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LNG FPSO MPDS (Multi-Purpose Dynamic Simulator) PROJECT (4) – Fractionation Unit & Fuel Gas Unit

The Fractionation Unit (F-5000) is to process the heavy hydrocarbons separated from the sweat and dry feed gas in the scrubber column (L-C-4001) of the liquefaction unit (L-4000). The fractionation unit consists of three (3) distillation columns in series, Deethanizer (F-C-5001), Depropanizer (F-C-5002) and Debutanizer

6. Fractionation Unit (F-5000)

6.1 Design Concept and Basis

The Fractionation Unit (F-5000) is to process the heavy hydrocarbons separated from the sweat and dry feed gas in the scrubber column (L-C-4001) of the liquefaction unit (L-4000). The fractionation unit consists of three (3) distillation columns in series, Deethanizer (F-C-5001), Depropanizer (F-C-5002) and Debutanizer (F-C-5003).

The Fractionation Unit produces the deethanizer overhead gas, propane, butane and stabilized condensate. In an actual natural gas liquefaction plant, the ethane and propane are mainly used to make-up the refrigerant of the propane and MR refrigeration systems, but this project assumes that the propane, butane and stabilized condensate produced from the fractionation unit are sent to the pressure boundaries as the LPG chilling, condensate cooling and storage units are not in scope of this project. This battery limit also assumes that the make-up refrigerants (methane, ethane and propane) are provided from the pressure boundary for each refrigerant.

The overhead of the Deethanizer column (F-C-5001) is routed to the Main Cryogenic Heat Exchanger where it will be liquefied in a dedicated low pressure tube circuit. The liquefied overhead product is mixed with the low pressure LNG from LNG expander (L-KG-4002) before entering the end flash vessel (L-V-4003).

The following figure shows the design heat and material balance of the fractionation unit (F-5000).

The Fractionation Unit consists of the following:

  • A Deethanizer column (F-C-5001) with a reboiler (F-E-5001), an LP propane cooled deethanizer condenser (L-E-4033), and associated reflux drum (F-V-5001) and pumps (F-P-5001 A/B).
  • A Depropanizer column (F-C-5002) with a reboiler (F-E-5003), air-cooled depropanizer condenser (F-E-5004), and associated reflux drum (F-V-5002) and pumps (F-P-5002 A/B).
  • A Debutanizer column (F-C-5003) with a reboiler (F-E-5005), air-cooled debutanizer condenser (F-E-5006), associated reflux drum (F-V-5003) and pumps (F-P-5003 A/B), and air-cooled condensate cooler (F-E-5007).

6.2 Deethanizer (F-C-5001) Unit

The Deethanizer column, F-C-5001, is designed to remove the lighter components such as Nitrogen, Methane and Ethane from the feed stream of natural gas liquid from scrubber column (L-C-4001) of Unit L-4000 to ensure the production of on-spec propane in the Depropanizer column, L-C-5002.

The Deethanizer takes feed from the bottoms of the Scrub Column, L-C-4001. The pressure of the feed is reduced over the flow control valve (L-FCV-4004) from 57.2 bara to 25.5 bara, before entering the Deethanizer.

The Deethanizer is design to meet the design specifications which requires the ethane content in the bottom product less than 0.5 mole%. The ethane concentration in the bottom product shall be sufficiently low to allow the production of export grade propane as overhead product from the next column. Based on the product specifications, the deethanizer is designed with 28 trays and 179.6 t/d of the reflux flow rate.

The natural gas liquid feeds to the 23rd tray of the deethanizer column (F-C-5001).

Heat is supplied to the column via the heating medium (steam) system which is treated as boundary to the kettle type reboiler, F-E-5001 and the design heat duty is 9,056 kW.

The column has a rectifying and a stripping section to limit the loss of propane and butane to the overhead product. The column overhead gas is partially condensed against low pressure propane in the Deethanizer condenser, L-E-4033. The liquid from the Deethanizer Reflux Drum, F-V-5001, is pumped by the Deethanizer Reflux Pumps, F-P-5001 A/B, to the column top tray as reflux. The vapour is routed to the Main Cryogenic Heat Exchanger via a dedicated low pressure circuit.

In normal operation, all the condensed overheads are refluxed to the top of column (F-C-5001). However, for operational flexibility, it is possible to route up to 25% of the maximum reflux to the HP fuel gas system through the flow control valve (F-FCV-5004).

Figure 10 shows the overall process of the Deethanizer unit.

6.3 Depropanizer (F-C-5002) Unit

The bottom product from Deethanizer column F-C-5001 is routed straight to the Depropanizer column F-C-5002.

The depropanizer column (F-C-5002) is designed with 38 trays and the butane contents in the overhead product less than 1 mole %. The liquid propane overhead product shall be adequate a refrigerant and export quality. The reflux flow rate is 1,817 t/d at the design operating condition and the feed enters just above tray 21.

The pressure of the feed is reduced over the level control valve (F-LCV-5001) from 25.64 bara to 16.4 bara before entering the Depropanizer (F-C-5002).

The heating medium is supplied to the Depropanizer Reboiler (F-E-5003) and the design heat duty is 6,941 kW to maintain the reboiler operating temperature at 108 °C.

The overhead propane product from the Depropanizer (F-C-5002) is totally condensed in the Depropanizer Condenser (F-E-5004), which is an air cooler. It accumulates in the Depropanizer reflux drum (F-V-5002).  A portion is used as reflux back to the column via the reflux pumps (F-P-5002 A/B), and the remainder is sent to the pressure boundary representing the storage system of an actual plant.

Figure 11 shows the overall process of the Depropanizer unit.

Figure 11 Overall Process of the Depropanizer and Debutanizer Unit

6.4 Debutanizer (F-C-5003) Unit

The bottom product from F-C-5002 is routed straight into the Debutanizer Column F-C-5003. The debutanizer is designed with 34 trays and i-pentane content in the overhead product less than 0.5 mole %. The Debutanizer Column shall be able to produce stabilised condensate at the specified maximum butane concentration, and butanes as an overhead product at the required C5+ specification. The reflux flow rate is 1,102.5 t/d at the design operating condition and the feed enters just above tray 17.

The pressure of the feed is reduced over the level control valve (F-LCV-5002) from 16.57 bara to 5.93 bara before entering the Debutanizer (F-C-5003).

The heating medium is supplied to the Debutanizer Reboiler (F-E-5005) and the design heat duty is 5,764 kW to maintain the reboiler operating temperature at 105 °C.

The Debutanizer Column overheads are totally condensed in the Debutanizer Condenser (F-E-5006), which is an air cooler, and accumulates in the Debutanizer Reflux Drum (F-V-5003).

The Debutanizer Reflux Pumps (F-P-5003 A/B) are used to provide reflux to the column and to send butane product to the pressure boundary representing the storage unit of an actual plant.

The bottom product from the Debutanizer Column is a stabilised condensate which is air-cooled in the Condensate Product Cooler (F-E-5007), and rundown to the pressure boundary representing a condensate storage system of actual plant.

Figure 11 shows the overall process of the Debutanizer.

7. Fuel Gas Unit (U-6000)

The HP fuel gas to the fuel gas KO drum (U-V-6001) is provided by three process units.

  • End Flash Compressor (L-K-4001)
  • Downstream of Feed Gas Heater (A-E-1006)
  • Discharge from Deethanizer Reflux Pump (F-P-5001 A/B)

In normal operation, the HP fuel gas comes from the End Flash Compressor (L-K-4001) and the pressure of fuel gas KO drum is 25.5 bara.

The main consumers of HP fuel gas are the MR compressor gas turbine (L-KT-4001) and Propane compressor gas turbine (L-KT-4002) and the remaining gas flows to the HP fuel gas header. The HP fuel gas feeds to consumers via the HP Fuel Gas Superheater (U-E-6001) which heats the HP fuel gas to 60 °C.

The LP fuel gas to the LP fuel gas header comes from the off-gas of 3-phase separator (D-V-2002) in the dehydration unit (D-2000).

Figure 12 shows the overall process of the Debutanizer.

Figure 12 Overall Process of the Debutanizer

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