Gas 2025 Issue

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.

T raditional wisdom is to be wary of liquid maldistri- bution 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 prob- lematic 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 pas- sage through the tray perforations.1 Weeping and nonu - niformity 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 simula- tion tool (ProTreat) applied to a hydraulic representation of 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

the ill-performing physical column using two parallel simu - lated 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 informa- tion 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 infor - mation, 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 pat - terns 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 maldistri - bution. The practitioner must be prepared to make assump- tions 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 con- nected 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 pat - tern is segregated. Evidence for this is abundant. If liquid

Gas 2025