| Popis: |
Glycol dehydration is a process that presents some unique challenges from 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 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 estimating height of packing used rules of thumb, not science. Mass transfer rate-based modeling, on the other hand, uses science and therefore offers greater reliability of design. The other challenge of dehydration using any glycol is thermodynamic. The dehydration of streams having very high concentrations of acid gases is hard to model reliably because the thermodynamics of vapor-liquid phase equilibrium involves water, one of nature’s most perversely-nonideal chemical species. Interactions between water and the acid gases CO2 and H2S, as well as with most hydrocarbons in the gas phase must be taken into account for a thermodynamic model to be reliable. Furthermore, in the liquid phase, aqueous glycol solutions themselves are quite nonideal because both water and glycol are polar molecules. 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 going from an environment in which it is saturated with the water contained in 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 humidification process extracts the necessary heat of vaporization as sensible heat from the liquid water phase and this can drop the wash water temperature by 30°F, 40°F or even more. Optimized Gas Treating, Inc. has recently released a new glycol dehydration model, currently for TEG, and being extended to MEG and DEG. This paper addresses the efficacy of the model in terms of (1) how well it reflects known phase behavior and (2) how closely it predicts known plant performance data using both bubble cap trays and packed columns without recourse to HETP or HTU estimates and other rules of thumb. |
