Sour water feeds
WWT plants feeds consist primarily
of ammonia and hydrogen sulfide dissolved
in water, with traces of carbon dioxide, phenol, cyanides and other contaminants
present.
WWT plants have typically
been designed for sour water with ammonia concentrations
between 0.3 and 6.0 Wt.% and hydrogen sulfide concentrations between 0.3 and
10.4 Wt.%. The process, however, is not limited to feeds in this composition
range; it can handle water with high ratios of ammonia to hydrogen sulfide
and also water with high carbon dioxide content. For Example, the
WWT Process
can handle the high ammonia-containing sour waters produced in shale oil and
tar sands upgrading plants and is capable of treating the carbon dioxide containing
sour water from shale oil retorting plants.
WWT plants have successfully processed sour water from many refinery sources:
- hydrocrackers and hydrotreaters
- fluid catalytic crackers
- cokers-delayed and fluid types
- crude units
- amine plants
- flare seals
[Back to top]
Process description
There are four distinct processing steps in the WWT
process: Degasification,
Acid-gas stripping, Ammonia stripping and Ammonia purification and recovery.
Figure 1 is a simplified flow scheme showing these processing.
Figure 1: WWT process |
[Back to top]
Degassing and feed storage
The sour water feed to the plant, from single or multiple sources, is combined
with a recycle stream from the ammonia stripper and is cooled and fed to a
degasser where dissolved hydrogen, methane and other light hydrocarbons are
removed. The release of acid gas and possible air pollution are minimized.
The degassed sour water is pumped to an off-plot storage tank that serves to
dampen flow rate and composition changes. The tank also facilitates removal
of entrained oil and solids.
Acid gas stripping
From the feed tank, the degassed sour water is pumped to the
WWT plant, where
it is heated by feed-bottoms exchange and fed to the acid gas or hydrogen sulfide
stripper. This stripper is a steam-reboiled distillation column. The hydrogen
sulfide, which is stripped overhead, is of high purity – an excellent
feed for sulfur recovery units or sulfuric acid plants. It contains negligible
ammonia and, because the plant feed has been degassed, only traces of hydrocarbons.
It does contain, however, any carbon dioxide that is present in the feed.
[Back to top]
Ammonia stripping
The hydrogen sulfide stripper bottoms stream, containing all the ammonia in
the feed and some hydrogen sulfide, is fed directly to the ammonia stripper,
which is a refluxed distillation column. In this column, essentially all ammonia
and hydrogen sulfide are removed from the water, which leaves as the column
bottoms stream. After exchanging heat with the hydrogen sulfide stripper feed,
this stripped water is cooled and sent off-plot for reuse or treating.
The ammonia and hydrogen sulfide stripped from the water in the ammonia stripper
are passed through an overhead condenser and are partially condensed.
Ammonia purification and recovery
The vapor product from the overhead condenser in the ammonia stripping section
is an ammonia-rich gas which may be handled in a variety of ways, as shown
in Figure 2. Selection of the appropriate ammonia recovery option will be totally
dependent on the site economics.
Figure 2: Ammonia recovery options |
[Back to top]
Ammonia incineration
For some plants, actual ammonia recovery may be neither desired nor economical.
In such cases, the ammonia product may be incinerated, either directly off
the reflux drum or after being scrubbed with water to reduce the hydrogen sulfide
content, or it may be further purified and recovered to produce either anhydrous
or aqueous ammonia suitable for sale or for further processing.
Anhydrous ammonia
For production of anhydrous ammonia, the gas is first passed through a two-stage
scrubbing system to remove hydrogen sulfide. It is then liquefied to produce
the anhydrous ammonia.
Aqueous ammonia
For production of aqueous ammonia, a one- or two-stage scrubbing system may
be used to remove the hydrogen sulfide. The ammonia gas is then dissolved in
water to yield the desired product grade.
[Back to top]