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Simulation of concentration changes in complex volatile mixtures during evaporation by using gas chromatography

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Summary

A method is proposed for the simulation of weathering and aging of mixtures of organic components of similar polarity, e.g. gasolines. Semi-empirical equations are derived and tested which correlate the composition of samples after evaporation with the composition of the original material. In this way, changes in composition during partial evaporation of the liquid mixture can be predicted. In the context of forensic investigations this procedure can be helpful for tracing back the composition of the liquid remaining to that of the original unweathered liquid. To perform this simulation, the relative volatilities of the individual components in the mixture have to be known. These are determined by gas chromatographic retention data simultaneously with the concentrations of the volatile components. The paper evaluates the semi-empirical equations employed and discusses the influence of the simplifying assumptions introduced on the accuracy of the simulation.

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Abbreviations

F:

error function

Gi(Δt) :

transport function for component i in the gas phase in the evaporation experiment

J1(ns) :

total interstage flow within cascades in a multistage distillation process

N:

theoretical plate number of the distillation process

Qi :

mass of component i

R:

gas constant

T:

temperature

V:

volume of the phase indicated by superscript

a:

parameter depending on the value of the transport function

b:

number of independent equilibration and diffusion steps in the evaporation experiment

b′:

parameter related to N in a distillation process

d:

parameter depending on the value of the transport function

k:

diffusivity related constant

n:

number of components submitted to regression

ns :

total number of moles in the still at a certain time

Poi :

vapour pressure of the pure component i at a given temperature

v:

average molar volume of the phase indicated

x:

mole fraction

α:

relative volatility

γ:

activity coefficient of the indicated component in the liquid phase

κ:

mass-distribution coefficient of the indicated component, capacity factor

(g):

gas phase

(L):

liquid phase

(m):

mobile phase

(s):

stationary phase

(GC):

indicating GC experiment

(v):

indicating evaporation experiment

exptl:

value obtained by experiment

calc:

calculated value

i:

individual component in the mixture

R:

reference component

(o):

indicating time 0, i.e. at the beginning of evaporation

(t):

indicating time t being elapsed after the start of the evaporation

(Δt):

indicating a small time increment

Qi (g) (o) :

mass of component i in the gas phase under equilibrium conditions at the beginning of the transport process

Qi (L) (t) :

mass of component i in the liquid phase at time t

xi (L) (o) :

mole fraction of i in the liquid remaining at the beginning of the distillation

xi (L) (t) :

mole fraction of i in the liquid remaining at time t

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Authors and Affiliations

  1. Department II/11, Federal Ministry of Internal Affairs, Liechtenwerderplatz 5, 1090, Vienna, Austria

    R. Hirz

  2. Institute for Analytical Chemistry, University of Vienna, Waehringerstrasse 38, 1090, Vienna, Austria

    A. M. Rizzi

Authors
  1. R. Hirz
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  2. A. M. Rizzi
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Hirz, R., Rizzi, A.M. Simulation of concentration changes in complex volatile mixtures during evaporation by using gas chromatography. Chromatographia 31, 224–232 (1991). https://doi.org/10.1007/BF02275741

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  • DOI: https://doi.org/10.1007/BF02275741

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