Modeling of PCB removal processes from natural gas transmission lines

PCBs exist in varying proportions within most gas transportation pipeline systems in the United States and elsewhere. There is strong interest on the part of the gas transmission companies to clean these PCB-contaminated pipeline systems, in order to reduce or eliminate the risk of environmental mishaps. The goal of eradicating PCBs from gas transmission systems is feasible with the appropriate technology. One such technology is solvent injection which is the focus of this study. In this study a model for tracking PCB migration under a variety of solvent injection scenarios is presented. The inherently transient nature of PCB migration in pipelines necessitates the development of a fully transient multiphase gas-liquid formulation. This formulation consists of a set of first-order nonhomogeneous, nonlinear hyperbolic partial differential equations (PDEs). Numerical solution of these equations requires robust numerical algorithms. A second-order accurate Total Variation Diminishing (TVD) scheme is implemented to transform the PDEs into ordinary differential equations (ODEs). The resulting ODEs are then solved using a 5th -order Runge-Kutta solver. The model serves as a tool for evaluating the efficacy and efficiency of terpene, a natural solvent, as a means of removing PCBs from contaminated natural gas pipelines. The study shows that PCBs migrate primarily in the liquid phase. In addition pipeline operational protocols significantly affect the length of times needed for decontaminating any particular pipeline system.