The Haskel 4AAD-2 is what is called an Air Amplifier. It was designed to boost compressed air pressure. Terp did some testing with one and was able to get some very good results. It does have a displacement of 60 cubic inches (3 times the displacement of an EXT420), however it can only increase the gas pressure to as high as the drive air pressure + the incoming vapor pressure. So if you have drive air of 80 psig and the butane or propane vapor pressure is 30 psig, the maximum output pressure is 80 + 30 psig = 110 psig. The other good thing is that it has a 4" air drive, so it uses half the drive air that the EXT420 uses (at the same cycle rate). So, theoretically the 4AAD-2 will move 3 times as much vapor as the EXT420 using half the air.
Sounds pretty good? Well, there are some drawbacks. The 4AAD-2 does not have internal separation like the EXT420 does, so it is possible (and likely) to get cross contamination between the drive air and the process gas (butane/propane). My biggest concern is that the drive air/compressor air will get into the solvent. That may not sound too bad, but the compressor air has oil (from the compressor) mixed in with it and it will contaminate the solvent with the compressor lubrication oil.
Terp told me that they are promoting it for use only with bottled nitrogen as the drive source. Bottled nitrogen is dry and will not add any contaminants to solvent, although it may be necessary to "burp" the nitrogen out of the solvent storage tank. Also, when driving off a nitrogen bottle, it is necessary to get a high pressure regulator that can drop the high pressure from the nitrogen bottle down to less than 125 psig (the maximum drive air pressure for the 4AAD-2).
Because of the good results that Terp did see with the 4AAD-2, Haskel is working on a pump with similar specifications to the 4AAD-2, but it will have separation to keep the drive air and the solvent vapor separate (just like the EXT420). We did get a prototype of the new pump in the field several months ago and it did provide very good results (about 3 times the recovery rate of the EXT420) however, it did have a failure. Haskel Engineering is working on a fix for the failure and I hope to have it back in test very shortly.
Sorry for the long answer. Let me know if you have any questions.