This document outlines and compares three common methods for desalination of seawater: multiple effect distillation (MED), multi-stage flash distillation (MSF), and reverse osmosis (RO). MED uses a series of distillation columns to purify water through evaporation and condensation. MSF uses a succession of chambers where portions of heated seawater flash into vapor due to pressure changes. RO uses pressure to force seawater through a permeable membrane to separate freshwater. The document discusses the process, advantages, and outputs of each method. MED is highlighted as being very power efficient compared to MSF and RO, with the capacity to produce 68,000 m3 of freshwater per day.

1. ChE 100
Hseen Baled
Design Project
Desalination of Seawater
Keerthi Gnanavel
Chen Xu
Blaine Bensur

2. Table of Contents
1. Abstract
2. Introduction
3. Methods
4. Results
5. Discussion
6. Conclusion
7. References
Appendix
Figure 3A: (MED)
Figure 3B: (MSF)
Figure 3C: (RO)

3. 1. Abstract
This project report outlines and illustrates the commonly used methods of desalination of
seawater in industry. The methods talked about in this project are multiple effect distillation (MED),
multi-stage flash distillation (MSF), and reverse osmosis (RO). These methods have their own advantages
and disadvantages in different environment conditions. The cost of these method can be varied based on
different factors such as size, efficiency, and budget. MED is the most reasonable method considering a
number of factors. It offers the highest production rate and lowest cost.
2. Introduction
Liquid water is often regarded as the key to life. Then, it is no surprise that Earth, a planet
teeming with life, has a surface of which 71 percent is water. But in modern times, this water is unusable
due to a high salt content and other waste contaminants. The next logical step in harvesting such a
bountiful resource is to desalinate and decontaminate it, effectively transforming the oceans into a nearly
unlimited supply of usable water.
Unfortunately, the desalination process is easier in theory than in application. The energy
requirement supercedes that of other methods to obtain freshwater. More specifically, the transportion of
freshwater from other parts of the world is sometimes easier than the desalination of seawater. That said,
if generaltransportation and energy costs are high in a certain region then desalination becomes
increasingly attractive. A potential breakthrough in desalination could give the answer to most of the
world’s water supply problems.
Desalination is popular in hot, arid regions that are close to seawater and have no other means of
freshwater. Places such as India,Middle East, California, and certain parts of Europe are working with
desalination to increase its efficiency in industry and commercial application.
This paper will outline three main methods of desalination, Multiple Effect Distillation (MED),
Multi-stage Flash Distillation (MSF), and Reverse Osmosis across membranes (RO). The classic methods
for desalination are MED and MSF, as they are more tolerant to extreme conditions and poor water
quality. Energy costs for MED are also favorable over RO, considering a higher quality product and an
easier scale-up process. However,development of new technology recently makes RO prevalent in areas
that can afford it, such as the developed parts of Europe, California, and the Central US.
3. Methods
Multiple Effect Distillation - 3.1
Multiple Effect Distillation (MED) implements a series of distillation columns to purify water.
Due to the mechanization and industrialization of this method, MED can be favorable over RO in climate
regions that are arid, hot, and sometimes difficult for RO. These regions, including South India and the
Middle East, have made considerable improvements in the efficiency of this process.
The feed water is heated by steam. Typically, the steam is brought to the distillation column
through tubes, but there are a multitude of pathways for this heat transfer. The steam tubes can be

4. submerged in the feed water,or the feed water can be sprayed onto the steam tubes. The steam is then
collected at the top of the column and recycled to the next evaporator. The water,now more dilute, is
collected at the bottom and led to the next column. The basic principle behind a distillation column is the
produce fresh steam from the feed water and use it to heat the next column.
Figure 3-A illustrates the overall setup of a multiple effect distillation plant. The feed water is fed
to the first column, along with some initial steam to heat the feed. The bottoms water will be more dilute
in salts than the feed, depending on how much steam is evaporated in the column. The steam is recycled
to the next column. The final steam output is condensed and cooled as the desired product, pure water.
Analysis of a single column of the plant shows the process in detail. The steam tubes are arranged
as layers in the column. The feed water is sprayed onto the tubes to create maximum surface area and
contact. Water is allowed to drip down the successive tubing until it is collected at the bottom of the
column. The steam produced from the evaporation will rise to the top of the column, where it is collected,
combined with the input steam,and sent to the next column.
Multistage Flash Distillation - 3.2
Multistage Flash Distillation (MSF) is a process similar to MED, but with slight variations. In
MSF there exists a succession of chambers,or stages,through which the feed water will flow. in these
stages,pressure and temperature are controlled to allow for portions of feed to evaporate or “flash” into
the vapor phase.
The principles of minimizing heat input and heat loss are applied to MSF through a counter-
current consisting of cold, feed water against steam. After successive heating through the stages,the feed
water enters a final heater to achieve maximum temperature. Then the heated feed enters stages that have
a gradient of pressures. The first stage has the highest pressure of all the stages,while the last stage has
the lowest pressure. The pressure is set depending on the vapor-pressure of the feed water. The salt
content of the seawater also has an effect on this vapor pressure. The water is fed into successive stages
using a valve system. It is important to regulate pressure in the chambers to allow for portions of the feed
to evaporate. As the feed cools, the valve allows flow into the next chamber. The cold feed water is piped
into the stages. This allows for simultaneous cooling and condensing of the steam product as well as
heating of the initial feed seawater. The condensed steam is the potable water product of interest.
Figure 3-B illustrates the overall process. Under steady state conditions, the feed water is heated
by the steam produced. The feed stream is piped into and out of the chambers and heated again by a
heater before entering the first stage as heated brine. The leaving product is the condensed steam as well
as some dilute brine saltwater. Some external heat input is required by the heater.
Analysis of a single stage will uncover more details. In the chambers, pressure will be lower than
the vapor pressure of the heated brine water. This allows vaporization of some water. As the brine cools,
its vapor pressure will reduce as well, then it is piped to another stage chamber where the pressure is
lower than the previous. The steam condenses and is as heat is passed to the incoming feed water. The
condensed steam is collected and piped out as product. This method conserves the majority of heat and
energy throughout the process and can be repeated until the desired product is achieved.

5. Reverse Osmosis - 3.3
Reverse osmosis is the process by which sea or brackish water is pushed through a permeable
membrane where freshwater is separated from dissolved salts. The liquid flowing through the membrane,
or permeate,flows through the membrane by the pressure difference between the feedwater and product
water. The product water is at almost exact atmospheric pressure. The higher the salinity of the water,the
more pressure is necessary for desalination to occur. For desalination of seawater,the operating pressure
should be somewhere between 800 and 1000 psi. As the remaining feed water is eliminated as brine, there
is no energy reactions or phase changes occurring.
There are four major separate components of the overall reverse osmosis process,as seen in
Figure 3-E. These are pretreatment,pressurization, membrane separation, and post-treatment
stabilization. Through pretreatment, the feedwater is adjusted to make sure that it will be usable with the
given membrane. This includes removing unneeded solids and changing the pH. The last part of
pretreatment is adding a threshold inhibitor to control the effects of certain constituents that could be
found in the feedwater. The second step,pressurization, is simply the process where the feedwater is
pumped into a closed container and then pressurized to accommodate the salinity of the feed.
Step three,membrane separation, is the most complex step of the entire operation. The
membranes are permeable to the freshwater while blocking passage of dissolved salts, and create two exit
streams. One of the streams is a freshwater product stream and the other is a brine reject stream. There are
different configurations of membranes with the most common being spiral wound. The membranes today
are made of thin polymer composites, contrary to the previous material, cellulose acetate. Through this
step, small amounts of salt will still cross the membrane since it is near impossible to create a completely
perfect membrane. The final step, stabilization, is to perfect the fresh water product stream so it can be
used for drinking water. This involves adjusting the pH from usually around 5 to as close to 7 as possible,
and degasifying the water. After this is complete the reverse osmosis process is finished and the water is
transferred to a holding area for further distribution.
4. Results
The input, output, cost, and maintenance of a plant vary based on a number of factors including
size, efficiency, and budget. Estimations of numerical results and date are shown below, read from
credible and current sources and documentation.
Multiple Effect Distillation - 4.1
Multiple effect distillation is a very power efficient process compared to MSF and RO. It also has
the capacity to produce more distillate water than either of the two. Result shown below are some data
from Veolia Water Treatment Technologies.
Product water output capacity 68,000 m3
/day = 2,833 m3
/hr
Product water salt content <2ppm
Seawater feed temperature Varies

6. Waste brine temperature 60o
C
Concentration of brine ~1.5 x feed concentration
Brine heater steam requirements 130o
C, ranges from 0.35 - 1 bar
Plant life ~25 years
Power consumption Ranges from 1.5KWh - 15.0 KWh
Overall annual cost ~1 Million USD
Multi-stage Flash Distillation - 4.2
In parts of India, namely Tamil Nadu, Gujarat, and Rajasthan, water shortage is a growing issue.
Over population as well as environmental regulations put a strain on industrialization. Research and
development in these states have proposed building an MSF plant near a nuclear/thermal power plant.
Even a relatively small plant could provide the necessary energy requirement for the heating in a
compatible MSF plant. MSF is a proven and well-developed solution in this region.
Product water output 4500 m3
/day = 187.5 m3
/hr
Product water salt content <50ppm
Seawater feed requirement 375 m3
/hr
Seawater feed temperature 29o
C
Waste brine temperature 40o
C
Concentration of brine ~2 x feed concentration
Brine heater steam requirements 130o
C, 2.8 bar
Performance ratio 9 kg water produced/kg steam input
Power consumption 500KWh (pumping) + 100KWh (lighting/other) = 600KWh
Overall annual cost 73.369 Million Indian Rupees = 1.1 Million USD
Reverse Osmosis - 4.3
RO is a relatively new desalination process. Also, there are many conditions that have to be met
before RO can take place. Shown below are some estimates under operation.

7. Product water capacity 50,600 m3
/day = 2,108 m3
/hr
Product water concentration 500 mg/solid solute
Pressure across membranes Ranges from 800psi - 1000psi
pH of product water ~5
Annual Cost ~365,000 USD/m3
5. Discussion
MED and MSF are very common methods of seawater desalination. They are prevalent mainly in
harsh climates because the processes can be run under the more extreme conditions. The cost comparison
between MED and MSF shows that MSF can be a little more efficient, using the seawater to continually
condense the product steam into freshwater. But MED surpasses MSF and RO in production capacity.
RO is a relatively new technology and is still developing. It is currently very costly and difficult
to maintain, due to frequent weather fluctuations, problems with the filter material, etc. The future of RO,
however, might be promising.
Desalination of seawater is not always to produce drinking water. Freshwater product can be used
to irrigate crops, cool industrial machines, and many more other applications. This allows some relatively
poor desalination techniques to be improved on under practice.
6. Conclusion
After researching these different methods, MED seems to be the most effective way of
desalinating seawater due to multiple reasons. Cost, overall production rate, and amount of power used
through the process are the biggest factors in making a comparison between the methods, all of which are
dominated by MED. The advantage originates in the heat transfer. The act of spraying water on heated
tubes to produce vapor is a much more efficient heat transfer method than submerging the hot pipe in
water,reducing pressure to vaporize, or pushing water through a membrane.

8. 6. References
1. Cohen-Tanugi, David, and Jeffrey C. Grossman. "Water Desalination across Nanoporous
Graphene." Nano Letters Nano Lett. 12.7 (2012): 3602-608. Web. (DOI:
10.1021/nl3012853) (NANO Graphene) (article)
2. Wade, Neil M. "Technical and Economic Evaluation of Distillation and Reverse Osmosis
Desalination Processes." Desalination 93.1-3 (1993): 343-63. Web. (DOI:
10.10160011916493801132) (RO MED MF) (article)
3. Manolakos, D., G. Papadakis, S. Kyritsis, and K. Bouzianas. "Desalination."
Experimental Evaluation of an Autonomous Low-temperature Solar Rankine Cycle
System for Reverse Osmosis Desalination 203.1-3 (Feb 2007): 366-74. Print. (DOI:
10.1016/j.desal.2006.04.018) (RO)(article)
4. https://www.oas.org/dsd/publications/Unit/oea59e/begin.htm#Contents -URL Secretariat,
General, comp. "2.1-2.5." Source Book of Alternative Technologies for Freshwater
Augmentation in Latin America and the Caribbean. Osaka, Japan: UNEP, 1998. N. pag.
Print.(RO)(print)
5. http://www.engr.uky.edu/~aseeched/SummerSchool/2007/session_handouts/Spreadsheets
/3%20Spreadsheet%20Applications%20Across%20The%20Curriculum/3%20Documents
/TripleEffect.pdf (MED)(paper-lecture)
6. Krishnan, Mangala Sunder, Professor, comp. "Modeling of Evaporators." Multiple Effect
Evaporator System (n.d.): n. pag. Web. 10 Oct. 2015.
<http://nptel.ac.in/courses/103107096/25>. (MED)(Paper-lecture)

9. Figure 3A

10. Figure 3C

11. Figure 3B