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# Selectivity Basic Question

## Selectivity Basic Question

(OP)
I know selectivity is = No of mole of desired product formed/ No of mole of reactant took part in the reaction (reactant Basis)

Ethylene Oxide example

Considering only the first two reactions (No further oxidation of EO)

Assume 153 Kmol/hr of EO forms

So if I have 90% selectivity that means I will have 34 Kmol/hr of CO2. (As every 1 mole ethylene produces 2 mol of CO2)

Another thing selectivity is for the overall process so with recycle right?

Much appreciated.

### RE: Selectivity Basic Question

Student question???

### RE: Selectivity Basic Question

The attachment doesnt work.

Selectivity usually means preference for one product over another undesired product. And usually expressed as a ratio, which is in this case, 153/(34*0.5) = 9. And selectivity references the actual feed to the reactor, so with recycle included, but product basis reference is reactor outlet, not the product outside the recycle loop.

Check your textbook also - the author may define selectivity differently. The actual definition and arithmetic expression for this may differ from author to author. What is important is to use the basis definition correctly.

### RE: Selectivity Basic Question

(OP)
@georgeverghese exactly. Your expression is what I knew but in the industry, I am seeing new selectivity calculations.

Selectivity= Mole of desired Product formed (feed basis)/ Mole of Reactant Feed)

I am quoting my supervisor "selectivity to EO on a per mole of inlet ethylene basis. That is the standard convention for the field."

Have you come across such references?

If we see the following post https://www.cheresources.com/invision/topic/27499-...
"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

Appreciate it.

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