(55a) Process Design of Light Naphtha Purification By Type III Deep Eutectic Solvents

Problem Statement: Sulfolane, though effective at removing benzene, toluene, and xylene (BTX) from naphtha, poses a serious risk to the environment. Type III deep eutectic solvents (DES) made from phosphonium or ammonium salts (hydrogen bond acceptors or HBA) paired with a hydrogen bond donor (HBD) may be able to replace sulfolane in the naphtha purification process through using similar absorption mechanisms. The experimental evaluation of Type III DES is problematic in that slight changes in concentration or functionality (e.g. changing the HBA from methyltriphenylphosphonium bromide to ethyltriphenylphosphonium) can have drastic effects on the properties of the DES. For the efficient screening of DES candidates for naphtha purification, a method that is based on knowledge of sulfolane absorption is needed. Sulfolane is an effective solvent for BTX removal from naphtha due to its high capacity for BTX (due to similar electron density distribution and favorable Van Der Waals interactions with BTX) and its high selectivity (due to its polarity differences with naphtha).

Proposed Solution: In this study, conductor like screening model (COSMO) was adopted to screen suitable HBA (quaternary ammonium and polyphenol phosphonium salts) and HBD (glycol, glycerol, and carboxylic acids) for the removal of BTX from light naphtha (modeled as pentane) based on selectivity and maximum capacity. The best performing DES where then evaluated in a technoeconomic analysis model and compared to sulfolane. A model process consisting of an absorber, desorption unit, solvent regeneration and heat recovery system was used with each of the selected DES’s thermophysical properties to obtain mass and energy balances for a naphtha purification process. Processing parameters such as maximum temperature and pressure were kept similar to sulfolane, with temperatures below 100°C and pressures 2 atm. This process was designed partially in ASPEN. A sensitivity analysis evaluating the effect of key parameters such as capacity, selectivity, heat recovery, etc., was used to determine which variables should be prioritized in guiding future endeavors to modify physical absorption units for use with DES.