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

Dr. Ola Abdelwahab

IntroductionIntroduction

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.

2Dr. Ola Abdelwahab

IntroductionIntroductionThe 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.

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Process DescriptionProcess DescriptionThe MVC system contains five major

elements, which include:1.Mechanical vapor compressor, 2.evaporator, 3.preheaters for the intake seawater,

brine.4.product pumps, and 5.venting system. Figure 1 shows a schematic diagram

for the systemDr. Ola Abdelwahab 4

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

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Process DescriptionProcess DescriptionIn 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.

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Process DescriptionProcess 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.

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Process DescriptionProcess 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.

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Process DescriptionProcess 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.

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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.

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Fig. 2. Cross section of the evaporator

Temperature profiles in Temperature profiles in MVCMVC 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.

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Temperature profiles in Temperature profiles in MVCMVC

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Self- Study TopicsSelf- Study Topics

Study the process description of one of the following evaporators

1.Single effect thermal vapor compression.

2.Single effect absorption vapor compression.

3.Single effect adsorption vapor compression.

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