Msfpt2.Les

8/10/2019 Msfpt2.Les

1/9

SWCC TRAINING CENTERAL-JUBAIL

ADVANCED OPERATIONS TRAINING COURSE

MSF DESALINATION PLANTS TECHNOLOGY AND SYSTEMS

LESSON No. 2.

SUBJECT/TOPIC : MSF DESALINATION PLANTS - PRINCIPLES

TIME : 6 ho !"

OBJECTIVE : To #$%&'() *h# +#"'&()'*(o) %!(),(%".

LOCATION : SWCC T!'()() C#)*#! A&-J '(&

TRAINING AIDS : T!')"%'!#),(#" 0 o1#!h#'+ %!o #,*o!

REF. MANUALS : MSF T#,h)o&o 3 Boo4

HAND-OUTS : M'*#!('& ,o1#!() *h# 5o&&o () ""o) o *&()#".

LESSON OUTLINE :

7. F&'"h() P!(),(%.

2. S() S*' # F&'"h() P!(),(%.

8. M &*("*' # F&'"h() P!(),(%.

9. MSF P&')*" Ch'!',*#!("*(,"

. H#'* T!')"5#! ')+ D("*(&&'*# %!o+ ,*(o) () MSF P&')*".

6. R#"("*'),#" *o *h# F&'"h() P!o,#"" () MSF P&')*".

7. Major Heat Transfer areas in MSF Plants.


2/9

LESSON

2LECTURE

MSF DESALINATION PLANTSPRINCIPLES

SUB-OBJECTIVE S

A* *h# #)+ o5 *h# L#""o) *h# T!'()##" (&& 4)o ')+ )+#!"*')+:-

7. F&'"h() P!(),(%.

2. S() S*' # F&'"h() P!(),(%.

8. M &*("*' # F&'"h() P!(),(%.

9. MSF P&')*" ,h'!',*#!("*(,".

. H#'* T!')"5#! ')+ D("*(&&'*# P!o+ ,*(o) () MSF P&')*".6. R#"("*'),#" *o *h# F&'"h() P!o,#"" () MSF P&')*".

;. M' o! H#'* T!')"5#! '!#'" () MSF P&')*".

7.< FLASHING PRINCIPLE

Vapors can be produced from a liquid which is at its boiling point, either by heataddition (boiling) or by pressure reduction (flashing).

Single and multistage flash processes use the second concept.

If water and vapor are in equilibrium in an enclosed space, their temperature andpressure are interrelated. Water can be made to flash (boil) ust as effectively byreducing the pressure as by raising the temperature. When pressure is reduced, theboiling temperature is correspondingly decreased. !he amount of energy which canbe stored in water (at its boiling point) decreases as pressure is decreased.!herefore, reduction is required in the sensible heat of the liquid. !his supplies heatfor production of a certain amount of vapor.

When the pressure is reduced, bubbles evolve from the whole liquid mass. Water continues to evaporate till it reaches equilibrium with its vapor at the prevailingpressure. "vaporation lowers the temperature of the remaining brine.

Separation of water from seawater in the #S$ process is based on the flashingprinciple. When seawater boils, the vapors produced are pure and free from anysalts. !hese vapors are led to a condenser where they are condensed and collectedas distilled water. Salts remain behind in the seawater which gets more concentrated.!he salts remain in the liquid phase because their vapor pressure is %ero at normaldistillation temperatures.


3/9

When hot brine at its boiling point flows into a flash stage, which is at a lower pressure, energy in e&cess of that which can be contained by brine produces vapors. !heloss of energy causes the brine temperature to decrease to its saturation temperature at thelower pressure.

In flash distillation evaporation ta'es place at a distance from the tubes. feature of the #S$ process is that all energy is added to the liquid before flashing is permitted tobegin. uring the process of vapor release, additional energy is not added.!herefore, it is necessary that all heat required for evaporation be inserted assensible heat. Since a large mass has to be heated sensibly to provide the energy for vapori%ation, this reduces the potential for heat transfer. large recirculation rate is acharacteristic feature of the #S$ process.

2.< SINGLE STAGE FLASHING PRINCIPLE

$eedwater is preheated in a condenser. It then flows to a brine heater where lowpressure steam is introduced from an e&ternal source. !he feedwater is maintainedunder pressure conditions which do not permit vapor formation. *o boiling ta'esplace in the pipe leading to the flash chamber. +ot feedwater from the brine heater isintroduced into the flash chamber, which is maintained under vacuum by an e ector.

F( . 2-7. S() S*' # F&'"h.

!he temperature in the stage is slightly below the boiling point (or saturationtemperature) of the feed at that pressure. When feed enters the stage, it is already atthe saturation temperature for a higher pressure. It becomes superheated and has togive off vapors (flash) to become saturated again. !ransfer of latent heat from brineto vapors causes brine to cool down to the saturation temperature equivalent to thestage pressure.

!he vapors, after passing through demisters, are condensed on the condenser tubes.!he heat of condensation supplies a large part of the heat required to raise the feedto its boiling point. istillate is collected in the distillate trough. It flows from thetrough to the product pump which pumps it to storage. nevaporated brine isre ected to the sea. $resh feedwater is added continuously.

8.< MULTISTAGE FLASHING PRINCIPLE

- *! !"/+*0 012 3 S2S!"#S #S$ 0-"4 !I0*S"SS0* 5 - 1" 5 V */" /0 4S"


4/9

!o increase the heat recovery efficiency of a single stage unit, the number of flashstages is increased. !he modified system recovers a considerable part of the wastedenergy and is 'nown as the #ultistage $lash (#S$) process. n #S$ evaporator canbe visuali%ed to be a single stage unit e&tended to * stages (usually 6789:) in series.

$or a given performance ratio, an increase in the number of stages reduces the e&pensiveheat transfer area (at the cost of relatively cheap steel partitions between stages).

!he pressure in each stage is lower than the pressure in the preceding stage. !heminimum pressure and temperature in the last stage are fi&ed by vapor volume andheat re ection considerations. !his temperature is usually in the ;/ range. !headdition of multiple stages reduces the amount of heat that has to be removed fromthe process. !he number of stages controls the amount of heat recovery possibleand this determines the amount of e&ternal energy required.

?rine is not re ected from the first stage as is done in the single stage flash process.It is sent to the second stage instead. When brine enters the second stage it flashesagain. Vapors are condensed on the condenser at the top of stage. 5. !hetemperature of unflashed brine drops to a value corresponding to the second stagepressure. !his brine flows into the third stage where it again undergoes flashing (asin the first two stages). !his continues till the *th stage. /oncentrated brine from this

stage is re ected (once through process) or recycled (recirculation process).In each stage, distillate is produced. !he amount of distillate produced in each stagevaries.

8.7 CONTROLLING PARAMETERS

6. !emperature drop in each stage.5. !otal flash range (difference between the top brine temperature and the brine

re ect temperature);. Stage heat transfer coefficients.

8.2 PLANT CHARACTERISTICS

In the #S$ system, various plant arrangements and operational techniques havebecome established. !he three main plant characteristics which can adequatelydescribe an #S$ system are@8

7. F&'"h() F&o S3"*#=

A 0nce throughA 4ecirculation

2. T3%# o5 Ch#=(,'& P!#*!#'*=#)*

A -olyphosphateA cidA +igh temperature additive

8. Co)+#)"#! * #" ,o)5( !'*(o)

A /rossA ong

#S$ 0-"4 !I0*S - *! !"/+*0 012 3 S2S!"#S V */" /0 4S" "SS0* 5 - 1"


5/9

F( . 2-2 V'%o! P!#"" !# - T#=%#!'* !# R#&'*(o) 5o! W'*#!

F( . 2-8 R#&'*(o)"h(% #* ##) T#=%#!'* !# ')+ P!#"" !# '* *h# Bo(&() W'*#!

- *! !"/+*0 012 3 S2S!"#S #S$ 0-"4 !I0*S"SS0* 5 - 1" = V */" /0 4S"


6/9

9.< HEAT TRANSFER AND DISTILLATE PRODUCTION IN MSF PLANTS

!he process of water production by distillation involves two basic steps@8

6. Vapor -roduction5. Vapor /ondensation

9.7 VAPOR PRODUCTION

#any substances can e&ist in more than one state under proper pressure andtemperature conditions. $or e&ample, at low temperatures, water e&ists as a solid(ice). t high temperatures, it e&ists as steam.

If a liquid is placed in a closed, partially filled vessel, the vapor molecules will e&ertpressure on the walls and on the water surface. !his pressure is 'nown as the vapor pressure. "very liquid e&erts a vapor pressure. !he magnitude of a liquidBs vapor pressure is a measure of its volatility. +igh vapor pressure liquids evaporate readily.!hose with low vapor pressure evaporate more slowly. !hey require an increase intemperature to speed the rate.

"nergy in liquid molecules is not evenly distributed. #ost molecules have energywhich is about equal to the average, some molecules have less and some have more.+eat input increases the 'inetic energy of these molecules. 0n heating, somemolecules accumulate sufficient energy to enable them to leave the surface. !heythen enter the space above the liquid (as vapor).

t equilibrium, in a closed vessel, a number of molecules leave the liquid but an equalnumber return to it. !he loss of molecules due to the heat of vapori%ation is balancedby the gain due to the heat of condensation. !herefore, the liquid temperatureremains the same. If equilibrium is upset for some reason (li'e heat input), thenumber of molecules which leave will be more than the number of molecules whichreturn. !herefore, vapor production ta'es place.

4apid evaporation or boiling ta'es place when a liquid reaches the temperature atwhich its vapor pressure equals the e&isting e&ternal pressure. ?oiling ischaracteri%ed by vapor bubbles forming in the interior and rising to the surface 8evaporation being a surface phenomenon. Since pressure within the bubbles equalsthe vapor pressure of the liquid at that temperature, the boiling point depends on thee&ternal pressure.

t sea level, atmospheric pressure is 6 atm (6=.< lbCin 5). t this pressure, water boilsat 6:: >/. t high altitudes (low pressures) or in closed vacuum systems, water boilsat a lower temperature. t higher pressures, water boils at a temperature above 6::>/.

!he amount of vapor produced is limited by the available energy. +owever, byma'ing more surface area available for the molecules to escape and by rapidlyadding heat, the rate of vapor formation can be increased. !he amount of energyrequired for vapori%ation is equal to the heat of vapori%ation. Since the liquid has tosupply this energy, evaporation results in the liquid being cooled. t 6:: >/, theamount of heat required to vapori%e 6 'g of water is 5,597 'D. !he same amount of heat is released if condensation ta'es place at the same temperature. !he totalamount of heat required, to convert 6 'g of water at :>/ into saturated vapor at t >/,is 'nown as the enthalpy of the vapor.It is the total heat required to raise the water from 08t >/ (enthalpy of the water) andthe heat of evaporation at t >/ (corresponding to the energy required to convert 6 'gof water at t >/ into 6 'g of vapor).

#S$ 0-"4 !I0*S - *! !"/+*0 012 3 S2S!"#S V */" /0 4S" "SS0* 5 - 1"


7/9

esalination by distillation ma'es use of the relationship between the boiling pointand applied pressure. Saline solutions are made to boil again and again, without theaddition of any heat, by successively reducing the pressure. !he most popular method for the production of vapor has been@8

F&'"h

In this process, brine is introduced into a chamber maintained at a lower pressure. Itboils, as soon as it enters the chamber, in order to reach equilibrium with the e&istingchamber pressure.

9.2 VAPOR CONDENSATION

!o promote vapor production, vapors have to be removed rapidly so as to reducevapor pressure above the boiling brine. Vapor removal ta'es place due to pressure or temperature differentials. Vapors are transported to condensers where they arecondensed. !he condensation process liberates heat equal to the heat of vapori%ation at that pressure. !he heat of condensation (which is removed) is usedeither to preheat or to vapori%e brine. !he product water (condensed vapors) isallowed to flow to collection points for removal.

.< RESISTANCES TO THE FLASHING PROCESS IN MSF PLANTS

.7 BOILING POINT ELEVATION

n important factor in distillation plant design is boiling point elevation. Seawater boils at a higher temperature than pure water at the same pressure. !he difference inthe boiling points, of a saline solution and pure water, is referred to as the boiling pointelevation (?-"). ?-" effects heat transfer and the temperature driving force. Itrepresents a degradation of the heat used to boil seawater and is a loss of heattransfer potential. It means that vapors produced from brine are at a lower temperature than the brine. !he vapors produced are superheated by an amountequal to the boiling point elevation. +ence, the ?-" is lost from the availabletemperature difference between stages. !his means that the driving force is reduced.

difference of even 6 >/ means a loss, especially for plants with high performanceratios. !he summation of the ?-" in all the stages of an #S$ plant adds up to asignificant loss. !he ?-" and pressure drop losses (due to obstructions in the vapor path) are the ma or sources of temperature reduction or thermal energy degradation.!hey must be accounted for in process design.

?oiling point elevation is a function of the dissolved salts concentration. s seawater is concentrated, vapor pressure is lowered and the boiling point is raised. Seawater containing ;9,::: ppm dissolved salts boils at 6::.7 >/, while seawater concentratedto /. 1reater the boiling point elevation, more thedeviations of saline water properties (specific heat, latent heat of evaporation, etc.)from those of pure water.

- *! !"/+*0 012 3 S2S!"#S #S$ 0-"4 !I0*S"SS0* 5 - 1" 7 V */" /0 4S"


8/9

.2 DRIVING POTENTIAL FOR VAPOR BUBBLE PRODUCTION

If vapor is able to e&ist as a separate phase, a surface of separation is necessary,which in the case of vapor bubbles is curved. Surface tension causes a liquid toresist the formation of such a curved surface. !herefore, in order to form a vapor bubble, wor' needs to be done. In a flash chamber this is accomplished by loweringthe vapor saturation temperature. !his is done by reducing pressure (through a flow

orifice) by an increment equivalent to the required driving potential.

.8 HYDROSTATIC PRESSURE

!his is another resistance for the flashing process. !he pressure increases withdistance below the vapor liquid interface. !he vapor saturation pressure andconsequently, saturation temperature will increase with depth. +ence, more flashingwill occur at the surface. $lashing will stop when the brine at any point equals or fallsbelow the required driving potential for bubble production. !his first occurs at thebottom of the brine pool.

6.< MAJOR HEAT TRANSFER AREAS IN MSF PLANTS

In distillation, saline water is evaporated and pure vapors are condensed to obtaindistillate. +eat transfer for this is accomplished in heat e&changers. /ooling water flows inside the heat e&changer tubes while steamCvapors condense outside thetubes. +eat transfer areas are a very e&pensive item and in an #S$ plant mayaccount for 59E of the capital cost. !he main heat transfer areas are @8

6.7 HEAT REJECTION SECTION CONDENSERS

!he tube condense vapors (from flashing brine) in the last few +4S stages to formproduct water. 4aw seawater is used as the coolant. fter being heated in the tubes,it is re ected but only after ma'eup has been tapped off. #a'eup feed is sent to thelast stage flash chamber. !he condensers are located at the top of the stages.

6.2 HEAT RECOVERY SECTION CONDENSERS

?rine circulates through the tubes of the stages comprising the hear recovery section.It is preheated by vapors released from flashing brine. !he temperature of recirculating brine increases from stage to stage. /ondensed vapors are collected asproduct water. !he condensers are located at the top of the stages.

6.8 BRINE HEATER

It increases the temperature of recirculating brine, which e&its from the last +1Sstage, upto the top temperature. +eat e&change is carried out by low pressure,e&ternally supplied steam, in a shell and tube heat e&changer. ?rine flows inside thetubes while steam condensers on the outside.

#S$ 0-"4 !I0*S - *! !"/+*0 012 3 S2S!"#S V */" /0 4S" "SS0* 5 - 1"


9/9

6.9 VENT CONDENSER

Steam and gases vented from the various evaporator stages are condensed in thisshell and tube heat e&changer. /ooling is usually done with raw seawater. 1ases,which are not condensed, are sent to the e ector condensers.

6. EJECTOR CONDENSERS

!hese condense steam and other gases (e&tracted from the various stages and thevent condenser) in a shell and tube heat e&changer. /ooling is usually done withma'eup water.

- *! !"/+*0 012 3 S2S!"#S #S$ 0-"4 !I0*S"SS0* 5 - 1" F V */" /0 4S"

Documents

Msfpt2.Les