PROCESS INTENSIFICATION BY UTILIZING MULTISTAGE MANIFOLD MICROCHANNEL HEAT AND MASS EXCHANGERS
Ohadi, Michael M
Al-Hajri, Ebrahim SA
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Much of research and development work has been dedicated to implement the heat and mass transfer using microchannel technology; however, it is not yet cost effective and is limited to higher end applications such as electronics cooling and selected applications in automotive and aircraft heat exchangers. The work on mass transfer application of micro channels also has been very limited, despite the very high potential contribution of micro channels for mass transfer enhancement. Scaling up of microchannel equipment presents several manufacturing and process organization challenges such as flow distribution inside microchannels, cost, fouling, high pressure drops etc. This thesis presents the development of a cost effective and compact tubular manifold microchannel heat and mass exchanger (MMHX) for industrial applications. A novel design for the flow distribution manifolds has been proposed. The proposed manifold helps in the enhancement of heat and mass transfer by creating better flow mixing. The MMHX is designed in such a way that the manifold causes the flow to break into multiple passes of very short flow lengths in the microchannels. These flow lengths are short enough such that the flow in the channels is always into entry zone (developing laminar zone) both hydrodynamically as well as thermally, resulting in higher heat transfer than that in the fully developed laminar flow in conventional microchannel heat exchangers. The pressure drop in the device is low as the fluid flow length into the microchannels is very short. While the manifold design helps in flow distribution, very short flow length inside the microchannels mitigates the problems of flow instability of two-phase heat transfer applications such as that in evaporators and condensers. The mass transfer in gas liquid reaction applications is enhanced due to the multiple passes where continuous breaking of the gas liquid interface as well as mixing of the bulk liquid occurs. A multi-pass microchannel heat and mass exchanger prototype was designed, fabricated and was experimentally tested for the performance as liquid-liquid heat exchanger, evaporator, condenser and gas- liquid absorber. Experiments were carried out by changing the liquid and gas flow rates, geometry of the microchannels and the size of the manifold. Flow visualization studies were also performed to study two phase flow distribution and flow pattern in the manifold. Experimental results have shown that the mass transfer coefficient (using CO2 and DEA-water solution) for the microchannel absorber is 1 to 2 orders of magnitude higher than the conventional absorber. This increase in mass transfer is mainly attributed to high interfacial area to volume ratio of microchannels and good mixing in the manifold. Similarly, heat transfer coefficient for the single phase heat transfer as well as for two phase heat transfer (evaporator and condenser) is about 3 to 8 times higher than the conventional heat exchangers such as shell and tube or plate type heat exchanger. High transfer rates enable us to design compact heat and mass transfer devices for the industrial applications. Industrial processes, such as carbon capture, which are not economically viable due to their high cost, can be feasible with the development of these next generation heat and mass transfer equipment. Due to the simplicity of the component design and the assembly, cost of the industrial scale equipment can be substantially lower as compared other compact heat exchangers. Current work is the continuation of heat and mass transfer work being carried out at the S2TS lab in University of Maryland. While Jha V.(2012) studied the first version of single pass manifold microchannel heat exchanger, Ganapathy H. (2014) studied the absorption of CO2 in DEA solution in single microchannel as well as in parallel microchannels. MMHX studied in this study builds on the previous work by introducing the multipass concept and utilizing commercially available fin tubes as microchannel surfaces.