Study On Fouling Characteristics During Horizontal Tube Falling Film Evaporation Under Vacuum

A major challenge of multiple-effect distillation desalination process is the fouling of the heater surface due to the precipitation of the dissolved salts,corrosion due to low pH of seawater,and fouling due to presence of organic matter in water.Therefore,this paper mainly study the mechanism of fouling outside the tube in the process of falling film evaporation of horizontal tube when the high-temperature and high-salt titanium tube and 3FL aluminum alloy tube are heat transfer tubes.In this paper,the horizontal tube falling film evaporation experiment bench under negative pressure is designed to completely simulate the fouling environment in real seawater desalination process.And choose seawater as experimental working medium,titanium tube and 3FL aluminum alloy tube are heat transfer tubes.The experimental conditions are as follows: spray seawater temperature are 60°C,70°C,80°C,and 90°C,the experimental system pressure are 22.9 kPa,35.5 kPa,53.5 kPa,and 78.5 kPa,the spray density are 0.03 kg/(m s),0.06 kg/(m·s)and 0.08 kg/(m·s),the seawater salinity are 3.5%,8%,10% and 12%.The growth curve of the fouling with time can be obtained by regularly measuring the weight gain of the heat transfer tube,and the influence of seawater temperature,salinity,and spray density on the fouling can be studied by analyzing the fouling growth characteristics and microscopic structure and composition of scale under different working conditions.When the heat transfer tube is a titanium tube,the seawater temperature is 90°C,and under the conditions of three types of seawater salinity,the asymptotic value of scale increases first and then decreases slowly with increasing spray density.At the seawater temperature of 70°C and 80°C and the spray density of 0.06 kg/(m·s),with the increase of seawater salinity,the asymptotic value of scale increases,and while at seawater temperature of 90°C and under the conditions of three types of seawater salinity,the asymptotic value of scale reduces with salinity increases.When the heat transfer tube is a 3FL aluminum alloy tube,the salinity is 3.5%,and as the spray density increases,the asymptotic value of the scale first increases and then slowly decreases.But when the spray seawater temperature is 90°C,as the spray density increases,the progressive value of scaling increases.At a salinity of 3.5% and under the conditions of three types of spray densities,the fouling asymptote increases as the temperature of the spray seawater rises.The scale on the surface of the heat transfer tube was analyzed from a microscopic point by scanning electron microscopy(SEM)and energy dispersive X-ray spectroscopy(EDX).The experiment mainly analyzes the microscopic appearance and composition of the scale when the heat transfer tube is titanium tube under different seawater salinities,and when the heat transfer tube is 3FL aluminum alloy tube under different seawater temperatures.When the heat transfer tube is a titanium tube,the morphology of the fouling under different sea salinity changes little,mainly for the columnar crystal scale.However,the fouling morphology under different seawater temperatures changes greatly with the increase of temperature.The needle-like crystal structure gradually develops to the regular polygonal crystal structure,and the size of the scale particles gradually increases.However,the fouling components are mainly magnesium hydroxide.When the heat transfer tube is 3FL aluminum alloy tube,the microscopic morphology of the fouling gradually develops from cellular crystal structure with pores to more dense scale layer as the seawater temperature increases.When the temperature of sprayed seawater rises to 90°C,a dense layer of dirt has formed on the surface of the heat transfer tube,and the scale component is still dominated by magnesium hydroxide.