MTDATA Demonstration : Gas-Solid-Aqueous Systems
Perhaps the most important industrial chemical
reaction is combustion. MTDATA is an ideal tool for studying reactions
of this kind. Consider the combustion of 1 mole of propane containing
0.03 moles of sulphur in air. How do the products vary with the amount
of air used?
This calculation uses data for the system "C, H,
O, N, S" from the SGTE database for substances. It can be done by
setting the temperature (1200 K) and pressure (1 atm)
and then stepping between two compositions, one for the fuel and one
for the fuel with air.
The default plot that MTDATA will produce shows
the amount of the various species produced at each step of the
calculation. The scale can be reduced and plotted on a log scale in
order to highlight the species with low amounts. This diagram is shown
to the right.
| Click to
It is useful to be
able to check the amounts of certain compounds, for instance here the
amount of NOx and other nitrogen compounds is of
interest. This can be done easily by focusing on a certain component,
in this case nitrogen. This diagram is shown left. This also shows that
substance numbers have been replaced with names, to make the diagram
easier to understand.
Combustion in a marine
What happens if the combustion takes place in a
marine environment or when there is salt on the road? This can be
simulated by adding NaCl to the overall composition of the previous
diagrams. The calculation is for a stoichiometric fuel-air mixture over
a wide range of temperature.
The diagram to the right shows the result by
"focusing" on the fate of sulphur. The results show that under some
circumstances sodium sulphate can form. This is important because
liquid sodium sulphate can dissolve the protective oxide on turbine
blades and cause rapid corrosion to occur.
Sodium chloride and sodium sulphate can mix and
make matters worse by increasing the range of liquid stability. Have a look at the section on salts.
Molybdenum disilicide can be used as a heating
element in air at high temperatures because an impervious layer of
glassy silica forms on the surface which inhibits further oxidation but
paradoxically it fails when used in neutral or reducing atmospheres.
Predominance area diagrams offer a useful way of
testing how the chemistry of components varies over a range of
conditions. The Module COPLOT allows the fate of more than one element,
in this case Mo and Si, to be investigated simultaneously. Note that
reactions at the surface of the MoSi2 can result
in volatilisation and separation of the molybdenum from the silicon, so
the overall result may be expected to be complex.
Let us begin by examining what happens to silicon
by itself using data for silicon and its oxides, nitride and oxynitride
from the SGTE substance database. This is shown in the diagram to the
In COPLOT the component to be studied is set by
amount and the axes of the plot are determined by the range command in
which the abscissa (in this case the partial pressure of O2<gas>
on a logarithmic scale) is fixed first and the ordinate (in this case
the partial pressure of N2<gas> on
a logarithmic scale) is fixed second.
Note the regions corresponding to Si, SiO2,
Si3N4 and Si2ON2
in the resultant diagram. The formation of SiO could also have been
investigated but that is another story.
Now we can do the
same for molybdenum. The diagrams to the left and below correspond to 1
mole and 0.001 mole Mo respectively. In both diagrams regions
corresponding to Mo, Mo2N, MoO2
can be found but, whereas in the first MoO3 is
predominant at the highest O2 partial pressure,
in the second MoO3 is replaced by Mo3O9.
The reason is that the partial pressure of Mo3O9
is greater than one third of 0.001 under the conditions for which the
diagram shows it to predominate, and the solid phase evaporates. This
feature of COPLOT allows gas-solid equilibria to be explored.
Having seen what
happens to Si and Mo separately, we can put them together under the
same conditions. For example we can make the amount of Mo substantially
less than that of Si (1 mole of Si and 0.001 moles of Mo). In the
resultant plot (left), note that the blue diagram for silicon is not
simply overlaid with the black diagram for molybdenum. The molybdenum
is strongly influenced by the presence of silicon and forms a series of
silicides, MoSi2, Mo5Si3
COPLOT is very suitable for exploring the complex
chemistry of gas solid reactions of this type.
Now let us see what MULTIPHASE does with the same
The diagram on the right show the results of
calculations carried out for a temperature of 1423 K, fixed
compositions of 0.1 mole of Mo, 1 mole of Si and 0.01 moles of N with
the number of moles of oxygen stepped from 1.39 to 2.71.
The result is exactly in accord with the
predominance area diagram plotted previously. The same sequence of
molybdenum silicides is observed. Note that coexistence of two
stoichiometric compounds causes other variables to become fixed. This
is why coexistences correspond to lines on the predominance area
MULTIPHASE adds detail to the broad picture
provided by COPLOT, for example it shows the partial pressure of SiO
and the various molybdenum oxides.
Aqueous systems and
Predominance area diagrams are much used for
aqueous systems. They often have axes defined by pH and Eh and are
called Pourbaix diagrams. In water circuits made of iron, corrosion is
inhibited by the formation of magnetite which offers a degree of
An interesting question to ask is, for example,
what happens when sulphur is present at 0.01 mol/kg for a temperature
For this calculation data for the
iron-sulphur-water system is taken from the hotaq database, remembering
that hydrogen and oxygen are both components and that, since the system
is open to charge, this too must be a formal component.
The amount of Fe in solution is set to a low
figure corresponding to a fairly low rate of corrosion, whereas the
sulphur amount is rather high. The activity of water is set to unity
and the gas volume is set to a very low value. The effect of this is
that partition of sulphur to the gas phase is ignored.
The default diagram
shown above contains a lot of information that is difficult to take in
at first glance. The diagram to the left has been edited to improve the
labelling. The brown lines delineate regions where the pressures of O2,
H2 would exceed one atmosphere and OH/- would exceed unit molarity. The
red lines delineate predominance areas for compounds of sulphur and the
black lines do the same for compounds of iron.
Notice that the sulphur has caused the region for
Fe3O4 partly to be
replaced by regions of FeS and FeS2 which are
non protective against corrosion. In this way the diagram, which takes
only seconds to calculate, gives a very useful feel for the overall
chemistry of the iron-sulphur-water system.
The THERMOTAB module is also useful in studying
gas-solid-aqueous reactions. For example it can be used to find the
condition for coexistence of ferrous sulphate and magnetite in terms of
a relation between the partial pressures of SO2
THERMOTAB OPTION ? define equation "FeSO4 = Fe3O4 + O2<g> + SO2<g>" !
THERMOTAB OPTION ? use sub_sgte default !
The equation is automatically balanced to give:
FeSO4 = 1/3 Fe3O4 + 1/3 O2<g> +
THERMOTAB OPTION ? step temperature
673.15 973.15 100 !
The equation for the equilibrium is:
+ log10(pSO2) = log10(K)
The values of "Beta" are obtained when the command
go is entered
THERMOTAB OPTION ? go
FeSO4 = 1/3 Fe3O4 + 1/3 O2<g> + SO2<> T Delta Cp Delta H Delta S Delta G Beta
K J/K mol J/mol J/K mol J/mol -G/RTln10
673.15 -7.2806 2.55811E+05 238.65 95165. -7.3844
773.15 -0.87374 2.55463E+05 238.15 71333. -4.8192
873.15 -12.434 2.55023E+05 237.63 47533. -2.8435
973.15 -21.968 2.53021E+05 235.47 23873. -1.2814
THERMOTAB could also be used to determine the
limits of validity of this equation by entering equations for the
coexistence of three compounds, eg by entering:
THERMOTAB OPTION? define equation "FeSO4 = Fe3(SO4)2 + Fe3O4 + O2<g>" !
THERMOTAB will balance this equation. In general
it would be wise to check which equations to use by means of COPLOT.