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Handling New Chemicals in AP-42’s Storage Tank Emissions Methodology

For many years, storage tank operators relied on USEPA’s TANKS software to estimate storage tank air emissions. As is well documented, TANKS miscalculates air emissions and is no longer supported by USEPA. Further, the AP-42 storage tank section has been amended which renders TANKS even further out-of-date. Commercial software is becoming the industry standard to estimate storage tank emissions. These commercial software packages typically come with the standard library of compounds found in the latest version of AP-42 (i.e., 13 petroleum liquids and nearly 200 organic liquids).

But how does one estimate emissions when storing material that is not one of the petroleum or organic liquids listed in AP-42? It requires adding the compound to the chemical library. Thermodynamic data on the material must be known. This includes the liquid and vapor molecular weights, liquid density, and vapor pressure constants to estimate vapor pressure across the entire range of temperatures where the material will be stored.

For chemicals (single component systems), the molecular weight and liquid density are easy to look up, either on a vetted and reliable source or from the manufacturer of the material. For petroleum liquids, the molecular weights are more problematic as the vapor molecular weight will be less than the liquid molecular weight due to lighter components evaporating more readily. Either the material supplier or the data in AP-42 can provide estimates for petroleum liquid vapor molecular weight.

Vapor pressure, however, is an entirely different matter. There is no one piece of data for vapor pressure across a full range of storage temperatures as a vapor pressure curve must be developed. This requires determining vapor pressure constants for appropriate vapor pressure equations.

In AP-42, the vapor pressure equation for petroleum liquids is:

ln P =A-BT

The vapor pressure equation for organic liquids is:


For the petroleum equation, the pressure is in units of psia and the temperature is in units of Rankine. For organic liquids, the pressure is in units of mm Hg and the temperature is in units of Celsius.

Deriving the vapor pressure constants requires a linear regression of vapor pressure (either log base 10 or natural logarithm depending on the liquid) vs temperature (either Rankine or Celsius depending on the liquid). The y-intercept of the linear regression is the vapor pressure constant A. The negative slope of the linear regression is the vapor pressure constant B. These are the constants to be added to the chemical database. C for organic liquids can be set to 273.15 if published constants cannot be found.

Be mindful of units when you do the regression as the petroleum equation uses English units and the organic liquids equation use metric units.

CFR Environmental has been performing these calculations for over 25 years and has software available for estimating AP-42 aboveground storage tank emissions. We can customize your chemical database to match the chemicals in your storage tanks. We have been working with storage tank operators for over 30 years, contact us to learn more about how we can support you.


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