Introduction
There are two parallel Selectox Sulfur Plants that treat acid gas from the two Amine Plants at the Gaviota Gas Plant. Both sulfur plants are identical. In the following discussion, we will refer to Plant 14 only. Plant 24 will operate exactly the same way. Plant 14 is designed to process 1.68 MMscfd of amine acid gas containing 15.8 vol% H2S. About 95% of the inlet H2S is recovered as liquid sulfur at design conditions, resulting in a sulfur production rate of about 9.5-long tons per day (LTPD) per plant. Each plant can also handle as little as 20% of the design flow with an H2S concentration as low as 5.0 vol%. In the turndown case, the sulfur recovery will be less than 95%. The tail gas from the Selectox Sulfur Plants is burned and scrubbed with a sodium sulfite/bisulfite solution in Plants 15 and 25.
Design Feeds and Products
Feeds
The acid gas feed to the Selectox Sulfur Plants comes from the Amine Plants’ regenerators. Each Sulfur Plant is designed to process as much as 1.68 MMscfd of amine acid gas containing 15.8 vol% H2S, 76.2 vol% carbon dioxide (CO2), 6.8 vol% water vapor, and a small amount of hydrocarbons.
There are two design basis cases for the plants: the maximum flow and the minimum flow cases. The feed rates and operating conditions for each case are summarized in Table 2.0-I.
The maximum flow case is based on the maximum volumetric flow rate and maximum sulfur production rate for the Sulfur Plants. The gas rate is 1.68 MMscfd and the H2S concentration is 15.8%. The sulfur production rate is 9.5 LTPD. This case was used to determine the size of all of the process equipment. The recycle blower is sized as a function of both the acid gas feed rate and the H2S concentration. The blower must be able to recycle enough gas back to the front end of the plant to dilute the feed to approximately 5% H2S to keep the Selectox converter outlet temperature below 700°F. Therefore, the blower might be a bottleneck even at lower-than-design acid gas rates if the H2S concentration is above 15.8%.
The minimum flow case is based on the least gas the plant can process and operate smoothly. In this case, the plant is turned down to 20% of the maximum flow rate or down to 0.34 MMscfd. The H2S concentration is 5.0% and the sulfur production rate is 0.6 LTPD. The Sulfur Plants cannot be brought on-line until the gas rate reaches 0.34 MMscfd and the H2S concentration is 5%. Until the gas reaches these values, the acid gas from Plant 11 or 21 is fed to the Tail Gas Plant, Plant 15 or 25. Acid gas does not have to go to the Sulfur Plant immediately upon reaching a rate of 0.34 MMscfd. The Tail Gas Plant can process the gas up to a total equivalent sulfur throughput of 2 LTPD.
The minimum flow case was used to determine the maximum turndown required from the recycle blower. As discussed above, the blower must recycle enough gas to control the temperature in the converter by reducing the H2S concentration in the acid gas to approximately 5%. But the H2S concentration in the acid gas in the minimum flow case is already 5%. In order to keep the recycle blower operating and the temperature in the converter under control, the temperature in the converter during minimum flow is 550°F. This forces more gas to be recycled to dilute the acid gas. This sets the minimum rate the recycle blower must process. The plant cannot operate with less than 0.34 MMscfd acid gas with 5% H2S.
The acid gas rate and H2S concentration will gradually increase from the minimum flow case as gas production increases and the gas becomes more sour. The CO2 concentration will decrease slightly as the H2S concentration increases. The day-to-day acid gas rate and composition will depend upon the Amine plant feed and operation. The acid gas is saturated with water (H2O), so the H2O content depends upon the pressure and temperature of the acid gas. The hydrocarbon concentration should not change much if the operation of the Amine plant is stable.
Products
At design conditions, Selectox Sulfur Plants produce 888.0 lb/hr (9.5 LTPD) of liquid elemental sulfur. The liquid sulfur is collected in the sulfur pit, where it is also degassed (stripped of dissolved H2S). Degassing lowers the H2S content of the product sulfur from 300 ppmw or less. The product sulfur is 99.9% pure, contains less than 30 ppmw ash, and contains less than 0.025% weight carbon. Approximately 95% of the sulfur present as H2S in the acid gas is recovered as liquid sulfur.
Approximately 2.20 MMscfd of tail gas containing 0.54 vol % H2S from the Sulfur plant is burned in Plant 15 or 25 to convert all sulfur compounds to SO2. The tail gas is then scrubbed to remove the SO2.
Each Sulfur Plant produces as much as 3305 lb/hr of 50 psig steam. The steam is generated in the first and second sulfur condensers and is used in the Amine Plant reboilers.
TABLE 2.0-I
OPERATING CONDITIONS
PLANTS 14 & 24 – SULFUR RECOVERY
GAVIOTA GAS PLANT
|
Feed
Acid Gas Rate, MMscfd H2S Concentration, vol% CO2 Concentration, vol%
Products Tail Gas Rate, MMscfd Sulfur Production Rate, LTPD Steam, Lb/Hr.
Operating Conditions Selectox Reactor Outlet Temperature, °F Recycle Gas to Acid Gas Feed Ratio, MMscfd / MMscfd
Recycle Blower Throughput, MMscfd
|
Maximum Flow Case
1.68 15.8 76.2
2.20 9.5 3305
700
2.77
4.65 |
Minimum Flow Case
0.34 5.0 87.0
0.38 0.6 310
550
1.27
0.43 |
1Flow rates in table are for one Sulfur Plant
Process Description
Process Objectives
The process objectives of the Selectox Sulfur Plants are to:
· Convert H2S to liquid sulfur
· Minimize the sulfur emissions to the environment
· Produce high purity, salable liquid sulfur
H2S is a poisonous, flammable gas that is removed from the produced gas in the Amine Plants. The H2S cannot be vented to atmosphere because it is extremely toxic. Therefore, the H2S from the Amine Plants is fed to the Selectox Sulfur Plants, which convert most of it into liquid elemental sulfur. Any sulfur compounds remaining in the Sulfur Plant tail gas are removed in the downstream Tail Gas Plant.
H2S and SO2 react to form sulfur and water in a Claus reaction. To convert as much of the H2S as possible, the ratio of H2S to SO2 should be 2:1 by volume. Since no SO2 is present in the acid gas feed, one-third of the incoming H2S must be converted to SO2 in the Selectox converter.
The Selectox Sulfur plant must operate properly to avoid:
· The release of poisonous gases to the environment
· Violation of air pollution control regulations
· A possible shutdown of all or part of the Gas Plant
Process Discussion
In this section, we discuss the important functions of the specific areas of the Selectox Sulfur Plant.
Feed Section
The acid gas feeding the Sulfur Plants can come from either Amine Plant. The plants are connected to allow Plant 14 to process acid gas from Plant 11 or 21. Similarly, Plant 24 can process acid gas from Plant 11 or 21. Both Sulfur Plants will not be fed from the same Amine Plant nor will two Amine Plants feed the same Sulfur Plant simultaneously.
The acid gas first enters the Selectox Sulfur Plant through the acid gas knockout drum, V-1415 which collects any liquids (water, hydrocarbons, or amines) that are carried over from the Amine Plant. The acid gas could contain these liquids during upset conditions in the Amine Plant. The drum is sized to handle large slugs of liquid and to prevent liquid entrainment from the drum to downstream equipment. This protects the catalyst in the Selectox converter and Claus converters from damage or deactivation. The liquids that are collected are pumped by the sour water pumps, P-1415 A and B, to either amine flash drum, V-1112 or V-2112, in the Amine Plants.
Downstream of V-1415, the acid passes through a Peco filter/separator, F-1415, which removes any liquid hydrocarbons that were not knocked out in the drum. Liquid hydrocarbons must be removed. Otherwise they could form coke (carbon) deposits on the Selectox catalyst and the catalyst could lose its activity. F-1415 is a two-stage filter. The first state consists of a 1-micron coalescer filter; the second stage consists of a wire mesh demister. Condensed liquid collected in F-1415 drains by gravity to P-1415 where it is pumped to the amine flash drums.
Acid gas from F-1415 is heated by the preheater, E-1413, to about 345°F and then mixed with process gas recycled from the first condenser, E-1420. The acid gas is heated to 345°F to prevent sulfur condensation in the lines after the acid gas and the recycle gas are mixed. The recycle gas is saturated with sulfur vapor at the exit temperature of E-1420 the first condenser (330°F design).
The combined gas stream of acid gas and recycle gas is further heated to 375°F in Reheater No. 1, E-1415. This is the minimum temperature necessary to initiate the reaction in the Selectox converter. Process air is added to the hot process gas stream which then passes through a static mixer, M-1415, to ensure complete mixing. The process gas/air mixture then enters the Selectox converter.
Selectox Converter Section
Process gas enters the Selectox converter, R-1420A, at 375°F. The Selectox converter oxidizes H2S to SO2. At the same time, the H2S and the newly formed SO2 react to form elemental sulfur and water vapor. These reactions are highly exothermic; therefore they increase the temperature of the process gas significantly. The maximum practical temperature for the Selectox reactor is 700°F. Operating below this temperature prevents excessive corrosion of the carbon steel vessel, takes advantage of the temperature effect on the Claus reaction, and keeps the catalyst from losing its selectivity. The Selectox converter outlet temperature is controlled by adjusting the amount of gas recycled from the first condenser back to Reheater No. 1, E-1415. Increasing the recycle rate lowers the outlet temperature. Decreasing the recycle rate increases the outlet temperature. During turndown operation, the outlet temperature of the converter is lowered to 550°F to allow the recycle blower to run above its minimum rate.
The process gas exits the Selectox converter and enters the first sulfur condenser, E-1420. The process gas is cooled to 330°F and the sulfur vapor that was formed in the Selectox converter condenses. The liquid sulfur flows out of the condenser tubes and drops to a bottom drain. The sulfur then passes through a sulfur seal and look box and into the sulfur pit. About 76% of the sulfur recovered in the Sulfur Plant is recovered in the first condenser. The process gas flows through a coalescer section fitted with a wire mesh demister pad that removes liquid sulfur entrained in the gas.
As the process gas is cooled on the tubeside of E-1420, 50 psig steam is generated from boiler feed water on the shellside. This steam is used in the Amine Plant regenerator reboilers.
A fraction of the gas exiting the first condenser goes through the recycle blower, K-1420, and is recycled back to the acid gas line feeding Reheater No. 1, E-1415. The discharge of the recycle blower could contain some condensed sulfur. A drain and sulfur seal on the blower discharge collects the condensed sulfur and routes it back to the sulfur rundown line of the first condenser, E-1420.
Claus Converter Section
The process gas exiting the first condenser is reheated to 430°F in E-1425. This prevents the sulfur vapor from condensing in the downstream converter bed. The gas passes through the first Claus converter, R-1420B, where the H2S and SO2 in the gas react in the Claus reaction to form sulfur and water vapor. Since the reaction is exothermic, the process gas temperature increases to about 460°F.
The process gas stream then passes from the converter to the second sulfur condenser, E-1430, where the sulfur vapor is condensed by cooling the stream to about 330°F. E-1430 operates similarly to E-1420. Steam generated shellside goes through the same header as steam for E-1420 and then through a common pressure controller. About 19% of the sulfur recovered in the Sulfur Plant is recovered in E-1430.
The process gas exiting the second condenser is reheated to 420°F in E-1435. This again prevents the sulfur vapor from condensing in the downstream converter bed. The gas passes through the second Claus converter, R-1420C, where the H2S and SO2 in the gas again react in the Claus reaction to form sulfur and water vapor. The process gas temperature increases to about 425°F.
The process gas stream then passes from the converter to the third sulfur condenser, E-1440, where it is cooled to about 280°F to condense the sulfur vapor. As in the first two condensers, the liquid sulfur goes through a drain, a sulfur seal, and a rundown line to the sulfur pit. About 5% of the sulfur recovered in the Sulfur Plant is recovered in E-1440. Heat is removed from the process gas by generating 15-psig steam. The generated steam is cooled and condensed in the steam condensers, E-1443A and B. The condensed steam is then fed back to the shellside of E-1440 to remove more heat. The process gas leaving E-1440 flows through a coalescer section fitted with a wire mesh demister pad to remove entrained liquid sulfur. The gas exiting the condenser flows to the Tail Gas Plant where it is burned and scrubbed to remove the remaining sulfur compounds.
Therminol System
Therminol 66 is used as the heat transfer medium in all three reheaters and the preheater. It is a synthetic fluid with physical properties that make it a good heat transfer fluid (i.e., high heat capacity, low viscosity, and good thermal stability). Therminol 66 has an operating temperature range of 15-650°F.
The Therminol is circulated through a closed-loop system by the Therminol pumps, P-1450A and B. Therminol is heated in a fired heater, F-1450, from about 469°F to 500°F. A constant flow rate of Therminol is maintained through the fired heater and the circulation pump to protect the equipment and to prevent high temperatures in the heater tubes. The Therminol then flows to all three reheaters and the preheater. These exchangers are all piped in parallel with respect to the Therminol flow. The Therminol is cooled by exchanging heat with the process gas in the reheaters and the preheaters. It then flows back to the circulation pump to begin the loop again.
A Therminol expansion drum, V-1450, handles the variations in liquid volume as the Therminol goes from ambient to operating temperature. The drum is blanketed with fuel gas to prevent oxidation of the Therminol. It is located at the highest point in the closed-loop system so air or other non-condensibles can be vented. Also, steam can be vented from V-1450 during startup of the Therminol system.
Sulfur Degassing and Storage
The degassing facilities for each Sulfur Plant are designed to process 9.5 LTPD of sulfur. There are three degassing stages to reduce the H2S concentration from 300 ppmw to less than 5 ppmw. Maximum H2S concentration in the pit vapor space is kept below the lower explosive limit of H2S (3.4 vol%), even during startup conditions. With a three-stage system, if one stage fails (e.g., circulation pump failure), the remaining two stages will still degas the sulfur to less than 10 ppmw H2S. This is adequate for short periods of time.
Product sulfur from the Selectox Plant enters the sulfur pit at 300°F and 300 ppmw H2S. The sulfur pit contains three separate compartments or stages in series for continuous degassing, plus a sulfur product storage compartment. The liquid sulfur passes through overflow and underflow baffles between the degassing compartments to prevent bypassing. In each compartment, the sulfur is circulated at a rate of 20-gpm with a sump pump. The circulated sulfur is sprayed into the air space above each degassing compartment through a full-cone spray nozzle. This spraying action agitates the liquid and releases H2S by breaking down the polysulfides in the liquid sulfur (most of the “dissolved” H2S in liquid sulfur is in the form of polysulfides). The residence time in each stage is eight hours. From the third degassing compartment, product sulfur overflows into the storage compartment. This provides seven days of surge volume. From the storage compartment, the degassed sulfur is pumped to the truck loading rack.
The degassing compartments and storage compartment are kept at 280°F by steam coils located just above the pit floor. The lowest viscosity of liquid sulfur is at about 280°F. Also, lower sulfur temperatures improve degassing and prolongs the life of the sulfur pumps. The pit temperature should never be allowed to exceed 310°F. Above this temperature, the viscosity of liquid sulfur increases dramatically, which makes pumping very difficult.
The pit temperature should be kept around 280°F to minimize sulfur vapor in the pit vapor space. This helps prevent sulfur condensation in the sweep gas outlet line.
A continuous air sweep in the vapor space above the liquid sulfur keeps the H2S concentration below the lower explosive limit (3.4 vol % H2S). The air sweep is produced by pulling air through the pit vapor space with a stream-jet eductor. This eductor draws air from a safety hood at the truck loading rack. The air sweep not only lowers the H2S content of the pit vapors, it also collects fumes given off while loading sulfur into trucks at the loading rack.
Equipment Description
The major equipment in Plants 14 and 24 is discussed in this section in the logical order of a plant walkthrough. The equipment in Plants 14 and 24 is identical. All references and discussions concerning equipment in Plant 14 apply to Plant 24 also.
Acid Gas Knockout Drum
The acid gas knockout drum, V-1415, is a standard Chevron knockout drum design. It measures 3 ft. 10 in. by 11 ft. 8 in. seam-to-seam. A tangential feed entry nozzle improves vapor-liquid separation. Centrifugal action throws liquid against the vessel wall. Vapor travels up the center of the vessel and out a nozzle in the top head, while liquid runs down the wall and collects in the bottom of the drum. There is a donut baffle ring between the tangential inlet and the bottom tangent line to reduce turbulence caused by liquid running down the wall. Another baffle ring is located 1 ft. below the top tangent line to prevent liquid from creeping up the wall if large slugs of liquid are carried into V-1415. The top baffle ring is welded to the wall of V-1415, but there is about 1 in. of space between the lower ring and the wall to allow room for liquid to pass through. The liquid outlet nozzle has a vortex breaker to prevent vortexing during liquid pump out. A pressure relief valve connected to the top head protects V-1415 and surrounding equipment from over-pressuring. The drum is designed for 50 psig and 650°F. The material of construction of the vessel is carbon steel.
Filter Separator
The filter separator, F-1415, is a Peco Series 75H two-stage filter. The first stage consists of seven tube-shaped, 1-micron mesh filter elements connected to a tubesheet. The acid gas enters the shellside of the first stage and passes through the wall of the filter elements. The acid gas then flows through the tubeside of the elements, past the tubesheet, and into the second stage. The second stage is a wire mesh mist eliminator. F-1415 has a hinged shell cover to allow water washing or change-out of the filter elements when the pressure drop across F-1415 gets too high. The filter vessel is designed for 50 psig and 650°F. The material of construction of the vessel is carbon steel.
Air Blowers
The air blowers, K-1415A and B, are Lamson Model 5110 blowers. They are identical ten-stage blowers (one blower is a spare). The design capacity is about 31,000 scfh at the design suction and discharge pressures of -0.2 psig and 9.5 psig, respectively. K-1415 requires a throughput of at least 50% of design capacity to operate properly. To get below 50% turndown, each blower comes equipped with flow element in the suction line and an atmospheric vent with flow control valve in the discharge line. When process air demand is below minimum blower throughput, the flow controller keeps blower throughput above 50% of capacity by venting the extra air to the atmosphere.
Recycle Blowers
The recycle blowers, K-1420A and B, are Buffalo-Forge Model 45-R Type 3 blowers. They are identical single-stage blowers (one blower is a spare). Each blower has a design capacity of about 200,000 scfh at normal suction and discharge pressures of 5.5 psig and 7.3 psig, respectively. K-1420 has to operate over a wide range of gas rates (10-100% of design), so it has a variable-speed motor. The Selectox bed temperature controller, TIC-14173, adjusts K-1420 speed to keep the Selectox bed temperature at the set point. Besides providing Selectox converter temperature control during normal operations, K-1420 is used during startup, shutdown, and standby operation to keep gas circulating through the plant. During normal operation, K-1420 takes suction from the outlet of E-1420. During startup, shutdown, and standby operation, the blower also takes suction from E-1440. In these cases, tail gas is completely recycled back through K-1420 to the front end of the plant.
Since the blowers handle gas that contains sulfur vapor at its dew point, K-1420A and B have a special mechanical design. The blower, as well as the piping to and from the blower, are steam traced.
The instrumentation around the blowers is purged with hot instrument air to prevent sulfur condensation in the leads. The discharge line has a rundown section with a seal leg that drains any accumulated condensed sulfur back to the sulfur seal of E-1420.
Preheaters and Reheaters
The preheater, E-1413, and the three reheaters, E-1415, E-1425, and E-1435, heat up the process gas before it enters each converter bed. Process gas entering each converter must be hot enough to initiate the reactions and prevent condensation of product sulfur within the converters. Process gas flows through the shellside of the heaters and hot Therminol flow through the tubes.
Each exchanger has a U-tube bundle with ¾-in. OD tubes. E-1415’s tubes are laid out on a 1-½ -in. square pitch. E-1413, E-1425, and E-1435 are all laid out on a 1-¼-in. square pitch. The tubes are made of carbon steel. The shells are all one pass and made of carbon steel. Each exchanger is designed for 75 psig pressure on the shell, 100 psig pressure in the tubes, and 650°F on both shell and tubes.
Selectox and Claus Converters
The Selectox and Claus converters are located in a common vessel, but they are completely separated by metal walls. This vessel is 7 ft. ID and 16 ft. tangent-to-tangent. The Selectox converter section is 7 ft. long and the Claus converter sections are each 4 ft. 6 in. long. The vessel has a design pressure of 50 psig and a design temperature of 725°F. The vessel shell is made of carbon steel.
Selectox Converter
The Selectox converter, R-1420A, is the first compartment of the converter vessel. It is the largest compartment of the converter, extending from one end of the vessel to a point 7 ft. 0 in. from the tangent line. The Selectox catalyst bed is 30 in. deep and centered in the converter. The catalyst sits on a 3 in. layer of 0.25 in. ceramic balls, and below that layer is a 3 in. layer of 0.5 in. balls. The 0.5 in. balls sit on a support grating covered with 6 mesh stainless steel screen. Above the catalyst is a 3 in. layer of 0.5 in. balls. R-1420A is lined with a 2 in. layer of refractory up to height 6 in. above the top of the catalyst.
Process gas enters R-1420A through a 14 in. nozzle on the top of the vessel and exits through a 20 in. nozzle on the bottom of the vessel. Seven thermowells extend into the Selectox catalyst bed from the side of the vessel. There are two 24 in. manways on R-1420A, one above the catalyst bed and one on the side of the vessel.
Claus Converters
The Claus converters, R-1420 B and C, are the second and third compartments of the converter vessel. Each is 4 ft. 6 in. long (R-1420 C also includes the dished head volume). Each has a 42 in. layer of Claus catalyst centered in the converter. A 24 in. half-pipe runs lengthwise along the bottom of each Claus converter. The half-pipes are perforated (about 40% open area) and covered with 6 mesh stainless steel screen. The space between the half-pipe and the catalyst bed is filled with catalyst support. The catalyst support is just large Claus catalyst (0.5 in. by 0.25 in. for the support versus 3 x 6 mesh for the normally sized catalyst).
Process gas enters R-1420B and C through 10 in. nozzles on top of the vessels and leaves through 10 in. nozzles on the bottom of the vessels. Five thermowells extend into each Claus catalyst bed from the side of the vessel. Like R-1420 A, R-1420 B and C each have two 24 in. manways, one above the catalyst bed and one on the side of the vessel.
Sulfur Condensers
Sulfur condensers, E-1420, E-1430, and E-1440, are fixed tubesheet heat exchangers with one tube pass. The purpose of each condenser is to cool the process gas from each converter stage by generating steam on the shellside. Most of the sulfur vapor formed in the upstream converter condenses and drains out the bottom of the condenser. Process gas flows through the condenser tubes. Cooled process gas and condensed sulfur separate from one another in the outlet channel. Each condenser is sloped to facilitate draining the sulfur. Process gas leaves through a nozzle on top of the channel, and condensed sulfur drains through a nozzle on the bottom of the channel. To minimize entrainment of liquid sulfur mist in the gas stream exiting each condenser, there is a 6 in. thick stainless steel demister pad just before each gas exit nozzle.
Steam is generated on the shellside of the condensers. The tubes are completely submerged in boiling water at all times. Water disengages from the generated steam in the space between the top tube and the top of the condenser shell. (This is a 19 in. space in E-1420 and a 17 in. space in E-1430 and E-1440). The blowdown from the condensers’ shellside goes to the steam blowdown drum, V-1430. The steam form E-1420 and E-1430 goes to a common pressure controller then into the 50 psig steam header. Steam from E-1440 goes to an overhead air cooler where the steam is condensed and returned to E-1440’s shellside.
The steam blowdown drum is a 30 in. ID by 6 ft. vessel, seam-to-seam. The vessel is made of carbon steel. The design pressure is 50 psig, and the design temperature is 650°F.
Sulfur Seals
Since the process gas flowing through the sulfur condensers is under pressure (2-6 psig), a liquid sulfur seal is required to keep the gas from blowing into the sulfur pit while allowing condensed sulfur to drain to the pit. Each sulfur seal is a pot of liquid sulfur 13 ft. deep. A sulfur drainpipe extends from the condensed sulfur outlet nozzle down the center of this sulfur pot. Liquid sulfur runs down through the center drain pipe and back up through the annular space between the drainpipe and the seal pot. The seals and piping to and from the seals are steam jacketed. When the sulfur gets to the top of the seal pot, it flows through an overflow nozzle and rundown line to the pit. Sulfur seal pots 13 ft. deep will prevent process gas from blowing through up to 10 psig gas pressure. Gas pressure should never reach 10 psig because high-pressure shutdown switch, PSH-14131, will shut down the Sulfur Plant at 8.5 - 9.0 psig.
Therminol Heater
F-1450 is a fuel gas fired heater with a design duty of 1.6 MMBH. It is designed to heat 225 gpm of Therminol from 570°F to 600°F. However, during normal operation, the Therminol inlet and outlet temperatures are 469°F and 500°F, respectively. F-1450 is shaped like a horizontal cylinder. Therminol flows through a helical (spiral-shaped) coil within the heater. The fuel gas burner is at one end of the heater. The burner flame is aimed down the center axis of the heater and Therminol coil. The hot combustion gas from the burner goes down through the axis of the Therminol coil, back along the outside of the coil, and out through an atmospheric vent stack. An air blower, K-1450, supplies combustion air for F-1450.
Therminol Expansion Drum
V-1450 is a horizontal vessel 3 ft. diameter by 9 ft. 8 in. long. It provides surge volume to accommodate changes in the liquid volume of the Therminol system. It is the highest point in the Therminol system, so it also acts as a high point vent for the Therminol system. V-1450 has a display system temperature indicator, a field level gauge with high and low level alarms, and a fuel gas blanket with pressure controls to keep blanket pressure at 2-3 psig.
The vessel is made of carbon steel. The design pressure is 50 psig and the design temperature is 650°F.
Sulfur Pit and Loading Rack
The sulfur pit is made up of essentially two sections: the degassing sections and the storage section. Each of the three degassing sections is a 4 ft. by 6 ft. by approximately 6 ft. deep. This corresponds to an eight-hour residence time in each compartment at design sulfur production. The storage section of the pit measures 13 ft. by 13 ft. 6 in. by approximately 12 ft. deep. This corresponds to a seven-day residence time.
Each degassing compartment has a 20 gpm pump that pumps liquid sulfur through a full-cone spray nozzle, which sprays the sulfur downwards into a laterally moving air stream (sweep air). The three compartments are in series separated by an underflow and an overflow weir to prevent sulfur from bypassing a stage.
The storage section of the pit has two 60 gpm sulfur transfer pumps, P-1410 A and B. The bottom of the pit is sloped to these transfer pumps to aid in pumping out the pit on a long-term shutdown.
All sections of the pit have steam coils along the bottom to keep the sulfur in a molten form and to melt sulfur after a shutdown. The coils are made of 304L stainless steel while the risers are made of Alloy 20. Steam at 50 psig is used in the steam coils to keep the sulfur in its pumpable range at around 300°F.
The floor, walls, and roof of the pit are made out of special, sulfur-resistant concrete. There is an explosion hatch on the roof of the pit that will relieve the pressure in the pit in the event of an explosion. This protects the pit structure. The pit is also provided with snuffing steam connections to snuff out any sulfur fires that occur in the pit.
The pit has an air inlet stack through which air is drawn to sweep the pit of degassed H2S. During sulfur loading at the sulfur loading rack, Plant 73A, air will be drawn from the loading rack to minimize sulfur mist emissions there. When the loading rack is not in use, air will be drawn in through a vent stack. The air is drawn into and across the pit by a steam eductor that uses superheated 80 psig steam as its motive fluid. The eductor discharges the steam/sweep air mixture into the tail gas incinerator, F-1500, where all the degassed H2S is burned.
There is also an emergency vent stack that allows venting the sulfur pit to atmosphere if the steam eductor fails. Sulfur degassing should stop immediately when the emergency vent stack is opened. Otherwise, there could be large quantities of H2S vented to the atmosphere, which would be a serious personnel hazard.
Sulfur Loading Rack
The sulfur loading rack, Plant 73A, is used to load sulfur from the pit storage compartment onto sulfur tank trucks. Each truck holds approximately 20 long tons of sulfur or the equivalent of one day’s production from two sulfur plants. The loading rack is sized to load a truck in approximately 45 minutes. The sulfur flows from the storage pit to the loading rack at 60 gpm. The rack has a safety hood, which pulls away fumes given off while loading the truck. A grounding cable connected to the tank truck during loading prevents sparks, which could cause sulfur fires.