Apr-2011

Glycol dehydration of high-acid gas streams

A model based on mass and heat transfer closely predicts plant performance data for glycol dehydration of high-acid gas streams

John Carroll, Gas Liquids Engineering
Nathan Hatcher and Ralph Weiland, Optimized Gas Treating

Article Summary

Glycol dehydration is a process that presents some unique challenges from both technical and computational standpoints. In the first place, modern designs almost invariably use tower internals consisting of structured packing rather than the more traditional bubble cap trays.

Structured packing offers a lower pressure drop and considerably higher capacity than trays, and it is well suited to handling the very low L/G ratios common in dehydration. However, until now, the height of the packing was estimated using rules of thumb, not good engineering science. Mass transfer rate-based modelling, on the other hand, uses science and therefore offers greater reliability of design. The other challenge of dehydration using any glycol is thermodynamic: water is the component of interest but it is probably nature’s most perversely non-ideal chemical, which makes it challenging to devise a truly accurate model for the vapour-liquid equilibrium, especially in systems with a high-acid gas content.

Traditionally, sweetened gases have been the main candidates for dehydration, mostly using triethylene glycol (TEG) prior to entering the transmission pipelines. More recently, there has been much interest in sour gas injection as a means of disposing of gas streams of too low quality for sulphur recovery. In the context of carbon capture, the recovered CO2 is compressed, liquefied and injected into underground formations. Handling these sour and high CO2 streams requires dehydration prior to compression and/or liquefaction.

There are other facets of glycol dehydration that are interesting just from an applied science viewpoint. One of them is the heat transfer situation that ensues in a regenerator using both stripping gas and a reboiler (Stahl column). When the hot gas hits the bottom of the packing in the wash section atop the column, it finds itself suddenly going from an environment in which it is saturated with water in equilibrium with a predominantly TEG stream into an environment where it is grossly under-saturated with respect to the pure water stream in the wash section. This causes extremely rapid humidification, and the humidification process extracts the necessary heat of vapourisation as sensible heat from the liquid water phase. Sudden humidification can drop the wash water temperature by 30–40°F or even more.

This article addresses the efficacy of a new mass and heat transfer rate-based model in terms of how well it reflects known phase behaviour and how closely it predicts known plant performance data using both bubble cap trays and packed columns without recourse to height equivalent to a theoretical plate (HETP) or height of transfer units (HTU) estimates and other rules of thumb.

Phase equilibrium
Our phase equilibrium concerns are twofold: accurate calculation of the equilibrium water content of high- and low-pressure gases containing very high levels of CO2 and/or H2S; and calculation of the solubility of all gases, including water, in the dehydration solvent itself. These two topics are inter-related because both require the accurate assessment of the interactions between components in the vapour phase; however, the solubility calculation is more demanding.

Normally, to model the solubility of a gas in a liquid, one uses the Henry’s Law approach. A thermodynamically complete version of Henry’s Law2,3 is:

a~ixiHij exp >v–i∞ (P – Pjo)H= γiPφˆiv              (1)                         RT             

where        
γi:    Activity coefficient
xi:    Mole fraction of component i in
    the solvent (its solubility)
Hij:    Henry’s constant for solute i in
     solvent j, kPa/mol frac
v-i∞    Partial molar volume of i in
     solvent j at infinite dilution,
    m3/kmol
Pjo:    Vapour pressure of the solvent,
    kPa
P:    Total pressure, kPa
R:    Universal gas constant,
    8.314 kJ/kmol·K
T:    Absolute temperature, K
yi:    Mole fraction of component i in
    the vapour
φˆiv Fugacity coefficient for
    component i in the vapour,
    unitless

Most people believe that Henry’s Law is only applicable to dilute solutions, but the form in Equation 1 can be applied without restriction. It would be unusual to do so, but this equation can even be applied to mixtures that are not typically considered to refer to the solubility of a gas in a liquid at all; for example, methanol and water.

The numerical value of the Henry’s constant is a function of both the solute and the solvent. Thus, the Henry’s Law constant, Hij, is different for methane in water than it is for methane in methanol, and Hij is different still for ethane in water. Furthermore, for every solute-solvent pair, the Hij is a function of the temperature. When comparing different solutes in the same solvent, the larger the Henry’s constant the lower the solubility of the solute.

The activity coefficient accounts for the effect of concentration on the activity of the component in the liquid phase. The infinite dilution definition of the activity coefficient is used here; its significance is that at infinite dilution, the activity coefficient defined this way is unity.