不同化學產品的結晶技術經濟評估比較

Abstract

化學工業程序中，結晶程序乃一歷史久遠且現今仍相當重要的分離技術。除了會消耗大量能量的傳統單效蒸發結晶程序外，程序設計或溶劑分離技術上的替代方案亦陸續被提出。本研究分析了以不同程序從水溶液連續生產數種定量晶體所需成本，其中包含蒸發結晶、以多孔性疏水膜進行的薄膜蒸餾、逆滲透薄膜結晶，以及共熔冷凍結晶。在分析溶質性質、各程序操作條件，以及各單元操作的性質後，結果顯示一般而言，用於移除水的成本影響程序經濟最劇。 在溶質性質中，溶解度因直接決定生產單位晶體所需移除水量而對程序經濟影響最大，其他性質則影響不大；另一方面，為了提供大量蒸發熱，加熱用公共流體的價格對程序經濟有重大影響：若可獲得成本低於產自煤或天然氣之低壓蒸氣的熱源，則其價格亦會大幅影響程序經濟。總的來說，薄膜蒸餾鮮為最經濟之程序，而若系統滲透壓夠小，逆滲透薄膜結晶會最為經濟。對於逆滲透法不適用之化學物質，共熔冷凍結晶會在熱源單價高及溶解度低時最為經濟，其餘狀況下，傳統的蒸發結晶則會成為最經濟的選擇，而最佳效數必須由溶解度、熱源單價、以及熱源溫度決定。基於文中結果推論，依程序參數而定的結晶技術建議也已呈現在本文最後。

Crystallization is an ancient unit operation that remains vital for the chemical process industry. Traditional single-effect evaporation consumes a great deal of energy and various process design or water removing technology alternatives have been proposed. In this work the total cost of producing a fixed quantity of different solid chemicals by crystallization from water is determined for several different technologies: evaporative crystallization (EC), membrane distillation (MD) with porous hydrophobic membranes, reverse osmosis membrane assisted crystallization, and eutectic freeze crystallization (EFC). After considering the physical properties of solute, operating conditions for each process, and characteristic of unit operations, the result shows that for most cases, cost related to water removal plays the most important role in the process economic. Among the solute properties, solubility has the greatest effect on the cost of the process since it determines the amount of water that must be removed per unit product produced while other properties’ effect on the process economic are limited. As for utility, due to the large amount of heat needed for evaporation, heating energy like low-pressure steam is of great importance. If waste heat is available at a unit price lower than that of low-pressure steam produced using coal or natural gas, then the assumed price of it also has a significant effect on the economics. The results indicate that generally, MD process possesses little advantage over other technology and RO will have the lowest cost when the osmotic pressure is low enough. Otherwise, when solubility is not too high (0.15–0.6 kg/kg solution) and heat source is of high unit price (higher than 2–7 USD/GJ), EFC will be the best choice. On contrary, a traditional EC process will have the lowest cost when producing highly soluble products or very cheap heating energy is available. Moreover, the optimal number of effect depends on three factors: solubility, heating energy unit price, and temperature of heating energy. Finally, heuristic suggestion for crystallization technology depending on process parameters is presented.