Untreated hydrocarbon
PC
Treated hydrocarbon
FC
FC
T hiolex
T hiolex
LC
LC
Hydrocarbon Caustic
FC
M
Fresh caustic
H S removal: H S + 2NaOH Na S + 2H O CO removal: CO + 2NaOH Na CO + H O Mercaptan extraction: RSH + NaOH RSNa + H O
Sulphidic caustic
Figure 1 Thiolex unit configuration
surface area, microscopic diffusion distance, and continu- ous renewal of the aqueous phase combine to yield mass transfer efficiencies far greater than possible with conven - tional dispersive treatments. Since it is highly customisable, fibre bundle geometry allows the treatment of a wide range of hydrocarbon types with different physical properties. The inherent low differential pressure drop across the fibre bundle and the fast phase separation time facilitate the ret- rofit of this treatment process into existing systems as a debottlenecking solution. The cocurrent flow is more forgiving during hydrocar - bon flow upsets, allowing treatment even under sub- optimal conditions. Carryover is virtually eliminated due to the avoidance of droplet formation as the aqueous phase adheres to the fibres in the contactor rather than being dispersed into the hydrocarbon phase. Emulsion formation is also effectively eliminated. Since efficient phase contact occurs without dispersion, stable emulsions rarely form in the unit. Similarly, the system does not depend on gravity settling for micro droplets or emulsion coalescence, which greatly reduces separation time. Processing vessels can be much smaller. In most cases, expensive downstream coalescers and other clean-up equipment are not required. However, coalescers can further enhance the separation, taking the ultimate separation down to almost non-detectable levels of carryover of aqueous treating liquids in the hydrocarbon product. The smaller size separation requirement translates to less expensive equipment cost, and with fewer pieces of smaller equipment, plant space is more efficiently utilised. Most importantly, this method of mass transfer achieves maximum removal of impurities from the hydrocarbon to meet today’s stringent standards as set forth by the US Environmental Protection Agency’s Clean Air Act. Challenges of other solutions Conventional caustic treating processes with dispersive mixing devices and phase separation were once the only option available to the industry. However, conventional dispersion and phase separation methods are subject to
numerous shortcomings, including lack of turndown capa - bility, plugging, flooding, channelling, unpredictable treat - ing results, long settling times, aqueous phase carryover, generation of dilute aqueous wastes, lower service factor, hydrocarbon losses, larger plot space, product contam - ination, and additional processing steps and equipment needed to separate phases. Even when the dispersion-based system provides ade - quate treatment, separating the two phases can be prob - lematic. The mixture must remain in the phase separator until the caustic droplets settle out by gravity, which can take hours. As the treating requirement becomes more stringent, mixing energy is increased to maximise interfa - cial surface area. This results in a greater dispersion of the aqueous phase, requiring exponentially longer separation times. Stable emulsions can form in the mixing device, result - ing in massive carryover from the separator vessel. Due to excessive carryover, expensive equipment such as knockout vessels, sand filters, and water wash units must be installed downstream to remove the dispersed aqueous phase from the treated product. Treatment is often interrupted if an emulsion develops. Excessive emulsion formation can dam - age downstream equipment and foul separators and other mass transfer equipment. Treating processes Merichem Technologies’ proprietary Fiber Film Contactors create a process for caustic extraction of mercaptans, naph - thenic acids, and H2S impurities from hydrocarbon streams. This mass transfer device creates an interfacial surface between hydrocarbon and caustic phases in a non-disper- sive manner. It consists of a multitude of thin fibres packed in a cylindrical column, where the hydrocarbon and aque- ous phases flow cocurrently downward, forming a thin film on the fibre surface. This eliminates problems associated with the principle of droplet formation and dispersion of one phase into the other, which are often encountered in conventional sul - phur extraction/sweetening units. The fibre pack creates
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PTQ Q1 2025
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