面對日益嚴峻的氣候變遷，直接空氣捕獲（Direct Air Capture, DAC）被視為達成負碳排放的關鍵技術之一。然而，大氣中的 CO2濃度僅約 400 ppm，遠低於燃燒後煙道氣，使得捕獲程序在熱力學驅動力、能耗與設備規模上皆面臨顯著挑戰。有鑑於膜分離技術具有結構簡單、模組化、免除吸收劑再生能耗等優勢，本研究提出系統性評估一套多級膜式直接空氣捕獲（membrane-based DAC, m-DAC）程序，藉由建模、經濟分析與最佳化的整合流程，探討其應用於超低濃度 CO₂ 捕獲之技術與經濟可行性。
本研究於 Aspen Custom Modeler（ACM）中，依據質量守恆與滲透傳輸方程，分別建構同向流、交叉流與逆向流三種膜分離單元模型，並以500個離散單元進行數值求解。模型於等溫、穩態、忽略壓降與固定滲透係數等假設下建立，並經網格獨立性測試確認其收斂性；同時以文獻數據交叉驗證，膜面積及兩側 CO2濃度之誤差均控制於5%以內。在此基礎上，本研究以Python建置遺傳演算法（Genetic Algorithm, GA），透過COM介面與Aspen Plus進行資料交換，並以年化總成本（TAC）最小化與CO2回收率最大化為雙目標，建立多級串聯之超結構最佳化架構；最終導入TOPSIS多準則決策法，自帕雷托前緣中篩選兼顧效能與經濟性之最適操作點。
分析結果顯示，流場配置為影響 m-DAC 效能的決定性因素：逆向流因能於膜模組軸向維持較高的局部濃度差，僅需5級串聯與約8bar 操作壓力即可達成目標純度，而交叉流與同向流則分別須提升至約12.7 bar 與30 bar。於商業膜材Polaris™ 的最佳化中，逆向流可同時使CO2出口流量提升6.63%、出口濃度提升5.92%，並使TAC 降低2.18%，展現效能與成本兼容的優勢。此外，本研究亦評估高透氣性Siloxane奈米薄膜之潛力，發現其雖能有效縮減膜面積並提高捕獲流量，卻因 N2、O2的伴隨穿透而推升後端壓縮能耗，凸顯膜材設計須在滲透率與選擇率間取得權衡。整體而言，本研究所建立之建模—最佳化整合方法，可作為未來 m-DAC 程序設計、膜材篩選與工業化評估之參考依據。

Direct air capture must contend with a CO2 concentration of only 400 ppm, at which a single membrane stage cannot generate a useful driving force, so a multistage cascade is unavoidable— and its stage pressures and areas form a large, strongly coupled design space.
This work couples rigorous membrane models to a genetic algorithm to optimise that space.Co-current, cross-flow and counter-current unit models were built in Aspen Custom Modeler,verified grid-independent at 500 cells and validated to within 5 %, then embedded in an Aspen Plus flowsheet and driven through a COM interface by a Python genetic algorithm using ratio-encoded chromosomes and a lexicographic purity constraint. TOPSIS selects one design from each Pareto front.
Flow configuration dominates: counter-current reaches the target purity in five stages at 8bar, whereas cross-flow needs 12.7 bar and co-current nearly 30 bar and a sixth stage. Cost is governed by the module frame (73.5–84.8 %), not the membrane (under 2 %). A high-permeance siloxane nanomembrane cuts the specific cost by 37–38 % in cross-flow and counter-current but roughly doubles the compression and cooling duty. The siloxane counter-current design strictly dominates the other five, at 20,738 USD per tonne CO2, 80.3 % recovery and 18.19 mol% purity.

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