Apr-2008

Acid gas enrichment part II: maximising selectivity

By recycling about 70% of enriched gas back to the front end of an acid gas enrichment unit, an almost two-fold improvement in SRU feed quality can be enjoyed simply for the minimal cost of providing a recycle line and no increase in operating costs

Ralph H Weiland
Optimized Gas Treating Inc

Article Summary

Beyond using a common regenerator for the  Shell Claus off-gas treating (SCOT) and acid gas enrichment (AGE) units previously described in part I of this two-part article (see PTQ Gas 2008), further improvements to the process configuration are possible. The rich solvent from the SCOT unit is always going to be fairly lightly loaded with acid gases. Rather than sending it directly to regeneration, this lightly loaded solvent might be better utilised by sending at least a portion of it to the AGE unit, where it could be used for the bulk removal of H2S in the lower part of the contactor, and would allow a smaller solvent stream to be sent to the top of the AGE unit for final gas cleanup. This would result in a reduction in the solvent circulation rate and, therefore, a concomitant reduction in regenerator size and energy consumption, as well as smaller peripheral equipment such as pumps and heat exchangers. This scheme is referred to as the Dual-Solve process by Johnson et al (1992), its key feature being the solvent cascade from the SCOT unit, as shown in Figure 1.

The process parameters shown in Figure 2 of the previously published part I of this article and Figure 1 in this article (part II) were kept the same so the effect of cascading could be scrutinised without masking effects from other parameters having been changed as well. For this example, the solvent flow to the SCOT unit was kept the same as in the original plant, and any solvent flow reductions made possible by the cascading were taken by the AGE absorber. Since only 30% of the total solvent was used by the SCOT column, all of it was subsequently fed to the AGE unit on tray 14 from the top. Simulation using Optimized Gas Treating Inc’s proprietary ProTreat software showed that the total solvent circulation rate could be reduced by 20% and the reboiler heat duty by 10%, while maintaining exactly the same H2S leak values from both the SCOT and AGE units. The area of the cross exchanger could be reduced by nearly 40% and additional savings, of course, would be realised in other peripherals as well. But this is not the only scheme that yields benefits.

Enriched gas recycle
The performance of the AGE unit is influenced by the composition of the gas being upgraded, in the sense that the higher the H2S content of the raw gas, the greater the H2S content of the sulphur-recovery unit (SRU) feed that can be produced. In the case of Figure 1, for example, the gas to the AGE unit is already quite high in H2S and the SRU feed produced is even higher still. Therefore, no further process improvement is warranted in this case. However, for AGE feed gases of much lower quality (eg, 8% H2S), the scheme in Figure 1 is not the only option.

In the case of a low-quality raw gas, if a portion of the gas produced from the AGE unit’s regenerator were recycled back to the AGE contactor feed, the feed stream to the AGE would automatically be richer in H2S, and you might anticipate that an even richer SRU feed would result. One possible PFD for this scheme is shown in Figure 2. This may or may not require a higher solution rate or a higher regenerator energy consumption, depending on specific conditions. The acid gas recycle approach could be what is referred to as a “special design feature” by Johnson and Wissbaum (1998) or the proprietary SupeR Enrichment process by Johnson et al (1992), although the literature does not really make this connection clearly. To illustrate the possible benefits of recycle, a lower quality sour gas, 8% H2S, is used for enrichment. The data shown in Figure 2 are simulated and compared with the same gases processed via Dual-Solve alone. All flows, temperatures, pressures, heat duties and vessel internals were maintained exactly the same between the recycle and proprietary Dual Flow configurations, and the percentage of SRU gross feed returned to the AGE column was varied from 0–75% to ascertain what effect recycle had on SRU feed quality as well as on the leak rates from the two absorbers. Results are shown in Table 1 and in Figures 3 and 4.

By recycling about 70% of the enriched gas back to the front end of the AGE unit, the simulations as shown in Figure 3 predict that an almost two-fold improvement in SRU feed quality, from 40–70% H2S, can be enjoyed simply for the cost of providing a recycle line. There is no increase in either solvent rate or regenerator heat duty required in this case. But you come hard up against a limit at around 71% recycle — H2S breaks through the AGE and the H2S leak begins to increase dramatically because the solvent rate is just not sufficient to absorb any more H2S. The contactor gradually goes from lean-end to rich-end pinched as the gas recycle (ie, the H2S load) increases, until suddenly H2S breaks through. Undoubtedly, more vigorous stripping or higher solvent rates could be used to achieve even greater enrichment. However, it is clear that acid gas recycling can be a viable means of greatly enhancing SRU feed quality 
with only a minimal additional 
capital investment and no increase in operating costs.

Processing conditions and contactor design
There are several questions that could be asked concerning the effect of process conditions such as operating pressures and temperatures, and types of tower internals on the performance of AGE contactors. Is higher pressure better than lower? Should the lean amine be warm or cool? Are trays superior to packing? How much packing and how many trays are best? And are there 
really general rules of thumb? To answer some of these questions, two extremes will be considered: upgrading very low H2S content (3%) sour gases and enriching already high H2S gases (30%) further using generic MDEA in a standalone absorber.

Low H2S gas
The base case is a 12-tray contactor with one-pass valve trays and 2.5in weirs enriching 3 MMscfd of 3% H2S, 97% CO2 gas at 3 psig and 120°F. The solvent is 100 USgpm of 35 wt% MDEA at 85°F. The regenerator takes 220°F rich amine and processes it across 22 valve trays also with 2.5in weirs and using a reboiler duty of 6.0 MMBtu/hr. For this base case, these conditions correspond to a molar stripping ratio of 4, where the stripping ratio is the ratio of moles water to moles total acid gas in the overhead vapour from the still column. Simulation gives a 4.7 ppmv H2S leak and 91% CO2 slip for the base case and a regenerator off-gas containing 25.9 mol% H2S. This is not a particularly wonderful SRU feed and it probably would benefit from acid gas recycle, although to what extent and at what cost is another story — the purpose of this example is to assess sensitivity to changes in process conditions and alternative tower internals. For the discussion, only a single parameter at a time is varied from this base case. The effect of varying parameters away from the base case is shown in Table 2.