Modelling of effluent dispersion from offshore natural gas desulfurization facilities

A proposed line-up for sour gas desulphurization considered an amine unit, a thermal oxidizer, a water scrubber column and an aeration tank. The water scrubber technology, as such, cannot provide an environmental solution unless the emissions of vent gas and efluent streams and how they disperse in the atmosphere and sea are properly studied. Dispersion modelling is an efficient way to predict the contaminants’ transport along with the concentration change occurring as they are leaving the source. Based on a straightforward formula that characterizes the three-dimensional concentration field produced by a point source under stationary meteorological emission conditions, the Gaussian Plume model is the most widely used. However, Computational Fluid Dynamics (CFD) has the benefit of fully capturing the system's transient behaviour, even though it can be difficult for non-experts to understand and time-consuming when modelling realistic scenarios. Although the Gaussian Plume model is much easier to use and can generate results in less than a minute, it is unable to represent the flow's temporal evolution and assumes steady-state conditions. Gaussian’s Plume’s simplicity has allowed it to be incorporated into industry-standard software packages that are used for monitoring and regulatory purposes. The use of such models is vital since the strict environmental regulations impost on pollutants’ emission should be met and therefore a realistic prediction of dispersion can benefit the design stage of the unit.

This study focuses on the efluent dispersion into the ocean coming from the water scrubber column, which was previously optimized using Aspen Plus software. COMSOL Multiphysics software was used to develop a comprehensive numerical model of efluent dispersion generated by the processing unit upon discharge to the marine environment. The evaluation of hydrodynamic transport, dilution behaviour, concentration distribution and pH evolution for efluent dispersion has been rendered by the numerical framework. The main aim of the CFD analysis was to confirm whether the regulatory pH limit of 6.5 is satisfied at a 4 meters radius from the discharge point. In recognition of the restricted availability of a completely parametrized and validated MATLAB Gaussian plume model, a CFD dispersion model was used to modify and fully parametrize a Gaussian Plume formulation for the efluent dispersion modelling.

The MATLAB-based Gaussian plume model for the marine environment has demonstrated the ability to reliably reproduce the CFD simulation results within acceptable uncertainty bounds through a methodical validation and verification approach. These models can now be confidently applied for further analyses for subsequent pH compliance assessments, thereby eliminating the need for parallel CFD simulations which are much more time-consuming