1) It was what the customer (client) specified;
2) It was what the engineering designer offered and was accepted;
3) No one cared about the units used and the computer simulation spit out lbmoles;
4) For convenience, if a gas process is involved; because the mole occupies a known quantity of gas (at "standard" conditions), it can be converted over to volumetric flow easily;
5) Because a chemist is involved and he/she wants molal relationships;
6) there are probably many other probabilities.
I have always constructed my PFDs in accordance with client wants or needs. It is no big deal to select either mass, volume, or molal and stick to the units. The biggest problem is usually getting a decision out of someone - especially business managers who are only versed in junk science and no engineering.
Way back when I started in the Chemical Industry our company and the biggest chemical company in the US used moles. Our impurity levels were moles/million moles (mpm) and the flows were called out in lbmoles and so on. It was a little tricky getting used to but as Montemayor states they will workout very nicely and we were lucky as our molecular weights were very close togather, so close we could change to parts per million without any loss of information. Mole units makes one familiar with their products.
One of our products had a total impurity level of less than 50 moles/million so an error of 1 mpm really meant nothing but was large percentage which management would jump on. We couldn't win.
ghamsa
Most of the reasons possible is already mentioned by Montemayor.
Just like to add one thing, for process engineers it usually is convinient to use moles but in reality PE's can live with whatever units that has been decided upon.
But for e.g. operations staff, moles doesnt mean that much. I had this issue up for discussion when it came to the operating manual and PFD was one of the documents in the manual. Here I couldnt see the rationality to use anything else but the engineering units to be used in the DCS, since this is what the operations staff refer to. However I lost the 'battle' and our operations staff have to live with the moles so I doubt that these PFD's will be as valuable as they could have been. I guess when the day comes, somebody of us PE's will 'translate' the PFD to be the one used by Ops. So why being proactive when you can be reactive!
