Apr-2025

Effect of maldistribution on tail gas treating unit absorber performance

A quantitative study of the effect of liquid and vapour maldistribution on the separation performance of a packed column used in a TGTU is described.

G Simon A Weiland, Prashanth Chandran and Ralph H Weiland
Optimized Gas Treating, Inc.

Article Summary

Traditional wisdom is to be wary of liquid maldistribution in packed towers because of its deleterious effects on column performance. However, it is hard to find quantitative information about just how negative its effect can be. Imbalanced liquid distribution is less problematic in trayed columns, so it is rarely mentioned in that context. Note, however, that most trays under severely turned down conditions, and dual flow trays under almost all conditions, will show severely nonuniform liquid passage through the tray perforations.1 Weeping and nonuniformity are both responsible for low efficiency. However, uneven vapour distribution can drastically decrease the performance of both types of columns.

A mechanical device (such as Shell’s proprietary Schoepentoeter, a vane-type inlet device) is sometimes used to help achieve more uniform vapour distribution in large-diameter columns where vapour maldistribution is prone to occur. Nevertheless, maldistribution caused by poorly introduced vapour has the same potential to cause serious performance loss as liquid maldistribution caused, for example, by an out-of-level or damaged distributor.

Although the initiating event may be associated with one phase or the other, liquid and vapour imbalance are tightly connected and always occur together. Maldistributed vapour causes the liquid to maldistribute and vice versa, so one must consider both phases in any deliberations. As discussed later, well-distributed countercurrent vapour and liquid flows in a packed column tend to be unstable. Nonuniformities, once initiated, usually grow, persist, and expand over the entire depth of the packed bed.

This study was done with the aid of a proprietary simulation tool (ProTreat) applied to a hydraulic representation of the ill-performing physical column using two parallel simulated columns.3 It revealed that a surprisingly small degree of unevenness in the distribution of either phase can cause a significant rise in the H2S leak from the tail gas treating unit (TGTU) without displaying the typical symptoms of column issues, for example, measurably affecting pressure drop across the column.

It is not possible to predict the extent of maldistribution. This complex hydraulics problem needs detailed information on such things as distributor out-of-levelness and the results of a computational fluid mechanics study of the flows in the tower sump and vapour entry. However, if maldistribution is suspected, thermal imaging of the column from several positions around its periphery can provide an estimate of where liquid flow is excessive and, by inference, where vapour flow is abnormally high. This kind of information, combined with an accurate simulation of a rough hydraulic representation of the tower with maldistribution in mind, can be useful in troubleshooting this challenging situation.

Maldistribution hydraulics
Almost all the literature dealing with the maldistribution of countercurrent gas and liquid flows in packed columns deals primarily with hydraulic modelling of the flow patterns within the packing, given the initial distribution of liquid.2 However, there appear to be no plausible methods for predicting the extent of liquid and/or vapour maldistribution. The practitioner must be prepared to make assumptions and propose estimates as to what fraction of the column cross-section carries excess liquid (or vapour) and the excess in liquid (or vapour) flow.
The maldistribution is modelled by segregating the two cross-sections into two separate parallel columns connected at the top and bottom according to the assumed cross-sectional areas. Once the specifications of area and excess flow are made, the rest of the parameters are fixed by enforcing the requirement of equal pressure drop across the two columns, which must behave as one when merged.

Maldistribution can be mitigated but cannot be prevented because uniform countercurrent two-phase flow through packed beds is inherently unstable. The preferred flow pattern is segregated. Evidence for this is abundant. If liquid is introduced at only one point at the top of a packed bed, it will tend to remain well segregated from the vapour as it flows down the column. It naturally takes (or even creates) the flow path of least resistance.

Likewise, the path of least resistance for the vapour is where there is little or no confronting liquid counterflow. As liquid and vapour flow through the column, they tend to segregate even after great pains have been taken to ensure these phases were introduced with very uniform distribution. This is why liquid redistributors can be such a critical part of a packed column.4

There are numerous causes of maldistributed flows. These include:
- Introducing vapour at high velocity through a single feed pipe at the base of a large diameter column (causing maldistributed vapour).
- Careless dumping of random packing into the column, leading to large voids.
- Using packing that is too large for the diameter of the column, causing preferential flows through the high voidage areas adjacent to the column walls. The conventional guideline that D/d >8 gives the maximum packing size (d) relative to the column diameter (D) to ensure negligible wall flow is, in our experience, optimistic – a value of 12-15 is minimal for surety.
- Wrong distributor type for the liquid flow rate.
- Poorly designed liquid distribution.⁴
- Out-of-level distributor.
- Lack of liquid redistributors (dumped packings) or wall wipers (structured packings).
- Packed bed too deep (single beds should not exceed L/D = 15-20 without liquid redistribution).

Case study
The TGTU absorber that forms the basis for this study contains 15ft of IMTP-40 random packing and uses 45 wt% methyldiethanolamine (MDEA) solvent. Other data are shown in Table 1. This packing depth maximises CO2 slip at almost 93% while removing H2S to 85 ppmv. Although a depth of 40ft will remove H2S to 31 ppmv, it will lower CO2 slip to only 82%. However, whether 15 or 40ft of packing are used, the conclusions reached in this study are qualitatively unaffected. To handle the flows, the column needs to be about 5.3ft in diameter (65% flood). With the bed depth (15ft) being less than three times the column diameter, no redistributor is needed.