(82e) Component Trapping with VLE Uncertainty: Principles and Troubleshooting

Intermediate components tend to accumulate near the middle of a distillation column in many chemical separations. Their accumulation may lead to off-spec products, corrosion, plugging, or periodic cycling. The most common method of removing these intermediate components is by taking them out in one or more a side draws. The addition of such side draws may not go far enough to achieve the desired product specs. In this case, the expensive addition of a new tower may be required.

The question of whether a side draw is sufficient to remove the accumulating intermediate components, or whether an additional column is needed, depends on the relative volatilities of the components at tower conditions. Often, there is a high degree of uncertainty in the thermodynamic prediction of the volatilities of these components. The Margules Uncertainty Analysis method (P. M. Mathias, J. Chemical & Engineering Data, 59, 1006-1015, 2014) is invaluable in analyzing these situations and in guiding engineers to the most effective path.

The case analyzed here is of an acetonitrile-water separation tower with t t-crotonaldehyde and propionitrile as the intermediate components. There are considerable uncertainties in the prediction of the volatilities of these intermediate components in this system. In the light of these uncertainties, our analysis addresses the question of whether the tight product specs can be achieved in one tower with a sidedraw or whether more than one tower is needed. The analysis shows that depending on the product specs, there would be situations where one can be certain that one tower will work, while in other cases more than one tower is needed to assure achievement of the required product specs.