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
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 enlarge
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
Combustion in a marine environment
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.
Predominance area diagrams
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 right.
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
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 and Mo3Si.
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 data.
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
Aqueous systems and Pourbaix diagrams
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 protection.
An interesting question to ask is, for example, what happens when sulphur is present at 0.01 mol/kg for
a temperature of 573K?
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
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 and
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> + SO2<g>
THERMOTAB OPTION ? step temperature 673.15 973.15 100
The equation for the equilibrium is:
1/3 log10(pO2) +
log10(pSO2) = log10(K) =
The values of "Beta" are obtained when the command go is
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.