EXPERIMENTAL CHARACTERIZATION OF CRYSTALLIZATION FOULING IN LIQUID-TO-AIR MEMBRANE ENERGY EXCHANGERS
Date
2018-04-03
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Thesis
Degree Level
Doctoral
Abstract
Liquid-to-air membrane energy exchangers (LAMEEs) are a recent development with potential for energy savings in heating, ventilation and air-conditioning (HVAC) applications. Unlike conventional heat exchangers that transfer only heat using plates or tubes, LAMEEs use semi-permeable membranes to transfer both heat and moisture between liquid desiccant and air streams, while preventing cross-contamination between the fluids. However, the porous membranes used in LAMEEs are prone to crystallization fouling from liquid desiccants, and this may compromise the performance of LAMEEs.
The research reported in this thesis aims to experimentally characterize crystallization fouling in LAMEEs. Experimental tests are performed to dehydrate MgCl2(aq) solution using two types of membranes. The objectives of the research are to (i) develop and calibrate indirect and non-invasive methods to detect the onset of crystallization fouling in a LAMEE, (ii) identify operating conditions that result in crystallization fouling in LAMEEs, and (iii) identify the evolution of crystallization fouling in membranes.
The objectives are met by using non-invasive and invasive methods. The non-invasive methods consist of indirect and direct methods. Three indirect and non-invasive methods are developed to detect fouling by analyzing two measured parameters (moisture transfer flux and resistance). One direct and non-invasive method is used to directly observe the onset of crystallization fouling in the LAMEE. The invasive methods consist of optical microscopy and scanning electron microscopy which are used to characterize the morphology of deposits on membranes, and energy dispersive X-ray spectroscopy which is used to analyze the composition of the deposits.
The direct and non-invasive method detects the start of fouling earlier than the three indirect and non-invasive methods by a factor of three to eight, and is used to calibrate and thereby augment the sensitivity of the indirect and non-invasive methods. Sensitivity studies indicate that crystallization fouling in the LAMEE is dependent on both the concentration of the desiccant solution and the relative humidity of air. Furthermore, the results show that reducing the moisture transfer rate through the membrane can minimize crystallization fouling in the LAMEE. Finally, the evolution and mechanisms of crystallization fouling in membranes are delineated.
Description
Keywords
Crystallization fouling, Membrane, Invasive methods, Non-invasive methods, Detection, Moisture transfer
Citation
Degree
Doctor of Philosophy (Ph.D.)
Department
Mechanical Engineering
Program
Mechanical Engineering