An investigation into the potential of NFM, DEG and TEG as replacement solvents for NMP in separation processes.

Abstract

Optimisation attempts within the petrochemical industry have led to interest in alternate solvents. The most widely used commercial solvents for the separation of hydrocarbons, by extractive distillation, are N-methylpyrrolidone and sulfolane. There has also been reference made to other solvents, such as N-formylmorpholine and the ethylene glycols [mono-, di-, tri- and tetra], being used. The alternate solvents proposed for this study were N-formylmorpholine, triethylene glycol and diethylene glycol. Infinite dilution activity coefficients, γ∞, provided a means of comparing the ease of separation of the different solutes using different solvents in extractive distillation. There is a substantial database of γ∞ measurements for systems involving N-methylpyrrolidone and hydrocarbons. A fairly large data set of γ∞ values of hydrocarbons in N-formylmorpholine has also been measured. Very little work has been conducted on the γ∞ values of hydrocarbons in either diethylene glycol or triethylene glycol. Gas liquid chromatography is one of the more common methods used to measure γ∞. To enable the measurement of γ∞ at higher temperatures, a pre-saturator was installed prior to the column. This ensured that the carrier gas entering the column was saturated with solvent and prevented the elution of solvent from the column. The γ∞ values of 25 solutes; including n-alkanes, alk-1-enes, alk-1-ynes, alcohols and aromatics; were measured at temperatures of 333.15, 348.15 and 363.15 K. The γ∞ measurements in N-formylmorpholine were used to verify this experimental set up and technique. Once the experimental set up had been proven, γ∞ in N-methylpyrrolidone, triethylene glycol and diethylene glycol were measured. Selectivities and capacities, at infinite dilution, of several solute combinations in the four solvents were then compared. In a few of these separation cases, the alternative solvents appeared to have better separation performance than N-methylpyrrolidone. The γ∞ values of three of the solutes in N-formylmorpholine and N-methylpyrrolidone were also measured using the novel cell design and measurement procedure suggested by Richon. It was found that this new technique required further development for the case of volatile solvents, since the results obtained using this technique did not compare favourably with the literature data.