Question

With two possible reactions in parallel, which type of reactor (CSTR, Batch, or Plug Flow) will you choose to maximize selectivity?

F-->P1 r1= k1(CF)^a1

F-->P2 r2= k2(CF)^a2

Answer 1

We are given two parallel reactions and their rate laws

  

Let us assume that P₁ is our desired reaction

Selectivity for a reaction can be defined as

Selectivity S = Rate of desired reaction/ Rate of undesired reaction

where n = a₁-a₂ .................................(1)

Now dependence of selectivity depend up on relative values of a₁ and a₂

There are three possibilities

Case 1) a₁=a₂ ;a₁-a₂ =0

Case 2) a₁>a₂ ;a₁-a₂  = n >0 (positive)

Case 3) a₁<a₂ ;a₁-a₂ =n<0 (negative)

Case 1) when a₁=a₂ , a₁-a₂ =0

In this case n=0 and selectivity S is independent of reactent concentration. Thus choice of reactor will depend on other process requirements such as production rate etc.

Case 2) when a₁>a₂ ;a₁-a₂  = n >0 (positive)   

Here n >0 ie, Positive, hence the selectivity S will be directely proportional to reactent concentration. That is if reactent concentration increases selectivity increases and thus rate of desired reaction will be more.

Thus it is required to keep reactent concentration high.

Therefore it is better to use a PFR since concentration fall in PFR is gradual but that in CSTR is sudden.

Use of Batch reacter will depend on production requirement. If batch reactor is used keep high feed concentration.

Case 3) when a₁<a₂ ;a₁-a₂ =n<0 (negative)

In this case n<0, Negative. thus selectivity is inversily proportional to reactent concentration. That is if recatent concentration increases then selectivity decreases. So it is required to keep a low reactent concentration.

Hence it is better to use a CSTR than PFR in this case.

If Batch reactor is used keep a low reactent concentration.