Date of Award

5-31-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Chemical Engineering - (Ph.D.)

Department

Chemical, Biological and Pharmaceutical Engineering

First Advisor

Sirkar, Kamalesh K.

Second Advisor

Iqbal, Zafar

Third Advisor

Khusid, Boris

Fourth Advisor

Mitra, S.

Fifth Advisor

Wang, Xianqin

Abstract

Direct contact membrane distillation (DCMD)-based desalination process is a thermally- driven separation process where a hydrophobic microporous membrane separates a hot brine feed and a cold distillate which condenses the water vapor from the hot brine passing through the membrane pores. So far, DCMD has been explored for hot brines and other aqueous solutions below 100oC. For feed solutions above 100oC, DCMD has an extra advantage over other conventional separation processes like reverse osmosis (RO) which requires cooling of the feed solution costing additional energy; further, RO can not utilize the heat available in the feed solution. Produced water obtained from steam assisted gravity drainage (SAGD) process is one such example where DCMD can potentially be a very useful process. In this dissertation, the DCMD technique is explored in the range of 80-130oC for brine solutions containing 10000 ppm sodium chloride with porous flat sheet polytetrafluoroethylene (PTFE) membranes. The pressure of solution went up to 2-3 atm. The performance of this membrane has also been explored with a saline feed containing 3000 ppm NaCl, 45 ppm phenol, 45 ppm cresol and 10 ppm naphthenic acid, simulating the composition of hot produced water obtained from the SAGD process. There was no leakage of salt under any conditions. The highest water vapor flux achieved was 195 L/m2-hr, which is a few times larger than that for seawater RO process. The water generated by the DCMD process may be used for steam generation in the SAGD process. Different types of hollow fiber modules were fabricated and explored with 1% NaCl solution and produced water at lower as well as higher temperatures.

Desalination by air gap membrane distillation (AGMD) combines in one device the processes of evaporation of water from hot brine through a porous hydrophobic membrane, condensation of the water vapor so generated on a cold condenser surface and recovery of heat in the condenser cooling liquid which may be the cold/cooled brine. Existing devices for AGMD are bulky due to the condenser arrangements adopted, even though evaporation from brine may be carried out with porous hydrophobic hollow fibers. In this research a two-hollow fiber-set based compact device was developed where all such functions are combined in a compact device. The first fiber set consists of porous hydrophobic hollow fibers of either polypropylene (PP) or polyvinylidene fluoride (PVDF). The second set of hollow fibers is of solid PP through the bore of each such hollow fiber the cooling liquid is passed and on the outside surface of which water vapor condensation takes place. A number of modules having different packing densities of hollow fibers have been studied for desalination of brine containing 1% NaCl. The performance of such modules have been investigated for a range of feed brine temperatures as well as the flow rates of the two liquid streams, hot brine and the cooling liquid. Enhanced water vapor productivity was achieved in small laboratory devices suggesting potential for scale up.

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