Salt Deposition in
FCC Gas Concentration Units
Michel Melin
Director
Technical Service
Grace Davison Refining
Technologies
Colin Baillie
Marketing Manager
Grace Davison
Refining Technologies
Europe, Middle East and Africa
Europe, Middle East and Africa
Various operational problems
can arise when ammonium chloride deposition occurs in FCC
gas concentration units, and
there is a range of likely causes.
34
ISSUE No. 107 / 2010
Director Marketing &
Business Development
Grace Davison
Refining Technologies
Europe, Middle East and Africa
main causes of salt deposition
so that the correct procedures
can be applied to manage this
phenomenon.
Introduction
Salt deposition in FCC gas concentration units can lead to various operational problems if it is
not dealt with in an appropriate
manner. It is therefore important
for refiners to be aware of the
Gordon McElhiney
Troubleshooting of FCCUs in
terms of cyclone problems, catalyst circulation issues or coking
has been discussed in much
detail.1 However, less informa-
tion has been reported about
ways of dealing with salt deposition issues. The salt that is
deposited most in FCC gas concentration units is ammonium
chloride (NH4Cl), but deposits
can also occur of the salts
ammonium hydrosulfide
(NH4)SH and iron sulfide (FeS),
although they are less common.
This article is intended to provide refiners with useful information regarding the most likely
causes of salt deposition, the
associated symptoms and resulting consequences, as well as
approaches that can be taken to
handle such situations. The
Grace Davison Refining
Technologies technical service
team has helped various refiners
manage the issue of salt deposition and this valuable experience
will be discussed.
Ammonium Chloride
Deposition: Likely Causes
There are two reasons for an
increasing occurrence of ammonium chloride deposits. First,
refiners are processing a higher
amount of residue feedstocks,
which typically have a higher
chloride content. Some refiners
are also bypassing the desalter
with imported atmospheric
residue feedstock, which contributes to higher feed chloride
levels. Second, due to the need
to produce low-sulfur gasoline, a
gasoline side cut is extracted
from the main fractionator (MF)
and subsequently hydrotreated.
This leads to main fractionator
top temperatures as low as
100°C (212˚F), compared to previous temperatures in the range
of 135-145°C (275-293˚F).
While these are the most likely
origins of ammonium chloride
deposits, there are other circumstances that can cause this
problem and a summary is listed
in Table I.
tion performed such a troubleshooting exercise, and the
problem was finally attributed to
the injection of slop to the main
fractionator. This slop was rich in
chloride and, together with the
effects of acidic crudes that were
being processed, resulted in
ammonium chloride deposition
on the fractionator (with severe
corrosion of the fractionator
packing, see Table III). The problem of salt deposition was
solved by water washing (see
Table IV).
Chloride Contribution from the
FCC Catalyst
In addition to the incorporation of
rare-earth chloride into FCC catalysts to stabilize the zeolite and
steer product selectivities, chlo-
ride is an integral feature of the
Grace Davison Al-sol binder system, which was first commercialised in the early 1980s, with
the Worms plant in Germany
being the pioneer site. This Alsol binder system provides the
basis for formulation flexibility
which generates the high performance associated with Grace
Davison FCC catalysts. Indeed
the uniqueness of this binder
system is one of the main reasons why Grace Davison FCC
catalysts have maintained a performance advantage over other
catalyst suppliers. The question
as to whether chloride from this
binder can contribute to salt deposition is occasionally raised,
and in this context the following
facts are relevant.
Table I
Most Likely Causes of Ammonium
Chloride Salt Deposition
Processing of imported atmospheric residue
Poor crude desalter operation
Recovery of a MF gasoline side cut (lower top temperature)
Reprocessing of slops in MF
During troubleshooting for a salt
deposition issue, all of these
possibilities should be considered, individually and in combination. For example, one
refinery that experienced issues
with ammonium chloride deposi-
Leaking overhead condenser (using sea water)
Overflowing overhead receiver water boot
Bad distribution of cold reflux stream (cold spot)
Feedstocks containing organic chloride from additives used to increase
the recovery of oil or for cleaning
GRACE DAVISON CATALAGRAM
35
During the FCC catalyst manufacturing process, the Al-sol
binder is “set” using a high temperature calcination to provide
attrition resistance over a wide
range of formulations. This hightemperature calcination step
cal temperatures in the FCCU
regenerator are significantly
higher than those used in calcination in the standard catalyst
manufacturing process, which in
turn are higher than typical reactor temperatures in the FCCU. In
Ammonium Chloride
Deposition - Symptoms and
Consequences
“The main type of salt deposited is
ammonium chloride and there is a range of
likely causes.”
also removes most (>80%) of
the chloride from the catalyst. If
necessary, additional processing
steps can be used to further
reduce the fresh catalyst chloride content. In use, the fresh
catalyst is added to the FCCU
via the regenerator, and it is
important to recognize that typi-
the FCCU flue gas, depending
on the regenerator design. It is
therefore recommended to avoid
adding the fresh catalyst to a
zone where it can bypass the
regenerator bed and travel
directly to the riser/stripper.
consequence, and accelerated
by the steam which is also present, chloride remaining on the
fresh FCC catalyst is very quickly removed in the regenerator
before the catalyst makes its first
transit to the reactor section.
Typically 80-95% of the fresh
catalyst chloride is removed in
Ammonium chloride deposition
takes place primarily at the top
of the main fractionator, although
it can be encountered to a lesser
extent in the overhead line,
where the gas is passed through
the air and water coolers, or the
downstream FCC gas plant.
Figure 1 shows a schematic diagram of where ammonium chloride deposition is most likely to
occur.
Figure 1
Diagram Highlighting Where Ammonium Chloride Deposition Can Occur
...and in the
Overhead overhead line
coolers
Deposition can
occur at
top of MF...
To wet gas compressor
Overhead
receiver
Water
Main
Fractionator
Wild naphtha to
primary absorber
Rich sponge oil
To sponge oil absorber
HCO
recycle
LCO
Stripper
Light cycle oil
(LCO) product
Hydrotreater
Steam
Reactor
vapours
Filter
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ISSUE No. 107 / 2010
Decanted oil product
The main symptom of ammonium chloride deposition is an
increase in pressure drop at the
top of the main fractionator.
Further symptoms are listed in
Table II.
Table II
Main Symptoms of Salt Deposition
Increase in MF delta P
Flooding of MF top section
Plugging of top products draws
Salt deposition can cause a
reduction in feedrate as well as
a slight deterioration of product
quality. This can be a consequence of the salt deposition
itself, but will also temporarily be
observed during any resulting
period of water wash applied to
reduce the salt deposition. In
addition, corrosion may also be
an issue especially for packed
columns. A summary of the consequences of salt deposition are
highlighted in Table III.
Managing and Solving the
Issues of Ammonium Chloride
Deposition
Loss of duty of pump around heat exchangers
Loss of separation efficiency between gasoline and LCO
Higher MF bottom temperature
Increase in reactor/regen pressure
Plugging of reflux/gasoline pump strainer
Reduced reflux/TPA rate
Wider opening of WGC suction valve
Difficulty when using HCN for reboiling depropanizer because the HCN
temperature is lower and salt may deposit in the tubes
Table III
Consequences of Salt Deposition
Corrosion of trays/packing
The Grace Davison Refining
Technologies technical service
team has worked with refiners to
help solve ammonium chloride
deposition issues, and the experience gained is shared in the
following main recommendations.
Reduced WGC capacity (lower suction P)
Reduced air blower capacity (higher regen P)
Increased unit delta coke (higher reactor P)
Fouling of slurry circuit (if higher bottom T)
Poor quality heavy gasoline
Cost associated with reduced feed rate and off spec products
during periodic water wash
Lower duty of depropanizer reboiler
To prevent ammonium chloride
deposition in the overhead line,
water is usually added, with typical quantities in the range of 6-7
vol.% water on a fresh feed
basis.
Addition of an anti-fouling additive in the reflux stream can prevent the formation of ammonium
chloride deposits on the trays
and packing. The salt is carried
instead with the gasoline stream,
in which it is insoluble. Such
additives have been used successfully to reduce ammonium
chloride salt deposition in various refineries over the last ten
years; for instance, at the
Lower duty of debutanizer reboiler
Pembroke refinery in south
Wales, UK, as well as the
Mongstad refinery in Norway.2,3
These additives are now considered established and effective
technology. They are also said to
protect against corrosion.
Another recommendation is
water wash the main fractionator. Water is injected either periodically or (more rarely)
continuously in the reflux
stream, and the main fractionator top temperature is reduced to
approximately 80°C (176˚F)
using the reflux rate or the tip
top pumparound, to allow water
to condense inside the column
to dissolve the salt. The water is
preferably removed on a dedicated tray, where it is separated
from the heavy cracked naphtha.
This procedure has been suc-
GRACE DAVISON CATALAGRAM
37
cessfully practised by Saudi
Aramco.4 Alternatively the main
fractionator top temperature can
be increased (for instance, to
above 135°C (275˚F)) for a
given period of time to enable
dissociation of the salt.
Obviously, this results in a fullrange gasoline leaving overhead
during the time period.
Other recommendations include
improving water settling in
imported feed tanks by allowing
more time and the use of additives.
Hardware modifications could
include the design of the main
fractionator’s reflux distributor to
avoid cold spots at the top of the
column. Alternatively the main
fractionator’s tray design could
be revised. For example, the
installation of a water boot in
one of the trays will allow water
(and the dissolved salt) to be
removed without contaminating
the heavy cracked naphtha. The
installation of a two-stage
desalter could also be considered to optimize the operation of
the crude desalter unit. Other
options include the installation of
a dedicated FCC feed desalter,5
or the installation of a gasoline
splitter and then collecting the
thermally cracked naphtha overhead of the main fractionator.
Finally, a very effective solution
is to hydrotreat the FCC feed, as
this removes most of the feed
chloride and significantly
improves the yield structure.
However, this requires a large
capital investment.
The main methods for managing
ammonium chloride deposition
are highlighted in Table IV.
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ISSUE No. 107 / 2010
Table IV
Methods for Managing Ammonium Chloride
Salt Deposition
Use of anti-fouling additives
Water washing of the MF
Increased MF top temperature
Improved water settling in imported feed tanks
Modification of MF reflux distributor to avoid cold spots at the top of the column
Installation of a water boot in one of the MF trays
Installation of a two-stage crude desalter
Installation of a FCC feed desalter
Installation of a gasoline splitter
Hydrotreating the FCC feed
Avoid adding FCC catalyst in a zone where it can bypass the regenerator bed
Ammonium Chloride Deposition in the Main Fractionator
Consider an FCC unit processing atmospheric residue feedstock under the following conditions:
•
•
•
•
•
•
•
•
•
Feed rate
Feed nitrogen content
Feed chloride content
MF top pressure
Steam to the MF = 50.5 tonne/h
Dry gas = 53584 Nm3/h
LPG = 100.3 tonne/h
LCN+reflux = 344.2 tonne/h
Total flow to the MF top
=
=
=
=
=
=
=
=
=
440 tonne/h (440 000 kg/h) (968,000 lb/hr)
1645 ppmw
1.93 ppmw
1.89 bara (27.8 psig)
2806 kmol/h
2392 kmol/h
1937 kmol/h
3843 kmol/h
10 978 kmol/h
The following example assumes that 15% of the feed nitrogen goes to NH3
• the production of nitrogen (from the feed)
= 440 000 × 0.1645 wt.%
= 723.8 kg/h
= 51.7 kmol/h
• the resulting production of ammonia
= 51.7 × 15%
= 7.79 kmol/h
• the partial pressure of NH3
= 1.89 × (7.79/10978)
= 1.34 × 10-3 bara
• the production of chloride (from the feed)
= 440 000 × 0.000193 wt.%
= 0.85 kg/h
= 2.4 × 10-2 kmol/h
• The partial pressure of HCl
= 1.89 × (2.4 × 10-2/10978)
= 4.13 × 10-6 bara
• ppNH3 × ppHCl
= 5.53 × 10-9 bara
Using the following formula:
ln (Kp) = - 21183.4/T + 34.17
where Kp = ppNH3 × ppHCl, and T is the minimum main fractionator top temperature required to
avoid salt deposition (measured in ˚K), the minimum top temperature required to avoid salt deposition under these conditons is 125°C (257˚F).
GRACE DAVISON CATALAGRAM
39