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Abstract
An analysis of heat and mass transfer in the dehydration of natural gas using solid desiccants has been carried out, leading to a mathematical model for the simulation of the dehumidification process of a wet gas stream flowing through a porous bed containing desiccant particles. The process is divided in two periods: one for adsorption, in which the gas stream is actually dehydrated, and another one for the regeneration of the desiccant material, both of which are cyclically applied. The mathematical model considers spatial dependence in the flow direction, such that heat and mass transfer resistances within particles are treated in a lumped capacitance fashion. The resulting formulation constitutes a system of four coupled equations for describing temperature and humidity fields within solid particles and in the gas stream. These equations are normalized in terms of physically meaningful dimensionless groups, and numerically solved for producing simulation results for different configurations. The results suggest that the duration of regeneration periods can be shorter than those for adsorption, and that this dependence is clearly influenced by the regeneration temperature. In addition, it is seen that larger operation pressures can lead to larger water concentration in the adsorbed phase, and hence higher dehumidification levels.
NOMENCLATURE
| = | cross-section area | |
| = | surface area | |
| c, cp | = | specific heats |
| C*r | = | thermal capacity ratio |
| hh | = | convective heat transfer coefficient |
| hm | = | convective mass transfer coefficient |
| i | = | specific enthalpy |
| isor | = | heat of sorption |
| j | = | mass flux |
| L | = | vessel height |
| m | = | mass |
| Ntu | = | number of transfer units |
| = | mass flow rate | |
| n | = | unit normal vector |
| = | specific surface area | |
| t | = | time |
| T | = | temperature |
| vi | = | process gas flow velocity |
| vb | = | bulk velocity |
| = | volume | |
| V*rat | = | capacity ratio between two periods |
| V*r | = | Volumetric capacity ratio |
| x | = | distance from the vessel entrance |
| W | = | adsorbed water concentration |
| Y | = | dry basis vapor concentration |
Greek Symbols
| ρ | = | density or specific mass |
| ε | = | porosity |
| Ω | = | dimensionless maximum water uptake |
| φ | = | relative humidity |
| ϕ | = | fraction of sensible energy to gas stream |
| τ | = | duration of a period |
| τdw | = | dwell time |
Subscripts
| adv | = | advective |
| ads | = | adsorption period |
| b | = | adsorbent bed |
| δ | = | closed pore |
| e | = | effective or apparent |
| g | = | natural gas |
| in | = | inlet |
| l | = | adsorbed water |
| op | = | operation |
| out | = | outlet |
| π | = | pore |
| reg | = | regeneration period |
| s | = | solid phase |
| v | = | water vapor |
Superscripts
| h | = | heat transfer |
| i | = | interparticle volume |
| m | = | mass transfer |
| p | = | particle |
| * | = | dimensionless quantity |
| ∼ | = | dry basis |
| = | space-averaged quantity |
Additional information
Notes on contributors
Jorge D. Benther
Jorge D. Benther received his B.S. (2009) and M.S. (2011) degrees, both in mechanical engineering, from Universidade Federal Fluminense, and he is currently a Ph.D. student, also in mechanical engineering, at the same university. He is the author of five conference papers, and his main research experience is related to modeling and simulation of the desiccant dehydration of natural gas.
Leandro A. Sphaier
Leandro A. Sphaier received his B.S. (1998) and M.S. (2000) degrees, both in mechanical engineering, from Universidade Federal do Rio de Janeiro, and his Ph.D., also in mechanical engineering, from the University of Illinois at Chicago (2005). After obtaining his doctorate degree, he joined the Department of Mechanical Engineering of Universidade Federal Fluminense (UFF), Brazil, in 2006, where he currently holds an associate professor position. He has co-authored more than 100 refereed journals and conference papers, besides being the author of an invited book chapter and a textbook (in Portuguese). His research is mainly focused on mathematical modeling and simulation of heat and mass transfer, which includes total heat exchangers, desiccant cooling cycles, adsorbed gas storage, and development of hybrid numerical–analytical methods. Some of his recent research activities also include thermal characterization of nanocomposite materials and modeling and simulation of heat transfer in microchannels. He is also a member of ASME and of the Brazilian Society for Mechanical Sciences and Engineering (ABCM).