(48d) Preventing Maldistribution in 4-Pass Trays

When the liquid load is high, the tray liquid is often split into two or more flow passes. Perfect symmetry is readily achievable in two pass trays, where the design of one pass is identical to the other, but not so where a further number of passes is involved. The non-symmetry renders multi-pass trays prone to vapor and liquid maldistribution. Three-pass trays are completely non-symmetrical and therefore uncommon. Four-pass trays retain the half-tray symmetry and are common and popular in industrial practice, but are still prone to maldistribution. Vapor and liquid maldistribution on 4-pass trays can lead to major loss of efficiency and capacity.

Tools for reliably predicting this maldistribution are scarce. Most of the available tools are proprietary and unreliable. In the absence of adequate predictive tools, achieving adequate pass distribution and keeping the distribution within the recommended distribution ratio is elusive and often unreachable. We are seeing far too many recent designs in which the distribution ratios were far worse than the recommended. We have seen this maldistribution causing major loss of efficiency, capacity, or both, in four-pass trays.

Our paper presents the key equations and procedures to calculate the vapor and liquid distribution on four pass trays. It then tests our method against two case studies in which the liquid distribution is known from gamma scans. Although focused on four-pass trays, the principles are readily extensible to other number of passes.