Hydrocarbon Processing - June 2021 - 57

Catalysts
version of HCN to NOx
. HCN levels tend to be very low when
, and HCN can be fully oxidized in this mode.
the regenerator is operating in full CO burn; i.e., there is an excess
of flue gas O2
FIG. 6 shows total cyanides in overhead drum water vs. some
regenerator operating variables for Refiner A's FCCU. Three
data sets are shown:
1. " Other supplier " period represents the baseline
when a non BASF catalyst was in use
2. " All " period documents the effects of the change to
a low-delta-coke catalysta
, which allowed Refiner A
to expand the envelope of its operations to feed
concarbon above the historical maximum of 4 wt%,
while increasing the catalyst circulation rate
3. " CN control " period shows how Refiner A was able to
mitigate the CN level while maintaining the advantages
offered by the low-delta-coke catalysta
.
Process variables interact on an FCCU, making it sometimes
difficult to isolate the effects of one variable. For example, higher
feed concarbon tends to increase regenerator bed temperature at
constant feed preheat and lead to a reduction in catalytic circulation
rate, as shown in FIG. 6. Although higher regenerator bed temperature
and lower catalytic circulation rate might be expected to
reduce HCN, the percentage of CO in flue gas was the dominating
variable for controlling HCN production. Note: Wash water
rates were maintained within the normal operating range.
This unit has some regenerator design shortcomings due to
expanding capacity over the years, emphasized by the high feed
concarbon intake with insufficient residence time to complete
HCN conversion, inefficient catalyst/air distribution leading to
localized regions of O2
deficiency/low bed temperatures, potential
short-circuiting of catalyst high in HCN directly to the outlet,
etc. At the same time, these issues highlighted the potential to further
optimize the unit design to enable the full concarbon intake
offered by the catalysta
.
Measures to reduce HCN production. Other operational
changes that may be considered for reducing HCN are to increase
O2
raising regenerator pressure and catalyst bed level. Use of additives
to promote the conversion of C to CO2
increase NOx
emissions.
Some experimental work indicates that use of a CO promoter
will reduce HCN as it is converted to N2
or NOx. The CO
promoter would be expected to be more effective for reducing
HCN in a full CO burn environment.
Changes to the regenerator design can also be considered
to reduce HCN by increasing conversion. Some examples are
providing more air near the spent catalyst inlet to increase O2
availability and regenerator bed temperature in this zone where
HCN is initially formed, and improving the distribution of
catalyst to achieve optimal contacting and minimize localized
zones of high CO.
Corrosion control and monitoring for wet H2
S damage.
As previously indicated, wash water is commonly injected into
the overhead gas streams to reduce cyanide and ammonium levels.
In addition, many North American FCCUs processing highN
feeds supplement water washing with ammonium polysulfide
(APS) injection, which has enabled satisfactory operation with
FIG. 5. Feed total N vs. feed.
very high cyanide levels above 100 ppm in water. Some FCCUs
also use filming inhibitors for corrosion control.
Various water wash schemes are used. Condensate is commonly
injected upstream of the first-and second-stage compressor
coolers. Some FCCUs also inject wash water upstream of the
catalytic fractionator overhead condensers to supplement condensed
reactor steam and water that may be recycled from the
overhead drum back to the condensers. To achieve optimal water/gas
contacting for maximum HCN removal, it is important to
provide suitable water spray nozzles and efficient water distribution
systems across multiple exchanger banks (if present).
Cyanides are more soluble in water at higher pressures; therefore,
injecting water upstream of the compressor coolers is the
best location to maximize HCN removal from gas. It also proHydrocarbon
Processing | JUNE 2021 57
partial pressure and gas/catalyst residence time by
is also known to
100
75
50
25
4
All
5
6
Catalyst/oil
CN control
Other supplier
FIG. 4. Processing heavier feed with increased cyanides.
2,500
2,000
1,500
1,000
500
2
3
4
Feed concarbon, wt%
5
6
7
8
10
100
75
50
25
2
All
3
4
Feed concarbon, wt%
CN control
5
Other supplier
6
Feed total N, ppm
Total CN, mg/l
Total CN, mg/l
Hydrocarbon Processing - June 2021

Table of Contents for the Digital Edition of Hydrocarbon Processing - June 2021
