Mar-2013

Choosing tower internals for LNG, shale gas and tail gas treating

It is well known that for best performance, tail gas treating requires the utmost in selectivity. Shale gas often contains a small amount of H2S in a much larger concentration of CO2. Sometimes this too demands highly selective treating to meet transmission pipeline specifications.

Ralph H Weiland, Nathan A Hatcher and Jenny Seagraves
Optimized Gas Treating

Article Summary

This paper shows that the inherent selectivity of trays in such applications can be greatly improved by simple changes in tray design parameters. Treating gas in an LNG plant to reduce CO2 to a level sufficient for liquefaction of the gas (typically 50 ppmv) is a completely non-selective process and one that often uses an absorber containing either random or structured packing. This paper compares structured packings of various sizes with trays, and shows the profound effect of packing type and size on absorber performance. There is an optimal packing size for each application. Specifying too large a packing will result in failure to meet the treating goal. Choosing a packing size that is unnecessarily small is not only costly because of higher packing cost; it invites plugging and reduces tower capacity, too. This paper provides a quantitative way to determine just what the right size is.

Introduction
One of the most neglected areas in gas treating is the proper selection of tower internals in both absorbers and regenerators. The literature is replete with data on vapour-liquid equilibrium in a host of aqueous amine systems containing both single amines and mixtures. Considerable data also exist on a variety of physical, thermal, and transport properties such as solution density, viscosity, heats of reaction, heat capacity, and diffusion coefficients. The kinetics of the reaction between dissolved CO2 and various amines has been measured and reported in terms of Arrhenius parameters numerous times for all the commonly-used amines. There is no question that such parameters are important in interpreting laboratory measurements of absorption rates and all of them have been used in that setting. Far fewer of these parameters have been applied in the design of commercial equipment using traditional methods such as equilibrium stage calculations, for the simple reason that ideal stages know nothing about what, if anything at all, is in the column. However, acid gas absorption and solvent regeneration are carried out commercially in columns with real internals. Apart from hydraulic considerations, the mass transfer performance of tower internals unfortunately has tended to be almost completely ignored. This is despite the fact that the mass transfer characteristics of the internals plays a central role, and at least as important a role in setting tower performance as do phase equilibrium and reaction heats. The translation from numbers of ideal stages to actual trays counts and to the required depths of structured or random packing has traditionally been done solely on the basis of experience. This approach seems to work fairly well in light hydrocarbon separations, for example, where tray efficiencies are fairly constant and well known and where vendor data exist on HETPs and HTUs for various packings for the systems of interest. Amine treating, however, is undoubtedly one of the processes least amenable to extrapolation into areas where experience is lacking.  When parts-per-million specifications must be met on product gases, selecting the wrong packing size or bed depth can result in a failed design.

For many years, packing has had a bad reputation in absorption and distillation at high pressure. Part of the reason is inattention to proper distributor design. Another is the persistent and still unresolved difficulty in translating ideal stages to actual packed bed depths and the selection of a particular commercial packing. However, packing is now being used increasingly in gas processing for several reasons. For a given size, packed columns tend to permit higher throughput than trays. And in offshore operations such as FPSO and FLNG, columns using structured packing are much less susceptible to the effects of rocking motion caused by wave action. It is particularly critical to select the right packing type and size in FLNG because of severe weight and footprint restrictions, so the need for reliability and accuracy is even higher.

It is no longer necessary to engage in any form of guesswork when it comes to designing towers containing one or more of a large range of packing types and sizes. The key is to simulate the column as the mass transfer device it really is. This entails a mass transfer rate approach. Although rate calculations require knowledge of the mass transfer characteristics of the internals (mass transfer coefficients for both phases, and the interfacial area) this kind of information is available in the literature as well as within the ProTreat amine treating simulator. As we shall see, ProTreat simulation makes packing performance just as easy to predict as trays. The emphasis here is on prediction because absolutely no parameters need to be guesstimated by the engineer to come up with the right answer. The simulation is out-of-the-box reliable and accurate.

In what follows, we briefly lay out the important factors that decide selectivity, then summarise previously reported findings on the effect of certain tray design parameters on selectivity. Attention is then turned to the performance of tower packings, first by comparing simulation with performance measured in an LNG plant in China, and then by studying the effect of packing size for a particular structured packing used in the same service.

Factors that affect selectivity
The solubility of H2S in a given amine is not enormously different from the solubility of CO2 in the same amine. Physical solubility is not what makes one amine selective and another not. There are two major factors that control selectivity. One is how fast CO2 reacts with the amine, and indeed, whether it reacts at all or not. The other is the mass transfer characteristics of the contacting device.

As soon as it absorbs into the solvent, H2S dissociates instantaneously into hydrogen and bi-sulphide ions. The reaction is so fast it throws the mass transfer resistance completely into the gas phase — the liquid offers no resistance to mass transfer. What is meant by the resistance being only in the gas phase is that the mass transfer resistance is caused by diffusional limitations with the mass transfer (absorption) rate determined by the gas-phase mass transfer coefficient. Both molecular and turbulent (eddy) diffusion contribute to the value of the gas-phase coefficient, although in commercial equipment the gas flow is usually quite turbulent, so the turbulence level sets the value of the mass transfer coefficient.

Primary and secondary amines are carbamate formers, with dissolved CO2 reacting directly with the amino group. The reaction lowers the CO2 concentration in the bulk liquid and it steepens the CO2 concentration gradient by providing an additional means for CO2 to disappear besides by diffusion. The faster the reaction, the lower the liquid phase resistance to CO2 absorption. On the other hand, tertiary amines such as N-methyldiethanolamine (MDEA) are capable only of accepting a proton such as is liberated by H2S dissociation and by CO2 hydrolysis in water.
Mopping up the hydrogen ion with MDEA shifts the hydrolysis reaction to the right; ie, it increases the solvent’s capacity for CO2; however, the hydrolysis reaction itself is slow and does essentially nothing to steepen the CO2 concentration gradient and speed up the absorption process2. The resistance to CO2 absorption remains completely in the liquid phase and is just as strong as it would be without MDEA. All the MDEA does is to increase greatly the absorption capacity of the solvent.