- 學位論文
直接接觸薄膜蒸餾─高鹽濃度進料之結垢分析與海水淡化之能源需求評估
Direct contact membrane distillation─Analysis of scaling for high NaCl conc. feeding and energy evaluation for seawater desalination
摘要
薄膜蒸餾至今未被工業界應用之主因，除熱能成本考量外，長時間操作穩定性瞭解有限亦是主因之一，於海水淡化長時間操作於後段高濃度溶液，薄膜結垢是通量衰退之主因。並了解結垢物產生現象及如何抑制結垢以減緩通量衰退為工業應用之重要課題，其實驗使用自行配製濃度(3及15wt.%)NaCl溶液做為進料，並操作不同之膜材、進料溫度及進料流量探討其滲透通量、鹽類阻擋率變化及結垢情形來探討通量衰退情形。最後利用實驗結果配合電腦模擬進行海水淡化能源整合系統之估算。 本研究所選用的膜材為中原薄膜中心 PTEE 0.12μm、Millipore PTFE 0.45μm及Millipore PVDF 0.2μm進行實驗，其結果顯示中原薄膜中心之PTFE薄膜操作在較高溫進料時，初始有明顯較高通量，相對造成濃度極化現象較為嚴重，進而導致滲透通量下降較明顯。本研究亦發現以進料為高濃度NaCl溶液(15wt.%)，此時提高進料端流量對於濃度極化及結垢現象是無太大幫助的。 為了減緩薄膜結垢，本研究於高濃度NaCl溶液中添加鹽酸及檸檬酸，結果顯示鹽酸與檸檬酸皆能減緩膜結垢的生成，尤以檸檬酸其阻垢效果較好，但經操作24小時後，由電導度量測發現，其中加入檸檬酸阻擋率降至僅約85%。本研究在長時間濃縮操作，當MD系統操作於進料溶液為70℃下，其進料濃度達約飽和狀態(28 wt.%)，已有大量結晶物附著薄膜表面，使通量大幅衰退，此操作程序下若加入抗垢劑也無顯著效果。 本研究也提出三段水回收之能源整合系統設計，本次實驗設計之DCMD模組大小為0.6 m × 0.6 m × 2(有效膜面積為0.72 m2)，日產100 m3純水之系統能源需求，若進料溶液以廢餘熱作為加熱熱源為主，其能源成本僅以電驅動泵浦為主之情況下，在總水回收率70%，以各段水回收率設定為(35, 25, 10)%，單位產水電能耗為5.36 MJ/m3，而總水回收率80%，以各段水回收率設定為(40, 30, 10)%時，單位產水電能耗為6.14 MJ/m3，其前者所需總膜面積為119.87m2，後者則為137.86m2。 最後探討倘若當工廠廢餘熱只可將進料溶液加熱至70℃，進行兩段水回收80%(50, 30)%，此能源消耗部分僅為泵浦電能10.14 MJ/m3，如能利用蒸氣加熱進料溶液至90℃進行三段水回收80% (40, 30, 10)% 操作，因此其蒸氣加熱能源與泵浦電能總消耗高達1095.61 MJ/m3，但所需總模組為333個較後者192個多出141個。
關鍵字
薄膜蒸餾並列摘要
Membrane distillation (MD) has been studied over 20 years, however, there is still very few applications in industries due to the concern of high energy consumption and lack of knowledge in scaling effect of the process. It is important to get a better understanding of membrane scaling and how to use antiscalants to limit scaling for MD desalination. In the study, direct contact membrane distillation experiments using high salt concentration solution as feed were conducted to investigate the effect of operation conditions on permeate flux﹐salts rejection and scaling phenomenon. In addition, energy cost and membrane area requirement for a three-stage direct contact membrane distillation process with a desalination capacity of 100 CMD were also simulated in the study. Three different membranes, CMT PTFE (0.12μm), Millipore PTFE (0.45μm) and Millipore PVDF (0.2μm), were used in the experiments. Experimental results of feeding 15 wt % NaCl solution at 70 oC showed that the initial permeate flux reaches 76.2 kg/m2.hr, but after 24 hours operation the flux has a 64% decline due to a scaling layer formed on the membranes surface. In order to limit membrane scaling in membrane distillation of high salt concentration solution, citric acid and hydrochloric acid as antiscalant were added into the feed solution for experiments. Although both the antisaclants can limit the rate of scaling formation, citric acid is seen to give a better performance in flux than that by hydrochloric acid, and after 24 hours operations, the salt rejection with the condition of adding hydrochloric acid will decrease to 80%. Experiments with continuous concentration of 15 wt % NaCl solution indicated that when the salt concentration in circulated stream increased to about 20~21 wt % will give a fast decline in flux. Under the critical NaCl concentration the addition of antiscalants will only give a slightly effect on enhancing flux. Base on the fluxes part from experiments and the other part from theoretical calculations, a three-stage process designed for 100 CMD seawater desalination was simulated. The flat-sheet membrane module each unit has an area of 0.72 m2 and waste heat is assumed to be available for heating the seawater feed to 90 oC. Simulated results showed that the energy consumption of pumps for per unit volume of water produced are 5.36 MJ/m3 for 70 % water recovery ( 35, 25, and 10 % for each stage). When the water recovery is increased to 80 % (40, 30 and 10 %, respectively), the energy consumption is 6.14 MJ/m3. The membrane areas requirement evaluated for the former is 120 m2 , while the latter is 138 m2. If the waste heat can only provide the feed solution to 70 ℃, simulation of two-stage MD process for 80 % recovery showed the pumping energy and the membrane modules requirement are 10.14 MJ/m3 and 333 units, respectively. When the seawater feed at 70 oC is further heated to 90 oC by another thermal source before entering the 1st stage MD modules, simulation results for three stage MD process showed that the thermal and pumping energy required increases to 1095.61 MJ/m3 for 80 % water recovery, but the membrane modules required is only 193 units.