4 replies to this topic

[chemeng_learner]

Say the selectivity of a chemical reaction is defined as the ratio of a by-product formed in the reactor to the desired product formed at the outlet, can the selectivity of this ever be 1 from an engineering point of view? If so, why? And if not, why? 

 

I am trying to identify a reasonable upper bound on the conversion from a selectivity vs conversion plot, and was wondering whether choosing the conversion at selectivity = 1 as the upper boundary would be considered as reasonable?

[Bobby Strain]

Selectivity is usually defined as the fraction, either molar or mass, clearly stated, of desirable product formed by the reaction. So your understanding of selectivity is not correct. Try Google where you should find the proper definition.

 

Bobby

[chemeng_learner]

Selectivity is usually defined as the fraction, either molar or mass, clearly stated, of desirable product formed by the reaction. So your understanding of selectivity is not correct. Try Google where you should find the proper definition.

 

Bobby

The definition I gave came from the brief given to me, so I'm not making it up.

[Bobby Strain]

Which only demonstrates that you should not accept everything you are told, or read, as the truth. Especially if it makes no sense. Best to confirm stuff independently where you lack knowledge. Many textbooks you use will contain such untruths, also. A plot of selectivity vs conversion in the proper sense would range from 0 to 1 (maximum) for the selectivity properly defined. If this is not the case for your source, then something is amiss. Selectivity at maximum may be <1 if mass based. Molar selectivity will be 1 if the reaction yields only the desired component(s).

 

Bobby

Edited by Bobby Strain, 29 January 2018 - 11:26 PM.

[MrShorty]

Ignoring the debate over exactly what selectivity is, if I was measuring a quantity s=n(byproduct)/n(product). If I then ask myself the generic question, "Is it possible for s to be 1 (meaning that there are equal quantities [mole in this case] of product and byproduct?", I would have to answer that it is almost certainly possible that there are reactions/processes out there where the amount of product and byproduct can be the same. Underlying the question would be a lot of situation specific details like reaction rates and equilibrium constants and what concentrations can I manipulate (Le Chatelier's principle) and so on, but it seems very possible to me to work through those things to get different ratios of byproduct to product (I seem to recall examples of this being discussed way back when in my Organic Chemistry courses). I will agree that this may not be the accepted industry standard definition of "selectivity". However, if your question is more about manipulating the amounts of product and byproduct created, and less about defining terms, then I would say that there must be cases out there where it is possible to manipulate a process to yield equal amounts of a product and byproduct.