Hydrocarbon Processing - June 2021 - 34

Process Optimization
TABLE 1. Typical carbon black feedstock properties
CSO market
Carbon black feedstock
Refinery fuel
Marine fuel
Pitch feedstock
Needle coke feedstock
Hydrotreater feedstock
Carbon fiber feedstock
The global consumption of carbon black feedstock is approxCSO
clarity, ppm
100-500
50-150
50-100
25-100
25-100
10-50
5-10
tivity to allow for more and heavier feed processing in a refinery.
This selectivity and the reduced demand for fuel oil have led the
way to the concept of increasing the amount of residual oil entering
the FCC unit (FCCU).
Catalyst coolers incorporated into FCC regenerators and
highly selective catalysts with the ability to give high conversion
with reduced coke are examples of technology improvements
that the refiner can use to dig deeper into the crude barrel. As
refiners introduce larger quantities of residual into the FCCU,
slurry oil yields increase, and the quality of the slurry oil decreases
due to a larger proportion of asphaltenes and heteroatoms
entering the FCCU. This is relevant because the level of
asphaltenes in the slurry oil becomes a factor in deciding which
technology is best for removing particulate solids. Asphaltenes
are the most hydrogen-deficient constituents of slurry oil. They
become more active and react with one another at higher temperatures-especially
in the presence of metal surfaces-to
form coke. To minimize downstream processing difficulties, the
removal of the contained catalyst will keep solids diluted below
recommended concentration levels.
The solution right under our reactors. Slurry oil yields differ
from each FCCU operation. Complex refineries can reduce
slurry generation to below 4% of the FCC yield for petrochemical
production. As a transportation fuel, FCC can see yields as
high as 12%. Regardless of the slurry make percentage, if it is not
handled properly, the result is profit loss and waste that must be
disposed of properly. Generally, refineries can mix slurry into
heavy fuel oil as a viscosity cutter. However, slurry oil's low API
gravity limits how much can be blended.4
Clarified slurry oil (CSO) applications and markets. The
use of slurry oil as cutter stock for heavy fuel oil blending has also
historically been a major outlet for slurry oil. However, trace metals
deposited on FCCU catalysts (e.g., nickel, vanadium and sodium)
or adhering to catalyst particles and FCCU catalysts themselves-
that contain aluminum and silicon as major components-can
combine with other elements to form high-melting-point compounds
that are corrosive to valve seats and exhaust valves in diesel
engines. Solids contents for marine and refinery use in the range of
50 ppmw-150 ppmw are generally permissible.5
Beyond fuel use, CSO is also sold to make carbon black,
which is used in automobile tires, belts, hoses and pigments,
among others. The carbon black industry has always used slurry
oil as feedstock, but this use is more prevalent in the Asia-Pacific
region than in North America. Typical carbon black feedstock
properties are provided in TABLE 1.
34 JUNE 2021 | HydrocarbonProcessing.com
imately 130,000 bpd, with most of the market focused on the
highly profitable low-ash specialty grades. The required density
for carbon black feedstock is high, and special attention is needed
to operate the FCCU fractionator at a high-enough temperature
to obtain the desired density. Some refiners do not have the
ability to obtain the required slurry oil density for carbon black
applications. In addition, specialized separation equipment is required
to meet the less than 50-ppm catalyst fines to obtain the
greatest profits.
Some refiners increase slurry consumption to increase value
in hydroprocessing unit process streams by feeding CSO to hydroprocessing,
primarily hydrocracking or severe hydrotreating.
Slurry oil solids contents should be reduced to very low levels
to prevent operational disruption. Downflow packed-bed reactors
will accumulate particulates near the entrance of the reactor,
which will eventually bridge the hydroprocessing catalyst
particles and cause plugging and premature shutdown. Reactors
operating in trickle flow do not have the velocity to carry catalyst
particles through the packed bed.3
Separating from the competition, but not from profits.
Catalyst particles (ash) are a particular problem for slurry, especially
for low-API and viscous oils needing long residence times
to allow for catalyst settling. Obtaining low ash (< 0.05 wt%)
requires special techniques such as heating, chemical additives,
filters, electrostatic precipitators, centrifuges and cyclones. The
selection of an attrition-resistant catalyst helps to a great extent,
and a few refiners buy higher-priced hard catalysts to alleviate
ash problems in slurry oil.5
Historically, holding tanks are used to allow solids to settle
out of the slurry oil. Many refiners de-ash with chemical settling
aids, which accelerates ash settling in storage. These chemicals
are polymeric compounds that adhere to the catalyst surface,
causing agglomeration of the fine particles to accelerate separation.
In most countries, sludge from slurry oil holding tanks
has been listed as a hazardous waste. Frequent and expensive
cleaning of settling tanks is required. Depending on the tank size
and rate of slurry oil production, cleaning costs can range from
$1 MM-$4 MM per cleaning. In the absence of countermeasures,
increasing residual feed to the FCCU tends to increase the
rates of slurry oil production and sludge formation.5
The first membrane filters entered the slurry oil service market
in 1990. One company's filtersa
operate at temperatures up to
315.56°C (600°F) and employ tubular porous metal elements.
The solids collect on the inside of the elements, while the filtrate
passes through to the outside. Another companyb
uses
porous sintered woven wire mesh metal filters and operates at
204.44°C-343.33°C (400°F-650°F). A third companyc
employs
2-µm to 5-µm woven wire filter elements, recommends a lightcycle
oil backwash at 176.67°C (350°F) and claims 85%-95%
solids removal from the feed slurry. A fourth companyd
uses sintered
porous metal filters with solids capture inside the tubes.
This company claims a product with < 50-ppmw solids, and, in
a departure from its competitors, uses a gas-assisted (pressureassisted)
backwash for rapid filter element cleaning.3
Electrostatic precipitators are routinely used to remove catalyst
fines from the FCCU stack, and a similar principle is used for
the removal of solids from liquids in an electrostatic slurry sepa
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