Gas Treating Processes
The processes summarized above are the more common ones to be included in a
fuel or energy refinery’s configuration. In addition to these, there will also be the
gas treating processes and often sulfur recovery processes. These are described and
discussed in the chapters “▶ Refinery Gas Treating Processes” and “▶ Environmental Control and Engineering in Petroleum Processing.”
Gas treating is always required to remove the H 2 S impurity generated by
hydrotreating or cracking from the refinery fuel gas or hydrogen recycle streams.
The removal of H 2 S for these purposes is accomplished by absorbing the hydrogen
sulfide into an amine or similar solution that readily absorbs H 2 S. Stripping the rich
absorbent solution removes the H 2 S from the system to be further reacted with air to
produce elemental sulfur. This latter reaction takes place in a specially designed
sulfur plant.
The rich H 2 S-laden gases from all the refinery sources enter below the bottom
tray (or packed bed) of the absorber tower. The lean, low-H 2 S absorbent solution
enters the tower above the top tray (or packed bed) to move down the tower
countercurrent to the gas moving upward. Mixing on the trays (or packed beds)
allows the H 2 S from the gas phase to be absorbed into the liquid solution phase. The
H 2 S-free gas (< ~ 100 vppm H 2 S) leaves the tower top to be routed to refinery fuel
or other prescribed destination.
The rich absorbent solution leaves the bottom of the absorber to be heat
exchanged with hot stripped absorbent solution before entering the feed tray of
the stripping column. The solution moving down the tower is stripped of most
H 2 S by a stripper vapor phase moving up the tower. This stripper phase is
generated by conventional reboiling of the bottom tray solution. The hot stripped
solution leaves the bottom of the tower to be cooled by heat exchange with the
feed and then by an air or water cooler before entering the absorber tower.
Conditions in the stripper column are maintained by partially condensing the
rich H 2 S overhead vapors and then returning the distillate as reflux to the top tray
of the rectifying section of the tower (which is the section of trays above the
feed tray).
The vapor not condensed leaves the reflux drum to be routed to a sulfur plant.
These vapors contain a high concentration of H 2 S (usually in excess of 90 % mol)
and enter the specially designed fuel “gun” of the sulfur plant heater. Here, about
one third is mixed with an appropriate concentration of air and “burned” in the
plant’s fire box to generate SO 2 . The gases generated are combined with the
remaining H 2 S and passed over a catalyst bed where almost complete conversion
to elemental sulfur occurs. This product, in molten form, enters a heated storage
pit or tank. The unconverted sulfurous vapors are further incinerated before
venting to atmosphere from an acceptably elevated location. Sulfur recovery
is detailed in the chapter on “▶ Environmental Control and Engineering in
Petroleum Processing.”
40
D.S.J. Jones
The processes summarized above are the more common ones to be included in a
fuel or energy refinery’s configuration. In addition to these, there will also be the
gas treating processes and often sulfur recovery processes. These are described and
discussed in the chapters “▶ Refinery Gas Treating Processes” and “▶ Environmental Control and Engineering in Petroleum Processing.”
Gas treating is always required to remove the H 2 S impurity generated by
hydrotreating or cracking from the refinery fuel gas or hydrogen recycle streams.
The removal of H 2 S for these purposes is accomplished by absorbing the hydrogen
sulfide into an amine or similar solution that readily absorbs H 2 S. Stripping the rich
absorbent solution removes the H 2 S from the system to be further reacted with air to
produce elemental sulfur. This latter reaction takes place in a specially designed
sulfur plant.
The rich H 2 S-laden gases from all the refinery sources enter below the bottom
tray (or packed bed) of the absorber tower. The lean, low-H 2 S absorbent solution
enters the tower above the top tray (or packed bed) to move down the tower
countercurrent to the gas moving upward. Mixing on the trays (or packed beds)
allows the H 2 S from the gas phase to be absorbed into the liquid solution phase. The
H 2 S-free gas (< ~ 100 vppm H 2 S) leaves the tower top to be routed to refinery fuel
or other prescribed destination.
The rich absorbent solution leaves the bottom of the absorber to be heat
exchanged with hot stripped absorbent solution before entering the feed tray of
the stripping column. The solution moving down the tower is stripped of most
H 2 S by a stripper vapor phase moving up the tower. This stripper phase is
generated by conventional reboiling of the bottom tray solution. The hot stripped
solution leaves the bottom of the tower to be cooled by heat exchange with the
feed and then by an air or water cooler before entering the absorber tower.
Conditions in the stripper column are maintained by partially condensing the
rich H 2 S overhead vapors and then returning the distillate as reflux to the top tray
of the rectifying section of the tower (which is the section of trays above the
feed tray).
The vapor not condensed leaves the reflux drum to be routed to a sulfur plant.
These vapors contain a high concentration of H 2 S (usually in excess of 90 % mol)
and enter the specially designed fuel “gun” of the sulfur plant heater. Here, about
one third is mixed with an appropriate concentration of air and “burned” in the
plant’s fire box to generate SO 2 . The gases generated are combined with the
remaining H 2 S and passed over a catalyst bed where almost complete conversion
to elemental sulfur occurs. This product, in molten form, enters a heated storage
pit or tank. The unconverted sulfurous vapors are further incinerated before
venting to atmosphere from an acceptably elevated location. Sulfur recovery
is detailed in the chapter on “▶ Environmental Control and Engineering in
Petroleum Processing.”
40
D.S.J. Jones