Vaccum Freeze Vapor Compression
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Transcript of Vaccum Freeze Vapor Compression
Vacuum-Freeze Vapor Compression ABUEL, Rachelle Jenine, ESTARIS, Mylene Chem 155 Report: Ma’am Luciana Ilao
I. Introduction
About 99% of the earth’s hydrosphere is distributed among the oceans and
the remaining 1% comes from freshwater sources such as river, lakes,
groundwater and glaciers. It is not that difficult to realize that this 99% may not be
usable to humans, primarily due to its salinity. Saline or brackish water, when
drank, can have its negative effects to the body such as dehydration and
imbalance of salts. With the world-wide dilemma for the search of clean and
potable water, the desalination process of salt water can be a very attractive
process for this purpose.
Desalination is a water-purifying process by removing salt from saline water.
Ocean water contains about 35,000 ppm of salt and for it to be considered
potable, the concentration of salt must be narrowed down to less than 1,000
ppm. There are numerous ways of desalinating salt water – reverse osmosis,
electrodialysis and thermal distillation. However, with the onset of new
technologies, new ways of removing salt have been proposed and these usually
include the state of the art freeze-melting processes.
The general claim of the freeze-melting process is the removal of a solvent
by freezing it out of the solution as crystals. The solvent crystals are physically
separated from the solute and are melted to produce the supposedly pure
solvent. In the case of desalination, fresh water is removed from saline water by
a refrigeration system that removes from the brine the heat of fusion of ice. The
ice is crystallized and remelted to produce fresh water. It is good to note that
under appropriate conditions, ice crystals formed can be very pure. There are
three kinds of freeze-melting processes – (i) direct contact freezing, (ii) indirect
contact freezing which can be internally or externally cooled and (iii) vaccum
freezing. All differ from how the solvent is crystallized in the process (Rahman
and Ahmed, 2006).
In this paper, the vacuum-freezing vapor-compression (VFVC) process of
desalination will be discussed. Generally, it uses the solvent (i.e. water) as its
own refrigerant. High vacuum is used to vaporize water which provides a
refrigeration effect for lowering the temperature of the product and causing
crystallization to occur. Water vapor contact is used to condense the ice crystals
back to its liquid state. Vapor is compressed to allow condensation as pure
crystals or on a heat-transfer surface (Rahman and Ahmed, 2006).
II. Description of the Process
A. Objectives
The main objective of VFVC is to separate the solvent from its
solute by freezing the solution and removing the crystallized solvent from
the concentrated solute. Recovery of the pure solvent is done by
condensation by direct contact with compressed solvent vapor. Its
objective is to concentrate the solute to produce a slurry solution and
remove solvent in its pure form.
VFVC is widely utilized for desalination of salt water, which is
needed mainly to increase potable water supply. Thus, when valuable and
susceptible substances are involved, crystallization becomes an attractive
and relevant method of separation and purification.
B. VFVC Process of Desalination
The VFVC process is based on a number of well-established
principles in physical chemistry and are summarized below:
1. Boiling seawater produces vapor that is pure water.
2. Freezing seawater produces individual ice crystals consist of pure
water. However, each crystal is coated with a layer of concentrated
brine adhering to the surface of the crystal and must therefore be
removed by washing.
3. The freezing point of standard seawater is not essentially affected
by reductions in pressure, but remains constant at 28.6oF.
4. The boiling point of seawater varies with pressure. By decreasing
the pressure to about 3.9 mm Hg absolute, the boiling point of
seawater is reduced until it is also 28.6oF, the same goes for the
freezing point. This means that at a pressure of 3.9 mm Hg
absolute, the seawater can vaporize and contain ice
simultaneously.
5. The ratio of heat applied in producing one pound of vapor into the
heat removed in producing one pound of ice is about 7.5 to 1. To
convert one pound of water into vapor, about 1070 Btu (British
thermal unit) must be introduced while 144 Btu must be removed to
convert one pound of water into ice.
6. The melting point of pure ice is 32.0% and the vapor pressure of ice
is the same as that of pure water at the same temperature. The
pressure is 4.58 mm Hg.
C. Specificity and Limitations of the Application
This process is specific for solutions with relatively concentrated
solute and limited to the separation of the solvent from the solute by
freezing under vacuum and remelting of pure solvent by condensation with
compressed vapor. The determining factor of this process is the removal
of the heat of fusion of the solvent in order to freeze. Also, the separated
supersaturated solution is not completely solvent-free.
III. Application of the Process
A. Flow Diagram of the Process
This is the general flowchart of a freezing-melting process, whish is also
involved in VFVC.
Figure 1. Freeze-Melting Process
1. Pretreatment of Feedwater and Passivation of Product Water
The VFVC process is done with the utilization of feedwater at low
temperatures and moderate concentration ratios. The feedwater lines are
periodically dosed with chlorine to avoid the formation of organic growth. The
other pretreatment is deaeration, which is employed to remove oxygen and
other non-condensables from the feedwater, which could possibly contribute
to corrosion or disrupt heat transfer.
Product salinity is usually ~300 ppm (500 ppm maximum). Typically, there
would be sufficient concentrations of calcium and bicarbonate ions in solution,
thus preventing attack of concrete. However, given that the water is stored in
a reservoir or if it becomes saturated with air, it might require chemical
treatment before entering the distribution system.
2. Vacuum Freezing-Vapor Compression Process
Figure 2. Flowsheet for a Typical Vacuum-Freeze Vapor Compression Process Plant
The seawater is pumped through a deaerator to eliminate air and non-
condensable gases. The deaerated water is then cooled via heat exchange
with the product-water and waste-brine streams. The cold deaerated
seawater is then introduced into the lower section of the chamber called
hydroconverter, which is maintained at a low pressure (3 mm Hg). This low
pressure causes the vaporization of the portion of water, hence, removing
heat from the seawater. The seawater boils so that part of it flashes into
vapor, until all of the sensible heat has been released and the seawater is at
its freezing point, where the removal of additional vapor causes a portion of it
to freeze and give up its heat of crystallization. Approximately half of the
seawater is frozen into ice crystals.
The slurry, which is the mixture of ice crystals and brine, is pumped to the
bottom of a separation column and the ice crystals are compacted forming a
porous bed of ice. The bed is moved upward by a slight positive pressure,
which is caused by the brine flowing through the bed and outward through the
screens located approximately at the middle of the column. The rising ice bed
is washed countercurrently with less than 5 percent of the total fresh water
product of the plant. The ice is then harvested by means of a mechanical
scraper at the top of the column. The ice scrapings are then dumped into the
melter, or the upper section of the hydroconverter.
Recalling that upon the entrance of seawater at the lower section of the
hydroconverter, some of the water flashes to vapor. This vapor is then
compressed by a specially designed compressor at the top of the
hydroconverter. The compressed vapor is condensed on the washed ice
entering the melter (upper portion of the hydroconverter). Since the
compressed vapor carries the heat originally removed from the sea water in
the freezer, the ice subsequently melts to freshwater.
Waste brine from the wash column and product water from the melter are
discharged through the heat exchangers to cool the incoming seawater.
However, due to the inefficiency of heat exchangers, some heat still enters
the system with the feed seawater. In addition, heat also enters the system
through pump work, compressor work and ambient heat leakage. To maintain
the system in thermodynamic balance, refrigeration is utilized to condense out
excess water vapor.
Because the operating pressure is far below atmospheric, some air
unavoidably enters the system. Since air interferes with heat transfer process
in ice melting, it is continually removed.
3. Analysis of Freezing Process
The VFVC operation shows that the major costs of converting seawater
are mainly centered in two areas: 1) water plant amortization costs and 2)
electric power costs. The production costs of water via freezing would be
lower when a brackish feed is used, which is mainly rooted in changes in
process pressures and pumping capabilities of the compressor when a lower
concentration feed is utilized. Therefore, for a given recovery and lower
concentration feed, the freezer may be operated at a higher pressure and the
compressor will move a greater mass of the more dense water vapor.
Alternatively, the product water recovery may be increased over the 34%
used for seawater. Either case, more water will be produced per module.
B. Advantages
Application of freezing in desalting seawater is comprised of three steps:
1) partial freezing of the feed stream to an ice-brine slurry, 2) separation of
the ice crystals from the brine, and 3) melting of the ice. The freezing process
possesses the following theoretical advantages: First, it requires the smallest
energy compared to any process involving a phase change (Figure 3).
Second, there is minimum corrosion and scaling because of the low
temperatures involved. The latter advantage also makes room for the use of
lower-cost construction materials and equipment as means of reducing capital
investment. Third, as seen earlier in the VFVC process, no specialized
cooling medium or heat-transfer surfaces are needed to attain freezing since
heat exchange is done using only the product water and waste brine from the
wash column.
Figure 3. Comparison of heat required in different phase-change processes. (BTU = British Thermal Unit)
In addition, VFVC requires only deaeration, unlike other processes that
require some form of chemical pretreatment for control of scale formation.
Acid injection for pH control is commonly employed in electrodialysis
plants and in conjunction with deaeration in distillation plants. Likewise,
polyphosphates are required in the reverse osmosis process and often in
electrodialysis process.
C. Disadvantages
Although the freezing method eliminates problems commonly
encountered in other techniques, it has the inherent problem of separating
ice crystals from the brine. Several methods including centrifugation,
compression, and counter-current washing (like that employed in VFVC)
have been developed, but none showed an entirely satisfactory result.
Experimental studies also showed that the cost of freshwater could be
substantial due to the cost and difficulty of the separation.
The VFVC process imposes difficulty for larger plants. For the
vapor compression system, a plant having a capacity of 227 m3/day uses
a compressor with a diameter of more than 3 m, needing a fairly high
moment of inertia for starting. For larger desalting plants of perhaps 4000
m3/day and above, it would be difficult to find a practical compressor.
D. Industrial Use
1. Recovering potable water from saline water.
2. Wastewater treatment. VFVC can be especially be applied in
concentrating solutions containing volatile organic compounds (VOCs),
which would otherwise be converted to dangerous and hazardous
vapour when heated, making VFVC a better option in treating
wastewater compounded with this hazardous materials.
3. Concentration of sugar in liquid foods such as fruit juices, coffee,
dairy products and other food products. The water component is
frozen and crystallized as ice so that a more concentrated solution will
be left behind. The ice crystals are supposed to be highly pure since
the small dimensions of the ice crystal lattice makes the inclusion of
any foreign compounds except for fluorohydric acid and ammonia,
hence leading to effective separation of water from the solution. In
addition, the VFVC process does involve heating, hence, most volatile
components remain in the concentrated solution which are important
contributors in the aroma of the liquid (coffee, juice, etc.).
4. Quantifying pharmaceutically active compounds (PhACs) in
various liquid samples. The concentration of PhACs in the unfrozen
liquid was increased 2 to 4 times of that in the feed water using
unidirectional freezing (UDF) and about ten times in the two-stage
UDF.
IV. References
Ahmed, M., Chen, D. & Rahman, M. S. (2006). Freezing-melting process and desalination: Review of the state-of-the-art. Separation and Purification Reviews, 35, 59-96. doi: 10.1080/15422110600671734.
Cerci, Y., Cengel, Y., Wood, B., Kahraman, N. & Karakas, E. S. (2003). Improving the thermodynamic and economic efficiencies of desalination plants: Minimum work required for desalination and case studies of four working plants. Mechanical Engineering, University of Nevada, Reno, Nevada, USA.
United States Department of the Interior. (1970). Office of saline water special report on status of desalting. Washington, D. C., USA: U.S. Government Printing Office.
United States Department of the Interior. (1968) Vacuum-freezing vapor compression desalting process. Washington, D. C., USA: U.S. Government Printing Office.
United States Department of the Interior. (1977). The A-B-C of desalting. Washington, D. C., USA: U.S. Government Printing Office.