Example 1
The following reaction is irreversible and first order:
Solution
The space time for a first order
reaction in a PFR is given by the following equation:
The mass flow in each tube is: (200/80) = 1.25kg/h
In molar basis:
Thus, the initial concentration of A is given by:
We can now calculate the initial
volumetric flow:
The volume of each tube can be calculated as follows:
The residence time can be
calculated as follows:
The expansion
parameter, εA can be determined as follows:
The rate constant, k,
can now be solved using equation 1:
Example 2
The following second order irreversible reaction is carried out in gas phase in a PFR reactor:
The PFR feed consists of 50%
by weight reactant A (MW = 40) and the rest is inert (MWinert = 20).
The reaction occurs at a constant temperature of 70C and pressure of 5.25atm.
The rate constant is 400m3/(kmol.ks).
For a production rate of R set at 30kmol/h with a 40% conversion, what should be the volume of the
reactor?
Solution
For a second-order reaction, the rate can be expressed as:
Substituting the PFR design equation into the above equation and integrating we obtain:
With a mass basis of 1.0g, we can calculate εA. The molar values for each feed components are 0.5/40 = 0.0125 mol of A and 0.5/20 = 0.025 mol of inert. Thus, the volume balance is as shown below:
Equation 1 can now be solved:
Given that the output molar flow FR = 30kmol/h, the volumetric flow rate, vo is:
