AVOID SALT PRECIPITATION
IN STRIPPER OVERHEAD SYSTEM
The Topsoe Hydroflex™ process converts renewable feedstocks into transportation fuels. A typical sketch of this process is shown in Appendix at the end of this document.
Renewable feedstocks contain some amounts of organic Chloride and Nitrogen. In the hydroprocessing reactors, these elements are converted into HCl and NH3. At low temperatures, these compounds can react and form NH4Cl, which precipitates as a salt that can cause plugging and corrosion.
After the reactor section, the liquid product is sent for separation in a fractionation section. Typically, the first column in the fractionation section is a steam stripper. In the stripper, the salts are sublimated and will precipitate in the overhead line (indicated in the drawing) if nothing is done to prevent this.
We have calculated that we need to keep the process temperature above 145ºC to avoid salt precipitation in the stripper overhead line. However, with some margin this temperature is suggested to be kept at minimum 155ºC. The high stripper overhead temperature means that a significant amount of jet boiling material is lost in the naphtha product stream – Around 2.4wt% relative to the renewable feed (jet boiling range is taken as 110-300ºC). Since jet and diesel products are the most valuable, it is not acceptable to lose this high amount of jet in the naphtha stream.
The loss of jet into the naphtha product can be minimized, preferably to less than 0.5wt% relative to the feed by sending stabilized naphtha to the fractionator, as shown in the sketch. The naphtha product from the stabilizer can be sent as reflux to the main fractionator or to the jet side cut stripper to recover the jet material. The naphtha product will then be the fractionator overhead liquid stream.
