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THERMALDESALINATIONPROCESS

A. MSFENGINEERING

Contents(ThisispartialcontentoftheoriginalsourceThermalDesalinationProcessMSFEngineeringforM.A.Darwish)

1. OncethroughMSFSystem

a. SinglestageFlashSystemb. TwoStagesFlashSystemc. nStageFlashSystem

2. RecirculationMSFSystem

B. MSFModeling

Contents(ThisispartialcontentoftheoriginalsourceMSFModelingandComputerPackageforH.Eltony)

1. OncethroughMSFSystem2. RecirculationMSFSystem

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THERMAL DESALINATION PROCESS

MSF ENGINEERING

M.A. Darwish Department of Mechanical and Industrial Engineering, Kuwait University, Kuwait

Contents (This is partial content of the original source)

1. Once-through MSF System 2. Recirculation MSF System

1. Once-through MSF system

1.1 Single-stage Flash System

The once-through MSF represents the limiting case of the ME system when the feed flow rate is increased to the extent that the vapor is generated by flashing only and the boiling heat transfer surface is entirely omitted. The flash vapor is condensed on the feed heater-type condensers.

In a single-stage flash desalting system (Figure 1), the feed Mf, after pre-treatment, is pre-heated from Tcw to Tf1 in the condenser tubes. It is then heated in the brine heater (called heat input section, HIS) to temperature Tbo by condensing the supply steam Ms (main energy source). The feed, leaving the brine heater at temperature Tbo and pressure Po, enters the first flashing chamber (or stage) kept at P1 of saturation temperature Tb1 < Tbo. The feed becomes unstable and part of it is spontaneously vaporized (i.e. flashed in order to reach stability by dropping its temperature to Tb1). The vapor released by flashing flows upward through demisters (to separate entrained liquid brine) to the condenser tubes in the upper portion of the chamber. The vapor condenses there and forms the product. The heat released by vapor condensation is gained (or recovered) by the heated feed. The temperature of the flashed vapor (from brine at Tb1) suffers some losses Tl due to non-equilibrium, boiling point elevation, and pressure drops through the demister and condenser tubes. The following energy balances can be carried out by assuming the following.

a) Constant and average specific heat, Cp ,for the feed Mf, distillate Md, and brine Mb, streams a

b) Constant and average latent heat value, ,for condensing vapor and supply steam. c) A completely insulated stage.

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Energy Balances:

(i) For the condenser tubes

1( )f p f cw dM C T T M = (1.1)

(ii) For flashing brine

1( )f p bo b dM C T T M = (1.2)

(iii) For the brine heater

1( )s f p bo fQ M M C T T= = (1.3) where Q is the heat input rate (kJ s-1) and Ms is the steam flow rate assumed to be supplied at saturated vapor condition and leaves at saturated liquid condition.

For the whole desalting unit

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1 1f p cw S b p b d p VM C T M M C T M C T+ + (1.4)

Since

f b dM M M= +

and

1 1V b lT T T=

then

1( )S F p b cwM M C T T

Equations (1.1), (1.2), and (1.4) give

1 1f cw bo bT T T T =

1

1

1d f cws b cw

M T TM T T

=

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The value of Mf /Md (=19.9) is large compared to Mf /Md (=2.5) for a single-effect or ME system when the feed total dissolved solids (TDS) = 42000 p.p.m. and brine blowdown TDS = 70 000 p.p.m.

The areas of heat transfer surface for both the brine heater and condenser heat exchangers can be determined by equating the thermal load of each exchanger with the overall heat transfer coefficient U, multiplied by the surface area multiplied by the effective temperature (LMTD). For the brine heater (with subscript b for the parameters involved)

s b b bM U A LMTD = (1.8) where

1 2

1

2

ln

b bb

b

b

T TLMTDTT

=

(1.9)

1 1

2

( )( )

b s f

b s bo

T T TT T T

= =

For the condenser (with subscript c)

d c c cM U A LMTD = (1.10)

1 2

1

2

ln

c cc

c

c

T TLMTDTT

=

(1.11)

1 1

2 1 1

( )( )

c V cw

c V f

T T TT T T

= =

Recalling Md/Ms = (Tf1 - Tcw)/(Tb1 - Tcw) and TV1 = Tb1 - Tl, the total specific area At/Md = (Ab + Ac)/Md is expressed by

1 1 1

1 1 1 1 1

ln ln( ) ( )

t b cw s f b l cw

d f cw b bo f s bo c f cw b l f

A T T T T T T TM T T U T T T T U T T T T T

= + (1.12)

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The typical example mentioned earlier, with

Ts = 100C, Tbo = 90C, Tb1 = 62C, Tcw = 30C, Tf1 = 58C, Tl = 1.2C and by assuming Ub = 3 kW m-2 C-1 and Uc = 2.5 kW m-2 C-1

has a specific total area At/Md = 119.6 m2 kg-1 s-1.

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1.2 Two-stage Flash System

Now a two-stage flash desalting system can be considered, as shown in Figure 2. The feed Mf entering the second stage condenser tubes at Tcw is successively pre-heated to Tf2 and to Tf1 in the second and first stage condensers, respectively, before it enters the brine heater. The supply steam Ms raises the temperature of the feed in the brine heater from Tf1 to Tbo before it enters the first stage as flashing brine which is kept at pressure P1 (of saturation temperature Tb1 < Tbo).

The vapor generated in the first stage D1 (due to the pressure drop of the flashing brine) is condensed on the outer surface of this stage condenser tubes and, thus, raises the temperature of the feed circulating in this condenser from Tf2 to Tf1. The unflashed brine (B1 = Mf - D1) and the formed distillate D1 proceed to the second stage kept at pressure P2 of saturation temperature Tb2 < Tb1. Again both flashing brine and distillate suffer a pressure drop, partially evaporated by flashing and their temperatures drop to the second stage saturation temperature. The vapor generated in the second stage is condensed on these stage condenser tubes and heats the feed flowing in the condenser from Tcw to Tf2. The unflashed brine (B2 = Mf - D1 - D2) leaves the second stage as brine blowdown at Tb2. The term D2 represents the vapor evaporated (flashed) from the flashing brine in the second stage only. The mass and energy balances for the two-stage flash desalting system give the following.

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Brine heater

1( )s f p bo fQ M M C T T= =

Flashing brine entering the first stage

1 1( )f p bo bD M C T T =

First stage condenser tubes

1 1 2

1 2

( )

where, f p f f f p c

c f f

D M C T T M C TT T T

= = =

Second stage flashing brine

2 1 1 2( ) ( )f p b bD M D C T T =

or

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( )f p bM D C TD =

where Tb = Tb1 Tb2 and D2 is the vapor released from flashing brine. Vapor flashed from distillate D1 entering the second stage, D1V, is equal to

1 1 2 11

( )p V V p bV

D C T T D C TD

= =

Accumulated vapor condensed on the second stage condenser tubes is equal to

12 1 2

p b f p cV

D C T M C TD D D

+ = + =

Second stage condenser

2 1 2( ) ( )f p f cw f p b bM C T T M C T T =

Overall system (assuming no heat losses)

2( )s f p b cwM M C T T =

The above equations give

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1 2bo f b cwT T T T =

1 1 2bo b f f b cT T T T T T T = = = 1 2 2b b f cwT T T T =

1 1 21 2 1

( ) ( )( )bo b b bd f p f pT T T TM D D M C M D C

= + = +

If (Tbo Tb1) is chosen equal to Tb1 Tb2, then

2 2bo bT T T =

and

f p f p pd f

M C T M C T C TM M

= +

Define

y = Cp T / Then

Md =Mf (1-y) y + Mf y

Md =Mf [1-(1-y)2]

2

11 (1 )

f

d

MM y

= (1.13)

Equations (3) and (13) give

2

1

1 (1 )( ) /

d

s p bo f

M yM C T T

= (1.14)

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If the temperature approaches of the two-stage condensers (Tb1 Tf1), and (Tb2 Tf2) are kept the same and equal to 4.2C and Tbo = 90C and Tcw = 30C, then

( ) ( )2 2bo cw bo b b cwT T T T T T = + 60 = (Tbo Tb2) + (Tbo Tf1) = (Tbo Tb2)+ (Tbo Tb1) + (Tb1 Tf1)

60 = 3 T + (Tb1 Tf1) This gives T = 18C, Tf2 = 48.6C, y = Cp T/ = 0.0335, Mf /Md = 15.16 and Md /Ms = 1.604.

Thus, the addition of another stage improves the performance ratio Md /Ms from 0.875 to 1.604, decreases Mf /Md from 19.5 to 15.615, and, thus, reduces the energy rejected with the blowdown brine (both brine blowdown flow and temperature are decreased).

The areas of heat transfer surfaces of the brine heater and stage condensers can be determined as before. For the brine heater

d b b bM U A LMTD =

1 2

1

2

ln

b bb

b

b

T TLMTDTT

=

1 1

2

( )( )

b s f

b s bo

T T TT T T

= =

For the first stage condenser area A1

( )1 2 1 1 =f p f f c CM C T T U A LMTD For the second stage condenser area A2

( )2 2 2 =f p f cw c CM C T T U A LMTD

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1.3 Multistage (n-stage) systems

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A flow sheet of a once-through MSF system is given in Figure 3a and 3b. The previous analysis can be extended to include n number of stages. Assumptions:

1) insulated stages

2) equal temperature drop of the flashing brine, i.e.

Tbo Tb1 = Tb1 Tb2 = Tb2 Tb3 = = Tb,n-1 Tb,n = Tb 3) equal temperature rise in the feed seawater in the condenser of stage, i.e.

Tf1 Tf2 = Tf2 Tf3 = Tf3 Tf4 = = Tf,n Tcw = Tf Accordingly,

nTb = Tbo Tb,n, and nTf = Tf1 Tcw So, using the fact that Tb = Tf = T, yield:

Tbo Tb,n = Tf1 Tcw.

The flashing brine enters the first stage of the flow rate Mf and leaves the first, second, third, and n stages at flow rates of :

Mf(1-y), Mf(1-y)2, Mf(1-y)3 and Mf(1-y)n, respectively,

where y = Cp T/ is the fraction of vapor released from the flashing brine. The flashing brine leaving the last stage n is the brine blowdown B, i.e. Bn = Mb = Mf (1-y)n.

The distillate generated from the flashing brine in the first, second, third, , and n stages is

y Mf, y(1-y) Mf, y(1-y)2 Mf, , and y(1-y)n-1 Mf , respectively.

Accumulated distillate Md in all stages is Md = Mf Mb and Md = Mf - Mf (1-y)n

11 (1 )

fn

d

MM y

= (1.15)

A simpler approximate relationship between Mf and Md can be obtained by considering that the heat gain by all condensers is equal to the heat loss by the average flashing brine due to a temperature drop from Tbo-Tb,n, i.e.

(Mf Md /2)Cp (Tbo-Tb,n) Md ,

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where (Mf Md /2) is the average flow rate of the flashing brine between entering at flow rate Mf and leaving at flow rate (Mf - Md). This gives

,

10.5 0.5 0.5( )

f

d p bo b n p

MM C T T n T C n y

= + = + = +

The heat gain in the brine heater Q is equal to Q = Mf Cp (Tbo Tf1) = Ms . A heat balance for the whole system gives

Ms . = Mf Cp (Tbo Tf1) = Mf Cp (Tb,n Tcw). The performance ratio D/S is then equal to

1

[1 (1 ) ]( )

nfd d

s f s p bo f

MM M yPRM M M C T T

= = = (1.16)

It is noticed that the terminal temperature difference (TTD) is Tbo Tf1, which is equal to

Tbo Tf1 = (Tbo Tb1) + (Tb1 Tf1) = T + Tb1 Tf1 and, consequently,

,1 1

[1 (1 ) ]

( )

nd

bo b nsp b f

M yPRT TM C T T

n

= = +

where T = (Tbo Tb,n)/n, and (Tb1 Tf1) is the temperature approach in the first-stage condenser. It is clear that T decreases with an increase in n and (Tb1 Tf1) depends on the condenser heat transfer surface and temperature losses. Thus, Md/Ms depends on n and the condenser's heat transfer surface area. It is also noticed that the temperature range (TR) is equal to

, ,

1 1 1

1 1

( ) ( )( ) ( )

( 1) ( )

bo cw bo b n b n cw

bo f b f

b f

TR T T T T T Tn T T T n T T T T

TR n T T T

= = = + = + +

= + +

The condenser and brine heater heat transfer surface areas can be determined as outlined before.

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For the brine heater

f p s b b bM C TTD M U A LMTD= =

For the stage i (see the temperature distribution in Figure 3b)

, , ,

, ,

,

( )ln

( )

ln( )

f p c i c i c i

f p c i c i

d d bi fi l

bi fi l

f pc i

d d c bi fi l

M C T U A LMTD

M C T U A TM M T T T T

T T T

M CA TM M U T T T

=

= +

=

Since temperature approach are the same for all stages, then Ac1 = Ac2 = = Ac,n = Ac. Therefore,

,c i cd d

A n AsAM M

= = The effects of the design parameters such as the number of stages n and top brine temperature (TBT) Tbo and operating parameters such as seawater temperature Tcw and temperature losses Tl on the performance ratio Md/Ms, feed to distillate ratio Mf/Md, and heat transfer surface areas are shown in Figures 4 and 5. The use of once-through MSF (compared to recirculation MSF which is discussed later) is very limited. Of 68 MSF plants reviewed by Birkett and Newton (1981), only four were once-though plants. The main reasons for not using once-through systems extensively compared to the recirculation systems are as follows.

(a) The system uses a high feed to distillate ratio Mf/Md (for Tbo = 90C, Tcw = 30C, n = 27, and Tb1-Tf1 = 4C, then Mf/Md = 10.2). This means that a large quantity of feed Mf (10.2 Md) is pre-treated (i.e. costly pre-treatment and a large quantity of brine Mb (9.2Md) is to be blown down at a temperature Tb,n of 6C above the seawater temperature (i.e. high energy loss).

(b) Large quantities of gas are released with flashing. This increases the danger of corrosion, decreases the condensation heat transfer coefficient due to non-condensing gases blanketing around the condenser tubes, and requires large ejector capacity with more steam (or energy) consumption.

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On the other hand, the once-through system is more simple than a recirculation MSF system. The brine concentration in the first stage at high temperature is lower than the corresponding one in the recirculation MSF system and this permits higher temperature in the once-through system and decreases the scaling potentiality.

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2. Recirculation MSF system

A recirculation MSF system of n stages, consists of three sections.

(a) The brine heater or the HIS. (b) The heat recovery section of (nr = nst-nj) stages. (c) The heat rejection section of nj stages.

Both recovery and rejection stages are trains of chambers similar to those of once-through MSF systems with condenser tube banks arranged in long-tube (LT) or cross-tube (CT) arrangements, as shown in Figure 7. In long-tube arrangements, the flow of the recirculation stream in the heat recovery section (or cooling water in the heat rejection section) inside the condenser tubes is parallel but in opposite direction (counterflow) to the flashing brine flow at the bottom of the chambers. In cross-tube arrangements the flow in the condenser tubes is in the traverse direction (or perpendicular) to the flashing brine flow. The incoming seawater, at flow rate Mc and temperature tc is pumped into the condenser tubes of the last heat rejection stage and pre-heated successively in the heat rejection stages. The cooling water stream leaving the rejection section is divided into two streams: the first (Mc - F) is rejected back to the sea and the second, the feed F, is chemically treated before joining part of the flashing brine leaving the last stage to form the recirculation stream. The recirculation stream R is pumped to the last recovery stage (n-j) condenser tubes and is successively heated by passing through the recovery stage heaters (i.e. condensers) and leaves the top (first) stage at temperature t1.

The recirculation flow then enters the HIS (i.e. brine heater) where it is heated to the TBT (To) by condensing the steam supply to the HIS as the main thermal energy input. The temperature increase of the recirculation flow in the brine heater (Tbo - Tf1) is called the terminal temperature difference (TTD). The recirculation flow leaving the HIS (e.g. at pressure Po and temperature Tbo) enters the first stage (kept at pressure P1 of the saturation temperature Tb1

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this stage condenser. The accumulated distillate in the second stage is cascaded to the third stage and so on to the last stage where it leaves at temperature Tn-Tl. The flashing brine leaves the last stage at temperature Tbn, highest concentration Xbn, and flow rate Mb equal to

Mb = Mr Md (2.1)

where D is the accumulated distillate product from all stages. Part of this outgoing brine, Bl, is rejected to the sea while the balance (B - Bl) forms a portion of the recirculation stream. As mentioned earlier, the other part of the recirculation stream is F which is taken from cooling seawater leaving the rejection section. This means that

(Mb Bl) + Mf = Mr

Bl = Mf Md (2.2)

3.1 Recirculation arrangements

In recirculation MSF system, the feed Mf (or make-up seawater) forms the portion of the recirculation flow by one of the following arrangements.

(a) The first arrangement. The treated feed is added to the flashing brine in the last rejection stage before blowdown takes place. This arrangement eliminates the need for a separate brine blowdown pump. The recirculation pump forces the recirculation from the last rejection stage to the condenser of last recovery stage and rejects the blowdown brine from its discharge side. The saving attained by not using a blowdown pump is encountered by waste in chemicals since the feed is dosed before blowdown and by waste in the pumping energy since the head of the recirculation pump is higher than (four or five times) the head required by a blowdown pump. (b) The second arrangement. The blowdown is rejected before mixing with the feed. A special pump is needed in this arrangement to reject the blowdown from the last rejection stage (operated and vacuum condition) to the environment with discharging pressure little higher than the atmospheric pressure.

For both arrangements, the mass and concentration balances give

Mb = Mr Md (2.3)

Xr Mr = Xb (Mr Md)

Xr = Xb (1- Md / Mr) (2.4)

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where Xr and Xb are the salt concentrations of the recirculation flow Mr at its entry to the first stage and the brine flow in the last stage when the feed is not added to the last stage.

The feed to distillate ratio Mf/Md can be determined for both arrangements in terms of the salt concentration of the feed Xf (depending on locality) and maximum brine concentration Xb (a design parameter usually taken as less than 70 000 p.p.m.) as follows.

In the first arrangement, where the feed F is mixed with the leaving brine in the last stage before brine blowdown Bl is rejected, a salt balance gives Xb (Mr-Md) + Xf Mf = (Bl + Mr)Xr.

The use of Eqs (2.3) and (2.4) gives

Mf/Md ={[(Mr/Md) 1] / [(Mr/Md) (1-(Xf/Xb) -1} (2.5)

The ratio Mr/Md is to be determined in the next section. Similarly, for the second arrangement, a salt concentration balance at the mixing point of the dosed feed F (at Xf) and the recirculated brine Mr-Mf (at Xb) gives Xb(Mr-Mf)+Xf Mf=XrMr. The use of Eqs (2.3) and (2.4) gives

Mf / Md = 1 / (1-(Xf/Xb) (2.6)

For a typical case of Tbo=90C, Tcw=30C, =2330 kJ/kg, Cp=4.2 kJ /kgoC, Xf=42000 p.p.m., and Xb=70000 p.p.m., there are values of Mr/Md=9.2 for both arrangements, Mf/Md=3.06 for the first arrangement, and Mf/Md=2.5 for the second arrangement. This shows that Mf/Md in the first arrangement is higher (i.e. more water to be treated at additional cost) than in the second arrangement.