Chapter 9 - Associated Processes

Abstract

This chapter analyzes the characteristics of major associated processes in thermal desalination, which includes: venting, steam Jet Ejectors, demisters, and MSF Weirs. Venting in the thermal desalination is driven by steam ejectors for removal of the non-condensable gases during startup and operation. During maintenance and other shut down procedures, the flashing chambers in MSF or the evaporation effects in MEE are opened to ambient air. Therefore, air removal is one of the main activities in the startup procedure. The startup ejectors have higher capacity and are capable of processing large air volumes over a short period of time. Conventional jet ejector has three main parts: the nozzle, the suction or mixing chamber, and the diffuser. The nozzle and the diffuser have the geometry of converging/diverging venturi. The diameters and lengths of various parts forming the nozzle, the diffuser, and the suction chamber together with the stream flow rate and properties define the ejector capacity and performance. The ejector capacity is defined in terms of the flow rates of the motive fluid and the entrained stream. Their sum gives the mass flow rate of the compressed gases. Further, wire mesh mist eliminator or demisters is a simple porous blanket of metal or plastic wire that retains liquid droplets entrained by the gas phase. Demisters are used in distillation or fractionation, gas scrubbing, evaporative cooling, evaporation and boiling, and trickle filters. Today, the wire mesh mist eliminator is widely used in thermal desalination plants. The main features of wire mesh mist eliminators are low-pressure drop, high separation efficiency, reasonable capital cost, minimum tendency for flooding, high capacity, and small size. The performance of wire mesh eliminators depends on many design variables such as supporting grids, vapor velocity, wire diameter, packing density, pad thickness, and material of construction.