The preheated feed plus hydrogen is now routed to a hydrotreating reactor to
convert any residual organic sulfur compounds quantitatively to H 2 S and to eliminate olefins. Organic chlorides that may be present are all converted to HCl.
Equations for these reactions are below. The reactions are driven almost totally to
completion and they are all exothermic:
Sulfur : RS g
ð Þ þ H 2 g
ð Þ ! RH g
ð Þ þ H 2 S g
ð Þ
(3)
Olefins : R À C ¼ C À R
0 g
ð Þ þ H 2 g
ð Þ ! R À C À C À R
0 g
ð Þ
(4)
Chlorides : R À Cl g
ð Þ þ H 2 g
ð Þ ! RH g
ð Þ þ HCl g
ð Þ
(5)
Because all the reactions are exothermic, the gas temperature can increase across
the hydrotreating reactor. However, since the starting sulfur and chloride contents
will be very low in the feeds, the main reaction generating heat is olefin saturation.
Olefins can generate 40
F rise per percent olefin in a gaseous feed, 15
F per
percent olefin in butane feed, and 10
F per percent olefin in a naphtha feed. These
rises place a limit on the amount of olefins in a feedstock. Hydrotreated product can
be recycled back through the feed compressors to help moderate the reaction, but
this consumes feed gas compressor capacity.
For gaseous feeds, the practical limit is about 5 % olefins in the combined feed
(including any recycle). To manage this concentration of olefins, you have to lower
the hydrotreater inlet temperature about 200
F, to about 500–550
F. If you go
lower in temperature, the hydrotreater reactions may not initiate. Higher temperature may result in a hydrocracking runway.
The pretreated gases, now at about 700–750
F, are passed through a bed of
chloride trap and beds of zinc oxide (ZnO). The chloride trap, normally activated
alumina (NaAlO 2 ), absorbs the chlorides quantitatively until it is nearly spent. The
zinc oxide beds absorb the sulfur quantitatively (<0.01 ppm residual is possible)
until the beds are nearly spent. Two ZnO beds are usually used in series with the
valves and piping to change out the beds on line and swing the order of the two
beds. The essential equations in the sorbent beds are below. These reactions are
slightly exothermic, but do not generate significant heat at the low impurity
concentrations seen in hydrogen plant feeds:
Chlorides : 2HCl g
ð Þ þ 2NaAlO 2 s
ð Þ ! NaCl s
ð Þ þ Al 2 O 3 s
ð Þ þ H 2 O g
ð Þ (6)
Sulfur : H 2 S g
ð Þ þ ZnO s
ð Þ ! ZnS s
ð Þ þ H 2 O g
ð Þ
(7)
Some units do not have separate hydrotreaters. This is not unusual where a
plant only feeds sweet natural gas. In these cases, combined function
hydrotreating/absorbing catalysts may be used. Some of the sweet feed units
may also not use a hydrogen recycle, but there is some risk of thiophene sulfur
getting through. Activated carbon has also been used instead of ZnO for sulfur
sorption, but this is uncommon today because activated carbon has such a low
capacity.
574
S.A. Treese
convert any residual organic sulfur compounds quantitatively to H 2 S and to eliminate olefins. Organic chlorides that may be present are all converted to HCl.
Equations for these reactions are below. The reactions are driven almost totally to
completion and they are all exothermic:
Sulfur : RS g
ð Þ þ H 2 g
ð Þ ! RH g
ð Þ þ H 2 S g
ð Þ
(3)
Olefins : R À C ¼ C À R
0 g
ð Þ þ H 2 g
ð Þ ! R À C À C À R
0 g
ð Þ
(4)
Chlorides : R À Cl g
ð Þ þ H 2 g
ð Þ ! RH g
ð Þ þ HCl g
ð Þ
(5)
Because all the reactions are exothermic, the gas temperature can increase across
the hydrotreating reactor. However, since the starting sulfur and chloride contents
will be very low in the feeds, the main reaction generating heat is olefin saturation.
Olefins can generate 40
F rise per percent olefin in a gaseous feed, 15
F per
percent olefin in butane feed, and 10
F per percent olefin in a naphtha feed. These
rises place a limit on the amount of olefins in a feedstock. Hydrotreated product can
be recycled back through the feed compressors to help moderate the reaction, but
this consumes feed gas compressor capacity.
For gaseous feeds, the practical limit is about 5 % olefins in the combined feed
(including any recycle). To manage this concentration of olefins, you have to lower
the hydrotreater inlet temperature about 200
F, to about 500–550
F. If you go
lower in temperature, the hydrotreater reactions may not initiate. Higher temperature may result in a hydrocracking runway.
The pretreated gases, now at about 700–750
F, are passed through a bed of
chloride trap and beds of zinc oxide (ZnO). The chloride trap, normally activated
alumina (NaAlO 2 ), absorbs the chlorides quantitatively until it is nearly spent. The
zinc oxide beds absorb the sulfur quantitatively (<0.01 ppm residual is possible)
until the beds are nearly spent. Two ZnO beds are usually used in series with the
valves and piping to change out the beds on line and swing the order of the two
beds. The essential equations in the sorbent beds are below. These reactions are
slightly exothermic, but do not generate significant heat at the low impurity
concentrations seen in hydrogen plant feeds:
Chlorides : 2HCl g
ð Þ þ 2NaAlO 2 s
ð Þ ! NaCl s
ð Þ þ Al 2 O 3 s
ð Þ þ H 2 O g
ð Þ (6)
Sulfur : H 2 S g
ð Þ þ ZnO s
ð Þ ! ZnS s
ð Þ þ H 2 O g
ð Þ
(7)
Some units do not have separate hydrotreaters. This is not unusual where a
plant only feeds sweet natural gas. In these cases, combined function
hydrotreating/absorbing catalysts may be used. Some of the sweet feed units
may also not use a hydrogen recycle, but there is some risk of thiophene sulfur
getting through. Activated carbon has also been used instead of ZnO for sulfur
sorption, but this is uncommon today because activated carbon has such a low
capacity.
574
S.A. Treese