Department of Chemical & Biomolecular

Engineering

THE NATIONAL UNIVERSITY

of SINGAPORE

CN3132: Separation Processes

Desalination by Flash Distillation

SEMESTER 1, AY2012/13

Name: Elvis Jeremy Dy Juanco Ayroso A0074996H

Evonne Ching Sze Yin A0069552X

Lau Shiyun Delci A0070156M

Lina Diyanah Bte Ramli A0070258H

Soon Yng Tyng A0070353N

Date of Submission of Portfolio : 10 October 2012

Table of Contents

Page1 Introduction 2

2 Theoretical Background 3

3 Design3.1 Assumptions

57

4 Methodology 7

5 Actual Procedure 10

6 Conclusion & Recommendations 14

7 References 15

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1. Introduction

Water is one of the most essential resources for the survival of mankind.

Currently, the natural supply of water comes from oceans, glaciers and ice caps,

lakes, rivers and even underground. However, the limited supply is unable to

support the growing demand for low-salinity water. A clean and safe supply of

water is needed for drinking and industrial purposes. Hence, this rapid demand

has spurred the advent of technology in the area of water treatment.

Desalination is able to curb the problem of increased demand by removing salts

from saline water. In other words, it is the process of separating fresh water

from saline water.

What is the composition of seawater?

Seawater i.e. saline water is actually pure water with dissolved solids and gases.

A 1kg sample of seawater contains approximately 35g of dissolved compounds,

including inorganic salts, organic compounds from living organisms, and

dissolved gases. The solid substances are also known as ‘salts’ and their

composition in seawater can be expressed by the term ‘salinity’ (units: parts per

million, or ppm). Oceanic salinities are generally within the range of 34000-

37000ppm. Fresh water (desalinated water) would exhibit a salinity content of

less than 1000ppm.

How is desalination carried out?

There are a few main methods by which desalination is carried out in industry:

multi-effect (ME) distillation, multistage flash (MSF) distillation, vapor-

compression (VC) distillation, reverse osmosis (RO) and electro dialysis (ED).

The first three distillation processes involve phase change while the latter two

takes place without any phase change. All the processes mentioned above

essentially separate seawater (saline water) into two streams: one with low

concentration of dissolved salts (fresh water) and the other that contains the

remaining dissolved salts (the brine stream).

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In the discussion that follows, we will investigate the effectiveness of flash

distillation in desalination.

What is flash distillation?

Distillation is the one of the oldest and most widely used desalination

techniques. In flash distillation, feed saline water (seawater) is evaporated or

flashed in the column to obtain water vapour, which is then condensed to form

the desired fresh water. This process produces water of better quality than that

obtained through crystallization and membrane processes.

2. Theoretical Background

Figure 2.1: Diagram of multi-stage distillation

Figure 2.1 above describes the multistage flash distillation process used in

industry. It works on the principle that seawater will evaporate when it is fed

into the 1st stage, as that stage is at a lower pressure than saturation pressure.

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Brine heater

1st stageP1

2nd stageP2

Nth stagePNSteam

Brine

Feed

Condensate

Fresh water

Flashing and heat recovery

Seawater is first fed into the brine heater in a bank of tubes, where steam

condenses on the outer wall. The heated seawater then flows to the first stage,

where it flashes upon entry to produce water vapour (steam) and the discharge

brine. The vapour obtained is condensed on the outside of the tubes

(represented by the black tubes in the diagram) carrying seawater feed to the

brine heater. The condensed water vapour is then collected in the desired fresh

water stream. The brine stream leaving the 1st stage has a higher salt

concentration and is sent to the 2nd stage for further flashing. P2 <P1 in order to

lower the boiling point of seawater.

At the second stage, more steam is obtained and recovered as fresh water by

condensation. The remaining seawater is then sent to the third stage and so on.

Each successive stage will be at a lower pressure than the previous stage. This is

to reduce the boiling point of the seawater as it becomes more concentrated as it

goes through the stages. Multiple boiling is thus possible without the additional

supply of heat after the brine heater.

For simplicity, we will only focus on a single stage for our calculations.

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3. Design

Figure 3.1: Single Stage Flash Distillation

As discussed above, we will be focusing on a simplified single stage flash

distillation.

According to a previous literature study, a single stage flash distillation can be

designed using the diagram above, Figure 3.1. However, this system involves

reflux. Hence, modifications will be made to this system for simplicity.

It is essential to know the operating conditions of a flash distillation as it affects

the F/V ratio. From our research, it was difficult to obtain the water-salt

equilibrium data and therefore, obtain the operating line. Hence, we are

modeling our simplified flash distillation system after the conditions of a

simulated distiller operating at high temperature based on a research done by

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Osman A. Hamed and his peers. Refer to Figure 3.2 for the operating conditions

at high temperature operations.

Figure 3.2: Operating conditions of Selected Distiller

Assumptions are made to further simplify the model and calculations for the

unknowns in Figure 3.3.

Figure 3.3: Simplified Model of Flash Distillation

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3.1 Assumptions

1. From our research, 1 kg of seawater contains 35 g of dissolved

compounds, such as inorganic and organic salts. It is assumed that these

salts have similar physical properties as NaCl.

2. Assume drum is well insulated. Hence, heat loss to the surrounding is

negligible.

3. Single stage flash system is assumed to be an ideal system. Hence, L and V

streams are in equilibrium

4. Distillate product is salt free. This assumption is valid since the boiling

temperature of water is much lower than that of the salt.

4. Methodology

This design question will make use of the basic principles of flash distillation. In

this adiabatic process, a sudden reduction in pressure across a valve, through

which the feed flows, causes the feed to flash from its liquid state to a vapor

state.

Known components: F, z, Tf, y, V, L, Tdrum and Pdrum

1. Calculate x

Component mass balance of more volatile species: Fz = Vy + Lx

Substitute all known values to find x.

2. Calculate T0

T0

o Energy balance: FhF + Qflash = VHv + LhL

Qflash = 0 (adiabatic)

Enthalpy of Liquid stream: hL=∑i

x iCLi (Tdrum−T ref )

Enthalpy of Feed stream: hF=∑i

z iCLi (T0−Tref )

Enthalpy of Vapor stream: H v=∑i

y i¿

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CL : liquid head capacity

λ : latent heat of vaporization

Cv : vapor heat capacity

o Substitute all known values into the energy balance to find TF (only

unknown).

3. Calculate heat load, QH

Energy balance: QH = F(hF – h1)

o Cal hF using the T0 determined above.

o h1=∑i

ziCLi(T f−T ref )

Substitute all known values into the energy balance to find QH.

From the energy balances, we can determine whether this design of the flash

process is energetically feasible.

4. Drum Sizing

We assumed a vertical flash drum design.

Calculate the permissible vapor velocity, uperm

o uperm =Kdrum √ ρL−ρ vρv

ρL: liquid density

ρ v : vapor density

Kdrum=exp¿¿

A = - 1.877478097

B = - 0.8145804597

C = - 0.1870744085

D = - 0.0145228667

E = - 0.0010148518

F lv=W L

W v √ ρ vρL W L: liquid mass flow rate

W v: vapor mass flow rate

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Calculate the cross sectional area, Ac

o Ac=V ×(MW v)

uperm×3600×ρ v

V : vapor flow rate

MW v: molecular weight of vapor

Calculate the size of the drum

o Diameter, D=√ 4 Acπ

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5. Actual Procedure

Properties of substances used:

Density of seawater = 1025kg/m3 (varies from 1020 to 1029)

Specific volume of water vapor at 112.8oC = 1136.6m3/kg

Details of the feed:

Lab preparation of the brine:

0.035kg of NaCl to 1kg of H2O

1. Determination of x

F = 9.95kg/hr

V = 1kg/hr

L = 8.95kg/hr

Mass balance of the water component:

Fz = Vy + Lx

9.95(0.9662) = 1(1) + 8.95(x)

x = 0.9624

2. Determination of T0

Tdrum = 385.95K

Energy balance: FhF + Qflash = VHv + LhL

QFlash = 0 (adiabatic)

Enthalpy of vapor stream, H v=∑i

y i¿

= 1[(2256) + (1.8892)(385.95 - 373.15)]

= 2280.18 kJ/kg

Enthalpy of liquid stream, hL=∑i

x iCLi (Tdrum−T ref )

= (4.028)(385.95- 373.15)

= 51.5584 kJ/kg

Enthalpy of feed stream, hF=∑i

z iCLi (T0−Tref )

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= (4.045)(TF – 373.15)

= 4.045TF – 1509.39 kJ/kg

Where,

Liquid heat capacity, CL

= 4.045 kJ/(kg.K) ---- for feed stream

= 4.028 kJ/(kg.K) ---- for liquid stream

[values differ due to the difference in salinity]

Latent heat of vaporization, = 2256 kJ/kgλ

Vapor heat capacity, CV = 1.8892 kJ/(kg.K)

Substituting the values into the energy balance equation,

FhF + Qflash = VHv + LhL

(9.95)(4.045T0 – 1509.39) + 0 = 1(2280.18) + 8.95(51.5584)

Hence, T0 = 441.27K = 168.12 o C

3. Determination of heat load, QH

From the energy balance equation around the heater,

QH = F(hF – h1)

hF = 4.045TF – 1509.39 kJ/kg

= 275.55kJ/kg

h1=∑i

ziCLi(T 0−Tref )

= (4.045)(303.15 – 373.15)

= -283.15kJ/kg

Substituting the values into the energy balance equation,

QH = F(hF – h1)

QH = 9.95(275.55-(-283.15))

= 5559.065 kJ

4. Drum Sizing

Assuming a vertical flash drum design,

To determine the permissible vapor velocity

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uperm =Kdrum √ ρL−ρ vρv

where,

ρL = 1025 kg/m3

ρ v = 0.0008798 kg/m3

Kdrum=exp¿¿

where,

A = - 1.877478097

B = - 0.8145804597

C = - 0.1870744085

D = - 0.0145228667

E = - 0.0010148518

F lv=W L

W v √ ρ vρLW L: 8.95 kg/hr

W v: 1kg/hr

Therefore, Flv = 0.008292

Hence, Kdrum = 0.2991

uperm = 322.84 ft/s

To determine cross sectional area, Ac:

Ac=V ×(MW v)

uperm×3600×ρ v

Where,

V = 1kg/hr

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MWv = 18kmol/kg

Therefore, Ac = 0.0176 ft2 = 0.00164 m2

To determine the size of the drum,

D=√ 4 Acπ

= 0.0457 m

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6. Conclusion & Recommendations

In conclusion we have determined the parameters for constructing a simple flash

distillation unit capable of separating salt from seawater. For the feasibility of

the setup, we have simplified the parameters of the unit to aid in the calculations.

Such assumptions include pure water being retrieved from the vapour phase. An

important assumption was considering the seawater mixture as a salt (NaCl)

water mixture. This helped in further simplifying the distillation to a 1

component flash process. Since the boiling point of NaCl is very high compared to

that of water, we are assured that the top product does not contain salt.

For the unit specifications, we have assumed negligible heat loss to surroundings

due to insulation of the unit. Operation conditions must also be considered for

the optimum separation efficiency of the unit. One has to note that certain tank

units have their pressure and temperature units; so it would be wise to check the

allowable limits of the operation first. For the feed flow specifics, we have also

assumed that the bottoms product contains a saturated amount of saltwater,

since precipitated salt might damage the unit. From all these assumptions, we

have computed all the specifications, inputs and outputs of the distillation unit.

We have also calculated the input heat requirements in order to sustain the

operation of the desalination unit.

There are many recommendations for this setup. Since NaCl is not the only salt

component in seawater mixture, experiments can be done to verify whether

ignoring the other salts indeed poses no significant change to the desalination

mixture. Other methods of separating seawater must also be explored on. Due to

the high latent heat and heat capacity of water, managing a desalination process

is very energy intensive, requiring a huge amount of energy and therefore

increasing operation costs.

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7. References

Cheah, S.M. (2000). Seawater Distillation. In Distillation. Retrieved from http://www.separationprocesses.com/Distillation/MainSet1.htm

Engineering Toolbox. (n.d.) Specific Heat of Water Vapor. Retrieved fromhttp://www.engineeringtoolbox.com/water-vapor-d_979.html

MIT. (n.d.) Thermophysical Properties of Seawater. Retrieved from http://web.mit.edu/seawater/

Osman A. Hamed, Mohammad AK. Al-Sofi, Monazir Imam, G. M. Mustafa Khalid Bamardouf, Hamad Al-Washmi. (n.d.) Simulation of Muiltistage Flash desalination Process. Retrieved from www.swcc.gov.sa/

Sharqawy, M.H., Lienhard J.H. , and Zubair, S.M. (2010, April). Thermophysical properties of seawater: A review of existing correlations and data. In Desalination and Water Treatment, Vol. 16 (354-380).

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