Knowledge for the Sulphuric Acid Industry
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Acid Plant Database
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Materials of Construction
DKL Engineering, Inc.
Theory of Operation
Early in 1976, Magna Corporation and Chemetics recognized that it would be beneficial if a tank protection system could be designed that would reflect the significant field experience gained by Chemetics from the anodically protected acid cooler. The result was a tank protection system to achieve a minimum corrosion allowance of less than 3 mpy.
Anodic protection is effective in minimizing corrosion and preventing hydrogen grooving of steel. At the same time, the quality of the acid is improved with respect to iron content which is an important criterion for some acid purchasers.
The anodic protection system consists of the cathodes, reference electrodes, controller and power stage.
Anodic protection depends upon the formation of a passive film on a metal immersed in an electrolyte by the application of a positive potential to the metal. The passive state is defined as a state of increase corrosion resistance of metals and alloys through anodic polarization. The inside of the tank that is in contact with the sulphuric acid is maintained at a predetermined potential against a reference electrode, and transmitting the potential value back to a controller. The control circuit compares this potential with a preset value and actuates the power stage to supply current to the tank via cathodes suspended in the acid from the tank roof. In this manner the potential of the tank is maintained within the passive range.
Tests show that the passive film will remain essentially intact when the tank is drained of acid. Consequently, when the tank is refilled the full protection of the original passive film is very rapidly recovered. It is when the tank is washed with water that the passive film is completely destroyed.
The cathode is basically a hollow tube which extends down to within 30 mm (1 ft) of the tank floor. The cathodes are inserted through special flanged openings in the roof of the tank. The cathode is electrically insulated from the flanged connection. The number of cathodes required depends on the capacity and the diameter of the storage tank. The cathode is internally pressurized and a pressure gauge is used to indicate any perforation of the cathode wall.
Reference electrodes are either inserted through nozzles in the roof or onto the tank side wall. The roof mounted electrodes consist of a short reference electrode welded to the end of a support pipe. The support pipe is flanged and mounted onto a nozzle on the roof of the tank.
Side wall mounted electrodes are mounted using an acid seal similar to the ones used on Chemetics acid coolers. This method of installation is mechanically simpler and easier to install.
There are at least three reference electrodes installed in a tank. The are positioned so that they will indicate when the tank is uniformly passivated. The electrode will also cause a alarm to sound and shutdown the system is if the electrode should malfunction for any reason. The electrode will also shutdown the system if a low level is reached in the tank.
The function of the controller is compare the tank potential fed back from the controlling reference electrode with a preset value and to control the current supply from the power stage to the cathodes. The controller indicates the current flow to the cathodes and the potential of the tank wall measured by the control reference electrode and the two auxiliary electrodes. In addition various fault conditions are indicated on the display panel of the controller.
The power stage consists of a step down transformer and a silicon rectifier The unit is normally immersed in oil for cooling and also as a means of protection against the corrosive atmosphere often found near acid plants.
The power output to the cathode is an appropriate DC current for the tank size, acid strength and temperature. Once passivation has been achieved in the tank, the power consumption is only a few hundred watts.