PROCESSES: BASICS OF PROCESS DESIGN - Biomass as a Sustainable Energy Source for the Future - page 222

The economic performance, expressed by the economic potential (EP 3 ), accounts
for the cost of capital investment. The ecological performance is the carbon atom
efficiency as defined in Equation (7.14). The water fraction in the effluent from
the FT conversion block reactor(s) is taken as a technological constraint specified
in Equation (7.15).
When comparing the outcomes of the enhanced conversion case with the base case
results, one sees that the higher CO conversion allows for slightly lower syngas intake
to still make the same amount of liquid product (C 4+ ). This reduction results in smaller
flows through the process and thus in smaller equipment. Consequently, the costs of
feed and investments go down for all process units: conversion, separator, and mixer.
It is seen from the data in Table 7.4 that the economic performance increases with
conversion but less than proportional. The carbon atom efficiency improves relative
to the base case but only marginally. The technological constraint (water fraction) is
nearly hitting its upper bound (0.22; see Equation (7.15)). This implies that CO
conversion cannot be increased much beyond 54% for this production rate. The con-
clusion of this evaluation is that economic performance and carbon atom efficiency
both become somewhat better when moderately increasing the CO conversion. The
ceiling for CO conversion is reached by hitting a technological constraint.

7.10 INTEGRATING PROCESS UNITS INTO THE
FUNCTIONAL NETWORK

The design levels discussed so far (1 3) have resulted in optional networks of required

processing functions jointly forming the process, as well as in targets for the functional
duties. Design level 4 deals with process integration of heat, solvents, and water,
between process units mutually and with the utility system. Then, at level 5, the design
focus will shift to the internal structuring of process units with the associated equipment
engineering aspects. This activity requires much insight into the nature of various exist-
ing specific processing technologies, equipment types, and the integration of a process
unit into the process. The next part of this topic extensively covers processing technol-
ogies for biomass as well as design aspects of process units. Development and design of
process units must take place within the overall frame of a process. As a prelude to the
integration of units into a process during design, one must ask a pertinent question:

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What kind of information does a process designer need about a process unit to be able to
properly integrate it into the functional network?

The range of conditions under which a process unit must be able to operate must lie
within its window of feasible operation. In other words, can the window(s) of feasible
operation of a process unit be matched with the windows of other units in the network?
Figure 7.13 shows some windows of unit variables that require attention when inte-
grating a unit into a process design. The boundaries of the windows of a process unit
are ideally provided as mathematical inequality constraints in a design model of a
process unit. We will now briefly discuss the conditions represented in Figure 7.13.