Process Description
Overview
The Sulfur Recovery Unit (SRU) converts hydrogen sulfide (H2S) in acid gas feeds to elemental liquid sulfur using a 3-stage Claus process. Two feed streams, one from an Amine Regenerator and one from a Sour Water Stripper (SWS), are combusted and reacted with process air in the Thermal Reactor. The Thermal Reactor is designed to destruct ammonia (NH3) contained in the feed from a Sour Water Stripper by achieving a high temperature in the specially designed burner. Process air is supplied from the Air Blower at a rate that achieves a near-stoichiometric ratio of residual H2S and sulfur dioxide (SO2) in the product tail gas from the SRU.
In the Thermal Reactor the feed H2S is converted to sulfur without the need for catalyst because of the high temperature achieved by the combustion of process air. However, the H2S conversion is limited by reaction equilibrium. High level heat from the Thermal Reactor is recovered in the Waste Heat Boiler by generating high pressure steam. The process gas leaving the Waste Heat Boiler is cooled in Sulfur Condenser No.1 by generating low pressure steam. Liquid sulfur is removed at the outlet of the condenser using a specially designed seal leg and is sent to the Sulfur Storage Pit.
Process gas is reheated in Reheater No. 1 and sent to Catalytic Reactor No. 1 where additional sulfur conversion takes place. The outlet gas from the reactor is cooled in Sulfur Condenser No. 2 and another seal leg removes liquid sulfur. Another reheater, catalytic reactor, and sulfur condenser helps achieve over 95% conversion of the feed H2S to sulfur. The product tailgas from the last sulfur condenser is sent downstream for additional sulfur recovery and/or incineration before discharge to atmosphere.
Acid Gas Feed Knockout Drums D-301 and D-302
Acid gases from the Amine Regenerator and the Sour Water Stripper flow into individual knockout drums to remove any entrained water or amine solution that may be contained in the feed. The presence of liquid in the acid gas sent to the Thermal Reactor can cause damage to the specially designed acid gas burner and the refractory-lined walls of the Thermal Reactor. Collected condensate is pumped to a disposal unit automatically. A high level sensor is installed in each Acid Gas Knockout Drum that will activate a shutdown of the Sulfur Recovery Unit (SRU) if the Condensate Pump cannot maintain the level.
Acid gas flow rate is determined by the operation of the source unit. Normally the Amine Regenerator produces 137.5 MSCF/H of acid gas with 89 % H2S and 5% CO2. The Sour Water Stripper produces 37.9 MSCF/H of acid gas with 35% H2S, 35% NH3 and the balance being water. The pressure in the SWS Acid Gas Knockout Drum normally floats on the Thermal Reactor’s pressure. If the emergency isolation
valve on the gas outlet line is closed by the shutdown system then an automatic pressure vent will release acid gas to the acid gas flare system. In the case of the Amine Acid Gas Knockout Drum a portion of the acid gas (68.9 MSCF/H) is routed to the Thermal Reactor’s burner. The balance of the acid gas is sent directly to the Thermal Reactor’s reaction chamber under pressure control. A pressure controller vents acid gas to the acid gas flare system if the emergency isolation valve on the outlet line is closed by the shutdown system.
Air Blower K-301
The Claus process requires oxygen to react with hydrogen sulfide (H2S) to form elemental sulfur. Air is used to provide the oxygen. A steam turbine-driven Air Blower is used to compress air to about 12.3 PSIG for use in the Claus process. The machine is normally run at constant speed of 3,000 RPM and excess air is vented to a blow-off valve via pressure control. The total air flow to the Claus Burner is 367.1 MSCF/H and the total flow through K-301 is controlled to 403.8 MSCF/H.
The air to the Thermal reactor is flow-controlled in two parallel control valves. The main valve FV-312 provides about 90 % of the total air and is controlled in a ratio to the theoretical air requirement of the process feeds. The theoretical air requirement is computed from the acid gas and fuel gas flows to the Thermal Reactor. The flow through the second valve FV-314 is adjusted by a feedback controller to obtain a precise stoichiometric ratio of H2S to sulfur dioxide (SO2) in the product tailgas. This combination of feedforward and feedback control stabilizes the tailgas composition to minimize upsets in downstream units (e.g., a Claus tailgas treater) and to avoid excessive sulfur emissions.
Thermal Reactor R-101
The Thermal Reactor consists of three parts: the burner, the thermal reactor front and rear reaction chambers, and the integrated Waste Heat Boiler.
Burner
Combustion air from the Air Blower, all of the Sour Water Stripper acid gas, and a portion of the acid gas from the Amine Regenerator are combusted in the burner. Fuel gas is also burned for startup heating and if it is required during normal operation to maintain a sufficiently high reactor temperature for ammonia destruction. The burner is specially designed for the combustion of acid gases and to produce a stable flame until low rate operation. Amine acid gas is split between the burner and the back reaction chamber to help maximize the amount of oxygen available for combusting ammonia (NH3) within the burner. Any uncombusted NH3 will thermally dissociate at the high temperatures attained by the burner. The normal operating temperature of the Claus Burner is around 2,345 DEG F. The temperature must be above 2,200 DEG F to ensure complete NH3 destruction.
Reaction Chambers
The product of the acid gas and fuel gas combustion in the burner flows into the front chamber and then through a checkered brick wall (open holes in the brick wall in a checkerboard pattern) into the back chamber of the Thermal Reactor. The checkered wall minimizes back-mixing of gases from the back thermal reaction chamber into the front chamber. The checkered wall also helps maintain the temperature in the front chamber by minimizing radiant heat loss to the thermal reaction chamber. By this design the temperature of the front chamber can be made high enough to destroy ammonia contained in the Sour Water Stripper acid gas feed. Also, by routing a portion of the Amine Regenerator acid gas to the back chamber the burner can maintain a near-stoichiometric oxygen level which ensures good combustion and minimizes soot formation if fuel gas is being fired. Also, because of the volume of the front chamber some reaction of the hydrogen sulfide (H2S) to sulfur occurs.
Additional reaction of H2S to sulfur occurs in the back chamber of the Thermal Reactor. A portion of the Amine Regenerator acid gas is mixed with high-temperature gas from the front chamber. The reaction of H2S to sulfur occurs without the need for catalyst because of the high temperatures in both the front and back chamber. However, only about 80% of the feed H2S is converted in the Thermal Reactor because of equilibrium limitations. The higher the reactor temperature the lower the conversion to sulfur. The outlet temperature of the Thermal Reaction Chamber is normally around 2,577 DEG F and is higher than the Claus Burner temperature due to the Claus reaction taking place in the chamber.
Because the Thermal Reactor typically operates at temperatures above 2,000 DEG F the reactor wall is lined with a refractory system. The pressure of the reactor is normally around 4.5 PSIG and depends on the total pressure drop through the downstream process equipment until the tail gas is discharged to atmosphere via an incineration stack. This pressure drop is dependent on the flow through the process and will increase due to plugging of and damage to equipment.
Waste Heat Boiler E-301
The Waste Heat Boiler is integrated into the back chamber of the Thermal Reactor to permit cooling the very hot reactor gas as quickly as possible. This design avoids an internally insulated transfer line which improves the mechanical reliability of the reactor.
The Waste Heat Boiler generates high pressure steam from heat available at the exit of the back chamber. The outlet gas passes through tubes which boil water from the Steam Drum. The Claus process gas is sent to Sulfur Condenser No. 1 at a temperature of around 595 DEG F.
Steam Drum D-303
The Steam Drum supplies boiler water to the Waste Heat Boiler via a set of vertical fire tubes to ensure good circulation of boiler water to the Waste Heat Boiler. The flow of makeup boiler water is controlled based on the water level in the Steam Drum. Generated steam is disengaged in the drum and sent off to the high pressure (650 PSIG) steam header.
Sulfur Condensers E-302, E-304 and E-306
The Claus process gas leaves the Waste Heat Boiler at about 595 DEG F and is cooled in Sulfur Condenser No. 1 by generating medium pressure (50 PSIG) steam. At this pressure the outlet temperature of the condenser is limited to the equilibrium temperature of water at that pressure so the sulfur will not solidify. The outlet temperature is typically around 30 DEG F above the saturation temperature of 50 PSIG steam (298 DEG F saturation temperature).
Much of the sulfur formed in the Thermal Reactor is condensed and collected in a specially designed outlet channel where it falls into Seal Leg No. 1. The channel design keeps gas velocities in a range so that fogging and entrainment of liquid sulfur in the exit gas is minimized. The cooled gas continues on to Reheater No. 1.
Water level is controlled by adjusting the makeup flow rate of boiler feed water. The shell side of Sulfur Condenser No. 1 is shared with Sulfur Condensers No. 2 and 3. This arrangement minimizes initial equipment cost and makes the operation of the unit somewhat simpler.
Sulfur Condenser No. 2 receives hot gas from Catalytic Reactor No. 1. Liquid sulfur is removed in Seal Leg No. 2. The cooled gas is passed on to Reheater No. 2 at a temperature of around 320 DEG F.
Sulfur Condenser No. 3 receives hot gas from Catalytic Reactor No. 2. Liquid sulfur is removed in Seal Leg No. 3. The cooled gas is passed on to the Coalescer at a temperature around 300 DEG F. The line to the Coalescer and the Coalescer itself is steam traced so that it does not cool below the solidification point of sulfur (around 246 DEG F).
Seal Leg Nos. 1, 2, and 3
The seal legs at the outlet of all three condensers consist of two concentric pipes that form a liquid seal to prevent process gas from escaping with the liquid sulfur being removed to the Sulfur Pit. The liquid trap principle is identical to that of water in an S-trap in the drainpipe of a kitchen sink. Liquid sulfur flows into the annular channel between the inner and outer pipes and then flows up from the bottom of the seal leg through the inner pipe to its top and then out to the sulfur pit. The level difference between the sulfur in the outer and inner pipes is roughly that height of sulfur which will provide a hydraulic pressure head equal to the difference of pressure between the condenser outlet and the sulfur pit which operates at atmospheric pressure. If the pressure of the process becomes high, then process gas can blow through the liquid sulfur seal.
To maintain sulfur in a flowing liquid state the seal legs are outfitted with a heating system that uses steam to keep the outside surface of the legs above the solidification temperature of sulfur (around 246 DEG F).
Reheaters E-303 and E305
After removal of sulfur from the process stream in the Sulfur Condenser No. 1 the remaining gas is passed to Catalytic Reactor No. 1 to convert more hydrogen sulfide (H2S) and sulfur dioxide (SO2) to sulfur. Reheater No. 1 is a heat exchanger that uses high pressure steam to bring the process gas up to the desired inlet temperature for Catalytic Reactor No. 1 (480 to 550 DEG F). A temperature controller on the outlet line adjusts the steam flow rate automatically.
High pressure steam (650 PSIG) is used because it can provide heat at a temperature suitable for the Catalytic Reactor No. 1 inlet temperature requirement.
Reheater No. 2 operates in the same fashion as Reheater No. 1. It heats gas from the outlet of Sulfur Condenser No. 2 that is destined for Catalytic Reactor No. 2.
Catalytic Reactor No. 1 R-302
To further react H2S and SO2 in the gas leaving the Thermal Reactor the gas from Reheater No. 1 is passed through Catalytic Reactor No. 1. This reactor contains a 48-inch deep bed of catalyst consisting of solid spheres of alumina (aluminum oxide) approximately 1/4-inch in diameter. Gas flow is from top to bottom.
The catalyst increases the rate of the Claus reaction at the lower temperatures in the Catalytic Reactors:
SO2 + 2H2S 2H2O + 3S + heat
The lower temperatures are desirable because they favor a higher conversion of H2S and SO2 to product sulfur. For maximum sulfur conversion the inlet temperature to Reactor No. 1 is normally around 460 DEG F.
Catalytic Reactor No. 1 R-302
One of the secondary functions of the reactor is to convert carbonyl sulfide (COS) and carbon disulfide (CS2) that are produced in the Thermal Reactor via the hydrolysis reaction:
CS2 + H2O COS + H2S
COS + H2O CO2 + H2S
Maximum conversion of CS2 and COS occurs at a higher temperature than what would be optimal for sulfur production alone. Therefore, when COS and CS2 are present in the tailgas the inlet temperature to Reactor No. 1 would be increased up to 50 DEG F to maximize the conversion of CO2 and CS2 back to hydrogen sulfide (H2S) while still achieving a reasonably high conversion of sulfur.
The Claus reaction produces heat so that the temperature increases as the reaction progresses through the reactor. Temperature rises in Catalytic Reactor No. 1 are typically, around 60 to 120 DEG F depending on the feed temperature and composition. About 15% of the total sulfur made in the process is produced in Catalytic Reactor No. 1 and an outlet temperature of around 560 DEG F is achieved (Approximately 100 DEG F temperature rise in Catalytic Reactor no. 1).
Catalytic Reactor No. 2 R-303
There usually is no need to elevate the inlet temperature of Catalytic Reactor No. 2 to convert COS and CS2 back to H2S so that the inlet temperature can be kept as low as possible to maximize sulfur conversion. However, at lower inlet temperatures a possibility exists that the temperature and sulfur concentration at the outlet of the reactor can cause sulfur condensation on the catalyst. This limits sulfur conversion because any produced liquid sulfur will coat the catalyst spheres at that point in the reactor and effectively render them inactive.
Catalytic Reactor No. 2 will normally operate at an inlet temperature ranging around 435 DEG F but can range from 420 to 460 DEG F. About 5 of the total sulfur made in the process is produced in Catalytic Reactor No. 2. The temperature rise in this reactor will consequently be much smaller than that of Catalytic Reactor No. 1. The normal temperature rise is 16 DEG F but can range from 10 to 25 DEG F depending on the activity of the catalyst and the performance of the upstream reactor.
Coalescer D-304
The Coalescer is designed to remove any entrained liquid sulfur in the tail gas from the unit so it does not overload the downstream tail gas treating unit or cause excessive sulfur emissions if discharged to atmosphere via an incinerator stack. Wire mesh and alumina balls help demist the tail gas. Liquid sulfur is collected in a seal trap and is taken off to the Sulfur Pit. The Coalescer wall and piping is kept warm by the use of steam so that sulfur does not solidify.
Instrumentation
FC-301 controls the acid gas flow from the Amine Regenerator to the Amine Acid Gas K.O. drum by adjusting the inlet control valve. A portion of the amine acid gas is taken off under flow control by FC-304 to the Claus Burner of the Thermal Reactor. The rate of FC-304 is determined by the temperature in the front chamber. If the temperature becomes too low for the destruction of ammonia then the rate is reduced manually. The balance of the amine acid gas is taken off to the back chamber of the Thermal Reactor by the K.O. Drum pressure controller PC-301. In case PC-301 cannot control this pressure PC-302 releases acid gas to the flare system. This can happen if the Claus Unit interlock closes the outlet isolation valve XV-301.
The liquid level of D-301 is indicated on LI-301. A high level alarm condition will cause the Amine Acid Gas Condensate Pump to start and cause the discharge valve to open to drain the drum. Upon reaching a low level alarm condition the pump is stopped and the discharge valve is closed.
FC-305 controls the acid gas flow from the Sour Water Stripper to the SWS Acid Gas K.O. drum by adjusting the inlet control valve. Normally the pressure of D-302 floats on the Thermal Reactor pressure. In case the outlet isolation valve of D-302 is closed PC-303 releases acid gas to the flare system. The position of outlet isolation valve XV-302 can be adjusted using HC-302 as long as the Claus Unit trip interlock is not active.
The liquid level of D-302 is indicated on LI-303. A high level alarm condition will cause the SWS Acid Gas Condensate Pump to start and cause the discharge valve to open to drain the drum. Upon reaching a low level alarm condition the pump is stopped and the discharge valve is closed.
SC-301 controls the speed of the Air Blower by adjusting the flow of medium pressure steam to the turbine. The total flow through the Air Blower is kept constant by FC-311 which blows off discharge air to the atmosphere.
The air flow to the Claus Burner is controlled by two controllers. FC-312 controls the main air. The setpoint of this controller comes from FY-307 which computes the theoretical air demand of all the combustible flows to the Claus Burner. FC-312 has a ratio setting to allow variation of the main air flow setpoint from the theoretical air demand. FC-314 is the second air flow controller. This controller adjusts the trim air flow based on a setpoint from the tail gas controller AC-301.
The natural gas flow to the Claus Burner of the Thermal Reactor is controlled by FC-321. Natural gas is combusted at startup to warm up the SRU equipment and is used to ensure the temperature of the front chamber is high enough to destroy ammonia. Nitrogen is also used at startup to warm up the equipment.
Nitrogen can be added to the process manually using HC-323. Normally, this is used at shutdown so the sulfur can be purged prior to cooling down the unit. The nitrogen valve is automatically opened on a one-shot of HC-323’s output in case of a Claus unit trip.
LC-321 controls the level of water in the Steam Drum by adjusting the setpoint of makeup boiler feedwater flow controller FC-323.
The level of the water side of the Sulfur Condensers in controlled by LC-323 which adjusts the setpoint of makeup boiler feedwater flow controller FC-325.
The outlet temperature of Reheater E-303 is controlled by TC-331 which throttles the steam high pressure steam valve. The outlet temperature of Reheater E-305 is controlled by TC-341. A steam trap located on the outlet of each Reheater automatically removes condensate from the heat exchangers.
The tailgas leaving the coalescer is analyzed for the concentration of SO2 and H2S. A special analyzer output signal is created to represent a linear process demand signal for oxygen. The tailgas air demand controller AC-301 uses this signal to attain an approximate 2:1 ratio of residual H2S to SO2 in the tailgas by adjusting the trim air flow to the Claus burner FC-314. Keeping this ratio near the ideal 2:1 stoichiometric ratio ensures the maximum sulfur conversion takes place in the SRU. A setpoint of 0.0 represents a demand for a 2:1 ratio of H2S to SO2. A setpoint less than 0.0 represents a demand for a ratio greater than 2:1. The normal setpoint of AC-301 is -1.0 so that the tailgas air demand is a little less than stoichiometric.