Most of the early works were conducted at lab scale in a...

Most of the early works were conducted at lab scale in a reaction tube which is basically a small diameter flow through tube with. The later focus has been on the FBR which I believe offers the highest level of selectivity towards morphology and product yields. Basically the reactor needs to allow sufficient gas solid contact and process condition control. Any configuration which allows this should function. 

The decision to take the alternate PTR path is likely due to MIN's longer game. MIN with their existing process plants will have plenty of low and high grade heat sources. A PTR should easily incorporate alternative heat source such as waste heat from existing turbines (probably run off tail gas) or perhaps molten salt, solar etc. Imagine the reactor as a worm drive, from the feed end you can add in different staged heat sources as the feed travels along the reactor, first inputs simply add heat with the final stage being where the temperature is finely controlled to achieve optimal methane conversion.