Economics of Sub-ambient MOF Fiber Sorbents
Having established that liquefaction is required to reach the target
delivery purity with the MOF sorbents we considered, the Pareto
frontiers were recomputed using a 17:83
CO2:N2 mixture to simulate the inlet to
the PSA unit when liquefaction and recycle are employed. The complete
Pareto front is given in Figures 6 and S16 with all PSA operating
conditions and fiber morphology parameters provided in Tables S21 and
S22. An inset in Figure 6 shows the points with >65%
purity and 92% recovery. MIL-101(Cr) shows a slightly more desirable
Pareto frontier than UiO-66. Both sorbents have a clear tradeoff between
the purity and recovery of the sub-ambient PSA unit. Ten separate PSA
conditions, five for each sorbent, were used as case studies in the
process model discussed. These conditions are indicated in Figure 6 as
starred points and their operating conditions, performance, and fiber
compositions are listed in Table 2. For each case, the temperature, PSA
pressure, PSA vacuum condition, and the PSA product purity and recovery
were used as conditions within the process model. The heat exchanger
system was then optimized to include a 5 °C temperature approach by
varying the liquefaction column pressure. The compressor prior to
liquefaction was always kept as a single-stage, which in cases where the
pressure ratio exceeded 10 may prove unrealistic and undesirable. In
cases where the liquefaction pressure was greater than the PSA operating
pressure, the same flowsheet adjustments were made as those described
previously. The Temperature-Enthalpy(T-H) diagrams for each of the cases
considered are given in Section S4.3 of the Supporting Information.
It is worth noting in Table 2 that both UiO-66 and MIL-101(Cr) show
productivities exceeding 0.015 mol kg-1sec-1, which based on previous discussion should lead
to reasonable costs. In all 10 cases considered, the optimizer yielded a
result close to the specified lower bound value of the vacuum level for
the PSA unit. This suggests there may be room for improvement in the
performance of these fiber sorbents if the PSA process could be operated
economically at even lower vacuum levels.