E-book: Basics of Petroleum Refining Units – Sulfur Recovery Units (VII)

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Chapter 7: Combined Unit for Aqueous Steam Stripping and Sulfur Recovery I. General Overview of the Unit and Sources of Raw Materials The combined sulfur recovery unit generally consists of three process units: an acidic aqueous steam stripping unit, a solvent regeneration unit, and a sulfur recovery unit (including exhaust gas treatment). Among them, the acidic water stripping unit is used to strip acidic water generated by units such as heavy oil catalytic cracking units and oil hydroprocessing units ； The solvent regeneration unit is used for the centralized regeneration of the desulfurized amine-rich liquid from the product refining unit, the desulfurized amine-rich liquid from the catalytic gasoline selective hydrogenation desulfurization unit, and the amine-rich liquid used for absorbing sulfur-containing off-gases ； The sulfur recovery unit processes regenerated acidic gas and stripped acidic gas to recover the chemical product sulfur, turning potential hazards into advantages through comprehensive utilization, thereby meeting exhaust emission standards. II. Process technical characteristics of the sulfur recovery combined unit: Solvent regeneration section: A conventional steam stripping regeneration process is employed. l Acidic water stripping section: A single-tank atmospheric pressure stripping process is used. l Sulfur recovery section: Technical features – a Claus process with external mixing and two-stage conversion, combined with a reduction-absorption process for treating exhaust gases. Environmental benefits – It deals with acidic gases, the main component of which is hydrogen sulfide, a colorless, highly toxic gas with a smell similar to rotten eggs. The process gas generated during the manufacturing process also contains toxic and harmful substances such as hydrogen sulfide and sulfur dioxide; these substances are harmful to human health and can even be life-threatening. Therefore, during regular inspections and operations, it is required that two people work together and carry portable hydrogen sulfide detectors to monitor for any leaks in the equipment at all times. The outdoor control room is equipped with protective gear such as air respirators and gas masks for emergency use, and operators must be proficient in how to use these devices. Features of equipment installation – During the production process, the product is liquid sulfur, and its temperature is maintained between 130 and 160 degrees. When the temperature drops below 130 degrees, the liquid sulfur condenses into solid sulfur, which can cause blockages in the equipment and pipelines. Therefore, the equipment and pipelines are installed at a certain inclination to facilitate the flow of liquid sulfur toward the sulfur seal tank and sulfur pool; meanwhile, jacket heating is used to maintain a temperature of 130–160 degrees. III. Principle of the sulfur recovery process: The sulfur production principle involves a thermal reaction section where H2S reacts partially with air, as well as two stages of conventional Claus catalytic reaction sections. After a two-stage Claus reaction, the sulfur recovery rate can reach 94%–95%. 1. Thermal reaction: H2S + 3/2 O2 → SO2 + H2O + Q
2H2S + SO2 → 3/XSX + 2H2O + Q
2. Catalytic reaction: 2H2S + SO2 → 3/XSX + 2H2O + Q

In an acidic gas combustion furnace, NH3 undergoes two reaction processes: oxidation and decomposition.
1. Oxidation reaction: 2NH3 + 3/2 O2 → N2 + 3H2O
2. Decomposition reaction: 2NH3 → 3H2 + N2

To ensure complete reaction of NH3, it is necessary to maintain: (1) a flame temperature of over 1250°C ； ⑵ A certain level of peroxide content ； ⑶Residence time of over 0.8S ； ⑷ Flue gas treatment for nozzles (burners) with excellent mixing effects and combustion performance: Sulfur-containing flue gas is mixed with hydrogen-rich gas; within the hydrogenation reactor, under the action of cobalt-molybdenum hydrogenation catalysts, sulfur dioxide and elemental sulfur in the flue gas are hydrogenated and reduced to hydrogen sulfide, while organic sulfur is hydrolyzed to produce hydrogen sulfide as well. Reduction reaction: SO2 + 3H2 → H2S + 2H2O + Q; H2 → H2S + Q. Hydrolysis reactions: COS + H2O → H2S + CO2 + Q; CS2 + 2H2O → 2H2S + CO2 + Q. The H2S and part of the CO2 in the exhaust gases are absorbed by a 25% ethanolamine solution; the purified exhaust gas is then subjected to thermal combustion to convert the remaining sulfur compounds into SO2, which is subsequently released into the atmosphere through chimneys ； The amine-rich liquid that has absorbed H2S is regenerated and returned to the exhaust gas absorption tower for reuse. At the same time, the regenerated H2S and CO2 are returned to the Claus system. After exhaust treatment, the total sulfur recovery rate can reach over 99.8%. IV. Process flows for various components of the combined acidic water stripping and sulfur recovery unit (flow diagrams are provided at the end of this chapter). In the acidic water stripping section, the acidic water from the heavy oil catalytic cracking unit and the oil hydroprocessing unit enters the raw water degassing tank where it is flashed and degassed; the light oil gases resulting from this process are sent to the flare system. The degassed acidic water first enters the raw water tank where it settles to remove oil; thereafter, it is pressurized by a raw water pressure pump and sent to the raw water oil remover for further oil removal. The oil removed in this process flows intermittently by gravity to the oil waste tank, from where it is transported intermittently to the factory’s oil waste tank using an oil waste pump. The degreased acidic water enters the raw water buffer tank, is pressurized by the raw water feed pump, then heated to 110°C through a raw water-purified water heat exchanger before entering the stripping tower. The bottom of the stripping tower is indirectly heated by a reboiler to maintain a bottom temperature of 124°C. The purified water from the bottom of the stripping tower is heated to 53°C through heat exchange with the feed water, and then pressurized by a purified water pump. Part of this water is sent outside the plant for reuse, while the excess purified water is cooled to 40°C using a purified water cooler before being discharged into the oily wastewater pipeline system. The acidic gas at the top of the stripping tower is condensed to 85°C using an overhead cooler, and then sent to the top reflux tank for separation. The acidic gas after separation is sent to the sulfur recovery unit for sulfur recovery, while the condensate is returned to the tower via a top reflux pump as reflux. A water seal tank and an adsorption tank are installed at the top of the raw water tank to prevent the release of harmful gases, adsorb the malodorous components in the released gases, and reduce environmental pollution. The solvent for regeneration is obtained by mixing the rich amine solution from the catalytic light oil selective hydrodesulfurization unit and the product refining unit – after being filtered through a rich liquid filter – with the rich amine solution derived from the absorption of sulfur off-gases. This mixture is then exchanged heat with lean liquid in a primary heat exchanger to reach a temperature of 65°C, after which it is sent to a flash tower to remove the dissolved hydrocarbons. The light hydrocarbon gas at the top of the tower is sent to the flare system, while the rich amine solution at the bottom of the tower is pressurized using a pump located at the bottom of the flash tower; it is then exchanged heat with lean amine solution in a secondary heat exchanger to reach a temperature of 88°C, after which it enters the regeneration tower. The bottom of the regeneration tower is heated by a reboiler, via indirect heating with 0.3 MPa (G) steam. After being condensed in the acidic gas air cooler and the acidic gas condenser, the gas at the top of the tower enters the reflux tank at the top of the regeneration tower for liquid separation. The acidic gas resulting from this separation is sent to the sulfur recovery unit for sulfur recovery, while the condensate is returned to the tower via the reflux pump at the top of the regeneration tower as reflux. The amine-poor liquid at the bottom of the tower is first heated in a two-stage heat exchanger for rich and poor liquids, then pressurized by a poor-liquid pump, heated again in a one-stage heat exchanger for rich and poor liquids, and cooled in an air cooler and a liquid cooler to 40°C. After that, it is divided into three streams: one stream is sent to the absorption tower as an absorption solvent, another stream is sent to the flash tower to remove H2S from the light hydrocarbon gas, and the third stream is sent outside the plant to be used as a desulfurization solvent in the catalytic light oil selective hydrogenation desulfurization unit and the product purification unit. In the sulfur recovery section, the stripping acidic gas obtained from the acid water stripping is separated in a stripping acidic gas separator tank; the regenerated acidic gas derived from solvent regeneration is separated in a regenerated acidic gas separator tank and then heated to 160°C in an acidic gas preheater. Both types of acidic gas proceed to the sulfur recovery acidic gas combustion furnace. The acidic liquids separated in the two acidic gas liquid separation tanks are sent to the acidic water stripping section for treatment via acidic hydraulic transfer, with intermittent pressurization by nitrogen. The air from the burner blower is preheated to 160°C using 1.0 MPa steam in an air preheater, before entering the acidic gas burner. The air supply for the combustion of acidic gases is controlled such that 80% of it corresponds to complete combustion of hydrocarbons and the conversion of 1/3 of the hydrogen sulfide into sulfur dioxide, while 20% of the air supply is adjusted based on the H2S/SO2 ratio of 2 in the Claus off-gases. After combustion, the high-temperature process gas enters the acidic gas waste heat boiler where it is cooled to 350°C and steam at 1.0 MPa is generated. It then proceeds to the primary condenser, where steam at 0.3 MPa is produced and the gas is cooled to 170°C. The condensed liquid sulfur flows from the bottom of the primary condenser through a liquid sulfur containment tank into the sulfur tank. The cooled process gas is mixed with the high-temperature gas flow inside the furnace via a first-stage mixing valve to reach a temperature of 240°C, after which it enters the first-stage reactor. There, under the action of the Claus catalyst, hydrogen sulfide reacts with sulfur dioxide to produce sulfur. The reaction gas at 295°C enters the secondary condenser, where it loses 0.3 MPa of steam and is cooled to 160°C. The condensed liquid sulfur flows from the bottom of the secondary condenser, through a liquid sulfur containment tank, into the sulfur tank. The cooled process gas is mixed with the high-temperature gas flow inside the furnace through a two-stage mixing valve to reach a temperature of 220°C, after which it enters the secondary reactor. There, under the action of the Claus catalyst, hydrogen sulfide and sulfur dioxide continue to react to produce sulfur. The gas from the reaction process at 235°C is cooled to 130°C by a three-stage condensation cooler; the condensed liquid sulfur flows from the bottom of the three-stage condensation cooler, through a liquid sulfur seal tank, into the sulfur tank. The cooled exhaust gas, after the sulfur mist is captured by a catcher, enters the exhaust gas treatment system. An online exhaust gas analyzer is installed on the exhaust pipeline at the outlet of the catcher to measure the levels of H2S/SO2 in the exhaust gas. This information is used to adjust the amount of air fed into the acidic gas combustion furnace by 20%, thereby ensuring that the ratio of H2S to SO2 in the process gas is 2:1. This approach helps to maximize the conversion efficiency of the Claus reaction, while also increasing sulfur recovery and reducing sulfur losses. In the sulfur tank, a liquid sulfur degassing pump is used to circulate and degas the liquid sulfur; the small amount of H2S released is pumped to an exhaust gas incinerator using a steam injector for combustion. The liquid sulfur product is pumped to the sulfur molding room using a sulfur pump, while solid sulfur is transported out of the facility by vehicle. The Claus off-gases, after sulfur mist has been captured, exchange heat with the flue gas from the off-gas incinerator in a gas-to-gas heat exchanger, thereby being heated to 300°C. After being mixed with externally supplied hydrogen-rich gas (provided by a dry gas hydrogen production unit), they enter the hydrogenation reactor; there, under the action of hydrogenation catalysts, SO2, COS, CS2, as well as liquid and gaseous sulfur, are all converted into H2S. The hydrogenation reaction is an exothermic reaction; the process gas at 330°C leaving the reactor enters the quench tower directly. The exhaust gas is cooled in the quench tower using circulating quench water. Quench water at 70°C flows out from the bottom of the quench tower, is pressurized by a quench water pump, filtered through a quench water filter, and then cooled to 40°C by a quench water air cooler and a quench water cooler before being circulated back to the top of the quench tower. Due to the exhaust gases, the water vapor in them is condensed by the quench water; the resulting acidic water is pumped by a quench water pump to the acidic water stripping section for treatment. Urgently, install an online pH analyzer for the cold water pipeline. The exhaust gas after rapid cooling leaves the top of the rapid cooling tower and enters the exhaust gas absorption tower, where it is absorbed by a 25% methyl diethanolamine solution to remove hydrogen sulfide from the exhaust gas, as well as to absorb some carbon dioxide. The purified exhaust gas exiting the top of the tower is sent to an exhaust gas incinerator for combustion; the temperature of the furnace is maintained at 700°C through control of the fuel gas flow, resulting in the complete conversion of any remaining hydrogen sulfide and other sulfur compounds into sulfur dioxide. After combustion, the flue gas has its heat recovered to produce 1.0 MPa steam in an exhaust waste heat boiler, after which its temperature drops to 430°C. Part of this gas exchanges heat with the Claus exhaust gas, while the remaining part is discharged through a chimney. During an accident, sulfur recovery involves burning the acidic gas outside the acidic gas delivery unit using a flare ； When the exhaust treatment unit is in an abnormal state, the Claus exhaust can flow directly through the crossover line into the exhaust incinerator for combustion, and then be discharged through the chimney. The 1.0 MPa steam generated by the acid gas waste heat boiler and the exhaust gas waste heat boiler is used, in part, for preheating the acid gas and air; the remaining steam is cooled and depressurized before being fed into the 0.3 MPa steam network. The 0.3MPa steam generated by the primary and secondary condensation coolers is used for insulation and heat tracing within the installation. The waste steam generated by the tertiary condensing cooler is cooled by the tertiary steam air cooler, and the condensed water is circulated back to the tertiary condensing cooler. The condensate from the device’s heat insulation and heating systems, the condensate generated by the stripping tower reboiler, and the condensate produced by the regeneration tower all flow into the condensate recovery tank where they undergo pressure-induced flashing. The vapor resulting from this flashing is cooled in a waste vapor air cooler before being returned to the condensate recovery tank. The condensed water in the tank is pressurized by a condensate pump and then sent as boiler feed water to the acid gas waste heat boiler, the exhaust gas waste heat boiler, and the primary and secondary condensation coolers to generate steam; the remaining portion is discharged from the system. IV. Impact of raw material properties on production: Hydrocarbons: 1. Accelerate the occurrence of side reactions, increase the production of COS and CS2, and reduce the sulfur conversion rate ； Increasing the combustion load of the furnace and raising its temperature can lead to severe overheating and damage to the furnace structure ； It increases the amount of air required for combustion as well as the volume of gases generated during combustion. If there isn’t enough air supply, carbon deposition can occur; this not only blocks equipment and pipes but also causes the catalyst to become clogged with carbon, leading to failure to function properly and the production of black sulfur ； At the start of hydrocarbon feeding, if the air supply cannot keep up, it will cause the furnace temperature to drop rapidly; backburning may occur in the exhaust furnace, which is also at risk of overheating. Yellow smoke will emerge from the chimney, leading to environmental problems. Ammonia content: The higher the amount of ammonia, the more air is required for combustion. The nitrogen and water produced by the combustion of ammonia are inert components in the Claus reaction, which reduces the partial pressure of the active components and thus lowers the sulfur yield. When combustion is incomplete, reactions occur to form ammonium hydrogen sulfide or ammonium polysulfide crystals, which can block equipment or pipelines and cause pressure buildup in the system. Ammonia reacts with alumina, leading to catalyst deactivation; the by-product nitrogen oxides cause air pollution. It also acts as a catalyst for the further oxidation of SO2, which can result in the formation of sulfuric acid and exacerbate corrosion of equipment and pipelines. With liquid present: Lowering the temperature of the furnace causes steam to vaporize, which increases the pressure inside the furnace and affects the combustion process, thereby reducing the conversion rate. A large amount of water can cause the furnace to shut down, damage the insulation lining, and disrupt production. Hydrogen sulfide concentration: A high concentration is favorable for the forward progression of the reaction ； At very low concentrations, the system temperature cannot be maintained, which may lead to sulfur accumulation and salt formation that can block pipes and equipment ； At low concentrations, in order to maintain the system temperature, an excessive amount of air needs to be supplied; as a result, all of the hydrogen sulfide is converted into SO2. This not only reduces the sulfur conversion rate but also increases the load on the exhaust gas treatment systems. When hydrogen reduction does not occur in a timely manner, sulfur is formed, leading to sulfur accumulation in the reactor and blockages in the quench tower, which prevents the exhaust gas from flowing properly and thus increases the SO2 content in the exhaust emissions. V. Product requirements: In the solvent regeneration section, the levels of H2S+CO2 in the lean amine solution should be ≤1 g/L, and the MDEA concentration should be 25%; these values meet the requirements of the product purification units, the catalytic gasoline selective hydrogenation desulfurization units, and the sulfur exhaust gas absorption systems regarding solvents ； l Acidic water stripping section: H2S ≤ 30 ppm, NH3 ≤ 80 ppm in the stripped and purified water ； l Sulfur recovery section: The quality of sulfur meets the first-class standards specified in GB2449-2006. Grade A: Purity ≥99.95%, ash content ≤0.03%, acidity ≤0.003%, organic matter ≤0.03%. Grade B: Purity ≥99.5%, ash content ≤0.1%, acidity ≤0.005%, organic matter ≤0.3%. Qualified grade: Purity ≥99.0%, ash content ≤0.2%, acidity ≤0.02%, organic matter ≤0.8%. VI. Main equipment for sulfur recovery: l Furnaces: Acid gas combustion furnaces, exhaust gas incineration furnaces. l Heat recovery boilers: Heat recovery boilers for acid gas, heat recovery boilers for exhaust gas. l Reactors: Primary and secondary Claus reactors, hydrogenation reactors. l Condensers: Primary and secondary sulfur condensers, tertiary sulfur condenser. l Sulfur production fans. l Exhaust gas fans. l Granulators. l Hydrogen sulfide ratio meters, hydrogen analyzers