Single-Effect Mechanical Vapour Compression (MVC) Lec. 6 Dr. Ola Abdelwahab

Introduction The MVC system was introduced in the 1980s. Early studies were motivated by the need to develop a thermal desalination process driven by electrical power. The MVC process was trial as a competitor to the newly introduced RO technology However, the test of time has shown the dominance of the RO process and a very limited growth in the reliability of MSF and MED processes. Dr. Ola Abdelwahab

Introduction The small size, and the fact that only electrical power is required, makes it feasible to operate using various forms of renewable energy, i.e. wind. On average MVC consumes 10–14 kWh/ m3of electrical power, to operate the system and other associated equipment including pumps, controls and auxiliaries. A 5,000 m3/day MVC would thus require 2–3 MW of electrical power, which can be easily provided by available renewable energy technologies. Dr. Ola Abdelwahab

Process Description The MVC system contains five major elements, which include: Mechanical vapor compressor, evaporator, preheaters for the intake seawater, brine. product pumps, and venting system. Figure 1 shows a schematic diagram for the system Dr. Ola Abdelwahab

A schematic diagram for a single effect mechanical vapor compression (MVC). Dr. Ola Abdelwahab

Process Description In the system, the compressor and evaporator form one single unit. The evaporator contains falling film horizontal tubes, spray nozzles, suction vapor tube, and wire-mesh mist eliminator. The feed preheaters are plate type heat exchanger, which operates on the intake seawater and the hot liquid streams leaving the evaporator. Dr. Ola Abdelwahab

Process Description The feed seawater enters the evaporator at a flow rate of Mf and a temperature of Tf. The feed seawater is sprayed over the horizontal tubes forming a falling film over succeeding tube rows. Formation of the thin film enhances the heat transfer rate and makes the evaporation process more efficient. The seawater temperature increases from Tf to Tb before evaporation commences. The formed vapors, Md, are at a temperature of Tb. The vapors transfer from the evaporator section to the compressor through the vapor suction tube, which is guarded by a wire-mesh mist eliminator. This is necessary to avoid entrainment of brine droplets in the vapor stream, which would result in damage of the compressor blades. Dr. Ola Abdelwahab

Process Description Limited temperature depression occurs as the vapors flow through the demister. The vapors flow through the compressor, where it is superheated from Tb to Ts. Upon compression, the vapors are forced inside the horizontal tubes, where it loses the superheat energy and its temperature drops from Ts to the saturation temperature Td. Condensation takes place at Td and the released latent heat is transferred to the brine film. The temperature difference Ts-Tb affects the compressor power consumption and is dictated by the temperature of the feed seawater. Dr. Ola Abdelwahab

Process Description The balance of energy within the system is maintained by recovery of the thermal load in the rejected brine and product streams. This is achieved in the feed preheater, which a plate type heat exchanger. In this unit, the intake seawater is at a low temperature, tcw, and a flow rate Mf. The rejected brine and product streams leaving the evaporator are at higher temperatures of Tb and Td, respectively. As heat is exchanged between the three streams the temperature of the seawater is increased to Tf and the temperature of the rejected brine and product streams is reduced to T0. Dr. Ola Abdelwahab

Fig. 2. Cross section of the evaporator As shown in Fig. 2., the tube bank is divided into two groups on either sides of the demister, which is placed in the centre of the evaporator. The demister arrangement is connected directly to the compressor intake, where the formed vapour is compressed and superheated to the desired temperature. Dr. Ola Abdelwahab

Temperature profiles in MVC The temperature of the feed seawater increases from Tcw to Tf in the preheater unit. Simultaneously, the temperatures of the rejected brine and the product stream decrease from Tb and Td, respectively, to the same temperature T0. Inside the evaporator, the temperature of the feed seawater increases from Tf to the boiling temperature Tb. The formed vapor is at the same boiling temperature, which is higher than the saturation temperature, Tv, by the boiling point elevation, Tb-Tv. The formed vapor is compressed and superheated to a temperature Ts. Condensation of the compressed vapor takes place at a temperature of Td. Dr. Ola Abdelwahab

Temperature profiles in MVC Dr. Ola Abdelwahab

Self- Study Topics Study the process description of one of the following evaporators Single effect thermal vapor compression. Single effect absorption vapor compression. Single effect adsorption vapor compression. Dr. Ola Abdelwahab