In "Closed Cycle" mode, the SW/FW diverter valves stop the supply of water to the scrubber and the discharge of wash water overboard. At the same time, it switches the valves to create a closed circuit for washing the water. In this case, the supply pumps supply the required amount of process fresh water to the nozzles in the jet and absorber sections. This scrubbing water is cooled in a heat exchanger before it enters the sprayers. Cooling water for the heat exchanger is obtained from an external cooling water unit.
In the circulation circuit, most of the water is re-circulated. The scrubbing water from the scrubber is degassed in a circulating tank and the exhaust gas is directed back to the scrubber through a pipe connected to the circulating tank.
Water purification and chemical treatment (caustic soda) are necessary to maintain sufficient washing water quality. An analytical unit with conductivity and pH measurements is used to control this water quality.
Part of the circulating water and / or fresh water may need to be discharged to ensure that the volume and quality of the circulating water does not exceed the specified limits. Evaporation or run-off of the wash water results in excess moisture in the flue gas, which makes it necessary to supplement the wash water with fresh water. If the alkalinity of the wash water (pH <12) decreases, it is necessary to add caustic soda solution (NaOH) to the system using a low power feed pump. Prior to discharge, the wash water is monitored for quality (pH and PAH) by the FW analysis unit.

NOTE:

  • Exhaust gases: the exhaust cloud in conventional wet scrubbers is characterized by a relatively high relative humidity. Offshore washing and cleaning equipment uses a function (Demister filter and heater) to reduce the amount of water vapor and water lost to the atmosphere, and thus the need for water replenishment.
  • Caustic soda: the concentration of the supplied solution reaches 50 %. Its density is 1.52 t/m3, pH value 14. It hardens at 12°C temperature and is normally transported warm. Caustic soda can be transferred from trucks using filling connections at bunker stations. Its storage tank can be made of ordinary steel used in shipbuilding.

Also, in order to save fresh water and space for the wash water in the Holding Tank, the wash water is treated by the Bleed-off Treatment Unit. The sludge extracted during the cleaning process is collected and stored in an on-board Sludge Tank.
The scrubber treatment plant can also be operated as a separate system from the rest of the flue gas cleaning system. By switching the scrubber from closed to open cycle mode, the water purifier can continue to clean in its circulation circuit until the preset time has elapsed.

Exhaust gas cleaning system Closed cycle fresh water (FW) mode

Last modified: Thursday, 26 January 2023, 6:22 PM