In the world of petrochemical plants, Claus sulfur recovery unit is a prime example of “turning waste into gold” – converting the originally pungent and highly corrosive acidic gas into elemental sulfur that is indispensable for chemical production and industrial applications. However, many people tend to focus on the core processes of reactors and condensers, but often overlook the seemingly “terminal” liquid sulfur system. In fact, this part not only affects the safety and stability of the plant, but also directly determines the economy of space occupation, investment, and operation and maintenance. In conventional designs, the layout of the four-stage sulfur containment tanks is like numerous “small hills”, with complex space occupation, pipelines, and high energy consumption. Is there a more ingenious solution? The answer is yes. By making a small “modification” to the flow direction of liquid sulfur, combined with the optimization of the sulfur tank structure and the heating system, the entire plant can “become leaner and more efficient”. RUICHANG will take an industry expert perspective to explain the new ideas for optimizing the design of the liquid sulfur system, and bring you truly implementable energy-saving and cost reduction, and efficiency improvement solutions.
With the development and utilization of sour crude oil and sour natural gas, the process of recovering elemental sulfur from acid gas using the Claus process has become an important form of processing sour natural gas or refinery gas. The Claus process for recovering elemental sulfur is mainly divided into two stages. One is the high-temperature thermal reaction stage, which is mainly completed in the acid gas combustion furnace. The furnace temperature is generally controlled to be not less than 980 ℃. The conversion rate of H2S in the furnace is 65% to 70%. The other is the catalytic reaction stage. The molar ratio of H2S and SO2 in the process gas at the outlet of the acid gas combustion furnace is controlled to be 2:1. Under the participation of the Claus catalyst in the reactor, H2S and SO2 complete the redox reaction to generate elemental sulfur. The yield of elemental sulfur in this stage is about 25% to 30%. It is generally believed that under appropriate operating conditions, the total sulfur conversion rate of the Claus sulfur production part is about 95%. To improve the conversion rate of elemental sulfur during the catalytic reaction, a sulfur condenser is used to cool and remove elemental sulfur vapor from the process gas, reducing the partial pressure of the product and promoting the positive reaction of H2S and SO2. After the sulfur vapor is condensed into liquid sulfur, it is collected by the liquid sulfur recovery system and returned to the sulfur pool for further degassing and forming. Figure 1 shows the principle flow of the Claus process.
The liquid sulfur recovery system consists of a sulfur seal tank, liquid sulfur pipelines, and underground sulfur storage tanks. If the settings of the sulfur seal tank or the sulfur storage tank are improper, problems such as uncoordinated layout of the liquid sulfur pipelines, large floor area occupation, liquid sulfur solidification and blockage, and even accidents like liquid sulfur backflow in the sulfur storage tank and sulfur tank fire may occur, which will affect the long-term stable operation of the equipment.
Optimization Design of Sulfur Seal Tanks
Principle and Function of Sulfur Seal Tanks
In the Claus sulfur production process, the liquid sulfur generated undergoes gas-liquid separation in the acid gas combustion furnace waste heat boiler or sulfur condenser. The liquid sulfur then flows freely into the sulfur seal tank. The sulfur seal tank is set at a certain height, and a liquid seal is formed by the static pressure generated by the self-weight of the liquid sulfur, preventing the process gas from escaping through the sulfur pool and entering the atmosphere, thereby achieving the separation of process gas and liquid sulfur as well as the isolation of the reaction system and the liquid sulfur storage system.
The entire sulfur seal tank is equipped with a 0.4 MPa steam jacket for heat tracing to prevent the solidification of liquid sulfur. The steam condensate is discharged from the bottom of the sulfur seal tank. After the liquid sulfur enters the sulfur seal tank, it flows by gravity through the inner pipe to the bottom of the sulfur seal tank, and then overflows through the outer pipe into the sulfur pool. The height difference H between the liquid level in the inner pipe and the overflow port is the actual effective height of the sulfur seal. The static pressure P of the liquid column formed by H is the maximum system pressure allowed by the device. When the system pressure is greater than P, the sulfur seal is breached, and the process gas enters the liquid sulfur pool, causing an accident. The static pressure value P of the sulfur seal liquid column in some devices is usually set at 1.25 to 1.30 times the maximum pressure at the outlet of the fan [5]. Even if the Claus sulfur production device system pressure reaches the designed maximum value due to a fan failure, there will be no phenomenon of process gas breaking through the effective liquid seal of the sulfur seal tank.
Optimization of Sulfur Sealing Tanks
Conventional Layout of Sulfur Sealing Tanks
In the conventional secondary Claus sulfur production process, a sulfur sealing tank is usually set after the acid gas combustion furnace waste heat boiler, the first, second, and third-stage sulfur condensers. The liquid sulfur collector and the third-stage sulfur condenser share one sulfur shoe tank. Some plants set up 4 cement wells after the condenser, and the fourth-stage sulfur sealing tanks are each installed in the wells. The cement well mouths are fixed to the sulfur sealing tanks with flanges; some plants specially set a sulfur sealing pit, concentrating the 4 sulfur sealing tanks in the sulfur sealing pit, and covering the pit with steel plates; some plants place the sulfur condenser on the second floor platform, and the 3 sulfur sealing tanks behind the first, second, and third condensers are suspended and installed. In the three layout schemes, when the sulfur sealing pit is used for centralized layout, a sealed space is formed in the pit, which easily leads to H2S deposition at the bottom of the pit, posing significant safety hazards. Moreover, it also has large floor area, and the liquid sulfur pipelines and corresponding steam tracing pipelines are difficult to be laid. Therefore, this scheme is not recommended. The other two layout schemes both have problems such as difficult pipeline laying for the liquid sulfur pipelines and steam tracing pipelines and inconvenient maintenance of the sulfur sealing tanks.
Optimization Design of Sulfur Sealing Tanks
As mentioned earlier, the maximum effective sulfur sealing height H of the liquid sulfur tank results in a liquid column static pressure P = ρgH, which is the maximum allowable working pressure for the reaction system. The liquid flow path of the sulfur is: condenser → sulfur sealing tank → sulfur pool. The maximum working pressure is at the condenser end, and the constant pressure is at the sulfur pool end. The liquid seal heights of the four sulfur sealing tanks are the same.
In the layout of the sulfur seal tanks, the first three levels of sulfur seal tanks can be omitted. The liquid flow direction is as follows: acidic gas combustion furnace waste heat boiler outlet → first condenser outlet → second condenser outlet → third condenser outlet → total sulfur seal tank → sulfur pool. Due to the pressure drop between the sulfur condenser and the reactor, on the liquid sulfur pipelines coming out from the acidic gas combustion furnace waste heat boiler, the first, second, and third condensers, a U-shaped bend is set. The static pressure value generated by the U-shaped bend with height H1 only needs to overcome the maximum pressure drop between adjacent equipment to meet the requirement that the process gas does not intermix between the various sulfur condensers (as shown by the red line in Figure 4). The liquid sulfur produced by the first three sulfur condensers converges together and enters the total sulfur seal tank, and the liquid sulfur flows from the total sulfur seal tank to the sulfur pool.
The effective liquid seal height H of the total sulfur seal tank remains unchanged, avoiding the situation where process gas breaks through the seal when the system pressure rises, and achieving the effect of gas-liquid isolation. For example, between the outlet of the first condenser and the outlet of the second condenser, there is a first reactor heater, the first reactor, and the second sulfur condenser. Assuming the pressure drop of each device is 5 kPa, the total pressure drop is 15 kPa. The density of the liquid sulfur is taken as 1780 kg/m3. By P = ρgH1, the theoretical height H1 of the U-shaped bend can be calculated as 0.86 m. Considering that abnormal operating conditions may cause the pressure drop of the equipment to increase, the total pressure drop is increased by 1.5 times, then the height of the U-shaped bend can be taken as 1.3 m. To avoid liquid sulfur pipeline blockage at the bottom of the U-shaped bend due to the accumulation of solid impurities, a blowdown valve can be added.
Optimization Design of the Sulfur Storage Tank
The liquid sulfur tank is made of reinforced concrete, and its inner lining is made of special acid-resistant bricks. The interior of the sulfur tank is separated by walls and divided into two parts: the degassing tank and the finished product tank. The liquid sulfur from the sulfur condenser contains H2S (with a mass fraction of 300-400 μg/g). Appropriate degassing measures are adopted to remove H2S from the liquid sulfur in the degassing tank. After degassing, the liquid sulfur is pumped into the finished product tank to proceed to the next step of shaping or liquid loading and transportation for export.
Problems and Improvements in the Method of Liquid Sulfur Injection into the Sulfur Tank
Problems: In the piping design for the total liquid sulfur pipeline entering the sulfur tank from the sulfur seal tank, some devices placed the total liquid sulfur pipeline in the trench and entered from the side of the sulfur tank, as shown in line a in Figure 5. Since the liquid sulfur pipeline is heated by a steam jacket, there is a temperature change in the pipeline, and thermal expansion and contraction lead to a loose seal at the contact surface between the pipeline and the sulfur tank wall when entering the sulfur tank, resulting in an gap. Once the liquid sulfur level in the sulfur tank exceeds the inlet position, liquid sulfur will reverse and enter the trench, posing a safety hazard. For example, in a 40 kt/a sulfur recovery device of a certain refining enterprise, the four-stage sulfur seal tanks were centrally arranged in the sulfur seal pit. The total liquid sulfur pipeline passed through the trench from the side of the sulfur tank and entered the tank, and there was a situation where liquid sulfur reversed and entered the sulfur tank pit from the trench. About 10 t of waste sulfur was removed from the sulfur seal pit once, and later, only fine sand was used to fill the trench, but this could not fundamentally prevent the liquid sulfur from reversing and entering the sulfur tank.
Optimization measures: Appropriately raise the height of the liquid sulfur outlet of the sulfur seal tank. Under the premise of meeting the self-flow inclination requirements of the liquid sulfur pipeline, the liquid sulfur pipeline enters from the top of the sulfur tank, as shown by the red line in Figure 5, avoiding the situation where the total liquid sulfur pipeline runs through the trench from the side of the sulfur tank to enter the tank.
Selection of Material for Liquid Sulfur Heating Pipes
Existing Issues: Steam heating pipes are installed inside the liquid sulfur tank to heat and maintain the temperature of the liquid sulfur within the range of 138 to 148 ℃. The material of these steam heating pipes is mostly 20# steel, but this material is prone to react with sulfur to form FeS. Currently, most facilities use non-purified air bubbling desulfurization for liquid sulfur degassing, maintaining a certain concentration of oxygen in the gas phase space of the sulfur tank. Even if other liquid sulfur degassing methods are adopted, due to the poor sealing of the liquid sulfur tank, the sulfur-containing exhaust gas generated from liquid sulfur degassing is drawn to the tail gas incinerator or acid gas furnace for treatment, causing the liquid sulfur tank to be in a slightly negative pressure state. This still easily leads to the entry of air into the sulfur tank, resulting in spontaneous combustion of FeS and causing a sulfur tank fire accident.
Optimization measures: All the pipe fittings exposed to the gas phase space of the sulfur tank, such as the steam inlet and outlet pipelines, the liquid sulfur pump body, the level gauge, the thermocouple sleeve, and the supporting angle steel, etc., should be made of more resistant-to-sulfur-corrosion stainless steel materials, such as 316L, to minimize the generation of FeS; To save investment costs, the steam pipes laid flat on the bottom of the sulfur tank, which are submerged below the liquid sulfur for a long time and isolated from the oxygen in the gas phase, rarely experience spontaneous combustion of FeS. For this part of the steam pipes, steel grade 20 can be used; However, during device maintenance and when the liquid sulfur in the sulfur tank needs to be emptied, it is necessary to strengthen the monitoring of FeS spontaneous combustion of the steam pipes; During normal production, attention should be paid to avoiding excessively low liquid levels in the sulfur tank to prevent the steam pipes at the bottom from being exposed to the oxygen-containing gas phase space.
Conclusion
- In the liquid sulfur recovery system of the Claus sulfurization process, there are four parallel sulfur sealing tanks. After optimization design, the parallel liquid sulfur lines are changed to series connections, and a U-shaped bend is added to the liquid sulfur line to overcome the pressure drop between the equipment and avoid the cross-contamination of process gas. By replacing the original sulfur sealing tanks with U-shaped bends, the number of sulfur sealing tanks can be reduced by 3, and various costs such as equipment costs and construction costs can be saved by approximately 400,000 yuan. Due to the reduction of static equipment, the liquid sulfur pipeline can be more easily configured, reducing the floor area and saving the heat supply steam consumption of the three sulfur sealing tanks.
- When the liquid sulfur pipeline enters the sulfur tank, it should preferably enter from the top to avoid the risk of liquid sulfur leakage from the sealing point’s annular gap when the liquid sulfur pipeline enters the sulfur tank from the side, especially when the liquid level of the sulfur tank is high.
- When the components in the sulfur tank are made of 20# steel, there is a possibility of generating FeS. When reacting with oxygen, a sulfur tank fire accident may occur. Pipes exposed in the gas phase space should be made of stainless steel materials such as 316L to reduce the generation of FeS; the flat tubes laid on the bottom of the sulfur tank are isolated from the gas phase space and do not come into contact with oxygen in the gas phase, so the probability of FeS self-ignition is lower. 20# steel can be used, but during the shutdown and maintenance of the sulfur tank for vacuuming, monitoring of FeS self-ignition should be strengthened.
By optimizing the liquid sulfur system of the Claus sulfur recovery unit, we not only addressed the problems such as complex pipelines, excessive floor space, and high energy consumption in the traditional design, but also achieved the reduction of equipment investment and the improvement of operational efficiency. The optimized solution has achieved a balance in terms of safety, economy, and environmental performance, creating a more efficient and green sulfur recovery unit for the enterprise. If you are planning a new project or wish to upgrade an existing unit, you might consider this optimization approach to make your unit stand out in the homogeneous competition. As a professional manufacturer specializing in environmental protection equipment, we will provide you with tailor-made solutions to help enterprises achieve real energy conservation, emission reduction, and value enhancement.
