 230 0, under high vacuum (< 1 kPa absolute), allowing recovery of the remaining aniline at the top and its recycle. The polyandries obtained at the bottom are cooled and stored at 10IPC for subsequent phosgenation. 

The term vacuum crystallization is capable of being interpreted in many ways any crystallizer that is operated under reduced pressure could be called a vacuum crystallizer. Some of the evaporators described above could be classified in this manner, but these units are better, and more correctly, described as reduced-pressure evaporating crystallizers. The true vacuum crystallizer operates through flash cooling-, supersaturation is achieved by simultaneous evaporation and cooling of the feed solution as it enters the vessel. 

Synthesis of (—)-0-methyl metazocine (39). To a cooled solution (—78 °C) of amine 37 (40 mg, 0.16 mmol) in THF (1 mL) was added diisopropyl amine (0.005 mL, 0.03 mmol), tetramethylene-diamine (0.010 mL, 0.066 mmol), and -BuLi (0.026 mL, 1.3 M in hexane). The reaction mixture was warmed to room temperature and stirred for 8 hours in an argon atmosphere. The solution was quenched with water, dried over Na2S04, and concentrated under vacuum. Flash column chromatography (5-10% MeOH in CH2CI2) provided 39 mg (98%) of the product 39. [a]D = —76.4 (c=1.3,EtOH). 

The production of styrene-butadiene rubber emulsions is one case where poly-merizabon is deliberately stopped at a low conversion, typically 70-80 %, in order to Hmit the crosslinking reaction from the pendant vinyl groups in the butadiene units. With such large amounts of residual monomer, economics force the recovery and recycling of both butadiene and styrene. After the polymerization stage, residual butadiene is flashed off under vacuum, compressed, cooled and returned to the reactor feed, and styrene is steam stripped in a column stripper, condensed and also returned to the reactor. 

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