Multiple modeling tools have been developed to estimate emissions from O&G sites. The first type is physio-chemical simulations that use equations of state, equipment design and settings, and related information to simulate process flows through a facility. In the context of production and midstream facilities considered here, these tools simulate varying gas/liquid compositions seen during physical separation, dehy-235 dration, and compression of produced product, including flows to atmosphere -i.e. air emissions. Common physio-chemical simulators include ProMax ® , widely used in upstream and midstream for facility design and diagnostics , and Aspen HYSYS, which is commonly used for optimizing hydrocarbon processes .