Reducing sulfur and anime carryover in treater off gas
If an ultra-low-sulfur HDS unit sends more H2S to the amine treaters than they can handle, one typical result is that H2S content in the hydrocarbon off gas from the treaters becomes excessive. (See Figure 11.) Thus, the absorber capacity will have to be increased. In some cases, this can be accomplished without major construction in either or both of two ways:
- Replace the absorber tower internals with trays or packing that provide higher capacity and efficiency, such as the Amistco structured packing shown before in Figure 4.
- Change to a different solvent that has higher absorption capacity for H2S. For instance, monoethanolamine (MEA) can be replaced with methyldiethanolamine (MDEA). There is another adverse result of operating an amine treater above its rated capacity, even if absorption might still be sufficient. Often, large quantities of amine liquids carry over with the absorber overhead gas due to poor performance of mist eliminators in the absorber tower and knockout drums.
There may be several costly consequences:
- Fuel gas compressors are damaged by pitting corrosion or corrosive amine salt deposits.
- When the fuel gas is burned in refinery heaters, even small amounts of amine solvent can plug the burner tips with salt deposits. Consequently, heat distribution in the firebox becomes uneven, possibly resulting in damage to the heater or inadequate heater performance. Due to the corrosive nature of amine salts, burner tips need to be replaced more frequently.
- Excessive amine losses mean higher makeup rates of fresh solvent, which in turn can increase the unit operating cost by hundreds of thousand of dollars per year.
The cost of amine losses alone can be unexpectedly high. For example, consider a situation in which the cost of amine solution makeup is $7.00 to $9.00 per gallon. Losses from using standard mist eliminators operated above design capacity are commonly 2 to 3 gallons per million standard cubic feet. In a typical refinery, the amine absorbers might put out 40 MMSCF of desulfurized off gas per day over a work year of 355 days. In such a case, the annual cost of amine carryover would be about 2.5 gal/MMSCF x $8.00/gal x 40 MMSCF/D x 355 days/year = $284,000 per year.
The optimum way to improve mist eliminator efficiency with increased throughput is the same as explained before for mist eliminators in knockout drums in hydrogen plants (Figure 6). Fine mist droplets are agglomerated by high-efficiency mesh pads with horizontal flow, and the resulting larger droplets are collected by vane units—preferably Amistco’s Double-Pocket variety. This solution can reduce amine losses to the range of 0.05 to 0.10 gallon per MMSCF, thus avoiding the problems that have been described.
Curing foam in anime treaters
Even if the existing amine treaters have enough spare capacity to accommodate the throughput increase generated by ultra-low-sulfur gasoline and diesel processes, the absorbers may begin to experience severe foaming. This can be caused by inadequate drying of the H2S-laden gas in the new clean-fuels units. The liquid that may arrive with that gas typically contains appreciable proportions of heavier hydrocarbons such as pentanes or hexanes, which tend to generate foam in the amine absorbers. Choking with foam reduces an absorber’s efficiency in scavenging H2S and also hampers the performance of the overhead mist eliminator. Thus, wet feed results in excessive carryover of both H2S and amine solvent with the off gas. Usually the throughput of the clean-fuels unit is cut back in an effort to reduce foaming and liquid carryover with the sour gas.
There are two relatively inexpensive ways to remedy this problem. The most obvious is to retrofit the applicable knockout drums with high-capacity, high-efficiency mist eliminator arrays as suggested before in connection with Figure 6. The vessels involved are the inlet knockout drums in the amine treaters (Figure 11) and the sour gas knockout drum in the clean-fuels plant (Figure 2). These improvements will minimize liquid carryover to the amine absorbers and avoid expensive upsets of the treater units due to foaming in the absorber.
The other possibility pertains to the solvent recirculation system in the amine treaters. Hydrocarbon liquid, including the heavier fractions that cause foaming, accumulates in the solvent as immiscible droplets. The hydrocarbon level in the solvent is held down by a liquid-liquid skimmer, shown following the absorber in Figure 11, which is typically a simple gravity settling tank. Improving the separation efficiency of this vessel will reduce the hydrocarbon content of the recirculating solvent and alleviate foaming. The best approach is to retrofit the skimmer with a properly designed high-capacity, high-efficiency liquid-liquid coalescer assembly as illustrated before in Figure 8. This will eliminate the burden of immiscible hydrocarbon liquid in lean solvent returning to the absorber towers, thus curing the foam problem.
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