Feb-2008

A new acid gas enrichment process

Patented HighSulf technology is one of the most effective methodologies for acid gas enrichment (AGE) and tail gas treatment (TGTU), and it can be highly beneficial when applied to shale gas treatment.

Tofik K Khanmamedov, TKK COMPANY
Ralph H Weiland, Optimized Gas Treating, Inc

Article Summary

Based on the use of generic N-methyldiethanolamine (MDEA), the technology enriches weak acid gas sulphur recovery unit (SRU) feed to much higher H2S concentrations than conventional AGE.  Distinct from all known AGE schemes, HighSulf also allows effective control of H2S concentration in the feed and more reliable operation of the SRU.  This leads to more stable and efficient operation of the SRU and a higher level of sulphur recovery.

A new member of the family of HighSulf technologies, called HighSulf Plus, is discussed by comparison with conventional AGE units. Also, a synergistic effect observed when HighSulf Plus is applied to the amine section of a TGTU is discussed. In this application, HighSulf Plus reduces the level of H2S going to the incinerator, increases H2S concentration in acid gas, and substantially reduces cooling requirements for the lean amine in the trim cooler and the condenser of the regenerator. In hot climates it can be very difficult to achieve low enough lean amine temperatures using ambient air.   HighSulf Plus  is one of the best ways to solve this problem.  Simulation and design numbers for a commercial unit are presented for discussion.

Introduction
many sour gases contain small enough concentrations of H2S to pose challenges because the acid gas produced in the treating plant is sub-quality for Claus sulphur plant feed, but it cannot be vented or incinerated either. Even in plants with a moderate CO2:H2S ratio of say 4:1, if complete acid gas removal is necessary (LNG for example), using a single contactor will necessarily produce an acid gas stream containing only 20% H2S. In other situations, if a significant proportion of the CO2 must be removed with the H2S, the resulting Claus plant feed may be too dilute for conventional processing.

As shown on Figures 1 and 2, sulphur plants are most efficiently operated when the feed contains 55% or more H2S. The balance of the SRU feed is CO2 and water, possibly with small amounts of hydrocarbons or other components. Lower concentrations of H2S result in greater sulphur plant complexity, larger equipment, and higher cost. Streams having less than 32% or so H2S are near the lower limit for a straight-through Claus process. In some cases even 45% H2S in the acid gas may be the lower limit for a straight-through Claus unit.

HighSulf is a general, patented, process strategy that can be applied incrementally in amine treating plants to increase the H2S concentration in the acid gas from the regenerator and produce an increasingly high quality Claus sulphur plant feed. HighSulf technology recognises that the higher the H2S content of the gas being treated in an amine unit, the greater will be the H2S concentration in the acid gas from its regenerator. HighSulf processing actually takes steps to increase the H2S content of the feed gas to the amine plant itself. As a result, the family of HighSulf processes produces a more concentrated product stream, as discussed by Khanmamedov^1-8. One such application is upgrading the acid gas from the main amine plant regenerator to higher H2S content 
by processing the regenerator acid gas in another, smaller amine plant. This is termed acid gas enrichment (AGE). It is almost always the case that this secondary treating or AGE unit can  apply HighSulf technology very profitably.

As discussed recently^{19, 20}, there are numerous flowsheet configurations that can be applied to AGE. We also recently examined several possible processing schemes applied to the enrichment of very low (8%) H2S-content gas and of marginally processable gas (34% H2S) and showed which HighSulf processing schemes are the best choices for consideration.

Selectivity of MDEA-based HighSulf
AGE depends critically on maximising selectivity for preferential absorption of H2S and for simultaneous rejection of CO2. Achieving the highest possible selectivity for H2S over CO2 starts by using the right solvent under the right process conditions in the right equipment. Detailed discussions of selectivity have been presented in many places including Khanmamedov3-4,19,20, Weiland et al^13,14,16, and Khanmamedov and Weiland9 so only a brief review is given here.

The equilibrium solubilities of H2S and CO2 in selective solvents such as MDEA do not differ radically from each other; in other words, chemical solvents in and of themselves do not have great inherent thermodynamic selectivity. In the final analysis, selectivity is really determined by the differences between H2S and CO2 absorption rates. Absorption rates are really controlled by (1) reaction kinetics and (2) the hydraulic and mass transfer characteristics of the contacting equipment (as expressed by the relative magnitudes of gas- and liquid-side mass transfer coefficients, for example). As shown in Figure 3, the driving force for selectivity of any solvent is the differences in absorption rates of H2S and CO2.

Greater differences in absorption rate lead to higher selectivity of the solvent, and absorption rate itself depends on the partial pressure of the desired component (H2S in this case). As described early^{1–9, 17–20}, HighSulf allows one to control the partial pressure of H2S in the feed to the absorber by the simple rerouting of piping. As a result, differences in absorption rates of H2S and CO2 in the MDEA-based HighSulf process is much higher than for any conventional MDEA-based process.  It should be pointed out that the HighSulf concept is not limited to MDEA as the solvent — any other selective solvent will exhibit similar differences but of varied degree. Table 1 shows how selectivity responds  to changes in various parameters.

The number of trays and the solvent circulation rate have opposite effects on selectivity when the amine unit operates at low pressure.  Leaner amine and higher H2S partial pressure in the feed gas increase the selectivity of MDEA or any other selective solvent.

Since 1930 when the first patent for regular amine process was filed by Bottoms²¹, practice has been fixated on amine units with conventional configurations.  For the most part, subsequent patents, published articles and publicly available project designs  have focused on minor rearrangements of equipment within a regular amine unit, but without significant overall improvement. Numerous patents have also been issued for new or formulated specialty solvents^22-24. During development of a large amount of fundamental  knowledge of such parameters as gas solubilities in different solvents and a good understanding of vapour-liquid contacting devices, the dramatic self-improvement of the basic flowsheet itself, as exemplified by HighSulf, have been  overlooked²².