|Ph.D Student||Lisitsin Marina|
|Subject||Investigation of High Recovery Desalination Processes|
|Department||Department of Chemical Engineering||Supervisors||Professor Emeritus Raphael Semiat|
|Professor Emeritus David Hasson|
|Full Thesis text|
The main efforts of this research were focused on the development of an innovative high recovery Multistage Flash Desalination (MSF) process. The MSF process is a robust system able to cope much better than membrane desalination processes for water recovery from contaminated saline feeds. Currently the application of MSF is limited by its high energy consumption and low recovery level which is less than 50% (concentration factor <2) due to the calcium sulfate precipitation concern. A high recovery MSF process can open the door for wide application of thermal desalination in treating contaminated saline wastes, notably produced water.
The development of a high efficiency MSF process was based on removal of a major portion of the calcium ions from the feed water by a viable electrochemical technique. The concept holds for all high calcium sulfate feed solutions and was examined in this work with sea water composition.
Ø Two routes were examined for the pretreatment: electrolytic removal of the calcium ions and Donnan dialysis separation of calcium ions. Electrolytic removal was the preferred technique.
Ø The Fabuss and Lu's model describing solubility limits of CaSO4 polymorphs was confirmed and extended to solutions of reduced calcium content.
Ø A single stage MSF pilot system was constructed to examine operational limits of a seawater feed from which 80% of the calcium ions were removed in long duration runs.
Ø Currently, the threshold of CaSO4 scale precipitation limits MSF seawater desalination to a top heating temperature of less than 120°C and a concentration factor less than 2 (maximum water recovery of 50%).
Ø Experimental results of this study indicated the feasibility of MSF design to a concentration factor as high as 4 and a top brine temperature as high as 1400C.
Ø Economic benefits rendered by these results are shown to be considerable.
A second topic investigated in this work was examining the feasibility of a novel concept for maintaining high recovery rates in brackish water desalination by a single strep process. Brackish water desalination represents an untapped potential for augmenting water resources by the reliable technology of reverse osmosis (RO). There is a significant economic incentive to operate the process at the highest possible fractional water recovery. The degree of water recovery possible with a given raw water is dictated by the scaling problem.
The basic concept of the novel technique is to inject the CO2 to the permeate stream and to induce the penetration of the CO2 from the permeate side of the membrane to the boundary level in the feed side. Carbonic acid solubilizes alkaline scale species such as CaCO3 so that permeation of CO2 gas through the backside of the membrane into the feed solution will alleviate the supersaturation conditions without the need to decrease the pH of the whole bulk of the feed solution. However, experimental results disclosed several unexpected obstacles which hinder application of the novel concept.