Advanced MultiEffect Distillation Plant: Novel Design
Authors: Mabrouk Abdelnasser 
Published: June 2019

The thermal desalination by Multi-Stage Flash (MSF) and Multi-Effect Distillation (MED) desalination plants are dominantly used in Qatar due to their maturity and reliability in dealing with the harsh seawater conditions of high (TDS, Seawater feed temperature, SDI, and HAB). Realizing the benefits and challenges of the thermal, there is a room of improvement to reduce energy consumption. Qatar Environment and Energy Research Institute (QEERI), Hamad Bin Khalifa University (HBKU) and Qatar Electricity and Water Corporation (QEWC) took initiative to improve the overall efficinecy and perfomance of thermal deslaintion plants in Qatar. In this work, a novel design and compact MED evaporator will be presented. The expected reduction in heat transfer area will reduce the capital cost of the evaporator as well as the footprint of the MED desalination plant. The present work describes a novel MED evaporator according to the GCC2016-31325 patent. The tubes are arranged in a way to allow generated vapor to flow in a smooth and minimized route to the following effect. The vapor route is designed to avoid shear losses and breakdown of film liquid around the tubes (dry zones) that will eliminate vapor entrainment, which will reduce brine carry over. Since there is no cross flow within tube bundle hence, eliminating the need for demister and vapor boxes. A well-developed Visual Simulation Program (VSP) is used to perform comparison between commercial MED desalination plant (63 MIGD, Rass Laffan, Qatar) and proposed novel design. The evaporator distillate production rate, seawater feed conditions (temperature and salinity), and heating steam conditions are specified as input to the software. The number of effects and the tube specifications are specified as input. The VSP software calculates the required steam consumption flow rate, the heat transfer area for each effect, the condenser and preheaters. The capital cost of the desalination plant was calculated using recent bidding of commercial desalination projects and the updated market material price. Process simulationm shows that, for a given gain output ratio of (GOR = 9), the required heat transfer area of novel evaporator is 20% less than of existing evaporators design due to thermal losses reduction. Moreover, the thermal losses reduction in novel evaporator were calculated via simulation and are in the range of 30% to 60%. Furthermore, removal of demister in novel MED evaporator decreases evaporator width by 65%. However, novel evaporator height is almost 1.25 times of the convnetional. In fact, the novel evaporator cost is 20 % less than that of the convnetional evaporator due to significant reduction of the evaporator width and vapor box. In addition to that, a 3D-CFD simulation have been conducted using COMSOL Multiphysics v.5.2a on the vapor route of the novel MED evaporator, to be compared with that of the convention MED evaporator. The engagement with QEWC to offer a site equipped with seawater intake/outfall, electricity will be highlighted. The detailed design of the pilot test, fabricate, installation, commission of MED pilot test of 25 m3/day at Dukhan will be used to develop the proof of concept.