Page 84 - Process simulation and control using Aspen

76 PROCESS SIMULATION AND CONTROL USING SPE N]
'
'Molarity basis. Accordingly, the
As directed in the problem statement, we use
Power law is expressed as:
E n
n 1
r= k exp (2.1)
[T0; R

K the reaction rate constant (kinetic factor in Aspen
where r is the rate of reaction,
Plus terminology), k the pre-exponential or frequency factor, T the temperature m degree
K Tn the datum temperature in degree K, n the tempera ture exponent S the activation
tant, C the molarity in kmol/m , a the concentration
energy R the universal gas cons
exponent, i the component in dex, and 0 the product operator.
If To is ignored, the Power law expression has the fo llowing form:
E
n
r= kT exp n(G) (2.2)
RT
where,
E
K = kTn exp (2.3)
RT
In most of our simple cases, the reaction rate constant is represented by the Arrhenius
law, that is
E N
K - k exp (2.4)
RT)
Notice that when the Arrhenius formula is used , we put zero for n and nothing for T0
in the Aspen Plus window. Also, the units of the pre-exponential factor are identical to
those of the rate constant and vary depending on the order of the reaction . As we
know, the dimensions of the rate constant for an nth order reaction are:

(time)-1 (concentration)1-'1
The kinetic data are required to provide in the above
Next come back to the problem.
sheet. Here we use the Arrhenius law to represent the reaction rate constant. It is
important to mention that the pre-exponential factor m ust be specified in SI unit. For
the pre-exponential factor and activation energy are given
the example CSTR problem ,
as 5 x 105 m3/kmol s and 20 ,000 Btu/lbmol respectively (see Figure 2.33).

Running the simulation
In the window shown in Figure 2 .33, the Status bar clearl
y indicates that all required
Hitting Next knob and clicking on OK we have the foUowing
mputs are now complete .
,
Control Panel (see Figure 2.34).