Environmental Engineering Reference
In-Depth Information
TABLE 17.1 Values required to calculate the temperature dependence of the Yates and
Satterfield expression for the FT reaction
Symbol
Meaning
Value
Units
8.8853310
−3
mols
−1
−1
bar
−2
a
0
Reaction rate coefficient at T = 493.15 K
kg
cat
3.737 × 10
4
Jmol
−1
E
a
Activation energy
bar
−1
b
0
Adsorption coefficient at T = 493.15 K
2.226
−6.837 × 10
3
Jmol
−1
Δ
b
H
Adsorption enthalpy
TABLE 17.2 Temperature dependence of the Yates and Satterfield
expression for the FT reaction rate
a
(mols
−1
−1
bar
−2
)
b
(bar
−1
)
r
(mmol kg
−1
s
−1
)
T
(K)
kg
cat
5.65 × 10
−3
470
5.07
0.324
1.00 × 10
−2
500
1.77
4.54
1.67 × 10
−2
530
0.697
44.8
original publication of Maretto and Krishna (a minus sign is missing in the exponent
of
a
0
); the values in Table 17.1 are correct.
Example 17.1 CO reaction rate as a function of temperature
Plot the reaction rate for CO Equation (17.1) as a function of temperature for a
relevant temperature range for FTS (470
530 K). Assume a partial H
2
pressure of
20 bar and a partial CO pressure of 10 bar and a catalyst multiplication factor
F
of 3.
You will find an increasing reaction rate with temperature. Why is the operating
temperature of Fischer
-
Tropsch (FT) reactors typically limited to about 500 K,
while a higher temperature would give a higher reaction rate?
-
Solution
Substitution of the values given in Table 17.1 in the expressions in Equation (17.2)
gives the values for
a
and
b
. Subsequently,
r
can be calculated. In this case, all
provided coefficients have the right units for use in the expressions. However, note
that that is often not the case: often you will have to convert values before the
expression can be used (e.g., the reactant pressures are given in MPa instead of
bar). To check your calculation, Table 17.2 gives the intermediate and final values
for three temperatures. Figure 17.2gives the plot for the complete temperature range.
The operating temperature of low-temperature FT reactors is typically limited to
around 500 K, since the reaction is strongly exothermal: a further increase of the tem-
perature will lead to a higher reaction rate and even more release of heat, making it very
hard to cool and control the reaction. Moreover, conversion is not the only thing that
counts: also the selectivity is very important. At higher temperatures, a larger amount
of less favorable products
—
such as methane
—
will be produced.
Search WWH ::
Custom Search