Effect of clay nano-particles on foam stability and mass transfer resistance
During the process of crude oil/gasoline loading and storage, significant amounts of volatile organic compounds (VOC) are emitted into the environment. VOC emission pollutes the environment, is a fire hazard and a threat to workplace and general safety. In this work novel aqueous foams have been formulated by incorporating a special type of clay nano-particles in an aqueous solution of surfactants and polymers and tested. These foams provide an effective and efficient mass transfer barrier to VOC emission during the process of loading of crude oil/gasoline. The use of special types of clays created continuous interlinked and overlapping layers of clay in the foam formulation. There was a decrease of the rate at which liquid is drained out of the foam lamellae and thus, a decrease of the rate of quality increase. There is also a significantly reduction of the rate of vapor diffusion through the thin lamellae and the whole foam column for the first 40 hours. These foams are found to be stable and lasted longer than 10 hours of loading operation in the presence of gasoline and crude oil. Increasing the polymer and clay concentrations significantly increased the foam stability. These foam systems are generally safe and technically and logistically easy to handle.
A mathematical model is developed for the diffusive mass transfer of volatile organic compounds (VOC) through a clay aqueous foam column. The model incorporates the lamella film thickness thinning due to the liquid drainage from the lamella, the bubble size growth which results from transfer of gas across gas-liquid interface mass transfer and the VOC concentration profile as a result of the overall diffusion across the foam column. Effect of gravity and capillary drainage from the plateau borders and the thinning of foam lamella thickness caused by the forces of capillary suction were considered. The model VOC emission simulation compares reasonable well with the experimental data of highly stable clay aqueous foam formulation developed using surfactants, stabilizer, polymer and clay and tested in the presence of gasoline. Higher clay loading foams gives better diffusive mass transfer resistance. The bubbles sizes do not change significantly in the model as compared to the experimental data due to the bubble collapse and coalescence not incorporated into the model.
The stability and mass transfer resistance of these novel foams were also investigated at temperatures up to 125°F. Increase in temperature increases the rate of foam breakage significantly. High temperature causes more vapors to be released from the oil layer creating bigger bubbles and eventual increase in drained liquid. High temperature causes the bubble collapse around the foam/oil interface and significant amount of evaporation at the foam/air interface leading to foam breakage at the top of the foam column. The foam system tested shows a very high degree of stability in the presence of gasoline and crude oil vapors. It lasted longer than 50, 20 and 10 hours at 75°F, 105°F and 125°F, respectively. Increasing temperature also reduced the half-life of the foams. The total VOC suppression within the 10 hours loading process for the 0.5% polymer and 0.5% clay foam was greater than 96% at 105°F and 125°F temperature conditions.
The bench-scale experiments are upscale to a large vessel that has an exposed surface area several orders of magnitude larger. Experiments are carried out in a continuous (dynamic) gasoline loading process. The total VOC emission decreases with increase in clay loadings. All the foams tested are found to suppress the VOC by more than 90%. The amount of drained liquid was found to increase with increase in clay loading. Laponite S showed a slightly higher degree of VOC suppression than Laponite RD and RDS, which showed about the same degree. The VOC suppression was higher than 96% for all the 0.5% polymer and 0.5% clay foams tested within the 10 hours of gasoline loading process and the drained liquid was less than 50%. Laponite RDS and S retained more liquid than Laponite RD. The foams were found to be stable and effective in suppressing VOC under dynamic loading in the presence of gasoline.
0775: Environmental engineering