Question

(a) Explain the concept of fractional yield and why it is important in reaction engineering.

[5 MARKS]

(b) A reactant A undergoes a reaction in the gas phase as per equation (1): A ® B+C (1)

1. (i) Thereactionissecondorderandthevaluesoftherateconstantparameterare k10=3x103 m3 mol-1 s-1 and Ea1= 50000 J mol-1 .

   If the reaction takes place at 300 °C, given that the feed is pure A at 1 atmosphere and that the molar flow rate is 1000 molA h-1, find the volume of a plug flow reactor needed to achieve a conversion of A of 95 %.

   [10 MARKS]

2. (ii) The stream leaving the plug flow reactor in (i) is cooled to 50 °C, a temperature at which A, B and C are liquids. The liquefied stream is fed to a mixed flow reactor where B experiences an isomerisation reaction as per equation (2):

   B ® D (2)
   The rate constant for this reaction is k2= 0.8 s-1.

   If the molar mass of A, B and C are 60, 42 and 18 g mol-1, respectively and given that the reactor volume is 0.5 m3, find the conversion of B at the outlet of the reactor.

   The densities of A and B in the liquid phase are similar at 900 kg m-3. The density of liquid C is 1000 kg m-3.

   [10 MARKS]

Answer 1

A) fractional yield is term used in parallel reactions or side reactions. In parallel reactions , one is desired product and another is undesired product. Fractional yield is defined as ratio of desired product formed to the product (desired) that would have been formed when there was no side reactions and limited reactant reacted completely.

It is important because it indicates what portion of limiting reactant reacted to form desired product . It should be high to get large amount of desired product.