Solar evaporation of saline water under vacuum

Abstract

The basic objective of this research was the desalination of sea or brackish water using solar energy and vacuum techniques. A process using the vacuum technique was devised for the desalination of sea or brackish water under desert conditions, and calculations were made to show the advantages of this process. To simulate a section of this overall process, an evaporator which may be operated at pressures ranging from relatively low vacuum to one atmosphere was designed. This evaporator was used to convert saline water into fresh water. Solar energy was simulated by using artificial lamps for the conversion process, and fresh water was used experimentally in place of saline water. The purpose was to obtain data on the relationship between the applied vacuum inside the evaporator, the fresh water yield, and the heat distribution in the system. The evaporator consisted of a hollow cylinder made of K-Monel with the bottom made of the same metal and painted black on the inside. Radiant energy from an infrared artificial lamp was transmitted through clear methyl-methacrylate (Plexiglas) which was fitted on the top of the cylinder. The vapor was drawn to a coil and shell condenser where it was condensed and collected. The vacuum applied on the system was controlled manually. Six runs were made with a constant batch feed of water, and each run was continued until the system reached a steady-state. A heat balance was made for each run, and the amount of heat available for water evaporation was compared to the weight of water evaporated and collected in the condenser chamber. Due to the application of vacuum and due to the absorptivity of the Plexiglas top, radiation always occurred from th Plexiglas top to the water basin The fresh water yield increased with a decrease in the pressure applied inside the evaporator. For a pressure of 0.46 lb/in^2 inside the evaporator, the rate of fresh water production was 72 g/hr while for a pressure of 1.28 lb/in^2 inside the evaporator, the rate of water production ws 55 g/hr. The heat available for evaporation increased with a decrease in the pressure applied on the system. When the applied pressure was 5 lb/in^2, 37.4 per cent of the incident radiation was available for water 2 production; and when the pressure decreased to 0.46 lb/in^2 the heat available for water evaporation was 52.6 per cent of the incident radiation. This investigation revealed the applicability of using vacuum techniques in saline water conversion. Recommended further research consisted mainly of investigating the productivity of the still when it is used under the sun in a desert site. Once these investigations are accomplished a main application of this process may be to furnish municipal water to communities. It may be also possible to develop a household desalting unit which could compete economically with the best household desalting units on the market today.