Thermally Conductive Polyethylene/Expanded Graphite Composites as Heat Transfer Surface: Mechanical, Thermo-Physical and Surface Behavior

Abstract

Composites of high-density polyethylene (HDPE) and expanded graphite (EG) are prepared for heat exchangers in multi-effect distillation (MED) desalination. At 50 wt.% EG loading, the thermal conductivity of HDPE was increased by 372%. Moreover, the surface wettability of the HDPE/EG composite was enhanced by corona and RF plasma treatment as demonstrated by the increase in surface free energy from 28.5 mJ/m2 for untreated HDPE/EG to 55.5 and 54.5 mJ/m2 for HDPE/EG treated by corona and RF plasma, respectively. This enhanced surface wettability was retained over a long time with only a 9% and 18% decrease in RF and corona plasma-treated samples’ surface energy after two months. The viscoelastic moduli and the complex viscosity profiles indicated that EG content dictates the optimum processing technique. At loading below 30 wt.%, the extrusion process is preferred, while above 30 wt.% loading, injection molding is preferred. The plasma treatment also improved the HDPE/EG composite overall heat transfer coefficient with an overall heat transfer coefficient of the composite reaching about 98% that of stainless steel. Moreover, the plasma-treated composite exhibited superior resistance to crystallization fouling in both CaSO4 solution and artificial seawater compared to untreated composites and stainless-steel surfaces.

Sustainability Assessment and Techno-Economic Analysis of Thermally Enhanced Polymer Tube for Multi-Effect Distillation (MED) Technology

“…In this section, the feasibility analysis is performed based on our technical results of fabricating thermal conductivity for polymer–graphite composite material published in [ 15 , 16 ]. The HDPE has been loaded up to 50 wt.% with expanded graphite (EG), and the results showed that the thermal conductivity of the composite HDPE/EG increased to 2.2 W/(m·K) [ 15 ]. In another work [ 16 ], the authors disclosed a new method of fabricating a large-size thermally enhanced thermoplastic sheet/pipe.…”

“…Total energy demands for the composite tube extrusion have been analyzed using an extrusion simulation software (Extruder module of the Compuplast CAE Virtual Extrusion Laboratory Software, Compuplast International, Zlín, Czech Republic). The simulator incorporates the physical/material properties (thermal conductivity, heat capacity, and viscosity) described in References [ 15 , 16 ] and assumptions of standing processing conditions (friction coefficient between screw and plastics = 0.2, and between plastics and barrel = 0.3, die pressure consumption of 20 MPa). Due to the high viscosity of highly polymer composites, the temperature profiles of 190, 220, 245, and 260 °C from the first heating zone to the die section were chosen.…”

“…Here, we systematically quantify the thermally enhanced polymer’s life-cycle effects (using our results on fabrication of the polymer–graphite composites [ 15 , 16 ] and titanium tubes for the MED plant by implementing the ReCiPe evaluation approach. We investigate the effect of changing the thermal conductivity from 3 to 5 W/(m·K) for PE/EG (70/30) and PE/EG (60/40) composite, respectively.…”

“…We recently reported the fabrication of HDPE/expanded graphite (EG) composites for MED evaporators [ 15 ]. The thermal conductivity of HDPE-50% EG was 372% higher than that of HDPE.…”

“…The plasma treatment HDEP/EG composite’s overall heat-transfer coefficient was about 98% of stainless steel. Moreover, the plasma-treated composite exhibited superior resistance to crystallization fouling in both CaSO 4 solution and artificial seawater than untreated composites and stainless-steel surfaces [ 15 ].…”