Length and density grading (LDG) technology is used to maximize
the recovery of reusable catalyst. Length grading is used
to selectively remove short catalyst particles that cannot
be removed with conventional screening. Length grading is
particularly valuable for units that are sensitive to pressure
drop.
Density grading separates catalyst particles of similar
size based on individual particle density. Particles with
density differences as small as 10% have been separated
effectively. One common application of density grading is
the separation of heel-contaminated reforming catalyst when
the catalyst must be dumped for reactor maintenance. (see
the September 18, 1995 issue
of the Oil & Gas Journal for more details).
Length
Grading
Length grading separates longer catalyst particles from
shorter ones. Conventional screening can only remove fines
and particles that are shorter than the catalyst diameter.
In many cases, the screened material does not meet the length
specifications for a given unit. Length grading can be used
to remove the short catalyst particles (typically those
with lengths of 1-2 times the catalyst diameter) and produce
material of acceptable length.
In
the example shown below, 1.6 mm cobalt molybdenum on alumina
catalyst was length graded to remove particles shorter than
2.5 mm. The figures show the particle length distributions
for the feed and the short and long fractions. About 24%
of the catalyst was rejected in the short fraction, while
the average length of the recovered catalyst was increased
from 3.2 to 3.7 mm.
Length
Grading of 1.6 mm CoMo Catalyst
$$
Feed
Short
Fraction
Long
Fraction
Wt%
$$
24
76
Average
Length, mm
3.2
1.9
3.7
Particles
<2.5 mm, wt%
19
92
5
Particle Length Distributions
Density Grading of Heel-Contaminated Reforming
catalyst
CCR reforming units continuously circulate catalyst through
the reactor. Catalyst exiting the reactor typicaly contains
about 5% carbon. It is regenerated online and fed back into
the reactor.
In
many units, a small fraction of the catalyst does not circulate
through the reactors. This non-flowing “heel”
catalyst is held up in the bottom and along the walls of
the reactors. Over time, the heel catalyst builds up carbon
levels as high as 50%.
When
the catalyst is unloaded for unit maintenance, the heel
catalyst along the reactor walls is released, contaminating
the low-carbon circulating catalyst. Typically, the last
10-20% of circulating catalyst that is unloaded is contaminated
with heel catalyst. The figure below shows carbon analyses
of catalyst removed from a refiner’s circulating catalyst
reactors. Near the end of the unloading, the carbon levels
increase as the catalyst is contaminated with high-carbon
heel catalyst.
Heel-contaminated
catalyst cannot be reused because the high-carbon heel catalyst
causes temperature excursions in the regenerator tower.
When a plant plans to reuse the catalyst removed in a turnaround,
there is a strong economic incentive to recover the low-carbon
circulating catalyst. If the low-carbon catalyst is not
recovered, it must be sent to platinum recovery and replaced
with fresh, makeup catalyst at considerable cost.
Density
grading can be used to recover the low-carbon catalyst.
Because the heel catalyst has a 40-80% higher density than
the low-carbon catalyst, a precise separation can be made.
One
refiner had shut down its reformer for a maintenance turnaround.
The refiner planned to reuse the catalyst, so the heel-contaminated
catalyst was sent for density grading.
Separation
Of Heel and Low-Carbon Catalyst
$$
Feed
Lights
Heavies
Total
Yields,lbs
$$
30,714
5,223
35,937
wt%
$$
85.5
14.5
100.00
Average
Carbon, wt%
7.3
2.5
43.1
$$
Peak
Carbon, wt%
47.8
12.4
47.1
$$
Compacted
Density, lb/ft3
$$
37
67
$$
The
table above demonstrates that the results of the separation
were excellent. The purity and recovery of the light fraction
were very good, and the amount of the low-carbon catalyst
in the heavy portion was minimal.
The
figure (above) shows carbon analyses on the light material
produced. The first 10 containers held high-quality material
with peak carbon levels less than 8% and average carbon
levels 2-3%. Peak carbon levels were determined by examining
the 5 darkest pellets from a population of 300-400 pellets.
This catalyst could easily be reused with satisfactory performance.
The
last two containers were contaminated with a small amount
(10-15%) of medium-carbon catalyst. This material is considered
marginal for reuse. However, the refiner had the early reactor
design, which allowed placement of this material behind
the catalyst withdrawal scoops. This minimized the impact
of the higher-carbon pellets.
Recovery
of reusable catalyst was high. By weight, 86.4% of good,
light catalyst was recovered from the mixture. Recoveries
of 60-90% are typical in the density grading of heel-contaminated
catalyst.
The
recovered catalyst was reloaded into the reformer and the
unit successfully started up. By using density grading,
the refiner recovered 30,714 pounds of catalyst that otherwise
would have been sent out for disposal. Considering the savings
in new catalyst purchases and platinum recovery costs, the
refiner saved more than $300,000.
