Distillation tray design techniques used for design and sizing of new columns are abundant in the literature. So too are the guidelines for modifying distillation towers for operation beyond their original design range. Reducing capacity of distillation trays merits at least as much consideration. Lack of knowledge and experience in this area causes many tower failures and misdesigned columns. The objective of this paper is to present some practical design considerations based on field experience that lead to effective operation of industrial equipment. The information presented covers design of tower trays for operation at loadings dramatically lower than normal for a particular design. General considerations cover liquid and vapor hydraulics and flow behavior. Detailed case studies are included for three typical units: a refinery vacuum crude still (lubricants service), a petrochemical superfractionator, and a steam stripper.
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