The sulfur recovery unit (SRU) is an extremely important component in many industrial settings, including oil refining and gas processing plants.
Hydrogen sulfide (H2S) is a byproduct of high-sulfur crude oil and natural gas processing, and is a very toxic gas at high concentrations. Because H2S is defined as both an environmental and industrial pollutant, plants are required to recover between 95 and 99.9 percent of total sulfur that arrives at the SRU. The SRU converts H2S to nontoxic elemental sulfur. Without the SRU, the toxicological and environmental consequences of H2S would be harsh and severe, including acid rain and ground-level sulfur dioxide contamination. Because the stakes are so high for refineries and other plants that must manage H2S, the SRU must be well maintained, reliable and able to meet the required recovery efficiency for the long term.
Unfortunately, SRUs are prone to fouling, as their nature depends on the fluids flowing within and over the tubes, and the reduction in heat transfer that results almost invariably has an impact on product cost. To reduce this impact, SRU performance should be intelligently monitored and the SRU cleaned at intervals determined by optimal economic criteria.
SRU fouling and its effects
Just as heat exchangers and condensers are prone to fouling, so are SRUs. In most cases, it is unlikely that fouling is exclusively due to a single mechanism, but in many situations, one mechanism will be dominant. Fouling tends to increase over time, with the trajectory being very site specific. Fouling has an economic impact, and determining when to clean often requires striking a balance between maximizing the quantity of finished product from the process and its cost. The sulfur buildup that accumulates in the tubes of SRUs can be hard and chalky, and it is not uncommon for tubes to be wholly obstructed with fouling.
Methods for cleaning the SRU
- TruFit® tube cleaners
- Liquid nitrogen
- HydroDrill™
Improving the performance of fouled SRUs requires cleaning the tubes periodically. Each time the tube deposits are removed, the tube surfaces are returned almost to bare metal, providing the tube itself with a new lifecycle.
TruFit mechanical tube cleaning
The most frequently chosen and fastest method to address light to moderate SRU fouling is the TruFit mechanical tube cleaner. The TruFit system is an offline method in which the mechanical tube cleaner is propelled through the tube with low-pressure water, flushing the debris out of the tube, leaving the tube free and clear of fouling. For offline mechanical cleaning, it is essential to select the appropriate cleaner. The TruFit cleaner manufactured by Conco has proven to be extremely effective.
HydroDrill
SRUs with more moderate to heavy fouling scenarios have been successfully cleaned with the Conco HydroDrill system. The HydroDrill uses a brush or drill bit mounted to the tip of a rotating extension or kelly rod, and as the rod rotates, the unit pumps water through the rod to the weep holes, flushing away hardened deposits as they are loosened.
HydroDrill cleaning is safe for all tube materials. Drill bits are sized to be 0.005 inches below the minimum tube ID, and they feature long shanks to ensure the axis of the bit and the axis of the tube are in complete alignment.
The bits are designed with carbide tips on the leading edge only and rounded corners to ensure sharp edges do not directly impact the tube wall. The small on-site footprint of the cleaning system (50 to 100 square feet) makes it an attractive and workable proposition.
Nitrolance™ liquid nitrogen cleaning
When SRU fouling is heavy to severe and tubes are filled and obstructed with deposition, Nitrolance™ liquid nitrogen cleaning can be used to restore flow. Using the power of liquid nitrogen, the super-cooled cryogenic jet emerges from the nozzle, filling cracks and crevices, expanding within the deposit, and causing the rapid breakup of all debris within the tube.
Liquid nitrogen cleaning is safe and ideally suited for the most challenging fouling scenarios, yielding quicker turnarounds for critical path components. Because liquid nitrogen readily dissipates, only the removed deposit is left behind, saving the plant thousands of dollars in cleanup costs.
Maintenance for life
There has never been a better time to consider maintenance options for your site's SRU. These units are prone to fouling, but with routine assessments and periodic cleaning interventions, the SRU can maintain its reliability and efficiency for the long term.
For more information, visit www.conco.net or contact Tim Meyer at tmeyer@conco.net.
