ChE-413-MSF Multi-Stage Flash Desal Ination

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Chemical Engineering Dep.

ChE-413: Desalination and Water treatment

King Saud University 1Prof. Ibrahim S. Al-Mutaz

Water DesalinationMulti-Stage Flash (MSF)

• Introduction

• Principle of MSF

• MSF Classification

• Stage Configuration: Cross / Long tube

• Once Through MSF

• Brine recirculation MSF

• Energy Balances




Introduction

� The MSF process is innovative where vapor formation takes place

within the bulk liquid instead of the surface of hot tubes.

� In other thermal processes, submerged tubes of heating steam are

used to perform evaporation. This always resulted in scale

formation on the tubes � less heat transfer & costly scale

inhibition or removal.




When saturated brine enters through slot at lower pressure stage. It senses the new lower pressure environment, and Flashes!

Principle of MSF




Weirs

Principle of MSF

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Principle of MSF




Principle of MSF




Principle of MSF - energyfrom the energy flow diagram, the great part of the heat input to the MSF system is returned back to the sea with the seawater drain stream




� Plant operation has improved with

• More efficient antiscalent

• Better corrosion control chemicals

• Construction material capable of standing harsh

conditions

Development in MSF

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Development in MSF

� Increase in the unit capacity from 454.4 m3/d to a current

capacity of 32731 m3/d. The largest unit was commissioned

in Al-Taweela with a capacity of 57,734 m3/d. Each capacity

doubling is associated with 24% reduction in unit product

cost.

� Decreasing the specific power consumption cost from 7-25

kW/m3 in 1955 to 4-10 kW/m3




� Treatment of the intake seawater is limited to screening and filtration

� Treatment of the feed seawater includes:

• Dearation

• Addition of antiscalent

• Addition of foaming inhibitors

Pretreatment




Inlet Distillate

Inlet Brine

Distillate Tray

Outlet Distillate

Outlet Brine

Submerged OrificeBrine PoolFlashed off Vapor

Distillate duct

Vent Line

Demister

Tube Bundle

Air Buffle

MSF Flashing stage




MSF Flashing stage

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Demister

Vent line

Brine Pool

Distillate Duct

Outlet Brine

Outlet Distillate Inlet Distillate

Inlet Brine

Air Baffle

Tube Bundle

Distillate Tray

Orifice Weir

MSF Flashing stage




MSF Flashing stage




Intake Seawater

Mf+Mcw= 115.44 kg/s

Xf= 42000 ppm

Tcw= 30 oC

Distillate Product

Md = 1 kg/s

Reject Brine

Mb = 10.54 kg/s

Xb = 45984.8 ppm

Tb = 40 oC

Cooling Seawater

Mcw = 103.9 kg/s

Xf = 42000 ppm

T1 = 35 OC

Heating Steam

Ms = 1.18 kg/s

Ts = 100 oC

Feed Seawater

Mf = 11.54 kg/s

Xf = 42000 ppm

To = 90 oC

Single Stage Flash Desalination




Single Stage Flash Desalination

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� Distillate product is salt free

� Cp is the same for all streams and equal 4.18 kJ/kg oC

� U is the same for preheater and brine heater and equal 2 kW/m2. oC

� Subcooling of condensate or superheating of the heating steam has negligible effect on the energy balance

� The heat loss is negligible

� Power consumption is not considered.

Assumptions




Top Brine

Temperature

To

TTDh

Brine HeaterFeed Preheater

Flashing Stage

To

T1

T1

Tv

Tcw

DTlossTTDc

Tb

DTst

Ts

Temperature profile




� The salinity of the rejected brine is much smaller than maximum allowable

� The heat transfer area for the brine heater and preheater is small due to the large temperature driving force.

� The specific heat transfer area in the brine heater is inversely proportional to the performance ratio.

Advantages of Single Stage




� PR is always less than 1.0

� The flow rate of the feed seawater is much larger than the

distillate produced ( about ten times) which means a large

amount of additives and chemicals needed for feed

treatment

� The flow rate of cooling water is very high which increases

the pumping power consumption

Drawbacks of single stage

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MSF Process Classification




MSF Process Classification




MSF Flow Configuration

Tube Sheet

Brine

Condenser Tubes

Demister

Partition Walls

Side Walls

Cross-tube MSF

Side Walls

Brine

Distillate

Condenser

Tubes

Demister

Long-tube MSF




Cross-tube MSF distiller

Cross-Tube Configuration

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Tube Sheet

Brine

Condenser Tubes

Demister

Partition Walls

Side Walls










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Cross-Tube Internal Layout




Long-tube MSF distiller

Long-Tube Configuration








Side Walls

Brine

Distillate

Condenser Tubes

Demister


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Long-Tube Internal Layout




Comparison between Cross Tube and Long Tube MSF Plant Parameters

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There are 2 types of Multi-stage MSF

• Once Through (OT) MSF

• Brine recalculated (recycled)

Multi-stage MSF




� To overcome the main drawbacks of the single

flash units

� To improve the system performance ratio

� This is achieved by adding more flash units and

reducing the temperature drop per stage

Objective of Multi-stage MSF




� N number of stages : all of the stages are identical

� Brine heater

� No recycle of any portion of brine

� Does not contain cooling water stream: because the

brine rejected with low temperature and large flow

rate contains the energy to be removed from the

system.

System Components




Once Through MSF

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Once Through MSF




Once Through MSF




Once Through MSF




Once Through MSF

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� The intake seawater at Tcw flow through the preheater of

stage n to 1. When it leaves stage 1 its temperature

increases to t1.

� The seawater leaving the condenser enters the brine

heater, where its temperature rises from t1 to To

� The heated brine flashes off as it flows through the

successive stages where its temperature decreases from Toto Tn

Process description




� Simultaneously, flashing vapor condenses around

the condenser tubes in each stage where it heats

the the seawater flowing through the tubes.

� The collected distillate leaves from stage n.

� The brine leaving the last stage is rejected to the

sea.

Process description … cont.




� Drastic increase in

performance ratio

� Does not use cooling water

for removing excess heat

added in the brine heater.

� Operation at low salinity of

both feed and flashed brine

� High flow rate of intake

seawater

� High total condenser

surface area

Advantages vs. Disadvantages




CondensateFeed Brine

Heat input Section

Heat Recovery Section

Heat Rejection Section

Heating Steam

Ms

BrinePool

Demister

CondenserTubes

BrineRecycle

Mr

Brine Blow-downMb

DistillateProduct

Md

Feed Seawater

Mf

Intake Seawater

Mf +Mcw

Cooling Seawater

Mcw

Cooling Seawater Recycle Winter

Temperature Control

1 2 3 19

20 21 22

23

24

WaterBoxes

Distillate Trays

Brine recirculation (BR) MSF

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� It is considered the industry standard

� The process consists of three section

• The heat recovery sections

• The heat rejection section (2-3 stages)

• The brine heater

� The system is driven by the heating steam which increases the temperature of the brine recycle or feed seawater to the desired value in the brine heater





� The heat rejection section is used to remove the excess

heat added to the system in the brine heater

� It is used to control the temperature of the of the recycled

brine

� This is achieved through recovery of a controlled amount of

energy from the flashing brine into the brine recycle and

rejection of the remaining energy into the cooling water

stream





� Experience in design and operation of MSF has led to use of inexpensive construction material capable of standing harsh conditions at high salinity

� The MSF systems does not include moving parts other than pumps.

� Construction is simple and contains a small number of tubes which limits leakage problems and simplify maintenance work

Advantage of brine recirculating MSF




� The hot brine flashes in the consecutive stages, where the

brine recycle flow inside the condenser tubes recover the

latent heat of the formed vapor.

� In the heat rejection section of brine circulation system,

the excess heat added to the system by the hating steam is

rejected to the environment by the cooling seawater stream

Description

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� The cooling water is introduced in the last stage of the heat recovery section.

� The cooling water leaving the first stage of the heat rejection section is divided into two parts

• Cooling water to the sea

• Feed water to the dearator to remove O2

� The feed is mixed with a portion of the brine from the last stage of the heat recovery section.

Process Description




� The brine recirculation pump pumps the water through the condenser/preheater tubes of the last stage of the heat recovery section

� Leaving the condenser in the first stage, the feed is introduced into the brine heater (steam-heated), where the temperature rises to the top brine temperature

� Flashing occurs in each stage and a small amount of water is generated and is cascaded through the stages

Process Description … cont.




� Vapor formation results because of the reduction of brine

saturation temperature

• The stage temperature changes from the hot to cold

• The brine flows across the stages without the aid of pumping

power (from high to low P).

� The flashed-off vapor is condensed on the preheater tubes


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� From the last stages

• Part of brine is rejected to the sea

• Another part is recycled

� The rejection of the brine is necessary to control the salt

concentration in the plant









Brine recirculation (BR) MSF Stream flow rate, salinity, and temperature




Brine recirculation (BR) MSF Material balances on the heat rejection section

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Temperature Profile




Energy Balances




Energy Balances




Energy Balances

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Energy Balances




Modelling a Flashing Stage




Modelling a Brine Heater




Interface with Rest of the Plant: Layout

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Multi Stage Flash




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King Saud University 73Prof. Ibrahim S. Al-Mutaz Chemical Engineering Dep.





King Saud University 75Prof. Ibrahim S. Al-Mutaz Chemical Engineering Dep.



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General view of low side flash chamber




General view of low side flash chamber




General view of Cu-Ni 70-30 alloy heat transfer tubes in heat recovery

section of Al-Kkafji MSF plant




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Operational experience show desalination plants operating

after 15 years successfully improved material selection suggests

that the design life can be extended to 40 years operation




Al-Taweelah Az Power & Desalination Plant



King Saud University 83Prof. Ibrahim S. Al-MutazSection 7

Umm Al Nar Desalination Station “B” Plant




Shoaiba Desalination Plant, Saudi Arabia

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Shoaiba Desalination Plant, Saudi Arabia

Process type: Multi-Stage Flash Distillation (MSF)Estimated project cost: $1.06 billionPopulation served: 1.5 millionProduct water output: 74,000m³/day (Phase 1); 450,000m³/day (Phase 2)Final total production capacity: 150 million m³/year

Project Timeline:Construction started: 1997Phase 1 completed: August 2000, Phase 2 completed: March 2003




ChE-413-MSF Multi-Stage Flash Desal Ination

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