Keywords: phosphoric acid, wet process, cooling water, cooling towers, plant design.
Wet process phosphoric acid plants require reliable cooling water facilities. At the reaction-filtration section reaction heat is removed as water vapour from the flashcooler. In the next stage the weak filtered acid is concentrated to a classic 52-54% P2O5 concentration using a closed loop with a heat exchanger and evaporator under vacuum. In both cases, water vapour, along with fluorine vapours are released from the flashcooler and the evaporator, which are then condensed with cold cooling water in a direct condenser. The hot acidic cooling water is collected in the condenser seal tank and sent back to the cooling tower.
Due to the acidic nature of the cooling water, containing traces of fluorine, chlorine, phosphoric acid and 1-2% solids, special precautions need to be taken while selecting the type and material of cooling towers, cooling water piping and cooling water pumps. Forced draft cooling towers are preferred since the fan will not be exposed to acidic vapours and droplets.
Cold cooling water is usually collected in an anti-acid protected underground concrete basin and pumped with vertical or horizontal pumps towards the various condensers inside the phosphoric acid plant. There are several ways of bringing the hot cooling water back to the cooling tower cells:
• By gravity via underground brick lined concrete channels towards a hot water basin from where it is pumped towards the cooling tower cells.
• By pumps located at the seal tanks and piping towards the hot water basin or cooling tower cells inlet.
• By gravity directly from the elevated seal tank via piping to the cells inlet.
• A combination of the above solutions.
Each way brings advantages and disadvantages, with a different cost and must be studied during the early design of the phosphoric acid plant, taking into account the layout constraints.
While selecting the number of cooling tower cells and their size it is always useful to match the number of cells with certain areas or number of parallel concentration lines in the plant.
Besides the thermal duty, which is the primary consideration, many important parameters need to be considered such as:
• Range: the delta T of the cooling water.
• Coinciding dry bulb and wet bulb temperature to be selected from ASHREA weather data.
• Approach: difference between wet bulb and cold water temperature.
• Water loading: rate of cooling water sprayed per square meter of cell area.
• Type of packing, to allow for 1-2% solids.
• Maximum allowable drift loss rate.
• L/G: ratio of mass flow of cooling water to the mass flow of air brought by the fans.
• Pumping head: pressure of hot water above the basin curb.
For phosphoric acid producers and plant designers it is important to understand the cooling tower design parameters used by cooling tower suppliers when selecting the optimal cooling tower design for the plant.