Flocculation behavior of asphaltenes in solvent/nonsolvent systems

Diffusion coefficients of asphaltenes dissolved in two aromatic (solvents, toluene-d₈ or ethylbenzene-d₁₀, were measured with the pulsed-field gradient spin echo nuclear magnetic resonance (PFG-SE NMR) technique upon addition of flocculant (pentane-d₁₂ or heptane-d₁₆). It was observed that the change in the diffusion coefficients, as a function of amount of added flocculant, was small in the concentration interval studied (up to 30 wt% alkane). Complementary kinetic flocculation studies were made at alkane additions above 55 wt%. The initial change in turbidity upon the addition of alkane was measured with an UV-VIS spectrophotometer. The obtained stability ratio, W, showed that asphaltenes were least stable in the ethylbenzene-pentane system and most stable in the toluene-heptane system. These findings were in agreement with the PFG-SE NMR. When combining the results from the two different techniques it appeared as if there was a dramatic increase in flocculation above a certain "threshold concentration" of added alkane. Furthermore, the flocculation appeared to be reaction controlled until as much as 63 wt% of n-pentane or, alternatively, 68 wt% of n-heptane had been added to the systems, after which the flocculation became primarily diffusion controlled. Finally, careful relaxation measurements showed that the asphaltenes displayed two distinctly different transverse (T₂) relaxation times (most probably averages), one at 0.6 ms and the other at 7 ms.