This section presents the basis for the design of the Tail Gas Treating Plants. The plant design is based upon four different feed rate and composition cases to cover possible plant operation. These different feed cases, as well as the product specifications and waste streams, are discussed in detail below.
The Tail Gas Plant processes the following waste gas streams:
The Tail Gas Plants are not designed to handle tank vents or emergency relief. The rate, composition, and conditions of waste gas feeds to the Tail Gas Plants can be found in Table 2.0-I.
There are four different feed cases for the Tail Gas Treating Plant. These cases are summarized in Table 2.0-II and outlined below:
Case I – Maximum Flow: This case is based on the maximum volumetric throughput of gas, 92% sulfur recovery in the Sulfur Plant, and a 1600°F operating temperature in the incinerator. This would occur when Sulfur Plant recovery efficiency is low at maximum Tail Gas Plant throughput. This could happen anytime, but it is most likely when the catalyst in the Sulfur Plant converters is deactivated. The incinerator is designed to operate at temperatures as high as 1600°F. This was the design case for these pieces of equipment:
This equipment would be the bottlenecks if we attempt to increase the plant’s throughput beyond this point.
Case II – Normal Operation: This case is based on maximum volumetric throughput of gas, 95% sulfur recovery in the Sulfur Plant, and a 1380°F operating temperature in the incinerator. This case occurs during normal operation of the Sulfur Plant with maximum flow. The same amount of gas is fed to the incinerator as in Case I, but there is less sulfur in the tail gas since the sulfur recovery efficiency is higher. This case reflects normal operation but it is not the design case for any equipment or lines in the Tail Gas Plant.
Case III – Maximum Sulfur: This case is based on the maximum gas flow, which can bypass a Sulfur Plant. This occurs when the acid gas flow is slightly less than the minimum acid gas flow required to operate one of the Sulfur Plants (roughly 20% of the design rate). Acid gas from the amine regenerator is processed directly in the Tail Gas Plant. This will occur during initial operation of the Amine Plant with low throughput. This was the design case for these pieces of equipment:
This equipment would be the bottlenecks if we attempt to increase the plant’s sulfur handling capacity beyond this point.
Case IV – Minimum Flow: This case is based on the first well production. As in Case III, there is not enough acid gas to operate a Sulfur Plant; so acid gas is sent directly to the Tail Gas Plant. This is the least amount of gas that the Tail Gas Plant will have to process. This was the design case for:
The design rates of sweep gas from the sulfur pit degassing facilities, and the vent streams from the Liquids Treating Plant, are the same for all four cases. These streams, which are predominantly air, may have lower rates and cause an increase in the air blower requirement for the incinerator. The blower will have to provide air to the incinerator that was once provided by the sweep gas and the vent streams.
The major “product” of each Tail Gas Plant is clean stack gas. At design gas and liquid rates, the stack gas will contain less than 50 ppmv SO2 and trace amounts of SO3. This SO2 concentration will meet environmental regulations for SO2 emissions. The maximum stack gas rate from each plant is 5.55 MMscfd at 160°F and occurs in Case I. The minimum stack gas rate is 0.67 MMscfd and occurs during Case IV.
Part of the solution used to scrub SO2 out of the flue gas must be purged from the circulating stream. This prevents buildup of salts in the system. The maximum purge rate is 13.0 gpm and occurs in Case III. The minimum purge rate is 1.4 gpm and occurs in Case IV. The purged solution has a specific gravity of roughly 1.13. It contains approximately 10% dissolved solids (i.e., NaOH, Na2SO3, NaHSO3, Na2SO4). The pH of the purge stream is 6.8.
The Tail Gas Plant also produces as much as 5800 lb/hr. of 350 psig steam. The waste heat boiler recovers heat from the incinerator flue gas. This steam is let down to 80 psig and is used in the Gas Plant.
TABLE 2.0-I
WASTE GAS FEEDS
PLANTS 15 AND 25 – TAIL GAS TREATING
GAVIOTA GAS PLANT
|
1 Waste Feed |
Tail Gas from Plants 14 and 24 Sulfur Recovery |
Eductor Effluent From Sulfur Degassing, Plants 14 and 24 Sulfur Recovery |
Process Vent Streams from Plant 17 6 Merox Treating |
|||||
|
2 Case I Maximum Flow |
3 Case II Normal Operation |
4 Case III Maximum Sulfur |
5 Case IV Minimum Flow |
From Caustic Settler V-1760 |
From Wash Water Degasser V-1780 |
From Caustic Degasser V-1790 |
||
|
7 Component Flow Rate, Lb/Hr Nitrogen Oxygen Water Hydrogen Sulfide Sulfur Dioxide Carbon Dioxide Sulfur S8 - Vapor S - Liquid Carbon Disulfide Carbonyl Sulfide Hydrocarbon Mercaptans
Total Lb/Hr MMscfd
Temperature, °F Pressure, psig |
1537 0 745 44 42 6190
5 8 Trace 5 30 14
8620 2.20
280 2.5
|
1537 0 755 25 23 6190
5 8 Trace 5 30 14
8592 2.20
280 2.5 |
Trace 0 46 199 0 1238
0 0 Trace 1 6 3
1493 0.34
120 2.5 |
Trace 0 5 21 0 128
0 0 Trace Trace 1 Trace
155 0.04
120 2.5 |
670 203 527 Trace 0 0
1 Trace 0 0 0 0
1401 0.54
357 2.5
|
12 3 2 0 0 0
0 0 0 0 10 0
27 0.01
120 2.5 |
1 1 Trace 0 0 0
0 0 0 0 0 0
2 Nil
120 2.5 |
303 91 27 0 0 0
0 0 0 0 0 0
421 0.14
120 2.5 |
1 Tail Gas Treating is not designed to handle tank vents or emergency relief
2 Case I – Maximum Flow – corresponds to maximum throughput and 92% sulfur recovery in the sulfur plants
3 Case III – Normal Operation – corresponds to maximum throughput and 95% sulfur recovery in the sulfur plants
4 Case III – Maximum Sulfur – represents a maximum turndown case for the sulfur plants. For this case, there is slightly less acid gas than required to operate one of the sulfur plants. Acid gas from one of the acid gas removal plants, 20% of the design rate, bypasses the sulfur plants and is sent directly to one of the Tail Gas Plants.
5 Case IV – Minimum Flow – corresponds to first well operation with acid gas bypassing the sulfur plants
6 Rates shown for process vent streams are design rates. The vent from the caustic settler is continuous; the vents from the wash water degasser and caustic degasser are intermittent.
7 Tail Gas Treating consists of two plants in parallel—Plants 15 and 25. Flow rates shown in this table are the feed rates per plant.
TABLE 2.0-II
OPERATING CONDITIONS
PLANTS 15 AND 25 – TAIL GAS TREATING
GAVIOTA GAS PLANT
|
Feeds to Plant Acid Gas Feed Rate, MMscfd Tail Gas Feed Rate, MMscfd Sulfur Feed Rate, LTD 5 Sulfur Pit Sweep Gas, MMscfd Liquids Treating Vent Gas, MMscfd
Operating Conditions Incinerator Operating Temperature, °F SO2 Concentration to Scrubber, ppm 25° Bč Caustic Injection, GPM Solution Temperature, °F
Products from Plant Stack Gas Rate, MMscfd Solution Purge Rate, gpm SOx Emissions to the Atmosphere, Lb/Hr.
|
Case I |
Case II |
Case III |
Case IV |
|
1 Maximum Flow |
2 Normal Operation |
3 Maximum Sulfur |
4 Minimum Flow |
|
|
0 2.20 0.67 0.54 0.01
1,600 5,000 1.7 160
5.55 6.0 2.8 |
0 2.20 0.38 0.54 0.01
1,350 4,000 1.2 160
4.71 4.1 1.1 |
0.34 0 2.0 0.54 0.01
1,350 37,500 3.6 180
1.49 13.0 0.7 |
0.04 0 0.21 0.54 0.01
1,350 9,200 0.4 190
0.67 1.4 0.2 |
1 Case I – Maximum Flow: Corresponds to maximum throughput, 1600°F incinerator operating temperature, and 92% sulfur recovery in the Sulfur Plants.
2 Case II – Normal Operation: Corresponds to maximum throughput, 1350°F incinerator operating temperature, and 95% sulfur recovery in the Sulfur Plants.
3 Case III – Maximum Sulfur: Represents a maximum turndown for the Sulfur Plants. For this case, there is slightly less acid gas than required to operate one of the Sulfur Plants. Acid gas from one of the Acid Gas Removal Plants, 20% of the design rate, bypasses the sulfur plants and is sent directly to one of the Tail Gas Plants.
4 Case IV – Minimum Flow: Corresponds to first well production with acid gas bypassing the Sulfur Plants.
5 Sulfur fed to plant in the form of H2S or SO2 in the tail gas or acid gas.
The Tail Gas Treating plant is the final cleanup step before venting the gas plant’s waste gas streams. For this reason, it is imperative to operate this plant well to minimize SO2, NOx, SO3, and volatile organic compounds (VOC) emissions. The Tail Gas Plant processes streams from the Sulfur Plant (or the Amine Plant), from the Liquids Treating Plant, from the VRU, and from the sulfur pit degassing facilities. Most of these streams contain H2S and other sulfur compounds. These must be removed from the gas before the gas is safe to emit to the atmosphere.
The Tail Gas Plant removes the sulfur compounds by first oxidizing them to SO2 in an incinerator and then scrubbing the SO2 out of the gas using a sodium sulfite/sodium bisulfite solution. The scrubber will reduce the SO2 concentration in the gas streams to less than 50-ppmv.
The following discussion will refer only to Plant 15, but Plant 25 will operate the same way.
The Tail Gas Plant treats the following streams:
Tail gas from the Sulfur Plants or acid gas from one of the Amine Plants
Sweep gas from the sulfur pit degassing facilities in the Sulfur Plants
Gas from the VRU
Process vents from the following vessels in the Liquids Treating Plant
The three vent streams from the Liquids Treating Plant come into the Tail Gas Plant through a single line. They immediately enter the vent gas knockout drum, V-1500, which removes any entrained or condensed liquids. Liquid from the knockout drum is pumped back to the caustic degasser, V-1790, in the Liquids Treating Plant. A single vent gas knockout drum is common to both Tail Gas Plants. The overhead gas can go to either plant or both.
The vent gas from V-1500 passes through a flame arrestor, M-1501. Steam is injected into the line upstream of the flame arrestor. The steam ensures that the gas velocity in the flame arrestor is above 20 ft/sec., which prevents flame propagation from the incinerator into the waste gas system.
Sulfur Plant tail gas (or Amine Plant acid gas) combines with sweep gas from the sulfur degassing facilities and passes through another flame arrestor, M-1500. Again, steam is injected into the line upstream of the flame arrestor. Enough steam is injected to prevent flame propagation from the incinerator to the Sulfur Plant or the sulfur pit.
Fuel gas for the incinerator comes from both the plant’s fuel gas header and the vapor recovery unit (VRU). The fuel gas passes through a knockout drum, V-1599, before entering the incinerator. The combustion air blower, K-1500, provides the necessary air to the incinerator.
The waste gas streams pass through the flame arrestors to the incinerator, F-1500. In the incinerator, the gases are burned at 1350°F. The incinerator operates at 25% excess air and approximately a 1.0-sec. residence time. During the normal operation, about 0.10 MMscfd of fuel gas is added to the incinerator to make sure all sulfur compounds are burned. Much less fuel gas is required when acid gas bypasses the Sulfur Plant and goes directly to the Tail Gas Plant. The heating value of the acid gas is higher than the heating value of the tail gas.
Flue gas from the incinerator passes through the waste heat boiler, E-1510, where it is cooled to 450°F. The heat recovered from the flue gas is used to generate 3850 lb/hr. of 350 psig steam. This steam pressure was chosen to keep the tube wall temperatures above the sulfuric acid dew point. The flue gas contains approximately 300 ppmv SO3 so the estimated sulfuric acid dew point is 370°F (the temperature of 350 psig steam is 436°F). Severe corrosion will take place if sulfuric acid does condense on the tubes.
The steam drum of the waste heat boiler, V-1510, is filled with boiler feed water by the boiler feed water booster pumps, P-1510A and B. The continuous blowdown goes to the continuous boiler blowdown drum, V-1501, and then to the Oil Plant. The intermittent blowdown from the bottom of the steam drum and the bottom of the waste heat boiler goes to the intermittent boiler blowdown drum, V-1502. Utility water is connected to V-1502 so that the blowdown can be cooled off and solids from the drum can be flushed out. V-1502 is flushed to the oily water sewer.
SO2 Scrubber
Cooled flue gas from the waste heat boiler flows through the SO2 scrubber, C-1520, where it contacts a circulating sodium sulfite/sodium bisulfite solution. The scrubber contains a quench section and eight valve trays. It cools the flue gas to 160°F and reduces the SO2 concentration in the flue gas to below 50 ppmv. At the design gas and liquid rates, approximately 85% of the SO3 in the gas will also be scrubbed out.
The pH of the scrubbing solution is kept at 6.8 by adding 25°Bč caustic solution (18.7 wt % NaOH solution) to it. During normal operation, about 4 GPM of the scrubbing solution is purged from the system and goes to disposal. During the maximum sulfur case, this purge rate can be as high as 13.0 GPM. The purge prevents build-up of salts in the solution and keeps the specific gravity of the recirculating solution at 1.13 (10 wt % dissolved solids). The solution can be purged directly to ocean disposal if there is enough produced water for adequate dilution (at least 85:1). However, a tank is provided to store the purged solution if the solution cannot be discharged to the ocean. During normal operation, Case II, with both plants operating, the tank will hold 10 days’ worth of spent solution. During the maximum sulfur case, Case III, with both plants operating, when the purge rate is greatest, the tank will hold three and one-half days’ worth of spent solution. The tank can be emptied into a tank truck for transport to an appropriate disposal site or the waste solution can be sent to an injection well.
The caustic feed and preparation system consists of three holding tanks: one for 50°Bč caustic, T-1580; one for process water, T-1570; and one for caustic mixing, T-1590. The 50°Bč caustic tank is filled from tanker trucks. The process water tank is filled with process water and with wash water from the wash water degasser, V-1780, in the Liquids Treating Plant. A 25°Bč caustic solution is prepared by diluting the 50°Bč caustic with process water in the caustic mixing tank. The 25°Bč caustic is then pumped to the SO2 scrubber to regenerate the scrubbing solution. Caustic and water from this system is also fed to the Liquids Treating Plant (Plant 17) and the Amine Plants (Plants 11 and 21).