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Tips & Tricks - Use the Claus Oxygen Calculator in Generic Combustion Applications 

The Claus Oxygen Calculator can save you time in combustion applications where the flow of oxygen or air needs to be calculated to meet a certain excess of oxygen with respect to the stoichiometric requirements. Although, the Oxygen Claus Calculator is located in the Claus stencil, this utility operation does not require the special Claus license to work.

In this example the flow of air needs to be calculated to combust a fuel stream with a specified excess of oxygen. The combustion furnace is conveniently modeled with an equilibrium reactor in Gibbs mode.

The use of the Claus Oxygen Calculator is compared against detailed stoichiometric calculations. While the Claus Oxygen Calculator can readily determine the flow of oxygen and air, the amount of oxygen required per combustion reaction has to be calculated separately for the detailed stoichiometric approach. For a small number of reactions, the detailed stoichiometric approach is straightforward and does not require too much additional work. However, for a large number of reactions, this approach becomes tedious and difficult to maintain because the model needs to be updated as new components are added to the flowsheet.

As can be seen in the figure below, from a flow sheet point of view, the two calculation options look very similar. The approach using the Claus Oxygen Calculator is shown in the top and the one using the process calculator is in the bottom:

TT_July_2012_1.png

The composition and the conditions of the Fuel streams are:

TT_July_2012_2.png

For the detailed stoichiometric approach the process calculator is used to calculate the flow of air needed. To calculate the moles of oxygen needed the combustion reactions of methane and ethane are required. The combustion reaction for methane is:

CH4 + O2 --> CO2 + 2H2O

And the combustion reaction for ethane is:

2C2H6 + 7O2 --> 4CO2 + 6H2O

From the combustion reactions it can be seen that for each molecule of methane, 2 molecules of oxygen are needed while ethane will need 3.5 molecules of oxygen to combust.

The following figure shows the calculation of the required air using the process calculator.

TT_July_2012_3.png

In this case, an air flow rate of 122.6 lbmol/h is required to meet the specification of 20% excess oxygen. Note that each of the combustible compounds in the fuel stream will require its own calculation for required oxygen, thus making this methodology tedious and prone to errors when the compounds in the flowsheet are changed.

Using the Claus Oxygen Calculator is much simpler, the fuel and the air streams are connected to the calculator and the required excess is specified in the form.

TT_July_2012_4.png

Note that since the Claus Oxygen Calculator was designed for a Claus process the fuel port is labeled as “AcidGasIn”.

It can be seen that the flow of air calculated is the same in both methodologies. However, using the Claus Oxygen Calculator is more convenient and ensures the calculation is up to date if the compounds change in the flowsheet.

The Claus Oxygen Calculator supports the complete combustion calculations for all hydrocarbons with the generic formula CxHy, all alcohols of the form CxHyOm, mercaptans CxHySm, as well as H2S, NH3, H4N2, CS2, COS, PH3, carbon and hydrogen. 

Please contact your local VMG office for more information.

Luis R. Duhne, Simulator Software Developer, VMG Calgary

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