“Soret-shifted” dew points for surfaces exposed to hydrocarbon vapors dilute in compressed CO 2

Abstract

We recently illustrated the strong effect of local Soret-transport-induced “enrichment” in raising condensation-onset temperatures for surfaces exposed to hydrocarbon vapors dilute in compressed gaseous N 2 at pressures up to ca. 100 atm [Rosner, D.E., Arias-Zugasti, M., 2007a. Chemical Engineering Science 62 (15), 3962–3969]. Because of current interest in the processing of compressed CO 2 containing more condensable hydrocarbon vapors, we generalize our previous methods to now include the phenomenon of carrier gas dissolution in the incipient condensate, which becomes especially important for systems like C 12 H 26 / CO 2 , even at pressures of the order of “only” 10 atm. As before, binary Soret factors and vapor-phase non-ideality are estimated using gas kinetic theory and a virial EOS, truncated at the binary encounter level [see e.g., Rosner, D.E., Arias-Zugasti, M., 2007b. A.I.Ch.E. Journal 53 (7), 1879–1890]. Carrier gas dissolution and liquid phase non-ideality are now accounted for using an extended form of regular solution theory shown to be successful for dilute CO 2 / n -alkane binary solutions by King et al. [1977. Chemical Engineering Science 32 (10), 1247–1252]. For the example of dodecane (our surrogate for jet- or diesel-fuel) vapor dilute in compressed CO 2 at, say, 20 atm near 1000 K, we predict condensation-onset surface temperatures which are higher than those expected on purely thermodynamic grounds (i.e., neglecting vapor-phase Soret enrichment) by ca. 24 K corresponding to 5.5% dew-point “shift” at a mainstream composition of 10 4 ppm ( v ) . Put another way, if measured dew-point temperatures were being used to infer mainstream vapor mole fractions ( y 1 , e ), neglect of this Soret enrichment effect would lead to an over-estimate by a factor of ca. 2.5. Under such “compressed gas” conditions, designers of partial condensation separation processes employing cooler-condensers can use the present formulation to account for all systematic departures from a familiar but degenerate limiting case: y 1 , e = p 1 , sat ( T dp ) / p , no longer valid. The present theory, leading instead to Eqs. (1) and (7) can also form the basis of an experimental method to extract high pressure Soret coefficient information from solid surface dew-point measurements in dilute undersaturated compressed gas systems of known vapor mole fraction, provided the system vapor/liquid equilibrium properties are well understood, and T mp , 1 < T dp ≪ T c , 1 .