Polymer film heat transfer elements for multi- effect and vapour compression desalination

Abstract

The continuing improved of existing desalination process- bith distillation and menbrane- is contributing significantly to reducing the cost of desalted water, and to the rapid growth of the desalination industry. Thus the word capacity has more than doubled during the two years 2000-2001, and desalination of seawatwe is at present the major source of potable watwr in arid coastal regions such as the Arabian Gulf region. Conventional multi- effect distillation ( MED) and multi- stage flash (MSF)desalinators use cupro-nickel and /or titanium heat transfer surfaces. Polyolefins such as high density polyethylene ( HDPE) and polypropylene (pp) have better corrosion resistance than these, which permits much thinner walls. Depending on the internal & external convection coefficients, 20-50µ thick HDPE and PP film heat tranfer elements have from 60-105% of the U value of 1mm cupro- nickel tubes. Experience has shown then to last as well as -and in some high-scaling water re-use applications better than-titanium elements. But per unit area they cost only about 1% as much. In chapter 2 we show how the low cost permits the installation of much more thermal conductance (UA) than is econimically feasible with metal heat transfer surfaces. This leads to a lower temperature difference ∆T1, between the condensing and evaporing sides, and a lower specific energy consumption. This thesis further describes the design, building and testing of a simple saline film mechanical vapour compression (MVC) desalinator. With air mattress-shaped polyolefin film heat transfer elements. Designed for operation (under vacuum) at various temperatures in the range 50-65ºC, with a small temperature difference ∆T1, between the condesing and evaporating sides. In chapter 3 we determine the pressure drop of the condensing vapour for laminar flow inside a film tube, to obtain the ralation batween film diameter , lenght, U value, temperature and the ratio RT = ∆T1/∆T1f temperature difference , ∆T1 to frictional temperature drop ∆Tf. We also determine , for ∆T1= 1k and RT = 8 , the relation between tensile stress and temperature for the HDPE and pp films that we have used to fabricare HTE's. For 39µ HDPE film elements up to 60-65ºC, and for 50µ PP ones to 90 – 95ºC, the stress is below 0.4 Mpa – and in most cases well creep strength of the materials for a 10 year design life. In chapter 4 we discuss the welding of thin HDPE and PP films on specially developed apparatuses to produce air mattress-like heat transfer elements(HTE's). some of these were pressure tested ( up to several bars at room temperature )to determine the strength of the weld lines. Chapter 5 discusses the sucessful vapour inlet manifolding of the heat transfer elements into heat transfer units. Also the design, construction and testing a vacuum vessel and a turbo vapour compressor. And of the other auxiliaries ( feed water heater, vacuum pump with protecting pre-condenser, water pumps, instrumentation..). It also discusses problems encountered, and the merits of various possible remedies. Chapter 6 discusses suitable surface treatments to increase their tension and wettabilityof the air mattress-like heat transfer elements (HTE's). As these are of a nonpolar hydrophobic material , such treatment – aimed at creating charged, polar or polarizable sites – is essential for film evaporation. As our original process – oxyfluorination – was only partially successful after several year's work, we have started another surface treatment- sulfonation – which is in the earty stages of evaluation.