Page 109 - Process simulation and control using Aspen

100 PROCESS SIMULATION ANnjWQlOLUS ING ASPEN'

Hitting on Next and clicking Kinetic button, we get Aeldn ics input form. A littl e
description is given below to understand the use of LHHW kmetxc model m Aspen
simulator.
The LHHW rate expression is represented by:

(kinetic factor) (driving force)
r = (2 .1)
(adsorption expression)
The kinetic factor (reaction rate constant) has the following form:

n ' 1 >
E \
K = k exp
R k T To) (2.8)
If Tq is ignored, Eq. (2.3) replaces the above expression. Note that all the notations
used in Eq. (2.8) have been de fined earlier.
The driving force is expressed by:
f N A
n c?

and the adsorption is modelled as:

M
N
nc"J
Li=i
where,
In (Ki) =Ai + Bi/T + Ci IniT) + D.T (2.9)
Here, m is the adsorption expression exponent , M the number of terms in the adsorption
expression, N the number of components , a the concentration exponent, K2, K, the
equilibrium constants [Eq . (2.9)], A,, fit, Q, the coeficients and I Notice that the
f
concentration term C used in the above discussion is dependent on the [CJ basis . Say
for example , when [CJ basis is selected as molarity , the concentration term represents
the component molar concentration (kmol/m3); similarly when [CJ basis is partial
pressure, the concentration term represents the component partial pressure (N/m2).
Providing required data we have the filled kinetic sheet shown in Figure 2.71.
,
,
Click on Driving Force to obtain a blank form as shown in Figure 2.72.
Select 'Term 1' and then 'Molarity' as [CJ basis . Under Concentration exponents for
reactants
, set ketene and methane
set acetone exponent to 1. Similarly for products ,
Also enter zero for all four driving force constants as mentioned in the
exponents to 0.
problem statement (see Figure 2 .73).
In the subsequent step (see Fi gure 2.74), select Term 2' from the pulldown Enter
Since the given reaction is first-order with respect to acetone and there is
term menu .
no second term , enter zero for all exponents and coefficients . Owing to the method
Aspen Plus uses to specif y a reaction, we should insert a very large negative value for
coetticient A (say , -106) t click
o make Term 2 essentially zero [see Eq (2 9)1 Finally,
on Next icon n- .
.