Boost performance, reliability with ceramic technologies at sulphur recovery plants

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The use of innovative ceramics can improve the reliability, performance and energy consumption of Sulphur Recovery Units in industrial processes like refining.  Read this article to discover products that offer maximum mechanically stability and efficiency, resulting in greater uptimes, throughputs and environmental performance for your operation.

Sulphur recovery

The Sulphur Recovery Unit (SRU) in a refinery or gas plant converts acid gas to elemental sulphur. The Reaction Furnace (RF) in the SRU poses the greatest design and operational challenges due to the complex role it serves both as the dominant equipment in the conversion of H2S to sulphur (about two-thirds of the sulphur conversion takes place in the thermal stage) while also playing a critical role in the processing and destruction of contaminants including hydrocarbons, ammonia and BTEX (Benzene, Toluene, Xylene and Ethylbenzene) that are present in the process gases fed to the SRU. 

An SRU RF is designed to provide adequate residence Time, Temperatures and mixing (Turbulence), the basic “3 Ts” of combustion, for the myriad of key reactions that take place in the unit. Its design and specific flow geometry are critical to achieving the necessary mixing and temperatures for the destruction of the aforementioned contaminants that would otherwise create plugging downstream of the RF. Besides the burner design, the temperature in the front end of the furnace can be further enhanced by the preheating of the process gases and the combustion air, the co-firing of fuel gas, and the redirecting of some of the acid gas flow towards the back of the RF to achieve a more stoichiometric combustion ratio. In terms of turbulence/mixing, even the best burners can’t maintain complete mixing across the entire length of the RF and internal structures such as checker walls or choke rings are often deployed to promote mixing and provide other benefits. 

Checker wall systems: The conventional roles of a checker wall in the RF are to help increase front zone temperatures, protect the tube sheet from radiant heat and improve mixing to some degree. Conventional checker walls in SRUs either have a cylindrical shape or a matrix box-shaped brick system jointed with mortar that can often fail due to thermal shock cycling, mortar failure and vibration. Figure 1 shows damage caused to both checker wall designs. The high failure rate of checker walls leads to high repair costs from frequent and prolonged shutdowns and lost production.

Damage to conventional checker walls due to loss of structural integrity can also lead to inadequate temperatures for ammonia destruction, causing deposits of ammonium salts in the downstream equipment. This is another factor in requiring shut down of the SRU at significant economic cost. 

Choke rings: A choke ring is used to reflect back some of the flow and create a degree of back mixing in the front zone of the furnace. However, a large portion of the flow jets through the center of the ring and jeopardizes the achievement of the minimum residence time requirements in the furnace.  

Development of the HexWall™ and VectorWall™

Blasch’s original objectives for a checker wall were to provide a structure that was more mechanically stable based on several customer complaints that their existing structures failed often and didn’t survive an entire campaign intact. The result was the development of the Blasch HexWall checker wall. The HexWall blocks range in depth from 9 to 18 inches, are designed to be stacked dry, and are mechanically engaged through a series of tabs and slots so that the wall is quite stable even when several meters in diameter.

No mortar is used between the blocks, allowing the interlocking assembly to accommodate the thermally driven expansion and contraction that comes with RF operation. The walls may either be erected into a slot in the existing lining or installed against the hot face brick with a course of brick on either side to lock it in.

As the stability and durability of the HexWalls were proven out, attention turned to additional benefits that could be provided by refinement of the structure.  With higher temperature benefits in the front zone proven, the focus shifted to improving mixing in the RF (turbulence) and thereby minimizing any channeling /stratification of the gases and thus ensuring sufficient residence time in the furnace. 

A vectoring hood was developed that would fit into the outlet ends of the existing blocks in the HexWall design, and that could be oriented in such a way as to redirect, or ‘vector’, the down-stream flow allowing the assembly to contribute to the creation of the overall desired flow. The objective was to make the best use of the furnace volume and create a flow pattern that yielded a long path length with a very tight residence time distribution – similar to the flow pattern of a plug flow reactor. This variation of the HexWall checker wall was christened the VectorWall.

It had been observed at VectorWall retrofits that the choke ring configuration seemed to allow a portion of the process flow out of the furnace well before any published minimum residence time numbers are met. VectorWalls solved this problem and subsequent CFD modeling discussed later in this article has confirmed this hypothesis.

Another advantage of the VectorWall, which is under constant refinement based on customer feedback, is that the size and angle of the vector hoods can be customized to provide the degree of mixing, shielding of the tube sheet, and limiting the heat loss from the front zone of the RF. There are now well over a hundred HexWalls and VectorWalls installed worldwide in both refinery and gas plant SRUs ranging in size from under 100 TPD to over 2,000 TPD.

Based on a deepened understanding of the various benefits of VectorWalls and client feedback, it is felt that gas plant SRUs which are often very large with the attendant concerns of stability of conventional checker walls and which also face the challenge of achieving adequate temperatures for BTEX destruction, can stand to benefit the most from this advancement in SRU RF internals.

Figure 3 compares the furnace temperatures before and after a revamp at a refinery that was experiencing serious problems with their ammonia destruction and decided to install a Blasch VectorWall. It can be seen that after the revamp, the Zone I temperature increased by about 100°C, thus solving the problem of ammonia destruction. The presence of the Vector hoods in the blocks also helped with reflecting more radiant heat back into zone 1. The refinery was also able to increase the sour water stripper gas (SWSG) feed by 30% as seen in Figure 4 and in addition, no more furnace vibrations were observed as was the case prior to the VectorWall revamp.

For lean gas feeds such as in gas plant SRUs, an important outcome of the front zone temperature increase with a VectorWall is that fuel gas co-firing used to provide adequate temperatures of about 1,050 to 1,100 C for adequate BTEX destruction can be reduced, thereby delivering valuable energy savings and incidental capacity increase.  Conservative calculations for a 500 LTPD SRU typical feed gas stream with a front zone temperature increase modeled on the results in Figure 3 and fuel gas valued at $3 /million BTU indicate energy cost savings in excess of $250,000 per year. This more than pays for the cost of a VectorWall. A capacity increase, fuel savings in the incinerator and overall CO2 emission reduction are additional benefits.

The VectorWall has also been deployed in Sulphuric acid plants to effect a more efficient and compact furnace design, and also address issues relative to NOx which is always of concern in Sulphuric Acid Regeneration (SAR) plants.

CFD Modelling of Choke ring and Vector Wall configurations

A detailed Computational Fluid Dynamics (CFD) analysis was performed recently by Porter McGuffie, Inc. to help Blasch characterize and compare the performance of an SRU RF at a refinery in Asia where the choke ring in the furnace had been replaced with a VectorWall. Prior to the revamp, when the reactor was equipped with a choke ring, the sulfur recovery unit (SRU) was limited in capacity by vibrations and insufficient ammonia (NH3) destruction.

After replacing the choke ring with a VectorWall, the SRU could process acid gas (AG) at a higher capacity, achieving a much better NH3 destruction and eliminating vibrations. The CFD analysis helped provide a theoretical understanding of this and other observations from the field by modeling the performance of the two reactor configurations with respect to velocity distribution, flame shapes, residence times and other parameters of interest.

Key insights gained from the CFD analysis included:

Improved flame characteristics and reduced vibrations: With the choke ring, pressure waves reflecting off the choke ring result in a flame shape within the burner that can likely cause the unstable vibrations during operation. As can be seen in Figure 5 the flame spreads to the ID of the burner can’s refractory and is not closed.  Instead, it is open, as shown by the centrally located streamlines released from the acid gas inlet passing relatively undisturbed through the core of the flame. This is unusual and indicative of poor combustion quality, possible refractory damage, and high in-service noise/vibrations. One of the important findings from the analysis was that the VectorWall caused a significant reduction in the interactions between the flow local to the burner and the separator.

Improved ammonia destruction: With the choke ring, a significant centerline jet formed through the separator as shown by the “bullet” shaped protrusion in the Choke Ring configuration versus the VectorWall configuration (Figure 6), resulting in a spread in the residence time distribution and some of the flow passing through the RF without adequate residence times for completion of the desired reactions.  One of the strongest manifestations of this is that  22.5 percent of the flow did not meet NH3 destruction residence time requirements with the choke ring configuration per the CFD analysis. (Figure 7). With the VectorWall, all of the Zone 1 gas flow meets retention time requirements for NH3 destruction. 

The low residence times in the choke ring configuration are consistent with field performance, as the choke ring configuration was found to have ammonium salts in downstream equipment, providing evidence of insufficient ammonia destruction.

Improved overall mixing and furnace operation: With the VectorWall, backpressure was reduced and a proper flame jet was formed from the burner throat with the flame extending out of the burner can (Figure 6), resulting in a higher intensity flame and better mixing. The low-temperature region resulting from the Zone 2 acid gas inlet just downstream of the VectorWall is smaller than that in the choke ring configuration due to improved mixing in Zone 2 with the VectorWall configuration.

Reduced pressure drop: Pressure drop through Blasch’s configuration is approximately 900 pascals lower than in the choke ring configuration, likely due to elimination of the flame’s spread in the burner can, which must be turned back to pass through the choke ring.

Ferrules

Blasch is also an internationally reputed major provider of ceramic ferrules for SRU waste heat boiler tube protection. Design features, manufacturing methods, and materials of construction all contribute to the proven reliability of Blasch’s suite of ferrule offerings over the last several decades. 

Blasch’s boiler tube ferrules are pre-engineered for each boiler tube installation to help protect the tube sheet from failure and costly downtime. They are properly sized, accurately molded and completely wrapped with all required fiber insulation. Castable refractory is not necessary between the ceramic ferrule, resulting in large savings in installation and curing time. Furthermore, by separating the ferrule head and stem, the company’s two-piece ferrules remove stress from the concentration point where the two connect in a one-piece design, enabling enhanced thermal expansion tolerance.

The heat flux just downstream of the ferrule outlet in a waste heat boiler (WHB) is many times that in the zone of well-developed flow because of flow channel discontinuity. This often leads to WHB tube failures and costly downtimes. Blasch has helped develop an improved ferrule design in collaboration with Citgo that eliminates this flow discontinuity (Figure 8). This design has provided reliable operation for over 10 years at a U.S. Gulf Coast refinery that was previously experiencing run lengths at times of under one year.

Conclusion

The reliability, performance and energy consumption of SRUs have been improved through the deployment of innovative ceramic products and manufacturing technologies. These products and overall systems continue to be refined through collaboration with process licensors, technology partners with product adjacencies, and operating companies to further enhance the performance of mature processes to realize higher uptimes, throughputs and environmental performance.

For the past 40 years, Blasch Precision Ceramics has developed and deployed highly engineered ceramic systems built on innovative design technologies and materials of construction to help deliver increased reliability and improved performance to a wide array of industries.

For more information, contact Market Manager Tim Connors at 518-436-1263 x21 or tconnors@blaschceramics.com. Visit the Blasch website for information on our products.

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