Cellulose Acetate versus Polyimide

Both CA and polyimides are susceptible to plasticization by CO2 [25-27]. When CO2 solnbihzes in the polymer matrix the polymer chains become more flexible and allow faster transport of CO2 molecules. If other gas molecules such as methane or nitrogen are present they also increase their transport rate often even more so than CO2. This means that mixed gas measurements often generate lower a than pure gas. Also, if pure gas measurements are done then methane is usually measured before CO2 since the polymer remembers exposure to CO2 and needs time to relax the extra free volume that has been created. Various studies have documented the response of CA permeation rates to tem-peratnre, pressure and CO2 concentration [28-31]. When pressure is increased more CO2 solubilizes in the polymer matrix increasing this plasticization. Going up in temperature also softens the polymer and causes loss of a. In addition, common heavy hydrocarbons in gas fields such as hexane or toluene can negatively impact performance of polyimides in treating natural gas [32-34]. 

Why do polyimides appear to be more impacted than CA on going into field conditions We have suggested [32] that the polyimides have a more ordered structure giving higher contributions to the diffusivity factor than CA. Therefore the polyimides are more susceptible to structural changes induced by plasticization by CO2 or higher hydrocarbons. 

Although CA was the original polymer system commercialized for natural gas treatment, CA still sets the bench mark against which other polymers can be compared. But if the natural gas stream is clean enough there are applications where the higher selectivities obtained by polyimides offer an advantage. 

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