An investigation of high operating temperatures in mechanical vapor-compression desalination

It is common practice to operate mechanical vapor-compression (MVC) seawater desalination systems at temperatures lower than 80EC. This study presents the detail engineering and economics of a MVC system operating at 172EC. The literature indicates that high overall heat transfer coefficients for the evaporator are possible at high temperatures with dropwise condensation on the steam side and pool boiling on the liquid side. Employing high operating temperature delivers the following advantages: (1) low compression work, (2) small latent heat transfer area, and (3) small compressor. These advantages potentially reduce operating costs and capital investment. The disadvantages follow: (1) pretreatment required to prevent fouling of heat exchangers, (2) careful selection of materials to prevent corrosion, and (3) larger sensible heat transfer area is required. A desalination plant is designed herein to produce 37,854 m3/d (10 mil gal/day), which is financed with a 5%, 30-y municipal bond. To maximize energy efficiency, combined-cycle cogeneration is employed. For the US ($5.00/GJ energy), the product water selling price is estimated to be $0.49/m3 ($1.86/thous gal). For the Middle East ($0.50/GJ energy) the product water selling price is estimated to be $0.38/m3 ($1.44/thous gal). These are attractive prices relative to competing technologies.

In this paper, higher operating temperatures for seawater desalination were explored. A novel sheet-shell heat exchanger/evaporator design was analyzed. According to literature data, dropwise condensation on the steam side and pool boiling on the liquid side makes it possible to obtain overall heat transfer coefficients of up to 182 kW/ (m2@K) [32,000 Btu/(h·ft2·EF)] under specific working conditions. To verify this value, an experimental investigation is being prepared by the authors. Because of the low temperature differential between water and steam due to dropwise condensation, the compression energy cost is low. The use of low-cost heat exchangers is required to achieve a low ΔT. A cost of $86.10/m2 ($8/ft2) for the heat evaporator/ condenser has been estimated based on the use of naval brass coated with a monolayer of PTF. Corrosion is prevented by keeping oxygen concentration low and by providing galvanic protection. Fouling is prevented by removing sulfate through ion exchange. Operating at elevated temperature and pressure reduces the size of the latent heat exchangers and compressor, which lowers capital costs. In addition, it reduces energy requirements, which lowers operating costs. For the US ($5.00/GJ energy), the product water selling price is estimated to be $0.49/m3 Cpf — gc — h hboiling — — hcomp — hcond — vap H2 H1vap — H1liq — Hc — Hs — j k — — knb — M — mb — mf — Specific heat of seawater feed [J/(kgm@K)] or [Btu/(lbm@ER)] Gravitational constant [kgm@m/ (N@s2)] or [(lbm@ft/(lbf@h2)] Empirical constant = !2.1609×10!4 Liquid-side heat transfer coefficient [W/(m2@K)] or [Btu/(h@ft2@EF)] Wall composite heat transfer coefficient [W/(m2@K)] or [Btu/(h@ft2@EF)] Steam-side heat transfer coefficient W/(m2@K)] or [Btu/(h@ft2@EF)] Vapor enthalpy at compressor exit (2) (J/kgm) or (Btu/lbm) Vapor enthalpy at compressor inlet (1) (J/kgm) or (Btu/lbm) Liquid enthalpy at compressor inlet (J/kgm) or (Btu/lbm) Specific enthalpy of condensate (J/kg) or (Btu/lbm) Specific enthalpy of steam (J/kg) or (Btu/lbm) Empirical constant = !3.5012×10!7 Liquid film thermal conductivity [W/(m@K)] or (Btu/(h@ft@EF)] Naval brass thermal conductivity [W/(m@K)] or (Btu/(h@ft@EF)] Molecular weight (lbm/lbmol) or (kgm/kmol) Rate of exiting brine flow (kg/h) or (lbm/h) Rate of seawater feed flow (kg/h) or (lbm/h) ms — mv — Pcritical Pl Pr Ps p — — — — — q/A qs R — — — Rp S Sf /S liq S — — — — S1vap — vap S2 — Tave Tb — — Tf — Ts Tl U — — — Vl Vs Vv — — — x — xnb ΔT — — Rate of steam flow and condensate (kg/h) or (lbm/h) Rate of vapor flow to the next effect (kg/h) or (lbm/h) Critical pressure (kPa) or (psi) Liquid pressure (kPa) or (psi) Reduced pressure Steam pressure (kPa) or (psi) Vapor pressure of salt water at the same temperature (N/m2) or (psi) Heat flux [Btu/(h@ft2)] or (W/m2) Rate of heat transfer (W) or (Btu/h) Gas constant (J/(kmol@K) or [ft@lbf / (lbmol@ER)] Surface roughness (µm) or (ft) Salinity (g salt/kg seawater) Fraction of droplet to total area Entropy of liquid water at compressor inlet [J/(kgm@K)] or [Btu/ (lbm@ER)] Entropy of steam at compressor inlet [J/(kgm@K)] or [Btu/ (lbm@ER)] Entropy of steam at compressor exit [J/(kgm@K)] or [Btu/ (lbm@ER)] 1/2(Ts+Tv) (K) or (ER) Boiling temperature of the seawater (K) or (EF) Temperature of the seawater feed (K) or (EF) Steam temperature (K) or (ER) Liquid temperature (K) or (EF) Overall heat transfer coefficient [W/(m2@K)] or [Btu/(h@ft2@EF)] Liquid velocity (m/s) or (ft/s) Steam velocity (m/s) or (ft/s) Specific volume of vapor (m3/kgm) or (ft3/lbm) Fraction amount of injection water that evaporates in the compressor Naval brass thickness (m) or (ft) Overall heat transfer temperature differential = ΔT + ΔTm + ΔTb (K) or (EF) Δ Tb — Δ Tm — ΔTs — Δx — Boiling side temperature difference (K) or (EF) Metal temperature difference (K) or (EF) Steam-side temperature difference (K) or (EF) Plate thickness (m) or (ft) Greek δc ηc μl — — — μs — σ — σ1 — Surface roughness (µm) or (ft) Compressor efficiency Liquid dynamic viscosity [kg/(m@s)] or [lbm/(h@ft)] Steam dynamic viscosity [kg/(m@s)] or [lbm/(h@ft)] Mass transfer accommodation coefficient Condensation factor Note This paper is to be presented at the IDA World Congress on Desalination and Water Reuse, Spain, 21–26 October 2007.