Purge gas stream

Constant-temperature decomposition or combustion, followed by trapping and weighing the volatilized gases, requires more specialized equipment. Decomposition of the sample is conducted in a closed container, and the volatilized gases are carried by a purge-gas stream through one or more selective absorbent traps. 

Table 4 summarizes commercial and precommercial gas separation appHcations (86,87). The first large-scale commercial appHcation of gas separation was the separation of hydrogen from nitrogen ia ammonia purge gas streams. This process, launched ia 1980 by Monsanto, was followed by a number of similar appHcations, such as hydrogen—methane separation ia refinery off-gases and hydrogen—carbon monoxide adjustment ia oxo-chemical synthetic plants. 

Metal Hydride Process for Ammonia Purge Gas, The metal hydride process will be illustrated using the case of hydrogen recovery from an ammonia purge gas stream generated during ammonia manufacture. 

N purge gas streams have been measured as a function of system... 

Purge-gas streams in ammonia, cyclohexane and methanol production,... 

A purge gas stream is withdrawn to control inerts buildup. This purge is normally dried and can be sent to direct chlorination for recovery of its ethylene content. 

Because the tapered element must vibrate freely, it is impossible to connect the sample cell outlet directly to a gas chromatograph (GC) or mass spectrometer (MS) for on-line analysis. Instead, a purge gas stream is passed along the outside of the tube to sweep the reactor effluent as it exits from the tapered element. 

The basic process as outlined above is very flexible, and modifications and variations can be easily incorporated into it to further improve the overall efficiency and/or to make it more suitable for specific types of feeds. Thus, external fuel, recycle gas, or liquid fuels can be easily introduced into the burner in the case of lean tar sands. By providing for a purge gas stream off the top of the combustion bed, one can adjust the flow rate of fluidizing gas to the pyrolysis bed. If desired, after recovery, gas produced in the pyrolysis bed can be recycled back to that bed and used instead of combustion gases to fluidize it. This is very important for lean tar sands which would otherwise have very low product concentration in the combined exit gas stream, making product recovery difficult. 

Calibrations could be made on the basis of the decomposition of solid standards, injections of volatile liquids, standard gas mixtures, injections of pulses of gas, or steady injection rates of gases into the purge gas stream. With proper normalisation procedures, all methods gave equivalent results.A good linear relationship between EGA peak area and amount of product was found for between a few and a few hundred microgrammes of gas when using a jet separator. 

The diagram of a plant that produces substitute natural gas (SNG) in addition to methanol is shown in Fig. 7.2. The entire purge gas stream from methanol synthesis is delivered to a methanation unit where the hydrogen and carbon oxides are reacted at a nickel catalyst to yield methane and water according to the following formulae... 

Fig. 7.7 Use of hydrogen-permeable membranes to recover and reuse hydrogen from hydrocracker purge gas streams.

Steam, which is introduced at the bottom of the bed. The hot adsorbent is returned to the top of the column by a gas lift system, and it is then cooled and dehydrated in the top section by heat exchange and contact with the purge gas stream. The process can thus be considered as a combined thermal swing-displacement desorption cycle. 

Hydrogen and helium have relatively small molecular sizes compared to other gases and exhibit high selectivity ratios in glassy polymers. Applications can be found in the recovery of H2 from purge gas streams in ammonia synthesis, petroleum refineries and methanol synthesis. 

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