Lessons learned in commercial scale-up of new chemical processes (3)
Stepwise vs. scale-up/scale-down commercialization
Two
extreme approaches exist in taking a process from the laboratory to the
commercial stage. Both are practiced, and both can lead to success.
In the
stepwise process, only the scale at hand is considered. For example, imagine
that a 1-tpd plant is built and operated, followed by a 10-tpd unit and finally
a 100-tpd commercial unit. No effort is made to think about the next step at
each stage. Startup companies that rely heavily on government funding often use
this model due to funding limitations imposed by these organizations.
In a
simplistic way, with the scale-up/scale-down approaches, the commercial scale
is always being examined. A concept plant for the commercial unit is designed
to evaluate the kinds of challenges that scale may impose on engineering and
design. The commercial unit is then scaled down, and the lab unit is scaled up.
This approach, when performed by experienced personnel, will quickly identify
ways in which scale will impact design, what elements can and cannot be piloted
in a practical manner, and what elements must be addressed using other methods,
such as cold-model testing. The author’s experience suggests that this approach
can reduce time to market by months or even years.
A number
of actual examples of potential drawbacks to the stepwise approach exist:
- A
12-in.-inside-diameter (ID) moving-bed waste-conversion reactor using
oxygen or enriched air was piloted successfully. The same concept on a
10-ft-ID commercial-scale (roughly 100:1 capacity scale) may pose a
serious heat removal challenge from the central section of the reactor,
possibly requiring a completely different design and raising questions
about the applicability of data collected on the smaller scale to date to
the larger scale (yield, selectivity, etc.). The technology developer can
opt to use multiple trains to keep reactor size small in the commercial
plant, but this carries a negative economic impact that will not be faced
until the commercial design stage is reached.
- Gasification,
combustion and many other processes produce solid byproducts (slag,
clinkers, spent solids, etc.) that require gravity removal from the
reaction zone. In a small unit, a few lb are removed daily and often
dropped into a 50-gal container that is sealed on top and purged with
nitrogen. The container is then emptied under safe procedures on a regular
basis. Pressure letdown and heat dissipation of material removed occurs in
the oversized container. On a commercial scale, the solids removal may be
thousands of lb/hr, requiring a large train of equipment to cool,
depressurize and inert the material at a significant cost. Often, this
added equipment also raises the elevation of the reactor, adding more cost
due to a taller structure, increased pipe run length and other elements.
These modifications pose a significant negative impact on process
economics that will not be identified until the commercial design is
started, possibly putting the project in jeopardy.
- Startup
and shutdown operations make up another
variable with potentially significant negative economic impacts, and must
be studied at the pilot scale. In the lab, an inert gas is passed through
a temperature-controlled heater to either heat or cool the reactor. This
once-through approach is infeasible on commercial scale, often requiring
the addition of a dedicated recycle compressor loop and associated
equipment, with negative economic impacts on the process.
CONTINUES
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