Mar-2025

Zero sulphur in refineries

The refining industry is pressured. Environmental regulations dominate investment budgets, and often take the top priority in operating expenditures.

Joseph C. Gentry
GTC Vorro Technology

Article Summary

Mandates on compliance rule over all else, and must be adhered as a license to operate regardless of the logic or cost/benefit analysis. The Gas Processing Industry faces some of the same challenges, and has found different ways to manage H₂S that may be useful in the downstream refinery industry.

Redundancy in SRU
Refiners typically have a sulphur recovery unit (SRU) as catch-all for sulphur from the various processing units. Organic sulphur is removed from the oil in hydrodesulphurisation units, where H₂S is generated; then removed in an off gas. This gas is treated in an amine contacting unit to remove the H₂S. The acid gas is then stripped and routed to the SRU, which uses the Claus technology to produce elemental sulphur. The SRU has long been a mainstay of the refinery operation; but now it is even more so required. New regulations require 100% redundancy of critical emissions control systems, including the sulphur recovery and tail gas treating (TGT) units. The goal is to completely eliminate flaring of the H₂S gas during outages of the SRU and TGT, even during startup and shutdown.¹

In times past, it was sufficient to have surplus capacity in the sulphur plant or multiple SRUs, in case one unit had to be taken offline. Now, there is need to coordinate the startup and shutdown sequences of the sulphur plant and the tail gas treating unit, which involve different types of unit operations and process dynamics. This requires a cumbersome coordination of process units just to make a normal startup or shutdown, which would otherwise require to temporarily flare a small amount of H₂S gas.

One interesting option is to use a solid scavenger system, such as SweetTreat to remove the H₂S during plant startup/shutdown, or during process upsets. This de-couples the main process units from emergency operation. The SweetTreat process is a passive, always-ready system to ensure that the acid gas is treated in every situation. The solid adsorbent converts 100% of the H₂S into an iron-sulphide, which is suitable for disposal as non-hazardous waste. SweetTreat is a fixed vessel operation that is activated simply by opening valves to the unit. When the adsorbent is spent, it is dumped and re-loaded for the next incident. This is far better than keeping a full-size redundant amine and incinerator within the TGT unit ready on hot standby, or a caustic scrubber that requires disposal of spent caustic.

Energy efficiency by burning sulphur-free gas
The SRU with TGT units are designed for certain sulphur recovery, to meet an overall site allowance of SO₂ emissions. The refinery-generated fuel gas contains some amount of H₂S that converts mainly to SO₂; but a small portion is further oxidised to SO₃, which condenses with water to form H₂SO₄. To avoid the corrosive effects of sulphuric acid on the process equipment, the furnace is operated to keep the flue gas at a temperature level above the acid-gas dew point. This limits the furnace efficiency, which would otherwise maximise the extraction of useful heat and result in a lower flue gas temperature. If the refinery fuel gas had zero H₂S, then the acid-gas dew point condition would be substantially lower, giving flexibility to optimise the furnace operation. Let’s look at an example in Table 1.

The SRU with TGT units are designed for certain sulphur recovery, to meet an overall site allowance of SO₂ emissions. The refinery-generated fuel gas contains some amount of H₂S that converts mainly to SO₂; but a small portion is further oxidised to SO₃, which condenses with water to form H₂SO₄. To avoid the corrosive effects of sulphuric acid on the process equipment, the furnace is operated to keep the flue gas at a temperature level above the acid-gas dew point. This limits the furnace efficiency, which would otherwise maximise the extraction of useful heat and result in a lower flue gas temperature. If the refinery fuel gas had zero H₂S, then the acid-gas dew point condition would be substantially lower, giving flexibility to optimise the furnace operation. Let’s look at an example in Table 1.

To guard against sulphuric acid being formed upon condensing the SO₃ with water, the stack outlet temperature would normally be kept at 150-160˚C or higher. This precludes use of low-level heat recovery into combustion air or boiler feed water.2,3 However, if the acid-gas dew point is not governing, the stack gases can be further cooled to the H₂O dew point or lower (subject to reasonable temperature approach), thereby increasing the furnace efficiency.4 See Table 2. By availing of the full air preheating potential, there may be energy savings of 10-15% in the furnace.  Higher combustion air temperature also makes it easier to control the level of excess O₂, to reduce this to the minimum level. The energy savings comes with reduction in CO₂ emissions from the refinery process heating, which can be useful to add to the refinery ESG scorecard.⁷

Even if the normal furnace operation maintains the stack temperature above the acid gas dew point, there can be times when the temperature may drop due to sudden weather change or turndown operation, bringing the system into the severe corrosion regime. Thus, it is important to avoid having H₂S in the fuel system in any case. Sulphur is not our friend.

Another example:
Many refinery gas systems operate at low pressure. In order to get a satisfactory removal of H₂S, they may use a strong, unhindered amine such as MEA. There are two issues with MEA which make it less efficient than hindered amines such as MDEA: 1) MEA holds the H₂S more strongly, and requires more energy for regeneration. The MEA also absorbs most of the CO₂, which requires regeneration cost. 2) CO₂ in the acid gas would be recycled back to the Claus unit and TGT units, which incurs operating costs that would not be present if CO₂ were excluded.

If the SweetTreat system were applied to the fuel gas, the refiner could relax the requirement for H₂S removal in the amine system and use a lower-cost system for bulk H₂S removal. SweetTreat removes the final quantity of H₂S in the fuel gas from the relaxed operation; nearly all of the CO₂ can be rejected into the fuel gas away from the Claus and TGT units; and energy required to operate these units is reduced.

Sulphur scavenging in hydrogen systems
The majority source of H₂S in the refinery is from hydrodesulphurisation units, which convert the organic sulphur into hydrogen sulphide. These units typically operate with hydrogen recycle gas to maintain the hydrogen partial pressure in the reactor. Some of the gas is purged to the fuel system, to maintain purity of the hydrogen and remove H₂S. Depending on the quantity of sulphur in the liquid feed, there may be need to operate an amine absorber on the hydrogen recycle gas stream to remove the bulk of H₂S. Even so, the amine will not capture all of the hydrogen sulphide, which returns to the reactor via the recycle hydrogen. H₂S is generally a catalyst poison and should be removed as low as possible. The SweetTreat system can remove all of the H₂S from the recycle hydrogen to improve the cycle length and operability of the hydrotreater.

One example is in the area of FCC gasoline selective hydrotreating. Here the objective is to remove the organic sulphur while avoiding to saturate the olefins. The ∆ RON between an olefin and its corresponding paraffin is about negative 50! If H₂S remains in the recycle hydrogen, it will recombine with olefins to create new mercaptan sulphur species, defeating the purpose of the hydrotreatment and requiring higher severity. Additionally, H₂S poisons the catalyst. Thus, it is important to eliminate all H₂S from the hydrogen gas.

We can apply the SweetTreat process within the hydrogen recycle loop to greatly improve the functional efficiency of the selective gasoline hydrotreatment system. This fixed bed adsorption system is installed on the recycle loop itself, or after the amine absorption unit if the sulphur content is extremely high. In either case the octane retention is improved, hydrogen consumption lowered, and operating costs outside the HDS unit are reduced.

Claus and TGT improvement

The first area to get attention for sulphur reduction was the Claus unit off gas, to add tail gas treating (TGT). One of the well-known options is the SCOT⁸ unit offered by Shell. The SCOT consists of a hydrogenation step to convert all of the sulphur species into H₂S, which is then re-absorbed into an amine for recycle to the sulphur recovery unit. In some cases, the remaining off-gas from the amine absorber may be sent to thermal oxidiser (TO) to convert all residual H₂S into SO₂, then absorbed with caustic wash to yield a final product suitable for venting. This is a complex arrangement to get the last traces of hydrogen sulphide removed from the vent.

One improvement that can be made to this arrangement is to use the SweetTreat process as replacement of the thermal oxidiser and caustic wash. This will eliminate the energy required in the TO, and the waste caustic to be handled. Alternatively, the SweetTreat process can be operated as a back-up to the amine and caustic wash / T.O. part of the TGT unit, if these needs to be shut down for a short duration.

A further enhancement would be to use a weaker amine in the TGT absorber. This permits easier and less costly regeneration of the H₂S, and could further reduce the CO₂ that is captured and recycled to the Claus plant. Di-isopropyl amine (DIPA) for example, may take 0.10-0.15 moles of CO₂ with each mole of H₂S that is recycled to the Claus reaction section. Optimising the amine choice and operating conditions may drop the CO₂ capture to 5% or less. Any elevated content of H₂S in the resulting off-gas would be removed in the SweetTreat adsorber, similar to the redundancy application of Figure 1.

Zero sulphur refiner

The logical progression of environmental activism is to have zero emission of whatever is offending. ESG initiatives provide the impetus to take action on reducing noxious waste to the atmosphere. In the case of sulphur, there is a pathway to get to zero sulphur leaving the refinery in the atmospheric emissions by removing all H₂S from the flare gas, TGT off-gas, and fuel gas streams. Traditional unit operations leave some small quantity of H₂S, which may require further treatment in an incinerator or caustic scrubbing system. SweetTreat is a solid adsorbent system that eliminates all of the H₂S in these streams.

SweetTreat can be used in an emergency or standby mode linked with the flare system; and used on a continuous basis on the refinery fuel gas and off gases. This is a way to boost public perception and community good, by going below the requirements set by a site limit on SOx emissions.

Conclusion

Sulphur is objectionable on account of equipment corrosion and acid rain, and general toxicity of H₂S and SO₂. Environmental regulations are squeezing every aspect of refinery emissions, including sulphur. Thus, refiners should avail of the best options to reduce sulphur emissions. The Gas Processing Industry extensively uses solid adsorption systems as scavengers for H₂S. Similar systems can also apply to its cousins in the downstream refining industry.

Solid adsorption scavenger systems are excellent means to provide redundancy to the TGT and eliminate flaring. They give continuous and complete removal of sulphur.

Removing all H₂S from the fuel gas allows the furnaces to operate with a lower stack temperature, extracting more heat from the fuel gas. Overall there could be an increase of 3-4% in refinery energy efficiency by recovery of the low-level heat from the fired heaters.

Scavenging sulphur in the hydroprocessing units reduces hydrogen consumption and improves operating performance such as gasoline octane value.

Claus/TGT and amine optimisation can be made by judicious use of fixed-bed scavenger system at key points in the process. This will reduce emissions going to the flare and directionally debottleneck the sulphur processing units.

A ‘Zero Sulphur Refiner’ will improve its ESG score, and be recognised as a leader that is sensitive to environmental issues.