(720g) 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

1.        
Mokhtari, B. and
Gmehling, J. (2010) '(Vapour and liquid) equilibria of ternary systems with
ionic liquids using headspace gas chromatography', J. Chem. Thermodynamics
42, pp. 1036-1038.

2.        
Wauquier, J.,
Trambouze, P. and Favennec, J. (1995) Petroleum Refining: Seperation
Processes, Volume 2, Paris: Editions Technip.

3.        
Olivier, E.,
Letcher, T.M., Naidoo, P. and Ramjugernath, D. (2010) 'Activity coefficients at
infinite dilution of organic solutes in the ionic liquid
1-ethyl-3-methylimidazolium trifluoromethanesulphonate using gas-liquid
chromatography at T=(313.15, 323.15, and 333.15)K', J. Chem. Thermodynamics
42, pp. 78-83.

4.        
Schult, C.J.
et.al. (2001) 'Infinite-dilution activity coefficients for several solutes in
hexadecane and in n-methyl-2-pyrrolidone (NMP): experimental measurements and
UNIFAC predicitions', Fluid Phase Equilibria 179, pp. 117-129.

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.