Matlab SimulationsCan we neglect side reactions?

To know the importance of side reactions, we compare the results of 3 cases:

Case 1: Reaction 1 only

Case 2: Reaction 1 & 2

Case 3: all three reactions

First, at the following reaction condition, T = 673 K (400 C) = constant and p = 0.115 torr (15.33 Pa) = constant, the results are:

(Hey, what's the definition of conversion, selectivity and yield?)

In case 2, when compared with case 1, although much more NH3 is consumed, the yield of NO is much less. That means the Reaction 2 is very competitive at the condition of T = 673 K and p = 0.115 torr. However, there is little difference between case 2 and 3, which tell us Reaction 3 is not important, at least in such conditions.

Now we know Reaction 2 cannot be neglected, but another question arises now: can we neglect Reaction 3 at any condition? Let us see what's happening at another condition: T = 1673 K(1400 C) = constant and p = 0.115 torr (15.33 Pa) = constant.

This time, case 1 and 2 have little difference, but not case 2 and 3, that is, at higher temperature, the Reaction 3 is more competitive.

At this point, we have the fact: at lower temperature, Reaction 2 is very important and Reaction 3 is negligible; at higher temperature, Reaction 3 is competitive , but Reaction 2 becomes less important. (Reason? When we look at the reaction rate expression, we notice that the exp term in the numerators in Reaction 2 and 3 are quite different. Reaction 2 has exp(26000/RT), which means it will decrease when T increases. But Reaction 3 has exp(-570/RT). The rate will increase with increasing T.) However, in the real situation, the temperature varies in the reactor, covering from low to high temperature. Therefore, we must consider both of the side reactions.

So the answer for the topic question is:

No, we cannot neglect either Reaction 2 or 3.

What we only can do is to optimize the reaction condition to minimize the side reaction effect.