A SIMULATION STUDY OF THE DRAW SOLUTION SEPARATION SYSTEM IN FORWARD OSMOSIS DESALINATION

A SIMULATION STUDY OF THE DRAW SOLUTION SEPARATION SYSTEM IN FORWARD OSMOSIS DESALINATION. Masters thesis, King Fahd University of Petroleum and Minerals.

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Arabic Abstract

نظرا لازدياد التعداد السكاني العالمي، مشاكل شح الماء في بعض المناطق والحاجة للماء النقي في نشاطات الحياة أصبحت أشد إلحاحاً. التحلية باستخدام الطاقة الحرارية أصبحت تقنية ناضجة إلى حد ما ومجالات تطويرها محدودة. لذلك لزم الباحثون التوجه وصب تركيز الدراسات على التحلية باستخدام المرشحات. التناضح الأمامي تعتبر تقنية واعدة في مجال تحلية المياه نظرا لقلة الطاقة الحرارية المستهلكة وجودة الماء الناتج من عماية التحلية. تتكون وحدة التناضح الأمامي من قسمين رئيسيين: 1) وحدة الترشيح و2) وحدة الفصل. مما جذب الباحثون للتوجه لهذه التقنية هو إمكانية فصل الماء باستهلاك طاقة حرارية منخفضة نسبياً وذلك باستخدام محلول بيكربونات الأمونيا لإتمام العملية. بعد عملية الفصل باستخدام أعمدة التقطير، ينتج من الأسفل الماء المحلى ومن أعلى العمود بخار ماء, ثاني أكسيد الكربون والنشادر في الحالة الغازية. و من ثم يتم امتصاص هذا التيار الغازي بواسطة جزء من محلول بيكربونات الأمونيا بواسطة عمود تقطير آخر. الهدف من هذه الرسالة هو دراسة وحدة الترشيح بشكل مبسط لحساب كمية الماء العابر من خلال المرشح وعمل محاكاة لجزئية فصل الماء من المحلول باستخدام برنامج Aspen Plus. إضافة إلى ذلك لتحسين أداء وحدة التحلية، تم إضافة ودراسة آلية استفادة من الطاقة الحرارية المهدرة لتخفيف إجمالي الطاقة المدخلة إلى النظام.

English Abstract

Due to the rapid increase in global population and the regional water scarcity issues, the demand of freshwater for life activates has become more significant. Thermal desalination technologies are mature to some extent, and they are not highly efficient in terms of energy consumption. Therefore, the global trend of research was turned to investigate more on membrane-based desalination. Forward osmosis is one of the promising alternative method for seawater or brackish water desalination since its remarkable capability of handling feed with different salinity level, high level of purity and low energy consumption. The FO system basically consists of two compartments i) membrane unit where thin-film composite was utilized and ii) separation system that comprises of distillation and absorption columns. The agent fluid utilized in the process is called the draw solution (DS). One of the potential candidate solutes is ammonia bicarbonate (AB) since it has a relatively high solubility in water and osmotic pressure, and most importantly it decomposes into water, CO2 and NH3 under moderate heating (60 oC at atmospheric pressure). Water flux highly depends on the osmotic pressure difference between feed and DS which is the driven- force for water solvent to pass through the membrane. Osmotic pressure was increasing with DS concentration while reached its peak value just when the AB starts the dissociation. The diluted DS was then introduced to RadFrac type distillation column with variable number of stages for the aim of separating water form DS solutes as liquid product at a rate of 1 L/min. Operating the column at high temperature and low temperature turned out to enhance the gain output ratio (GOR), because AB decomposition process occurs more rapidly at these conditions. The ammonia level in product water was assigned as controlling parameter for the distillation process and ensuring not to exceed 1 ppm. The specific energy consumption was found to be 180 kWh/m3 at normal pressure and temperature. Part of the DS coming out of the membrane has been utilized as agent stream in the absorber to recover the draw solute leaving the distillation column. The gas dissolving process (absorption) in liquid operates better at high pressure and low temperature which is the opposite conditions of what distillation column. Therefore, the concept of adding a gas compressor was necessary for achieving good recovery ratio. In order to enhance the overall system performance, a heat recovery system was implemented to utilizes the compressed gas stream as a heating source at certain column stages. The performance improvement was significant ranging from 25 to 57 % in term of GOR, and it was concluded that as the compressed gas enters at stages closer to the reboiler lead to a better result. To evaluate the contribution of the compressor work, the performance ratio (PR) concept was applied to account for both thermal and electrical energy utilization. Overall, the FO desalination system was not as low energy system as expected that agrees to the recent studies published. However, it is still under development stages which implies further advanced studies have to be conducted, and operating the system at large scale to acquire a more accurate performance values.

Item Type: Thesis (Masters)
Subjects: Chemical Engineering
Mechanical
Department: College of Engineering and Physics > Mechanical Engineering
Committee Advisor: Antar, Mohamed Abdelkarim Mohamed
Committee Members: Zahid, Umer and Khalifa, Atia and Falath, Wail
Depositing User: MOHAMMED AL-TAIRI (g201471500)
Date Deposited: 07 Oct 2021 10:34
Last Modified: 07 Oct 2021 10:34
URI: https://eprints.kfupm.edu.sa/id/eprint/141965