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MTDATA Demonstration : Gas-Solid-Aqueous Systems

Contents

  1. Combustion
  2. Combustion in a marine environment
  3. Predominance area diagrams
  1. MULTIPHASE calculations
  2. Aqueous systems and Pourbaix diagrams
  3. THERMOTAB

Please click the diagrams below to see a full size version.

 

Click to enlarge

Combustion

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 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 understand.


Click to enlarge

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 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 and Mo3Si.

COPLOT is very suitable for exploring the complex chemistry of gas solid reactions of this type.


MULTIPHASE calculations

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 diagram.

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 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 system.


THERMOTAB

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 O2.

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) = "Beta"

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.


 

Updated 28 April 2010