PROCESSES: BASICS OF PROCESS DESIGN - Biomass as a Sustainable Energy Source for the Future - page 210

Subprocess 2

Waste

C

Oxygen

Conditioning
and
puriication
of feeds

Recovery and
recycle of
unconverted
feed

Chemical
conversion
(e.g., to syngas)

Puriication
of
products

Produc t
syngas

Biomass

Waste

FIGURE 7.10 Expansion of subprocess 2 in Figure 7.8 with sequential order of processing
functions.

The chemical conversion, being the heart of the process, can take place only if
the reactant feeds are delivered under conditions at which the reactions will actu-
ally run. Thus, upstream of a reactor, the feeds must be purified and conditioned
for use in the reactor. This purification involves the removal of abrasive and cor-
rosive components, catalyst inhibitors, and solid materials that will accumulate
in a reactor (like sand and ash). Downstream of a chemical conversion function,
the unconverted reactants can be recovered by physical separations and recycled.
That is, the value of the recovered material must well exceed the cost of sepa-
ration . When a recycle occurs, the accumulation of the inert components in
the system must be prevented by creating an exit or purge (by venting (G) or
bleeding (L)). The product can be purified from side products and is brought
to customer quality specs. This generic order of processing functions is shown
in Figure 7.10. Here, subprocess block 2 in Figure 7.8 is expanded into a block
diagram with a common order of processing activities. Mixing and splitting of
streams are essential functions to direct and control the streams in a process.
Subprocess 3 is expanded in Figure 7.11, showing a sequential - parallel

ordering.
In subprocess 3, syngas from biomass and from natural gas is partially mixed
to match the inlet syngas composition (H 2 : CO = 2.1) for the FT conversion to
hydrocarbon fuels. The remainder of biomass syngas is fed to a power generator,
including some unconverted syngas from the FT synthesis. An alternative is not
to recycle the unconverted syngas but combust it directly for power generation.
In summary, the key activities in the synthesis step are to establish:

￿ Which processing functions are needed per subprocess and in which order

￿ Targets for duties of the various functions (yield, recovery, and separation

factors)

￿ Links between duties and costs of processing functions, allowing to balance

the duties for a good overall (economic) performance of the subprocess