In the last article we took a broad look at the overall Inert Gas system which consisted of the inert gas plant as well as the inert gas distribution system. Now we will study about the scrubber and its role in the overall plant. I would suggest that in case you havent studied about the basic concepts behind the IG plant namely the fire triangle, flammability graph and gas exchange, take a quick look at them before you proceed further. The IG plant might become a mandatory equipment on board vessels not only in tankers but also possibly in other types of ships.
What would you normally do if you have worked very hard and are full of sweat, dust and soot before you attend a party? Obviously you would like to clean yourself with lots of water (soap, shampoo etc) and the same case applies to flue gases. Since flue gases refer to the left over of the combustion process in the boilers and engines they obviously are very hot, have lot of impurities both in the form of suspended particles of soot and other harmful substances such as sulphur dioxide. Hence even though the oxygen content of such a gas could be as low as 5%, it would not be feasible to use it directly for inerting the cargo tanks on the ship.
Hence these gases are thoroughly washed with seawater which is freely available in abundance in the proximity of the ship. The process consists of having a large tall structure known as the scrubber. There is an isolating valve between the boiler uptake and the scrubber tower which serves to isolate the two portions when required for purposes of maintenance.
Sea water flows downwards in the scrubber while the flue gases rise upwards and during this process of cross-flow the sea water dissolves the soot and sulphur dioxide as well as cools the gas. There is an extensive arrangement of spray nozzles, perforated plates and venture nozzles inside the tower to maximize the contact between the water and gases. Therefore it acts like a heat exchanger and the exact design of the scrubber tower could vary depending on the make of the machine and the intended purpose of use.
The working conditions require that the scrubber is made out of highly corrosion resistant material which can withstand the hot corrosive gases that pass through it and there should be ample provision for viewing and cleaning the inside portions of the chamber during maintenance routine.
After the scrubber the gas passes through a demister which removes excess of water from the flue gases which is done using materials such as polypropylene or cyclone dryers. This gas is now ready to be sent to inert gas blowers which pump this gas further to the required regions.
Apart from being used in inert gas systems, scrubbers would become compulsory in ships in the coming days once relevant IMO regulations take effect. Currently the Marpol Act and relevant European Union directives requires sulphur oxides concentrations as low as 0.1% in certain cases for various type of ocean going vessels.
We will learn about the next step in the process namely the inert gas blowers in the next article.