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Design of a thermal desalination system with spray flash drum and condenser under vacuum based on droplet dynamics

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Abstract

A thermal desalination system with a flash drum, a spray-nozzle system, and a condenser under vacuum is designed. Theoretical yield of water vapor in the vacuum flash drum has been evaluated based on conservation equations of mass and energy in the range from 0.35 % to 8.37% at temperature of saline feed water from 40 °C to 60 °C, salinity from 0.035 to 0.04 kg/kg, feed flow rate of 40000 kg/day, and supersaturation from 2 °C to 50 °C. A thermal-fluid model has been adopted to simulate the dynamics of saline water droplets for flash evaporation under vacuum. At a moderate droplet velocity, the model predicts time scales for evaporation from 0.5 s to 1 s with yield of flashing vapor from 69.24 % to 84.59 % at droplet radius from 100 μm to 150 μm. The yield increases with the increase in supersaturation and feed temperature, whereas it increases appreciably with the decrease in droplet size. The design is based on the evaporation time scale and the theoretical yield. Both droplet dynamics for flash evaporation and theoretical yield have been validated with results from the literature.

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Abbreviations

A :

orifice area of the nozzle, m2

B :

liquid flow rate, kg h-1

c p :

specific heat, J kg-1 °C-1

D fd :

diameter of flash drum, m

D saut :

Sauter mean diameter of droplet, m

D s :

shell diameter, m

Dc v :

diffusion coefficient of water vapor in air, m2 s-1

D :

diameter of droplet, mm

F :

feed flow rate, kg h-1

f dis :

friction factor of discharge pipeline

H :

specific enthalpy, kJ kg-1

\({H}_{V}^{sw}\) :

latent heat of vaporization of seawater, kJ kg-1

\({H}_{V}^{w}\) :

latent heat of vaporization of water, kJ kg-1

h :

heat transfer coefficient, W m-2 °C-1

h B :

height of saturated saline water at flash drum, m

h c :

condensation film coefficient, W m-2 °C-1

h m :

mass transfer coefficient, m s-1

ht fd :

height of flash drum, m

K valve :

loss coefficient of discharge valve

k :

thermal conductivity, W m-1 °C-1

kf :

discharge coefficient of nozzle

L :

length of tube, m

M :

molecular weight, kg kmol-1

m :

mass of droplet, kg

ṁ :

mass flowrate, kg h-1

N d :

number of droplets

N r :

average number of tubes in a vertical tube row

N t :

total number of tubes in the bundle

P :

pressure, Pa, N m-2

Pr :

Prandtl number, dimensionless

Re :

Reynolds number, dimensionless

r :

radius of droplet, μm

Sc :

Schmidt number, dimensionless

s :

salinity, kg kg-1

T :

temperature, °C

U :

overall heat transfer coefficient, W m-2 °C-1

V :

vapor flow rate, kg h-1

W c :

condensate flowrate, kg s-1

Δ :

differential operator

Γ :

condensate flow per unit length of tube, kg m-1 s-1

λ :

latent heat of vaporization, kJ kg-1

μ :

viscosity, kg m-1 s-1

θ :

cone angle of nozzle, rad

ρ :

density, kg m-3

σ :

Surface tension of saline water, mN m-1

τ e :

time scale of evaporation, s

Ω D :

collision integral

dis :

discharge

F :

feed

fd :

flash drum

i :

inside

L :

liquid

l :

liquid

o :

outside

s :

surface

sat :

saturation

V :

vapor

v :

vapor

vac :

vacuum

sw :

saline water

w :

water

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Acknowledgments

This work is based on a project proposed by Clean Energy Resources Pvt Ltd. Kolkata, India. The authors would like to thank Prof Mandira Mukherjee and Mr. Yuvraj Aggarwal for the fruitful discussions and tips.

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Authors and Affiliations

  1. Department of Chemical Engineering, Birla Institute of Technology, Mesra, Jharkhand, 835215, India

    Arnab Karmakar, Dipanjan Kar, Adil Ahmad & Sumit Kumar Jana

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  1. Arnab Karmakar
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Correspondence to Arnab Karmakar.

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Karmakar, A., Kar, D., Ahmad, A. et al. Design of a thermal desalination system with spray flash drum and condenser under vacuum based on droplet dynamics. Heat Mass Transfer 58, 933–948 (2022). https://doi.org/10.1007/s00231-021-03152-4

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