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.