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New calculation method provides better way to distribute flowpaths in distillation column
S. D. JAGUSTE and J. V. KELKAR, Reliance Industries Ltd., Mumbai, India
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Poorly designed trays can impact flow distribution and hydraulic capacity of multipass trays; thus, significantly adversely affecting the vapor to liquid (V:L) ratio. Consequently, the total separation is compromised.
In a distillation column, flowpaths can be designed to have similar efficiency and similar capacity. The presented calculations devise a way to equalize flowpath for three- or four-pass trays. The model provides simple rules in which these flowpaths can be balanced.
BACKGROUND
Multipass trays are used in large-diameter columns or where liquid loadings are high. While each flowpath for a two-pass tray is identical, the individual flowpaths of three- and four-pass trays can be substantially different, if not designed properly. Vapor distribution is governed by the hole or open area in each flowpath. Liquid distribution among the passes is determined by the upper tray pressure drop and downcomer exit loss, which in turn depends on the weir height and length, and under the downcomer area.
In a two-pass tray, design of the individual pass is identical; hence, equal distribution of vapor and liquid is easily achieved. On a three- and four-pass trays, this is difficult, because the weir length of side pass differs significantly from that of the center and off-center downcomers. Maldistribution of liquid flow will considerably lower tray efficiency. The hydraulic capacity of individual path will also be different and can cause premature flooding of column.
The model for overall efficiency of multipass trays has been presented by Bolles. 1 He correlated the tray inefficiency in terms of distribution ratio, i.e., ratio of maximum to minimum L:V among all flowpaths. Optimum efficiency and capacity is obtained when each flowpath is identical in parameters such as V:L ratio, weir loading, weir crest and tray pressure drop. In a recent article, Pilling discussed the importance of flow balancing and offered some design guidelines. 2 However, there is no literature providing equations to estimate flow distribution for a given design.
On a three- and four-pass tray, the liquid flow from off-center downcomer distributes to the side and center/ off-center downcomers of the tray below. While evaluating design for a...