Flowsheet Description
The overall process shown in Figure 2 can be considered in four
subsections: 1. gas treatment and energy recovery, 2. sub-ambient heat
exchange, 3. pressure driven CO2 separation, and 4.
product liquefaction. Figure 2 shows one exemplar separation process
which could be utilized, a VPSA unit. A second version of this flowsheet
is reported in Figure S1 for process configurations where the
liquefaction pressure exceeds the separator operating pressure. In that
configuration, one of the compressors (COMP4) is replaced with a
turboexpander (TURB3) and its feed is passed through the sub-ambient
heat exchanger network prior to expansion. The complete flow sheet
including all unit operations for the case shown in Figure 2 is given in
Figure S2, with its accompanying stream table reported in Table S1.
Specific values discussed throughout this section come from this
flowsheet and accompanying stream tables.
An important feature of this design is the fact that it enables
steady-state operation, including the secondary liquefaction-based
separation without the need for any external cooling utility beyond
cooling water. This is similar to the Linde cryogenic distillation case
for air separations53-57 in this process, mechanical
compression and expansion provide the cooling for the process and
sufficient driving force to enable the separation. Variation of process
conditions to enable a reasonable minimum temperature approach and those
subsequent effects on energetics and economics are considered in the
following discussion.