PROCESSES: BASICS OF PROCESS DESIGN - Biomass as a Sustainable Energy Source for the Future - page 197

of operation, and one sets targets for the degree of conversion of feeds. The impact of
decision variables on economics decreases when addressing the increasingly finer
details of the design at the lower levels: process unit, compartments, and equipment
internals.
Along with the hierarchical approach comes top-down targeting: the design at a
particular level establishes targets for a processing task, which must be achieved
by the design at a subsequent level of finer detail. For example, one can target for
a certain product yield of a reactor when dealing with the integration of that reactor
in the entire process. When going one level down in actually designing the reactor, the
product yield target becomes a fixed quantity, and the reactor variables, such as tem-
perature and residence time, can be varied to meet this target. Then, when setting the
residence time as new target, the geometric size of the reactor can be varied to meet
this residence time target.

7.5.4 Design Levels for a Biomass Conversion Process

A hierarchical decomposition approach is presented here, in which the process design
decisions are distributed over several levels in a top-down manner. Decisions made at
an upper level restrain the choices for making decisions at the lower levels. The inter-
actions in the opposite direction are weaker, although there is always some iterative
feedback: if at a lower level only bad solutions are possible due to an apparently poor
decision at an upper level, this latter decision has to be revised.
For the conceptual design of a biomass conversion process, seven levels will be
considered. The first three levels are treated in this introductory chapter on design,
because they offer the basis of a design with the highest impact on performance.
The remaining four levels are essential to arrive at a finished conceptual process
design. They are mentioned here for the sake of completeness but not further
explained due to restrictions on the size of this chapter.
Level 1: Input output type of exchanges between the process and its environment.

This environment is often a

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of materials, energy, and information,
from which the process extracts feedstocks and to which the products are delivered.
Level 2: Splitting the process into subprocesses, where a subprocess can buy feed
from or sell product to the market, or when technologies of different license holders
are used, or when subprocesses are at different locations.
Level 3: Creating a network of process units per subprocess. The focus here is on the
selection of suitable process unit operations and the setting of targets for processing
duties and changes in the flows of resources in a balanced way to enhance overall
performance. The targets act as set points for the control system design.
Level 4: Process integration of energy, solvents, water, and common reactants (e.g.,
hydrogen). Such integration is done by matching internal sources and sinks. When a
resource is generated in some excess in a unit, it can be transferred to units where
shortages of that resource arise, provided that the quality level of the available
resource is or can be made fit for the identified sink(s).

“ supply chain ”