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.