Check if the answer is correct regarding the issue of excessive C5 levels in the debutanizing tower of the natural gas processing plant

Attachment 1: Process Overview for Fault Analysis of Light Hydrocarbon Recovery System. The non-condensable gases from the production separators, electric dehydrators, crude oil buffer tanks, and crude oil stabilizers in the crude oil processing system are sent to the desulfurization plant for hydrogen sulfide removal. Thereafter, the associated gas along with the low-pressure gas (0.5 MPa) coming from Pipeline 12 converge and pass through the first-stage inlet separator V-B01, where the liquids contained in the non-condensable gases are separated out; these liquids are then sent to the oily wastewater treatment system. The gas proceeds to compressor C-B02, where it is pressurized at the first stage and cooled by the water cooler HE-B03. Some of the heavier hydrocarbons in the natural gas are separated out in the second-stage inlet separator V-B04. After further compression and cooling by the water cooler HE-B06, all hydrocarbons with a molecular weight of C5 or higher, as well as some C3 and C4 components, are condensed in the second-stage outlet separator V-B07. The gas separated by the export separator V-B07 enters the dehydration unit to merge with the pipeline gas. The heavy hydrocarbons separated from the secondary inlet separators V-B04A/B and the secondary outlet separator V-B07 are heated to 60°C by the heavy hydrocarbon preheater HE-B08, then flashed in the heavy hydrocarbon flash tank V-B09. Subsequently, the flashed heavy hydrocarbons are pumped using a heavy hydrocarbon feed pump to the debutanizing tower in the distillation unit for further processing. Note: The heavy hydrocarbons mentioned here are relative terms; they refer to the heavier components that are separated from the uncondensed gas obtained in crude oil processing systems, and hence are called heavy hydrocarbons. The natural gas from offshore oil fields is transported onto land via 8\" sea pipes and then enters the ball collector PR-B29 and the trap V-B30A. In the trap, the condensate is separated out, and this condensate is sent to the crude oil processing system for treatment. The natural gas coming out of the collector enters the pre-separator V-B31 to further remove liquids and moisture from it; thereafter it goes to the molecular sieves V-B32A/B for dehydration. After that, a dust filter FT-B33 is used to remove impurities from the natural gas, which is then sent to the cold separation system. There are two types of molecular sieves: one for dehydration and one for regeneration. During dehydration, natural gas flows from the top to the bottom; during regeneration, regenerating gas flows from the bottom to the top. The two molecular sieves alternate between dehydration and regeneration. A small stream of natural gas (2600 m3/h) coming out of the dust filter is heated to 300°C in the regenerating gas heater HE-B36, and then used as regenerating gas to regenerate the molecular sieve. The regenerating gas flows from the bottom to the top of the molecular sieve; after being cooled in the water cooler HE-B34, the gas rich in water vapor enters the regenerating gas separator V-B35 where the water is removed. The resulting regenerating gas is then sent to the gas distribution station for use by the turbine units. After dehydration and drying, the natural gas flows in two streams to the primary heat exchanger HE-B37 and the feed gas/fuel gas heat exchanger HE-B38. The natural gas entering HE-B38 exchanges heat with the ethane dry gas coming from the deethanizer, heating the ethane gas to 20°C; at the same time, the natural gas itself is pre-cooled. The flow rate of natural gas entering HE-B38 is adjusted to meet the heating requirements for the ethane dry gas, and a temperature controller TI-B381 is used to regulate this flow rate. The majority of the remaining natural gas flows into HE-B37, where it cools down by exchanging heat with the dry gas coming from the expander. These two streams combine, and their temperature is reduced to 4°C before they enter the propane evaporator HE-M02. There, they are cooled further by the propane refrigeration system, until their temperature reaches –34°C at which point most of the C3 and higher molecular weight components condense. Gas-liquid separation takes place in the primary low-temperature separator V-B40; the liquid portion goes back to the deethanizer, while the gas portion enters the secondary heat exchanger HE-B41 to cool down further by exchanging heat with the dry gas from the expander. After reaching a temperature of –61°C, all components with molecular weights above C3, as well as most of the C2 components, condense. Gas-liquid separation occurs again in the secondary low-temperature separator V-B42; the separated liquid goes back to the deethanizer, while the gas proceeds to the expansion stage of the expansion compressor, where it undergoes throttling expansion to lower its temperature further to –102°C and 0.4 MPa(A). The low-temperature methane dry gas is then used to provide cooling in the secondary heat exchanger HE-B41 and the primary heat exchanger HE-B37, after which it enters the compression stage of the expansion compressor, where its pressure is increased to 0.55 MPa(A). It is then cooled to 40°C using the water cooler HE-B45 before being sent to the gas distribution station, where it serves as fuel gas for heaters and is also supplied to downstream customers such as XinAo and Beiguan. The condensate (-34°C) separated from the primary low-temperature separator (V-B40) enters the deethanizer as the feed to the lower section of the deethanizer (TW-B05), while the condensate (-60°C) separated from the secondary low-temperature separator (V-B42) enters the deethanizer as the feed to the upper section of the same device. The top temperature of the deethanization column is controlled at –20°C, with a pressure of 1.7 MPa. The C1, C2, and C3 fractions separated at the top of the tower are cooled to –32°C in the deethanizer top condenser (HE-M03), after which they enter the deethanizer top reflux tank (V-M06). All of this condensed liquid serves as tower top reflux; it is pressurized to 2.1 MPa by the deethanizer reflux pumps (P-M07A/B) and then sent back to the top of the tower. The non-condensable gas from the tower top reflux drum enters the feed gas/fuel gas heat exchanger (HE-B38), where it is heated; thereafter, it is used as gas for the regeneration of molecular sieves. After regeneration is complete, it is sent to the gas distribution station to serve as fuel gas for the turbine generators. The heat source at the bottom of the tower is provided by the reboiler at the bottom of the deethanizer (HE-M04), with the temperature controlled at 63°C. The bottom stream from the deethanizer flows automatically into the debutanizer as one of its feed streams. The heavy hydrocarbons from the heavy hydrocarbon flash tank (V-B09) serve as the second feed stream to the butane removal tower; the temperature at the top of the tower is maintained at 57°C, while the pressure is kept at 1.2 MPa. The C3 and C4 fractions separated at the top of the tower emerge from there and enter electric heaters (HE-BS01A/B; note: heat transfer oil is used to heat the gas). After being heated to a gaseous state, they proceed to the liquefied gas desulfurization tower (TW-BS02A/B) where hydrogen sulfide is removed. Subsequently, they pass through air coolers (HE-BS03A/B) to cool down, then through the water cooler at the top of the old butane removal tower (HE-B54) for further cooling. After reaching 40°C, they enter the reflux tank at the top of the butane removal tower (V-M11). This condensed liquid is pressurized to 1.6 MPa by the butane removal tower reflux pump (P-M12A/B); part of it is sent back to the top of the butane removal tower as reflux, while the other part is sent to the liquefied gas storage tanks (T-B72A/B) as liquefied gas product (with components above C5 accounting for no more than 3%, and C3 and C4 together accounting for 95% or more). The heat source at the bottom of the tower is provided by the reboiler at the bottom of the butane removal tower (HE-M09), with the temperature controlled at 153°C. The distillate oil from the bottom of the tower enters the unstable light hydrocarbons flash tank (V-B50); the gaseous phase resulting from flashing then enters the flasher condenser (HE-M53) where it is cooled, after which it merges with the liquid phase from the stabilized reflux tank (V-A16) and enters the heavy hydrocarbons flash tank (V-B09). In abnormal situations, it can be directed to the crude oil storage tank system. The liquid phase resulting from the flashing in the unstable light hydrocarbons flash tank enters the bottom heat exchanger (HE-B51) of the butane removal tower to be cooled; thereafter, it goes to the water cooler (HE-B52) where it is cooled to room temperature before being sent as a light oil product to the light oil storage tanks (T-B76A/B). The analysis data of the liquefied gas components entering the tank on a certain day are as follows: Component C1 C2 C3 C4 C5 CO2 CC; Content: 0.39 5.29 54.19 34.42 5.71 0 1a. Questions: (1) What are the problems with the liquefied gas entering the tank? (2) How can it be adjusted to normal operating conditions?

The component in question is C5, and its level in the liquefied gas obtained from natural gas has exceeded the specified limit by 5.71%. C2 is 5.29, while C3 and C4 are approximately 98.61%

Please, someone who understands this topic can explain why clauses 2 and 4 regarding C5 exceeding the limits apply, and whether there are any errors in them. Is it too concise? I misunderstood.

You understand correctly. But in my opinion, a high liquid level at the bottom of the tower has nothing to do with the composition of the material at the top of the tower.

Finally someone answered. I watched it again today; I really don’t understand the second point – I can’t figure it out. Does Article 4 indicate that the pump pressure is high, resulting in less return fluid going back to the tower? But I still don’t understand.

Should answer ② be: The temperature at the top of the butane tower is too high; lower this temperature； Answer ④: The pressure at the top of the butane column is too low, causing an increase in the C5 fraction; therefore, the pressure should be increased appropriately.

Still not quite understanding? ! ! Please provide a detailed explanation