Infinite Dilution Activity Coefficient Measurements of Various Hydrocarbon Solutes In NMP, NFM, DEG and TEG by Gas Liquid Chromatography and the Inert Gas Stripping Technique

Optimisation attempts within the petrochemical industry have led to interest in alternate solvents. The separations of interest involve the production of alpha-olefins from synthetic petroleum containing alkanes and alkenes (C₅'s – C₉'s) and alcohols (C₁'s – C₃'s).  The most widely used solvents for this purpose, commercially, are NMP (N-methylpyrrolidone) and sulfolane, although there are references to other solvents being used ^{1, 2}.  These less commonly used solvents include NFM (N-formylmorpholine) and the ethylene glycols [mono-, di-, tri- and tetra-].  The alternate solvents proposed in this study are n-formylmorpholine (NFM), triethylene glycol (TEG) and diethylene glycol (DEG).  Detailed and accurate equilibrium data is required for the investigation into the use of different solvents in this separation process.  Infinite dilution activity coefficients provide a means of comparing the ease of separation of the different solutes using extractive distillation ^{3, 4}.  The selectivity and the capacity of the solvents for the different solutes will be used to determine which hydrocarbons will be most difficult to separate from the alpha-olefins in a theoretical synthetic petroleum stream. 

There is a substantial database of infinite dilution activity coefficient measurements for systems involving NMP and hydrocarbons ^{4, 5}.  A fairly large data set of infinite dilution activity coefficients of hydrocarbons in NFM has also been measured ^{5, 6}.  Very little work has been conducted on the infinite dilution activity coefficients of hydrocarbons in either DEG or TEG ⁷.  Previously measured infinite dilution activity coefficients for hydrocarbons in NMP and NFM were mostly evaluated at temperatures in close proximity to room temperature.  This was due to the complication of the measurement techniques instigated by the increase of volatility of the solvents with temperature.  To enable measurement of infinite dilution activity coefficients at higher temperatures, a modification of the gas-liquid chromatography technique was recommended ⁸.  A pre-saturator was included prior to the column, to ensure saturation of the carrier gas entering the column, and thus prevent elution of the solvent from the column. 

Infinite dilution activity coefficients can also be measured using an inert gas stripping method/dilutor cell technique.  An improved cell design and measurement procedure suggested by Richon⁹ will be used to measure the infinite dilution activity coefficients of the same systems as measured by gas-liquid chromatography.  This new design and procedure provides rapid and simplified measurements, as well as improving accuracy.  The results obtained from the gas-liquid chromatography method will be used to verify the accuracy of measurements obtained using the new dilutor cell design and measurement procedure. Selectivities and capacities will be calculated for various separation problems/combinations and compared to literature values for the commonly used solvents.

REFERENCES

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5.         Weidlich, U., Rohm, H.J. and Gmehling, J. (1987) 'Measurement of gamma infinite using GLC. 2. Results for the Stationary Phases N-formylmorpholine and N-methylpyrrolidone', J. Chem. Eng. Data, vol. 32, pp. 450-453.

6.         Knoop, C., Tiegs, D. and Gmehling, J. (1989) 'Measurement of gamma infinite using gas-liquid chromatography. 3. Results for the stationary phases 10-nonadecanone, N-formylmorpholine, 1-pentanol, m-xylene, and toluene', J. Chem. Eng. Data, vol. 34, pp. 240-247.

7.         Sun, P., Gao, G. and Gao, H. (2003) 'Infinite Dilution Activity Coefficients of Hydrocarbons in Triethylene Glycol and Tetraethylene Glycol', J. Chem. Eng. Data, vol. 48, pp. 1109-1112.

8.         Kwantes, A. and Rijinders, G.W.A. (1958) Gas Chromatography 1958, 1^st edition, London: Butterworths.

9.         Richon, D. (2011) 'New equipment and a new technique for measuring activity coefficients and Henry's constants at infinite dilution', Review of Scientific Instruments, vol. 82, In press.