Define conversion, selectivity, and yield in reactor operations.

Conversion (X) is the fraction of limiting reactant that has reacted: X = (moles reacted)/(moles fed). Selectivity (S) is the ratio of desired product to undesired product formed, or desired product to total products. Yield (Y) is the amount of desired product formed relative to amount that could form if all reactant converted to desired product: Y = conversion x selectivity. High conversion is not always desired if selectivity suffers - optimization balances both.