Gas Liquid Separation & Gas Decomposition Process

Ammonia Removal System

The function of Ammonia Removal System is to process waste water containing ammonia (NH3) and oother contaminants prior to discharge the environment. The ammonia sourced is from the chemical injection in Boiler Turbine to maintain the pH around 8-9. Ammonia content may not dispose of into environment because the concentration exceed is the limits (90 mg/m3). So that is need to remove the ammonia content before discharging to the environment. The ammonia removal process have combination in two main process, that are removal from the water and removal from the gas (feed capacity 8.8 m3/hr). Ammonia Removal System consist of two main process, that are Gas Liquid Separation & Gas Decomposition Process.

Let's start discuss about gas liquid separation in Ammonia Removal System.

figure 1. gas liquid separation block diagram

Sodium Hydroxide 30% is dosing to pH Adjuster  for achieving pre-set pH ( 12 ). By pH 12, the stripper efficiency will increase due to both ammonium ions and dissolved gas exist together as the following graphic: 

figure 2. ammonia pH equilibrium

According the above graphic:

In a waste stream, ammonia ions exist in equilibrium with ammonia.

• pH < 7, virtually all ammonia will be soluble ammonia ions
• pH > 12, virtually all ammonia will be present as a dissolved gas
• 7< pH <12, both ammonium ions and dissolved gas exist together

Before going to the stepper, feed waste water is through the Ammonia waste water heater. The function is to increase temperature until 45 C so the solubility of gas in the waste water will decrease. Low solubility means easy to be separated. The graphic below can illustrate: 

figure 3. ammonia solubility at various temperature

The next step is going to Stripper which the main process in gas liquid separation. After the pH ( 12 ) and temperature ( 45 C), the waste water enters the above of the tower (Stripper). Air from Fan enters the bottom of the Stripper and travel upward through the packing. Since the ammonia is partially present as a dissolved gas, some of the ammonia transfer from the water to the air. Below is a simple image to easily know: 

figure 4. stripper

The function packing media is place that happen separation between waste water and ammonia. Mist Eliminator is to prevent moisture out from stripper.

figure 5. liquid gas separation inside packing media

Let's start discuss about gas decomposition process in Ammonia Removal System.

The simplified block diagram of gas decomposition process is in the following:

figure 6. gas decomposition process block diagram

The decomposition of ammonia is happen in Catalytic Oxidation Reactor. In the present of high temperature condition ammonia (400 C) reacts with free oxygen to form Nitrogen, Nitrogen Oxide and Water (H2O). The reaction is helped by a catalyst to accelerate the reaction (decrease activation energy). The hot gas out from Catalytic Oxidation prior pass through the stack are cool down by utilized the heat to increase temperature of residual gas (311 C), output from Stripper. Ammonia and NO2 analyzer are provided for monitoring purpose. The reaction is following :