Can We Count on Good Turndown in Two-Pass Moving-Valve Trays?

At turndown, the efficiency of two-pass moving-valve trays may be lower than expected due to maldistribution of the vapor within the trays. This article explores this maldistribution and describes ways to alleviate it.

The moving-valve tray is one of the most common high-capacity tray types in distillation and absorption columns in the oil refining, petrochemical, and chemical industries. It provides excellent turndown — typically down to 20–25% of the maximum operating loads (1) — with good efficiency, because at low vapor loads the valves close, preventing liquid weep that is detrimental to tray efficiency. Excellent turndown is a common reason for designers and operators to favor moving-valve trays.

Recently, a few cases were reported where turndown of two-pass moving-valve trays fell short of expectations (2). This is of great concern, as most trays larger than 8 ft in diameter are two-pass or multipass. Although a few designers believe that maldistribution is the main culprit, the nature and mechanism of this maldistribution have remained a mystery.

To explore this maldistribution mechanism, this article applies the Fluor Multipass Maldistribution Model (MMM) (3) to discover multiple steady-state vapor/liquid distributions in two-pass moving-valve trays at turndown. Due to the symmetry of two-pass trays, a perfect split of both vapor and liquid between the passes is always a possible and well-known steady-state distribution. The other, previously unknown, steady states have uneven vapor distribution, in some cases highly so, with wide variations in liquid/vapor (L/V) ratios from pass to pass. This severe maldistribution is detrimental to tray efficiency.

The maldistribution described here is due to neither poor initial distribution of vapor or liquid from an inlet, nor differences in tray geometry between passes. The cause of the maldistribution is that at turndown, the valves close, and while closing, the dry (friction) pressure drop becomes independent of the vapor flowrate. This makes it easy for the vapor to swing from one pass to another, forming several alternative “preferred-path” steady states.

Using our model, we found that varying weir loads and uniform weeping have only a slight effect on the steady-state multiplicity. The multiplicity appears to be a vapor-driven phenomenon with little effect on or influence by the liquid. The magnitude of this turndown maldistribution can be largely reduced by using valves of different weights. This technique, practiced in many facilities, should be considered when potential for this type of maldistribution exists.

This article describes our investigation and findings. Additional details are available in Ref. 4.