Pyrolysis cold trapping

In a world increasingly conscious of the dangers of contact with chemicals, a process that is conducted within the walls of a vacuum chamber, such as the VDP process for parylene coatings, offers great advantages. Provided the vacuum pump exhaust is appropriately vented and suitable caution is observed in cleaning out the cold trap (trace products of the pyrolysis, which may possibly be dangerous, would collect here), the VDP parylene process has an inherently low potential for operator contact with hazardous chemicals. 

GC-TEA Analysis. A Bendix model 2200 GC and Thermo Electron model 502 TEA were used. The GC injector temperature was 210 C. The TEA pyrolysis furnace was operated at 450 C and the cold trap was held at -150 C in isopentane slush. Oxygen flow to the ozonator was 20 cc/min and indicated pressure was 1.5 torr at a helium flow rate of 20 cc/min. TEA output was processed by a digital integrator (Spectra Physics System I). 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C and is usually a vapor phase reaction. In the laboratory this is done by using a glass tube in the heating zone of a small furnace. The vapors of the reactant are swept through the hot chamber by an inert gas and into a cold trap. Similar reactions occur with esters derived from long-chain acids. If the boiling point of the ester is above the decomposition temperature, the reaction can be carried out in the liquid phase, with distillation of the pyrolysis product. 

On the other hand, numerous examples are already known in which monomeric metaphosphoric esters are generated by thermolysis reactions. Most worthy of mention in this context is the gas phase pyrolysis of the cyclic phosphonate 150 which leads via a retro-Diels-Alder reaction to butadiene and monomeric methyl metaphosphate (151) 108,109, no). While most of the phosphorus appears as pyrophosphate and trimeric and polymeric metaphosphate, a low percentage (<5%) of products 152 and 153 is also found on condensation of the pyrolyzate in a cold trap containing diethylaniline or N,N,N, N,-tetraethyl-m-phenylene-diamine. The... 

Hertler16 was the first to report the preparation ofpoly(tetrafluoro-p-xylylene) by a multistep synthesis as shown in Scheme 2. Pyrolysis (330°C, 0.025 Torr) of dibromotetrafluoro-p-xylene (B CgFL,) over zinc led to deposition of the polymer film in a cold trap. 

Receiver D is a 250-mL, round-bottomed flask with two, 15-cm long necks with a diameter of 3.5 to 5 cm (entrance neck) and 2.5 cm (exit neck) (see Figure 1). If a receiver D with a narrower entrance neck than indicated is used the neck can become plugged by condensate. A wide-bore connection is used between receiver D and the cold trap E to prevent a pressure drop during pyrolysis, which might be caused by a restriction in the HCI-gas flow. 

In the HPLC mode, most solvent vapors and pyrolysis products are similarly trapped in a series of two cold traps (dry ice + ethanol). This has been found to be quite efficient in removing most organic solvent vapors, but is unworkable with aqueous mobile phases because of freeze-up and blockage of the cold traps. Also, traces of moisture entering the detection chamber interfere with the chemiluminescence detection. Furthermore, mobile phases containing in-... 

Heterocyclic phosphorus ylides (e.g., 114, R = Me) containing an azepinedione nucleus have been prepared, although in low yield (13-26%), by flash vacuum pyrolysis (FVP) of the corresponding open-chain ylide precursor 113 (Equation 15) <2001TL141>. The authors noted, however, that the azepinediones were isolated from the inlet residue rather than from the cold trap after the pyrolysis tube. 

The residence time is calculated based on the fluidizing gas velocity, assuming that the "free volume" (i.e. the volume of the expanded bed minus the volume of the sand) is fully utilized. At the temperature, total reactor gas flow rates, and sand bed volumes used, the residence time was about 0.5-1.0 sec. A typical operation began by washing the sand in 10% HNO3 and distilled water to remove impurities, such as iron, which may act as catalysts, and then calcined at 850° C for at least 12 hours to remove any sulfides and carbonates. The coal feed is then begun and pyrolysis products then exit the pyrolyser to a set of two cold traps fitted with cellulosic thimble filters maintained at 0° C. The outlet gas temperature after the first trap is 30-34° C. Much of the light char formed is entrained in the exit gas and carried into these traps, with most of it in the first trap. 

The conversion of appropriate precursors to condensed PAHs at elevated temperatures is the classical synthesis of numerous pure aromatic compounds. Depending on the reactivity of the starting materials temperatures between 300°C and 1300°C have been applied [31]. The reactions are normally performed in an inert quartz tube placed in a furnace with resistance heating (see Scheme 1). The precursor is transferred into the gas phase in a temperature controlled evaporation zone and swept through the pyrolysis tube by a carrier gas and/or a pressure gradient produced by a vacuum pump. The process has been referred to as flash vacuum pyrolysis (FVP) with typical reaction times of 2 -50 ms or as flow pyrolysis (FP, typical reaction times 0.2-2 s) if the conversion is carried out under normal pressure. The pyrolysis products condense immediately behind the oven in a cold trap. 

The first such 1,4-shift was observed in the gas-phase pyrolysis of 1-methyl-benzotriazole, where the N-phenylformimine was isolated in the form of its trimer 32). Since the trimer collected in the cold-trap at —196 °C — and not in the warmer air-cooled part of the apparatus — trimerization must have taken place in the trap after condensation. 

The solid is placed in a large sublimation apparatus with a high capaci ty cold trap. Evacuation of the system results in collection of a substantial amount of water and some organic liquid in the trap. The organic material, which contains a small amount of codistilled product, is extracted with ht2C), concentrated, and returned to the sublimer. The residue of diisopropylethanediol is sublimed at 60-80 "C/0.1 mbar, yield 100 g [85 % based on estimated diisopropyldioxolane from pyrolysis, 62 % based on (4/f.5/f)-4,5-bis(l-ace-toxy-l-methylethyl)-2,2-dimethyl-l,3-dioxolane, 41-47% based on tartaric acid] mp 74 76 C [a] 5 + 1.3 (c = 2.7. toluene) [2] -3.78 (c = 4.2, CH3OH). 

C and the volatile pyrolysis products collected in a cold trap. 

Chemiluminescence detectors possess considerable selectivity for nitrosamines because the light emitted from the NO-ozone reaction is in the near infrared region, whereas other known chemiluminescent reactions with ozone emit light in the visible or near UV region (17,20,26,27). An optical filter eliminates response to emissions occurring below 600 nanometers. Selectivity is additionally provided by a cold trap between the pyrolysis chamber and the NO-ozone reaction chamber which removes all but... 

In addition to these techniques, analytical pyrolysis experiments were performed where 0.5-mg samples were batch pyrolyzed under flowing helium gas in a tube furnace connected to a hquid nitrogen cold trap. By wanning the cold trap, the pyrolysis products were transferred directly to a GC/MS system for identification. 

The pyrolysis of esters has usually been done with acetate esters. The thermal requirement for the reaction is very high, with temperatures above 500° usually required. The pyrolysis is thus a vapor-phase reaction. In the laboratory, this is usually accomplished in a packed glass tube heated with a small furnace. The reacting vapors and product are swept through the hot chamber at an appropriate rate by an inert gas such as nitrogen, and into a cold trap or other system for condensation. 

The temperature of the pyrolysis tube affects the selectivity and must therefore be reproducibly controlled. The nitric oxide radical is cleaved from N-nitrosoa-mines at relatively low temperatures owing to the weak N-NO bond. Cold traps and packed columns of a solid phase such as Tenax are placed between the pyrolysis tube and the detector to remove potentially interfering compounds, while allowing nitric oxide to pass. 

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