預測離子液體水溶液互溶間隙及下方臨界溶液溫度於正滲透海水淡化程序之應用

Prediction of Miscibility Gap and Lower Critical Solution Temperature of Aqueous Ionic Solutions for Forward Osmosis Desalination of Seawater

蔡亞叡(Ya-Ruei Tsai)

指導教授 ： 林祥泰

國立臺灣大學/工學院/化學工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201802012

摘要

目前海水淡化相關程序中，逆滲透(reverse osmosis)程序仍佔大宗，但高耗能的特性限制了其經濟效益，相對於逆滲透須施加高壓做為質傳驅動力，近年被提出的正滲透(forward osmosis)程序是透過加入抽取溶質(draw solute)利用溶液本身之滲透壓差分離的技術，可避免高壓所需的高耗能，因此具有取代或輔助現有逆滲透程序的潛力。

根據熱力學理論推導，正滲透相對於逆滲透，多進行了混合抽取溶質的程序，在欲達到相同分離效果的前提下，理論上不可能達到較小的系統能耗，但程序設計端，有研究者提出使用下方臨界溶液溫度(lower critical solution temperature, LCST)型的離子液體做為抽取溶質進行前處理，先利用正滲透將海水中的純水驅動至離子液體溶液中，再利用廢熱加熱LCST-型離子液體使之分為水相及離子液體相，後續再經奈米過濾(nanofiltration)或逆滲透將分離純水，因此有機會降低高單價能源(如電)的使用。

在此程序中，離子液體抽取溶質的選擇非常關鍵，在程序端我們傾向選擇具有高滲透壓、較少的所需加熱能量、較大的溶解度間隙(miscibility gap)以及略低於室溫之LCST，以得到較高的水通量、水處理量以及較低的能耗，並滿足室溫附近的操作條件。由於實驗上LCST型離子液體的數據數量仍十分有限，因此在此研究中，我們建立了一套快速篩選離子液體抽取溶質的方法。我們使用熱力學模型COSMO-SAC 2010 搭配 Pitzer-Debye–Hückel (PDH)模型。此模型僅需個別分子的化學結構資訊，即可得到混合物的熱力學性質。此研究驗證了COSMO-SAC模型即便未經任何迴歸，仍可有效分辨19種上方臨界溶液溫度型(UCST-positive)及18種LCST型離子液體的液液相平衡型態，同時可提供離子液體在不同濃度下之滲透壓、混合焓(enthalpy of mixing)、互溶間隙以及LCST。儘管和實驗相比，我們預測得到的LCST(ARD% = 40.00%)和互溶間隙(ARD% = 35.22%)定量上的預測表現並不佳，定性上互溶間隙及LCST結果和實驗值整體趨勢接近，並可再經由實驗值溫度的相互線性迴歸關係，得到校正之LCST。同時我們討論離相曲線(spinodal curve)附近之溫度效應，發現在LCST-型溶液中氫鍵項的貢獻遠大於在一般UCST型溶液中最重要的理想混合熵(ideal entropy of mixing)項，亦即是分子間作用力和混合熵項之間的競爭導致此型相分離。我們最後利用此模型分析程序所需之能量，驗證了FO + RO和RO程序相比，確實需耗費較大的總能，但經FO前處理後所需的理論電能也大幅下降，亦即確實有應用廢熱降低成本的潛力。我們相信此研究提出的方法可應用於更大規模的抽取溶質快速篩選，協助實驗者更有效率地找出可行的正滲透抽取溶質，並可有效降低實驗所需成本。

關鍵字

正滲透 ； 抽取溶質 ； 離子液體 ； 液液相分離 ； 下方臨界溶液溫度 ； 滲透壓 ； COSMO-SAC ； 能耗分析

並列摘要

As an alternative for high energy-intensive reverse osmosis (RO) in seawater desalination, the novel approach known as forward osmosis (FO) has recently attracted much attention. Unlike RO which uses the high pressure difference as the driving force, FO utilizes the chemical potential gradient, or osmotic pressure, to drive the mass transfer. However, some studies showed that FO is actually not an energy-efficient approach, which may require more thermal energy and lead to higher cost in total. A new desalination scheme using lower critical solution temperature (LCST) ionic liquids as the draw solutes is presented. Despite higher energy consumption, the approach could replace the high cost electricity with low cost waste heat in plants or solar heat in the overall process. The osmotic pressure, required work, LCST and the miscibility gap are the most significant factors for the evaluation of the feasibility of ionic liquid draw solutes. In this work, we use COSMO-SAC 2010 + Pitzer-Debye–Hückel (PDH) model to perform a priori and efficient prediction of phase behavior. In the model the molecular structures are the only input in the model. We show that COSMO-SAC is able to differentiate the LCST and upper critical solution temperature (UCST) mixtures. Though our model provides poor quantitative prediction of LCSTs and the miscibility gaps (ARD%-LCST = 40.00%; ARD%-LLE 35.22%), the model appears to have great performance in qualitative agreement with experimental data. The thermodynamic properties including osmotic pressure, enthalpy of mixing, and the theoretical minimum work for the process are also evaluated. We analyze the temperature effect near the spinodal boundaries in order to have a molecular insight into LCST-positive phase behavior. Our results show that it is the preference of high entropy at high temperature that dominate in LCST-negative phase separations, while competition between molecular interactions, especially the hydrogen bonding interactions, play a more important role in the temperature dependence in the LCST-type phase separation. It is also demonstrated by our model that though with much higher required thermal energy, hybrid FO + RO process consume less electrical duty theoretically, allowing an energy-efficient use of waste heat. We propose a quick and feasible procedure for the screening ionic liquid draw solutes for FO processes.

並列關鍵字

forward osmosis ； draw solute ； ionic liquid ； liquid-liquid equilibrium ； lower critical solution temperature ； osmotic pressure ； COSMO-SAC ； minimum work

參考文獻

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