Hydrocarbon Streams are contaminated by soluble polar (or ionic) contamination in a variety of ways (viz., caustic treatment, amine treatment, acid catalysts etc). The contaminant typically exists within the hydrocarbon phase in a soluble form, and in some cases it may not even be ionized within the non-polar hydrocarbon. Nevertheless, it is generally necessary to remove these contaminants. In some cases, the contaminants may poison downstream process equipment; or example, diesel fired turbines are sensitive to alkali metal contaminants; selective hydrogenation units (SHU) are sensitive to sodium). In other cases, these soluble contaminants represent an ongoing operating cost (i.e., amine dissolved in a C3/C4 stream needs to be made up with fresh (expensive) amine).
Conventional Approaches to removing the dissolved polar components, utilize washing the hydrocarbon with water. Typical approaches involve:
- Water-Wash Towers
- Water Injection followed by Coalescer or Knock-Out
In all cases, the process involves dispersing the water into the hydrocarbon stream followed by a separation downstream. Since the mixing and the separation are not very efficient, typical water-washes utilize large quantities of water (20 – 40%) that are run in a re-circulating loop. A small fraction of water is continually added, and a small fraction of water is purged. A typical tray, or packed-column, offers 60% stage-wise efficiency for mass-transfer and consequently requires a large wash column for mass transfer. Additionally, multiple stages may need to be applied to overcome the low stage-wise efficiencies. Finally, these systems do not have high aqueous-hydrocarbon separation efficiencies; thereby, increasing the likelihood of the extracted component being re-entrained back into the hydrocarbon.