Techniques for the design and sizing of new trayed distillation columns are abundant in the literature. So, too, are the guidelines for modifying towers for operation beyond their original design range. Reducing capacity of distillation trays merits at least as much consideration. Indeed, lack of knowledge and experience in this area causes many tower failures and misdesigned columns. In this article, we present some practical design considerations, based on field experience, for tower trays operating at loadings dramatically lower than normal for a particular design. General considerations cover liquid and vapor hydraulics and flow behavior. Case studies are included for three typical units: a refinery vacuum crude still, a petrochemical superfractionator, and a steam stripper.
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