Question 5.30: Acombined gas stripping and scrubbing (absorption) process s...

1. This special combined gas stripping and scrubbing (absorption) process system shown in Fig. 24 is for simultaneous ammonia removal and recovery and is a significant advance in the state of the art of nitrogen management. The new process overcomes most of the limitations of a conventional gas stripping process and has the advantage of recovering ammonia as a byproduct.

It appears that the improved process (Fig. 24) includes an ammonia-stripping unit and an ammonia-absorption unit (or scrubbing unit). Both units are essentially sealed from the outside air but are connected by appropriate ducting. The stripping gas, which initially is air, is maintained in a closed cycle. The stripping unit operates essentially in the same manner that is now being or has been used in a number of conventional gas stripping systems, except that this system recycles the gas stream rather than using single-pass outside air.

It can been seen from Fig. 24 that most of the ammonia discharged to the gas stream from the stripping unit is removed in the absorptio n unit. Because ammonia is an alkaline substance, the absorbing liquid should be maintained at a low pH to convert absorbed ammonia gas to soluble ammonium ion. This technique may effectively trap the ammonia and also may have the effect of maintaining the full driving force for absorbing the ammonia, because ammonia gas does not build up in the absorbent liquid.

The absorption unit can be a slat tower, packed tower, or sprays similar to the stripping unit, but will usually be smaller owing to kinetics of the absorption process.

The absorbent liquid initially should be water with acid added to obtain a low pH, usually below 7.0. In the simplest case, as ammonia gas is dissolved in the absorbent and converted to ammonium ions, acid should be added to maintain the desired pH. If sulfuric acid is added, for example, an ammonium sulfate salt solution is formed. This salt solution may continue to build up in concentration and the ammonia may be finally discharged from the absorption device as a liquid or solid (precipitate) blowdown of the absorbent. With current shortages of ammonia-based fertilizers, a salable byproduct may result. This is the advantage of this new process system.

Other methods of removing the ammonia from the absorbent may also be applicable, depending on the acid used and the desired byproduct. Ammonia gas or aqua ammonia could be produced, for example, by steam stripping the absorbent. In this case, acid makeup would be unnecessary.

It is believed that the usual scaling problem associated with ammonia-stripping towers will be eliminated by the improved process system (Fig. 24) because the carbon dioxide that normally reacts with the calcium and hydroxide ions in the water to form the calcium carbonate scale will be eliminated from the stripping air during the first few passes. The freezing problem can be eliminated owing to the exclusion of nearly all outside air. The treatment system may normally operate at the temperature of the wastewater.

2. The process system introduced in Example 22 involves only one process unit in which the emission stream treats contaminated groundwater, and the groundwater serves as a scrubbing liquid for purifying the emission stream at the same time. From an air pollution control perspective (for treatment of the contaminated air emission stream), it is a scrubbing (absorption) process, whereas from a water pollution control perspective (for treatment of the contaminated groundwater), it is a gas stripping process. The process system introduced here employs two separate but almost identical process units connected to each other. A gas-stripping process unit removes ammonia from a contaminated wastewater and a scrubbing (absorption) process unit recovers the stripped ammonia gas from the emitted gas stream for reuse.