(176a) Tray Design Techniques at Low Liquid Load Conditions

We (as an industry) have a difficult time describing minimum liquid load for many trayed distillation applications. For example, we prescribe two criteria at present for standard downcomers; minimum weir loading and spray (blowing) factor. For truncated downcomers there is a third term which has to deal with keeping the downcomer sealed against vapor by-pass. The problem is that the first two are ill defined and little is understood about them. In addition, the industry lacks the technical background as to why we even look at these values at all. Wash sections of towers, upper sections of low pressure distillation towers and separation of high boilers all can lead to tray designs that have very low liquid loads. Today many of these applications use random or structured packings as the preferred tower internal. However, as engineers we are to find the economical solution to a problem, and many times the economical solution may still be trays. This paper is intended to address this issue and help define a design approach with Picket Fence outlet weirs that can accommodate low liquid load tray design.