Process Technical Specifications for Sulfur Recovery Units

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I have here the technical specifications for sulfur recovery units that I’d like to share with everyone. Since I don’t have the permission to upload them, I’ve copied them down; I hope someone will be able to use them. Section 1: Process Principle This sulfur recovery unit employs the traditional Clous method for sulfur production, which involves introducing the acidic gases from the desulfurization unit directly into the reaction furnace, along with air. The ratio of acid gas to air is strictly controlled to ensure that all the hydrocarbons in the acid gas are oxidized; part of the H2S burns to produce SO2 and water. The reaction is as follows: CnHm + (n+m/4)O2 → nCO2 + m/2H2O ① (where n is the number of carbon atoms in the hydrocarbon) ； (m is the number of hydrogen atoms in the hydrocarbon) H2S + 3/2 O2 → SO2 + H2O + heat. ② The remaining H2S in the acidic gas reacts with the generated SO2 according to the following reaction: 2 H2S + SO2 → 3/Sx + 2H2O + heat. ③ The reactions in equations ①, ②, and ③ above mainly take place in the reaction furnace; the heat is released by the reaction gases in the waste heat boiler. Elemental sulfur formed during this cooling process is condensed and separated. In the gas exiting the waste heat boiler, there is still a considerable amount of H2S and SO2 present, which is why two stages of catalytic conversion reactors are used. By using an acid gas reheater and a gas/gas heat exchanger, the temperature of the process gas entering the reactor is controlled; elemental sulfur is produced through catalytic reaction at the optimal temperature, ultimately achieving a sulfur recovery rate of 95%. Section 2: Process Flow and Overview. As can be seen from the flow diagram, this unit employs a typical direct-current acid gas reheating sulfur recovery process. At the same time, this device generates a large amount of heat energy; the heat carrier, MEG solution, absorbs this heat energy through the waste heat boiler E–2250, as well as the sulfur condensation coolers E–2450 and E–2550. This heat energy is then transferred to other units for use in purposes such as insulation, heat tracing, and heating, and it circulates within its closed system, over and over again. The acidic gases from the desulfurization unit are introduced directly into the reaction furnace (E—2650) after further separation, and an appropriate amount of air is added in proportion. This allows all of the CH4 and other hydrocarbons present in the acidic gases to be burned; 1/3 of the H2S is burned to produce SO2. The heat released during these reactions is transferred to an MEG (ethylene glycol) aqueous solution through a waste heat boiler, as the reaction gases pass through the double-pass furnace tubes, resulting in a temperature drop to around 190°C. The elemental sulfur produced in the reaction is condensed into a liquid phase, separated from the process gas, and sent to the liquid sulfur tank. The cooled process gas still contains approximately 30–40% unreacted H2S and SO2 (as a percentage of the feed gas volume); therefore, it is introduced into the acid gas reheater. There, it mixes in the mixing chamber with the high-temperature SO2 gas generated by the combustion of acid gas in the reheater, thereby reaching the optimal reaction temperature of 270°C. It then enters converter A, where catalytic reactions take place to produce elemental sulfur. The process gas containing large amounts of sulfur vapor enters the heat exchanger (E—2150), where it exchanges heat with the process gas coming from the primary condenser. The gas, now at a lower temperature, is further fed into the primary condenser (E—2450), where its temperature is reduced below the sulfur dew point; as a result, the sulfur vapor condenses into a liquid phase and is separated out. The remaining process gas then goes to the heat exchanger E—2150, where it exchanges heat with the high-temperature gas coming from converter A, thereby being heated up. It subsequently enters the second converter, where, through the catalytic reaction in converter B, any remaining H2S and SO2 are converted back into elemental sulfur. This sulfur-rich gas then passes through the secondary condenser (E—2550), where it exchanges heat with ethylene glycol; again, the sulfur vapor condenses and is separated out. The remaining process gas is sent to an exhaust gas incineration furnace, where the small amount of H2S still present in the exhaust gas is burned into SO2, which is then released into the atmosphere along with the exhaust gas. The liquid sulfur separated from the waste heat boilers and various stages of condensation coolers is collected and sent to the shaping unit for flake packaging before being shipped out. As can be seen from the process flow, this technique involves two conversion stages: the first stage is high-temperature conversion, and the second stage is catalytic conversion. Section 3: Design Basis ① Acid gas treatment capacity: 18.18 Kmol/h, approximately 432 m3/h. ②Acid gas components: H2S > 82% (V), CH4 ≤ 0.3% (V). ③Sulfur recovery rate: >95%. ④Sulfur purity: >99.5%. ⑤Sulfur content in exhaust gas: 24.4 Kg/h. Section 4: Auxiliary Facilities – Refer to the sulfur shaping unit. This unit is equipped with an exhaust gas combustion furnace designed to convert H2S present in the recovered exhaust gases into SO2 through combustion, which is then released through a chimney. The attached ethylene glycol circulation system flows through the waste heat boiler E—2250 ； Sulfur condensers E—2450 and E—2550 remove the heat of reaction during the sulfur production process. Section 5 Key Points of Process Operation and Control Instructions I. Stable Operation of the Unit The acid gas that enters the sulfur recovery unit comes from the regeneration tower of the desulfurization unit; therefore, the stable operation of the regeneration tower is a fundamental condition for the proper functioning of the sulfur recovery unit. The stability of the regeneration tower depends to a large extent on the stable operation of the entire desulfurization unit. Hence, to ensure the stable operation of the sulfur recovery unit, it is first necessary to maintain stable operation in the desulfurization unit, especially by avoiding significant changes and fluctuations in the flow rate of the absorption tower, flash tower, lean liquid circulation, and the flow rate of ethylene glycol entering the reboiler. II. Relationship between the pressure in the regeneration tower system and the amount of acid gas recovered: The pressure in the regeneration tower is maintained at a certain level by controlling the amount of acid gas fed into the sulfur recovery process, and this is achieved through a pressure control loop PIC–600A/B located on the acid gas pipeline at the outlet of the acid gas separator. The function of the PIC—600A control circuit is to introduce the acid gas into a flare for combustion, when the acid gas pressure in the regeneration tower system rises above the safe set value and the sulfur recovery unit does not permit any additional acid gas to be introduced. This prevents overpressure in the regeneration tower system, which could lead to serious accidents. The function of the PIC—600B control loop is primarily to maintain the acid gas pressure in the regeneration tower at a set value, thereby supplying a steady amount of acid gas to the sulfur recovery unit and achieving stable operation. III. Operation and control of the reaction furnace The reaction furnace is part of the high-temperature conversion unit in sulfur recovery systems, and its overall sulfur conversion capacity can reach around 60%–70%. Therefore, the quality of operation of the reaction furnace directly determines the level of sulfur recovery. When the acid gas entering the reactor stabilizes, as can be seen from the main reaction equations ①–③, the amount of air supplied determines whether the substances participating in the reaction can achieve a relatively high equilibrium conversion rate. As can be seen from the reaction equation, maintaining a ratio of H2S to SO2 in the process gas or exhaust gas at 2:1 is key to ensuring that the reaction reaches theoretical equilibrium and that the device operates at a high conversion rate. To maintain this 2:1 ratio, the ratio between the acidic gas and air entering the reactor is particularly important. Therefore, when selecting the appropriate ratio of sour gas to air, it is necessary to take into account both the concentrations of H2S and CH4 in the sour gas, as well as the levels of H2S and SO2 in the process gas and exhaust gas determined through random analyses, in order to determine the suitable air supply ratio (sour gas to air ratio). The maintenance of the air-fuel ratio in the reaction furnace is primarily achieved through the proportional control loop FIC-2650; the amount of air changes proportionally to the amount of acid gas, thereby ensuring stability in the air-fuel ratio. This unit uses an air-to-acid ratio of around 2.3 (on a volume basis) based on the composition of the acid gas; this value is not fixed, as it can be determined or adjusted by the operator depending on changes in the acid gas composition. IV. Operation and Control of the Waste Heat Boiler The heat generated by the reaction furnace is introduced into the tubes of the waste heat boiler as the reaction gases flow through them. This waste heat boiler features a two-tube design; sulfur mist capture screens are installed at the ends of these tubes. The heat carrier, an ethylene glycol solution, is pressurized by a pump in the ethylene glycol system and then fed into the shell side of the waste heat boiler, flowing along the baffle plates throughout the entire shell side before exiting from the outlet on the other end. During this process, the high-temperature gas flowing inside the furnace tubes transfers heat to the ethylene glycol solution through the tube walls, raising the temperature of the solution. As a result, the temperature of the gas stream after the reaction is reduced below the sulfur dew point; the elemental sulfur produced is condensed from a gaseous state to a liquid state and is then discharged into the sulfur pipeline at the end of the second tube side of the waste heat boiler. Due to the high temperatures generated by the reaction, the reactor operates in a harsh environment characterized by high temperatures, severe sulfur corrosion, and rapid temperature fluctuations. Therefore, while maintaining the liquid level in the waste heat boiler, ensuring thorough heat exchange between the high-temperature gas stream and the heat carrier is a necessary condition for the long-term operation of the waste heat boiler. It is thus essential to strictly control the maximum feed rate of the sulfur recovery unit in order to maintain a full liquid level, preventing the formation of vapor spaces in the shell side of the waste heat boiler, which could otherwise lead to overheating and damage of the boiler tubes. V. Operation and Control of the Reheater The main function of the reheater is to raise the temperature of the reaction gas stream entering the catalytic converter to an optimal level for catalytic conversion, thereby achieving the highest conversion rate. Since this reheater uses acid gas for reheating, its operation involves fully burning the acidic gases introduced into the furnace to produce high-temperature gases, which are then mixed with the process gas in order to increase its temperature. The control of the reheat furnace is also achieved by adjusting the amount of acid gas fed into the furnace based on the volume of material to be processed, in order to maintain the temperature of the gas stream entering the converter. During the operation of the reheat furnace, it is necessary to prevent an excessive air ratio, in order to avoid an excess amount of oxygen in the process gas, which could lead to the combustion of sulfur and cause overheating of the converter as well as sulfation of the catalyst. Secondly, during shutdown and startup, the reheat furnace can be operated using fuel gas, thereby meeting the requirements of processes such as desulfurization and temperature raising. VI. Operation control of the converter: The catalyst used in the converter is active alumina. The grade employed by our factory is S–201, which belongs to the series of catalysts produced by Caesar Aluminum Company in the United States. This catalyst features a large specific surface area, high compressive strength, and good reaction activity; it is one of the most efficient catalysts currently used in sulfur recovery systems, with Al2O3 as its main component. During normal operation, H2S and SO2 react more rapidly in the catalyst, generating some reaction heat, which is manifested as a gradual increase in the bed temperature from the upper to the lower parts. When the components in the process gas remain relatively stable, the temperature rise in the converter bed remains at a relatively constant level. According to the original documentation, the temperature rise in the first-stage converter bed is approximately 39–49°C, while that in the second stage is around 22–28°C. If the temperature rise exceeds these ranges, it is necessary to consider whether the air distribution in the system is appropriate. VII. Operation control of sulfur condensation coolers: For two-stage sulfur condensation coolers, their function is primarily to use ethylene glycol solution as a cooling medium in order to condense the process gas containing large amounts of sulfur vapor after it passes through the converter, thereby reducing the temperature of the process gas below the sulfur dew point and allowing the elemental sulfur formed to be separated out. At the outlet of the two-stage condensation cooler, mist traps are installed to capture tiny sulfur droplets. This helps to collect liquid sulfur, thereby increasing the sulfur recovery rate, reducing resource waste, and preventing air pollution. Therefore, it is important to control the temperature of the process gas at the outlet of the sulfur condensation cooler. VIII. Process Control: When operating the sulfur recovery unit, it is necessary to regularly observe and analyze the entire process as well as any changes that occur. By referring to the test results of exhaust gases and process gases, adjustments should be made in a timely manner to ensure the stable operation of the unit. Due to frequent fluctuations, the sulfur recovery rate can decrease by 5-10%, and the resulting losses and pollution are considerable. To determine the operating condition of sulfur recovery units, it is necessary to regularly rely on test results, combine them with operational data and carry out necessary calculations in order to arrive at an accurate assessment, formulate appropriate countermeasures, and take timely actions. Carefully observing the operating condition of the exhaust gas burning furnace also helps with operation. Section 6: Inspection and Preparation for Starting Up the Equipment The starting up of equipment must be carried out in accordance with a specific procedure, and sufficient time must be allocated for this process. Pursuing speed recklessly will exacerbate damage to the equipment, **shorten its lifespan, and lead to accidents. Therefore, the start of work should proceed in the following order: ①. Inspection, confirmation, and preparation. ②. Purging, airtightness test. ③. Warm pots (including waste heat boilers, condensers, liquid sulfur seals, and liquid sulfur pipelines). ④. Ignite and heat up. ⑤. Feeding material. ⑥The ignition of each furnace should be carried out in sequence: first ignite the burning furnace, then the reheating furnace, and finally the reaction furnace. The inspections and preparations for putting the sulfur recovery unit into operation should begin during the equipment maintenance and acceptance process. This process consists of two steps: the first is the inspection and preparation of the process dynamic and static equipment ； The second is the inspection and preparation of the instrument control system. Secondly, the commissioning of the facility should proceed in accordance with a strictly prepared and reviewed commissioning plan, to avoid arbitrary changes to the plan, disruption of procedures, and reckless actions that could lead to accidents. I. Inspection of moving and stationary equipment ① Inspection of operating equipment (see the section on operating equipment) ② Inspection of stationary equipment. The inspection of stationary equipment involves checking whether the acid gas water separators, reaction furnaces, waste heat boilers, condenser coolers, converters, heat exchangers, as well as the pipelines and valves connected to them, have been properly installed and secured after maintenance; whether the manholes and access holes on the equipment itself are correctly fitted; whether the bolts have been treated with grease to prevent rust; and whether all necessary accessories such as safety valves and check valves are present. Check for leaks in other valves, whether packing has been added, whether the valves have been properly maintained, and whether the handwheels rotate smoothly. If there are any issues, corrective actions must be taken. Some pressure vessel equipment, such as waste heat boilers, sulfur condensation coolers, and inner tubes of sulfur liquid seals, operates under high pressures and temperatures; therefore, after each maintenance task, it is necessary to carry out a hydrostatic test to ensure there are no issues before putting them back into use. After the maintenance work is completed, as a precaution, an air blower should be used to introduce air into the device, and the exhaust pipeline should be temporarily blocked with a blind flange. The pressure inside the device is then raised to around 0.04–0.05 MPa in order to conduct a leak test and check for any leakage points. The inspection should include all parts that have been removed as well as any areas where leakage may occur. ③ Inspection of control instruments. Contents of the inspection for control instruments: A. Check whether the engineering parameters of each control loop have been altered, and whether such changes are necessary or reasonable. B. Whether the regulator’s output value corresponds to the changes in the field actuator, whether the actuator (control valve) is flexible, and whether there is any leakage from the packing. C. The on-site instruments, including valve positioners, compressed air pressure regulators, electrical converters, instrument cables, and transmitters, have all been installed; they are firmly fixed with no visible defects, and there is no blockage in the air supply signal. D. Check whether the drain hole of the air pressure reducing valve is unobstructed, and remove any accumulated water or dust. E. Power and air supply are accurate and functioning properly. ④ The final step in the preparation work is to check and confirm the switch positions that all control valves, such as those in the process units and instrumentation systems, should be in when operations begin; this step must not be overlooked. Therefore, this step must be checked and confirmed step by step by the unit operators, team leaders, unit supervisors, and workshop technical supervisors, with the results of each check to be reported at each level; not the slightest negligence is allowed. ⑤ Once all the aforementioned tasks have been completed and the corrective actions have met the required standards, the next step is to provide insulation for the heat transfer medium MEG circulating fluid that is introduced into the waste heat boiler, sulfur condensation cooler, sulfur liquid seal, and liquid sulfur pipelines. The temperature of the MEG system is gradually adjusted to a value near normal. After an appropriate amount of time, once the waste heat boiler, sulfur condensation cooler, sulfur liquid seal, and liquid sulfur pipelines have reached their normal temperatures, flake-shaped sulfur product is filled into the sulfur liquid seal. Once it is fully filled and melted, the furnaces can be prepared for ignition. The criterion for determining whether the insulation has reached the normal temperature is that solid sulfur, when brought into contact with the liquid sulfur pipelines and the outer tubes of the sulfur liquid seal, can melt them. In the waste heat boiler and sulfur condensation cooler, the MEG temperature exceeds 100°C and remains at this level for more than 2 hours. Inspect and shut off each valve through which fuel gas enters the device, and remove the blind flanges from the exhaust pipeline. Once all the tasks are completed, start the blower to introduce 300–600 NM3/h of air into the device, purge for 3–5 minutes, and then stop the purging. Prepare tools such as torches, pipe wrenches, and adjustable wrenches to carry out the ignition. Section 7: Ignition and Heating I. Ignition ① Before ignition, the fuel gas pipelines of each furnace should be checked again, and the fuel gas control valves must be fully closed to prevent fuel gas from leaking into the furnaces. Proceed to the next step only after confirming there is no leakage. ② Start the recovery fan and put the supply air pressure PIC—2650 control circuit into operation, with the pressure set at 49 KPa. ③ Compressed air is introduced into the reaction furnace and reheating furnace through air pipelines; a high flow rate of air is used to purge the entire recovery system, with the purging time being no less than 3 minutes, typically maintained between 3 and 5 minutes, so that all combustible gases in the system are expelled through the exhaust chimney. ④ Turn off the purge air and open the ignition port. Insert the torch into the furnace, slightly open the air supply valve, and maintain a low airflow. Once stabilized, gradually open the fuel gas supply valve to ignite the burner, and adjust the ratio of fuel gas to air to maintain a stable combustion at the minimum level. If ignition fails, the fuel gas must be cut off, an air purge must be performed to displace the fuel gas, and then ignition attempted again. This process is repeated until ignition is successful. ⑤ No specific requirements are set for the ignition sequence of each furnace; however, in line with the principles of convenience and safety, ignition can be carried out in reverse order, starting from the exhaust gas combustion furnace. ⑥ Subsequent temperature increases should be carried out in accordance with the specified temperature rise curve, while maintaining the required rate of temperature increase and holding time at each temperature level. It should be noted that strict control over the temperature rise process is of great significance for extending the equipment’s service life, reducing the amount of maintenance work required, and increasing the period during which the device can operate safely. ⑦ To effectively control the furnace temperature at the beginning of ignition, and on the premise of flame stability, an excess amount of air equal to 2–10 times the chemical equivalence can be used to control the temperature rise. But keep in mind that this operation must be carried out on the basis of proper shutdown and desulfurization procedures; it is not suitable for use after a short-term shutdown. ⑧ During super-stoichiometric air supply operation, it should also be noted that as the bed temperature of the converter rises over time, the amount of excess air supplied should be gradually reduced in order to achieve the desired temperature increase. The fuel gas flow rate can also be increased simultaneously, but the excess oxygen level should be maintained such that when the highest temperature in the converter bed approaches 190°C, the air-fuel ratio is reduced to the chemical equivalent, thereby preventing catalyst overheating, sulfation, and equipment corrosion. II. Constant temperature and heating: After a prolonged period of shutdown for maintenance, the temperature inside each furnace has approached normal levels. Due to various construction activities, the lining materials inside the furnaces may become damp; under localized high temperatures, this can lead to cracking or collapse of the lining, thereby reducing the furnace’s service life and even causing damage to the furnace. Therefore, after each furnace is ignited, a low-temperature operating period must be maintained, and the temperature must be increased gradually according to a prescribed heating curve under controlled conditions, until it reaches or comes close to the normal operating temperature. Temperature rise curve: The catalyst in the converter bed must be maintained at the appropriate reaction temperature for it to function properly; therefore, it is necessary to raise the temperature gradually to the operating level before feeding material in. Otherwise, a sharp increase in temperature, sulfur condensation, or blockages may occur, leading to failure in feeding the material. Therefore, the heating of the converter also needs to be carried out gradually; otherwise, it will cause difficulties in the operation of the device, and the catalyst may suffer severe damage leading to a decrease in conversion efficiency. Section 8: Feed Conditions The feed conditions described in this section refer to the state that the recovery unit should be in before it starts operating with acid gas, including the operation of upstream units and the specifications of the acid gas supplied. ① Requirements for the raw acid gas: The natural gas processing capacity of the desulfurization unit must be ≥30×104 m3/d ； It can also provide a steady amount of acid gas to the recovery device ； H2S in sour gas ≥82% (V) ； CH4