Hydrocarbon Processing - December 2021 - 41

Process
Optimization
B. JAZAYERI, Contributing Editor,
Irvine, California
Design and scale-up of gaseous Group A fluid bed
systems for chemical synthesis
More than 100 fluid bed reactors for chemical synthesis and
comparable processes using Group A powders have been installed
and operated successfully since the late 1940s, with some
reactors having an inside diameter at much larger than 7 m (23
ft). This article discusses developmental milestones leading to
their success, the key design differences between chemical synthesis
circulating fluid beds and fluid catalytic cracking (FCC),
and the author's recipe for successful scale-up. Root causes of
typical operational issues of this class of beds are also discussed.
Introduction. Types of gas fluid beds used typically include:
* Fixed fluid bed (FFB), also called bubbling or
conventional fluid bed. Examples include phthalic
anhydride, acrylonitrile, ethylene dichloride, maleic
anhydride and Sasol gas-to-liquid (GTL).
* Dual FFB circulating bed. Examples includes Model IV
FCC and others.
* Circulating fast fluid bed (CFFB), using a riser reactor.
Examples include Sasol Synthol GTL, DuPont maleic
anhydride, TRIG coal gasification, catalytic waste
conversion, gas-to-olefin and chemical looping.
They are characterized by a riser velocity of
< 10 m/sec and very high riser solids holdup.
* Circulating fast bed (CFB), using a riser reactor.
Examples include FCC, pyrolysis and coal combustion.
They are characterized by a riser velocity of
approximately 6 m/sec and low riser solids holdup.
* Moving beds and downers.
Fluid bed chemical synthesis reactions convert gas-phase hydrocarbons
to high-value-added products. Reaction is typically
limited to around 450°C (840°F). The more common mode of
operation is with gas-phase oxygen supplied as air, enriched air
or oxygen. When using gas-phase oxygen, operating pressure
is usually limited to around 3 barg (45 psig) due to safety concerns
and also because gas-phase oxygen reactions are favored
by reduced pressure. Higher pressures may be used safely when
oxygen is provided in solid form or when there is no oxygen, as
occurs in some processes with comparable characteristics, such
as the Sasol circulating or bubbling GTL process.
Since the mid-1940s, more than 100 FFB reactors using
Group A powders have been installed and operated successfully
for chemical synthesis and comparable processes around
* Figuring out how to perform research at the lab
to obtain data for scale-up correctly
* Correct fluidization regime(s) to use
* Proper gas distributor design (many holes distributed
evenly with adequate pressure drop)
* Placement of coils inside the reactor and the process
and mechanical design parameters of the coils.
Use of CFFB systems using Group A powders for chemical
synthesis also dates back to the 1940s, but their use has been
less successful than FFBs due to more complex design, higher
risk when gaseous oxygen is present, and limited window of
operation when internal coils are needed (metal erosion).
CFFBs have been used or assessed for several types of largescale
processes:
* GTL by iron oxide reduction (solid-phase oxygen)-
Sasol Synthol process
* Hydrocarbons to oxygenated chemicals by adsorbed
oxygen (solid-phase oxygen)-DuPont butane
to maleic anhydride
* Hydrocarbons to oxygenates with air, enriched air
or oxygen (gas-phase oxygen)
* Natural gas to olefins by adsorbed oxygen
(solid-phase oxygen)
* Natural gas to olefins with air (gas-phase oxygen)
* Non-slugging gasification of coal with oxygen
(gas-phase oxygen)-TRIG process
* Conversion of solid waste to chemicals or syngas
* Chemical looping.
FIG. 1 depicts two CFFB systems presented in literature.1
The sketch on the left represents the Sasol Synthol process for
conversion of syngas to gasoline using iron oxide as catalyst.
The iron oxide is reduced as conversion progresses. The sketch
on the right is the concept piloted and commercialized by DuPont
to convert butane to maleic anhydride using adsorbed
oxygen. Both of these systems circulate a solid between two
or three vessels, consisting of a riser reactor, a stripper/solid
separator and, when required, a regenerator. The regenerator
may operate as an FFB or a riser.
Hydrocarbon Processing | DECEMBER 2021 41
the world. Their success was based on the following developmental
milestones:
* Correct overall range for catalyst size (wide size
distribution, high levels of 0 micron-40 micron)
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