PTQ Gas 2022 Issue

Stahl columns – an alternative to molecular sieves?

Case study and independent cost estimations lay out what may someday become the standardway to achieve deep dehydration of produced gas streams

JEFFREY A WEINFELD, NATHAN A HATCHER, DARYL R JENSEN, RALPHWEILAND and CHRIS VILLEGAS Optimized Gas Treating, Inc.

G as dehydration is used to remove water vapour from gas streams for applications such as pipeline transportation and cryogenic processing. Absorption into glycols is the favoured dehy- dration method for pipeline transportation, where typically a moisture content between 80 and 140 ppmv is required, although 7 lb/ MMSCF is the often-quoted gas-in- dustry standard.¹ However, for deep dehydration applications such as LNG pro- cessing, where a moisture content of usually less than 0.1 ppmv is required, glycol dehydration for bulk water removal followed by adsorption onto molecular sieves is currently the most common method.² However, even though the molecular sieve process can achieve deep dehydration, it is economically expensive. Disadvantages such as high pres- sure drop, maintenance due to bed changes, and maintaining switching valves can amount to high costs. On the contrary, the glycol absorption process is economically favoura- ble, but the inability to adequately strip the wet solvent with a reboiled column has until now prevented it from being used in really deep dehydration applications. Deep water removal In the glycol process, the achieva- ble moisture content of a gas being dehydrated is almost entirely con- trolled by the dryness of the lean solvent (glycol) and the temperature of the wet inlet gas. On the solvent side, dried lean solvent is obtained by regeneration of the wet glycol. Concerning the controlling temper-

that dissolved into the glycol in the dehydration column (usually at high operating pressure). However, they are released near the top of the column, where they are swept out immediately. They, therefore, do the least good because they have such a small volume of the column in which to operate. In any case, their concen- trations are usually far too low to have a significant dilution effect. They may also contain compo- nents with a significant sales-gas value, so it is desirable to keep them with the sales gas. In addi- tion, there may be components with serious environmental concerns if released into the atmosphere with the stripped water vapour. Incidentally, most glycol regenera- tors are refluxed. This provides no benefit to dehydration because put - ting some of the already stripped water back into the column is counterproductive. Condensate is recycled to recap- ture glycol from the vapour via a water wash, not to enhance dehy- dration. The boiling of solvent in the reboiler is chiefly responsible for stripping water from the wet glycol. The column itself contributes very little. What small benefit it has is, to a considerable extent, destroyed by returning reflux water to the top of the regenerator to recover glycol vapour before it escapes from the system. At best, the reboiler is a single ideal stage of contact. The rest of the regeneration system, mostly the col- umn, is functionally dormant as far as water removal is concerned. To activate the column itself requires use of a stripping gas to dilute the stripped water vapour and encour-

ature, only a small solvent flow is needed to treat a large gas flow, so the L/V ratio in a dehydration col- umn is usually quite small. The thermal mass of the solvent flow relative to the gas is therefore too small to affect the gas temper - ature significantly in most of the absorber. Thus, contrary to popu- lar belief, most of the dehydration column is usually close to the tem- perature of the entering gas, not the temperature of the lean solvent (except at the very top of the column where the gas rapidly cools or heats the glycol).³ From a process standpoint, the conventional reboiled regenerator has a cripplingly serious, inherent weakness in the context of deep water removal. The dehydrat- ing agent is saturated steam, but water is the very component that is desired to be removed from the solvent. There is no carrier or dilu- ent for the removed water. In other words, there is no place for the stripped water to go except into the already saturated steam. The driving force for stripping out the water is the difference between the equilibrium and actual water content of the vapour. These quan- tities are nearly equal throughout most of the column; thus, there is little or no driving force for strip - ping water from the solvent when the vapour is already almost all water. This flaw in the solvent regeneration side of the process can be overcome by providing a diluent gas. Reboiler inefficiencies To a limited extent, this diluent is already provided by the gases

22 Gas 2022