Page 316 - Air Pollution Control Engineering

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2. The difference between the process system introduced in Example 22 and the process
system introduced in this example (see Fig. 24).
Solution

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 ini-
tially 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 con-
vert absorbed ammonia gas to soluble ammonium ion. This technique may effec-
tively 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 strip-
ping 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, usu-
ally 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 sul-
furic 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 ammo-
nia 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 tow-
ers 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