Continuous-mode pyrolysis

Pyrolysis may be performed using continuous-mode or pulse-mode instruments. In the first instance, the material must be introduced rapidly into the furnace at a predetermined temperature, which is maintained throughout the pyrolysis. Common modem pulse-mode instruments allow very rapid sample heating within a specified time. There are three different types of pulse-mode pyrolyzers ... 

There are several procedures to perform pyrolysis flash pyrolysis (pulse mode), slow gradient heating pyrolysis (continuous mode), step pyrolysis, etc. Commonly, the pyrolysis for analytical purposes is done in pulse mode. This consists of a very rapid heating of the sample from ambient temperature, targeting isothermal conditions at a temperature where the sample is completely pyrolysed. Controlled slow temperature gradients are also possible in pyrolysis, but their use in analytical pyrolysis is limited. Step pyrolysis heats the sample rapidly but in steps, each step following a plateau of constant temperature kept for a limited time period. 

Depending on the heating mechanism, pyrolysis systems have been classified into two groups continuous-mode pyrolyzers (e.g., furnace pyrolyzer) and pulsemode pyrolyzers (e.g., heated filament. Curie point, and laser pyrolysis). All of them are extensively used in polymer characterization and degradation smdies. 

Continuous-Mode Method. May involve tube furnaces or microreactors. In this mode the heated wall of the reactor is at a higher temperature than the sample and secondary reactions of pyrolysis products will most likely occur. 

The screw pyrolyzer, with its lower cost of construction and operation, has great prospects in the future. A screw pyrolysis conveyer is provided with internal and external heating modes. It has a special configuration for the removal of the coke formed during the process, which is a threat to heat transfer, and continuous operation. Figure 27.4 shows a screw pyrolyzer. 

Lephardt s original work was on the pyrolysis and combustion of tobacco, for which he used Ig samples. We have adapted the techniques to work with 5-lOmg samples of and have used it successfully to characterise a variety of polymers. Our standard conditions are IC C/min ramp rate, 50ml/min purge gas (usually N2), 10s data collection time (25 scans), continuous (GC mode). We use both identity and evolution profile of the evolved gases to evaluate the sample. 

Since the 1980s, chemical vapor deposition has been widely adopted in the continuous production of with F-doped Sn02 (Sn02 F) [13]. By far, the majority of TCO films are currently produced in this way. Although ITO was first made by spray pyrolysis, sputtering has become the preferred mode for its production. Significant achievements have also been made to obtain low-resistivity and textured ZnO films using CVD [50, 53, 54, 56-59]. 

In this type of system a resistively heated platinum or nichrome wire coil or ribbon is used to rapidly heat the sample. The wire is continuously swept with carrier gas, whereupon the pyrolysis vapors are transported into the chromatographic column. Heating times are relatively large (up to 20 s) for this system, which may lead to nonrepeatable pyrograms and secondary reactions. The pyrolysis conditions, sample size, and location must be carefully controlled to obtain repeatable data. Two possible heating modes are available for this system pulse mode or programmed mode. For most forensic applications the pulse mode has been used. 

Continuous or pulse mode (flash pyrolysis) step mode (temperature-programmed pyrolysis)... 

The simplest MS technique is the linear programmed heating of the sample in the vacuum chamber of the mass spectrometer. The evolved gases are analyzed continuously as the mass spectrometer scans the mass range of interest. Either a plot of ion current versus time or temperature for each ionic mass may be made or a single ionic mass may be monitored as a function of time or temperature of the pyrolysis. Both modes are commonly employed. 

Pyrolysers can be divided into two main categories on the basis of their mode of operation, i.e. the continuous type, where the sample is supplied to a furnace preheated to the final temperature, and pulse mode reactors in which the sample is introduced into a cold furnace which is then heated to the final pyrolysis temperature. In the analytical pyrolysis of solid and some liquid materials mainly pulse mode pyrolysers are used and the following sections will focus on a few of the most popular pyrolysis techniques utilizing this mode of operation. However, for pyrolytic studies of liquid and gaseous samples continuous pyrolysers are applied. 

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